Organic Geochemistry of Neogene Sediments from the Walvis Ridge, Deep Sea Drilling Project Leg 74

Abstract

Two samples of Miocene sediments from Site 525 and four samples of sediments ranging in age from Pleistocene to Miocene from Site 528 have been analyzed for concentrations of organic and carbonate carbon, carbon/nitrogen ratios of organic matter, and extractable hydrocarbons and fatty acids. Organic carbon concentrations average 0.32% and show a diagenetic decrease with greater sediment age. Distributions of n-alkanes and n-alkanoic acids give evidence of considerable microbial reworking and of eolian contributions of terrigenous components. Organic contents of these sediments reflect a history of low marine productivity and poor preservation of organic matter in the eastern South Atlantic since middle Miocene times.

Organic geochemistry of late Cenozoic sediments from the subtropical South Atlantic Ocean

“…Because the Leg 74 sites are on the Walvis Ridge and have low clay MARs (Table II), transport of terrigenous lipids by downslope flows is unlikely; hence, air-borne and hemipelagic transport apparently are responsible for such inputs. Comparison of n-alkane distributions in Quaternary and early Pliocene sediments at Site 528 with those of late to middle Miocene sediments from Site 525 shows a proportionally larger fraction of terrigenous components in the younger sediments (Meyers and Keswani, 1984). Because this shift in source character is contrary to that expected from diagenetic effects, it may indicate substantial modification of organic matter transport patterns in the South Atlantic since mid-Miocene times.…”

“…Sediments analyzed from these sites are of similar ages but differ in their compositions. Samples from Legs 72 and 74 represent low-productivity regions with relatively similar, low concentrations of organic carbon (Meyers and Dunham, 1983;Meyers and Keswani, 1984), whereas Leg 75 samples are from an area of high productivity associated with the Benguela upwelling system off South-West Africa (Hay et al, 1982;Meyers and Dunham, 1984).…”

Geolipids of Late Cenozoic Sediments from the Western Flank of the East Pacific Rise, Deep Sea Drilling Project Leg 92

“…Distributions similarly dominated by n-C22 have been found in organic-carbon-poor sediments from Legs 72 and 74 (Meyers and Dunham, 1983;Meyers and Keswani; and may result from postdeposi- . , .…”

“…Because of our earlier studies in the South Atlantic, which showed major contributions of land-plant hydrocarbons to sediments from DSDP Sites 516 and 517 on the Rio Grande Rise and Sites 525 and 528 on the Walvis Ridge (Meyers and Dunham, 1983;Meyers and Keswani, 1984), we were interested in examining geolipid distributions for evidence of eolian inputs in sediments from locations where marine productivity was low and where sediment gravity flows could not contribute terrigenous material. DSDP Leg 92 obtained samples from such locations, on the western flank of the East Pacific Rise in the subtropical South Pacific (Fig.…”