NEEM stable water isotope consortium
Names and contact details for chairs:
Point of contact and procedure for joining the consortium mailing list
Please go to http://mailman.nbi.ku.dk/listinfo to sign up for the NEEM-isotopes mailing list. The list is managed by the consortium chairs.
A brief description of the work of the consortium.
The isotopic composition of ice core water provides an integrated tracer of past climate conditions. Water molecules have naturally different stable isotope forms, which differ by their mass (number of neutrons) and by their diffusivity (due to the symmetry of the molecules). The most abundant forms are H216O, H218O, HDO and H217O. Their relative proportions can be accurately measured using mass spectrometers.
During phase changes (evaporation, condensation), the liquid or solid phases tend to be enriched in heavy molecules, while the water vapour phase tends to be depleted in heavy molecules. As a result, when air masses travel from moisture sources to Greenland, their cooling induces a progressive depletion in heavy molecules. This distillation process results in a strong relationship between the isotopic composition of rain or snowfall and temperature (the "isotopic thermometer"). Regional and global atmospheric models can include the explicit representation of the different water isotopes, and they are used to explore the relationships between climate and polar snowfall isotopic composition.
Of course, Greenland temperature is not the single control on the isotopic composition of snowfall : it can be affected by changes in evaporation conditions, moisture origin, air mass trajectory, condensation temperature. The combined measurements of all water stable isotope ratios makes it possible to identify the relative importance of these different factors. The climate information is acquired along the atmospheric water cycle, from the evaporation to the cloud condensation conditions. It is deposited within snowfall events at the surface of the ice sheet. If snowfall occurs preferentially over one season, then the isotopic composition of the snow will mostly inform us on the climate variability of that season ("seasonal bias").
Post-deposition factors also affect this climatic information. Isotopic diffusion erases the initial variability between one snowfall event and the next, and produces smooth seasonal cycles. These seasonal cycles can be used for identifying annual layers and, combined with other chemical markers, date the ice cores. Past variations in ice isotopic composition have been used to characterise Greenland climate dynamics, on time scales ranging from the interannual variations, linked with the North Atlantic Oscillation, to rapid climate shifts, and glacial-interglacial variations.
Field work will be conducted to characterise the isotopic composition of water vapour, snowfall, and the diffusion processes.
During the 2007 field season, 3 shallow cores have been drilled upstream and at the NEEM site. The main core and 2 other shallow cores have been drilled at the NEEM site during the 2008 field season, with lengths up to 106 m. These ice cores cover the past 3-4 centuries.
Over these ice cores, two pieces have been cut for isotopic measurements: first, 55 cm long samples ("bag samples") and second, 2.5 cm samples ("fine cuts"), giving access to a monthly resolution. There are therefore already 15 000 samples to be measured. These samples will be distributed amongst the different laboratories. A constant inter-calibration effort will be conducted to warrant the quality of the measurements.
The comparison between the different shallow ice cores at NEEM will bring key information on the signal to noise ratio and the reliability of the main ice core. The comparison with upstream ice cores is necessary to understand how the ice flow affects the isotopic signals in the deepest parts of the main core (which had been initially formed upstream of NEEM).
A specific effort will be dedicated to the comparison between the meteorological, satellite information and the recent isotopic variability, in order to understand the climate information preserved in NEEM ice cores.
The very high resolution of isotopic measurements planned on the main core (2.5 cm for the first ~800 m, and then 5 cm) will give access to the seasonal to annual variability over the current interglacial period, and to a temporal resolution of 20-30 years for the previous interglacial.