NORTH ATLANTIC SEA SURFACE AND THERMOCLINE TEMPERATURE VARIATIONS DURING THE MIDDLE-LATE MIOCENE (16-8 MA)

The Middle to Late Miocene (16-5.3 Ma) was associated with a period of global warming known as the Miocene Climatic Optimum (MCO) that spanned roughly 17 to 14.7 Ma and was followed by a period of global cooling and the formation of a permanent Antarctic ice sheet. However, only sparse data from this interval exists regarding sea surface temperatures of the North Atlantic, which plays a key role in controlling ocean water circulation and global climates. We reconstructed the surface and thermocline structure of the central subtropical North Atlantic gyre during the MCO and the subsequent period of cooling and increase in ice volume (14.7-10 Ma) by analyzing three species of planktonic foraminifera from DSDP Site 558 (37.8° N latitude) for carbon and oxygen stable isotopes: surface-dwelling D. altispira and G. quadrilobatus and thermocline-dwelling G. venezuelana. Mean increase in δ¹⁸O of seawater from 14.7-10 Ma was estimated using Mg/Ca to be ~0.5‰ due to the development of a permanent east Antarctic ice sheet. Thus, a net ~1.0‰ increase in δ¹⁸O of benthic foraminifera and G. venezuelana over this time interval was due to deep water and thermocline cooling of ~2°C. In contrast, a 0.5‰ increase from 14.7-10 Ma in surface dwellers can be entirely ascribed to an increase in δ¹⁸O_sw, suggesting no change in surface water temperatures in the subtropical gyre of the North Atlantic. This is similar to previous reports of Middle to Late Miocene surface temperatures from higher latitudes in the northern North Atlantic, and implies that the development of a strong thermocline was linked to events associated with development of a permanent Antarctic ice sheet and that surface waters were largely unaffected.