Holocene optimum events inferred from subglacial sediments at Tschierva Glacier, Eastern Swiss Alps

Abstract

This study investigates the subglacial sedimentary archive at Tschierva Glacier, Eastern Swiss Alps. Subfossil wood remains found at the retreating glacier tongue indicate that their emergence results from recent transport from an upvalley basin. A confluence-basin-like structure was found to exist by georadar measurements underneath the present glacier. In combination with high resolution age determinations based on dendrochronology and radiocarbon dating it is implied that a retreated Tschierva Glacier allowed vegetation growth and sediment accumulation in that basin. Three periods of glacier recession were detected, which occurred around 9200 cal yr BP, from 7450 to 6650 cal yr BP and from 6200 to 5650 cal yr BP. These periods are called Holocene optimum events (HOE). Accordingly, an equilibrium line rise >220 m compared to the reference period from 1960 to 1985 was inferred from digital elevation models of former glacier extents. Since glacier mass balance depends on summer (June–July–August) temperature and precipitation, an equilibrium line altitude (ELA) rise of 220 m implies a summer temperature increase of about 1.8 °C assuming unchanged precipitation during the dated HOE. Alternative calculations point to probable temperature increase in a broad interval between +1.0 °C taking into account a precipitation change of −250 mm/a to +2.5 °C with +250 mm/a precipitation change, supporting earlier paleotemperature estimates. It is proposed that higher mean summer insolation caused a stronger seasonality during the mid-Holocene as compared to late Holocene conditions.

Introduction

A broadly stable Holocene climate is revealed by studies on oxygen isotopes as a proxy of annual temperature in Greenland ice cores (Johnsen et al., 1997) and northern Alpine lake sediments (von Grafenstein et al., 1999). However a growing number of studies (Mayewski et al., 2004 and references there) have demonstrated that climatic variations occurred repeatedly throughout the Holocene on a multi-centennial scale. Typically, a change to cooler conditions is associated with glacier advances and deposition of moraines. The impact of such events was previously reported by Denton and Karlén (1973) and since then in more detailed studies from many mountain ranges, e.g. in Scandinavia (Matthews et al., 2000, Matthews et al., 2005; Nesje and Dahl, 2003), in North America (Calkin et al., 2001; Reyes and Clague, 2004; Miller et al., 2005), the Alps (Furrer, 1991; Nicolussi and Patzelt, 2000b). However, information is generally sparse on periods of glacier retreat, because subsequent glacier advances overrode smaller moraines resulting in a tendency to overestimate the frequency and magnitude of glacier advances. Additional information is archived in lake sediments, the analysis of which could help to overcome these difficulties (Dahl et al., 2003). For example, in the Alps, lake sediments have provided insights into the Holocene environmental conditions based on physical parameters (Leemann and Niessen, 1994), pollen (Tinner et al., 1996; Haas et al., 1998), tree-line studies (Tinner and Theurillat, 2003; Nicolussi et al., 2005) and chironomid assemblages (Heiri et al., 2003). Since the problem of identifying directly the extent of glacier retreat has persisted, the question of the full amplitude of Holocene glacier fluctuations remains open (Furrer, 1991). As a consequence, difficulties exist when glacier records are compared to other proxies or against the influence of forcing factors on longer time scales. Therefore, assessing glacier recessions, in particular the amplitude of glacier fluctuations is an important paleoclimate issue.

Alpine glaciers adapt their geometry, particularly glacier length, in response to variations in climate (Meier and Post, 1962; Oerlemans, 2005). Indeed, the ongoing rapid retreat of glaciers is well documented (WGMS, 1998; Dyurgerov, 2002) and is one of the consequences of recent global warming (IPCC and Climate Change, 2007). Field experiments and modelling studies have demonstrated that changes in glacier length mainly reflect variations in summer temperatures (Kerschner, 1997; Oerlemans, 2001; Vincent, 2002). Thus a recent study by Oerlemans (2005) demonstrated that measured glacier length records are an independent indicator of temperatures for the last 500 yr. The expansion further back in time requires reconstructions of glacier extent based on the geologic record. In the Alps, earlier studies concentrated on the dating of lateral or terminal moraines, which are associated with glacial advances during cold periods (Röthlisberger, 1986). Due to the discontinuous deposition of moraines such reconstructions are incomplete, in particular with regard to the extent and timing of glacier recessions. To overcome this problem, the discovery of wood and peat fragments associated with meltwater outburst events directed attention to the paleoclimate record of glacier fluctuations archived in subglacial basins (Nicolussi and Patzelt, 2000a; Hormes et al., 2001). Although the abundance of dating has improved the correlation of different records towards a general chronology (Joerin et al., 2006), the extent of glaciers during recessions has resisted a conclusive answer.

Here, we examine the Holocene recessions of Tschierva Glacier (eastern Swiss Alps) to provide a first constraint on minimum glacier extent that leads to a first-order estimate of the amplitudes of past glacier fluctuations. The exceptional discovery of well-preserved wood samples at Tschierva Glacier allows us to establish a dendrochronology of early Holocene events. We infer the area of original tree growth and subsequent subglacial preservation from georadar measurements and glacier bed topography in combination with stratigraphic evidence from the samples. Based on these results we reconstruct former glacier extents, which we use to calculate equilibrium line altitudes (ELA) in comparison to reference conditions from 1965 to 1985. Finally, the results are tested against independent reconstructions of paleotemperatures and discussed in the context of studies on natural climate variability during the Holocene.