Why would the loss of H₂ to space mean that the Earth's geochemistry would be based on acid/base reactions?

Question

I am reading a paper[1] that contains the line:

Most of the $\small H_2$ in the Earth's mantle escaped to space early in the Earth's history; consequently, the overwhelming majority of the abiotic geochemical reactions are based on acid/base chemistry, i.e., transfers of protons without electrons.

The connection between the loss of $\small H_2$ and acid/base chemistry is not clear to me.

Why would the loss of $\small H_2$ to space make acid/base chemistry the default?

1. Falkowski, P. G., T. Fenchel, and E. F. Delong. (2008) The microbial engines that drive Earth's biogeochemical cycles. Science 320:1034-1038.

Answer 1

The statement you cite contains a reference to “How Earth’s atmosphere evolved to an oxic state: A status report”, D. C. Catling and M. W. Claire, Earth and Planetary Science Letters 237 (2005) 1–20. This paper in turns give some more details on this assertion, which I'll summarize below.

It all turns around the question of how reducing the prebiotic atmosphere was. The paper I cited above contains this quote:

before life existed the atmosphere contained mainly N₂, with negligible O₂, probably more CO₂ than today, and only minor levels of reducing gases such as H₂.

So, in the absence of widely available gases such as oxidative O₂ or reducing H₂, oxygen was mainly present as H₂O and CO₂. This means redox reactions are not big players in the overall chemical scheme of the periodic atmosphere, and acide-base reactions dominate (by default).