ORGANIC GEOCHEMISTRY OF AN ANOXIC CYCLE - A CASE-HISTORY FROM THE OLIGOCENE SECTION, HUNGARY

Total organic carbon, extractable organic matter, sulfur content, kerogen type and saturated hydrocarbons have been determined in 25 core samples from an approximately 100 m thick section of the Oligocene Tard Clay, near Budapest. The Tard Clay records a five million year long anoxic cycle initiated by isolation of the sea, development of a positive water balance and resulting density stratification, and terminated by the re-opening of access to the sea. The lower, non- and moderately laminated shales record the build-up of anoxia, they contain 0.5-5.0% of organic carbon and Type III kerogen. Their saturated hydrocarbon fractions show high abundance of phyllocladanes, biomarkers characterising Coniferae-derived organic matter. The overlying moderately and strongly laminated sediments containing 1.5-5.0% organic carbon, a predominance of hopanes over the n-alkanes and Type II kerogen show the imprint of increased planktonic productivity. The nutrients needed for this high productivity were liberated by alteration processes of contemporaneous volcanic glass. The upper part of the strongly laminated shales containing a maximum of 2% organic carbon and Type III kerogen, deposited after the termination of volcanic ash falls, records a drop of planktonic productivity. At the top of the Tard Clay the weak lamination and the low content (<1%) of organic carbon signifies the end of the bottom water oxygen depletion due to the destruction of density stratification of the water column. © 1990.