Abstract
Mercury geochemistry is emerging recently as a hot topic in chemostratigraphical and facies research, owing to the diagnostic character of Hg enrichments as a proxy of volcanic activity (crucial in the context of assumed causal links between volcanic cataclysms and mass extinctions). Thus, as a prerequisite to such far-reaching interpretations, reliable analytical determinations of Hg concentrations are necessary. In conventionally performed analyses in sedimentary geochemistry, Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) is usually applied, as an analytical standard for trace elements, including Hg. However, with a detection limit (DL) of 10 ppb, such measured values have been questioned as a conclusive geochemical indicator of Hg anomalies, and, instead, far more accurate techniques, such as Atomic Absorption Spectrometry (AAS; DL = 0.2 ppb), are requested. As a preliminary test of this view, we present comparative analysis of 91 samples from three sections encompassing the key Frasnian-Famennian and Famennian-Tournaisian boundary intervals in Morocco (Lahmida), Germany (Kahlleite) and Uzbekistan (Novchomok), for which Hg concentrations were determined by both methods in the same samples. Despite some differences, especially at low Hg concentrations, both analytical methods reveal the same 12 extraordinarily enriched samples in excess of 1 ppm (with one exception, the determination error is <20%), as well as similar overall chemostratigraphic patterns characterized by a few prominent Hg spikes, with a top value of 5.8 ppm. The Hg concentrations determined by ICP-MS and AAS are significantly correlated, as high as r = 0.98 (Novchomok), even if the first method reveals a general tendency toward slightly heightened values (by ~15 to 30% for medians). Therefore, ICP-MS results can conclusively be used in mercury chemostratigraphy in order to recognize extraordinary volcanic (or other) signals, at least in the Devonian geological record. False Hg anomalies were not generated by these conventional ICP-MS determinations.
Highlights
Despite great advances in sedimentary elemental geochemistry in the last few decades, focused on the use of trace elements to refine palaeoenvironmental reconstruction (e.g., Sagemann and Lyons, 2003; Tribovillard et al, 2006; Calvert and Pedersen, 2007; Ramkumar, 2015), mercury, in contrast to iridium, molybdenum, uranium, vanadium, barium and zirconium, used to be largely ignored in event chemostratigraphy and facies analysis
We provide comparative Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrometry (AAS) analyses of 91 samples from three sections encompassing the key Frasnian-Famennian (F-F) and Devonian-Carboniferous (D-C) boundary interval in Morocco, Germany and Uzbekistan (Fig. 1) for which Hg concentrations were determined by both methods
A preliminary test of two analytical methods of Hg determination negates the view that ICP-MS values are unreliable, and generate of false signals
Summary
Despite great advances in sedimentary elemental geochemistry in the last few decades, focused on the use of trace elements to refine palaeoenvironmental reconstruction (e.g., Sagemann and Lyons, 2003; Tribovillard et al, 2006; Calvert and Pedersen, 2007; Ramkumar, 2015), mercury, in contrast to iridium, molybdenum, uranium, vanadium, barium and zirconium, used to be largely ignored in event chemostratigraphy and facies analysis. Prior to deposition of oxygenated reactive Hg via rain, it can be distributed worldwide in the atmosphere because of the Hg residence time of ~0.5–2 years. Hg is scavenged during depositional processes by sedimentary organic matter, and by absorption on clay minerals, sulphides (in oxygen-deficient conditions), and hydrous iron oxides (see Sanei et al, 2012; Percival et al, 2015; Sial et al, 2016; Bergquist, 2017; Sabatino et al, 2018). In the stratigraphical record, Nascimento-Silva et al (2011) and Sanei et al (2012) concurrently highlighted Hg anomalies as a record of catastrophic volcanic eruptions during the end-Cretaceous and end-Permian mass extinctions, respectively, while Hg was used as a marker of Neoproterozoic volcanism in Cryogenian deposits by Sial et al (2010)
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