Extraterrestrial iridium, sediment accumulation and the habitability of the early Earth's surface

We have measured the concentrations of iridium and platinum in ancient meta sediments from Akilia Island, southern West Greenland, to obtain the first direct constraints on the extraterrestrial mass flux at the Earth's surface before 3.8 Gyr. The lunar impact record indicates this was a time of heavy bombardment; the average lunar cratering rate between 4.15 and 3.8 Gyr was elevated similar to 150 times the modern value. Hence, heightened Ir and P t abundances are possible in sediments from this time. However, the observe d concentrations of Ir and Pt in the Akilia metasediments are extremely low . In most samples, Ir <3 ppt and Pt <20 ppt. In contrast, the average moder n crustal abundances of Ir and Pt are similar to 50 ppt and similar to2 ppb , respectively. The very low concentrations observed in the Akilia metasedi ments (cherts and banded iron formations) can be reconciled with the lunar record by quantifying the effects of sedimentation rate on the efficiency w ith which the extraterrestrial mass flux is sampled. The observations are e xplained if rapidly accumulating sediments sampled stochastic bombardment b y an impactor population governed by a power law mass distribution, such th at exogenous Ir and Pt are concentrated in rare horizons. The required sedi mentation rates are geologically plausible. This explanation has two conseq uences. First, evidence of massive impact events may be rare in sediments o lder than 3.8 Gyr despite heavy bombardment. Second, conditions at the Eart h's surface may have been conducive for extensive accumulation of sedimenta ry sequences and for microbial life during most of the period 4.15-3.8 Gyr. It is possible that the Earth's surface hosted microbial habitats during m ost of this time if microbes living in deep-sea or deep-crustal environment s survived photic zone vaporizing impacts that would have occurred every si milar to 30 Myr, Hence, evidence of life is expected even in the earliest s edimentary sequences.