Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

Abstract. The Southern Ocean is known as the largest High-Nutrient, Low-Chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, which is sustained partly by iron released from the sediments deposited onto the margins. In the framework of the KEOPS-2 project, we used radium isotopes (224Ra, T1/2 = 3.66 d; 223Ra, T1/2 = 11.4 d; 228Ra, T1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sediment-derived inputs (including iron and other micronutrients) towards offshore waters. Significant 223Ra and 223Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224Ra and 223Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the Polar Front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales in the order of several days up to several weeks) and (ii) that the Polar Front is not a physical barrier for the chemical elements released from the sediments of Kerguelen Plateau. The Ra dataset suggests that iron and other micronutrients released by the shallow sediments of the Kerguelen margins may contribute to fuel the phytoplankton bloom downstream of the islands, despite the presence of the Polar Front. However, the heterogeneous distribution of the 224Ra and 223Ra activities in surface waters suggests that this supply across the front is not a continuous process, but rather a process that is highly variable in space and time.

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Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

Biogeosciences ◽

10.5194/bg-12-1415-2015 ◽

2015 ◽

Vol 12 (5) ◽

pp. 1415-1430 ◽

Cited By ~ 13

Author(s):

V. Sanial ◽

P. van Beek ◽

B. Lansard ◽

M. Souhaut ◽

E. Kestenare ◽

...

Keyword(s):

Phytoplankton Bloom ◽

Chemical Elements ◽

Continuous Process ◽

Polar Front ◽

Global Ocean ◽

Data Set ◽

Radium Isotopes ◽

Heterogeneous Distribution ◽

Kerguelen Islands ◽

Rapid Transfer

Abstract. The Southern Ocean is known to be the largest high-nutrient, low-chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, a bloom which is sustained partly by iron released from the sediments deposited onto the shelves. In the framework of the KEOPS-2 project, we used radium isotopes (224Ra, T1/2 = 3.66 d; 223Ra, T1/2 = 11.4 d; 228Ra, T1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sediment-derived inputs (including iron and other micronutrients) towards offshore waters. Significant 224Ra and 223Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224Ra and 223Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the polar front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales on the order of several days up to several weeks) and (ii) that the polar front is not a physical barrier for the chemical elements released from the sediments of the Kerguelen Plateau. The Ra data set suggests that iron and other micronutrients released by the shelves of the Kerguelen Islands may contribute to fueling the phytoplankton bloom downstream of the islands, despite the presence of the polar front. However, the heterogeneous distribution of the 224Ra and 223Ra activities in surface waters suggests that this supply across the front is not a continuous process but rather a process that is highly variable in space and time.

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Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean

Biogeosciences ◽

10.5194/bg-12-1925-2015 ◽

2015 ◽

Vol 12 (6) ◽

pp. 1925-1940 ◽

Cited By ~ 10

Author(s):

L. Farías ◽

L. Florez-Leiva ◽

V. Besoain ◽

G. Sarthou ◽

C. Fernández

Keyword(s):

Nitrous Oxide ◽

Southern Ocean ◽

Greenhouse Gases ◽

Phytoplankton Bloom ◽

Polar Front ◽

Austral Spring ◽

Kerguelen Islands ◽

The North ◽

Ch4 Production ◽

Natural Iron

Abstract. The concentrations of greenhouse gases (GHGs), such as nitrous oxide (N2O) and methane (CH4), were measured in the Kerguelen Plateau region (KPR). The KPR is affected by an annual microalgal bloom caused by natural iron fertilization, and this may stimulate the microbes involved in GHG cycling. This study was carried out during the KEOPS 2 cruise during the austral spring of 2011. Oceanographic variables, including N2O and CH4, were sampled (from the surface to 500 m depth) in two transects along and across the KRP, the north–south (TNS) transect (46°–51° S, ~ 72° E) and the east–west (TEW) transect (66°–75° E, ~ 48.3° S), both associated with the presence of a plateau, polar front (PF) and other mesoscale features. The TEW presented N2O levels ranging from equilibrium (105%) to slightly supersaturated (120%) with respect to the atmosphere, whereas CH4 levels fluctuated dramatically, being highly supersaturated (120–970%) in areas close to the coastal waters of the Kerguelen Islands and in the PF. The TNS showed a more homogenous distribution for both gases, with N2O and CH4 levels ranging from 88 to 171% and 45 to 666% saturation, respectively. Surface CH4 peaked at southeastern stations of the KPR (A3 stations), where a phytoplankton bloom was observed. Both gases responded significantly, but in contrasting ways (CH4 accumulation and N2O depletion), to the patchy distribution of chlorophyll a. This seems to be associated to the supply of iron from various sources. Air–sea fluxes for N2O (from −10.5 to 8.65, mean 1.25 ± 4.04 μmol m−2 d−1) and for CH4 (from 0.32 to 38.1, mean 10.01 ± 9.97 μmol−2 d−1) indicated that the KPR is both a sink and a source for N2O, as well as a considerable and variable source of CH4. This appears to be associated with biological factors, as well as the transport of water masses enriched with Fe and CH4 from the coastal area of the Kerguelen Islands. These previously unreported results for the Southern Ocean suggest an intense microbial CH4 production in the study area.

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Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off the Kerguelen Islands

Biogeosciences ◽

10.5194/bg-12-1983-2015 ◽

2015 ◽

Vol 12 (6) ◽

pp. 1983-1992 ◽

Cited By ~ 11

Author(s):

I. Obernosterer ◽

M. Fourquez ◽

S. Blain

Keyword(s):

Trace Metal ◽

Bacterial Production ◽

Bacterial Abundance ◽

Heterotrophic Bacteria ◽

Kerguelen Islands ◽

High Nutrient ◽

Limiting Nutrient ◽

Heterotrophic Metabolism ◽

Heterotrophic Production

Abstract. It has been univocally shown that iron (Fe) is the primary limiting nutrient for phytoplankton metabolism in high-nutrient, low-chlorophyll (HNLC) waters, yet the question of how this trace metal affects heterotrophic microbial activity is far less understood. We investigated the role of Fe for bacterial heterotrophic production and growth at three contrasting sites in the naturally Fe-fertilized region east of the Kerguelen Islands and at one site in HNLC waters during the KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2) cruise in spring 2011. We performed dark incubations of natural microbial communities amended either with iron (Fe, as FeCl3) or carbon (C, as trace-metal clean glucose), or a combination of both, and followed bacterial abundance and heterotrophic production for up to 7 days. Our results show that single and combined additions of Fe and C stimulated bulk and cell-specific bacterial production at the Fe-fertilized sites, while in HNLC waters only combined additions resulted in significant increases in these parameters. Bacterial abundance was enhanced in two out of the three experiments performed in Fe-fertilized waters but did not respond to Fe or C additions in HNLC waters. Our results provide evidence that both Fe and C are present at limiting concentrations for bacterial heterotrophic activity in the naturally fertilized region off the Kerguelen Islands in spring, while bacteria were co-limited by these elements in HNLC waters. These results shed new light on the role of Fe in bacterial heterotrophic metabolism in regions of the Southern Ocean that receive variable Fe inputs.

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Iron stable isotopes track pelagic iron cycling during a subtropical phytoplankton bloom

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1421576112 ◽

2014 ◽

Vol 112 (1) ◽

pp. E15-E20 ◽

Cited By ~ 33

Author(s):

Michael J. Ellwood ◽

David A. Hutchins ◽

Maeve C. Lohan ◽

Angela Milne ◽

Philipp Nasemann ◽

...

Keyword(s):

Stable Isotopes ◽

Mixed Layer ◽

Phytoplankton Bloom ◽

Iron Bioavailability ◽

Global Ocean ◽

Minor Role ◽

Surface Mixed Layer ◽

Spring Phytoplankton Bloom ◽

Dissolved Iron ◽

Particulate Iron

The supply and bioavailability of dissolved iron sets the magnitude of surface productivity for ∼40% of the global ocean. The redox state, organic complexation, and phase (dissolved versus particulate) of iron are key determinants of iron bioavailability in the marine realm, although the mechanisms facilitating exchange between iron species (inorganic and organic) and phases are poorly constrained. Here we use the isotope fingerprint of dissolved and particulate iron to reveal distinct isotopic signatures for biological uptake of iron during a GEOTRACES process study focused on a temperate spring phytoplankton bloom in subtropical waters. At the onset of the bloom, dissolved iron within the mixed layer was isotopically light relative to particulate iron. The isotopically light dissolved iron pool likely results from the reduction of particulate iron via photochemical and (to a lesser extent) biologically mediated reduction processes. As the bloom develops, dissolved iron within the surface mixed layer becomes isotopically heavy, reflecting the dominance of biological processing of iron as it is removed from solution, while scavenging appears to play a minor role. As stable isotopes have shown for major elements like nitrogen, iron isotopes offer a new window into our understanding of the biogeochemical cycling of iron, thereby allowing us to disentangle a suite of concurrent biotic and abiotic transformations of this key biolimiting element.

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Watermasses as a unifying framework for understanding the Southern Ocean carbon cycle

Biogeosciences Discussions ◽

10.5194/bgd-7-3393-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 3393-3451 ◽

Cited By ~ 3

Author(s):

D. Iudicone ◽

I. Stendardo ◽

O. Aumont ◽

K. B. Rodgers ◽

G. Madec ◽

...

Keyword(s):

Southern Ocean ◽

Large Scale ◽

Thermohaline Circulation ◽

Model Simulation ◽

Polar Front ◽

Global Ocean ◽

Inversion Method ◽

Co2 Fluxes ◽

Meridional Transport ◽

Regional Patterns

Abstract. A watermass-based framework is presented for a quantitative understanding of the processes controlling the cycling of carbon in the Southern Ocean. The approach is developed using a model simulation of the global carbon transports within the ocean and with the atmosphere. It is shown how the watermass framework sheds light on the interplay between biology, air-sea gas exchange, and internal ocean transport including diapycnal processes, and the way in which this interplay controls the large-scale ocean-atmosphere carbon exchange. The simulated pre-industrial regional patterns of DIC distribution and the global distribution of the pre-industrial air-sea CO2 fluxes compare well with other model results and with results from an ocean inversion method. The main differences are found in the Southern Ocean where the model presents a stronger CO2 outgassing south of the polar front, a result of the upwelling of DIC-rich deep waters into the surface layer. North of the subantarctic front the typical temperature-driven solubility effect produces a net ingassing of CO2. The biological controls on surface CO2 fluxes through primary production is generally smaller than the temperature effect on solubility. Novel to this study is also a Lagrangian trajectory analysis of the meridional transport of DIC. The analysis allows to evaluate the contribution of separate branches of the global thermohaline circulation (identified by watermasses) to the vertical distribution of DIC throughout the Southern Ocean and towards the global ocean. The most important new result is that the overturning associated with Subantarctic Mode Waters sustains a northward net transport of DIC (15.7×107 mol/s across 30° S). This new finding, which has also relevant implications on the prediction of anthropogenic carbon redistribution, results from the specific mechanism of SAMW formation and its source waters whose consequences on tracer transports are analyzed for the first time in this study.

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Validation of Thorpe-scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

Biogeosciences ◽

10.5194/bg-11-6927-2014 ◽

2014 ◽

Vol 11 (23) ◽

pp. 6927-6937 ◽

Cited By ~ 15

Author(s):

Y.-H. Park ◽

J.-H. Lee ◽

I. Durand ◽

C.-S. Hong

Keyword(s):

Bottom Topography ◽

Water Layer ◽

Polar Front ◽

Surface Mixed Layer ◽

Kerguelen Islands ◽

Seasonal Thermocline ◽

Thorpe Scale ◽

Microstructure Measurements ◽

High Degree ◽

O 10

Abstract. The Thorpe scale is an energy-containing vertical overturning scale of large eddies associated with shear-generated turbulence. This study investigates indirect estimates of vertical diffusivities from the Thorpe scale method in the polar front region east of the Kerguelen Islands based on fine-scale density profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivities are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler. The results are sensitive to the choice of the diffusivity parameterization and the overturn ratio Ro, and the optimal results have been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion, rather than the parameterization by Osborn (1980) and the Ro = 0.2 criterion originally suggested by Gargett and Garner (2008). The Thorpe-scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5) m2 s−1 in the Winter Water layer and in the area immediately north of the polar front to a high value of O(10−4) m2 s−1 in the seasonal thermocline between the surface mixed layer and the Winter Water. The latter high diffusivities are found especially over the shallow plateau southeast of the Kerguelen Islands and along the polar front that is attached to the escarpment northeast of the islands. The interaction of strong frontal flow with prominent bottom topography likely causes the observed elevated mixing rates.

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Validation of the Thorpe scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

Biogeosciences Discussions ◽

10.5194/bgd-11-12137-2014 ◽

2014 ◽

Vol 11 (8) ◽

pp. 12137-12157 ◽

Cited By ~ 6

Author(s):

Y.-H. Park ◽

J.-H. Lee ◽

I. Durand ◽

C.-S. Hong

Keyword(s):

Surface Waters ◽

Water Layer ◽

Polar Front ◽

Surface Mixed Layer ◽

Kerguelen Islands ◽

Seasonal Thermocline ◽

Thorpe Scale ◽

Microstructure Measurements ◽

High Degree ◽

O 10

Abstract. The Thorpe scale is an energy containing vertical overturning scale of large eddies associated with shear generated turbulence. We make here indirect estimates of vertical diffusivities from the Thorpe scale method in the Polar Front region east of the Kerguelen Islands based on fine scale density profiles gathered during the 2011 KEOPS2 cruise. These are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP microprofiler. The results are sensitive to the choice of the diffusivity parameterization and the Gargett and Garner's (2008) overturn ratio Ro, with the optimal results showing an agreement within a factor of 4, on average, having been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion. The Thorpe scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5 m2 s−1) in the Winter Water layer and in the Subantarctic surface waters immediately north of the Polar Front to a high value of O(10−4 m2 s−1) in the seasonal thermocline just below the surface mixed layer. The latter values are found especially over the shallow plateau southeast of the Kerguelen Islands and in the Antarctic surface waters associated with the Polar Front attached to the escarpment northeast of the islands.

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Mesozooplankton structure and functioning during the onset of the Kerguelen phytoplankton bloom during the KEOPS2 survey

Biogeosciences ◽

10.5194/bg-12-4543-2015 ◽

2015 ◽

Vol 12 (14) ◽

pp. 4543-4563 ◽

Cited By ~ 13

Author(s):

F. Carlotti ◽

M.-P. Jouandet ◽

A. Nowaczyk ◽

M. Harmelin-Vivien ◽

D. Lefèvre ◽

...

Keyword(s):

Spring Bloom ◽

Phytoplankton Bloom ◽

Size Structure ◽

Early Stage ◽

Distribution Patterns ◽

Zooplankton Biomass ◽

Zooplankton Abundance ◽

Mesozooplankton Community ◽

Remarkable Feature ◽

Kerguelen Islands

Abstract. This paper presents results on the spatial and temporal distribution patterns of mesozooplankton in the naturally fertilized region to the east of the Kerguelen Islands (Southern Ocean) visited at early bloom stage during the KEOPS2 survey (15 October to 20 November 2011). The aim of this study was to compare the zooplankton response in contrasted environments localized over the Kerguelen Plateau in waters of the east shelf and shelf edge and in productive oceanic deep waters characterized by conditions of complex circulation and rapidly changing phytoplankton biomass. The mesozooplankton community responded to the spring bloom earlier on the plateau than in the oceanic waters, where complex mesoscale circulation stimulated initial more or less ephemeral blooms before a broader bloom extension. Taxonomic compositions showed a high degree of similarity across the whole region, and the populations initially responded to spring bloom with a large production of larval forms increasing abundances, without biomass changes. Taxonomic composition and stable isotope ratios of size-fractionated zooplankton indicated the strong domination of herbivores, and the total zooplankton biomass values over the survey presented a significant correlation with the integrated chlorophyll concentrations in the mixed layer. The biomass stocks observed at the beginning of the KEOPS2 cruise were around 1.7 g C m−2 above the plateau and 1.2 g C m−2 in oceanic waters. Zooplankton biomass in oceanic waters remained on average below 2 g C m−2 over the study period, except for one station in the Polar Front zone (F-L), whereas zooplankton biomasses were around 4 g C m−2 on the plateau at the end of the survey. The most remarkable feature during the sampling period was the stronger increase in abundance in the oceanic waters (25 × 103 to 160 × 103 ind m−2) than on the plateau (25 × 103 to 90 × 103 ind m−2). The size structure and taxonomic distribution patterns revealed a cumulative contribution of various larval stages of dominant copepods and euphausiids particularly in the oceanic waters, with clearly identifiable stages of progress during a Lagrangian time series survey. The reproduction and early stage development of dominant species were sustained by mesoscale-related initial ephemeral blooms in oceanic waters, but growth was still food-limited and zooplankton biomass stagnated. In contrast, zooplankton abundance and biomass on the shelf were both in a growing phase, at slightly different rates, due to growth under sub-optimal conditions. Combined with our observations during the KEOPS1 survey (January–February 2005), the present results deliver a consistent understanding of patterns in mesozooplankton abundance and biomass from early spring to summer in the poorly documented oceanic region east of the Kerguelen Islands.

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Sea Level Anomalies in the Southern Ocean due to Thermohaline Variability

10.5194/egusphere-egu21-14301 ◽

2021 ◽

Author(s):

Fabien Roquet ◽

Marlen Kolbe ◽

Etienne Pauthenet ◽

David Nerini

Keyword(s):

Southern Ocean ◽

Sea Level Rise ◽

Sea Level ◽

Salt Content ◽

Principal Component ◽

Polar Front ◽

Functional Principal Component Analysis ◽

Global Ocean ◽

Regional Variability ◽

Steric Height

<div> <div> <div> <p>The Southern Ocean is responsible for the majority of the global oceanic heat uptake which contributes to global sea level rise. At the same time, ocean temperature does not change everywhere at the same rate and salinity changes are also associated with sea level variability. Changes in heat and salt content drive together variations in the steric height that differ importantly in both time and space. This study investigates steric height variability in the Southern Ocean from 2008 to 2017 by analysing temperature and salinity variations obtained from global ocean reanalyses. The thermohaline variability is decomposed on so-called thermohaline modes using a functional Principal Component Analysis (fPCA). Thermohaline modes provide a natural basis on which to decompose the joint temperature-salinity vertical profiles into a sum of vertical modes weighted by their respective principal components. Steric height was computed in the reanalyses and related to the principal component using a Multiple Linear Regression (MLR) model. Trends in steric height are found to differ significantly between subtropical and subpolar regions, simultaneously which with a shift from a thermohaline stratification dominated by the first "thermocline" mode in the North to the second "saline" mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized. Since 2008, steric height has dropped close to the Antarctic continent, while subtropical waters farther north have mostly risen due to increased heat storage. While the dominant cause for the significant sea level rise south of 30S remains freshwater discharge from glaciers and ice sheets, thermohaline variability produces sizeable regional variability in the rate of sea level rise.</p> </div> </div> </div>

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