Silicon isotopic signatures of granitoids support increased weathering of subaerial land 3.7 billion years ago

Introduction

Several lines of evidence suggest that between 25¹ and 75%² of continental crust already existed 3.0 billion years ago (Ga); however, it is unclear if this crust had emerged above sea level or was covered by epicontinental seas, and also how much earlier in Earth’s history the continental crust started to become established. Continents impact the flow of nutrients to the ocean and allows for the existence of ponds, lakes and continental shelves³; such warm ponds and lakes are considered potential settings for the origin and evolution of life, as they likely contained high concentrations of essential biogenic elements^{4, 5–6}. Meanwhile, continental shelves provided shallow benthic zones and ecological niches that facilitated the diversification of microbial communities^7,8. Furthermore, it is likely that even a small area of subaerial land would have a positive impact on a planet’s habitability by strengthening climate weathering feedbacks^9,10. In particular, weathering and erosion of emerged crust consumes CO₂, balancing its outgassing^11,12. Below 1−1.5% subaerial land area, it is likely that silicate weathering of emerged crust becomes supply-limited, which describes a situation where weathering reactions run to completion due to a lack of weatherable rocks^{9, 10–11}. This condition prevents an efficient and continuous weathering feedback between atmosphere and land, making seafloor weathering the dominant factor to balance the carbon cycle¹¹. Once the area of land overcomes this threshold, continental weathering becomes the primary driver of CO₂ regulation at steady-state¹¹. This bears importance, as a better understanding of the weathering feedback mechanisms of the early Earth enhances our understanding of the environmental conditions that supported the development of early life¹¹.

While there is no agreement on the precise timing, geochemical evidence and zircon age distributions point to a rise in the surface area of emerged crust on Earth between 3.0 Ga and 2.5 Ga^{13, 14, 15, 16–17}, with land exposure eventually approaching its present value between 2.8 Ga and 2.5 Ga^16,17. However, the observation of ca. 3.7 to 3.8 Ga clastic sediments^18,19, light oxygen isotopes in zircon^20,21, the Sr isotopic composition of 3.52 to 3.20 Ga baryte²², Ge/Si ratios in banded iron formations²³