An examination of episodic dolomite cementation in an Early-Miocene Eger Rift lake deposits (Czech Republic)

This study aims to enhance the understanding of geomicrobiological processes and low-temperature dolomite formation, a field significantly developed by Judith A. McKenzie¹. It focuses on shallow burial diagenetic dolomite from the onset of the Miocene Climatic Optimum (MCO, 16.9-14.7 Ma)². Our investigation entailed examining interstitial ferroan and calcian dolomite in claystone (dolomitic mudstone), and integrates bulk-rock stable isotopes (C, N, Δ₄₇ and Δ₄₈) and elemental concentration analyses. The mineral is partially ordered (mean I₍₀₁₅₎/I₍₁₁₀₎=0.42), microcrystalline (2 to 12 μm) and predominantly subhedral (planar-s). Isotopic data revealed it formed under substantial benthic microbial activity, as evidenced by δ¹⁵N values suggestive of sustained N₂ losses (+8.59 ± 2.51 ‰, median 9.50 ‰, N=19)—as typically observed in sedimentary settings featuring high rates of denitrification and anammox. Dolomite δ¹³C values (+1.41 to +11.44 ‰, median 7.58‰, N=19) record a mixture of dissolved inorganic carbon sources, dominated by methanogenic CO₂. In the ca. 70 m-thick, partially eroded lacustrine succession, conspicuous correlation between dolomite abundances and bulk-rock potassium and barium levels provides evidence of episodic increases in chemical weathering during the warm and humid MCO climate². The results reveal complex causal interactions linked to fluctuating pore-water dolomite precipitation potentials. Accordingly, Miocene oscillations in pCO₂ levels accelerated silicate weathering in catchment areas dominated by alkaline igneous bedrocks, including  trachybasalt³, K-rich peridotite and granitoids, thus enriching nearby prevalently anoxic rift paleolake with dolomite-ankerite reactants (i.e., reducible Fe³⁺, Mg²⁺ and Ca²⁺)³ and macronutrients (e.g., iron(III) oxide-bound PO₄^3-). Overall, these sedimentary dynamics, coupled with the influx of soil-derived oxidized nutrients, enhanced benthic ferric iron-based respiration and the sediment redox buffering capacity, which was conducive to punctuated, interstitial dolomite cementation. The dolomite-bearing claystone levels, characterized by Post-Archean Shale-normalized positive europium anomalies, challenge traditional hydrothermal interpretations of dolomitization in rift lakes. Their coupled isotope values (∆₄₇ and ∆₄₈, Bernecker et al., 2024: EGU24-11056) point to formation at temperatures below 40°C, and in a shallow-burial diagenetic realm where the pore fluids and dolomite interacted within a closed system⁴.  Climate variability during the onset of the MCO, along with changes in precipitation and weathering regimes, likely played a fundamental role in establishing the internal boundary conditions necessary for lacustrine dolomite formation.

References

1. Vasconcelos, C., McKenzie, J., Bernasconi, S. et al. Nature 377, 220–222 (1995).
2. Kříbek, B., et al. J. Paleolimnol. 58, 169–190 (2017).
3. Rapprich, V. et al. Depos. Rec. 9, 871–894 (2023).
4. Staudigel, P., et al. Geochemistry, Geophysics, Geosystems 24, e2023GC011117 (2023).