The origin of Jurassic FeMn deposits and their reliability as geochemical archives for Tethian seawater

Modern to Cenozoic hydrogenetic FeMn nodules and crusts are reliable geochemical archives of past seawater chemistry. In this study, we report the first petrographic and geochemical data of Jurassic FeMn nodules and crusts from the Calcaereous Alps of the Pyhrntal area (Austria) that were formed ca. 170 million years ago and, thus, ~ 10 million years after the Toarcian extinction event. The combined approach of petrographic data, including XRD and SEM+BSE, with major and trace element signatures and stable U-Mo isotopes of individual FeMn nodule and crust layers obtained by tandem ICP-MS and MC-ICP-MS, respectively, is used to extend the geochemical record of marine FeMn deposits roughly 100 million years back in time and evaluate their reliability as archives for Jurassic seawater. Trace elements and redox-sensitive U-Mo isotopes aid in reconstructing the origin of the FeMn nodules and redox conditions of Tethian seawater in the aftermath of the Toarcian extinction event.

The FeMn deposits of the Pyhrntal area can be subdivided into four types: Type I nodules rich in carbonates (< 90wt %; calcite, rhodochrosite) with minor Fe-oxides (10 wt%; hematite, goethite) and clays (< 20 wt %). Manganese-rich type II nodules (< 75 %; todorokite, ranceite) contain fewer carbonates (< 47 wt %), Fe oxides (<40 wt %), and clays (< 10 wt %). Type III nodules and crusts rich in Fe oxides (< 60 wt %) and carbonates (< 60 wt %) with minor Mn oxides and type IV nodules with Fe- (< 50 wt %) and Mn- oxides (10 wt %), carbonates (< 30 wt %) and < 12 wt % of clay. Despite their different mineralogy, all four FeMn deposit types show sub-parallel shale-normalized rare earth elements and yttrium (REY_SN) patterns that are typical of (modern) hydrogenetic FeMn deep-sea nodules and crusts, suggesting a seawater-derived origin. Typical REY_SN features include strong positive Ce_SN anomalies due to the oxidation of Ce³⁺ to Ce⁴⁺ on (hydr)oxide surfaces and a negative Y_SN anomaly related to higher complex stability of Y in seawater relative to neighboring REY. Furthermore, stable U and Mo isotope compositions of all four types show a narrow range in δ^98/95Mo (-0.97 to -0.56 ‰) and δ^238/235U (-0.75 to -0.47 ‰), consistent with isotopic values observed in modern and Cenozoic FeMn deposits, suggesting an overall oxic water column at latest 18 Ma after the Toarcian extinction event.

The approach using petrography with major and trace element systematics in combination with stable U-Mo isotope signatures highlights the Tethian FeMn deposits as unique geochemical archives of Jurassic seawater that enable a reliable reconstruction of the origin of the Alpine FeMn deposits and the ambient redox conditions in Tethian paleo-environments.