Anoxic and iron-rich seawater conditions facilitated reverse weathering: Evidence from the Mesoproterozoic siliceous rocks

Enhanced reverse weathering is pivotal for sustaining the persistent ice-free climate during the mid-Proterozoic despite low solar irradiance. Silicon-rich seawater is considered the primary factor driving reverse weathering; however, authigenic clay minerals, the major products of reverse weathering, are not consistently more abundant in siliceous rocks, implying the existence of additional controlling factors. Here, we analyzed Mesoproterozoic siliceous rocks from the ∼1.1 Ga Songziyuan Formation of South China, using multiple techniques. The results show that berthierine and stilpnomelane, the dominant authigenic clay minerals, are enriched solely under silica-rich, anoxic, and iron-rich micro-environments. These data support the hypothesis that anoxic, iron-rich conditions may have played a crucial role in promoting reverse weathering, specifically the authigenesis of Fe-bearing clay minerals in silica-rich environments. The predominantly short-term, iron-rich conditions [e.g., Fe(II) > 50 μM] in the mid-Proterozoic silica-rich ocean may have enhanced reverse weathering and thereby facilitated long-term ice-free climate conditions.