Water column anoxia, enhanced productivity and concomitant changes in delta C-13 and delta S-34 across the Frasnian-Famennian boundary (Kowala Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Fras nian-Famennian boundary section at Kowala (Holy Cross Mountains/Poland) sho ws that the lower water column was oxygen-deficient during late Frasnian an d early Famennian times. The abundance and carbon isotopic composition of d iaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove t hat euxinic waters reached into the photic zone, at least episodically. Tot al organic carbon (TOC) contents show two maxima that are time-equivalent t o the Kellwasser horizons deposited in shallower water settings. Enhanced T OC concentrations are explained by a higher primary productivity, presumabl y as a consequence of an enhanced nutrient supply from the continent. The i ncrease in the abundance of hopanes and bituminite suggests that the bacter ial contribution to TOC increased at the Frasnian-Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27 parts per thousand excursion across the boundary. The observation th at the delta S-34 values of organic-bound sulfur closely resemble that of p yrite sulfur indicates a common sulfur source, likely early diagenetic sulf ide. A change in the delta C-13 of total dissolved inorganic carbon as a co nsequence of an enhanced burial of C-12-enriched organic carbon is indicate d by a + 3 parts per thousand excursion measured for TOC as well as for ind ividual n-alkanes and isoprenoids. The burial of large amounts of organic c arbon is expected to result in a decrease in pCO(2) and should affect the p hotosynthetic carbon isotope fractionation (epsilon (p)) The fact that we o bserve no change in epsilon (p) can be explained by the circumstance that e psilon (p) was most probably at maximum values, as a consequence of high at mospheric and oceanic-dissolved CO2 concentrations during the Devonian. (C) 2001 Elsevier Science B.V. All rights reserved.