Reassessment of the Rates at which Oil from Natural Sources Enters the Marine Environment

Previous estimates of the world-wide input of oil to the marine environment by natural seeps ranged from 0 ·2 to 6 ·0 million (metric) tonnes per year with a \u27best estimate\u27 of 0 ·6 million tonnes per year. Based on considerations of the availability of oil for seepage from the world\u27s known and assumed oil resources, we believe that the world-wide natural oil seepage over geological time should be revised to about 0 ·2 million tonnes per ),ear with a range upward or downward of a factor of ten leading to estimates between 0 ·02 and 2 million tonnes per year. Our estimate of the amount of oil eroding from the land and being transported to the oceans is about 0 ·05 million tonnes per year with an order of magnitude uncertainty. Therefore, while the uncertainties are large, we estimate that the total amount of oil entering the marine environment by natural, geological processes, is about 0 ·25 million tonnes per year, and the estimate may range from about 0 ·025 to 2 ·5 million tonnes per year

Geochemical evidence for gas hydrate in sediment near the Chile Triple Juction

Coring at ODP Sites 859, 860, and 861 near the Chile Triple Junction failed to recover anticipated gas hydrate that was inferred to be present from two lines of geophysical evidence: pre-cruise observation of a weak to strong bottom simulating reflector (BSR) marking the predicted base of the gas-hydrate stability zone, and post-cruise interpretation of the velocity and resistivity logs at Site 859 that suggests the presence of gas hydrate. In contrast to other gas-hydrate occurrences observed during previous DSDP and ODP drilling, this Chile Margin sediment is very low in contents of total organic carbon (TOC < 0.5%) and residual methane (C₁), that are inconsistent with an in-situ source of methane for gas-hydrate formation. However, methane/ethane (C₁/C₂) ratios (>200) and δ¹³ C, values (<-60‰) show little evidence that the methane came from deeper thermogenic sources. Pore-fluid freshening does, however, suggest that gas hydrate is present, disseminated thinly and heterogeneously throughout the stability zone, and occupies less than 25% of the available pore space. The environment of the gas hydrate in sediment near the Chile Triple Junction has unique characteristics relative to known gas-hydrate occurrences elsewhere

Analyses of gashydrates from DSDP Sites 76-533 and 84-568 (Table 1)

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO2. Residual methane in sediments at both sites usually exceeded 10 ml/l of wet sediment. Carbon isotopic compositions of methane, CO2, and SumCO2 followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments

Results from controlled decomposizion of gas hydrates from ODP Leg 112 (Table 1)

Gas hydrates were recovered during coring by Ocean Drilling Program (ODP) Leg 112 at Sites 685 and 688 on the Peruvian outer continental margin at latitudes of 9° and 11.5°S, where water depths are 5070 and 3820 m, respectively. In addition, nearby Sites 682 and 683 yielded compelling evidence that gas hydrates are present, but gas hydrates were not directly observed there. Anomalous acoustic reflectors, known as bottom-simulating reflectors (BSRs), on marine seismic profiles from the region also provided inferential evidence that gas hydrates are present. Geothermal gradients of about 43 and 49°C/km were calculated on the basis of relations between depths to BSRs, bottom-water temperatures, and the pressure-temperature stability field of gas hydrates. Geochemical studies revealed that methane concentrations increase rapidly with depth after pore-water sulfate concentrations have been depleted. The relationship between methane and sulfate suggests that microbial processes account for the generation of methane, and the relationship between the carbon isotopic composition of methane and dissolved carbon dioxide supports this suggestion. We believe that decreasing chlorinity in pore water from squeezed sediment at the four sites results mainly from the decomposition of gas hydrates and is a dilution artifact observed as a result of the squeezing procedure. Maximum chlorinity values at or near the surface result from excess salt that comes from the formation of gas hydrates composed of freshwater. Record alkalinity attests to the intensity of diagenetic processes and has significant effects on salinity profiles at these sites. Gas hydrates were recovered at 99 and 166 meters below the seafloor (mbsf) at Site 685, and at 141 mbsf at Site 688 in Pleistocene diatomaceous mud. Methane constitutes more than 99% of the hydrocarbon gas mixture in the gas hydrates. The volumetric ratio of methane to water in the sample from Site 685 is 100, indicating that the sampled gas hydrate is either undersaturated with respect to methane or had partially decomposed during core recovery or both. The discovery of gas hydrates in lower slope deposits of the Peruvian outer continental margin extends our knowledge of gas-hydrate formation and occurrence in the Circum-Pacific region

(Table 1, page 765) D/L Ratios of Amino Acids in Bone Nuclei of Manganese Nodules from the North Pacific Ocean

The extent of racemization of isoleucine and aspartic acid has been measured on cetacean ear bones from the center of 8 manganese nodules in order to estimate the ages of the nuclei of the nodules and the rates of accretion of ferromanganese oxide layers. Age estimates obtained range from about 0.4 to 6 m.y., and rates of growth from <1 to about 10 mm/m.y. These rates are consistent with those obtained by other nucleus dating methods (K-Ar and fission track) and by layer dating methods (230Th, 231Pd, 10Be and 26Al)

Organic geochemistry of ODP Leg 104 holes

The Leg 104 organic geochemistry program consisted of monitoring (a) hydrocarbon gases, (b) organic and inorganic carbon, and (c) parameters resulting from Rock-Eval pyrolysis at three sites on the Voring Plateau. The results amplify some of those obtained earlier on Deep Sea Drilling Project (DSDP) Leg 38. In a regional sense there is an inverse correlation between amounts of hydrocarbon gas and organic carbon. For example, significant concentrations of methane are present only at Site 644 in the inner part of the plateau where organic carbon contents are always less than 1%; in contrast, at Site 642 on the outer plateau, methane concentrations are very low (ppm range) whereas amounts of organic carbon approach 2%. Only at Site 644 are the environmental conditions such that methanogenesis is an active diagenetic process. Because of the importance of routine gas analyses to the Ocean Drilling Program (ODP), a procedure was devised to improve the use of Vacutainers for collection of gas samples. Comparison of methods for determining organic carbon showed that at Sites 643 and 644 Rock-Eval TOC could be used as a measure of organic carbon, but not at Site 642. Although no liquid or solid hydrocarbons were encountered at any of the sites, a catalog of potential organic geochemical contaminants was developed in anticipation of such a discovery

(Table 3) Gas hydrates at DSDP Leg 84

On DSDP Leg 84, gas hydrates were found at three sites (565, 568, and 570) and were inferred, on the basis of inorganic and organic geochemical evidence, to be present at two sites (566 and 569); no evidence for gas hydrates was observed at Site 567. Recovered gas hydrates appeared as solid pieces of white, icelike material occupying fractures in mudstone or as coarse-grained sediment in which the pore space exhibited rapid outgassing. Also a 1.05-m-long core of massive gas hydrate was obtained at Site 570. Downhole logging indicated that this hydrate was actually 3 to 4 m thick. Measurements of the amount of methane released during the decomposition of these recovered samples clearly showed that gas hydrates had been found. The distribution of evolved hydrocarbon gases indicated that Structure I gas hydrates were present because of the apparent inclusion of methane and ethane and exclusion of propane and higher molecular weight gases. The water composing the gas hydrates was fresh, having chlorinities ranging from 0.5 to 3.2 per mil. At Sites 565, 568, and 570, where gas hydrates were observed, the chlorinity of pore water squeezed from the sediment decreased with sediment depth. The chlorinity profiles may indicate that gas hydrates can often occur finely dispersed in sediments but that these gas hydrates are not recovered because they do not survive the drilling and recovery process. Methane in the gas hydrates found on Leg 84 was mainly derived in situ by biogenic processes, whereas the accompanying small amounts of ethane likely resulted from low-temperature diagenetic processes. Finding gas hydrates on Leg 84 expands observations made earlier on Leg 66 and particularly Leg 67. The results of all of these legs show that gas hydrates are common in landward slope sediments of the Middle American Trench from Mexico to Costa Rica