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Oxygen isotope variations in cherts from the Rhenish Massif record their diagenetic history, not climatic variations

Cherts are sturdy and ubiquitous throughout Earth history and therefore potentially ideal climatic archives, yet their diagenetic formation challenges straightforward interpretations. Previous work shows that oxygen isotope ratios (δ18Ochert) are not only controlled by seawater T and δ18Oseawater, but also by basal heat flow (Q) and burial rates, i.e. the diagenetic T-t path (Tatzel et al., 2022).

To strengthen the empirical evidence for the control of Q and burial rates on δ18Ochert we exploit the geological framework of the Rhenish Massif: During the Lower Carboniferous siliceous sediments were deposited onto stretched continental crust with variable Q across the basin. The opal mud then transformed during burial diagenesis into chert while tectonic nappes were stacked onto the autochthonous sediments during the Armorica- Laurussia collision.

To isolate the effects of the T-t-path on δ18Ochert we sampled isochronous cherts and siliceous shales from the Northern and Eastern margin of the Rhenish Massif. A decreasing trend in δ18Ochert from West-to-East (by >6 ‰) testifies to differences in burial rates and paleo-Q. We derive constraints on burial rates from peak diagenetic temperatures using Raman Spectrometry of carbonaceous matter and deconvolve for paleo-Q. Triple oxygen isotope compositions (Δ’17O) show increasing variations with increasing δ18Ochert – presumably a reflection of a wider range of diagenetic conditions in low-Q settings.

Tatzel, M., Frings, P.J., Oelze, M., Herwartz, D., Lünsdorf, N.K., Wiedenbeck, M., 2022. Chert oxygen isotope ratios are driven by Earth’s thermal evolution. Proc. Natl. Acad. Sci. 119.


Michael Tatzel1, Katharina Ebert1, Patrick Frings2, Tommaso di Rocco1, Andreas Pack1, N. Keno Lünsdorf1
1Universität Göttingen, Germany; 2GFZ Potsdam, Potsdam, Germany
GeoBerlin 2023