REDOX REACTIONS AND CARBONATE PRESERVATION IN DEEP-SEA SEDIMENTS

Geochemical analyses of long sediment cores in turbidite sequences from two abyssal plains in the North Atlantic have provided evidence for a relationship between post-depositional oxic decay of organic matter and the dissolution of carbonates. Palaeo-oxidation zones in the upper part of individual turbidites were initially identified by distinct colour changes in the sediments. These zones were found to have distinctly lower amounts of organic carbon, carbonate minerals and labile metals such as Fe, Mn, Ni and Zn, as compared with the underlying unoxidized parts of the same turbidite. It appears that the initial oxidation of organic matter by oxygen results in an undersaturation of the pore water with respect to carbonates, while at the same time some metals are precipitated as easily reducible and soluble oxyhydroxides. After burial under suboxic conditions the palaeo-oxidation zones are subjected to mildly reducing pore waters that leach the oxyhydroxides, resulting in a depletion in labile metal contents in the palaeo-oxidized zones. It is important that these characteristics be identified in the sedimentary record as the result of diagenetic processes and not as a reflection of primary production in palaeo-oceans. © 1990.