scholarly journals Spatial hydrochemical structure in surface waters of the Southern ocean between Africa and Antarctica

2021 ◽  
Vol 67 (4) ◽  
pp. 328-347
Author(s):  
K. V. Artamonova ◽  
I. A. Gangnus ◽  
L. A. Dukhova ◽  
V. V. Maslennikov ◽  
N. A. Lavinen
Keyword(s):  
Surface Layer ◽  
Southern Ocean ◽  
Interannual Variability ◽  
High Latitude ◽  
Polar Front ◽  
Hydrochemical Characteristics ◽  
The South ◽  
The North ◽  
The Antarctic ◽  
Subantarctic Front

Some hydrochemical characteristics and, first of all, the main nutrients (phosphorus, nitrogen, silicon) can be used as markers for distinguishing different types of water masses and positions of the main fronts of the Southern Ocean. The seasonal and interannual variability of these characteristics also reflects the character of biological processes in the surface layer of the ocean, which is important for assessing biological productivity. The aim of this study was to analyze the main features of the spatial distribution of hydrochemical characteristics in the surface layer in the Atlantic and Indian Ocean sectors of the Southern Ocean between the Subantarctic Front and the shores of Antarctica and assess their seasonal (spring–autumn) and interannual variability for the observation period from 2008 to 2020. We describe the surface nutrient concentrations between Africa and Antarctica along the transects that cross the Subantarctic Front (SAF) in the north, the Polar Frontal Zone (PFS), Polar Front (PF) and Antarctic Zone water in the south. The findings revealed an increase in dissolved oxygen and nutrients towards the south. Nitrates changed values within the SAF from 15 μM to 24 μM, whereas values from 1.2 μM to 1.7 μM were observed for phosphates. Silicate increased considerably within the Polar Front, from 6.6 μM to 20.8 μM. An analysis was carried out of the seasonal and interannual variability of the hydrochemical conditions in the surface layer of the Southern Ocean. The interannual variability of the nutrients was determined by the spatial variability of the main fronts of the Antarctic Circumpolar Current (ACC) and the intensity of the largescale Weddell Gyre (WG). Since 2017, there has been an increase in the meridional transfer of waters: in the Antarctic Summer 2017–2018, there was a spreading of high-nutrient WG waters toward the north, and in the Summer 2019–2020, the low-nutrient waters anomaly was transferred far to the south (up to 60°S).According to the data obtained, the seasonal dynamics of the nutrients in the surface layer of the Southern Ocean was rather weakly expressed. An exception is the high-latitude waters of the Cooperation and Davis Seas, where maximum seasonal variability of the hydrochemical characteristics was observed. The highest rate of nutrient consumption was observed in the coastal area of the Cooperation Sea near the fast ice edge from mid–December to early January and reached 3.2 μM per day for silicate, 1.8 μM per day for nitrates, and 0.12 μM per day for mineral phosphorus. The results of the long-term monitoring of the hydrochemical conditions in the Cooperation Sea made it possible to distinguish conditionally “warm” years with early vegetation (at the end of December) and intensive consumption of nutrients by phytoplankton, and “cold” years, when the formation of high-latitude “oases” in December–January was not observed.

Australasian Southern Ocean frontal stucture during summer 1976-77

1982 ◽  
Vol 33 (1) ◽  
pp. 3 ◽  
Author(s):  
RJ Edwards ◽  
WJ Emery
Keyword(s):  
Southern Ocean ◽  
Frontal Zone ◽  
Polar Front ◽  
Temperature Structure ◽  
Surface Salinity ◽  
Mode Water ◽  
Polar Frontal Zone ◽  
Eastern Australia ◽  
The Antarctic ◽  
Subantarctic Front

Fourteen north-south expendable bathythermograph temperature sections are used to define the positions of the Subtropical Convergence, the Subantarctic Front and the Polar Front in the Australasian sector of the Southern Ocean. The data were collected by supply vessels sailing to the Antarctic mainland during the austral summer of 1976-77. Frontal features are identified on the basis of both temperature structure and observations of surface salinity. The Polar Front is found to be more convoluted than the other fronts, leading to wide and narrow sections of the Antarctic Polar Frontal Zone between it and the Subantarctic Front. South-west of Australia the Subantarctic Front shifts north along with the Subtropical Convergence, and the Antarctic Polar Frontal Zone is at its widest. East of a southward shift of all three fronts, south of eastern Australia, they separate to give almost equal separations of about 700 km between fronts south-east of New Zealand. The homogeneous Subantarctic Mode Water between the Subantarctic Front and the Subtropical Convergence is found to be progressively warmer towards the west. Though the data is not truly synoptic, being spread over 3½ months, it does reflect the frontal structure during the period of observation.

Particle export and the biological pump in the Southern Ocean

2004 ◽  
Vol 16 (4) ◽  
pp. 501-516 ◽  
Author(s):  
SUSUMU HONJO
Keyword(s):  
Organic Carbon ◽  
Southern Ocean ◽  
Carbon Particle ◽  
Polar Front ◽  
Antarctic Polar Front ◽  
Biological Pump ◽  
The South ◽  
Regeneration Rate ◽  
Particle Export ◽  
The Antarctic

The organic carbon particle export to the interior layers in the Southern Ocean in the New Zealand–Tasmania Sector was approximately 170 mmolC m−2 yr−1. The export of particulate inorganic carbon in CaCO3 was 110 mmolC m−2 yr−1 and was contributed mostly by pteropods shells in the Antarctic Zones. The Si flux from biogenic opal at the sub-Antarctic Zone was 67 mmolSi m−2 yr−1 and rapidly increased to the south up to nearly 1 molSi m−2 yr−1 in the Antarctic Zone. The Antarctic Polar Front clearly demarcated the area where the biological pump was driven by CaCO3 to the north and biogenic SiO2 particle export to the south. Summer stratification caused by the sub-zero winter water layer in the Seasonal Ice Zone (SIZ) curtails the zooplankton community and hinders the replenishment of Fe. This hypothesis explains the large organic carbon export with large f- and export ratios at the SIZ and extremely large opal production at the Antarctic Circumpolar Zone. Estimated regeneration rate of CO2 from the export production and settling particulate fluxes of organic carbon in the water column between 100 m to 1 km was about 13 mmolC m−2 d−1 in the Antarctic Zone and Polar Frontal Zone.

scholarly journals Application of sediment core modelling to understanding climates of the past: An example from glacial-interglacial changes in Southern Ocean silica cycling

2006 ◽  
Vol 2 (6) ◽  
pp. 1371-1386 ◽  
Author(s):  
A. Ridgwell
Keyword(s):  
Southern Ocean ◽  
Sediment Core ◽  
Polar Front ◽  
Antarctic Polar Front ◽  
Sea Ice Extent ◽  
Silica Cycling ◽  
The North ◽  
Late Neogene ◽  
Primary Driver ◽  
The Antarctic

Abstract. Paleoceanographic evidence from the Southern Ocean reveals an apparent stark meridional divide in biogeochemical dynamics associated with the glacial-interglacial cycles of the late Neogene. South of the present-day position of the Antarctic Polar Front 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 is an important model validation target for testing hypotheses of glacial-interglacial change, particularly with respect to understanding the causes of the variability in atmospheric CO2. Here, I illustrate a time-dependent modelling approach to helping understand past climatic change by means of the generation of synthetic sediment core records. I find a close match between model-predicted and observed down-core changes in sedimentary opal content is achieved when changes in seasonal sea-ice extent is imposed, suggesting that the cryosphere is probably the primary driver of the striking features exhibited by the paleoceanographic record of this region.

Climate Response to External Sources of Freshwater: North Atlantic versus the Southern Ocean

2007 ◽  
Vol 20 (3) ◽  
pp. 436-448 ◽  
Author(s):  
Ronald J. Stouffer ◽  
Dan Seidov ◽  
Bernd J. Haupt
Keyword(s):  
Southern Ocean ◽  
Southern Hemisphere ◽  
North Atlantic ◽  
Surface Waters ◽  
Real World ◽  
The Other ◽  
Sea Surface ◽  
The North ◽  
The North Atlantic ◽  
The Antarctic

Abstract The response of an atmosphere–ocean general circulation model (AOGCM) to perturbations of freshwater fluxes across the sea surface in the North Atlantic and Southern Ocean is investigated. The purpose of this study is to investigate aspects of the so-called bipolar seesaw where one hemisphere warms and the other cools and vice versa due to changes in the ocean meridional overturning. The experimental design is idealized where 1 Sv (1 Sv ≡ 106 m3 s−1) of freshwater is added to the ocean surface for 100 model years and then removed. In one case, the freshwater perturbation is located in the Atlantic Ocean from 50° to 70°N. In the second case, it is located south of 60°S in the Southern Ocean. In the case where the North Atlantic surface waters are freshened, the Atlantic thermohaline circulation (THC) and associated northward oceanic heat transport weaken. In the Antarctic surface freshening case, the Atlantic THC is mainly unchanged with a slight weakening toward the end of the integration. This weakening is associated with the spreading of the fresh sea surface anomaly from the Southern Ocean into the rest of the World Ocean. There are two mechanisms that may be responsible for such weakening of the Atlantic THC. First is that the sea surface salinity (SSS) contrast between the North Atlantic and North Pacific is reduced. And, second, when freshwater from the Southern Ocean reaches the high latitudes of the North Atlantic Ocean, it hinders the sinking of the surface waters, leading to the weakening of the THC. The spreading of the fresh SSS anomaly from the Southern Ocean into the surface waters worldwide was not seen in earlier experiments. Given the geography and climatology of the Southern Hemisphere where the climatological surface winds push the surface waters northward away from the Antarctic continent, it seems likely that the spreading of the fresh surface water anomaly could occur in the real world. A remarkable symmetry between the two freshwater perturbation experiments in the surface air temperature (SAT) response can be seen. In both cases, the hemisphere with the freshwater perturbation cools, while the opposite hemisphere warms slightly. In the zonally averaged SAT figures, both the magnitude and the pattern of the anomalies look similar between the two cases. The oceanic response, on the other hand, is very different for the two freshwater cases, as noted above for the spreading of the SSS anomaly and the associated THC response. If the differences between the atmospheric and oceanic responses apply to the real world, then the interpretation of paleodata may need to be revisited. To arrive at a correct interpretation, it matters whether or not the evidence is mainly of atmospheric or oceanic origin. Also, given the sensitivity of the results to the exact details of the freshwater perturbation locations, especially in the Southern Hemisphere, a more realistic scenario must be constructed to explore these questions.

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.

A fission track thermochronological study of King George and Livingston islands, South Shetland Islands (West Antarctica)

2004 ◽  
Vol 16 (2) ◽  
pp. 191-197 ◽  
Author(s):  
I. SELL ◽  
G. POUPEAU ◽  
J.M. GONZÁLEZ-CASADO ◽  
J. LÓPEZ-MARTÍNEZ
Keyword(s):  
South Shetland Islands ◽  
West Antarctica ◽  
The South ◽  
North West ◽  
The North ◽  
Early Oligocene ◽  
Barton Peninsula ◽  
Bransfield Basin ◽  
Back Arc ◽  
The Antarctic

This paper reports the dating of apatite fission tracks in eleven rock samples from the South Shetland Archipelago, an island arc located to the north-west of the Antarctic Peninsula. Apatites from Livingston Island were dated as belonging to the Oligocene (25.8 Ma: metasediments, Miers Bluff Formation, Hurd Peninsula) through to the Miocene (18.8 Ma: tonalites, Barnard Point). Those from King George Island were slightly older, belonging to the Early Oligocene (32.5 Ma: granodiorites, Barton Peninsula). Towards the back-arc basin (Bransfield Basin), the apatite appears to be younger. This allows an opening rate of approximately 1.1 km Ma−1 (during the Miocene–Oligocene interval) to be calculated for Bransfield Basin. Optimization of the apatite data suggests cooling to 100 ± 10°C was coeval with the end of the main magmatic event in the South Shetland Arc (Oligocene), and indicates slightly different tectonic-exhumation histories for the different tectonic blocks.

scholarly journals The South Atlantic Subtropical High: Climatology and Interannual Variability

2017 ◽  
Vol 30 (9) ◽  
pp. 3279-3296 ◽  
Author(s):  
Xiaoming Sun ◽  
Kerry H. Cook ◽  
Edward K. Vizy
Keyword(s):  
Interannual Variability ◽  
Annual Cycle ◽  
Large Scale ◽  
Austral Summer ◽  
Southern Annular Mode ◽  
South Atlantic ◽  
Subtropical High ◽  
Interannual Variations ◽  
The South ◽  
The North

ERA-Interim and JRA-55 reanalysis products are analyzed to document the annual cycle of the South Atlantic subtropical high (SASH) and examine how its interannual variability relates to regional and large-scale climate variability. The annual cycle of the SASH is found to have two peaks in both intensity and size. The SASH is strongest and largest during the solstitial months when its center is either closest to the equator and on the western side of the South Atlantic basin during austral winter or farthest poleward and in the center of the basin in late austral summer. Although interannual variations in the SASH’s position are larger in the zonal direction, the intensity of the high decreases when it is positioned to the north. This relationship is statistically significant in every month. Seasonal composites and EOF analysis indicate that meridional changes in the position of the SASH dominate interannual variations in austral summer. In particular, the anticyclone tends to be displaced poleward in La Niña years when the southern annular mode (SAM) is in its positive phase and vice versa. Wave activity flux vectors suggest that ENSO-related convective anomalies located in the central-eastern tropical Pacific act as a remote forcing for the meridional variability of the summertime SASH. In southern winter, multiple processes operate in concert to induce interannual variability, and none of them appears to dominate like ENSO does during the summer.

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.

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