Using fluorescence microscopy to discern in situ from reworked palynomorphs in dynamic depositional environments — An example from sediments of the late Miocene to early Pleistocene Caspian Sea
Résumé
Knowledge of the ecological affinities of fossil palynomorphs found in sediments allows palynologists to reconstruct environmental change through time. However, the journey from source to sink is not straightforward and it is often difficult to discern whether palynomorphs are in situ, especially when working on sediments deposited in dynamic environments such as deltas. Palynomorphs may be buried for long periods of time before being re-suspended and transported to secondary depositional sites, sometimes without visible changes in their appearance. In such cases, the palynological assemblages are not representative of the contemporary environment, as they contain a mixture of in situ and reworked palynomorphs. Here we use fluorescence microscopy as a tool to assess levels of reworking in a spatiotemporally highly dynamic setting, the Pliocene hydrocarbon reservoir rocks of the South Caspian Basin. The results highlight trends in the fluorescence properties of palynomorphs before, during and after the Pliocene that were not apparent using transmitted light microscopy. The late Miocene and early Pleistocene marine-influenced samples show predominantly unaltered assemblages, which may be considered in situ. The Pliocene samples, however, show variable signals, from relatively unaltered to highly altered assemblages. Potential causes of alteration include strong heating and exposure to oxygen/microbial decay. Due to low burial temperatures at the sites sampled, exposure to oxygen during transport is likely the primary mechanism of change in fluorescence properties. This paper emphasises use of the fluorescence method as a tool to characterise palynomorph properties and separate reworked from in situ palynomorph assemblage fractions. The fluorescence approach could also be applied to palynological studies in dynamic depositional settings worldwide.