Jean-Jacques Pichon

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.

Alkenones and alkenes in surface waters and sediments of the Southern Ocean: Implications for paleotemperature estimation in polar regions

Abstract The concentration of C37-C39 long-chain alkenones and alkenes were determined in surface water and surface sediment samples from the subpolar waters of the Southern Ocean. Distributions of these compounds were similar in both sample sets indicating little differential degradation between or within compound classes. The relative amounts of the tri- to tetra-unsaturated C37 alkenones increased with increasing temperature for temperatures below 6°C similar to the di- and tri-unsaturated C37 alkenones. The C37 di-, tri-, and tetra-unsaturated methyl alkenones are used in paleotemperature calculations via the U37K and the U37K ratios. In these datasets, the relative abundances of the C37:2 and the C37.3 alkenones as a proportion of the total C37 alkenones were opposite and strongly related to temperature (the latter with more scatter), but the abundance of the C37:4 alkenone showed no relationship with temperature. The original definition of U37K includes the abundance of 37:4 in both the numerator and denominator, and thus it is perhaps not surprising that there is considerable scatter in the values obtained for U37K at low temperatures. Of the two, we suggest that U37K′ is the better parameter for use in paleotemperature estimations, even in cold locations. U37K′ values in the sediments fall on virtually the same regression line obtained for the water column samples of Sikes and Volkman (1993), indicating that their calibration is suitable for use in Southern Ocean sediments. The comparison of water column data with sedimentary temperature estimates suggests that the alkenone distributions are dominated by contributions from the summer when the biomass of Emiliania huxleyi and presumably flux to the sediment, is expected to be high.

Surface water temperature changes in the high latitudes of the southern hemisphere over the Last Glacial‐Interglacial Cycle

A set of numerical equations is developed to estimate past sea surface temperatures (SST) from fossil Antarctic diatoms. These equations take into account both the biogeographic distribution and experimentally derived silica dissolution. The data represent a revision and expansion of a floral data base used previously and includes samples resulting from progressive opal dissolution experiments. Factor analysis of 166 samples (124 Holocene core top and 42 artificial samples) resolved four factors. Three of these factors depend on the water mass distribution (one Subantarctic and two Antarctic assemblages); factor 4 corresponds to a “dissolution assemblage”. Inclusion of this factor in the data analysis minimizes the effect of opal dissolution on the assemblages and gives accurate estimates of SST over a wide range of biosiliceous dissolution. A transfer function (DTF 166/34/4) is derived from the distribution of these factors versus summer SST. Its standard error is ± 1°C in the −1 to +10 °C summer temperature range. This transfer function is used to estimate SST changes in two southern ocean cores (43°S and 55°S) which cover the last climatic cycle. The time scale is derived from the changes in foraminiferal oxygen and carbon isotopic ratios. The reconstructed SST records present strong analogies with the air temperature record over Antarctica at the Vostok site, derived from changes in the isotopic ratio of the ice. This similarity may be used to compare the oceanic isotope stratigraphy and the Vostok time scale derived from ice flow model. The oceanic time scale, if taken at face value, would indicate that large changes in ice accumulation rates occurred between warm and cold periods.

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.

Transfer functions between diatom assemblages and surface hydrology in the Southern Ocean

Interpretation of the distribution of 31 marine diatom species and two silicoflagellate genera in 28 core tops from the Atlantic and West Indian sectors of the Southern Ocean by Q-mode factor analysis allows definition of three significantly different floral assemblages, one associated with Subantarctic and two with Antarctic waters. These three areas are clearly delineated by prominent oceanographic limits: the Antarctic Convergence and the maximum retreat of sea-ice during the summer season. A set of palaeoecological transfer functions is derived by comparison between the distribution of these associations and surface water hydrological parameters. Using these equations, estimated summer temperatures from core top floral assemblages range from 12°C in the north of the study area to −2.0°C near the southernmost cores. The standard errors of estimate are ±1.1°C for summer temperatures, ±1.5 months/year for sea ice cover, ±0.2 mg-atP/l for sea-surface phosphate concentrations. Numerical tests of these paleoecological equations demonstrate their usefulness to reconstruct the evolution of the surface oceanography of the Austral Ocean at different periods of the last climatic cycle.

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.

Microfossil and stable-isotope evidence for changes in Late Holocene palaeoproductivity and palaeoceanographic conditions in the Prydz Bay region of Antarctica

Microfaunal and microfloral data from two gravity cores, and stable-isotope results from one of these cores taken on Fram Bank near Prydz Bay, Antarctica indicate changes in sea-ice patterns and oceanographic conditions which may have been linked to regional and global climate changes that occurred over the past 8000 yr. Modern diatom assemblages indicative of ice free conditions 1–2 months of the year, and dominated by Nitzschia species became prevalent some time around 2000–2700 yr B.P. The occurrence of Chaetoceros spp. diatom spore-dominated assemblages, and increased abundances of benthic and planktonic foraminifera, ostracods, and diatoms, coupled with carbon- and oxygen-isotope changes around 2700–3400 yr B.P. strongly suggests that this area of the shelf experienced conditions conducive to increased productivity during this time period. Based on diatom assemblage associations recognized in modern environments, the upper water column of Fram Bank shelf waters was probably stratified during that time due to the presence of low-salinity melt water and a very shallow mixed layer, protected from storms, with sea-ice cover for less than 10 months per year. The close correspondence of δ13C and δ18O values from N. pachyderma (r2 = 0.74) throughout one core suggests that whatever oceanographic conditions influenced δ18O also influenced δ13C. Changes in Prydz Bay microfossil abundances and isotopes may result from alterations in circulation patterns, upwelling conditions, or sea-ice patterns which significantly affected benthic and planktonic productivity in the area. The environmental conditions indicated by older sediments in the cores are less clear, but an ice tongue may have been in place 3200–3800 yr B.P., and minor fluctuations in productivity are indicated around 6000 and 7000–7500 yr B.P. The lower 100 cm of both cores may not represent in situ deposition, but if the corrected AMS dates reflect the actual age of the sediments analyzed, microfossil and isotope data indicate that productivity, and oceanographic conditions were very variable around 8500-8600 yr B.P. The chronology adopted herein must be regarded as tentative until further study of the area has been undertaken.

Quantification of the biogenic silica dissolution in Southern Ocean sediments

Abstract A transfer function has been established to quantify the dissolution of diatom silica in Southern Ocean sediments. The relationship between the amount of silica dissolution and changes in diatom species distribution is built by controlled progressive dissolution of biogenic silica in five recent sediment samples from box-core tops, each representative of a modern diatom species sediment assemblage. The amount of dissolved silica was measured for each experiment. The resulting data set of species abundances (42 samples containing 32 diatom species and 2 silicoflagellate genera) was added to the modern data base of diatom species distributed over the Southern Ocean (124 core tops). Q-mode factor analysis individualizes four factors explaining 83% of the variance. The first three factors are controlled by surface water properties (mostly temperature). The fourth factor is the only one correlated with loss of silica in the reference samples ( R = 0.900). We quantified the dissolution factor using this correlation: superficial sediments of the Southeast Indian Ocean are characterized, from low to high latitudes, by a decrease in silica loss by dissolution (from >50 to 10%) from the Subantarctic Zone (40°S) to around 55°S, followed by an increase of silica loss to values larger than 60% between 63° and 68°S. Application of the dissolution factor in two cores from the Southern Ocean (≈44° and 55°S) shows enhanced opal dissolution during the last glaciation, particularly during Emilianis stage 3 (from 40,000 to 30,000 yr B.P.).

Biogenic silica accumulation rate during the Holocene in the southeastern Indian Ocean

Three box-core transects were selected in the southeastern Indian Ocean to establish the variability of biogenic silica accumulation rate in the main subsystems of the Southern Ocean. Cycladophora davisiana and δ18O chronologies, previously calibrated by 14C dates, and biogenic silica contents determined by X-ray diffraction analysis were used to calculate accumulation rates. Concurrently, a transfer function was used to quantify the silica loss during the biogenic particulate accumulation in the deep-sea sediments. The average rate of biogenic silica rain on the sea-floor, calculated from the accumulation rate and the amount of dissolved biogenic silica, ranges from less than 0.1 to 16 g opal cm−2 ka−l. During the Holocene, biogenic silica has accumulated at the highest rates on the Southeast Ridge, south of the Polar Frontal Zone. To the north and south, the accumulation rate drops where summer sea surface temperatures are above 8°C or lower than 2°C. Biogenic silica dissolution is maximum in marginal sea ice zone. Accumulation rates of biogenic silica can be a useful index to estimate changes of palaeoproductivity in the southeastern Indian Ocean, although there is no strict proportionality between accumulation and silica rain rates.

Intense summer Si‐recycling in the surface Southern Ocean

[1] Si-cycle in surface waters was investigated in summer 2003 during a transect conducted from south-Australia to Antarctica. Diatoms dominated the microphytoplankton. Silicic acid was depleted up to 60� S; a subsurface maximum of biogenic silica (= biosilica) was observed in the Permanent Open Ocean Zone. In the 100–0.01% light zone, the ratio of depth-integrated biosilica dissolution rate (D) to depth-integrated biosilica production rate (P) ranged between 0 to 3.1, being >1 for 5 of our 6 stations. The biosilica dissolution was related to the percentage of dead diatoms but not to the temperature and might be, at least partially, under bacteria mediation. This study shows that during summer the Southern Ocean silicate pump can be much less efficient than usually expected. Existence of scenarios with intense surface Si-recycling in the Southern Ocean has major consequences both for modelers and paleoceanographers. INDEX TERMS: 4805 Oceanography: Biological and Chemical: Biogeochemical cycles (1615); 1615 Global Change: Biogeochemical processes (4805); 4855 Oceanography: Biological and Chemical: Plankton; 4207 Oceanography: General: Arctic and Antarctic oceanography; 4870 Oceanography: Biological and Chemical: Stable isotopes.