Research articlePaleoceanographic changes in the East China Sea during the last ~400 kyr reconstructed using planktic foraminifera
Introduction
The East China Sea (ECS) receives a large discharge from the Changjiang Basin rivers, which occupies the major part of south China and is influenced by the East Asian Summer Monsoon (EASM) during the summer. The EASM at present greatly influences the sea surface temperature (SST) and sea surface salinity (SSS) conditions in the East China Sea (Kubota et al., 2010) affecting the population abundance of planktic foraminifera and their diversity. The warm and saline Kuroshio Current (KC), which is associated with the subtropical North Pacific Ocean, enters the ECS through a passage between Taiwan and Iriomote Island (Qiu, 2001), and supplies warm oxygen-rich, saline water to this site, influencing planktic foraminiferal population. The influence of Taiwan Warm Current is primarily restricted to the western part of the ECS, which flows to the north throughout the year, even during winter with strong northerly winds (Ji-lan et al., 1994; Qi et al., 2017). High temperatures thus characterise the Kuroshio and Taiwan currents even during the winter (Fig. 1).
The last 400 kyr time interval witnessed four termination events, covering five major glacial and interglacial transitions (Cheng et al., 2016), which were driven by the orbital changes (Das et al., 2018; Matsuzaki et al., 2019). The glacial/interglacial cycles had significantly impacted the ECS due to the lowering of the sea level during glacial periods, exposing the continental shelf and thus rivers (Yangtze and Yellow rivers) migrated farther south (Gallagher et al., 2015) with the change in the strength of the KC in the ECS. Recent studies have suggested that stronger coastal current pushed the KC southward in the ECS and thus influenced only southern part of the ECS during the Last Glacial Maximum (LGM) (Ijiri et al., 2005; Matsuzaki et al., 2019).
Various planktic foraminifera respond to variations in the characteristics of surface water current in the ECS over the year. These planktic foraminiferal species may be grouped based on their environmental preferences (Bé and Hutson, 1977; Kennett and Srinivasan, 1983). Planktic foraminifer like Pulleniatina obliquiloculata is a characteristic species of KC in the ECS (Baohua et al., 1997; Ijiri et al., 2005; Ujiié et al., 2016). Other planktic foraminiferal species associated with the tropical warm KC are Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, etc. (Gallagher et al., 2015). In addition to stable isotope compositions (δ18O), the Mg/Ca ratio in planktic foraminiferal tests is a commonly used proxy to reconstruct paleo SST and past ocean conditions (e.g., Hollstein et al., 2017) including vertical mixing. This study aims to assess the paleoceanographic changes in the ECS and variations in the strength of the KC over the last 400 kyr focussing termination events using newly generated planktic foraminifer species associations along with published Mg/Ca ratio derived paleo SST and δ18O variations in planktic foraminifera.
Section snippets
Site location and oceanographic settings
Integrated Ocean Drilling Program (IODP) Site U1429 (31°37.04′N, 128°59.85′E) located on the western continental slope of the northern Okinawa Trough in water depths of 732 m (Fig. 1a and b) drilled up to 182 m core composite depth below seafloor (CCSF-D) (Tada et al., 2015). Surface waters of the ECS strongly influenced by the warm and saline KC and Taiwan Warm current (Fig. 1; Matsuzaki et al., 2016) as well as Changjiang Diluted Water (CDW), which is cooler, less saline and shows seasonal
Materials and methods
We have analysed 196 sediment core samples (10 cc) collected from IODP Site U1429, from the top of the sediment succession down to 182 m below seafloor (mbsf). Samples were processed using standard methods outlined by Das et al. (2018). We have generated planktic foraminiferal census data from an aliquot of ~300 or more specimens from the 125 μm + size fraction of each sample and calculated their percentages (Annexure-I). Six samples of which four from tephra layers (Sagawa et al., 2018) and
Planktic foraminifera
Forty-two species of planktic foraminifera were identified, of which some species are grouped based on their morphological similarities and identical environmental preferences (Haddam et al., 2016; Li et al., 2001; Schiebel and Hemleben, 2017): Neogloboquadrina pachyderma (D) and Neogloboquadrina pachyderma (S) into Neogloboquadrina pachyderma (total), Globorotalia tumida, Globorotalia flexuosa and Globorotalia menardii into Globorotalia menardii (total), Globigerinoides trilobus and
Environmental preferences of dominant planktic foraminifera and their associations in the ECS
The environmental preferences of planktic foraminifera were assessed using published literature (e.g., Ujiié et al., 2016) and planktic foraminiferal influx from the sediment trap experiment in the ECS (Yamasaki and Oda, 2003). The planktic foraminifera count per gram dry sediment has a wide range and planktic foraminifera count decreases near the tephra layer and at even MIS stages (Fig. 6). Factor 1 comprises of N. incompta, N. pachyderma (total) and N. dutertrei. Neogloboquadrina incompta is
Conclusions
The environmental preferences of planktic foraminiferal species assemblages suggest that four different paleoceanographic conditions were prevalent in the East China Sea (ECS) during the last 400 kyr. These conditions were modulated by differential inflow of surface Kuroshio Current, upwelling of subsurface Kuroshio water and Yangtze River discharge. KC influx was higher in the ECS and path was similar to that of the modern-day during the MIS 11 that weakened towards the MIS 10. KC inflow in
Declaration of Competing Interest
The authors declare that there is no Conflict of Interest with this study.
Acknowledgements
Integrated Ocean Drilling Program acknowledged for providing core samples to RKS (Request No. #4201 and 13522). RKS, NV, SM, and DKP acknowledge the financial support given by the ESSO-National Centre for Polar and Ocean Research, Ministry of Earth Sciences, under the IODP initiative to undertake this research (RP-077) and IIT Bhubaneswar (SP-053). NV acknowledges CSIR for providing CSIR-JRF and SRF (FILE NO: 09/1059(0013)/2017-EMR-I). AKG thanks DST, New Delhi for J.C. Bose Fellowship (Grant
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