The Ocean Chemistry of Water Worlds
Abstract
The chemistry of planetary oceans is determined by hydrothermal and riverine inputs and by the sedimentary output. The ejection temperature of submarine hydrothermal fluids increases with gravity and water depth, while the tectonic regime and gravity constrain the proportion of the planetary surface subject to weathering. Weathering is the main proton sink, whereas hot hydrothermal fluids are the proton source. The pH of ocean worlds increases when planetary gravity decreases. pH is controlled by seawater alkalinity and primarily Cl- contents. The abundance of each cation M in hydrothermal fluids is buffered by a series of reactions [Mn+]/[H] n+= K(T). 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 oxidized 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 water worlds must be very high. 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 reacts with atmospheric CO2 to produce CH4 and a variety of mixed Fe2+/Fe3+ minerals such as magnetite and ferrihydrite. 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P14A..07A
- Keywords:
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- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 5210 Planetary atmospheres;
- clouds;
- and hazes;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 5220 Hydrothermal systems and weathering on other planets;
- PLANETARY SCIENCES: ASTROBIOLOGY;
- 5419 Hydrology and fluvial processes;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS