PhD defence by Hans Christian Steen-Larsen – Niels Bohr Institute - University of Copenhagen

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PhD defence by Hans Christian Steen-Larsen

The stable water isotope climatic method: Enhancing our understanding of the present-day isotope processes for the Greenland Ice Sheet

Supervisor: Sigfus J. Johnsen

Samples of precipitation and atmospheric water vapor were collected together with shallow rn/ice cores as part of the new deep drilling project in North-West Greenland { the NEEM project. These samples were analyzed for their isotope composition to understand the processes a ecting the climatic signal archived in the water stable isotope records from the NEEM deep ice core.

The dominant moisture source for the snow deposited at the NEEM-site may be originating as far south as 35°N from the western part of the Atlantic Ocean. The surface atmospheric water vapor appears in isotopic equilibrium with the snow surface indicating a large water exchange between the atmosphere and snow pack. The interannual variability of NEEM shallow rn/ice cores stable isotope data covering the last ~40 years shows an unexpectedly weak NAO signal. Regional to global atmospheric models simulate a dominant summer precipitation in the NEEM area, suggesting that the intermittency of modern winter precipitation is responsible for the lack of a strong NAO imprint. The interannual variability of NEEM isotope data however shows a strong correlation with interannual variations of Baffin Bay sea ice cover, a relationship consistent with air mass trajectories. NEEM deep ice core isotopic records may therefore provide detailed information on past Baffin Bay sea ice extent. Over the past 40 years, NEEM stable water isotope content increasing trend points to a local warming trend of ~1.3°C over the last 40 years.

Surface snow samples from Summit station in Greenland collected every 1-2 weeks over the last about 4 years was analyzed for their water stable isotope content.
The analysis of these samples show good agreement with the conclusions based on the NEEM samples. A robust correlation between the monthly mean isotopic composition of the snow surface samples and the surface temperature gives a slope of 0.42 ‡/K. This is en excellent agreement with previous work. Also a good correlation between the mean monthly temperature at Ilulissat, Greenland, and the mean monthly isotopic content was found.

Very precise measurements of 17O-excess of shallow core samples together with precipitation and vapor samples are presented. These samples show the potential of using isotopes to understand the hydrological cycle. However they do not support the conclusion of previous studies on the NEEM samples. This points to a possible slightly wrong fractionation factor for the vapor-ice transition. It was found from analysis of vapor samples by the use of back trajectory models that moisture originating from the Arctic Ocean had a high d-excess and 17O-excess.

A continuous record of the water vapor isotopic composition above the snow surface at NEEM through 70 days is presented. This record clearly shows the e ects of the local processes and the external signal on the water vapor isotopic composition.
It was found that local processes made the isotope signal follow the humidity very closely. Conceptual and simple 1D models were applied to explain the observed signal.

If interested please contact Hans Christian Steen-Larsen for a copy of the thesis.
E-mail: hanschr @