A. Kaiser

Magnetostratigraphy and tephrochronology of the Upper Quaternary sediments in the Okhotsk Sea: implication of terrigenous, volcanogenic and biogenic matter supply

Seven lithological units, alternating between horizons enriched in biogenic opal (diatoms) and carbonate (foraminifera) and units composed largely of terrigenous sediment with very low biogenic admixture, were distinguished in a series of Late Quaternary sediment cores from the Sea of Okhotsk. Sediments were characterised using on-board visual description, smear-slides analyses, grain-size composition and magnetic susceptibility (MS) records. Five tephra marker layers, identified as Ko, TR, K2, K3 and K4, were distinguished using petrological, mineralogical and geochemical analyses. Age models were developed by comparing lithological units, tephra stratigraphy and MS records with oxygen isotope curves and with records of biogenic CaCO3, opal content, and sediment grain-size composition in three AMS radiocarbon dated Okhotsk cores. This chronological framework allowed us to investigate climate control over the sedimentation regime and productivity, and to clarify the formation times of the lithological and tephra units. The data show that sediment MS variations in the south-eastern area of the glacial Okhotsk Sea arose primarily from volcanic material input. The sedimentation in all other parts of the sea was mainly controlled by the supply of terrigenous material. The cores show a tight correlation between MS, sediment grain size and climate change in the Okhotsk Sea: coarse sediments with a high MS were accumulated during the cold period-oxygen isotope stages (MIS) 2, 4, 6; fine sediments with a low ice-rafted debris (IRD) content and MS were formed in the warm isotope stages. According to the oxygen isotope stratigraphy and radiocarbon data, the Ko, TR, K2, K3 and K4 tephra were deposited at 7.7, 8.0, 26.0 kyr BP, in MIS 4 about 60–70 kyr ago, and near MIS 5.4, respectively. Comparison of tephra mineralogy, petrology, spatial distributions and ages with the history of Kurile–Kamchatka volcanism allows us to identify likely candidates for the tephra source eruptions. The combined use of MS records, tephrochronology and lithological unit sequences provides a detailed basis for Okhotsk Sea sediment stratigraphy.