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:
- Thermal maturation of organic-rich deposits has increased Hg content
> 2-fold
- Hg/TOC ratios from mature sediments are inflated because of organic
carbon loss from the host rock during maturation
- Thermal history of sediments must be considered when using Hg as a
proxy for volcanism
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).