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The 18O/16O ratio of cherts (δ18Ochert) increases nearly monotonically by ~15‰ from the Archean to present. Two end-member explanations have emerged: cooling seawater temperature (TSW) and increasing seawater δ18O (δ18Osw). Yet despite decades of work, there is no consensus, leading some to view the δ18Ochert record as pervasively altered. Here, we demonstrate that cherts are a robust archive of diagenetic temperatures, despite metamorphism and exposure to meteoric fluids, and show that the timing and temperature of quartz precipitation and thus δ18Ochert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ18Ochert is influenced by heat flow through the sediment column. Heat flow has decreased over time as planetary heat is dissipated, and reasonable Archean-modern heat flow changes account for ~5‰ of the increase in δ18Ochert, obviating the need for extreme TSW or δ18Osw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ18OSW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ18Ochert record as driven by solid Earth cooling over billion-year timescales that is compatible with Precambrian glaciations and biological constraints and satisfyingly accounts for the monotonic nature of the δ18Ochert trend.

The processes governing dolomite [CaMg(CO3)2] formation remain among the most debated topics in sedimentary geology. Although primary dolomite can precipitate at low temperatures in certain modern environments, its scarcity today contrasts sharply with its abundance in ancient rocks—a discrepancy known as the ‘dolomite problem’. Dolomite typically forms through two pathways: (1) primary precipitation during early diagenesis, often influenced by microbial activity and organic matter and (2) secondary replacement of preexisting carbonates during burial at higher temperatures. In this study, we investigate Mg isotope fractionation in a modern sabkha in southern Qatar to evaluate its potential as a tracer of dolomite formation processes. We analysed δ26Mg and δ44Ca in surface‐ and pore waters, authigenic clays and organic‐ and leached dolomite‐containing fractions. Ca isotopes reveal an 1‰ fractionation between pore water–organic matter and dolomite, consistent with a two‐step, biologically mediated formation pathway. Contrary, only minor 26Mg enrichment in the organic fraction relative to pore water suggests that Mg isotopes alone provide limited evidence for such microbial mediation. Dolomite δ26Mg values ( −2.15‰) align with predictions for temperature‐dependent inorganic precipitation. Overall, the results indicate that microbial activity probably influences dolomite formation indirectly by altering local water chemistry rather than having a distinct Mg isotopic fractionation. These findings refine the application of Mg isotopes as proxies for dolomite genesis and offer new insights into carbonate diagenesis in saline environments. Modern sabkha sediments from southern Qatar show ca. 1‰ Ca isotope fractionation between pore water, organic matter and dolomite, supporting microbial mediation. In contrast, minimal Mg isotope fractionation and inorganic‐like dolomite δ26Mg indicate that microbial influence is indirect, expressed through subtle pore water shifts driven by Mg partitioning into coexisting phases.