ICE SHEET SOURCES OF SEA LEVEL RISE AND FRESHWATER DISCHARGE DURING THE LAST DEGLACIATION Anders E. Carlson1,2 and Peter U. Clark3 Received 22 August 2011; revised 22 September 2012; accepted 25 September 2012; published 22 December 2012. [1] We review and synthesize the geologic record that of which 2–7 m represents an excess contribution above that constrains the sources of sea level rise and freshwater dis- derived from ongoing ice sheet retreat. Widespread retreat of charge to the global oceans associated with retreat of ice Antarctic ice sheets began at 14.0–15.0 ka, which, together sheets during the last deglaciation. The Last Glacial Maxi- with geophysical modeling of far-field sea level records, sug- mum (26–19 ka) was terminated by a rapid 5–10 m sea gests an Antarctic contribution to this meltwater pulse as level rise at 19.0–19.5 ka, sourced largely from Northern well. The cause of the subsequent Younger Dryas cold event Hemisphere ice sheet retreat in response to high northern lat- can be attributed to eastward freshwater runoff from the Lake itude insolation forcing. Sea level rise of 8–20 m from 19 to Agassiz basin to the St. Lawrence estuary that agrees with 14.5 ka can be attributed to continued retreat of the Lauren- existing Lake Agassiz outlet radiocarbon dates. Much of the tide and Eurasian Ice Sheets, with an additional freshwater early Holocene sea level rise can be explained by Laurentide forcing of uncertain amount delivered by Heinrich event 1. and Scandinavian Ice Sheet retreat, with collapse of Lauren- The source of the abrupt acceleration in sea level rise at tide ice over Hudson Bay and drainage of Lake Agassiz basin 14.6 ka (meltwater pulse 1A, 14–15 m) includes contri- runoff at 8.4–8.2 ka to the Labrador Sea causing the 8.2 ka butions of 6.5–10 m from Northern Hemisphere ice sheets, event. Citation: Carlson, A. E., and P. U. Clark (2012), Ice sheet sources of sea level rise and freshwater discharge during the last deglaciation, Rev. Geophys., 50, RG4007, doi:10.1029/2011RG000371. 1. INTRODUCTION with periods of reduced Atlantic meridional overturning circulation (AMOC) and a colder climate in the North [2] Ever since the landmark study of Fairbanks [1989] on the last deglacial relative sea level (RSL) history from Bar- Atlantic region such as the Younger Dryas cold period from bados, paleoclimatologists have debated the timing, rates, 12.9 to 11.7 ka [Boyle and Keigwin, 1987; Broecker, and ice sheet sources of sea level rise (Figure 1). In partic- 1990; McManus et al., 2004], which, if these events origi- ular, Fairbanks [1989] identified two intervals during the nated from Northern Hemisphere ice sheets, questioned the last deglaciation where sea level rise accelerated, which he commonly held hypothesis that increased meltwater dis- termed meltwater pulses (MWPs) 1A and 1B, that occurred charge to the North Atlantic reduced the AMOC [Birchfield at 14.6 and 11.3 ka, respectively [Bard et al., 1990]. What and Broecker, 1990; Manabe and Stouffer, 1995; Stocker became readily apparent, however, was that these two and Wright, 1991]. Alternatively, retreat of the Antarctic intervals of more rapid ice sheet melting did not correspond Ice Sheet (AIS) may have contributed a significant compo- nent of one or more of these MWPs [Clark et al., 1996; Peltier, 1994], or the relationship between meltwater dis- charge and AMOC may not be as sensitive as climate models 1Department of Geoscience and Center for Climatic Research, University of Wisconsin–Madison, Madison, Wisconsin, USA. suggest [Stanford et al., 2006; Tarasov and Peltier, 2005]. 2Now at College of Earth, Ocean, and Atmospheric Sciences, Oregon [3] A different mechanism that could explain reductions State University, Corvallis, Oregon, USA. in AMOC and North Atlantic cold events involves the 3College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA. location of continental runoff discharge relative to regions of deep water formation. In another landmark study building Corresponding author: A. E. Carlson, College of Earth, Ocean, and on previous considerations [Johnson and McClure, 1976; Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA. ([email protected]) Rooth, 1982], Broecker et al. [1989] suggested that routing ©2012. American Geophysical Union. All Rights Reserved. Reviews of Geophysics, 50, RG4007 / 2012 1of72 8755-1209/12/2011RG000371 Paper number 2011RG000371 RG4007 RG4007 CARLSON AND CLARK: SEA LEVEL RISE AND FRESHWATER DISCHARGE RG4007 [4] In the 23 years since the Fairbanks [1989] and Broecker et al. [1989] studies, a large amount of new data have become available that better constrain the timing and magnitude of sea level rise as well as ice sheet margin and runoff histories. Similarly, significant advances in climate and ice sheet modeling allow for further testing of hypoth- eses on the relationship between MWPs, runoff routing, AMOC strength, and abrupt climate change. Here we review the RSL records for the last deglaciation and the geologic data constraining potential ice sheet sources of sea level rise, focusing on periods of rapid sea level rise at 19 ka and during MWP-1A, MWP-1B, and the early Holocene.We also examine the history of meltwater discharge and fresh- water routing for Northern Hemisphere and Antarctic ice sheets and their relationship to periods of abrupt climate change such as the Oldest and Younger Dryas cold events, the Bølling and Allerød warm periods, and the 8.2 ka event. [5] The term “eustatic” sea level has conventionally been used to refer to a uniform global sea level change either due to changes in the volume of water in the world oceans or net changes in the volume of the ocean basins. Because these are two separate controls on sea level (albeit a change in the volume of water affects the volume of the ocean basin through isostasy), however, referring to sea level change without distinguishing between the controls is ambiguous. Moreover, several factors cause regional sea level to differ from the global mean on a range of time scales [Milne et al., 2009], so the concept of a uniform rise of sea level is invalid. “ ” Figure 1. Ice sheet extent at the LGM [Clark and Mix, In the Birch Lecture titled Eulogy for eustasy presented at 2002; Denton et al., 2010] with ice sheets discussed in text the 2009 AGU meeting, Jerry Mitrovica recommended that labeled by their first initial. Ice also covers Greenland and the term “eustatic” no longer be used. We follow this rec- British Isles, but these are not labeled nor discussed as they ommendation, and we use instead the term global mean sea only contributed a small fraction of the sea level change level (GMSL) to refer to spatially averaged sea level that across the last deglaciation [Clark and Mix, 2002]. Also reflects the combined signals of (largely) mass and basin shown are the LIS runoff outlets (arrows), portions of the volume. To specifically identify the ice sheet volume con- LIS discussed (numbered as follows: 1, James and Des tribution to GMSL, we refer to ice-equivalent sea level Moines lobes; 2, Green Bay, Lake Michigan, and Lake [Yokoyama et al., 2000]. Huron lobes; 3, New England; and 4, Canadian Arctic Archi- [6] Of the various factors that contribute to regional sea pelago), and locations of relative sea level records (numbered level variability on glacial-interglacial time scales, the glacial as follows: 5, Barbados; 6, Bonaparte Gulf; 7, Sunda Shelf; isostatic adjustment (GIA) process had the largest influence 8, New Guinea; 9, Tahiti; and 10, Ireland Coast). and caused sea level at nearly any location in the world’s oceans to differ from the global mean during the last deglaci- of North American runoff from the Mississippi River to the ation [Clark et al., 1978; Lambeck and Chappell,2001;Milne St. Lawrence River (Figure 1) from retreat of the southern and Mitrovica, 2008] (Figure 2). RSL records thus cannot be Laurentide Ice Sheet (LIS) north of the Great Lakes reduced compared directly to each other without accounting for GIA AMOC and caused the Younger Dryas cold event. Such effects. We further illustrate this by comparing RSL at far- routing events do not necessarily cause changes in sea level field sites to ice-equivalent sea level derived from a model and may have caused other deglacial AMOC reductions (Figure 3a) [Bassett et al., 2005], which shows that RSL at [Clark et al., 2001; Obbink et al., 2010; Thornalley et al., any particular site differs relative to each other as well as to 2010]. This routing hypothesis with respect to the Youn- the global mean, particularly in the earlier half of the dela- ger Dryas (and other cold events as well) has been ques- ciation. We then adjust the RSL data for GIA (Figure 3c) tioned based on interpretations of St. Lawrence salinity using a model prediction of RSL for each site (Figure 3b) proxies [de Vernal et al., 1996; Keigwin and Jones, 1995], [Bassett et al., 2005], which now shows good agreement the southern LIS margin chronology [Fisher et al., 2009; between the adjusted sea level data and ice-equivalent sea Lowell et al., 2009], and ice sheet model simulations level. [Tarasov and Peltier, 2005], posing a fundamental problem [7] This review discusses the major intervals or episodes of what caused deglacial AMOC reductions if not MWPs or of sea level rise and freshwater discharge through the last routing events [Broecker, 2006; Steig, 2006]. deglaciation, with a broad summary of geological data 2of72 RG4007 CARLSON AND CLARK: SEA LEVEL RISE AND FRESHWATER DISCHARGE RG4007 Younger Dryas cold interval, most of which involve an increase in freshwater flux either from routing or a flood.