Planktonic foraminifera in the sea of Okhotsk : population and stable isotopic analysis from a sediment trap

Abstract

The relationship of planktonic foraminiferal populations and stable isotopes to synoptic sea surface temperatures (SST's) was examined in the Sea of Okhotsk, using sediment trap samples (trap deployed Aug. '90 - Aug. '91; 53°19'N, 149°50'E) and AVHRR satellite sea surface temperature data. Synoptic SST's for the deployment ranged between -1.2 and 14.8°C. Two species dominated total yearly foraminiferal flux: N. pachyderma (left) and G. bulloides, with 57% and 31% of yearly flux, respectively. Calcification depths (from δ180eq. calcite) for these species ranged between 20 and 40 meters. Due to the highly stratified water column in the Sea of Okhotsk, these foraminifera experienced temperatures much colder than SST, and use of standard paleotemperature proxies significantly underestimates SST. The average paleotemperatures calculated from both δ180pachy and δ18Dbull were 5-6°C lower than average synoptic SST. Faunal indicators were consistent with this finding, with the N. pachyderma left-to-right coiling ratios >98% for the entire trap deployment. These ratios would suggest maximum seasonal SST's no greater than 10°C, nearly 5°C colder than the actual sea surface temperature maximum. The results from this study indicate that in highly stratified water columns, standard paleotemperature proxies may yield results as much as 5°C cooler than actual sea surface conditions.