资源环境科技发展态势分析平台

The emergence of Earth's continental crust above sea level is debated. To assess whether emergence can be observed at a regional scale, we present zircon U-Pb-Hf-O isotope data from magmatic rocks of the Coorg Block, southern India. A 3.5-Ga granodiorite records the earliest felsic crust in the region. Younger phases of magmatism at 3.37-3.27 and 3.19-3.14 Ga, comprising both reworked crust and juvenile material, record successive crustal maturation. We interpret an elevation in delta O-18 through time as an increase in both the amount of sediment recycling and hence, crustal thickening, as well as an increase in the emerged area of continental crust available for weathering. Geochemical signatures do not point to any apparent change in geodynamic regime. We interpret the isotopic evolution of these rocks as solely reflecting regional emergence and thickening of the continental crust, assisted by the increasing strength of the lithosphere.

Plain Language Summary The Earth has not always been a mixture of oceans, continents, and mountain ranges. It is generally agreed that early in Earth history the oceans covered the continents almost entirely, but when large landmasses rose above sea level is hotly debated. This process not only altered the chemistry of the oceans and atmosphere but the evolution of life itself. To examine the emergence of the continents, we have studied very small crystals (of the mineral zircon) collected from a suite of rocks in southern India. Using the radioisotopic decay of uranium, we measure their age, and through their chemical composition we learn about the Earth's crust at that time. Over the time period of three and a half to three billion years ago, our data indicate that regionally the continental crust got thicker, and it became significantly emerged above sea level. Critically for understanding Earth evolution, we do not interpret this to be a change in tectonic processes, such as the onset plate tectonics, but simply due to the crust becoming stronger over time.