Investigating the origin of a Greenland ice core geochemical anomaly near the Bølling-Allerød/Younger Dryas boundary

Abstract

The source of a platinum peak identified in the Greenland Ice Sheet Project 2 (GISP2) ice core that occurs almost synchronously with Younger Dryas (YD) cooling is poorly understood. The GISP2 Pt spike is associated with high platinum/iridium (Pt/Ir) and platinum/aluminium (Pt/Al) ratios, and previous research attributed the anomaly to an unusual iron-rich Ir-poor meteorite impact. The Pt spike timing is also broadly contemporaneous with the Laacher See eruption (LSE), Germany, suggesting a possible source. However, this link is understudied because of perceived chronological mismatches (reconciled recently), and the lack of Pt and Ir data from the Laacher See tephra (LST). To explore this link further, proximal tephra deposits from the LSE were sampled at localities around the volcano and the relevant geochemistry analysed. This report presents evidence that the LSE is not the Pt spike source because: i) the LST has low Pt concentrations, ii) the LST’s geochemical ratios are dissimilar to the GISP2 Pt spike and iii) conversion of the Pt spike timing to the newest ice core age-depth model shows a chronological offset of ~60 years between the two events. Further comparison of the Pt spike’s geochemical ratios against magmas and meteorites, YD Boundary sediments and others suggests that the Pt spike geochemistry is incomparable to any known source. Therefore, the Pt spike origin is interpreted as either: i) a noncataclysmic impact of an undiscovered iron meteorite or ii) an unidentified Pt-rich volcanic eruption contemporaneous with the anomaly, whose aerosol fractionated in the atmosphere or ice. This report supports the LSE as the simplest explanation for the YD trigger, because the LSE was synchronous with Greenland Stadial-1 (GS-1) and contained enough sulfur to cause substantial cooling. The event resulting in the Pt spike occurred ~60 years after GS-1 cooling, and was therefore not the primary trigger. The Pt spike may however represent a subsequent radiative cooling event that prolonged the YD.