Ostracoda (marine/nonmarine) and palaeoclimate history in the Upper Jurassic of Central Europe and North America

Abstract

The role of Ostracoda in determining climate developments in the Upper Jurassic of Central Europe and North America is reviewed, based upon two different studies. The Late Jurassic is a period of time for which a change from a humid to a more arid climate has been suggested for several parts of the world. However, a correlation, though almost logical, between the arid phase in the middle and late Tithonian and a generally cooler climate, combined with a sea-level lowstand and lower water temperatures, has only been demonstrated in very few papers. This study intends to seeks palaeoclimatic evidence for the Late Jurassic by the use of Ostracoda. Development of marine ostracod palaeobiogeography in Central and Western Europe parallels a suggested cooling trend. During the Oxfordian, Kimmeridgian, Tithonian, and Berriasian, a growing diversification of marine ostracod biogeography and an increase in the degree of endemism can be observed. In addition, there is a gradual southward migration of many species from the subboreally influenced areas in the northwest towards the margin of the Tethys in the southeast, mainly from the Kimmeridgian into the Tithonian. Moreover, the genus Cytherelloidea, which has a long tradition (despite some recent contradictions) as an indicator for relatively warm water temperatures, shifts its northern boundary of occurrence towards the south during the Kimmeridgian, Tithonian, and Berriasian. This occurs independently from the general change of facies, because conditions principally suitable for Cytherelloidea (salinity, water depth) have at times also existed in some northern basins during the Tithonian/Berriasian. These data are in correspondence with a suggested increase in the influx of cold boreal waters into central Europe with the beginning of the Tithonian, leading to slightly colder water temperatures in the shallow seas, reduced atmospheric moisture, cool-arid conditions on the neighbouring continents, and also to the diversification of biogeographic regions as a reaction to the conflicting boreal (increased) and Tethyan (still steady) influences. For the Kimmeridgian/Tithonian nonmarine Morrison Formation in the Western Interior Basin, USA, both temporal and latitudinal trends from warm climates in the south and near the base of the formation to slightly cooler climates in the north and in the upper parts of the sections have been detected on the basis of the charophyte floras. Monospecific δ 18O isotope data from charophytes and ostracods have revealed a trend towards lighter compositions towards the top of the formation. This might be interpreted as a general cooling, but other interpretations must also be discussed. From preserved vital effects and δ 18O/δ 13C plots of charophyte/ostracod associations showing coherent groupings similar to those from modern lakes, it is likely that the calcite shells have preserved a high degree of isotopic fidelity. Different scenarios to explain the observed patterns are discussed. A global change towards colder temperatures and arid conditions near the end of the Jurassic period is in good correspondence with all the results. More local factors, however, would also be able to explain the data.