OS51C-01
Physical and Acoustic Properties of Gas-bearing Sediments in Jinhae Bay, the South Sea of Korea
High-resolution seismic survey and sediment core sampling were conducted to investigate acoustic characteristics of gas-bearing sediments in Jinhae Bay, the southeast of Korea. . The sediment in the Jinhae Bay is mostly homogenous mud deposited after Holocene transgression. Along with the 410 km of chirp seismic profiling, 13 piston core samples were collected on the track lines. Gassy sediments are common and widely occurred in the bay. Core samples were analyzed for physical properties (porosity, water content, bulk density, grain density and shear strength), acoustic properties (compressional wave velocity and attenuation), and electrical resistivity. Pulse transmission technique (1 MHz) that programmed to pick up first arrival signal was used to calculate accurate velocity. Electrical resistivity was measured by the standard four electrode method. Sediment texture was also analyzed using sedimentation method. Subsampling interval was 10 cm. X- radiograph image analysis was also performed to observe the shape of degassing cracks. Both quantitative and composition analyses of free gas were carried out for the cores of gas zone. Carbon isotope was also determined for the origin of the gas. There is no significant downcore variation on physical and sediment textures regardless of existence of gas bubbles. However, velocity dramatically decreases from average 1480 to 1350~911 m/s for the cores that penetrate the gas-bearing zones. This is probably due to degassying cracks which developed by escaping gases and free gas bubbles that still trapped in the cores. Electrical resistivity is the only geotechnical property increase in the gas-bearing zone where velocity decreases abruptly. This indicates the possibility of electrical resistivity as an index variable as well as velocity for sediment microstructure because there is little changes in texture and composition of sediment. Methane is a dominant component with about 5% average content in the gassy zones. The carbon isotope analysis suggests that the shallow gas in Jinhae Bay is biogenic origin.
OS51C-02
The use of ADCPs for Detecting Turbulence
It is generally assumed that the backscatter utilized by Acoustic Doppler Current Profilers (ADCPs) originates from particulate or biological matter in the water column. Based on a 70-day data set from the deeper reaches of the Faroe-Bank Channel, it is, however, demonstrated that under pronounced shear-flow conditions turbulence- induced density inhomogeneities appear to be capable of also serving this purpose. Measurements took place at three different stations representing varying hydrographic situations; across the channel the density stratification associated with the shear zone increases significantly between the northeastern side and the southwestern side. The recorded magnitude of the acoustic-return intensity proved to be highest at the level of maximum vertical shear. A large number of data losses were furthermore found in the shear layer, most likely due to the fish- rejection option included in the signal processing. A closer investigation showed a correlation between the data losses and the predominant semi-diurnal tide which supports the hypothesis that the high return signals are due to turbulence-induced density inhomogeneities; it is unlikely that particulate/biological matter suspended in the free water mass would show any tidal dependence. This suggested use of ADCPs is based of the same principles as those underlying Sonic Detection and Ranging (SODAR) used in atmospheric research.
OS51C-03
Multi-biomarker Characterization of Sedimentary Organic Carbon along the Mullica River, NJ
Located in southeastern New Jersey, the Mullica River is approximately 90 km long, extending from the Indian Mills headwaters, through the Pinelands National Reserve to the Great Bay Estuary. The land cover vegetation of the Mullica River watershed (~ 1700 km2) encompasses pine-oak forests (50%) predominantly in the Pinelands followed by wetlands (36%) especially towards the estuary, representing one of the most pristine areas in the state. Sources and potential transformations of sedimentary organic carbon along the Mullica River were assessed using a natural multi-biomarker approach. Sediment samples were collected from the Pinelands to the bay and analyzed as silylated total extracts by gas chromatography-mass spectrometry. The primary biomarker classes found in the samples included n-alkanoic acids, n-alkanols, phytosterols, triterpenoids and saccharides. In general, sediment extracts composition reflected changes in the characteristic main cover vegetation along the river. The major biomarker contributions in the Pinelands sediments were phytosterols (sitosterol, stigmasterol, campesterol), their reduced products (i.e., stigmastanol, campestanol) and triterpenoids (β-amyrin, oleanoic acid) derived from higher plants detritus, followed by n-alkanoic acids and n- alkanols from epicuticular plant wax. Diterpenoids, mainly dehydroabietic acid (a biomarker for conifers), as well as the monosaccharide glucose and the fatty acids 16:0, 16:1, 18:0, 18:1, 18:2 (ubiquitous in biota) were important organic tracers observed in these sediments. The higher plants biomarkers tended to decrease downstream to trace levels in the bay extracts. An exception was the triterpenoid taraxerol (previously identified as a mangrove biomarker) derived mainly from the salt marsh vegetation draining the Great Bay. The higher abundances of the low molecular weight fatty acids (< C19) observed in the estuarine sediments are likely derived from marine phytoplanktonic inputs.
OS51C-04
Tsunami Deposit Data Base
A digital database has been established describing tsunami deposits around the world (3 phases; 15 months). The projects involved the review and tabulation of data derived from books, catalogs, journals, preprints, citations and abstracts (currently 1000 references), into a database designed to provide a comprehensive review of the types of tsunami deposits, their geographic distribution and location, sedimentary characteristics, fossil content, age, preservation, run-up, wave height and inundation observations, etc. (34 parameters). The tsunami occurrences can be divided into many subjects, e.g., Volcanogenic (N=375), Seismites (N=49), Co-seismic (N=258), K/T Boundary Impact-triggered debris flows (N=97), Landslides (N=43), etc. Numerous publications compare tsunami deposits to storm deposits (N=38), or analyze the origin of megaboulders (N=22). Tsunami deposits occur throughout geologic time (Pre-Cambrian to present day), and because of plate tectonics, they occur along plate margins (primarily subduction zones) as well as interior to plates. In addition, they occur in epi-continental seas, fjords, etc. Few publications describe depositional processes. Deposits generated by tsunamis occur in multiple environments such as the marine, fresh water, and subaerial. Common characteristics of tsunami deposits include: 1) Deposition of thin sand sheets (can be normal, massive, inversely graded, chaotic or bimodal). 2) Erosional: basal uncomformity, mud balls, rip-up clasts, reworked fossils produced by scouring. 3) Lithology: Stacks of couplets reflecting marine incursions (often sands) into fresh water or subaerial environments (mud, soil, peat). 4) Fossil: Couplets reflects marine fossils, fresh water fossils or a mixed assemblage. 5) Geomorphology: The sand sheets taper landward and can rise in elevation. 6) Deformation: syn-depositional (soft sediments) and intraformational (stiff sediments).
OS51C-05
The Contribution of Various Types of Settling Particles to the Flux of Organic Carbon in the Gulf of St. Lawrence, CAN
The contents of 32 free-drifting sediment traps deployed in the Gulf of St. Lawrence (GSL) during the Canadian JGOFS program in 1993-1994 were analyzed for the total carbon flux (essentially POC) and the C contribution attributable to the different types of sedimenting particles. Two kinds of trap were used: small traps (0.03 m2 diam.) deployed at 50 m (which showed a consistent bias towards higher fluxes) and large traps (1/8 m-2) at 50 and 150 m. Mean POC fluxes amounted to 42 and 149 mg C m-2d- 1 for the larger and small traps, respectively. Fecal pellets were the major component of this flux (6 and 60 mgC m-2d-1), while phytoplankton contributed 3.2 - 42.9 mgC m-2d-1. These are in the same range as fluxes encountered in regions of moderate productivity. Microzooplankton were also an important contributor of C, particularly during the summer of 1994. Combined with the C associated with fecal pellets and dinoflagelates, the vertical flux of attributable C in the GSL is dominated by components of animal origin. The contribution of marine snow was estimated by difference. This appears to be the most import vector of carbon flux but its true importance remains uncertain because of cumulative uncertainties in the estimates of each of the other components.