Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Cardigan Bay Basin, Wales

Abstract

The Late Pliensbachian Event (LPE), in the Early Jurassic, is associated with a perturbation in the global carbon cycle (positive carbon isotope excursion (CIE) of ∼2 ‰), cooling of ∼5 ∘C, and the deposition of widespread regressive facies. Cooling during the late Pliensbachian has been linked to enhanced organic matter burial and/or disruption of thermohaline ocean circulation due to a sea level lowstand of at least regional extent. Orbital forcing had a strong influence on the Pliensbachian environments and recent studies show that the terrestrial realm and the marine realm in and around the Cardigan Bay Basin, UK, were strongly influenced by orbital climate forcing. In the present study we build on the previously published data for long eccentricity cycle E459 ± 1 and extend the palaeoenvironmental record to include E458 ± 1. We explore the environmental and depositional changes on orbital timescales for the Llanbedr (Mochras Farm) core during the onset of the LPE. Clay mineralogy, X-ray fluorescence (XRF) elemental analysis, isotope ratio mass spectrometry, and palynology are combined to resolve systematic changes in erosion, weathering, fire, grain size, and riverine influx. Our results indicate distinctively different environments before and after the onset of the LPE positive CIE and show increased physical erosion relative to chemical weathering. We also identify five swings in the climate, in tandem with the 405 kyr eccentricity minima and maxima. Eccentricity maxima are linked to precessionally repeated occurrences of a semi-arid monsoonal climate with high fire activity and relatively coarser sediment from terrestrial runoff. In contrast, 405 kyr minima in the Mochras core are linked to a more persistent, annually wet climate, low fire activity, and relatively finer-grained deposits across multiple precession cycles. The onset of the LPE positive CIE did not impact the expression of the 405 kyr cycle in the proxy records; however, during the second pulse of heavier carbon (13C) enrichment, the clay minerals record a change from dominant chemical weathering to dominant physical erosion.

Full text

Clim. Past, 19, 979–997, 2023
https://doi.org/10.5194/cp-19-979-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
Environmental changes during the onset of the
Late Pliensbachian Event (Early Jurassic)
in the Cardigan Bay Basin, Wales
Teuntje P. Hollaar1,2, Stephen P. Hesselbo2,3, Jean-François Deconinck4, Magret Damaschke5, Clemens V. Ullmann2,3,
Mengjie Jiang2, and Claire M. Belcher1
1WildFIRE Lab, Global Systems Institute, University of Exeter, Exeter, EX4 4PS, UK
2Camborne School of Mines, Department of Earth and Environmental Sciences,
University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
3Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
4Biogéosciences, UMR 6282 CNRS, Université de Bourgogne/Franche-Comté, 21000 Dijon, France
5Core Scanning Facility, British Geological Survey, Keyworth, NG12 5GG, UK
Correspondence: Teuntje P. Hollaar (t.p.hollaar@exeter.ac.uk)
Received: 8 November 2022 – Discussion started: 30 November 2022
Revised: 24 March 2023 – Accepted: 7 April 2023 – Published: 15 May 2023
Abstract. The Late Pliensbachian Event (LPE), in the Early
Jurassic, is associated with a perturbation in the global car-
bon cycle (positive carbon isotope excursion (CIE) of ∼
2 ‰), cooling of ∼5◦C, and the deposition of widespread
regressive facies. Cooling during the late Pliensbachian has
been linked to enhanced organic matter burial and/or disrup-
tion of thermohaline ocean circulation due to a sea level low-
stand of at least regional extent. Orbital forcing had a strong
influence on the Pliensbachian environments and recent stud-
ies show that the terrestrial realm and the marine realm in
and around the Cardigan Bay Basin, UK, were strongly in-
fluenced by orbital climate forcing. In the present study we
build on the previously published data for long eccentricity
cycle E459 ±1 and extend the palaeoenvironmental record
to include E458 ±1. We explore the environmental and de-
positional changes on orbital timescales for the Llanbedr
(Mochras Farm) core during the onset of the LPE. Clay min-
eralogy, X-ray fluorescence (XRF) elemental analysis, iso-
tope ratio mass spectrometry, and palynology are combined
to resolve systematic changes in erosion, weathering, fire,
grain size, and riverine influx. Our results indicate distinc-
tively different environments before and after the onset of the
LPE positive CIE and show increased physical erosion rela-
tive to chemical weathering. We also identify five swings in
the climate, in tandem with the 405 kyr eccentricity minima
and maxima. Eccentricity maxima are linked to precession-
ally repeated occurrences of a semi-arid monsoonal climate
with high fire activity and relatively coarser sediment from
terrestrial runoff. In contrast, 405 kyr minima in the Mochras
core are linked to a more persistent, annually wet climate,
low fire activity, and relatively finer-grained deposits across
multiple precession cycles. The onset of the LPE positive
CIE did not impact the expression of the 405 kyr cycle in the
proxy records; however, during the second pulse of heavier
carbon (13C) enrichment, the clay minerals record a change
from dominant chemical weathering to dominant physical
erosion.
1 Introduction
The Early Jurassic is a period marked by large climatic fluc-
tuations and associated carbon isotope excursions (CIEs) in
an overall warmer than present and high-pCO2world (McEl-
wain et al., 2005; Korte and Hesselbo, 2011; Steinthorsdottir
and Vajda, 2015; Korte et al., 2015; Robinson et al., 2016).
A series of small and medium-sized CIEs have recently been
documented for the Sinemurian and Pliensbachian, which
have mainly been from European, North African, and North
American records (Korte and Hesselbo, 2011; Franceschi et
Published by Copernicus Publications on behalf of the European Geosciences Union.

980 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
al., 2014; Korte et al., 2015; Price et al., 2016; De Lena et al.,
2019; Hesselbo et al., 2020a; Mercuzot et al., 2020; Storm
et al., 2020; Silva et al., 2021; Cifer et al., 2022; Bodin et
al., 2023). Notable is the pronounced positive CIE in the late
Pliensbachian, which has been called the Late Pliensbachian
Event (LPE) and is linked to climatic cooling (Hesselbo and
Korte, 2011; Korte et al., 2015) and a supra-regional/global
sea level lowstand (Hallam, 1981; de Graciansky et al., 1998;
Hesselbo and Jenkyns, 1998; Hesselbo, 2008). The LPE has
been recognized by a positive shift in benthic marine oxy-
gen isotopes (∼1.5 ‰–2 ‰) (Bailey et al., 2003; Rosales et
al., 2004, 2006; Suan et al., 2010; Dera et al., 2011a; Ko-
rte and Hesselbo, 2011; Gómez et al., 2016; Alberti et al.,
2019, 2021), coeval with a positive shift in marine and ter-
restrial carbon isotopes (∼2 ‰) (Jenkyns and Clayton, 1986;
McArthur et al., 2000; Morettini et al., 2002; Quesada et al.,
2005; Rosales et al., 2006; Suan et al., 2010; Korte and Hes-
selbo, 2011; Silva et al., 2011; Gómez et al., 2016; De Lena
et al., 2019).
A cooler late Pliensbachian climate has been suggested
based on low pCO2values inferred by leaf stomatal in-
dex data from eastern Australia (Steinthorsdottir and Vajda,
2015), the presence of glendonites in northern Siberia (Ka-
plan, 1978; Price, 1999; Rogov and Zakharov, 2010), vegeta-
tion shifts from a diverse flora of different plant groups to one
mainly dominated by bryophytes in Siberia (Ilyina, 1985; Za-
kharov et al., 2006), and possible ice-rafted debris found in
Siberia (Price, 1999; Suan et al., 2011). Whilst the presence
of ice sheets is strongly debated, a general cooling period
(∼5◦C lower; Korte et al., 2015; Gómez et al., 2016) is evi-
dent from several temperature reconstructions from NW Eu-
rope. A cooling is hypothesized via enhanced carbon burial
in the marine sediments, leading to lower pCO2values and
initiating cooler climatic conditions (Jenkyns and Clayton,
1986; Suan et al., 2010; Silva et al., 2011; Storm et al.,
2020). Direct evidence of large-scale carbon burial in up-
per Pliensbachian marine deposits has not yet been docu-
mented (Silva et al., 2021). Alternatively, cooling has been
suggested to be caused by a lower sea level which would
have disrupted ocean circulation in the Laurasian Seaway,
reducing poleward heat transport from the tropics (Korte et
al., 2015). In the UK region, a dome structure in the North
Sea has been linked to the shedding of sediments during sea
level lowstands from the late Toarcian and possibly before
(Underhill and Partington, 1993; Korte et al., 2015; Archer
et al., 2019). Disruption of the ocean circulation between the
western Tethys and the Boreal realm is supported by ma-
rine migration patterns (Schweigert, 2005; Zakharov et al.,
2006; Bourillot et al., 2008; Nikitenko, 2008; Dera et al.,
2011; van de Schootbrugge et al., 2019) and numerical mod-
els (Bjerrum et al., 2001; Dera and Donnadieu, 2012; Ruval-
caba Baroni et al., 2018); however, the net direction of the
flows remains debated.
An additional factor to be considered is that a strong or-
bital control exists on the Pliensbachian sedimentary succes-
sions (Weedon and Jenkyns, 1990; Ruhl et al., 2016; Hin-
nov et al., 2018; Storm et al., 2020; Hollaar et al., 2021).
Previous studies have indicated that sea level changes, pos-
sibly coupled to glacioeustatic rise and fall, occurred during
the LPE on a 100 kyr (short eccentricity) timescale (Korte
and Hesselbo, 2011). A high-resolution record of charcoal,
clay mineralogy, bulk-organic carbon isotopes, total organic
carbon (TOC), and CaCO3encompassing approximately one
405 kyr cycle from the Llanbedr (Mochras Farm) borehole,
Cardigan Bay Basin, NW Wales, UK, suggested that the long
eccentricity orbital cycle had a significant effect on back-
ground climatic and environmental change during the late
Pliensbachian, particularly affecting the hydrological regime
of the region (Hollaar et al., 2021). This previous research fo-
cussed on orbital forcing of environmental change for a time
lacking any large excursion in δ13Corg and so unaffected by
perturbations to the global carbon cycle. Here, we expand on
the record of Hollaar et al. (2021) to cover two long eccen-
tricity cycles (which we identify as spanning the time from
cycle E459 ±1 to the start of E457 ±1 of Laskar et al., 2011,
and Laskar, 2020), where the final parts of E458 and the
start of E457 are interrupted by the onset of the Late Pliens-
bachian Event (Fig. 1). This longer record allows us to more
robustly examine the influence of the long eccentricity cycle
and the potential impact of a global carbon cycle perturbation
on the palaeoclimate and depositional environment. We find
that the long-eccentricity forcing continued to dictate the pre-
cise timing of major environmental changes in the Cardigan
Bay Basin, including the initial step of the positive carbon
isotope excursion.
2 Material
2.1 Palaeo-location and setting
Associated with the break-up of Pangaea, connections be-
tween oceans via epicontinental seaways were established
during the Early Jurassic, such as the Hispanic Corridor,
which connected the north-western Tethys and the east-
ern Panthalassa, and the Viking Corridor which linked the
north-western Tethys Ocean to the Boreal Sea (Sellwood and
Jenkyns, 1975; Smith, 1983; Bjerrum et al., 2001; Dambore-
nea et al., 2013). The linking passage of the NW Tethys
Ocean and the Boreal Sea (south of the Viking Corri-
dor) is the palaeo-geographical location of the Llanbedr
(Mochras Farm) borehole, Cardigan Bay Basin, NW Wales,
UK (Fig. 2) – referred to hereafter as Mochras. Due to the
location of the Mochras succession during the late Pliens-
bachian, it was subject to both polar and equatorial influences
allowing the study of variations in the circulation in the N–
S Laurasian Seaway (including the Viking Corridor) prior to
and across the LPE. Mochras was located at a mid-palaeo-
latitude of ∼35◦N (see Torsvik and Cocks, 2017).
The depositional environment of Mochras is likely char-
acterized by a rift setting, which is reflected by the relatively
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T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 981
Figure 1. Stratigraphic framework of the Mochras borehole. (a) The 405 kyr metronome (Laskar, 2020), which shows that this study spans
E459 ±1 and E458 ±1. (b) The 2.4 Myr and 405kyr filters derived from the δ13Corg record from Storm et al. (2020). A slight offset in
pacing is observed in the 405 kyr metronome based on an assumed fixed 405kyr cycle length (a), versus filtering of the 405 kyr signal from
the orbital solution (b).(c) δ13Corg curve from the Mochras borehole (Storm et al., 2020), showing the ∼1.8 ‰ +CIE that marks the LPE.
High-resolution data are visualized in light grey and a 10-step moving average in black. The blue bar marks interval in the Mochras borehole
considered in this study. The three grey shaded bars represent the three pulses in the positive CIE of the LPE. (d) Close-up of the δ13Corg
(Storm et al., 2020) and δ18Obulk and δ18Ofossil (Ullmann et al., 2022) from the late Pliensbachian of the Mochras core. A pre-LPE gradual
rise is recorded in the δ13Corg, followed by the initiation of the LPE positive CIE, which consists of three pulses. After the LPE positive CIE,
δ13Corg values drop, recorded starting at ∼910 m.b.s., and the Spinatum negative CIE is recorded. The δ18Obulk of the Mochras core (blue)
is diagenetically altered and unlikely to preserve a palaeoclimatic imprint (Ullmann et al., 2022). Also, shown are δ18Ofossil values (red).
open and deep marine facies and the evidence for below-
storm wave-base and contourite deposition (Pie´
nkowski et
al., 2021) but always with a strong terrestrial influence
(van de Schootbrugge et al., 2005; Riding et al., 2013) from
the nearby landmasses (Dobson and Whittington, 1987). The
Cardigan Bay Basin fill was downthrown against the early
Paleozoic Welsh Massif by a major normal fault system,
probably comprising the Bala, Mochras, and Tonfanau faults
at the eastern and south-eastern margins of the basin in late
Paleozoic–early Mesozoic time (Woodland, 1971; Tappin et
al., 1994). The main source of detrital material is understood
to be the Caledonian Welsh Massif, followed by the Irish and
Scottish landmasses (Deconinck et al., 2019). Other massifs
that could have influenced the provenance are the London–
Brabant Massif to the south-east and Cornubia to the south
(van de Schootbrugge et al., 2005), depending on the marine
circulation and sediment transport at the time.
2.2 Core location and material
The Llanbedr (Mochras Farm) borehole was drilled onshore
in the Cardigan Bay Basin (52◦4803200 N, 4◦0804400 W) in
1967–1969, North Wales (Woodland, 1971; Hesselbo et al.,
2013). The borehole recovered a 1300 m thick Early Jurassic
sequence (601.83–1906.78 m. b.s. – metres below surface),
yielding the most complete and extended Early Jurassic suc-
cession in the UK, being double the thickness of same-age
strata in other UK cores and outcrops (Hesselbo et al., 2013;
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982 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
Figure 2. Palaeo-location of the Mochras borehole in the context
of potential North Sea doming. Figure reprinted and adapted from
Korte et al. (2015), which is open access (https://creativecommons.
org/licenses/by/4.0/, last access: 1 November 2022). The Mochras
borehole was located at a palaeo-latitude of ∼35◦N in the Cardi-
gan Bay Basin (Torsvik and Cocks, 2017). Circulation in the Tethys
Ocean and between it and the Boreal region influenced the deposi-
tional environment of the Mochras core (Pie´
nkowski et al., 2021).
Late Pliensbachian sea level fall potentially resulted in the occlusion
of the Viking Corridor as the topography of the North Sea dome
structure disrupted circulation in the seaway (Korte et al., 2015).
Ruhl et al., 2016). The Lower Jurassic is biostratigraphi-
cally complete at the zonal level (Ivimey-Cook, 1971; Copes-
take and Johnson, 2014), with the top truncated and un-
conformably overlain by Cenozoic strata (Woodland, 1971;
Dobson and Whittington, 1987; Tappin et al., 1994; Hesselbo
et al., 2013). The lithology is dominated by argillaceous sed-
iments, with alternating muddy limestone, marl, and mud-
stone (Woodland, 1971; Sellwood and Jenkyns, 1975).
The Pliensbachian Stage in the Mochras borehole occurs
between ∼1250 and ∼865 m. b.s., with the Margaritatus
Zone between ∼1013 and 909 m. b.s. (Kevin N. Page in
Copestake and Johnson, 2014). The Pliensbachian interval
comprises alternations of mudstone (with moderate TOC)
and organic-poor limestones, with a pronounced cyclicity
at a ∼1±0.5 m wavelength (Ruhl et al., 2016). The up-
per Pliensbachian contains intervals that are silty and locally
sandy, whilst levels of relative organic enrichment also occur
through the Pliensbachian (Ruhl et al., 2016). Overall, the
upper Pliensbachian is relatively rich in carbonate (Ruhl et
al., 2016; Ullmann et al., 2022).
3 Methods
For this study, samples were taken at a ∼30 cm resolution
from slabbed core from 934–918 m. b.s. for X-ray diffrac-
tion (XRD) and mass spectrometry, as well as palynofacies
and microcharcoal analysis. X-ray fluorescence (XRF) anal-
yses were made at a 1 cm resolution from 934–918 m. b.s.
(complete dataset deposited as Damaschke et al., 2021).
These new samples complement samples and data at a
10 cm resolution from 951–934 m. b.s. published in Hollaar
et al. (2021).
3.1 TOC, CaCO3, and bulk organic carbon isotope
mass spectrometry
TOC and δ13Corg were measured to track the changes in the
total organic fraction and the bulk organic carbon isotope ra-
tios in relation to the other palaeoenvironmental proxy data.
Powdered bulk rock samples (∼2 g) were decarbonated
in 50 mL of 3.3 % HCl. After this, the samples were trans-
ferred to a hot bath of 79 ◦C for 1 h to remove siderite and
dolomite. Subsequently, the samples were centrifuged and
the liquid decanted. The samples were rinsed repeatedly with
distilled water to reach neutral pH. After this, the samples
were oven-dried at 40◦C, re-powdered, and weighed into tin
capsules for mass spectrometry using the Sercon Integra 2
stable isotope analyser at the University of Exeter Environ-
ment and Sustainability Institute (ESI), stable isotope facility
on the Penryn Campus, Cornwall. Samples were run along-
side in-house reference material (bovine liver: δ13C−28.61;
alanine: δ13C−19.62) which was used to correct for instru-
ment drift and to determine the δ13C values of the samples.
The δ13Corg values are reported relative to V-PDB (Vienna
Peedee Belemnite) following a within-run laboratory stan-
dard calibration. Total organic carbon was determined us-
ing the CO2beam area relative to the bovine liver standard
(% C 47.24). Replicate analysis of the in-house standards
gave a precision of ±<0.1 ‰ (2 SD).
The carbonate content was measured by the dry weight
sample loss before and after decarbonation. The carbon per-
centage content derived from the mass spectrometer was cor-
rected for carbonate loss to derive TOC.
3.2 XRD to determine clay mineralogy
Clay mineral analysis was performed to gain insight into
the relative importance of physical erosion versus chemical
weathering and related changes in the hydrological cycle.
About 2–3 g of gently powdered bulk rock was decar-
bonated with a 0.2 M HCl solution. The clay sized fraction
(<2 µm) was extracted with a syringe after decantation of
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T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 983
the suspension after 95 min following Stokes’ law. The ex-
tracted fraction was centrifuged and oriented on glass slides
for XRD analysis using a Bruker D4 Endeavour diffrac-
tometer (Bruker, Billerica, MA, USA) with Cu Kαradi-
ations, LynxEye detector, and Ni filter under 40 kV volt-
age and 25 mA intensity (Biogéosciences Laboratory, Uni-
versité Bourgogne/Franche-Comté, Dijon). Following Moore
and Reynolds (1997), the clay phases were discriminated
in three runs per sample: (1) air-drying at room tempera-
ture; (2) ethylene–glycol solvation for 24h under vacuum;
(3) heating at 490 ◦C for 2 h.
Identification of the clay minerals was based on their main
diffraction peaks and on comparison of the three diffrac-
tograms obtained. The proportion of each clay mineral on
glycolated diffractograms was measured using the MACD-
IFF 4.2.5. software (Petschick, 2000). Identification of the
clay minerals follows the methods in Deconinck et al. (2019)
and Moore and Reynolds (1997).
3.3 Palynofacies and microcharcoal
Palynofacies were examined to explore shifts in the terrestrial
versus marine origins of the particulate organic matter. Each
∼20 g bulk rock sample was split into 0.5cm3fragments,
minimizing breakage of charcoal and other particles, to op-
timize the surface area for the extraction of organic matter
using a palynological acid maceration technique. The sam-
ples were first treated with cold hydrochloric acid (10 % and
37 % HCl) to remove carbonates. Following this, hydrofluo-
ric acid (40 % HF) was added to the samples to remove sili-
cates. Carbonate precipitation was prevented, by adding cold
concentrated HCl (37 %) after 48 h. The samples were neu-
tralized via multiple dilution–settling–decanting cycles using
DI (distilled) water, after which five droplets of the mixed
residue were taken for the analysis of palynofacies prior to
sieving. The remaining residue was sieved through a 125 and
10 µm mesh to extract the microcharcoal fraction.
A known quantity (11 µL) out of a known volume of liquid
containing the 10–125 µm sieved residue was mounted onto
a palynological slide using glycerine jelly. This fraction, con-
taining the microscopic charcoal, was analysed and the char-
coal particles counted using an Olympus (BX53) transmitted
light microscope (40 ×10 magnification). For each palyno-
logical slide, four transects (two transects in the middle and
one on the left and right side of the coverslip) were followed
and the number of charcoal particles determined. Charcoal
particles were identified with the following criteria: opaque
and black, often elongated lath-like shape with sharp edges,
original anatomy preserved, and a brittle appearance with a
lustrous shine (Scott, 2010). These data were then scaled up
to the known quantity of the sample according to the method
of Belcher et al. (2005).
Palynofacies were grouped broadly according to Oboh-
Ikuenobe et al. (2005): sporomorphs, fungal remains, fresh-
water algae, marine palynomorphs, structured phytoclasts,
unstructured phytoclasts, black debris, amorphous organic
matter (AOM), and charcoal (further described in Hollaar et
al., 2021). The palynofacies were quantified on a palynolog-
ical slide using the optical light microscope (40 ×10 mag-
nification) and counting a minimum of 300 particles per
slide. Because the samples are AOM-dominated, counting
was continued until a minimum of 100 non-AOM particles
were observed. We used the percentage of terrestrial phyto-
clasts, which includes sporomorphs, and structured and un-
structured phytoclasts to examine changes in terrestrial or-
ganic particle content.
3.4 XRF to determine detrital elements
Detrital elemental ratios were examined to analyse changes
in relative terrestrial influx and the type of material trans-
ported from the land to the marine realm. The slabbed archive
halves of the Mochras borehole were scanned via automated
XRF at a 1 cm resolution for the interval 951–918 m. b.s.,
with the The Itrax multi-core (MC) at the British Geological
Survey Core Scanning Facility (CSF), Keyworth, UK (Dam-
aschke et al., 2021). The measurement window was 10s and
long-term drift in the measurement values was counteracted
by regular internal calibration with a glass reference (NIST-
610). Duplicate measurements were taken every 5m for a
50 cm interval to additionally verify the measured results.
3.5 Statistical analysis
Principal component analysis (PCA) was performed to ex-
amine a potential change in the proxy data before and after
the positive CIE. This was executed in the software PAST
on the normalized dataset including microcharcoal, TOC,
CaCO3,δ13Corg, S/I, K/I, primary clay mineralogy, Si/Al,
and Zr/Rb. The samples before the positive CIE (951.0–
930. m. b.s.) and the samples after the positive CIE (930.3–
918.0 m. b.s.) are grouped to examine a potential difference
in the sedimentary composition before and after the posi-
tive CIE. A Pearson’s correlation was executed in MATLAB
R2017b. The pvalue tests the hypothesis of no correlation
against the alternative hypothesis of a positive or negative
correlation (significance level at p≤0.05).
4 Results
4.1 TOC, CaCO3, and bulk organic carbon isotope ratio
mass spectrometry
Alternating TOC-enhanced and Ca-rich lithological couplets
occur on a metre scale through the studied interval, with TOC
and CaCO3having a strong negative correlation (r= −0.64,
p=0.001). TOC content fluctuates in the range of 0.17–
1.72 wt % (mean 0.8 wt %), and the highest fluctuations in
TOC content are found from 939–930 m. b.s. The CaCO3
content fluctuates in opposition to TOC and varies between
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984 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
Figure 3. Detrital ratios over the Ca-rich and TOC-enhanced lithological couplets for the studied interval. Overview of Ca (black, derived
from Ruhl et al., 2016), CaCO3(blue), and TOC content of the studied interval 951–918m. b.s. The grey shading represents the TOC-
enhanced beds and the unshaded bands mark the Ca-rich (limestone) beds. The detrital ratios reflect the silt to fine sand fraction (Si, Zr)
versus the clay fraction (Rb, Al, K). Two increasing-upward cycles are observed in the Si/Al and Zr/Rb ratios. The pattern observed in all
detrital ratios (except Ti/Al) is similar and likely reflects overall upward coarsening.
14 % and 89 %. The studied interval is generally high in
CaCO3(mean 58 %) (Fig. 3). The δ13Corg displays a minor
(∼0.5 ‰) shift towards more positive values at ∼944 m. b.s.
(as reported in Storm et al., 2020; Hollaar et al., 2021). At
∼930 m. b.s. an abrupt shift of ∼1.8 ‰ (Figs. 1 and 4; Storm
et al., 2020) indicates the onset of the Late Pliensbachian
Event (LPE) in the Mochras core. In agreement with this, the
results of the present study show a shift from ∼ −27 ‰ to
∼ −25.15 ‰ between 930.8 and 930.4 m. b.s. (Fig. 4). The
δ13Corg data presented here have been divided into three
phases: the pre-LPE gradual rise, followed by the positive
CIE, which is subdivided into pulses 1, 2, and 3 (Fig. 1).
After the onset of the positive δ13Corg excursion, the TOC
content drops to the lowest values (from 0.85% before and
0.6 % after the positive CIE on average), but the 1 m fluc-
tuations continue (Figs. 3 and 4). No overall change in the
CaCO3content is observed through the positive carbon iso-
tope excursion (Fig. 3).
4.2 Clay minerals
XRD analysis shows that the main clay types found in
this interval are illite, random illite–smectite mixed lay-
ers (I-S R0) (hereafter referred to as smectite), and kaoli-
nite. Illite and kaolinite co-fluctuate in the interval studied
here and are directly out of phase with smectite abundance.
Chlorite and I-S R1 are present in minor proportions but
reach sporadically higher relative abundance (>10 %) from
∼932 m. b.s. upwards, with sustained >10 % abundance at
∼925–918 m. b.s. (Figs. 4 and S1 in the Supplement). The
relative abundances of smectite and illite and of kaolinite and
illite are expressed by the ratio S/I and K/I, respectively.
These ratios were calculated according to the intensity of the
main diffraction peak of each mineral.
4.3 Organic matter
The type of particulate organic matter, and more specifi-
cally the abundance in the marine versus terrestrial origin
of the particles, fluctuates on a metre scale from 18 %–42 %
(Figs. 4 and S2). Palynofacies indicate that the type of or-
ganic matter does not change in relation to the metre-scale
lithological facies cycles (no correlation between percent-
age terrestrial phytoclasts and TOC or CaCO3). No large
and abrupt changes are recorded in the terrestrial/marine
proportions, but the proportion of terrestrial phytoclasts has
four high phases: between 944.6 and 942.0 m. b.s., 937.5
and 934.9 m. b.s., 930.4 and 925.4m. b.s., and 920.3 and
918.0 m. b.s. (Fig. S2). The first and second high phase falls
within the +0.5 ‰ positive swing in the δ13Corg, whilst the
latter two high phases correspond to pulse 1 and pulse 2 in
the positive CIE. Amorphous organic matter (AOM) is very
abundant, followed by unstructured phytoclasts, with lower
proportions of structured phytoclasts and charcoal (Fig. S3).
Charcoal particles make up a relatively large proportion of
the terrestrial particulate organic matter (∼10 % on average)
and ∼3.5 % on average of the total particulate organic mat-
ter fraction (Fig. S3). Only sparse marine and terrestrial pa-
lynomorphs were observed (Fig. S3).
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T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 985
Figure 4. Synthesis diagram showing the climatic swings observed in tandem with the long eccentricity cycle. The studied interval (upper
Pliensbachian Margaritatus Zone) comprises part of the pre-LPE gradual rise, the initiation of the LPE positive CIE, and pulses 1 and 2
(δ13Corg data in black from this study and in light grey from Storm et al., 2020). Five climatic phases (A–E) are interpreted from Si/Al,
smectite / illite, kaolinite / illite, chlorite, and I-S R1 abundance, and the microcharcoal abundance. In tandem with the 405 kyr cycle (Storm
et al., 2020), the climatic state of a year-round wet climate, low fire activity, and fine-grained sediments across multiple precession cycles
(phases A and C) alternates with a climatic state that includes repeated precessionally driven states that are semi-arid, with high fire activity
and coarser sediments (phases B and D). The top of the record (phase E) indicates increased physical erosion (chlorite +I-S R1, kaolinite)
relative to chemical weathering. In the terrestrial phytoclast column, the grey line shows the 10-step moving average.
To assess the character of the observed fluctuations in mi-
crocharcoal abundance, whether changes in microcharcoal
can be related to enhanced runoff from the land and/or or-
ganic preservation, or whether the microcharcoal signifies
changes in fire activity on land, the charcoal record was
corrected for detrital influx. We adjust the charcoal parti-
cle abundances using the XRF elemental record, normalizing
to the total terrigenous influx following Daniau et al. (2013)
and Hollaar et al. (2021). The stratigraphic trends in the nor-
malized microcharcoal for Eter/Ca, Si/Al, Ti/Al, and Fe/Al
remain the same (Fig. S4). The absolute number of mi-
crocharcoal particles decreases, with 1.06 ×105per 10 g of
raw mean charcoal particles and Eter/Ca normalized mean
9.7×104number of charcoal particles (hereafter denoted n)
per 10 g, Ti/Al normalized 6.4×104nper 10g, Si/Al nor-
malized 7.7×104nper 10 g, and Fe/Al normalized 9.8×104
mean number of microcharcoal particles per 10 g (Fig. S4).
The number of microcharcoal particles per 10 g of processed
rock decreases when correcting for terrestrial runoff changes,
hinting that perhaps part of the “background” microcharcoal
is related to terrestrial influx; the normalization also shows
that the observed patterns in microcharcoal abundances are
not influenced by changes in terrestrial runoff and taphon-
omy. Hence, the highs and lows in the microcharcoal record
can be interpreted as representing changes in the fire regime
on land. The microcharcoal abundance fluctuates strongly in
the record presented here; however, no clear difference in mi-
crocharcoal content has been observed before and after the
onset of the positive CIE.
4.4 Detrital elemental ratios (XRF)
Strong similarities are observed between the fluctuating ra-
tios of Si/Al, Si/K, Zr/Rb, Zr/Al, and Zr/K (Fig. 3). The
elements Al, Rb, and K sit principally in the clay fraction
(e.g. Calvert and Pederson, 2007), whereas Si and Zr are of-
ten found in greater abundance in the coarser fraction related
to silt and sand grade quartz and heavy minerals (Calvert
and Pederson, 2007). The ratios all show clear metre-scale
fluctuations, and these are superimposed on two increasing-
upward trends observed in both Si/Al and Zr/Rb, followed
by a drop and rise to peak values in the latest part of phase D
and phase E above the onset of the positive CIE (Figs. 3
and 4). A parallel trend is observed between the clay ratios
(XRD) and elemental ratios Si/Al and Zr/Rb (Fig. 3). Phases
of high S/I correspond to the peaks in the two coarsening-
upward sequences, whereas phases of high K/I correspond
to the low phases in the two coarsening-upward sequences.
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986 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
After the positive CIE onset (in phase E) this relationship
turns around, and an enrichment in the kaolinite /illite ratio
corresponds to the elemental ratios, where highest kaolinite
relative abundance is observed in parallel with elemental ra-
tios suggesting a maximum coarse fraction.
4.5 PCA
The proxy datasets (δ13Corg, TOC, percentage of terrestrial
phytoclasts, microcharcoal, smectite / illite, kaolinite / illite,
abundance of chlorite and I-S R1, Si/Al, Zr/Rb, Zr/Al) were
normalized between 0–1 and run for PCA in PAST. Sixty-
four percent of the variance is explained by the first three
axes (PCA-1 27.7 %, PCA-2 19.7 %, PCA-3 15.3 %) inside
the 95 % confidence interval.
PC-1 (PC – principal component) mainly explains the
anti-correlation of TOC and CaCO3. PC-2 shows the anti-
correlation of K/I and S/I. Positive loadings were observed
for S/I, microcharcoal, macrocharcoal, and CaCO3. For PC-
2, negative loadings were observed for K/I, and abundance
of chlorite and I-S R1. PC-3 shows strong positive loadings
(>0.3) for δ13Corg, Si/Al, and Zr/Al.
Plotting PC-1 (yaxis) over PC-3 (xaxis) shows that after
the onset of the positive CIE the samples are grouped to the
top of the yaxis (more associated with S/I compared to K/I)
and to the right of the xaxis (more associated with primary
minerals, phytoclasts, and higher Si/Al, Zr/Rb, and Zr/Al)
(Fig. 5).
5 Discussion
Figure 1 provides the context for the LPE “cooling event”
at Mochras set within the background record. Shifts in bulk
δ18Ocarb are coeval to the δ13Corg change to heavier isotopic
values (∼930 m. b.s.) and reach a maximum in the Margari-
tatus Zone (>1 ‰) (Ullmann et al., 2022). The bulk oxygen-
isotope excursions of Mochras are affected by diagenesis
and are deemed unlikely to reflect environmental conditions
(Ullmann et al., 2022). However, oxygen-isotope data from
marine benthic and nektobenthic molluscs and brachiopods
show heavier values during the late Margaritatus Zone con-
current with a positive shift in δ13Corg, indicating cooling
during the LPE in the nearby Cleveland Basin (Robin Hood’s
Bay and Staithes) (Korte and Hesselbo, 2011), and this trend
is also observed in several European sections (e.g. Korte et
al., 2015). The duration of the positive CIE has been esti-
mated as ∼0.4–0.6 Myr in the Cardigan Bay Basin (Ruhl et
al., 2016; Storm et al., 2020; Pie´
nkowski et al., 2021).
5.1 Background sedimentological and environmental
variations
The Mochras succession shows metre-scale, alternating
TOC-enhanced and Ca-rich lithological couplets (mud-
stone/limestone; Fig. 3). Previous assessments of the
palaeoenvironmental signature of these TOC-enhanced and
Ca-rich couplets indicate strongly that the different deposi-
tional modes are driven by orbital precession (Ruhl et al.,
2016; Hinnov et al., 2018; Storm et al., 2020; Hollaar et al.,
2021; Pie´
nkowski et al., 2021). Precession-driven changes
in monsoonal strength have been suggested to influence the
deposition and preservation of TOC and carbonate in the
Cardigan Bay Basin (Ruhl et al., 2016), although the im-
pact may have been expressed, at least partially, by changes
in the strength of bottom currents in the seaway as a whole
(Pie´
nkowski et al., 2021).
The preservation of primary carbonate is poor in the
Mochras borehole, making it complex to determine in detail
the relative importance of carbonate producers for the bulk
carbonate content (Ullmann et al., 2022). However, Early
Jurassic, pelagic settings in the Tethys region often received
abiotic fine-grained carbonate from shallow marine carbon-
ate platforms (Weedon, 1986; Cobianchi and Picotti, 2021;
Krencker et al., 2020) and partly via carbonate producing
organisms (such as coccolithophores in zooplankton pellets)
(Weedon, 1986; van de Schootbrugge et al., 2005, e.g. Wee-
don et al., 2019; Slater et al., 2022). Coccolithophores are of-
ten poorly preserved and recrystallized (Weedon, 1986; Wee-
don et al., 2019; Slater et al., 2022). The organic matter found
in the studied section of the Mochras borehole varies be-
tween 18 % and 42 % of terrestrial phytoclasts (Fig. 4). Phy-
toclasts are common, but palynomorphs are relatively sparse
and poorly preserved. Marine amorphous organic matter is
the main constituent in the present study of particulate or-
ganic matter in unsieved macerated samples in the interval
studied here (951–918 m. b.s.). An examination of variations
in the terrestrial/marine proportions of organic matter shows
no correspondence between the type of organic matter and
the TOC-enhanced or Ca-rich lithological alternations. How-
ever, previous research has indicated that the percentage of
terrestrial phytoclasts shows precession forcing independent
of the lithological couplets (so out of phase with precession-
scale changes in Ca-TOC content) between 951–934 m. b.s.
in the Mochras core (Hollaar et al., 2021). Such orbital forc-
ing of the terrestrial vs. marine proportions of organic mat-
ter was also found in Early Jurassic sediments of Dorset and
was similarly independent of the lithological facies (Water-
house, 1999). Terrestrial phytoclastcontent shows a weak ex-
pression of long-eccentricity-driven variations in the section
studied (Fig. 4).
Fossil charcoal makes up a substantial proportion of the or-
ganic fractions (11 % of the terrestrial fraction) and has pre-
viously been shown to vary considerably over long eccentric-
ity cycle 459 ±1 peaking in abundance during the phase of
maximum eccentricity (Hollaar et al., 2021). Micro-charcoal
also appears to be most abundant during the maximum phase
of the subsequent long eccentricity cycle 458 ±1 (Fig. 4).
Additionally, K/I and S/I clay mineral ratios appear to al-
ternate in response to long-eccentricity drivers (Fig. 4) up to
931 m. b.s., where the clay mineral signature changes. De-
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T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 987
Figure 5. PCA shows a distinctly different depositional signature before and after the onset of the LPE positive CIE in the Mochras core.
PCA plot of PC-1 and PC-3: all samples before the onset of the LPE positive CIE are marked by closed black circles and the samples after
the onset of the LPE positive CIE are marked by open blue squares.
trital clays form in soil weathering profiles and/or physical
weathering of bedrock. Chemical weathering is enhanced in
a high-humidity environment with relatively high tempera-
tures and rainfall, when clays are formed in the first stages of
soil development. In the modern day, kaolinite is primarily
formed in tropical soils, under year-round rainfall and high
temperatures (Thiry, 2000). Smectite also occurs in the trop-
ics but is more common in the subtropical to Mediterranean
regions, where humidity is still high but periods of drought
also occur (Thiry, 2000). Hence, smectite forms predomi-
nantly in soil profiles under a warm and seasonally dry cli-
mate (Chamley, 1989; Raucsik and Varga, 2008) and kaoli-
nite in a year-round humid climate (Chamley, 1989; Ruffell
et al., 2002). Similarly, alternating intervals of kaolinite and
smectite dominance were observed for the late Sinemurian
(Munier et al., 2021) and the Pliensbachian of Mochras (De-
coninck et al., 2019).
The predominantly detrital character of these clay min-
erals has been confirmed by TEM (transmission electron
microscopy) scanning of Pliensbachian smectite minerals,
which revealed the fleecy morphology and lack of over-
growth (Deconinck et al., 2019). Therefore, the alternations
of smectite and kaolinite are interpreted as reflecting palaeo-
climatic signatures of a changing hydrological cycle, with a
year-round wet climate evidenced by high K/I ratios and a
more monsoon-like climate with seasonal rainfall with high
S/I (Deconinck et al., 2019; Hollaar et al., 2021; Munier et
al., 2021) (see Figs. 3 and 6). The intervals with a signal
for weaker seasons appear to correspond to phases of low
eccentricity in the 405 kyr cycle and signals of greater sea-
sonality with periods of high more pronounced eccentricity
(Fig. 4) in the 405 kyr cycle. Between 951 and 930m. b.s.
high K/I occurs during phases of low long eccentricity sug-
gesting an enhanced hydrological cycle (Hollaar et al., 2021)
with more intense weathering and enhanced fine-grained ter-
restrial runoff to the marine record (Deconinck et al., 2019).
In contrast, phases of maximum long eccentricity appear to
be smectite-rich, indicating seasonal rainfall, enhanced fire
(Hollaar et al., 2021) and thus periods of droughts, and lower
terrestrial runoff and subsequent lower dilution (Deconinck
et al., 2019).
Detrital elemental ratios increase accordingly during the
smectite-rich phases and are lower during kaolinite-rich
phases between 951 and 930 m. b.s. Detrital elemental ra-
tios can be used to explore changes in sediment composi-
tion (e.g. Thibault et al., 2018; Hesselbo et al., 2020b), and
the similarity of the long-term trend in Zr/Rb and Si/Al
(Fig. 3) indicates that these elemental ratios reflect grain
size. The clay fraction (hosting Al, and Rb; Chen et al.,
1999), diminishes upwards, whereas the coarser silt to sand
fraction (associated with Si: Hesselbo et al., 2020b; associ-
ated with Zr: Chen et al., 2006), increases upward (Figs. 3
and 4). The grain size changes inferred here reflect two
overall coarsening-upward sequences (Figs. 3 and 4). These
sequences may reflect changes in clastic transport due to
changes in the proximity to the shore/siliciclastic source,
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988 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
Figure 6. Scheme of four environmental scenarios under the influence of eccentricity on a precessional timescale. (a) The most extreme sea-
sonal contrast in the Northern Hemisphere occurs during maximum precessional forcing (i.e. low-precession index) and maximum amplitude
modulation by eccentricity. The seasonal contrast leads to a wet season that allows biomass to build up, high terrestrial runoff, and relatively
enhanced organic burial in marine settings. During the dry season, fuel moisture levels are lower and fires are rapidly ignited and spread.
Intensified monsoonal rains may lead to enhanced coarse-grained terrestrial runoff. Overall, less terrestrial runoff during this dry season
results in less dilution of carbonate production and/or less primary productivity of organic plankton. (b) Minimum precessional forcing and
maximum amplitude modulation of eccentricity leads to the least seasonal contrast. Chemical weathering on land is more intense during this
year-round humid climate. And although biomass is abundant, fire is suppressed due to the high moisture status. Both seasons are humid
and have considerable terrestrial runoff, resulting in marine organic burial. (c) Moderate seasonality occurs during maximum precessional
forcing and minimum amplitude modulation of the eccentricity cycle. During the wet season biomass grows, and during the dry season fires
can occur due to drier fuel conditions. However, due to a lesser seasonal contrast, the dry conditions are less pronounced and fire is not
widespread. Runoff includes coarse- and fine-grained sediments and charcoal during the dry season. (d) The seasonal contrast is low during
minimum precessional forcing and the minimum amplitude modulation of the eccentricity cycle. Both seasons were humid and experienced
runoff of fine-grained sediments and organic burial in marine settings. Moderately thick soil profiles could develop under this humid climate
(figure developed from Martinez and Dera, 2015).
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T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 989
changes in runoff due to a changing hydrological cycle,
changes in the intensity of weathering of the bedrock, or ac-
celerated bottom currents with a greater carrying capacity for
coarser sediments.
5.2 Depositional and environmental changes before and
after the LPE positive CIE
5.2.1 Climate forcing of the hydrological cycle
The LPE positive CIE begins around 930m. b.s. in the
Mochras core and encompasses the remaining part of the
studied section (Fig. 4). We contrasted all the pre-CIE sedi-
ment signatures with those of the positive CIE signatures us-
ing principal components analysis, which indicates distinctly
different sedimentary composition and environmental signa-
ture before and after the onset of the positive CIE in Mochras
(Fig. 5).
Before the positive CIE onset, the clay mineral assem-
blage shows alternating phases of smectite and kaolinite, in-
dicating pedogenic weathering. The relative abundance of
the detrital clay types observed in the studied interval has
the potential to hold important palaeoclimatic information
regarding the hydrological cycle and the relative proportion
of chemical weathering and physical erosion. The hydrolog-
ical cycle was forced by the 405 kyr eccentricity before the
positive CIE, with alternating eccentricity maxima linked to
enhanced seasonality (smectite) and eccentricity minima to
an equitable wet climate (kaolinite) (Figs. 3 and 6). Higher-
frequency cycles are not observed in the clay mineral ratios,
with no precession or obliquity forcing detected in the high-
resolution part of the study at 951–934 m.b.s. (Hollaar et al.,
2021) and no expression of the 100 kyr cycle in the record
presented here. The formation of developed kaolinite-rich,
and to a lesser extent smectite-rich soil profiles, requires a
steady landscape for many tens of thousands of years, al-
though the ∼1 Myr timescale of Thiry (2000) seems exces-
sive in our case given the clear expression of clay mineral
changes through long eccentricity cycles. Also, the trans-
portation and deposition of continental clays will occur af-
ter soil formation and add further time between formation
and final deposition (Chamley, 1989; Thiry, 2000). Thus,
there is likely to be a lag of the climatic signal observed in
the marine sediments (Chamley, 1989; Thiry, 2000). How-
ever, we note that high-frequency climatic swings have been
recorded in the clay mineral record in some instances, such
as in the Lower Cretaceous in SE Spain (Moiroud et al.,
2012). The limestone–marl alternations there are enhanced
in smectite versus kaolinite and illite, respectively, reflecting
precession-scale swings from a semi-arid to a tropical hu-
mid climate (Moiroud et al., 2012). Precession and higher-
frequency shifts in the clay record are likely caused by fluc-
tuations in runoff conditions rather than the formation of soils
with a different clay fraction.
Directly after the initial positive CIE shift from 930–
924 m. b.s. (Phase 1 of Fig. 4) little seems to change, and
the system evidently continued to respond as before to the
long-eccentricity forcing, despite the predicted cooling (Ko-
rte and Hesselbo, 2011; Korte et al., 2015; Gómez et al.,
2016). However, from around 924 m. b.s. up to the top of the
studied section (Phase 2 of Fig. 4) the clay mineral assem-
blage displays a distinctly different composition, with kaoli-
nite dominating especially the early part of phase 2 of the
LPE (Fig. 4). At the same time there is an enhancement of
the primary minerals illite and chlorite and I-S R1 (Figs. 4
and S1). Although an enhancement in detrital kaolinite indi-
cates an acceleration of the hydrological cycle, detrital kaoli-
nite is dual in origin and can also be derived from a reworking
of the primary source material (Deconinck et al., 2019). If the
climate is cooler, chemical weathering becomes less domi-
nant and physical erosion of the bedrock becomes the main
detrital source of clay minerals. In the Cardigan Bay Basin,
the bedrock of the surrounding Variscan massifs (such as the
Scottish, Welsh, and Irish massifs) were a likely source of
these clays. In the Early Jurassic of the NW Tethys region,
lower Paleozoic mudrocks bearing mica illite and chlorite
were emergent (Merriman, 2006; Deconinck et al., 2019);
hence the enhancement of illite and chlorite likely indicates
physical erosion in the region surrounding the study site.
Finally, authigenic clay particles could have been formed
during burial diagenesis. At temperatures between 60–70 ◦C
smectite illitization occurs and I-S R1 is formed; however,
the high abundance of smectite in Mochras indicates limited
burial diagenesis at that location (Deconinck et al., 2019).
Weak to moderate thermal diagenesis is confirmed for the
Pliensbachian of Mochras, with Tmax from pyrolysis analysis
between 421 and 434 ◦C (van de Schootbrugge et al., 2005;
Storm et al., 2020). Therefore, I-S R1 in Mochras is inter-
preted as being derived from chemical weathering of illite
(Deconinck et al., 2019). The coeval increase in these pri-
mary clay minerals, I-S R1, and kaolinite indicates that dur-
ing this period physical erosion dominated over soil chemi-
cal weathering (Deconinck et al., 2019; Munier et al., 2021).
This is similar to what has been observed for the latest Pliens-
bachian in Mochras previously (Deconinck et al., 2019).
Erosion of weathering profiles transports clay minerals
(including kaolinite and smectite) to the marine realm. In the
ocean, the differential settling of kaolinite (near the shore)
and smectite (more distal) could occur based on the morphol-
ogy and size of clay particles (Thiry, 2000). However, a com-
parison of long-term inferred regional sea level changes from
surrounding UK basins (Hesselbo, 2008) suggests that the
relative proportions of smectite and kaolinite are not influ-
enced by changes in relative sea level in the Pliensbachian of
Mochras (Deconinck et al., 2019). On the assumption that the
coarsening-upward sequences at Mochras are indicative of
relative sea level change, it can also be argued that the prox-
imity to the shore did not impact the proportions of smectite
and kaolinite. Instead, we observe enhanced smectite dur-
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990 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
ing “proximal” deposition and enhanced kaolinite at times
of more “distal” deposition, the opposite of what might be
expected (Fig. 4).
We suggest that the first phase of the LPE (Fig. 4, phase 1)
was characterized by repeated periods of rainfall in a sea-
sonal climate forced by precession in which chemical weath-
ering (smectite formation) dominated the sedimentary signa-
tures. This corresponds to maximum long eccentricity and
shows the same climatic signature as during maximum ec-
centricity phases before the positive CIE. This is then fol-
lowed by a second phase (Fig. 4, phase 2) where the climate
is generally cooler and overall potentially more arid but with
rainfall throughout the year over multiple precession cycles.
This appears to have favoured deep physical erosion, owing
to the abundance of primary clay minerals, kaolinite, and I-
S R1. This interval corresponds to a minimum phase in the
405 kyr eccentricity based on Storm et al. (2020). This in-
terpretation is further supported by decreasing and then low
microcharcoal abundance, pointing to the suppression of fire
activity at this time.
5.2.2 Climate forcing of sedimentary changes
Two coarsening-upward cycles that predate the onset of the
positive CIE and continue for a few metres after its initi-
ation are present in the detrital elemental ratios (best ex-
pressed in Si/Al and Zr/Rb records) (Figs. 3 and 4), and in-
dicate a changing sediment influx over the studied interval.
A previous study of the lithofacies of the Mochras borehole
has also shown the coarsening-upward sequences of 0.5–3m
thickness, which are observed to be followed upwards by a
thinner fining-upward succession (Pie´
nkowski et al., 2021).
This reported fining-upward part is not reflected in the el-
emental ratios of the two sequences shown in this study.
Furthermore, the coarsest phases of these sequences are ap-
proximately coeval with decreasing trends in the K/I ratio
and increasing trends in S/I. This could indicate that pe-
riods of a strong monsoonal/seasonal climate (indicated by
S/I) brought coarser-grained material to the basin, whereas
periods of year-round humidity (K/I) are associated with
higher chemical weathering (low Si/Al). Therefore, these
two coarsening-upward cycles appear to link to increasing
long eccentricity. A similar mechanism has been inferred for
the northern South China Sea region in the Miocene, where
coarser-grained material is found during periods of a strong
summer monsoon and relatively lower chemical weathering
(Clift et al., 2014). Present-day studies show that bedrock
erosion and associated sediment transport is greater in ar-
eas with high seasonal contrast (Molnar, 2001, 2004). Hence,
the Si/Al record also appears to reflect weathering and ero-
sion conditions on land (Clift et al., 2014, 2020), driven
by long-eccentricity-modulated climate (Fig. S5). However,
other scenarios that would influence the grain size on this
timescale cannot be dismissed and include changes in prox-
imity to siliciclastic sources or changes in sediment transport
via bottom-water currents.
Changes in bottom-water current strength and direction
likely affected the depositional site of the Mochras core
(Pie´
nkowski et al., 2021) although there is as yet no consen-
sus on the processes that likely controlled these palaeoceano-
graphic parameters. In the UK region, the North Sea tectonic
dome structure may have disrupted the circulation in the N–S
Laurasian Seaway (including the Viking Corridor) in the late
Pliensbachian when global sea levels are suggested to have
been low (Haq, 2018) and therefore diminished the connec-
tivity between the western Tethys and the Boreal realm, hy-
pothetically reducing poleward heat transport from the trop-
ics (Korte et al., 2015). This mechanism has also been argued
to explain the later cooling observed in NW Europe during
the transition of the warmer Toarcian to the cooler Aalenian
and Bajocian (Korte et al., 2015). Late Pliensbachian occlu-
sion of the Viking Corridor is supported by the provincialism
of marine faunas at this time, showing a distinct Euro-Boreal
province and a Mediterranean province (Dera et al., 2011).
During the Toarcian, a northward expansion of invertebrate
faunal species has been found (Schweigert, 2005; Zakharov
et al., 2006; Bourillot et al., 2008; Nikitenko, 2008), indicat-
ing a northward (warmer) flow through the Viking Corridor
(Korte et al., 2015). More recently, a southward expansion
of Arctic dinoflagellates into the Viking Corridor was sug-
gested for the termination of the T-OAE (Toarcian Oceanic
Anoxic Event) (van de Schootbrugge et al., 2019), which is
in agreement with a N to S flow through the Viking Corridor
suggested by numerical models (Bjerrum et al., 2001; Dera
and Donnadieu, 2012; Ruvalcaba Baroni et al., 2018) and
sparse Nd isotopes (Dera et al., 2009).
Over the European epicontinental shelf (EES) and the
Tethys as a whole, a clockwise circular gyre likely brought
oxygenated warm Tethyan waters to the south-west shelf,
with a progressively weaker north and eastward flow due to
rough bathymetry and substantial island palaeo-geography
(Ruvalcaba Baroni et al., 2018). This predominantly sur-
face flow is modelled to have extended to shelfal sea floor
depths. Only episodically might nutrient-rich boreal wa-
ters have penetrated south onto the EES in these coupled
ocean–atmosphere general circulation model (GCM) scenar-
ios (Dera and Donnadieu, 2012). The modelling also sug-
gests – counter-intuitively – that the clockwise surface gyre
of the Tethys extended further northwards and impacted the
EES more effectively when the Hispanic Corridor was more
open. The timing of the opening of the Hispanic Corridor
is debated and varies from the Hettangian to Pliensbachian
(Aberhan, 2001; Porter et al., 2013; Sha, 2019).
An alternative bottom current configuration was discussed
for Mochras specifically, wherein changes in north-to-south
current strength (see Bjerrum et al., 2001) are proposed for
the changes in grain size and silt or sand versus clay content
via contour currents (Pie´
nkowski et al., 2021). A strong flow
from the cooler and shallow boreal waters is hypothesized
Clim. Past, 19, 979–997, 2023 https://doi.org/10.5194/cp-19-979-2023

T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) 991
to have brought a coarser grain size fraction in suspension
and as bedload, which was then deposited in the Cardigan
Bay Basin while flowing to the deeper and warmer waters of
the peri-Tethys (Pie´
nkowski et al., 2021). Times of a strong
north-to-south current are proposed to be associated with
more oxygenated bottom waters (Pie´
nkowski et al., 2021).
In contrast, when the north-to-south current became weaker,
less coarse material will have been carried in suspension and
as bedload and a relatively higher clay proportion will have
been deposited in the Cardigan Bay Basin (Pie´
nkowski et al.,
2021). In this scenario, times of sluggish currents are associ-
ated with low bottom-water oxygenation (Pie´
nkowski et al.,
2021) and thus climate forcing of current strength could ex-
plain the deposition of alternating coarser and finer fractions
in the Mochras borehole (Pie´
nkowski et al., 2021).
Our research suggests that orbital cycles both before and
during the onset of the positive CIE have a significant in-
fluence on seasonality and hydrology, affecting both fire
regimes and sediment depositional character. Further re-
search is required to consider how long eccentricity and
obliquity cycles might interact with north–south flow in the
Cardigan Bay Basin and circulation processes. What is clear
is that orbital cycles have an impact on terrestrial processes
in the terrestrial sediment source areas (Hollaar et al., 2021)
and led to differences in deposition within the marine sedi-
ments in the Mochras core (Ruhl et al., 2016; Pie´
nkowski et
al., 2021). Our data indicate that periods of coarser-sediment
deposition correspond to periods that include more seasonal
climates before the onset of the positive CIE (low kaolin-
ite), which is in line with the hypothesized grain size changes
caused by contour currents (Pie´
nkowski et al., 2021). How-
ever, after the onset of the positive CIE, although we suggest
that the chemical weathering rate decreased, enhanced runoff
and physical erosion are indicated by a peak in primary clay
minerals and K/I. Enhanced runoff could be expected to im-
pact the thermohaline contour currents (Dera and Donnadieu,
2012). Simultaneously, an increasingly cold climate (as indi-
cated by enhanced physical erosion over chemical weather-
ing) indicates a boreal influence. It remains to be determined
to what extent orbital cycles might have the power to influ-
ence ocean circulation in the basin.
Relatively coarse sediments in the late Pliensbachian
have also been related to shallower sediment deposition in
UK basins (Hesselbo and Jenkyns, 1998; Hesselbo, 2008;
Korte and Hesselbo, 2011). Around the UK area, these re-
gressive facies are plausibly related to enhanced sediment
shedding from the North Sea dome structure during sea
level lowstand across the region (Korte and Hesselbo, 2011).
Sequence stratigraphy of the Lower Jurassic of the Wes-
sex, Cleveland, and Hebrides basins (Hesselbo and Jenkyns,
1998; Hesselbo, 2008; Archer et al., 2019) shows relative sea
level changes and sand influxes in the late Margaritatus Zone
in the studied basins. Noteworthy in the Mochras borehole
are phases of low δ18O of macrofossils which seem to corre-
spond to high phases of macrofossil wood concomitant with
low sea level, suggesting a possible control of relative sea
level on the oxygen-isotope record and the source of detrital
material (Ullmann et al., 2022). The broad spatial distribu-
tion of these basins suggests that associated regression and/or
sediment influx is of at least regional scale (Hesselbo, 2008).
The results presented here fall within this phase of regression
(Hesselbo and Jenkyns, 1998; Hesselbo, 2008).
In the context of the North Sea topographic dome structure
(occlusion of the Viking Corridor in regional ocean flow) as a
possible cause of the late Pliensbachian cooling, these facies
can be interpreted as representing shallowing upward in rel-
atively shallow water or the supply of coarser sediment into
a deep-water system. The doming is hypothesized to have
minimized or prohibited the southward flow of cooler wa-
ters from the Boreal and northward flow from warmer wa-
ters from the Mediterranean area (Korte et al., 2015). The
Mochras borehole is situated on the south-western flank of
the dome and would have been cut off from the northern parts
of the Laurasian Seaway, including the Hebrides Basin and
Cleveland Basin (Korte et al., 2015). This change in seaway
circulation could have impacted the source area of the detrital
sediments in the Mochras borehole.
Superimposed on these larger-scale factors affecting grain
size, orbital forcing clearly also had a strong impact. The
Cardigan Bay Basin (Mochras) is positioned about 290 km
to the SW of the Cleveland Basin and at a similar latitude
but to the west of the Wessex Basin (Ziegler, 1990; Torsvik
and Cocks, 2017) and is therefore expected to be impacted by
the same regional changes in sea level and/or sediment flux.
In the late Pliensbachian of the Cleveland Basin, the detrital
ratios of Si/Al, Zr/Al, and Zr/Rb show similar coarsening-
upward sequences, which have been interpreted as reflecting
changes in riverine transport of siliciclastic grains and grain
size (Thibault et al., 2018). The inferred changes in sea level
in the Cleveland Basin occur at a 100kyr pacing (Huang and
Hesselbo, 2014; Hesselbo et al., 2020b), potentially linking
the regression cycles to short eccentricity (Huang et al., 2010,
and references therein) and long eccentricity (Thibault et al.,
2018). This would mean that eccentricity-driven changes in
inferred sea level change could be linked to glacioeustatic
cycles during these times (Brandt, 1986; Suan et al., 2010;
Korte and Hesselbo, 2011; Krencker et al., 2019; Ruebsam et
al., 2019, 2020b; Ruebsam and Schwark, 2021; Ruebsam and
Al-Husseini, 2021). Glacioeustatic sea level changes are dis-
cussed for the Early Jurassic and Middle Jurassic (Krencker
et al., 2019; Bodin et al., 2020; Ruebsam and Schwark, 2021;
Nordt et al., 2022). A recent study on the rapid transgression
observed at the Pliensbachian–Toarcian boundary ruled out
other mechanisms that could force sea level at this timescale,
such as aquifer eustasy, and emphasized that glacioeustatic
changes in sea level are a likely possibility at times in the
Early Jurassic (Krencker et al., 2019). Therefore, our find-
ings overall are compatible with the episodic occurrence of
continental ice at the poles (Brandt, 1986; Price, 1999; Suan
et al., 2010; Korte and Hesselbo, 2011; Korte et al., 2015;
https://doi.org/10.5194/cp-19-979-2023 Clim. Past, 19, 979–997, 2023

992 T. P. Hollaar et al.: Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic)
Bougeault et al., 2017; Krencker et al., 2019; Ruebsam et al.,
2019, 2020a, b; Ruebsam and Schwark, 2021; Ruebsam and
Al-Husseini, 2021).
6 Conclusions
The terrestrial environment adjacent to the Cardigan Bay
Basin was strongly influenced by orbitally driven climate
forcings (particularly precession and eccentricity) and colder
climate linked to the Late Pliensbachian Event (LPE). Long-
eccentricity forcing remained strong both prior to and during
the LPE. Our results identify five swings in the climate in the
study interval in tandem with the 405 kyr eccentricity minima
and maxima. Eccentricity maxima are linked to precession-
ally repeated occurrences of a semi-arid monsoonal climate
with high fire activity and relatively coarser sediment from
terrestrial runoff. In contrast, 405 kyr minima in the Mochras
core are linked to a more persistent, annually wet climate,
low fire activity, and relatively finer-grained deposits across
multiple precession cycles. Although the 405 kyr cycle in the
proxy records persists through the onset of the LPE positive
CIE, the expression in the clay mineralogical record changes
to indicate year-round relatively cool and wet climate ex-
tended over multiple precession cycles driving significant
erosion of bedrock. Therefore, both the Milankovitch forc-
ings and larger climatic shifts operate in tandem to govern
changes in the terrestrial environment.
Data availability. Supplementary data are available at the
National Geoscience Data Centre at Keyworth (NGDC) at
https://doi.org/10.5285/1461dbe5-50a8-425c-8c49-ac1f04bcc271
(Hollaar, 2022) for the interval 934–918m. b.s. All data pre-
sented for the interval 951–934 m. b.s. are available at the
National Geoscience Data Centre at Keyworth (NGDC) at
https://doi.org/10.5285/d6b7c567-49f0-44c7-a94c-e82fa17ff98e
(Hollaar et al., 2021b). The full Mochras XRF dataset is in
Damaschke et al. (2021) (https://doi.org/10.5285/c09e9908-6a21-
43a8-bc5a-944f9eb8b97e).
Supplement. The supplement related to this article is available
online at: https://doi.org/10.5194/cp-19-979-2023-supplement.
Author contributions. CMB, SPH, and TPH designed the re-
search. TPH conducted the laboratory measurements, with JFD con-
tributing to the XRD measurements and MD, CVU, and MJ to the
XRF measurements. TPH, CMB, and SPH wrote the paper, with
contributions from all authors.
Competing interests. The contact author has declared that none
of the authors has any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. This is a contribution to the JET
project funded by the Natural Environment Research Council
(NERC) (grant number NE/N018508/1). Stephen P. Hesselbo,
Claire M. Belcher, Jean-François Deconinck, Clemens V. Ullmann,
Mengjie Jiang, and Teuntje P. Hollaar, acknowledge funding
from the International Continental Scientific Drilling Program
(ICDP) and TPH acknowledges funding from the University of
Exeter. We thank the British Geological Survey (BGS), especially
James Riding and Scott Renshaw, for facilitating access to the
Mochras core. We also thank Simon Wylde and Charles Gowing
for their contribution to the XRF scanning and discussion of the
results. We further thank Chris Mitchell for help with the TOC
and δ13Corg analyses. Finally, we thank Ludovic Bruneau for
technical assistance with the XRD analysis. We thank referees
Stéphane Bodin and Wolfgang Ruebsam for their very helpful
comments and Mathieu Martinez for informal discussion.
Financial support. This research has been supported by the Nat-
ural Environment Research Council (grant no. NE/N018508/1) and
the University of Exeter.
Review statement. This paper was edited by Luc Beaufort and
reviewed by Stéphane Bodin and Wolfgang Ruebsam.
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