Cryptotephra investigation was performed at a sample spacing of 4 cm in the interval between 3020 and 3180 cm in core MD99-2284. This interval comprises the GS9-GI8 transition, for which several, mainly basaltic tephra layers have been identified in Greenland ice cores and geochemically characterized (42, 43). To extract basaltic glass shards, freeze-dried and homogenized sediments were immersed in 10% HCl to remove carbonate material and subsequently wet-sieved in three size fractions (>125, 80 to 125, and 25 to 80 μm) (44). For specific cryptotephra analysis, the 25- to 80-μm fraction was further separated into different density fractions (<2.3, 2.3 to 2.5, and >2.5 g/cm3) to isolate the basaltic glass shards using a heavy liquid technique (45). Furthermore, the >2.5 g/cm3 fraction underwent a magnetic separation procedure to separate paramagnetic basaltic from minerogenic material (46). The samples were then mounted in Canada Balsam on microscope slides, and basaltic glass shards were quantified using optical microscopy.

Geochemical analysis was performed on basaltic shards from 3040 to 3041 cm, where a distinct peak in shard concentration was found (fig. S2). Selected shards were mounted in epoxy resin on a frosted microscope slide, which was then grinded using decreasing grades of silicon carbide paper and polished using diamond suspension. Electron probe microanalysis was performed at the Tephra Analytical Unit, University of Edinburgh. A Cameca SX-100 electron microprobe with five wavelength-dispersive spectrometers was used to obtain the major element composition of 37 individual shards (47). The instrumental drift, precision, and accuracy were determined by running BCR2g basalt secondary standards, and error bars in fig. S2 (B and C) represent two SDs of replicate analyses of the BCR2g reference glass. To compare the tephra layer in MD99-2284 with published tephra layers from Greenland ice cores, the raw geochemical data (provided online) were normalized to an anhydrous basis (i.e., 100% total oxides). The geochemistry of the marine and ice core tephra layers was statistically evaluated using the similarity coefficient (SC) (48) and the statistical distance test (D2) (49).

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