Concentration of C2-C6 hydrocarbon gases in dry sediments of ODP Sites 180-1109 and 180-1115

Low molecular weight hydrocarbon (LMWH) distributions were examined in sediments from Sites 1109 and 1115 in the western Woodlark Basin using purge-trap thermal adsorption/desorption gas analysis. A number of different hydrocarbon components >C1, which were not detected during shipboard gas analysis, were detected at both sites using the purge-trap procedure. Concentrations of ethane, propane, and butane remained relatively low (<100 pmol/g) throughout Site 1109 and had no consistent trend with depth. In contrast, the longer-chain components increased in concentration with depth. Hexane concentrations rose to 716 pmol/g at the base of the site with a concomitant increase in both 2-methyl- and 3-methylpentane. At Site 1115, concentrations of ethane, propane, butane, and isobutylene + 1-butene remained low (<60 pmol/g) throughout the site and again had no consistent trend with depth. 2-Methylpentane, 3-methylpentane, and hexane concentrations had a subsurface maximum that coincided with sediments containing abundant plant-rich material. The LMWH downhole profiles plus low in situ temperatures suggest that the LMWH components were formed in situ by low-temperature biological processes. Purge-trap analysis has indicated the presence of some unexpected deep low-temperature bacterial reactions, which demonstrates that further analysis of LMWH may provide valuable information at future Ocean Drilling Program sites.

Free and dissolved gases in the area of the Calypso hydrothermal vent, Bay of Plenty, New Zealand

Gas composition and hydrochemistry of bottom waters of the Bay of Plenty in the hydrothermally active zone of the Pacific island arc are investigated. Methane content in underwater vents is an order of magnitude greater than that in volcanic exhalations on the land. Salinity, pH, total content of CO2, its partial pressure, and silica content also differ. Correlations between gas parameters, hydrochemical parameters, and biological and microbiological parameters are identified.

Geochemistry of interstitial gases in sedimentary deposits at DSDP Leg 64 Holes

We analyzed interstitial gases from holes at Sites 474, 477, 478, 479, and 481 in the Gulf of California, using gas chromatography and stable isotope mass spectrometry to evaluate their composition in terms of biogenic and thermogenic sources. The hydrocarbon gas (C1-C5) concentrations were comparable to the shipboard data, and no olefins could be detected. The ?13C data for the CH4 confirmed the effects of thermal stress on the sedimentary organic matter, because the values were typically biogenic near the surface and became more depleted in 12C versus depth in holes at Sites 474, 478, and 481. The CH4 at Site 477 was the heaviest, and in Hole 479 it did not show a dominant hightemperature component. The CO2 at depth in most holes was mostly thermogenic and derived from carbonates. The low concentrations of C2-C5 hydrocarbons in the headspace gas of canned sediments precluded a stable carbon-isotope analysis of their genetic origin.

Hydrocarbon gases in ODP Leg 104 samples

Geochemical studies at three ODP Leg 104 sites on the Wring Plateau help define the distribution of hydrocarbon gases in sediment of this prominent feature of the Norwegian continental margin. Low levels of hydrocarbon gas were encountered in sediment of the outer part of the plateau, but sediment of the inner part of the plateau is very gassy. The molecular composition of inner plateau gases (>99.9% methane) and the carbon isotopic composition of the methane (avg. = -76 per mil relative to the PDB standard) clearly show that the gas is biogenic. Heavier hydrocarbon gases accompany this methane, and their presence is probably a result of both chemical and microbial low-temperature diagenesis. Although these heavier hydrocarbons were not detected in sediment of the outer part of the plateau during shipboard analyses, subsequent shore-based analyses showed that these compounds are present at very low concentrations. Methane in the gassy sediment of the inner part of the plateau may be present as gas hydrates, judging from sedimentological and inorganic geochemical considerations, but no discernible gas hydrates were recovered during drilling.

Sm-Nd, Rb-Sr, and He isotopic composition and hydrocarbon gases in rocks and minerals from the Markov Deep, Mid-Atlantic Ridge rift valley

The paper presents characteristics of the Nd and Sr isotopic systems of ultrabasic rocks, gabbroids, plagiogranites, and their minerals as well as data on helium and hydrocarbons in fluid inclusions of the same samples. Materials presented in this publication were obtained by studying samples dredged from the MAR crest zone at 5°-6°N (U/Pb zircon dating, geochemical and petrological-mineralogical studies). It was demonstrated that variations in the isotopic composition of He entrapped in rocks and minerals were controlled by variable degrees of mixing of juvenile He, which is typical of basaltic glass for MAR (DM source), and atmospheric He. Increase in the atmospheric He fraction in plutonic rocks and, to a lesser degree, in their minerals reflects involvement of seawater or hydrated material of the oceanic crust in magmatic and postmagmatic processes. This conclusion finds further support in positive correlation between the fraction of mantle He (R ratio) and 87Sr/86Sr ratio. High-temperature hydration of ultrabasic rocks (amphibolization) was associated with increase in the fraction of mantle He, while their low-temperature hydration (serpentinization) was accompanied by drastic decrease in this fraction and significant increase in 87Sr/86Sr ratio. Insignificant variations in 143Nd/144Nd (close to 0.5130) and 87Sr/86Sr (0.7035) in most of gabbroids and plagiogranites as well as the fraction of mantle He in these rocks, amphibolites, and their ore minerals indicate that the melts were derived from the depleted mantle. Similar e-Nd values of gabbroids, plagiogranites, and fresh harzburgites (6.77-8.39) suggest that these rocks were genetically related to a single mantle source. e-Nd value of serpentinized lherzolites (2.62) likely reflects relations of these relatively weakly depleted mantle residues to another source. Aforementioned characteristics of the rocks generally reflect various degrees of mixing of depleted mantle components with crustal components (seawater) during metamorphic and hydrothermal processes that accompanied formation of the oceanic crust.

Determination of hydrocarbon gases in mud volcanoes in the Sorokin Trough

Hydrocarbon gases were determined in sediments from three mud volcanoes in the Sorokin Trough. In comparison to a reference station outside the mud volcano area, the deposits are characterized by an enrichment of high-molecular hydrocarbons (C2-C4), an absence of unsaturated homologues, a predominance of iso-butane in comparison with n-butane, and the presence of gas hydrate. The molecular composition of the hydrocarbon gases suggests their deep sources and thermogenic origin. In the pelagic sediments at the reference station, the methane concentration is relatively low (up to 49 ml/l); maximum concentrations are reached in deposits of the Dvurechenskii mud volcano (up to 400 ml/l). It was the first time that gas hydrate was sampled at the Dvurechenskii mud volcano. The gas extracted by dissociation of hydrate samples was dominated by methane (99.5%) with low amounts of ethane and propane (less than 0.5%). The isotopic composition of the methane varies between -62 and -66 per mill PDB in d13C, and between -185 and -209 per mill SMOW in dD, indicating a mainly biogenic origin with an admixture of thermogenic gas.

Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source. There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases. Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The 3He/4He in the East Pacific Rise shows a remarkably uniform ratio of (1.21 +/- 0.07)*10**-5, while the40Ar/36Ar ranges from 700 to 5600.

Distribution of organic matter and gases in sediments at DSP Leg 63 Holes

As part of our continuing organic geochemical studies of sediments recovered by the Deep Sea Drilling Project, we have analyzed the types, amounts, and thermal alteration indices of organic matter in samples collected from the California continental margin on Leg 63. Some of the samples were frozen core; others were canned on site. Canned samples were analyzed for gas content using methods described by Mclver (1972). Our main objective was to see if the changes in surface circulation that had occurred through time off the California coast were reflected in changes in the type and amount of organic matter accumulating on the sea floor.