Extraterrestrial 3He shows that Mesozoic marl-limestone alternations are mainly driven by CaCO3 variations at the astronomical timescale
Résumé
Introduction Marl-limestone alternations are well known rhyth-mical inter-bedded deposits that commonly occur inmany hemipelagic to pelagic deposits of the Phanerozoic. It is quite well established that the origin of these lithological variations are astronomically-driven climatic variations (22, 41, 100 and 405 ka being the main periods) e.g. [1]. However, the exact sedimentological control is not clear: several models attribute these alternations to cyclic changes in the carbonate flux, whereas the terrigenous silicoclastic flux remained relatively constant. On the opposite, some models suggest that the carbonate flux was constant while the silicoclastic flux changed cyclically. Material and methods To disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary succession of Bajocian (3 marl-limestone couplets over 3.5 m) and Valanginian (1 marl-limestone couplet over 1 m) age from the Southern French Alps (Barles). We analyzed their carbonate contents as well as their extraterrestrial 3He (3HeET) concentrations in ~200 mg decarbonated aliquots. Results and discussion The carbonate content ranges from 45% in marls to 86% in limestones. Importantly, for all samples, measured 3HeET concentrations are constant in the silicoclastic fractions, within uncertainties. Hence, our results indicate that sedimentation rates at the astronomical timescale in the examined examples were mainly controlled by large changes in the CaCO3 fluxes, leading to variable dilution of the terrigenous and 3HeET fractions. Finally, assuming a constant 3HeET flux of 0.1 pcc/cm2/ka [2], and the whole thickness of Bajocian and Valanginan strata in this region, the measured 3HeET concentrations imply sedimentation rates that are broadly compatible with current duration estimates of these two stages. References: [1] Eldrett J. S. et al. (2015) Earth. Plan. Sci. Let., 423, 98-113. [2] Farley K.A. et al. (2012) GCA, 84, 314-328.
