A single-substrate model to interpret intra-annual stable isotope signals in tree-ring cellulose.

The carbon and oxygen stable isotope composition of wood cellulose (delta(13)C(cellulose) and delta(18)O(cellulose), respectively) reveal well-defined seasonal variations that contain valuable records of past climate, leaf gas exchange and carbon allocation dynamics within the trees. Here, we present a single-substrate model for wood growth to interpret seasonal isotopic signals collected in an even-aged maritime pine plantation growing in South-west France, where climate, soil and flux variables were also monitored. Observed seasonal patterns in delta(13)C(cellulose) and delta(18)O(cellulose) were different between years and individuals, and mostly captured by the model, suggesting that the single-substrate hypothesis is a good approximation for tree ring studies on Pinus pinaster, at least for the environmental conditions covered by this study. A sensitivity analysis revealed that the model was mostly affected by five isotopic discrimination factors and two leaf gas-exchange parameters. Modelled early wood signals were also very sensitive to the date when cell wall thickening begins (t(wt)). Our model could therefore be used to reconstruct t(wt) time series and improve our understanding of how climate influences this key parameter of xylogenesis.

GRIP deuterium excess reveals rapid and orbital-scale changes in Greenland moisture origin.

The Northern Hemisphere hydrological cycle is a key factor coupling ice sheets, ocean circulation, and polar amplification of climate change. Here we present a Northern Hemisphere deuterium excess profile covering one climatic cycle, constructed with the use of delta18O and deltaD Greenland Ice Core Project (GRIP) records. Past changes in Greenland source and site temperatures are quantified with precipitation seasonality taken into account. The imprint of obliquity is evidenced in the site-to-source temperature gradient at orbital scale. At the millennial time scale, GRIP source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events, and these rapid shifts are associated with large-scale changes in atmospheric circulation.

More than 200 meters of lake ice above subglacial Lake Vostok, Antarctica.

Isotope studies show that the Vostok ice core consists of ice refrozen from Lake Vostok water, from 3539 meters below the surface of the Antarctic ice sheet to its bottom at about 3750 meters. Additional evidence comes from the total gas content, crystal size, and electrical conductivity of the ice. The Vostok site is a likely place for water freezing at the lake-ice interface, because this interface occurs at a higher level here than anywhere else above the lake. Isotopic data suggest that subglacial Lake Vostok is an open system with an efficient circulation of water that was formed during periods that were slightly warmer than those of the past 420,000 years. Lake ice recovered by deep drilling is of interest for preliminary investigations of lake chemistry and bedrock properties and for the search for indigenous lake microorganisms. This latter aspect is of potential importance for the exploration of icy planets and moons.

Design and performance of a mass spectrometric facility for measuring helium isotopes in natural waters and for low-level tritium determination by the 3He ingrowth method.

The design and performance of a mass spectrometric system for the measurement of helium isotopes and very low tritium concentrations in natural waters are described and discussed in the light of analytical precision and accuracy. The system consists of a VG 3000 mass spectrometer with a fully automated inlet system for preparation and purification of the samples. Along with this mass spectrometric system, different custom-fabricated units are described, especially designed for taking samples, extracting helium or degassing tritium samples prior to the mass spectrometric analysis. The 3He detection limit of the system is close to 10(-16) cm3 STP corresponding to a tritium level of 0.003 TU for a 500 g water sample stored six months for 3He regrowth. A vertical oceanic tritium profile from the south hemisphere is presented as an illustration of the system's capability to detect very low tritium concentrations in the environment.