Uranium (U) associated with coal can be an important source of U and result in environmental pollution during coal combustion. In this study, we developed a method for measurement of U isotope ratios in coals using multiple‐collector inductively coupled plasma‐mass spectrometry. The 233 U‐ 236 U double‐spike was utilised to calibrate the instrumental isotopic fractionation. High‐pressure bomb and dry ashing were adopted to digest the coal samples. The δ 238 U CRM‐145 values obtained from the two different digestion procedures were in good agreement. The δ 238 U CRM‐145 of seven coal and one fly ash reference materials are reported. Furthermore, the results of fly ash, bottom ash and feed coal samples reveal that the combustion processes lead to relatively small U isotopic fractionation between the samples within the same coal‐fired power plant, indicating that U isotope data can be used as a tracer for heavy metal pollution resulting from coal combustion. The U isotope measurement method of coal established in this study provides technical support to understand the behaviour of U during coal formation and combustion.

Oceanic Anoxic Event-2 (OAE-2, ∼94 Ma ago) is marked by a significant perturbation to the carbon and nutrient cycles. Despite the general idea of widespread expansion of the oceanic anoxia during OAE-2, various localities likely experienced heterogeneous redox fluctuations throughout the roughly 500-thousand-year-long event. In certain localities, redox-sensitive elements imply persistent anoxic-to-euxinic conditions, notwithstanding paleontological data that indicate short-term hypoxic-to-anoxic oscillations in bottom waters. Such discrepancies likely result from sampling resolution, local proxy sensitivity, and/or time-integration of signals, particularly for severe hypoxic conditions. We applied vanadium (V) isotopes to three well-studied OAE-2 localities in the proto-North Atlantic Basin. Under the relatively stable anoxic-to-euxinic conditions throughout OAE-2 at Well S75 and Site 367, the sedimentary δ51V values present consistent variations analogous to modern anoxic-to-euxinic environments. However, at Site 1258, the sedimentary δ51V values present greater perturbations toward more negative values that are indicative of short-term oxygenation in bottom waters. These negative perturbations of δ51V values covary with the abundances of the low-oxygen-tolerant benthic foraminifers. Together, these data refine the heterogeneous redox fluctuations in individual localities, revealing occasional short-term weak oxygenation under the generally anoxic conditions throughout OAE-2. This study documents the unique utility of V isotopes to track bottom water redox fluctuations, particularly short-term variations from anoxic to mildly oxic conditions that are hard to track with other methods.

Understanding and constraining redox conditions surrounding "Oceanic Anoxic Events" (OAEs), as they represent recent intervals of major climate upheaval, are of significant interest as they provide insight into the cascade of events and timeframe leading to natural deoxygenation variability during climate perturbation.These ancient events bury a significant portion of organic matter and are recorded by large carbon isotope excursions (δ 13 C).The enhanced burial of organic matter contributes to the consumption of oxygen, global cooling, and hydrocarbon reserves.The term OAE suggests that the dominant and pervasive global marine redox conditions during these events were anoxia.Additionally, it suggests that these reducing conditions were restricted to these intervals as defined by δ 13 C. Here, we have utilized two new geochemical redox proxies, vanadium (V) and thallium (Tl) isotopes, to better constrain the local and global redox variability during OAE-2 (~94 million years ago).Recently, V isotopes have been shown to be a reliable proxy to record changes in low oxygen conditions, particularly at concentrations important for biota.Importantly, V isotopes document local stratigraphic redox variability at a site with laminated, organic-rich mud deposition -more consistent with previous biological evidence.This is likely recording a vacillation between the result of the expansion and contraction of the oxygen minimum zone (OMZ).Thallium isotopes provide a global record of marine oxygenation through the burial of manganese oxides, which require the presence of dissolved oxygen.Thus, perturbations in seawater Tl isotopes provide a record of the earliest changes in global marine oxygenation.Two long-term Tl isotope records surrounding OAE-2 suggest progressive deoxygenation well before the OAE.Thus, the OAE represents the culmination of the maximum extent of deoxygenation.Using a combination of redox proxy data suggest that OAEs represent the punctuation of a longer-term climate scenario, and also that local redox conditions are more variable and dynamic -i.e., expansion/contraction of OMZs.The wealth of data during this time interval provides a unique window to constrain the impacts affecting marine oxygen conditions.The utility of combining redox proxies provides unique spatiotemporal records that can differentiate aspects of the redox ladder.

It has been hypothesized that vanadium (V) isotopes have the potential to track sedimentary redox conditions due to multiple valence states occurring in nature, which might induce variable V isotope fractionation as a function of sedimentary redox state. These characteristica could make V isotopes a useful paleo-redox proxy. However, in order to understand the mechanisms driving V isotope fractionation, it is crucial to build a framework for the depositional and post-depositional controls on sedimentary V isotope records from a diverse set of sedimentary environments. This study, for the first time, investigates the V isotope variations of modern marine sediments deposited under a range of redox environments. Our results document that changes in local redox conditions impart a significant isotopic fractionation from seawater as recorded in the local sedimentary V isotopic signature. Importantly, there is a significant difference between the V isotope composition of sediments deposited in the open ocean setting with oxygen-deficient bottom waters compared to less reducing environments, whereby oxic sediments (benthic oxygen contents > 10 mu M) exhibit Delta(oxic) = -1.1 +/- 0.3 parts per thousand and anoxic sediments exhibit Delta(anoxic) = -0.7 +/- 0.2 parts per thousand. Combined with previous studies on seawater particulate and sediment pore fluid analysis, our results indicate that V is mainly delivered and enriched in anoxic sediments through settling particulates. Authigenic V isotope compositions in marine sediments are likely controlled by isotope fractionation between V species bound to particulates and dissolved in seawater, which likely varies with the speciation and adsorption properties of V that are strongly controlled by local redox conditions. In addition, the euxinic Cariaco Basin sediments exhibit distinctive Delta(euxinic) = -0.4 +/- 0.2 parts per thousand, which is likely influenced by the relationship between the seawater V removal rate and the seawater renewal rate. Our results highlight the direct link between authigenic marine sedimentary V isotope compositions and the overlying local redox conditions. This investigation of V isotopes in modern marine environments provides an initial framework for the utilization of V isotopes to reconstruct ancient redox fluctuations, which has the potential to track subtle redox variations of local oxygen-deficient to low oxygen environments. (C) 2020 Elsevier Ltd. All rights reserved.