We conclude that the putative link between Ba accumulation and export production may be obscured by changes in particle composition even within a dominantly biogenic sedimentary regime, as well as by early aliagenetic transfer between phases, and that the non-barite elemental Ba inventory may complicate the use of elemental Ba as a quantitative proxy for barite in the bulk sediment.

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Molecular and isotopic compositions of void gases from Sites 618 and 619 were determined. The gases were predominantly methane with the C L /C 2 ratios averaging 4,000 and 26,000 at Sites 618 and 619, respectively. The δ 13 C-Cj values at Site 618 were nearly constant, ranging from -70.1 to -73.7‰. δ 13 C-CHj values for Site 619 between 76 and 178 meters sub-bottom ranged from -94.8 to -70.8‰, becoming progressively heavier with depth. The molecular and isotopic compositions at both sites are characteristic of biogenic methane.Small spheres (~ 1-2 mm diameter) of gas hydrates were observed in the Orca Basin between 20 and 40 m sub-bottom and were often associated with sandy sections. The hydrate was predominantly methane with a δ 13 -Cj of -71‰ and trace amounts of ethane, propane, and carbon dioxide. The isotopic value for the methane was similar to that of the nearest gas pocket, indicating that hydrate was formed from predominantly biogenic methane without isotopic fractionation.

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%o 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.

Methane carbon-isotopic compositions (δ 13 C values relative to the PDB standard) at Sites 565, 566, 567, and 569 were lighter (enriched in 12 C) than-60‰, indicating a biogenic origin. In the deeper sections at Sites 568 and 570, δ 13 C values were heavier, approaching-40‰, and therefore suggest a thermogenic source. A significant thermogenic source was discounted, however, because the carbon dioxide δ 13 C values in these sections were also anomalously heavy, suggesting that the methane may have formed biogenically by reduction of the heavy carbon dioxide. δ 13 C values of ethane and higher hydrocarbons were measured in several sections from Sites 566 and 570 that contained sufficient C 2-C 4 hydrocarbon concentrations. Ethane values in six sections (245-395 m sub-bottom) from Site 570 were fairly uniform, ranging from-24 to-26%o. These values are among the heaviest ethane values reported for natural gases. The isobutane/n-butane and isopentane/«-pentane ratios of the core gases suggested that the C 2-C 5 hydrocarbons are thermally produced by low-temperature chemical diagenesis of indigenous organic matter. This process apparently generates isotopically heavy C 2-C 5 hydrocarbons. High gas concentrations in the serpentinite basement rocks at Sites 566 and 570 appear to have resulted from migrated biogenic methane gas containing small amounts of immature C 2-C 5 hydrocarbons.