Xufeng Zheng

ITCZ and ENSO pacing on East Asian winter monsoon variation during the Holocene: Sedimentological evidence from the Okinawa Trough

Deep-sea fan sediments provide an excellent geological archive for paleoenvironment reconstruction. Grain size, clay mineral and elemental (Ti, Fe, Ca) compositions were measured for a core retrieved from a submarine fan in the Okinawa Trough. Varimax-rotated Principal Component Analysis (V-PCA) on time-evolution of grain size spectrum reveals that, since the Holocene, sediment was transported mainly by the benthic nepheloid layer (33%) and upper layers (33%) which is driven by the East Asian winter monsoon (EAWM). The intensification of the Kuroshio Current during the Holocene, masks the fluvial signal of the summer monsoon and obstructs clay minerals derived from the Yellow River, a major contributor prior to 12 ka BP. A new grain size index (GSI), which represents the EAWM well, exhibits a negative correlation with the delta O-18 record in Dongge Cave, China during the Holocene when sea level was relatively steady. This anti-correlation suggests the southward migration of the Intertropical Convergence Zone (ITCZ). The consistency among our records and rainfall records in Peru, Ti counts in the Cariaco Basin, monsoon records in Oman and the averaged summer insolation pattern at 30 degrees N further support the ITCZs impact on monsoon systems globally. Cross-Correlation Analyses for GSI and log(Ti/Ca) against delta O-18 record in Dongge Cave reveal a decoupling between the East Asian winter and summer monsoon during 5500-2500 cal yr BP, with greater complexity in the last 2500 years. This can be attributed to exacerbated ENSO mode fluctuations and possibly anthropogenic interference superimposed on insolation and ITCZ forcing.

Deepwater circulation variation in the South China Sea since the Last Glacial Maximum

Deepwater circulation plays a central role in global climate. Compared with the Atlantic, the Pacific deepwater circulation’s history remains unclear. The Luzon overflow, a branch of the North Pacific deep water, determines the ventilation rate of the South China Sea (SCS) basin. Sedimentary magnetic properties in the SCS reflect millennial-scale fluctuations in deep current intensity and orientation. The data suggest a slightly stronger current at the Last Glacial Maximum compared to the Holocene. But, the most striking increase in deep current occurred during Heinrich stadial 1 (H1) and to a lesser extent during the Younger Dryas (YD). Results of a transient deglacial experiment suggest that the northeastern current strengthening at the entrance of the SCS during H1 and the YD, times of weak North Atlantic Deep Water formation, could be linked to enhanced formation of North Pacific Deep Water.Deepwater circulation plays a central role in global climate. Compared with the Atlantic, the Pacific deepwater circulations history remains unclear. The Luzon overflow, a branch of the North Pacific deep water, determines the ventilation rate of the South China Sea (SCS) basin. Sedimentary magnetic properties in the SCS reflect millennial-scale fluctuations in deep current intensity and orientation. The data suggests a slightly stronger current at the Last Glacial Maximum compared to the Holocene. But, the most striking increase in deep current occurred during Heinrich stadial 1 (H1) and, to a lesser extent during the Younger Dryas (YD). Results of a transient deglacial experiment suggest that the Northeastern current strengthening at the entrance of the SCS during H1 and the YD, times of weak North Atlantic Deep Water formation, could be linked to enhanced formation of North Pacific Deep Water.

Contribution of Asian dust and volcanic material to the western Philippine Sea over the last 220 kyr as inferred from grain size and Sr-Nd isotopes

Asian dust and volcanogenic materials are two major components in the northwestern Pacific. Quantitatively distinguishing them and estimating their mass accumulation rates (MARs) are very important for understanding regional and global climate change. Here we present the grain-size composition of detrital sediments and the radiogenic strontium (Sr) and neodymium (Nd) isotopic compositions of different grain-size fractions of detrital sediments that were recovered from the western Philippine Sea. These new records show that the different grain-size distributions can be associated with 1) Asian dust from the western and central Chinese deserts and Chinese loess and 2) volcanogenic materials that were derived from the Luzon Islands. The MARs of this Asian dust and volcanic materials are obtained by using Weibull-function fitting. The MARs of Asian dust and volcanic materials are coupled with the glacial-interglacial cycle; these values are found to have been higher and more variable during the glacial period than during the interglacial period. We argue that the strengthening aridity of the Asian continent, which is connected to solar insolation and ice volume variations from orbital eccentricity, constitutes an important mechanism that drives the high MARs of glacial dust in the western Philippine Sea. The internal positive feedback of dust may be another important mechanism. The significant increase in volcanic material during the glacial period was caused by sea level changes, which were driven by the ice volume and solar insolation at high latitudes, and by strengthened precipitation from the El Nino/Southern Oscillation (ENSO), which is driven by orbital eccentricity and precession cycles on the Luzon Islands.

Distinct control mechanism of fine‐grained sediments from Yellow River and Kyushu supply in the northern Okinawa Trough since the last glacial

High-resolution multi-proxy records, including clay minerals and Sr-Nd-Pb isotopes of the clay-sized silicate fraction of sediments from IODP Site U1429 in the northern Okinawa Trough, provide reliable evidence for distinct control mechanism on fine-grained sediments input from the Yellow River and the southern Japanese Islands to the northern Okinawa Trough since 34 ka BP. Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu. Since the last glacial, clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation. During low sea-level stage (∼34‒14 ka BP), the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough, favoring large fluvial discharge or even direct input of detrital sediments, which resulted about four times more flux of clay-sized sediments supply to the study area as during the relatively high sea-level stage (∼14‒0 ka BP). The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current and Tsushima Warm Current intensity, with increased input in phase with weakened Kuroshio Current/Tsushima Warm Current. Our study suggests that the Kuroshio Current was very likely flowed into the Okinawa Trough and thus influenced the fine-grained sediment transport in the area throughout the last glacial and deglacial. During ∼34‒11 ka BP, the Kyushu clay-sized sediment input was mainly controlled by the Kuroshio Current. Since ∼11 ka BP, the occurrence of Tsushima Warm Current became important in influencing the Kyushu fine-grained sediment input to the northern Okinawa Trough.

Formation of the modern current system in the East China Sea since the early Holocene and its relationship with sea level and the monsoon system

The Okinawa Trough is a natural laboratory for the study of air-sea interaction and paleoenvironmental change. It has been demonstrated that present offshore export of particles in the bottom nepheloid layer occur primarily with downwelling from the northeast winter monsoon, which is inhibited by a transverse circulation pattern in summer. This current system was very different during the Last Glacial Maximum owing to low sea level (−120 m) and exposure of a large shelf area. We collected sediment core Oki01 from the middle Okinawa Trough during 2012 using R/V Kexue No. 1 to elucidate the timing and cause of the current system transition in the East China Sea. Clay mineral, dry density, and elemental (Ti, Ca) composition of core Oki01 was analyzed. The results indicate that clay minerals derived mainly from the Huanghe (Yellow) and the Changjiang (Yangtze) Rivers during 16.0–11.6 ka, and the modern current system in the East China Sea formed beginning in the early Holocene. Therefore, mixing of East China Sea continental shelf, Changjiang River and partially Taiwan Island sediment are the major contributors. The decrease of log(Ti/Ca) and alternating provenance since the early Holocene indicate less sediment from the East China in summer because of resistance of the modern current system, i.e., a “water barrier” and upwelling. Conversely, sediment delivery persists in winter and log(Ti/Ca) indicates the winter monsoon signal since the early Holocene. Our evidence also suggests that sediment from Taiwan Island could be transported by the Kuroshio Current to the middle Okinawa Trough, where it mingles with winter monsoon-induced export of sediment from the Changjiang River and East China Sea continental shelf. Although the present research advances understanding of the evolutionary history of paleoenvironmental change in the Okinawa Trough, more sediment cores should be retrieved over wide areas to construct a larger scenario.

Magnetocaloric Effect in ErSi Compound

The magnetic properties and magnetocaloric effects (MCEs) have been investigated in ErSi compound, which crystallizes in the orthorhombic CrB-type structure. This compound is antiferromagnetic (AFM) with a Néel temperature <i>TN</i> of 12.5 K. The measurements of the temperature and field dependences of the magnetization clearly show that the ErSi compound undergoes a field-induced metamagnetic transition from AFM to ferromagnetic (FM) state below <i>TN</i>. The isothermal magnetic entropy change (Δ<i>SM</i>) and refrigerant capacity (RC) of ErSi have been measured for various applied magnetic fields and their values are found to be -14.8J ·kg ^-1·K ^-1 and 210 J ·kg ^-1 for a field change of 0-5 T, respectively. The large MCE is attributed to the field-induced AFM to FM metamagnetism occurring in this compound.

Millennial-scale ocean dynamics controlled export productivity in the subtropical North Pacific

The integrated effects of ocean-climate dynamics on export production in the North Pacific have remained elusive. We present a 91 k.y. export productivity (EP) record based on sedimentary reactive phosphorus from the western subtropical North Pacific. On a millennial time scale, EP decreased during Northern Hemisphere cold events when atmospheric dust loading was high, and increased during warm episodes. The inferred antiphase relation between dust and EP suggests that the supply of macronutrients to the sunlit surface ocean, modulated by the penetration depth of North Pacific Intermediate Water and not eolian Fe, exerted a major control on EP in the subtropical North Pacific. A compilation of global EP records suggests that eolian Fe most likely played a role in stimulating EP regionally only in the Subantarctic zone of the Southern Ocean. Over the past 91 k.y., during the cold-south–warm-north phase of the bipolar seesaw, the biological pump in both hemispheres was enhanced synchronously, yet by different drivers; atmospheric Fe input for the Subantarctic and subsurface macronutrient supply for the North Pacific, including the tropical and/or subtropical Pacific, and the Antarctic zone of the Southern Ocean.

Rare earth elements and yttrium in ferromanganese deposits from the South China Sea: distribution, composition and resource considerations

Ferromanganese nodules and crusts contain relatively high concentration of rare earth elements (REE) and yttrium (REY), with a growing interest in exploitation as an alternative to land-based REY resources. On the basis of comprehensive geochemical approach, the abundance and distribution of REY in the ferromanganese nodules from the South China Sea are analyzed. The results indicate that the REY contents in ferromanganese deposits show a clear geographic regularity. Total REY contents range from 69.1×10–6 to 2 919.4×10–6, with an average value of 1 459.5×10–6. Especially, the enrichment rate of Ce content is high, accounting for almost 60% of the total REY. This REE enrichment is controlled mainly by the sorption of ferromanganese oxides and clay minerals in the nodules and crusts. Moreover, the total REY are higher in ferromanganese deposits of hydrogenous origin than of diagenetic origin. Finally, Light REE (LREE) and heavy REE (HREE) oxides of the ferromanganese deposits in the study area can be classified into four grades: non-enriched type, weakly enriched type, enriched type, and extremely enriched type. According to the classification criteria of rare earth resources, the Xisha and Zhongsha platform-central deep basin areas show a great potential for these rare earth metals.

Footprints of obliquely incident internal solitary waves and internal tides near the shelf break in the northern South China Sea: FOOTPRINTS OF ISWS AND INTERNAL TIDES

A mooring system and two sites of bottom currents were deployed over the slope and near the shelf break on the propagating paths of internal solitary waves (ISWs), west off Dongsha Atoll in the northern South China Sea. Data indicated that energetic ISWs obliquely shoaled onto the shelf west off Dongsha Atoll in an approximately 290 degrees direction, causing strong reversing currents (some exceeding 80 cm/s) near the bottom. Two types of sandwaves and short scour channels are discernible on the seafloor near the shelf break, which have reasonable correlations with the obliquely incident ISWs and internal tides. Type 1 sandwaves, featured by ISWs at the depths of 130-150 m, have flat crests interacting with the isobaths at an angle of nearly 45 degrees which slightly incline and migrate upslope. Type 2 sandwaves are associated with internal tides, which have crests parallel to the isobaths and distinctly incline and migrate downslope. Short channels are parallel to the depth contours and truncate the strata, which could be formed and maintained by along-slope currents that are probably produced by the obliquely ISWs on a large gradient (gamma>0.8 degrees). The ISWs can move coarse grains or suspend fine grains but do not change the long-term trend of sediment transport on the seabed with larger gradients (gamma/c>1), which is dominated by internal tides. These features are likely widespread near the shelf break in the northern South China Sea and other seas but are limited on mild slopes where ISWs do not break.