David D. Sheu

Marine geology in the arc-continent collision zone off southeastern Taiwan: Implications for late neogene evolution of the coastal range

Abstract Marine geology in the region between Taiwan and Luzon is characterized by a convergent tectonics in the south and an oblique arc-continent collision in the north. In the collision zone six geologic provinces are recognized. They are the Manila Trench, the accretionary prism of the Hengchun Ridge, the sutural basin of the Southern Longitudinal Trough, the subduction melange of the Huatung Ridge, the forearc basins of the Taitung and the North Luzon Troughs, and the volcanic arc of the North Luzon Ridge. The oceanic crust of the South China Sea is subducting eastward beneath the Philippine Sea plate along the Manila Trench south of 20°N. Further to the north, the Trench has changed progressively its axis toward the west from the subduction melanges off southeastern Taiwan at 3 Ma to the Hengchun peninsula of southern Taiwan at 2-1 Ma, and then to the present-day location off southwestern Taiwan. Because of the continuing collision and the westward shifting of the Manila Trench, sediments deposited in the region are passively incorporated into the accretionary prism and deformed to the Hengchun Ridge. After the collision the Southern Longitudinal Trough, consisting of a 2 km thick of orogenic sediments, represents a sutural basin superposed on the plate boundary between the accretionary prism of the Hengchun Ridge (Asian continent) and the subduction melange of the Huatung Ridge (Philippine Sea plate). The Huatung Ridge is a bathymetric high connected to the southern Coastal Range and is characteristic of low, negative free-air gravity and magnetic anomalies. The Ridge contains deeply weathered sedimentary blocks with abraded deep-water late Pliocene foraminifera and identical clay mineral compositions as those in the onshore Lichi Melange. This implies that at least a portion of the Huatung Ridge is composed of melange materials and represents a former subduction zone extending from the Lichi Melange of the southern Coastal Range. The North Luzon Trough and the Taitung Trough are the constructed foreac basins with 2 km of orogenic sediments unconformably overlying the volcanic islands and the subduction melange. The Taitung Trough is southward connected to the North Luzon Trough through a bathymetric passage, which was formed by narrowing the North Luzon Trough as a consequence of clockwise rotation of the Lutao volcanic island during the late Pleistocene. A detailed study of the four Neogene tectonostratigraphic units in the Coastal Range shows that the overall history of the formation of the Range can be related to the present-day geologic provinces and the on-going collision processes in the region off southeastern Taiwan. The Range had evolved from a southward accretion of the Shuilien-Loho remnant forearc basin and the Chimei volcanic pair at 2 Ma, followed by the Chengkung-Taiyuan remnant forearc basin and the Chengkuangao volcanic island pair at 1 Ma, and the Taitung Trough and the Lutao volcanic island pair presently in process. By analogy, the northern part of the North Luzon Trough and the Lanhsu volcanic island will be accreted to the Coastal Range in the next million years. The paired forearc basin-volcanic island accretion model proposed in this study may thus have implications for mountain ranges that evolved from similar tectonics of oblique arc-continent collision.

Surface distributions of carbon chemistry parameters in the East China Sea in summer 2007

[1] Comprehensive carbon chemistry data including total alkalinity (TA), dissolved inorganic carbon (DIC), pH, fugacity of CO 2 and other pertinent data (i.e., temperature, salinity, and levels of nitrate and chlorophyll a) were measured in surface waters of the East China Sea (ECS) shelf in July 2007. The results show that spatial variations in these parameters closely correspond to the distributions of various water types. The Changjiang Diluted Water (CDW) and Yellow Sea Water (YSW) areas are the two major sinks of atmospheric CO 2 ; the Coastal Upwelling Water (CUW) area is the most important CO 2 source, whereas the Kuroshio Water and Taiwan Current Warm Water areas are weak sources. The entire ECS acted as a sink for atmospheric CO 2 , with a flux of -2.4 to - 4.3 mmol C m -2 d -1 during the study period. Identification of the CUW source area suggests that previous studies might have overestimated CO 2 uptake by the ECS in summer. Our results further suggest that high biological production might be responsible for the strong sink in the CDW area but that high input of TA from the Huanghe River, which led to an elevated TA/DIC ratio, could have resulted in formation of a significant CO 2 sink in the YSW area. The present data set represents the most comprehensive CO 2 survey in the ECS to date and can thus be used as a baseline for monitoring future changes in the CO 2 system arising from the construction of the Three Gorges Dam in the middle stretch of the Changjiang River.

Evaluation of Cellulose Extraction Procedures for Stable Carbon Isotope Measurement in Tree Ring Research

Abstract Evaluation of cellulose extraction procedures for preparing tree ring samples for carbon isotopic analysis reveals that cellulose can be best extracted with a C6H6-C2H5OH mixture in a Soxhlet apparatus and then soaked and bleached in NaClO2 + CH3COOH solutions. Moreover, systematic measurements of δ13C of tree rings show that they vary considerably across rings and also vary around rings in different radial directions, a fact of great importance in interpretation of tree ring δ13C time series records for past climatic conditions.

Variability of tree-ring δ13C in Taiwan fir: Growth effect and response to May–October temperatures

We have analyzed the β13C of tree-ring cellulose in three radial directions from a 120-year old (1873–1992) Taiwan fir (Abies Kawakamii (Hayata) Ito) growing on the margin of a relatively open forest at an elevation of approximately 3,844 m in southern Taiwan. The overall δ13C trend increases from approximately −24.0‰ to −22.4‰ to 1950, then drops sharply and fluctuates around an averaged value of −23.2‰ to 1992. Moreover, values of δ13C in different radial directions reveal a greater variability before 1950 than those after 1950 with the maximal and averaged amplitudes of 1.62‰ vs. 1.30‰ and 0.70 ± 0.36‰ vs. 0.55 ± 0.31‰, respectively. A plot of annual increase in ring-area further shows that the growth of Taiwan fir has essentially stabilized in 1950. This synchronous change in 1950 points to a possible growth control on the observed variability of δ13C before 1950. The variation in δ13C from 1950 to 1992, however, is found to correlate inversely to the mean May–October temperatures and a response coefficient of −0.46‰ °C−1 is derived. Additional analysis of δ13C across five consecutive rings reveals that δ13C can vary up to 1.5‰ and becomes heavier from the early to the late woods. Such seasonal variations further appear to be different among individual rings. In view of these results, we conclude that apart from the commonly observed increase in tree-ring δ13C of forest trees due to the canopy effect, a similar increasing trend can also be seen in the coniferous trees and may be attributed to an accompanying decrease in the stomatal conductance with tree age. We further propose that an adequate estimate of the response of tree-ring δ13C to climatic and/or environmental changes must be performed on trees during stable growth.

High‐resolution paleoclimate records from monsoon Asia

Studying high-resolution climate records of the past 2000 years has been identified as a high research priority by the IGBP PAGES program (International Geosphere-Biosphere Program Past Global Changes) [Eddy, 1992] as well as by the U.S. Global Change research community [Bradley et al., 1991]. Monsoon Asia, particularly south and southeast Asia, has great potential for expanding our understanding of climatic fluctuations over the past 2 millennia. However, relatively little is known about climatic changes in that area over the last 2000 years. The lack of information is particularly significant because of the areas very large population and its vulnerability to climatic anomalies. At the same time, a real opportunity exists to document climatic changes in monsoon Asia by analyzing a number of types of high-resolution paleoclimatic record.

Optimization of Heating Programs of Gfaas for the Determination of Cd, Cu, Ni and Pb in Sediments Using Sequential Extraction Technique

Abstract The volatilization behaviors of Cd, Cu, Ni and Pb in different extracting media have been studied using graphite furnace atomic absorption spectrometry (GFAAS) to determine metal concentrations in natural sediments. Considerable interference was found for Cd, Ni, and Pb in the extracts of high matrix concentrations, and could be largely reduced by carefully selecting the ashing and atomization temperatures of GFAAS. Optimal heating programs are proposed to suit a variety of extracts containing different matrices.

Geochemical behavior of 210Pb and 210Po in the nearshore waters off western Taiwan.

Dissolved and particulate (210)Pb and (210)Po were determined at 15 stations along the coastline off western Taiwan in April 2007. The (210)Pb activities in dissolved and particulate phases fell within a relatively small range of 2.4-5.2 dpm 100 L(-1) and 1.0-3.2 dpm 100 L(-1), respectively. The dissolved and particulate (210)Po activities also fell within a small range of 0.8-3.4 dpm 100 L(-1) and 1.1-2.9 dpm 100 L(-1), respectively. The correlation of the distribution coefficients (K(d)) of (210)Pb and (210)Po with particle concentration in turbid waters are not as evident as in the open ocean. The mass balance calculation shows that the residence times of (210)Pb and (210)Po with respect to particle removal from the nearshore waters ranges from 3 to 15 days and from 14 to 125 days, respectively. The flux of particulate organic carbon was estimated by (210)Po proxy and ranged from 4.8 to 33.7 mmol-C m(-2) d(-1).

Removal fluxes of Mn and Fe from the nearshore waters along the west coast of Taiwan

Dissolved and particulate Mn and Fe in the nearshore waters were determined at 27 stations along the west coast of Taiwan during 19-23 November 2004. The latitudinal distributions of Mn and Fe were very similar; however, the concentration in the dissolved phase was lower, whereas that in the particulate phase was higher in the northern regime. The higher percentage of Fe than Mn that was associated with particles resulted in a two-order of magnitude higher particle-water partition coefficient for Fe, K(d)(Fe), than that for Mn, K(d)(Mn). The removal fluxes of Mn and Fe could be estimated by multiplying the particulate (234)Th removal flux with the Mn/(234)Th and Fe/(234)Th ratios in suspended particles, which ranged from 0.1 to 3.8 mmol m(-2) d(-1) for Mn, and from 3.4 to 194.5 mmol m(-2) d(-1) for Fe. Residence times ranged from 1 to 20 days for Mn and Fe were estimated in the nearshore.

Distributions and removal fluxes of trace metals in the water column of the Hung-Tsai Trough off southwestern Taiwan.

Vertical profiles of dissolved and particulate Mn, Fe, Ni, Cu, Zn, Cd, Pb, and (234)Th were determined in the Hung-Tsai Trough off southwestern Taiwan during 19-23 November, 2004. Except in the case of Cd, the distribution coefficient (K(d)) of the trace elements showed a negative correlation with the suspended particle concentration. Based on the average K(d) values, the general sequence of particle affinities for the eight trace elements is, from highest to lowest, Fe>Mn≈Pb>Zn≈Th>Cd≈Cu≈Ni. The trace metal data was coupled with the particle removal flux estimated from (234)Th/(238)U disequilibrium to investigate metal removal by particle sinking from the euphotic layer. The residence time of trace elements with respect to particle removal from the euphotic layer was estimated. A negative correlation between the residence time and the distribution coefficient for the trace metals was found.

Influence of El Niño on the sea-to-air CO2 flux at the SEATS time-series site, northern South China Sea

[1] The eastern equatorial Pacific is the predominant source area for atmospheric CO 2 , and the size of this source is significantly reduced during El Nino events. Here we apply a newly constructed 1999-2008 time series carbon chemistry trend to show a similar reduction can also be identified in the northern South China Sea. The net sea-to-air CO 2 flux during the 2002-2003 El Nino event (1.57 ± 0.13~1.61 ± 0.28 g C m -2 a -1 ) at the SEATS time series site was significantly lower than that during the 2007-2008 La Nina event (10.35 ± 0.66~10.67± 0.93 g C m -2 a -1 ). The appreciable reduction (by ~85%) is a direct response to the diminished vertical mixing of CO 2 -rich subsurface waters from below and, possibly, an increase in the lateral flow transport from the western Pacific during the weakening winter monsoon in the El Nino years. Thus, the suppression of CO 2 efflux during the El Nino events is a basin-wide phenomenon across from the eastern equatorial Pacific to the western subtropical Pacific including the northern South China Sea.