Extremely depleted radiocarbon impact on estimation of Glacial North Pacific Intermediate Water ventilation

Abstract

Reconstructing the ventilation history of intermediate-depth Pacific Ocean remains challenging due to the influence of geologic carbon release, which biases radiocarbon (¹⁴C) ventilation age estimates. These uncertainties complicate the interpretation of past North Pacific Intermediate Water (NPIW) dynamics and its responses to climate variability. Here, we investigate NPIW changes during the last glaciation and subsequent deglaciation by analyzing benthic and planktonic ¹⁴C age offsets (B-P ¹⁴C age offsets), along with stable oxygen (δ¹⁸O_bf) and carbon isotope (δ¹³C_bf) records of benthic foraminifera in a sediment core from the southwestern Okhotsk Sea. The results indicate that B-P ¹⁴C age offsets were more than ∼9000 yrs during the last glaciation, suggesting a significant release of ¹⁴C-free geologic carbon, likely sourced from mixed gas clathrates in the southwestern Okhotsk Sea. These highly depleted ¹⁴C values have, therefore, led to an overestimation of ¹⁴C ventilation ages for Glacial North Pacific Intermediate Water in both the Okhotsk Sea and the Northwest Pacific during the Last Glacial Maximum (LGM). A compilation of δ¹⁸O_bf records from the North Pacific suggests that intermediate water formed in both the Okhotsk and Bering Seas during the LGM, extending to depths of at least ∼1590 m and ∼1000 m, respectively. Compared to the LGM, enhanced intermediate water formation in the Bering Sea during Heinrich Stadial 1 played an important role in intensifying NPIW, effectively flushing ¹⁴C-free geologic carbon from the Okhotsk Sea. This process might lead to a more uniform ventilation pattern at intermediate depths between the Northwest and Northeast Pacific. This study advances our understanding of NPIW evolution and reconciles previous discrepancies in ventilation reconstructions between the Northwest and Northeast Pacific during the LGM.