Organic matter composition and sulfate reduction intensity in Oman Margin sediments

Petrographical and geochemical studies of Neogene marine sediments from the Oman Sea (Leg 117, Sites 720, 724, 726 and 730), show a close relationship between the nature and amount of the organic matter, and the degree of degradation of organic matter by sulfate reduction, i.e. pyritization. Petrographically, three major pyritization types were observed: (1) Finely dispersed pyrite framboids in sediments from Oman Margin and Indus Fan, enriched in autochthonous marine organic matter. (2) Infilling of pores by massive pyrite crystals in Oman Margin sediments with a low TOC and a high microfossil content. (3) Pyrite mineralization of lignaceous fragments in organic-depleted sediments from the Indus Fan leading to more massive pyrite. Geochemically, we can define a sulfate reduction index (SRI) as the percentage of initial organic carbon versus that of residual organic carbon. Finely laminated Pliocene-Pleistocene sediments from the Oman Margin exclusively contain organic matter deriving from organic phytoplankton, for which the quantity (TOC) positively correlates with the geochemical quality (Hydrogen Index). We think that the occurrence of this residual organic matter is linked mainly to a high primary paleo-productivity. The intensity of sulfate reduction is constant for sediments with TOC up to 2% and becomes more important when organic input decreases. This degradation process can destroy up to 50% of the initial organic matter, but is not sufficient to explain some of the encountered very low TOC values. It can be seen that sharp increases of certain plankton species (with mineral skeletons) are responsible for a pronounced degradation of organic matter, due to increased sulfate reduction. In that case, the organic matter may be strongly degraded (high SRI), although deposited in an oxygen-depleted environment. Conversely, Miocene-Pliocene sediments contain an autochthonous organic matter that is typical of both low productivity and oxic processes; their very low sulfate reduction index indicates that very little metabolizable organic matter was initially present.

Petrology and isotope characteristics of basalts from ODP Hole 111-504B

Petrography and isotope geochemical characteristics of H, O, S, Sr, and Nd have been described for basalts recovered from Hole 504B during Leg 111 of the Ocean Drilling Program. The petrographic and chemical features of the recovered basalts are similar to those obtained previously (DSDP Legs 69, 70, and 83); they can be divided into phyric (plagioclase-rich) and aphyric (Plagioclase- and clinopyroxene-rich) basalts and show low abundances of TiO2, Na2O, K2O, and Sr. This indicates that the basalts belong to Group D, comprising the majority of the upper section of the Hole 504B. The diopside-rich nature of the clinopyroxene phenocrysts and Ca-rich nature of the Plagioclase phenocrysts are also consistent with the preceding statement. The Sr and Nd isotope systematics (average 87Sr/86Sr = 0.70267 ± 0.00007 and average 143Nd/144Nd = 0.513157 ± 0.000041) indicate that the magma sources are isotopically heterogeneous, although the analyzed samples represent only the lowermost 200-m section of Hole 504B. The rocks were subjected to moderate hydrothermal alteration throughout the section recovered during Leg 111. Alteration is limited to interstices, microfractures, and grain boundaries of the primary minerals, forming chlorite, actinolite, talc, smectite, quartz, sphene, and pyrite. In harmony with the moderate alteration, the following alteration-sensitive parameters show rather limited ranges of variation: H2O = 1.1 ±0.2 wt%, dD = - 38 per mil ± 4 per mil, d180 = 5.4 per mil ± 0.3 per mil, total S = 562 ± 181 ppm, and d34S = 0.8 per mil ± 0.3 per mil. Based on these data, it was estimated that the hydrothermal fluids had dD and d180 values only slightly higher than those of seawater, the water/rock ratios were as low as 0.02-0.2, and the temperature of alteration was 300°-400°C. Sulfur exists predominantly as pyrite and in minor quantities as chalcopyrite. No primary monosulfide was detected. This and the d34S values of pyrite (d34S = 0.8 per mil) suggest that primary pyrrhotite was almost completely oxidized to pyrite by reaction with hydrothermal fluids containing very little sulfate.

(Table, page 233) Comparative analysis of manganese nodules

A preliminary report on the manganese nodule field southwest of Western Australia published in this Journal recently (Frakes, Exon and Granath, 1977) quoted chemical analyses which were carried out on air-dried material. Significantly higher metal values have been recorded in some later analyses done on nodules dried at 105°C. Tests have shown that the ground, air-dried material retains considerable moisture, which accounts for the higher metal values of the later analyses. The average water content (after drying at 105°C) has been determined at 16 percent. The relevant chemical data now available on this material are summarised in the accompanying table: in this table metal values (by atomic absorption spectrophotometry) have been recalculated assuming a moisture content of 16 percent.