Radiocarbon age differences between benthic-planktonic foraminifera in sediment cores from the Shatsky Rise, central North Pacific

Abstract

Benthic-planktonic foraminiferal radiocarbon age differences in deep-sea sediments are a useful paleo-proxy to estimate deep-water ventilation age from the last glacial maximum to the Holocene. However, it is very difficult to collect reasonable sediment cores for ventilation age reconstruction in the carbonate-corrosive Pacific Ocean. The Shatsky Rise, which is located in the central part of the North Pacific, is a peculiar spot in that carbonate sediments can be collected from abyssal depths. We analyzed benthic-planktonic foraminiferal radiocarbon age differences in two gravity core samples (Core NGC102 and Core NGC108) collected from the Shatsky Rise. The two cores consisted mostly of bioturbated calcareous ooze. Sedimentation rates were 1.4-5.3cm/ka in Core NGC102 (2,612m water depth) and 2.3-6.6cm/ka in Core NGC108 (3,390m water depth). Benthic-planktonic foraminiferal age differences showed large variances, from 7,010±90 to 180±130 years in Core NGC102 and from 2,730±80 to 580±110 years in Core NGC108. In Core NGC102, the age differences (6,440 years on average) in the mixed layer were approximately 4,700 years older than the modern North Pacific deep-water age, reflecting the upward mixing of old benthic foraminifera and low sedimentation rates. On the other hand, the age difference (2,470 years) at 14-16cm depth may be approximately similar to the past deep-water age at that time. The age differences (180±130 to 280±140 years) at 22-28cm depths (16-17 ka, corresponding to Heinrich stadial event 1) were remarkably smaller than the modern deep-water age. In Core NGC108, the benthic-planktonic age difference (990±200 years) at 15,700 cal BP was estimated to be 760 years younger than the modern deep-water age. This small age difference suggests an active ventilation event of Pacific deep waters during Heinrich stadial event 1; however, causes such as atmospheric ¹⁴C decrease and/or sediment mixing (e.g., bioturbation) cannot be excluded.