Lower Cretaceous Climate and Non-marine Stratigraphy (LCCNS)

Studies on climate cycles and changing paleoenvironments in non-marine (continental) archives of the Mesozoic greenhouse earth are hitherto rare, primarily a result of the lack of high-resolution stratigraphy and correlations to the marine record. This project uses the Lower Cretaceous European record as a test site for the integration of ostracod biostratigraphy and assemblage changes, and cyclostratigraphy (orbitally/climate driven cycles). Uppermost Jurassic-Lower Cretaceous non-marine ostracod biostratigraphy has a long tradition, particularly concerning the Purbeck-Wealden interval (latest Tithonian to earliest Aptian) in Europe and contemporaneous deposits worldwide, but has, at the same time, always been affiliated with considerable problems and limitations. Notably with regard to supraregional (i.e., inter-basinal) to global scales, serious attempts at correlating the respective deposits, as well as dating and chronological linking of events documented by these, remain problematic let alone correlation to the marine standard sections. Nevertheless, ostracods are definitely the most useful biostratigraphical and palaeoenvironmental tool in Purbeck-Wealden sequences. The apparent cyclic changes in non-marine ostracod assemblages of the Wealden of southern England (the faunicycles of F.W. Anderson) will be revised and analysed for orbitally(?) driven cyclicities based on a multi-proxy study and multivariate statistical analyses. The approach is multiple: Biostratigraphy in the supraregional to global context, cyclostratigraphy using ostracods, lithologic parameters and sediment geochemistry, stable isotope geochemistry, and magnetostratigraphy for chronological control. A selected interval of the Lower Weald Clay of the Weald Sub-basin will be resampled at high resolution and the samples analysed for ostracod assemblage composition and changes, stable oxygen and carbon isotopes of ostracod valves, as well as TOC and XRF, magnetic susceptibility, gamma log, and lithology of the sedimentary rock. These data will be integrated and statistically tested for cyclicities (and the results compared with an accordant statistical analysis of F.W. Andersons faunicycles). The proposed integrative methodology targets the correlation of the faunal composition change with the variation of geochemical and sedimentological parameters through time and inferences on controlling (palaeoenvironmental) factors and their regulating mechanisms (climate changes, orbital cycles?). The central approach is to turn the tables in that the strong facies control thus far widely considered a substantial drawback as to supraregional biostratigraphical utility of non-marine ostracods shall be (re-)analysed and tested for cyclostratigraphic use. The combination of different methods and data allows the evaluation of their utility on different chronological and geographical scales. These methods can then be efficiently applied on larger scopes and to larger datasets. The proposed integrated project represents a pioneering study in this area of research, and will also be an important contribution towards progress in chemo- and magnetostratigraphy of the English Wealden and the nature of its cycles, as well as Early Cretaceous climate change. Formatiert: Englisch (USA)

Dating and correlating sedimentary rocks - stratigraphy - is a fundamental discipline in the geologic sciences, as these are essentially historical sciences using the rock record as sources of information. To arrange sediment record in a correct temporal succession, we need the absolute age (age in years), which is not always possible, and the relative age (older/younger than) of beds of successions. One newer tool that can be used (in combination with others) is 'cyclostratigaphy'. Recurring ('cyclic') natural global climate changes are controlled by changing rates of solar energy received on Earth's surface, which are ultimately controlled by astrophysically determined changes in Earth's orbital parameters. Such recurring changes (Milankovitch-cycles, such as 100,000 or ~20,000-year periodicities) partially explain cyclic natural climate changes of the past that in turn led to changes in the geological signals at the same frequencies: Changes in sediment composition and changes in fossils contained in those sediments should both correspond to cyclic climate changes and to each other on the same Milankovitch frequencies, as they represent the climate-dependent ecosystems and depositional system changes in the past. This concept has successfully been applied to marine sediments, but in non-marine deposits (such as those of rivers and lakes) such studies are rare. This mainly results from the nature of non-marine deposits, in contrast to marine ones: they often are laterally and vertically limited and discontinuous, which means they represent discontinuous small geographic areas and (geologically) 'short' time intervals, respectively. Since the essential geologic reference time-scales are largely based on marine deposits and fossils and non-marine deposits contain totally different fossils and record different sedimentary events, marine to non-marine correlations remain difficult. Therefore, the quality (temporal resolution and completeness) of age determinations of non-marine successions largely remains insufficient as does the integration into the marine standard time-scales. However, the same cyclic global climate changes should be recorded by marine and non-marine sediments and their fossil assemblages on the same recurring time intervals. This project aimed at testing an interval of a well-known non-marine succession (Lower Cretaceous Wealden of southern England, ~130 million years before present) for such cyclicities and their application potential for improved non-marine stratigraphy (e.g. dating the Wealden dinosaurs). Fossil assemblage changes of microfossils (ostracods) and changes of different physical and chemical sediment parameters were recorded in high resolution sample intervals and statistically analysed for frequencies and their correlation. Although the exact geochronologic age constraints require further investigation, the results show that there indeed seem to be real Milankovitch cyclicities in this succession, which leads to the conclusion that climate changes were the dominant control factor of these cycles, and that these, in turn, can be utilized for cyclostratigraphic correlation.