Gilles Delaygue

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica

The recent completion of drilling at Vostok station in East Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial–interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.

Hydrological conditions over the western Mediterranean basin during the deposition of the cold Sapropel 6 (ca. 175 kyr BP)

A new oxygen isotope record is reported from a stalagmite collected in the Argentarola Cave located on the Tyrrhenian coast of Italy. As shown from observations and numerical modeling of N 18 O in modern precipitation, the recorded N 18 O variability for this zone is dominated by the amount of precipitation (so-called ‘amount effect’). The N 18 O profile measured in the stalagmite is characterized by a prominent negative excursion (ca. 2^3x) between 180 and 170 kyr BP. This paleoclimatic feature is interpreted as being due to a relatively wet period which occurred during the penultimate glacial period, more precisely, during Marine Isotope Stage 6.5. This pluvial phase is shown to correspond chronologically to the deposition of the sapropel event 6 (S6). Although this particular sapropel event occurred during a cold phase, the N 18 O excursion is similar to those corresponding to other sapropels (S4, S3 and S2). The evidence for humid conditions during S6 in the western Mediterranean basin agrees with previous studies based on deep-sea sediment cores. Taken collectively, the data suggest that during sapropel events dilution of ocean surface waters was not restricted to the output of the river Nile but was rather widespread over the entire Mediterranean Sea due to increased rainfall. < 2002 Elsevier Science B.V. All rights reserved.

Oxygen isotope/salinity relationship in the Northern Indian Ocean

We analyze the surface δ18O-salinity relationships of the Bay of Bengal and the Arabian Sea, in the northern Indian Ocean, known for their contrasting hydrological conditions. New measurements of these tracers show a very low δ18O-salinity slope associated with the strong dilution in the Bay of Bengal, but a slope more typical of this latitude in the Arabian Sea. Although this region is marked by a complex monsoonal regime, numerical modeling using a box model and a general circulation model is able to capture the δ18O-salinity slope and its geographical variation. Both models clearly show that the low δ18O-salinity slope is due to the evaporation-minus-precipitation balance, with an important contribution of the continental runoff in the Bay of Bengal. Although the low value of these slopes (∼0.25) makes past salinity reconstructions uncertain, insight into the Last Glacial Maximum conditions shows a probable stability of these slopes and limited error on paleosalinity.

The origin of Antarctic precipitation: a modelling approach

The contribution of different moisture sources to Antarctic precipitation for present-day and glacial conditions is estimated with the NASA/GISS Atmospheric General Circulation Model. Despite its low horizontal resolution (8°×10°), this model simulates reasonably well the broad features of the observed present-day hydrological cycle. Simulated present-day Antarctic precipitation is dominated throughout the year by moisture from a subtropical/midlatitude band (30°S-60°S). The moisture supplied to a given coastal area of Antarctica originates mostly in the adjacent oceanic basin; closer to the pole, other oceanic basins can also contribute significantly. Replacing the present-day sea surface temperatures (SSTs) and sea ice cover in the GCM with those from the CLIMAP oceanic reconstruction for the last glacial maximum (LGM), greatly increases the simulated latitudinal temperature gradient, with the consequence of slightly enhancing the contribution of low latitude moisture to Antarctic precipitation. It also changes the seasonality of the different contributions and thus their budget, particularly in coastal regions. Because the nature of LGM tropical SSTs is still under debate, we performed an additional LGM simulation in which the tropical SSTs are reduced relative to those of CLIMAP. The resulting decrease in the latitudinal gradient brings the relative contributions to Antarctic precipitation more in line with those of the present-day simulation.

A 420,000 year deuterium excess record from East Antarctica: Information on past changes in the origin of precipitation at Vostok

Here we present and analyze the complete Vostok ice cores deuterium excess record which spans the last four climatic cycles, back to ∼420,000 years B.P. To extract paleoclimate information from this record, we use isotopic modeling showing that changes in deuterium excess (d = δD-8δ18O) of Vostok precipitation reflect changes in the average temperature of oceanic moisture sources. Over the last 250,000 years the deuterium excess is strongly anticorrelated with changes in Earths obliquity. This reflects changes in the relative contribution of low and high latitudes to the Vostok precipitation, resulting from changes in the latitudinal annual mean insolation gradient governed by the obliquity. However, this modulation by obliquity is not observed prior to 250,000 years B.P. We attribute this difference to the ice flow, the deeper ice under Vostok station having accumulated in a location upstream Vostok, receiving precipitation from different oceanic origins. Despite this difference between the earlier and the later portions of the record, the deuterium excess changes during all the glacial inceptions are similar in amplitude and timing relative to the deuterium decrease. The glacial inceptions are characterized by high deuterium excess, indicating the significant role of the tropics in supplying moisture to the already cold poles.

Oxygen 18–salinity relationship simulated by an oceanic general circulation model

The distribution of H218O in the ocean is simulated using an oceanic general circulation model (GCM). Surface isotopic fluxes prescribed to this model are derived from an atmospheric GCM, which ensures their consistency. The oceanic GCM is initialized with a homogeneous composition, and redistributes isotopic gradients so as to equilibrate with the atmosphere. A good agreement is found between the observations and the simulation concerning (i) the interbasin H218O gradients, especially the Atlantic being more enriched than the Pacific, as well as the meridional gradients; and (ii) the H218O–salinity relationship.

Validity of the isotopic thermometer in central Antarctica: Limited impact of glacial precipitation seasonality and moisture origin

The classical interpretation of water stable isotopes (deuterium or oxygen 18) retrieved from ice cores into past local temperature relies on the use of the spatial isotope/temperature slope as a surrogate of the temporal slope. Whereas this assumption has been challenged by independent methods in central Greenland, it is still considered as valid in central Antarctica. We use an atmospheric General Circulation Model (GCM) to study two parameters highlighted by previous studies as being among the most important with respect to this assumption. We show that in the GCM, between present-day and Last Glacial Maximum, the change in precipitation seasonality and the cooling of the moisture sources have limited and opposite effects on the isotopic record of the Antarctic local temperature. This conclusion strengthens the validity of the classical interpretation of isotope records in central Antarctica.

Reconstruction of Lamb weather type series back to the eighteenth century

The Lamb weather type series is a subjective catalogue of daily atmospheric patterns and flow directions over the British Isles, covering the period 1861–1996. Based on synoptic maps, meteorologists have empirically classified surface pressure patterns over this area, which is a key area for the progression of Atlantic storm tracks towards Europe. We apply this classification to a set of daily pressure series from a few stations from western Europe, in order to reconstruct and to extend this daily weather type series back to 1781. We describe a statistical framework which provides, for each day, the weather types consistent enough with the observed pressure pattern, and their respective probability. Overall, this technique can correctly reconstruct almost 75% of the Lamb daily types, when simplified to the seven main weather types. The weather type series are described and compared to the original series for the winter season only. Since the low frequency variability of synoptic conditions is directly related to the North Atlantic Oscillation (NAO), we derive from the weather type series an NAO index for winter. An interesting feature is a larger multidecadal variability during the nineteenth century than during the twentieth century.