EAPS

The chemistry of the Archean ocean and atmosphere: A tribute to John Edmond and Wally Broecker

Abstract:

The abundance of phosphorus in the sedimentary record and strontium isotopes in ancient carbonates indicate that in the Archean most of our planet was covered by the ocean. Seawater chemistry is determined by the balance between hydrothermal and riverine inputs and sedimentary output. Weathering is the main proton sink, whereas hot hydrothermal fluids are the proton source. The pH of the ocean is controlled by seawater alkalinity and by its Cl- content. The abundance of each cation in hydrothermal fluids is tightly buffered by a series of mineral reactions with seawater. The sedimentary output is controlled by mineral solubility (sulfates, sulfides, hydroxides, carbonates, ferric iron), which themselves are controlled by the overall electron balance of the environment. Carbonates, nitrates, and sulfates dominate in oxidised environments at the surface of the modern Earth, but only because of loss of organic carbon by subduction over geological time. Electron availability is higher in abiotic environments, such as the surface of the Earth prior to 2.4 Ga and other planets, which is translated by the dominance of sulfide over sulfate, of ferrous compounds, and by the lack of nitrates and carbonates. The seawater of reducing oceans is dominated by Fe2+, Na+, Ca2+, and Cl-. Since the only oceanic Mg2+ input is runoff, the Fe2+/Mg2+ ratio of seawater must have been very high, which contrasts withe the modern ocean. Likewise, riverine PO4 being the only P input, seawater of water worlds cannot be propitious to the emergence of life. Finally, Fe2+ dissolved in seawater reduces atmospheric CO2 to produce CH4 and a variety of iron oxides. The ocean composition therefore varies with planetary radius, the depth of the ocean, the tectonic regime, and to a lesser extent the composition of the mantle.