Rhenium–osmium and molybdenum isotope systematics of black shales from the Lower Cambrian Niutitang Formation, SW China: Evidence of a well oxygenated ocean at ca. 520 Ma

Abstract

The Early Cambrian was a key period in Earth's history, and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period. New Early Cambrian data for Re–Os and Mo isotopic compositions and redox-sensitive elements (RSE) are reported here for the Niutitang Formation, SE Chongqin, SW China. A Re–Os age of 520 ± 30 Myr (Model 3, 2σ, n = 21, MSWD = 62) and a (187Os/188Os)i ratio of 0.79 ± 0.11 were obtained, consistent with previously reported data for equivalent strata from other areas. Separate regression of samples with (187Os/188Os)i of 0.72–0.79 and 0.81–0.89 produce more precise ages of 520.1 ± 9.5 Myr (Model1, 2σ, n = 7, MSWD = 1.0) and 513 ± 10 Myr (Model 1, 2σ, n = 8 MSWD = 0.96) respectively, suggesting heterogeneity in the primary source of Os as the dominant cause of scatter and uncertainty. High initial 187Os/188Os of 0.79 ± 0.11 may indicate a high Os input from oxidative weathering of upper continental crust for Early Cambrian ocean.Based on RSE data such as Mo, U, V enrichment and Ni/Co, V/Cr, V/(V + Ni), MoEF/UEF, Re/Mo ratios, two geochemically distinct zones (upper and lower) can be delineated in the sedimentary sequence, as reflected in drillcore sections. The upper section (n = 9) is characterized by low total organic carbon (TOC) contents (mean 1.8 ± 1.5 wt%), low RSE contents (Mo (0.008 ± 0.006) × 10−3 g/g; V (0.22 ± 0.15) × 10−3 g/g; U (0.008 ± 0.004) × 10−3 g/g), and low V/Cr (2.0 ± 1.7), Ni/Co (5.9 ± 3.5), MoEF/UEF (2.6 ± 1.8), and Mo/TOC (6.0 ± 3.9) ratios; in contrast, the lower core section (n = 13) is characterized by high TOC contents (4.4 ± 2.2 wt%), high RSE contents (Mo (0.10 ± 0.08) × 10−3 g/g; V (1.0 ± 1.0) × 10−3 g/g; U (0.05 ± 0.04) × 10−3 g/g), high V/Cr (6 ± 5), Ni/Co (24 ± 14), MoEF/UEF (6.3 ± 2.2), and Mo/TOC (21 ± 9) ratios (uncertainties are ±1SD here). Significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic/euxinic to oxic condition. All samples display extremely low Re/Mo ratios of 0.2 × 10−3–1.9 × 10−3, indicating the absence of intermediate reducing conditions, possibly reflecting rapid ocean oxygenation.The Mo isotopic composition in the sedimentary succession of the YC9 core exhibits a large variation, with δ98/95Mo values of 0.04–2.00‰. Upper section δ98/95Mo values display an opposite trend to Mn content and negative correlation with Mo/TOC ratios, indicating Mo fractionation associated with adsorption on Mn oxides or oxyhydroxides. Lower section δ98/95Mo values are positively correlated with Mo/TOC ratios and negatively with Re/Mo ratios, suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions.High δ98/95Mo of 2.00‰ and high average Mo/TOC ratios of 21 (10−6 g/g)/wt% were observed in euxinic shales in the Niutitang Formation, suggesting extensive ocean oxygenation may have taken place at ca. 520 Ma.