The inorganic geochemistry of Cretaceous black shales (DSDP Leg 41) in comparison to modern upwelling sediments from the Gulf of California

Abstract

Summary: Recent trace element data for sea water and plankton reflect the importance of biological activity for the distribution of these elements in the water column and for their accumulation in sediments underlying areas of high productivity. Many trace metals in sea water are highly correlated to nutrients like phosphate, nitrate, or silica. By considering the ability of an element to be involved in nutrient cycling, as demonstrated by its deep/surface sea water concentration ratio, and its availability in sea water, a relationship between plankton and sea water chemistry is evident. Fifty sediment samples from the Gulf of California, an area of exceptionally high primary productivity, have been analysed for all major and 15 minor elements, including Cd, Zn, Cu, Ni, Ba, Se, Cr, As, V, Mo, Co, Pb, Tl, and Bi. The ‘excess’ of nutrient and trace elements in upwelling sediments from the Gulf of California reflects the plankton chemistry. Among those elements found enriched in the diatomaceous oozes from the Guaymas Basin Slope relative to average shale are P, S, Mo, Se, Cd, and possibly Bi. For Mo and V an additional accumulation process besides bioconcentration seems to be active during early diagenesis. For these two elements their concentration ratios to aluminum increase with burial depth in the upper 30 cm of the sedimentary column, indicating that sea water seems to be a major source, besides plankton. The chemistry of Ba in pore-waters and solids is controlled by barite solubility. A front of barite precipitation and dissolution (the ‘barite front’) is moving through the sedimentary column in the depth range of near zero pore-water sulphate, which at a drastic reduction of the sediment accumulation rate would lead to the formation of larger barite crystals or even nodules in this depth range. Cretaceous black shales from the Cape Verde Basin (Site 367, Cores 17 to 21) are characterized by extremely high (see maximum values in brackets) concentrations of Mo (280 p.p.m.), V (3600 p.p.m.), Zn (7000 p.p.m.), and a number of other trace elements, besides S (7.4%) and organic carbon (35.6%) which cannot be explained by upwelling. If these enrichments are of primary origin, then complete stagnation of the Cape Verde Basin during intervals in Cretaceous time seems likely. The marine environment is metal-poor, therefore stagnation and corresponding low accumulation rates of detrital material would favour the accumulation of organic carbon and trace metals.