Host phases of redox-sensitive trace metals and an evaluation of trace metal ratios as redox proxies in Ordovician organic-rich sedimentary rocks.

Abstract

Methods for classifying bottom water redox conditions using trace metal ratios are currently being developed, which has increased the importance of identifying primary host phases. Past studies are not in agreement regarding the primary host phases of Thallium (Tl), Rhenium (Re), Molybdenum (Mo), Uranium (U), and Vanadium (V) in organic-rich sediments. Tl is thought to be predominately hosted by pyrite and clay minerals, particularly illite. The central debate for Re and Mo host phases is between pyrite and organic matter, with clay minerals being a less important host. U is found in a wide variety of host phases including carbonates, pyrite, phosphates, or organic matter. Finally, V enrichment is hypothesized to be influenced by organic matter and clay minerals. The concentrations of redox-sensitive trace metals, when normalized to Al content, have been used to identify the bottom water redox conditions during sediment deposition. In a few recent studies, researchers developed enrichment parameters of V, Re, U, and Mo, as well as Re/Mo ratios, which could be used to classify ancient depositional environments. However, it is unclear to what extent ocean redox classification schemes developed based on observations of modern marine sediments can be applied to ancient oceans.

A 2M HNO3 acid leach was the procedure used in this study and is based on methods summarized in past work. The 2M HNO3 leach is very effective at dissolving pyrite and can target reactive forms of organic matter and some clay minerals (especially metals adsorbed to clay mineral surfaces). For a subset of samples, total digestions were performed on the residue left behind following 2M nitric acid digestions to evaluate the amount of trace metals remaining. After the 2M nitric acid leach and subsequent total digestion of the residues, the multi-elemental analysis was done using the Agilent 8800 Triple Quadrupole Inductively Coupled Plasma Mass Spectrometer (QQQ-ICP-MS). A total of 19 rock powder samples were sent to Activation Laboratories Ltd (ActLabs) for quantitative X-ray diffraction analysis. All samples were also previously analyzed using whole rock digestions. The mineral analysis was used in tandem with elemental data (organic and inorganic carbon, sulfur, aluminum, phosphorus) to identify the primary host phase for each redox-sensitive trace metal.

The prominent host phases for Tl are organic matter and pyrite, with some hosted in clay minerals. Re can be found within organic matter and pyrite but also exhibits the potential to be hosted in certain clay minerals. The results from this study support the previous studies that organic matter is the primary host of Mo in non-euxinic environments. U was found to have been primarily hosted by organic matter and pyrite, with some being hosted in clay minerals. The primary host phases for V within these shales are organic matter and pyrite. Regarding local redox conditions, the trace metal ratios did not agree with each other for most cores. All cores had Mo/Al values of <5 µg g-1/% which indicates the sediments were deposited in settings that could have been oxic, suboxic, or dysoxic. However, the V/Al and Re/Mo ratios contradict this interpretation, suggestive of anoxic and/or euxinic local redox conditions at multiple localities. The U/Al values that are between 1-5 µg g-1/% ambiguously indicate either euxinic, suboxic, or dysoxic conditions. Only one core from the Collingwood Member showed consistent agreement among proxies, pointing to oxygenated bottom waters in this case. These thresholds probably should not be used to establish the local redox conditions of bottom waters during the Early Paleozoic or Precambrian when paleogeography and global ocean redox states were significantly different from the modern ocean.