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 (δ¹⁸O_chert) are not only controlled by seawater T and δ¹⁸O_seawater, 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 δ¹⁸O_chert 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 δ¹⁸O_chert we sampled isochronous cherts and siliceous shales from the Northern and Eastern margin of the Rhenish Massif. A decreasing trend in δ¹⁸O_chert 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 δ¹⁸O_chert – 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.