Figure 1. Location of the Lower Saxony Basin (LSB) in the Early
Jurassic (ca. 182 Ma). Modern shorelines are shown as dashed lines.
TOC—total organic carbon.
Geological Setting
The Lower Saxony Basin (LSB), located in north-western Germany (Fig. 1),
is the country’s most important oil province (Betz et al., 1987). It is
a 300-km long and 65-km wide E–W-oriented basin formed during the
breakup of the supercontinent Pangaea through extension and subsidence
(Betz et al., 1987; Brink et al., 1992; Bruns et al., 2013). The Lower
Toarcian Posidonienschiefer is a distinct organic-rich unit preserved in
the LSB. For more than a century, this unit has been of scientific
interest due to its importance as a source rock, including sedimentology
(Littke et al., 1991; Röhl et al., 2001; Schmid-Röhl et al., 2002),
stratigraphy (Riegraf et al., 1984; Frimmel et al., 2004; Schwark &
Frimmel, 2004), petrology and geochemistry (Jenkyns, 1985, 1988;
Leythaeuser et al., 1988; Rullkötter et al., 1988; Rullkötter & Marzi,
1988; Littke et al., 1988, 1991; Wilkes et al., 1998).
Evidence of persistent euxinia in the water column, and specifically
indicators of free H2S in the photic zone, comes from
the identification of aryl isoprenoids and other carotenoids
(isorenieratene) reported in the Posidonienschiefer from the LSB
(Blumenberg et al., 2019) and from the Posidonia shales from other NW
European basins (Schwark & Frimmel, 2004; French et al., 2014; Song et
al., 2017). Redox conditions are important to consider when interpreting
the variability of Hg records, which may result in enhanced Hg or
subdued Hg sequestration and/or changes in sedimentary host-phase (e.g.,
Grasby et al., 2016; Them et al., 2019, Shen et al., 2019; Grasby et
al., 2019; Frieling et al., 2022 PREPRINT). However, since the
Posidonienschiefer was likely deposited under basin-wide euxinic
conditions (Blumenberg et al., 2019), we do not expect differences in
the Hg sequestration pathway between the analysed core localities.
Because of high marine productivity and relatively minor influences of
terrestrial OM (Rullkötter et al., 1988; Littke et al., 1991), major
differences in OM-type between cores are also unlikely, excluding
influences of variable proportions of marine and terrestrial OM on Hg
loading.
Since its deposition in the Early Jurassic the Posidonienschiefer has
been buried to various depths within the LSB, due to the effect of
locally variable fault-bounded basin subsidence, followed by basin
inversion during the Paleogene (Betz et al., 1987). As a result, the
thermal maturity of the OM contained within the Posidonienschiefer
varies over relatively short horizontal distances of tens of kilometres,
making it a good candidate for testing whether thermal maturation can
change the concentration of metals in organic-rich sedimentary rocks
(Dickson et al., 2022).
Three ~30-m-long drilled cores containing
stratigraphically equivalent sections of the Posidonienschiefer were
retrieved from different parts of the LSB. Two of the cores (core A and
B) have been studied previously to explore the maturation-dependent
changes in stable-isotope compositions and concentrations of Mo, Zn, Cd
and U (Dickson et al., 2020, 2022) and the growth of the bed-parallel
and oblique calcite veins known as ‘beef’ (Hooker et al., 2020). The
thermal maturity of each core was established by reflectance
measurements (%Ro) on terrestrial organic macerals
(vitrinite~0.5% for core A (immature) and
~1.5% and ~3.5% (post-mature) for core
B and core C, respectively. Because cores B and C lack a
biostratigraphic framework, a core-to-core correlation is achieved via
distinct basin-wide chemostratigraphic trends in the TOC and trace-metal
records. Using the δ13Corg data for
core A and subsequent correlation using the TOC of all three cores, we
identify the interval of the Posidienschiefer that was deposited after
the T-OAE, to avoid the Hg-cycle perturbation that characterises the
T-OAE (Percival et al., 2015; Fantasia et al., 2018; Them et al., 2019).
3. Methods
A total of 647 samples (478 for core A, 121 for core B, and 48 for core
C, with a stratigraphic resolution of 10 to 30 cm) were powdered and
analysed for Hg content. Mercury content was measured on a Lumex RA-915
Portable Mercury Analyser coupled to a PYRO-915 pyrolysis unit at the
University of Oxford (Bin et al., 2001). Powdered samples of between 50
and 100 mg were introduced into a sample boat, heated to
>700 °C and left for up to 120 seconds to fully volatilize
the Hg present. The instrument was calibrated before each run using
NIST-SRM2587 (National Institute of Standards and Technology – Standard
Reference Material: Trace Elements in Soil Containing Lead from Paint),
with a Hg content of 290 ppb. The same standard was run for every ten
samples to correct instrument drift. We calculate an external
reproducibility of ±8.7% based on the repeat standard measurements (1
standard deviation (S.D.), n=199).
Hydrogen and oxygen indices (HI, OI) and TOC content data of all three
cores have been published previously in Hooker et al. (2020). An
additional 39 new samples (15 for core A, 21 for core B, and 3 for core
C) were analysed with a Rock-Eval 6, following the methods in Behar et
al. (2001), at the University of Oxford. The in-house standard SAB134
(Blue Lias organic-rich marl, 2.74 % TOC) was measured every 8 to 10
samples. The standard deviation of the in-house standard (SAB134) was
~0.03 % TOC (1 S.D.).
Analyses of total sedimentary sulfur (%TS) were undertaken on 155
samples (57 for core A, 50 for core B, and 48 for core C). An aliquot of
each sample was wrapped in a tin capsule and then combusted using an
Elemental Cube Elemental Analyzer Vario El III at the Department of
Earth Sciences, Royal Holloway University of London (Carvajal-Ortiz et
al., 2021; Fadeeva et al., 2008). A sulfanilamide reference standard
(18.62 % S) was analysed at the start and end of each run and between
every ten samples to monitor instrument drift. Within-run
reproducibility calculated from the Eocene shale standards (1.24 % S)
run as unknowns was ±0.12 % S (1 S.D., n=12).
High-resolution δ13Corg data from core
A was measured by Celestino (2019).
δ13Corg for core B and core C were
analysed at the Open University (U.K). The samples powders were prepared
by decarbonating bulk sediments in 1M HCl before rinsing with de-ionised
water until a neutral pH was reached. Samples were dried and re-powdered
before being weighed into tin capsules and analysed using a Thermo
MAT253 mass spectrometer coupled to a Flash HT combustion system.
δ13Corg is expressed relative to the
Vienna Pee Dee Belemnite (VPDB) scale via within-run calibration using
NIST 8572 glutamic acid (-26.39 ‰), International Atomic Energy Agency
(IAEA) CH-6 sucrose (-10.45 ‰) and L-Alanine (-23.33 ‰). Analytical
precision, calculated from the 1 S.D. of L-alanine, is better than ±0.1
‰.
4. Results
We focus on an interval of Posidonienschiefer that is stratigraphically
above the negative carbon-isotope excursion characteristic of T-OAE
(hereafter referred to as Posidonienschiefer). The stratigraphic
successions of the cores in this study were tied together based on
distinctive patterns in total organic carbon (TOC) following Dickson et
al. (2022). The average TOC values of the Posidonienschiefer show an
increase from core A to core B and from core B to core C (Table 1).
Rock-Eval determined Hydrogen Index (HI) values in the section in core A
average 687 mg hydrocarbons /g TOC (Fig. 2). By contrast, in cores B and
C, the HI values throughout the Posidonienschiefer are relatively
constant at near 0 (Fig. 3 and 4). Total sulfur (TS) content is ranging
from 1.5 to 6 % for cores A, B, and C through the Posidonienschiefer,
(Fig. 2, 3, and 4).