Amino acids and hydrocarbons ∼3,800-Myr old in the Isua Rocks, southwestern Greenland

Abstract

The Isua supracrustal belt consists of metamorphic rocks, including metamorphosed sediments, and are the oldest known rocks on Earth. It has been postulated^1,2 that hydrocarbons recently detected in these rocks are remnants of organisms that lived ∼3,800 Myr ago. This popular interpretation is inconsistent with the high-temperature history of the Isua rocks. Various types of hydrocarbons were liberated from these rocks after repeated solvent extraction by matrix dissolution or pyrolysis. However, the Isua rocks have been metamorphosed to the upper greenschist or amphibolite facies (400–600 °C); the metamorphic episodes lasted >10⁶ yr. Consequently, it is very important that the origins of the key organic constituents be established. In the study reported here both hydrocarbons and amino acids were detected in Isua rock samples. In addition to the common biological amino acids, geologically unstable amino acids and biologically uncommon ones were detected (such as sarcosine). The extent of amino acid racemization suggests the apparent continuous diffusion of biochemicals into the Isua rocks from encrusting lichens since the end of the last ice age. Also, the cold-temperature history (which retards racemization) suggests that the amino acids may be modern to a few tens of thousands of years old. n-Alkanes lacked odd/even carbon chain preference which may indicate some antiquity. However, the hydrocarbons also resemble a recent petroleum distillate fraction. Laboratory experiments, supported by kinetic and thermodynamic calculations, show that amino acids and aromatic and saturated aliphatic hydrocarbons, including pristane, could not have survived the known metamorphic history of the Isua rocks.