WAS THERE A METEORITE IMPACT OR VOLCANIC ERUPTION AT THE ONSET OF THE YOUNGER DRYAS?

초록

The Younger Dryas (YD; 12,900 ? 11,700 yr) was an abrupt cooling period following the termination of a warm period, Allerød. It is recorded in Greenland ice cores as Greenland Stadial-1 (GS-1) [1,2]. The YD cooling event is thought to have resulted from freshwater flooding of the northeast Atlantic and/or the Arctic Oceans that prevented deep water formation [3,4]. The proposed cooling triggers include 1) catastrophic freshwater drainage of proglacial Lake Agassiz [5], 2) meteorite-induced destabilization and/or melting of the Laurentide Ice Sheet [6], and 3) large sulfate aerosol loading in the stratosphere from the explosive eruption of Laacher See volcano in Eifel, Germany. Evidence for the latter two triggers comes from the Greenland Ice Sheet Project 2 (GISP2) ice core. Petaev et al. [7] observed an anomalous Pt spike with a high Pt/Ir ratio of ~1200 at the onset of YD (~12,870 years before A.D. 2000; b2k, henceforth referred to as yr). They attributed the Pt anomaly to the impact of a 0.8 km diameter Sikhote-Alin type iron meteorite. Baldini et al. [8] synthesized recent revisions in ice-core chronology and connected a large sulfate peak observed in the late Allerød (12,917 yr) in GISP2 to the recently revised age of Laacher See volcanic eruption at 12,930 ± 40 yr [9,10]. They suggested that the contemporaneity of the late Allerød sulfate peak with the Laacher See eruption shows possible volcanic aerosol cooling as the YD initiation trigger. However, the physical evidence for each of the above two mechanisms remains elusive [11-13]. Here, we investigate whether there was a one-time catastrophic event or a series of events that led to the YD cooling using chemical and Os and Pb isotopic signatures in the Greenland Ice Core Project (GRIP) ice core transecting the YD onset. In addition, we analyzed Laacher See tephra for Os and Pb isotopes. If there was a meteorite impact at the YD onset, the GRIP samples should display a signific