Element enrichment and U-series isotopic characteristics of the hydrothermal sulfides at Jade site in the Okinawa Trough

The geochemical and U-series isotopic characteristics of hydrothermal sulfide samples from the Jade site (127A degrees 04.5'E, 27A degrees 15'N, water depth 1300-1450 m) at Jade site in the Okinawa Trough were analyzed. In the hydrothermal sulfide samples bearing sulfate (samples HOK1 and HOK2), the LREEs are relatively enriched. All the hydrothermal sulfide samples except HOK1 belong to Zn-rich hydrothermal sulfide. In comparison with Zn-rich hydrothermal sulfides from other fields, the contents of Zn, Pb, Ag, Cd, Au and Hg are higher, the contents of Fe, Al, Cr, Co, Ni, Sr, Te, Cs, Ti and U lower, and the Pb-210 radioactivity ratios and Pb-210/Pb ratios very low. In the hydrothermal sulfide mainly composed of sphalerite, the correlations between rare elements Hf and U, and Hf and Mn as well as that between dispersive elements Ga and Zn, are strongly positive; also the contents of Au and Ag are related to Fe-sulfide, because the low temperature promotes enrichment of Au and Ag. Meanwhile, the positive correlations between Fe and Bi and between Zn and Cd are not affected by the change of mineral assemblage. Based on the Pb-210/Pb ratios of hydrothermal sulfide samples (3.99x10(-5)-5.42x10(-5)), their U isotopic composition (U-238 content 1.15-2.53 ppm, U-238 activity 1.07-1.87 dpm/g, U-234 activity 1.15-2.09 dpm/g and U-234/U-238 ratio 1.07-1.14) and their Th-232 and Th-230 contents are at base level, and the chronological age of hydrothermal sulfide at Jade site in the Okinawa Trough is between 200 and 2000 yr.

Benthic foraminifera and bottom water evolution in the middle-southern Okinawa Trough during the last 18 ka

A piston sediment core E017 from the middle-southern Okinawa Trough was investigated. A preliminary study of the deep-water evolution since 18 cal. ka BP was performed based on the quantitative census data of benthic foraminiferal fauna, together with planktonic foraminiferal oxygen and carbon isotope, AMS(14)C dating, and the previous results achieved in the southern Okinawa Trough. The result shows that the benthic fauna was dominated by Bulimina aculeata (d'Orbigny), Uvigerina peregrina (Cushman), Hispid Uvigerina and Uvigerina dirupta (Todd) during the glaciation-deglaciation before 9.2 cal. ka BP, while Epistominella exigua (Brady), Pullenia bulloides (d'Orbigny), Cibicidoides hyalina (Hofker), Sphaeroidina bulloides (d'Orbigny) and Globocassidulina subglobosa (Brady) predominated the fauna in the post-glacial period after 9.2 cal. ka BP. The benthic foraminifera accumulation rate (BFAR), paleoproductivity estimates and benthic foraminiferal assemblage conformably indicate that surface water paleoproductivity and organic matter flux during the glaciation-deglaciation were higher than those of the post-glacial period in the middle-southern Okinawa Trough, and gradually enhanced from the southern to the central Okinawa Trough during the glaciation-deglaciation, which could be caused by the discrepancy of the terrigenous nutrients supply. High abundances of E exigua, an indicator of pulsed organic matter input, after 9.2 cal. ka. BP may indicate that the intensity of seasonally riverine pulsed flux during the post-glacial period was stronger than that of the glaciation-deglaciation period, and the seasonal influx in the central trough might be stronger than in the south. The temporal distributions of the typical species indicating bottom water oxygen content and ventilation condition show that the ventilation of the bottom water during the post-glacial period is more active than the glaciation-deglaciation, which reflects that the evolution of the intermediate and deep waters of the northwestern Pacific during the last glaciation has no evident influence on the deep-water of the middle-southern Okinawa Trough. Additionally, the variations in agglutinated benthic foraminiferal abundance and other carbonate dissolution proxies indicate that carbonate dissolution gradually increased since the last 18 ka in the Okinawa Trough and rapidly enhanced at 9.2 cal. ka BP. The modern shallow carbonate lysocline could form at 3 cal. ka BP.

Gas seepage on the sea floor of Okinawa Trough Miyako Section

A marine geophysical survey was carried out, on the RN Science 1 of the Institute of Oceanography, Chinese Academy of Sciences (IOCAS), in 2000, at the Miyako Section of Okinawa Trough. Here we present seismic and acoustic evidence of a gas seep on the sea floor on the western part of the Okinawa Through, near the lower slope of the East China Sea Slope and discuss the possibility of related formation of gas hydrate. A gas column reflection was observed in echo-sounder data above a section where the sea floor reflector was missing, on both the echo-sounder and the seismic data for line H14. The seismic data also show an acoustic curtain reflection and a turbidity reflection at this section. These anomalies are the evidence of the existence of a gas seep, which occupies an area 2.2 km in diameter. Based on the acoustic curtain on line H14, we believe that the amount of gas contained in the sediments below the gas seep is larger than 1 % by volume of sediment. Tectonically, the gas seep developed in a small basin controlled by basement uplift in the north, south and east. The thickness of the sediment layer can be greater than 3.5 km. A mud diapir structure was found in layer D beneath the gas seep. Over-pressure may occur due to the large sediment thickness and also the tectonic basement uplift in the north, south, and east. The mud diapir could be the preferential pathway for methane-rich fluids. The acoustic curtain may indicate that free gas related to the gas seep can be formed on the sea floor. We also note that the layer above the acoustic curtain on profile H14 may contain gas hydrate.

Research on crustal structure and lithosphere property in the Okinawa Trough area

A mass of geological, geophysical and geochemical data and information from the Okinawa Trough area are collected for comprehensive research in the study area from East China to Okinawa Trough and then to Ryukyu Island Are region. According to the seismic tomography result (P and S wave) and the processing result of free-air and Bouguer gravity anomaly and magnetic anomaly data in the study area, the comprehensive interpretation is carried out. The Moho depth distribution of the study area is obtained by the inversion calculation based on gravity data using the Harmonious Series method. The crust properties are analyzed. Meantime, some Cenozoic basalt data from Kuandian (NE China), Hannuoba (North China), Minxi (South China), Penghu Islands (Taiwan Strait), Okinawa Trough and Japan Island Arc regions are chosen to make the comparison research on element- isotopes. The result indicates that the lithosphere thickness in the Okinawa Trough area has obviously decreased, where a Low -velocity layer of upper-mantle has reached the Moho interface and the metasometized asthenosphere has formed. The research result on element- isotopes shows that the characteristic of the crust in the Okinawa Trough area is different from that in East China area and the Ryukyu Island Arc area. It is considered that the crust in the Okinawa Trough area belongs to the transition type, which is quite similar to the feature of the oceanic crust.

Paleoceanographic records in the sedimentary cores from the middle Okinawa Trough

Two gravity piston cores (Cores 155 and 18) involved in this study were collected from the middle Okinawa Trough. Stratigraphy of the two cores was divided and classified based on the features of planktonic foraminifera oxygen isotope changes together with depositional sequence, millennium-scale climatic event comparison, carbonate cycles and AMS(14)C dating. Some paleoclimatic information contained in sediments of these cores was extracted to discuss the paleoclimatic change rules and the short-time scale events presented in interglacial period. Analysis on the variation of oxygen isotope values in stage two shows that the middle part of the Okinawa Trough may have been affected by fresh water from the Yellow River and the Yangtze River during the Last Glacial Maximum (LGM). The oxygen isotope value oscillating ranges of the cores have verified that the marginal sea has an amplifying effect on climate changes. The delta(13)C of benthic foraminifera Uvigerina was lighter in the glacial period than that in the interglacial period, which indicates that the Paleo-Kuroshio's main stream moved eastward and its influence area decreased. According to the temperature difference during the "YD" period existing in Core 180 and other data, we can reach the conclusion that the climatic changes in the middle Okinawa Trough area were controlled by global climatic changes, but some regional factors had also considerable influence on the climate changes. Some results in this paper support Fairbanks's point that the "YD" event was a brief stagnation of sea level rising during the global warming up procession. Moreover, the falling of sea level in the glacial period weakened the exchange between the bottom water of the Okinawa Trough and the deep water of the northwestern Pacific Ocean and resulted in low oxygen state of bottom water in this area. These procedures are the reasons for carbonate cycle in the Okinawa Trough area being consistent with the "Atlantic type" carbonate cycle.

Benthic foraminiferal distribution in surface sediments along continental slope of the southern Okinawa Trough: dependance on water masses and food supply

Benthic foraminiferal analysis of 29 samples in surface sediments from the southern Okinawa Trough is carried out. The results indicate that benthic foraminiferal abundance decreases rapidly with increasing water depth. Percentage frequencies of agglutinated foraminifera further confirm the modem shallow carbonate lysocline in the southern Okinawa Trough. From continental shelf edge to the bottom of Okinawa Trough, benthic foraminiferal fauna in the surface sediments can be divided into 5 assemblages: (1) Continental shelf break assemblage, dominated by Cibicides pseudoungerianus, corresponds to subsurface water mass of the Kuroshio Current; (2) upper continental slope assemblage, dominated by Cassidulina carinata, Globocassidulina subglobosa, corresponds to intermediate water mass of the Kuroshio Current; (3) intermediate continental slope assemblage, dominated by Uvigerina hispida, corresponds to the Okinawa Trough deep water mass above the carbonate lysocline; (4), lower continental slope - trough bottom assemblage, dominated by Pullenia bulloides, Epistominella exigua and Cibicidoides hyalinus, corresponds to deep water mass of the Okinawa Trough; and (5) trough bottom agglutinated assemblage, dominated by Rhabdammina spp., Bathysiphon flavidus, corresponds to I strongly dissolved environment of the trough bottom. The benthic foraminiferal fauna in the southern Okinawa Trough are controlled jointly by water masses and food supply. Water temperature, oxygen concentration and carbonate dissolution of the water masses are important controlling factors especially for the continental shelf break and trough bottom assemblages. The food supply also plays an important role in these benthic foraminiferal assemblages along the western slope. of the Okinawa Trough. Both the abundance and the 5 assemblages of benthic foraminifera correspond well to the organic matter supply along the continental slope and a lateral transport of TSM (total suspended matter) and POC (particulate organic carbon) from the shelf break to the deep water is also an important food supply for benthic fauna in this region.

Geological records of marine environmental changes in the southern Okinawa Trough

Indexes of sediment grain size, sedimentation rates, geochemical composition, heavy minerals, benthic foraminiferal fauna, indicator species of the Kuroshio Current, paleo-SST and carbonate dissolution of core E017 conformably suggest a great marine environmental change occurring at about 10.1-9.2 cal. kaBP in the southern Okinawa Trough, which may correspond to the strengthening of the Kuroshio Warm Current and re-entering the Okinawa Trough through the sea area off northeast Taiwan. The invasion of Kuroshio current has experienced a process of gradual strengthening and then weakening, and its intensity became more fluctuation during the last 5000 years. Compared to the transition of sediment grain size, geochemical composition and heavy minerals, the foraminiferal faunas show a 900-year lag, which may indicate that the invasion of Kuroshio Current and the consequent sea surface and deep-water environmental changes is a gradual process, and fauna has an obvious lag compared to environment altering. The carbonate dissolution of the Okinawa Trough has had an apparent strengthening since 9.2 cal. kaBP, and reached a maximum in the late 3000 years, which may be caused by the deep-water environmental changes due to the invasion of Kuroshio Current.

A broad deglacial delta C-13 minimum event in planktonic foraminiferal records in the Okinawa Trough

The equatorial Pacific upwelling zone has been suspected of playing an important role in the global atmospheric CO2 changes associated with glacial-interglacial cycles. In order to assess the influencing scope of the surface water deglacial delta(13)C minimum in the tropical low-latitude Pacific, the core DGKS9603, collected from the middle Okinawa Trough, was examined for 4513 C records of planktonic foraminifera N. dutertrei and G. ruber. The planktonic foraminiferal delta(13)C records show a clear decreasing event from 20 to 6 cal. kaBP., which is characterized by long duration of about 14 ka and amplitude shift of 0.4 x 10(-3). Its minimum value occurred at 15.7 cal kaBP. The event shows fairly synchrony with the surface water deglacial delta(13)C minimum identified in the tropical Pacific and its marginal seas. Because there is no evidence in planktonic foraminiferal fauna and 45180 records for upwelling and river runoff enhancement, the broad deglacial delta(13)C minimum event in planktonic foraminiferal records revealed in core DGKS9603 might have been the direct influencing result of the deglacial surface water of the tropical Pacific. The identification for the event in the Okinawa Trough provides new evidence that the water evolution in the tropical low-latitude Pacific plays a key role in large regional, even global carbon cycle.

P wave velocity of the pumice from the Okinawa Trough at high temperature and high pressure.

P wave velocity of the pumice sample from the middle Okinawa Trough and andesite sample from vicinity Yingdao volcanic island, Kyushu Japan were measured at temperature (from room temperature to 1500 C) and pressure (from room pressure to 2.4GPa) using a multi-anvil pressure apparatus called the YJ-3000 press. The measured data shows that at low temperature and low pressure (<1GPa, <800degreesC), the P wave velocity of pumice is lower than that of andesite, while at high temperature and high pressure (>1GPa, >800degreesC) the P wave velocity of pumice and andesite. becomes consistent (5.9km/s). The paper points out that 1GPa/800degreesC is the point of thermodynamic phase transformation Okinawa Trough pumice and vicinity andesite, and the point is deeper than 18km.

The paleoclimatic events and cause in the Okinawa Trough during 50 kaBP

Planktonic foraminiferal delta O-18 record for core DGKS9603 from the Okinawa Trough shows a series of climatic fluctuations and sudden cooling events in short time scale during 50 kaBP, which appear to correlate closely to the Younger Dryas and Heinrich events H1-5 recorded in Chinese loess, the South China Sea, the North Atlantic cores and the Greenland ice cores. Three polarity reversal events, correlating to Gothenburg, Mungo and Laschamp events, approximately correspond to Heinrich events H1, H3 and H5 respectively, which could be a cause of global climate changes. The delta O-18 curve of the Okinawa Trough is well associated with the grain size record of the Lijiayuan loess profile in northwestern China and is somewhat different from the climate fluctuations documented in the Greenland ice cores. These correlation results indicate that regional factors play an important role in controlling the climate changes in the East Asia, and the East Asian Monsoon could be the prominent regional controlling factor.

Millennial-scale paleoceanography in Okinawa Trough during Late Quaternary period

Based on the analyses of foraminifer and accelerator mass spectrometer radiocarbon dating in DGKS9603 core from mid-Okinawa Trough close to bottom, oscillation curve, which expressed the relation between the surface water temperature and the depth, has been obtained by using foraminifer analysis and calculation of FP-12E transfer function. The whole core indicated seven cold phases and eight warm phases. Obvious expression of low temperature event during Middle and Late Holocene, YD,H1,H2,H3 and H4 events, as well as the short cold phase during the middle last glacial period, implied that short shifts since 50 kaBP would have been global significance. Sedimentation rate during cold phases is usually faster than that in warm stages, with the lowest rate in Holocene, which may be connected with rising sea level and principal axial of Kuroshio Current moving to west. Volcanic activities highly developed in Okinawa Trough during the Quaternary period, thus abundant volcanic glass and pumice were well preserved.

ISOTOPIC CHARACTERISTICS AND AGE DATING OF THE PUMICE IN OKINAWA TROUGH

The data on the isotope compositions of rubidium, strontium and oxygen in the pumice of Okinawa Trough are reported for the first time. The ages of the pumice were successfully dated with the method of U-series disequilibrium. Then, the material source, crystallization evolution of magma and activity cycles of volcanos are explored. Isotopic data show that pumice magma was originally from the mantle, but had undergone a full crystallization differentiation and had been contaminated to a fair extent by crust-derived materials before the magma was erupted out of the sea floor. According to the dating results available so far, the earliest volcanic eruption in Okinawa Trough occurred about 70,000 a ago and the latest eruption was about 10,000 a B.P. During this period, there were three volcanic eruption cycles which were respectively corresponding to the middle Late Pleistocene, the late Late Pleistocene and the Early Holocene.

Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation: Evidence from the sedimentation rate and planktonic foraminiferal record

Well-dated, high-resolution records of planktonic foraminifera and oxygen isotopes from two sediment cores, A7 and E017, in the middle Okinawa Trough reveal strong and rapid millennial-scale climate changes since similar to 18 to 17 thousand years before present (kyr B.P.). Sedimentation rate shows a sudden drop at similar to 11.2 cal. kyr B.P. due to a rapid rise of sea level after the Younger Dryas (YD) and consequently submergence of the large continental shelf on the East China Sea (ECS) and the retreat of the estuary providing sediment to the basin. During the last deglaciation, the relative abundance of warm and cold species of planktonic foraminifera fluctuates strongly, consistent with the timing of sea surface temperature (SST) variations determined from Mg/Ca measurements of planktonic foraminifera from one of the two cores. These fluctuations are coeval with climate variation recorded in the Greenland ice cores and North Atlantic sediments, namely Heinrich event 1 (H1), Bolling-Allerod (B/A) and YD events. At about 9.4 kyr B.P., a sudden change in the relative abundance of shallow to deep planktonic species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough, which was synchronous with a rapid sea-level rise at 9.5-9.2 kyr B.P. in the ECS, Yellow Sea (YS) and South China Sea (SCS). The abundance of planktonic foraminiferal species, together with Mg/Ca based SST, exhibits millennial-scale oscillations during the Holocene, with 7 cold events (at about 1.7, 2.3-4.6, 6.2, 7.3, 8.2, 9.6, 10.6 cal. kyr BP) superimposed on a Holocene warming trend. This Holocene trend, together with centennial-scale SST variations superimposed on the last deglacial trend, suggests that both high and low latitude influences affected the climatology of the Okinawa Trough. (c) 2006 Elsevier B.V. All rights reserved.