Drivers of episodic carbonate cementation during the Miocene Climatic Optimum in a paleolake of the Eger Rift

Abstract

Lacustrine carbonates are sensitive records of climate-driven environmental changes, with carbonates in terrigenous lake successions capturing variations in hydrochemistry, paleoproductivity, and weathering interactions, all potentially influenced by fluctuating atmospheric carbon dioxide levels (pCO₂). This complexity underscores the need to explore how various interlinked drivers impact lacustrine carbonate formation and alteration during pivotal climatic periods like the Miocene Climatic Optimum (MCO). Here we investigated ferroan dolomite and siderite as episodic pore-space filling cements within decimeter-scale claystone horizons. The targeted horizons are interbedded with carbonaceous claystone in the Lower Miocene lacustrine succession of the Sokolov sub-basin, Eger Graben, Czech Republic. During the MCO, intensified weathering of shale, K-rich basalt, mafic and granitic bedrocks enriched the paleolake with base cations and soil-derived nutrients. Microbial Fe- and Mn-respiration linked to an active nitrogen cycle sustained elevated shallow burial pore-water alkalinity and pH, driving episodic (Ca, Mg)-Fe carbonate cementation. High dissolved inorganic carbon (DIC), derived from organic matter remineralization and magmatic CO₂ degassing in the continental rift setting, along with favorable (Mg²⁺ + Fe²⁺)/Ca²⁺ ratios, facilitated interstitial ferroan dolomite growth in central lake facies, while siderite dominated transitional littoral-to-palustrine environments. The δ¹³C signatures (median = 8.3 ‰, mean = 9.0 ‰, range = [+1.8, +18.5]‰) provide evidence for significant methanogenesis influencing dissolved inorganic carbon (DIC) in the pore water–sediment system. Additionally, the lowest δ¹³C values observed reflect the admixture of ¹²C-enriched DIC derived from magmatic CO₂ or modified by attendant dissimilatory iron reduction. Bulk δ¹⁵N values suggest important nitrogen losses across the paleolake, possibly via denitrification and ammonia volatilization. Based on its clumped isotopologue contents, dolomite cements stabilized in near isotopic equilibrium with diagenetic pore waters at relatively low temperatures, T(Δ₄₇, Δ₄₈) ≤ 58 °C. Rare earth element (REE) patterns and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd indicate transport of groundwater evolved after interaction with Paleogene basalts and Paleozoic shale and granitic bedrocks, while Ce anomalies revel a redox-buffered environment favorable to diagenetic carbonate precipitation. These findings highlight complex interactions regulating pore-water carbonate equilibrium in rift lakes. Early Miocene pCO₂ fluctuations intensified silicate weathering in alkaline igneous rocks of the catchment areas, delivering dolomite–ankerite–siderite reactants (Fe³⁺, Mg²⁺, Ca²⁺) into stagnant paleolakes. Concurrently, soil-derived oxidized nutrients (e.g., phosphate bound to iron oxides) altered the lakes trophic states, driving episodes of elevated productivity that were followed by heightened but stratigraphically localized benthic heterotrophy and element cycling, which had a role in sustained alkalinity generation and pH buffering during cementation.