R U Shelley

N J Wyatt

G A Tarran

A P Rees

P J Worsfold

M C Lohan

A tale of two gyres: Contrasting distributions of dissolved cobalt and iron in the Atlantic Ocean during an Atlantic Meridional Transect (AMT-19)

, suggesting the potential for a similar biological control of dCo distributions. Inverse correlations between dCo and Prochlorococcus abundance in the North Atlantic gyre provinces, combined with extremely low dCo where nitrogen fixation rates were highest (~20-28° N), suggests the dominance of biological controls on dCo distributions. The contrasting dCo and dFe distributions in the North and South Atlantic gyres provides insights into the differences

between the dominant controls on the distribution of these two bioactive trace metals in the central Atlantic Ocean.

INTRODUCTION

Cobalt (Co), like iron (Fe), is essential for phytoplankton growth (e.g. [START_REF] Morel | Zinc and carbon co-limitation of marine phytoplankton[END_REF]Saito et al. 2002;Sunda and Huntsman, 1995a;1995b;[START_REF] Timmermans | Not all eukaryotic algae can replace zinc with cobalt: Chaetoceros calcitrans (Bacillariophyceae) versus Emiliania huxleyi (Prymnesiophyceae)[END_REF][START_REF] Rodriguez | Influence of Co and B12 on the growth and nitrogen fixation of Trichodesmium[END_REF]. Cobalt is the metal centre in the vitamin B12.(cobalamin) complex which is essential for the synthesis of amino acids, deoxyriboses, and the reduction and transfer of single carbon fragments in many biochemical pathways. Cobalt is required for the de novo synthesis of vitamin B12 by marine prokaryotes [START_REF] Bonnet | Vitamin B12 excretion by cultures of the marine cyanobacteria Crocosphaera and Synechococcus[END_REF]. However, the majority of eukaryotic marine phytoplankton are B vitamin auxotrophs, acquiring their vitamin B12 requirements through a symbiotic relationship with bacteria [START_REF] Croft | Algae acquire vitamin B12 through a symbiotic relationship with bacteria[END_REF]Cruz-Lopez and Maske, 2016), although this pathway might not be a simple linear flux from producer to consumer [START_REF] Helliwell | Cyanobacteria and eukaryotic algae use different chemical variants of vitamin B12[END_REF]. Cobalt is also the metal co-factor in the metalloenzyme, carbonic anhydrase (CA), which is required for inorganic carbon acquisition by Prochlorococcus, and Synechococcus (Sunda and Huntsman, 1995a;Saito et al., 2002).

In addition, Trichodesmium require Co for nitrogen fixation [START_REF] Rodriguez | Influence of Co and B12 on the growth and nitrogen fixation of Trichodesmium[END_REF], and Co can substitute for zinc (Zn) as the metal co-factor of the protein PhoA in the enzyme alkaline phosphatase (AP) [START_REF] Gong | Characterization of a thermostable alkaline phosphatase from a novel species Thermus yunnanensis sp. nov. and investigation of its cobalt activation at high temperature[END_REF]Sunda and Huntsman, 1995a). The production of AP facilitates acquisition of phosphorus (P) from the organic-P pool by phytoplankton and bacteria (e.g. [START_REF] Mahaffey | Alkaline phosphatase activity in the subtropical ocean: insights from nutrient, dust and trace metal addition experiments[END_REF]. In addition, the strong correlation between dissolved Co (dCo) and inorganic-P (phosphate,PO4) in the upper water column, across diverse oceanic regimes (Saito and Moffett, 2002;[START_REF] Noble | Cobalt, manganese, and iron near the Hawaiian Islands: a potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals[END_REF]2012;[START_REF] Bown | The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa[END_REF]Dulaquais et al., 2014a;[START_REF] Baars | Dissolved cobalt speciation and reactivity in the eastern tropical North Atlantic[END_REF], indicates the nutritive role of Co.

The role of iron (Fe) as an essential requirement for phytoplankton growth is well documented (e.g. [START_REF] Martin | Glacial-interglacial CO2 change: the iron hypothesis[END_REF][START_REF] Coale | A massive phytoplankton bloom induced by an ecosystem-scale iron fertilzation experiement in the equatorial Pacific Ocean[END_REF][START_REF] Boyd | Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions[END_REF]. For example, photosystems I and II are Fe intensive, and Fe is required for enzymatic process at nearly all stages of the microbial nitrogen cycle, including nitrogen fixation [START_REF] Morel | The biogeochemical cycles of trace metals in the oceans[END_REF][START_REF] Küpper | Iron limitation in the marine cyanobacterium Trichodesmium reveals new insights into regulation of photosynthesis and nitrogen fixation[END_REF][START_REF] Richier | Abundances of iron-binding photosynthetic and nitrogen-fixing proteins of Trichodesmium both in culture and in situ from the North Atlantic[END_REF]. Despite Fe being the fourth most abundant element in the Earth's crust, dissolved Fe (dFe) is often only present at trace concentrations (< 0.5 nM) in oxygenated surface waters of the open ocean (Blain et al., 2008;[START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. Consequently, primary production is limited by low Fe-availability in 30 -40% of the world's oceans [START_REF] Moore | Iron cycling and nutrientlimitation patterns in surface waters of the World Ocean[END_REF][START_REF] Boyd | The biogeochemical cycle of iron in the ocean[END_REF]. In the Atlantic Ocean, a number of studies have demonstrated that primary production can be under Fe-stress or limitation, seasonally in association with the spring bloom. [START_REF] Moore | Iron limits primary productivity during spring bloom development in the central North Atlantic[END_REF][START_REF] Nielsdottir | Iron limitation of the post bloom phytoplankton communities in the Iceland Basin[END_REF], as well as in regions where subsurface nutrient supply is enhanced (Moore et al., 2013, and references therein). The supply of Aeolian Fe is also a key control on the distribution of diazotrophs (Mills et al., 2004;[START_REF] Nielsdottir | Iron limitation of the post bloom phytoplankton communities in the Iceland Basin[END_REF]. In addition to Fe, light, macronutrients (N, P, Si), vitamins (e.g. B12) and micronutrients (e.g. Co, Zn) may also (co-)limit marine productivity [START_REF] Bertrand | Vitamin B12 and iron co-limitation of phytoplankton growth in the Ross Sea[END_REF]Saito et al., 2008;[START_REF] Moore | Processes and patterns of oceanic nutrient limitation[END_REF][START_REF] Browning | Nutrient regimes control phytoplankton ecophysiology in the South Atlantic[END_REF].

A major vector of trace elements (TEs) to Atlantic surface waters is atmospheric deposition [START_REF] Jickells | Global iron connections between desert dust, ocean biogeochemistry and climate[END_REF][START_REF] Baker | Trends in the solubility of iron, aluminium, manganese and phosphorus in aerosol collected over the Atlantic Ocean[END_REF]2007;[START_REF] Sarthou | Influence of atmospheric inputs on the iron distribution in the subtropical North-East Atlantic Ocean[END_REF][START_REF] Buck | The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: Observations from the A16N CLIVAR/CO2 repeat hydrography section[END_REF][START_REF] Evangelista | Inferring episodic atmospheric iron fluxes in the western South Atlantic[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF][START_REF] Shelley | Elemental ratios and enrichment factors in aerosols from the US-GEOTRACES North Atlantic transects[END_REF], much of which originates from Northwest Africa (Prospero and Carlton, 1972). An estimated 240 ± 80 Tg of dust is transported westwards annually [START_REF] Kaufman | Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean[END_REF], primarily during the summer months. Approximately 40% of annual global dust deposition occurs in the North Atlantic Ocean [START_REF] Jickells | Global iron connections between desert dust, ocean biogeochemistry and climate[END_REF]; the majority of this into waters beneath the Saharan dust plume (~ 5 -30° N) [START_REF] Mahowald | Dust sources and deposition during the last glacial maximum and current climate: a comparison of model results with paleodata from ice cores and marine sediments[END_REF][START_REF] Prospero | Environmental characterization of global sources of atmospheric dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product[END_REF][START_REF] Kaufman | Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean[END_REF]. Hence, it is between these latitudes that surface Fe concentrations are highest [START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Fitzsimmons | Dissolved iron in the tropical North Atlantic Ocean[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. Wet deposition in the Intertropical Convergence Zone (ITCZ) scavenges aerosols from the atmosphere, effectively preventing the southwards transport of North African aerosols [START_REF] Schlosser | Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide[END_REF]. Thus the seasonal migration of the ITCZ drives the latitudinal gradient in aerosol dust loading [START_REF] Prospero | Vertical and areal distribution of Saharan dust over the Equatorial North Atlantic Ocean[END_REF][START_REF] Doherty | Control of Saharan mineral dust transport to Barbados in winter by the Intertropical Convergence Zone over West Africa[END_REF][START_REF] Doherty | Role of the convergence zone over West Africa in controlling Saharan mineral dust load and transport in the boreal summer[END_REF][START_REF] Tsamalis | The seasonal vertical distribution of the Saharan Air Layer and its modulation by the wind[END_REF], and hence surface water Fe concentrations and results in a concomitant shift in the latitudinal distribution of diazotrophy and corresponding dissolved inorganic-P depletion [START_REF] Schlosser | Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide[END_REF]. Despite Co being less abundant in crustal material than Fe (Fe 3.9%, Co 0.002%; [START_REF] Rudnick | Composition of the continental crust[END_REF], atmospheric deposition is a source of Co to surface waters. [START_REF] Shelley | Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum[END_REF]Dulaquais et al., 2014a). Consequently, we anticipated that Co concentrations would also be highest under the Saharan plume due to the sheer volume of dust that is deposited.

Another important source of trace metals to remote Atlantic surface waters is through vertical mixing. This mechanism reportedly provides ~ 5-35 % of the dFe input flux to the Atlantic mixed layer [START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. Vertical mixing is particularly important in the tropics where elevated sub-surface dFe concentrations are associated with low oxygen, upwelled water [START_REF] Bergquist | Dissolved iron in the tropical and subtropical Atlantic Ocean[END_REF][START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Fitzsimmons | Dissolved iron in the tropical North Atlantic Ocean[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. On the other hand, lateral advection of Fe from shelf regions to the remote Atlantic Ocean is reported to range from minimal [START_REF] Laes | Sources and transport of dissolved iron and manganese along the continental margin of the Bay of Biscay[END_REF][START_REF] Ussher | Distribution and redox speciation of dissolved iron on the European continental margin[END_REF][START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF][START_REF] Fitzsimmons | Dissolved iron in the tropical North Atlantic Ocean[END_REF] to significant in the vicinity of 20 °N [START_REF] Rijkenberg | Fluxes and distribution of dissolved iron in the eastern (sub-) tropical North Atlantic Ocean[END_REF]. For Co, understanding the contribution of these sources is hindered by a relative paucity of data. However, lateral transport has recently been reported in both the eastern and western basins of the Atlantic [START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF]Dulaquais et al., 2014a;2014b).

Iron and Co distributions are also strongly influenced by both redox speciation and organic complexation. Although Fe 2+ is the more bioavailable form of Fe [START_REF] Shaked | Dissassembling iron availability to phytoplankton[END_REF], the thermodynamically favoured species of Fe in oxic seawater (pH 8) is Fe 3+ . However, Fe 3+ is relatively insoluble under these conditions, and is rapidly scavenged from the water column and forms insoluble Fe 3+ oxyhydroxides [START_REF] Liu | The solubility of iron in seawater[END_REF].

Chelation by organic ligands increases the solubility of Fe in seawater; both strong (e.g. siderophores) and weaker ligand classes (e.g., humics) have been shown to be play a role in maintaining Fe in solution [START_REF] Mawji | Hydroxamate Siderophores: Occurrence and Importance in the Atlantic Ocean[END_REF][START_REF] Croot | The importance of kinetics and redox in the biogeochemical cycling of iron in the surface ocean[END_REF][START_REF] Heller | Basin scale survey of marine humic fluorescence in the Atlantic: Relationship to iron solubility and H2O2[END_REF][START_REF] Buck | The organic complexation of dissolved iron along the U.S.GEOTRACES (GA03) North Atlantic Section[END_REF]. Similarly, Co 2+ in also thermodynamically favoured in oxic seawater, and Co forms strong organic complexes [START_REF] Ellwood | Determination of organic complexation of cobalt in seawater by cathodic stripping voltammetry[END_REF][START_REF] Saito | Complexation of cobalt by natural organic ligands in the Sargasso Sea as determined by a new high-sensitivity electrochemical cobalt speciation method suitable for open ocean work[END_REF][START_REF] Baars | Dissolved cobalt speciation and reactivity in the eastern tropical North Atlantic[END_REF].

The primary removal mechanism for Co and Fe from the euphotic zone is through biological uptake [START_REF] Martin | Northeast Pacific iron distributions in relation to phytoplankton productivity[END_REF][START_REF] Moffett | Oxidation of cobalt and manganese in seawater via a common microbially catalyzed pathway[END_REF]. In addition, adsorptive scavenging on to particles [START_REF] Moffett | Oxidation of cobalt and manganese in seawater via a common microbially catalyzed pathway[END_REF][START_REF] Johnson | What controls dissolved iron concentrations in the world ocean?[END_REF][START_REF] Wu | Soluble and colloidal iron in the oligotrophic North Atlantic and North Pacific[END_REF][START_REF] Bruland | Controls of trace metals in seawater[END_REF] and aggregation and sinking [START_REF] Croot | Short residence time for iron in surface seawater impacted by atmospheric dry deposition from Saharan dust events[END_REF]

MATERIALS AND METHODS

Sampling

Twenty nine stations were sampled during cruise AMT-19 (13/10/09-28/11/09) from Falmouth, UK to Punta Arenas, Chile, on board the R.R.S. James Cook (Fig. 1). Stations were sampled from the six biogeographical provinces listed in Figure 1, described by [START_REF] Longhurst | Ecological Geography of the Sea[END_REF]. In this study, the distribution of salinity, temperature, dCo, dFe and macronutrients (nitrate and phosphate) were used to identify the province boundaries (Table 1). The assigned province boundaries are subject to small-scale variations due to their seasonal drift, as is the ITCZ, a region that forms the boundary between the atmospheric hemispheres which migrates seasonally from a position centred at ~5°N in boreal winter to ~10°N in the boreal summer [START_REF] Sultan | Abrupt shift of the ITCZ over West Africa and intra-seasonal variability[END_REF].

Figure 1. AMT-19 cruise track, showing the 29 water column stations and the biogeochemical provinces [START_REF] Longhurst | Ecological Geography of the Sea[END_REF] Samples for the determination of dCo and dFe were collected from 10 L trace metalclean Teflon coated Ocean Test Equipment (OTE) bottles, attached to a titanium CTD rosette. Samples for macronutrients were collected from ten depths during each titanium CTD rosette deployment to correspond with trace metal sampling, and additionally from standard 20 L Niskin bottles fitted to a stainless steel CTD rosette (Seabird), thus providing high resolution profiling along the cruise track. All ship-based trace metal sample handling was conducted in a pressurised clean van. Seawater samples for dCo and dFe were filtered into acid-cleaned, low density polyethylene (LDPE) bottles (Nalgene) using a 0.2 μm Sartobran 300 filter capsule (Sartorius) and acidified to pH 1.7-1.8 (0.024 M) with ultraclean hydrochloric acid (HCl, Romil SpA) inside a class-100 laminar flow hood. Samples for the determination of TdFe were not filtered prior to acidification to 0.024 M HCl. All samples were then double zip-lock bagged for storage prior to analysis in the home laboratory.

Dissolved cobalt determination

Dissolved Co was determined in the ISO accredited clean room facility (ISO 9001) at Plymouth University, UK by flow injection with chemiluminescence detection (FI-CL; [START_REF] Shelley | Determination of total dissolved cobalt in UV-irradiated seawater using flow injection with chemiluminescence detection[END_REF]. Briefly, the flow injection manifold was coupled with a photomultiplier tube (Hamamatsu,. The dCo was determined in UV-irradiated samples (3 h; 400 W medium-pressure Hg lamp, Photochemical Reactors) from the chemiluminescence produced from the catalytic oxidation of pyrogallol (1,2,3-trihydroxybenzene), the chemiluminescence emission was recorded using LabVIEW v.7.1 software. Due to the extremely stable nature of organic complexes of Co in seawater, several studies have demonstrated the requirement to UV irradiate samples prior to analysis in order to liberate strongly-complexed Co (Vega and van den Berg, 1997;[START_REF] Donat | Direct determination of dissolved cobalt and nickel in seawater by differential pulse cathodic stripping voltammetry preceded by adsorptive collection of cyclohexane-1,2-dione dioxime complexes[END_REF][START_REF] Saito | Production of cobalt binding ligands in a Synechococcus feature at the Costa Rica upwelling dome[END_REF][START_REF] Shelley | Determination of total dissolved cobalt in UV-irradiated seawater using flow injection with chemiluminescence detection[END_REF]. During all analytical runs UV-irradiated SAFe D2 reference samples were analysed (n = 4; measured value, 50 ± 2 pM; consensus value 46± 3 pM).

Typically, blank values were 4 ± 1 pM (n = 8), with a detection limit of 3 pM (blank + 3σ).

Dissolved and total dissolvable iron determination

Dissolved Fe and total dissolvable Fe (TdFe; unfiltered seawater) were also determined using FI-CL in the same clean room facility as the dCo. The Fe FI-CL method used in this study was based on the method originally described by [START_REF] Obata | Automated determination of iron in seawater by chelating resin concentration and chemiluminescence detection[END_REF] and modified by [START_REF] De Baar | Titan: A new facility for ultraclean sampling of trace elements and isotopes in the deep oceans in the international GEOTRACES program[END_REF]. Briefly, measurements were made based on the catalytic oxidation of luminol (5-amino-2,3-dihydrophthalazine-1,4-dione; Aldrich) by hydrogen peroxide (H2O2) in the presence of Fe. As this method detects Fe(III), this study used a H2O2 oxidation step whereby H2O2 (10 nM) was added to each sample 1 h prior to the determination of Fe(III) [START_REF] Lohan | Determination of iron and copper in seawater at pH 1.7 with a new commercially available chelating resin[END_REF]. Chemiluminescence emission was detected by a Hamamatsu photomultiplier tube (model H 6240-01) and recorded using LabVIEW v.7.1 software. The accuracy of the method was assessed for every analytical run by the determination of dFe in SAFe S and D1 seawater reference materials. The concentrations of dFe measured in the SAFe reference samples were in good agreement with the consensus values (measured value, S = 0.12 ± 0.04 nM, n = 13; D1 = 0.72 ± 0.08 n = 14; consensus value, S = 0.093 ± 0.008 nM; D1 = 0.67 ± 0.04).

Consensus values for dCo and dFe were reported to the GEOTRACES Intercalibration Committee in 2010 (dCo) and 2011 (dFe), and are available at:http://geotraces.org/science/intercalibration/322-standards-and-reference-materials.

Nutrients, temperature, salinity and chlorophyll-a

Dissolved inorganic macronutrients, phosphate (PO4 3-) and nitrate (NO2 -+ NO3 -= Σ NO3) were analysed on-board within 3-4 h of collection using a 5-channel segmented flow autoanalyser (Bran and Luebbe, AAII AutoAnalyzer) following standard colorimetric procedures [START_REF] Grashoff | Methods in Seawater Analyses[END_REF]) modified by [START_REF] Woodward | The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea[END_REF]. Low-level nutrients were not determined using liquid wave guides during AMT-19.

Salinity, temperature and dissolved O2 were measured using a CTD system (Seabird 911+). Dissolved O2 was determined by a Seabird SBE 43 O2 sensor. Salinity was calibrated on-board using discrete samples taken from the OTE bottles using an Autosal 8400B salinometer (Guildline), whilst dissolved O2 was calibrated using an automated photometric Winkler titration system [START_REF] Carritt | Comparison and evaluation of currently employed modifications of the Winkler method for determining dissolved oxygen in seawater; a NASCO Report[END_REF]. Chlorophyll fluorescence and beam attenuation were determined using an Aquatraka MkIII fluorometer and Alphatraka MkII transmissometer (Chelsea Instruments), respectively. Sampling depths were determined by reference to the in situ fluorescence, temperature, salinity and irradiance (photosynthetically active radiation, PAR, 400-700 nm) profiles, to include 97%, 55%, 33%, 14%, 1% and 0.1% PAR levels. For chlorophyll-a determination, samples were filtered (0.2 μm polycarbonate) and the filters extracted in 10 mL of 90 % acetone overnight at 4° C [START_REF] Welschmeyer | Fluorometric Analysis of chlorophyll a in the presence of chlorophyll b and pheopigments[END_REF]. The chlorophyll-a extract was measured on a pre-calibrated (pure chlorophyll-a standard, Sigma-Aldrich) Turner Designs Trilogy 700 fluorimeter.

Prochlorococcus and Synechococcus were enumerated by flow cytometry using a Becton Dickinson FACSort (Oxford, UK) flow cytometer equipped with an air-cooled laser providing blue light at 488 nm [START_REF] Tarran | Latitudinal changes in the standing stocks of nano-and picoeukaryotic phytoplankton in the Atlantic Ocean[END_REF].

The trace metal (dCo, dFe and TdFe) data, ancillary data and a full station list are available at: http://www.bodc.ac. uk/ projects/uk/amt/

RESULTS

Hydrographic setting and macronutrient distributions

The six biogeographical provinces used in this study are shown in Figure 1. Note that the North Atlantic gyre is divided into two separate provinces; the North Atlantic subtropical gyre (NAST) and the North Atlantic tropical gyre (NATR). In these provinces, the thermohaline structure of the upper water column (Fig. 2) is primarily determined by the water masses that occupy each region and the relative evaporation and precipitation rates.

In the North Atlantic, the lowest upper water column temperatures (12-22°C) were observed in the NADR. Here, the water column displayed weak thermohaline stratification, characteristic of high wind stress in the NADR during boreal autumn [START_REF] Longhurst | Ecological Geography of the Sea[END_REF]. In the NAST, the introduction of warmer (> 20° C), more saline (> 36.5), water from the Gulf Stream enters via the Azores Current (AC, centred at 35-36 °N) [START_REF] Aiken | The Atlantic Meridional Transect: overview and synthesis of data[END_REF] resulting in a mixed layer depth of between 40 and 50 m. Further south in the NATR, the North Equatorial Current (NEC, centred at 15 °N) supplies water with salinity > 37, due to the high rates of evaporation at these latitudes. Consistent with previous AMT observations [START_REF] Aiken | The Atlantic Meridional Transect: overview and synthesis of data[END_REF][START_REF] Robinson | The Atlantic Meridional Transect (AMT) Programme: A contextual view 1995-2005[END_REF], the NEC was observed to depths of ~ 150 m between 20 and 26° N during AMT-19.

Towards the southern extent of the NATR province, a plume of cooler (< 20° C), fresher (< 36), lower oxygen (< 150 µM) upwelled water was clearly visible below 60 m (Fig. 2). This oxygen minimum zone (OMZ), which extended throughout the tropical Atlantic to the southern boundary of the WTRA, results from the divergence between the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC) at ~ 10° N, and the divergence between the NECC and the South Equatorial Current (SEC) at ~ 2° S [START_REF] Hastenrath | Annual cycle of subsurface thermal structure in the tropical Atlantic Ocean[END_REF][START_REF] Longhurst | Ecological Geography of the Sea[END_REF][START_REF] Aiken | The Atlantic Meridional Transect: overview and synthesis of data[END_REF] (Fig. 1). Mixed layer depths (defined as the depth at which potential density differed by 0.05 kg m -3 from the surface) in the WTRA varied between 9 and 95 m. Throughout the upper 150 m of the WTRA low salinity (< 36.5) water, relative to the sub-tropical gyres, was observed caused by dilution through excess precipitation over evaporation [START_REF] Aiken | The Atlantic Meridional Transect: overview and synthesis of data[END_REF].

A surface salinity minimum (< 35) was observed in the WTRA between ~6 and 10° N to a depth of 30 m (Fig. 2), a common feature that can arise from either converging air masses and subsequent high precipitation rates in the ITCZ, or from Amazon Water transported eastwards across the Atlantic by the NECC [START_REF] Aiken | The Atlantic Meridional Transect: overview and synthesis of data[END_REF]. However, no elevation in surface silicate concentration (data not shown), which would be indicative of Amazon Water, was observed during AMT-19. In addition, two intense rainfall events were recorded between 6 and 9 °N during the cruise, suggesting that the high rates of precipitation that characterise the ITCZ could be the cause of the WTRA salinity minimum.

As observed during earlier AMT studies [START_REF] Robinson | The Atlantic Meridional Transect (AMT) Programme: A contextual view 1995-2005[END_REF], a gradual latitudinal decrease in sea surface temperature and salinity was observed in the SATL (10-33° S) and into the SSTC (33 -38° S), a manifestation of the decrease in evaporation rates associated with lower temperatures at higher latitudes. An increase in the westerly winds as the ship travelled south, coupled with increased downwelling associated with the anti-cyclonic circulation of the sub-tropical gyre [START_REF] Longhurst | Ecological Geography of the Sea[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]

Dissolved Co and Fe distributions

Surface water (upper 25 m) dCo and dFe distributions during AMT-19 displayed distinct differences between the North and South Atlantic (Fig. 4). Surface dCo concentrations during AMT-19 were highly variable (10-93 pM). The lowest concentrations were observed in the northern gyre provinces (NAST 25 ± 14 pM and NATR 21 ± 2.8 pM, respectively, n = 6), whilst higher concentrations were observed in the upwelling region (WTRA 51 ± 38 pM, n = 9) and the South Atlantic gyre (SATL 60 ± 31 pM, n = 3) (Fig. 4) This trend is similar to that previously reported for PO4, with very low concentrations of PO4 (0.01-0.05 µM) observed in the North Atlantic gyre regions and higher concentrations (0.2-0.5 M) in the South Atlantic gyre [START_REF] Mather | Phosphorus cycling in the North and South Atlantic Ocean subtropical gyres[END_REF]. At approximately 28° S the SATL is sub-divided into two cells separated by the subtropical counter-current. To the south of this front (25-30° S) the Brazil Current (BC) forms the southern extent of a recirculation cell [START_REF] Mémery | The water masses along the western boundary of the south and equatorial Atlantic[END_REF] and references therein). The high surface dCo in this region (89 ± 4 pM at 28.8°S, 26.1°W, Fig. 4) is attributed to offshore advection of continental Co mobilised by the western boundary current and a declining gradient is observed to the south of this frontal region. The surface water (upper 25 m) dFe and TdFe distribution is in complete contrast to dCo, as dFe and TdFe were relatively high in the NATR and NAST, and low in the SATL (Fig. 4). The highest surface dFe and TdFe concentrations were observed in the NATR (dFe, 0.68 ± 0.28 nM; TdFe, 1.1 ± 0.25 nM, n = 12 and 10, respectively) and the WTRA (dFe, 0.76 ± 0.61 nM; TdFe 1.3 ± 0.33 nM, n = 6) provinces between ~ 5 and 30° N, corresponding to the latitudinal extent of the Saharan plume (5-30° N) [START_REF] Prospero | Environmental characterization of global sources of atmospheric dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product[END_REF][START_REF] Kaufman | Dust transport and deposition observed from the Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) spacecraft over the Atlantic Ocean[END_REF]. Here, two distinct surface dFe maxima were observed. The first, located between ~ 20 and 28° N (dFe, 0.88 ± 0.14 nM, n = 6), was in the vicinity of the elevated rates of surface nitrogen fixation (0.85-1.1 nmol L -1 d -1 ) determined during this study (data not shown, but available from www.bodc.ac.uk). The second, at ~10-14° N (0.74 ± 0.58 nM, n = 7), overlapped with the ITCZ surface salinity minimum (Fig. 2), which is consistent with the observation that high rainfall rates associated with the ITCZ contributes to high wet deposition fluxes of Fe in the south NATR/north WTRA [START_REF] Kim | Wet deposition of trace elements and radon daughter systematics in the South and equatorial Atlantic atmosphere[END_REF][START_REF] Powell | Estimation of the atmospheric flux of nutrients and trace metals to the eastern tropical North Atlantic Ocean[END_REF]. The locations of these two surface dFe maxima coincided with high TdFe concentrations (1.1 ± 0.17 nM and 1.3 ± 0.28 nM, respectively) between 4 -30° N, and are in excellent agreement with observations from previous North Atlantic studies [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF][START_REF] Bergquist | Dissolved iron in the tropical and subtropical Atlantic Ocean[END_REF][START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. Combined with the low dFe in the SATL, the peaks in dFe and TdFe in the North Atlantic gyre provinces indicate the importance of atmospheric deposition in controlling surface dFe concentrations (e.g., [START_REF] Schlosser | Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide[END_REF]. North of ~ 30° N, surface dFe concentrations were lower (0.34 ± 0.14 nM, n = 14) and less variable (Fig. 4), most likely due to a reduced Saharan dust input and strong winter mixing in the NAST and NADR, compared with weak seasonal mixing in the NATR [START_REF] Longhurst | Ecological Geography of the Sea[END_REF].

In sub-surface waters (deeper than 25 m), the dCo distribution was also a tale of sharp contrasts. Extremely low concentrations were observed throughout the North Atlantic gyre provinces, with the lowest concentrations (16 ± 3.4 pM, n = 8) observed at the base of the mixed layer. The maximum abundances of Prochlorococcus (> 4 x 10 5 cells mL -1 ), a cyanobacteria with an absolute requirement for Co (Sunda and Huntsman, 1995a) At the dynamic SATL/SSTC boundary (33.3°S, 34.2°W), a slight increase in dCo was observed at 80 m relative to the surrounding water (58 pM at 80 m, 44 pM at 45 m and 29 pM at 100 m). The source of this high dCo is not immediately clear, but may result from spinoff of eddies containing higher dCo water from the south. The presence of eddies in this region is confirmed by the sea surface anomaly image, Fig. S1 in the Supplementary Material. As concentrations of dCo can be highly variable over scales of ~10 km (Saito and Moffett, 2002;[START_REF] Noble | Cobalt, manganese, and iron near the Hawaiian Islands: a potential concentrating mechanism for cobalt within a cyclonic eddy and implications for the hybrid-type trace metals[END_REF][START_REF] Shelley | Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum[END_REF], the low dCo observed at the adjoining station (15.5 ± 0.3 pM at 35.3°S, 37.1°W) may be just as characteristic of this province (reflecting seawater that has had no contact with the continental shelf and low atmospheric inputs) as water with high dCo. Regardless of the dCo concentration, in all gyre provinces dCo exhibited a broadly nutrient-type distribution (lower concentrations in the mixed layer than below it) in the upper 150 m.

The sub-surface distribution of dFe also displayed strong latitudinal gradients (Fig. 4.) In a reversal of the trend for dCo, sub-surface dFe concentrations in the SATL were low and relatively uniform (0.26 ± 0.06 nM, n = 12) compared with the northern gyre provinces (0.40 ± 0.17 nM, n = 25) where atmospheric deposition is much higher. Below 100 m in the northern NATR/southern NAST waters between 23 and 31° N, the dFe and TdFe concentrations were 0.48 ± 0.14 nM (n = 5) and 0.72 ± 0.11 nM (n = 5), respectively and could be a relic of a previous atmospheric deposition event. Interestingly, we observed a similar feature at the same depth for dCo (36 ± 3.4 pM; Fig. 4).

For both dCo and dFe, elevated sub-surface concentrations were associated with the low oxygen waters. Maximum sub-surface dCo and dFe concentrations (62 ± 16 pM and 0.62 ± 0.20 nM, respectively) were observed between 0-10 °N, coincident with an oxygen minimum of 100 -150 µM (Fig. 2). Observations of elevated dFe in this OMZ are consistent with previous studies [START_REF] Bergquist | Dissolved iron in the tropical and subtropical Atlantic Ocean[END_REF][START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Fitzsimmons | Dissolved iron in the tropical North Atlantic Ocean[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF] suggesting that the elevated dFe may be a steady-state feature in this region, sustained by either remineralisation of high Fe:C organic matter formed in the Fe-rich surface and/or lateral mixing of high dFe water from sedimentary sources. However, in contrast to dFe, the elevated dCo concentrations were not confined to the OMZ, but extended over a broader latitudinal range (southwards) and wider depth range, suggesting that mechanisms other than remineralisation and low dissolved oxygen concentrations were sustaining the elevated dCo concentrations in this region.

DISCUSSION

Given that there are a number of similarities in the redox and organic speciation of Co and Fe, the difference in the distributions of these two elements in the Atlantic Ocean is stark. In the northern gyre provinces (NATR and NAST), where deposition and dissolution of atmospheric aerosols is the dominant source of Fe (e.g. [START_REF] Duce | Atmospheric transport of iron and its deposition to the ocean[END_REF][START_REF] Duce | Atmospheric transport of iron and its deposition to the ocean[END_REF][START_REF] Sarthou | Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean[END_REF][START_REF] Jickells | Global iron connections between desert dust, ocean biogeochemistry and climate[END_REF][START_REF] Baker | Trends in the solubility of iron, aluminium, manganese and phosphorus in aerosol collected over the Atlantic Ocean[END_REF][START_REF] Buck | The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: Observations from the A16N CLIVAR/CO2 repeat hydrography section[END_REF][START_REF] Evangelista | Inferring episodic atmospheric iron fluxes in the western South Atlantic[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF], the extremely low concentrations of dCo contrast strongly with the relatively high concentrations of dFe. A number of studies have alluded to an atmospheric source of Co which could influence surface dCo concentrations in regions of high atmospheric deposition [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF]Dulaquais et al., 2014a;[START_REF] Knauer | Cobalt in north-east Pacific waters[END_REF][START_REF] Thuroczy | Dissolution of cobalt and zinc from natural and anthropogenic dusts in seawater[END_REF][START_REF] Wong | Cobalt in the West Philippine Sea[END_REF]. Furthermore, aerosol Co is significantly more soluble than aerosol Fe (Dulaquais et al., 2014a;[START_REF] Mackey | Rapid and gradual modes of aerosol trace metal dissolution in seawater[END_REF]e.g. 8-10% fractional solubility for Co and 0.44-1.1% fractional solubility for Fe for the same Saharan dust samples, [START_REF] Shelley | Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum[END_REF], further supporting the assertion that atmospheric supply may play a pivotal role in controlling surface distributions of dCo and hence influence phytoplankton community dynamics.

For dFe, the sharpest gradient was observed at the NAST/NATR boundary, and is almost certainly linked to atmospheric inputs and the approximate location of the northern extent of the Saharan plume. Indeed the relationship between dFe in the upper water column and atmospheric supply are well documented (e.g. [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF][START_REF] Baker | Trends in the solubility of iron, aluminium, manganese and phosphorus in aerosol collected over the Atlantic Ocean[END_REF][START_REF] Baker | Dry and wet deposition of nutrients from the tropical Atlantic atmosphere: Links to primary productivity and nitrogen fixation[END_REF]2013;[START_REF] Rijkenberg | Fluxes and distribution of dissolved iron in the eastern (sub-) tropical North Atlantic Ocean[END_REF][START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF], which makes the low dCo in the same latitudinal band somewhat of a paradox. One explanation could be that the Co is being scavenged in the water column following oxidation by manganese (Mn) oxidising bacteria, which oxidise both Mn and Co via a common microbial pathway (Moffet and Ho, 2001). However, significant removal via the Mn co-oxidation pathway is not supported by the literature in open ocean environments, as it is driven by competitive inhibition [START_REF] Moffett | Oxidation of cobalt and manganese in seawater via a common microbially catalyzed pathway[END_REF][START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF] and dCo is low (this study; A. Noble, pers. comm.) and dMn is high [START_REF] Wu | The distribution of dissolved manganese in the tropical-subtropical North Atlantic during US GEOTRACES 2010 and 2011 cruises[END_REF]Hatta et al., 2015) in the northern gyre provinces.

In the vicinity of the ITCZ, both dFe and TdFe were significantly inversely related to salinity in the mixed layer (r 2 = 0.89 and 0.82 respectively; p < 0.05, n = 5) suggesting that the scavenging of dust incursions into the ITCZ [START_REF] Adams | CALIPSO-Derived Three-Dimensional Structure of Aerosol over the Atlantic Basin and Adjacent Continents[END_REF] as it migrated south towards to its boreal winter position (centred at ~ 5° N) could be a source of Fe to surface waters at the NATR/WTRA border, as described by [START_REF] Kim | Wet deposition of trace elements and radon daughter systematics in the South and equatorial Atlantic atmosphere[END_REF]. However, the small number of samples (n = 5) make any links tenuous at best, particularly as this relationship is driven by the high dFe and TdFe values (both 1.1 nM) at 1.5 m depth at 10.6 °N, 32.0 °W. Similarly, the relatively sparse dCo dataset for mixed layer waters influenced by the ITCZ (n = 4) makes assessing a link between dCo and precipitation unrealistic, and is further complicated by the limited literature on dCo in rainwater of the ITCZ and the contrasting conclusions reached; i.e. either precipitation dilutes surface dCo (Helmer and Schremms, 1995;[START_REF] Pohl | Synoptic transects on the distribution of trace elements (Hg, Pb, Cd, Cu, Ni, Zn, Co, Mn, Fe, and Al) in surface waters of the Northern and Southern East Atlantic[END_REF], or it is a source of dCo [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF]. In this study, two modest enrichments of dCo (relative to the underlying water and to adjoining stations) coincided with rain events at ~ 31 °N, and the intense rain events in the ITCZ at 6 and 9 °N (M. Chieze, pers. Comm; www.giovanni. sci.gsfc.nasa.gov). At 31 °N, for example, the concentration of dCo was 46.4 pM at 2 m depth, whereas at 25 m depth dCo had been drawn down to 21.2 pM. In addition, wet deposition has been estimated to account for >90% of the total atmospheric deposition flux of Co, compared with just 20% for Fe, based on data from Bermuda (T. Church, unpublished data). In the eastern tropical Atlantic (in September-November), [START_REF] Powell | Estimation of the atmospheric flux of nutrients and trace metals to the eastern tropical North Atlantic Ocean[END_REF] estimate that wet deposition may be a relatively more important source of Fe than in the western North Atlantic gyre, contributing up to 70% of the total atmospheric flux.

We have estimated the soluble Co and Fe deposition fluxes for 20 °N and 20 °S from dry deposition data published in [START_REF] Shelley | Elemental ratios and enrichment factors in aerosols from the US-GEOTRACES North Atlantic transects[END_REF] and Dulaquais et al. (2014a) (20 °N) and [START_REF] Chance | Atmospheric trace metal concentrations, solubility and deposition fluxes in remote marine air over the south-east Atlantic[END_REF] (20 °S) (Table 1). For Co, in the NATR, under the Saharan outflow, dry deposition contributes only 1.4% of the mixed layer depth (MLD) concentration of dCo (assuming permanent stratification of the water column). In contrast, atmospheric deposition may supply twice the amount of dFe observed in the mixed layer over the course of the year.

In the SATL, where atmospheric deposition may be orders of magnitude lower, atmospheric supply alone cannot account for the concentrations of either metal observed (<<0.5% and 21% of mixed layer dCo and dFe, respectively). It is noted that these atmospheric deposition fluxes do not account for wet deposition, and thus, the estimates presented in Table 1 may be rather conservative. Nonetheless, these data highlight the role of atmospheric deposition in controlling the dFe concentrations in surface waters of the two gyre regions. For Co, the impact of atmospheric deposition is more subtle.

Our calculations are sensitive to the percentage of the metal that is soluble in seawater. Unfortunately, aerosol metal solubility is poorly constrained. In Table 1, a Co solubility value of 9.0% is used for the NATR (Dulaquais et al., 2014a). However, Co solubility is a function of the composition of the bulk aerosol, which in turn is a function of aerosol provenance, and may be up to threefold higher (i.e., ~30%, R. Shelley, unpublished data, available at: www.bco-dmo.org) in aerosols sourced from Europe as opposed to those from North Africa, due to a higher component of industrial emission aerosols in the former. This will result in a higher flux of soluble Co, and given the extremely low concentrations of dCo in the northern gyre provinces, suggests that atmospheric supply may still have an important role in supplying Co to surface waters [START_REF] Thuroczy | Dissolution of cobalt and zinc from natural and anthropogenic dusts in seawater[END_REF]. If, as our data suggests, aerosols are indeed a source of Co to the northern gyre provinces, how can the contrasting distributions of dCo and dFe be reconciled? We hypothesise that biological uptake primarily by the dominant components of the bacterial assemblage, such as the cyanobacteria Prochlorococcus and Trichodesmium, is exceeding supply, leading to a dCo deficit in the northern gyre provinces (NAST, NATR).

Biological controls on dissolved Co distributions

Although Prochlorococcus are ubiquitous in tropical and sub-tropical oceans, their range extends throughout the Atlantic from ~50 °N -40 °S [START_REF] Heywood | Prokaryoplankton standing stocks in oligotrophic gyre and equatorial provinces of the Atlantic Ocean: Evaluation of inter-annual variability[END_REF].

Prochlorococcus thrive in oligotrophic conditions and have an obligate requirement for Co for carbon fixation (Sunda and Huntsman, 1995a;Saito et al., 2002). During AMT-19, Prochlorococcus dominated the picoplankton assemblage, with Synechococcus only proliferating where Prochlorococcus abundance was less than 10 5 cells mL -1 (Fig. 5), i.e., the temperate margins of this AMT transect (NADR and SSTC), and in the low-salinity (<35)

surface waters of the ITCZ (upper 30 m at 6-10 °N; Fig. 2). Our data are consistent with the observation that Prochlorococcus typically outnumber Synechococcus by one to two orders of magnitude in stratified, oligotrophic waters [START_REF] Durand | Phytoplankton population dynamics at the Bermuda Atlantic time-series station in the Sargasso Sea[END_REF]. In this study, the highest abundances of Prochlorococcus were observed in the high-dCo tropical upwelling region (~5 °N-5 °S) (Fig. 5). This contrasts with the phytoplankton dynamics in another high dCo upwelling region, the Costa Rica upwelling dome (CRD),

where Synechococcus dominated the picoplankton assemblage (Ahlgren et al., 2014).

In terms of Prochlorococcus abundance, the northern gyre was divided in two (at approximately the boundary between the NATR and NAST). The NAST and NATR, were both characterised by extremely low dCo concentrations, with the dCo minima (NAST = 15 ± 3.8, NATR = 15 ± 1.4 pM, at 28-45 m) generally corresponding with the maximum abundances of Prochlorococcus in these provinces (Fig. 5). The Prochlorococcus maxima were at shallower depths than the DCM (e.g., 4 x10 5 cells mL -1 at 29 m at 11.5 °N, compared to a DCM of 0.41 µg L -1 chl-a between 46-50 m ). The relationship between dCo distributions, Prochlorococcus abundance, and the relative position of the DCM has previously been observed in the Sargasso Sea (western NAST/NATR; [START_REF] Shelley | Controls on dissolved cobalt in surface waters of the Sargasso Sea: Comparisons with iron and aluminum[END_REF].

In the NATR, Prochlorococcus abundance was high (>3x10 5 cells mL -1 ), even though dCo was extremely low (22 ± 15 pM). In the NAST, dCo was similarly low (22 ± 3.8 pM), but

Prochlorococcus abundance was lower than in the NATR (generally <2x10 5 cells mL -1 ). As atmospheric deposition decreases northwards from the NATR to NAST, we hypothesise that aerosol supply indirectly impacts Prochlorococcus abundance via its role as a key source of Co and Fe. Moreover, the sub-surface dCo minimum coincides with the region of maximum rates of nitrogen fixation during AMT-19 (21 -23°N), consistent with a Co requirement for nitrogen fixation by Trichodesmium [START_REF] Rodriguez | Influence of Co and B12 on the growth and nitrogen fixation of Trichodesmium[END_REF], which are abundant in the tropical to subtropical North Atlantic, but almost entirely absent between 5 and 30° S [START_REF] Tyrrell | Large-scale latitudinal distribution of Trichodesmium spp. in the Atlantic Ocean[END_REF][START_REF] Schlosser | Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide[END_REF]. In the SATL, dCo concentrations and Prochlorococcus abundance were decoupled to the extent that the opposite trend was observed, with high dCo and high abundances of Prochlorococcus occurring together. This occurred in concert with a near absence of Trichodesmium, suggesting that the presence/absence of Trichodesmium may also have an important role in driving the dCo distribution.

Moreover, the presence/absence of other bacteria may influence dCo distributions.

Although the bacterial abundance was roughly equivalent in the SATL and NATR/NAST during AMT-19 (M. Zubkov, pers. comm.) differences in the bacterial community composition have been reported between the two gyres (Schattenhofer et al., 2009;Friedline et al., 2012). In addition to the cyanobacteria, the marine bacteria SAR11, require Co for vitamin B12 [START_REF] Carini | Nutrient requirements for growth of the extreme oligotroph 'Candidatus Pelagibacter ubique' HTCC1062 on a defined medium[END_REF] and are more abundant in the northern gyre provinces and WTRA compared to the SATL (Schattenhofer et al., 2009;[START_REF] Bown | The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa[END_REF]Friedline et al., 2012). Recent work has demonstrated that bacteria are the first to directly respond to Saharan dust inputs of trace elements and nutrients, and these authors argue that the bacterial assemblage is a key mediator of trace metal distributions following dust deposition [START_REF] Westrich | Saharan dust nutrients promote Vibrio bloom formation in marine surface waters[END_REF]. Thus, the differences in bacterial community composition can impact the biogeochemical cycle of Co and hence explain the differing dCo distributions between the northern and southern gyres.

In addition to active uptake, Trichodesmium, which are abundant in the subtropical/ tropical North Atlantic due to the delivery of atmospheric Fe [START_REF] Richier | Abundances of iron-binding photosynthetic and nitrogen-fixing proteins of Trichodesmium both in culture and in situ from the North Atlantic[END_REF] and P [START_REF] Ridame | Does phosphate adsorption onto Saharan dust explain the unusual N/P ratio in the Mediterranean Sea?[END_REF], can scavenge both Fe [START_REF] Rubin | Dust-and mineral-iron utilization by the marine dinitrogen-fixer Trichodesmium[END_REF] and P from solution (Sañudo-Wilhelmy et al., 2001). Could the same removal mechanism be an important sink for Co? Although we do not have particulate Co or TdCo data for AMT-19, TdCo was determined in surface samples (7 m depth) on AMT-3 (a similarly grye-centred AMT; [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF], where low concentrations of ~ 30 pM dCo (AMT-19) and TdCo (AMT-3) were observed between 3 and 17 °N. In addition, recent studies of particulate Co in the Atlantic

Ocean, demonstrated that it was ~ 5% the concentration of dCo in a full-depth transect along ~12 °S [START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF] and 12 ± 12 in the West Atlantic (Dulaquais et al., 2014a), suggesting that scavenging may only be a minor sink for Co under a range of open ocean environmental conditions.

Lastly, dCo distributions can be influenced by dissolved organic phosphorus (DOP) acquisition. The region where extremely low dCo was observed is also where chronically low PO4 concentrations are observed [START_REF] Mather | Phosphorus cycling in the North and South Atlantic Ocean subtropical gyres[END_REF]. In the North Atlantic gyre provinces the DOP pool is 5-10 times higher than inorganic phosphorus and phytoplankton and bacteria must utilise AP to acquire their essential phosphorus requirement [START_REF] Mahaffey | Alkaline phosphatase activity in the subtropical ocean: insights from nutrient, dust and trace metal addition experiments[END_REF]. Zinc is the metal co-factor in the protein PhoA used for AP activity and, while Co can substitute for Zn as the metal centre in PhoA (Sunda and Huntsman 1995a), the preference is for Zn (Saito and Goepfert 2008). A recent study in the sub-tropical Atlantic has demonstrated that Zn concentrations, which are very low in this region, could limit AP activity [START_REF] Mahaffey | Alkaline phosphatase activity in the subtropical ocean: insights from nutrient, dust and trace metal addition experiments[END_REF]. Therefore, the low Co concentrations may arise from uptake by cyanobacteria and also from its substitution for Zn in AP. Using Co uptake results from freshwater phytoplankton grown under PO4 limitation, [START_REF] Ji | Differential effects of phosphorus limitation on cellular metals in Chlorella and Microcystis[END_REF] hypothesised, that the very high demand for Co in the tropical North Atlantic may be the result of persistent PO4 stress in this region. However, the discovery of a calcium (Ca)-based AP [START_REF] Kathuria | Prevalence of a calcium-based alkaline phosphatase associated with the marine cyanobacterium Prochlorococcus and other ocean bacteria[END_REF] suggests that at least some Prochlorococcus ecotypes and bacteria are able to bypass the need for Co in AP, which may reduce the potential for Co-Zn-P co-limitation.

However, field based evidence in this region clearly shows that the AP activity is limited by Zn [START_REF] Mahaffey | Alkaline phosphatase activity in the subtropical ocean: insights from nutrient, dust and trace metal addition experiments[END_REF]) as a result of the extremely low dZn concentrations in the North Atlantic [START_REF] Conway | The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic Ocean[END_REF][START_REF] Roshan | Water mass mixing: The dominant control on the zinc distribution in the North Atlantic Ocean[END_REF] and with the low dCo concentrations observed in this study AP activity may also be limited by Co

In the SATL, Trichodesmium is largely absent [START_REF] Tyrrell | Large-scale latitudinal distribution of Trichodesmium spp. in the Atlantic Ocean[END_REF][START_REF] Schlosser | Seasonal ITCZ migration dynamically controls the location of the (sub)tropical Atlantic biogeochemical divide[END_REF], and Procholorococcus abundance was lower, with maximum abundances deeper than in the northern gyre provinces likely due to significantly lower dFe concentrations, and a deeper MLD in the northern section of the SATL compared with the NATR/NAST (Figs. 4 and5). The positive correlations between dCo and Prochlorococcus abundance in the South Atlantic (Fig. S2, Supplemental Material) may be linked with higher inorganic phosphorus availability, as well as higher dCo. In the South Atlantic, where atmospheric deposition is low, a combination of highly efficient internal cycling (85% of the dCo uptake rate in the SATL may be accounted for by remineralisation of organic matter, Dulaquais et al., 2014a), lateral inputs [START_REF] Bown | The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa[END_REF][START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF] and relatively low biological demand results in higher dCo concentrations compared with the northern gyre provinces.

The different relationship between dCo and bacterial dynamics in the northern gyres and the SATL suggests that dCo availability has the potential to influence both the bacterial and phytoplankton community structure, or vice versa, through a complex interplay with other factors, such as Fe and inorganic phosphorus availability. However, the northern gyre provinces appear unique in the sense that biotic removal dominates and controls dCo distributions (Moffett and Ho, 1986). In future decades increased stratification and predicted increases in nitrogen supply [START_REF] Behera | Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies[END_REF] could exacerbate the disparity between the northern and southern gyres in terms of trace metal distributions as a result of proximal nutrient limitation and, thus, the potential for changes to the bacterioplankton community structure.

Low oxygen waters

Upwelling (vertical transport) can deliver macro-and micronutrient-enriched deep water to the mixed layer of the tropical North Atlantic, although for Fe the dominant flux is from the atmosphere [START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF]. Using the average dCo and dFe concentrations from below the surface mixed layer of the WTRA during AMT-19 (64 and 421 nmol m -3 , respectively) and an upward vertical mixing rate of 14.3 m y -1 (based on the method presented by [START_REF] Ussher | Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean[END_REF] for a similar cruise track, AMT-16), we estimate an upward vertical mixing flux of 2.5 and 16.5 nM m -2 d -1 for dCo and dFe, respectively. The combination of this upward vertical transport of nutrient-rich water and atmospheric supply sustains relatively high algal biomass in surface waters of the tropical Atlantic (e.g., the maximum chl-a concentration of 0.41 µg L -1 was observed at 11.5 N at 46-50 m just above the thermocline and oxycline). These high levels of primary productivity result in a large amount of sinking detritus. Bacterial degradation of this detritus consumes oxygen which, in turn, contributes to the development of OMZs. In the productive eastern equatorial Atlantic, a broad OMZ extends from ~100-900 m depth [START_REF] Karstensen | Oxygen minimum zones in the eastern tropical Atlantic and Pacific Oceans[END_REF].

Both high dCo and dFe have previously been reported in the oxygen deficient waters of the WTRA [START_REF] Bowie | Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean[END_REF][START_REF] Measures | High-resolution Al and Fe data from the Atlantic Ocean CLIVAR-CO2 repeat hydrography A16N transect: extensive linkages between dust and upper ocean geochemistry[END_REF][START_REF] Pohl | Synoptic transects on the distribution of trace elements (Hg, Pb, Cd, Cu, Ni, Zn, Co, Mn, Fe, and Al) in surface waters of the Northern and Southern East Atlantic[END_REF]) and during AMT-19 elevated dCo (> 60 pM) and dFe (> 0.60 nM) were observed in the OMZ of the WTRA.

However, while elevated dFe in the sub-surface WTRA was associated with the OMZ (150 μM contour positioned at depths > 40-100 m depending on latitude), elevated sub-surface dCo covered a much wider depth range and was not confined to the WTRA, spilling over into the SATL at depths below ~ 100 m (Fig. 4). It is unlikely that the WTRA is supplying dCo to the SATL, as the two provinces are separated by the South Equatorial Current (SEC), and there is no evidence of elevated dFe to the south of the upwelling zone. Rather, preferential scavenging of Fe with respect to Co, in the Benguela and South Equatorial Currents [START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF], which feed into the South Atlantic gyre, provides the most likely explanation for the difference in dCo and dFe concentrations to the south of the upwelling zone.

During AMT-19, the 100 μM O2 contour was observed to shoal to depths as shallow as 100 m, and in the WTRA as a whole the DCM was positioned just above the 150 μM O2 horizon. In these productive waters bacterial degradation of sinking organic particles is evidenced by the apparent oxygen utilisation (AOU). Furthermore, the bacteria that consume the oxygen during the bacterial degradation of particles may be an additional source of highaffinity, metal binding ligands [START_REF] Barbeau | Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands[END_REF][START_REF] Sarthou | Atmospheric iron deposition and sea-surface dissolved iron concentrations in the eastern Atlantic Ocean[END_REF] which also retain remineralised Co and Fe in solution. While a positive relationship between dFe and AOU (r 2 = 0.6, p = 0.03, n = 7) in the latitudinal band 1-17 °N, was observed, for dCo and AOU the relationship was weak and not significant (r 2 = 0.2, p = 0.3, n = 8) (Fig. 6), suggesting that other sources of Co (e.g., vertical transport, lateral advection) are relatively more important in this region.

Lateral transport

In this study, surface dFe concentrations in the NADR of 0.20-0.58 nM were similar to the 0.14-0.60 nM reported by [START_REF] Ussher | Distribution and redox speciation of dissolved iron on the European continental margin[END_REF] for Northeast Atlantic surface waters.

These authors observed a dFe concentration gradient over a relatively short distance spanning the shelf break, and concluded that minimal lateral transport of dFe from the shelf to the open ocean occurred in this region, despite severe winter storms. In this study, there was little evidence for the lateral transport of dFe from the European shelf margin to the open ocean. In contrast, in the NADR, a sharp gradient in dCo was observed at the boundary with the NAST with the highest concentrations of dCo appearing to be transported offshore along the 26.0 kg m -3 isopycnal. In the South Atlantic, although the Falkland/Malvinas Current could potentially be a vector for the offshore transport of dFe, here, too, we saw no evidence for the offshore transport of dFe Lateral advection may however, be a more important source of dCo. Indeed, [START_REF] Bown | The biogeochemical cycle of dissolved cobalt in the Atlantic and the Southern Ocean south off the coast of South Africa[END_REF] report evidence of just such a mechanism in the Southeast Atlantic Ocean.

Furthermore, [START_REF] Noble | Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean[END_REF] also observed a large-scale (> 2000 km), offshore dCo plume in the SATL. These authors also noted offshore advection of dFe, but that the plume covered a far smaller distance (< 500 km) than the dCo plume, and despite no evidence for offshore advection of dMn (a tracer for sedimentary inputs), they concluded that reducing sediments on the African margin were a likely source of all three metals. However, dFe and dMn were scavenged preferentially to dCo, which explained the difference in the extent of the offshore plumes. Dulaquais et al. (2014b) also argue that scavenging is a fairly insignificant removal term for dCo in the western Atlantic, as they were unable to resolve dCo removal, via scavenging, from dilution by mixing.

To the south of the SATL, the cruise track passed through a dynamic frontal region, the confluence of the Brazil and the Falkland/Malvinas Currents. Both western boundary currents flow along the continental shelf until they meet and are deflected offshore. Indeed, [START_REF] Boebel | Kinematic elements of Antarctic Intermediate Water in the western South Atlantic[END_REF] and [START_REF] Jullion | Circulation and water mass modification in the Brazil-Malvinas Confluence[END_REF] report cross frontal mixing in the Argentine Basin of the subtropical surface waters of the Brazil Current and sub-Antarctic Surface Water from the Southern Ocean at the Brazil-Falkland/Malvinas confluence. Furthermore, as only about 3% of fluvial Co is estimated to be retained within river systems [START_REF] Sholkovitz | The coagulation, solubility and adsorption properties of Fe, Mn, Cu, Ni, Cd, Co and humic acids in a river water[END_REF], the northward flowing Falkland/Malvinas Current may also transport organically-complexed fluvial Co offshore, contributing to the elevated surface concentrations in this frontal region, as has previously been reported for Fe (Rijkenberg et al., 2014).

CONCLUSIONS

Dissolved Co and Fe distributions showed strong, and often contrasting, regional differences during AMT-19. Extremely low concentrations of dCo (NATR/NAST; ~20-30 pM)

were observed in the northern gyre provinces where dFe was high, whereas the opposite trend was observed in the SATL. Both dCo and dFe distributions were generally nutrient-like;

highlighting the nutritive role of these two bioactive elements. However, the extremely low dCo of the northern gyre provinces is somewhat of a paradox given the seemingly plentiful supply of trace elements from Saharan dust. In these regions, we propose that dCo distribution in waters shallower than ~ 100 m is controlled predominantly by biological uptake by the bacteria (primarily Prochlorococcus, Trichodesmium and SAR11), and other organisms that utilise a Co analogue of AP for DOP uptake. This has important implications in the context of climate change, where stratification is predicted to increase, thus reducing phosphate inputs from below to surface waters. This situation may be further exacerbated by predicted increases in nitrogen deposition [START_REF] Behera | Ammonia in the atmosphere: a review on emission sources, atmospheric chemistry and deposition on terrestrial bodies[END_REF] as a result of increasing urbanisation/ industrialisation. Future studies should assess the potential for Co-Zn-P limitation in the North Atlantic.

ACKNOWLEDMENTS

With many thanks to the Captain and crew of RRS James Cook, and Carolyn Harris, Malcolm Woodward and Claire Widdicombe for kindly providing the nutrient, and the chl-a data, respectively. Thank you also to Mike Zubkov and Manuela Hartmann for discussion of bacterial abundance during AMT-19. We thank two anonymous reviewers for their valuable comments and suggestions. Funding for this work was provided through a Marine Institute (Plymouth University) Studentship to RUS, a Natural Environment Research Studentship (NERC) to NJW and NERC grant number NE/G016267/1 to MCL. Thank you also to the Atlantic Meridional Transect Programme co-ordinators who provided a berth on AMT-19 and therefore made this study possible. This study is a contribution to the international IMBER project and was supported by the UK NERC National Capability funding to Plymouth Marine Laboratory and the National Oceanography Centre, Southampton. This is contribution number 277 of the AMT programme.

  are also important removal pathways for both Co and Fe. The Atlantic Meridional Transect (AMT) programme provides an ideal platform to investigate Co and Fe cycling in the upper Atlantic Ocean and the role of these metals on climate-relevant biological processes. Here we report the geographical distribution and biogeochemistry of Co and Fe in the upper water column along a 12,000 km, gyre-centred transect of the Atlantic Ocean (AMT-19) between ~ 50° N and 40° S. As our knowledge of Fe biogeochemistry is arguably more advanced than for Co, the following discussion aims to develop our understanding of Co biogeochemistry in the upper water column (≤ 150 m) of the Atlantic Ocean between 50° N and 40 °S by making comparisons with dissolved Fe distributions from this and earlier studies.

  defined in this study: North Atlantic Drift (NADR,38-56°N), North Atlantic Gyre (NAST,30-38°N), North Atlantic Tropical Gyre (NATR,12-30°N), Western Tropical Atlantic (WTRA,12°N-10°S), South Atlantic Gyre (SATL,10-33°S), and South Atlantic Subtropical Convergence (SSTC, 33-55°S), and Atlantic Ocean surface currents: AzC = Azores Current, NEC = North Equatorial Current, NECC = North Atlantic Counter Current, SEC = South Equatorial Current, BC = Brazil Current, SAC = South Atlantic Current, FC = Falklands Current. The approximate position of the ITCZ (4-9°N with the most intense rain activity between 4-5°N) during November 2009 was identified from the Giovanni data product (http://giovanni.sci.gsfc. nasa.gov), and is marked by the shaded box.

Figure 2 .

 2 Figure 2. The distributions of temperature (top), salinity (middle) and dissolved oxygen (bottom) in the upper 150 m of the Atlantic Ocean during AMT-19, with the biogeochemical provinces marked above (refer to Figure 1 for acronyms). Stations were sampled approximately every 1-1.5° of latitude at a 1 m depth resolution.

  , resulted in a deepening of the SATL mixed surface layer down to 61 m, and a fully homogeneous upper water column (T ~ 16 °C, S ~ 35.5) in the SSTC.The distribution of macronutrients along the transect (Fig.3; NO3 data is not shown due to the similarity with the distribution of PO4) revealed extremely low mixed layer concentrations (PO4 < 0.05 µM) in the NAST and NATR and three distinct regions where concentrations below the mixed layer were elevated. Firstly, in the NADR, macronutrient concentrations were elevated below 60 m (PO4 = 0.2-0.9 µM, NO3 = 2.5-12 µM). These elevated concentrations continued into the northern section of the NAST before becoming depleted. Secondly, macronutrient concentrations were elevated in waters associated with the equatorial upwelling (PO4 =0.2-1.5 µM, NO3 = 2.5-23 µM). Thirdly, macronutrient concentrations in the SSTC were elevated below 100 m (PO4 = 0.2-0.5 µM; NO3 = 2.5-5 µM), values similar to those reported for the Southwest Atlantic at 40° S by[START_REF] Wyatt | Biogeochemical cycling of dissolved zinc along the GEOTRACES South Atlantic transect GA10 at 40°S[END_REF].

Figure 3 .

 3 Figure 3. Distribution of phosphate (PO4) in the upper 150 m of the Atlantic Ocean during AMT 19 with the biogeochemical provinces marked above (refer to Figure 1 for acronyms). Note the higher concentrations in the SATL compared to the NAST and NATR.

Figure 4 .

 4 Figure 4. The distribution of dCo (pM) overlaid with potential density anomaly (kg m -3 ; top panel), dFe (nM) overlaid with the TdFe (nM; bottom panel) in the upper 150 m of the Atlantic Ocean during AMT-19, with the approximate depth of the mixed layer marked (MLD) shown as a solid white line. The biogeochemical provinces are displayed above the top panel (refer to Figure 1 for acronyms).

  , in the North Atlantic gyre provinces were observed in the southern NATR in concert with a shoaling of the MLD, and were accompanied by very low dCo concentrations (13-17 pM at 35-40 m depth), suggesting biological drawdown as an important control of dCo distribution in this region. Higher dCo concentrations were observed in the provinces adjoining the northern gyre provinces, e.g., in the NADR (dCo = 59 ± 23 pM, n = 10) Prochlorococcus were less abundant and dCo appears to be advected southwards along the 26 kg m -3 isopycnal (Fig. 4, top panel) to ~ 40°N and the boundary with the NAST.The highest sub-surface dCo concentrations (e.g. 89 ± 4 pM at 28.8°S, 26.1°W) were observed in the SATL. Between 25-150 m, the SATL was characterised by relatively high dCo (52 ± 15 pM, n = 10), and decreasing temperature and salinity with increasing latitude.

Table 1 .

 1 Estimation of the contribution of atmospheric dry deposition to the mixed layer (ML) inventories of dCo and dFe. The values used are from: a = Shelley et al. (2015); b = Dulaquais et al., 2014a; c= this study; d = Chance et al. (2015), respectively.

Figure 5 .

 5 Figure 5. Prochlorococcus and Synechococcus distributions in the upper 150 m during AMT-19. The white line depicts the approximate depth of the mixed layer.

Figure 6 .

 6 Figure 6. dCo (left) and dFe (right) plotted against the apparent oxygen utilisation (AOU; µM) in the region of low dissolved oxygen (> 150 µM dissolved oxygen; 1-17 °N)


Marceau Métillon
email: marceau.metillon@ls2n.fr

Saman Lessanibahri
email: saman.lessanibahri@ls2n.fr

Philippe Cardou
email: pcardou@gmc.ulaval.ca

Kévin Subrin
email: kevin.subrin@ls2n.fr

Stéphane Caro
email: stephane.caro@ls2n.fr

Caro A Stéphane

Cable- Driven

Marceau M Étillon

K Évin Subrin

St Éphane Caro

A Cable-Driven Parallel Robot with Full-Circle End-Effector Rotations

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

INTRODUCTION

A Cable Driven Parallel Robot (CDPR) belongs to a particular class of parallel robots where a moving platform is linked to a base frame using cables. Motors are mounted on a rigid base frame and drive winches. Cable coiled on these winches are routed through exit points located on the rigid frame to anchor points on the moving platform. The pose (position and orien- * Address all correspondence to this author. tation) of the moving platform is determined by controlling the cable lengths.

CDPRs have several advantages compared to classical parallel robots. They are inexpensive and can cover large workspaces [START_REF] Hussein | Geometric optimization of a large scale cdpr operating on a building facade[END_REF]. The lightweight cables contribute to the lower inertia of the moving platform and consequently to a better dynamic performance over classical parallel robots [2]. Another characteristic of the CDPRs is their reconfigurability. Changing the overall geometry of the robot can be done by changing the exit points and anchor points.

Reconfigurability of the CDPRs is suitable for versatile applications especially in an industrial context [START_REF] Gagliardini | Discrete reconfiguration planning for cable-driven parallel robots[END_REF]4,5]. CDPRs have drawn researchers' interests towards robotic applications such as pick-and-place operations, robotic machining, manipulation, intralogistics measurements and calibration systems [6].

CDPRs can offer an extremely large three-degrees of freedom translational workspace, but their orientation workspace is usually limited due to cable/cable and cable/moving platform collisions. Cable interferences can be divided into cable-cable and cable-environment interferences [7]. For a given pose of the moving platform, cable-cable collision may occur by changing the moving platform orientation. Cable-environment collisions refer to the interferences between the moving platform and its surrounding environment.

This paper presents the concept of a hybrid manipulator with decoupled translation and orientation motions of the moving platform. It is called hybrid since two parallel mechanisms

A 1 A 2 A 3 A 4 A 5 A 6 A 7 A 8 F 0 F 1 F 2 x 1 y 1 z 1 x 2 y 2 z 2 x 0 y 0 z 0 O 0 O 1 O 2 rigid frame motor gearbox winch

Cables moving platform exit points

Figure 1: CDPR with full-circle end effector rotations are connected in series, namely, a CDPR and a parallel spherical wrist [START_REF] Merlet | Parallel robots[END_REF]. In [START_REF] Vikranth Reddy | Planar cable-driven robots with enhanced orientability[END_REF], a bi-actuated cable is used in order to improve the orientation capacity of the end-effector.

In [START_REF] Lessanibahri | Kinetostatic analysis of a simple cable-driven parallel crane[END_REF] a hybrid CDPR uses a cable-loop for remote actuation of an embedded hoist mechanism on the moving platform. In [START_REF] Lessanibahri | A cabledriven parallel robot with an embedded tilt-roll wrist[END_REF] a moving platform embedding a two-DOFs differential gear set mechanism is presented allowing the two-DOFs unlimited rotational motions of the end-effector. Thanks to cable-loops (bi-actuated cables), the embedded mechanisms are actuated by transmitting power through cables from motors, which are fixed on the ground, to the moving platform. Therefore, by remote actuation through cable-loops, the tethering of the power cable to the moving platform is eliminated [12]. Moreover, a lower mass and a lower inertia of the moving platform are obtained due to the remote actuators.

In this paper the mechanical design of the manipulator under study is presented in Section 2. Its kineto-static model is described in Section 3. The static workspace of the manipulator is studied in Section 4. The methodology followed to determine the optimal cable arrangement of the system is explained in Section 5. The developed prototype of the mechanism at hand and some experimental results are shown in Section 6. Conclusions and future work are drawn in Section 7.

DESCRIPTION AND PARAMETRIZATION OF THE MANIPULATOR

The kinematic architecture of the manipulator consists of two parallel manipulators mounted in series. A CDPR grants a large translation workspace and a Parallel Spherical Wrist (PSW) grants an unlimited orientation workspace. This hybrid manipulator is able to combine advantages of both mechanisms in terms of large translation and orientation workspaces.

OVERALL ARCHITECTURE

Figure 1 shows the overall architecture of the manipulator with its main components, namely, winches, exit-points and the moving platform. The winches control the cable lengths, which move and actuate the moving platform. Cables are routed through exit-points located on the rigid frame and connected to anchor-points located on the moving platform.

Figure 2 shows the moving platform, which hosts a top plate assembly and embeds the PSW. The end-effector of the wrist is a sphere actuated by three cable-loops, which transmit the required power from motors fixed on the ground to the end-effector of the moving platform, namely the sphere.

The PSW is described in detail in Sec. 2.2 while the cableloop system is presented in Sec. 2.3. The top plate has the six-DOFs and the PSW grants a large orientation workspace to the sphere providing an overall nine-DOFs workspace to the spherical end-effector regarding the base frame F 0 .

THREE-DOFs PARALLEL SPHERICAL WRIST

The concept of the PSW relies on the Atlas platform principle [13]. An end-effector is linked to the base of the wrist using a spherical joint. Three omni-wheels are linked to the wrist base with revolute joints. The end-effector is a sphere actuated by the top plate Sphere (End Effector) arm cables anchor points spring double omni-wheel

F 1 x 1 y 1 z 1 O1 B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 x 2 y 2 z 2 O 2 F 2 Figure 2:
The moving platform rotation of the omni-wheels. The position of the wheel relative to the sphere surface was defined to allows a singular-free and unlimited orientation of the sphere around the global x 0 , y 0 and z 0 axes as discussed in [14].

Here, the top plate of the CDPR amounts to the wrist base. Three carriage sub-assemblies are rigidly attached to the top plate. Every carriage hosts an omni-wheel. A three rigid arm platform hosts the sphere using three caster balls. Three antibacklash compression springs are mounted on threaded rods using nuts, thus ensuring the connection of the arm platform to the top plate. The springs ensure an adjustable contact force of the omni-wheels on the sphere. The omni-wheels transmit torque to the sphere thanks to friction. Each omni-wheel is independently driven by a cable-loop system.

CABLE-LOOP PRINCIPLE

Figure 3 represents a simplified form of the cable-loop system. The latter consists of a single cable of which both ends are actuated by two motors while passing through exit-points, namely, A 1 , A 2 , and anchor-points, namely, B 1 , B 2 . The cableloop is coiled up around a drum on the moving platform. The cable-loop drum then acquires one rotational DOFs with respect to the moving platform used to actuate a tool or to control additional degrees of freedom such as rotations over wide ranges. The purpose of the cable-loop is double. Firstly, its aim is to motor 1 motor 2 base top plate

t 1 ω τ t 2 drum r B 1 B 2 A 1 A 2
Figure 3: Representation of a cable-loop translate the top plate as two single cables would do when the coiling directions of both motors are the same. Secondly, it actuates the embedded pulley by circulating the cable when coiling directions of the two actuators are different.

By controlling the difference of tension in both ends of the cable loop, namely δt 12 = t 1t 2 , it is possible to transmit torque τ to the pulley. This capacity can be used to increase the orientation workspace of the end-effector when the pulley is used to rotate the end-effector or the pulley can actuate an embedded mechanism without having the drawbacks of embedding the actuators on the platform. Here, three cable loops are used to actuate independently the three omni-wheels in contact with the sphere as illustrated in Fig. 3.

OVERALL PARAMETERIZATION

As shown in Fig. 1, F 0 denotes the frame fixed to the base of origin point O 0 and axes x 0 , y 0 and z 0 . F 1 is the frame attached to the top-plate of origin point O 1 and axes x 1 , y 1 and z 1 . F 2 is the frame attached to the end-effector, i.e., the sphere, of origin point O 2 , the geometric center of the sphere and axes x 2 , y 2 and z 2 .

It is noteworthy that F 0 and F 1 have a translational and orientational relative movement while F 1 and F 2 only have a relative rotational movement.

The exit points A i are the points belonging to the frame through which the cables are routed between the winch and the top plate. The anchor points B i are the points belonging to the TP where the cables are connected. It is noteworthy that the cables are connecting exit point and anchor points accordingly and a unit vector expresses the cable direction. Therefore, the loop-closure equations associated with each cable are expressed as follows:

0 l i = 0 a i -0 p -0 R 1 1 b i (1) with i ∈ [[1, . . . , 8
]] where 0 l i is the i-th cable vector, 0 a i is the corresponding anchor point expressed in the base frame, 1 b i is the coordinate vector of exit point in the platform frame, 0 p is the position vector of the platform frame and 0 R 1 is the rotation matrix from F 0 to F 1 .

We can then write the i-th unit cable vector as:

0 u i = 0 l i l i (2)
with l i being the i-th cable length.

KINETOSTATIC MODEL OF THE MANIPULATOR

In this section, we proceed to the kinetostatic modelling of the overall manipulator. We write the static equation of the manipulator as follows:

Wt + w e = 0 9 (3)
with W being the wrench matrix, t being the cable tension vector and w e being the external wrenches applied on the platform. In our case, we only consider the action of the weight of the moving platform as external wrench.

The wrench matrix of the manipulator W is the concatenation of the wrench matrices of both mechanisms:

W = W T P W SW 9×8 (4)
where W T P is the wrench matrix associated to the top plate and W SW is the wrench matrix related to the PSW. Similarly, the wrench vector of the manipulator consists of the wrench vector of both mechanisms:

w g = w T P g w SW g 9×1 (5)
where w T P g is the wrench vector associated to the top plate and w SW g is the wrench vector exerted on the PSW. W T P and w T P g are defined in Section 3.1 while W SW and w SW g are defined in Section 3.2.

KINETOSTATIC MODEL OF THE TOP PLATE

In this section we write the kinetostatic model of the top plate. The static equilibrium of the platform can be written as:

W T P t + w T P g = 0 8 (6)
W T P takes the following form:

W T P = u 1 0 u 2 0 u 3 0 u 4 0 u 5 0 u 6 0 u 7 0 u 8 0 d 1 0 d 2 0 d 3 0 d 4 0 d 5 0 d 6 0 d 7 0 d 8 6×8 (7)
with 0 d i being the cross-product matrix of vector b i and u i

0 d i = 0 R 1 1 b i × 0 u i (8)
We define the wrench vector of the top plate w T P as follows:

w T P g = f T P m T P 6×1 (9)
where f T P is the 3-dimensional force and m T P is the 3dimensional moment exerted on the top plate.

KINETOSTATIC MODEL OF THE PARALLEL SPHERICAL WRIST

The parametrization of the parallel spherical wrist is shown in Figure 4 and expressed as follows:

C i : Contact point between the omni-wheel and the sphere π i : Plane passing through the contact point C i and tangent to the sphere µ i : Unit tangent vector of i-th omni-wheel along line t α : Angle associated to the position of the contact points C i (α ∈ [0, π]), on the sphere β : Angle between tangent to the sphere and the actuation force of the omni-wheel (β ∈ [-π 2 , π 2 ]) γ 2 : Angle between the contact points C 1 and C 2 γ 3 : Angle between the contact points C 1 and C 3 r s : Radius of the sphere r o : Radius of the omni-wheels φi : Angular velocity of the i-th omni-wheel f ni : Actuation force produced by the i-th omni-wheel on the sphere

r s r o O 2 H C 1 µ 1 C 2 C 3 ϕ 1 α β γ 2 γ 3 F 2 x 2 y 2 z 2 π 1 f n1
omni-wheel sphere The optimal set of the parameters of the wrist was defined in [14] in order to maximize the amplitudes of its orientation as well its dexterity. The following hypotheses are taken into account for the further analysis and modelling:

1. The omni-wheels are normal to the sphere. 2. The contact points between the omni-wheels and the sphere belong to the circumference of a circle. The latter is the base of an inverted cone, its tip being the centre of the sphere. The angle between the vertical axis and the cone is named α. 3. In the plane containing the cone base, the three contact points form an equilateral triangle.

By taking into account the hypotheses, α and β are calculated as follows [14]:

α = 35.2 • β = 0 • (10)
The angular velocity vector of the sphere ω = [ω x , ω y , ω z ] T is expressed as a function of the angular velocity vector of the omni-wheels φ = [ φ1 , φ2 , φ3 ] T , as follows:

Aω = B φ (11)
A and B are the forward and inverse Jacobian matrices of the PSW, which take the form:

A = r s 2   -2C α C β -2S β 2S α C β C α C β + √ 3S β S β - √ 3C α C β 2S α C β C α C β - √ 3S β S β + √ 3C α C β 2S α C β   (12)
and,

B = r o 1 3×3 (13)
where, r s is the sphere radius, r o is the omni-wheel radius, S i = sin(i) and C i = cos(i), i = α, β. Notice that the radii of the omniwheels are identical. Equation ( 11) is then rewritten as follows:

ω = J ω φ ( 14
)
where J ω = A -1 B, is the Jacobian matrix of the wrist, i.e., J ω is the mapping from angular velocities of the omni-wheels into the required angular velocity of the end-effector. Based on the theory of reciprocal screws [15], it turns out that:

m T SW ω = τ T φ ( 15
)
where m SW = [m x , m y , m z ] T is the output moment vector of the sphere and τ = [τ 1 , τ 2 , τ 3 ] T is the input torque vector, namely the omni-wheel torque vector. By substituting Eq. ( 14) into Eq. ( 15), we obtain:

τ = J T ω m SW = W ω m SW ( 16
)
The wrench matrix W ω = J T ω maps the output torque of the sphere to the omni-wheels torques. As shown in Figure 5, it is possible to express τ as a function of cable tensions so that:

τ 1 = r o (t 1 -t 2 ) (17) τ 2 = r o (t 3 -t 4 ) (18) τ 3 = r o (t 5 -t 6 ) (19)
From Equations ( 17) to ( 19), the omni-wheel torque vector τ is expressed as a function of the the cable tension vector as follows:

τ = W c t (20)
where:

W c =   r o -r o 0 0 0 0 0 0 0 0 r o -r o 0 0 0 0 0 0 0 0 r o -r o 0 0   ( 21
)
and t = [t 1 , . . . , t 8 ] T is the cable tension vector. The equilibrium of the wrench applied on the PSW is written in the following:

m SW = W SW t (22)
with m SW being the external moments applied by the environment onto the PSW. The wrench matrix expresses the relation between cable tensions and the wrist moments as:

W SW = W ω W c ( 23
)
In Figure 6, the orientation of the sphere with respect to the base frame is parametrized with angles pitch, θ, yaw, ψ, and roll, χ angles. Those three angles are the components of the orientation vector q SW of the wrist, namely,

q SW =   θ ψ χ   (24)

WORKSPACE ANALYSIS

The static workspace of the manipulator consists of the set of positions and orientations of the moving platform and the orientations of the end-effector, namely, 0 p and 0 R T P and 0 q SW , which satisfies the static equilibrium of the manipulator.

The cable tension set T amounts to a hyper-cube in an eightdimensional space: The static workspace of the manipulator is defined as follows:

T = {t ∈ R 8 : t min ≤ t ≤ t max } (25) t i+ t i- r o ω i τ i
S = {( 0 p, 0 R T P , 0 q SW ∈ R 3 × SO(3) × R 3 : ∃t ∈ T , Wt + w e = 0 9 } (26)
where SO(3) is the group of proper rotation matrices. The static workspace in a nine-dimensional space is expressed using the Equation (26). As the visualization of such high-dimensional space is impossible with common human perception in 3D, we define the static workspace of the manipulator for a simplified case. We define S for a constant orientation of the top-plate. The former subset, namely, S AO is a set for a given orientation of the top-plate while the wrist is free to rotate:

S AO = { 0 p ∈ R 3 | 0 R 1 = I 3 | -π ≤ θ, ψ, χ ≤ π : ∃t ∈ T , Wt + w e = 0 9 } (27)
The discretization of the Cartesian space is made so that n x , n y and n z are the numbers of discretized points along x 0 , y 0 and z 0 axes, respectively. R S is defined as the proportion of the static workspace to the overall space occupied by the manipulator:

R S = N S (n x + 1)(n y + 1)(n z + 1) (28)
with N S being the number of points inside the discretized static workspace S.

OPTIMAL CABLE ARRANGEMENT

This section deals with the cable arrangement of the manipulator for obtaining the maximum size of the static workspace. The ith cable arrangement is the association of anchor points B i to exit points A i . The top plate has fifteen points for anchor points and the manipulator has a maximum of eight actuators. Therefore, the optimal cable arrangement is defined as the association of eight anchor points to eight exit points such that the workspace is maximized.

The number of exit point combinations, N e is:

N e = n e n c ( 29
)
with n e and n c being the numbers of available exit-points and cables, respectively. The number of anchor-points combinations, N a , consists in the number of permutations of the set of points, which is given by:

N a = n a n c n c ! (30)
n a being the number of selected anchor-points. S C is the set of possible cable configuration, the number of configuration N C = dim(S C ) is thus given by: Figure 7 shows all the available anchor points, namely, r i , i = 1, 2, . . . , 15, which are divided into two groups: S CL = {r 3 , r 4 , r 8 , r 9 , r 13 , r 14 } which is the set of points associated to cable-loops and S SC = {r 1 , r 2 , r 5 , r 6 , r 7 , r 10 , r 11 , r 12 , r 15 } which is the set of points associated to the simple cables. Six points amongst the fifteen points are selected to make the three cable loops. Therefore, nine remaining anchor points host the remaining two single-actuated cables.

n c = n SC + n CL ( 32
)
The number of available anchor points for the single and biactuated cables are denoted as n aSC and n aCL :

n a = n aSC + n aCL (33)
The number, N CL , of combinations considering cable-loop is given by:

N CL = n e n c n aSC n SC n c ! (34)
Six points are associated to the wrist actuation and consequently to the three cable-loops: n aCL = 6. The remaining nine anchor points are to be assigned to simple cables n aSC = 9. Finally, N CL , expressed in Eq. ( 34) is given by: The static workspace is computed for the 120 960 cable arrangements. Figure 8 illustrates the cable arrangement corresponding to the largest workspace. Figure 8a shows a schematic of the anchor-plate as well as the optimal cable arrangement.

N CL =
Figure 8b shows the corresponding static workspace with R S = 65%.

EXPERIMENTAL DEMONSTRATOR

An experimental demonstrator of the manipulator was manufactured as shown in Figure 9. The architecture of the manipulator with an embedded PSW is presented in Figure 10. The main hardware of the CREATOR demonstrator consists of a PC (equipped with c MATLAB and c ControlDesk software), c PARKER SME60 motors and TPD-M drivers, a c dSPACE DS1007-based real-time controller and custom made winches.

The experimental setup allows us to evaluate the capabilities of the manipulator in operation. However, there is one crucial constraint imposed by the demonstrator setup in terms of number of actuators. Since the manipulator has nine-DOFs and the demonstrator is equipped with only eight motors, the overall system is underactuated. Her the performance of the manipulator is assessed by considering the following cases:

Case 1: The moving platform is over-actuated with six controllable DOFs of the top-plate. Hence, the desired trajectory includes the translational motion of the top-plate within

CONCLUSIONS AND FUTURE WORK

This paper addressed the novel design of a cable driven parallel robot with an embedded parallel spherical wrist. The mechanism has nine degrees of freedom. A complete kinetostatic 3 https://metillon.net/3T3R_3 model of the overall moving platform was expressed. The cable arrangement was optimized in order to maximise the static workspace. Moreover a set of preliminary experiments illustrates the capabilities of the mechanism in term of translation and orientation.

The overall system is underactuated because the movingplatform has nine degree of freedom and eight actuators only are used. Later on, the parasitic inclinations of the end-effector will be studied in order to find some strategies to compensate them.

Figure 4 :

 4 Figure 4: Parametrization of the Parallel Spherical Wrist

Figure 5 :Figure 6 :

 56 Figure 5: Representation of a cable-loop drum

1 Figure 7 :

 17 Figure 7: The fifteen points for the anchor points on the movingplatform

  As N CL is very large, for computing-time sake, the number of available points for anchor points is supposed to be equal to three. Thus S SC = {r 1 , r 6 , r 11 }. By substituting n aSC = 3 into Eq. (34) we obtain:

Figure 8 :

 8 Figure 8: Results of the optimization algorithm

Figure 9 :Figure 10 :

 910 Figure 9: The moving platform with full-circle end-effector

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ACKNOWLEDGMENT

This work was supported by both the ANR CRAFT project, grant ANR-18-CE10-0004 and the RFI AtlanSTIC2020 CRE-ATOR project.