GEOCHEMISTRY OF RECENT OXIC AND ANOXIC MARINE-SEDIMENTS - IMPLICATIONS FOR THE GEOLOGICAL RECORD

The distributions of certain minor and trace elements in marine sedime nts should potentially provide forensic tools for determining the redo x conditions of the bottom waters at the time of deposition. The abili ty to identify such conditions in the geological past is important bec ause (1) current models of the conditions of formation of organic-rich rocks require re-examination, (2) a method to determine whether the a real extent of anoxic waters expanded or retracted in response to pala eoceanographic changes is required, and (3) the effects of such enviro nmental changes on the geochemical balance of these elements in the oc ean need to be understood. Recent research has suggested that some min or and trace elements are precipitated where free dissolved sulphide i s present (Cu, Cd, Ni, Zn) without undergoing a valency change, wherea s others undergo a change in valency and are either more efficiently a dsorbed onto solid surfaces under oxic (1) or anoxic (V) conditions or are precipitated under anoxic conditions (Cr, Mn, Mo, Re, U, V). Henc e, the enrichment of these minor and trace elements relative to their crustal abundances indicates that the host sediments accumulated under anoxic conditions, although not necessarily under anoxic bottom water s. Examination of the chemical composition of the sediments of anoxic basins, continental margin sediments and oxidized deep-sea sediments s hows that I and Mn enrichments are reliable indicators of bottom water oxygenation, whereas enrichments of the remaining elements reflect ei ther bottom water anoxia or element uptake by subsurface anoxic sedime nts below a relatively thin surficial oxic veneer. Hence, the absence of metal enrichment in these cases can be taken as firm evidence that the bottom waters of a basin of sedimentation were not anoxic. These b ehaviours may be used to propose, for example, that the Holocene sapro pel in the Black Sea accumulated under oxic bottom waters, whereas the modern facies reflects its formation under the prevailing intensely a noxic conditions, and that the Panama Basin bottom waters were not ano xic during the Last Glacial Maximum when the rate of accumulation of o rganic carbon increased. Likewise, the enrichment of Mn as a mixed car bonate phase in some ancient black shales strongly suggests that they formed under oxic bottom waters rather than anoxic conditions as is co mmonly assumed.