Geochemistry and mineralogy of Paleoproterozoic metasediments in the Imandra-Varzuga Greenstone Belt: Implications for sediment provenance, tectonic settings and weathering intensity at the transition to oxygenated surface environments

Abstract

The oxygenation of the atmosphere during the Great Oxygenation Event (GOE) at around 2400 Ma triggered an intensification of chemical weathering on Earth’s surface and caused an acidic attack related to the oxidation and dissolution of sulphidic minerals in surface rocks. The oxidation and selective mobilization of specific elements or, vice versa, their retention in weathering crusts during the GOE is well exemplified in the behaviour of redox sensitive elements such as chromium (Cr), uranium (U) and molybdenum (Mo). Intensification in overall silicate weathering rates during the GOE, however, has remained poorly constrained. The Paleoproterozoic volcano-sedimentary succession of the Imandra-Varzuga Greenstone Belt (deposited ca. 2450–2000 million years ago; Ma) on the northern Fennoscandian Shield records the transformation of Earth’s surface conditions from anoxic to oxic. However, the silicate weathering indices of the sediments deposited in the Imandra-Varzuga Basin across the anoxic–oxic transition do not signal a measureable effect of this transition on the weathering regime. Weathering intensity of the Seidorechka Sedimentary Formation and the Umba Sedimentary Formation – deposited prior and after the GOE, respectively – are similarly moderate. A multitude of physical and chemical factors including the sediment protolith, climate, topography and atmospheric pCO2 control overall weathering intensity. Our results suggest that the oxygenation of the atmosphere and the punctuated (sulphuric) acidic weathering did not significantly overwhelm the former controls over the long term. Silicate weathering following the GOE was, either, tempered by removal of weathered materials by the Huronian global glaciations and dropping atmospheric pCO2 levels, or the felsic protolith and higher latitudional position led to locally muted weathering intensities to be recorded in the post-GOE Umba Sedimentary Formation despite increased background weathering rates.