Search

Abstract: Between May 2004 and March 2005, samples of suspended particulate matter (SPM) were collected from the top 200m on five cruises to the South-East Asia time-series study (SEATS) Station. Isotopic and elemental analyses of the organic matter in these samples gave δ 13C values ranging from −25.2‰ to −21.3‰ with a decreasing trend downward, and C/N ratios ranging from 5.5 to 11.4 with a weighted mean value of 6.74, which is very close to the Redfield ratio, suggesting a predominantly marine origin. The temporal isotopic variation in the surface layer has been successfully simulated with the algorithm based on diffusion-controlled carbon uptake during photosynthesis. The calculation of the carbon isotopic composition of phytoplankton was based on observed values of hydrographic, isotopic and chemical variables. It is noted that variations in the biological parameters, including the specific growth rate, enzymatic isotope fractionation during carbon fixation, cell size, and cell wall permeability, within the normal ranges may have contributed significantly to the observed isotope variability. According to simulation using the same algorithm, isotopically very light particulate organic carbon (POC) could be produced in the subsurface euphotic zone due to the much reduced specific growth rate, but the contribution of the subsurface production to the sinking flux was probably not significant. Sediment traps deployed from September 2001 to May 2002 in the northern South China Sea (SCS) provided samples for isotopic and elemental analyses of the organic matter. The measurements gave δ 13C values ranging from −25‰ to −20.8‰ and C/N ratios ranging from 5.5 to 18. The isotopic variation of organic carbon in the sediment trap samples was successfully explained by the mixing of terrigenous organics (δ 13C=−25.5‰ and C/N=22) and marine organics (δ 13C=−22.1±1.1‰ and C/N=6.63±1). The latter composition is very close to the weighted mean composition of the suspended particulate organic matter (POM) from the top 20m, implying the surface water as the major source of organic matter in sinking particles. Compared to previously reported results of samples collected from the seafloor in the SCS, the inferred δ 13C values of the marine organics in the sinking flux are lower than those (δ 13C=−22.9‰ to −20.1‰) of the POC in the nepheloid layer, which are in turn lower than those of the organics (δ 13C=−21.5‰ to −18.8‰) in surficial sediments. The progressively heavier POC below the top 200m is contrary to the trend of decreasing δ 13CPOC in the top 200m. We have demonstrated that the Suess effect and the elevated concentration of aqueous CO2 in the surface water due to the increasing atmospheric CO2 partial pressure may cause depletion of 13C more than enough to account for the observed depression of δ 13C values in the progressively younger POM. In the past, diagentic isotopic alteration has been proposed as the process responsible for both the decreasing trend of δ 13CPOC in the surface layer and the increasing trend in the subsurface layer. Although the diagenetic effect cannot be ruled out, this study shows that other processes are sufficient to explain the observed trends of isotopic variation of POC. [Copyright &y& Elsevier]

Abstract: The southwestern-most Okinawa Trough (SOT), characterized by high sedimentation rates (>0.1cm/yr), has the potential for recording high-resolution episodic events, such as storm floods and seismic activities, at least on a regional scale. To retrieve data on past climate change from nearby sediment cores and quantitatively reconstruct it, particularly with respect to precipitation (or typhoon-induced flood events), a linkage between fluvial sediment discharge and terrigenous sediment flux is warranted. Apparent sediment fluxes, observed with four arrays of sediment traps deployed in the SOT, were found to vary with fluvial sediment discharges. Empirical equations for individual arrays of sediment traps are site-dependent and related to the scenario of initial supply, transport and final deposition of terrigenous sediments (i.e. land–sea interaction). Using these equations and hydrological data from 1950 to 2000, the long-term temporal and spatial variations of settling sediment fluxes were simulated. Simulation results agree well with sediment mass accumulation rates derived from literature data on 210Pb and 137Cs chronology. The simulated spatial patterns of sediment fluxes along a slope–trough section illustrate that sediment plumes can disperse concurrently in two manners, namely near-bottom and mid-depth plumes, and the flood-driven plumes can travel very long distances, approaching 125°E or beyond. The sediment burial budget in the SOT was estimated to be approximately 5.2Mt/yr, representing about 80% of riverine exports from the Lanyang Hsi, Taiwan. This is the first study dealing qualitatively and quantitatively with two parameters, namely terrigenous sediment flux and fluvial sediment discharge. [Copyright &y& Elsevier]

We used multi-decadal pollen and diatom records from sediment core TFL-1 from Tsuifong Lake to reconstruct the vegetation dynamics and hydroclimate in northeastern Taiwan during the past 3500 cal BP. Coarse grained sediments in association with higher percentages of wetland pollen (Cyperaceae) and upper conifer pollen ( Tsuga and Pinus ) in the lower part of the core indicate low lake levels and a relatively cold/dry climate between 3500 and 2030 cal BP, reflecting a decline of the East Asian summer monsoon (EASM). Muddy sediments coupled with reduction of wetland pollen represent the rise of lake levels, implying the re-strengthening of the EASM during the past 2000 years. Paleotemperature was inferred from the variation of pollen origin from the upper and lower mountain forest, indicating the global temperature anomalies of the Medieval Warm Period (MWP) and the Little Ice Age (LIA). In comparison to the main climate forces in the North Pacific, we suggest that the long-term climatic trend in Taiwan was controlled by variations in EASM intensity, while increased precipitation over the past 2000 years may also be linked to warmer sea surface temperature (SST) of the western Pacific Warm Pool (WPWP) and increased El Niño-Southern Oscillation (ENSO) events, which increased typhoon intensity. Higher diatom-inferred pH during 2930–2030 cal BP and the LIA suggest strong hydrological disturbances, reflecting more typhoons passing over Taiwan. The frequent typhoon events could be linked by an abrupt shift of typhoon track, due to the reduction of the WPWP and expansion of the Northwestern Pacific High, which move typhoons in a more westerly direction. [ABSTRACT FROM AUTHOR]

Highlights: [•] We measured REE compositions of the fluvial sediment in Taiwan. [•] The average REE concentrations of fluvial sediment in Taiwan are higher than those of the Changjiang and Huanghe, but lower than the Zhujiang. [•] The REE patterns could be a useful tool for tracing sediment provenances. [•] Sediment provenance and chemical weathering exert important controls over the REE composition. [Copyright &y& Elsevier]

Abstract: We present a mineralogical and geochemical study of core MD012404, retrieved from the central Okinawa Trough (OT) of the East China Sea. Our studies reveal that the sediment sources of the core have been changed through time during the past 100 ka. Our mineralogical proxies indicate that the sediments source from the Yangtze River correlates well sea-level changes before 24 ka. Our Ti/Al ratios otherwise indicate an increase of sediment supply from eastern Taiwan after 26 ka. The cooler climate of the Last Glacial Maximum (LGM, 23∼19 ka) led to a reduction in fluvial sediments from the Yangtze River. However, subsequent climate warming (after ∼19 ka) resulted in an abrupt increase in fluvial sediments. After the LGM, the Kuroshio intrusion flow into the OT may have increased. We also infer anomalous changes in eolian sources transported by winter monsoons during the LGM and at 80 ka based on an eolian mineralogical indicator (feldspar). We conclude that the sediment source of core MD012404 is primarily of terrestrial origins, but influenced by sea-level changes and variations in the East Asian monsoon. [Copyright &y& Elsevier]

To evaluate the potential impact of geological processes and depositional history on shaping the subsurface biosphere, the bacterial community structures in a sediment core of the South China Sea was investigated by molecular approaches that target 16S rRNA gene fragments. Samples were obtained from different lithologic intervals at site U1433 during the International Ocean Discovery Program (IODP) Expedition 349. Bacterial abundance decreased rapidly with depth, with nearly three orders of magnitude decline within the first 100 m below seafloor (mbsf). Community diversity displayed a similar decreasing pattern, yet, a slight increase in diversity emerged in the early to middle Miocene. Such excursion might reflect enhanced cell activity in response to increasing temperature due to a steep geothermal gradient. Non-metric multidimensional scaling ordination revealed that the bacterial communities along the sediment core represent four clusters based on depth and geological time. There were distinct bacterial community shifts among clusters at 4.50–98.93 mbsf (Pleistocene), 108.15–273.20 mbsf (Pleistocene), 296.09–709.13 mbsf (Pliocene and late Miocene), and 732.10–789.91 mbsf (early to middle Miocene). Classification analysis revealed a striking pattern: the relative abundance of microorganisms affiliated with Gammaproteobacteria , Actinobacteria , and Cyanobacteria overall consistently decreased with depth, whereas those affiliated with Chloroflexi , candidate division OP9, candidate phylum BHI80-139, and Nitrospirae increased; these findings correspond to different clusters. Total organic carbon content and ratio of total organic carbon: total nitrogen, along with pore water phosphate concentration and salinity, were the statistically most significant variables that explained the bacterial community cluster pattern, which indicates potential linkages of bacterial communities to changes in quality and quantity of buried organic matter over geological time scales. Geographic isolation across depth was more important than environmental condition and geological age for the development of unique community structure in marine deep biosphere, although environmental variables partially shaped bacterial community composition. [ABSTRACT FROM AUTHOR]

In 2010 two moorings each configured with a CTD and an ADCP, one with an additional non-sequential sediment trap (NSST), were deployed in the head region of the Gaoping Submarine Canyon 3 days after the typhoon-induced peaks of the runoff and suspended sediment concentration (SSC) of the Gaoping River in southern Taiwan. Our data show a demarcation between tidal and hyperpycnal regimes in the temperature, salinity, and flow fields. The latter existed in the first 5 days out of the 18-day deployment, as defined by higher water density due to high SSC. Several lines of evidence indicate the presence of the tail end of a hyperpycnal turbidity current (HTC), including the retention of warm water near the canyon floor, high SSC, down-canyon directed residual flow and its vertical structure, and high terrestrial fraction (larger than 70%) of the organic particles carried by the flow. The decreasing mass flux during the deployment is also an indication of a waning HTC. Our findings also show that the vertical flow structure and the direction of the gravity-driven down-canyon HTC were retarded by the instantaneous up-canyon-directed tidal oscillations in the submarine canyon. [ABSTRACT FROM AUTHOR]

Abstract: Surficial grain-size and down-core clay mineralogical data show that sediment along the inner-most part of the continental shelf in East China Sea is mainly derived from the Yangtze River (Changjiang), spanning from the Yangtze mouth (33°N) ~1000km southward to the southwestern corner of the Taiwan Strait (24°N). High-resolution CHIRP seismic profiles reveal an elongated mud wedge extending along the inner shelf, with a northern depocenter on the modern Yangtze delta and a southern depocenter at 27.5°N. Four distinct acoustic units are delineated within the mud wedge (from bottom up): unit I (late-Pleistocene, mainly valley fills), unit II (formed by transgressions, thin strata), unit III (11–2kyr BP, downlapping strata) and unit IV (2–0kyr BP, flat and opaque strata). Incised valleys, up to 15-m deep, are filled by flat-lying or inclined strata in unit I. The thin (<3m) and acoustically transparent unit II is only seen between 30 and 26°N in water depths between 40 and 90m. Separated by acoustically opaque strata or unconformities, units III and IV are widely distributed. During the past 11kyr Yangtze sediment accumulation has been unsteady, showing two high and one low accumulation-rate periods. The high-accumulation period at 5–8kyr BP may be related to maximum East Asian summer monsoon precipitation in the Yangtze basin; the other high-accumulation period, 0–2kyr BP, probably reflects intensive human activities in the river''s watershed. The low-accumulation-rate period at 2–5kyr BP, which is seen in both northern and southern Yangtze depocenters, is probably related to low river discharge and/or intensified Taiwan Warm Current and China Coastal Current. [Copyright &y& Elsevier]

Abstract: Changes in paleorainfall intensity linked to the strength of the East Asian (EA) summer monsoon since 21cal kyr BP are inferred from the organic matter contents of a 15-m sediment core from Tung-Yuan Pond in southern Taiwan. High total organic carbon/total nitrogen (TOC/TN) values in association with increased TOC content suggest that more soil-derived material containing terrestrial organic matter (OM) was delivered to the lake during periods of increased runoff associated with extensive precipitation that resulted from intensified summer monsoons, whereas low values indicate OM possessing a dominant algal origin during weakened summer monsoons. Rainfall intensity in terms of the proportion of terrestrial OM was high in four periods: the last deglaciation (∼17.2 to ∼12.2ka), the early Holocene (∼10.6 to ∼8.6ka), the middle Holocene Thermal Optimum (∼7.7 to ∼5ka) and the late Holocene (∼4.2 to ∼2ka), whereas it was low in the intervening time periods. High TOC/TN values coincide with peak values of summer insolation, and thus the strongest EA summer monsoon during the early and middle Holocene; small drops in these ratios correspond to increasing and decreasing solar radiation in the deglacial period and the late Holocene, respectively. The four intervals with low TOC/TN ratios, as well as episodic drops of the ratios during the deglaciation and the early and late Holocene are concordant with the late last glacial (∼21 to ∼17.2ka), the Oldest (∼14.8ka), the Older (∼13.3ka) and the Younger Dryas (∼13 to ∼11.5ka), the 8.2 cold event (∼8.6–7.7ka) and a late Holocene cold event (∼5–4.2ka), and suggest a weakened EA summer monsoon during these times. Moreover, high frequency hydrological variability occurred during the early Holocene, heavy rainfall persisted during the middle Holocene, and precipitation intensity generally diminished after ∼5ka. The Tung-Yuan Pond sediment record indicates that the TOC/TN ratio can be used as a paleorainfall intensity proxy to trace variations in the EA summer monsoon strength in other small lakes. [Copyright &y& Elsevier]

Abstract: In this study, polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations and depositions in ambient air, water column and sediment were measured at a coupled reservoir-watershed system in northern Taiwan. The atmospheric PCDD/F concentration measured in the vicinity of the reservoir ranged from 4.9 to 39fgI-TEQm−3 and the Asian dust storm in February accounted for the peak value, which corresponded to a total suspended particle concentration of 128μgm−3. The atmospheric PCDD/F deposition ranged from 1.4 to 19pg I-TEQm−2 d−1, with higher deposition occurring during winter and spring (long-range transport events). During summer, when atmospheric deposition is lower, consecutive tropical cyclones (typhoons) bring heavy rainfall that enhances soil erosion and creates turbidity-driven intermediate flow. This results in significantly higher PCDD/F deposition in water column of the reservoir at 70m water depth (179pg I-TEQ m−2 d−1) than at 20m (21pg I-TEQm−2 d−1) during typhoon event. The accumulation rate of PCDD/Fs (9.1ngI-TEQm−2 y−1) in the reservoir sediments (depth: 0–2cm) was consistent with PCDD/F deposition obtained from water column (6.1 and 8.3ngI-TEQm−2 y−1); however, it is significantly higher when compared to the atmospheric deposition (2.0ng I-TEQm−2 y−1). Based on the mass balance between the measurements of atmospheric deposition and sinking particles in water column, around 54–74% of PCDD/F inputs into the reservoir were contributed by the catchment erosion during normal period. However, the PCDD/F input contributed by the enhanced catchment erosion significantly increased to 90% during intensive typhoon events. [Copyright &y& Elsevier]