NOAA/WDS Paleoclimatology - Sulu Sea Core MD97-2141 Termination I Globoratalia tumida d18O and Mg/Ca Data

The meridional migration of the bifurcation latitude of the Pacific North Equatorial Current (NEC) in the western boundary of the tropical Pacific modulates the strength of the Kuroshio Current. Using salinity reanalysis data, we show the NEC bifurcation latitude also acts as the dominant control on thermocline salinity of the Sulu Sea, just west of the Philippine archipelago, by regulating influx of western Pacific thermocline water via the Luzon Strait. We used oxygen isotopes (d18O) and Mg/Ca in the thermocline-dwelling foraminifera Globorotalia tumida from Sulu Sea sediment core MD97-2141 to determine past thermocline d18Ow and salinity variability spanning ~20-5 ka with an average sampling interval of ~50 years and infer past changes in the NEC bifurcation latitude. Our Sulu Sea thermocline reconstruction reveals high salinity from ~18.8-15.5 ka, ~12.2-11.5 ka, and from ~9.5-8.5 ka indicating the NEC bifurcation latitude was shifted north and the Kuroshio was weak at those times. Low Sulu Sea thermocline salinity from ~13.0-12.4 ka, ~11.5-10.9 ka, and from ~8.5 ka until the end of the record at ~5.6 ka indicates the NEC bifurcation latitude was shifted south and the Kuroshio Current was relatively strong. Comparison to other paleoclimate records suggests the observed northward (southward) shifts of the NEC bifurcation latitude were driven by southward (northward) shifts of the Indo-Pacific ITCZ, consistent with modern mechanisms controlling interannual NEC bifurcation variability. The NEC bifurcation latitude shifts likely modulated northward energy transport via the Kuroshio Current and the mean temperature and salinity of the Indonesian Throughflow.

热带太平洋西边界处，太平洋北赤道流（North Equatorial Current, NEC）的分叉纬度经向迁移，可调控黑潮的强度。本研究借助盐度再分析数据证实，NEC分叉纬度还可通过吕宋海峡调控西太平洋温跃层水的输入通量，进而成为菲律宾群岛西侧苏禄海温跃层盐度的主导控制因子。我们对苏禄海沉积岩芯MD97-2141中栖息于温跃层的有孔虫瘤状圆球虫（Globorotalia tumida）的氧同位素（δ¹⁸O）与镁钙比（Mg/Ca）进行分析，重建了距今约20~5 ka的古温跃层水氧同位素组成与盐度变化序列，平均采样间隔约50年，并据此反演了NEC分叉纬度的历史演化过程。苏禄海温跃层盐度重建结果显示，在约18.8~15.5 ka、12.2~11.5 ka以及9.5~8.5 ka三个时段，温跃层盐度偏高，表明彼时NEC分叉纬度向北偏移，黑潮势力较弱；而在约13.0~12.4 ka、11.5~10.9 ka以及8.5 ka至记录截止的约5.6 ka时段，苏禄海温跃层盐度偏低，对应NEC分叉纬度向南偏移，黑潮相对强盛。将本研究结果与其他古气候记录对比后发现，观测到的NEC分叉纬度向北（向南）偏移，由印太热带辐合带（Intertropical Convergence Zone, ITCZ）向南（向北）偏移驱动，这与调控年际尺度NEC分叉变化的现代机制相一致。NEC分叉纬度的变化可能通过黑潮调控北向能量输送，同时影响印度尼西亚贯穿流的平均温度与盐度。