Rekonstruktion von Cenoman-zeitlichen Meeresoberflächentemperaturen mittels integrierter Rudistenschalen-Sklerochemie (O-Isotopie, Mg/Ca, clumped isotopes)
Zusammenfassung der Projektergebnisse
Shallow marine carbonate platforms are a valuable archive for the reconstruction of deeptime environmental and climatic conditions due to their quick response to environmental stressors. Rudists, an order of gregarious, often bizarrely shaped sessile bivalves were the main carbonate producers on these carbonate platforms during the Cretaceous. If wellpreserved, the outer low-Mg calcite shell layer of rudists has been shown to reliable record geochemical changes of the ambient water mass. The main objective of the proposal was to reconstruct the evolution of neritic sea surface temperatures (SST) from the Mid-Cenomanian cool episode towards the Cenomanian- Turonian hothouse by use of different palaeothermometers (δ18O, Δ47, Mg/Ca). Due to unfavourable outcrop conditions and a lack of suitable (diagenetically unaltered) shell material, alternative rudist-bearing sections in the Apennine sector had to be targeted and dated first. Therefore, two expanded sections representing a transect through the Apennine carbonate platform (Monte La Costa: platform margin; Santa Lucia: lagoonal inner platform) had to be stratigraphically precisely constrained by establishing an integrated biostratigraphic (green algae and benthic foraminifera) and chemostratigraphic framework (carbonate bulk δ13C, 87Sr/86Sr depicted from bivalve shell calcite). Notably, several major exposure surfaces and their related temporal gaps were detected at both sections and dated precisely. Most importantly, a late Aptian platform-wide shift to (bacinelloid) microbial carbonate production was observed. Considering the newly established integrated stratigraphic framework, the initial phase of microbial proliferation coincides with the final stage of the so-called late Aptian “cold snap” and the subsequent temperature increase, which was paralleled by a significant sea-level rise. These results contrast with observations from the early Aptian Oceanic Anoxic Event 1a, where a similar shift toward microbial “bacinelloid” carbonate production has been linked to exceptionally warm conditions and hypoxia. Unfortunately, the chosen sections did not yield rudist material ascribed to the Cenomanian. Moreover, the Aptian-Albian portions of the sections are mainly dominated by the more problematic radiolitid rudist family (characterized by a porous often diagenetically altered low-Mg calcite shell). Therefore, one important task was to characterize shell microstructures and to evaluate which parts of the shells are suitable sclerochemical archives. On the basis of a thorough petrographic analysis, selected (best-preserved) shells derived from similar stratigraphic horizons were sclerochronologically analysed for their major and trace elemental compositions using high-resolution quantitative μXRF line scans and subsequently for their carbon and oxygen-isotope composition. Preliminary results are promising: A smooth sine functional δ18O (anti-correlating with Sr concentrations) pattern in the compact left valve is observed. The more problematic right valve provides a more complex sclerochronological elemental and stable isotope pattern, characterized by various cyclic changes. Established dense sclerochemical records will be the subject to a thorough statistical analysis protocol (double PCA approach: in collaboration with Rute Coimbra, Aveiro University). Once this is done, the best preserved shell materials will be analyzed for clumped isotopes and Mg/Ca ratios.
Projektbezogene Publikationen (Auswahl)
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(2016): Reconstruction of Cenomanian shoal-water temperatures using integrated rudist shell sclerochemistry (δ18O, Mg/Ca, Δ47), 32nd IAS Meeting of Sedimentologists 2016, Marrakech
Schmitt, K., Heimhofer, U., Huck, S.
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(2017): An integrated bio-chemostratigraphic framework for Lower Cretaceous (Barremian-Cenomanian) shallow-water carbonates of the Central Apennines. 10th International Symposium on the Cretaceous System, Vienna
Schmitt, K., Heimhofer, U., Frijia, G., Huck, S.
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(2017): An integrated bio-chemostratigraphic framework for Lower Cretaceous shallow-water carbonates of the Central Apennines (Italy). Geophysical Research Abstracts Vol. 19, EGU General Assembly, Vienna
Schmitt, K., Heimhofer, U., Frijia, G., Huck, S.
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(2019): Platform-wide shift to microbial carbonate production during the Late Aptian cold snap. Geology
Schmitt, K., Heimhofer, U., Frijia, G., Huck, S.
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(2019): Radiolitid rudists: an underestimated palaeoclimatic archive? Geophysical Research Abstracts Vol. 21, EGU General Assembly, Vienna
Huck, S., Schmitt, K., Heimhofer, U., de Winter, N.
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(2020): Deciphering the fragmentary nature of Cretaceous shallow-water limestone archives: A case study from the subtropical Apennine carbonate platform. Newsletters on Stratigraphy
Schmitt, K., Heimhofer, U., Frijia, G., Di Lucia, M., Huck, S.
