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Glacier Extent During the Younger Dryas and 8.2-ka Event on Baffin Island, Arctic Canada

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Science  14 Sep 2012:
Vol. 337, Issue 6100, pp. 1330-1333
DOI: 10.1126/science.1222759

Abstract

Greenland ice cores reveal that mean annual temperatures during the Younger Dryas (YD) cold interval—about 12.9 to 11.7 thousand years ago (ka)—and the ~150-year-long cold reversal that occurred 8.2 thousand years ago were ~15° and 3° to 4°C colder than today, respectively. Reconstructing ice-sheet response to these climate perturbations can help evaluate ice-sheet sensitivity to climate change. Here, we report the widespread advance of Laurentide Ice Sheet outlet glaciers and independent mountain glaciers on Baffin Island, Arctic Canada, in response to the 8.2-ka event and show that mountain glaciers during the 8.2-ka event were larger than their YD predecessors. In contrast to the wintertime bias of YD cooling, we suggest that cooling during the 8.2-ka event was more evenly distributed across the seasons.

Summit Greenland ice cores record several abrupt cold excursions that occurred throughout the last glacial period and into the early Holocene (1, 2). Characterized by rapid onset and termination times, the Younger Dryas (YD) and the excursion that occurred 8.2 thousand years ago (ka) are two of the most dramatic examples of abrupt climate change, and consequently their intra- and interhemispheric signature has been the subject of intense focus [e.g., (3, 4)]. Characterizing the extent and magnitude of abrupt climate events beyond their central Greenland stratotypes is critical for understanding the mechanisms that drive abrupt climate change and for understanding how regional climate-system changes are transmitted globally via atmospheric and oceanic teleconnections.

The expression of the YD outside central Greenland has proven particularly puzzling. Even on Greenland, independent terrestrial proxy records that track changes in summer climate do not replicate the extreme temperature depression of the YD depicted in Greenland ice [e.g., (5)]. This disparity has led to the hypothesis that the YD and other abrupt cold reversals in the North Atlantic region were typified by strong seasonality, with cooling primarily restricted to the wintertime and only minimal to moderate summer cooling (6, 7). To test this hypothesis, proxies for terrestrial climate change need to be generated from elsewhere in the North Atlantic region.

Located directly adjacent to Greenland, Baffin Island hosts remnants of the Laurentide Ice Sheet (LIS) and numerous mountain glaciers whose late glacial to early Holocene histories have remained relatively unexploited compared with their counterparts in Europe [e.g., (8)]. An extensive moraine system deposited along eastern Baffin Island fjord heads was originally believed to demarcate the late Wisconsin maximum extent of the LIS (Fig. 1) (9, 10). Recent work, however, has demonstrated that during the late Wisconsin, the LIS extended to at least the fjord mouths and likely out onto the Baffin Bay continental shelf, with regional deglaciation commencing ~15 to 16 ka (11, 12). Thus, fjord-head moraines represent an advance of the LIS superimposed on deglaciation. Considered correlative with this ice limit are moraines that were deposited by local mountain glaciers that became independent of the LIS during deglaciation (10). Fjord-head moraines are currently dated with conventional radiocarbon ages from marine fauna that broadly constrain moraine deposition to ~8 to 9.5 thousand calibrated years before the present (cal yr B.P.) (Cockburn Substage) (13); this chronology has remained relatively unchanged for three decades. Here, we precisely date mountain-glacier moraines deposited during the Cockburn Substage on Baffin Island using 10Be surface exposure dating and compare our new chronology to 10Be- and 14C-dated fluctuations of nearby LIS outlet glaciers (Fig. 1).

Fig. 1

(A) Late Wisconsin extent of the LIS [dashed line; modified from (12, 35)] and the extent of the fjord-head Cockburn moraine system (9, 10, 13). Green and red on basemaps are lowest and highest elevations, respectively. (B) LIS Cockburn limit in central Baffin Island. AYR, Ayr Lake valley; CI, Clyde Inlet; SFF, Sam Ford Fiord; CF, Clyde Foreland. Red bull–s eyes mark the position of 8.2-ka ice-contact deltas in Sam Ford fjord and Clyde Inlet (Fig. 3).

Our chronology arises from mountain-glacier moraines deposited in Ayr Lake valley after Laurentide ice, which occupied the valley during the last glaciation, retreated during deglaciation. 10Be ages from the Clyde Foreland and Ayr Lake valley, calculated with a locally constrained 10Be production rate (14), indicate that the LIS retreated from the Clyde Foreland 14.1 ± 1.2 ka (n = 7 10Be ages; mean ± 1 SD), through Ayr Lake valley at 13.7 ± 0.1 ka (n = 3 10Be ages), and well inland of the study area by 12.7 ± 0.3 ka (n = 1 10Be ages) (Fig. 2) (14). Thus, tributary glaciers extending from adjacent ice caps flanking Ayr Lake valley disconnected from the LIS outlet glacier ~13.7 ± 0.1 ka. We dated boulders from two correlative moraines deposited in Ayr Lake valley by mountain glaciers during the Cockburn Substage (Fig. 2). Both moraines, ~20 km apart, have mature lichen cover and are situated immediately down-valley of unvegetated, fresh-appearing moraines that we assume were deposited during the late Holocene. 10Be ages from moraine boulders range from 17.2 ± 0.3 to 7.9 ± 0.2 ka (n = 13 10Be ages). Two boulder ages of 17.2 ± 0.3 and 14.2 ± 0.3 ka are >3 SD older than the remaining 10Be ages and likely contain 10Be inherited from previous exposure, which is widespread in this region outside of valley bottoms (12, 14). The remaining 11 10Be ages indicate that an advance of mountain glaciers in Ayr Lake valley culminated ~ 8.2 ± 0.2 ka (Figs. 2 and 3).

Fig. 2

(A) The Clyde Foreland with recessional ice limits (11) and Ayr Lake valley with 10Be ages (ka) (1 SD analytical uncertainty) that constrain retreat of the LIS outlet glacier through Ayr Lake valley. (B and C) 10Be ages from moraine boulders deposited by local mountain glaciers.

Fig. 3

(A) Ayr Lake valley 10Be ages plotted as distance from the valley head. The normal kernel density estimate depicts individual and summed 10Be ages from Ayr Lake moraine boulders (red) and boulders atop the Clyde Inlet ice-contact delta (gray). Results from central Baffin Island are compared to Greenland isotopic (North Greenland Ice Core Project) and elevation-corrected temperature records (27, 36) and to a local summer temperature reconstruction (Lake CF8) (19). (B and C) The full deglaciation histories of Sam Ford fjord and Clyde Inlet (1416). 14C ages that directly relate to 8.2-ka ice-contact deltas are highlighted and also presented in (A) (14).

Additional 10Be and 14C ages from the heads of Clyde Inlet and Sam Ford fjord constrain early Holocene fluctuations of the LIS (Figs. 1B and 3). In Clyde Inlet, 10Be ages from boulders atop an ice-contact delta range from 8.6 ± 0.6 to 7.5 ± 0.8 ka and have a mean age of 8.0 ± 0.4 ka (n = 7 10Be ages) (Fig. 3) (15). This age assignment agrees with independent 14C ages that bracket deposition of the ice-contact delta to between 8435 ± 30 and 7950 ± 45 cal yr B.P. (1 SD) (14). In Sam Ford fjord, numerous 10Be ages indicate that the majority of the fjord rapidly deglaciated 9.5 ± 0.3 ka but did not deglaciate completely until after ~7.7 ka (16). Radiocarbon ages of 8330 ± 30 and 8210 ± 190 cal yr B.P. from an ice-contact delta deposited during the Cockburn Substage mark the timing of a readvance by the LIS outlet glacier occupying Sam Ford fjord (14).

Taken together, 10Be and 14C ages from Ayr Lake valley, Clyde Inlet, and Sam Ford fjord indicate that a synchronous advance of mountain glaciers and LIS outlets occurred between ~8.3 and 8.0 ka, probably driven by the 8.2-ka abrupt cold reversal displayed in Greenland ice cores (Fig. 3). In some locations, several distinct ice limits associated with the Cockburn Substage are identifiable (e.g., Sam Ford fjord) (17), indicating that the 8.2-ka event glacier response may be superimposed upon a broader pattern of glacier advance and retreat between ~9.5 and 8 thousand cal yr B.P. whose climatic importance, if any, remains unclear. Nonetheless, the 8.2-ka event cooling was sufficient to trigger a widespread response of eastern Baffin Island glaciers, perhaps correlative with early Holocene advances of the western Greenland Ice Sheet (18). In addition, the 8.2-ka event triggered an advance of the eastern LIS indicating that ice sheets are capable of an extremely rapid glaciological response (i.e., centennial-scale or less) to a short-lived climate perturbation.

Because the 8.2-ka moraines rest directly on the Ayr Lake valley floor that deglaciated ~14 to 13 ka, 8.2-ka event moraines mark the most extensive limit of local mountain glaciers since ~14 ka. Therefore, our chronology indicates that mountain glaciers were larger during the 8.2-ka event than during the YD. Although our chronology does not preclude a YD-triggered advance of Ayr Lake mountain glaciers, the morphostratigraphic relationship between the 8.2-ka moraines and the Ayr Lake valley floor requires glaciers during the YD to have been less extensive than they were during the 8.2-ka event. The larger response of Baffin Island glaciers to the 8.2-ka event relative to the YD is surprising considering the significantly longer duration and greater amplitude of YD temperature change compared with the 8.2-ka event.

The absence of a distinct YD moraine on Baffin Island is not necessarily unexpected. For example, unequivocal YD moraines in the Northern Hemisphere have not been identified outside northern Europe, and in the Southern Hemisphere mid-latitudes, glaciers appear to have advanced before, and then retreated during, the YD interval (4, 19). On eastern Greenland, a major moraine complex brackets the YD but, perhaps more importantly, indicates that YD summertime cooling was only 3.9° to 6.6°C relative to today (7), much less than the ~15°C mean annual cooling recorded in central Greenland ice cores by gas-fractionation paleothermometry (2).

An independent proxy record from Lake CF8 in our study area indicates that summer temperatures during the 8.2-ka event lowered by ~3.5°C (20) (Figs. 2 and 3). At high northern latitudes, ~90% of the variability in glacier mass balance is controlled by ablation-season (summer) temperature (21), and thus summertime cooling of ~3.5°C during the 8.2-ka event helped drive eastern Baffin Island mountain glaciers to advance beyond their YD limit. Precipitation rates during the YD, however, were up to ~50 to 100% less than early Holocene values (22, 23) and likely contributed to a restricted YD ice extent. Because a ~40 to 50% change in precipitation is equivalent to a ~1°C temperature change (24, 25), we constrain the magnitude of summer cooling during the YD on eastern Baffin Island to have been no more than ~4.5° to 5.5°C. This level of summer cooling during the YD is consistent with strong seasonality when considering the ~15°C mean annual cooling recorded in Greenland ice cores (6).

In contrast, the 3.5°C of summer cooling recorded on Baffin Island during the 8.2-ka event is indistinguishable from central Greenland mean annual temperatures depicting a peak 8.2-ka temperature depression of ~3° to 4°C (26, 27) (Fig. 3). Furthermore, terrestrial proxy records and model simulations of the 8.2-ka event climate [e.g., (3)] typically assert that maximum cooling in the North Atlantic region occurred downstream (east) of the 8.2-ka event’s epicenter in the Labrador Sea due to westerly atmospheric and oceanic circulation patterns. Deposition of the Cockburn moraines on Baffin Island, combined with the independent summer temperature reconstruction from the Clyde Foreland, indicates that significant summer cooling during the 8.2-ka event extended west of the Labrador Sea. Unlike YD cooling, cooling during the 8.2-ka event included a significant summer component, and we suggest that the difference in mountain-glacier response between the YD and the 8.2-ka event was due to contrasting seasonality. What remains unclear, however, is the driving mechanism(s) that would allow for the 8.2-ka event to have a proportionally stronger summer-based regional cooling signature than the YD, because both cold reversals are thought to have shared similar reorganizations of North Atlantic thermohaline circulation and concomitant expansion in sea-ice coverage (3, 6, 28).

One explanation may involve each cold period’s triggering mechanism. The 8.2-ka event is linked to the catastrophic drainage of Laurentide-dammed lakes and routing of meltwater through the Hudson Strait directly into the Labrador Sea (29). Accordingly, the sharp isotopic onset of the 8.2-ka event in Greenland ice cores is consistent with rapid North Atlantic freshening. The origin of the YD, however, remains debated [e.g., (28)]. Although one of several possible YD triggers invokes the sudden release of North American meltwater into the North Atlantic, no geomorphic evidence lending support to this hypothesis has been identified (30), and compared to the 8.2-ka event’s onset, the beginning of the YD is less notably abrupt and succeeds a millennial-scale cooling trend (28). It has also been suggested that a Heinrich ice armada outburst (i.e., Heinrich event H0) occurred in Baffin Bay at the onset of the YD (31). However, because multiple Heinrich events occurred through the last glacial period, a Heinrich-related YD triggering mechanism would indicate that the YD is not the product of a one-time catastrophic event (28) but rather the result of a reoccurring pacemaker. Moreover, stalagmite δ18O records suggest that the YD and previous YD-like events are inherent, nonstochastic components of ice-age terminations (32).

Each cold reversal’s unique isotopic expression suggests differing causations and possibly different hemispheric climatic imprints. The sudden influx of freshwater into the Labrador Sea resulted in significant summer cooling in the Baffin Bay region during the 8.2-ka event, whereas an alternative YD triggering mechanism may have led to comparatively restricted YD summer cooling in Baffin Bay. Indeed, a recent climate model simulation of the 8.2-ka event depicts a strengthened North Atlantic subpolar gyre with maximum sea-surface cooling occurring along the western edge of the gyre (33). Furthermore, an open Nares Strait channeling frigid Arctic waters into the region, and multiple pre–8.2-ka freshwater outbursts into the Labrador Sea, likely resulted in a preconditioned Baffin Bay climate system that crossed a threshold during the 8.2-ka event, resulting in strong regional cooling (34).

Our results provide direct age limits for the Cockburn moraine system deposited by the LIS and local mountain glaciers and highlight the importance of generating regional records of climate variability spanning intervals of abrupt climate change. The severity of the YD compared with the 8.2-ka event is obvious in central Greenland, yet our results indicate that the latter resulted in more extended Baffin Island mountain glaciers, perhaps due to different triggering mechanisms and unique pre–8.2-ka event climatic baseline conditions in the Baffin Bay region. In addition, the comparatively restricted YD summer cooling on Baffin Island reinforces the broader pattern of mild YD summers in Greenland-proximal locations (5, 7). The amplitude of Baffin Island summer temperature depression during the 8.2-ka event is similar to the drop in Greenland mean annual temperature, indicating a fundamental contrast in seasonality between YD and 8.2-ka event climates.

Supplementary Materials

www.sciencemag.org/cgi/content/full/337/6100/1330/DC1

Materials and Methods

Figs. S1 to S4

Tables S1 to S3

References (3755)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online
  2. Acknowledgments: We thank M. Badding, D. Bedard, S. McGrane, N. Michelutti, and E. Thomas, who aided in sample collection. We also thank G. Miller for insightful discussions and the helpful comments from two anonymous reviewers. This work was funded in part by U.S. National Science Foundation awards ARC-909334, BCS-1002597, and BCS-752848 to J.P.B.
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