Chronicle of regional volcanic eruptions recorded in Okhotsk Sea sediments over the last 350 ka

• High-resolution ISP MS and lithological analyses of Okhotsk Sea sediments.
• Determination of most sensitive and contrasting indicators of cryptotephra.
• Identification of 31 new cryptotephras in the sediments dating to 350 ka.
• Ages and chronology of 31 new cryptotephras were developed.

Recovered from the central Okhotsk Sea, the dated marine sediment core MR PC-7R was used to reconstruct the history of volcanic eruptions taking place over the last 350 ka in the Kuril–Kamchatka area, one of the most active volcanic regions in the world. Three visible tephra layers previously identified in Okhotsk Sea sediments were identified in this core. A novel approach was applied to detect traces of tephra fallout from ancient eruptions, namely cryptotephras, based on the elemental content of bulk sediment and a new lithological index. The concentrations of 63 elements in 1723 samples of bulk sediment sampled every 1 cm throughout core length were measured via ICP-MS analysis, with an average resolution time of 200 years. Factor analyses of these 63 elements plus 2 further variables enabled the identification of Factor 1 elements, which had lowered content in tephra, and those in the Factor 2 group, with elevated content in tephra. A Volc/Ter ratio determined under a binocular microscope by means of visual assessment of volcanic and terrigenous grains in the >63 μm sediment fraction was used as an independently measured index of cryptotephra occurrence. The factor analysis and Volc/Ter ratio were then employed to facilitate the detection of cryptotephras in the sediments, with 31 detected in addition to the three visible tephra for the last 350 ka. The previously constructed age model for the core, based on marine isotope stages/sub-stages and millennial-scale cold events, allowed us to determine the age and stratigraphy of the newly-identified cryptotephras and therefore also of ancient regional eruptions.

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