Comparing thallium isotopes to paleosalinity and Mo-U isotope paleoredox proxies: A case study of the Upper Devonian Kettle Point Formation, Ontario

Abstract

Black shale formations are utilised to reconstruct ancient global ocean redox conditions by analyzing and inferring their geochemical and isotopic compositions. However, the accuracy of these reconstructions can decrease if regional and local influences overprint global signals. This study utilizes the novel thallium (Tl) isotope paleoredox proxy, in conjunction with the well-studied molybdenum (Mo) and uranium (U) isotope redox proxies, to reconstruct the depositional environment of black shales from the Famennian Kettle Point Formation of southern Ontario. The Mo-U isotope ratios of these black shales have been previously inferred by Kendall et al. (2020) to reflect a local redox change from strongly to weakly euxinic conditions within the bottom-waters of the Chatham Sag sub-basin. Paleosalinity proxies were also interpreted by Kendall et al. (2020) as an increase in the Chatham Sag’s basin restriction during the Late Famennian. Authigenic Tl concentrations were obtained through leaching of black shale rock powder using 2 M nitric acid and have a range of 0.72 - 14 μg/g. Units 1 to 3 contained low Tl concentrations (0.72 - 4.3 μg/g) and Unit 4 had relatively high Tl concentrations (3.2 - 14 μg/g). Authigenic Tl isotope ratios were measured against NIST SRM 3158 (0 ε units; statistically identical to NIST SRM 997) and range between –7.1 and –2.0 ± 0.4 ε units (2SD). Authigenic Tl concentrations have a strong negative correlation (r = –0.79; p < 0.001) with authigenic Tl isotope ratios, which indicate low concentrations contain heavier Tl isotope compositions and high concentrations contain lighter Tl isotope compositions. Thallium isotopes have a very strong negative correlation with U isotope ratios (r = –0.89; p < 0.001) and a strong positive correlation with Mo isotopes (r = 0.71; p < 0.001). The positive correlation with Mo isotopes suggests that the Fe-Mn oxyhydroxide particulate shuttle did not influence Tl and Mo isotope expression. Three distinct groups of samples are apparent in the cross plots of Tl - U and Tl - Mo isotopes. In stratigraphically ascending order, these groups are described as the high Tl - low U - high Mo group of Unit 1 to lower Unit 3 (ε205Tlmean = –2.7 ± 0.4, 2SD, n = 13), the transitional group of upper Unit 3 to lower Unit 4 (ε205Tlmean = –4.1 ± 0.5, 2SD, n = 5), and the low Tl - high U - low Mo group of upper Unit 4 (ε205Tlmean = –6.1 ± 0.6, 2SD, n = 17). Together with the paleosalinity proxies, these groups represent redox and salinity changes within the Chatham Sag throughout the Famennian. Unit 1 to lower Unit 3 experienced increased basin restriction that promoted strong euxinia in the deep basin and the predominance of river inputs as the main source of Tl to the Chatham Sag, which is reflected by average ε205Tl values comparable to modern rivers. Low trace metal (TM) accumulation and marine paleosalinity during this time suggests that the Chatham Sag was still somewhat connected to the open ocean, and received infrequent inputs of oceanic water to replenish the TM reservoir and maintain marine salinity at the sediment-water interface. During upper Unit 4 deposition, the Chatham Sag’s source of TMs shifted to mainly oceanic inputs, indicated by average ε205Tl values comparable to the modern ocean, and suggests a relative decrease in basin restriction. Increased hydrographic connectivity to the open ocean during Unit 4 deposition also increased sedimentary TM accumulation. Additionally, decreased basin restriction and increased freshwater inputs, which is indicated by brackish paleosalinity, diluted aqueous hydrogen sulfide concentrations of the bottom-waters to weakly euxinic conditions. The transitional period between these two environmental regimes occurred during upper Unit 3 to lower Unit 4 deposition, which was coincident with the Late Famennian eustatic transgression of Algeo et al. (2007). The decrease in paleosalinity during the transitional period may have been caused by glacial meltwater outwash from the Acadian highlands to the paleo-south, but an increase in regional precipitation frequency and intensity caused by a shift in regional climate is also considered a potential mechanism. Overall, the application of the Tl isotope paleoredox proxy to the Kettle Point Formation black shales reveals that local and regional environmental factors have a strong influence on Tl isotope expression, and global-scale redox reconstructions using this proxy can be widely inaccurate if smaller scale factors are not considered.