A DYNAMIC KUROSHIO CURRENT EXTENSION DURING THE PLIOCENE-EARLIEST PLEISTOCENE

Understanding western boundary currents is important as these features play a critical role in ocean/atmosphere exchange and are responsible for regional weather and climate patterns. This project investigates the behavior of the Kuroshio Current Extension (KCE) across several major climate shifts [the closure of the Central American Seaway (CAS, ~3.6 Ma), the mid-Piacenzian Warm Period (mPWP, 3.2-2.9 Ma), and initiation of Northern Hemisphere glaciation (NHG, 2.7 Ma)] to determine better how the KCE came into its modern configuration and how it may behave under elevated atmospheric CO₂ levels. This study is the first to characterize in detail how the KCE responded to past tectonic events and major climate shifts.

We used deep-sea sediments from Ocean Drilling Program (ODP) Holes 1207A, 1208A, and 1209A, located on the northern edge, underneath, and to the south of the modern-day position of the KCE, respectively, as the source of samples to investigate the behavior of the current. Stable isotopic analyses (δ¹³C and δ¹⁸O) from surface-dwelling planktic foraminifera Globigerinoides ruber and Gs. obliquus were used to characterize sea surface conditions. In addition, foraminiferal counts were conducted on samples with extremely high (cool) or low (warm) δ¹⁸O values and are used to further understand the signals inferred from the geochemical records. Modern δ¹⁸O values were modeled at each site to infer when the KCE came into its modern configuration. With CAS closure the records indicate warming followed by slow cooling of the KCE. Within the mPWP interval isotopic data from Holes 1207A and 1208A indicate warming, whereas data indicate cooling at Hole 1209A and a large δ¹⁸O gradient between the sites develops around 3.06 Ma. These data indicate a sharpening of the water mass boundaries along the northern edge of the KCE during maximum warming of the mPWP, while cooling at the southernmost site might be explained by greater seasonality of sea surface temperatures or increased upwelling along the southern edge of the KCE. During initiation of NHG all three records converge and the gradients between sites resemble modeled modern δ¹⁸O values indicating the KCE came into its modern configuration during initiation of NHG ~2.7 Ma.