Clim. Past, 9, 119–133, 2013 www.clim-past.net/9/119/2013/ Climate doi:10.5194/cp-9-119-2013 of the Past © Author(s) 2013. CC Attribution 3.0 License. Past climate changes and permafrost depth at the Lake El’gygytgyn site: implications from data and thermal modeling D. Mottaghy1, G. Schwamborn2, and V. Rath3 1Geophysica Beratungsgesellschaft mbH, Aachen, Germany 2Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany 3Department of Earth Sciences, Astronomy and Astrophysics, Faculty of Physical Sciences, Universidad Complutense de Madrid, Madrid, Spain Correspondence to: D. Mottaghy ([email protected]) Received: 6 July 2012 – Published in Clim. Past Discuss.: 16 July 2012 Revised: 12 December 2012 – Accepted: 13 December 2012 – Published: 22 January 2013 Abstract. This study focuses on the temperature field ob- perature, an estimate of steady-state conditions is possible, served in boreholes drilled as part of interdisciplinary sci- leading to a meaningful value of 14 ± 5 K for the post-glacial entific campaign targeting the El’gygytgyn Crater Lake in warming. The strong curvature of the temperature data in NE Russia. Temperature data are available from two sites: shallower depths around 60 m can be explained by a compar- the lake borehole 5011-1 located near the center of the lake atively large amplitude of the Little Ice Age (up to 4 K), with reaching 400 m depth, and the land borehole 5011-3 at the low temperatures prevailing far into the 20th century. Other rim of the lake, with a depth of 140 m. Constraints on per- mechanisms, like varying porosity, may also have an influ- mafrost depth and past climate changes are derived from ence on the temperature profile, however, our modeling stud- numerical simulation of the thermal regime associated with ies imply a major contribution from recent climate changes. the lake-related talik structure. The thermal properties of the subsurface needed for these simulations are based on labo- ratory measurements of representative cores from the qua- ternary sediments and the underlying impact-affected rock, 1 Introduction complemented by further information from geophysical logs ◦ 0 and data from published literature. The crater Lake El’gygytgyn in NE Russia (67 30 N, ◦ 0 The temperature observations in the lake borehole 5011-1 172 5 E, 492 m above sea level) was formed by an Aster- are dominated by thermal perturbations related to the drilling oid impact 3.6 Myr ago (Fig. 1). Since it is believed to have process, and thus only give reliable values for the lowermost never been covered by ice during the glacial cycles since value in the borehole. Undisturbed temperature data recorded then, it provides the unique possibility to investigate Arctic over more than two years are available in the 140 m deep climate and environmental changes back to the impact. For land-based borehole 5011-3. The analysis of these observa- this reason, in 2008/2009 an interdisciplinary drilling cam- tions allows determination of not only the recent mean an- paign targeted this area. General information on this Interna- nual ground surface temperature, but also the ground sur- tional Continental Drilling Program (ICDP) project is given face temperature history, though with large uncertainties. Al- by Melles et al. (2011), while the most important paleocli- though the depth of this borehole is by far too insufficient for matological results were published by Melles et al. (2012). a complete reconstruction of past temperatures back to the In the course of the project, boreholes were drilled at Last Glacial Maximum, it still affects the thermal regime, and two sites, one near the deepest part of the lake (ICDP thus permafrost depth. This effect is constrained by numer- site 5011-1), and the other on land close to its shore- ical modeling: assuming that the lake borehole observations line (ICDP site 5011-3). From borehole 5011-1, 315 m of are hardly influenced by the past changes in surface air tem- sediment cores were retrieved, complemented by 200 m from the underlying volcanic bedrock, while from 5011-3 a Published by Copernicus Publications on behalf of the European Geosciences Union. 120 D. Mottaghy et al.: Lake El’gygytgyn site: thermal modeling a Siberia Alaska Fig. 2. Basin scheme of the Lake El’gygytgyn site with the digital elevation model in the background. The numerical model is based on this conceptual model by Melles et al. (2011). b crater rim 2007; Lemke et al., 2007; Serreze et al., 2007; Miller et al., 2010a,b; Serreze and Barry, 2011). When studying heat transfer in the Earth’s upper crust, the upper boundary condition of heat transport is determined by the local climatic conditions. The variations of this bound- 5011-1 ary condition induces a transient signal which diffuses into 5011-3 the subsurface. Thus, ground temperatures may be seen as an Lake El`gygytgyn archive of past climate signals. Reconstructing past changes is of major interest, since one of the most important com- ponents of climatic change is the variation of temperature at the Earth’s surface. However, the diffusive character implies that the older the signal is, the more it is attenuated, with N a corresponding larger uncertainty in magnitude and timing. Analyzing the variation of temperature with depth, past fluc- 18 km tuations at the Earth’s surface can be reconstructed to a cer- tain extent (see, e.g. the recent review by Gonzalez-Rouco´ Fig. 1. (a) Geographic position of the El’gygytgyn Crater in NE et al., 2009, and the references therein). This reconstruc- Russia (red box). (b) A Landsat image showing the crater (dotted tion can be performed by numerical forward modeling with line) and the drill sites 5011-1 and 5011-3. models of varying complexity, or inverse methods in the nar- row sense (amongst others, Shen and Beck, 1991; Beltrami and Mareschal, 1991; Beck et al., 1992; Rath and Mottaghy, 2007). A prerequisite for such a reconstruction is the avail- 141.5-m-long permafrost core composed of frozen deposits ability of sufficient data, the most relevant of which is tem- was recovered (Fig. 2). perature observations. Here, we limit ourselves to forward In this paper, we focus on the characterization of the ther- modeling for determining the present-day temperature pro- mal field beneath and around the lake, studying the influence file, because the available temperature data is not sufficient of variations of thermal properties and past ground surface for a reliable reconstruction by inversion of the ground sur- temperature changes using numerical modeling techniques face temperature history (GSTH) at the study area. Neverthe- (e.g. Osterkamp and Gosink, 1991; Galushkin, 1997; Mot- less, we propose that the combination of forward modeling taghy and Rath, 2006; Holmen´ et al., 2011). In the recent and sensitivity studies allow some meaningful statements to years, the impact of climate change on permafrost forma- be made concerning the local GSTH. tion and evolution has become a particular subject of inter- As our approach relies on forward modeling, we must as- est. The understanding of the response of permafrost regimes sume a certain GSTH entering the simulations. In particular, to transient changes in surface temperatures is a key issue it has been shown that the amplitude of the Last Glacial Max- regarding the prediction of the influence of global warm- imum (LGM) and the following warming from Pleistocene to ing on permafrost areas (amongst many others, Saito et al., Holocene influence the temperature distribution at all depths Clim. Past, 9, 119–133, 2013 www.clim-past.net/9/119/2013/ D. Mottaghy et al.: Lake El’gygytgyn site: thermal modeling 121 0 0 T 5.5 ºC 1 T2 100 T3 T4 water column 50 borehole 5011-3 T5 200 z(m) 300 100 25.1 ºC lake sediments total depth (m) 400 T avg 150 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 500 T (°C) 0 Breccia 2 0102030 temperature (ºC) 4 Fig. 4. Temperature logs in borehole ICDP site 5011-1. T5 is the 6 most reliable, and thus is used for estimating the temperature gradi- 8 ent (data by ICDP-OSG, 2009). 10 z(m) julian days 12 15 165 315 matic changes affect the ground surface temperature (Mot- 14 45 195 345 taghy and Rath, 2006; Rath and Mottaghy, 2007). This im- 75 225 365 plies that the reconstruction of past temperatures by forward 16 105 255 or inverse methods must include these processes. 18 -5135 285 0 20 -15 -10 -5 0 5 10 2 Available data T (°C) The data used for this study originate from the aforemen- Fig. 3. Selection of the temperatures logs in the permafrost borehole 5011-3 during the year 2010. Note the different scales in the two tioned boreholes, one in the deepest part of the lake (ICDP panels. site 5011-1, depth 517 m below lake bottom), and one close to its shoreline (ICDP site 5011-3, depth 140 m below sur- face). Their locations are shown in Fig. 2, which provides a schematic sketch of the basin. Our investigations rely on up to the surface (Rath et al., 2012). In the arctic region the (a) temperature measurements in both boreholes; and (b) de- values of this amplitude are not well known, but the avail- termination of thermal properties from laboratory measure- able data suggest a 18 ± 7 K cooler mean temperature than ments on cores and γ -ray logging in borehole 5011-1. today (Miller et al., 2010a,b). There have been some efforts The temperature data from the lake borehole ICDP site to determine the spatial distribution of this parameter (De- 5011-1 do not represent steady-state conditions.