Xavier Crosta

Application of modern analog technique to marine Antarctic diatoms: Reconstruction of maximum sea‐ice extent at the Last Glacial Maximum

Modern analog technique (MAT) applied to Antarctic diatoms is a new approach for quantitative sea-ice paleoreconstructions in the Southern Ocean. In a first step we show that MAT is a better approach than the Imbrie and Kipp Method to reconstruct the modern sea-ice pattern. We then use this approach to reconstruct sea-ice presence in number of months per year during the last glacial maximum (LGM). At this time, sea-ice presence was greater than today, leading to a shorter diatom growing season. The maximum sea-ice extent, inferred from quantitative values of sea-ice presence, was located 5–8° north of its actual position, leading to double the surface of modern winter sea ice. This greater sea-ice extent may have played a significant role on atmospheric and surface oceanic circulations and therefore on southern mid-latitude and high-latitude climates. It may also have reduced the amount of heat, moisture, and CO2 from the ocean to the atmosphere, thus participating in the lowering of atmospheric CO2 during the LGM.

Late Quaternary sea ice history in the Indian sector of the Southern Ocean as recorded by diatom assemblages

Abstract A Modern Analog Technique (MAT5201/31) has been applied to fossil diatom assemblages to provide down-core estimates of February sea-surface temperatures (SSTs) and of sea ice duration over the past 220 000 years at 56°40′S, 160°14′E. At the core location, sea ice progression lagged the SST drop by ∼1 ka at interglacial–glacial transitions, and sea ice retreat was almost synchronous to the SST increase at glacial–interglacial terminations. Sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times, although SSTs were almost constant during glacials. This indicates that SSTs are the major parameter determining the advance and retreat of sea ice at transitions, but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo, air temperature and meridional wind stress. The strong correlation (r=0.75) between sea ice duration at the core location and the Vostok CO2 record argues for a control of Antarctic sea ice extent on atmospheric CO2 concentration via the modification of the ocean-to-atmosphere gas balance.

Sequence of events during the last deglaciation in Southern Ocean sediments and Antarctic ice cores

variation depends on the age model as 14 C determinations cannot be obtained for the time interval of 29.5–14.5 ka. Assuming a constant sedimentation rate for this interval, our data suggest that sea ice and nutrient changes at about 19 ka B.P. lead the increase in atmospheric pCO2 by approximately 2000 years. Our diatom-based sea ice record is in phase with the sodium record of the Vostok ice core, which is related to sea ice cover and similarly leads the increase in atmospheric CO2. If gas exchange played a major role in determining glacial to interglacial CO2 variations, then a delay mechanism of a few thousand years is needed to explain the observed sequence of events. Otherwise, the main cause of atmospheric pCO2 change must be sought elsewhere, rather than in the Southern Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4267 Oceanography: General: Paleoceanography; 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4870 Oceanography: Biological and Chemical: Stable isotopes; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: biogenic opal, stable isotopes, diatoms, IRD, last glacial, Southern Ocean

Reappraisal of Antarctic seasonal sea‐ice at the Last Glacial Maximum

We used modern analog technique applied to Antarctic diatoms to quantitatively reconstruct seasonal sea-ice extent at the Last Glacial Maximum. Winter maximum sea-ice limit occurred around 48°S in the Atlantic and western Indian sectors, around 55°S in the eastern Indian and western Pacific sectors, and around 58–60°S in the eastern Pacific sector of the Southern Ocean. Summer maximum sea-ice extents during the last ice age and today are similar, which contradicts CLIMAPs findings. This implies a reduced summer albedo feedback of the Southern Hemisphere and a greater transfer of heat and moisture from the ocean to the atmosphere than shown by previous qualitative studies.

Relationship between Antarctic sea ice and southwest African climate during the late Quaternary

Here we compare late Quaternary southwest African climate records from the west coast of southern Africa (published winter rainfall and trade wind intensity records from a core off the coast of Namibia) to records of Antarctic sea-ice extent. This comparison reveals coherent changes between Antarctic sea-ice extent and the southwest African winter rain region since 45 k.y. B.P., with enhanced winter rainfall and trade-wind vigor during periods of increased sea-ice presence. We propose an oceanic and atmospheric coupling between Antarctic sea ice and the winter rainfall zone of southwest Africa that may lead to increased desertification in the region if global warming persists.

Distribution of Chaetoceros resting spores in modern peri-Antarctic sediments

Abstract One hundred and sixty-five surface sediment samples from the Southern Ocean were examined for distribution and relative abundance of Chaetoceros resting spores. The contribution of resting spores to the total diatom assemblage ranges from 0% in the Subantarctic Zone to 95% in the Antarctic Peninsula sector. On the basis of both absolute and relative abundances four ‘biogeographic’ zones are distinguished: (1) the Antarctic Peninsula sector, (2) the Embayment Systems (Ross Sea and Weddell Sea), (3) the Continental Shelf zone (water depth 2000 m). Chaetoceros resting spores abundance reaches up to 900 × 10 6 valves/g of dry sediment in the Gerlache Strait, southwest of the Antarctic Peninsula. The hydrology of this region is characterized by an intense stratification of the water column due to sea-ice meltwater inputs, continental glacial runoffs and thermal warming of the surface water layer. The availability of nutrients, the lack of vertical mixing in those surface waters having low salinity ( 2.4 °C) is thought to be the main pre-condition for development of large Chaetoceros species blooms. We propose that increased relative abundances of Chaetoceros resting spores in fossil diatom assemblages from the Southern Ocean can therefore be used as tracers of water-column stratification due to glacial melt water.

Holocene climate in the Atlantic sector of the Southern Ocean: Controlled by insolation or oceanic circulation?

ABSTRACTThe Holocene climate of the Southern Ocean is not well un-derstood, mainly because of the lack of high-resolution reconstruc-tions of ocean surface properties. Here we present a 12,500-yr-long,decadal-scale record of Holocene sea-surface temperatures and sea-ice presence from the Polar Front of the East Atlantic SouthernOcean. The record shows gradual climate change, with no abruptNeoglacial cooling, and an unprecedented late Holocene warming.The dominant forcing factor appears to be precessional insolation;Northern Hemisphere summer insolation correlates to at least theearly to middle Holocene climate trend. Spectral analysis revealscentennial-scale cyclic climate changes with periods of 1220, 1070,400, and 150 yr. The record shows good correlation to East Ant-arctic ice cores and to climate records from South Georgia andBunger Oasis. However, the record shows out-of-phase behaviorwith regard to climate records from the western Antarctic Penin-sula and the Peru-Chile Current; such behavior hints at a climaticdivide through Patagonia, the Drake Passage, and between Westand East Antarctica.Keywords: Southern Ocean, paleoclimate, diatoms, modern analogtechnique, Holocene, insolation, Antarctic Polar Front.INTRODUCTIONThe Holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period. The Holoceneis generally assumed to be a stable interglacial period when climatechanges in the North Atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation (Imbrie et al., 1992). Thelarge fluctuations of the last glacial period appear to continue into thisinterglacial, although in a subdued manner (Bond et al., 2001).The Southern Ocean has received even less attention than the highnorthern latitudes; oceanic records are few and coarse. They generallydisplay a warm early Holocene followed by cooling (Labracherie etal., 1989; Pichon et al., 1992; Hodell et al., 2001), similar to NorthAtlantic climate (e.g., Koc¸ and Jansen, 1994). High-resolution recordsare restricted to Antarctic ice cores, which mostly display little climatevariability during the Holocene (Jouzel et al., 2001). An exception isthe Taylor Dome deuterium record, in which the Holocene appears tobe split by an abrupt cooling event ca. 5.5–5 ka (Steig et al., 1998).This cooling appears to fit the late Holocene global cooling trend (Por-ter, 2000) as a nonlinear response from the climate system to insolationchanges (deMenocal et al., 2000; Hodell et al., 2001).High-resolution Holocene records available for Southern Oceanclimate reconstruction are rarely from the Antarctic Circumpolar Cur-rent proper (e.g., Domack et al., 2001; Stenni et al., 2001; Lamy et al.,2002), with the exception of a multiproxy approach to Holocene cli-mate that includes a high-resolution description of lithic fragments(Hodell et al., 2001).We present data from site TN057-17 near the Polar Front in theEast Atlantic Southern Ocean. It is based on quantitative estimates ofsummer sea-surface temperature (SSST) and sea-ice presence (SIP),utilizing the down-core variability of diatom assemblages. The recordshows good correlation to Antarctic ice cores, but a gradual change inphase relationships. During the early Holocene, the record shows anin-phase relationship with the Ross Sea region ice cores; during thelate Holocene, the record shows an in-phase relationship with the EastAntarctic region ice cores.Antarctic Polar FrontThe Antarctic Polar Front (APF, Fig. 1) is an important climateboundary in terms of air-sea fluxes and the heat and salt budgets ofthe oceans. At the surface, the APF is a steep sea-surface temperaturegradient, while the subsurface expression is the southern limit for sub-duction of cold, fresh Antarctic Surface Water below warmer and moresaline Subantarctic Surface Water (Orsi et al., 1995). Globally, the APFcould be the southern limit of synchrony with Northern Hemisphereclimate (Broecker, 1996). The APF is subject to large spring and sum-mer blooms of diatoms (Fischer et al., 2002), and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the SouthernOcean (Burckle and Cirilli, 1987). It is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the APF region suitable for pro-ducing high-resolution records of climate change.

The 7–13 March 2006 major Saharan outbreak: Multiproxy characterization of mineral dust deposited on the West African margin

Mineral dust deposits were collected at Mbour, Senegal, throughout the spring of 2006 and especially during the well-documented March 7-13 large Saharan dust outbreak. During this 7-day period, significant changes in mass flux, grain-size, clay mineralogy and Sr and Nd isotopic compositions were recorded, indicating distinct provenances for the dust transported and deposited during and outside the event. All these terrigenous proxies, as well as freshwater diatom taxa, also showed significant temporal variations during the outbreak, implying contributions from at least two different provenance regions. Tri-dimensional back-trajectories and satellite imaging enabled us to link those distinct signatures to regions increasingly to the southeast within a large area covering Mauritania, Mali and southern Algeria, identified by the Total Ozone Mapping Spectrometer (TOMS) as the main source of the prominent winter/spring plume over the tropical Atlantic. The multiproxy characterization of the March 7-13 dust fall therefore enables us to typify the terrigenous signature of two different regions supplying dust off West Africa, and provide valuable clues for the interpretation of Northeastern Tropical Atlantic Ocean dust sedimentary records in terms of changes in provenance regions and transport systems. Additionally, because dust deposition data are scarce, flux and grain size data obtained in this study, among other parameters such as clay assemblages, provide important constraints for atmospheric transport models and dust deposition budget estimates in this area.

Seasonal progression of diatom assemblages in surface waters of Ryder Bay, Antarctica

Phytoplankton assemblages from seasonally sea-ice covered Ryder Bay (Adelaide Island, Antarctica) were studied over three austral summers (2004–2007), to link sea-ice variability and environmental conditions with algal speciation. Typical of near-shore Antarctic waters, biomass was dominated by large diatoms, although the prymnesiophyte Phaeocystis antarctica was numerically dominant. Although there was considerable interannual variability between main diatom species, high biomass of certain species or species groups corresponded consistently to certain phases of seasonal progression. We present the first documentation of an extensive bloom of the late-season diatom Proboscia inermis in February 2006, accounting for over 90% of diatom biomass. At this time, water column stratification and nutrient drawdown were high relative to other periods of the study, although carbon export was relatively low. Melt water flux in this region promotes well-stratified surface waters and high chlorophyll levels, but not necessarily concurrent increases in export production relative to seasons with lower freshwater inputs.

Highly branched isoprenoids as proxies for variable sea ice conditions in the Southern Ocean

Abstract Concentrations of a highly branched isoprenoid (HBI) diene determined in over 200 sediment samples from the Arctic co-vary with those of an HBI monoene (IP25) shown previously to be a sedimentary sea ice proxy for the Arctic. The same diene, but not monoene IP25, occurred in nine sea ice samples collected from various locations around Antarctica. The diene has been reported previously in Antarctic sea ice diatoms and the 13C isotopic compositions of the diene determined in two Antarctic sea ice samples were also consistent with an origin from sea ice diatoms (δ13C -5.7 to -8.5‰). In contrast, HBIs found in two Antarctic phytoplankton samples did not include the diene but comprised a number of tri- to pentaenes. In sediment samples collected near Adélie Land, East Antarctica, both the diene and the tri- to pentaenes often co-occurred. 13C isotopic compositions of the tri- to pentaenes in three sediment samples ranged from -35 to -42‰ whereas that of the diene in a sediment sample was -18‰. We propose the presence of this isotopically 13C enriched HBI diene in Antarctic sediments to be a useful proxy indicator for contributions of organic matter derived from sea ice diatoms. A ratio of the concentrations of diene/trienes might reflect the relative contributions of sea ice to phytoplanktonic inputs of organic matter to Antarctic sediments.