Sunspot cycles drove decadal fluctuations in shallow water oxygenation in the Mesoproterozoic

The mid-Proterozoic (∼1.8–0.8 Ga) shallow ocean was dominantly anoxic and ferruginous, and has long been considered to have been redox-stable (the so-called “Boring Billion”), but recently recognized million-year-scale redox variability raises the question of whether even higher-frequency fluctuations occurred. Decadal-scale solar cycles drive climate variability, yet their influence on iron cycling and associated redox dynamics in mid-Proterozoic shallow marine settings remains unexplored. Here, we report sunspot cycles recorded in ∼1.3 Ga marine red beds of the Luanshigou Formation, South China. The studied carbonate laminites consist of alternating coarse, hematite-poor intraclastic laminae, and fine, hematite-rich microbial mat laminae, reflecting seasonal fluctuations in hydrodynamic energy and redox conditions driven by wet–dry seasonal variability. The preservation of tufted microbial mats indicative of months-long growth supports an annual origin for each couplet. Long-term sedimentation rates (∼14.3–34.9 μm/yr) are lower than the annual rate inferred from mean couplet thickness (∼206 μm/yr), consistent with accumulation-rate underestimation over longer, incomplete stratigraphic records. Spectral analysis of laminite thickness, grayscale and geochemical data (Al, Fe/Al, P/Al) reveals periodicities of 11-yr Schwabe, 22-yr Hale, 35-yr Brückner and 70–90-yr Gleissberg cycles. We thus propose that in anoxic and ferruginous shallow marine settings of the mid-Proterozoic, sunspot activity modulated climate and thereby the development of microbial mats. This, in turn, formed localized oxygen oases that mediated Fe(II) oxidation and drove substantial short-term redox variability, indicating pervasive dynamic fluctuations across decadal to million-year timescales and further challenging the traditional “Boring Billion” paradigm.