Temperature and circulation changes in the eastern North Atlantic during the last 150,000 years: Evidence from the planktonic foraminiferal record

Abstract

During the last 150,000 years the circulation of the subtropical gyre in the eastern North Atlantic has changed in response to glacial-interglacial variations in climate. Maps and time series constructed from planktonic foraminifera recovered from 18 piston cores show the nature of this response. Paleontological, chemical, and lithologic indices are used to establish a stratigraphy in the cores. Numerical analysis of the paleontological data yields estimates of the sea-surface temperatures (SST) and salinity. Maps of these estimates and of the interpreted circulation have been constructed for 18, 28, 73, 93, 125, and 150,000 years B.P. ( 18O Stages 2, 3, 4, 5-b, 5-e, 6). The analysis yielded six major results: 1. (1) In the southern Sargasso Sea, faunal abundances and SST have remained essentially constant for the last 150,000 years. 2. (2) In the northwestern part of the study area, between 30°N and 40°N, faunal variations were large but SST variations small (2–3°C) during the last 150,000 years. SST remained stable due to different ecological water types of similar temperature range exchanging dominance downcore. 3. (3) In the northern part of the study area, between 40–45N, SST remained warm in the west and decreased to glacial-level minima in the east between 120 and 80,000 years B.P. ( 18O substages 5-d to 5-a). The large west-east gradients may have been maintained by a meridional flow of the North Atlantic Current around the Azores Plateau. 4. (4) Along the Canary Current, there were generally monotonic decreases in SST during the last climatic cycle. 5. (5) The advance of ice that culminated in the Wisconsin maximum at 18,000 years B.P. was accompanied by rising SST in both the vicinity of the Azores and along the southern part of the Canary Current. 6. (6) Inferred increases in the velocities of the North Atlantic Current occurred at approximately the same time as all seven of the major polar advances of the last 150,000 years, and are associated with stable or rising temperatures. These SST patterns may have resulted from increased North Atlantic Current velocities advecting warm source waters more rapidly into the mid-latitudes. The increased velocities may have been due to a combination of reinforcing factors: increased wind stress, steepening of the thermocline due to increased cooling of 18°C water south of the Gulf Stream, and increased baroclinicity that resulted when the low-temperature North Atlantic polar front migrated southward toward the stable gyre center.