scholarly journals Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

2011 ◽  
Vol 8 (5) ◽  
pp. 10155-10185 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
A. de Brauwere ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Isotopic Composition ◽  
Water Column ◽  
Mixed Layer ◽  
Biogenic Silica ◽  
Fractionation Factor ◽  
Tropical Zone ◽  
Weddell Gyre ◽  
Elemental Cycling ◽  
Si Isotopes

Abstract. Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial pCO2 variation. The natural silicon isotopic composition (δ30Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratio in the surface waters. We present a new dataset in the Southern Ocean which includes for the first time summer δ30Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the sub-tropical zone up to the Weddell Gyre. In general, the biogenic silica isotopic composition at depth reflected a mixed layer origin and seemed not affected by any diagenetic effect in the water column, even if in the northern part of the Weddell Gyre an effect of biogenic silica dissolution cannot be ruled out. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh) should satisfactorily describe the δ30Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ30Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production is low. This results from: (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted together with a large vertical silicic acid gradient.

scholarly journals Processes controlling the Si-isotopic composition in the Southern Ocean and application for paleoceanography

2012 ◽  
Vol 9 (7) ◽  
pp. 2443-2457 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
A. de Brauwere ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Isotopic Composition ◽  
Mixed Layer ◽  
Biogenic Silica ◽  
Polar Front ◽  
Fractionation Factor ◽  
Subtropical Zone ◽  
Weddell Gyre ◽  
Elemental Cycling ◽  
Rayleigh Distillation

Abstract. Southern Ocean biogeochemical processes have an impact on global marine primary production and global elemental cycling, e.g. by likely controlling glacial-interglacial pCO2 variation. In this context, the natural silicon isotopic composition (δ30Si) of sedimentary biogenic silica has been used to reconstruct past Si-consumption:supply ratios in the surface waters. We present a new dataset in the Southern Ocean from a IPY-GEOTRACES transect (Bonus-GoodHope) which includes for the first time summer δ30Si signatures of suspended biogenic silica (i) for the whole water column at three stations and (ii) in the mixed layer at seven stations from the subtropical zone up to the Weddell Gyre. In general, the isotopic composition of biogenic opal exported to depth was comparable to the opal leaving the mixed layer and did not seem to be affected by any diagenetic processes during settling, even if an effect of biogenic silica dissolution cannot be ruled out in the northern part of the Weddell Gyre. We develop a mechanistic understanding of the processes involved in the modern Si-isotopic balance, by implementing a mixed layer model. We observe that the accumulated biogenic silica (sensu Rayleigh distillation) should satisfactorily describe the δ30Si composition of biogenic silica exported out of the mixed layer, within the limit of the current analytical precision on the δ30Si. The failures of previous models (Rayleigh and steady state) become apparent especially at the end of the productive period in the mixed layer, when biogenic silica production and export are low. This results from (1) a higher biogenic silica dissolution:production ratio imposing a lower net fractionation factor and (2) a higher Si-supply:Si-uptake ratio supplying light Si-isotopes into the mixed layer. The latter effect is especially expressed when the summer mixed layer becomes strongly Si-depleted, together with a large vertical silicic acid gradient, e.g. in the Polar Front Zone and at the Polar Front.

scholarly journals Unveiling the Si cycle using isotopes in an iron-fertilized zone of the Southern Ocean: from mixed-layer supply to export

2016 ◽  
Vol 13 (21) ◽  
pp. 6049-6066 ◽  
Author(s):  
Ivia Closset ◽  
Damien Cardinal ◽  
Mathieu Rembauville ◽  
François Thil ◽  
Stéphane Blain
Keyword(s):  
Southern Ocean ◽  
Isotopic Composition ◽  
Mass Balance ◽  
Silicic Acid ◽  
Surface Waters ◽  
Polar Front ◽  
Biogeochemical Cycle ◽  
Reference Station ◽  
Kerguelen Plateau ◽  
Fractionation Factor

Abstract. A massive diatom bloom forms annually in the surface waters of the naturally iron-fertilized Kerguelen Plateau (Southern Ocean). In this study, silicon isotopic signatures (δ30Si) of silicic acid (DSi) and suspended biogenic silica (BSi) were investigated through the whole water column with unprecedented spatial resolution, during the KEOPS-2 experiment (spring 2011). We used δ30Si measurements to track the sources of silicon that fuelled the bloom, and investigated the seasonal evolution of the Si biogeochemical cycle in the iron-fertilized area. We compared the results from stations with various degrees of iron enrichment and bloom conditions to an HNLC reference station. Dissolved and particulate δ30Si signatures were highly variable in the upper 500 m, reflecting the effect of intense silicon utilization in spring, while they were quite homogeneous in deeper waters. The Si isotopic and mass balance identified a unique Winter Water (WW) Si source for the iron-fertilized area that originated from southeast of the Kerguelen Plateau and spread northward. When the WW reached a retroflection of the Polar Front (PF), the δ30Si composition of the silicic acid pool became progressively heavier. This would result from sequential diapycnal and isopycnal mixings between the initial WW and ML water masses, highlighting the strong circulation of surface waters that defined this zone. When comparing the results from the two KEOPS expeditions, the relationship between DSi depletion, BSi production, and their isotopic composition appears decoupled in the iron-fertilized area. This seasonal decoupling could help to explain the low apparent fractionation factor observed in the ML at the end of summer. Taking into account these considerations, we refined the seasonal net BSi production in the ML of the iron-fertilized area to 3.0 ± 0.3 mol Si m−2 yr−1, which was exclusively sustained by surface water phytoplankton populations. These insights confirm that the isotopic composition of dissolved and particulate silicon is a promising tool to improve our understanding of the Si biogeochemical cycle since the isotopic and mass balance allows resolution of processes in the Si cycle (i.e. uptake, dissolution, mixing).

scholarly journals Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

2018 ◽  
Vol 15 (16) ◽  
pp. 4973-4993 ◽  
Author(s):  
Christian Schlosser ◽  
Katrin Schmidt ◽  
Alfred Aquilina ◽  
William B. Homoky ◽  
Maxi Castrillejo ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Water Column ◽  
Mixed Layer ◽  
Surface Mixed Layer ◽  
Pore Waters ◽  
Phytoplankton Blooms ◽  
Faecal Pellets ◽  
South Georgia ◽  
Shelf Sediments ◽  
Bottom Waters

Abstract. The island of South Georgia is situated in the iron (Fe)-depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this, we present an inventory of Fe, manganese (Mn), and aluminium (Al) in shelf sediments, pore waters, and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes, and provide estimates of the relative dissolved Fe (DFe) fluxes from these sources. Seafloor sediments, modified by authigenic Fe precipitation, were the main particulate Fe source to shelf bottom waters as indicated by the similar Fe ∕ Mn and Fe ∕ Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface-adsorbed (labile) Fe and therefore potentially available to organisms. Pore waters formed the primary DFe source to shelf bottom waters, supplying 0.1–44 µmol DFe m−2 d−1. However, we estimate that only 0.41±0.26 µmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and via zooplankton egestion and excretion processes. On average 6.5±8.2 µmol m−2 d−1 of labile particulate Fe was supplied to the surface mixed layer via faecal pellets formed by Antarctic krill (Euphausia superba), with a further 1.1±2.2 µmol DFe m−2 d−1 released directly by the krill. The faecal pellets released by krill included seafloor-derived lithogenic and authigenic material and settled algal debris, in addition to freshly ingested suspended phytoplankton cells. The Fe requirement of the phytoplankton blooms ∼ 1250 km downstream of South Georgia was estimated as 0.33±0.11 µmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep winter mixing, and aeolian dust estimated as ∼0.12 µmol m−2 d−1. We hypothesize that a substantial contribution of DFe was provided through recycling of biogenically stored Fe following luxury Fe uptake by phytoplankton on the Fe-rich shelf. This process would allow Fe to be retained in the surface mixed layer of waters downstream of South Georgia through continuous recycling and biological uptake, supplying the large downstream phytoplankton blooms.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean inferred from Si-isotopes

Ocean Science ◽  
2011 ◽  
Vol 7 (5) ◽  
pp. 533-547 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Polar Front ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Silicon isotopic signatures (δ30Si) of water column silicic acid (Si(OH)4) were measured in the Southern Ocean, along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). This provides the first reported data of a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional water transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determine different mixing interfaces: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products and with the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration northward, across the different interfaces, without significantly changing the AASW δ30Si composition. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, the δ30Si composition of the silicic acid pools is getting slightly, but significantly lighter from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This isotopic trend is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Through the use of δ30Si constraints, net biogenic silica production (representative of annual export), at the Greenwich Meridian is estimated to be 5.2 ± 1.3 and 1.1 ± 0.3 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively. This is in good agreement with previous estimations. Furthermore, summertime Si-supply into the mixed layer of both zones, via vertical mixing, is estimated to be 1.6 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

scholarly journals Silicon pool dynamics and biogenic silica export in the Southern Ocean, inferred from Si-isotopes

2011 ◽  
Vol 8 (2) ◽  
pp. 639-674 ◽  
Author(s):  
F. Fripiat ◽  
A.-J. Cavagna ◽  
F. Dehairs ◽  
S. Speich ◽  
L. André ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Silicic Acid ◽  
Mixed Layer ◽  
Deep Water ◽  
Biogenic Silica ◽  
Water Masses ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Isotopic Signatures ◽  
The Antarctic

Abstract. Water column silicon isotopic signatures (δ30Si) of silicic acid (Si(OH)4) in the Southern Ocean were measured along a meridional transect from South Africa (Subtropical Zone) down to 57° S (northern Weddell Gyre). These data are the first reported for a summer transect across the whole Antarctic Circumpolar Current (ACC). δ30Si variations are large in the upper 1000 m, reflecting the effect of the silica pump superimposed upon meridional transfer across the ACC: the transport of Antarctic surface waters northward by a net Ekman drift and their convergence and mixing with warmer upper-ocean Si-depleted waters to the north. Using Si isotopic signatures, we determined different mixing interfaces between ACC water masses: the Antarctic Surface Water (AASW), the Antarctic Intermediate Water (AAIW), and the thermoclines in the low latitude areas. The residual silicic acid concentrations of end-members control the δ30Si alteration of the mixing products. With the exception of AASW, all mixing interfaces have a highly Si-depleted mixed layer end-member. These processes deplete the silicic acid AASW concentration across the different interfaces northward without significantly changing the AASW δ30Si. By comparing our new results with a previous study in the Australian sector we show that during the circumpolar transport of the ACC eastward, there is a slight but significant Si-isotopic lightening of the silicic acid pools from the Atlantic to the Australian sectors. This results either from the dissolution of biogenic silica in the deeper layers and/or from an isopycnal mixing with the deep water masses in the different oceanic basins: North Atlantic Deep Water in the Atlantic, and Indian Ocean deep water in the Indo-Australian sector. This eastward lightening is further transmitted to the subsurface waters, representing mixing interfaces between the surface and deeper layers. Using the Si-isotopic constraint, we estimate for the Greenwich Meridian a net biogenic silica production which should be representative of the annual export, at 4.5 ± 1.1 and 1.5 ± 0.4 mol Si m−2 for the Antarctic Zone and Polar Front Zone, respectively, in agreement with previous estimations. The summertime Si-supply into the mixed layer via vertical mixing was also assessed at 1.5 ± 0.4 and 0.1 ± 0.5 mol Si m−2, respectively.

scholarly journals Autonomous profiling float observations of the high biomass plume downstream of the Kerguelen plateau in the Southern Ocean

2014 ◽  
Vol 11 (12) ◽  
pp. 17413-17462 ◽  
Author(s):  
M. Grenier ◽  
A. Della Penna ◽  
T. W. Trull
Keyword(s):  
Southern Ocean ◽  
Water Column ◽  
Mixed Layer ◽  
Dissolved Inorganic Carbon ◽  
Mixed Layer Depth ◽  
Light Limitation ◽  
Clear Correlation ◽  
Kerguelen Plateau ◽  
Layer Depth ◽  
Chl A

Abstract. Natural iron fertilisation from Southern Ocean islands results in high primary production and phytoplankton biomass accumulations readily visible in satellite ocean colour observations. These images reveal great spatial complexity with highly varying concentrations of chlorophyll, presumably reflecting both variations in iron supply and conditions favouring phytoplankton accumulation. To examine the second aspect, in particular the influences of variations in temperature and stratification, we deployed four autonomous profiling floats in the Antarctic Circumpolar Current near the Kerguelen plateau in the Indian sector of the Southern Ocean. Each "bio-profiler" measured more than 250 profiles of temperature (T), salinity (S), dissolved oxygen, chlorophyll fluorescence (Chl a), and particle backscatter in the top 300 m of the water column, sampling up to 5 profiles per day along meandering trajectories extending up to 1000 km. Comparison of surface Chl a estimates (top 50 m depth; analogous to values from satellite images) with total water column inventories revealed largely linear relationships, suggesting that dilution of chlorophyll by mixed layer depth variations plays only a minor role in the spatial distributions observed by satellite, and correspondingly that these images provide credible information on total and not just surface biomass accumulations. Regions of very high Chl a accumulation (1.5–10 μg L-1) were associated predominantly with a narrow T–S class of surface waters, which appears to derive from the northern Kerguelen plateau. In contrast, waters with only moderate Chl a enrichments (0.5–1.5 μg L-1) displayed no clear correlation with water properties, including no dependence on mixed layer depth, suggesting a diversity of sources of iron and/or its efficient dispersion across filaments of the plume. The lack of dependence on mixed layer depth also indicates a limited influence on production by light limitation. One float became trapped in a cyclonic eddy, allowing temporal evaluation of the water column in early autumn. During this period, decreasing surface Chl a inventories corresponded with decreases in oxygen inventories on sub-mixed layer density surfaces, consistent with significant export of organic matter and its respiration and storage as dissolved inorganic carbon in the ocean interior. These results are encouraging for the expanded use of autonomous observing platforms to study biogeochemical, carbon cycle, and ecological problems, although the complex blend of Lagrangian and Eulerian sampling achieved by the floats suggests that arrays rather than single floats will often be required.

scholarly journals Exploring interacting influences on the silicon isotopic composition of the surface ocean: a case study from the Kerguelen Plateau

2013 ◽  
Vol 10 (7) ◽  
pp. 11405-11446 ◽  
Author(s):  
N. Coffineau ◽  
C. L. De La Rocha ◽  
P. Pondaven
Keyword(s):  
Steady State ◽  
Isotopic Composition ◽  
Mixed Layer ◽  
Biogenic Silica ◽  
Box Model ◽  
Kerguelen Plateau ◽  
Silica Dissolution ◽  
Surface Ocean ◽  
Basalt Weathering ◽  
State Growth

Abstract. This study presents 6 new water column profiles of the silicon isotopic composition (δ30Si) of dissolved silicon (DSi) from the Atlantic and the Indian sectors of the Southern Ocean and a variable depth box model of silica cycling in the mixed layer constructed to illuminate the evolution of surface ocean δ30Si over the full course of a year. In keeping with previous observations, δ30Si values ranged from +1.9 to +2.4‰ in the mixed layer (ML), +1.2 to +1.7‰ in Winter Water (WW), and +0.9 to +1.4‰ in Circumpolar Deep Water (CDW). These data also confirmed the occurrence of diminished values for ML δ30Si at low DSi concentrations in early austral autumn on the Kerguelen Plateau. The box model was used to investigate whether these low, post-growing season values of δ30Si were related to input of DSi to the ML from basalt weathering, biogenic silica dissolution (with or without isotopic fractionation), the onset of winter mixing, or some combination of the three. Basalt weathering and fractionation during biogenic silica dissolution could both lower ML δ30Si below what would be expected from the extent of biological uptake of DSi. However, the key driver of the early autumn decrease in δ30Si appears to be the switch from bloom growth (with net removal of DSi and net accumulation of biogenic silica (BSi) biomass) to steady state growth (when slow but continuing production of BSi prevented significant net increase in DSi concentrations with diffusive input of DSi from WW but not decrease in ML δ30Si towards WW values). Lastly, fractionation during dissolution had only a negligible effect on the δ30Si of BSi exported throughout the course of the year, implying that seasonal changes in export efficiency (e.g., favoring the export of bloom BSi vs. the export of BSi produced during other times of the year) strongly influence the δ30Si of BSi accumulating in marine sediments. Altogether, these results suggest that as a paleoceanographic proxy, δ30Si may more reflect the dominant mode of production of the BSi that is exported (i.e. bloom vs. steady state growth) rather than strictly the extent of DSi utilization by diatoms.

scholarly journals Exploring interacting influences on the silicon isotopic composition of the surface ocean: a case study from the Kerguelen Plateau

2014 ◽  
Vol 11 (5) ◽  
pp. 1371-1391 ◽  
Author(s):  
N. Coffineau ◽  
C. L. De La Rocha ◽  
P. Pondaven
Keyword(s):  
Steady State ◽  
Isotopic Composition ◽  
Mixed Layer ◽  
Biogenic Silica ◽  
Box Model ◽  
Kerguelen Plateau ◽  
Silica Dissolution ◽  
Surface Ocean ◽  
Basalt Weathering ◽  
State Growth

Abstract. This study presents six new water column profiles of the silicon isotopic composition (δ30Si) of dissolved silicon (DSi) from the Atlantic and Indian sectors of the Southern Ocean and a variable depth box model of silica cycling in the mixed layer that was constructed to illuminate the evolution of surface ocean δ30Si over the full course of a year. In keeping with previous observations, δ30Si values ranged from +1.9 to +2.4‰ in the mixed layer (ML), +1.2 to +1.7‰ in Winter Water (WW), and +0.9 to +1.4‰ in Circumpolar Deep Water (CDW). These data also confirmed the occurrence of diminished values for ML δ30Si at low DSi concentrations in early austral autumn on the Kerguelen Plateau. The box model was used to investigate whether these low, post-growing season values of δ30Si were related to input of DSi to the ML from basalt weathering, biogenic silica dissolution (with or without isotopic fractionation), the onset of winter mixing, or some combination of the three. Basalt weathering and fractionation during biogenic silica dissolution could both lower ML δ30Si below what would be expected from the extent of biological uptake of DSi. However, the key driver of the early autumn decrease in δ30Si appears to be the switch from bloom growth (with net removal of DSi and net accumulation of biogenic silica (BSi) biomass) to steady state growth (when slow but continuing production of BSi prevented significant net increase in DSi concentrations with diffusive input of DSi from WW but not decrease in ML δ30Si towards WW values). Model results also indicated that fractionation during dissolution has only a negligible effect on the δ30Si of BSi exported throughout the course of the year. However, seasonal changes in export efficiency (e.g. favouring the export of bloom BSi versus the export of BSi produced during other times of the year) should strongly influence the δ30Si of BSi accumulating in marine sediments. Finally, the choice for the parameterisation of the mixing between the ML and the WW in terms of δ30Si (i.e. constant or allowed to vary with the seasonal migration of the thermocline) is critical to take into account in box model simulations of the silica biogeochemical cycle. Altogether, these results suggest that as a paleoceanographic proxy, δ30Si may more reflect the dominant mode of production of the BSi that is exported (i.e. bloom versus steady state growth) rather than strictly the extent of DSi utilisation by diatoms.

scholarly journals Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

2017 ◽  
Author(s):  
Christian Schlosser ◽  
Katrin Schmidt ◽  
Alfred Aquilina ◽  
William B. Homoky ◽  
Maxi Castrillejo ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Water Column ◽  
Mixed Layer ◽  
Surface Mixed Layer ◽  
Pore Waters ◽  
Phytoplankton Blooms ◽  
Faecal Pellets ◽  
South Georgia ◽  
Shelf Sediments ◽  
Bottom Waters

Abstract. The island of South Georgia is situated in the iron (Fe) depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this we present the first inventory of Fe, manganese (Mn) and aluminium (Al) in shelf sediments, pore waters and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes. The seafloor sediments were the main particulate Fe source to shelf bottom waters as indicated by Fe / Mn and Fe / Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface adsorbed (labile) Fe, and therefore potentially available to organisms. Pore waters formed the primary dissolved Fe (DFe) source to shelf bottom waters supplying 0.1–4 μmol DFe m−2 d−1. However, only 0.41 ± 0.26 μmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and zooplankton excretion processes. On average 6.5 ± 8.2 μmol m−2 d−1 of labile particulate Fe was supplied to the surface mixed layer via krill faecal pellets, with further DFe released by krill at around 1.1 ± 2.2 μmol m−2 d−1. The faecal pellets released by krill constituted of seafloor derived lithogenic material and settled algae debris, in addition to freshly ingested suspended phytoplankton specimen. The phytoplankton Fe requirement in the blooms ca. 1250 km downstream the island of South Georgia was 0.33 ± 0.11 μmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep winter mixing and via aeolian dust estimated as ~ 0.12 μmol m−2 d−1. We suggest that additionally required DFe was provided through recycling of biogenically stored Fe following luxury Fe uptake by phytoplankton on the Fe rich shelf. This process would allow Fe to be retained in the surface mixed layer of waters downstream of South Georgia through continuous recycling and biological uptake, and facilitate the large scale blooms.

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