Glacial melting in West Antarctica has intensified with the increased intrusion of warm ocean water beneath ice shelves, but the processes controlling the export of meltwater-associated micronutrient iron (Fe) to Southern Ocean surface waters remain unclear. Here, we report Fe concentrations and dissolved Fe (dFe) isotope ratios in the inflowing deepwater layer that drives melting of the Dotson Ice Shelf and in the meltwater-enriched outflow to determine meltwater-derived dFe. Isotopic mass balance points to an anoxic Fe-reducing region of the upstream subglacial hydrologic system as the dominant source of meltwater dFe, rather than ice shelf melt itself. Remarkably, total meltwater contributes only similar to 10% of outflowing dFe, with the majority contributed by inflowing deep water (62%), augmented by inputs from shelf sediments (28%). Outflowing suspended particulate Fe exceeds inflow by 46%, at 100 times the dFe concentration, with 25% in reactive phases. Predictive models of future ecosystem effects should consider that the primary role of ice shelf melting is to provide buoyancy that transports Fe from deep sources to the Fe-limited surface ocean, stimulating phytoplankton growth.

The flux of nutrients from continents to the oceans sustains oceanic primary productivity and is a fundamental component of the carbon cycle. In most regions of the world's oceans primary productivity is limited by the supply of nutrients. In particular, iron can become limiting in the open-ocean due to its low solubility. Glaciated continents have been suggested as an underappreciated source of iron to the high-latitude oceans. Yet, uncertainty remains regarding the magnitude and spatial variability of glacially derived nutrient fluxes, and the extent to which these nutrients impact open-ocean ecosystems. To quantify lithogenic fluxes at the West Greenland margin, we measured Th-232 and Th-230 in seawater and core-top sediments across the shelf and slope. Our results highlight a negative correlation between low-salinity waters and dissolved and particulate Th-232, suggesting a glacial source for this lithogenic isotope. We calculated dissolved Th-232 fluxes 5-24 mu g m(-2) yr(-1) (100-500 m depth), and sedimentary Th-232 fluxes 105-711 mu g m(-2) yr(-1), higher than typical open-ocean settings and similar to margin sites influenced by large inputs from aeolian dust and rivers. A sampling transect shows that dissolved Th-232 fluxes increase toward Greenland, confirming that lithogenic inputs are sourced laterally from the margin. Using our Th-232 fluxes, we estimate an elevated supply of dissolved Fe which extends over the continental slope toward the open ocean. This Fe flux is large enough to support much of the local primary productivity, highlighting the importance of lithogenic fluxes in supporting the marine ecosystem in high-latitude oceans.

Iron is an essential micronutrient for phytoplankton and plays an integral role in the marine carbon cycle. The supply and bioavailability of iron are therefore important modulators of climate over glacial-interglacial cycles. Inputs of iron from the Antarctic continental shelf alleviate iron limitation in the Southern Ocean, driving hotspots of productivity. Glacial meltwater fluxes can deliver high volumes of particulate iron. Here, we show that glacier meltwater provides particles rich in iron(II) to the Antarctic shelf surface ocean. Particulate iron(II) is understood to be more bioavailable to phytoplankton, but less stable in oxic seawater, than iron(III). Using x-ray microscopy, we demonstrate co-occurrence of iron and organic carbon-rich phases, suggesting that organic carbon retards the oxidation of potentially-bioavailable iron(II) in oxic seawater. Accelerating meltwater fluxes may provide an increasingly important source of bioavailable iron(II)-rich particles to the Antarctic surface ocean, with implications for the Southern Ocean carbon pump and ecosystem productivity.

The role of suspended particulate matter in the biogeochemical cycling of neodymium (Nd) in the ocean is not well understood. This study reports the measurement of dissolved Nd concentrations and isotopes (<0.2 um), and particulate Nd concentrations along the Eastern Pacific Zonal Transect (EPZT, GP16) as part of the GEOTRACES program. The western part of the EPZT cruise track is influenced by a hydrothermal plume, observed between 2200 and 2800 m, originating at the southern East Pacific Rise. This setting allows for the investigation of the role of particulate matter in Nd biogeochemical cycling and its contribution in modifying the distribution of dissolved Nd. Results show that the highest removal of dissolved Nd (∼ 21 %) was observed near the hydrothermal ridge crest, decreasing to ∼ 8 % at the westernmost station. Here, we report the partition coefficient (Kd) of each particle constituent (e.g., lithics, CaCO3, POM, Opal, Fe-phase, Mn-phase) to show that particle composition plays an important role in Nd removal alongside particle mass. Along the plume, both particulate Mn and Fe contributed to the removal of Nd, with Mn playing the dominant role in sites closest to the vent. At the western stations, below the plume, the concentration of both dissolved and particulate Nd increased with depth, with particulate Nd one to two orders of magnitude lower than measured in the dissolved phase. The increase in dissolved Nd with depth in the interval below the plume could result either from the continuing remineralization of raining particles or the influx of dissolved Nd from bottom sediments. Close to the ridge crest, where the hydrothermal Nd input is at its highest, there is a hint of local modification of dissolved εNd signature, however not large enough to affect the use of εNd as a water mass tracer.

The Amundsen Sea in the Pacific sector of West Antarctica receives meltwater from the fastest retreating Antarctic glaciers, and its coastal polynyas host the highest primary productivity per unit area observed on the Antarctic continental shelf. Polynya productivity provides the base for a robust, diverse ecosystem and is controlled primarily by light and the availability of the micronutrient iron (Fe). While the sources of Fe in the region are not yet certain, Fe could be transported within modified Circumpolar Deep Water (mCDW) that intrudes onto the retrograde shelf and into ice shelf cavities, where it gains buoyancy through the addition of glacial meltwater and is injected into the upper water column when it exits the cavity. Thus, fluxes of dissolved Fe from the seafloor into in-flowing mCDW may ultimately be a source of Fe to the euphotic zone in the Amundsen Sea. To investigate the surface sediment biogeochemistry and the potential for a significant benthic flux of Fe to the waters on the Amundsen Sea shelf, sediment cores were collected at two sites close to the calving fronts of the Pine Island and Thwaites Glacier ice shelves. Pore water was analyzed for trace element content, and sediment was analyzed for physical and chemical properties including organic carbon and trace elements. Using a novel approach based on hypothesized Fe speciation and colloidal particle radius, theoretical Fe fluxes were calculated from pore water gradients and porosity. The fluxes reveal a spatially variable Fe input to the lower water column that could ultimately fertilize primary productivity. Supported by geochemical and physical evidence, we conclude that submarine weathering of volcanic glass grains observed and quantified in seabed sediments at the Pine Island site drives nonreductive Fe fluxes that are 100-fold higher than at the Thwaites site. This study highlights the need for further investigations of benthic-pelagic coupling in the Amundsen Sea region, which will likely be impacted in coming decades by accelerating glacial melting.

Recent studies, including many from the GEOTRACES program, have expanded our knowledge of trace metals in the Arctic Ocean, an isolated ocean dominated by continental shelf and riverine inputs. Here, we report a unique, pan-Arctic linear relationship between dissolved copper (Cu) and nickel (Ni) present north of 60 degrees N that is absent in other oceans. The correlation is driven primarily by high Cu and Ni concentrations in the low salinity, river-influenced surface Arctic and low, homogeneous concentrations in Arctic deep waters, opposing their typical global distributions. Rivers are a major source of both metals, which is most evident within the central Arctic's Transpolar Drift. Local decoupling of the linear Cu-Ni relationship along the Chukchi Shelf and within the Canada Basin upper halocline reveals that Ni is additionally modified by biological cycling and shelf sediment processes, while Cu is mostly sourced from riverine inputs and influenced by mixing. This observation highlights differences in their chemistries: Cu is more prone to complexation with organic ligands, stabilizing its riverine source fluxes into the Arctic, while Ni is more labile and is dominated by biological processes. Within the Canadian Arctic Archipelago, an important source of Arctic water to the Atlantic Ocean, contributions of Cu and Ni from meteoric waters and the halocline are attenuated during transit to the Atlantic. Additionally, Cu and Ni in deep waters diminish with age due to isolation from surface sources, with higher concentrations in the younger Eastern Arctic basins and lower concentrations in the older Western Arctic basins.

The Amundsen Sea (AS) hosts the highest primary productivity per unit area observed on the Antarctic continental shelf, concentrated in coastal polynyas near the outlets of the fastest melting glaciers on Antarctica. These polynyas are highly dynamic carbon sinks where production is controlled by light and the availability of iron (Fe). Previous studies indicate that the majority of the Fe in the polynya surface waters is delivered through upward transport of modified Circumpolar Deep Water that invades the deep shelf, acquires additional Fe, and enters the ice shelf cavities, where it is injected into the upper water column by the addition of meltwater buoyancy (the “meltwater pump”). While an Fe source from shelf sediments is suggested by observations and modeling to exceed a direct glacial meltwater source, the distribution and magnitude of the purported benthic Fe flux is unknown. To investigate the surface sediment biogeochemistry and the potential for a dominant benthic flux of Fe to the AS, sediment cores were collected at two sites close to the calving face of the Pine Island and Thwaites ice shelves in late summer 2020, and subsampled for pore water and solid sediment. Pore water was analyzed for Fe and other trace elements. Solid sediment was analyzed for porosity, along with grain size, organic carbon content, and total trace metal contents. Dissolved Fe fluxes were calculated from porewater gradients and porosity, revealing a spatially variable but potentially important input to the lower water column that could ultimately fertilize primary productivity in the polynyas via the meltwater pump. High flux and burial of labile organic carbon in the rapidly deposited sediments may increase benthic Fe reduction, creating a positive feedback loop between primary production and sediment Fe release. The

The Arctic Ocean is unique, connecting the Atlantic and Pacific basins and being especially vulnerable to the impacts of a changing climate. Iron stable isotopes (δ56Fe) provide a unique window into the biogeochemical cycling of Fe in the Arctic. Here we present the first seawater δ56Fe for the Western Arctic Ocean, from the 2015 U.S. GEOTRACES GN01 transect. Samples analyzed for δ56Fe include seawater dissolved (<0.2 μm), soluble (<∼0.003 μm), and leachable particulate phases. Several key processes were explored using Fe isotopes, each characterized by a distinct combination of δ56Fe and Fe concentrations. Input of Fe from reducing continental shelf sediments was characterized by high dissolved Fe concentrations (6.18 ± 4.84 nmol kg−1) and low δ56Fe (−1.57 ± 0.66‰). Riverine Fe input observed in the Transpolar Drift Current was characterized by high Fe concentrations corresponding to a riverine end‐member Fe concentration of 19 nM, and near‐zero δ56Fe (0.02 ± 0.23‰) that was similar to that of average crustal material. The deep Arctic was mostly characterized by low Fe concentrations (0.33 ± 0.14 nmol kg−1) and slightly higher δ56Fe (0.05 ± 0.30‰), except for samples taken near continental slopes that were affected by sedimentary Fe input with lower δ56Fe, and in the Amundsen Basin which showed possible hydrothermal Fe input. Our data thus illuminate Fe biogeochemical cycling processes in the modern Arctic Ocean, and serve as a baseline for understanding how the Arctic Fe cycle responds to climate change.

The role of suspended particulate matter (SPM) in modulating dissolved Nd gains importance in hydrothermal settings where particle dynamics and scavenging play important roles in trace metal behaviors. Here we use seawater Nd isotopes and concentrations ([Nd]) from a Southern East Pacific Rise (SEPR) hydrothermal plume, sampled during the US-GEOTRACES GP16 Eastern Pacific Zonal Transect, to report how they are influenced by particles. Within the plume (2200-3000 m, identified by 3 He) dissolved Nd isotope ratios at both vent and distal stations are ~0.3-0.4 ε Nd - units more positive than shallower samples, approximately the analytical uncertainty. These results support previous conclusions that hydrothermal activity is not a significant source of seawater Nd [1]. In contrast, seawater [Nd] near vent stations exhibit a prominent decrease, which attenuates with increasing distance from the SEPR [2]. Within the SEPR plume there is a loss of 7-12% of the dissolved Nd inventory compared to proximal stations outside the plume, consistent with a 6-10% loss seen at the TAG site in the North Atlantic [3]. This cannot be explained by SPM concentration alone, possibly due to SPM

The surface waters of the Arctic Ocean include an important inventory of freshwater from rivers, sea ice melt, and glacial meltwaters. While some freshwaters are mixed directly into the surface ocean, cryospheric reservoirs, such as snow, sea ice, and melt ponds act as incubators for trace metals, as well as potential sources to the surface ocean upon melting. The availability and reactivity of these metals depends on their speciation, which may vary across each pool or undergo transformation upon mixing. We present here baseline measurements of colloidal (similar to 0.003-0.200 mu m) iron (Fe), zinc (Zn), nickel (Ni), copper (Cu), cadmium (Cd), and manganese (Mn) in snow, sea ice, melt ponds, and the underlying seawater. We consider both the total concentration of colloidal metals ([cMe]) in each cryospheric reservoir and the contribution of cMe to the overall dissolved metal phase (%cMe). Notably, snow contained higher (cMe) as well as higher %cMe relative to seawater for metals such as Fe and Zn across most stations. Stations close to the North Pole had relatively high aerosol deposition, imparting high (cFe) and (cZn), as well as high %cFe, %cZn, %cMn, and %cCd (>80%). In contrast, surface seawater concentrations of Cd, Cu, Mn, and Ni were dominated by the soluble phase (<0.003 mu m), suggesting little impact of cMe from the melting cryosphere, or rapid aggregation/disaggregation dynamics within surface waters leading to the loss of cMe. This has important implications for how trace metal biogeochemistry speciation and thus fluxes may change in a future ice-free Arctic Ocean.

Size-segregated particulate air samples were collected during the austral summer of 2016–2017 at Palmer Station on Anvers Island, western Antarctic Peninsula, to characterize trace elements in aerosols. Trace elements in aerosol samples – including Al, P, Ca, Ti, V, Mn, Ni, Cu, Zn, Ce, and Pb – were determined by total digestion and a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS). The crustal enrichment factors (EFcrust) and k-means clustering results of particle-size distributions show that these elements are derived primarily from three sources: (1) regional crustal emissions, including possible resuspension of soils containing biogenic P, (2) long-range transport, and (3) sea salt. Elements derived from crustal sources (Al, P, Ti, V, Mn, Ce) with EFcrust<10 were dominated by the coarse-mode particles (>1.8 µm) and peaked around 4.4 µm in diameter, reflecting the regional contributions. Non-crustal elements (Ca, Ni, Cu, Zn, Pb) showed EFcrust>10. Aerosol Pb was primarily dominated by fine-mode particles, peaking at 0.14–0.25 µm, and likely was impacted by air masses from southern South America based on air mass back trajectories. However, Ni, Cu, and Zn were not detectable in most size fractions and did not present clear size patterns. Sea-salt elements (Ca, Na+, K+) showed a single-mode distribution and peaked at 2.5–4.4 µm. The estimated dry deposition fluxes of mineral dust for the austral summer, based on the particle-size distributions of Al measured at Palmer Station, ranged from 0.65 to 28 mg m−2 yr−1 with a mean of 5.5±5.0 mg m−2 yr−1. The estimated dry deposition fluxes of the target trace elements in this study were lower than most fluxes reported previously for coastal Antarctica and suggest that atmospheric input of trace elements through dry deposition processes may play a minor role in determining trace element concentrations in surface seawater over the continental shelf of the western Antarctic Peninsula.

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is hypothesized to be an important source of cryospheric iron, fertilizing the Southern Ocean, yet its trace-metal composition and factors that control its dispersal remain poorly constrained. Here we characterize meltwater iron sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord. Using dissolved and particulate ratios of manganese to iron in meltwaters, porewaters, and seawater, we show that surface glacial melt and subglacial plumes contribute to the seasonal cycle of iron and manganese within a fjord still relatively unaffected by climate-change-induced glacial retreat. Organic ligands derived from the phytoplankton bloom and the glaciers bind dissolved iron and facilitate the solubilization of particulate iron downstream. Using a numerical model, we show that buoyant plumes generated by outflow from the subglacial hydrologic system, enriched in labile particulate trace metals derived from a chemically modified crustal source, can supply iron to the fjord euphotic zone through vertical mixing. We also show that prolonged katabatic wind events enhance export of meltwater out of the fjord. Thus, we identify an important atmosphere–ice–ocean coupling intimately tied to coastal iron biogeochemistry and primary productivity along the WAP.

The bioavailability and scavenging fate of dissolved iron (dFe) in the ocean is dependent on its physicochemical speciation, yet there are no analytical methods that reveal the general character of marine dFe, especially basic differentiation of organically-complexed Fe from inorganic nanoparticulate phases. Here, we propose that Fe isotopes can be used to infer this speciation, as long as the system meets certain assumptions. We demonstrate this using a case study of the Southern East Pacific Rise (SEPR) non-buoyant hydrothermal plume at 15°S. We objectively assign Fe isotope signatures for each of three dFe species that are present in the hydrothermal plume: background organic dFe-ligand (FeL) complexes, hydrothermal organic dFe-ligand complexes, and Fe oxyhydroxide nanoparticles. We then apply these isotope signatures to an isotope mass balance model, which also includes the soluble and colloidal Fe concentrations, to reveal the complete physico-chemical speciation of dFe in this plume. We show that hydrothermally-sourced Fe-oxyhydroxide nanoparticles dominate dFe near the ridge-axis but are rapidly removed down-plume (net residence times: <10 y soluble, <20 y colloidal). In contrast, hydrothermally-sourced dFe-ligand complexes are present only as soluble-sized species near the ridge-axis, followed by aggregation/exchange into larger colloidal ligand complexes that begin to dominate dFe concentrations down-plume (residence time 10-60 y). Colloidal-sized ligand-stabilized dFe persists farthest among all

Cold-Water Corals (CWCs), and most marine calcifiers, are especially threatened by ocean acidification (OA) and the decrease in the carbonate saturation state of seawater. The vulnerability of these organisms, however, also involves other global stressors like warming, deoxygenation or changes in sea surface productivity and, hence, food supply via the downward transport of organic matter to the deep ocean. This study examined the response of the CWC Desmophyllum dianthus to low pH under different feeding regimes through a long-term incubation experiment. For this experiment, 152 polyps were incubated at pH 8.1, 7.8, 7.5 and 7.2 and two feeding regimes for 14 months. Mean calcification rates over the entire duration of the experiment ranged between −0.3 and 0.3 mg CaCO3 g−1d−1. Polyps incubated at pH 7.2 were the most affected and 30% mortality was observed in this treatment. In addition, many of the surviving polyps at pH 7.2 showed negative calcification rates indicating that, in the long term, CWCs may have difficulty thriving in such aragonite undersaturated waters. The feeding regime had a significant effect on skeletal growth of corals, with high feeding frequency resulting in more positive and variable calcification rates. This was especially evident in corals reared at pH 7.5 (ΩA = 0.8) compared to the low frequency feeding treatment. Early life-stages, which are essential for the recruitment and maintenance of coral communities and their associated biodiversity, were revealed to be at highest risk. Overall, this study demonstrates the vulnerability of D. dianthus corals to low pH and low food availability. Future projected pH decreases and related changes in zooplankton communities may potentially compromise the viability of CWC populations.

A major surface circulation feature of the Arctic Ocean is the Transpolar Drift (TPD), a current that transports river-influenced shelf water from the Laptev and East Siberian Seas toward the center of the basin and Fram Strait. In 2015, the international GEOTRACES program included a high-resolution pan-Arctic survey of carbon, nutrients, and a suite of trace elements and isotopes (TEIs). The cruises bisected the TPD at two locations in the central basin, which were defined by maxima in meteoric water and dissolved organic carbon concentrations that spanned 600 km horizontally and similar to 25-50 m vertically. Dissolved TEIs such as Fe, Co, Ni, Cu, Hg, Nd, and Th, which are generally particle-reactive but can be complexed by organic matter, were observed at concentrations much higher than expected for the open ocean setting. Other trace element concentrations such as Al, V, Ga, and Pb were lower than expected due to scavenging over the productive East Siberian and Laptev shelf seas. Using a combination of radionuclide tracers and ice drift modeling, the transport rate for the core of the TPD was estimated at 0.9 +/- 0.4 Sv (10(6) m(3)s(-1)). This rate was used to derive the mass flux for TEIs that were enriched in the TPD, revealing the importance of lateral transport in supplying materials beneath the ice to the central Arctic Ocean and potentially to the North Atlantic Ocean via Fram Strait. Continued intensification of the Arctic hydrologic cycle and permafrost degradation will likely lead to an increase in the flux of TEIs into the Arctic Ocean. Plain Language Summary A major feature of the Arctic Ocean circulation is the Transpolar Drift (TPD), a surface current that carries ice and continental shelf-derived materials from Siberia across the North Pole to the North Atlantic Ocean. In 2015, an international team of oceanographers conducted a survey of trace elements in the Arctic Ocean, traversing the TPD. Near the North Pole, they observed much higher concentrations of trace elements in surface waters than in regions on either side of the current. These trace elements originated from land, and their journey across the Arctic Ocean is made possible by chemical reactions with dissolved organic matter that originates mainly in Arctic rivers. This study reveals the importance of rivers and shelf processes combined with strong ocean currents in supplying trace elements to the central Arctic Ocean and onward to the Atlantic. These trace element inputs are expected to increase as a result of permafrost thawing and increased river runoff in the Arctic, which is warming at a rate much faster than anywhere else on Earth. Since many of the trace elements are essential building blocks for ocean life, these processes could lead to significant changes in the marine ecosystems and fisheries of the Arctic Ocean.

Dissolved iron (Fe) and manganese (Mn) share common sources and sinks in the global ocean. However, Fe and Mn also have different redox reactivity and speciation that can cause their distributions to become decoupled. The Arctic Ocean provides a unique opportunity to compare Fe and Mn distributions because the wide Arctic continental shelves provide significant margin fluxes of both elements, yet in situ vertical regeneration inputs that can complicate scavenging calculations are negligible under the ice of the Arctic Ocean, making it easier to interpret the fate of lateral gradients. We present here a large-scale case study demonstrating a three-step mechanism for Fe and Mn decoupling in the upper 400 m of the Western Arctic Ocean. Both Fe and Mn are released during diagenesis in porewaters of the Chukchi Shelf, but they become immediately decoupled when Fe is much more rapidly oxidized and re-precipitated than Mn in the oxic Chukchi Shelf water column, leading to Fe hosted primarily in the particulate phase and Mn in the dissolved phase. However, as these shelf fluxes are transported toward the shelf break and subducted into the subsurface halocline water mass, the loss rates of all species change significantly, causing further Fe and Mn decoupling. In the second decoupling step in the shelf break region, the dominant shelf species are removed rapidly via particle scavenging, with smallest soluble Fe (sFe < 0.02 µm) being least subject to loss, while colloidal Fe (0.02 µm < cFe < 0.2 µm), dissolved Mn (dMn), and non-lithogenic particulate Fe (pFexs) are all lost at similarly rapid rates. In the third decoupling step, once these species are swept >1000 km offshore with the prevailing current into the low-particle waters of the open Arctic, cFe and dMn appear conserved, while pFe, dFe, and sFe are very slowly removed with variable log-scale distances of transport: pFe ≪ dFe < sFe. To assess the role of physicochemical speciation on these trends, we observed that Fe(II) was a small (∼7%) fraction of total dFe in the upper 400 m of the Arctic, even over the shelf (∼2%). Also, colloidal contribution to dFe was very low (∼20%) in the open Arctic, in contrast to dFe in the North Atlantic, which is composed much more by colloids (≥50%). Throughout the Western Arctic Ocean, Fe and Mn are thus decoupled as a result of distinct oxidation kinetics and different scavenging rates within high- and low-particle regimes. As the “scavengers of the sea”, the relative distribution of particulate Fe and Mn phases across the Arctic Ocean shelf and slope, respectively, will play an important role in determining the distribution and ultimate sediment burial site for other scavenging-prone trace elements. Additionally, we suggest that the future effects of climate change, including loss of sea ice that could impact the formation of the halocline, might change distributions of Fe and Mn species in the future Western Arctic.