THE EFFECT OF SEAWATER REDOX STRATIFICATION ON EARLY METAZOANS FROM THE TERMINAL EDIACARAN KHATYSPYT FORMATION OF ARCTIC SIBERIA

Ediacara fossils – unusually preserved as carbonaceous compressions or in concretions in thinly-bedded bituminous limestone – occur sporadically throughout the 160 m thick Khatyspyt Formation (ca. 545 Ma) in the Olenek Uplift of arctic Siberia. The local FAD of these enigmatic soft-bodied organisms coincides with a negative-to-positive δ¹³C anomaly in carbonates and the first of three pronounced δ³⁴S oscillations preserved in disseminated pyrite through the Khatyspyt stratigraphy. The first anomaly begins near the base of the formation with sulfur isotope compositions as low as -20 that rise through the FAD to values as high as +40 at around 60 meters. From this peak δ³⁴S values fall to +15 and then rise to a maximum of +55 at around 80 meters; values fall again to near 0 and then rise to the third peak at +50 near the top of the formation. These isotopic transitions are associated with an up-section decrease in CAS abundance and sulfur isotope fractionation, which may be the result of sulfate limitation, redox stratification of the water column, and a pronounced sulfur isotope gradient in seawater. Consistent with this interpretation, total organic carbon in most of the samples is additionally depleted in ¹³C suggesting the presence of chemoautotrophic or methanotrophic communities living along the oxic-anoxic interface within the water column or in sediments. Neither the stratigraphically coherent fluctuations in sulfur isotope abundances nor the three positive anomalies recorded in the Khatyspyt Formation have yet been identified elsewhere in equivalent strata, suggesting redox heterogeneity of terminal Ediacaran ocean basins. While the local FAD of the Ediacara biota may be associated with shallowing of sea level and the oxygenation of bottom waters, the organisms also occur higher in the formation where both carbon and sulfur isotope compositions oscillate between positive and negative extremes. These observations suggest the possibility that the Ediacara biota in the Siberian basin was tolerant of anoxic and potentially euxinic bottom waters, and thus may have harbored chemosymbionts like modern deep vent tube worms.