Chromium isotopes in carbonates — A tracer for climate change and for reconstructing the redox state of ancient seawater

Abstract

Strontium and carbon isotopes of marine carbonates are routinely applied for chemostratigraphic cross correlations of time-equivalent sedimentary sequences and for calibration of the compositional evolution of seawater throughout Earth's history, mainly for the purpose of reconstructing ancient climatic changes. We here present results of a new isotopic tracer system – stable chromium isotopes – applied to a late Ediacaran marine carbonate sequence exposed in the Calera de Recalde syncline, Arroyo del Soldado Group, Uruguay. The aim was to compare Cr isotope signatures directly to δ 13C, 87Sr/ 86Sr and 143Nd/ 144Nd fluctuations in a well defined stratigraphic profile comprising sediments that were deposited during cold–warm periods accompanied by sea-level changes in response to glaciation–deglaciation at higher latitudes. The studied section is characterized by a pronounced negative (down to − 3.3‰) δ 13C excursion in carbonates paralleled by a decrease of 87Sr/ 86Sr values. Chromium isotope signatures over this section also show a correlated decrease in δ 53Cr (δ 53Cr = [(( 53Cr/ 52Cr) sample / ( 53Cr/ 52Cr) SRM979) − 1] × 1000) values from ~+0.29 to − 0.17‰ which mirrors a decrease in positively fractioned seawater signatures to slightly negative values characteristic of high-temperature magmatic sources. Linear correlations between δ 53Cr and εNd(570 Ma), 87Sr/ 86Sr and Cr concentrations can be explained by mixing between two major input sources of Cr, Nd and Sr into the shallow seawater: 1) a source characterized by negative δ 53Cr values of ~−0.2‰ , low 87Sr/ 86Sr values of ~ 0.707, and elevated 147Sm/ 144Nd values of ~ 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated δ 53Cr values of ~+0.2‰, higher 87Sr/ 86Sr values of ~ 0.708, and lower 147Sm/ 144Nd values of ~ 0.11, a source which is strongly affected by continentally derived input. Chromium isotopes provide a powerful tool for reconstructing the redox state of ancient seawater since positive values indicate that, at least locally, Neoproterozoic shallow ocean waters were sufficiently oxidized to fractionate chromium and/or that oxygen levels of the atmosphere were sufficient to transform Cr(III) into the more mobile hexavalent Cr(VI) formed during weathering processes on land. The fact that 87Sr/ 86Sr values, despite δ 13C fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO 2 limitation was the cause of negative δ 13C and δ 53Cr excursions in otherwise nutrient rich late Neoproterozoic basins, and that glaciation is only one more consequence of a tectonically driven, biologically mediated system. In such a scenario, glaciation acts as an amplifier of δ 53Cr signals. These signals in marine carbonates are a sensitive tracer for redox processes in the ocean and/or on land and have the potential to contribute significantly, in combination with the other commonly used isotopic tracers, to the reconstruction of climatic changes, particularly those that are associated with major glaciation periods in Earth's history.