Chih-An Huh

SEDIMENTATION DYNAMICS IN THE EAST CHINA SEA ELUCIDATED FROM 210PB, 137CS AND 239.240PU

Sedimentation dynamics in the East China Sea was investigated based on profiles and inventories of 210 Pb , 137 Cs and ex 239,240 Pu in sediment cores collected from various settings in this marginal sea. The derived sedimentation rates vary by two orders of magnitude, from ; 2 to 0.02 cm yr y1 , and generally decrease southward along the inner shelf and eastward offshore. This pattern is consistent with the dispersal of Yangtze River-derived sediments by the current and tidal systems. y2 y1 . The weighted mean sedimentation rate over the shelf ca. 0.3 g cm yr appears to be higher than fluvial inputs from the y2 y1 . Yangtze River and Taiwan 0.14 g cm yr . The discrepancy can be reduced by considering the effect of sediment . mixing which renders sedimentation rates overestimated and the possibility of additional sediment input from the Yellow Rivers dispersal system. Spatial variations of the observed nuclide inventories in sediments suggest extensive sediment redistribution, boundary scavenging of 210 Pb, and large inputs of 137 Cs and 239,240 Pu from Yangtze Rivers drainage basin.

210Pb, 137Cs and 239,240Pu in East China Sea sediments: sources, pathways and budgets of sediments and radionuclides

Profiles of 210 Pb, 137 Cs and 239;240 Pu measured in 83 sediment cores collected from various sedimentary regimes in the East China Sea were analyzed to elucidate the sources, routes and budgets ofsediments as well as these radionuclides. Distributions ofsedimentation rates and nuclide inventories reveal alongshore transport ofsediments, 137 Cs and 239;240 Pu from the mouth of the Yangtze River toward the south, largely confined to the inner-shelf area (water depth 6 70 m). Mass balance calculations suggest that the East China Sea is a sink for the particle-reactive 210 Pb and 239;240 Pu, with about one-sixth oftheir sedimentary budgets supplied via boundary scavenging. In contrast, due to lower affinity of 137 Cs for particles and rapid turnover of the shelf water, the East China Sea serves as a source for 137 Cs. About two-thirds ofthe cumulative input of 137 Cs have been transported out ofthe East China Sea, leaving the remaining one-third stored in the bottom sediments and the overlying water column. As for the sediment budget, mass balance cannot be established due to a shortfall in sediment supply of more than 30% based on a comparison between input terms documented thus far and the sedimentation flux derived from this study. It is very likely that we have overestimated the sediment burial flux or that long-distance transport from the Yellow River’s dispersal system to the East China Sea is underestimated. Alternatively, the imbalance could be explained by the discrepancy between sediment input and output on decadal to centennial timescales. 7 2002 Elsevier Science B.V. All rights reserved.

Sedimentation dynamics in the Santa Monica-San Pedro Basin off Los Angeles: radiochemical, sediment trap and transmissometer studies

Abstract A large number of sediment cores and sediment trap samples collected from different parts of the Santa Monica-San Pedro (SM-SP) Basin during 1985–1988 were studied for radionuclides, trace metals and other sedimentary components. The radiochemical data are presented here to give a basin-wide view of the sedimentation dynamics. 210 Pb stratigraphy indicated that sedimentation rates were higher and more variable (30–80 mg cm −2 y −1 ) in the more dynamic slope region, but were uniformly low (13.4–18.8 mg cm −2 y −1 ) in the flat, deep basin. From the sediment record, it was suggested that sedimentation rates were decreasing and the area of anoxia had been expanding, at least during the past one to two centuries. Turbidite layers found in the sediment cores suggested higher frequency and more recent occurrence toward the basin margins. 210 Pb geochronologies indicated that the recent turbidites might be related to major storms which occurred during the past two decades. Sediment traps deployed in the basin recorded very large short-term spatial and temporal variabilities of mass flux, with unusually high fluxes corresponding to recorded large wave events. Trap-measured near-bottom mass fluxes averaged over all collection periods (622 days) were consistent with 210 Pb-based sediment accumulation rates. With few exceptions, trap-measured fluxes decreased offshore but increased with depth at any location, strongly suggesting lateral input of materials. Transmissometer data demonstrated the existence of nepheloid plumes off the eastern slope of the SM-SP Basin. The offshore decrease of sedimentation rate in the eastern part of the SM Basin was consistent with the fact that nepheloid plumes were confined to the basin slopes and that the major transport pathway of suspended particles, as indicated via progressive vector analysis, was alongshore from the SP Basin toward the SM Basin. Concentrations and fluxes of radionuclides measured in the near-bottom trap in the deep basin compared favorably with those registered in bottom sediments. Based on water column, sediment trap and sediment core data, self-consistent flux balances can be constructed for 228 Th and 210 Pb. Flux balances for 234 Th were less well defined. The cyclic pattern of uranium profiles in deep basin sediments appeared to be in phase with the sedimentary record of CaCO 3 and the historical record of primary production and anchovy biomass. It is suggested that the removal of uranium from the water might be regulated by longterm regional changes in biological processes and sedimentation environments.

Vertical profiles of some natural radionuclides over the Alpha Ridge, Arctic Ocean

Concentration profiles of 228 Ra, 234 Th, 230 Th, 232 Th, 228 Th, and 231 Pa were measured in the CESAR Ice Camp water column (85°50′N, 108°50′W). Thorium isotopes were determined in both dissolved and particulate forms. The results show that rates of scavenging of the reactive nuclides are unusually low, even in comparison with other oligotrophic oceans. Application of a reversible exchange model to the Th isotope data suggests that Th cycling is measurably faster in the surface water than it is at depth.

Fluid seepage along the San Clemente Fault scarp: basin-wide impact on barium cycling

Material fluxes associated with fluid expulsion at cold seeps and their contribution to oceanographic budgets have not been accurately constrained. Here we present evidence that the barium released at cold seeps along the San Clemente Fault zone may significantly impact the geochemical budget of barium within the basin. Barium fluxes at seep localities on the fault scarp, measured with benthic chambers, reach values as high as 5 mmol m 32 day 31 . This is the largest dissolved barium flux measured to date at a cold seep. The discharge of barium-rich fluids results in formation of massive barite deposits along the escarpment wall. The deposits are young (approximately 8 yr) and appear to grow at a minimum rate of 0.2 cm yr 31 . This rapid growth rate requires a barium efflux rate that is about two orders of magnitude higher than the measured dissolved flux. We believe that the discrepancy reflects a highly localized seepage system and that chambers positioned as close as possible to the growing chimneys did not sample the foci of fluid discharge. Transport of fine barite particles from the seeps may be responsible for excess rates of barium accumulation throughout the San Clemente Basin, relative to other basins in the California Margin. Based on a preliminary budget, we estimate that cold-seep barite is accumulating at the basin floor in San Clemente at a rate of 2 Wmol m 32 day 31 , a value that is comparable to the total barium accumulation rates driven by detrital and biogenic components in neighboring basins. Remobilization of cold-seep barite on the basin floor adds to that driven by the biogenic barium flux and results in benthic barium recycling rates (effluxes) within the San Clemente Basin that are as much as seven times higher than the effluxes from surrounding borderland basins. Our estimates imply that processes associated with fluid seepage along the San Clemente Fault significantly contribute to the basin’s barium cycle. The strontium isotopic composition of the seep barite is significantly different from marine ‘biogenic’ barite, which is known to accurately record seawater composition. In addition, the seep deposits are depleted in 226 Ra relative to their modern biogenic counterparts, and are likely to be a source of radium-depleted particulate barium to the basin. Thus the impact of barite transport from seeps on the San Clemente escarpment to the basin floor might also have implications for the geochemistry of elements other than barium.

Profiles of dissolved and particulate thorium isotopes in the water column of coastal Southern California

Abstract Profiles of 234 Th, 232 Th, and 230 Th in the Santa Monica Basin were measured. 234 Th in the upper part of the water column suggests that the residence time of dissolved 234 Th, with respect to adsorption onto and removal by particles, increases with depth, from 3–4 weeks at the surface to > 5 months below the euphotic zone. The partition of 234 Th between dissolved and particulate forms points to a residence time of 4–7 days for particulate 234 Th in the surface euphotic zone. Rapid removal of particulate 234 Th from the surface water precludes the possibility of an adsorption-desorption equilibrium state between dissolved and particulate 234 Th. Based on the 230 Th data, the overall residence time of dissolved Th in the water column is estimated at 5–6 years. Both dissolved 230 Th and 232 Th profiles exhibit higher concentrations at the surface than immediately underneath, suggesting a terrestrial input for them. From the surface to the bottom of the water column, the concentration of dissolved 232 Th decreases by a factor of 3–4, and the dissolved 230 Th/ 232 Th activity ratio increases by a factor of about five (from 2–3 to 9–12). This reflects the surface source of the terrestrial end-member and its removal at depth. On the average about 6% of the 234 Th, 25% of the 230 Th, and 50% of the 232 Th were found in the particulate form, which is as expected and relates to their respective sources, to the difference in radioactive decay half-life, and to the kinetics of the scavenging process.

Oceanic 232Th: A reconnaissance and implications of global distribution from manganese nodules

Abstract 232 Th concentrations in seawater reported in the literature since 1961 are compiled and evaluated, and new data are presented. In general, recent data are lower than old ones by 1 to 2 orders of magnitude, with nearly all post-1980 data lower than 0.1 dpm/10 3 −1 and pre-1980 values higher than 0.1 dpm/10 3 −1. We believe this is caused by improvements in sampling and analytical techniques. Reliable profiles of 232 Th in coastal and open ocean waters have been measured in recent years, some by the newly developed neutron activation analysis and mass spectrometry techniques. The data indicate external sources of 232 Th to the surface and its scavenging throughout the water column. We suggest that aeolian input dominates in the open ocean; but, fluvial input may be more important in coastal regions if mechanisms exist for recycling Th from estuarine sediments. Some information on the global distribution pattern and marine geochemistry of 232 Th in seawater can be inferred from initial 230 Th/ 232 Th activity ratios observed on surfaces of manganese nodules and crusts. The rather distinct interocean variation of the 230 Th/ 230 Th activity ratio in the central gyre region (averaging ~ 160 in the South Pacific, ~ 60 in the North Pacific and ~ 17 in the Atlantic) may reflect higher aeolian input of materials (including 232 Th) to the Atlantic and to the northern hemisphere. Superimposed on the large-scale global trend are some small-scale variations which are indicative of the following effects: (1) abrupt change of water depth, and hence 230 Th concentration, at regions of irregular seafloor topography; (2) higher 232 Th input at ocean boundaries; and (3) enhanced supply of 232 Th from hydrothermal vents. It is still unclear whether seafloor sediments act as a net source or sink of dissolved 232 Th (and 230 Th) in bottom waters. Pore water study should be undertaken to address this important issue. This task can be accomplished by the mass spectrometric technique.

History of Lead Pollution Recorded in East China Sea Sediments

Box cores collected from the shelf and slope regions of the East China Sea during 1996‐1997 were analyzed for 210 Pb, 137 Cs, 239;240 Pu and Pb concentrations to establish sediment chronologies and unravel the history of lead pollution. The data show pronounced increases of anthropogenic lead during the past two decades that can be attributed to rapid economic growth and the lack of waste control practices in China. From the circulation pattern and spatial variations of sedimentation rates and fluxes of anthropogenic Pb, it is clear that the Yangtze River is the main source of sediments and pollutants in this marginal sea. ” 1999 Elsevier Science Ltd. All rights reserved

Gravity Flows Associated with Flood Events and Carbon Burial: Taiwan as Instructional Source Area

Taiwans unique setting allows it to release disproportionately large quantities of fluvial sediment into diverse dispersal systems around the island. Earthquakes, lithology, topography, cyclone-induced rainfall, and human disturbance play major roles in the catchment dynamics. Deep landslides dominate the sediment-removal process on land, giving fluvial sediment distinct geochemical signals. Extreme conditions in river runoff, sediment load, nearshore waves and currents, and the formation of gravity flows during typhoon events can be observed within short distances. Segregation of fresh biomass and clastic sediment occurs during the marine transport process, yet turbidity currents in the Gaoping Submarine Canyon carry woody debris. Strong currents in the slope and back-arc basin of the Okinawa Trough disperse fine-grained sediments rapidly and widely. Temporal deposition and remobilization may occur when the shallow Taiwan Strait acts as a receptacle. Taiwan can therefore serve as a demonstration of the episodic aspect of the source-to-sink pathway to both the coastal and deep-ocean environments.

Linkages between turbidites in the southern Okinawa Trough and submarine earthquakes

[1] Turbidite layers in surficial ( 6.8) submarine earthquakes recorded in the region since the 20th century. The repeating pattern of turbidite layers reported here on decadal to centennial time scales suggests what may be the long-term rhythm of seismic activities at this tectonically active plate boundary. INDEX TERMS: 0932 Exploration Geophysics: Radioactivity methods; 1035 Geochemistry: Geochronology; 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 3025 Marine Geology and Geophysics: Marine seismics (0935); 7221 Seismology: Paleoseismology. Citation: Huh, C.-A., C.-C. Su, W.-T. Liang, and C.-Y. Ling (2004), Linkages between turbidites in the southern Okinawa Trough and submarine earthquakes, Geophys. Res. Lett., 31, L12304, doi:10.1029/2004GL019731.