The use of Ostracoda to reconstruct the oxygen levels of Late Palaeozoic oceans

Abstract

It has been recently demonstrated that, in post-Palaeozoic and modern oceans, filter-feeding ostracods totally dominate over deposit feeders during times of dysaerobia. In modern oceans, the filter-feeding Platycopida, the sole extant group with this feeding strategy, in terms of abundance completely dominate the oxygen minimum zone, despite occurring there in low specific diversity. The percentage of filter feeders has been used previously to locate kenoxic events and to determine palaeoxygen levels in Liassic, Cenomanian and Palaeogene oceans. (The term kenoxic is used to denote an event when oxygen levels were reduced. Most so-called anoxic events, which for the name implies total deprivation of oxygen were, in fact, kenoxic. A kenoxic event produces dysaerobic rather than anaerobic conditions.) In the present paper, Upper Palaeozoic Ostracoda are analysed and it is concluded that the Palaeocopida, Metacopina, Platycopina, Kloedenellacea and the Paraparchitacea were filter-feeders. Filter-feeders were, therefore, much more diverse in the Palaeozoic than subsequently. In the palaeopsychrosphere, all ostracods, even podocopids, acquired peculiar adaptations to allow them to live in the oxygen minimum zone. Using the percentage of filter-feeding ostracod species of shelf seas as a guide, relative oxygen levels for the seas which deposited the carbonate platforms of northwest Europe during the Emsian to Visean interval are calculated. Changes in oceanic oxygen are related to global climatic changes, especially in temperature and are, therefore, important in any considerations of palaeoclimatology. Our reconstructions for both dissolved oxygen and sea water temperatures from the late Early Devonian to the Early Carboniferous, based upon the percentage of filter-feeding species, clearly depict the Late Devonian Events, as a time of low oxygen, high temperatures and high filter-feeders (into the Frasnian). There was a subsequent cooling and improved oxygenation (into the Famennian) and a glacial maximum marked by a low percentage of filter-feeders in the Lower Tournaisian. This was followed by a slight increase in the percentage of filter-feeders into the Visean; a signal of lower levels of oxygen but continuing in a contrasted climate. A model which yields reference points giving absolute values of oxygen is proposed for Late Palaeozoic benthic marine environments.