Clim. Past, 11, 1–12, 2015 www.clim-past.net/11/1/2015/ doi:10.5194/cp-11-1-2015 © Author(s) 2015. CC Attribution 3.0 License. Glacier response to North Atlantic climate variability during the Holocene N. L. Balascio1,2, W. J. D’Andrea1, and R. S. Bradley3 1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA 2Department of Geology, The College of William & Mary, Williamsburg, VA 23187, USA 3Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA Correspondence to: N. L. Balascio ([email protected]) Received: 16 April 2015 – Published in Clim. Past Discuss.: 27 May 2015 Revised: 23 October 2015 – Accepted: 27 October 2015 – Published: Abstract. Small glaciers and ice caps respond rapidly to cli- Atlantic Ocean and periods of regional ice cap expansion, mate variations, and records of their past extent provide in- which confirms their regional significance and indicates that formation on the natural envelope of past climate variabil- considerable glacier activity on these timescales is a normal ity. Millennial-scale trends in Holocene glacier size are well feature of the cryosphere. The data provide a longer-term per- documented and correspond with changes in Northern Hemi- spective on the rate of 20th century glacier retreat and indi- sphere summer insolation. However, there is only sparse and cate that recent anthropogenic-driven warming has already fragmentary evidence for higher-frequency and continuous impacted the regional cryosphere in a manner outside the nat- variations in glacier size because in many Northern Hemi- ural range of Holocene variability. sphere regions glacier advances of the past few hundred years were the most extensive and destroyed the geomorphic evi- dence of ice growth and retreat during the past several thou- sand years. Thus, most glacier records have been of lim- 1 Introduction ited use for investigating centennial-scale climate forcing and feedback mechanisms. Here we report a continuous record of Glaciers and ice caps represent a small but important portion 2 glacier activity for the last 9.5 ka from southeast Greenland, of the cryosphere (∼ 785 000 km ; Dyurgerov and Meier, derived from high-resolution measurements on a proglacial 2005). Their mass wasting during the 20–21st century is lake sediment sequence. Physical and geochemical param- responsible for 60 % of the sea-level rise unattributable to eters show that the glaciers responded to previously docu- ocean warming (Meier, 2007) and they continue to retreat mented Northern Hemisphere climatic excursions, including at an exceptional rate (Zemp et al., 2012). Moreover, be- the “8.2 ka” cooling event, the Holocene Thermal Maximum, cause small glaciers and ice caps respond rapidly to climate Neoglacial cooling, and 20th century warming. In addition, changes and there is a strong relationship between glacier the sediments indicate centennial-scale oscillations in glacier mass balance and summer temperature (Oerlemans, 2005), size during the late Holocene. Beginning at 4.1 ka, a series of past glacier extent can inform us about past climate variabil- abrupt glacier advances occurred, each lasting ∼ 100 years ity. and followed by a period of retreat, that were superimposed Holocene glacier activity in the Arctic is reasonably on a gradual trend toward larger glacier size. Thus, while well documented at millennial timescales (Miller et al., declining summer insolation caused long-term cooling and 2010). Northern Hemisphere glaciers receded in the early glacier expansions during the late Holocene, climate sys- Holocene and were smaller than present during the mid- tem dynamics resulted in repeated episodes of glacier expan- Holocene. Centennial-scale variations, however, are not well sion and retreat on multi-decadal to centennial timescales. constrained because there are few high-resolution and con- These episodes coincided with ice rafting events in the North tinuous records, and because in many regions the most ex- tensive glacier advances since the early Holocene took place Published by Copernicus Publications on behalf of the European Geosciences Union. 2 N. L. Balascio et al.: Glacier response to North Atlantic climate variability within the past few hundred years, destroying geomorphic evidence of intervening glacier positions. This is generally the case in Greenland, where historical (AD 1200–1940) ad- vances of local glaciers were generally the most extensive since at least the early Holocene (Kelly and Lowell, 2009). Evidence from the Greenland Ice Sheet (Kobashi et al, 2011), marine sediments (Bond et al., 1997; Thornalley et al., 2009; Moffa-Sánchez et al., 2014 TS1 ; Jiang et al., 2015 TS2 ), and terrestrial archives (D’Andrea et al., 2011; Larsen et al., 2012; Olsen et al., 2012) indicate that abrupt changes in atmospheric circulation and ocean dynamics, including abrupt cooling events, have punctuated the Holocene. These episodes have alternately been attributed to solar variabil- ity, freshwater forcing, volcanic activity, and/or changes in Atlantic Meridional Overturning Circulation (Wanner et al., 2011). How sensitive were glaciers to these abrupt episodes, and did glaciers throughout the North Atlantic respond uni- Figure 1. Location and catchment setting of Kulusuk Lake. The formly? During the period from AD 1250–1900, often re- white dashed line marks the watershed boundary and the red dashed ferred to as the “Little Ice Age”, well-resolved records from line defines the crest of moraines in front of both glaciers mapped the North Atlantic region suggest coherence in ice cap activ- in the field. Cores were collected in the deepest basin (red circle). ity that was potentially driven by volcanic activity coupled Image: Google, NASA. with sea-ice/ocean feedbacks (Miller et al., 2012). However, prior to the last 1000 years there are sparse data for use in in- vestigating the synchrony of glacier response to climate vari- 2 Study site ability in the North Atlantic region. Lakes that receive meltwater from temperate glaciers can Kulusuk Lake (0.8 km2, 69 m maximum depth) is located be- be used to develop continuous records of glacier activity. low a cirque with two small glaciers and is within a low arctic Bedrock erosion at the base of glaciers provides sediment maritime region (MAT −1 ◦C, MAP 900 mm). Characteris- supply for meltwater transport to proglacial lakes. In catch- tic erosional features indicate that local glaciers have tem- ments where other sources of sediment are limited, such perate thermal structures (Humlum and Christiansen, 2008). as from mass wasting, paraglacial effects, or the release of Distinct moraines defined by sharp crests are located in front stored sediment, there is a strong relationship between sed- of both glaciers, and the recently glaciated area accounts for iment properties and glacier size (Nesje et al., 2000; Dahl ∼ 50 % of the catchment, which is composed of Archaen et al., 2003; Jansson et al., 2005), which also follows from gneisses (Bridgwater, 1976) (Fig. 1). Kulusuk Lake is ideally the assumption that large glaciers produce more minerogenic situated to capture and preserve a clear sedimentary record of material and meltwater than small glaciers. Measurements glacier activity because (i) it only receives runoff from a very of physical and geochemical properties of proglacial lake small catchment (catchment : lake area ratio of ∼ 2 V 1), mini- sediments can therefore be used to reconstruct records of mizing the potential for long-term storage of sediments prior past glacier size. Here we report a continuous 9.5 ka record to deposition and limiting sediment input from non-glacial of glacier activity on Kulusuk Island, southeast Greenland processes, (ii) the proximity of the glaciers to the lake results developed using sediment cores recovered from Kulusuk in minimal sediment transport distance, and (iii) the small Lake (65.56◦ N, 37.11◦ W; 202 m) (Fig. 1). We character- size of the glaciers makes them sensitive to minor climate ize changes in sedimentation using measurements of phys- variations (Fig. 1). Therefore, bedrock erosion by the glaciers ical sediment properties, including the following: bulk den- provides the primary source of minerogenic sediment to the sity, organic matter content, magnetic susceptibility, and ac- lake and changes in glacier size should clearly be reflected in cumulation rates. We also measured the relative elemental sediment properties. compositions of the sediment using scanning X-ray fluores- cence (XRF) to characterize minerogenic changes at higher 3 Methods resolution and with greater sensitivity. These data provide de- tailed information on sedimentation in Kulusuk Lake related 3.1 Sediment core collection and analysis to glacier input. Sediment cores were recovered from Kulusuk Lake in April 2010 when the lake was ice covered. Bathymetric mea- surements were made manually through holes drilled in the ice, and sediment cores were collected using Uwitec grav- Please note the remarks at the end of theClim. manuscript. Past, 11, 1–12, 2015 www.clim-past.net/11/1/2015/ N. L. Balascio et al.: Glacier response to North Atlantic climate variability 3 ity and percussion coring devices from the deepest location, from 150 to 5 ppm (Croudace et al., 2006). All of the cores which has a water depth of 69 m. A composite 3.5 m record were scanned at 200 µm intervals with an exposure time of was compiled by matching the physical stratigraphy and 10 s, voltage of 30 kV, and current of 55 nA. scanning XRF profiles from a 26