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).