Exploring seafloor alteration as a viable mechanism to sustain Earth’s earliest biosphere

On the modern Earth, oxidative weathering of continental crust constitutes the dominant source of most nutrient elements to the ocean that ultimately sustains the biosphere over geological timescales. However, continental crust exposed above sealevel may have been scarce on the early Earth, and oxidation was limited prior to the rise of atmospheric O₂ at ca. 2.4-2.3 billion years ago (Ga). Several experimental and modelling studies have therefore suggested that anoxic seafloor weathering and hydrothermal alteration provided the major sources of bioessential elements such as phosphate and transition metals. Here, these datasets are reviewed, and new supportive evidence is presented from the Paleoarchean North Star and Mount Ada basalts (3.5-3.47 Ga) in the Pilbara craton, Western Australia. Alteration gradients reveal depletion in key nutrients, supporting the idea that this process contributed to sustaining microbial ecosystems at that time. Direct evidence of a Paleoarchean seafloor biosphere is preserved in the form of microbialites found in an offshore marine setting with no evidence of felsic material influx. Collectively, these findings show that life could be maintained on an ocean-dominated planet; however, continental emergence was perhaps important for biological diversification and innovation over the later course of Earth’s history.