Preservation and organic chemistry of Late Cenozoic organic-walled dinoflagellate cysts: A review

Abstract

Within the last decade considerable information has become available on the effects of early diagenesis on the taphonomy of organic-walled dinoflagellate cysts. Here, we review the information currently available on this topic. After discussing organic matter degradation in general, an overview on the effects of different laboratory treatments on the dinoflagellate cyst association is given. Hereafter, the rates and amount of species-selective degradation in modern and fossil natural environments are discussed. It appears that the availability of oxygen in the sediments is the most important diagenetic variable. Some of the modern dinoflagellate cyst species survive thousands of years in well oxygenated sediments and are as such among the most refractory types of organic matter. Most (but not all) of these refractory species are phototrophic gonyaulacoids. However, the refractory cysts form only a part of the modern gonyaulacoid or phototrophic cyst producing taxa. The modern species most vulnerable to degradation are often produced by heterotrophic peridinioids. Again, these vulnerable species form only a part of the heterotrophic species and species with a peridinioid plate configuration. To get insight in the intrinsic properties of the cysts bringing about the selective preservation, we continue with reviewing the understanding of algal cell walls and dinoflagellate cyst walls at the molecular level. The review documents that cysts of Mesozoic age have different preservation characteristics than Late Cenozoic to Modern species. We propose that over long periods, taphonomic processes on a molecular level substantially change the cyst wall macromolecular structure and herewith cyst degradability. Having described the impact of selective preservation on the dinoflagellate cyst assemblages, we continue summarising the methods presently available for the recognition of and correction for this diagenetic overprint. Subsequently, we take advantage of the selective preservation by using it for reconstructing past export production. Since the rates of dinoflagellate cyst degradation are strongly related to the bottom water oxygen concentration, this opens the way for a new proxy to reconstruct deep-ocean oxygen concentrations. The importance of the rate of deep-ocean ventilation within the marine global carbon cycle and its relationship with climate change, make this use of selective dinoflagellate cyst preservation an important though unexpected application.