The stable U-Mo-Fe isotope records of Middle Jurassic hydrogenetic ferromanganese deposits

verfasst von

Viona Klamt, Dennis Kraemer, Oscar Fernandez, Ingo Horn, Simon V. Hohl, Stefan Weyer, Sebastian Viehmann

Abstract

Ferromanganese (Fe-Mn) nodules are marine chemical sediments that represent unique archives for the reconstruction of ambient seawater conditions. This record is limited from modern to Cenozoic times due to the instability of Fe-Mn oxides during burial and subduction of the oceanic crust and overlying sediments. However, almost uncharacterized Fe-Mn nodules exist in Jurassic sedimentary strata throughout the ancient Tethyan region approximately 100 Ma older than the oldest yet-investigated Cenozoic nodules. Their reliability as geochemical archives for the reconstruction of ancient seawater is, however, poorly understood. In this study, Fe-Mn nodules from the Pyhrntal area (Austria) are geochemically and mineralogically characterized and subdivided into four types with variable amounts of carbonates (calcite, rhodochrosite), todorokite, and hematite as major phases. Shale-normalized rare earth element and yttrium systematics of all types indicate a hydrogenetic origin with characteristic positive Ce anomalies and negative Y anomalies. In-situ Fe isotope measurements of the Fe-Mn nodules display a δ

^56/54Fe range between −0.32 and −0.02 ‰ for the Jurassic Tethyan Ocean, similar to values from modern Atlantic nodules. Stable Mo (δ

^98/95Mo = −0.97 to −0.56 ‰) and U (δ

^238/235U = −0.75 to −0.47 ‰) isotope compositions resemble those of modern and Cenozoic Fe-Mn nodules, suggesting that Middle Jurassic oceans were similarly well‑oxygenated as modern oceans. Our results demonstrate the reliability of fossil Fe-Mn nodules in the Pyhrntal as geochemical archives for the composition of paleo-seawater, encouraging the investigation of other ancient Fe-Mn deposits which may significantly improve and complement the picture of the redox evolution of Phanerozoic oceans.

Organisationseinheit(en)

Abteilung Mineralogie
AG Geochemie
Institut für Erdsystemwissenschaften

Externe Organisation(en)

Bundesanstalt für Geowissenschaften und Rohstoffe (BGR)
Tongji University
State Key Laboratory of Marine Geology

Typ

Artikel

Journal

Chemical Geology

Band

684

ISSN

0009-2541

Publikationsdatum

11.04.2025

Publikationsstatus

Elektronisch veröffentlicht (E-Pub)

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Geologie, Geochemie und Petrologie

Ziele für nachhaltige Entwicklung

SDG 14 – Lebensraum Wasser

Elektronische Version(en)

https://doi.org/10.1016/j.chemgeo.2025.122795 (Zugang: Offen)