SEAWATER AS THE SOURCE OF MINOR ELEMENTS IN BLACK SHALES, PHOSPHORITES AND OTHER SEDIMENTARY-ROCKS

Many of the minor elements in seawater today have a concentration-dept h profile similar to that of the biologically essential nutrients, NO3 - and PO43-. They show a relative depletion in the photic zone and enr ichment in the deep ocean. The difference between their surface- and d eep-ocean values, normalized to the change in PO43-, approaches the av erage of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plan kton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is i n the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies , particularly of sediments which accumulate in shelf-slope environmen ts where biological productivity in the photic zone is exceptionally h igh and organic carbon contents of the underlying sediment elevated. T he interelement relations observed for some of these sediments approac h the average values of plankton. These same interelement relations ar e observed in many marine sedimentary rocks such as metalliferous blac k shales and phosphorites, rocks which have a high content of marine f ractions (e.g., organic matter, apatite, biogenic silica and carbonate s). Many previous studies of the geochemistry of these rocks have conc luded that local hydrothermal activity, and/or seawater with an elemen tal content different from that of the modern ocean, was required to a ccount for their minor-element contents. However, the similarity in se veral of the minor-element ratios in many of these formations to minor -element ratios in modem plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal se as, of the ocean today. The accumulation of the marine fraction of min or elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorpti on from seawater. The suite of elements in this fraction reflects redo x conditions in the bottom water, as determined by bacterial respirati on. For example, high Mn, high Cr + V and high Mo concentrations, abov e those which can be attributed to the accumulation of planktonic matt er, characterize accumulation under bottom-water oxidizing, denitrifyi ng and sulfate-reducing conditions, respectively.