The onset of continental weathering recorded in Archean banded iron formations

Early landmass emergence impacted marine chemistry and biosphere evolution by supplying nutrients through continental weathering. However, uncertainties persist regarding the timing of emergence and onset of early continental weathering. We present a compilation of germanium (Ge) and silicon (Si) in banded iron formations (BIFs) to assess Archean weathering. By leveraging the inverse correlation between Ge/Si ratios and Si contents in BIFs, we interpolate the evolution of Archean seawater Ge/Si for the first time. Our analyses reveal a stepwise decrease in Ge/Si, from near-hydrothermal values to those resembling continental input, at ca. 3.5 Ga. Based on a Ge-Si mass balance model, we propose that this shift is attributable to increased riverine fluxes, coinciding with the rapid emergence of continental crust. Significantly, this implies that two crucial prerequisites for the establishment of a marine biosphere were fulfilled in the early Archean: (1) the presence of marginal marine environments suitable for benthic and planktonic communities; and (2) a supply of terrestrially sourced nutrients.