# Protracted carbon burial following the Early Jurassic Toarcian Oceanic Anoxic Event (Posidonia Shale, Lower Saxony Basin, Germany)

R. F. S. Celestino<fnref n="1" /> · M. Ruhl<fnref n="2" />,<fnref n="3" /> Ⓓ · A. J. Dickson<fnref n="3" />,<fnref n="4" /> · E. Idiz<fnref n="3" /> · H. C. Jenkyns<fnref n="3" /> · M. J. Leng<fnref n="5" /> · E. Mattioli<fnref n="6" /> · D. Minisini<fnref n="7" />,<fnref n="8" /> · S. P. Hesselbo<fnref n="1" />

Received: 2 August 2023 / Accepted: 16 October 2024
© The Author(s) 2024

## Abstract

Lower Jurassic marine basins across the northwest European epicontinental shelf were commonly marked by deposition of organic-rich black shales. Organic-carbon burial was particularly widespread during the Toarcian Oceanic Anoxic Event (T-OAE: also known as the Jenkyns Event) with its accompanying negative carbon-isotope excursion (nCIE). Lower Toarcian black shales in central and southern Germany are known as the Posidonia Shale Formation (Posidonienschiefer) and are thought to have formed during the T-OAE nCIE. Here, we present stratigraphic (carbon-isotope, Rock-Eval, calcareous nannofossil) data from the upper Pliensbachian and lower Toarcian strata from a core drilled on the northern flank of the Lower Saxony Basin, north-west Germany. The bio- and chemostratigraphic framework presented demonstrates that (i) the rock record of the T-OAE at the studied locality registered highly condensed sedimentation and/or multiple hiatuses and (ii) the deposition of organic-rich black shale extended significantly beyond the level of the T-OAE, thereby contrasting with well-studied sections of the Posidonia Shale in southern Germany but showing similarities with geographically nearby basins such as the Paris Basin (France). Prolonged and enhanced organic-carbon burial represents a negative feedback mechanism in the Earth system, with locally continued environmental perturbance accelerating the recovery of the global climate from T-OAE-associated hyperthermal conditions, whilst also accelerating a return to more positive δ¹³C values in global exogenic carbon pools.

**Keywords** Posidonia Shale · Toarcian · Black shale · Lower Saxony Basin

## Introduction

The early Toarcian was characterized by major environmental changes, most notably the geographically widespread development of marine anoxia, such that this time-interval is commonly taken to record the so-called

Toarcian Oceanic Anoxic Event (T-OAE or Jenkyns Events (Gambacorta et al. 2024)). The development of globally widespread marine (and lacustrine) anoxia was expressed by the formation of organic-rich, commonly laminated black shales in interconnected but hydrographically restricted sub-basins across the Laurasian Seaway of

<footnote>M. Ruhl
micha.ruhl@tcd.ie</footnote>

<footnote>S. P. Hesselbo
s.p.hesselbo@exeter.ac.uk</footnote>

<footnote n="1">¹ Camborne School of Mines and Environment and Sustainability Institute, University of Exeter, Penryn Campus, Treliever Road, Penryn, COR TR10 9FE, UK</footnote>

<footnote n="2">² Department of Geology, School of Natural Sciences, Trinity College Dublin, The University of Dublin, College Green, Dublin, Ireland</footnote>

<footnote n="3">³ Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK</footnote>

<footnote n="4">⁴ Centre of Climate, Ocean and Atmosphere, Department of Earth Sciences, Royal Holloway University of London, Egham, SRY TW20 0EX, UK</footnote>

<footnote n="5">⁵ National Environmental Isotope Facility, British Geological Survey, Nottingham, UK</footnote>

<footnote n="6">⁶ Université Claude Bernard Lyon 1, ENS de Lyon, UCBL, ENSL, UJM, CNRS, LGL-TPE, 69622 Villeurbanne, France</footnote>

<footnote n="7">⁷ ExxonMobil, 22777 Springwoods Village Pkwy, Spring, TX 77389, USA</footnote>

<footnote n="8">⁸ Department of Earth, Environmental, and Planetary Sciences, Rice University, 6100 Main St, Houston, TX 77005, USA</footnote>
epicontinental northern Europe during the Early Jurassic. The characteristic facies include the Posidonia Shale in southern Germany, the Jet Rock in the Cleveland Basin (Yorkshire, UK) and the Schistes Carton in the Paris Basin (France) (Fig. 1) (Jenkyns 1988, 2010; Ziegler 1992; Röhl et al. 2001; Hesselbo et al. 2000; Schmid-Röhl et al. 2002, 2003; Frimmel et al. 2004; Cohen et al. 2004; Schwark and Frimmel 2004; Hermoso et al. 2009, 2013; Xu et al. 2017; Kemp et al. 2022a, b; Gambacorta et al. 2024).

The Toarcian OAE is globally associated with a – 4‰ to – 8‰ negative carbon-isotope excursion (CIE), recorded in organic and inorganic, marine and terrestrial substrates from geographically widespread locations (e.g., Jenkyns and Clayton 1987; Jenkyns 1988; Harries and Little 1999; Hesselbo et al. 2000; McArthur et al. 2000; Röhl et al. 2001; van Breugel et al. 2006; Hesselbo et al. 2007; Woodfine et al. 2008; Sabatino et al. 2009, 2013; Al-Suwaidi et al. 2010; Caruthers et al. 2011; Kafousia et al. 2011; Kemp and Izumi 2014; Them et al. 2017; Xu et al. 2017, 2018a; Fantasia et al. 2019; Baghli et al. 2020; Ullmann et al. 2020; Al-Suwaidi et al. 2022; Bodin et al. 2023; Storm et al. 2024). The characteristic T-OAE negative carbon-isotope excursion (CIE) likely resulted from the massive injection of isotopically light (¹²C enriched) carbon into the ocean-atmosphere system from magma degassing linked to Karoo-Ferrar Large Igneous Province (LIP) emplacement, and/or the release of thermogenic methane from sill-intruded subsurface organic-rich sediments in the Karoo Basin, and/or the dissociation of sub-seafloor methane clathrates (Hesselbo et al. 2000; McElwain et al. 2005; Svensen et al. 2007; Percival et al. 2015; Jones et al. 2016; Heimdal et al. 2021; Ruhl et al. 2022). The associated perturbations to the global carbon cycle resulted in climatic, environmental and biotic/ ecosystem changes, such as significant global warming (characterizing this event as a hyperthermal), an accelerated hydrological cycle (leading to enhanced global continental weathering rates), the widespread development of anoxic conditions, and a major extinction event (Jenkyns 1985; Harries and Little 1999; McArthur et al. 2000; Röhl et al. 2001; Bailey et al. 2003; Cohen et al. 2004; Hesselbo et al. 2007; Gómez et al. 2008; Mattioli et al. 2009; Sabatino et al. 2009; Al-Suwaidi et al. 2010; Hesselbo and Pienkowski 2011; Caswell and Coe 2014; Ikeda and Hori 2014; Percival et al. 2016; Them et al. 2017; Dickson et al. 2017; Xu et al. 2017, 2018a; Fantasia et al. 2018; Ruebsam et al. 2020; Ullmann et al. 2020; Erba et al. 2022; Kemp et al. 2022a, b). The T-OAE was preceded by a smaller carbon-cycle perturbation at the Pliensbachian-Toarcian boundary, which was probably linked to early phases of volcanic activity within the Karoo-Ferrar LIP region, as shown by sedimentary Hg enrichment, magnetostratigraphy, and U-Pb radio-isotopic dating, suggesting that

volcanic activity was initiated in the late Pliensbachian (Hesselbo et al. 2007; Littler et al. 2010; Percival et al. 2015; Bodin et al. 2016; Xu et al. 2018b; De Lena et al. 2019; Al-Suwaidi et al. 2022; Ruhl et al. 2022).

One of the organic-rich sedimentary units associated with the development of anoxia in basins of the Laurasian Seaway was the Posidonia Shale Formation (Posidonienschiefer), which crops out in Southern Germany and is present in the subsurface of northern Germany and the western Netherlands (Küpert 1982; Riegraf et al. 1984; Littke et al. 1991; Ziegler 1992; Röhl et al. 2001; Schmid-Röhl et al. 2002; Frimmel et al. 2004; Schwark and Frimmel 2004; Montero-Serrano et al. 2015; Song et al. 2017). Studies conducted across different NW European basins, show that the expression of the T-OAE differs between localities, suggesting a strong local/regional control on the depositional environment, superimposed on global climatic and environmental perturbances (e.g., Remirez and Algeo 2020a and b and references therein). Within Germany, most studies on the T-OAE, integrating high-resolution carbon-isotope stratigraphy and biostratigraphy, have focused on the SW German Basin, with few detailed stratigraphic datasets available from the Lower Saxony Basin. We present here a high-resolution carbon-isotope dataset combined with calcareous nannofossil biostratigraphy, pyrolysis, and sedimentological data obtained from a core drilled in the Lower Saxony Basin (LSB) of NW Germany. The studied succession stratigraphically covers the Posidonia Shale Formation and the T-OAE interval. With this framework, we characterize the lithostratigraphic expression of the T-OAE and the Pliensbachian-Toarcian boundary event in the LSB, suggest stratigraphic correlation to well-documented reference sections elsewhere, and provide evidence for the stratigraphic diachroneity of Posidonia Shale successions between adjacent basins. Our objective is to differentiate and constrain the timing of the depositional mechanisms leading to the formation of organic-rich black shales in geographically separated, but locally adjacent basins during the T-OAE.

## Regional geological setting

The Lower Saxony Basin (LSB) (Fig. 1) is an E-W-trending 350-km-long and 100-km-wide basin that developed through extension and subsidence during the breakup of the supercontinent Pangaea (Betz et al. 1987; Brink et al. 1992; Ziegler 1992; Senglaub et al. 2005; Bruns et al. 2013). Thickness variations of the sedimentary units in the LSB, identified through seismic reflection profiles and well-data, suggest that the basin formed initially as a large, asymmetric, and internally faulted graben system (Bruns et al. 2013). The Jurassic sediments of the LSB are composed of calcareous claystone, limestone,

<figure><img src="image_1.png" /></figure>
<figure><img src="image_2.png" /><figcaption>**Fig. 1** a Early Jurassic palaeogeography. b Palaeogeographic location of Lower Saxony Basin (LSB), and nearby basins marked by total organic carbon (TOC) accumulation in the Laurasian Seaway. CB Cleveland Basin; PB Paris Basin; SWGB Southwest Germanic Basin; LSB Lower Saxony Basin (modified after Korte et al. 2015;</figcaption></figure>

Ruebsam et al. 2018). c Geographical map of NW Germany showing the extent of the Lower Saxony Basin in the subsurface (modified after Betz et al. (1987)), with marked the geographic location of the here-studied Core A

sandstone, and shale. The eastern part of the basin was dominated by a shallow-water deltaic depositional environment, with sandy deposits, while the western part was characterized by deeper water shales and marls, including the organic-rich Toarcian Posidonia Shale Formation (Fig. 1) (Senglaub et al. 2005). The thickness of the Posidonia Shale Formation varies between 25 and 70 m across

the basin because of spatially varying sedimentation rates and/or stratigraphic variation in hiatuses at its base and top. Across the LSB, the Posidonia Shale reached varying burial depths, with current burial depths of 1–3 km, but possibly as much as 3–8 km during the Early Cretaceous, with resultant degradation of sedimentary organic matter, hydrocarbon formation and expulsion, and formation
of ‘beef’ calcite veins (McCann 2008; Bruns et al. 2013; Hooker et al. 2020; Dickson et al. 2022).

# Materials and methods

## Materials

The samples studied here from an industry core, Core A, in the Lower Saxony Basin (LSB) span a ~ 70 m stratigraphic interval (Fig. 1). Sediments in Core A are thermally imma- ture and present the most complete and expanded upper Pliensbachian and Toarcian sedimentary succession of the LSB, thereby arguably presenting the best reference core for the Posidonia Shale. Core A is referred to as Well A in Gorbanenko and Ligouis (2014), Core A in Dickson et al. (2022), and Core 1 in Hooker et al. (2020).

## Bulk organic δ¹³C analysis

The carbon-isotope composition (δ¹³C) of total sedimentary organic carbon was measured on 340 decarbonated bulk-rock samples, which were crushed and ground to a fine powder using an agate pestle and mortar. An aliquot of ~0.5 g was weighed in to 50-ml centrifuge tubes, and 10% hydrochlo- ric acid was added and the resultant mixture subsequently heated to 60 °C in a water bath. The samples were then rinsed three times with distilled water and dried in an oven at 40 °C. Between 5 and 100 mg of decarbonated sediment, depending on organic-carbon content, was weighed into tin capsules to be measured for δ¹³C. ¹³C/¹²C analyses were performed by combustion in a Costech ECS4010 Elemen- tal Analyser (EA) online to a VG TripleTrap and Optima dual-inlet mass-spectrometer at the British Geological Sur- vey, Keyworth (UK). The δ¹³C values were calibrated to V-PDB, using a within-run laboratory standard calibrated against NBS18, NBS-19 and NBS-22. Replicate analysis of well-mixed samples indicated a precision of <0.1‰ (1 SD).

## Calcareous nannofossil biostratigraphy

Calcareous nannofossil identification was carried out on 59 samples from Core A, at the Université Claude Bernard, Lyon 1, France. Stratigraphic sample resolution is approxi- mately 2 m within the Posidonia Shale Formation, and down to 10 cm around the formation boundaries and the interval covering the T-OAE. Sample positions for calcareous nan- nofossil biostratigraphy are shown in Fig. 2. Smear slides of rock powder (Bown and Young 1998) were mounted using Rhodopass mounting medium and then studied under an optical polarizing microscope at 1000×. All the encountered

nannofossil specimens were counted along 3 transverses of the slide (3 mm²), using an Axioskop 40 Zeiss polarizing microscope. NJT zonation after Ferreira et al. (2019), modi- fied after Mattioli and Erba (1999) and Bown and Cooper (1998), was used. Preservation degree of nannofossils was estimated according to etching and overgrowth of specimens (Roth 1984): poor nannofossil assemblages show strong etching, overgrowth or fragmentation; moderate and good have limited alteration.

## Rock-Eval pyrolysis

Between 2 and 100 mg of powdered bulk-rock samples (n=287) were weighed into stainless steel crucibles and measured using a Rock-Eval 6 Unit (Behar et al. 2001), at the Department of Earth Sciences, University of Oxford. A temperature profile of 300–650 °C for pyrolysis and 300–850 °C for oxidation was applied to the samples. S1, S2, Tmax, S3, S4 and S5 peaks were determined to calculate the Hydrogen Index (HI, in mg HC/g TOC), Mineral Carbon (weight %), Oxygen Index (OI, in mg CO₂/g TOC), Residual Organic Carbon (%) and total organic carbon (TOC, %). An in-house standard SAB134 (Blue Lias, Jurassic; Hesselbo et al. 2004) was measured every ten samples through the analytical programme. One standard deviation on repeated TOC and HI analyses of the SAB134 in-house standard is 0.065% (TOC) and 22.65 mg HC/g TOC (HI), respectively.

# Results

## Sedimentology and lithostratigraphy

The studied sedimentary succession in Core A of the LSB covers part of the Lias delta, Lias epsilon and Lias zeta, also known as the Amaltheenton, Posidonienschiefer (Posidonia Shale) and Jurenismergel formations, respectively (Fig. 2, Supplementary Fig. 1). The Amaltheenton Formation (Fm), at the base of the studied core, is composed of quartz- and clay-rich, dark-grey mudstone with abundant sideritic nod- ules and pyritized burrows. The carbonate content is low through this stratigraphic interval, with only a few samples containing calcareous nannofossils; the abundant scours are filled with silt to fine sand. The transition from the Amaltheenton to the Posidonia Shale Formation is charac- terized by a succession of condensed horizons and carbon- ate-rich concretionary intervals (interpreted as hiatus con- cretions; cf. Voigt 1968) between 45 and 43 m in the studied succession (Figs. 2, 3a–c and 5; relative depths with respect to top of the studied interval). The first carbonate-rich unit occurs at 45 m and is composed of a 30-cm-thick network of septarian nodules with cracks filled with multiple gen- erations of calcite (Fig. 3d), with the top showing abundant

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2** Lithological log of Core A with First Occurrences of calcareous nannofossils, stratigraphic location of main hiatus horizons, and sample positions for calcareous nannofossil biostratigraphy (red lines). An enlarged view of the 40–50 m cored section highlights the more detailed sedimentology and higher resolution sampling of this interval</figcaption></figure>

reworked carbonate nodules and sediment infilling of cracks at a hiatus concretion level (Fig. 3c, Hiatus A), similar to observations on the Sinemurian mudstone successions in the Wessex Basin, Dorset, UK (Hesselbo and Palmer 1992). The hiatus concretion at 45 m is taken here as the boundary between the Amaltheenton and Posidonia Shale formations and marks an upward change to overall dark-grey and dark-brown (sub-) laminated marl. Stratigraphically upward,

identified nodules change from sideritic to calcitic (Fig. 3) and become abundant throughout the lower part of the Posidonia Shale.

A second level of condensed sedimentation and/or erosion is evident at 43.5 m, showing reworked calcite nodules as well as belemnite rostra and shell fragments overlying an erosional surface (Fig. 2, Hiatus B). Sediments between hiatuses A and B comprise dark grey-brown mudstone with
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Core photographs showing sedimentological features in Core A. **a** Top of the Posidonia Shale Formation showing burrowed firm-ground and overlying debris of belemnite rostra with microborings (Hiatus D). **b** Erosional surface on pale yellow-grey very fine sand lamina overlain by condensed stratigraphic horizon with belemnite and shell debris as well as reworked nodule fragments (Hiatus B). **c** Partially broken-up septarian carbonate nodule with sediment-filled</figcaption></figure>

cracks. The top shows reworked nodules (Hiatus A). **d** Septarian crack network with two phases of calcite growth. **e** Brown mudstone soft-sediment deformation with common silt and very fine sand-sized clastic and shell material with truncated base and top (Hiatus C). **f** Two horizons with very fine sand-sized clastic material, followed by a 10 cm-thick shell debris layer. **g** Siderite nodule (Amaltheenton Formation). **h** Calcareous nodule (Posidonia Shale Formation)

multiple well-defined mm-thick silty layers, suggesting episodic sediment redeposition. Between 43.5 and 42.5 m in the studied succession the dark grey-brown mudstone shows no clear lamination and contains abundant pyrite-filled burrows. From 42.6 m upwards the sedimentary facies changes with the onset of visible lamination and common horizons of silt and very fine sand and terrestrial plant debris (Fig. 3e). A 10-cm thick unit at 42.5 m shows clear soft sediment deformation, with the sharp base and top suggesting a hiatus (Fig. 3e, Hiatus C). Coarser grained (turbiditic) silty

levels decrease in abundance from 41 m upwards, and the subsequent deposits to the top of the Posidonia Shale Formation are dominated by brown-grey mudstones with diffuse lamination and an absence of clearly visible bioturbation. Abundant thin-shelled bivalves, macroscopically visible pyrite, as well as small pieces of fish bone, can be found in the lower half of the formation.

Calcite concretions occur at several levels, but mostly below 26 m in the studied succession. At 26 m, a 1 cm thick layer consisting of sand-sized detrital grains, is followed by

<figure><img src="image_6.png" /></figure>
a 10 cm-thick layer of shell debris (Fig. 3f). The upper half of the Posidonia Shale in Core A is homogenous and very poor in macrofossil material, with also only a few small burrows and varying degrees of lamination. The top of the Posidonia Shale is marked by a condensed horizon (Fig. 2, Hiatus D) containing abundant debris of belemnite rostra with microborings as well as larger burrow structures with pale-coloured sediment infill. The overlying Jurensismergel is characterized by the absence of lamination and signs of bioturbation with regular occurrence of pyrite replacement in burrow structures.

## Integrated carbon-isotope and calcareous nannofossil stratigraphy

Stable bulk sedimentary organic carbon-isotope analyses of Core A, with an overall sample resolution of 5–15 cm

and a ~2 cm sample resolution of the T-OAE stratigraphic interval, show values of ~25.8‰ for the lowermost part of the core (at 67–70 m in the studied succession), followed by a sharp negative shift to ~26.5‰ at 66 m, possibly indicating a hiatus in the record (Fig. 4). Values gradually increase up-section to ~26‰ at 58 m, before decreasing to ~29.5‰ at 48 m. The δ¹³CₗTOC record shows a narrow negative CIE of ~1.5‰ between 50 m and 46 m. More elevated values of ~28‰ at 47–44 m are subsequently followed by an abrupt negative shift in δ¹³CₗTOC down to ~30‰ before values are truncated and sharply increase to ~26‰ at 43 m. The δ¹³CₗTOC values thereafter decrease over half a metre to ~32‰ at 42.5 m, representing a negative excursion, with values subsequently increasing more gradually to ~26.5‰ at 41.5 m. From 41.5 m to 34 m in the studied section the δ¹³CₗTOC data show a decreasing trend to a point where values remain relatively

<figure><img src="image_7.png" /><figcaption>**Fig. 4** Lithological log of Core A with First Occurrences of calcareous nannofossils, stratigraphic location of main hiatus horizons, and the generated δ¹³CₗTOC (this study), Total Organic Carbon (TOC: this study, Hooker et al. 2020; Dickson et al. 2022), Hydrogen Index (HI: this study), Oxygen Index (OI: this study) and the calculated cal-</figcaption></figure>

cium carbonate (inferred from the mineral carbon content: this study) curves with the identified Toarcian Oceanic Anoxic Event (T-OAE) and the likely stratigraphic position of the Pliensbachian–Toarcian boundary
stable at approximately – 29‰ up to 10 m, where another
sharp increase in values to – 26.5‰ can be observed. The
uppermost 10 m of the studied succession in the core is
characterized by values of around – 27‰ to – 28‰ with
only minor variation.

Calcareous nannofossil data from the same core iden-
tify Lower Jurassic biozones, and permit assignment of
the boundary between the NJT5b-c Zone and the NJT6
Zone (Figs. 4, 5 and Supplementary Fig. 2), with the First
Occurrence of *Carinolithus superbus* at 46 m, within the
uppermost part of the Amaltheenton Formation. Samples
studied from the Amaltheenton Formation generally con-
tain very few calcareous nannofossils, whereas samples
from the Posidonia Shale show abundant nannofossils from
41 m upwards. The nannofossil *Crepidolithus crassus*, typi-
cally found in the recovery phase of the T-OAE in succes-
sions elsewhere (Mattioli et al. 2008), is dominant in this
interval. The First Occurrence of *Discorhabdus striatus*,
at 25 m, combined with a reduced dominance of *C. cras-
sus* and an increase of *Schisosphaerella* at this level defines
the boundary between NJT6 and NJT7 in Core A. The top
of the NJT7 Zone is assigned to the stratigraphic level at
9.3 m, coinciding with Hiatus D, and the formation bound-
ary between the Posidonia Shale and the Jurensismergel.
The overlying Jurensismergel is characterized by the First
Occurrence of *Retecapsa incompta*, which defines the base
of the NJT8a nannofossil Zone. The *Schisosphaerella* abun-
dance decreases in the Jurensismergel, whilst larger *Lothar-
ingius* nannofossils become more dominant. Overall, the
calcareous nannofossil biostratigraphy allows assignment
of the Amaltheenton Formation to the upper Pliensbachian,
and the Posidonia Shale and Jurensismergel to the lower
to middle Toarcian. The combined calcareous nannofossil
biostratigraphy and carbon-isotope chemostratigraphy sug-
gest the Pliensbachian–Toarcian boundary to coincide with
the ~1.5‰ negative CIE at ~48 m in the studied succession
of Core A. The First Occurrence of *C. superbus* and the
shifts to δ¹³CₗₚC values of ~ 32‰ suggest the sedimentary
succession associated with the T-OAE negative CIE to be
constrained between 42 and 44.5 m in Core A, where it is
truncated and/or strongly condensed.

Organic matter type

Rock-Eval pyrolysis was used to determine organic-matter content (total organic carbon: TOC) and kerogen type (hydrogen index: HI) in the studied sedimentary succession. The HI value is commonly used to evaluate the impact of possible source mixing on the bulk organic δ¹³CₗₚC record. Total organic carbon (TOC) content varies substantially within the studied stratigraphic interval, between 1% and 15% [this study (Fig. 4); Hooker et al. 2020; Dickson et al. 2022]. Low TOC values of ~1% characterize the

Amaltheenton Formation, except for a stratigraphically short
increase up to 14% TOC at ~46.5 m. Sedimentary TOC con-
tent varies between 1% and 5% in the lower 2.5 m of the
Posidonia Shale, followed by a significant rise to 13% TOC
at the stratigraphic level of Hiatus C (Fig. 4). The TOC con-
tent subsequently remains generally high, between 5% and
15% throughout the remainder of the Posidonia Shale. TOC
concentrations significantly reduce again to ~1% at Hiatus
D, the boundary between the Posidonia Shale and overlying
Jurensismergel Formation.

Hydrogen index values generally mimic the TOC record, with values of 100–300 mg HC/gTOC in the Amaltheenton Formation and 500–800 mg HC/gTOC in the Posidonia Shale (Figs. 4 and 6). High HI and low oxygen index (OI) in the Posidonia Shale suggest a well-preserved, elevated marine algal contribution to the bulk sedimentary organic matter. Hence, small variations in HI and OI in the main body of the Posidonia Shale possibly reflect minor changes in the relative contribution of pelagic algal marine organic matter vs terrestrial organic matter. The generally poor correlation between δ¹³CₗₚC and HI values in the studied succession of Core A (Fig. 6) suggests that changes in organic-matter source were not the main driver of stratigraphic changes in bulk organic carbon-isotope values.

Discussion

Protracted deposition of the posidonienschiefer
after the T-OAE

The Lower Saxony Basin is one of the main basins of the north European epicontinental seaway. Compared to the well-studied Toarcian records of the Southern German Basin (e.g., Röhl et al. 2001; Schmid-Röhl et al. 2002; Schwark and Frimmel 2004), to date only limited Lower Jurassic bio- and carbon-isotope stratigraphic data exist for the Lower Saxony Basin (e.g., Gorbanenko and Ligouis 2014; Gor- banenko 2015; van de Schootbrugge et al. 2019; Marten et al. 2024; Schwark and Ruebsam 2024), with much data originating from the Hils Syncline (e.g., Vandenbroucke et al. 1993, and references therein). Comparison of the geo- graphic and stratigraphic variation in geochemical, physi- cal, and biological characteristics of the Posidonia Shale within and between the LSB and adjacent basins provides constraints on the mechanisms leading to the intra- and inter- basinal variation in organic-carbon burial and deposition of black shale in the Laurasian seaway during the Toarcian.

Many of the well-studied sections and cores in NW
Europe and elsewhere, covering the T-OAE interval, show
organic-carbon enrichment predominantly during the T-OAE
negative CIE stratigraphic interval (e.g., Jenkyns et al. 2002;
Kemp et al. 2005; Hesselbo et al. 2007; Xu et al. 2018a; Erba
<figure><img src="image_8.png" /><figcaption>**Fig. 5** Selection of the most important calcareous nannofossils identified in Core A. Images with black background were taken with cross-polarized light and the white scale is 5 µm</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6** Cross-plot of δ¹³Cₗ TOC and corresponding Hydrogen Index (HI) showing the absence of a strong correlation, suggesting that stratigraphic variations in δ¹³Cₗ TOC occur independently from organic-carbon source mixing, but rather because of exogenic carbon cycle changes at that time. Amaltheenton Formation data in grey; Posidonia Shale Formation data in black</figcaption></figure>

et al. 2022; Kemp et al. 2022a, b). Integrated calcareous nannofossil, carbon-isotope and lithostratigraphic analyses of Core A enabled correlation to sections elsewhere, including the expanded reference section of the Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin (UK), hereafter referred to as Mochras, providing the first detailed stratigraphic framework for the upper Pliensbachian and Toarcian sediments of the Lower Saxony Basin (Germany). Stratigraphical age assignment in the Lower Saxony Basin is, however, challenging for the Amaltheenton Formation, e.g., particularly below 50 m in Core A, as this part of the core does not contain sufficient calcareous nannofossils to establish a nannofossil zonation. The first nannofossil Zone to be reliably determined is NJT5b-c, based on the concomitant presence of *Lotharingius crucicentralis*, *Lotharingius velatus*, *Etmorhabdus crucifer* and *Discorhabdus ignotus*. Such a nannofossil assemblage in this stratigraphic interval suggests an age at around the Pliensbachian–Toarcian boundary as these species have their First Occurrence very close to the base of the Toarcian in stratigraphically well-constrained sections (Supplementary Fig. 2) (Bucefalo Palliani and Mattioli 1998; Mattioli and Erba 1999; Mattioli et al. 2013; Fraguas et al. 2015; Menini et al. 2018; Ferreira et al. 2019). At the same stratigraphic level, the δ¹³Cₗ TOC signal shows a small-amplitude negative excursion, which is tentatively interpreted to record the Pliensbachian–Toarcian boundary event (cf. Hesselbo et al. 2007; Littler et al. 2010; Da Rocha et al. 2016; Xu et al. 2018a).

Stratigraphic correlation between the upper Pliensbachian strata in Core A, with the Sancerre core of the Paris Basin, which includes a high-resolution calcareous nannofossil biostratigraphic zonation (Peti et al. 2017) and the Schandelah core from the Lower Saxony Basin (van de Schootbrugge et al. 2019) is challenging due to the lack of a characteristic and easily identifiable carbon-isotope event during this time period. The stratigraphically lowest large-amplitude excursion in the δ¹³Cₗ TOC record of the Sancerre core is a negative shift close to the top of the *P. margaritatus* ammonite Zone, which is also observed in both the Schandelah and Mochras cores (Peti et al. 2017; van de Schootbrugge et al. 2019; Storm et al. 2020). As no large amplitude δ¹³Cₗ TOC negative shift is visible in Core A of the Lower Saxony Basin, a substantial hiatus is inferred (Hiatus A?), and the lowermost part of the core tentatively assigned to the NJT5b, NJT5a and NJT4b nannofossil zones, corresponding to the time period equivalent to the *A. margaritatus* and upper *P. spinatum* ammonite zones. A similar pattern of truncation of upper Pliensbachian strata is inferred from carbon-isotope stratigraphy in other basins (Bodin et al. 2023).

Above the assigned Pliensbachian–Toarcian boundary at 48 m, a gradual change to more positive δ¹³Cₗ TOC values can be observed, reaching – 26‰ at the transition between the Amaltheenton and the Posidonia Shale, representative of an overall positive excursion up to 35 m. This overall positive excursion is interrupted by two negative shifts of – 3‰ and – 7‰, respectively, with the latter falling to
**Core A, Lower Saxony Basin, Germany**

*(This study, blue)*

<figure><img src="image_10.png" /><figcaption>**Fig. 7** Integrated stratigraphic record for Core A, with the d.¹³CₗTOC record from Core A, superimposed on the stratigraphic record of the Mochras core. Ammonite zonation from Ivimey-Cook (1971), Page</figcaption></figure>

**Mochras Borehole, Cardigan Bay Basin, UK**

(Xu et al. 2018a,b; Storm et al. 2020; grey/black)

<figure><img src="image_11.png" /></figure>

in Copestake and Johnston (2013) and Pieńkowski et al. (2024); carbon isotopes and magnetostratigraphy from Xu et al. (2018a,b) and (Storm et al. 2020); nannofossil zonation from Ferreira et al. (2019)

values of – 32‰. This positive excursion, with the superimposed ~– 7‰ negative shift, is here taken as the chemostratigraphic signature of the Toarcian Oceanic Anoxic Event (T-OAE). These two observed negative excursions in δ¹³CₗTOC are separated by even more positive δ¹³CₗTOC values and may reflect the stepped δ¹³CₗTOC record typical for the onset of the T-OAE negative carbon-isotope excursion (CIE), albeit probably intersected by multiple hiatuses. The overall condensed nature of this stratigraphic interval in the Lower Saxony Basin, combined with the occurrence of stratigraphic hiatuses and the lack of ammonite biostratigraphy, challenges correlation of specific steps in the δ¹³CₗTOC record to time-equivalent strata in sedimentary

archives at other localities. Nevertheless, the recognition of the T-OAE in this part of the core, based on the occurrence of very negative carbon-isotope values, superimposed on an overall long-term positive CIE is supported by nannofossil biostratigraphy. The First Occurrence of the nannofossil *C. superbus* at 46 m assigns the overlying stratigraphic interval to the NJT6 Zone (Ferreira et al. 2019).

The NJT6 Zone encapsulates the negative carbon-isotope excursion and TOC maximum characterizing the T-OAE in many Tethyan sections and In the Euro-Boreal domain (e.g., Mattioli et al. 2004; Menini et al. 2018, 2021; Ferreira et al. 2019), the *C. superbus* nannofossil Zone is thus proven to be reliable for inter-regional correlations. In the Euro-Boreal
a

<figure><img src="image_12.png" /></figure>

b

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
◄ **Fig. 8** a Cd/Mo versus Co*Mn concentrations and b Mo/TOC ratios of Posidonia Shale sediments in Core A (Dickson et al. 2022), compared to sedimentary values in present-day and Toarcian anoxic–euxinic depositional environments and to modern deepwater renewal times (Algeo and Lyons 2006; Fernandez-Martínez et al. 2023)

domain, the First Occurrence of *C. superbus* occurs in few cases at the base or within the *H. serpentinum* (equivalent to *H. falciferum*) ammonite Zone (e.g., the Schandelah core in North Germany; van de Schootbrugge et al. 2019). However, the diachroneity in the FO of *C. superbus* is only apparent and likely due to discrepancies in the sample density or in the ammonite record. In fact, in some localities the ammonite record is very scarce in the organic-rich interval. Alternatively, the occurrence of condensed levels or hiatuses in the lower Toarcian of many Tethyan sections may also account for some differences in terms of ranking of nannofossil bioevents, as discussed by Menini et al. (2018).

Carbon-isotope values remain relatively stable following the overall $\delta^{13}C_{TOC}$ positive excursion, similar to the trend observed in the Mochras core (Xu et al. 2018a), up to the top of the Posidonia Shale Formation and covering the NJT6 and NJT7 nannofossil zones. Bio- and chemostratigraphic correlation to the Mochras core suggests that the top of the preserved Posidonia Shale succession in the LSB correlates to a *H. bifrons* or *H. variabilis* biozone age (Fig. 7). Such an age is consistent with the one provided by nannofossil data. In fact, the NJT8 Zone correlates to the *H. (B). gradadata* (*H. variabilis*) ammonite Zone (Ferreira et al. 2019). At Dotternhausen, the Posidonia Shale also corresponds to the *C. superbus* (NJT6) and the *D. striatus* (NJT7) nannofossil biozones; however, the Posidonia Shale is much thinner there (~4.5 m of ‘Oil Shale’, overlain by ~5.5 m of ‘Bituminous Mudstone’ at Dotternhausen (Schmid-Röhl et al. 2002), compared to ~35 m in Core A.

The top of the Posidonia Shale in the LSB is characterized by a 2‰ positive shift in carbon-isotope values, condensed sediment, with abundant micro-bored belemnite debris and a hiatus (Hiatus D), with calcareous nannofossils assigning the strata of the overlying Jurensismergel to the *R. incompta* Zone (NJT8a) of middle to late(?) Toarcian age.

## Reduced sediment supply and erosion during the T-OAE in the Lower Saxony Basin

The Posidonia Shale is commonly considered to have formed coevally with the T-OAE and associated negative CIE in the Germanic basins, with an observed ~4 m thick negative CIE in ~14 m thick Posidonia Shale successions in locations in SW Germany (Rohl et al. 2001; Schmid-Röhl et al. 2002; Schwark and Frimmel 2004). In the Lower Saxony

Basin, however, the T-OAE negative CIE interval (including hiatuses) is stratigraphically restricted to the lower ~3 m of the Posidonia Shale Formation, which is on average ~35 m thick. This stratigraphy suggests that early Toarcian sedimentation rates in the SW Germanic basins were significantly reduced because of either limited accommodation space or sediment starvation, or both, whereas higher subsidence rates likely caused the deposition of a much thicker, and possibly more complete (bar the hiatus-riddled T-OAE interval) Posidonia Shale in the Lower Saxony Basin.

The onset of the T-OAE is commonly associated with the development of condensed sedimentation and/or the occurrence of stratigraphic gaps in geographically widespread sedimentary deposits such as the Neuquén Basin in Argentina (Al-Suwaidi et al. 2010, 2016), sections in France, Spain, Portugal, and the UK (Hesselbo and Jenkyns 1998; Gómez et al. 2008; Hesselbo 2008; Mailliot et al. 2009; Kemp et al. 2011; Sandoval et al. 2012; Suan et al. 2015; Martinez et al. 2017; Bodin et al. 2023), western Canada (Caruthers et al. 2011), as well as in northern Siberia (Suan et al. 2011; Ruebsam et al. 2020). The globally recognized condensed nature of sedimentation at the base of the T-OAE interval has been interpreted as a consequence of eustatic sea-level rise leading to sediment starvation in some basins (Hallam 1997; Al-Suwaidi et al. 2010; Pittet et al. 2014; Montero-Serrano et al. 2015; Thibault et al. 2018). However, such an interpretation may be complicated by local Early Jurassic tectonism. Short-term relative sea-level changes around Pliensbachian–Toarcian boundary time occurred superimposed on a late Pliensbachian sea-level low-stand that preceded major sea-level rise during the early Toarcian (e.g., Hallam 1997; Hesselbo and Jenkyns 1998; Hesselbo 2008; Haq 2017; Bodin et al. 2023). This inferred widespread, potentially largely global, sea-level evolution is corroborated by carbonate platforms showing initial subaerial exposure followed by early Toarcian drowning (Blomeier and Reijmer 1999; Woodfine et al. 2008; Léonide et al. 2012; Sandoval et al. 2012).

The Lower Saxony Basin sedimentary record suggests long-term relative sea-level rise from the depositional interval of the Amaltheenton Formation to the Posidonia Shale, with decreasing grain-size and the loss of thin turbiditic horizons and shell debris, as well as the more consistent occurrence of calcareous nannofossils. However, superimposed on the longer-term sedimentary evolution, inferred Hiatus A (at 45 m) and Hiatus B (43.5 m) suggest sediment winnowing, potentially from rapid relative sea-level fall during the earliest Toarcian and associated erosion or non-deposition, possibly modulated by irregular basin morphology and local bathymetric highs, causing the T-OAE to have only been partially recorded in the Lower Saxony Basin. However, the origin of hiatuses in mudstone is a long-standing point of debate going back for example to Fürsich
et al. (1992), Hallam (1999), Wetzel and Allia (2000), and Coe and Hesselbo (2000); condensation and erosion could, in principle, result from either sediment starvation due to sea-level rise, or increased energy on the seafloor due to sea-level fall, and it is often difficult to tell the causes apart.

Erosional and condensed horizons for a similar stratigraphic interval were also reported for the Schandelah core from northern Germany (van de Schootbrugge et al. 2019), as well as in basins further afield (Ruebsam et al. 2020), supporting the role of eustatic sea-level changes in controlling the local depositional environment. Subsequent relative sea-level rise at the onset of the T-OAE then led to the development of deeper water conditions. Combined, our data suggest that eustatic sea-level change, as well as local bathymetric conditions, played an important role in controlling sediment supply, deposition and erosion, particularly around the Pliensbachian–Toarcian transition and during the early Toarcian in the LSB, as recorded in Core A. In addition, global climatic and environmental forcing associated with the T-OAE controlled water-column chemistry (Dickson et al. 2017, 2022) and the deposition of the characteristic organic-rich black shales of the Posidonia Shale.

## Prolonged basin restriction and water-column anoxia as the cause for protracted Toarcian organic-matter burial in the Lower Saxony Basin

Deposition of the Posidonia Shale in the Lower Saxony Basin continued significantly after the T-OAE negative CIE, which suggests a prolonged period of perturbed environmental conditions promoted the deposition of organic-rich black shale long after the organic-rich sediment formation had ceased in other basins across the NW European seaway. High sedimentary TOC values through the T-OAE interval are commonly linked to elevated organic primary productivity in the surface waters inducing water-column anoxia–euxinia, and enhancing sedimentary organic matter preservation with, in addition, possibly reduced sedimentation rates (Kemp et al. 2022a, b; Ruebsam and Schwark 2024). Redox- and productivity-sensitive trace and major elemental concentrations (Co*Mn) and ratios (Cd/Mo) are commonly used to infer palaeo water-column redox states (Sweere et al. 2016; McArthur 2019). Utilizing available data from the Posidonia Shale in the studied core of the Lower Saxony Basin it appears that Toarcian organic-carbon burial here was largely driven by elevated preservation, under conditions of basin restriction (marked by water-column stratification, anoxia and elevated carbon burial fluxes) that were extreme relative to such modern-day settings (Fig. 8) (Algeo and Lyons 2006; Sweere et al. 2016; Dickson et al. 2022). The observed sedimentary molybdenum-isotope and Mo/TOC values in the wider Germanic Basin (Dickson et al. 2017) and the core studied here, respectively, suggest that the lower part

of the Posidonia Shale in the Lower Saxony Basin formed under anoxic–euxinic conditions with a relatively long (10²–10³ years) deep-water age or renewal time (Algeo and Lyons 2006; Dickson et al. 2022). Observed sedimentary Mo/TOC values for the upper part of the Posidonia Shale suggest an even more restricted environment, similar to the present-day Black Sea (Algeo and Lyons 2006; Dickson et al. 2022), in line with observed Mo/TOC values for lower Toarcian black shales in adjacent basins across the north European seaway (Thibault et al. 2018; Remirez and Algeo 2020a and b; Fernandez-Martínez et al. 2023). Importantly, the thermally early mature nature of the Posidonia Shale in core A may have enhanced the Mo/TOC ratio (Lewan et al. 1983; Dickson et al. 2020); therefore, the basinal deep-water age or renewal times suggested by the Mo/TOC values of the here studied Posidonia Shale sediments, and in comparison to modern-day marine sediment values, may in actuality have been longer. However, the global ocean Mo inventory (and by inference water-column Mo concentrations) were likely significantly lower during (parts of) the early Toarcian, impacting on the interpreted water-column renewal times for individual basins.

The overall Lower Saxony Basin history and persistence of anoxic conditions well after the T-OAE negative CIE was not limited to this basin alone, nor even the European epicontinental seaway, but has also been reported from, for example, the Cleveland Basin (U.K., e.g., Atkinson et al. 2023), the Western Canada Sedimentary Basin (Them et al. 2018) and, to some extent, the Paris Basin (van Breugel et al. 2006; Hermoso et al. 2013).

## Conclusions

High-resolution carbon-isotope stratigraphy combined with calcareous nanofossil biostratigraphy in Core A of the Lower Saxony Basin (LSB), Germany, constrains deposition of the studied sedimentary succession to the Lower Jurassic Pliensbachian and Toarcian stages. The upper Pliensbachian Amaltheenton Formation and Toarcian Posidonia Shale Formation in the Lower Saxony Basin record the Pliensbachian–Toarcian boundary event as well as the Toarcian oceanic anoxic event (T-OAE), and the associated carbon-isotope excursions (CIEs). Similarly to other sections in NW Europe, the onset of the T-OAE interval was characterized by condensed sedimentation and hiatuses, possibly because of a sea-level low-stand during the latest Pliensbachian and earliest Toarcian. The T-OAE negative CIE in core A of the LSB, albeit interrupted by hiatuses, is recorded over a 3-m stratigraphical interval at the base of the Posidonia Shale, with TOC values between 1% and 14%. This interval is stratigraphically followed by another ~27 m of organic-rich black shales of the Posidonia Shale Formation, with TOC

<figure><img src="image_15.png" /></figure>
values of on average ~10% (varying between 4% and 14.5%), that were laid down stratigraphically following the T-OAE. The observed pattern of sedimentation suggests protracted local perturbation of the basinal depositional environment, with hydrographic restriction and likely longer basin-water renewal times having contributed to continued, relatively elevated carbon burial in the Lower Saxony Basin, setting it apart from adjacent basins across the north European sectors of the Laurasian seaway.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00531-024-02477-9.

**Acknowledgements** This study was funded by Shell Global Solutions International B.V. ExxonMobil Production Deutschland GmbH provided access to core materials. RS was funded by the University of Exeter as part of the NERC Oil and Gas Doctoral Training College. We thank Mrs Ghislaine Broillet for preparation of smear slides for calcareous nannofossil analyses. We sincerely thank the guest-editor of this Special Issue, Christoph Hartkopf-Fröder, reviewer Mariano Remirez, and one anonymous reviewer for their thorough and constructive comments and suggestions, which significantly improved this manuscript. This is a contribution to UNESCO IGCP Project 739—The Mesozoic and Palaeogene Hyperthermal Events.

**Funding** This study was funded by Shell Global Solutions International B.V.

**Data availability** All here published data are available in the Online Supplementary Information files provided with this paper.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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