Tracing Craton emergence with Boron isotopes in Archean–Proterozoic marine deposits

Emergence and formation of continental crust profoundly impacted Earth's evolution. Ensuing continental erosion enriched the oceans with terrestrial nutrients and set the stage for the evolution of complex life. Many crustal growth models indicate significant continental volume increases between 3.0–2.5 Ga, marking this period as critical for crustal evolution, possibly linked to the transition towards subduction tectonics.

Here we present a novel approach to trace continental emergence by reconstructing oceanic boron isotope composition from marine deposits (chert, iron formations, and shales). Boron enrichment in continental crust means runoff directly influences ocean boron concentration and isotopic composition, with continental runoff representing the largest modern source.

Our comprehensive marine B isotope record reveals a major compositional shift at 3.0 Ga: pre-3.0 Ga deposits show mean δ¹¹B values of -23.1 ± 2.7 ‰, whereas post-3.0 Ga sediments are more variable with mean δ¹¹B of -8.9 ± 3.1 ‰, projecting to seawater δ¹¹B = +16 ‰ at 2.4 Ga. This change reflects enhanced continental emergence and subaerial erosion after 3.0 Ga, substantially increasing boron flux and driving seawater towards higher δ¹¹B in the Proterozoic. A second elevation to modern values (δ¹¹B = +39.6 ‰) occurred throughout the Phanerozoic due to increased chemical weathering following land plant appearance, paralleling the increase in seawater δ⁷Li.