Environment for the origin of life on the Earth and search for its geological record
by
Norman Sleep(Stanford University)
→
Europe/Stockholm
William-Olssonsalen, Earth Science Building
William-Olssonsalen, Earth Science Building
Description
The Earth teemed with anoxygenic photosynthetic life by the time of the first sedimentary record 3.8-3.9 billion years ago. Highly metamorphosed black shales and banded iron formations provide record. By then the oceanic pH was the modern value of ~8. It is thus convenient to use planet-scale physics and rock thermodynamics to appraise putative environments for the origin of life. Submarine hydrothermal vents above serpentinite produce chemical potential gradients of aqueous and ionic hydrogen, thus providing a very attractive venue for the origin of life. This environment was most favourable before the Earth's massive CO2 atmosphere was subducted into the mantle which occurred tens to ~100 million years after the moon-forming impact. During this time, thermophile to clement conditions persisted for several million years while atmospheric pCO2 dropped from ~25 bar to below 1 bar. The ocean was weakly acid (pH ~6), and a large pH gradient existed for nascent life with the pH 9 to 11 fluids venting from serpentinite on the seafloor. Total CO2 in water was significant so the vent environment was not carbon limited as in modern oceans. Biologically important phosphate and Fe(II) were somewhat soluble during this period of geologic history. Serpentinite existed by 3.9 Ga, but older rocks that might retain evidence of its presence have not been found. A Hadean record of last record thus may persist in hard rocks. The mantle of the Earth sequesters extensive evidence of Archaean and younger subducted biological material, but has yet to be exploited for the Hadean record.
