The Hadean-Archaean Environment

Abstract

A sparse geological record combined with physics and molecular phylogeny constrains the environmental conditions on the early Earth. The Earth began hot after the moon-forming impact and cooled to the point where liquid water was present in $10 million years Subsequently, a few asteroid impacts may have briefly heated surface environments, leaving only thermophile survivors in kilometer-deep rocks. A warm 500 K, 100 bar CO 2 greenhouse persisted until subducted oceanic crust sequestered CO 2 into the mantle. It is not known whether the Earth's surface lingered in a $708C thermophile environment well into the Archaean or cooled to clement or freezing conditions in the Hadean. Recently discovered $4.3 Ga rocks near Hudson Bay may have formed during the warm greenhouse. Alkalic rocks in India indicate carbonate subduction by 4.26 Ga. The presence of 3.8 Ga black shales in Greenland indicates that S-based photosynthesis had evolved in the oceans and likely Fe-based photosynthesis and efficient chemical weathering on land. Overall, mantle derived rocks, especially kimberlites and similar CO 2 -rich magmas, preserve evidence of subducted upper oceanic crust, ancient surface environments, and biosignatures of photosynthesis. Earth scientists ask whether inferences from molecular phylogeny make geological and ecological sense. They also use the Earth's geological record to tie phylogenic events to the Earth's absolute time scale. The lack of ancient preserved rocks hinders all these efforts to the point of grasping at straws. One may use the pittance of ancient rock samples working back from younger better recorded times. A parallel

Origin of the RNA world: The fate of nucleobases in warm little ponds

“…Annual rainfall varies sinusoidally [25], creating seasonal wet and dry environments. The increased heat flow from greater abundances of radiogenic sources at this time [33] causes temperatures of around 50-80…”

Astrobiology: An astronomer's perspective

“…In fact, the formation of the Earth was not a quick process, but over the first few hundred million years we suffered from constant bombardment from space. Some of these impacts would have been enough to vaporize any young oceans on the surface, perhaps sterilizing incipient life [17,46]. Determining the history of these impacts requires high fidelity images of the Moon and counting the number of craters with different sizes.…”

“…3, the geological evidence points toward the emergence of life on our planet as the frequency of energetic impacts decayed. Southam et al [50] argues that the Earth was essentially habitable throughout the Hadean, while Sleep [46] makes an argument for photosynthesis as early as 3.8 Byr. The important point here is not the details, which are complex, but that simple life did not take billions of years to take root.…”

Mathematical modelling of molecule evolution in protocells

“…The problem of the emergence of life is difficult to solve; up to now direct evidence, of the earliest life formation, has not been found, and because life emerged over 4 billion years ago all traces of this process probably have vanished (Lazcano, 2010). Moreover, the exact conditions on the early Earth are unknown (Lunine, 2006;Sleep, 2010;Mojzsis et al, 1999).…”

“…There are many speculations and theories concerning the values of environmental parameters such as chemical composition of pre-biotic atmosphere (Zahnle et al, 2010), oceans (Feig and Uhlenbeck, 1999) and rocks (Sleep, 2010), distribution of temperature (Gaucher et al, 2010) and pressure.…”