Nuna supercontinent assembly linked to carbon cycling in shear zones 1.9–1.7 billion years ago

Abstract

A peak in organic carbon burial approximately 2.1–1.8 billion years ago led to the prolific growth of graphite-rich orogens internal to the supercontinent Nuna. The temporal history of graphitic carbon in such orogens, however, is poorly understood, which limits our understanding of the timescales of carbon cycling during orogenesis. Here we examine the graphitic roots of two North American (Laurentian) orogens associated with Nuna supercontinent assembly using coupled rhenium–osmium and uranium–lead dating. These are the Taltson–Snowbird orogeny, approximately 1.94–1.89 billion years ago, and the Trans-Hudson orogeny, approximately 1.84–1.72 billion years ago. The geochronology results show that the studied graphite, hosted in shear zones, predominantly reflects biogenic carbon that was hydrothermally remobilized during the final exhumation stages of orogenesis approximately 1.92–1.67 billion years ago. Our results demonstrate that graphite deposition occurred with a periodicity of 31 ± 4 million years and probably involved carbon cycling on the scale of 50 million tonnes or more over a 200 million year interval. Comparisons with other contemporaneous shear zones suggests that late-orogenic, fluid-mediated graphite deposition was common among Laurentian, and more broadly Nuna, mountain belts. These findings imply that the mass movement of graphitic carbon is an endemic feature of the orogenic cycle. Graphitic carbon was hydrothermally cycled through shear zones during late-stage orogenesis associated with Nuna supercontinent assembly, according to a coupled rhenium–osmium and uranium–lead dating study.