Are the Kimmeridge Clay deposits affected by “burn-down” events? Palynological and geochemical studies on a 1 metre long section from the Upper Kimmeridge Clay Formation (Dorset, UK)

Two independent analytical approaches, palynology and inorganic geochemistry, were applied to identify potential oxygen “burn-down” events in the Late Jurassic Kimmeridge Clay Formation (KCF). The KCF interval of the rotunda ammonite zone, spanning 121.82–122.72 m depth was sampled from the Swanworth Quarry 1 borehole (Dorset, UK) at 2.5–5.0 cm resolution. Samples were analysed for total organic carbon (TOC), concentrations of elements that are known to be productivity- and/or nutrient-related (e.g. Cu, P), detrital (e.g. Al, Ti, Zr) and redox-sensitive/sulphide-forming (e.g. V, Mo, Fe, Mn, S), and palynofacies components including analysis of organic-walled dinoflagellate cysts (dinocysts) on a species level.The TOC contents generally exceed 2 wt.%, with a maximum of 8.8 wt.% at 122.37 cm depth and elevated values in the central part of the investigated interval. This interval of relatively higher TOC values correlates well with the maximum recovery of marine palynomorphs and low Al values, suggesting that the TOC is primarily of marine organic matter (OM).Changes in V/Al, Mo/Al, Fe/Al, Mn/Al and S patterns at 122.37 m depth mark a shift from anoxic conditions in the lower part of the studied interval to more oxic conditions in its upper part. Such a shift could explain the relatively high TOC and marine palynomorph concentrations in the lower part of the studied interval as a result of better preservation, and the subsequent decrease as an effect of a post-depositional “burn-down”, i.e. OM oxidation. As the amount of marine palynomorphs and TOC content diminishes from the middle part of the section upwards, species-specific changes in dinocyst assemblages can be observed. In particular, concentrations of Circulodinium spp., Cyclonephelium spp., Sirmiodinium grossi, Senoniasphaera jurassica and Systematophora spp. decrease rapidly in comparison to other species, such as Glossodinium dimorphum and Cribroperidinium sp. 1, which may suggest selective degradation of dinocysts due to oxidation.We suggest that post-depositional oxygenation of bottom and pore waters within the sediment was most probably the cause for decreasing TOC values and reduced recovery of marine palynomorphs towards the top of the studied interval in comparison to high TOC and marine palynomorph values in the central part of the studied interval due to anoxic conditions.