Quantitative estimation of aerobic diagenetic overprint of palaeoproductivity signals

Abstract

Species-selective aerobic decomposition affects fossil organic-walled dinoflagellate cyst (dinocyst) records and hence dinocyst-based interpretations of primary productivity and oceanographic conditions. However, since the recognition of dinocyst species sensitive and resistant to oxic degradation (S- and R-cysts, respectively) it has become apparent that R-cysts may still serve as reliable productivity and oceanographic conditions proxies. On the other hand S-cysts provide a way to quantify aerobic degradation of organic matter (OM) and past bottom-water O2 concentrations. OM degradation plays a key role in global carbon cycling and is important for global climate change. Therefore dinocysts are a valuable tool for estimating the rate of diagenetic process. Questions concerning species-selective aerobic degradation still remain and will be adressed here. To obtain information on the rate of S-cyst decomposition, the relationship between S-cyst degradation and O2 concentrations, and the aerobic degradation of extinct dinocyst species, a natural exposure experiment has been conducted and studies of both Quaternary and pre-Quaternary material from sediment cores were executed. The exposure experiment was conducted in the natural setting of the Eastern Mediterranean. During a 15 month exposure period to oxic water masses, concentrations of S-cysts (Brigantedinium spp. and Echinidinium granulatum) decreased by 24 to 57%. However, taxa such as Nematosphaeropsis labyrinthus, Echinidinium aculeatum, Operculodinium israelianum and Impagidinium aculeatum demonstrated a slight increase in concentration, indicating resistance to aerobic degradation. These results show that even short-term exposure to oxygen may cause considerable changes in the dinocyst assemblage and thus overprint the primary signal, leading to misinterpretation of the environmental conditions. No degradation was observed during exposure of S-cysts to anoxic water masses. Analysis of two short cores from the Atlantic sector of the Southern Ocean permitted calculation of the degradation constant (k) for S-cysts (Brigantedinium spp. and Selenophemphix antarctica). Calculated k decreases exponentially with increasing oxygen exposure time (OET), supporting earlier findings that OM degradation depends on labile organic component concentrations. Constant k also shows a positive correlation with pore-water O2 concentrations, implying that degradation is dependent not only on OET and OM concentration, but also on O2 concentrations in bottom and pore waters. O2 seems to be a limiting factor at lower concentrations, whereas at higher O2 concentrations the availability of labile OM seems to be more important. A study of the Jurassic Kimmeridge Clay Formation provided information on the selective degradation of extinct dinocyst species. This is the first attempt to investigate aerobic degradation of in situ Jurassic dinocysts. Several taxa (i.e. Circulodinium spp., Cyclonephelium spp., Sirmiodinium grossi, Senoniasphaera jurassica and Systematophora spp.) decrease in abundance during post-depositional oxidation of sediments. Reconstruction of depositional redox conditions was based on coupled independent methods, combining palynofacies analysis with organic and inorganic geochemical proxies (total organic carbon and Fe, Mn, S, Cu, P, Al respectively). Further research is necessary to establish a list of extinct dinocyst species sensitive to oxidation. These results show that both extant and extinct dinocysts may be affected by species-selective aerobic degradation, making interpretations of fossil records difficult. Species-selective degradation is shown to be a rapid process and therefore cannot be neglected on any time scale. The dependence of S-cyst degradation on pore water O2 concentrations has implications for aerobic OM decomposition, indicating that OM decay is dependent not only on OET and availability of labile OM but also on bottom- and pore-water O2 concentrations.