THE DECOUPLING OF BIOLOGICAL AND INORGANIC GEOCHEMICAL SIGNALS IN MICROSTRATIGRAPHIC SHALE STUDIES

The utility of high-resolution studies of black shale intervals is becoming widely recognized in part due to the variability of depositional conditions preserved in this lithology. Despite this increasing awareness of fluctuating depositional conditions, the magnitude of variability that is recognized through microstratigraphic analysis and the utility of different proxies to capture this range are poorly constrained. In order to quantify at high resolution and statistically analyze the variability in biological and chemical signals, over 250 samples from 5 localities were collected and analyzed from Middle and Upper Devonian black shales in the Appalachian basin. A range of gray to black shales representing what are inferred to be oxic to anoxic conditions were sampled continuously and processed at cm scale resolution. The trace fossil signal recorded as ichnofabric index (ii1-5) was used to capture the biological variability, and a range of redox sensitive trace metals (Mo, Mn, U, V, Cr, Ni, Pb, Cu, Zn) were analyzed to characterize variation in the preserved inorganic geochemical signal. The biological signal can in rare cases, though typically does not, fluctuate dramatically (i.e. more than one ii value) on the cm scale through these intervals, indicating that the bottom water oxygen levels can change rapidly. The trace metal concentrations do not closely correlate stratigraphically with the biological signal; this disconnect is interpreted to reflect the variation in the redox signal, which is preserved and potentially modified in the sediment as a result of subsequent changes in bottom water chemistry. These results support that significant variation in depositional conditions can be recognized on the cm scale, but care must be taken to recognize potential post-depositional mixing and overprinting of the original chemical signal in sediment.