ORGANIC-CARBON ACCUMULATION AND PRESERVATION IN SURFACE SEDIMENTS ON THE PERU MARGIN

Concentrations and characteristics of organic matter in surface sedime nts deposited under an intense oxygen-minimum zone on the Peru margin were studied in samples from deck-deployed box cores and push cores ac quired by submersible on two transects spanning depths of 75 to 1000 m at 12 degrees and 13.5 degrees S. The source of organic matter to the seafloor in these areas is almost entirely marine material as confirm ed by the narrow range of delta(13)C of organic carbon obtained in the present study (-20.3 to -21.6%; PDB) and the lack of any relationship between pyrolysis hydrogen index and carbon isotope composition. Orga nic carbon contents are highest (up to 16%) on the slope at depths bet ween 75 and 350 m in sediments deposited under intermediate water mass es with low dissolved oxygen concentrations (<5 mu mol/kg). Even at th ese low concentrations of dissolved oxygen, however, the surface sedim ents that were recovered from these depths are dominantly unlaminated. Strong currents (up to 30 cm/s) associated with the poleward-flowing Peru Undercurrent were measured at depths between 160 and 300 m on bot h transects. The seafloor in this range of water depths is characteriz ed by bedforms stabilized by bacterial mats, extensive authigenic mine ral crusts, and (or) thick organic flocs. Constant advection of dissol ved oxygen, although in low concentrations, active resuspension of sur ficial organic matter, activity of organisms, and transport of fine-gr ained sediment to and from more oxygenated zones all contribute to gre ater degradation and poorer initial preservation of organic matter tha n might be expected under oxygen-deficient conditions. Dissolved-oxyge n concentrations ultimately may be the dominant affect on organic matt er characteristics, but reworking of fine-grained sediment and organic matter by strong bottom currents and redeposition on the seafloor in areas of lower energy also exert important controls on organic carbon concentration and degree of oxidation in this region. (C) 1998 Elsevie r Science B.V. All rights reserved.