The influence of sediment thermal maturity and hydrocarbon formation on Hg behaviour in the stratigraphic record
A. O. Indraswari1,2, J. Frieling1, T. A. Mather1, A. J. Dickson3, H. C. Jenkyns1, E. Idiz1
1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, UK.
2Geoscience Study Program, Faculty of Mathematics and Natural Sciences (FMIPA), Universitas Indonesia, Depok 16424, Indonesia.
3Centre of Climate, Ocean and Atmosphere, Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK.
Corresponding author: Asri O. Indraswari (asri.indraswari@exeter.ox.ac.uk)
Key Points:
Abstract
While Hg in sediments is increasingly used as a proxy for deep-time volcanic activity, the behaviour of Hg in OM-rich sediments as they undergo thermal maturation is not well understood. In this study, we evaluate the effects of thermal maturation on sedimentary Hg contents and, thereby, the impact of thermal maturity on the use of the Hg/TOC proxy for large igneous province (LIP) volcanism. We investigate three cores (marine organic matter) with different levels of thermal maturity in lowermost Toarcian sediments (Posidonienschiefer) from the Lower Saxony Basin in Germany. We present Hg content, bulk organic geochemistry, and total sulfur in three cores with different levels of thermal maturity. The comparison of Hg data between the three cores indicates that Hg content in the mature/overmature sediments have increased > 2-fold compared to Hg in the immature deposits. Although difficult to confirm with the present data, we speculate that redistribution within the sedimentary sequence caused by the mobility and volatility of the element under relatively high temperatures may have contributed to Hg enrichment in distinct stratigraphic levels of the mature cores. Regardless of the exact mechanism, elevated Hg content together with organic-carbon loss by thermal maturation exaggerate the value of Hg/TOC in mature sediments, suggesting that thermal effects have to be considered when using TOC-normalised Hg as a proxy for far-field volcanic activity.
1. Introduction
Mercury (Hg) is highly toxic, which means that understanding its behaviour in shallow- and deep-earth environments and how it cycles through ecosystems is of considerable importance. Emissions of Hg into the atmosphere include those from natural sources, such as volcanic exhalations, and anthropogenic sources, such as artisanal and small-scale mining, fossil-fuel combustion, non-ferrous metal smelting, and cement production (UN Environment, 2019). Critically, a substantial part of these emissions is in gaseous form, and the relatively long atmospheric lifetime of Hg (0.5–2 years) means that it can be globally dispersed before deposition (e.g., Lindqvist et al., 1991; Mason et al., 1994; Lamborg et al., 2002a,b). It is generally assumed that most of the Hg will be finally sequestered in sediments containing organic matter (OM) as OM-Hg complexes. Indeed, field-data show that OM is usually the dominant Hg carrier, both in the water column and sediment (Wallace, 1982; Benoit et al., 2001; Outridge et al., 2007). In addition to OM, sulfides (e.g., HgS and Hg-inclusions in pyrite) and clays may prove significant sedimentary Hg hosts (Percival et al., 2018; Shen et al., 2020; Wang et al., 2020).
The behaviour of Hg once deposited into sedimentary archives is of interest for several reasons. The presence and levels of Hg in oils and their organic-rich petroleum source rocks is important as it is considered a contaminant in hydrocarbon fields. Mercury is found in hydrocarbons in highly varying concentrations (Wilhelm & Bloom, 2000). For example, fuel oils contain Hg with values ranging from 7 to 30,000 ppb, with a typical value being 3500 ppb (Wilhelm, 2001; Mukherjee et al., 2009). Knowledge about the presence and level of Hg in these hydrocarbon streams is essential because it can determine, amongst other things, decisions regarding processing facility design (e.g., the inclusion of costly removal units) to mitigate Hg pollution (Wilhelm & Bloom, 2000; Gajdosechova et al., 2016). The observed variable and potentially very high Hg content in hydrocarbons implies that it is critical to understand which sedimentary strata are likely to be enriched in Hg during deposition, and what processes might move and concentrate Hg into expelled fluids during thermal maturation of sediments.
Further to concerns regarding Hg behaviour in hydrocarbons, there has been much recent interest in the potential of Hg as a proxy for large-scale volcanism (namely large igneous provinces (LIP)) in the sedimentary record since volcanoes are amongst the largest natural sources (Pyle & Mather, 2003; Sanei et al., 2012; Percival et al., 2015; Scaife et al., 2017; Percival et al., 2021). Hg records are usually normalised to Total Organic Carbon (TOC) to correct for increases in Hg content associated with greater TOC contents (Sanei et al., 2012; Grasby et al., 2019). However, Hg can also be bound to sulfides and clay minerals and, in some environments, Hg deposition with such geochemical species may confound the usual sedimentary Hg-OM relationship (see e.g., Sanei et al., 2012; Charbonnier & Föllmi, 2017; Percival et al., 2018). Thus, several works (e.g., Grasby et al. (2019) and Shen et al. (2020)) have argued that it is critical to look at the relationship of Hg with TOC, as well as Hg variance with clay (Al) and total sulfur levels. Moreover, several previous studies acknowledge the potential of changes to geological deep-time sedimentary Hg records induced by OM sources or types that, for example, may be related to coastal proximity (i.e., marine- vs terrestrial-derived OM; Grasby et al., 2017; Wang et al., 2018; Them et al., 2019). Various fixation mechanisms for Hg in organic-rich mudrocks have been investigated, including adsorption onto OM and clay minerals and incorporation of Hg into the crystal structures of other host minerals, particularly sulfides (Krupp, 1988; Shen et al., 2020). Pyrite-hosted Hg might become a more dominant phase when sediments are deposited under sulfidic conditions, where free H2S occurs in the water column or sediment pore waters (Shen et al. 2020). However, whether these factors also lead to enhanced Hg sequestration or how these conditions affect proportioning between sedimentary host phases is not well constrained. While various potential confounding factors on sedimentary Hg distributions have been investigated (Grasby et al., 2019; Shen et al., 2020), the effects of thermal maturity on Hg content in deep-time sediments have not been tested systemically. This is however a critical knowledge gap as sedimentary Hg can be mobilised at temperatures known to be relevant for post-depositional sediment alteration and resulting oil and gas generation (e.g., Rumayor et al., 2013; Liu et al., 2022) and is clearly enriched in some hydrocarbon sources (e.g., Wilhelm, 2001; Mukherjee et al., 2009).
Thermal maturation of labile sedimentary OM occurs due to increasing temperature with increasing burial depth, typically on the order of ~30°C rise in temperature per kilometre of overburden. in catagenesis, that is conversion of kerogen, stable at lower temperatures, through thermal cracking into lower molecular weight components, loosely defined as bitumen. In the early stage of maturation, this bitumen contains a large proportion of high-molecular-weight compounds such as resins and asphaltenes. With increasing maturation, bitumen undergoes further cracking and disproportionation, resulting in lower molecular-weight hydrocarbon molecules and an insoluble coke-like C-rich residue: pyrobitumen (Tissot & Welte, 1984; Sanei, 2020). As the kerogen becomes more thermally ‘mature’ it progresses through the stages of oil and gas generation and, depending on the type of kerogen, potentially loses up to 60% of its organic matter mass due to migration of the generated products away from the host rock (Lewan et al., 1979; Tissot & Welte, 1984; Raiswell & Berner, 1987).
Thermal maturation may play a key role in altering the geochemical signature of metals (e.g., Mo, V, and Ni) due to the close association of these geochemical species with sedimentary OM and the transformations that OM undergoes (Lewan & Maynard, 1982; Chappaz et al., 2014; Dahl et al., 2017). For example, thermal maturation has been shown to lead to progressive enrichment in both the concentration of metals and their TOC-normalised values in sedimentary rocks for Mo, Zn, U and Cd (Dickson et al., 2020, 2022). Such increases can be attributed to the loss of mass caused by the removal of bitumen during thermal maturation and the minor partitioning of metals into mobilized organic fluid phases (Dickson et al., 2020, 2022). The generation of H2O, CO2, and H2S from both the organic and inorganic phases in the rocks also might play a role in causing additional mass losses (Abarghani et al., 2020; Dickson et al., 2020). However, it is difficult to directly measure metals bound to kerogen since the primary method to isolate this organic component is by digesting the mineral matrix using strong acids such as HF and HCl, which can also leach metals from the kerogen itself. Therefore, the mechanistic behaviour of metals, including Hg, in OM-rich sediments as they undergo thermal maturation is still not well understood.
Unlike most metals, there is evidence from analytical methods that utilise thermal desorption that sediments exposed to temperatures during burial typical of sedimentary basins (60–225 °C) could mobilise some Hg compounds (e.g., elemental Hg, weakly absorbed Hg, Hg-halides) (Rumayor et al., 2013, Liu et al. 2022), but the combined effects of prolonged exposure to the pressure and temperature regimes that typically exist during maturation remain untested.
Our study explores the influence of thermal maturation on sedimentary Hg using three cores covering a wide range of thermal maturity from the Lower Saxony Basin, Germany. By investigating Hg, bulk OM characteristics and total sulfur contents in a stratigraphically constrained interval from a single basin, we examined the role of thermal maturation as a key factor in post-depositional Hg mobility in sediments. We focus on the part of the Posidonienschiefer that stratigraphically sits above the negative carbon-isotope excursion characteristic of the Toarcian Oceanic Anoxic Event (T-OAE).