PRIMARY LIGHT-ISOTOPE ENRICHMENT OF THE ORGANIC CARBON IN HOLOCENE SEDIMENTS OF THE BLACK SEA

Abstract

The Black Sea is a present-day analog of a typical past situation that promoted the development of sedimentary deposits rich in organics, including oil shales and the parent rocks of oil. In many cases the organic matter from such rocks is greatly enriched in the lighter carbon isotope. Identifying the causes and mechanisms for the formation of marine organic matter with anomalously light organic carbon is one of the chief tasks of organic geochemistry. Isotopic analysis of Holocene sapropelic oozes, a possible diagenetic analog of such sedimentary rocks, appears to be an important route to this goal. We investigated the organic matter of a three-member sedimentary sequence typical of the Black Sea, consisting of Recent sediments, ancient Black Sea sediments, and Neoeuxine deposits. Our data indicate that the enrichment of the organics of Holocene sapropels with the light carbon isotope is not the result of a large influx of terrigenous sediments or of isotopic changes attendant on diagenesis. The enrichment is probably due to the assimilation of CO2 under the distinctive gas and hydrogeochemical conditions that have developed in the Black Sea in recent millennia.