Nearshore euxinia in the photic zone of an ancient sea

Evidence for photic zone euxinia (PZE) in ancient oceans is widespread and commonly associated with global climate change and biotic events. However, evidence of PZE in marginal settings is commonly extrapolated to infer basin-scale characteristics. To explore the relationship between euxinia in marginal settings and the wider basin in one Palaeozoic context, we have conducted a detailed spatial and temporal examination the palaeowater-column redox state from the Polish sector of the Southern Permian Basin (SPB) during deposition of the Zechstein (Lopingian) second carbonate (Main Dolomite) cycle. Sediments from toe-of-slope, slope and hypersaline lagoonal settings of the northeastern margin of the SPB contain abundant chlorobactane, isorenieratane and their likely degradation products (C15 to C31 2,3,6-aryl isoprenoids). These indicate that part of the photic zone was euxinic during significant intervals of the Main Dolomite deposition. Further evidence for strongly anoxic conditions includes the occurrence of C28-bisnorhopane, high concentrations of pentakishomohopanes (i.e. high homohopane indices), and the occurrence of framboidal and native sulphur crystals in slope facies. However, a lack of these signatures in basinal settings indicates that strongly reducing conditions were restricted to the slope, and toe-of-slope locations. Even in these settings, however, highly variable homohopane indices and concentrations of isorenieratene and chlorobactene derivatives, as well as the presence of benthic fauna and bioturbation, indicate that anoxia fluctuated throughout deposition of the Main Dolomite, likely on a variety of timescales. Sterane distributions are also variable, suggesting a dynamic ecosystem, likely exhibiting a strong response to environmental forcing. Overall, it appears that high but episodic primary bioproductivity of organic matter was concentrated on the northeastern slope and restricted lagoons leading to the formation of source rocks for petroleum; however, the temporal and geographical restriction of anoxia appears to have prevented the accumulation of large or more widespread quantities of organic matter, and in fact TOC contents exhibit poor correlation with ecological and anoxia indicators. This model indicates that strong evidence for PZE in shelf and slope facies need not be associated with widespread, basin-scale anoxia, with implications for organic matter burial, carbon cycling and biotic crises.