Past climate changes and permafrost depth at the Lake El'gygytgyn site: implications from data and thermal modelling

Abstract

Abstract. We present results of numerical simulations of the temperature field of the subsurface around and beneath the crater Lake El'gygytgyn in NE Russia, which is subject of an interdisciplinary drilling campaign within the International Continental Drilling Program (ICDP). This study focuses on determining the permafrost depth and the transition between talik and permafrost regimes, both, under steady-state and transient conditions of past climate changes. Thermal properties of the subsurface are deduced from measurements on three representative core samples taken from the quaternary sediments and the underlying impact rock. Further information is derived from the available geophysical logs and literature data. The temperature data from the lake borehole ICDP site 5011-1 down to 400 m depth below lake bottom are dominated by thermal perturbations related to the drilling process, and thus only give reliable values for the lowermost value in the borehole. Undisturbed temperature data recorded over more than two years in the 140 m deep land-based borehole ICDP site 5011-3 allow to determine the mean annual ground surface temperature (GST), as well as its history (GSTH) to a certain extent. Although the borehole's depth is by far not sufficient for a complete reconstruction of past temperatures back to the last glacial maximum (LGM), the temperature data and our modelling results show that there is still an influence of the LGM on the thermal regime, and thus on the permafrost depth. Whereas the latter result is obtained from the deeper part of the temperature profile, the rather strong curvature of the temperature data in shallower depths around 30 m can be explained by a comparatively large amplitude of the Little Ice Age (LIA), with a subsequently persistent cool period. Other mechanisms like varying porosity may also have an influence on the temperature profile, however, our modelling studies imply a major contribution from recent climate changes.