Deep current evolution in the central part of the South China Sea since the Middle to Late Pleistocene
The Pacific western boundary current(WBC)is a crucial oceanic circulation that transports heat and nutrients between the Northern and Southern Hemispheres.The deep current in the South China Sea(SCS)originates from intermediate-deep waters in the western Pacific Ocean,entering through the Luzon Strait.The long-term relationship between the WBC and the deep current in the SCS over geological timescales remains uncertain.In this study,we selected a sedimentary core(located at 116.75°E and 15.80°N,at a water depth 3000 m,with a length of～580 cm)from the central basin of the South China Sea to illustrate the evolution of the deep current in the central SCS.The core sediments primarily comprise silty clay,clayey silt and fine silt,exhibiting colors ranging from light greenish gray to dark gray.Notably,there are no distinct signs of erosion or bioturbation.A total of 290 paleomagnetic samples and an equivalent number of powder samples were continuously obtained for paleomagnetic measurements and debris grain size analysis,respectively.Additionally,twenty samples were selected for hysteresis loop determination and first-order reversal curve(FORC)diagram tests.The paleomagnetic and rock magnetic results indicated that single-domain(SD)and pseudodomain(PSD)soft magnetic minerals faithfully record changes in the geomagnetic field.Stable and reliable natural remanence could be achieved after alternating demagnetization in the laboratory.The relative paleointensity(RPI)proxy can be reconstructed using normalized natural remanent magnetization(NRM)with saturation isothermal remanent magnetization(SIRM).The core chronology was established through AMS14C dating of three planktonic foraminifera samples and correlation of the relative paleointensity with the stacked master curve of the geomagnetic field intensity.To gain insights into hydrodynamics,we conducted End-member decomposition of the grain-size composition of detrital sediments.Combining these data with changes in magnetic fabric parameters,we aimed to establish an indicator reflecting the deep current in the South China Sea and its evolution.The results showed that strong hydrodynamic conditions are supported by the presence of sortable particles(10-63 μm)(SS％),End Member 2(EM2)with a median size of approximately 17.26 μm,and the anisotropy degree P of the sediments.Principal component analysis of these three parameters indicated that the PC1 fraction can represent deep current activity in the South China Sea.Over the last 250 ka,the evolution of the deep current in the South China Sea has consistently mirrored that of the western boundary current in the Pacific Ocean.Its activity has been high,with notable fluctuations during the oxygen isotope phases of MIS6 and MIS2 and lower fluctuations during other periods.The intensity of deep current activity is closely related to changes in the temperature of the thermocline in the western Pacific.High deep current activity coincides with temperature decreases.One possible mechanism is that the robust western boundary current enters the Philippine Sea,is reversed,and rises.This causes the thermocline temperature to decrease and the density to increase.Consequently,the density difference between seawater on either side of the Luzon Strait increases.This leads to larger water volume entering the South China Sea and an increased flow rate.Ultimately,this results in heightened deep current activity in the South China Sea.
Pacific western boundary currentdeep current of the South China Seagrain size of detrital sedimentsanisotropy degree of magnetic susceptibility
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