Lithium isotopic compositions of the Neoarchean Huoqiu BIF and the Paleoproterozoic Yuanjiacun BIF from North China Craton: Implications for the triggering mechanism of the Great Oxidation Event

Abstract

It has been generally agreed that changes in Earth surface systems are the key factors to trigger the Great Oxidation Event (GOE) at about 2.4 Ga. In the present study, we selected banded iron formation (BIF) samples that formed at ∼2.54 Ga (before the onset of the GOE) and ∼2.28 Ga (after the onset of the GOE) from the North China Craton (NCC) for Li isotope investigations, which represent an effective tool for studying continental silicate weathering processes, to reveal possible changes in Earth’s surface systems before and after the onset of the GOE. Using the Li isotope fractionation coefficients between iron oxide and fluid first obtained by our synthetic experiments, we demonstrate that the Li isotopic composition (δ⁷Li values) of ancient seawater increased from ∼+10.5 ‰ at 2.54 Ga to ∼+15 ‰ at 2.28 Ga. Monte Carlo simulations based on mass balance show that changes in the riverine Li isotopic composition are the main reason for the variations in the δ⁷Li values of seawater during this period. In view of the present data, enhanced erosion rates caused by uplifted mountains and increased secondary mineral formation in floodplains are the main reasons for this riverine change. The close coupling of δ⁷Li values with the ⁸⁷Sr/⁸⁶Sr values of seawater, εHf values and δ¹⁸O values of zircon and preserved orogeny length over time indicate the assembly of supercratons before GOE initiation. The assembly of small and scattered landmasses could be the cause of mountain uplift, and thus enhanced erosion rates bring more nutrients to the ocean through collisional orogenic processes accompanied by these convergences, promoting aerobic photosynthesis, e.g., the participation of cyanobacteria, disrupting the balance between oxygen production and consumption, and finally triggering the GOE.