Organic geochemistry of sediments at DSDP Legs 56-57 Holes

The quantity, type, and maturity of the organic matter of Quaternary and Tertiary sediments from the Japan Trench (DSDP Leg 56, Sites 434 and 436; and Leg 57, Sites 438, 439 and 440) were determined. The hydrocarbons in lipid extracts were analyzed by capillary- column gas chromatography and combined gas chromatography/ mass spectrometry. Kerogen concentrates were investigated by microscopy, and vitrinite-reflectance values were determined. Measured organic-carbon values were in the range of 0.13 to 1.00 per cent. Extract yields, however, were extremely low. Normalized to organic carbon, total extracts ranged from 4.1 to 15.7 mg/g Corg. Gas chromatography of non-aromatic hydrocarbons showed that all sediments, except one Oligocene sample, contained very immature, mainly terrigenous organic material. This was indicated by n-alkane maxima at C29 and C31 and high odd-carbon-number predominances. Unsaturated steroid hydrocarbons were found to be major cyclic compounds in lower- and middle-Miocene samples from the upper inner trench slope (Sites 438 and 439). Perylene was the dominating aromatic hydrocarbon in all but the Oligocene sample. Microscopy showed kerogens rich in terrigenous organic particles, with a major portion of recycled vitrinite. Nevertheless, almost all the liptinite particles appeared to be primary. This is a paradox, as the bulk of the samples were composed of hemipelagic mineral matter with a major siliceous biogenic (planktonic) component. A trend of reduced size and increased roundness can be seen for the vitrinite/ inertinite particles from west to east (from upper inner trench slope to outer trench slope). All sediments but one are relatively immature, with mean huminite-reflectance values (Ro)in the range of 0.30 to 0.45 per cent. The oldest and deepest sediment investigated, an Oligocene sandstone from Site 439, yielded a mean vitrinitereflectance value of 0.74 per cent and a mature n-alkane distribution. This sample may indicate a geothermal event in late Oligocene time. It failed to affect the overlying lower Miocene and may have been caused by an intrusion. Boulders of acidic igneous rocks in the Oligocene can be interpreted as witnesses of nearby volcanic activity accompanied by intrusions.

(Table 1) Composition of bituminous substances in sediments from the East Pacific Rise

Bitumoid contents in metalliferous sediments from axial parts of the East Pacific Rise (EPR) have been studied. Maximal concentrations of bitumoid matter and aromatic hydrocarbons have been found in the axial zones of the EPR as a result of hydrothermal processes and accumulation of endogenous matter in the sediments. Distribution of bitumoids and hydrocarbons coincides with deposition of ore minerals that indicates similarity of their sources.

(Table 1) Organic geochemistry at DSDP Leg 71 Holes

Forty-three core samples from Sites 511 through 514 of DSDP Leg 71 were analyzed geochemically. The black shales at the bottom of Hole 511, in the basin province of the Falkland Plateau, contain an average of 1590 ppm extractable organic matter (EOM) and 120 ppm hydrocarbons. Whereas molecular type-carbon number distributions of mono- and polynuclear aromatic hydrocarbons and their sulphur and oxygen analogues in the black shale "aromatic hydrocarbon" fractions are very similar to those of many crude oils, other data - gas chromatography (GC) fingerprint, pyrolysis GC, visual kerogen analysis, H/C ratio - suggest the black shale section is thermally immature. Together, these observations imply that many of the hydrocarbons were deposited with the original sediments or are diagenetic products of other biological compounds. Pyrograms of the humic acid and kerogen fractions from the black shale interval are typical of geopolymers derived from marine algal material. It appears that these humic acids and kerogens are derived from the same lipid stock.

(Table 1) Lithological and organic-geochemical compositions of bottom sediments from the Southwestern Indian Ocean (Mozambique Basin)

Geochemical changes in organic matter of bottom sediments from the Mozambique Basin at the river-sea barrier from the mouths of the Zambezi and Limpopo rivers toward the pelagic zone are discussed. Changes in bitumen, hydrolyzable material, humic acids, amino acids, n-alkanes, and polycyclic aromatic compounds resulting from genetic and diagenetic factors are described. This information is significant for paleoceanology reconstructions and for knowing ways of organic matter transformation into fossil forms.