Ages, sedimentation rates and geochemical composition of ODP Site 162-983 sediments

A geochemical study of sediments from Ocean Drilling Program Site 983 was conducted to examine low-frequency variations in carbonate content as expressed by blue-band reflectance (450-500 nm) over the last 1.2 Ma. Sedimentary percent organic carbon, percent carbonate, and excess barium (Ba[ex]) were used as the primary tools to evaluate the factors responsible for these long-term changes. We observe positive correlation between the mass-accumulation rate of various biogenic components and the mass-accumulation rate of Ba(ex), especially in sediments younger than ~600 ka. Deeper in the section (~600-1200 ka), the correlation between Ba(ex) and the other biogenic tracers is weak. The lack of correlation between Ba(ex) and biogenic carbonate likely results either from a higher supply of terrigenous material at that time (which confounds Ba[ex] estimation), or remobilization of Ba resulting from low pore-water sulfate ion concentrations, or both. Nonbiogenic sediments at Site 983, represented by Th, K2O, and the molar Ti/Al ratio, exhibit cyclic variations that represent mixing between continental and oceanic (i.e., basaltic) terrigenous sources. The timing of these cycles matches that of the major glacial-interglacial cycles, which suggests that they result from the supply of continental material as ice-rafted debris during glacial periods and fine-grained basaltic material by bottom currents during interglacial periods. Given these observations, the most likely causes for the low-frequency carbonate variations observed in the Site 983 sediments are shifts in surface productivity and, to a lesser extent, dilution by the input of terrigenous material.

Mineral composition of sediments from ODP Leg 190 sites and DSDP Hole 31-297

This data report documents the acquisition of two new sets of normalization factors for semiquantitative X-ray diffraction analyses. One set of factors is for bulk sediment powders, and the other applies to oriented aggregates of clay-sized fractions (<2 µm). We analyzed mixtures of standard minerals with known weight percentages of each component and solved for the normalization factors using matrix singular value decomposition. The components in bulk powders include total clay minerals (a mixture of smectite, illite, and chlorite), quartz, plagioclase, and calcite. For clay-sized fractions, the minerals are smectite, illite, chlorite, and quartz. We tested the utility of the method by analyzing natural mudstone specimens from Site 297 of the Deep Sea Drilling Project, which is located in the Shikoku Basin south of Site 1177 of the Ocean Drilling Program (Ashizuri transect).

Mineralogy of silts from the Bengal Fan

Early Miocene to Quaternary sediments drilled from the Bengal Fan are divided into six zones by modal proportions of heavy minerals. The sediments were mostly derived from the Himalayas. Detritus from the Indian subcontinent is found sporadically in clay-rich sediments that were deposited during periods of slow sedimentation, when the deep-sea channel migrated away from the drilled sites. The oldest sediments, ranging from 17 to about 15 Ma, were derived mostly from the Precambrian and Paleozoic sedimentary rocks of the lower Himalayas. At about 15 Ma, metamorphic terrains were eroded in the source area. Further large-scale unroofing of metamorphic rocks occurred around 11 Ma. After 10 Ma, the major constituents in the drainage basin or the drainage pattern changed a few times. Between 3.5 and 0.5 Ma, a large peridotite body was unroofed by uplift and successive erosion of the central Himalayas. At this time, the single large river that had supplied detritus to the early Bengal Fan was divided into the Indus and Ganges rivers.