Glacial to deglacial ventilation and productivity changes in the southern Okhotsk Sea

As a source region of North Pacific Intermediate Water, the Okhotsk Sea plays an important role in the ventilation of the North Pacific. To understand the detailed oceanographic changes in this marginal sea since the last glaciation, we studied decadal to centennial scale proxy records from new sediment cores from the southwestern Okhotsk Sea. Glacial to Holocene δ¹⁴C records of benthic foraminiferal shells suggested enhanced ventilation in the Okhotsk Sea during the early deglacial period between 18 and 15ka, corresponding to Heinrich Event 1. Although the δ¹⁴C reconstruction has considerable uncertainties, the Okhotsk Sea may have acted as a source for vigorous ventilation of the subarctic Pacific during this period. CaCO₃ preservation events appear to be better explained by the ventilation history of the Okhotsk Sea than by coccolithophores and foraminifera production. CaCO₃ preservation started to improve during 18 to 15ka, and pronounced peaks in the CaCO₃ content corresponded to the Bølling-Allerød (15 to 13ka) and Preboreal (11.5 to 10ka) warm periods. Diatom and coccolithophore productivity remained low in the Okhotsk Sea throughout the glacial to deglacial periods, different from the situation in the open subarctic Pacific, where high productivity was observed during the Bølling-Allerød period. After the Preboreal period, biogenic opal gradually increased and δ¹⁵N decreased in the southern Okhotsk Sea, suggesting that productivity was enhanced by a relaxation of the nitrate limitation.