PP41A-05: Triggering of the Younger Dryas Cooling by Extraterrestrial Impact
* Kennett, J P (kennett@geol.ucsb.edu), Dept. of Earth Science, Univ. of California Santa Barbara, Santa Barbara, CA 93106, United States
Becker, L (lbecker@crustal.ucsb.edu), Inst. for Crustal Studies, Univ. of California Santa Barbara, Santa Barbara, CA 93106, United States
West, A (Allen7633@aol.com), Geoscience Consulting, Geoscience Consulting, Dewey, AZ 86327, United States
The enigmatic Younger Dryas (YD) cooling episode (12.9-11.5 ka) is unique to late Quaternary deglacials; its timing is unexplainable by orbital forcing because Northern Hemisphere insolation was increasing at that time. The abrupt onset of YD cooling coincided with, and was likely partially in consequence of reduction in north Atlantic thermohaline circulation (THC). Triggering of YD cooling has often been attributed to major diversion of freshwater outflow from the continental interior via newly opened outlets resulting from ice sheet melt-back. However, an alternative primary trigger for YD cooling is actively being sought because of problems related to the timing of outlet openings and the relative importance of the magnitude and timing of freshwater input into the ocean. We present evidence in support of the hypothesis that the primary trigger for YD cooling may have instead been an extraterrestrial impact (the YD impact event) over northern North America at 12.9 ka. This impact event is recorded in a diverse range of impact-related proxies within the widely distributed YDB layer that indicates continent-wide impact effects. YD cooling likely resulted both from a combination of short-term atmospheric processes due to the impact itself that reduced solar radiation and from reorganization of ocean circulation. The hypothesis also posits that the impact caused partial destabilization of northern ice sheets and the opening of freshwater conduits; ice sheet melting due in part to the deposition of impact related dust (i.e. albedo) on the ice sheets; freshening of Arctic and northern Atlantic surface waters, and resulting strong reduction in north Atlantic THC. The impact hypothesis for YD triggering explains the timing enigma of the YD episode and is consistent with much existing data: 1) Abrupt and dramatic switch in Lake Agassiz outflow at 12.9 ka away from the Mississippi River's southern outlet to newly-opened outlets to the east and/or north; 2) A major and abrupt drop in ice-margin lake levels at precisely the time of the impact, based on the chronostratigraphy at Lake Hind and supported by Lake Agassiz sediment records; 3) Evidence at the onset of YD cooling in northern Atlantic and Arctic sediment cores of widespread IRD (during Heinrich event H0) reflecting ice-rafting armadas, freshwater discharges and reduced sea-surface salinity that resulted from the partial destabilization and melting of the ice sheet; 4) Evidence for an anomalous peak in Greenland ice sheet margin melting at the onset of the YD during a time of abrupt ocean and atmosphere cooling. The YD impact event supports the concept that extraterrestrial impacts, even of such limited geographic extent, can significantly and abruptly affect global climate change. Comparison of the YD event with other even larger meltwater outflow events of the last deglacial that had limited affects on the THC, suggests that the YD cooling was not triggered solely by salinity-driven ocean circulation changes. Instead, short-term climate pertubations due to the impact likely played a key role in triggering initial cooling that was reinforced or enhanced by the ocean circulation changes. Thus, the YD climate episode would not have occurred in the absence of the YD extraterrestrial impact event.