Millennial-Scale North Pacific Intermediate Water Ventilation Changes during Dansgaard-Oeschger Events of the Last 60,000 Years
Abstract:A prominent feature of the modern North Pacific is the presence of intermediate water with higher oxygen content and lower salinity (centered at σθ = 26.8 isopycnal). This North Pacific Intermediate Water (NPIW) is the only “fresh” water mass formed in the North Pacific Ocean today in which its formation is physically coupled to dense water formation by brine rejection in seasonal sea-ice regions of the Okhotsk Sea. Differences in past NPIW ventilation have been proposed as one important mechanism to understand millennial-scale ocean oxygenation changes observed during the past 60,000 years in the North Pacific (Behl and Kennett, 1996). However, the role of NPIW changes in the past is still under debate. In particular, sufficient proxy data from the North Pacific (including the Okhostk Sea) to study the ventilation of NPIW during the past 60,000 years are not available, yet. Here we present epibenthic stable carbon isotope records (δ13C) from high-resolution sediment cores of the Okhotsk Sea (LV29-108-4, 52° 01.29’N, 153° 34.48’E, 625m water depth) and Bering Sea (SO201-2-101KL, 58° 52.52’N, 170° 41.45’E, 630m water depth), the two potential source regions of past NPIW, used to trace the NPIW ventilation history of the last 60,000 years. We found several short-term events of high nutrient and low oxygen conditions in the mid-depth Okhotsk Sea and Bering Sea. These high nutrient and low oxygen events correlates remarkably well with several (but not all) Dansgaard-Oeschger interstadials in Greenland ice-cores (Dansgaard et al., 1993) and associated brief interstadial low-oxygen events found in the northeast Pacific during the past 60,000 years (Behl and Kennett, 1996). We suggest that the intrusion of high nutrient and low oxygen water masses from the Okhotsk Sea and Bering Sea into the northwest Pacific to form NPIW might have facilitated the widespread, nearly synchronous ocean oxygenation changes observed in the North Pacific during Dansgaard-Oeschger interstadials of the past 60,000 years.
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