Boron isotopes in Archean-Proterozoic marine deposits as a tracer of continental evolution and emergence

One of the major and most impactful changes the Earth has experienced is the formation, evolution and volumetric accumulation of continental crust through geologic time. The transition of continental crust from being mostly submerged to subaerial greatly affected the evolution of complex life by inducing the enrichment of oceans with nutrients via erosion. The transition from stagnant/squishy-lid dynamics to modern-style plate tectonics was probably facilitated by the growth and accumulation of thick continental crust. Many ideas of crustal growth curves were previously suggested, each employing different types of constraints. Many of these curves point to the Archean–Proterozoic transition as an important point of infliction in the evolution and accumulation of continental crust. We adopted a new approach aiming to indirectly trace the growth in subaerial exposure of continental crust by observing the change in elemental and isotopic composition of boron in the ocean, preserved in Archean–Proterozoic marine deposits. Boron is a continental element that is concentrated in the continental crust over time. The oceanic boron isotopic composition is controlled by the balance between the different sources and sinks of boron in and out of the ocean, amongst which the largest source is continental runoff. The onset of widespread continental emergence initiated the largest boron influx into the ocean, thus greatly affecting the oceanic B concentration and B isotopic budget. Box modelling of oceanic B concentration over time employing different crustal growth scenarios also shows that the mode of crustal growth greatly affects the time dependant change in oceanic B concentration. We analyzed samples of Archean and Proterozoic cherts, iron formations and marine shales for B isotopes using in-situ LA-MC-ICP-MS in order to construct the oceanic B isotopic record across this critical period. We observe a significant increase both in B concentration and in the range and variation of B isotopic values across the Archean–Paleoproterozoic transition that may suggest a transition of oceanic boron towards modern values (modern ocean δ¹¹B = +39.6 ‰) at that time, suggesting a large increase in the area of exposed continental crust.