Antarctic Polar Front migrations in the Kerguelen Plateau region, Southern Ocean, over the past 360 kyrs

2021 ◽  
pp. 103526
Author(s):  
M. Civel-Mazens ◽  
X. Crosta ◽  
G. Cortese ◽  
E. Michel ◽  
A. Mazaud ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Polar Front ◽  
Antarctic Polar Front ◽  
Kerguelen Plateau ◽  
Plateau Region ◽  
The Past

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 Southern Ocean phytoplankton turnover in response to stepwise Antarctic cooling over the past 15 million years

2016 ◽  
Vol 113 (25) ◽  
pp. 6868-6873 ◽  
Author(s):  
James S. Crampton ◽  
Rosie D. Cody ◽  
Richard Levy ◽  
David Harwood ◽  
Robert McKay ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Atmospheric Carbon Dioxide ◽  
Large Scale ◽  
Species Turnover ◽  
Polar Front ◽  
Extinction Rate ◽  
The Past ◽  
Extinction Rates ◽  
Phytoplankton Communities ◽  
Global Extinction

It is not clear how Southern Ocean phytoplankton communities, which form the base of the marine food web and are a crucial element of the carbon cycle, respond to major environmental disturbance. Here, we use a new model ensemble reconstruction of diatom speciation and extinction rates to examine phytoplankton response to climate change in the southern high latitudes over the past 15 My. We identify five major episodes of species turnover (origination rate plus extinction rate) that were coincident with times of cooling in southern high-latitude climate, Antarctic ice sheet growth across the continental shelves, and associated seasonal sea-ice expansion across the Southern Ocean. We infer that past plankton turnover occurred when a warmer-than-present climate was terminated by a major period of glaciation that resulted in loss of open-ocean habitat south of the polar front, driving non-ice adapted diatoms to regional or global extinction. These findings suggest, therefore, that Southern Ocean phytoplankton communities tolerate “baseline” variability on glacial–interglacial timescales but are sensitive to large-scale changes in mean climate state driven by a combination of long-period variations in orbital forcing and atmospheric carbon dioxide perturbations.

scholarly journals Chemometric perspectives on plankton community responses to natural iron fertilization over and downstream of the Kerguelen Plateau in the Southern Ocean

2014 ◽  
Vol 11 (9) ◽  
pp. 13841-13903 ◽  
Author(s):  
T. W. Trull ◽  
D. M. Davies ◽  
F. Dehairs ◽  
A.-J. Cavagna ◽  
M. Lasbleiz ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Growth Rates ◽  
High Growth ◽  
Large Cell ◽  
Polar Front ◽  
Community Responses ◽  
Kerguelen Plateau ◽  
Austral Spring ◽  
Particulate Nitrogen ◽  
Natural Iron

Abstract. We examined phytoplankton community responses to natural iron fertilisation at 32 sites over and downstream from the Kerguelen Plateau in the Southern Ocean during the austral spring bloom in October–November 2011. Community structure was estimated from chemical and isotopic measurements (particulate organic carbon POC, 13C-POC, particulate nitrogen PN, 15N-PN, and biogenic silica BSi) on size-fractionated samples from surface waters (300, 210, 50, 20, 5, and 1 μm fractions). Higher values of 13C-POC (vs. co-located 13C-DIC source values) were taken as indicative of faster growth rates, and higher values of 15N-PN (vs. co-located 15N-NO3 source values) as indicative of greater nitrate use. Community responses varied in relation to both regional circulation and the advance of the bloom. Iron fertilised waters over the plateau developed dominance by very large diatoms (50–210 μm) with high BSi / POC ratios, high growth rates, and significant ammonium recycling as biomass built up. In contrast, downstream Polar Frontal waters with similar or higher iron supply were dominated by smaller diatoms (20–50 μm) and exhibited greater ammonium recycling. Stations in a deep water bathymetrically trapped recirculation south of the Polar Front with lower iron levels showed the large cell dominance observed on the plateau, but much less biomass. Comparison of these communities to surface water nitrate (and silicate) depletions as a proxy for export shows that the low biomass recirculation feature exported similar amounts of nitrogen to the high biomass blooms over the plateau and north of the Polar Front. This suggests that trophodynamic and export responses differed between regions with persistent low levels vs. punctual high levels of iron fertilisation.

scholarly journals Winter Biogeochemical Cycling of Dissolved and Particulate Cadmium in the Indian Sector of the Southern Ocean (GEOTRACES GIpr07 Transect)

2021 ◽  
Vol 8 ◽  
Author(s):  
Ryan Cloete ◽  
Jean C. Loock ◽  
Natasha R. van Horsten ◽  
Susanne Fietz ◽  
Thato N. Mtshali ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Biogeochemical Cycling ◽  
Polar Front ◽  
Water Masses ◽  
Antarctic Polar Front ◽  
Intermediate Water ◽  
Subtropical Zone ◽  
Near Surface ◽  
The Antarctic ◽  
Indian Sector

Winter distributions of dissolved cadmium (dCd) and particulate cadmium (pCd) were measured for the first time in the Indian sector of the Southern Ocean thereby contributing a unique spatial and seasonal dataset. Seven depth profiles, between 41°S and 58°S, were collected along the 30°E longitude during the 2017 austral winter to investigate the biogeochemical cycling of cadmium during a period characterized by contrasting upper water column dynamics compared to summer. Our results support an important role for biological uptake during winter months albeit weaker compared to summer. Distinct, biologically driven changes in cadmium cycling across the transect were observed. For example, surface ratios of pCd to phosphorus (P; pCd:P) increased from 0.37 to 1.07 mmol mol–1 between the subtropical zone (STZ) and the Antarctic zone (AAZ) reflecting increased Cd requirements for diatoms at higher latitudes which, in turn, was driven by a complex relationship between the availability of dCd and dissolved iron (dFe), zinc (dZn) and manganese (dMn). Vertical profiles of pCd:P displayed near-surface maxima consistent with (1) P occurring in two phases with different labilities and the lability of Cd being somewhere in-between and (2) increasing dCd to phosphate (PO4; dCd:PO4) ratios with depth at each station. North of the Antarctic Polar Front (APF), a secondary, deeper pCd:P maximum may reflect an advective signal associated with northward subducting Antarctic Intermediate Water (AAIW). The strong southward increase in surface dCd and dCd:PO4, from approximately 10–700 pmol kg–1 and 40–400 μmol mol–1, respectively, reflected the net effect of preferential uptake and regeneration of diatoms with high Cd content and the upwelling of Cd enriched water masses in the AAZ. Furthermore, distinct dCd versus PO4 relationships were observed in each of the intermediate and deep water masses suggesting that dCd and PO4 distributions at depth are largely the result of physical water mass mixing.

scholarly journals Sourcing the iron in the naturally-fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics

2014 ◽  
Vol 11 (9) ◽  
pp. 13389-13432 ◽  
Author(s):  
P. van der Merwe ◽  
A. R. Bowie ◽  
F. Quéroué ◽  
L. Armand ◽  
S. Blain ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Water Temperature ◽  
Surface Waters ◽  
Coastal Waters ◽  
Polar Front ◽  
Reference Station ◽  
Kerguelen Plateau ◽  
Lower Productivity ◽  
Winter Water Temperature ◽  
Shallow Coastal Waters

Abstract. The KEOPS2 project aims to elucidate the role of natural Fe fertilisation on biogeochemical cycles and ecosystem functioning, including quantifying the sources and processes by which iron is delivered in the vicinity of the Kerguelen Archipelago, Southern Ocean. The KEOPS2 process study used an upstream HNLC, deep water (2500 m), reference station to compare with a shallow (500 m), strongly fertilised plateau station and continued the observations to a downstream, bathymetrically trapped recirculation of the Polar Front where eddies commonly form and persist for hundreds of kilometres into the Southern Ocean. Over the Kerguelen Plateau, mean particulate (1–53 μm) Fe and Al concentrations (pFe = 13.4 nM, pAl = 25.2 nM) were more than 20-fold higher than at an offshore (lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM). In comparison, over the plateau dissolved Fe levels were only elevated by a factor of ∼2. Over the Kerguelen Plateau, ratios of pMn/pAl and pFe/pAl resemble basalt, likely originating from glacial/fluvial inputs into shallow coastal waters. In downstream, offshore deep-waters, higher pFe/pAl, and pMn/pAl ratios were observed, suggesting loss of lithogenic material accompanied by retention of pFe and pMn. Biological uptake of dissolved Fe and Mn and conversion into the biogenic particulate fraction or aggregation of particulate metals onto bioaggregates also increased these ratios further in surface waters as the bloom developed within the recirculation structure. While resuspension of shelf sediments is likely to be one of the important mechanisms of Fe fertilisation over the plateau, fluvial and glacial sources appear to be important to areas downstream of the island. Vertical profiles within an offshore recirculation feature associated with the Polar Front show pFe and pMn levels that were 6-fold and 3.5-fold lower respectively than over the plateau in surface waters, though still 3.6-fold and 1.7-fold higher respectively than the reference station. Within the recirculation feature, strong depletions of pFe and pMn were observed in the remnant winter water (temperature-minimum) layer near 175 m, with higher values above and below this depth. The correspondence between the pFe minima and the winter water temperature minima implies a seasonal cycle is involved in the supply of pFe into the fertilized region. This observed association is indicative of reduced supply in winter, which is counterintuitive if sediment resuspension and entrainment within the mixed layer is the primary fertilising mechanism to the downstream recirculation structure. Therefore, we hypothesise that lateral transport of pFe from shallow coastal waters is strong in spring, associated with snow melt and increased runoff due to rainfall, drawdown through summer and reduced supply in winter when snowfall and freezing conditions predominate in the Kerguelen region.

scholarly journals Oxygen depletion recorded in upper waters of the glacial Southern Ocean

2016 ◽  
Vol 7 (1) ◽  
Author(s):  
Zunli Lu ◽  
Babette A. A. Hoogakker ◽  
Claus-Dieter Hillenbrand ◽  
Xiaoli Zhou ◽  
Ellen Thomas ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Planktonic Foraminifera ◽  
Critical Role ◽  
Oxygen Depletion ◽  
Polar Front ◽  
Ice Age ◽  
Upper Ocean ◽  
Antarctic Polar Front ◽  
Near Surface ◽  
Poor Ventilation

Abstract Oxygen depletion in the upper ocean is commonly associated with poor ventilation and storage of respired carbon, potentially linked to atmospheric CO2 levels. Iodine to calcium ratios (I/Ca) in recent planktonic foraminifera suggest that values less than ∼2.5 μmol mol−1 indicate the presence of O2-depleted water. Here we apply this proxy to estimate past dissolved oxygen concentrations in the near surface waters of the currently well-oxygenated Southern Ocean, which played a critical role in carbon sequestration during glacial times. A down-core planktonic I/Ca record from south of the Antarctic Polar Front (APF) suggests that minimum O2 concentrations in the upper ocean fell below 70 μmol kg−1 during the last two glacial periods, indicating persistent glacial O2 depletion at the heart of the carbon engine of the Earth’s climate system. These new estimates of past ocean oxygenation variability may assist in resolving mechanisms responsible for the much-debated ice-age atmospheric CO2 decline.

scholarly journals Sea-ice extent in the Southern Ocean during the Last Glacial Maximum: another approach to the problem

1998 ◽  
Vol 27 ◽  
pp. 302-304 ◽  
Author(s):  
Lloyd H. Burckle ◽  
Richard Mortlock
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Surface Sediments ◽  
Open Water ◽  
Ice Cover ◽  
Polar Front ◽  
Age Dating ◽  
Antarctic Polar Front ◽  
Sea Ice Extent ◽  
Biogenic Opal

Determining past sea-ice distribution is an important goal of paleocean-ographers. Here, we present a possible approach to determining past sea-ice distribution in the Southern Ocean during the Last Glaciol Maximum (LGM). Diatoms are the prin-cipal opal-forming organisms south of the Antarctic Polar Front; their productivity is partly mediated by the presence/absence of sea ice. We reasoned that there should be good coherence between percentage biogenic opal in surface sediments and percentage annual sea-ice cover. This hypothesis was tested by comparing percentage biogenic opal in surface sediments against modern-day sea-ice cover in surface waters directly above each core site. The chronology for each core was determined by various means (biostratigraphy, 14C age dating, and carbonate and opal stratigraphy). With the resulting curve we estimate that yearly concentration of sea ice can be determined to within 30%. Using these data, we estimated percentage sea-ice cover during the LGM for a number of sediment sites (50-66°S) from the Southern Ocean. Core sites now beneath 100% open water witnessed some 25-60% sea ice during the LGM, while core sites presently beneath sea ice during half of the year witnessed more than 75% sea-ice cover during the LGM.

scholarly journals Application of sediment core modelling to interpreting the glacial-interglacial record of Southern Ocean silica cycling

2007 ◽  
Vol 3 (3) ◽  
pp. 387-396 ◽  
Author(s):  
A. Ridgwell
Keyword(s):  
Southern Ocean ◽  
Sediment Core ◽  
Polar Front ◽  
Antarctic Polar Front ◽  
Sea Ice Extent ◽  
Atlantic Sector ◽  
Silica Cycling ◽  
The North ◽  
Late Neogene ◽  
Primary Driver

Abstract. Sediments from the Southern Ocean reveal a meridional divide in biogeochemical cycling response to the glacial-interglacial cycles of the late Neogene. South of the present-day position of the Antarctic Polar Front in the Atlantic sector of the Southern Ocean, biogenic opal is generally much more abundant in sediments during interglacials compared to glacials. To the north, an anti-phased relationship is observed, with maximum opal abundance instead occurring during glacials. This antagonistic response of sedimentary properties provides an important model validation target for testing hypotheses of glacial-interglacial change against, particularly for understanding the causes of the concurrent variability in atmospheric CO2. Here, I illustrate a time-dependent modelling approach to helping understand climates of the past by means of the mechanistic simulation of marine sediment core records. I find that a close match between model-predicted and observed down-core changes in sedimentary opal content can be achieved when changes in seasonal sea-ice extent are imposed, whereas the predicted sedimentary response to iron fertilization on its own is not consistent with sedimentary observations. The results of this sediment record model-data comparison supports previous inferences that the changing cryosphere is the primary driver of the striking features exhibited by the paleoceanographic record of this region.

scholarly journals Sourcing the iron in the naturally fertilised bloom around the Kerguelen Plateau: particulate trace metal dynamics

2015 ◽  
Vol 12 (3) ◽  
pp. 739-755 ◽  
Author(s):  
P. van der Merwe ◽  
A. R. Bowie ◽  
F. Quéroué ◽  
L. Armand ◽  
S. Blain ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Water Temperature ◽  
Surface Waters ◽  
Coastal Waters ◽  
Polar Front ◽  
Reference Station ◽  
Kerguelen Plateau ◽  
Lower Productivity ◽  
Winter Water Temperature ◽  
Shallow Coastal Waters

Abstract. The KEOPS2 project aims to elucidate the role of natural Fe fertilisation on biogeochemical cycles and ecosystem functioning, including quantifying the sources and processes by which iron is delivered in the vicinity of the Kerguelen Archipelago, Southern Ocean. The KEOPS2 process study used an upstream high-nutrient, low-chlorophyll (HNLC), deep water (2500 m), reference station to compare with a shallow (500 m), strongly fertilised plateau station and continued the observations to a downstream, bathymetrically trapped recirculation of the Polar Front where eddies commonly form and persist for hundreds of kilometres into the Southern Ocean. Over the Kerguelen Plateau, mean particulate (1–53 μm) Fe and Al concentrations (pFe = 13.4 nM, pAl = 25.2 nM) were more than 20-fold higher than at an offshore (lower-productivity) reference station (pFe = 0.53 nM, pAl = 0.83 nM). In comparison, over the plateau dissolved Fe levels were only elevated by a factor of ~ 2. Over the Kerguelen Plateau, ratios of pMn / pAl and pFe / pAl resemble basalt, likely originating from glacial/fluvial inputs into shallow coastal waters. In downstream, offshore deep-waters, higher pFe / pAl, and pMn / pAl ratios were observed, suggesting loss of lithogenic material accompanied by retention of pFe and pMn. Biological uptake of dissolved Fe and Mn and conversion into the biogenic particulate fraction or aggregation of particulate metals onto bioaggregates also increased these ratios further in surface waters as the bloom developed within the recirculation structure. While resuspension of shelf sediments is likely to be one of the important mechanisms of Fe fertilisation over the plateau, fluvial and glacial sources appear to be important to areas downstream of the island. Vertical profiles within an offshore recirculation feature associated with the Polar Front show pFe and pMn levels that were 6-fold and 3.5-fold lower, respectively, than over the plateau in surface waters, though still 3.6-fold and 1.7-fold higher respectively than the reference station. Within the recirculation feature, strong depletions of pFe and pMn were observed in the remnant winter water (temperature-minimum) layer near 175 m, with higher values above and below this depth. The correspondence between the pFe minima and the winter water temperature minima implies a seasonal cycle is involved in the supply of pFe into the fertilised region. This observed association is indicative of reduced supply in winter, which is counterintuitive if sediment resuspension and entrainment within the mixed layer is the primary fertilising mechanism to the downstream recirculation structure. Therefore, we hypothesise that lateral transport of pFe from shallow coastal waters is strong in spring, associated with snow melt and increased runoff due to rainfall, drawdown through summer and reduced supply in winter when snowfall and freezing conditions predominate in the Kerguelen region.

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