Hui-g Lin

THE EARLY MATUYAMA DIATOM MAXIMUM OFF SW AFRICA, BENGUELA CURRENT SYSTEM (ODP LEG 175)

Abstract Several sites of Ocean Drilling Program Leg 175 between 20° and 30°S (1081, 1082, 1083, 1084 and 1085) in the Benguela Current system show a distinct opal (and diatom) maximum within the late Pliocene and early Quaternary, spanning the lower half of the Matuyama reversed polarity Chron (MDM, Matuyama Diatom Maximum). This maximum is centered around 2.0–2.6 Ma, and follows a rapid increase of diatom deposition near 3.1 Ma. Within the MDM, we recognized intervals dominated by the antarctic/subantarctic species Thalassiothrix antarctica which have the aspect of diatom mats. The MDM (and the mats) developed during a time of sustained Southern Ocean influence (3.2–1.6 Ma), probably reflecting a period of seasonally-pulsed continuous advection of subantarctic waters into the Benguela Oceanic Current. The onset and cessation of the MDM raises questions about modes in the operation of the silica cycle and the relation of these to major oceanographic events. A depositional model for the late Pliocene is proposed which involves positional changes in a longshore system of fronts, substantial equatorward excursions of subantarctic waters during MDM time, and subsurface waters greatly enriched in silicate.

Cyclone-induced hyperpycnal turbidity currents in a submarine canyon

[1]xa0Density currents such as turbidity currents are major transport agents in various terrestrial, lacustrine, and marine environments worldwide. However, a gap exists between those who study the deposits by turbidity currents (turbidite) on a field scale, and those who study turbidity currents using small-scale laboratory experiments and theoretical/numerical models. We report two typhoon-triggered hyperpycnal turbidity current events observed in a submarine canyon. Our findings verify turbidite sequences with the characteristics of suspended sediment carried by passing turbidity currents that displayed distinct waxing and waning phases. Our study also confirms the direct link between typhoon-triggered hyperpycnal flows in a small mountainous river and turbidity currents in a nearby submarine canyon that transport sediment to the deep-sea efficiently.

Late Pleistocene nutrients and sea surface productivity in the South China Sea: a record of teleconnections with Northern hemisphere events

Surface nutrient variations during the last 200,000 years have been reconstructed based on the Cd=Ca ratios in the tests of planktonic foraminifera (Neogloboquadrina dutertrei ) and the biogenic opal content from two gravity cores retrieved from the northern and northwestern parts of the South China Sea (SCS). The Cd=Ca ratios obtained from N. dutertrei in Recent sediments from fourteen box cores distributed over the SCS basin reflect present nutrient levels in the overlying surface waters. Areas with relatively high Cd=Ca values in the surface sediments are in the region of high chlorophyll-a in the northern South China Sea near the Dongsha Islands, consistent with previous observation. High values of Cd=Ca found in sediments east of Vietnam and east of the Nansha Islands may be a result of increased nutrients due to local upwelling induced by the summer monsoon. Core 17940-2, raised from the northern continental slope of the SCS, exhibits exceptional high sedimentation rates (25‐85 cm=kyr) for the last 35 kyr. The Cd=Ca ratios in this core are relatively higher during the periods of late d 18 O stage 3 and the mid-Holocene. Core 17950-2, with an average sedimentation rate of 5 cm=kyr, offers a continuous long record back to d 18 O stage 7. In general, Cd=Ca values in the downcore records are higher during interglacials and progressively increase as the interglacials proceed. Values peak around the times of optimum climate conditions, and correlate with periods of increased opal wt%. Correlations in the interval of overlap between the two records support the interpretation that variations in Cd=Ca ratios in core 17940-2 are not an individual signal but can be replicated. Of special interest in this study is the tentative correlation between Cd=Ca peaks in the SCS record and the timing of iceberg discharge events (Heinrich events) in the North Atlantic. Mass accumulation rate (MAR) of opal data from core 17950-2 indicate high surface productivity during glacial intervals corresponding to oxygen isotope stages 6, 4, and 2. The increase in surface productivity during glacials from the northern SCS is contemporaneous with an intensified winter monsoon as recorded by the terrestrial loess deposits. The agreement with wind strength recorded in the Loess Plateau from mainland China supports the

Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

Taiwan [NSC 98-2116-M-001-005]; Academia Sinica Thematic Program AFOBi; China (973 Program) [2009CB421200]; Introducing Talents of Discipline to Universities [B07034]

Temporal variations of the trace fossil Zoophycos in a 425 ka long sediment record from the South China Sea: implications for the ethology of the Zoophycos producer

A 425 ka long record from piston core GIK17925-3 taken in the northeastern South China Sea was used to study the environmental conditions controlling the distribution of the Zoophycos trace fossil. The 12 m long core offers a unique opportunity to study the response of the Zoophycos -producing animal to environmental variations over four glacial–interglacial cycles. The trace fossils show a strong glacial to interglacial variation in their abundance with a special preference for intervals with low sedimentation rates. Additional X-ray radiograph studies of piston cores from the Southeast Asian Marginal Seas show that the trace fossil Zoophycos is widespread in slope and deep-marine sediments, with the highest abundances encountered in low sedimentation rate settings. The preference of the Zoophycos producer for low sedimentation rates in a setting with strong seasonal fluctuations in food supply due to the shift between winter and summer monsoons, is interpreted to be the result of a cache-model behaviour, where food is collected during rich times and squirrelled away for poor times. Core GIK17925-3 also offers an opportunity to assess the impact of Zoophycos bioturbation on various palaeoenvironmental proxies. In this core, more than 30 % of the measured data points were more or less strongly affected by Zoophycos bioturbation. Together with the widespread occurrence this percentage indicates that Zoophycos may pose a serious threat to palaeoclimatic reconstructions in cores from low to moderate sedimentation rate sites.

A late Pliocene diatom Ge/Si record from the Southeast Atlantic

Abstract As a result of both culture and sediment core studies, the ratio of germanium (Ge) to silicon (Si) in diatom shells has been proposed as a proxy for monitoring whole-ocean changes in seawater Ge/Si, a ratio affected by changes in continental weathering. However, because of the difficulties of extracting and cleaning diatom frustules from deep-sea sediments, only samples from highly pure diatom oozes in the Antarctic region have been previously analyzed. Here we present data on diatom Ge/Si ratios, (Ge/Si) opal , for the time interval between 3.1 and 1.9 Ma from a mid-latitude, coastal upwelling area where significant terrigenous sediment input complicated the sample processing and analyses. In general, our (Ge/Si) opal values show the same decreasing trend after 2.6 Ma than previously measured in Antarctic sediments (Shemesh et al., 1989. Paleoceanography 4, 221–231), but with a noisier background that may reflect the local imprint of proximal continental input superimposed upon global changes in the ocean reservoir. The time of initiation of large-scale North Hemisphere glaciation at ∼2.6 Ma is characterized by a declining pattern of diatom Ge/Si ratios, which could have resulted from a global increase in the input of riverine Si due to enhanced silica weathering and/or equatorward (northward) intrusions of subantarctic waters enriched in silica. High (Ge/Si) opal ratios are associated with high opal contents from the same sediment samples and with warm climate as indicated by depleted benthic foraminiferal δ 18 O values from the North and Equatorial Atlantic. Cold periods signified by enriched benthic δ 18 O values, on the contrary, are associated with lower (Ge/Si) opal ratios. We interpret diatom Ge/Si values to reflect the prevailing weathering state on the continents, with greater chemical weathering during warm and wet periods of the Pliocene and less during cooler and drier intervals.

Vertical distribution of δ13C of dissolved inorganic carbon in the northeastern South China Sea

Abstract The δ13C of ΣCO2 along with dissolved oxygen and nutrients in seawater collected near the Luzon Strait were analyzed to provide new data regarding water exchange between the South China Sea and the Pacific. Sampling covered the area 20–22°N and 118–120°E in April, June, and October 1995. Three stations were repeated during these cruises. In addition, sections along 119° and 120°E near the entrance of Luzon Strait were constructed. The depth profiles of δ13C from the nine stations presented in this study are similar to those reported from the open ocean. Correlation with chemical parameters (apparent oxygen utilization and phosphate) aids in the interpretation of the δ13C data. An apparent shift of δ13C profiles between cruises but at the same site was observed and was concluded to be related not to the seasonal change, but to the presence of different water masses exhibiting distinct θ-S relationships.

Examining N cycling in the northern South China Sea from N isotopic signals in nitrate and particulate phases

Nitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here, we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new dataset of N isotopic compositions (δ15N) of nitrate, zooplankton and sinking particles. Average δ15N in subsurface nitrate is 4.8±0.3‰, similar to that of sinking particles (δ15Nsink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long-term mooring traps at 200 m. This along with oft-observed shallow nitracline (< 100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsink at the bottom of the EZ resembles those of suspended particles (4.2±1.0‰) and zooplankton (5.4±1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000–3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N-depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsink in the deep waters. The significant δ15Nsink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleo- nitrogen processes.