Abstract: Organic Carbon Sedimentation and Preservation Patterns in Modern Oxygen-Deficient Environments: Is the Present an Analogue for the Past?

M. A. Arthur

Recent studies of a number of modern oxygen-deficient environments have failed to establish a clear link between water column dissolved oxygen concentrations and organic matter preservation. Our studies of Holocene sediments deposited under an intense oxygen-minimum zone across the Peru margin and in the euxinic Black Sea suggest that this may, in part, be the result of depositional complexities that strongly overprint other preservational patterns. For example, offshore Peru, organic carbon contents (primarily marine autochthonous material) were highest (up to 16 weight %) in association with low dissolved oxygen concentrations (<5 micro moles/kg) at depths between 150 and 450 meters. However, organic matter preservation indices are quite variable. Sediment transport and reworking by strong bottom currents (as high as 30 cm/sec) is suggested as a secondary control on organic carbon preservation. Likewise, although deep basinal sediments in the Black Sea are enriched inorganic carbon (to 12 weight % as mixed marine and terrestrially derived material), preservational indices are variable and somewhat lower than expected for a euxinic setting. In addition, organic carbon accumulation rates are not significantly higher than those for oxic environments with similar sedimentation rates. A significant proportion of the organic material appears to have been reworked, perhaps by basinward transport of fine-grained material resuspended from surrounding shelves.

Studies of ancient black shales, on the other hand, appear to indicate a strong association between oxygen depletion and enhanced organic carbon preservation. Was there something fundamentally different about ocean chemistry and circulation at times in the past that enhanced the role of anoxia? We suggest several possible characteristics of ancient earth systems that enhance the role of anoxia as a means of increasing organic matter accumulation and preservation. However, most of these mechanisms involve strong feedbacks to nutrient concentrations and a first-order control of surface-water productivity and organic carbon flux to the sediment/water interface on amount and type of organic matter preserved.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994