Abstract
Our young Earth operated under conditions fundamentally distinct from today, with reduced solar luminosity, elevated CO₂, intense UV radiation, and acidic, low-pH oceans. Sustained by strong greenhouse forcing, atmospheric CO₂ declined from Hadean levels to progressively lower Archaean–Proterozoic concentrations, coinciding with the emergence and diversification of prokaryotic life by ~3.8 Ga, or possibly earlier. This talk explores how coupled ocean–crust–atmosphere processes governed early ocean chemistry and sustained long-term habitability. Palaeoarchaean records reveal dominantly submarine volcano-sedimentary successions shaped by pervasive hydrothermal alteration and dynamic biogeochemical cycling. Silicification and carbonatization served as key alteration pathways, with near-seafloor rocks crosscut by hydrothermal, chert-filled fractures and overlain by bedded cherts that seal alteration zones and preserve carbonaceous matter‒potential archives of early life. Volcanic massive sulfide deposits and altered oceanic crust further attest to nutrient-rich, hydrothermally driven ecosystems.
Before significant continental emergence (~3.0 Ga), submarine ecosystems were largely sustained by nutrients derived from anoxic alteration of komatiitic and basaltic crust. The subsequent expansion of continental landmasses, onset of oxygenic photosynthesis, and development of shallow-marine platforms during the Meso- to Neoarchean initiated fundamental shifts in seawater chemistry and biosphere structure, possibly punctuated by early icehouse conditions. By integrating field-based, sedimentological, geochemical, and alteration signatures, this keynote highlights the co-evolution of ocean chemistry and Earth-system processes across deep time, emphasizing feedbacks among tectonics, hydrothermal fluxes, and the early biosphere, and constraining the limits of planetary habitability.