An examination of episodic dolomite cementation in an Early-Miocene Eger Rift lake deposits (Czech Republic)
Authors: Petrash Daniel A. , Staudigel Philip T. , Bernecker Miguel , Roeser Patricia , Della Porta Giovanna 
Published: March 2024

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, &#916;47 and &#916;48) and elemental concentration analyses. The mineral is partially ordered (mean I(015)/I(110)=0.42), microcrystalline (2 to 12 &#956;m) and predominantly subhedral (planar-s). Isotopic data revealed it formed under substantial benthic microbial activity, as evidenced by &#948;15N values suggestive of sustained N2 losses (+8.59 &#177; 2.51 &#8240;, median 9.50 &#8240;, N=19)&#8212;as typically observed in sedimentary settings featuring high rates of denitrification and anammox. Dolomite &#948;&#185;&#179;C values (+1.41 to +11.44 &#8240;, median 7.58&#8240;, 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 &#160;trachybasalt3, K-rich peridotite&#160;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 PO43-). 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 (&#8710;47 and &#8710;48, Bernecker et al., 2024: EGU24-11056) point to formation at temperatures below 40&#176;C, and in a shallow-burial diagenetic realm where the pore fluids and dolomite interacted within a closed system4. &#160;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.References1. Vasconcelos, C., McKenzie, J., Bernasconi, S. et al. Nature 377, 220&#8211;222 (1995).2. K&#345;&#237;bek, B., et al. J. Paleolimnol. 58, 169&#8211;190 (2017).3. Rapprich, V. et al. Depos. Rec. 9, 871&#8211;894 (2023).4. Staudigel, P., et al.&#160;Geochemistry, Geophysics, Geosystems&#160;24, e2023GC011117 (2023).