A Test of Models for the Evolution of Water-Column Anoxia in the Black Sea Over the Past 8.5 Years

Project: Research project

Project Details

Description

9628344 Arthur This proposal seeks to resolve a debate as to the intensity and depth distribution of oxygen deficiencies in the water column of the Black Sea and to provide a test of the efficacy of new techniques for elucidating paleoenvironments of ancient 'black shales'. Previous workers have suggested, on the basis of interpretation of the fabric and geochemical characteristics of sediments deposited from about 8 ka to 2 ka, that finely laminated, organic-carbon (OC) rich sediments of Unit II, often referred to as sapropels, either were deposited under an anoxic water column for which initiation of anoxia was time transgressive, beginning earlier in deeper water, or were produced under an anoxic water column as the result of high surface-water productivity. New results from the study of Black Sea cores recovered during the 1988 R/V Knorr expedition suggest that the onset of water-column anoxia at about 7.5 ka was virtually synchronous across the basin over a depth range of about 200 m to 2250 m, and that anoxia prevailed throughout deposition of Unit II. However, the concentration, accumulation rate, and degree of preservation of organic matter all decreased in the upper part of Unit II, perhaps as a result of decreasing productivity and possible erosion of the top of the chemocline. Various redox-sensitive geochemical indicators have been interpreted by different workers in contrasting ways. In the proposed study, we will use two new techniques, one involving pyrite (framboid) size distributions that constrain the site of pyrite precipitation (water column vs. within sediment), and the other a set of organic biomarker compounds indicative of the presence of photosynthetic sulfide-oxidizing bacteria in the water column. These techniques will be used in analysis of a depth transect of available Black Sea gravity cores that recovered complete sequences of Unit II in order to test models of the Holocene evolution of the Black Sea and the origin(s) of sediments enriched in organic car bon in general.

StatusFinished
Effective start/end date2/1/978/30/98

Funding

  • National Science Foundation: $85,000.00

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