Using Planktonic Foraminifera as Monitors of the Tropical Surface Ocean

Abstract

The thermocline is a dynamic feature of tropical Pacific and Atlantic Oceans that responds to, as well as influences, wind-driven circulation and tropical climatic conditions. Methods for reconstructing the thermocline can be applied to understanding changes in tropical winds, the interactions between tropical climate and extratropical conditions, mechanisms for sea surface temperature change, and the role of the tropics in global climate change. Here, we review methods that employ depth stratified planktonic foraminifera to estimate thermocline depth in the tropical Atlantic and Pacific Oceans. The first method uses transfer functions to relate foraminifera assemblages to thermocline depth. The second method uses oxygen isotopic measurements of species that calcify at different depths in the water column to estimate changes in thermocline depth. These methods are used to reconstruct the thermocline depth across the Pacific and Atlantic Oceans in the Last Glacial Maximum (LGM). In the tropical Pacific, the thermocline was deeper in the west, and shallower in the central equatorial Pacific, resulting in a more steeply dipping cross-basinal tilt, possibly indicating that Walker circulation was enhanced during the LGM. In the tropical Atlantic, the thermocline was shallower across the northern region, deeper across the basin just north of the equator, and more steeply dipping on the equator and across the southern region. These patterns indicate stronger northeasterly and southeasterly trades during the LGM. Sea surface temperature (SST) was estimated for one location in the western tropical Atlantic using oxygen isotopic data on populations of single foraminifera shells. There was higher SST seasonality and 2–3°C cooler average SST in the LGM relative to today. Cooler northwest SSTs in the Atlantic could be explained by enhanced mixing of cool thermocline waters into the mixed layer.