A review of Glacial and Holocene paleoclimate records from southernmost Patagonia (49–55°S)

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Antarctic palaeo-ice streams

Ross Sea paleo-ice sheet drainage and deglacial history during and since the LGM

Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth's orbit Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 degrees C This landscape is likely to have developed during the initial phases of Antarctic glaciation According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years

The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet

We investigate the evolution of Cenozoic climate and ice volume as evidenced by the oxygen isotopic composition of seawater (δ18Osw) derived from benthic foraminiferal Mg/Ca ratios to constrain the temperature effect contained in foraminiferal δ18O values. We have constructed two benthic foraminiferal Mg/Ca records from intermediate water depth sites (Ocean Drilling Program sites 757 and 689 from the subtropical Indian Ocean and the Weddell Sea, respectively). Together with the previously published composite record of Lear et al. [Science 287 (2002) 269–272] and the Neogene record from the Southern Ocean of Billups and Schrag [Paleoceanography 17 (2002) 10.1029/2000PA000567], we obtain three, almost complete representations of the δ18Osw for the past 52 Myr. We discuss the sensitivity of early Cenozoic Mg/Ca-derived paleotemperatures (and hence the δ18Osw) to assumptions about seawater Mg/Ca ratios. We find that during the middle Eocene (∼49–40 Ma), modern seawater ratios yield Mg/Ca-derived temperatures that are in good agreement with the oxygen isotope paleothermometer assuming ice-free conditions. Intermediate waters cooled during the middle Eocene reaching minimum temperatures by 40 Ma. The corresponding δ18Osw reconstructions support ice growth on Antarctica beginning by at least 40 Ma. At the Eocene/Oligocene boundary, Mg/Ca ratios (and hence temperatures) from Weddell Sea site 689 display a well-defined maximum. We caution against a paleoclimatic significance of this result and put forth that the partitioning coefficient of Mg in benthic foraminifera may be sensitive to factors other than temperature. Throughout the remainder of the Cenozoic, the temporal variability among δ18Osw records is similar and similar to longer-term trends in the benthic foraminiferal δ18O record. An exception occurs during the Pliocene when δ18Osw minima in two of the three records suggest reductions in global ice volume that are not apparent in foraminiferal δ18O records, which provides a new perspective to the ongoing debate about the stability of the Antarctic ice sheet. Maximum δ18Osw values recorded during the Pleistocene at Southern Ocean site 747 agree well with values derived from the geochemistry of pore waters [Schrag et al., Science 272 (1996) 1930–1932] further highlighting the value of the new Mg/Ca calibrations of Martin et al. [Earth Planet. Sci. Lett. 198 (2002) 193–209] and Lear et al. [Geochim. Cosmochim. Acta 66 (2002) 3375–3387] applied in this study. We conclude that the application of foraminiferal Mg/Ca ratios allows a refined view of Cenozoic ice volume history despite uncertainties related to the geochemical cycling of Mg and Ca on long time scales.

Application of Benthic Foraminiferal Mg/Ca Ratios to Questions of Cenozoic Climate Change

Clay minerals in Cenozoic sediments off Cape Roberts (McMurdo Sound, Antarctica) reveal palaeoclimatic history

The mineralogy and geochemistry of the clay fraction of Victoria Land Basin (Ross Sea, Antarctica) sediments was investigated, to determine the origin of clay minerals and the features of authigenic smectite. The investigated core (CRP-3) is ~800 m long, mostly of Oligocene age. The clay fraction of the upper sequence consists of mica, chlorite and detrital smectite, while that of the central and lower part is largely made up of authigenic smectite. Authigenic smectites are ditrioctahedral, with a composition close to saponite, while detrital smectites such as Al-Fe beidellites are dioctahedral. Authigenic smectites have no illite mixed layers, show a higher degree of crystallization, higher MgO, Fe2O3, V, Cr, Co, Ni and Sc contents and lower SiO2, Al2O3, K2O, TiO2, Ba, Rb and Zr contents with respect to detrital clay minerals, and a clear depletion of LREE with respect to HREE. Authigenic smectite formed from the alteration of volcanic materials and clay minerals.

Mineralogical and geochemical characteristics (major, minor, trace elements and REE) of detrital and authigenic clay minerals in a Cenozoic sequence from Ross Sea, Antarctica

A new classification of opals through X-ray powder diffraction (XRPD) methodology, by analysing 75 new samples of opal came from different worldwide areas, is introduced. A brief historical summary of the application of XRPD analysis on opals and the most important XRPD results reported in literature were compared with the newly obtained XRPD data. A simple method for the classification of opals on the basis of their degrees of structural order-disorder calculated from the diffraction data is proposed. In addition, a clear boundary, which has not been previously described by others in literature, related to the presence (or absence) of two-peak characteristic of the cristobalite phase is identified. This boundary allows for a discrimination of opals C from CT.

XRPD patterns of opals: A brief review and new results from recent studies

The aim of this work is to evaluate the origin of smcctitcs in sediments of CRP-3 core (Victoria Land Basin, Antarctica), through TEM observations and microanalyses on smectite microparticles and XRD analyses of clay fractions. Smcctites in the lower sedimentary section (between 602.05 and 815.86 mbsf). and in some levels of the central section of the core, are intermediate members of the beiclellite-saponite series, with lower K and higher M g contents in interlayer and Iiighcr cristallinity values. The clay fraction largely consists of smectite. that is regarded as authigenic. Smectites in the upper (between 6.60 and 101.69 mbsf) and in some other levels of the central section of the core are Al-Fe beidellites. with higher K and lower Mg contents in interlayer sites and lower values of crystallinity. In the clay fraction illite and chlorite also occur. These characteristics suggest that smectites in these sediments are detrital. Smectites in an igneous body (between 901.20 and 919.06 mbsf) have a homogeneous composition and are Al-rich beidellite-montmorillonites; the clay fraction is mostly composed by kaolinite with mixed-layer smcctite-illite or kaolinite-smectite. Authigenic smectites probably formed from the alteration of volcanic material (pyroxenes, glasses) andlor through precipitation from fluids of possible hydrothermal origin.

Luigi Marinoni

Papers

Clay minerals in Cenozoic sediments off Cape Roberts (McMurdo Sound, Antarctica) reveal palaeoclimatic history

Mineralogical and geochemical characteristics (major, minor, trace elements and REE) of detrital and authigenic clay minerals in a Cenozoic sequence from Ross Sea, Antarctica

XRPD patterns of opals: A brief review and new results from recent studies

Crystal-chemistry of smectites in sediments of CRP-3 drillcore (Victoria Land Basin, Antarctica): preliminary results

Late Quaternary glacial marine to marine sedimentation in the Pennell Trough (Ross Sea, Antarctica)