Revisiting Holocene North Pacific Intermediate Water ventilation through the lens of radiolarian morphometric analysis

Understanding intermediate-depth ventilation processes in the North Pacific during past warm periods is essential for assessing the climatic role of ocean circulation dynamics, which significantly influence global climate change and the carbon cycle. Over the last two decades, considerable efforts have focused on understanding the evolution of North Pacific Intermediate Water (NPIW) during the Holocene (~11,700 years to present), providing the background climate state for modern anthropogenic global warming. While modern NPIW primarily ventilates from the Okhotsk Sea, the Holocene ventilation history of Okhotsk Sea Intermediate Water (OSIW) still remains unresolved: epibenthic δ¹³C records suggest a 30–50% reduction in oxygenation during the Holocene optimum, whereas most benthic foraminiferal-based oxygen concentrations and radiolarian assemblages indicate well-ventilated conditions in the mid-Holocene. To resolve this discrepancy, this study reconstructs the OSIW evolution pattern from its source region (the Okhotsk Sea northern shelf) using the radiolarian assemblages, revealing an evolution pattern consistent with prior radiolarian reconstructions. Meanwhile, we introduce a novel quantitative approach—Cycladophora davisiana morphometric parameters—providing, to our knowledge, the first time series of C. davisiana size distributions. New radiolarian size data demonstrate that mid-Holocene peaks in C. davisiana abundance are not primarily driven by food supply (vital effects), supporting the hypothesis of well-ventilated OSIW due to reduced freshwater input and saltier surface water. These findings not only advance quantitative methods in radiolarian-based micropaleontology but also help reconcile the intermediate-water ventilation conundrum in the Okhotsk Sea since the Holocene.