OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS

OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS     

Modeling and interpreting the seismic-reflection expression of sandstone in an ancient mass-transport deposit dominated deep-water slope environment

Subsurface mass-transport deposits (MTDs) commonly have a chaotic seismic-reflection response. Synthetic seismic-reflection profiles, created from a precise lithological model, are used to interpret reflection character and depositional geometries at multiple frequencies. The lithological model was created from an outcrop of deep-water lithofacies where sandstone deposition was influenced by mass-transport deposit topography. The influence of MTD topography on sandstone distribution should be considered in reservoir characterization and modeling when MTDs underlie the reservoir, especially if the reservoir is thin relative to the scale of the topography. MTD topography up to several tens of meters in both the horizontal and vertical dimensions (relative to local elevation) compartmentalizes significant quantities of sandstone and is not resolved at lower seismic-reflection frequencies. The resolvability of thick (up to 70 m) sandstone packages is hindered when they are encased in MTDs of at least equivalent thickness. Lateral and vertical changes in seismic-reflection character (e.g., amplitude, polarity, geometry) of sandstone packages in the synthetic profiles are due to lithology changes, tuning effects, resolution limits, and depositional geometries, which are corroborated by the lithological model. Similar reflection-character changes are observed in an actual seismic-reflection profile, of comparable scale to the synthetic profiles, from the Gulf of Mexico, which demonstrates similar lithofacies distributions. Synthetic profiles, when constrained by a precise lithological model, are particularly useful analogues for interpretation of lithofacies relationships, and depositional geometries, in complicated depositional environments, such as deep-water slope deposits.     

Polar Compounds from the Dissolution of Weathered Diesel

Hydrocarbon and nonhydrocarbon components dissolving in water from fresh diesel and field samples of highly weathered diesel (spilled up to 50 years ago) from two sites were investigated. The fresh and weathered diesels were equilibrated with water using a slow-stirring method, and the product and equilibrated aqueous water samples analyzed by a range of analytical procedures. The water phase equilibrated with weathered diesels had higher total dissolved organics concentrations (96 and 8.6 mg/L at the two sites) compared to the water phase equilibrated with fresh diesel (average of 3.4 mg/L). Compound class characterization of dissolved organics in water from the weathered diesel showed that polar components were a significant compound class (98% and 42% at the two sites) and appeared largely as an unresolved complex mixture (UCM) in the total ion chromatograms (TICs). Identification of 1-adamantanol in the polar fraction of both weathered diesel samples (3.6 and 0.3 μg/L at the two sites) suggested that at least some of the associated polar components are from a petroleum source. The analysis of total petroleum hydrocarbons (TPH) is aimed at measuring only dissolved carbon and hydrogen-containing compounds, and dissolved polar compounds present as a UCM are often assumed to be from natural organic matter (NOM) and removed. This may result in a gross underestimation of the total soluble organic material in water associated with weathered diesels. In addition, the risk posed by these fuel-derived polar compounds is unknown.     

A two-dimensional probabilistic acute human-health risk assessment of insecticide exposure after adult mosquito management

Ultra-low-volume (ULV) aerosol applications of insecticides are used to manage high densities of adult mosquitoes. We used two-dimensional probabilistic risk assessment methodologies to evaluate three pyrethroid insecticides (phenothrin, resmethrin, and permethrin), pyrethrins, and two organophosphate insecticides (malathion and naled), applied by truck-mounted ULV sprayer. Piperonyl butoxide, a synergist commonly used in pyrethroid and pyrethrins formulations, was also assessed. The objective of our study was to evaluate probabilistically if a deterministic human-health risk assessment of mosquito insecticides was sufficiently conservative to protect human-health. Toddlers and infants were the highest risk groups while adult males were the lowest risk group assessed in this study. Total acute exposure ranged from 0.00003 to 0.0003 mg/kg day⁻¹ for the chemicals and subgroups assessed examining inhalation, dermal, oral, and hand-to-mouth exposure. We used the risk quotient (RQ) method for our risk assessment, which is calculated by dividing the total potential exposure for each subgroup and chemical by its ingestion toxic endpoint value (RfD). Mean RQs ranged from 0.000004 to 0.034 for all subgroups and chemicals, with none exceeding the RQ level of concern. Naled had the highest RQs of any chemical assessed while PBO had the lowest. Sensitivity analysis demonstrated that the exposure from inhalation and deposition contributed the largest variance to the model output. Results support the findings of previous studies that the risks from adult mosquito management are most likely negligible, and that the human-health deterministic risk assessment is most likely sufficiently conservative.     

Limits of tidal energy dissipation by fluid flow in subsea formations

Tidal loading causes fluid flow through permeable seafloor and between regions of contrasting elastic properties or porosity within subsea formations. We examine theoretically the dissipation of energy by these flows and its global significance as a mechanism for tidal energy dissipation. Expressions are given for energy dissipation rates in layered formations due to vertical flow caused by tidal loading, but the results can be used to constrain dissipation by other flow patterns. We consider flow near the seafloor, in gas-bearing sediments, and in highly fractured permeable igneous crust. Energy dissipation by the first two mechanisms is negligibly small globally, although it may be locally significant under extreme conditions. Under favourable conditions, flow in fractured crust may have greater energy dissipation, but the total amount is limited by the thickness of the permeable layer. Based on our current understanding of subsea hydrogeology, tidally induced flow in subsea formations appears to make little contribution to the observed global tidal energy dissipation.     

Sedimentology,stratigraphy and architecture of the Nicobar Fan (Bengal–Nicobar Fan System), Indian Ocean: Results from International Ocean Discovery Program Expedition 362

Drill sites in the southern Bay of Bengal at 3°N 91°E (International Ocean Discovery Program Expedition 362) have sampled for the first time a complete section of the Nicobar Fan and below to the oceanic crust. This generally overlooked part of the Bengal–Nicobar Fan System may provide new insights into uplift and denudation rates of the Himalayas and Tibetan Plateau. The Nicobar Fan comprises sediment gravity-flow deposits, mostly turbidites, that alternate with hemipelagite drapes and pelagite intervals of varying thicknesses. The decimetre-thick to metre-thick oldest pre-fan sediments (limestones/chalks) dated at 69 Ma are overlain by volcanic material and slowly accumulated pelagites (0.5 g cm⁻² kyr⁻¹). At Expedition 362 Site U1480, terrigenous input began in the early Miocene at ca 22.5 Ma as muds, overlain by very thin-bedded and thin-bedded muddy turbidites at ca 19.5 Ma. From 9.5 Ma, sand content and sediment supply sharply increase (from 1–5 to 10–50 g cm⁻² kyr⁻¹). Despite the abundant normal faulting in the Nicobar Fan compared with the Bengal Fan, it offers a better-preserved and more homogeneous sedimentary record with fewer unconformities. The persistent connection between the two fans ceased at 0.28 Ma when the Nicobar Fan became inactive. The Nicobar Fan is a major sink for Himalaya-derived material. This study presents integrated results of International Ocean Discovery Program Expedition 362 with older Deep Sea Drilling Project/Ocean Drilling Program/International Ocean Discovery Program sites that show that the Bengal–Nicobar Fan System experienced successive large-scale avulsion processes that switched sediment supply between the Bengal Fan (middle Miocene and late Pleistocene) and the Nicobar Fan (late Miocene to early Pleistocene). A quantitative analysis of the submarine channels of the Nicobar Fan is also presented, including their stratigraphic frequency, showing that channel size/area and abundance peaked at ca 2 to 3 Ma, but with a distinct low at 3 to 7 Ma: the intervening stratigraphic [sub]unit was a time of reduced sediment accumulation rates.     

The paleoecology and archaeology of long‐term water storage in a Hohokam reservoir,southwestern Arizona,U.S.A.

Water storage reservoirs were an important feature of economic organization among ancient societies in the North American Southwest. Analyses of reservoir sediments from a Hohokam archaeological site in the Sonoran Desert yielded taxonomic species of ostracodes (microscopic crustaceans) and pollen grains that are indicative of a past water‐rich environment. The discovery that this reservoir was capable of storing water on a long‐term basis indicates that archaeological models for the region, which have relied on direct historic analogy, must be reexamined. In contrast to the local ethnographic record, paleoecological data generated by this study imply that the Hohokam could establish permanent desert settlements with water storage reservoirs away from perennial rivers and streams. Moreover, residents of these areas were geographically positioned to facilitate the circulation of marine resources (i.e., salt and shell) from the Gulf of California to territories within and beyond the Hohokam region. © 2004 Wiley Periodicals, Inc.