Biomarkers as environmental indicators in a carbonate complex, example from the Middle to Upper Devonian, Holy Cross Mountains, Poland

Autochthonous biomarker distributions were studied in an array of different subfacies of a Devonian carbonate complex (represented in platform, reef a nd off-platform shelf-basinal systems). It revealed that biomarkers are sen sitive indicators of depositional environments and may serve to refine pala eoenvironmental interpretations of carbonate complexes based on "classical" sedimentological/facies analyses. The contrasting biomarker composition of the carbonate platform and reef complex versus shelf-basin deposits is her e interpreted as the effect of both more diverse biotic sources of a primar y organic matter in the former environments (highly scattered results of st erane distribution) and contrasting styles of decay and bacterial reworking of the primary biomass. Peritidal dolostones deposited under anoxic to sub oxic conditions, and with scarce sedimentological evidence of elevated sali nity, reveal the highest potential of organic matter preservation among oth er platform/reefal facies. Their biomarker composition includes several com pounds indicating hypersaline conditions and anoxia whereas others, in part icular gammacerane, indicate that the water-column was stratified during se dimentation. This observation constrains environmental interpretation of th ese deposits, pointing to shallow-subtidal lagoons with elevated salinity a nd low-oxygen near-bottom waters most probably originating from a salinity- controlled stratification of the water-column. Biomarkers diagnostic of gre en sulphur bacteria were found in both peritidal/lagoonal dolostones and sh elf-basinal facies. This indicates the existence of photic zone anoxia in t he latter environment, thus suggesting a shallow position of the redoxeline . The Frasnian/Famennian mass extinction event(s) apparently had no impact on the biomarker distribution in the shelf-basinal system continuous across the stage boundary. (C) 2000 Elsevier Science B.V. All rights reserved.