AN ASSESSMENT OF THE SUITABILITY OF LIMESTONE-MARL ALTERNATIONS FOR ASTROCHRONOLOGICAL APPLICATIONS

Limestone–marl alternations are widespread and typical sediments of ancient shelf environments and are present in varying abundance throughout the Phanerozoic. In many cases, their rhythmic appearance has been interpreted as a direct response to orbital forcing. However, it is a challenge to unequivocally prove a sedimentary origin of the rhythmic intercalation of the two lithologies. This difficulty arises from differential diagenesis that alters limestone beds in different ways than interlayers (marls), causing a loss of comparability between the final lithologies. Differential diagenesis causes passive enrichment of the inert non-carbonate fraction in interlayers, where calcium carbonate is dissolved, as well as passive dilution in limestone beds, which get cemented by imported calcium carbonate. Therefore, reliable information about primary systematic differences in the precursor sediments is preserved only in constituents that are not modified during diagenesis. Such diagenetically inert components include the spectra of organic-walled microfossils (but not their absolute concentration in the bulk sediment) and the ratios of certain trace elements (also not their absolute concentrations). Systematic differences in diagenetically inert components can provide unequivocal proof of primary differences. Only if such differences are observed data from limestone-marl alternation should be applied for astrochronology.