Showing papers by "Sigfus J Johnsen published in 2000"

Diffusion of stable isotopes in polar firn and ice : the isotope effect in firn diffusion

Abstract: Ice core records are often affected by post-depositional processes that need to be better understood to prevent wrong interpretation of the data. Records of stable isotopes are affected by diffusion both in the firn and in the deeper ice. We present a quantitative theory for diffusion in firn that applies the measured tortuosity factors for O2 and CO2 in fim to the diffusing water vapor. Because of different fractionation factors, the theory predicts stronger smoothing for 8 0 than for bD, in excellent agreement with our data. This effect opens up the possibility for using detailed isotope records to estimate paleotemperatures in deeper strata. We show that this differential smoothing can create an artificial annual cycle in deuterium excess, which was not present at the time of deposition. It also increases the slope observed in high resolution data series between bD and 8 0 variations. For the annual cycles, we observe that this slope can increase from 8 at the surface up to II in deeper firn. In the Holocene ice for the GRIP core, we observe much stronger smoothing than predicted from diffusion in solid ice; this suggests an anomalous diffusion process in glacier ice. Possible models for this excess diffusion are discussed i.a., in terms of the thickness of water films on grain boundaries and in veins.

Land-ice teleconnections of cold climatic periods during the last Glacial/Interglacial transition

Abstract: Independent calendar year chronologies are a basic requirement for the establishment of high resolution land-ice teleconnections. The annually laminated Meerfelder Maar record provides both an independent chronology, established by varve counting, and high resolution lithological proxy data for the period of the last Glacial/Interglacial transition. These data reveal a series of four periods of climatic deterioration coinciding with negative isotopic deviations in the GRIP record signal, thus demonstrating the synchronicity of environment changes in Western Germany and temperature shifts in Greenland. The terrestrial data supports a further sub-division of the event stratigraphy based on the GRIP core, by introducing the cold event GI-1c2 between 13 500 and 13 400 calendar years BP. Multiproxy analyses reveal that the environmental response at Meerfelder Maar was not linear throughout the Lateglacial but was modified by local processes. A change in the response of the lake environment to climate deterioration was observed during sub-stage GI-1b (Gerzensee oscillation), the only event with gradual rather than abrupt transitions. The two-fold character of the Younger Dryas as seen in the GRIP record is more pronounced in the Meerfelder Maar record. This lithological signal occurred with a delay of 60 years to the GRIP signal, and has been linked to a shift in the catchment. It is proposed that the trigger for this shift was a trend towards a more humid second half of the Younger Dryas.

Sticking deep ice core drills: Why and how to recover

Abstract: The GISP deep drill became stuck in 1981, but was free the following year. The NGRIP/EPICA deep drill has suffered from two big setbacks: The drill is stuck both at NGRIP in Greenland and at Dome C in Antarctica. Both events occured in a period with routine drilling and high productivity. The two reasons for the two events are believed to be different, but the chosen bore-hole liquid seems to be problematic. The densifier can adhere to the surface of the ice cuttings, making fine cuttings to sink in the liquid, in spite of a liquid density of 935 kg/m**3. In spite of changed procedures and modified constructions, the drill became stuck again at NGRIP. It was freed using glycol, making use of both the temperature and temparature gradient in the hole.