Large Pt anomaly in the Greenland ice core points to a cataclysm at the onset of Younger Dryas

Abstract

One explanation of the abrupt cooling episode known as the Younger Dryas (YD) is a cosmic impact or airburst at the YD boundary (YDB) that triggered cooling and resulted in other calamities, including the disappearance of the Clovis culture and the extinction of many large mammal species. We tested the YDB impact hypothesis by analyzing ice samples from the Greenland Ice Sheet Project 2 (GISP2) ice core across the Bølling-Allerød/YD boundary for major and trace elements. We found a large Pt anomaly at the YDB, not accompanied by a prominent Ir anomaly, with the Pt/Ir ratios at the Pt peak exceeding those in known terrestrial and extraterrestrial materials. Whereas the highly fractionated Pt/Ir ratio rules out mantle or chondritic sources of the Pt anomaly, it does not allow positive identification of the source. Circumstantial evidence such as very high, superchondritic Pt/Al ratios associated with the Pt anomaly and its timing, different from other major events recorded on the GISP2 ice core such as well-understood sulfate spikes caused by volcanic activity and the ammonium and nitrate spike due to the biomass destruction, hints for an extraterrestrial source of Pt. Such a source could have been a highly differentiated object like an Ir-poor iron meteorite that is unlikely to result in an airburst or trigger wide wildfires proposed by the YDB impact hypothesis.

Keywords: ICP-MS analysis; PGE; climate change; meteorite impact.

Fig. 1.

(A and B) Ir (A) and Pt (B) variations in the GISP2 ice samples across the Bølling-Allerød/YD boundary. Open symbols indicate upper limits. During the transitional period the annual rate of ice accumulation changes from high to low values (1). The Ir contents in the ice (A) show some variations with no clear substantial anomalies. On the contrary, a large Pt anomaly (B) occurs in the middle of the Bølling-Allerød–YD transition period. The anomaly coincides with a sharp drop in the δ¹⁸O values in ice (black curve). The δ¹⁸O curve is calculated from the data of ref. averaged for 40 cm of ice thickness. (C) The time of the Pt anomaly differs from those of the volcanic SO₄²⁻ spikes (19) and the Laacher See eruption (LSE) (9) and precedes the onset of the NH₄⁺ spike (2) by ∼30 y.

Fig. 2.

(A and B) Pt/Ir (A) and Pt/Al (B) ratios in the GISP2 ice samples across the Bølling-Allerød/YD boundary vary between the chondritic and continental crust values. The only exception is the Pt anomaly with both Pt/Ir and Pt/Al ratios greatly exceeding chondritic and crustal values, with three top points also exceeding Pt/Ir ratios of low-Ir iron meteorites. Such a large fractionation of both Pt/Ir and Pt/Al ratios within the anomaly most likely results from atmospheric processing of the Pt-rich material.