Iron biogeochemistry in Fram Strait and on the Northeast Greenland Shelf

Fram Strait is the major gateway for Arctic Ocean sea-ice export, and the only deep-water connection between the Arctic Ocean and high latitude North Atlantic. The region is confined by the NE Greenland Shelf to the west and Svalbard to the east; approximately half is covered by summer sea-ice. The bioessential micronutrient iron (Fe) limits primary production across much of the high latitude ocean, including parts of the sub polar North Atlantic south of Fram Strait. Whilst primary production in the Arctic Ocean is generally thought to be controlled by a combination of light and fixed nitrogen availability, the potential role of trace elements as co-limiting factors for phytoplankton growth and their role in ecosystem dynamics has scarcely been investigated. What factors control the supply of Fe to the dynamic Fram Strait region and how does this affect marine primary production? To answer these questions, in late summer 2016, we performed a detailed investigation into the macronutrient and trace element micronutrient distribution across Fram Strait as part of the GEOTRACES GN05 cruise, including full Fe speciation analysis, and conducted nutrient addition bioassay experiments to assess spatial patterns in limiting nutrients. Surface dissolved Fe (dFe), the biologically most accessible form of Fe, showed an east-to-west gradient across Fram Strait. Concentrations were elevated near the Greenlandic coast in proximity to the marine-terminating glaciers Nioghalvfjerdsbrae and Zachariæ Isstrøm, and depleted in the West Spitsbergen Current near Svalbard. Fixed nitrogen (N), the sum of nitrate, nitrite and ammonium, and dFe were deficient in seawater relative to typical phytoplankton requirements. An east to west trend in the relative deficiency of N and Fe was apparent and aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in +N treatments near the Greenland continental margin, and +N+Fe near Svalbard. Collectively, our results suggest primary N limitation of phytoplankton growth in the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. The supply of Atlantic-derived subsurface N and Arctic-derived Fe therefore exerts a strong control on summertime primary production in the Fram Strait region. Analyses of Fe species immediately adjacent to the Nioghalvfjerdsbrae glacier terminus revealed a decoupling of dFe from labile particulate Fe inputs likely due to a prolonged residence time of meltwater beneath Greenland’s largest floating ice-tongue. Subglacial removal in combination with limited stabilization from organic material (i.e. Fe-binding ligands) leads to restricted supply of ‘new’ Fe from meltwater. Water exchange between the subglacial cavity, formed by the 80 km long floating ice tongue, and the shelf, is driven by the cavity overturning circulation, and exerts a strong control on subglacial dFe discharge to the NE Greenland Shelf. Comparison of findings at Nioghalvfjerdsbrae to observations in Antarctica suggests a more universal role for cavity overturning circulation in determining the extent of lateral dFe fluxes to broad glaciated shelf regions. Future retreat of deep-grounded marine termini may result in increased export of glacial dFe to shelf environments and more direct connectivity between meltwater discharge and marine primary production. Fluxes of dissolved trace elements across Fram Strait was dominated by the southward-directed East Greenland Current and the northward-directed West Spitsbergen Current, and comprised ⁓80% of gross dFe, dMn, dCo and dZn transport across Fram Strait. Dissolved Fe, Mn, Co and Zn fluxes on the NE Greenland Shelf that includes Greenland Ice Sheet discharge were of only local importance and contributed ⁓10% to gross northbound and southbound transport. The advection of Central Arctic Ocean waters including the trace element-rich Transpolar Drift, feeding into the East Greenland Current, plays a fundamental role in dFe, dMn and dCo supply to surface Fram Strait. Comparison of fluxes to estimates from the Barents Sea Opening and Davis Strait stresses the importance of Fram Strait as the main gateway for Arctic-Atlantic dFe, dMn, dCo and dZn exchange, a consequence of intermediate and deep water transport between Svalbard and Greenland. Fluxes of all three gateways combined suggests the Arctic is exporting 2.7 ± 2.4 Gg dFe per year to the North Atlantic Ocean. Arctic export of dMn (2.8 ± 4.7 Gg per year) and dCo (0.3 ± 0.3 Gg per year) appears more balanced and within uncertainty. For dZn, Arctic-Atlantic exchange was balanced with an insignificant net northbound flux of 3.0 ± 7.3 Gg per year. More observational data, particularly from non summer months, is needed to project changes in seasonal and interannual Arctic-Atlantic micronutrient exchange and potential effects on ecosystem services sensitive to (micro)nutrient stoichiometry

Unprecedented Fe delivery from the Congo River margin to the South Atlantic Gyre

Rivers are a major supplier of particulate and dissolved material to the ocean, but their role as sources of bio-essential dissolved iron (dFe) is thought to be limited due to rapid, efficient Fe removal during estuarine mixing. Here, we use trace element and radium isotope data to show that the influence of the Congo River margin on surface Fe concentrations is evident over 1000 km from the Congo outflow. Due to an unusual combination of high Fe input into the Congo-shelf-zone and rapid lateral transport, the Congo plume constitutes an exceptionally large offshore dFe flux of 6.8 ± 2.3 × 108 mol year−1. This corresponds to 40 ± 15% of atmospheric dFe input into the South Atlantic Ocean and makes a higher contribution to offshore Fe availability than any other river globally. The Congo River therefore contributes significantly to relieving Fe limitation of phytoplankton growth across much of the South Atlantic

The influence of Arctic Fe and Atlantic fixed N on summertime primary production in Fram Strait, North Greenland Sea

Climate change has led to a ~ 40% reduction in summer Arctic sea-ice cover extent since the 1970s. Resultant increases in light availability may enhance phytoplankton production. Direct evidence for factors currently constraining summertime phytoplankton growth in the Arctic region is however lacking. GEOTRACES cruise GN05 conducted a Fram Strait transect from Svalbard to the NE Greenland Shelf in summer 2016, sampling for bioessential trace metals (Fe, Co, Zn, Mn) and macronutrients (N, Si, P) at ~ 79°N. Five bioassay experiments were conducted to establish phytoplankton responses to additions of Fe, N, Fe + N and volcanic dust. Ambient nutrient concentrations suggested N and Fe were deficient in surface seawater relative to typical phytoplankton requirements. A west-to-east trend in the relative deficiency of N and Fe was apparent, with N becoming more deficient towards Greenland and Fe more deficient towards Svalbard. This aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in + N treatment near Greenland and + N + Fe near Svalbard. Collectively these results suggest primary N limitation of phytoplankton growth throughout the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. We suggest that the supply of Atlantic-derived N and Arctic-derived Fe exerts a strong control on summertime nutrient stoichiometry and resultant limitation patterns across the Fram Strait region.S.K. was financed by GEOMAR and the German Research Foundation (DFG Award Number AC 217/1-1 to E.P.A). T.J.B. acknowledges funding from a Marie Skłodowska-Curie Postdoctoral European Fellowship (OceanLiNES; Project ID 658035). Open Access funding provided by Projekt DEAL

Sediment release in the Benguela Upwelling System dominates trace metal input to the shelf and eastern South Atlantic Ocean

Upwelling of subsurface waters injects macronutrients (fixed N, P and Si) and micronutrient trace metals (TMs) into surface waters supporting elevated primary production in Eastern Boundary Upwelling Regions (EBUR). The eastern South Atlantic features a highly productive shelf sea transitioning to a low productivity N-Fe (co)limited open ocean. Whilst a gradient in most TM concentrations is expected in any off-shelf transect, the factors controlling the magnitude of cross-shelf TM fluxes are poorly constrained. Here, we present dissolved TM concentrations of Fe, Co, Mn, Cd, Ni and Cu within the Benguela Upwelling System (BUS) from the coastal section of the GEOTRACES GA08 cruise. Elevated dissolved Fe, Co, Mn, Cd, Ni, Cu and macronutrient concentrations were observed near shelf sediments. Benthic sources supplied 2.22 ± 0.99 μmol Fe m-2 d-1, 0.05 ± 0.03 μmol Co m-2 d-1, 0.28 ± 0.11 μmol Mn m-2 d-1 and were found to be the dominant source to shallow shelf waters compared to atmospheric depositions. Similarly, off-shelf transfer was a more important source of TMs to the eastern South Atlantic Ocean compared to atmospheric deposition. Assessment of surface (shelf, upper 200 m) and subsurface (shelf edge, 200 - 500 m) fluxes of Fe and Co indicated TM fluxes from subsurface were 2 - 5 times larger than those from surface into the eastern South Atlantic Ocean. Under future conditions of increasing ocean deoxygenation, these fluxes may increase further, potentially contributing to a shift towards more extensive regional limitation of primary production by fixed N availability. Key Points Shelf sediments release redox-sensitive trace metals (TMs) to overlying oxygen-depleted waters in the Benguela Upwelling System (BUS) Sediment-derived TMs are upwelled and laterally transported constituting a major source to shelf waters and to the eastern South Atlantic Subsurface fluxes of dissolved Fe and Co from the shelf edge play an important role in supplying Fe and Co to the eastern South Atlanti

Arctic – Atlantic exchange of the dissolved micronutrients Iron, Manganese, Cobalt, Nickel, Copper and Zinc with a focus on Fram Strait

The Arctic Ocean is considered a source of micronutrients to the Nordic Seas and the North Atlantic Ocean through the gateway of Fram Strait. However, there is a paucity of trace element data from across the Arctic Ocean gateways, and so it remains unclear how Arctic and North Atlantic exchange shapes micronutrient availability in the two ocean basins. In 2015 and 2016, GEOTRACES cruises sampled the Barents Sea Opening (GN04, 2015) and Fram Strait (GN05, 2016) for dissolved iron (dFe), manganese (dMn), cobalt (dCo), nickel (dNi), copper (dCu) and zinc (dZn). Together with the most recent synopsis of Arctic-Atlantic volume fluxes, the observed trace element distributions suggest that Fram Strait is the most important gateway for Arctic-Atlantic dissolved micronutrient exchange as a consequence of Intermediate and Deep Water transport. Combining fluxes from Fram Strait and the Barents Sea Opening with estimates for Davis Strait (GN02, 2015) suggests an annual net southward flux of 2.7 ± 2.4 Gg·a-1 dFe, 0.3 ± 0.3 Gg·a-1 dCo, 15.0 ± 12.5 Gg·a-1 dNi and 14.2 ± 6.9 Gg·a-1 dCu from the Arctic towards the North Atlantic Ocean. Arctic-Atlantic exchange of dMn and dZn were more balanced, with a net southbound flux of 2.8 ± 4.7 Gg·a-1 dMn and a net northbound flux of 3.0 ± 7.3 Gg·a-1 dZn. Our results suggest that ongoing changes to shelf inputs and sea ice dynamics in the Arctic, especially in Siberian shelf regions, affect micronutrient availability in Fram Strait and the high latitude North Atlantic Ocean

Die Biogeochemie des Eisens in der Framstraße und auf dem Schelf vor Nordostgrönland

Fram Strait is the major gateway for Arctic Ocean sea-ice export, and the only deep-water connection between the Arctic Ocean and high latitude North Atlantic. The region is confined by the NE Greenland Shelf to the west and Svalbard to the east; approximately half is covered by summer sea-ice. The bioessential micronutrient iron (Fe) limits primary production across much of the high latitude ocean, including parts of the sub polar North Atlantic south of Fram Strait. Whilst primary production in the Arctic Ocean is generally thought to be controlled by a combination of light and fixed nitrogen availability, the potential role of trace elements as co-limiting factors for phytoplankton growth and their role in ecosystem dynamics has scarcely been investigated. What factors control the supply of Fe to the dynamic Fram Strait region and how does this affect marine primary production? To answer these questions, in late summer 2016, we performed a detailed investigation into the macronutrient and trace element micronutrient distribution across Fram Strait as part of the GEOTRACES GN05 cruise, including full Fe speciation analysis, and conducted nutrient addition bioassay experiments to assess spatial patterns in limiting nutrients. Surface dissolved Fe (dFe), the biologically most accessible form of Fe, showed an east-to-west gradient across Fram Strait. Concentrations were elevated near the Greenlandic coast in proximity to the marine-terminating glaciers Nioghalvfjerdsbrae and Zachariæ Isstrøm, and depleted in the West Spitsbergen Current near Svalbard. Fixed nitrogen (N), the sum of nitrate, nitrite and ammonium, and dFe were deficient in seawater relative to typical phytoplankton requirements. An east to west trend in the relative deficiency of N and Fe was apparent and aligned with phytoplankton responses in bioassay experiments, which showed greatest chlorophyll-a increases in +N treatments near the Greenland continental margin, and +N+Fe near Svalbard. Collectively, our results suggest primary N limitation of phytoplankton growth in the study region, with conditions potentially approaching secondary Fe limitation in the eastern Fram Strait. The supply of Atlantic-derived subsurface N and Arctic-derived Fe therefore exerts a strong control on summertime primary production in the Fram Strait region. Analyses of Fe species immediately adjacent to the Nioghalvfjerdsbrae glacier terminus revealed a decoupling of dFe from labile particulate Fe inputs likely due to a prolonged residence time of meltwater beneath Greenland’s largest floating ice-tongue. Subglacial removal in combination with limited stabilization from organic material (i.e. Fe-binding ligands) leads to restricted supply of ‘new’ Fe from meltwater. Water exchange between the subglacial cavity, formed by the 80 km long floating ice tongue, and the shelf, is driven by the cavity overturning circulation, and exerts a strong control on subglacial dFe discharge to the NE Greenland Shelf. Comparison of findings at Nioghalvfjerdsbrae to observations in Antarctica suggests a more universal role for cavity overturning circulation in determining the extent of lateral dFe fluxes to broad glaciated shelf regions. Future retreat of deep-grounded marine termini may result in increased export of glacial dFe to shelf environments and more direct connectivity between meltwater discharge and marine primary production. Fluxes of dissolved trace elements across Fram Strait was dominated by the southward-directed East Greenland Current and the northward-directed West Spitsbergen Current, and comprised ⁓80% of gross dFe, dMn, dCo and dZn transport across Fram Strait. Dissolved Fe, Mn, Co and Zn fluxes on the NE Greenland Shelf that includes Greenland Ice Sheet discharge were of only local importance and contributed ⁓10% to gross northbound and southbound transport. The advection of Central Arctic Ocean waters including the trace element-rich Transpolar Drift, feeding into the East Greenland Current, plays a fundamental role in dFe, dMn and dCo supply to surface Fram Strait. Comparison of fluxes to estimates from the Barents Sea Opening and Davis Strait stresses the importance of Fram Strait as the main gateway for Arctic-Atlantic dFe, dMn, dCo and dZn exchange, a consequence of intermediate and deep water transport between Svalbard and Greenland. Fluxes of all three gateways combined suggests the Arctic is exporting 2.7 ± 2.4 Gg dFe per year to the North Atlantic Ocean. Arctic export of dMn (2.8 ± 4.7 Gg per year) and dCo (0.3 ± 0.3 Gg per year) appears more balanced and within uncertainty. For dZn, Arctic-Atlantic exchange was balanced with an insignificant net northbound flux of 3.0 ± 7.3 Gg per year. More observational data, particularly from non summer months, is needed to project changes in seasonal and interannual Arctic-Atlantic micronutrient exchange and potential effects on ecosystem services sensitive to (micro)nutrient stoichiometry