(Appendix) Chemical composition of bottom sediments at DSDP Leg 72 Holes

The distributions of calcium carbonate, of amorphous silica, and of 21 chemical compounds and elements in sediments of Holes 515A, 515B, 516, 516F, 517, and 518 are highly nonuniform; they change depending on the sediment types, grain size, and mineral composition. The main source of the lithogenous elements (K, Li, Rb, Fe, Ti, Zr, Ni, Cr, Sn) is terrigenous matter of South America. These elements correlate well or at least satisfactorily with each other and with the sum of clay minerals. CaCO3, amorphous SiO2 and organic C form a second group, the main source of which is biota of the ocean. Zn, Cu, Ba, Mo, (V, Na) are a third group, which is supplied by both terrigenous and biogenic matter. Judging by the distribution of chemical elements and components in sediments of Site 515, this area of the Brazil Basin is characterized by the rather constant conditions of pelagic terrigenous sedimentation from upper Eocene till Holocene. Small changes in chemical composition of sediments throughout the section are linked mainly to the evolution of subaerial source provinces, changes in hydrodynamic regime, and fluctuations of the ocean level. The chemical composition of sediments from the Rio Grande Rise sites suggests the existence of three main stages of sedimentation in this area. The first stage is the initial period of sediment accumulation on basalts at the beginning of the Late Cretaceous. Then followed sedimentary conditions notable for their sharp changes in chemical composition and type. Beginning in the middle Eocene and persisting into the Holocene, stable conditions of sedimentation characterize a third stage, represented by the formation of approximately 700 m of nannofossil oozes of rather monotonous chemical composition.

(Table 1) Major and trace elements of basalts at DSDP Hole 72-516F

The geochemistry of basalts recovered during Leg 72 is described with emphasis on trace elements. Only Hole 516F penetrated basement; the basalts recovered are plagioclase-phyric and olivine-phyric and pervasively altered. Chemically, the basalts from Hole 516F are rather uniform in composition. However, four distinct geochemical units can be recognized, although the chemistry of two of the units appears to be controlled by chemical mobility associated with alteration. The two less-altered units cannot be related by fractional crystallization processes. Hole 516F basalts have a trace element chemistry characteristic of T-type mid-ocean ridge basalt; rare-earth element patterns (as indicated by Ce/Y ratios) are mildly fractionated flight rare-earth element enriched), and a number of incompatible element ratios are close to chondritic.