Widespread tephra layers in the Bering Sea sediments: distal clues to large explosive eruptions from the Aleutian volcanic arc

摘要

Tephra layers within marine sediments provide information on past explosive eruptions, which is especially important in the case of remote island arcs where data on proximal pyroclastic deposits can be scarce. Three Alaska-Aleutian tephras (labeled Br2, SR2, and SR4) were found in the late Pleistocene-Holocene sediments of the Bering Sea (north Pacific). We fingerprint glass from these tephras with the help of single-shard electron microprobe and LA-ICP-MS analyses and provide microprobe data on minerals from two of these tephras. The large compositional variability of the Alaska-Aleutian volcanoes permits the use of ratios of highly incompatible trace elements (Ba/Nb, Th/Nb, Th/La, La/Nb) for identification of distal tephra sources by comparison of these ratios in tephra glass and proximal bulk rock analyses. This method, along with mapped tephra dispersal, has allowed us to link tephras under study to Aniakchak, Semisopochnoi, and Okmok volcanoes, respectively. Our results indicate that tephra Br2 was derived from the 3.6ka Aniakchak II caldera-forming eruption (Alaska, USA). This is the first ever finding of the Aniakchak II tephra in Bering Sea sediments, which permits enlargement of its tephra volume and eruption magnitude to 100km(3) and 6.8, respectively. Tephra SR2, dated at 12.2ka, is likely associated with a post-glacial caldera on the Semisopochnoi Island, Aleutians (USA). Tephra SR4 (dated at 64.5ka), likely was derived from an earlier undocumented eruption from Okmok volcano (Aleutians). All three regionally spread tephra layers are valuable isochrones, which can be used for correlating and dating of Bering Sea sediments.