Evaluating the flux of extraterrestrial osmium at the onset of Younger Dryas in the GRIP ice core

초록

The Younger Dryas (YD: 12.911.6 ka) was an abrupt cooling event during the last deglaciation. The mechanism behind the cooling is suggested to be a temporary slowdown of North Atlantic thermohaline circulation due to catastrophic release of meltwater from proglacial Lake Agassiz during the retreat of the Laurentide Ice Sheet [1]. An alternative hypothesis states that the cooling was directly/indirectly triggered by one or more cosmic airbursts/impacts [2]. While several papers have documented evidence for a YD extraterrestrial impact including microspherules, nanodiamonds, magnetic grains, and glasslike carbon [47], this hypothesis remains controversial [810]. In a recent study by Petaev et al.[11], an unusually high Pt/Ir ratio of ~1200 was discovered in the GISP2 ice core at the onset of YD, indicating a large Pt enriched iron meteorite impact. Such a high Pt/Ir in extraterrestrial materials has not been documented [12]. Thus, Petaev et al. [11] acknowledge that the interpretation of the Pt anomaly is based on circumstantial evidence. The distinct Os isotopic composition (187Os/188Os ratio) of the terrestrial (=1.26) and extraterrestrial (= 0.13) sources should allow us to evaluate if there was a meteorite impact at the YD boundary. These analyses are technically challenging owing to rather low concentration of Os in icemelts (~1x10 g/g). Here, we will present Os isotope data from the GRIP ice core spanning the time period through YD to shed light on the meteorite/comet impact hypothesis.