Are the Oldest Milanković Cycles Preserved in the 3.22 Ga Barberton Greenstone Belt (south Africa)?

Knowledge of Paleoarchean climatic processes remains scarce, yet insight in these processes is crucial to understand the environment in which early life evolved. Through the detection of Milanković cyclicity, we can infer climate responses to external forcing on timescales of tens of thousands to hundreds of thousands of years. The ICDP BASE drilling project recovered 2.9 kilometres of unweathered continuous core material from the 3.22 Ga Moodies Group (Barberton Greenstone Belt). The Moodies Group represents some of the Earth’s oldest well-preserved strata, which contains remarkable sedimentary rhythmicity from the millimetre to the decametre-scale. BASE Site 5A is especially suited for a cyclostratigraphic approach, given its overall finer grain size and inferred deeper setting compared to other BASE drilling localities. We use cm-scale X-Ray Fluorescence (XRF) and Natural Gamma-Ray (NGR) data in order to investigate the spectral signature of this core. Frequency ratios between the dominant waveforms in these datasets, combined with amplitude modulation patterns, as well as field- and core-based sedimentological observations allowed us to identify Milanković signatures from which an age-depth model could be constructed for part of the core. This age-depth model covers ~110 m out of ~451 m total core depth and is estimated to represent ~319 kyr. Geochemical proxies (Fe/Al, NGR) in this interval show interference patterns consistent with predicted Palaeoarchean eccentricity-precession dynamics. Independent accumulation rate estimates derived from a U-Pb-based age model support this cyclostratigraphic interpretation. The depositional environment of the studied interval is tentatively interpreted as a submarine fan system, although different depositional models cannot be ruled out. These sedimentary systems are susceptible to autogenic forcing such as channel avulsion and compensational stacking, which have the potential to mask allogenic astronomical signals. Nevertheless, statistical testing indicates the presence of non-random rhythmicity in the studied interval. We further differentiate Milanković from autogenic cycles through facies analysis, a consistent frequency ratio between eccentricity and precession, and amplitude modulation patterns consistent with astronomical theory. Our observations along multiple lines of evidence indicate that the Moodies Group may preserve the oldest stratigraphic expression of Milanković forcing yet identified and opens a new scale in the interpretation of greenstone belt chronostratigraphy.