Dating of weathering and oxygenation events in the Keonjhar Paleosol, Singhbhum Craton, East India

The chemical inventory of Archean paleosols provides crucial constraints on early paleoclimate and paleoredox conditions. However, it often remains ambiguous whether recent weathering has also affected such early signals. This is especially crucial for understanding when free oxygen was present in Earth’s atmosphere. Although there is multi-proxy geochemical evidence for episodes of early oxygenation prior to the GOE, some claims for early oxygenation signals in near-surface rocks may have resulted from modern terrestrial weathering. Among the rare occurrence of Archean paleosols, the ca. 3.0 Ga Keonjhar Paleosol in the Singbhum Craton of East India has received particular attention in this regard, as the occurrence of Ce anomalies has been interpreted to originate from the presence of free oxygen in Earth’s Mesoarchean atmosphere [1]. To test this hypothesis, we applied 138La-138Ce geochronology [2] to various sections of this paleosol that reveal a 138La-138Ce age of 115±54 Ma, which precludes Archean oxygenation and rather reflects late Mesozoic weathering. In addition, we have also determined 147Sm-143Nd and 176Lu-176Hf ages for this paleosol, of 1058±56 Ma and <84 Ma, respectively. This result is surprising, as the 176Lu-176Hf system seems to be fully overprinted by young weathering compared to the 147Sm-143Nd system. The Sm-Nd age may reflect partial REE remobilization during Proterozoic orogenic overprint [3].

[1]Mukhopadhay et. al., GEOLOGY, 2014, 42, 10, 923–926. [2]Tanaka and Masuda, Nature, 1982, 300, 515-518. [3]Ghosh and Bose, JMPS, 2020, 115, 70-87.