S. J. Kao

Water and sediment discharge from small mountainous rivers, Taiwan: The roles of lithology, episodic events, and human activities

Taiwan’s natural setting creates highly vulnerable watersheds whose rivers discharge disproportionately large quantities of sediment to the coastal ocean. The 16 Taiwanese rivers analyzed in this article discharge ∼180 Mt yr-1 of sediment to the coastal ocean, although totals over the past 20 years have varied between 16 and 440 Mt yr-1. The mean annual sediment yield of 9500 t km-2 yr-1 for the 16 rivers is 60-fold greater than the global yield of 150 t km-2 yr-1, but mean yields for the individual rivers vary by more than 2 orders of magnitude, from 500 to 71,000 t km-2 yr-1. Most sediment erosion and delivery occur in response to typhoon-generated floods, as evidenced by the fact that >75% of the long-term flux occurs in < 1% of the time, about one-third of which reaches hyperpycnal concentrations. Detailed analysis of the 16 watersheds reveals little evidence of any single environmental factor that controls sediment load. The Erren, the highest-yield river on Taiwan, drains an erodible but low-gradient watershed with relatively low runoff. In contrast, three east coast rivers, the Hoping, the Hualien, and the Beinan, have high sediment yields that may be explained by relatively frequent earthquakes coupled with high runoff. Farming and urbanization also have elevated sediment yields in eastern watersheds, whereas Holocene sediments buried in the Taiwan Strait suggest that present-day sediment loads of the western rivers may be no higher than prehuman levels.

Organic carbon and nitrogen contents and their isotopic compositions in surficial sediments from the East China Sea shelf and the southern Okinawa Trough

Abstract The southern Okinawa Trough has been noted as an important depocenter for the particulate organic matter from the East China Sea shelf. In order to determine the major sources of sedimentary organic matter in the southern Okinawa Trough, we analyzed surficial sediments from the East China Sea shelf and the southern Okinawa Trough for organic carbon and nitrogen contents and their isotopic compositions in the decarbonated fraction. Distributions of total organic carbon content, δ13Corg and nitrogen content all show a similar spatial pattern, resembling the distribution of fine-grained sediments. The coastal belt of elevated organic carbon content extends southward from the Changjiang mouth and veers offshore towards the southern Okinawa Trough just north of Taiwan, implying a pathway for channeling fine-grained sediments from the inner shelf to the depocenter. This distribution pattern is consistent with the shelf circulation. The isotope compositions (δ13Corg, δ15N) of sediments from the southern Okinawa Trough fall between those of riverine particulate organic matter and the mid-outer shelf sediments, but overlap with those of the inner shelf sediments. In addition, the previously reported δ13Corg values of sinking particles collected by sediment traps in the southern Okinawa Trough are also close to those of the inner shelf sediments, especially during high-flux conditions. Therefore, the isotopic evidence strongly supports the notion that a major fraction of the sedimentary organic matter in the southern Okinawa Trough may originate from the inner shelf.

Short-term Changes in Seafloor Character due to Flood-Derived Hyperpycnal Discharge: Typhoon Mindulle, Taiwan, July 2004

During Typhoon Mindulle in early July 2004, the Choshui River (central-western Taiwan) discharged similar to 72 Mt of sediment to the eastern Taiwan Strait; peak concentrations were g= 200 g/L, similar to 35%-40% of which was sand. Box-core samples and CHIRP (compressed high-intensity radar pulse) sonar records taken just before and after the typhoon indicate that the hyper-pycnal sediment was first deposited adjacent to the mouth of the Choshui, subsequently resuspended and transported northward (via the Taiwan Warm Current), and redeposited as a patchy coastal band of mud-dominated sediment that reached thicknesses of 1-2 m within megaripples. Within a month most of the mud was gone, probably continuing its northward transit, but more mud appeared in late August in response to Typhoon Aere. Following autumn and winter storms, the entire nearshore area was again sand dominated by early spring.

Distribution of terrigenous lipids in marine sediments off northeastern Taiwan

Surface sediments on the continental margin off northeastern Taiwan have been analyzed for terrigenous lipids including n-alkanes, n-fatty alcohols, and sterols. Marine input to the sediments is particularly low based on the average n-C17/n-C29 alkane and n-C16/n-C28 fatty alcohol ratios, 0.15±0.13 and 0.13±0.06, respectively; this may be due to the fact that marine lipids are more prone to degradation than terrestrial ones. The study area has the highest plant wax n-alkane contribution (average carbon preference index 3.9±1.2) among the coastal marine areas surrounding Taiwan; lateral particle transport from the southern East China Sea shelf and river runoff from the east Taiwan coast are considered to be the major contributors. The distributions of plant wax n-alkane and n-alkanol concentrations normalized to total organic carbon (TOC) in the study area generally show maximum values on the upper slope of the southernmost Okinawa Trough, but not for phytosterols. Linear regression of TOC versus plant wax n-alkane concentrations show a weak relationship (r=0.64, p=0.001), and an even weaker relationship (r=0.42, p=0.05) between TOC and plant wax n-fatty alcohol concentrations is found. This could be attributed to several factors: (1) a complex input (not a point source) of terrigenous organic matter to the study area, (2) TOC also including marine organic matter, (3) temporal variations in river flow due to flooding, and (4) different rates of degradation for TOC and individual biomarkers. However, in spite of those factors, TOC and phytosterol concentrations are positively linearly correlated (r=0.85, p<0.001), implying that the dilution of phytosterols in terrigenous organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio, which is remarkable. In addition, the predominant source of diploptene in the sediments does not appear to be of higher plant origin.

High wintertime particulate matter pollution over an offshore island (Kinmen) off southeastern China: An overview

Both the Yangtze River Delta (YRD) and the Pearl River Delta (PRD), the two most rapidly developing areas in eastern China, have suffered from serious air pollution, and thus, numerous investigations were devoted to studying these problems. Other areas in eastern China have received less attention despite similar rapid development in ntheir industries and economy. In this study, we analyzed air‐quality data from Kinmen Island (24°27′26′′N, 118°19′36′′E) located off Fujian Province and between the two above‐mentioned deltas. Our results clearly show that the study area is experiencing serious air quality deterioration. Particularly, high levels of suspended particulate matter (PM) were observed during winter, when the northeasterly monsoon prevails. For example, concentrations of wintertime PM10 (particles ≤ 10 mm in diameter) frequently exceeded 100 mg/m3 in the last three years. In addition to the air‐quality data analysis, aerosol samples were collected between 22 November 2007 and 6 March 2008 and subjected to chemical analyses of various species. Our findings show that the three principal PM components include organic, mineral, and sulfate species with moderate to minor fractions of nitrate, sea salt, elemental carbon, and trace metal oxides. The high PM levels observed over the island may be partly attributed to the transport from a mixed‐type industrial area located ∼40 km northeast of Kinmen. Our study could partially fill the air quality data gap between the YRD and PRD regions, and highlight the alarming fact that air pollution has gradually expanded along eastern China’s coastal zone.