Alkaline vents and steep Na+ gradients from ridge-flank basalts-Implications for the origin and evolution of life

摘要

Life may have emerged on early Earth in serpentinizing systems, where ultramafic rocks react with aqueous solutions to generate high levels of dissolved H-2 and CH4 and, on meeting seawater, steep redox, ionic, and pH gradients. Most extant life harnesses energy as ion (e.g., H+, Na+) gradients across membranes, and it seems reasonable to suggest that environments with steep ion gradients would have also been important for early life forms. The Strytan Hydrothermal Field (SHF) is a mid-ocean ridge-flank submarine hydrothermal (similar to 70 degrees C) vent in Iceland that produces steep Na+ (<3-468 mM) and pH (8.1-10.2) gradients, concomitant with enrichments in methane (0.5-1.4 mu M) and hydrogen (0.1-5.2 mu M), relative to seawater. Large (up to 55 m) saponite towers create ideal "incubators" similar to other proposed origin-of-life analogs (e.g., Lost City hydrothermal field in the mid-Atlantic). However, the SHF is basalt hosted. We suggest that the observed conditions are generated by (1) plagioclase hydrolysis, coupled with calcite precipitation, and (2) hydration of Mg in pyroxene and olivine in basalt. Along with microbial activity, aqueous reactions of Fe in olivine and pyroxene are possible sources of the observed H-2. Although the delta C-13-CH4 values were highly variable (-53 parts per thousand to -8 parts per thousand), isotopically heavy CH4 suggests possible abiotic formation or the imprint of methane oxidation. If environments similar to SHF occurred on the early Earth, they should be considered as potential origin-of-life environments.