Andrés S Rigual-Hernández

Thomas W Trull

Stephen G Bray

Ivia Closset

Leanne K Armand

Andrés S Rigual-Hernandez
email: andres.rigualhernandez@mq.edu.au

Seasonal dynamics in diatom and particulate export fluxes to the deep sea in the Australian sector of the southern Antarctic Zone

Keywords: diatom flux, particulate flux, sediment traps, Southern Ocean

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Diatoms are unicellular algae with an absolute requirement for silicic acid to form their frustules. They constitute a major component of phytoplankton communities, being responsible for ~40% of all marine carbon fixation [START_REF] Nelson | Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation[END_REF]. Diatoms are the main contributors to the silica-rich deposits in deep-sea sediments and are thought to influence the present and past global climate via their influence on the biological pump of CO 2 from the atmosphere into the ocean interior [START_REF] Nelson | Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation[END_REF][START_REF] Sarmiento | Simulated response of the ocean carbon cycle to anthropogenic climate warming[END_REF][START_REF] Matsumoto | Silicic acid leakage from the Southern Ocean: A possible explanation for glacial atmospheric pCO2[END_REF]. The composition of phytoplankton communities and abundance of diatoms within them are related to specific ecological parameters of the water masses where they live (e.g., temperature, sea-ice cover and nutrient availability), and hence in the case of diatoms, their frustules can be used as biotic proxies for palaeoenvironmental and palaeoceanographic reconstructions.

In order to evaluate the role of diatoms in the biological pump and the cycling of silicon, it is essential to thoroughly understand their ecology and the processes that the living biocoenoses undergo from their initial production in the euphotic zone to their eventual preservation in the ocean sediments (e.g. [START_REF] Varela | Biological fractionation of silicon isotopes in Southern Ocean surface waters[END_REF]Grigorov et al., 2014). This knowledge is also required to validate paleorecontructions based on the diatom sedimentary record (e.g. [START_REF] Leventer | Diatom Evidence for Late Holocene Climatic Events in Granite Harbor, Antarctica[END_REF][START_REF] Taylor | Postglacial marine diatom record of the Palmer Deep, Antarctic Peninsula (ODP Leg 178, Site 1098) 2. Diatom assemblages[END_REF][START_REF] Armand | Palaeo sea ice distribution and reconstruction derived from the geological record[END_REF].

The Southern Ocean is regarded as having one of the highest diatom biomasses of the global ocean. Despite its high-nutrient low chlorophyll (HNLC) regime, massive diatom blooms occur every year during spring and summer associated with specific areas, such as oceanographic fronts (e.g. [START_REF] Honjo | Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170 degrees W. Deep-Sea Res. (2 Top[END_REF]Moore and Abbott, 2002), coastal areas of Antarctica (e.g. [START_REF] Wefer | Seasonal particle flux in the Bransfield Strait, Antarctica[END_REF][START_REF] Bathmann | Short-term variations in particulate matter sedimentation off Kapp Norvegia, Weddell Sea, Antarctica: relation to water mass advection, ice cover, plankton biomass and feeding activity[END_REF][START_REF] Arrigo | Phytoplankton community structure and the drawdown of nutrients and CO 2 in the Southern Ocean[END_REF] and the retreating sea ice edge (e.g. [START_REF] Smith | Importance of ice edge phytoplankton production in the Southern Ocean[END_REF][START_REF] Sullivan | Phytoplankton standing crops within an Antarctic ice edge assessed by satellite remote sensing[END_REF]. As a result of this relatively high diatom productivity [START_REF] Pondaven | Resolving the `opal paradox' in the Southern Ocean[END_REF], large amounts of biogenic silica accumulate in the Southern Ocean sediments, mainly south of the Antarctic Polar Front (APF), where about 30% of the global opal marine accumulation occurs [START_REF] Tréguer | The world ocean silica cycle[END_REF].

Moored sediment traps are one of the few available tools for monitoring particle fluxes in the open ocean over extended periods of time. They provide a means to determine the magnitude and timing of phytoplankton blooms, document species succession and estimate the remineralization of labile components throughout the water column. The use of sediment traps has contributed significantly to our understanding of diatom ecology in the Southern Ocean and coastal Antarctic systems (e.g. [START_REF] Leventer | Diatom flux in McMurdo Sound, Antarctica[END_REF][START_REF] Wefer | Seasonal particle flux in the Bransfield Strait, Antarctica[END_REF][START_REF] Leventer | Factors influencing the distribution of diatoms and other algae in the Ross Sea[END_REF]Abelmann and Gersonde, 1991;[START_REF] Ishikawa | Variation in the diatom community under fast ice near Syowa Station, Antarctica, during the austral summer of 1997/98[END_REF][START_REF] Suzuki | Short-term variability in the flux of rapidly sinking particles in the Antarctic Marginal Ice Zone[END_REF][START_REF] Pilskaln | Geochemical particle fluxes in the Southern Indian Ocean seasonal ice zone: Prydz Bay region, east Antarctica[END_REF][START_REF] Ichinomiya | Temporal variations in the abundance and sinking flux of diatoms under fast ice in summer near Syowa Station, East Antarctica[END_REF]Romero and Armand, 2010). However, these studies are scattered in space and time, and large regions of the Southern Ocean, including the Australian Sector, remain undocumented.

During the Australian multidisciplinary ACE CRC SAZ Project (Trull et al., 2001a), the main hydrological zones of the Australian sector of the Southern Ocean were instrumented with sediment trap mooring lines. The central goal of this experiment was to determine the origin, composition and fate of particulate matter transported to the ocean interior. This research yielded important results, including the demonstration that particulate organic carbon (POC) export in the Southern Ocean is similar to the global ocean median [START_REF] Bray | SAZ Project Moored Sediment Traps: Results of the 1997-1998 Deployments[END_REF]Trull et al., 2001b).

Here, as part of the ACE CRC SAZ project, we report on the biogenic particle fluxes registered by two sediment traps deployed in the southern Antarctic Zone (60° 44.43´S; 139°E 53.97´S) over a year (November 2001to September 2002) in order to [START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF] document the magnitude, composition and seasonal distribution patterns of the settling particle fluxes, with particular focus on diatoms and their specific composition; and (2) assess the effects of dissolution and physical processes in the water column on the diatom assemblage composition by comparing the assemblages registered by the 2000 and 3700 m sediment traps. An improved understanding of diatom ecology and changes that the diatom assemblages undergo during their sinking through the water column should lead to a better interpretation of proxy records in the Southern Ocean.

Oceanographic setting

The Antarctic Circumpolar Current (ACC) flows eastward around Antarctica driven by strong westerly winds connecting the Pacific, Atlantic and Indian Oceans. Several circumpolar jets or fronts divide the ACC into distinct zones (Fig. 1a), each one characterized by specific hydrological and biochemical properties [START_REF] Orsi | On the meridional extent and fronts of the Antarctic Circumpolar Current[END_REF].

The fronts coincide with strong current cores of the ACC defined by contours of sea surface height (SSH). Each of these fronts consists of multiple branches or filaments, where their position varies rapidly over time [START_REF] Sokolov | Structure of southern ocean fronts at 140 E[END_REF], 2007and 2009a, b). From north to south, these fronts and zones are the Subtropical Front (STF), the Subantarctic Zone (SAZ), the Subantarctic Front (SAF), the Polar Frontal Zone (PFZ), the Polar Front (PF), the Antarctic Zone (AZ) and the Southern ACC Front (SACCF) (Sokolov and Rintoul, 2009a, b).

The surface waters of the Australian sector of the Southern Ocean are nitrate and phosphate rich and their concentrations remain fairly uniform across the ACC [START_REF] Bostock | A review of the Australian-New Zealand sector of the Southern Ocean over the last 30 ka (Aus-INTIMATE project)[END_REF]. In contrast, silicic acid (Si) content shows a marked south to north gradient. Highest Si concentrations are reached south of the Polar Front Zone (up to 70 μM), whereas the Subantarctic Zone waters exhibits low Si values (1 to 5 μM) [START_REF] Coale | Southern Ocean iron enrichment experiment: Carbon cycling in high-and low-Si waters[END_REF][START_REF] Bostock | A review of the Australian-New Zealand sector of the Southern Ocean over the last 30 ka (Aus-INTIMATE project)[END_REF]. Despite the relatively high macronutrient concentrations, Southern Ocean surface waters are often characterized by relatively low phytoplankton biomass. Light limitation related to deep mixing [START_REF] Sakshaug | Factors governing pelagic production in polar oceans[END_REF] and extremely low concentrations of trace metals such as iron [START_REF] De Baar | Importance of iron for phytoplankton blooms and carbon dioxide drawdown in the Southern Ocean[END_REF][START_REF] Johnson | What controls dissolved iron concentrations in the world ocean?[END_REF]Fitzwater et al., 2000;[START_REF] Martin | Iron deficiency limits phytoplankton growth in Antarctic waters[END_REF][START_REF] Boyd | Phytoplankton bloom upon mesoscale iron fertilisation of polar Southern Ocean waters[END_REF] seem to be the main causes for this "high-nitrate, low-chlorophyll" (HNLC) regime.

Sea ice seasonality off East Antarctica is considered linked to patterns of oceanic currents, which in turn are related to sea floor topography [START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF].

Seasonal sea-ice advance occurs from early autumn through early spring followed by retreat from late spring through summer [START_REF] Comiso | Concentration gradients and growth/decay characteristics of the seasonal sea ice cover[END_REF][START_REF] Kimura | Large-scale processes governing the seasonal variability of Antarctic sea-ice area[END_REF][START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF].

Our study site, station 61 S (60° 44.43´S; 139°E 53.97´S), is located within the southern Antarctic Zone (AZ-S; [START_REF] Parslow | A sub-surface chlorophyll maximum in the Polar Frontal Zone south of Australia: seasonal evolution and implications for phytoplankton-light-nutrient interactions[END_REF], between the southern branch of the PF (59°S) and the southern front of the SAACF [START_REF] Rintoul | A late winter Hydrographic section from Tasmania to Antarctica[END_REF][START_REF] Rintoul | Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3)[END_REF]. The mooring site is within the same region where the first open-ocean iron enrichment experiment in the Southern Ocean (Southern Ocean Iron Release Experiment -SOIREE) was conducted [START_REF] Boyd | Phytoplankton bloom upon mesoscale iron fertilisation of polar Southern Ocean waters[END_REF] and can be considered representative of the region between the PF and the SACCF (between 54°S and 62°S) (Trull et al., 2001c). Despite surface waters rich in macronutrients (i.e. silicate, phosphate and nitrate), the algal biomass accumulation is considered low (<0.5 μg/L) [START_REF] Parslow | A sub-surface chlorophyll maximum in the Polar Frontal Zone south of Australia: seasonal evolution and implications for phytoplankton-light-nutrient interactions[END_REF][START_REF] Popp | Controls on the carbon isotopic composition of Southern Ocean phytoplankton[END_REF]Trull et al., 2001c). Copepods, mainly large calanoid copepodites, dominate the zooplankton community at the study site. Grazing pressure is considered low (<1% of the phytoplankton standing stock removed per day) and is thought not to greatly influence the development of the annual bloom [START_REF] Zeldis | Mesozooplankton community composition, feeding, and export production during SOIREE[END_REF]. Very low iron concentrations (0.1-0.2 nM; [START_REF] Sohrin | The distribution of Fe in the Australian sector of the Southern Ocean[END_REF][START_REF] Boyd | Phytoplankton bloom upon mesoscale iron fertilisation of polar Southern Ocean waters[END_REF] appear to be responsible for the low primary production. The study area is far from the influence of coastal waters and just north of the maximum winter sea-ice extent (Fig. 1b; [START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF].

MATERIAL AND METHODS

Field experiment

Site 61 S was instrumented with a mooring line equipped with three McLane Parflux time series sediment traps [START_REF] Honjo | Large aperture time-series sediment traps; design objectives, construction and application[END_REF] placed at 1000, 2000 and 3700 m depth in a water column of 4393m (Fig. 1c). Each trap was paired with an Aanderaa current meter and temperature sensor. The trap sampling cups were filled with a buffered solution of sodium tetraborate (1 g L -1 ), sodium chloride (5 g L -1 ), strontium chloride (0.22 g L -1 ), and mercury chloride (3 g L -1 ). Cup rotation intervals were synchronized between traps and were established based on anticipated mass fluxes. The shortest sampling intervals were 8 days and correspond with the austral summer and autumn, whereas the longest interval was 55 days corresponding with austral winter (Table 1). No samples were recovered from the shallowest trap owing to equipment malfunction. The two deeper traps completed their collection sequence as programmed without any instrumental failures providing a continuous time-series for 317 days (November 30, 2001to September 29, 2002) divided into 21 collecting intervals. Owing to the low particle fluxes registered at the onset and end of the experiment insufficient material remained for diatom analysis of cup 1 of the 2000 m trap and cups [START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF]2,19,20 and 21 of the 3700 m trap (Table 1). After recovery, sediment trap cups were removed, capped on board and stored at 4°C in the dark until they were processed. The original samples were sieved through a 1mm nylon mesh in order to remove the largest swimmers, and only the fraction <1 mm was analyzed. Then, they were split into 10 equal fractions using a McLane WSD-10 wet-sample divider. One complete split was used for microplankton analysis. A detailed description of the geochemical analytical procedures is given by Trull et al. (2001b) and [START_REF] Bray | SAZ Project Moored Sediment Traps: Results of the 1997-1998 Deployments[END_REF]. Component fluxes are reported for individual cups along with average values over the collection or deployment period for each component (Table 1). As the collection period was shorter than a calendar year, annual mean estimates were determined and are presented in Table 1. These annual estimates take into account the fact that the unobserved days occurred in winter when fluxes were low, and were obtained by using the flux for the last winter cup (#21 in 2002) to represent mean daily fluxes during the unobserved period.

In order to investigate the correlation between time series, a correlation matrix was calculated (Table 2).

Siliceous microplankton sample preparation and analysis

Each diatom fraction sample was refilled with distilled water to 40 ml, from which 10 ml was subsampled and buffered with a solution of sodium carbonate and sodium hydrogen carbonate (pH 8) and kept refrigerated for future calcareous nannoplankton analysis. The remaining 30 ml was treated with hydrogen peroxide, potassium permanganate and concentrated hydrochloric acid in order to clean the sample of organic matter and calcareous components following the methodology of [START_REF] Romero | Variability in export production documented by downward fluxes and species composition of marine planktonic diatoms: observations from the tropical and equatorial Atlantic[END_REF][START_REF] Romero | Siliceous phytoplankton of the western equatorial Atlantic: sediment traps and surface sediments[END_REF]. The resulting sediment slurry was stored in bottles filled with distilled water. Microscopic slides were prepared following the decantation method outlined by Flores and Sierro (1997). This method produces random settling of the diatoms for quantitative microscopic purposes. The dried cover-slip was permanently mounted onto a glass slide with Norland optical adhesive 61 mounting medium (Refractive index:

1.56).

Qualitative and quantitative analyses were performed using a Olympus BH-2 compound light optical microscope at 1000x magnification with phase-contrast illumination. A minimum of 400 specimens were counted per sample. Each diatom valve was identified to the lowest taxonomic level possible. Scanning Electron Microscope observations of selected samples were used to verify taxonomic identifications made with the LM. The recommendations of [START_REF] Schrader | Diatoms and silicofagellates. Micropaleontological counting methods and techniques: an exercise on an eight metres section of the Lower Pliocene of Capo Rosello, Sicily[END_REF] were used as a basis for the counting of diatom valves. We did not include the counts of the girdle bands of Dactyliosolen antarcticus in the determination of relative abundances.

The microplankton counts were transformed into daily fluxes of specimens m -2 d -1 following the formula of [START_REF] Sancetta | The annual cycle of sedimentation in Saanich Inlet, British Columbia: implications for the interpretation of diatom fossil assemblages[END_REF] and [START_REF] Romero | Seasonal and interanual dynamics in diatom production in the Cariaco Basin, Venezuela[END_REF]:

𝐹 = 𝑁𝑁 � 𝐴 𝑎 � 𝑁 𝑉 𝑁 𝑆 𝑑 𝑁 𝑇
where "F" is the daily flux of specimens, "N" the number of specimens, "A" the total area of a Petri dish, "a" the analysed area, "V" the dilution volume, "S" the split of the cup, "d" the number of days of collection and "T" the aperture area of the sediment trap.

The diatom species diversity was estimated applying Shannon's diversity index (H´) to the relative abundance data (H´= -∑ p i log 2 p i , where p = n1/N, n1 = number of specimens of one taxon and N the total number of specimens). Large values of H indicate greater diversity.

The annualized diatom valve flux for the 2000 m sediment trap was estimated following the same procedure as for the component fluxes (Table 1).

The temporal overlap of diatom fluxes between 2000 m and the 3700 m traps occurred between December-May (172 days) (Table 1). This overlap was used to estimate the sinking velocities of diatom valves and to track the alteration of settling diatom assemblages between traps. The sinking settling velocities for the main diatom taxa were calculated from the time lag between associated peaks in both traps and the vertical distance between traps. We used a squared-chord distance as the metric for assessing the similarity between the diatom assemblages of both traps [START_REF] Ortiz | Comparison of Imbrie-Kipp transfer function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas[END_REF]. Squared chord distance values can range between 0.0 and 2.0, with 0.0 indicating identical proportions of species within the assemblages being compared.

Satellite imagery, meteorological and oceanographic data

Weekly mean sea surface temperature (SST) data for the period 2001 to 2002 was extracted from the NOAA Optimum Interpolation Sea Surface Temperature Analysis database [START_REF] Reynolds | An improved in situ and satellite SST analysis for climate[END_REF]. The monthly averages of the upper 300 m thermal structure for the 2001-2002 period (Fig. 2) were obtained for the sampling location from the World Ocean Atlas 2009 (Locarnini et al., 2010). The maximum weekly SST mean during the field experiment was 2.94 °C degrees occurring in March 2002, whereas the minimum was 0.12 °C at the beginning of October 2002. Changes of the weekly mean SST mirrored the seasonal changes in the vertical structure of the water column temperature profile. A pattern of stratification during austral summer months and vertical homogeneity in autumn and winter was noted during the field experiment (Fig. 2).

Satellite-derived monthly chlorophyll-a concentration and photosynthetically active radiation (PAR) estimates were obtained from NASA's Giovanni program (Acker and Leptoukh, 2007) (Fig. 2). The pigment concentration record was then used as a proxy for surface algal biomass accumulation. The overall chlorophyll-a levels were low (0.07-0.30 mg/m 3 ) and similar to previous observations in the study area (Trull et al., 2001c). The increase in the algal biomass commenced in September 2001 and reached its highest levels in November 2001, coinciding with a maximum in the insolation levels (Fig. 2). Pigment concentration declined through summer and reached its lowest levels in autumn and winter (March to August 2002).

RESULTS

Bulk composition

Total fluxes of particulates at both traps were highly seasonal, with maxima registered during the austral summer (up to 1151 mg m -2 d -1 at 2000 m and 1157 mg m -2 d -1 at 3700 m) and almost negligible fluxes during winter (up to 42 mg m -2 d -1 at 2000 m and below detection limits at 3700 m) (Fig. 2). The fluxes of total particulates at 2000 and 3700 m depth showed similar seasonal variations and were closely correlated (R 2 = 0.82; Fig. 4). Particulate fluxes were slightly higher at 2000 m (261 ± mg m -2 d -1 = deployment average ± standard deviation) than at 3700 m (216 ± 337 mg m -2 d -1 ). Biogenic silica (SiO 2 ) was the dominant bulk component, regardless of the sampling period or depth (deployment average = 76% at 2000 and 78% at 3700 m). The highest relative contribution of opal was registered from the end of summer through early-autumn at both depths (Table 1). Secondary contributors were carbonate (CaCO 3 ) (7% at 2000 m and 9% at 3700 m) and particulate organic carbon (POC) (1.4% at 2000 m and 1.2% at 3700 m). The relative concentration of carbonate and POC was at its highest in austral spring and summer (Table 1). The fluxes of all components were closely correlated. The results of the correlation matrix are given in Table 2. Total mass flux showed a strong correlation with POC at both sediment traps (R 2 = 0.86 at 2000 m and R 2 = 0.87 at 3700 m). BSiO 2 fluxes were also highly correlated with POC at both depths (R 2 = 0.83),

whereas CaCO 3 and POC fluxes exhibited the lowest values (R 2 = 0.24 at 2000 m and R 2 = 0.63 at 3700 m).

The molar POC/PN ratios were relatively low (Table 1), and thus similar to surface Southern Ocean particles and phytoplankton [START_REF] Copin-Montegut | Stoichiometry of carbon, nitrogen, and phosphorus in marine particulate matter[END_REF].

Composition of the biogenic opal fraction

The biogenic opal fraction was composed of diatoms, silicoflagellates, radiolarians and the dinoflagellate Actiniscus pentasterias. Diatoms were, numerically, the dominant siliceous plankton group registered by the traps with a mean flux between 67 x 10 6 and 76 x 10 6 valves m -2 day -1 at 2000 m (annualized mean and deployment average, respectively). Mean diatom flux at the 3700 m trap yielded higher values (deployment average = 132 x 10 6 valves m -2 day -1 ) due to the lower sampling duration (172 days) over the winter season (Table 1). Silicoflagellates and radiolarians were three and four orders of magnitude lower than diatoms (not shown), whereas only one specimen of A.

pentaserias was indentified in the lower trap. At both depth levels, total diatom fluxes showed strong seasonal variations that closely followed total mass seasonality with

Pearson correlation coefficients of r = 0.88 for 2000 m and r = 0.81 for 3700 m (Fig. 4).

Diatom frustules from 61 taxa were identified over the entire experiment and are listed in Table 3 together with their relative contribution for the whole sampling period. The seasonal changes in the diatom flux and main species at mooring site 61°S are plotted in Figure 5.

Diversity index values (H´) followed the same seasonal trend as total diatom flux at both depths, with highest values registered during austral summer and lowest during winter (Fig. 5). The squared-chord distance between the sediment trap diatom assemblages at 2000 and 3700 m was 0.003, indicating highly similar proportions of species within depths.

The dominant species of the diatom assemblage was Fragilariopsis kerguelensis with a mean flux between 53 x 10 6 and 60 x 10 6 valves m -2 day -1 at 2000 m (annualized mean and deployment average, respectively). This species contribution ranged from 37% to 96% of the total assemblage at 2000 m (average relative contribution for the overlapping period between traps = 72%; Table 3) and from 31% to 82% at 3700 m (average relative contribution for the overlapping period between traps = 68%) (Table 3). The diatom flux peaks at both depths can be attributed to an increased flux of F.

kerguelensis, with exception to a peak in late January at 2000 m when Fragilariopsis pseudonana dominated the settling assemblage (39%). Secondary contributors to the diatom assemblage at 2000 and 3700 m were Thalassiosira lentiginosa (average relative contribution for the overlapping period between traps = 5% and 7%, respectively), Thalassiosira gracilis var. gracilis (6% and 7%), Fragilariopsis separanda (3% and 2%), Fragilariopsis pseudonana (4% and 1%), Fragilariopsis rhombica (2% and 2%),

Fragilariopsis curta (1% and 1%) and Azpeitia tabularis (1% and 1%) (Table 3 and Fig. 5). It is worth noting that some large and/or entangled frustules of some diatom taxa, such as Thalassiothrix [START_REF] Hallegraeff | Taxonomy and morphology of the marine plankton diatoms Thalassionema and Thalassiothrix[END_REF], could have been be retained in the 1 mm screen mesh, and therefore, underrepresented in this study.

Scanning electron microscope (SEM) pictures of some of the most relevant taxa are shown in Fig. 6.

Diatom settling velocities

The calculation of the sinking velocities of the main diatom taxa was only possible for the "particle bloom" period, i.e. from December to March, when distinct peaks were registered in both sediment traps (Fig. 5). The precision of the calculations is limited by the duration of the sampling intervals during this period (8 days). The majority of the taxa exhibited an offset of a single sampling interval (8 days) between the 2000 and 3700 m traps, suggesting and average settling speed of 210 m d -1 .

DISCUSSION

Quality check of downward particle fluxes

The use of sediment traps has greatly enhanced our understanding of particle transfer in open ocean environments. However, laboratory and field experiments have shown that the measurement of downward particle fluxes can be subject to several hydrodynamic biases (e.g. Gardner, 1980;[START_REF] Baker | Field assessment of sediment trap efficiency under varying flow conditions[END_REF][START_REF] Yu | Trapping efficiency of bottom-tethered sediment traps estimated from the intercepted fluxes of 230 Th and 231 Pa Deep-Sea Res[END_REF]. Therefore, an assessment of the trapping efficiency is needed prior to interpreting the results of any sediment trap experiment.

Our mooring line at site 61°S was maintained taut by the distribution of floats along the line and at the mooring head. Measured mean current speeds at both trap levels were always lower than 10 cm s -1 that is, as a rule of thumb, considered the threshold at which trapping efficiency decreases significantly (e.g. [START_REF] Baker | Field assessment of sediment trap efficiency under varying flow conditions[END_REF][START_REF] Honjo | A sediment trap intercomparison experiment in the Panama Basin, 1979[END_REF][START_REF] Yu | Trapping efficiency of bottom-tethered sediment traps estimated from the intercepted fluxes of 230 Th and 231 Pa Deep-Sea Res[END_REF][START_REF] Heussner | Spatial and temporal variability of downward particle fluxes on a continental slope: Lessons from an 8-yr experiment in the Gulf of Lions (NW Mediterranean)[END_REF]. On the basis of global comparisons, these conditions suggest that fluxes registered by the traps were unlikely to be significantly biased by under-or over-trapping [START_REF] Honjo | Fluxes of particles to the interior of the open oceans[END_REF][START_REF] Yu | Trapping efficiency of bottom-tethered sediment traps estimated from the intercepted fluxes of 230 Th and 231 Pa Deep-Sea Res[END_REF].

Variability of total mass flux and composition of particles

The seasonal variability of the vertical particle transfer at site 61°S appears principally controlled by seasonal changes in the euphotic zone productivity. A significant increase in chlorophyll-a concentration from October 2001, approximately two months before any significant warming or stratification occurs (Fig. 2), is in line with an increase in incident insolation from the beginning of spring. Taking into account that pigment concentration at the sea surface reached its highest levels during November 2001 (Fig.

2), and that maximum total particle and diatom fluxes were registered at the beginning of January 2002 (Figs 3 and 5), a time lag of about two months between peak production in the surface waters and onset of particle export in the study area can be assumed. This feature is in agreement with the observations of [START_REF] Buesseler | Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 170°W[END_REF] in the Pacific Sector of the Southern Ocean who reported a similar delay in the delivery of the surface bloom to the ocean interior. Contrastingly, algal biomass reached its annual minimum during the austral winter (June-September) (Fig. 2) and very low particle and diatom export fluxes were registered by the traps (Fig. 3 and5). These low chlorophylla concentration and flux values appear to be driven by two factors: (i) insufficient sunlight as a result of the low solar angle and shortened day length that reduced the ability to increase biomass and (ii) intense vertical mixing that transported phytoplankton below their critical depth, i.e. depth at which the rate of photosynthesis equals the rate of respiration (Fig. 2).

In terms of particle composition, silica-rich and carbonate-poor particulate fluxes registered by the traps mirrored the dominance of diatoms in the waters south of the Antarctic Zone, south of Tasmania. The composition of the settling material is consistent with that of the surface sediments in the region. Opal content in the surface sediments of the Southern Ocean increases from north to south, being the dominant component south of the APF. In contrast, CaCO 3 is dominant north of the SAF and decreases southward [START_REF] Honjo | Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170 degrees W. Deep-Sea Res. (2 Top[END_REF].

Total mass and POC fluxes provided the strongest correlation at both sediment traps (Table 2). As BSiO 2 dominated the mass fluxes throughout the sampling period, changes in the BSiO 2 flux directly affected POC fluxes, suggesting that diatom valve sedimentation plays an important role in controlling the organic carbon export to depth at the AZ site. On the other hand, the small CaCO 3 fluxes and lower correlation values with POC at both sediment traps suggest that CaCO 3 had a lesser influence on the POC export. However, as the carrying capacity of POC per CaCO 3 unit could not be quantified, the role of CaCO 3 in controlling the transfer of organic carbon to depth remains unknown.

Seasonal trend of diatom fluxes and species composition

The annual diatom fluxes registered by the 2000 m sediment trap (Table 1) fall within the range of those estimated by Fischer et al. (2002) in the AZ of the eastern Atlantic (20 x 10 6 valves m -2 d -1 ) and those reported by Grigorov et al. (2014) in the Ross Sea Gyre (93 x 10 6 valves m -2 day -1 ). The diatom fluxes recorded at site 61°S are therefore comparable in magnitude to previously reported diatom data sets of the AZ in the Southern Ocean.

Although several factors, such as grazing, dissolution and lateral advection, can influence diatom flux [START_REF] Boltovskoy | Annual flux of Radiolaria and other shelled plankters in the eastern equatorial Atlantic at 853 m: seasonal variations and polycystine species-specific responses[END_REF], the similar seasonal patterns, with a two month offset between primary productivity and diatom flux variations (Figs. 2 and5a)

suggest that the primary signal of the phytoplankton bloom in the overlying water masses is registered by the traps. About two-thirds of the annual diatom export fluxes at each trap depth were registered during January and February (Fig. 5). A markedly seasonal pattern is characteristic of high latitude systems (e.g. [START_REF] Wefer | Seasonal particle flux in the Bransfield Strait, Antarctica[END_REF]Dunbar et al., 1998;[START_REF] Honjo | Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170 degrees W. Deep-Sea Res. (2 Top[END_REF]Fischer et al., 2002;[START_REF] Pilskaln | Geochemical particle fluxes in the Southern Indian Ocean seasonal ice zone: Prydz Bay region, east Antarctica[END_REF]Romero and Armand, 2010) and illustrates the opportunistic character (r selection) of the dominant species during the summer bloom. As a consequence of this rapid diatom biomass increase, silica in the AZ-S is often stripped out the mixed layer by midsummer (Trull et al., 2001c).

The temporal variations in the composition of diatom assemblages mirrored the changes of the hydrographic conditions in the AZ-S, south of Tasmania. Overall, the major diatom taxa recorded at our mooring site are typical of living and fossil assemblages representing open ocean waters of the Antarctic Zone (Romero and Armand, 2010;Crosta et al., 2005, respectively).

The seasonal diatom flux was mainly driven by changes in the flux of the large, heavilysilicified and bloom-forming F. kerguelensis, a prominent member of the diatom assemblages in the iron-limited Southern Ocean waters [START_REF] Smetacek | Diatoms and the ocean carbon cycle[END_REF]Abelmann et al., 2006). Large abundances of F. kerguelensis in phytoplankton blooms have also been previously reported in our study area during the SOIREE experiment (Gall et al., 2001;[START_REF] Trull | Insights into Southern Ocean Carbon export from the δ 13 C of particles and dissolved inorganic carbon during the SOIREE iron release experiment[END_REF], in the AZ of the south-west Atlantic [START_REF] Hart | On the phytoplankton of the South-West Atlantic and the Bellinghausen Sea, 1929-31[END_REF] and in the vicinity of open-ocean fronts in the Atlantic and Pacific sectors (Bathmann et al., 1997;[START_REF] Laubscher | Phytoplankton production and biomass at frontal zones in the Atlantic Sector of the Southern Ocean[END_REF]Grigorov et al., 2014). The relative contribution of F.

kerguelensis at both 61 S traps (68-80%; Table 1) is consistent with its distribution in the Southern Ocean surface sediments where its maximum abundances (70-83%) are found between the Polar Front and the maximum summer sea-ice edge [START_REF] Crosta | The biogeography of major diatom taxa in Southern Ocean sediments: 2. Open ocean related species[END_REF]. ) and in the Ross Sea Gyre (22%; 20 x 10 6 valves m -2 d -1 ), respectively. However, these sites were under the influence of seasonal ice, where seaice affiliated taxa such as smaller Fragilariopsis species and Chacetoceros spp., are known to often dominate the diatom assemblages [START_REF] Armand | The biogeography of major diatom taxa in Southern Ocean surface sediments: 1. Sea ice related species[END_REF].

At the onset of the summer particle "bloom", H´ rises significantly due to the burst of reproduction and sedimentation of most of the diatom taxa (Fig. 5a). The small and rapidly dividing diatoms Chaetoceros group, Fragilariopsis pseudonana, Fragilariopsis rhombica, Fragilariopsis separanda and Pseudo-nitzschia spp., together with the cool open ocean species Thalassiosira gracilis, are major contributors to the bulk of the spring-summer maximum (Fig. 5). The majority of these diatoms correspond to the Group 1 defined by [START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF], consisting of slightly silicified and fast-growing species that undergo rapid species succession during productive periods. The biomass accumulation of such diatoms is thought to be controlled principally by nutrient availability rather than grazing pressure. This concept agrees well with the observations of [START_REF] Zeldis | Mesozooplankton community composition, feeding, and export production during SOIREE[END_REF] who reported low grazing impact over the bloom development during the SOIREE experiment. At the end of their growth season, these bloom-forming species are considered to undergo mass mortality resulting in the formation of aggregates that rapidly sink from the euphotic zone [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF].

The AZ-S site is remote from the direct influence of sea-ice in summer [START_REF] Government Printing | Change and Variability in East Antarctic Sea Ice Seasonality[END_REF]Fig. 1b), which makes the occurrence of the sea-ice affiliated diatoms

Fragilariopsis cylindrus and Fragilariopsis curta during January (Fig. 5b) puzzling.

The distribution of F. cylindrus in the modern Southern Ocean is constrained to the north by the maximum summer sea ice edge [START_REF] Semina | SEM-studied diatoms of different regions of the World Ocean[END_REF][START_REF] Armand | The biogeography of major diatom taxa in Southern Ocean surface sediments: 1. Sea ice related species[END_REF]Esper et al. 2010), whereas F. cylindrus appears limited by the maximum extension of sea ice during winter [START_REF] Armand | The biogeography of major diatom taxa in Southern Ocean surface sediments: 1. Sea ice related species[END_REF]Esper et al., 2010). Likely explanations for the presence of these species at site 61°S are either (i) the occurrence of an iceberg in the vicinity of our study area or (ii) the advection of a transient bloom produced in a region under the influence of sea ice. Rich sea-ice affiliated diatom communities have been found in association with free-drifting icebergs (e.g. [START_REF] Smith | Carbon export associated with free-drifting icebergs in the Southern Ocean[END_REF], however this explanation is unlikely since our site is remote from any known iceberg pathway (Gladstone et al., 2001). Taking into account a time lag of about two months between peak production and particle export, a healthy and neutrally buoyant bloom could cover a much larger distance than that estimated for a sinking particle (detailed in 4.5). Sea-ice coverage west of ~90°E can reach latitudes well above 61°S (Massom et al., 2013; Fig. 1b). We hypothesize that the pulses of F. curta and F. cylindrus could correspond to the sedimentation of a diatom bloom that was either advected from an area upstream the ACC under the influence of seasonal sea ice or transported northward by Antarctic Surface Water currents (AASW) produced by the seasonal retreat of the sea ice, south of our study.

Enhanced fluxes of the large and thick-walled F. kerguelensis persisted for a longer period (until April) than those of Group 1 (sensu Quéguiner, 2013) most likely contributing to the progressive reduction of silicic acid concentrations in the mixed layer throughout the summer. A similar pattern was reported by [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] both inside and outside the iron-fertilized bloom during the European Iron Fertilization Experiment (EIFEX). Fragilariopsis kerguelensis is considered a "sinking-silica species" [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF] largely responsible for the decoupling of silicon and carbon cycles in the iron-limited ACC.

At the end of the diatom export maximum (i.e. February-March), Thalassiothrix antarctica contributed more significantly to the flux via increased abundances (Fig. 5b).

This species is a large, slow-growing diatom distributed within discrete layers of the water column [START_REF] Kemp | The "Fall dump" a new perspective on the role of a "shade flora" in the annual cycle of diatom production and export flux[END_REF]. Such diatoms are considered k-strategists [START_REF] Kemp | High diatom production and export in stratified waters -A potential negative feedback to global warming[END_REF] and fall within Group 2 defined by [START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF]. High abundances of Thalassiothrix antarctica have been reported within and south of the APF (Laubscher et al., 1995;[START_REF] Bracher | Primary production, physiological state and composition of phytoplankton in the Atlantic Sector of the Southern Ocean[END_REF] and associated to a subsurface chlorophyll maximum in the PFZ south of Tasmania [START_REF] Kopczynska | Phytoplankton and microzooplankton variability between the Subtropical and Polar Fronts south of Australia: Thriving under regenerative and new production in late summer[END_REF][START_REF] Parslow | A sub-surface chlorophyll maximum in the Polar Frontal Zone south of Australia: seasonal evolution and implications for phytoplankton-light-nutrient interactions[END_REF]Gomi et al., 2010) and in Prydz Bay [START_REF] Quilty | A seasonally recurrent patch of Antarctic planktonic diatoms[END_REF]. We suspect that Thalassiothrix antarctica develops in parallel to Group 1 diatoms, as a result of them inhabiting the pycnocline discontinuity, exploiting deep nutrients and undergoing low rates of primary productivity under low-light conditions [START_REF] Kemp | The "Fall dump" a new perspective on the role of a "shade flora" in the annual cycle of diatom production and export flux[END_REF][START_REF] Kemp | Production of giant marine diatoms and their export at oceanic frontal zones:implications for Si and C flux from stratified oceans[END_REF][START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF].

The abrupt drop of the photosynthetically active radiation levels to less than half their peak values from February to March (Fig. 2a) and/or the occurrence of a vertical mixing event may have produced both light and/or nutrient limitation leading to the rapid flocculation and sinking of T. antarctica during the summer-autumn transition.

Post-March diatom export decreases significantly (Fig. 5a) due to the reduction to lowest levels of light and the concomitant intensification of mixing of the water column (Fig. 2a,b). The "post-bloom" diatom assemblage is characterized by low diversity values (Fig. 5a) mainly due to the high relative abundance of F. kerguelensis (up to 96% in September). The remaining assemblage is subsequently composed of large and heavily silicified centrics such as Thalassiosira lentiginosa, Azpeitia tabularis and Thalassiosira oliveriana. These are typical open ocean diatoms [START_REF] Crosta | The biogeography of major diatom taxa in Southern Ocean sediments: 2. Open ocean related species[END_REF][START_REF] Romero | The biogeography of major diatom taxa in Southern Ocean sediments: 3. Tropical/Subtropical species. Palaeogeogr., Palaeoclimatol[END_REF] with presumed lower nutrient requirements than the summer bloom-forming species.

Transfer to depth

Our results suggest that a fast and relatively undisturbed downward transport of particles occurred between 2000 and 3700 m (Fig. 3 and4). The similar BSi and POC content in both traps (Table 1) indicates that little silica dissolution and remineralization occurred between 2000 and 3700 m. Such a result is consistent with previous sediment trap studies that reported minimal alteration of the silica and organic carbon fluxes below the mesopelagic-bathypelagic boundary (~2000 m) [START_REF] Takahashi | Seasonal fluxes of pelagic diatoms in the subarctic Pacific, 1982-1983[END_REF][START_REF] Honjo | Biological pump and particulate fluxes[END_REF]. However, the high Si/C at both depth levels also implies that organic matter is recycled faster than opal before reaching the traps, and this must occur at mesopelagic depths. We interpret this as the remineralization of the settling material by the microbial community and by the zooplankton that preferentially feed on organic matter [START_REF] Honjo | Biological pump and particulate fluxes[END_REF]. High Si/C ratios are characteristic of the iron-limited systems of the Southern Ocean where large and highly silicified species (e.g. Fragilariopsis kerguelensis)

dominate the diatom assemblages. In this regard, the average POC content (1.4%) of our samples at 2000 m is very low, and the average BSi/POC molar ratio for the entire collection period was less than 0.1 for both traps, in comparison to an average of ~1 for a compilation of deep Southern Ocean traps [START_REF] Honjo | Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesis of global sediment trap programs since 1983[END_REF] and a median of ~2 for a global compilation (Lampitt and Antia, 1998). This makes it clear that the trap samples have experienced very strong losses of organic carbon, consistent with previous studies suggesting that Southern Ocean waters south of the Polar Front can be described as low carbon export regimes [START_REF] Lam | High biomass low export regimes in the southern ocean[END_REF].

The sinking velocities of the major diatom taxa during the "particle bloom" indicate rapid sedimentation of the diatom assemblages between the upper and lower traps (210 m d -1 ). These settling speeds are very similar to those estimated by [START_REF] Honjo | Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170 degrees W. Deep-Sea Res. (2 Top[END_REF] in the AZ of the Pacific Sector and are equal, or greater than, those observed in high productivity areas at lower latitudes [START_REF] Honjo | Fluxes of particles to the interior of the open oceans[END_REF]. In contrast, based on the time delay between samples with similar silicon isotopic signatures at different depths at site 61 S,

Closet et al. (unpublished results

) estimated the settling velocities outside the production period to be 120 m d -1 or less. These observations suggests that sinking rates of diatom valves at site 61 S are related to flux size, perhaps because higher fluxes lead to the formation of fast-sinking aggregates (e.g. [START_REF] Alldredge | Direct observations of the mass flocculation of diatom blooms: characteristics, settling velocities and formation of diatom aggregates[END_REF]. This concept is consistent with the observations of Grigorov et al. ( 2014) on sediment trap material from the Pacific Sector of the Southern Ocean who reported abundant diatom aggregates at times of peak flux and mainly individual cells and small chains outside the production period. Moreover, the formation of aggregates would also explain the enhanced POC export (Table 1) in association with high fluxes of Group 1 diatoms during the production period [START_REF] Assmy | Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current[END_REF].

The pulse of Thalassiothrix antarctica in early March at both depths coincides with an upturn of the POC fluxes (Table 1). Thalassiothrix antarctica cells are about 60 times larger (in estimated biovolume) than F. kerguelensis [START_REF] Cornet-Barthaux | Biovolume and biomass measurements of key Southern Ocean diatoms[END_REF], and therefore, have the potential to contribute significantly to the annual organic carbon export even at background concentrations (Goldman 1993;Goldman and McGillicuddy, 2003;[START_REF] Quéguiner | Iron fertilization and the structure of planktonic communities in high nutrient regions of the Southern Ocean[END_REF].

During winter, the lower sinking rates and high area/volume ratios of single diatom cells may facilitate the recycling of the valves and remineralization of organic matter in the upper water column. The enrichment of dissolution-resistant diatoms (e.g.

Fragilariopsis kerguelensis, Thalassiosira oliveriana, Azpeitia tabularis) during the winter months observed in our traps could, therefore, be partially related to enhanced, selective dissolution of more lightly silicified species (e.g. Pseudo-nitzschia spp.,

Fragilariopsis pseudonana, Chaetoceros spp.).

With regard to the comparison of the annual diatom assemblage between traps, their highly similar abundance proportions, as revealed by the low squared chord distance score, indicate that both traps registered diatom assemblages from the same source.

Moreover, the latter results further support the idea that silica BSi dissolution below 2000 m is minimal at the AZ-S site.

CONCLUSIONS

The main objective of our study was to document the variability in the magnitude, Annual flux 69 g m -2 y -1 53 g m -2 y -1 6 g m -2 y -1 0.9 g m -2 y -1 23 10 9 valves m -2 y -1

  Fischer et al. (2002) andGrigorov et al. (2014) reported lower relative abundance and fluxes of this species in sediment traps in the AZ of the Atlantic sector (29%; 6 x 10 6 valves m -2 d -1

Figure 1 .

 1 FIGURE CAPTIONS

Figure 2 :

 2 Figure 2: (a) Monthly mean chlorophyll-a concentration (mg m -3 ), sea surface temperature (SST) (°C) and photosynthetically active radiation (Einstein m -2 d -1 ) for the

Figure 3 .

 3 Figure 3. Temporal variability of the total mass and major biogenic fluxes for <1 mm

Figure 4 .

 4 Figure 4. Correlation between (a) mass flux (mg m -2 day -1 ) at 2000 and 3700 m, and (b) mass flux (mg m -2 day -1 ) and diatom valve flux (10 6 m -2 day -1 ) at 2000 and 3700 m.

Figure 5 .

 5 Figure 5. Seasonal variation of (a) total diatom flux and Shannon's diversity index (H´)

Figure 6 :

 6 Figure 6: SEM photos of some of the most relevant diatom taxa collected by the

Table 1 :Table 3 :

 13 Daily export fluxes of total mass flux, biogenic silica (BSiO 2 ), calcium carbonate (CaCO 3 ), particulate organic carbon (POC) and diatom valves registered at the 61°S site from November 2001 through October 2002. Mass fluxes listed as zero were too small to measure (<1 mg). *Average diatom fluxes for the 2000 and 3700 m traps have been estimated for different sampling intervals (309 and 172 days, respectively). Table 2: Correlation matrix (R 2 ) for the total mass and bulk compound fluxes at both sediment traps. Integrated fractional abundances of the diatom taxa found at the 2000 and 3700 m sediment traps at station 61°S. REFERENCES Abelmann, A., Gersonde, R. (1991) Biosiliceous particle flux in the Southern Ocean, Mar. Chem., 35, 503-536. Abelmann, A., Gersonde, R., Cortese, G., et al. (2006) Extensive phytoplankton blooms in the Atlantic Sector of the glacial Southern Ocean, Paleoceanography, 21, PA1013 Acker, J.G. and Leptoukh, G. (2007) Online analysis enhances use of NASA Earth science data. Eos, Transactions. AGU, 88, 14-17. Dunbar, R.B., Leventer, A.R. and Mucciarone, D.A. (1998) Water column sediment fluxes in the Ross Sea, Antarctica: atmospheric and sea ice forcing. J. Geophys. Res. 103 (30) 741-59 Esper, O., Gersonde, R., Kadagies, N. (2010). Diatom distribution in southeastern Pacific surface sediments and their relationship to modern environmental variables. Palaeogeogr., Palaeoclimatol., Palaeoecol., 287, 1-27. Fischer, G., Fütterer, D., Gersonde, R., et al. (1988) Seasonal variability of particle flux in the Weddell Sea and its relation to ice cover. Nature, 335, 426-8. Fischer, G., Gersonde, R., Wefer, G. (2002) Organic carbon, biogenic silica and diatom fluxes in the marginal winter sea-ice zone and in the Polar Front Region: interannual variations and differences in composition. Deep-Sea Res. (2 Top. Stud. Oceanogr.), 47, 1721-1745. Fitzwater, S.E. Johnson, K.S., Gordon, et al. (2000) Trace metal concentrations in the Ross Sea and their relationship with nutrients and phytoplankton growth. Deep-Sea Res. (2 Top. Stud. Oceanogr.), 47, 3159-3179. Flores, J.A., Sierro, F.J. (1997) A revised technique for the calculation of calcareous nannofossil accumulation rates. Micropal., 43, 321-324. Gall, M.P., Boyd, P.W., Hall, J., et al. (2001) Phytoplankton processes. Part 1: Community structure during the Southern Ocean Iron Release Experiment (SOIREE) Deep-Sea Res. (1 Oceanogr. Res. Pap.), 48, 2551-2570. Gardner, W.D. (1980) Sediment trap dynamics and calibration: a laboratory evaluation. J. Mar. Res., 38, 17-39. Gardner, W.D. (1985). The effect of tilt on sediment trap efficiency. Deep-Sea Res. (1 Oceanogr. Res. Pap.) 32, 349-361. Gladstone, R.M., Bigg, G.R., Nicholls, K.W. (2001) Iceberg trajectory modelling and meltwater injection in the Southern Ocean. J. Geophys Res., 106 (9), 19903-19915. Goldman, J.C. (1993) Potential role of large oceanic diatoms in new primary production. Deep-Sea Res. (1 Oceanogr. Res. Pap.) 40 (1), 159-168. Goldman, J.C., McGillicuddy, D.J. (2003) Effect of large marine diatoms growing at low light on episodic new production. Limnol. Oceanogra., 48 (3), 1176-1182. Gomi, Y., Fukuchi, M., Taniguchi, A. (2010) Diatom assemblages at subsurface chlorophyll maximum layer in the eastern Indian sector of the Southern Ocean in summer. J. Plank. Res. 32 (7), 1039-1050. Grigorov, I., Rigual-Hernandez, A.S., Honjo, S., et al. (2014) Settling fluxes of diatom frustules to the interior of the Antarctic Circumpolar Current along 170 o W. Deep-SeaRes.(1 Oceanogr. Res. Pap.), 93,1-13.

  Figure 1

Figure

  Figure(s) Click here to download Figure(s): Fig. 5. Black and white.eps

Figure

  Figure(s) Click here to download Figure(s): Fig. 5. Colour.eps

Figure

  Figure(s) Click here to download Figure(s): Fig. 6.eps

Table 1

 1

	BSiO 2	CaCO 3	POC

Table ( s) Click here to download Table(s): Table 1.eps

 (

	2000 m	Total Mass Flux BSiO 2	CaCO 3	POC
	Total Mass Flux	-
	BSiO 2	0.99	-
	CaCO 3	0.37	0.30	-
	POC	0.86	0.83	0.24	-
	3700 m	Total Mass Flux BSiO 2	CaCO 3	POC
	Total Mass Flux	-
	BSiO 2	0.99	-
	CaCO 3	0.61	0.54	-
	POC	0.87	0.83	0.63	-

Table 2

 2

Table ( s) Click here to download Table(s): Table 2.eps

 (

				3700 m
		Average 309 days Average 172 days	Average 172 days
	Actinocyclus spp.	0.01	0.01	0.03
	Actinocyclus actinochilus (Ehrenberg) Simonsen	0.02	0.01	0.00
	Asteromphalus hookeri Ehrenberg	0.17	0.20	0.48
	A. hyalinus Karsten	0.16	0.26	0.29
	A. parvulus Karsten	0.19	0.32	0.35
	Asteromphalus spp.	0.01	0.01	0.01
	Azpeitia tabularis (Grunow) Fryxell et Sims	0.68	0.57	0.74
	Chaetoceros aequatorialis var. antarcticus Manguin	0.02	0.03	0.06
	Ch. atlanticus Cleve	0.16	0.29	0.37
	Ch. dichaeta Ehrenberg	0.11	0.19	0.16
	Ch. peruvianus Brightwell	0.00	0.00	0.01
	Chaetoceros sp. A	0.08	0.14	0.11
	Chaetoceros sp. B	0.07	0.13	0.19
	Chaetoceros spp.	0.15	0.27	0.32
	Chaetoceros Hyalochaete resting spores	0.14	0.25	0.14
	Corethron spp.	0.01	0.01	0.00
	Dactyliosolen antarcticus Castracane
	Eucampia antarctica (Castracane) Mangin	0.13	0.14	0.20
	Fragilariopsis curta (Van Heurck) Hustedt	0.57	1.00	1.11
	F. cylindrus (Grunow) Krieger	0.16	0.29	0.10
	F. kerguelensis (O'Meara) Hustedt	79.91	72.41	68.34
	F. obliquecostata (van Heurck) Heiden	0.02	0.03	0.05
	F. pseudonana (Hasle) Hasle	2.03	3.64	1.43
	F. rhombica (O'Meara) Hustedt	0.93	1.58	2.13
	F. ritscheri Hustedt	0.07	0.13	0.11
	F. separanda Hustedt	2.10	2.85	2.03
	F. cf. sublineata (Van Heurck) Heiden	0.03	0.04	0.03
	Haslea trompii (Cleve) Simonsen	0.01	0.01	0.02
	Navicula directa Smith) Ralfs in Pritchard	0.33	0.58	1.06
	N. sicula var. bicuneata (Castracane) Grunow	0.06	0.10	0.14
	Pleurosigma directum Grunow in Van Heurck	0.02	0.03	0.03
	Table 3

Acknowledgements

This project is supported through funding from the Australian Government´s Australian Antarctic Science Grant Program (Project number 4078) and Macquarie University. The SAZ Project is grateful for logistical and equipment support from ANARE and the Australian Antarctic Division (T. Trull, ASAC grants 1156 and 2256), the CSIRO Division of Marine Research Oceans and Climate Program, Geosciences Australia, and New Zealand's National Institute of Water and Atmospheric Research (NIWA). The officers, crews, and scientific staff of the Aurora Australis voyages are thanked for their


Benjamin Reig

Véronique Bardinal
email: bardinal@laas.fr

Jean-Baptiste Doucet

Emmanuelle Daran

Thierry Camps

Maëlenn Aufray

Alain Lamure

Claire Tendero

Study of SU-8 reliability in wet thermal ambient for application to polymer micro-optics on VCSELs

We present experimental data on the reliability of SU-8 polymer when used as a core material for the integration of microlenses on vertical-cavity surface-emitting lasers (VCSELs). The respective effects of a hot and humid environment on structural, mechanical and optical properties of this epoxy resist are investigated. High aspect-ratio SU-8 micropillars are found to keep a good surface morphology and a stable optical transmission, as well as a good adherence on the wafer. Thermal cycling is also studied to check material stability under electro-thermal actuation in SU-8 microopto-electro-mechanical system (MOEMS). These results are of great importance for the collective integration of low-cost SU-8-based passive or active microlens arrays onto VCSELs wafers for optical interconnects and optical sensing applications.

Introduction

Beam shaping is one of the key issues for vertical-cavity surface-emitting lasers (VCSELs) integration in highlyminiaturized optical systems and for coupling them to single-mode optical fibers. [1][2][START_REF] Michalzik | VCSELs: A Research Review[END_REF][4] It is indeed mandatory to associate these laser devices to micro-optical elements to meet most of systems requirements on beam divergence reduction or on beam focusing. 5) Early reports on microoptics association to VCSELs were mainly based on hybrid assembly methods. Nonetheless, these methods require tricky alignment steps and are often not compatible with high density VCSELs arrays. Direct integration of microlenses inside the devices or at their surface was therefore showed to be more efficient and more collective. 6,7) In particular, polymer-based technologies using SU-8 material at VCSEL surface were proven to lead to simple and low temperature fabrication process. [8][START_REF] Nallani | Proc. SPIE[END_REF][START_REF] Gracias | Proc. SPIE[END_REF][START_REF] Bardinal | [END_REF][12] SU-8 is a negative-tone epoxy resist that was first used for MEMS molding and for microfluidics fabrication because it allows for high aspect-ratio patterning at a micrometric scale. 13) This key feature constitutes a great advantage for the precise fabrication of thick transparent pedestals and of associated microlens necessary for VCSEL beam collimation [Fig. 1(a)] [START_REF] Bardinal | [END_REF] or for fabricating optical MEMS for dynamic focusing [Fig. 1(b)]. [START_REF] Bardinal | Inter. Conf. Optical MEMS and Nanophotonics[END_REF] One important condition for the future industrial use of such lensed-VCSELs consists in checking reliability in presence of moisture, as it is done for bare VCSELs. [START_REF] Xie | [END_REF][16][17] In particular, the stability of optical transmission and adherence of SU-8 on the semiconductor wafer has to be verified.

SU-8 series resist reliability was already investigated for MEMS applications. This resist was first found to be independent to humidity, provided its polymerization process is fully completed. 18) However, Ali et al. reported that absorption of water in the material can lead to swelling and to a residual strain in SU-8 cantilevers. 19) This phenomenon was also observed by Feng et al. who assumed a reversible mechanism. 20) These authors concluded that the resulting strain can be minimized by applying a drying process. Park et al. led an extensive reliability study on silicon-based MEMS including SU-8 spacers (made of SU-8-2000). 21) Their actuators were submitted to mechanical stresses (shocks and vibrations), to hot and humid storage conditions and to thermal cycling. Storage at 85 °C with a relative humidity (RH) of 85% was found to cause SU-8 delaminating at a level that depends on the resist version and on storage duration. Finally, Salm et al. highlighted sensitivity of SU-8 to humid environments (30 °C, 95% RH), reporting a decrease of SU-8 adherence on CMOS chips. 22) However, to our knowledge, there is no data available for the last version of the resist for which the formulation was improved in terms of adherence (SU-8 3000 series). Moreover, only few studies were led on SU-8 reliability over long storage durations and none of them considered the dimensions involved for VCSELs beam shaping, neither the evolution of optical properties after aging. To this end, we investigate here the evolution of structural, mechanical and optical properties of 100-µm-thick SU-8 micro-optical structures submitted to hot and humid environment and to thermal cycling.

Experimental methods

To carry out this study, accelerated aging tests were performed using a climatic chamber in which temperature and humidity levels could be real-time controlled with reduced transient times (Vötsch VCL 0010). Thanks to an experimental Taguchi plan, 23) it was possible to lead a comprehensive study including three parameters at three different values (temperature: 33, 50, and 85 °C, RH level: 50, 65, and 85% and duration: 100, 500, and 1000 h) while reducing the number of tests conditions (from 27 to 9). As seen in Fig. 2, the most severe conditions were: 85 °C, 95% relative humidity and 500 h, which are the typical aging conditions to check optoelectronic devices reliability for fiber optic telecommunications.

A batch of 10 samples, corresponding to the reference state and to the 9 different aging conditions, was prepared. We have used 4 in. silicon wafers with a thin silicon dioxide layer (thickness ³2.5 µm) deposited at the surface by low temperature plasma enhanced chemical vapor deposition (PECVD), in order to reproduce well the surface of a SiO 2 -passivated VCSEL wafer, onto which we usually integrate our SU-8 static [START_REF] Bardinal | [END_REF] or tunable 12) lens, the thermal properties of GaAs and Si wafers being very close. A similar batch was also fabricated on transparent substrates (Corning Pyrex 7740) and simultaneously aged, to make possible optical transmission measurements. A 100-µm-thick SU-8 layer (SU-8 3050 Microchem™) was deposited on each wafer by standard spin-coating. Cylindrical patterns corresponding to microlens pedestals were then defined with diameters ¤ equal to 50, 100, and 200 µm using photolithography (Fig. 3). A standard hard-bake (2 min at 125 °C) was performed to make sure that complete polymerization was reached. 500-µmlength rectangular patterns with widths W equal to 50, 80, and 100 µm were also included to make possible mechanical tests (shear-tests) on patterns corresponding to lens pedestals and to MOEMS anchors. After fabrication, the samples were successively placed at the center of the climatic chamber. They were then systematically characterized two days after the end of the aging test.

Results and discussion

3.1 Surface characterization SU-8 surface profile was characterized before and after each aging process using an atomic force microscope (AFM; Bruker Dimension 3100). Two areas of 5 © 5 = 25 µm 2 were scanned for each considered pattern. As seen on images measured after aging No. 7 and No. 9 (Fig. 4), nano-sized bubbles appear randomly at the surface. The origin of these bubbles is not clearly stated. As bubbles distribution is found to systematically vary with the substrate type (silicon or Pyrex), it could be correlated to the large difference of thermal properties between the two kinds of substrates (for example their thermal conductivities: respectively 149 and 1.1 W m ¹1 K ¹1 ) leading to a difference of thermal transients during baking steps on a hot plate. In both cases anyway, bubbles size and density increase with simultaneous increase of temperature and humidity levels, with maximal diameters than can reach locally 60 nm for silicon and 80 nm for Pyrex for the most drastic test (aging No. 9: 85 °C, 95% RH, and 500 h). However, it is worth noting that the corresponding average surface roughness remains in all conditions lower than 4 nm (RMS), as seen in Table I. These values remain acceptable for the aimed application.

Optical characterizations

Optical transmission spectra of SU-8 micro-patterns in the visible-NIR spectral range (600-1100 nm) were measured thanks to a FTIR spectrometer equipped with a microscope stage (spatial resolution ³20 µm). Whatever the considered aging test, the variation of polymer transmission was found to be smaller than the measurement error of the set-up used (1%). As seen in Fig. 5 for the case of aging No. 9, SU-8 transmission remains higher than 98% in the VCSEL operation wavelength range, i.e., around 850 nm. Moreover, we have checked that no significant optical diffusion could be measured in the structures despite the presence of the nanobubbles at the surface.

Finally, to evaluate water absorption in SU-8 material, we also measured its absorption spectrum in the MIR spectral range (from 4000 to 600 cm ¹1 ) using attenuated total reflexion (ATR). The ratio of the peaks areas measured respectively in the 3686-3621 cm ¹1 region (corresponding to O-H bonds) and in the 3500-2766 cm ¹1 region (corresponding to C-H bonds) has been compared before and after aging test. We noticed a significant increase of this ratio just after aging: from 1.13 to 2.2 for the highest humidity level. However, it returns rapidly to 1.6 after a short storage in ambient air (less than one day) and to 1.3 after two days in a nitrogen storage box. These results demonstrate that water absorption in SU-8 due to exposition to wet ambient is largely reversible.

Mechanical characterizations

Two adherence tests were used in order to characterize the SU-8/SiO 2 assembly. First, mechanical peeling experiments were performed to measure the propagation of a crack at the interface. 24) An Instron 3367 machine was used at 10 mm/min, and 90°for 1 © 8 cm 2 bands of SU-8. These measurements showed that SU-8 exhibit a very good adherence on SiO 2 : an average minimal peeling force of 6.8 N/cm was indeed obtained; this value being only limited by the used set-up. Secondly, initiation tests were performed as shear-tests on micro-scale patterns. The principle is based on the measurement of the pressure applied to a blade for leading to a lateral delamination of the microstructure [Fig. 6(a)]. Adherence forces values extracted from these measurements are reported in Table II for small cylinders and in Table III for larger rectangular patterns. As seen in Fig. 6(b), the annular footprint of the released cylinders is clearly visible at the surface, showing that SU-8 adheres more at the pillar periphery than at the rest of the surface. As the ratio periphery-on-surface evolves as 1/R (R being the radius), this bounding effect linked to the periphery is more preponderant in the case of small diameters. We divided the force by the total surface to highlight this size effect (Tables II andIII) and to estimate the mechanical shear stress (in MPa). We obtained a higher value for small cylinders (diameter 50 µm) than for larger patterns. In all cases anyway, a value at least higher than 20 MPa was obtained, showing a good stability of the SU-8/SiO 2 mechanical interface. Results are even better after long, hot and humid aging tests. This tends to indicate that there is an incomplete polymerization of the samples at the reference state. Data from mechanical tests were exploited for the analysis of the aging test matrix (Fig. 7). This study indicates that the most influent factor on adherence evolution is the temperature whatever the considered size, the impact of the others factors variation (RH% and time) being much lower and we account it to an incomplete polymerization at the reference state.

To further explore temperature individual influence, we submitted new samples to positive thermal cycling tests in normal humidity conditions (40%RH). The following thermal steps were applied: 10 min at 20 °C, ramp up to 120 °C at +10 °C/min, 10 min at 120 °C and finally decrease at ¹10 °C/min down to 20 °C for a total cycle duration of 40 min. The maximal temperature chosen here is higher than 85 °C and corresponds to the operation conditions of the electro-thermal MOEMS we recently developed for tunable lens integration on VCSELs. [START_REF] Gracias | Proc. SPIE[END_REF]12) The surface structure in this case was found to be close to the one of the reference state (roughness <0.5 nm), confirming that bubbles size and density mainly increase when humidity and heat levels simultaneously increase. Series of 10, 50, and 100 cycles were performed and their respective effect on mechanical shear stress was analyzed. In all these cases, we observed a similar behavior than previously: shear stress remains higher than 20 MPa and increases with tests duration, with a maximum value of 45 MPa for 100 cycles, demonstrating that SU-8 microstructures tolerate well positive thermal cycling.

Conclusions

We have studied the reliability of SU-8 polymer as a functional material for the fabrication of integrated microoptical elements on VCSELs, thanks to aging tests performed in a hot and wet ambient. When increasing temperature and moisture, polymer morphology is found to slightly degrade, with the progressive apparition of "nano-bubbles" at the surface. However, the average surface roughness measured by atomic force microscopy (AFM) remains low: less than 4 nm for the worst case (85 °C, 95%RH, 500 h). Optical transmission measured at VCSEL emission wavelength was also checked and found to be not significantly changed. Besides, the analysis of optical absorption in the mid infrared range shows that water absorption occurring in the material is largely reversible. Finally, adherence strength of SU-8 on the substrate was also studied. It is found to be high, with measured shear stresses at least equal to 20 MPa. Due to periphery effects, this adherence stress is even twice higher for patterns dimensions lower than 100 µm, that correspond to those involved in typical lens pedestals or MOEMS anchors that we have recently integrated on VCSELs devices. This study also highlighted a higher sensitivity of adherence evolution to temperature than to humidity level or to test duration. Finally, positive thermal cycling tests performed between 20 and 120 °C demonstrate a good mechanical stability of SU8 properties for a use in conditions of electrothermal actuation. Now that SU-8 microstructures reliability for integrated micro-optics is assessed, our future work will concern a more complete study of the reliability of lensed-VCSELs devices under laser operation.

Fig. 1 .

 1 Fig. 1. (Color online) Principle of operation (left) and corresponding scanning electron microscope (SEM) images (right) of two different types of SU-based microlenses collectively integrated on VCSELs by the authors: (a, b) cylindrical pillars for static collimation; (c, d) optical MEMS with rectangular anchors for dynamic focusing.

Fig. 2 .

 2 Fig. 2. (Color online) Aging test matrix.

Fig. 3 .Fig. 4 .

 34 Fig. 3. (Color online) Top: Description of the SU-8 microstructures fabricated on SiO 2 /silicon to reproduce the passivatived surface of a VCSEL wafer (a) and on Pyrex for optical transmission measurements (b). Bottom: SEM images of SU-8 cylindrical (c) and rectangular (d) patterns designed for aging tests.

Fig. 5 .Fig. 6 .

 56 Fig. 5. (Color online) Transmission spectrum measured on a 100-µmdiameter SU-8 pillar on Pyrex before and after aging test No. 9.

Fig. 7 .

 7 Fig. 7. Taguchi plan analysis: evolution of shear stress in function of aging parameters for cylindrical microstructures: (a) ¤ = 50 µm, (b) ¤ = 80 µm, and (c) ¤ = 100 µm.

Table I .

 I Average surface roughness (RMS) measured on the reference and after each aging test for samples fabricated on silicon and on Pyrex substrates.

	No.	T (°C)	RH (%)	t (h)	RMS Si (nm)	RMS Pyrex (nm)
	Ref.	-	-	-	0.53	0.67
	1	33	50	100	0.57	0.53
	2	33	65	500	0.45	0.52
	3	33	95	1000	0.39	0.42
	4	50	50	500	0.92	3.82
	5	50	65	1000	2.11	2.18
	6	50	95	100	0.6	1.62
	7	85	50	1000	0.87	1.29
	8	85	65	100	0.48	0.49
	9	85	95	500	3.42	3.93

Table II .

 II Results of shear-test measurements performed before and after aging on SU-8 cylindrical microstructures fabricated on SiO 2 /Si.

	No.	Shear force (N) for ¤ = 50 µm	Force on surface ratio (MPa) for ¤ = 50 µm	Shear force (N) for ¤ = 100 µm	Force on surface ratio (MPa) for ¤ = 100 µm	Shear force (N) for ¤ = 200 µm	Force on surface ratio (MPa) for ¤ = 200 µm
	Ref.	0.117	59.5	0.302	38.4	1.080	34.4
	1	0.130	66.2	0.395	50.3	1.241	39.5
	2	0.119	60.2	0.309	39.3	0.938	29.9
	3	0.123	62.5	0.358	45.5	1.005	32
	4	0.129	65.2	0.471	60	1.343	42.7
	5	0.130	66.4	0.381	48.4	1.090	34.7
	6	0.127	64.5	0.358	45.5	1.157	36.8
	7	0.173	87.7	0.553	70.4	1.789	56.7
	8	0.132	67.4	0.414	52.6	1.332	42.4
	9	0.177	89.9	0.555	70.6	1.635	52.1

Table III .

 III Results of shear-test measurements performed before and after aging on SU-8 rectangular microstructures fabricated on SiO 2 /Si.

	No.	Shear force (N) for W = 50 µm	Force on surface ratio (MPa) for W = 50 µm	Shear force (N) for W = 80 µm	Force on surface ratio (MPa) for W = 80 µm	Shear force (N) for W = 100 µm	Force on surface ratio (MPa) for W = 100 µm
	Ref.	0.976	39	1.512	37.8	1.666	33.3
	1	0.736	29.4	1.431	35.8	1.431	28.6
	2	0.849	33.9	1.253	31.3	1.483	29.7
	3	0.535	21.4	1.127	28.2	1.563	31.3
	4	0.800	32	1.350	33.7	1.797	35.9
	5	0.861	34.5	1.402	35	1.732	34.6
	6	0.734	29.4	1.116	27.9	1.394	27.9
	7	1.131	45.2	1.964	49.1	2.495	49.9
	8	0.757	30.3	1.308	32.7	1.718	34.3
	9	1.113	44.5	1.758	43.9	2.162	43.2

Acknowledgments

The authors gratefully acknowledge Région Midi Pyrénées for financial support (FIAB-SU8 project) and RENATECH (French Network of Major Technology Centres) within LAAS-CNRS for technological support. The authors would also like to thank FIABLAB for their contribution to mechanical measurements and Airbus Defense and Space for fruitful discussions.


Opitz, R N Poirier

Nuninger L Ostir

LIDAR IN MEDITERRANEAN AGRICULTURAL LANDSCAPES -REASSESSING LAND USE IN THE MAUGUIO

Keywords: lidar, Mediterranean, agricultural landscapes, visualization

The value of lidar data for the study of forested areas has been repeatedly demonstrated, recording large numbers of previously unknown sites and features where most survey methods are ineffective. However, the question of the value of lidar in cultivated areas already investigated through historical mapping and archaeological studies such as fieldwalking survey, aerial photographic survey and excavation remains open. This paper summarizes the results of recent research in the open and heavily cultivated Mauguio region of southern France and reflects on the challenges of integrating information from lidar survey into an existing body of knowledge on the development of an agricultural landscape.

Introduction

This paper presents an assessment of the 'added value' of lidar data for the study of a Mediterranean landscape in the Mauguio area of the French Languedoc which has been intensively studied by archaeologists since the 1980s, primarily through the substantial fieldwalking surveys led by F. Favory and C. Raynaud [START_REF] Favory | L'habitat gallo-romain autour de l'étang de l'Or (Hérault). Hiérarchie, dynamique et réseaux du 2è s. av. au 5è s. ap. J.-C[END_REF]. This area was further studied through a number of thematic projects at national and European level, enhancing the information about land use and settlement patterns in this area through regional and inter-regional comparanda.

In focusing on an agricultural Mediterranean landscape this paper highlights a regional divide in what might be termed the fundamental building blocks of knowledge about landscape history. Earthworks, ranging from ridge and furrow field systems to massive Bronze and Iron Age hillforts have long been essential to the character and archaeological description of landscapes throughout northern and central Europe. Landscape archaeology and the practice of survey in the Mediterranean, on the other hand, developed within a different research tradition -one heavily dependent on the ceramic and stone remains found through fieldwalking. Given a tradition of studying rural landscapes that does not take as much account of topography, what kind of role can lidar data play and how should its evidence be balanced with that from traditional data sources?

While many lidar surveys are commissioned, or existing data acquired, with the primary aim of identifying new sites (in the traditional sense of the term) or documenting existing ones, a lidar survey in 2007, covering a block of about 40 km², was undertaken with the specific aim of creating an accurate DTM to better understand changes in fluvial patterns, drainage systems and the evolution of the lagoon coastline. Following the completion of an initial palaeo-hydrological study, a second project was established to explore the relationship between archaeological features visible in the lidar, many of which were interpreted as related to agriculture or drainage, and land use patterns as understood from previous archaeological research. The relationship between areas characterized by extensive agrarian manuring spreads, indicative of intensive or long term agricultural use, and the visibility of features in the lidar data was of particular interest. Both projects focused on the agricultural landscape and the evolution of land use, rather than the settlement pattern (the usual focus of survey in Mediterranean archaeology) setting lidar up to play a complementary rather than augmentary role in landscape scale research in the region. In establishing the lidar survey as a source of complementary knowledge, technical challenges were posed by visual noise in the data caused by deep ploughmarks, and interpretive challenges arose from the process of integrating the detected features into existing datasets and assessing the value of such information in a well studied area.

Geographic Context

The Mauguio study area is located in the Languedoc region of southern France, part of a plain bordering the Mediterranean Sea. It is very flat, lying just a few meters above sea level, surrounding a coastal lagoon. This marshy area was heavily managed during the past and is currently occupied by scrubland, vineyards and agricultural land (Figure 1).

History of Research

Knowledge of local settlement history and land use is primarily based on the work of F. Favory and C. Raynaud [START_REF] Favory | L'habitat gallo-romain autour de l'étang de l'Or (Hérault). Hiérarchie, dynamique et réseaux du 2è s. av. au 5è s. ap. J.-C[END_REF] and the Archaeomedes Project undertaken in the 1990s ( [START_REF] Van Der Leeuw | Etudes multiscalaires sur la vallée du Rhône dans le programme ARCHAEOMEDES[END_REF]. The area was also studied through limited excavation, and satellite and aerial imagery were used to identify field systems and roads [START_REF] Favory | La production du paysage en Langudoc oriental dans l'Antiquité et au Moyen Age : Etude de Mauguio (Hérault)[END_REF]. In the 2000s this region was studied by L. Nuninger & K. Oštir, using remote sensing data to detect palaeo-channels and develop a reconstructed palaeo-DTM [START_REF] Nuninger | Contribution à la modélisation des paléo-reliefs de la plaine littorale de l'Etang de Mauguio (Languedoc, France) : premières approches par télédétection[END_REF].

The results of all these projects allow us to trace the general trends of long-term land use. While the earliest remains date from the Palaeolithic period, it is in the early Iron Age that settlement and activity along the banks of the coastal lagoon increases significantly. The Roman period saw further agricultural intensification on the plain, with the introduction of a cadastral land division system, an extensive settlement network and widespread drainage infrastructure installed. These developments totally changed the environmental dynamics of the area. The Roman drainage systems, roads and land divisions established the framework of the agricultural landscape for subsequent periods, and they remain apparent in the contemporary landscape.

DTM preparation in areas of intensive agriculture

The high resolution of the lidar DTM (0.5 m) is appropriate for the detection of microtopographic features such as field boundaries, drainage ditches or ancient roads. However, this precision can be problematic in fields under cultivation because plough furrows, vineyards, and other artefacts of modern land use which are not easily separated from the 'interesting' ground surface (Figure 2) are also recorded in great detail. And where the agricultural noise is greatest in areas of heavy ploughing archaeological relief will be at its most subtle due to heavy erosion. Identifying subtle traces of past activity in the present-day agricultural landscape therefore required careful filtering and visualization to remove noise and improve visibility.

To facilitate the visual recognition of archaeological features in the plough zone, the removal of linear 'plough noise' using established image processing techniques was attempted. An initial effort (Nuninger et al., 2008: p. 36-39) demonstrated that Sobel edge detection, low-pass filters and directional filters, as implemented in Erdas Imagine, did not significantly improve the visibility of archaeological features.

However, an approach based on Fourier analysis was more successful in removing the traces of ploughing using operations (Figure 3). Two approaches were taken. The application of an 'Automatic Periodic Noise Removal' algorithm in Erdas Imagine, requiring a minimum of input from the user, was applied in the first instance. In cases where this did not improve the appearance of the image, manual editing in Fourier space to remove the periodic plough noise was performed. Removing the linear pattern created by the most recent phase of ploughing sometimes revealed evidence of earlier phases of cultivation and differences in land use in the form of changes in field size, row spacing and orientation of cultivation.

Visualisation in the ploughzone

The hillshading model applied always influences the visibility of features in a lidar DTM, and in heavily cultivated landscapes this effect is magnified by the shadows created by ploughmarks. Because fields may be ploughed in variety of directions, any directional hillshading model applied over a reasonably large area will cause serious shadowing effects in some fields where it is near-perpendicular to the direction of the ploughing. Consequently, while simple directional hillshades are often most effective, in this situation we tested two complex hillshading models: the Swiss Hillshade Model and the MDOW (Multi-Directional Oblique Weighting) Hillshade Model (Erreur ! Source du renvoi introuvable.).

The Swiss Hillshade Model is designed to, "emphasize the major geographic features [and] minimize the minor features, smooth irregularities on the slopes, but maintain the rugged characteristics of ridge tops and canyon bottoms…You can then simulate an aerial perspective that makes the higher elevations lighter and the lower elevations darker" [START_REF] Barnes | Using ArcMap to Enhance Topographic Presentation[END_REF]. On first assessment, this model may not seem suitable for processing digital terrain models to detect archaeological entities, since it is supposed to highlight major geographic features, minimize the margins and smooth out slope irregularities. However, in an area dominated by ploughing it ameliorates the problems introduced by a directional hillshade. Further, by lightening shading on the highest altitudes, and darkening shading on lower elevations, absolute elevation differences are highlighted. In an area with a large elevation range such shading will not show up archaeological features, but it can be effective on very flat terrain where relief is very low (a few centimetres elevation change across a feature of several metres extent) and the total elevation variation is very small.

In contrast, the Multi-Directional Oblique Weighting (MDOW) Hillshade Model is theoretically an answer to the problem of, "traditional computer-generated shaded-relief maps [which] emphasize structures that happen to be obliquely illuminated, but wash out structures that are illuminated along the structural grain. This […] technique, which emphasizes oblique illumination on all surfaces, provides more detail in areas of an image that would otherwise be illuminated by direct light or left in darkness by a single source illumination." (Robert Mark, USGS, http://pubs.usgs.gov/of/1992/of92-422/of92-422.pdf) Testing this model revealed that rather than improve visibility in areas with multi-directional ploughing, noise associated with the plough lines in the image increased because more fields were affected by shading perpendicular to the direction of their ploughing.

While the Swiss Hillshades were judged to be useful, initial assessment did not indicate that any single visualization of the terrain consistently provided the best results. Following the practice of many projects a combined approach was taken, including using directional hillshades and other visualizations of the terrain in the course of interpretation. Successfully working with a variety of visualizations relies on experience interpreting topography and archaeological knowledge (see Halliday this volume) more than it depends on technical prowess or excessive computing power. Hillshades and other terrain models can be easily produced using batch scripts and storage space for the files is increasingly inexpensive and readily obtained. The challenge is in knowing which visualization is likely to be most useful for any area of landscape, in being able to look at a visualization and, based on its utility, select the next one to apply to the same area, avoiding having to systematically look through every rendering.

Integrating Lidar with existing Knowledge

After processing to remove noise and create good visualizations as described above, a large number of features associated with agriculture, drainage and land division were identified. An essential first step is discriminating between features that document past activity and those which are modern. Establishing chronology is always problematic when working with aerially acquired data. In this case, orthophotographic coverage (Figure 5) and the Napoleonic cadastre were used to assess relatively recent changes in the landscape and attempt to identify features associated with these changes before undertaking a comparison with data from fieldwalking survey. On the basis of this exercise 85 features were identified as being archaeological and previously undocumented. These were dominated by linear features such as field boundaries, drainage ditches and paths.

Comparison with the Napoleonic cadastre and orthophotographs

The Napoleonic cadastre was compiled for France and many other countries under the Napoleonic Empire during the course of the 19 th century, continuing even after the end of the Empire. In this part of southern France these maps were made in 1811 (AD34, 3P3424) and for part of the study area the maps are available in digital form. These were georeferenced and all the field boundaries were digitized. Within the area covered by the digital maps, six of the 24 lidar features (25%) are depicted on the cadastral map, corresponding to established field boundaries. One of these is also visible on the orthophoto coverage (Figure 6).

Evidence for early field systems

Fieldwalking campaigns between 1986 and 1992 recorded both site (settlements) and off-site (manuring scatters) materials. In the eastern Languedoc almost 300 archaeological features were identified, the majority of which are characterized as settlements. The manuring scatters, while fewer in number, occupy a greater area and consequently play an important role in characterizing the land use history of the region. These scatters cover over 900 hectares and date from the Iron Age to the Modern Era (Figure 7).

In the study area defined by the lidar survey, two field systems dating from the Roman period were identified from aerial imagery, historic maps and excavations. These systems, known as Sextantio-Ambrussum and Nîmes A, strongly influenced the development of subsequent field systems. The earlier arrangement, Sextantio-Ambrussum, is oriented at 22°30'W while the Nîmes A is oriented at 30°30'W (Figure 8).

Global Orientation

Since the features detected from the lidar data were almost all linear features interpreted as field boundaries, their individual and global orientations were studied. The directional spectrum of the features identified in the lidar data was compared with those of the field systems shown on the 19th century cadastral map and the two Roman cadastres discussed above (Figure 9). The directionality of these field systems was assessed by identifying the orientation of each linear feature in the field systems (i.e. individual field boundaries) and the results for each system were compared graphically (Figure 10). Based on the global similarities in orientation we conclude that many of the features identified in the lidar models are related to the Roman systems. For the lidar-derived features the most frequent orientation [66°;70°] matches that of the Sextantio-Ambrussum (67.5°) cadastre and the second most common orientation corresponds to the primary direction of the Nîmes A cadastre. While it appears that the majority of these features respect the orientations of the Roman cadastres, there are also important differences. For example, features oriented between 1° and 10° appear frequently in the lidar data, but not in the Napoleonic cadastre, the Roman cadastres or contemporary field systems.

Based on the graphical analysis, we have noted a global correlation between the Napoleonic cadastre and the models proposed for two Roman cadastres. Based on this similarity we suggest that elements of the system of land division established in the Roman period persisted into the 19 th century, and its global orientation and structure continue to influence the current landscape. Alternatively, it can be proposed that the various landscape organizations are all influenced by the similar environmental and topographic factors, leading to broadly similar solutions for landscape organization.

Proximity to sites identified through fieldwalking

Following the assessment of the orientation of the field systems, the locations of individual features identified in the lidar were compared with data from fieldwalking. Here we assume that consistent spatial proximity, compared to a random distribution, may suggest a link between the presence of features in the lidar data and sites found through field walking. That is not to say that individual features identified in the lidar should be specifically associated with nearby remains from field walking, but that the overlap between the overall patterns from the two data sources allows us to identify more and less active areas of the landscape. Thirty sets of random points with the same count as the lidar-derived feature set were generated within the spatial extent of the lidar coverage and features identified in the lidar data were assigned centroids. The minimum distance between each centroid and the closest point indicating a feature identified through field walking was calculated (Figure 11). The analysis was then repeated measuring the minimum distance between each point feature identified through field walking and the nearest point in each random set.

The results of this analysis show a difference between the mean minimum distances observed for features seen in the lidar and those for random point sets. The smaller average minimum distance for the lidar features indicates a potential link between the location of these features and field walking sites. Based on this analysis it was suggested that concentrations of features from the two datasets, representing increased activity in the landscape, should be expected to appear together.

Areas with evidence of long term agricultural activity

To further investigate long term land use dynamics a second analysis was conducted comparing the average durability of agrarian occupation, based on evidence from manuring scatters (as defined by the Archaedyn project, [START_REF] Poirier | Measurements of Diachronic Stability of Agrarian Exploitation[END_REF] and the density of lidar features (Figures 12 and13).

Based on a global comparison there is no demonstrable correlation between the average durability of agricultural activity as represented by manuring scatters and the density of lidar features detected (Figure 13). However, further analysis based on refined chronologies of both the lidar features and manuring areas, or analysis based on another metric for concentrations of lidar features, may provide different results.

Conclusions and future directions

The project described here sought to improve knowledge of past agricultural and land management systems in the contemporary agricultural landscape of the Mauguio, combining lidar data, aerial photographs, field walking data and historic maps. Based on the history of research in the Mauguio area, and no doubt influenced by the general habits of researchers who work on Mediterranean landscapes, the approach taken here was one of making the lidar data 'fit' with existing evidence from field walking and historical map sources on which rural Mediterranean studies often depend, ( and noting where it differed. The testing of 'fit' between lidar and other data sources was carried out primarily through formal and graphical spatial analysis. This approach, while contributing some new knowledge in this case, presents two notable problems.

First, through spatial analyses concentrations of undated features identified in the lidar data were (loosely) related to concentrations of dated archaeological or historical features. This kind of proximity based analysis is inherently problematic. It is well known that individual features that appear together are not necessarily related, and it may be that even general trends in concentrations of features should not be correlated. Even where spatial analysis legitimately suggests how features identified through lidar survey might fit with patterns identified from historical maps and field walking data, it is usually suggested that further study in the field is needed to understand the character, function and chronology of the lidar features. While recognizing the problems involved in interpreting lidar data partly on the basis of spatial analyses, precluding spatial analysis entirely does not seem the way forward, as doing so would seriously limit the possibilities for formally integrating evidence from lidar surveys with other datasets and the creation of new knowledge.

Equally problematic is the implicit treatment of evidence from the lidar survey as secondary to field walking and historic mapping data, and the underlying expectation that it will augment, rather than contradict, existing datasets. In wooded areas where little previous archaeological research has been undertaken, evidence from field walking, historic maps and lidar may easily find themselves on equal footing as new knowledge is developed. Landscapes with long research histories start us off with a certain set of prejudices and ideas. By sheer quantity field walking data dominates the available information for the Mauguio study area, and as it has been the subject of extended study across several projects it is unsurprising that as interpreters we are more confident in our assessment of this material. But does this mean that we should necessarily be surprised if the evidence derived from the lidar survey points to different, or just partially divergent, conclusions than those drawn by previous studies? Clearly not, and the need to be willing to revise our interpretations over time is well argued by Rog Palmer (this volume). At the same time, it is important to consider whether it is our original interpretation or our understanding of the new evidence that should be questioned. The challenge of integrating new evidence and reassessing our confidence in different data sources occurs whenever an area or subject is restudied because new information has come to light. Meeting this challenge is essential if we wish to use lidar data to study multifaceted topics like long term land use and if we plan to include lidar in ongoing research on the rural Mediterranean, where multiple sources of information and complex histories of research abound.

Figure 1 :

 1 Figure 1 : Location of the study area

Figure 9 :

 9 Figure 9 : Comparing the orientation of the Roman cadastres, Napoleonic cadastre and features identified through the lidar survey

Figure 12 : Dispersion graph showing durability index values and lidar features density