Geochemistry

A Cooling Off Period

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Science  19 Apr 2002:
Vol. 296, Issue 5567, pp. 431
DOI: 10.1126/science.296.5567.431b

The earliest stage of Earth's history from 4.56 to 4.45 billion years ago (Ga) was hot and has been referred to as Hadean. The oldest known rocks date from 4.0 Ga and the oldest known water-derived sedimentary rocks from 3.8 Ga, consistent with a cooled Earth consisting of a stable crust and liquid water oceans. Whether the intervening stage from 4.4 to 4.0 Ga was hot, with boiling oceans and a steamy atmosphere, is not yet settled.

To estimate surface conditions during this period, Valley et al. compiled all of the oxygen isotopic data for detrital zircon grains with U-Pb crystallization ages between 4.4 and 2.6 Ga. All of the grains were enriched in the heavier 18O isotope relative to standard mean ocean water, which is consistent with zircon crystallization in a liquid water environment and suggests that, as early as 4.4 Ga, the steamy Earth was already condensing into cool blue oceans. A cooling Earth would have been subject to the late heavy bombardment (LHB), a peak of bolide impacts in the inner solar system between 3.8 to 4.0 Ga (no oxygen isotopic concentrations are available from zircons within this period). Nevertheless, the heating of Earth's surface caused by the LHB could have been brief and might not necessarily have destroyed the nascent hydrosphere and the first inklings of life.—LR

Geology30, 351 (2002).

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