Isabelle Basile-Doelsch

Another continental pool in the terrestrial silicon cycle

Silicon is the second most abundant element on Earth. It is an important nutrient for phytoplankton and is readily absorbed by terrestrial vegetation; it also assists the removal of carbon dioxide from the atmosphere through the weathering of silicates. But the continental cycle of silicon is not well known, and only a few studies have attempted to use silicon stable isotopes (28Si, 29Si and 30Si) to quantify the continental silicon reservoirs. Dissolved silicon in sea and river waters forms a reservoir of mean isotopic value +1.1‰ (refs 7, 10). It is enriched in 30Si with respect to the igneous rocks reservoir, which has a mean isotopic value of -0.3‰ (refs 4, 9). This enrichment can only be produced by a major fractionation during weathering, and should result in the formation of a continental 30Si-depleted reservoir. Such a reservoir, however, has not been identified to date. Here we analyse silicon isotopes of in situ quartz from a sandstone series in France, using a new-generation secondary ion mass spectrometry apparatus. We show that quartz that precipitates as siliceous cements forms a strongly 30Si-depleted reservoir with isotopic values down to -5.7‰, a more negative value than any previously published for terrestrial samples. Our findings suggest that quartz re-precipitation plays an important role in the biogeochemical cycle of silicon.

Aeolian dust in East Antarctica (EPICA‐Dome C and Vostok): Provenance during glacial ages over the last 800 kyr

Aeolian mineral dust archived in Antarctic ice cores represents a key proxy for Quaternary climate evolution. The longest and most detailed dust and climate sequences from polar ice are provided today by the Vostok and by the EPICA-Dome C (EDC) ice cores. Here we investigate the geographic provenance of dust windborne to East Antarctica during Early and Middle Pleistocene glacial ages using strontium and neodymium isotopes as tracers. The isotopic signature of Antarctic dust points towards a dominant South American origin during Marine Isotopic Stage (MIS) 8, 10, 12, and back to MIS 16 and 20 as deduced from EDC core. Data provide evidence for a persistent overall westerly circulation pattern allowing efficient transfer of dust from South America to the interior of Antarctica over the last 800 kyr. Some small but significant dissimilarity between old and recent glacial ages suggests a slightly reduced Patagonian contribution during ancient glaciations.

6. Late quaternary interglacials in East Antarctica from ice-core dust records

Abstract Aeolian dust records from deep East Antarctic ice cores evidence extremely low dust fluxes during the last five interglacials (10 to 25 times lower than in glacial periods), related to reduced primary production and mobilization on the Southern Hemisphere continents, to changes in atmospheric transport and hydrological cycle. The Sr-Nd isotope fingerprint of aeolian dust in Antarctica suggests a dominant southern South America provenance during Quaternary glacial times, but the first geochemical data for Stage 5.5 and the Holocene presented in this work show significant differences and open the possibility for a different source mixing. Dust-size variability in the EPICA-Dome C ice core suggests shorter transport time for dust or more direct air mass penetration to the site during interglacials with respect to cold periods and a clear multisecular scale mode of atmospheric circulation variability during the Holocene.

First Characterization and Dating of East Antarctic Bedrock Inclusions from Subglacial Lake Vostok Accreted Ice

Environmental Context.Lake Vostok is a large subglacial lake trapped below the East Antarctic ice sheet. The meteoric ice from deep Vostok ice cores has been used to document the climatic history of the Earth over hundreds of millennia, while the deeper part of the core preserves some basal rock fragments. These rock fragments represent unique geological samples of the inhospitable, ice-covered East Antarctic Plateau. Abstract.The Vostok (East Antarctica, 78°S, 106°E) ice core preserves, below the meteoric ice keeping the climatic memory of the last 420u2007000 years, ice formed by freezing of subglacial Lake Vostok water. This latter contains some bedrock fragments representing unique samples for the geological investigation of the East Antarctic Plateau, covered by ~2–4 km of ice. The first geochemical (87Sr/86Sr versus 143Nd/144Nd) and mineralogical characterization of these inclusions as well as the dating of one of them (Nd model age on whole-rock sample) has given evidence for a Mid-Proterozoic age of the basement lying below the ice sheet, consistent with recent geophysical data. The geochemical characteristics of bedrock inclusions within the accreted ice zone are markedly different from those of the mineral dust of aeolian origin archived in the uppermost part of the Vostok ice core and originating from deflation of the Southern Hemisphere continents, and easily discriminates between the two contributions.

Distribution of PAHs and trace metals in urban stormwater sediments: combination of density fractionation, mineralogy and microanalysis

Sediment management from stormwater infiltration basins represents a real environmental and economic issue for stakeholders due to the pollution load and important tonnages of these by-products. To reduce the sediment volumes to treat, organic and metal micropollutant-bearing phases should be identified. A combination of density fractionation procedure and microanalysis techniques was used to evaluate the distribution of polycyclic aromatic hydrocarbons (PAHs) and trace metals (Cd, Cr, Cu, Ni, Pb, and Zn) within variable density fractions for three urban stormwater basin sediments. The results confirm that PAHs are found in the lightest fractions (du2009<u20091.9, 1.9u2009<u2009du2009<u20092.3xa0gxa0cm−3) whereas trace metals are equally distributed within the light, intermediary, and highest fractions (du2009<u20091.9, 1.9u2009<u2009du2009<u20092.3, 2.3u2009<u2009du2009<u20092.6, and du2009>u20092.8xa0gxa0cm−3) and are mostly in the 2.3u2009<u2009du2009<u20092.6xa0gxa0cm−3 fraction. The characterization of the five fractions by global analyses and microanalysis techniques (XRD and MEB-EDX) allowed us to identify pollutant-bearing phases. PAHs are bound to the organic matter (OM) and trace metals to OM, clays, carbonates and dense particles. Moreover, the microanalysis study underlines that OM is the main constituent responsible for the aggregation, particularly for microaggregation. In terms of sediment management, it was shown that density fractionation is not suitable for trace metals but could be adapted to separate PAH-enriched phases.