Javier Tesán Onrubia

Mariia V Petrova

Viena Puigcorbé

Erin E Black

Ole Valk

Aurélie Dufour

Bruno Hamelin

Ken O Buesseler

Pere Masqué

Javier A Tesán Onrubia

Aurelie Dufour

Frederic A C Le Moigne

Jeroen E Sonke

Michiel Rutgers Van Der Loeff

Lars-Eric Heimbürger- Boavida

Mercury Export Flux in the Arctic Ocean Estimated from 234 Th/ 238 U Disequilibria

Keywords: GEOTRACES, particulate mercury, Arctic Ocean, thorium export, particle export flux

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

Main text

Elevated Hg levels in arctic biota [START_REF] Dietz | What are the toxicological effects of mercury in Arctic biota?[END_REF][START_REF] Wang | Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic[END_REF][START_REF] Dudarev | Traditional Diet and Environmental Contaminants in Coastal Chukotka III: Metals[END_REF] have been explained by enhanced inorganic Hg inputs to the Arctic Ocean [START_REF] Outridge | A mass balance inventory of mercury in the Arctic Ocean[END_REF][START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF]. Several recent studies aimed at refining Hg inputs from the atmosphere [START_REF] Schroeder | Arctic springtime depletion of mercury[END_REF][START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF], rivers [START_REF] Sonke | Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean[END_REF], coastal erosion [START_REF] Leitch | The delivery of mercury to the Beaufort Sea of the Arctic Ocean by the Mackenzie River[END_REF], Schuster et al., 2019[START_REF] Lim | A revised northern soil Hg pool, based on western Siberia permafrost peat Hg and carbon observations[END_REF] and other oceans [START_REF] Cossa | Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect[END_REF][START_REF] Petrova | Arctic Ocean exports methylmercury to the Atlantic Ocean[END_REF]. Once delivered to the Arctic Ocean, Hg can undergo complex biotic and abiotic reactions, including the transformation into the bioaccumulative neurotoxin methylmercury (MeHg). Because MeHg is produced at shallow depth in the Arctic Ocean [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF], it is important to know the residence time of Hg, which is largely driven by two removal mechanisms: evasion to the atmosphere and downward export flux with settling particles.

D

Hg' h gh ff y f p c c g g p f p y major role in removing Hg from the surface ocean [START_REF] Lamborg | An examination of the role of particles in oceanic mercury cycling[END_REF]. A fraction of this downward Hg export flux is finally buried in the marine sediments and in this way removed from the oceanic Hg budget for millennial time scales. Providing the first comprehensive Arctic Hg budget [START_REF] Outridge | A mass balance inventory of mercury in the Arctic Ocean[END_REF] estimated a burial rate of Hg to the marine sediments of 108 Mg y -1 . Later on, [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF] established a box model which estimates the downward pHg flux to be 37 Mg y -1 and a burial rate of 28 Mg y -1 based on the few observations available at the time. Both studies also differ in their estimation of the exchange flux with the atmosphere (Tab. S1). [START_REF] Outridge | A mass balance inventory of mercury in the Arctic Ocean[END_REF] find a low Hg evasion flux of 10 Mg y -1 , which results in a net atmospheric deposition of 98 Mg y -1 to the Arctic Ocean. On the contrary the box model developed by [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF] implies that the Arctic Ocean is a Hg source with a net evasion flux to the atmosphere of 23 Mg y -1 . The downward Hg export flux from the surface to the deep Arctic Ocean, as well as Hg burial rate in marine sediments, remain poorly constrained (Tab. S1). Sedimentation rates in the central Arctic Ocean are thought to be very low, and sediment mixing and diagenesis bias the Hg sediments records [START_REF] Gobeil | Mercury Profiles in Sediments of the Arctic Ocean Basins[END_REF]. Most sediment traps deployed in the Arctic Ocean use HgCl 2 for sample preservation [START_REF] Kraft | Amphipods in sediment traps of the eastern Fram Strait with focus on the life-history of the lysianassoid Cyclocaris guilelmi[END_REF] and cannot be used for Hg analysis. Thus far only one study could provide particulate Hg (pHg) measurements in the central Arctic Ocean [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF].

Previous estimates of Hg fluxes in the Arctic Ocean [START_REF] Outridge | A mass balance inventory of mercury in the Arctic Ocean[END_REF][START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF][START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF] had to be based on available information of pHg in riverine particles [START_REF] Graydon | Mercury in the Mackenzie River delta and estuary: Concentrations and fluxes during open-water conditions[END_REF], ice algae [START_REF] Burt | Mercury uptake and dynamics in sea ice algae, phytoplankton and grazing copepods from a Beaufort Sea Arctic marine food web[END_REF] and zooplankton (P ćk .

2014), sediments [START_REF] Lim | A revised northern soil Hg pool, based on western Siberia permafrost peat Hg and carbon observations[END_REF][START_REF] Gobeil | Mercury Profiles in Sediments of the Arctic Ocean Basins[END_REF], or by using partition coefficients (K d ) from the North Atlantic (1.1 x 10 5 L kg -1 , [START_REF] Mason | Mercury in the North Atlantic[END_REF] or the Pacific Ocean (1 x 10 6 L kg -1 , [START_REF] Mason | The distribution and biogeochemical cycling of mercury in the equatorial Pacific Ocean[END_REF]. [START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF] used the GEOS-Chem model to explain atmospheric Hg observations in the Arctic. In the absence of pHg observations for the Arctic Ocean, this study used the K d from the North Atlantic [START_REF] Mason | Mercury in the North Atlantic[END_REF] for all types of particles in the Arctic Ocean.

We used new observations, to develop two different approaches to provide a new estimate for the pHg export flux in the central Arctic Ocean and in the outer shelf (> 100 m).

First, we used new data to calculate pHg normalized to suspended particulate matter (Hg SPM )

and K d for the Arctic Ocean. Then, we used pHg and 234 Th observations, to estimate the pHg export flux based on the 234 Th/ 238 U disequilibrium [START_REF] Coale | 234Th:238U disequilibria within the California Current[END_REF]. Finally, we reestimated the net Hg burial rates from our Arctic sediment cores. Particulate Hg concentrations on GN04 averaged 0.097 ± 0.039 pmol L -1 (n = 9), and are similar those measured in the Western Arctic Ocean during the GN01 cruise (0.072 ± 0.051 pmol L -1 , n = 10, [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF]), but about 3 times higher than pHg measured in the North Atlantic [START_REF] Mason | Mercury in the North Atlantic[END_REF][START_REF] Bowman | Mercury in the North Atlantic Ocean: The U.S. GEOTRACES zonal and meridional sections[END_REF]. The highest pHg concentrations were observed over the shelf (GN04: 0.137 ± 0.046 pmol L -1 , n = 3; GN01: 0.204 pmol L -1 , n = 1) and lowest off-shore (GN04: 0.076 ± 0.008 pmol L -1 n = 6; GN01: 0.058 ± 0.022 pmol L -1 , n = 9). The pHg concentrations were converted from water volume (pHg in pmol L -1 ) to particle mass-specific concentrations (Hg SPM in ng g -1 ), using SPM concentrations from the GN01 cruise for each station at depths between 80 and 120 m [START_REF] Lam | Size-fractionated major and minor particle composition and concentration from the US GEOTRACES Arctic cruise (HLY1502) on USCGC Healy from August to October 2015[END_REF]. In the absence of SPM measurements on GN04, we applied average values from the GN01 cruise, 9.29 ± 2.28 µg L -1 for the central Arctic Ocean and of 146 ± 8.46 µg L -1 for the outer shelf (Table S2). The Hg SPM concentrations at 100 m depth were 211 ± 69 ng g -1 on the shelf and 1484 ± 467 ng g -1 in the central Arctic Ocean (Tab. S2). The Hg SPM concentrations in the central Arctic Ocean were comparable to those measured by [START_REF] Lamborg | An examination of the role of particles in oceanic mercury cycling[END_REF] in the North Atlantic (240 -1080 ng g -1 ), but 4 to 30 times higher than ones [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF] for their model. From Hg SPM and dissolved Hg observations we calculated K d for each station. The K d values range from 1.2 x 10 6 to 1.6 x 10 7 L kg -1 . Our lowest K d values are one order of magnitude higher than the value applied by Fisher et al.

(2012) in their model (1.1 x 10 5 L kg -1 ), and 3 times higher than those determined in the North Atlantic (4.5 x 10 6 L kg -1 , Lamborg et al., 2016, Tab. S2). Our Hg SPM and K d values suggest that previous studies may have underestimated Hg export fluxes.

To test this, we used the 234 Th / 238 U deficit measurements to quantify the Hg export flux in the Arctic Ocean. Several studies have employed the 234 Th proxy to estimate the particulate flux of trace metals and other compounds (Gustafsson et al., 1997a, Gustafsson et al., 1997b[START_REF] Dulaquais | Contrasting biogeochemical cycles of cobalt in the surface western Atlantic Ocean: Surface biogeochemical cycles of cobalt[END_REF][START_REF] Weinstein | Vertical flux of particulate Al, Fe, Pb, and Ba from the upper ocean estimated from 234Th/238U disequilibria[END_REF][START_REF] Hayes | Flux of Particulate Elements in the North Atlantic Ocean Constrained by Multiple Radionuclides[END_REF], Black et al., 2019). Here we apply for the first time the radionuclide pair 234 Th / 238 U, combined with pHg / 234 Th ratios, to estimate the downward Hg export flux. The half-life of 234 Th is 24.1 days and it is highly particle reactive with K d values (~10 6 -10 7 , IAEA, 1985) similar to those of Hg in oxygenated sea water [START_REF] Lamborg | An examination of the role of particles in oceanic mercury cycling[END_REF]. The relatively short half-life of 234 Th makes it a suitable tracer to examine biologically mediated temporal changes in POC production and export, by using the POC/ 234 Th ratio in the particles to convert the 234 Th export flux to POC export flux [START_REF] Coale | 234Th:238U disequilibria within the California Current[END_REF][START_REF] Buesseler | Carbon and nitrogen export during the JGOFS North Atlantic Bloom experiment estimated from 234Th: 238U disequilibria[END_REF]. Similarly, here we measured the pHg were lower in the permanently ice-covered central Arctic Ocean and higher over the outer shelf (Tab. S2). Applying the average pHg / 234 Th ratio measured during GN01 and GN04 to the 234 Th fluxes reported in the current and by previous studies [START_REF] Amiel | 234Th/238U Disequilibrium as an Indicator of the Seasonal Export Flux of Particulate Organic Carbon in the North Water[END_REF][START_REF] Coppola | Calibration of Sediment Traps and Particulate Organic Carbon Export Using 234 Th in the Barents Sea[END_REF][START_REF] Chen | Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria[END_REF][START_REF] Qiang | Regional estimates of POC export flux derived from thorium-234 in the western Arctic Ocean[END_REF][START_REF] Lalande | Export Fluxes of Particulate Organic Carbon in the Chukchi Sea: A Comparative Study Using 234Th/238U Disequilibria and Drifting Sediment Traps[END_REF][START_REF] Amiel | Terrestrial and Marine POC Fluxes Derived from 234 h D b δ 13 m h ck z h f[END_REF][START_REF] Lalande | 234Th-Derived Particulate Organic Carbon Fluxes in the Northern Barents Sea with Comparison to Drifting Sediment Trap Fluxes[END_REF], Cai et al., 2010[START_REF] Roca-Martí | Carbon export fluxes and export efficiency in the central Arctic during the record sea-ice minimum in 2012: a joint 234 Th/ 238 U and 210 Po/ 210 Pb study: POC fluxes in the central Arctic in 2012[END_REF] we estimated the Hg export fluxes below 100 m depth for different locations in the central Arctic Ocean and outer shelf zones (Fig. 1). collected. The other data points have been calculated using previously published 234 Th fluxes [START_REF] Amiel | 234Th/238U Disequilibrium as an Indicator of the Seasonal Export Flux of Particulate Organic Carbon in the North Water[END_REF][START_REF] Coppola | Calibration of Sediment Traps and Particulate Organic Carbon Export Using 234 Th in the Barents Sea[END_REF][START_REF] Chen | Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria[END_REF][START_REF] Qiang | Regional estimates of POC export flux derived from thorium-234 in the western Arctic Ocean[END_REF][START_REF] Lalande | Export Fluxes of Particulate Organic Carbon in the Chukchi Sea: A Comparative Study Using 234Th/238U Disequilibria and Drifting Sediment Traps[END_REF][START_REF] Amiel | Terrestrial and Marine POC Fluxes Derived from 234 h D b δ 13 m h ck z h f[END_REF][START_REF] Lalande | 234Th-Derived Particulate Organic Carbon Fluxes in the Northern Barents Sea with Comparison to Drifting Sediment Trap Fluxes[END_REF], Cai et al., 2010[START_REF] Roca-Martí | Carbon export fluxes and export efficiency in the central Arctic during the record sea-ice minimum in 2012: a joint 234 Th/ 238 U and 210 Po/ 210 Pb study: POC fluxes in the central Arctic in 2012[END_REF] and the average pHg / 234 Th ratio obtained in this study.

The lowest pHg export fluxes were found in the permanently ice-covered central Arctic Ocean (average 95 ± 195 pmol m -2 d -1 , n = 49) while the highest were found in the outer shelf (average 635 ± 480 pmol m -2 d -1 , n = 50, Fig. 1). Our pHg export flux estimates for the central Arctic Ocean are comparable to observations in the Pacific Ocean based on sediment trap deployments (157 ± 48 pmol m -2 d -1 , [START_REF] Munson | Mercury species concentrations and fluxes in the Central Tropical Pacific Ocean: Central Pacific mercury[END_REF]. However, we observe higher pHg export flux on the productive outer shelf. Our assessment might be slightly biased by seasonal and long term variability, because some of the used literature 234 Th export fluxes were obtained during different seasons and years. Similar to our study, some of the previously reported values indicated negligible 234 Th export fluxes [START_REF] Amiel | 234Th/238U Disequilibrium as an Indicator of the Seasonal Export Flux of Particulate Organic Carbon in the North Water[END_REF][START_REF] Coppola | Calibration of Sediment Traps and Particulate Organic Carbon Export Using 234 Th in the Barents Sea[END_REF][START_REF] Chen | Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria[END_REF][START_REF] Qiang | Regional estimates of POC export flux derived from thorium-234 in the western Arctic Ocean[END_REF][START_REF] Lalande | Export Fluxes of Particulate Organic Carbon in the Chukchi Sea: A Comparative Study Using 234Th/238U Disequilibria and Drifting Sediment Traps[END_REF][START_REF] Amiel | Terrestrial and Marine POC Fluxes Derived from 234 h D b δ 13 m h ck z h f[END_REF][START_REF] Lalande | 234Th-Derived Particulate Organic Carbon Fluxes in the Northern Barents Sea with Comparison to Drifting Sediment Trap Fluxes[END_REF], Cai et al., 2010[START_REF] Roca-Martí | Carbon export fluxes and export efficiency in the central Arctic during the record sea-ice minimum in 2012: a joint 234 Th/ 238 U and 210 Po/ 210 Pb study: POC fluxes in the central Arctic in 2012[END_REF].

To estimate the annual export flux of Hg, we used a primary production period of 85 days for the central Arctic Ocean and 190 days for the outer shelf [START_REF] Strass | Seasonal shifts in ice edge phytoplankton blooms in the Barents Sea related to the water column stability[END_REF][START_REF] Niebauer | An update on the climatology and sea ice of the Bering Sea, in: The Bering Sea: A Summary of Physical, Chemical and Biological Characteristics and a Synopsis of Research[END_REF] and surface areas for the central Arctic Ocean and the outer shelf of 5 x 10 6 km 2 and 3 x 10 6 km 2 , respectively. Based on this assumption we estimated that 8 ± 17 Mg y -1 of Hg are exported below 100 m depth in the central Arctic Ocean and 73 ± 55 Mg y -1 over the outer shelf (Fig. 2). The fact that most of the Hg export takes place over the outer shelf is likely related to the enhanced primary productivity compared to the central arctic basin waters (Sakahaug et al., 2004). Several studies already reported high uncertainties of 234 Th-derived C and trace-metal fluxes [START_REF] Le Moigne | Global Database of Surface Ocean Particulate Organic Carbon Export Fluxes Diagnosed from the 234 h ch q[END_REF], Puigcorbé et al, 2017, Black et al., 2019). We estimated a lower boundary for the pHg export flux from surface water with 81 ± 58 Mg y -1 . If we assume that fluxes on the inner shelf (< 100 m depth), which represents about 25% of the Arctic Ocean surface and receives direct Hg and nutrients inputs via river runoff and coastal erosion [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF], Sonke et al., 2018), were similar to those of the outer shelf, extrapolated the inner shelf, annual flux for the entire Arctic Ocean is about 156 Mg y -1 .

Settling particles are remineralized during their downward flux and only a small portion is finally buried in marine sediments [START_REF] Heimbürger | Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)[END_REF] To check how this affects Hg export flux we measured pHg concentrations in sediments collected during the GN04 cruise on the outer shelf and central Arctic Ocean (Fig. 1). We used our data and previous pHg observations (upper 0.5 -2 cm, [START_REF] Gobeil | Mercury Profiles in Sediments of the Arctic Ocean Basins[END_REF][START_REF] Trefry | Trace metals in sediments near offshore oil exploration and production sites in the Alaskan Arctic[END_REF][START_REF] Hare | Natural and Anthropogenic Mercury Distribution in Marine Sediments from Hudson Bay, Canada[END_REF][START_REF] Fox | Mercury in the northeastern Chukchi Sea: Distribution patterns in seawater and sediments and biomagnification in the benthic food web[END_REF], Canário et al., 2014, Tab. S3) to estimate the net Hg burial rates.

Based on sedimentation rates typically found in the Arctic Ocean (0.03 mm y -1 ) and for the open ocean and outer shelf (0.3 mm y -1 ) following [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF], we calculated average Hg burial rates of 11 ± 8 and 51 ± 35 pmol m -2 d -1 , respectively. Considering the surface area of the central Arctic Ocean and outer shelf with 5 x 10 6 and 3 x 10 6 km 2 , respectively, we estimated Hg burial rates of 4 ± 3 Mg y -1 in the central Arctic Ocean and 12 ± 7 Mg y -1 in the deep shelf (Tab. S3). Our new estimates are slightly lower than those by [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF] (3 and 25 Mg y -1 , respectively). In the absence of pHg observations on the inner shelf, our Hg burial rate of 16 ± 10 Mg y -1 for the Arctic Ocean is a lower boundary. If we were to compare our results (51 ± 35 pmol m -2 d -1 ) to another semi-enclosed basin, the Mediterranean Sea, with sedimentation rates (0.4 mm y -1 ) similar to the arctic outer shelf (0.3 mm y -1 ), we find lower Hg burial rates than 115 pmol m -2 d -1 [START_REF] Heimbürger | Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)[END_REF]. The high range of observed sedimentation rates, 0.0025 -0.05 mm y -1 for the central Arctic Ocean and 0.1 -2.5 mm y -1 for the shelf (Pirtle- [START_REF] Pirtle-Levy | Chlorophyll-a in Arctic sediments implies long persistence of algal pigments[END_REF][START_REF] Polyak | Quaternary sedimentation in the Arctic Ocean: Recent advances and further challenges[END_REF][START_REF] Polyak | Quaternary sedimentation in the Arctic Ocean: Recent advances and further challenges[END_REF] also results in a high uncertainty in the Hg burial rates estimates.

We used our new estimates of pHg downward export and the Hg burial fluxes combined with current observations and modelling studies [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF], Sonke et al., 2018[START_REF] Petrova | Arctic Ocean exports methylmercury to the Atlantic Ocean[END_REF] to refine our understanding of the Hg cycle in the Arctic Ocean (Fig. 2). Based on our Hg observations on SPM and sediments, we calculate that 5.6 ± 2.9% of the exported pHg from the upper 100 m depth is ultimately buried in the central Arctic sediments. Most of the pHg downward flux is remineralized below 100 m depth which is also suggested by the 234 Th/ 238 U profiles (Puigcorbé pers. comm., data not shown here). We hypothesize that pHg exported below 100 m depth may contribute to the shallow production MeHg in the Arctic Ocean [START_REF] Wang | Total and Methylated Mercury in the Beaufort Sea: The Role of Local and Recent Organic Remineralization[END_REF][START_REF] Wang | Subsurface seawater methylmercury maximum explains biotic mercury concentrations in the Canadian Arctic[END_REF][START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF][START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF]. Post depositional diagenetic processes lead to epibenthic mobilization of dissolved Hg species to overlaying sea water. [START_REF] Heimbürger | Natural and anthropogenic trace metals in sediments of the Ligurian Sea (Northwestern Mediterranean)[END_REF] found that epibenthic mobilization of Hg can explain 73% of the observed difference between water column export flux and Hg burial. Our study could not address the possible influence of nycthemeral zooplankton dynamics, convective SPM and diffusive fluxes. Cascading processes, which take place on the shelf by the downwelling of waters [START_REF] Roeske | Deep water circulation and shelf water inputs to the Arctic Ocean by dissolved Ba[END_REF] may also be an important pathway for pHg and 234Th displacement from shallow to deep waters. About 76 Mg y -1 of Hg have been suggested to be transported to the Arctic shelves by rivers or sourced from coastal erosion [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean: Arctic Ocean Hg and MeHg mass budget[END_REF], Sonke et al., 2018). The high sensitivity of the Arctic to climate change can perturb Arctic Hg cycling, increasing land to ocean Hg export fluxes since permafrost thawing and extreme weather events are expected to deliver important amounts of Hg to Arctic Ocean [START_REF] Schuster | Permafrost Stores a Globally Significant Amount of Mercury[END_REF]. In addition, the increase of the open water season due to ice-retreat is leading to an increase of net primary production [START_REF] Arrigo | Continued increases in Arctic Ocean primary production[END_REF] enhancing also Hg scavenging and downward export, which would reduce the amount of Hg available for MeHg production. However, shift to smallsized phytoplankton, may lead to increased MeHg production [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF]. Only continued seasonal and long term arctic surveys will allow building a first time series and make coherent future projections.

About half of the Arctic shelf is shallower than 100 m (inner shelf), and could not be addressed by our study. We can only provide approximate pHg export fluxes below 100 m depth for the outer shelf and the central Arctic Ocean. The use of pHg / 234 Th ratio in particles to obtain Hg export flux should be further developed and expanded to other oceans. In particular, future research should address the spatial and temporal variability of the pHg / 234 Th ratio considering biogeochemical regions and seasonality. Ongoing climate change impacts to the Arctic Ocean ecosystem should also be assessed, including changes in sea ice cover, particle and nutrient supply, primary productivity, and hydrological circulations will likely impact pHg export and overall Hg budget in the Arctic Ocean.

Methods

We used pHg from the GN01 cruise for which information on the measurements details are given by [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF]. We obtained the data set upon request from Alison

Agather by personal email and should become available on https://www.bcodmo.org/dataset/738307. Particles were collected using McLane in situ pumps (LV08) equipped with a pre-filter (screen) of 51 or 53 µm pore size (for GN01 and GN04, respectively) followed by a pre-combusted QMA filter from where the samples for this study were collected (i.e., particle size sampled 1 -51 or53 µm). For both cruises, particulate 234 Th and pHg were measured from the exact same filter. Punches (25 mm) were subsampled from each filter for particulate 234 Th, dried overnight at 50°C and counted on board and recounted again 6 months later for background corrections. After the second counting, POC content was analyzed in the same filters with an EuroVector Elemental Analyzer (Euroanalysator EA).

During the GN04 cruise pHg the remainder of the filters were kept frozen at -20°C until analyses for pHg. Twelve sediment cores were recovered using the trace metal clean NIOZ minicorer. Sediment cores were sliced every 5 mm using a Plexiglas spatula. Only the upper 5 mm were considered for the estimation of Hg burial rates. The sediment samples were kept frozen at -20°C until analyses for pHg. All samples were freeze-dried (Christ Gamma 1-16 LSCplus) and 25 mm diameter punch-out samples were analysed using a CV-AAS (LECO AMA 254) equipped with a low Hg optical cell. The limit of detection, estimated as three times standard deviation of the blank samples, was 1.2 pg, which was equivalent to about 0.0001 pM considering the filtered amount of sea water (289 -599 L).

Total 234 Th activities were determined from 4 L of sea water collected at 18 -20 depths (GN04) or 10 -15 depths (GN01) in the upper ~ 400 m of the water column.

Replicates of deep samples (GN04: 3000 m, GN01: 2000 m) were collected for calibration purposes [START_REF] Van Der Loeff | A review of present techniques and methodological advances in analyzing 234Th in aquatic systems[END_REF]. The samples were processed following the MnO 2 coprecipitation technique [START_REF] Buesseler | An Intercomparison of Small-and Large-Volume Techniques for Thorium-234 in Seawater[END_REF]) using 230 Th as a chemical yield tracer [START_REF] Pike | Quantification of 234 Th recovery in small volume seawater samples by inductively coupled plasma-mass spectrometry[END_REF]. The counting was done on board using low background beta counters and measured again 6 months later for background quantification, as per particulate 234 Th samples. Recoveries for 234 Th were determined on all filters by inductively coupled plasma mass spectrometry with an average recovery of 0.87 ± 0.03 (n = 225, GN04).

  to estimate the Hg export flux of 43 Mg y -1 in the Arctic Ocean. Typically, the GEOS-Chem model uses an average K d of 3.2 x 10 5 L kg -1 for simulations in other oceans. A recent study, conducted in the North Atlantic by Lamborg et al. (2016), showed that observed K d values are an order of magnitude higher (4.5 x 10 6 L kg -1 ). Soerensen et al. (2016) conducted a box model study that suggested that about 37 Mg y -1 of Hg are exported below 200 m depth. The authors estimated Hg export fluxes based on particulate fluxes of solids from rivers and erosion (660 Tg y -1 ) and primary production (1,100 Tg y -1 of which 35 Tg y -1 are exported below 200 m depth). In the absence of pHg data the authors used a best estimate of 50 ng g -1

  /234 Th ratios on filtered particulates (1 -51 or 53 µm) to convert the 234 Th flux to the pHg flux, assuming that the pHg / 234 Th ratios in those particles are equivalent to the ratios on the sinking particles driving pHg export. We chose to sample at 100 m depth within the halocline, well below the polar mixed layer (typically 20 m) and above the Atlantic-sourced waters (typically 150 -700 m depth). The particulate 234 Th activities measured during the GN04 and GN01 cruises averaged 0.24 ± 0.16 dpm L -1 (n = 19, range 0.067 -0.76 dpm L -1 , Tab. S2). We calculated234 Th export flux at 100 m depth, which ranged between -300 ± 100 and 569 ± 207 dpm m -2 d -1 in the central Arctic Ocean and between 221 ± 175 and 700 ± 100 dpm m -2 d -1 on the shelf (Tab. S2). Negative values are considered to be negligible and equal to zero, which might be caused by remineralization, lateral advection[START_REF] Savoye | 234 Th deficit and excess in the Southern Ocean during spring 2001: Particle export and remineralization: S.O. particle export and remineralization[END_REF][START_REF] Maiti | Insights into particle formation and remineralization using the short-lived radionuclide, Thorium-234: particle remineralization using Th-234[END_REF] and/or inputs of 234 Th -enriched ice algae[START_REF] Rodriguez Y Baena | 234Th-based carbon export during an ice-edge bloom: Sea-ice algae as a likely bias in data interpretation[END_REF].Low (or negligible) 234 Th export fluxes are common in the Arctic Ocean(Roca Marti et al., 2016) particularly at the end of summer when the cruises took place, due to low productivity and a community structure that does not promote efficient export. The average pHg / 234 Th ratio was 0.48 ± 0.31 pmol dpm -1 (n = 19, range 0.072 -1.05 pmol dpm -1 , Tab. S2). Based on the measured pHg / 234 Th ratios, we estimated the Hg export fluxes at each station, which

Fig. 1 .

 1 Fig. 1. Particulate Hg export fluxes at 100 m (pmol m -2 day -1 ) derived from the 234 Th approach and the pHg / 234 Th ratios from this study. Negative fluxes were considered equal to

Fig. 2 :

 2 Fig. 2: Mass balance of Hg in the Arctic Ocean with fluxes (Mg y -1 ) components

Acknowledgements

The authors would like to thank the chief scientists Ursula Schauer, Dave Kadko and William Landing; Captains, officers and crew of the FS Polarstern and the USCGC Healy.

We thank the CNRS Chantier Arctique Français funding via the Pollution in the Arctic System project, European Research Council (ERC-2010-StG_20091028) to JES and the AXA Research Fund to LEHB (postdoc grant Levering Knowledge Gaps To Understand And Anticipate Risk From Methylmercury Exposure; and outreach grant Arctic Mediterranean Mercury). We also thank a NASA Earth and Space Science Fellowship Program grant (NNX13AP31H) and the National Science Foundation (OCE-1458305) funding to EEB. We are thankful to Sandra Gdaniec for her support with the in situ pump sampling and to Walter Geibert for the thorium recovery analyses. The IAEA is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. PM acknowledges the support of the Generalitat de Catalunya (MERS 2017 SGR -1588). This work contributes to the ICTA Unit of Excellence (MinECo, MDM2015-0552). We also thank Itziar Tesán for the support with a graphical design.


Matthew A Charette
email: mcharette@whoi.edu

Lauren E Kipp

Laramie T Jensen

Jessica S Dabrowski

Laura M Whitmore

Jessica N Fitzsimmons

Tatiana Williford

Adam Ulfsbo

Elizabeth Jones

Randelle M Bundy

Sebastian M Vivancos

Katharina Pahnke

Seth G John

Yang Xiang

Mariko Hatta

Mariia V Petrova

Lars-Eric Heimbürger-Boavida

Dorothea Bauch

Robert Newton

Angelica Pasqualini

Alison M Agather

Rainer M W Amon

Robert F Anderson

Per S Andersson

Ronald Benner

Katlin L Bowman

R Lawrence Edwards

Sandra Gdaniec

J A Loes

Gerringa

Aridane G González

Brian Haley

Chad R Hammerschmidt

Dennis A Hansell

Paul B Henderson

David C Kadko

Karl Kaiser

Phoebe J Lam

Carl H Lamborg

Martin Levier

Xianglei Li

Andrew R Margolin

Chris Measures

Frank J Millero

Willard S Moore

Ronja Paffrath

Hélène Planquette

Benjamin Rabe

Robert Rember

Matthieu Roy-Barman

Michiel Rutgers Van Der Loeff

Mak Saito

Ursula Schauer

Peter Schlosser

Robert M Sherrell

Alan M Shiller

Hans Slagter

Jeroen E Sonke

Colin Stedmon

Ryan J Woosley

Ole Valk

Jan Van Ooijen

Ruifeng Zhang

The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean

Keywords: Arctic Ocean, Transpolar Drift, trace elements, carbon, nutrients, GEOTRACES Plain

come

Introduction

Of all the major oceans on Earth, the Arctic Ocean is the most heavily influenced by processes occurring over continental shelves, which cover over 50% of its area [START_REF] Jakobsson | Hypsometry and volume of the Arctic Ocean and its constituent seas[END_REF]. The Arctic Ocean also has the lowest salinity surface waters, a result of limited evaporation, high riverine inputs, the annual sea-ice freeze/melt cycle, and restricted exchange with other ocean basins [START_REF] Serreze | Perspectives on the Arctic's shrinking sea-ice cover[END_REF]. These factors combine to impart a shelf-derived biogeochemical signature over much of the polar mixed layer, the low salinity surface layer influenced by sea-ice and freshwater, even in the central basin.

In the western Arctic's Canada Basin, hydrographic fronts serve as barriers to rapid shelf-basin exchange processes, thereby eddies and wind-induced upwelling or downwelling constitute the primary mechanisms for off-shelf water and material transport and exchange [START_REF] Muench | An Arctic Ocean cold core eddy[END_REF][START_REF] Pickart | Flow of winter-transformed Pacific water into the Western Arctic[END_REF][START_REF] Pickart | Dynamics of upwelling in the Alaskan Beaufort Sea and associated shelf-basin fluxes[END_REF]. In the eastern Arctic, however, the Transpolar Drift (TPD) is a major current that directly transports shelf water and sea ice directly from the Laptev and East Siberian Seas toward the central basin and Fram Strait, a major outlet for Arctic waters [START_REF] Ekwurzel | River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean[END_REF][START_REF] Mclaughlin | Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin[END_REF][START_REF] Rigor | Response of Sea Ice to the Arctic Oscillation[END_REF][START_REF] Rudels | Arctic Ocean circulation, processes and water masses: A description of observations and ideas with focus on the period prior to the International Polar Year 2007-2009[END_REF][START_REF] Schlosser | Arctic river-runoff: mean residence time on the shelves and in the halocline[END_REF]. The timescale for the trans-Arctic crossing of this current is on the order of 1-3 years [START_REF] Pfirman | Potential for rapid transport of contaminants from the Kara Sea[END_REF][START_REF] Steele | Circulation of summer Pacific halocline water in the Arctic Ocean[END_REF]; as such, the TPD is currently a mechanism for the rapid transport of shelf-derived materials including nutrients and carbon to the deep Arctic basin [START_REF] Kipp | Radium isotopes as tracers of boundary inputs of nutrients and trace elements to the coastal and open ocean[END_REF][START_REF] Letscher | Rapid removal of terrigenous dissolved organic carbon over the Eurasian shelves of the Arctic Ocean[END_REF][START_REF] Opsahl | Major flux of terrigenous dissolved organic matter through the Arctic Ocean[END_REF][START_REF] Wheeler | Nutrients, organic carbon and organic nitrogen in the upper water column of the Arctic Ocean: implications for the sources of dissolved organic carbon[END_REF], with potential biogeochemical impacts detected as far downstream as the North Atlantic Ocean (e.g. [START_REF] Amon | Dissolved organic carbon distribution and origin in the Nordic Seas: Exchanges with the Arctic Oean and the North Atlantic[END_REF][START_REF] Gerringa | Organic complexation of iron in the West Atlantic Ocean[END_REF][START_REF] Noble | Coastal sources, sinks and strong organic complexation of dissolved cobalt within the US North Atlantic GEOTRACES transect GA03[END_REF][START_REF] Torres-Valdés | Export of nutrients from the Arctic Ocean[END_REF]. At present, primary production in the largely icecovered central Arctic is light limited; however, surface warming has led to reductions in ice cover, as well as increases in river discharge and permafrost thawing [START_REF] Frey | Impacts of permafrost degradation on arctic river biogeochemistry[END_REF][START_REF] Mcclelland | Increasing river discharge in the Eurasian Arctic: Consideration of dams, permafrost thaw, and fires as potential agents of change[END_REF][START_REF] Peterson | Increasing river discharge to the Arctic Ocean[END_REF][START_REF] Schuur | Climate change and the permafrost carbon feedback[END_REF][START_REF] Spencer | Detecting the signature of permafrost thaw in Arctic rivers[END_REF]. With reduced ice cover, the TPD-derived transport of ice-rafted materials might be interrupted [START_REF] Krumpen | Arctic warming interrupts the Transpolar Drift and affects long-range transport of sea ice and ice-rafted matter[END_REF], though [START_REF] Newton | Increasing transnational sea-ice exchange in a changing Arctic Ocean[END_REF] have shown that in the near term (~several decades) long distance ice transport will accelerate as the ice thins and is more responsive to the winds. Together, these changes are expected to modify the ecosystem dynamics of the Arctic Ocean, with shelf-basin exchange processes playing a significant role.

In 2015, three nations led cruises to the Arctic Ocean as part of the international GEOTRACES program, a global survey of the distributions of oceanic trace element and isotopes (TEIs). The Arctic GEOTRACES program represented an unprecedented effort in sampling of the Arctic water column from a biogeochemical perspective. High-resolution coverage of waters above 84°N captured the TEI fingerprint of the TPD, and will serve as an important reference for future studies that focus on climate change impacts in the Arctic.

Radium isotopes measured during the Arctic GEOTRACES cruises have already been used to show that the chemical composition of the TPD is modified during passage over the Laptev Shelf, and to suggest that potentially significant changes in the flux of nutrients and carbon from the Siberian shelves are already underway [START_REF] Kadko | The residence times of trace elements determined in the surface Arctic Ocean during the 2015 US Arctic GEOTRACES expedition[END_REF][START_REF] Kipp | Radium isotopes as tracers of boundary inputs of nutrients and trace elements to the coastal and open ocean[END_REF]Rutgers van der Loeff et al., 2018). Additionally, [START_REF] Rijkenberg | Dissolved Fe in the deep and upper Arctic Ocean with a focus on Fe Limitation in the Nansen Basin[END_REF] found higher dissolved Fe and [START_REF] Slagter | Organic Fe speciation in the Eurasian Basins of the Arctic Ocean and its relation to terrestrial DOM[END_REF] found increased concentrations of Fe-binding organic ligands in the path of the TPD relative to adjacent sampling stations. These ligands and the associated Fe on the one hand were found to correlate strongly with terrestrial sources, which are projected to increase in a changing Arctic. On the other hand, [START_REF] Rijkenberg | Dissolved Fe in the deep and upper Arctic Ocean with a focus on Fe Limitation in the Nansen Basin[END_REF] found a local occurrence of Fe limitation over the Nansen basin and hypothesized that retreating ice could further exacerbate this nutrient limitation. This paper is a synthesis of the distributions of TEIs in the central Arctic Ocean associated with the TPD. We examine the origin and fate of TEIs in this important trans-Arctic conduit and provide a first estimate of the mass transport rate for the TPD, based on ice drift trajectories and radionuclide tracers. By combining the TPD mass transport estimate with the TEI inventories reported herein, fluxes of these elements to the central Arctic Ocean via the TPD are estimated. Finally, we discuss the biogeochemical implications of the changing climate on TEI concentrations and fluxes to the Arctic and North Atlantic Oceans.

Study Area

The characteristics of water masses in the Arctic Ocean are controlled by bathymetry and inflows from the Atlantic and Pacific Oceans. The Arctic has two major basins, the Eurasian and Amerasian Basins, which are separated by the Lomonosov Ridge [START_REF] Rudels | Arctic Ocean circulation, processes and water masses: A description of observations and ideas with focus on the period prior to the International Polar Year 2007-2009[END_REF]. The Lomonosov Ridge is an underwater ridge of continental crust that emerges north of the Siberian shelves at approximately 140°E. Here we refer to the Amerasian Basin as the "western Arctic", while the Siberian shelves and Eurasian Basin are referred to as the "eastern Arctic". The Eurasian Basin is further divided into the Nansen and Amundsen Basins by the Gakkel Ridge, and the Amerasian Basin is divided by the Alpha-Mendeleev Ridge into the large Canada Basin and the Makarov Basin. Surrounding these basins are wide, shallow continental shelves that occupy over 50% of the Arctic Ocean's area [START_REF] Jakobsson | Hypsometry and volume of the Arctic Ocean and its constituent seas[END_REF]. Pacific water flows into the Arctic through the narrow and shallow Bering Strait, while Atlantic water enters through the Barents Sea and the Fram Strait [START_REF] Rudels | Arctic Ocean Circulation[END_REF]. The major outflows of Arctic waters are through the Canadian Arctic Archipelago and Fram Strait, on either side of Greenland, into the North Atlantic [START_REF] Carmack | Freshwater and its role in the Arctic Marine System: Sources, disposition, storage, export, and physical and biogeochemical consequences in the Arctic and global oceans[END_REF] (Fig. 1).

Salinity is the largest control on water density, and therefore large-scale pressure gradients, in the Arctic Ocean. Between the (relatively fresh) North Pacific and the (salty) North Atlantic waters, there is a steric height gradient of about a meter, creating a pressure gradient across the Arctic from the Pacific down to the Atlantic. Large inputs of freshwater along the Arctic coastlines create a sea-surface height gradient from the coasts to the central basins, which drives a series of boundary currents in the coastal seas and over the continental slope that move water eastward (counter-clockwise) around the Arctic [START_REF] Rudels | Radium Isotopes Across the Arctic Ocean Show Time Scales of Water Mass Ventilation and Increasing Shelf Inputs[END_REF][START_REF] Rudels | Arctic Ocean circulation, processes and water masses: A description of observations and ideas with focus on the period prior to the International Polar Year 2007-2009[END_REF].

Overprinted on these perennial pressure gradients, the surface circulation is strongly impacted by winds. Predominant atmospheric circulation causes the average sea level pressure to be high over the Canada Basin and low over the Eurasian Basin, Barents Sea, and Nordic Seas [START_REF] Hunkins | Laboratory simulation of exchange through Fram Strait[END_REF][START_REF] Serreze | Characteristics of the Beaufort Sea high[END_REF]. The resulting winds draw relatively fresh water over the Amerasian Basin, and set up the anti-cyclonic Beaufort Gyre, and a weaker cyclonic gyre in the Eurasian Basin [START_REF] Alkire | Variability in the meteoric water, sea-ice melt, and Pacific water contributions to the central Arctic Ocean, 2000-2014[END_REF][START_REF] Bauch | Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007[END_REF][START_REF] Carmack | Freshwater and its role in the Arctic Marine System: Sources, disposition, storage, export, and physical and biogeochemical consequences in the Arctic and global oceans[END_REF][START_REF] Ekwurzel | River runoff, sea ice meltwater, and Pacific water distribution and mean residence times in the Arctic Ocean[END_REF][START_REF] Newton | Baroclinic eddies in the Arctic Ocean[END_REF][START_REF] Proshutinsky | Two circulation regimes of the winddriven Arctic Ocean[END_REF]. These two circulation cells converge just north of Siberia to form the Transpolar Drift [START_REF] Rudels | Arctic Ocean circulation, processes and water masses: A description of observations and ideas with focus on the period prior to the International Polar Year 2007-2009[END_REF]. The TPD extends from the Siberian shelves to the Fram Strait, as inferred from ice motion [START_REF] Rigor | Response of Sea Ice to the Arctic Oscillation[END_REF] and water mass characteristics [START_REF] Mclaughlin | Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin[END_REF].

The position of the TPD is determined by the Arctic Oscillation (AO), a large-scale Arctic climate pattern characterized by sea level pressure anomalies (Fig. 1). The AO is highly correlated with the North Atlantic Oscillation (NAO) [START_REF] Mysak | Patterns of Arctic Circulation[END_REF], sea level pressure over the central Arctic, and with sea surface height anomalies along the coastal Arctic [START_REF] Newton | A simple model of the Arctic Ocean response to annular atmospheric modes[END_REF].

During a low or negative AO and NAO, a strong Arctic High exists over the Canada Basin, expanding the anticyclonic Beaufort Gyre. In this case, the TPD originates from the Laptev and East Siberian Seas and flows over the Lomonosov Ridge [START_REF] Morison | Relaxation of central Arctic Ocean hydrography to pre-1990s climatology[END_REF][START_REF] Woodgate | A year in the physical oceanography of the Chukchi Sea: Moored measurements from autumn 1990-1991[END_REF] (solid red arrows in Fig. 1). Positive AO and NAO indices produce a weak Arctic High, resulting in a smaller Beaufort Gyre [START_REF] Mysak | Patterns of Arctic Circulation[END_REF]. In a persistently positive phase of the AO, the TPD shifts eastward towards the Bering Strait, entraining more Pacific water from the Chukchi Sea while still receiving a contribution from the East Siberian Shelf waters, which are transported farther east along the shelf before entering the TPD [START_REF] Morison | Changing Arctic Ocean freshwater pathways[END_REF][START_REF] Mysak | Patterns of Arctic Circulation[END_REF] (dashed red arrows in Fig. 1). During the years preceding the 2015 Arctic GEOTRACES sampling, the annual average AO was neutral to negative, and thus during the expeditions the TPD was located over the Lomonosov Ridge [START_REF] Kipp | Radium isotopes as tracers of boundary inputs of nutrients and trace elements to the coastal and open ocean[END_REF]Rutgers van der Loeff et al., 2018). Monitoring of atmospheric circulation [START_REF] Morison | Changing Arctic Ocean freshwater pathways[END_REF][START_REF] Proshutinsky | Beaufort Gyre freshwater reservoir: State and variability from observations[END_REF] as well as biogeochemical and water mass properties on previous hydrographic transects [START_REF] Falck | Disappearance of Pacific Water in the northwestern Fram Strait[END_REF][START_REF] Morison | Changing Arctic Ocean freshwater pathways[END_REF][START_REF] Steele | Circulation of summer Pacific halocline water in the Arctic Ocean[END_REF] [START_REF] Jones | Tracing Pacific water in the North Atlantic Ocean[END_REF][START_REF] Jones | Is the global conveyor belt threatened by arctic ocean fresh water outflow?[END_REF] and extends north of Greenland to the Fram Strait [START_REF] Dmitrenko | Variability of the Pacific-Derived Arctic Water Over the Southeastern Wandel Sea Shelf (Northeast Greenland) in 2015-2016[END_REF][START_REF] De Steur | Hydrographic changes in the Lincoln Sea in the Arctic Ocean with focus on an upper ocean freshwater anomaly between 2007 and 2010[END_REF]. Over the Eurasian Basin, the Pacificinfluenced layer is absent, with Atlantic waters occupying the entire water column [START_REF] Bauch | Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007[END_REF].

Methods

Sampling and Analyses of TEIs

The data presented herein was collected primarily during two cruises in 2015 associated with the Arctic GEOTRACES program. The U.S. GEOTRACES GN01 (HLY1502) cruise was held aboard the USCGC Healy, while the German GEOTRACES GN04 (PS94) cruise was on the R/V Polarstern (Fig. 1). All sampling and analyses were conducted according to pre-established GEOTRACES approved protocols (for TEIs) [START_REF] Cutter | Sampling and Sample-handing Protocols for GEOTRACES Cruises[END_REF] and/or GO-SHIP approved protocols (for non-TEIs) (Hood et al., 2010). To further ensure quality of TEI data across participating laboratories, extensive intercalibration efforts were taken in accordance with GEOTRACES protocols [START_REF] Cutter | Intercalibration in chemical oceanography-Getting the right number[END_REF]. For example, the GN01 and GN04 cruises both occupied the same station within two weeks of each other (GN01 station 30 and GN04 station 101), which enabled investigators to intercompare results for their respective TEIs. CTD/rosette data and methodologies for PS94 are available via the PANGEA database (Ober et al., 2016a[START_REF] Ober | Physical oceanography measured on water bottle samples with ultra clean CTD/Water sampler-system during POLARSTERN cruise PS94 (ARK-XXIX/3)[END_REF][START_REF] Pangaea Rabe | Physical oceanography measured on water bottle samples during POLARSTERN cruise PS94[END_REF]Rabe et al., , 2016a)). The GN01 CTD/rosette procedures are stored on the BCO-DMO database (Landing et al., 2019a[START_REF] Landing | Bottle data from the GEOTRACES Clean Carousel sampling system (GTC) on the Arctic Section cruise[END_REF]. Detailed methodologies can be found in the publications where the original TEI data were first reported (in case of Hg: Lamborg et al., 2016, Heimbürge et al., 2015[START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF]. The GN01 and GN04 Hg species data were intercalibrated at the crossover station.

Total Hg concentrations agreed well for the majority of sampling depths. The tHg and MeHg data have been intercalibrated and validated by the GEOTRACES standards & intercalibration committee.

Linear Mixing Model

In order to study the provenance and pathways of TEIs carried by the TPD, we must quantitatively parse the fraction (f) of source waters in each collected GEOTRACES sample.

To do so, we use the relatively well-studied distribution of salinity (S), δ 18 O-H 2 O ratios, and the Arctic N-P tracer (ANP; see [START_REF] Newton | Canadian Basin freshwater sources and changes: Results from the 2005 Arctic Ocean Section[END_REF]. These can be used to identify fractions of Pacific (Pac)-and Atlantic (Atl)-sourced sea water, sea-ice melt (SIM), and meteoric water (Met). The latter includes runoff and net in-situ precipitation. Along the cruise transects, in-situ precipitation is expected to be small in comparison with the continental runoff; hence f Met will be our primary proxy for determining the water masses most influenced by the TPD. The value for each in a sample is expressed as a linear combination of the values in its constituent water masses:

f Atl [S Atl ] + f Pac [S Pac ] + f Met [S Met ] + f SIM [S SIM ] = [S] Obs f Atl [δ 18 O Atl ] + f Pac [δ 18 O Pac ] + f Met [δ 18 O Met ] + f SIM [δ 18 O SIM ] = [δ 18 O] Obs f Atl [ANP Atl ] + f Pac [ANP Pac ] + f Met [ANP Met ] + f SIM [ANP SIM ] = [ANP] Obs f Atl + f Pac + f Met + f SIM = 1
This constitutes a 4-dimensional linear system that can be solved in matrix form:

[f] = {C} -1 [y],
where [f] is a vector of water-mass fractions, [y] is a vector of the parameter values in the sample, and {C} is a matrix of values in the 'end members', i.e. the source waters. The model assumes 4 end members (Table 1) and 4 equations, so will yield an exact solution.

Table 1. Endmember parameter values for the water mass analysis linear mixing Fortunately, our focus here is on the concentration of meteoric waters and this fraction is insensitive to nutrient concentrations. Rather, it depends on salinity and δ 18 O with the error originating primarily from seasonal and geographical variability in the δ 18 O endmember of Arctic rivers [START_REF] Cooper | Flow-weighted values of runoff tracers (δ18O, DOC, Ba, alkalinity) from the six largest Arctic rivers[END_REF]. Monte Carlo analysis across a reasonable range of estimated mean δ 18 O values for runoff yielded errors of about 1% on the meteoric fractions.

The relationship between each TEI and the meteoric water fraction was determined using a linear regression model. The slope, intercept, r 2 value, and p value for each Hg species are reported in Table 2. The effective shelf endmember concentrations of the TEIs were calculated using their respective linear regressions at 20% meteoric water, assuming that this is the meteoric water fraction of the TPD when it leaves the shelf and that there was no significant TEI removal or addition during transport. Meteoric water fractions of 10 -35%

have been observed at the point of origin of the TPD in the Laptev Sea [START_REF] Bauch | Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007[END_REF] and its terminus at the Fram Strait [START_REF] Dodd | The freshwater composition of the Fram Strait outflow derived from a decade of tracer measurements[END_REF]. During the 2015 GEOTRACES expeditions, fractions up to 25% were observed near the North Pole, thus 20% is a conservative estimate. [START_REF] Boyle | On the chemical mass-balance in estuaries[END_REF][START_REF] Shiller | The effect of recycling traps and upwelling on estuarine chemical flux estimates[END_REF], but they still provide a first approximation to compare with sparse existing river and shelf sea data. There is some data on TEI concentrations in the Eurasian rivers that ultimately feed into the TPD. Most are derived from the Arctic Great Rivers Observatory (A-GRO), which began as the Pan-Arctic River Transport of Nutrients, Organic Matter and Suspended Sediments (PARTNERS) project [START_REF] Holmes | Arctic Great Rivers Observatory. Water Quality Dataset[END_REF]. The weighted averages reported by the A-GRO provide a useful comparison for many of the elements discussed in this manuscript, but could be improved with measurements of more TEIs in each of the Arctic rivers and knowledge of the relative influence of each river in the TPD at a given time. Due to the shelf circulation patterns (Fig. 1), the major Eurasian rivers (Lena, Ob', Yenisey, and Kolyma) will exert a stronger influence on the TPD than the North American rivers (Mackenzie and Yukon). As such, we report herein the discharge weighted average TEI concentrations for the Eurasian rivers only.

Most importantly, any differences between the effective river endmember and the mean river concentrations should not be interpreted in a quantitative manner; rather, this analysis is meant only to give the reader a sense of the relative influence of rivers and/or estuarine removal/addition processes on the TEIs that are transported to the central Arctic Ocean via the TPD.

Results and Discussion

We define the lateral extent of the TPD as ~84°N (in the Canada Basin) to 87°N (in the Eurasian Basin) for waters in the top 50 m. These boundaries were chosen qualitatively based on the distributions of the meteoric water fraction and TPD-influenced TEIs (Fig. 2).

For example, there is a sharp concentration gradient for chromophoric dissolved organic matter (CDOM), dissolved organic carbon (DOC), dissolved Fe, and 228 Ra at stations north of 84°N 150°W, which coincides with a front between high and intermediate meteoric water fractions (~250 km along the section distance in Fig. 2). On the Eurasian side of the transect, there is minimal meteoric water influence south of 87°N (~1100 km along the section distance in Fig. 2). The TPD can be characterized generally by this high meteoric water component, which is due to large river contributions to the Siberian Arctic shelves. However, the meteoric water fraction alone cannot be used to delineate the western boundary of the TPD because the Beaufort Gyre in the Canada Basin contains a significant and growing freshwater component sourced from eastern Arctic rivers [START_REF] Giles | Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre[END_REF][START_REF] Morison | Changing Arctic Ocean freshwater pathways[END_REF][START_REF] Rabe | An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006-2008 period[END_REF][START_REF] Rabe | Arctic Ocean basin liquid freshwater storage trend 1992-2012[END_REF].

As a function of depth, the elevated TEI concentrations and meteoric water fractions are confined to the upper 50 m. The 50 m cutoff also serves to exclude the halocline from our analysis, which is rich in certain TEIs and nutrients like silicate (Fig. 2g), and is influenced by different ventilation processes and source water masses than the TPD [START_REF] Aagaard | On the halocline of the Arctic Ocean[END_REF]. The data presented herein are shown mainly as a function of the meteoric water fraction and were collected in the upper 50 m of the water column for all stations north of 84°N, which includes the polar mixed layer and the TPD.

Trace Element and Isotope Distributions, Sources, and Sinks

Mercury

Among all stations located north of 84°N and shallower than 50 m, total mercury (tHg) ranged from ~0.5-2.5 pmol L -1 , methyl-mercury (MeHg, the sum of mono-and dimethyl-mercury) ranged from <0.05-0.22 pmol L -1 , monomethyl-mercury (MMHg) ranged from <0.05-0.20 pmol L -1 , and dimethyl-mercury (DMHg) ranged from <0.05-0.12 pmol L -1 (Fig. 3). Samples with elevated meteoric water fractions (>15%) were characterized by higher tHg concentrations (up to ~2 pmol L -1 ), though there was no significant correlation between the two variables. This might be because rivers are not the only source of tHg to the water column. Mercury also enters the Arctic Ocean via atmospheric deposition and oceanic inputs, mostly from the Atlantic Ocean [START_REF] Cossa | Mercury distribution and transport in the North Atlantic Ocean along the GEOTRACES-GA01 transect[END_REF][START_REF] Outridge | A mass balance inventory of mercury in the Arctic Ocean[END_REF][START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF][START_REF] Sonke | Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean[END_REF]. However, the lack of correlation between total Hg and meteoric water input in the TPD is surprising given the substantial input flux predicted from measurements of Hg in Arctic rivers. [START_REF] Sonke | Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean[END_REF] derived a discharge-weighted tHg concentration of 46 pmol L -1 for the monitored Eurasian rivers, with values of up to 191 pmol L -1 in the spring freshet (Yenisei River). This result implies a large loss of Hg in estuaries and shelves, which may be the result of atmospheric evasion [START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF][START_REF] Sonke | Environmental science: Mercury in flux[END_REF]. A more recent box model study reveals that a portion of the evading Hg is in the form of DMHg [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF]. Estuarine and shelf sediments might also act as sinks for Hg entering from pan-Arctic rivers (e.g., [START_REF] Amos | Global biogeochemical implications of mercury discharges from rivers and sediment burial[END_REF], but this idea remains to be tested for this basin.

The MeHg species had no significant correlation to meteoric water fraction above 84°N. Since shelf sediments can be sources of MeHg (e.g., [START_REF] Hammerschmidt | Methylmercury cycling in sediments on the continental shelf of southern New England[END_REF][START_REF] Hollweg | Mercury and methylmercury cycling in sediments of the mid-Atlantic continental shelf and slope[END_REF], we might expect a correlation to meteoric water inputs. The lack of such a correlation suggests that either MeHg produced on the Eurasian shelves was lost to demethylation processes during the ~6-18-month transit from the shelf-break to the central Arctic Ocean, or that production in the mixed layer is a stronger source than the shelves.

Large subsurface maxima in methylated Hg species [START_REF] Agather | Distribution of mercury species in the Western Arctic Ocean (U.S. GEOTRACES GN01)[END_REF][START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF] suggests a third source for MeHg in the TPD could be diffusion from the MeHg species-rich halocline [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF].

In the future, climate warming is expected to increase Hg inputs to the Arctic drastically as permafrost contains large Hg stocks [START_REF] Schuster | Permafrost Stores a Globally Significant Amount of Mercury[END_REF]. The Arctic reservoir with the highest relative proportion of MeHg, often representing more than 40%, is generally open ocean sea water [START_REF] Heimbürger | Shallow methylmercury production in the marginal sea ice zone of the central Arctic Ocean[END_REF]. It is primarily the ocean-sourced MeHg that bioamplifies to harmful levels, putting Arctic wildlife and human health at risk. The additional input of Hg and DOC might further stimulate MeHg production in the Arctic Ocean. Future coupled ocean-atmosphere numerical models (e.g. [START_REF] Fisher | Riverine source of Arctic Ocean mercury inferred from atmospheric observations[END_REF][START_REF] Zhang | Biogeochemical drivers of the fate of riverine mercury discharged to the global and Arctic oceans[END_REF] and box model assessments (e.g. [START_REF] Soerensen | A mass budget for mercury and methylmercury in the Arctic Ocean[END_REF] designed to constrain Arctic Hg cycling will need to consider Hg cycling and transport associated with the TPD.

Conclusions

Intensification of the hydrologic cycle and permafrost degradation may result in the release of about 25% of the carbon stored in Arctic soils in the next 100 years [START_REF] Gruber | The vulnerability of the carbon cycle in the 21st century: an assessment of carbonclimate-human interactions[END_REF]. According to the NOAA Arctic report card [START_REF] Osborne | Arctic Report Card[END_REF], the 2018 summer/autumn discharge for the largest rivers flowing into the Arctic was 20% greater than in the 1980-89 period and will continue to increase. These changes will have a substantial effect on the riverine supply of DOM into the Arctic Ocean, as well as the long-distance transport of TEIs within the Transpolar Drift that are likely complexed by this organic matter, including Fe, Co, Ni, Cu, Th, and possibly the REEs.

While the halocline contains ample nutrient concentrations, the increased freshwater inputs are strengthening water column stratification, which could further limit nutrient inputs via vertical mixing processes (Rudels et al., 1991). Hence, increased macro-and micronutrient concentrations delivered to the central Arctic Ocean via the TPD may play an important role in upper ocean productivity in the coming decades, since, for example, nitrate already limits primary production in some Arctic locations [START_REF] Tremblay | The effects of irradiance and nutrient supply on the productivity of Arctic waters: a perspective on climate change[END_REF], as does Fe in the case of one large under ice bloom [START_REF] Rijkenberg | Dissolved Fe in the deep and upper Arctic Ocean with a focus on Fe Limitation in the Nansen Basin[END_REF]. In the case of Fe, whether limitation will occur in the changing Arctic will depend on the interplay between nutrient utilization ratios [START_REF] Rijkenberg | Dissolved Fe in the deep and upper Arctic Ocean with a focus on Fe Limitation in the Nansen Basin[END_REF] and projected increases of ligand-borne, specifically humic-borne, terrestrial dFe [START_REF] Slagter | Organic Fe speciation in the Eurasian Basins of the Arctic Ocean and its relation to terrestrial DOM[END_REF][START_REF] Slagter | Fe-Binding Organic Ligands in the Humic-Rich TransPolar Drift in the Surface Arctic Ocean Using Multiple Voltammetric Methods[END_REF].

The complexity of physical and biochemical factors and their interplay, such as the effect of increased river runoff and stratification on the saturation state of aragonite [START_REF] Yamamoto-Kawai | Aragonite undersaturation in the Arctic Ocean: effects of ocean acidification and sea ice melt[END_REF], combine with scarcity of data to make future effects of TPD influence on the central Arctic difficult to predict [START_REF] Carmack | Towards recognition of physical and geochemical change in Subarctic and Arctic Seas[END_REF]. However, DOM is strongly related to hydrographic parameters and biogeochemical cycles in the shelf seas and TPD [START_REF] Amon | Dissolved organic carbon distribution and origin in the Nordic Seas: Exchanges with the Arctic Oean and the North Atlantic[END_REF][START_REF] Granskog | Characteristics of colored dissolved organic matter (CDOM) in the Arctic outflow in the Fram Strait: Assessing the changes and fate of terrigenous CDOM in the Arctic Ocean[END_REF], but has the advantage of relatively simple measurement via remote sensing in ice-free waters [START_REF] Fichot | Pan-Arctic distributions of continental runoff in the Arctic Ocean[END_REF][START_REF] Juhls | Dissolved organic matter at the fluvial-marine transition in the Laptev Sea using in situ data and ocean colour remote sensing[END_REF][START_REF] Matsuoka | Pan-Arctic optical characteristics of colored dissolved organic matter: Tracing dissolved organic carbon in changing Arctic waters using satellite ocean color data[END_REF] or in-situ instrumentation capable of high vertical resolution such as the fluorometers deployed on these cruises. Looking to the future, this makes CDOM a powerful tracer of climate change impacts on a multitude of Arctic system processes (e.g. [START_REF] Stedmon | An approach to estimate the freshwater contribution from glacial melt and precipitation in East Greenland shelf waters using colored dissolved organic matter (CDOM)[END_REF].

For some TEIs, the sediments within the broad and shallow eastern Arctic shelves play a dominant role in their cycling and signature within the TPD. Radium isotopic ratios and a mass balance calculation point to shelf sediments as the dominant source of 228 Ra carried by the TPD [START_REF] Kipp | Radium isotopes as tracers of boundary inputs of nutrients and trace elements to the coastal and open ocean[END_REF]. While 228 Ra is not a biologically important TEI, it acts as a quasi-conservative tracer of other shelf-derived materials like Ba, which has TPD concentrations that cannot be fully explained by a river source. The Ra-derived evidence of active sediment-water exchange processes in the eastern Arctic coastal zone supports the apparent strong sinks for Pb and V, which are known to be removed by particle scavenging and/or reduction processes in shelf sediments. Increased 228 Ra levels in the TPD therefore suggest that the concentrations of these other TEIs may be affected both positively (Ba) or negatively (Pb, Al, Ga, and V) under a changing climate where shelf sediments are exposed to wind-driven mixing under reduced ice cover.

The TEIs that have the strongest correlation with meteoric water fraction are those that are known to form complexes with organic matter. As a result, other than dilution via mixing, their concentrations, which are significantly elevated relative to other ocean basins, are preserved in the TPD over distances >1000 km and timescales of up to 18 months. It is therefore reasonable to expect that this TEI "fingerprint" of the TPD would be carried beyond the ice covered central Arctic Ocean, through Fram Strait, and into the ice-free surface waters of the North Atlantic Ocean as seen for Arctic river DOM [START_REF] Amon | Dissolved organic carbon distribution and origin in the Nordic Seas: Exchanges with the Arctic Oean and the North Atlantic[END_REF][START_REF] Benner | Terrigenous dissolved organic matter in the Arctic Ocean and its transport to surface and deep waters of the North Atlantic[END_REF][START_REF] Granskog | Characteristics of colored dissolved organic matter (CDOM) in the Arctic outflow in the Fram Strait: Assessing the changes and fate of terrigenous CDOM in the Arctic Ocean[END_REF]. In the present day, the TEIs transported in the TPD may become participants in biogeochemical processes of this ocean basin, or in the future be utilized closer to their source as the pan-Arctic ice cover is reduced with warming temperatures. This new utilization would apply to an increasingly ice-free Arctic Ocean including the Canada Basin, where the Beaufort Gyre (Fig. 1) is known to entrain and store an increasing amount of freshwater sourced from eastern Arctic rivers [START_REF] Giles | Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre[END_REF][START_REF] Morison | Changing Arctic Ocean freshwater pathways[END_REF][START_REF] Rabe | An assessment of Arctic Ocean freshwater content changes from the 1990s to the 2006-2008 period[END_REF][START_REF] Rabe | Arctic Ocean basin liquid freshwater storage trend 1992-2012[END_REF].

Lastly, our understanding of the effects of the changing climate on Arctic Ocean TEI concentrations and fluxes has been greatly hampered by a lack of data, mainly due to the logistics and expense of conducting oceanography at high-latitudes where icebreakers are required for sampling. Geopolitical issues have resulted in large data gaps for the eastern Arctic shelf seas. In the near future, international collaboration through long term observatories at key locations and Arctic gateways, synoptic surveys (e.g. http://www.synopticarcticsurvey.info) and advances in technology (e.g. floats, gliders, ice tethered sensors and samplers) may provide the temporal and spatial coverage needed to address some of the pressing unanswered questions posed herein.

  provide evidence that this position has remained relatively stable over the past ca. 30 years. The characteristics of the upper water column differ on either side of the TPD because it generally acts as a boundary between Atlantic and Pacific contributions to the Arctic pycnocline. High nutrient, high DOM, low salinity Pacific water is typically observed as an "upper halocline" over the Canada and Makarov Basins, where it separates surface waters from the Atlantic boundary currents below about 200 meters. Sub-surface distributions of nitrate, phosphate and silicate indicate that a layer of nutrient-rich shelf-modified Bering Strait Inflow thins and shoals northward from the Chukchi continental slope and dissipates in the vicinity of the TPD. Pacific influence is dominant in the Canadian Arctic Archipelago

Fig. 1 .

 1 Fig. 1. Map of the Arctic Ocean with the major upper ocean circulation features

  model. References: a Newton et al. (2013); b Pacific Water: slope = 14; intersept = -11; Atlantic Water: slope = 17; intersept = important sources of error, which are discussed in detail by Newton et al. (2013) in the context of the 2005 Arctic Ocean Section. Briefly, the least-constrained fractions are those of Pacific-and Atlantic-influenced ocean water, which suffer from the non-conservative nature of nutrients in the ocean, large scatter in the values in the source waters, and potentially from drift in the end-member means with time[START_REF] Newton | Canadian Basin freshwater sources and changes: Results from the 2005 Arctic Ocean Section[END_REF].

Fig. 2 .

 2 Fig. 2. Section plots for key trace element and isotope concentrations along a

Fig. 3 .

 3 Fig. 3. Total Hg (a) and MeHg (b) concentrations as a function of the meteoric

Table 2 .

 2 Linear curve fit data and statistics for the Hg species vs. meteoric water

	relationship plots.
	Property	slope	y-int	r2	p
	total Hg (pM)	0.0057	1.16	0.00	0.619
	total MeHg (pM)	-0.0054	0.16	0.29	0.005
	MMHg (pM)	0.0026	0.04	0.07	0.371
	Initial estimates of river endmember concentrations were calculated by extrapolating

the linear regression to 100% meteoric water (regression intercept). These estimates have a high statistical uncertainty associated with them due to the extrapolation beyond the measured range and other factors that violate the assumptions of the standard estuarine mixing model

Acknowledgements

This study would not have been possible without the dedication of the captains and crews of the USCGC Healy and R/V Polarstern. The authors also thank the many members of the shipboard scientific parties, including the chief scientists, who enabled sample collection via the various CTD rosette and in situ pumping systems, as well as the sample analyses on the ship and back in their home laboratories. Funding for Arctic GEOTRACES was provided by the U.S. National Science Foundation, Swedish Research Council Formas, French Agence Nationale de la Recherche and LabexMER, Netherlands Organization for Scientific Research, and Independent Research Fund Denmark.

Data from GEOTRACES cruises GN01 (HLY1502) and GN04 (PS94) have been archived at the Biological & Chemical Oceanography Data Management Office (BCO-DMO; https://www.bco-dmo.org/deployment/638807) and PANGAEA (https://www.pangaea.de/?q=PS94&f.campaign%5B%5D=PS94) websites, respectively. The inorganic carbon data are available at the NOAA Ocean Carbon Data System (OCADS; doi:10.3334/CDIAC/OTG.CLIVAR_ARC01_33HQ20150809).