Persistent sea surface temperature and declined sea surface salinity in the northwestern tropical Pacific over the past 7500years

To understand Holocene climate evolutions in low-latitude region of the western Pacific, paired δ¹⁸O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber (250-300μm, sensu stricto, s.s.) from a marine core ORI715-21 (121.5°E, 22.7°N, water depth 760m) underneath the Kuroshio Current (KC) off eastern Taiwan were analyzed. Over the past 7500years, the geochemical proxy-inferred sea surface temperature (SST) hovered around 27-28°C and seawater δ¹⁸O (δ¹⁸O_W) slowly decreased 0.2-0.4‰ for two KC sites at 22.7° and 25.3°N. Comparison with a published high-SST and high-salinity equatorial tropical Pacific record, MD98-2181 located at the Mindanao Current (MC) at 6.3°N, reveals an anomalous time interval at 3.5-1.5kyr ago (before 1950 AD). SST gradient between the MC site and two KC site decrease from 1.5-2.0°C to only 0-1°C, and δ¹⁸O_W from 0.1-0.3‰ to 0‰ for this 2-kyr time window. The high SST and low gradient could result from a northward shift of the North Equatorial Current, which implies a weakened KC. The long-term descending δ¹⁸O_W and increasing precipitation in the entire low-latitude western Pacific and the gradually decreasing East Asian summer monsoonal rainfall during middle-to-late Holocene is likely caused by different land and ocean responses to solar insolation and/or enhanced moisture transportation from the Atlantic to Pacific associated with the southward movement of ITCZ.