ALBERT

Description: A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard–Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age–gas age difference (Δage) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Δage model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from δ15N of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.

Description: In the Eurasian Arctic, the archipelago of Severnaya Zemlya is the most eastern one which is covered by a considerable ice cap, giving the opportunity to study regional climate signals from the Holocene period. The Academii Nauk ice cap (Komsomolets Island) was chosen for drilling a deep ice core because it is the thickest and coldest ice cap on Severnaya Zemlya. A suitable drilling site was found at 80°31'N 94°49'E by the help of airborne radio-echo sounding data and SAR interferometry. The ice thickness was 724 m at this location. Drilling was carried out between 1999 and 2001 reaching bedrock. It was a joint project of the Alfred Wegener Institute (Germany), the Arctic and Antarctic Research Institute, and the Mining Institute (Russia, St.Petersburg both). An electromechanical ice core drill (KEMS-112M) was used, the same type as at Vostok Station, Antarctica. The paper presents the results of electrical conductivity measurements (DEP) of the whole ice core. Several zones with high conductivity were assumed to be caused by major volcanic eruptions. By help of catalogues of historical volcanic events we used these signals for core dating of the upper 245 meters. The time scale developed this way is in good agreement with horizons of enriched radioactivity caused by nuclear weapon tests in the early 1960's and by the Chernobyl accident. The d180 record fits almost perfectly to values published earlier by Klementev et al. for Akademii Nauk, however, we have no evidence for the age model used in this Russian paper. We found annual accumulation rates in the isotope record and in the electrical data indicating none-steady state conditions of this glacier in the past. Hence, the core ages are overestimated by flow models. There seems to be an age discordance in the deepest part of the core. For Akademii Nauk ice cap the isotope data indicate a climate warming since app. 1860 which is much higher than found at central Greenland (GRIP/GISP2), Devon lsland or Hans Tausen ice cap. REFERENCE Klementev, O.L.; Potapenko, V. Yu.; Savatyugin, L.M. & Nikolaev, V.l.: Studies of the intemal structure and thennal-hydrodynarnic state of Vavilov Glacier, Archipelago Sevemaya Zernlya. IAHS Publ. 208, 1991, p.49-59