Early cretaceous shoal water carbonates from the Central Apennines

sedimentology, chemostratigraphy and sclerochemistry

authored by: Katharina Elena Schmitt
supervised by: Ulrich Heimhofer
Abstract: During the Cretaceous (145.0 to 66.0 Myr) elevated atmospheric CO2 levels resulted in Oceanic Anoxic Event (OAE) causing widely spread “black shales” and an abrupt increase in temperature. The climate of this period was disturbed by a few short-lived but prominent cooling events, interrupting the prevailing greenhouse conditions. An outstanding high global sea level, the submergence of several continental plates, and the evolution of large shelf areas favoured the development of isolated shallow marine carbonate platforms. The latter were exceptionally widespread, particularly in and around the Tethyan realm. There they built large and complex sedimentary systems, which have been repeatedly devastated by fast-changing environmental conditions. Rudists, an order of gregarious, sediment dwelling, heterodont, bizarrely shaped, sessile bivalves were the main carbonate producers. Shallow marine carbonate platforms are a valuable archive for the reconstruction of deep-time environmental and climatic conditions due to their quick reaction to environmental changes. Their often-low resolution of biostratigraphic schemes makes a precise chronostratigraphic classification difficult, resulting in a fragmentary record. In order to reconstruct palaeoenvironmental conditions, a precise chronostratigraphic framework was established. Two sections, the platform marginal Monte La Costa and the inner lagoonal Santa Lucia sections, located on the Apennine carbonate platform, were analysed in great detail, using an integrated bio- (calcareous algae and benthic foraminifera) and chemostratigraphic (C-, O- and Sr-isotope analysis) approach. Furthermore, both sections were correlated with well-dated pelagic reference curves and with each other. Several exposure surfaces and their related temporal gaps were detected at both sections and dated precisely. A microbial episode appearing on the Apennine Carbonate Platform was detected in both sections. Its coeval onset was dated, using the aforementioned framework, to the OAE1b, between the Jacob and Kilian black-shale level especially. The main phase of bacinelloid growth can be linked to the subsequent temperature rise after the Late Aptian cold snap as well as an increase in sea level. In order to use low-Mg calcite radiolitid shells as a palaeoenvironmental archive, a first approach of high-resolution sampling, using isotopic and elemental data, was applied. Shells were selected using literature based thresholds. Sclerochronological profiles were created and various shell structures (compact and porous) described. The results of selected shells are promising: The smooth, sine functional δ18O pattern in the compact left valve may be used for palaeotemperature and palaeosalinity reconstructions. In the right valves two microstructure types were classified: Shells, where compact and non-compact parts alternating, might be used for palaeoenvironmental reconstructions, while those that contain a celluloprismatic structure, can be used only for habitat reconstructions, as the isotopic and elemental dataset shows the impact of diagenetic alteration.
Organisation(s): Institute of Geology
Type: Doctoral thesis
No. of pages: 166
Publication date: 2019
Publication status: Published
Sustainable Development Goals: SDG 13 - Climate Action, SDG 14 - Life Below Water
Electronic version(s): https://doi.org/10.15488/4968 (Access: Open)