Geochemistry of basalts at DSDP Site 89-462

At Site 462 in the Nauru Basin, western Pacific Ocean, 56 lithologic units have been recovered from an extensive flood basalt province. Fossil evidence suggests that the lavas were emplaced during the interval 100-115 Ma, some 30 m.y. after formation of the underlying Jurassic ocean crust. The lithologic units can be broadly divided into three chemical units, the lowermost two of which are chemically monotonous, suggesting rapid eruption of basalt from a compositionally homogeneous magma chamber. All the basalts are hypersthene- (hy-) rich tholeiites, with approximately chondritic La/Sm, La/Yb, Zr/Nb, La/Ta, and Th/Hf ratios. Chemically they resemble, in part, "transitional" mid-ocean ridge basalts (MORB) from areas such as the Reykjanes Ridge, although Rb, Ba, and K contents are very low and similar to those of "normal" MORB. Their 87Sr/86Sr ratios are higher than in N-type MORB (Fujii et al., 1981). The chemistry of the Nauru basalts differs from that of continental flood basalts, which tend to be strongly enriched in large-ion lithophile (LIL) elements, although the extent to which the differences result from sialic contamination or source variability is not clear.

(Table 1) Concentrations and carbon isotopic compositions of CH4 and CO2 at DSDP Site 76-533

The principal gaseous carbon-containing components identified in the first 400 m of sediment at Deep Sea Drilling Project Site 533, Leg 76, are methane (CH4) and carbon dioxide (CO2). Below a sub-bottom depth of about 25 m, sediment cores commonly contained pockets caused by the expansion of gas upon core recovery. The carbon isotopic composition (d13C per mil relative to PDB standard) of CH4 and CO2 in these gas pockets has been measured, resulting in the following observations: (1) d13C-CH4 values increase with depth from approximately -94 per mil in the uppermost sediment to about -66 per mil in the deepest sediment, reflecting a systematic but nonlinear depletion of 12C with depth. (2) d13C-CO2 values also increase with depth of sediment from about -25 per mil to about -4 per mil, snowing a depletion of 12C that closely parallels the trend of the isotopic composition of CH4. The magnitude and parallel distribution of d13C values for both CH4 and CO2 are consistent with the concept that the formation of the CH4 resulted from the microbiological reduction of CO2 from organic substances. These results imply that CH4 and CO2 incorporated in gas hydrates at this site are biogenic.

(Table 2) Distribution of phytoliths in selected samples from DSDP Site 90-591

Phytoliths are described from deep sea sediments at Site 591 in the southwestern Pacific. Their regional distribution is related to the arid and semiarid regions of Australia, from where they were blown by westerly winds into the Tasman Sea area. The stratigraphic record ranges from the middle Miocene, at about 14.4 m.y., until the early Pleistocene. A distinct increase in frequencies observed during the Pliocene and a maximum at about 2.5 m.y. coincide with important trends in paleogeography and paleoclimatology: the development of the Antarctic ice cap, the northward drift of the Australian Plate, and the generation of arid conditions on the Australian continent.