Geochemistry of Recent oxic and anoxic marine sediments: Implications for the geological record

The distributions of certain minor and trace elements in marine sediments should potentially provide forensic tools for determining the redox conditions of the bottom waters at the time of deposition. The ability to identify such conditions in the geological past is important because (1) current models of the conditions of formation of organic-rich rocks require reexamination, (2) a method to determine whether the areal extent of anoxic waters expanded or retracted in response to palaeoceanographic changes is required, and (3) the effects of such environmental changes on the geochemical balance of these elements in the ocean need to be understood.

Recent research has suggested that some minor and trace elements are precipitated where free dissolved sulphide is present (Cu, Cd, Ni, Zn) without undergoing a valency change, whereas others undergo a change in valency and are either more efficiently adsorbed onto solid surfaces under oxic (I) or anoxic (V) conditions or are precipitated under anoxic conditions (Cr, Mn, Mo, Re, U, V). Hence, 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 waters. Examination of the chemical composition of the sediments of anoxic basins, continental margin sediments and oxidized deepsea sediments shows that I and Mn enrichments are reliable indicators of bottom water oxygenation, whereas enrichments of the remaining elements reflect either bottom water anoxia or element uptake by subsurface anoxic sediments 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 behaviours may be used to propose, for example, that the Holocene sapropel in the Black Sea accumulated under oxic bottom waters, whereas the modern facies reflects its formation under the prevailing intensely anoxic conditions, and that the Panama Basin bottom waters were not anoxic during the Last Glacial Maximum when the rate of accumulation of organic carbon increased. Likewise, the enrichment of Mn as a mixed carbonate phase in some ancient black shales strongly suggests that they formed under oxic bottom waters rather than anoxic conditions as is commonly assumed.