Copper, zinc, chromium and osmium isotopic compositions of the Teplá-Barrandian unit black shales and implications for the composition and oxygenation of the Neoproterozoic-Cambrian ocean

Abstract

Two Neoproterozoic-Cambrian shale suites that consist of normal (NBS) and metal-rich (HMBS) black shales from the Teplá-Barrandian Unit, Bohemian Massif (Czech Republic) are studied in terms of their Cu, Zn, Cr and Re–Os isotopic compositions to improve our knowledge of elemental and isotopic mass balance of these elements in Earth's oceans in the past and the extent of surface oxygenation at the Neoproterozoic–Cambrian boundary. The rock compositions indicate that the inputs of Cu and Zn have been controlled by variable proportions of metal-rich authigenic and low-metal terrigenous sources, the latter having exceptionally low δ65Cu and partially δ66Zn. In this respect, our data confirm previous studies conclusions that organic-rich sediments with high a terrigenous flux play an important role in the sinking of light Zn in the oceans and provide new evidence that these types of rocks also represent the currently unidentified burial of very light Cu in the ocean. The chromium isotopic composition reveals a complex history of Cr uptake and fractionation through the dominant Cr input from different terrigenous sources with variable δ53Cr followed by Cr fractionation that was controlled by redox states (NBS). On the other hand, mixing and/or continuous equilibrium isotopic exchange between seawater and hydrothermal Cr was the dominant process for the HMBS. The Re–Os contents of the NBS were controlled by hydrogenous components, which yield a poorly defined Re–Os age for the least disturbed samples of 555 ± 60 Ma, indicating an open-system behavior, while the HMBS Re–Os composition was largely influenced by hydrothermal fluids during deposition. The calculated authigenic δ65Cu, which is similar to that in the present-day ocean, and highly positive δ53Cr values of the HMBS suggest high levels of surface oxygenation during the Neoproterozoic–Cambrian transition. Furthermore, the authigenic δ66Zn, which is indistinguishable from that in the present-day ocean, may imply a constant Zn isotopic composition of the oceans from the Neoproterozoic because of the well-balanced cycle of phosphates, which were strongly adsorbed by Fe-hydroxides. The radiogenic nature of the initial 187Os/188Os (~0.6), although with a high associated error, seems to not confirm previous indications of an abrupt increase of seawater 187Os/188Os during the late Neoproterozoic due to the large-scale radiogenic Os flux into the ocean.