Kuroshio Current Research Articles

Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

Abstract Phytoplankton productivity and community structure in the East China Sea (ECS) play an important role in marine ecology and carbon cycle, but both have been changing rapidly in response to recent oceanic and atmospheric circulation changes. However, the lack of long-term records of phytoplankton productivity and community structure variability in the region hinders our understanding of natural forcing mechanisms. Here, we use the phytoplankton biomarker (brassicasterol, dinosterol and alkenones) contents as well as the ratios between these biomarkers in three sediment cores from the ECS shelf to reconstruct the spatiotemporal variations of productivity and community of diatoms, dinoflagellates and coccolithophores during the Holocene, respectively. During 9–7 ka, the ECS shelf was characterized by low phytoplankton productivity with low coccolithophore contribution, caused by the oligotrophic condition mainly owing to the restricted Kuroshio Current (KC) intrusion under low sea-level conditions, thus the lack of nutrient input. Phytoplankton productivity generally increased during 7–4.6 ka, in response to the initial intrusion of the Yellow Sea Warm Current (YSWC, a branch of the KC), bringing nutrient from the subsurface KC to the upper layer of the ECS for phytoplankton growth. Phytoplankton productivity continuously increased during 4.6–1 ka, due to an enhanced circulation system (YSWC and Yellow Sea Coastal Current (YSCC)) driven by strong East Asia Winter Monsoon (EAWM). Significantly, high alkenone contents and coccolithophore contribution in the eastern core F11A was associated with its location closer to the warm and saline YSWC, which was suitable for coccolithophore growth. Beyond diagenetic processes which could partly account for higher biomarker contents near core tops, elevated phytoplankton productivity during the last 1 ka might be induced by more nutrient supply from the intensified circulation system driven by enhanced KC and anthropogenic activities. The latter also resulted in high dinoflagellate proportions in all three cores. These temporal and spatial changes of phytoplankton productivity and community structure in the ECS during the Holocene corresponded to different mechanisms by the air-sea interaction, providing insights into distinguishing natural forcing and anthropogenic influences on marine ecology.

Dynamics of The FKT System with Different Mooring Lines

Abstract To harness the endless hydrokinetic energy of the Kuroshio current, the joint research team of the National Taiwan University and the National Taiwan Ocean University has developed a floating Kuroshio turbine (FKT) system in Taiwan. In normal operation, the system floats at a certain small depth from the ocean surface to reduce the wave effects and take advantage of faster current speeds. In the present study, the effect of the mooring line on the system dynamics is investigated computationally. Two different auxiliary mooring line designs and, for each design, three different common mooring lines (polyester ropes of neutral buoyancy, iron chains, and 6×19 wires ropes with wire core) are examined. The study makes use of several commercial and in-house packages, integrated to find various coefficients. It is found that the mooring line, the auxiliary mooring line design, and the gravity centre can have a significant effect on system fluctuations in normal operation if the combination of these factors is not properly matched.

Paleoceanographic changes in the Northern East China Sea during the last 400 kyr as inferred from radiolarian assemblages (IODP Site U1429)

The East China Sea (ECS) is a shallow marginal sea that is sensitive to glacio-eustatic sea-level changes and is influenced by warm oligotrophic water of the Kuroshio Current (KC), the nutrient-rich Taiwan Warm Current, and freshwater discharges from rivers in southern China during the East Asian summer monsoon season. In this area, local paleoceanographic changes for times prior to 40 ka remain poorly studied because of high sediment accumulation rates on the seafloor. During Integrated Ocean Drilling Program Expedition 346, long sediment cores representing the last 400 kyr were retrieved from the northern part of the ECS (Site U1429). In these cores, radiolarians are abundant and well-preserved, thus using the ecological properties of radiolarians, we analyzed how glacio-eustatic sea-level variations have influenced the paleoceanography of the ECS over the last 400 kyr, with a focus on changes in water properties at intermediate depths. Additionally, the summer sea surface temperature (SST) and intermediate water temperature at about 500 m were quantified by means of data on selected radiolarian species. The KC influenced the shallow water at Site U1429 during each interglacial period over the last 400 kyr (marine isotope stages [MISs] 1, 5, 7, 9, and 11), causing a high summer SST (about 27 °C), although inflow of the KC into the ECS was probably delayed until after the sea-level maximum of interglacial MIS 1 and MIS 5. During this lag time, ECS shelf water was the dominant influence on the system. During glacial periods (MISs 2–4, 6, and 10), our data suggest that coastal conditions prevailed, probably because of a sea-level drop of more than 90 m. At these times, the summer SST was colder, ca. 20 °C. Changes in the relative abundance of Cycladophora davisiana indicate that the most significant changes in the bottom water occurred during MIS 6, when the bottom water likely became poorer in oxygen. An increase in the shallow-water primary productivity during MIS 7 and MIS 6 was probably the key factor causing the oxygen-poor conditions.

Improvements in polycyclic aromatic hydrocarbon contamination in the Japan Sea: An interannual survey from 2008 to 2014

The concentrations and distributions of polycyclic aromatic hydrocarbons (PAHs) were examined from 2008 to 2014 in the surface waters of the Japan Sea region. PAH concentrations decreased in the Japan Sea and Tsushima Strait; consistent with improvements in atmospheric pollution from Japan. There were no changes in the Korean Strait, suggesting a continual influx of contamination separate from the Japan Sea. At all sites, PAHs in the dissolved phase were greater than in the particulate phase. A separate survey in 2011 studied whether contamination in the Kuroshio current accounted for the contamination in the Tsushima strait and Japan Sea. PAH concentrations increased as waters moved into the Tsushima strait and the Japan Sea, suggesting inputs were local and not from the Kuroshio current. This was however confounded by the greater influence of particulate phase PAHs, which are likely deposited by seasonal desert storms.

Dual effects of reversed winter–spring temperatures on year‐to‐year variation in the recruitment of chub mackerel (Scomber japonicus)

AbstractTo clarify the effects of temperature on the recruitment of chub mackerel (Scomber japonicus) in the North Pacific, we investigated the influence of winter surface temperature (WST) on spawners at the time of maturity around the spawning grounds and the influence of ambient spring temperature on larvae using estimated temperature (ET) obtained from particle tracking experiments. We found a significant positive correlation between ET approximately 10 days following hatching and the recruitment per spawning stock biomass (RPS) after 2000. The closer (more meandering) the Kuroshio Current (KC) axis was in relation to the spawning ground, the higher (lower) the spring surface temperature and the higher (lower) RPS was in the spawning ground. In contrast, WST inside KC near the maturity/spawning ground was significantly negatively correlated with RPS. A significant negative correlation between the temperatures in winter and spring was detected in the area after 2000, when the conditions of the Pacific decadal oscillation index and the stability of the Kuroshio Extension were synchronous, indicating that KC shifted northward during this time. The reversed temperature pattern was consistent with the winter–spring movement of KC axis in the offshore direction and was correlated with the winter–spring difference in the intensity of the Aleutian low. These results suggest that the annual variation in chub mackerel recruitment after 2000 was strongly affected by the combined effects of ambient temperature because of the reversal of conditions that occurred between winter and spring around the maturity/spawning ground, which was related to the KC path.

Persistent intensification of the Kuroshio Current during late Holocene cool intervals

The Kuroshio Current (KC), which redistributes heat and moisture from the tropics toward mid-latitudes, considerably affects ocean–atmosphere–land interactions along its pathways. However, our knowledge about the KC strength and associated forcing, based on investigations of instrumental data and model simulations, is controversial, impeding assessment of its possible change in the near future. In this study, we investigated KC strength over the past 4400 yr by analyzing alkenone (U37K′) sea surface temperature (SST) signal in a sediment core under the influence of the Yellow Sea Warm Current (YSWC), a branch of the KC in the western North Pacific, along with a coastal SST record over the last 1000 yr outside of the YSWC effect. Our SST record in the warm current pathway, opposite to the patterns of coastal SST and solar irradiance changes at multi-centennial timescales, effectively manifests variations of YSWC strength. In conjunction with published records, we show a remarkably intensified YSWC, and probably the KC as well, during the cool interval between 1400–1850 AD (the Little Ice Age, LIA). We suggest that cold climate conditions, perhaps in relation to strengthened East Asian winter monsoon and Pacific Walker circulation, were more favorable for KC enhancement.

Spatial and vertical distribution of 129I and 127I in the East China Sea: Inventory, source and transportation

Iodirne-129 is useful for tracking water mass movement in the ocean. In this study, the concentration of iodine isotopes in seawater of the East China Sea (ECS) in October 2013 were analyzed to investigate the spatial and vertical distribution of 129I and 127I to understand water mass exchange. Results showed that the 129I/127I atomic ratios varied with the water mass, with higher values of (10–20) × 10−11 in the coastal regions and lower values of <8 × 10−11 offshore. Inventories of 129I were estimated to be (0.23–1.7) × 1012 atoms m−2 (n = 18) in upper 100 m waters, which is comparable to those of other regions without being contaminated by the nuclear accidents or nuclear reprocessing facilities. The total amount of 129I in the ECS water column was estimated to be 88 g in which over 90% is attributed to the oceanic input (e.g., West Pacific) via the Kuroshio Current (KC). The contributions of 129I from Changjiang (Yangtze River) terrestrial watershed (<7.5%) and atmospheric fallout (<2.7%) were small. Those from the Fukushima accident were negligible during this investigation. The 129I/127I ratios versus salinity distribution showed the range and stratification of the Changjiang, Yellow Sea, and KC waters in the ECS. Our study shows that the Changjiang fresh water could be transported to the North Jiangsu coast in October; the Taiwan Warm Current water could intrude to Northern part of the Changjiang Estuary (32°N). Besides, our results suggest that the 129I/127I profile is useful to indicate the seawater mixing process in ocean marginal systems.

High-resolution reconstruction of sedimentary organic matter variability during the Holocene in the mud area of the Yellow Sea using multiple organic geochemical proxies

As a result of terrestrial input and local circulation, an area containing continuously deposited mud with abundant organic matter (OM) has formed in the Yellow Sea (YS) off the Shandong Peninsula, China, during the Holocene. The understanding of temporal variations in sedimentary organic matter (SOM) from this mud area helps to evaluate the influences of Yellow River discharge and marine productivity on the burial of organic matter in the YS. In this study, multiple organic geochemical proxies were used to reconstruct Holocene changes in SOM in a sediment core (A02C) recovered from the mud area of the YS. Higher total organic carbon (TOC) and biomarker contents during the early Holocene relative to the middle and late Holocene are attributed to sea-level fluctuations. The δ13C values of TOC (δ13CTOC) range from −21.8‰ to −23.5‰, becoming more positive from the bottom to the top of the core. TMBR' (terrestrial and marine biomarker ratio) values vary from 0.43 to 0.80, with a decreasing trend during the Holocene. The decrease in TOM contribution was related to elevated sea-level, shifting of the mouth of the Yellow River, and weakening of the East Asian Winter Monsoon (EAWM) during the Holocene. A mixing model with three end-members, based on δ13CTOC and TMBR', reveals that the contribution of soil OM (average 42%) is higher than that of plant OM (average 17%). Simultaneous decreases in the plant OM fraction and the marine OM fraction occurred around 10.3, 8.2, 5.9, 4.2, and 2.8 kyr BP, corresponding to drift-ice events in the North Atlantic. Spectral analyses also reveal that all three OM fractions exhibit 1015-year periodicity and that the soil and plant OM fractions display 1420-year periodicity, consistent with the 1000-year cycle of solar activity and the 1500-year cycle of the Kuroshio Current (KC), respectively.

Seasonal changes in the abundance of bacterial genes related to dimethylsulfoniopropionate catabolism in seawater from Ofunato Bay revealed by metagenomic analysis

Ofunato Bay is located in the northeastern Pacific Ocean area of Japan, and it has the highest biodiversity of marine organisms in the world, primarily due to tidal influences from the cold Oyashio and warm Kuroshio Currents. Our previous results from performing shotgun metagenomics indicated that Candidatus Pelagibacter ubique and Planktomarina temperata were the dominant bacteria (Reza et al., 2018a, 2018b). These bacteria are reportedly able to catabolize dimethylsulfoniopropionate (DMSP) produced from phytoplankton into dimethyl sulfide (DMS) or methanethiol (MeSH). This study was focused on seasonal changes in the abundances of bacterial genes (dddP, dmdA) related to DMSP catabolism in the seawater of Ofunato Bay by BLAST+ analysis using shotgun metagenomic datasets. We found seasonal changes among the Candidatus Pelagibacter ubique strains, including those of the HTCC1062 type and the Red Sea type. A good correlation was observed between the chlorophyll a concentrations and the abundances of the catabolic genes, suggesting that the bacteria directly interact with phytoplankton in the marine material cycle system and play important roles in producing DMS and MeSH from DMSP as signaling molecules for the possible formation of the scent of the tidewater or as fish attractants.

Spatiotemporal temperature variations in the East China Sea shelf during the Holocene in response to surface circulation evolution

The Holocene environment evolution in the East China Sea (ECS) is characterized by the gradual establishment and strengthening of its shelf circulation system, but knowledge about temperature responses in temporal and spatial scales is limited due to the lack of continuous high-resolution records. Here, we compare U37K' and TEX86 temperature records for three cores from the ECS shelf, which provide the temporal and spatial patterns of Holocene temperature structure variations. These temperature records revealed broadly consistent temporal trends with three intervals characterized by two distinct shifts. During the early Holocene (10.0–6.0 ka), the modern-type circulation system was not established, which resulted in strong water column stratification; and the higher sea surface temperature (SST) might be associated with the Holocene Thermal Maximum (HTM). The interval of 6.0 to 1.0/2.0 ka displayed a weaker stratification caused by the intrusion of the Yellow Sea Warm Current (YSWC) and the initiation of the circulation system. A decreasing SST trend was related to the formation of the cold eddy generated by the circulation system in the ECS. During 1.0/2.0 to 0 ka, temperatures were characterized by much weaker stratification and an abrupt decrease of SST caused by the enhanced circulation system and stronger cold eddy, respectively. Thus, the temperature structure in the shelf of ECS was closely related with circulation system changes during the mid-late Holocene, which was most likely driven by the intrusion of Kuroshio Current (KC). The significant asynchrony of temperature decreases in the three locations during the late Holocene was likely caused by the gradual expansion of the ECS cold eddy area.

Effect of ENSO events on larval and juvenile duration and transport of Japanese eel (Anguilla japonica).

Spawning ground of Japanese eel (Anguilla japonica) is located near the West Mariana Ridge seamount. The species travels through the North Equatorial Current (NEC) and then enters the Kuroshio Current (KC) on the migration toward East Asian growth habitats. Therefore, El Niño–Southern Oscillation (ENSO) events serve as the potentially important drivers of interannual variability across the equatorial Pacific. Because the NEC bifurcation and salinity profiles are related to ENSO events, we investigated the influence of locations of the NEC bifurcation and salinity front on the success of larval entry to the KC by numerically modeling particle transport in ocean currents from 1972 to 2013 and possible effects on the size of glass eels at continental recruitment and, via otolithometry on the duration of larval migration. Circulation and hydrography used for particle tracking were obtained from the results of the Model for Interdisciplinary Research on Climate (MIROC) high-resolution forecasting experiment. Our results demonstrated that during El Niño years, (1) the southward movement of the salinity front might cause the larvae to experience slower currents and (2) the northward movement of the NEC bifurcation might broaden the separation between their spawning ground and NEC bifurcation, thus prolonging the time needed for the larvae to enter the KC from their spawning ground, because of which the duration of entrainment in the water column and body size increase when eels reach estuarine waters. In addition, this might cause more water to flow into the Mindanao Current (MC), leading to a decline in the rate at which larvae get entrained into the KC.

Sources and distribution of tetraether lipids in sediments from the Zhejiang–Fujian coastal mud area, China, over the past 160 years: Implications for paleoclimate change

Proxies based on glycerol dialkyl glycerol tetraether (GDGT) lipids in surface sediments have been successfully used to reconstruct sea surface temperature (SST), mean annual air temperature (MAAT), and soil pH in the East China Sea and its adjacent ocean margins. In this study, two cores from the Zhejiang–Fujian coastal mud area (ZFCMA) were studied in detail to investigate temporal changes in the source and distribution of tetraether lipids in the area, and the implications of such data for paleoclimate change. The results indicate that isoprenoid GDGTs (isoGDGTs) in the ZFCMA were derived mainly from Thaumarchaeota, with a minor contribution from in situ and allochthonous methanogenic archaea in upwelling areas, whereas branched GDGTs (brGDGTs) originated mainly from terrigenous bacteria and in situ organisms. However, the minor methanogen-derived and allochthonous isoGDGTs were considered to have had no effect on inferred SSTs in this study. The presence of brGDGTs produced in situ might corrupt the relationship between the methylation index of branched tetraethers or the cyclization index of branched tetraethers (MBT/CBT) and MAAT or pH, particularly in hypoxic areas. However, #ringtetra data, used here as an index to evaluate the sources of the brGDGTs, indicated that the brGDGTs buried in the sediments are predominantly derived from soil erosion. Furthermore, temporal changes in the GDGT-based proxy records indicate that climate changes and events (e.g., relating to the East Asian Monsoon, Pacific Decadal Oscillation (PDO)/Kuroshio Current (KC) intensity, and flood events), as well as anthropogenic influences (e.g., dam construction, soil conservation, fertilizer usage, and discharge of industrial waste water and domestic sewage) affect the signal in the core sediments. SSTs fluctuated over the past 160 years in the ZFCMA, with higher values in the 20th century than in the 19th century. In addition, the humidity, as inferred from the relationship between precipitation/soil water content and pH, and continental temperature in the mid–lower Yangtze River basin switched between dry–hot/dry–cool and wet–hot/wet–cool over the last 160 years. The results also indicate that PDO/KC intensity plays a key role in temperature changes and has a larger impact on terrestrial ecosystems than on ocean ecosystems in the ZFCMA and the adjacent continent.

Typhoon Effect on Kuroshio and Green Island Wakes: A Modelling Study

Green Island, located in the typhoon-active eastern Taiwan coastal water, is the potential Kuroshio power plant site. In this study, a high resolution (250–2250 m) shallow-water equations model is used to investigate the effect of typhoon on the hydro-dynamics of Kuroshio and Green Island wakes. Two typhoon–Kuroshio interactions—typhoon Soulik and Holland’s typhoon model—are studied. Simulation results of typhoon Soulik indicate salient characteristics of Kuroshio, and downstream island wakes seems less affected by the typhoon Soulik, because the shortest distance of typhoon Soulik is 250 km away from Green Island and wind speed near Green Island is small. Moreover, Kuroshio currents increase when flow is in the same direction as the counterclockwise rotation of typhoon, and vice versa. This finding is in favorable agreement with the TOROS (Taiwan Ocean Radar Observing System) observed data. Simulations of Kuroshio and Holland’s typhoon model successfully reproduces the downstream recirculation and vortex street. Numerical results reveal that the slow moving typhoon has a more significant impact on the Kuroshio and downstream Green Island wakes than the fast moving typhoon does. The rightward bias phenomenon is evident—Kuroshio currents increase (decrease) in the right (left) of the moving typhoon’s track, due to the counterclockwise rotation of typhoon.

Spatial distribution and controlling factors of planktonic foraminifera in the modern western Pacific

Studies of the spatial distribution of living planktonic foraminifera in modern water columns are still lacking in the western Pacific. In this study, 40 plankton tows at 8 stations were conducted, focusing on the terminus of the North Equatorial Current (NEC) and its two bifurcated branches in the western Pacific. The results show that planktonic foraminifera were in high abundance around the Kuroshio Current (KC) source region and the Mindanao Current (MC) system but in low abundance in the NEC. The planktonic foraminiferal assemblage was dominated by G. sacculifer and G. ruber, with relative abundances of 44.0% and 27.3%, respectively. According to a cluster analysis of the results of seven stations, the stations can be divided into three groups, namely, Group NEC, Group KC and Group MC, which displayed different features of planktonic foraminiferal assemblages. The low abundance of planktonic foraminifera that characterized the NEC stations may be induced by the thick mixed layer and deep thermocline/nutricline in this region. However, stations in the KC region, which also had thick mixed layers and deep thermoclines, showed higher abundances of planktonic foraminifera than the NEC stations. This result was probably caused by the higher terrigenous input. The planktonic foraminifera show the highest abundance at the MC stations, which may be the result of existing upwelling conditions.

Understanding the Compositional Variability of the Major Components of Hydrothermal Plumes in the Okinawa Trough

Studies of the major components of hydrothermal plumes in seafloor hydrothermal fields are critical for an improved understanding of biogeochemical cycles and the large-scale distribution of elements in the submarine environment. The composition of major components in hydrothermal plume water column samples from 25 stations has been investigated in the middle and southern Okinawa Trough. The physical and chemical properties of hydrothermal plume water in the Okinawa Trough have been affected by input of the Kuroshio current, and its influence on hydrothermal plume water from the southern Okinawa Trough to the middle Okinawa Trough is reduced. The anomalous layers of seawater in the hydrothermal plume water columns have higher K+, Ca2+, Mn2+, B3+, Ca2+/SO42-, and Mn2+/Mg2+ ratios and higher optical anomalies than other layers. The Mg2+, SO42-, Mg2+/Ca2+, and SO42-/Mn2+ ratios of the anomalous layers are lower than other layers in the hydrothermal plume water columns and are consistent with concentrations in hydrothermal vent fluids in the Okinawa Trough. This suggests that the chemical variations of hydrothermal plumes in the Tangyin hydrothermal field, like other hydrothermal fields, result in the discharge of high K+, Ca2+, and B3+ and low Mg2+ and SO42- fluid. Furthermore, element ratios (e.g., Sr2+/Ca2+, Ca2+/Cl−) in hydrothermal plume water columns were found to be similar to those in average seawater, indicating that Sr2+/Ca2+ and Ca2+/Cl− ratios of hydrothermal plumes might be useful proxies for chemical properties of seawater. The hydrothermal K+, Ca2+, Mn2+, and B3+ flux to seawater in the Okinawa Trough is about 2.62–873, 1.04–326, 1.30–76.4, and 0.293–34.7 × 106 kg per year, respectively. The heat flux is about 0.159–1,973 × 105 W, which means that roughly 0.0006% of ocean heat is supplied by seafloor hydrothermal plumes in the Okinawa Trough.

Sedimentary evolution of the Yangtze River mouth (East China Sea) over the past 19,000 years, with emphasis on the Holocene variations in coastal currents

Water mass variations in the early Yangtze estuary during the postglacial transgression have not been thoroughly studied. This research gap has hampered the understanding of riverine sediment ‘source to sink’ processes in this region. This study presents the results of integrated analyses of grain size, pollen-spore-dinocyst assemblages, benthic foraminifera and ostracods from a borehole, H5, located at the northern Yangtze mouth, to investigate the complete sedimentary history and associated variations in coastal currents since 19ka. At the core site, a fluvial floodplain developed from ca. 19–11.3ka, whereas an estuary to shelf to subaqueous delta prevailed during most of the Holocene. Based on changes in the dominant microfauna species, which mainly consist of Chinese coastal water species, two main current patterns were distinguished: one dominated by the Yellow Sea coastal current (YSCC) before ca. 7.9ka and one dominated by the Yangtze dilute water (YDW). Marked increases in open-sea saline species, including Bolivina robusta, and stratified water dinocyst species Spiniferites ramous and Spiniferites spp. were interpreted as the encroachment of the Taiwan warm current (TWWC) and the intensification of the YDW, respectively. The YDW and TWWC penetrated the area dominated by the YSCC before 7.9ka, which was likely related to intensity variations in the Kuroshio Current (KC) and Asian summer monsoon (ASM). Under maximal ASM and KC conditions, upwelling resulted from the intensified intrusion of the TWWC, as indicated by the blooming of Bolivina robusta from 7.9–6.3ka. After ca. 6ka, the YDW clearly weakened due to the decline of the ASM, yet the Yangtze plume prevailed due to the rapid seaward migration of the Yangtze mouth. Considering the dominance of the YSCC in the early Holocene, Yellow River-derived sediment likely represented a large proportion of the material delivered to the early Yangtze estuary.

Seasonal fluctuations of ichthyoplankton assemblage in the northeastern South China Sea influenced by the Kuroshio intrusion

AbstractThe distribution and assemblage of ichthyoplankton related to local hydrographic features, especially intrusion of the Kuroshio Current (KC) and Guangdong Coastal Current (GCC) in the northeastern South China Sea (NESCS) were investigated in this study. A total of 193 species of ichthyoplankton belonging to 141 genera and 66 families were recognized in summer 2015 and late‐winter 2016. The ichthyoplankton assemblage was identified with distinct seasonal differences, and the differences were responded well with the oceanography conditions. In summer, the assemblages were identified into offshore and inshore groups, as controlled by the South China Sea Warm Current (SCSWC) in off‐shelf, upwelling, and the diluted water from the Pearl River in inshore. In winter, three groups were divided, representing the southwestern inshore, intrusion of the KC in the eastern offshore, and the GCC in the northern inshore area, respectively. Furthermore, Sigmops gracilis and Vinciguerria sp.3 were probably brought into the NESCS by intrusion of the KC from the eastern Luzon Strait. In addition, high ichthyoplankton abundance was found in high chlorophyll a located in upwelling areas in the northern coastal area in summer and in the southwestern Taiwan Bank in winter, respectively. The nutrition outcropped by the upwelling together, and those brought from the Pearl River probably supported high food availability of the ichthyoplankton in the northern coast of the NESCS.

Coral record of variability in the upstream Kuroshio Current during 1953–2004

AbstractThe Kuroshio Current (KC), one of the most important western boundary currents in the North Pacific Ocean, strongly affects regional hydroclimate in East Asia and upper ocean thermal structure. Limited by few on‐site observations, the responses of the KC to regional and remote climate forcings are still poorly understood. Here we use monthly coral δ18O data to reconstruct a KC transport record with annual to interannual resolution for the interval 1953–2004. The field site is located in southern Taiwan on the western flank of the upstream KC. Increased (reduced) KC transport would generate strong (weak) upwelling, resulting in relatively high (low) local coral δ18O. The upstream KC transport and downstream transport, off Tatsukushi Bay, Japan, covary on interannual and decadal time scales. This suggests common forcings, such as meridional drift of the North Equatorial Current bifurcation, or zonal climatic oscillations in the Pacific. The intensities of KC transport off southeastern and northeastern Taiwan are in phase before 1990 and antiphase after 1990. This difference may be due to a poleward shift of the subtropical western boundary current as a response to global warming.

New evidence for Kuroshio inflow and deepwater circulation in the Okinawa Trough, East China Sea: Sedimentary mercury variations over the last 20 kyr

AbstractThe evolution of the Kuroshio Current (KC) in the Okinawa Trough (OT) has long been of paleoceanographic interest and has been the focus of many investigations because of its large impacts on both the ocean environment and the climate in East Asia. Here we present a new high‐resolution mercury (Hg) record from the OT sediments that provides robust evidence for the glacial‐interglacial variabilities of KC inflow. The Hg levels are anomalously high in the Holocene deposits, which feature a large and abrupt Hg increase at ~9.6 ka and a significant drop at 4.5–3.2 ka. Our observations show that most Hg was derived from seafloor hydrothermal plumes via active lateral transportation. Thus, the generally high Hg levels since ~9.6 ka are associated with enhanced basin‐wide deepwater circulation triggered by the KC inflow. This driving mechanism for the deposition and widespread of Hg is also evidenced by an abrupt drop in Hg levels during the late Holocene neoglacial cold period (~5–3 ka) when the KC inflow was temporarily interrupted and deepwater circulation was consequently weak or even absent in the OT. The changes in sedimentary Hg appear to be synchronous with those in other geochemical proxies that suggest that bottom water conditions switched from anoxic to oxic due to increased upwelling from the bottom in tandem with the strengthening KC and reduced freshwater effect since ~9.6 ka. This study shows that the hydrothermal Hg is a sensitive and independent tool for the reconstruction of paleodynamics of KC in the OT.

Late Quaternary water temperature variations of the Northwest Pacific based on the lipid paleothermometers TEXH86, UK´37 and LDI

The Kuroshio Current (KC) and Oyashio Current (OC) are the two major western boundary currents of the Pacific Ocean and their interplay exerts a major control on the climate evolution of the northwestern Pacific region as well as East Asia. Although millennial scale variations in the strength and flow pattern of the KC are well documented, only little is known on the long-term evolution of this ocean current and its role in affecting regional and global climate over geological time scales. Here, we present surface and thermocline temperature records covering the last two glacial-interglacial cycles of IODP (Integrated Ocean Drilling Program) Site C0011, SE of Japan, using the lipid paleothermometers TEXH86 (tetraether index of tetraethers consisting of 86 carbon atoms), UK´37 (unsaturated ketone index) and LDI (long-chain diol index). Lower average water temperatures (20.1–20.7°C in TEXH86, 21.6–22.0°C in UK´37, and 20.7–21.9°C in LDI) during marine isotope stages (MIS) 2 and 6 are considered to indicate a reduction in warm water mass export from the Western Pacific Warm Pool (WPWP) to northern mid-latitudes via the KC due to decreased subtropical gyre circulation in the North Pacific. A synchronous southward displacement of the KC/OC interfrontal zone resulted in an overall stronger influence of colder and more polar waters at Site C0011. MIS 1, 3 and 5 are characterized by generally higher water temperatures (21.7–22.1°C in TEXH86, 23.2–24.3°C in UK´37, and 23.1–24.3°C in LDI), likely reflecting an increased northward transport of subtropical waters to the study site. Higher Holocene than Eemian water temperatures are attributed to a stronger KC and the formation of its short meander south of Japan, whereas a less strong KC during the Eemian likely favored the formation of the large meander path. Better correlations between the different lipid paleothermometers during cold MIS are considered to indicate more similar production seasons and habitat depths of the biological proxy sources with the latter factor likely driven by thermocline shoaling.