Water column anoxia, enhanced productivity and concomitant changes in δ¹³C and δ³⁴S across the Frasnian-Famennian boundary (Kowala - Holy Cross Mountains/Poland)

The investigation of the trace element and organic geochemistry of the Frasnian-Famennian boundary section at Kowala (Holy Cross Mountains/Poland) shows that the lower water column was oxygen-deficient during late Frasnian and early Famennian times. The abundance and carbon isotopic composition of diaryl isoprenoids, biomarkers indicative for green sulfur bacteria, prove that euxinic waters reached into the photic zone, at least episodically. Total organic carbon (TOC) contents show two maxima that are time-equivalent to the Kellwasser horizons deposited in shallower water settings. Enhanced TOC concentrations are explained by a higher primary productivity, presumably as a consequence of an enhanced nutrient supply from the continent. The increase in the abundance of hopanes and bituminite suggests that the bacterial contribution to TOC increased at the Frasnian-Famennian transition. The sulfur isotopic composition of pyritic- and organically bound sulfur shows a +27‰ excursion across the boundary. The observation that the δ³⁴S values of organic-bound sulfur closely resemble that of pyrite sulfur indicates a common sulfur source, likely early diagenetic sulfide. A change in the δ¹³C of total dissolved inorganic carbon as a consequence of an enhanced burial of ¹²C-enriched organic carbon is indicated by a + 3‰ excursion measured for TOC as well as for individual n-alkanes and isoprenoids. The burial of large amounts of organic carbon is expected to result in a decrease in pCO₂ and should affect the photosynthetic carbon isotope fractionation (ε_p). The fact that we observe no change in E_p can be explained by the circumstance that E_p was most probably at maximum values, as a consequence of high atmospheric and oceanic-dissolved CO₂ concentrations during the Devonian.