Sedimentology of cores from the shelf of Lazarev Sea, East Antarctica

Distinct facies types, classified in radiocarbon-dated sediments from the shelf of the Lazarev Sea, East Antarctica, reveal a detailed history of processes that have controlled sedimentation during the deglaciation over the last 10,000 yr. The ice retreat on this part of the Antarctic shelf started 9500 yr BP, marked by the deposition of laminated sediments, deposited from a floating ice shelf. These laminites, which occur on top of diamictons laid down from a grounded ice sheet, are the basal sediments of the postglacial sequence. The intensity of the Antarctic Coastal Current (ACC), directed by shelf morphology, controlled sedimentation of the postglacial facies. A residual glaciomarine sediment with the fine fraction winnowed by strong currents developed from 9000-8000 yr BP in the western part of the investigation area and from 9000-5000 yr BP in the eastern part, closer to the prominent 'Fenno Deep' trough. Current velocities apparently decreased between 8000 and 2000 yr BP due to a deflection of the ACC by advancing ice tongues to the east of the investigation area during the 'Hypsithermal'. This led to a deposition of fine-grained sediments, and clay mineralogy suggests a continental source, possibly near the grounding line of the Nivl Ice Shelf, rather than a winnowing of sediments near the shelf break or advection from deeper water. Current velocities intensified after 2000 yr BP, removed fine material from these sediments and led to a relict sediment, consisting of coarse bryozoan and molluscan debris.

Calcium carbonate content in surface sediments and benthic fauna on Antarctic shelves

We analyzed 214 new core-top samples for their CaCO3 content from shelves all around Antarctica in order to understand their distribution and contribution to the marine carbon cycle. The distribution of sedimentary CaCO3 on the Antarctic shelves is connected to environmental parameters where we considered water depth, width of the shelf, sea-ice coverage and primary production. While CaCO3 contents of surface sediments are usually low, high(> 15%) CaCO3 contents occur at shallow water depths (150-200 m) on narrow shelves of the eastern Weddell Sea and at a depth range of 600-900 m on the broader and deeper shelves of the Amundsen, Bellingshausen and western Weddell Seas. Regions with high primary production, such as the Ross Sea and the western Antarctic Peninsula region, have generally low CaCO3 contents in the surface sediments. The predominant mineral phase of CaCO3 on the Antarctic shelves is low-magnesium calcite. With respect to ocean acidification, our findings suggest that dissolution of carbonates in Antarctic shelf sediments may be an important negative feedback only after the onset of calcite undersaturation on the Antarctic shelves. Macrozoobenthic CaCO3 standing stocks do not increase the CaCO3 budget significantly as they are two orders of magnitude lower than the budget of the sediments. This first circumpolar compilation of Antarctic shelf carbonate data does not claim to be complete. Future studies are encouraged and needed to fill data gaps especially in the under-sampled southwest Pacific and Indian Ocean sectors of the Southern Ocean.

Clay mineral and heavy metal composition of sea-bed surface sediments and pre-Holocene sediment samples taken during POLARSTERN cruise ANT-XXIII/9

Surface mineralogical compositions and their association to modern processes are well known from the east Atlantic and south-west Indian sectors of the Southern Ocean, but data from the interface of these areas - the Prydz Bay-Kerguelen region - is still missing. The objective of our study was to provide mineralogical data of reference samples from this region and to relate these mineralogical assemblages to hinterland geology, weathering, transport and depositional processes. Clay mineral assemblages were analysed by means of X-ray diffraction technique. Heavy mineral assemblages were determined by counting of gravity-separated grains under a polarizing microscope. Results show that by use of clay mineral assemblages four mineralogical provinces can be subdivided: i) continental shelf, ii) continental slope, iii) deep sea, iv) Kerguelen Plateau. Heavy mineral assemblages in the fine sand fraction are relatively uniform except for samples taken from the East Antarctic shelf. Our findings show that mineralogical studies on sediment cores from the study area have the potential to provide insights into past shifts in ice-supported transport and activity and provenance of different water masses (e.g. Antarctic slope current and deep western boundary current) in the Prydz Bay-Kerguelen region.

Sediment and pore water geochemistry of sediment cores, Potter Cove, King George Island

Redox-sensitive trace metals (Mn, Fe, U, Mo, Re), nutrients and terminal metabolic products (NO3-, NH4+, PO43-, total alkalinity) were for the first time investigated in pore waters of Antarctic coastal sediments. The results of this study reveal a high spatial variability in redox conditions in surface sediments from Potter Cove, King George Island, western Antarctic Peninsula. Particularly in the shallower areas of the bay the significant correlation between sulphate depletion and total alkalinity, the inorganic product of terminal metabolism, indicates sulphate reduction to be the major pathway of organic matter mineralisation. In contrast, dissimilatory metal oxide reduction seems to be prevailing in the newly ice-free areas and the deeper troughs, where concentrations of dissolved iron of up to 700 µM were found. We suggest that the increased accumulation of fine-grained material with high amounts of reducible metal oxides in combination with the reduced availability of metabolisable organic matter and enhanced physical and biological disturbance by bottom water currents, ice scouring and burrowing organisms favours metal oxide reduction over sulphate reduction in these areas. Based on modelled iron fluxes we calculate the contribution of the Antarctic shelf to the pool of potentially bioavailable iron (Feb) to be 6.9x10**3 to 790x10**3 t/yr. Consequently, these shelf sediments would provide an Feb flux of 0.35-39.5/mg/m**2/yr (median: 3.8 mg/m**2/yr) to the Southern Ocean. This contribution is in the same order of magnitude as the flux provided by icebergs and significantly higher than the input by aeolian dust. For this reason suboxic shelf sediments form a key source of iron for the high nutrient-low chlorophyll (HNLC) areas of the Southern Ocean. This source may become even more important in the future due to rising temperatures at the WAP accompanied by enhanced glacier retreat and the accumulation of melt water derived iron-rich material on the shelf.

Porosity and density of the AND-1B sediment core

A study of density and porosity is presented for the 1285-m-long AND-1B core recovered from a flexural moat in the McMurdo Sound (Antarctica) in order to interpret sediment consolidation in an ice-proximal location on the Antarctic shelf. Various lithologies imply environmental changes from open marine to subglacial, and are numerically expressed in high-resolution whole-core wet-bulk density (WBD). Grain density data interpolated from discrete samples range from 2.14 to 3.85 g/cm3 and are used to calculate porosity from WBD in order to avoid the 5%-15% overestimation and underestimation of porosities obtained by standard methods. The trend of porosity extends from 0.5 near the top (Pleistocene) to 0.2 at the bottom (Miocene). Porosity fluctuations in different lithologies are superimposed with 0.2-0.3 in sequences younger than ca. 1 Ma and 0.5-0.8 in Pliocene diatomites. The AND-1B porosities and void ratios of Pliocene diatomites and Pleistocene mudstones exhibit a large negative offset compared to modern lithological analogs and their consolidation trends. This offset cannot be explained in terms of the effective stress at the AND-1B site. The effective stress ranges from 0 to 4000 kPa in the upper 600 m, and reaches 13,000 kPa at the base of the AND-1B hole. We suggest an excess of effective overburden stress of ~1700 and ~6000 kPa to explain porosities in Pliocene diatomites and Pleistocene mudstones, respectively. This is interpreted as glacial preconsolidation by subsequently grounded ice sheets under subpolar to polar, followed by colder polar types of glaciations. Information on Miocene consolidation is sparse due to alteration by diagenesis.

(Table 1) Characteristics of air temperature, sea surface temperature, sea surface salinity, nutrients, and total particulate matter for Admiralty Bay and Potter Cove, King George Island, Antarctica

Since the early 1990s, phytoplankton has been studied and monitored in Potter Cove (PC) and Admiralty Bay (AB), King George/25 de Mayo Island (KGI), South Shetlands. Phytoplankton biomass is typically low compared to other Antarctic shelf environments, with average spring - summer values below 1 mg chlorophyll a (Chl a)/m**3. The physical conditions in the area (reduced irradiance induced by particles originated from the land, intense winds) limit the coastal productivity at KGI, as a result of shallow Sverdrup's critical depths (Zc) and large turbulent mixing depths (Zt). In January 2010 a large phytoplankton bloom with a maximum of around 20 mg Chl a/m**3, and monthly averages of 4 (PC) and 6 (AB) mg Chl a/m**3, was observed in the area, making it by far the largest recorded bloom over the last 20 yr. Dominant phytoplankton species were the typical bloom-forming diatoms that are usually found in the western Antarctic Peninsula area. Anomalously cold air temperature and dominant winds from the eastern sector seem to explain adequate light : mixing environment. Local physical conditions were analyzed by means of the relationship between Zc and Zt, and conditions were found adequate for allowing phytoplankton development. However, a multiyear analysis indicates that these conditions may be necessary but not sufficient to guarantee phytoplankton accumulation. The relation between maximum Chl a values and air temperature suggests that bottom-up control would render such large blooms even less frequent in KGI under the warmer climate expected in the area during the second half of the present century.

At surface behaviour of emperor penguins from Pointe Géologie, Adélie Land, Antarctica, from expedition DDU 2005

The foraging distributions of 20 breeding emperor penguins were investigated at Pointe Géologie, Terre Adélie, Antarctica by using satellite telemetry in 2005 and 2006 during early and late winter, as well as during late spring and summer, corresponding to incubation, early chick-brooding, late chick-rearing and the adult pre-moult period, respectively. Dive depth records of three post-egg-laying females, two post-incubating males and four late chick-rearing adults were examined, as well as the horizontal space use by these birds. Foraging ranges of chick-provisioning penguins extended over the Antarctic shelf and were constricted by winter pack-ice. During spring ice break-up, the foraging ranges rarely exceeded the shelf slope, although seawater access was apparently almost unlimited. Winter females appeared constrained in their access to open water but used fissures in the sea ice and expanded their prey search effort by expanding the horizontal search component underwater. Birds in spring however, showed higher area-restricted-search than did birds in winter. Despite different seasonal foraging strategies, chick-rearing penguins exploited similar areas as indicated by both a high 'Area-Restricted-Search Index' and high 'Catch Per Unit Effort'. During pre-moult trips, emperor penguins ranged much farther offshore than breeding birds, which argues for particularly profitable oceanic feeding areas which can be exploited when the time constraints imposed by having to return to a central place to provision the chick no longer apply.

Fatty acid content of POM, sediment, foraminifer and peracarid samples obtained during POLARSTERN cruise ANT-XXIV/2 (PS71)

In order to investigate the diversity of diet composition in macrobenthic peracarid crustaceans from the Antarctic shelf and deep sea, the fatty acid (FA) composition of different species belonging to the orders Isopoda, Amphipoda, Cumacea and Tanaidacea was analysed. Multivariate analyses of the FA composition confirmed general differences between the orders, but also distinct differences within these orders. To gain information on the origin of the FAs found, the potential food sources sediment, POM and foraminiferans were included in the study. Most of the analysed amphipod species displayed high 18:1(n-9)-18:1(n-7) ratios, widely used as an indicator for a carnivorous component in the diet. Cumaceans were characterised by increased phytoplankton FA markers such as 20:5(n-3) (up to 29% of total FAs), suggesting a diet based on phytodetritus. High values of the FA 20:4(n-6) were found in some munnopsid isopods (up to 21% of total FAs) and some tanaidacean species (up to 19% of total FAs). 20:4(n-6) also occurred in high proportions in some foraminiferan samples (up to 21% of total fatty acids), but not in sediment and POM, possibly indicating the ingestion of foraminiferans by some peracarid crustaceans.

(Table 1) Foraging trip characteristics of male Adélie penguins (Pygoscelis adeliae) during their first incubation trip at Pointe Géologie

Knowledge of habitat use by top marine predators in response to environmental conditions is crucial in the current context of global changes occurring in the Southern Ocean. We examined the at-sea locations of male Adelie penguins (Pygoscelis adeliae) breeding at Dumont d'Urville during their first, long incubation trip. Compared with the chick-rearing period, penguins performed longer trips, going to oceanic waters as far as 320 km from the colony. We observed 3 strategies: (1) five individuals covered large distances to the north, targeting open-ocean areas and following the currents of two persistent eddies; (2) five individuals foraged to the north-west, close to the Antarctic shelf slope at the limit of the pack ice; and (3) three individuals covered much shorter distances (northwards or eastwards). The foraging range also seemed to be limited by the body condition of the penguins before their departure to sea.

Chemistry of organic matter of glacial and Cretaceous sediments from the Prydz Bay

Organic matter in Miocene glacial sediments in Hole 739C on the Antarctic Shelf represents erosional recycled continental material. Various indications of maturity in bulk organic matter, kerogens, and extracts imply that an exposed section of mature organic carbon-rich material was present during the Miocene. Based on biomarker, n-alkane, and kerogen analysis, a massive diamictite of early Eocene/Oligocene age at Hole 739C contains immature organic matter. Visual and pyrolysis analyses of the kerogens suggest a predominance of terrestrial organic matter in all samples from Hole 739C. A reversal of thermal maturities, i.e., more-mature overlying less-mature sections, may be related to redeposition generated from glacial erosion. Siliciclastic fluviatile sediments of Lower Cretaceous age from Hole 741A were analyzed. The organic matter from this hole contains immature aliphatic and aromatic biomarkers as well as a suite of odd carbon number-dominated nalkanes. Visual examination and pyrolysis analysis of the kerogen suggests that predominantly immature terrestrial organic matter is present at Hole 741A. The similarities between Hole 739C Unit V and Hole 741A suggest that the source of the organic matter in the glacial sediments in Unit V at Hole 739C could be Cretaceous in age and similar to sediments sampled at Hole 741A in Prydz Bay.