GEOLOGY | January 2014 | www.gsapubs.org 75 INTRODUCTION Pronounced ongoing changes of the Green- land Ice Sheet have important ramifications for global climate and sea-level change (Alley et al., 2010). Predicting future Greenland Ice Sheet change is challenging, however, because the spatial and temporal patterns of recent and ongoing ice sheet response to Arctic warming are highly variable (e.g., Joughin et al., 2010). Reconstructions of past ice margin changes can constrain the spatial expression of Greenland Ice Sheet response to climate variability on lon- ger time scales (Long, 2009). The Holocene is an important period over which to evaluate ice margin response to climate change because the temperature history is relatively well known and includes conditions that were warmer than the 20 th century during the Holocene thermal maxi- mum (e.g., Kaufman et al., 2004). The overall pattern of ice sheet retreat during early Holocene warming followed by ice sheet advance during late Holocene cooling is generally known from field and modeling results (e.g., Funder et al., 2011). However, reconstructing the timing of maximum ice sheet retreat during warm times in the Holocene is challenging because the geo- logic record of former ice margins is mostly obscured by subsequent ice advances during the late Holocene Neoglacial period (Kelly, 1980; Long et al., 2009). Varieties of approaches have been used to re- construct past glacier change, but few tools are available for reconstructing smaller-than-present ice extents. Sediments in adjacent marine and lake basins have been used successfully to re- construct past ice margin changes, but ideal sites are sparse (e.g., Briner et al., 2010; Colville et al., 2011; Larsen et al., 2011). Numerical mod- els driven by paleoclimate reconstructions of- fer insight into interglacial ice sheet extent (e.g., Simpson et al., 2009), but these are often poorly constrained by geologic data. Additional evidence for the former extent of smaller-than- present ice margin positions can be gleaned from marine faunal remains that have been reworked by glaciers that readvanced across marine sediment, as has been described for the Greenland Ice Sheet during the late Holocene (e.g., Tarr, 1897; Weidick and Bennike, 2007; Bennike, 2008). Radiocarbon dating of re- worked shells and bones of marine animals has been used to date the time of smaller-than-pres- ent ice sheet extent (e.g., Weidick et al., 2004; Weidick and Bennike, 2007). The great abundance of reworked marine fos- sils in deposits that fringe much of the Green- land Ice Sheet attests to both the extent and timing of former ice-free marine environments. However, radiocarbon dating is a cost-limiting method for generating the large number of ages needed to determine the age-population distri- bution for the plethora of reworked material. Here we report a new approach to dating a large suite of reworked bivalve shells collected from around Greenland’s perimeter to constrain ice margin positions through the Holocene (Fig. 1). The cost-effective technique is based on the enantiomeric composition of amino acids pre- served in mollusk shells. We used a mathemati- cal procedure to model the relationship between the extent of aspartic acid (Asp) racemization and radiocarbon age from three different sites spanning western Greenland. We then compare the populations of Asp-inferred shell ages with independent climate and ice margin reconstruc- tions and discuss how amino acid geochronol- ogy can be used to constrain the ice margin his- tory of western Greenland. Amino acid ratios in reworked marine bivalve shells constrain Greenland Ice Sheet history during the Holocene Jason P. Briner 1 , Darrell S. Kaufman 2 , Ole Bennike 3 , and Matthew A. Kosnik 4 1 Department of Geology, University at Buffalo, State University of New York, Buffalo, New York 14260, USA 2 School of Earth Sciences & Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA 3 Geological Survey of Denmark and Greenland, Øster Voldgade 10, 1350 Copenhagen-K, Denmark 4 Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia ABSTRACT Reconstructions of ice sheet fluctuations during the Holocene, which encompassed cooler and warmer conditions than those that are captured in the historic record, help to elucidate ice margin sensitivity to climate change. We used amino acid geochronology to constrain the history of the western Greenland Ice Sheet margin during intervals of relative warmth in the middle Holocene. We measured the extent of amino acid racemization in 251 ice sheet– reworked marine bivalve shells from three locations spanning western Greenland. A signifi- cant relationship between shell age and the ratio of aspartic acid (Asp) isomers (Asp D/L) was revealed using Bayesian model fitting on 20 radiocarbon-dated shell fragments. The range of Asp-inferred bivalve ages at each site corresponds well with independent records of early Holocene ice retreat and late Holocene ice advance. Furthermore, the frequency of Asp- inferred bivalve ages from the three widely separated locations is nearly identical, with most ages between 5 and 3 ka, coinciding with optimum oceanic conditions. Because ice margin changes in western Greenland are tightly linked with oceanographic conditions, the distri- bution of reworked bivalve ages provides important information about relative ice margin position during smaller-than-present ice sheet configurations. This approach adds a new chro- nometer to our toolkit for constraining smaller-than-present ice sheet configurations and may have wide applicability around Greenland. GEOLOGY, January 2014; v. 42; no. 1; p. 75–78; Data Repository item 2014017 | doi:10.1130/G34843.1 | Published online 22 November 2013 © 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Figure 1. A: Greenland and three ice margin locations where we col- lected reworked marine bivalve shells from his- torical moraines. B–D: Il- lustrations showing how advances of Greenland Ice Sheet into marine set- tings can rework fossil- iferous marine sediment into Neoglacial moraines. on January 5, 2014 geology.gsapubs.org Downloaded from