Reassessment of foraminiferal-based tropical sea surface δ18O paleotemperatures

The possibility exists that the magnitude of Glacial/Holocene δ¹⁸O change in the tropical oceans was previously under-estimated due to the dampening effect of bioturbation on bulk foraminiferal δ¹⁸O measurements. We have investigated this possibility by isotopically analyzing suites of individual planktonic foraminifera from box core samples from the equatorial Atlantic. We present oxygen isotopic results for individual specimens of two surface-dwelling planktonic foraminifera Globigerinoides ruber (white) and Globigerinoides sacculifer (no final sac) from glacial-age box core samples from seven sites located between about 15°N and 15°S in the tropical Atlantic. These data are used to constrain the mimimum glacial temperatures recorded by tropical surface-dwelling planktonic foraminifera during the last glacial maximum. The individual foraminiferal δ¹⁸O temperatures reveal minimum sea surface temperatures (SSTs) that ranged from 1°C colder to as much as 5° to 6°C colder than present-day summer SSTs after correcting for an "ice volume effect". However, these minimum temperatures represent less than 10% of the foraminiferal population measured from glacial horizons. Approximately 80% of the glacial specimens record temperatures that were within 2°C of the mean Holocene SST. We conclude that either the tropical Atlantic was not pervasively colder during the Glacial or that there was a systematic negative shift in the isotopic composition of the surface waters that offset a more signficant temperature change.