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 sea-water 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 delta C-13, Sr-87/Sr-86 and Nd-143/Nd-144 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%.) delta C-13 excursion in carbonates paralleled by a decrease of 87Sr/86Sr values. Chromium isotope signatures over this section also show a correlated decrease in delta Cr-53 (delta Cr-53 = [((Cr-53/Cr-57)(sample)/(Cr-53/Cr-52)(SRM979)) - 1] x 1000) values from similar to+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 delta Cr-53 and epsilon Nd(570 Ma), Sr-87/Sr-86 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 delta Cr-53 values of similar to-0.2 parts per thousand, low Sr-87/Sr-86 values of similar to 0.707, and elevated Sm-147/Nd-144 values of similar to 0.13, recognized as a subaqueous hydrothermal dominated input source, and 2) a source characterized by positively fractionated delta Cr-53 values of similar to+0.2 parts per thousand, higher Sr-87/Sr-86 values of similar to 0.708, and lower Sm-147/Nd-144 values of similar to 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 Sr-87/Sr-86 values, despite delta C-13 fluctuations, remain low (indicative of a strong hydrothermal input into the basin at his time) implies that CO2 limitation was the cause of negative delta C-13 and delta Cr-53 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 delta Cr-53 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 glaciationperiods in Earth's history. (C) 2011 Elsevier B.V. All rights reserved.
