Title: Reassessing evidence of Moon-Earth dynamics: No evidence of shorter lunar months from tidal bundles at 3.2 Ga (Moodies Group, Barberton Greenstone Belt)

Christoph E. Heubeck, Tom Eulenfeld

Institut für Geowissenschaften, Friedrich Schiller Universität Jena, Germany

Event: GeoKarlsruhe 2021

Date: 2021

DOI: 10.48380/dggv-0s3d-3811

The sole Archean data point to reconstruct past orbital parameters of the Earth’s moon is from the Moodies Group (ca. 3.22 Ga) of the Barberton Greenstone Belt. From time-series analysis of tidal bundles of a subaqueous sand wave, Eriksson and Simpson (2000) suggested that the Moon’s anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although our reexamination of the site confirmed that the sandstone bed in question is likely a subaqueous dune, mud clasts, channel-margin slumps, laterally aggrading channel fills and bidirectional paleocurrents suggest that this bedform was likely located in a major nearshore channel; it thus risks incompleteness. Remeasurements of foresets along the published traverse, perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along additional traverses parallel to bedding along the 20.5 m-wide exposure are affected by zones of minor faulting, uneven outcrop weathering, changing illumination, weather, and observer bias. Our most robust measurements show a distinct periodicity peak of approximately 14, removed by Eriksson and Simpson (2000) in the original data, and are interpreted to be due to a lunar month of about 28 Earth days, as today. This estimate agrees well with Earth-Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a nearer orbit, rotating faster around a faster-spinning Earth.

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