A DYNAMIC KUROSHIO CURRENT EXTENSION DURING THE PLIOCENE

This mid-Pliocene Warm Period is often studied as an analog to determine how our oceans will behave under increased warming, and understanding western boundary currents are important since they play an essential role in ocean/atmospheric exchange and are responsible for regional weather patterns and climate. This project combines both topics by examining the degree to which the subtropical southern section of the Kuroshio Current Extension (KCE) warmed during the mid-Pliocene Warm Period (mPWP; ~3.2-2.9 million years ago). Over this interval atmospheric CO₂ is thought to have been slightly elevated compared to today’s values. To date, no studies have characterized in detail how the KCE responded to warming events of the geologic past.

We used deep-sea sediments from Ocean Drilling Program (ODP) Hole 1209A (32°39.10’N, 158°30.35’E), located just south of the KCE to investigate the behavior of the current during the mid-Pliocene Warm Period. Stable isotopic analyses (δ¹³C and δ¹⁸O) from surface-dwelling planktic foraminifera Globigerinoides ruber and Gs. obliquus are used to characterize sea surface conditions. These new data are combined with other stable isotopic records from sites located under and to the north of the KCE. Isotopic data from ODP Holes 1207A and 1208A indicate an increase temperature gradient among sites during the mid-Pliocene warm period, whereas data indicate cooling at Hole 1209A. These changes may indicate a sharpening of the water mass boundaries along the northern edge of the KCE during maximum warming of the mPWP between Holes 1207A and 1208A, but cooling at the warmest-water site might be explained by altered seasonality of sea surface temperatures or increased upwelling along the southern edge of the KCE. After the mid-Pliocene Warm Period interval, all three records indicate cooler sea surface temperatures and values converge during initiation of Northern Hemisphere Glaciation. These mid-latitude regions have not been studied in great detail for this interval of global change, and this research project is providing valuable information about the evolution of the NW Pacific, specifically, and western boundary current migration, in general, in response to warming and cooling climate.