Pieńkowski et al.
Progress in Earth and Planetary Science(2024) 11:18
https://doi.org/10.1186/s40645-024-00612-3

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Early Jurassic extrinsic solar system dynamics
versus intrinsic Earth processes: Toarcian
sedimentation and benthic life in deep‑sea
contourite drift facies, Cardigan Bay Basin, UK
Grzegorz Pieńkowski1^, Alfred Uchman2* , Krzysztof Ninard2, Kevin N. Page3 and Stephen P. Hesselbo3

Abstract
The Cardigan Bay Basin (UK) may have functioned as a deep and narrow strait, and thereby influenced Early Jurassic oceanic circulation through the northern and southern Laurasian Seaway, and between Boreal and Peri-Tethys
domains. Toarcian hemipelagic deposits of the basin in the Mochras borehole show strongly bioturbated contourite facies. Trace fossils are strongly dominated by Phycosiphon incertum (represented by four morphotypes), which
was produced by opportunistic colonizers. Thalassinoides, Schaubcylindrichnus and Trichichnus are common (thelatter is a deep-tier trace fossil produced by filamentous sulfide-oxidizing bacteria with a high tolerance for dysoxia),
accompanied by less common Zoophycos, Planolites, Palaeophycus, Teichichnus, Rhizocorallium, Chondrites, and dwelling and resting structures, such as cf. Polykladichnus, Siphonichnus, Skolithos, Arenicolites, Monocraterion and Lockeia.
Ichnological and lithological signals suggest repetitive fluctuations in benthic conditions attributed to a hierarchyof orbital cycles (precession and obliquity [4th order], short eccentricity [3rd order], long eccentricity [2nd order]and Earth–Mars secular resonance [1st order]). The Pliensbachian–Toarcian transition appears to be a significant palaeoceanographic turning point in the Cardigan Bay Basin, starting a ­CaCO3 decline, and with the most severe oxygencrisis of the Tenuicostatum Zone (here dysoxic but not anoxic) ending at the onset, in the early Serpentinum Zone
(Exaratum Subzone), of the Toarcian negative carbon isotope excursion (To-CIE—linked with the Toarcian Oceanic
Anoxic Event occurring in the lower part in the Serpentinum Zone). This trend contrasts with the prevalence of anoxia
synchronous with the To-CIE in many other settings. Minor dysoxia returned to the Mochras setting in the latest Thouarsense to Dispansum zone interval. Extreme climate warming during the To-CIE may have enhanced
and caused a reversal in the direction of deep marine circulation, improving oxygenation of the sea floor. Spectral
analysis of binary data on ichnotaxa appearances gives high confidence in orbital signals and allows refined estimation of ammonite zones and the duration of the Toarcian (minimum ~ 9.4 Myr).
Keywords Hemipelagic contourites, Deep sea circulation, Climate, Trace fossils, Orbital forcing, Toarcian
astrochronology^

Grzegorz Pieńkowski: died in April 2023.

*Correspondence:
Alfred Uchmanalfred.uchman@uj.edu.pl
Full list of author information is available at the end of the article© The Author(s) 2024. 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 theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother 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 thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 2 of 34

1 Introduction
The impact of orbital forcing versus internal Earth processes on climate change through Earth history is an
important topic of debate (e.g., Lourens 2021). Here, theinfluence of these factors on the Earth system in the deep
geological past (the Toarcian age, Early Jurassic epoch),over the course of about 10 Myr, is discussed in relation
to major climate changes and deep-sea circulation. The
Llanbedr (Mochras Farm) borehole, hereafter referred
to as Mochras, is situated in the Cardigan Bay Basin(Wales, UK) and cored an unusually thick (~ 1300 m)
Early Jurassic age succession, with relative completenessof ammonite chronozones (Woodland 1971). During
the Early Jurassic, this extensional basin was located at a
mid-palaeolatitude (c. 35–40°N), on the NW fringe of theEuropean shelf and within the Laurasian Seaway (Fig. 1).
The Toarcian part of the core has been studied in many
respects, including biostratigraphy (Woodland 1971;
Page in Copestake and Johnson 2014; McArthur et al.
2016), lithology (Woodland 1971; Dobson and Whittington 1987; Hesselbo et al. 2013; Copestake and Johnson
2014; McArthur et al. 2016), carbon cycle and climatedriven local sedimentary processes (Jenkyns et al. 2002;
Xu et al. 2018a), ostracod and foraminiferal assemblages(Boomer 1991; Copestake and Johnson 2014; Reolid et al.
2019; Rodríguez-Tovar et al. 2020), magnetostratigraphy (Xu et al. 2018b), and calcareous nannofossils andprimary productivity (Bown 1987; Menini et al. 2021;
Paulsen and Thibault 2023). A chemostratigraphic reference record shows a major ∼ 7‰ negative excursion inδ13CTOC, associated with the Toarcian Oceanic Anoxic
Event (T-OAE) preceded and followed by early Toarcian
positive excursions (Jenkyns et al. 2002; Xu et al. 2018b).
Climate-driven, accelerated continental weathering andhydrological cycling was suggested as the cause of intensified sediment supply to the Cardigan Bay Basin (Xuet al. 2018a, b). However, a more focussed characterization of sedimentary environment, ichnology, and benthic
life conditions has not yet been published.
The prominent expression of Carbon Isotope Excursions (CIEs) in Mochras with an amplitude of about
one per mil in δ13CTOC (Xu et al. 2018a) testifies to the
expanded and relatively complete stratigraphic record
therein, particularly in the early Hettangian and Sinemurian age part of the section, allowing for a distinction
between intrinsic Earth processes and extrinsic solar system dynamics as the driving mechanisms for the Early
Jurassic δ13C fluctuations (Xu et al. 2018a; Storm et al.
2020; Hollaar et al. 2023; Hesselbo et al. 2023).
The duration of the whole Toarcian, based on astronomical calibration is not well constrained to date. Time
series of magnetic susceptibility (Huret et al. 2008) andXRF-derived Ti content from the Sancerre borehole,Paris Basin, France, has been used, which yielded a minimum duration of 8.3 Myr from 405-kyr long eccentricity
cycles (Boulila et al. 2014, 2019), a result largely confirmed by Huang (2018). Nonetheless, numerous studiesof the T-OAE CIE interval itself have not yet resulted ina consensus on duration, with estimates varying widely. It
has been estimated to be ~ 300 to ~ 500 kyr (Kemp et al.
2005, 2011; Boulila et al. 2014, 2019; Boulila and Hinnov 2017; Ikeda et al. 2018) or ~ 900 kyr (Suan et al. 2008;
Huang and Hesselbo 2014).
In this paper, detailed sedimentological and ichnological data are presented. They allow recognition of ecological parameters, as trace fossils are sensitive in situ
indicators of even very subtle environmental changes
such as current strengths, oxygen content, food supply, and stability of the environment, allowing for new
interpretations and correction of existing ones, basedon other proxies. Furthermore, deep-sea environmental
changes using both new and existing data (δ13CTOC, calcium content, organic geochemistry, benthic fossils—Xu
et al. 2018a; Reolid et al. 2019) are discussed to determine
which changes are caused by intrinsic Earth processes
and which can be attributed to extrinsic solar system
dynamics, also based on spectral analyses of trace fossil
appearances. Benthic invertebrates respond to various
environmental changes driven by orbital forcing (e.g.,
Wetzel 1991; Erba and Premoli Silva 1994; Hüneke andStow 2008; Pervesler et al. 2008; Rebesco et al. 2014),
although the mechanisms of control are still poorly
understood. In the present study we document andanalyse lithological and ichnological cycles that can be
related to orbital controls, and by this means provide a
new estimate of the duration of the Toarcian and ammonite zones within this stage.

2 Material and methods
The Mochras slabbed core (in 1 m core sections) is heldat the British Geological Survey, Keyworth, Nottingham,
UK. A nearly continuous 261 m-thick section of core
was investigated using standard visual sedimentologicaldescription at centimetre scale accuracy (Fig. 2a, b) withspecial attention paid to the integration of sedimentological and ichnological observations. Depths in Figs. 2a, b
and 3 are referred to metres below surface (mbs). Alternative descriptive classifications were adopted—for
siliciclastic sections, the classical array of claystone-mudstone-siltstone-sandstone-conglomerate was used, while
in sections with high carbonate content (over 30–40%)
the classification of carbonate rock lithotypes basedon Dunham (1962) was used (mudstone-wackestonepackstone-grainstone; Figs. 2a, b and 4, 5, 6). Based onammonite occurrences intervals have been assigned tochronozones as documented most recently in McArthurPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 3 of 34

Fig. 1 Location maps. a Toarcian palaeogeography with localization of the Laurasian Seaway and Mochras borehole. b Enlargement of the areain the frame a showing major basins and elevated areas in the Toarcian times (after Cope et al. 1992, amended) and enhanced thermohaline
circulation from the North from the late Falciferum Subzone on, with generally improved oxygenation of the bottom (circulation intensity
and average oxygenation achieved its highest level in the late Bifrons and Variabilis to earliest Thouarsense zones. CBB Cardigan Bay Basin. c The
same map showing the inferred current flow paths during the Tenuicostatum Zone–Exaratum Subzone and), the latest Thouarsense–Dispansum
Zone and (to a lesser extent) Pseudoradiosa Zone thermal anomalies related to a strengthening of warm equatorial Tethyan westward currents
and associated with the conspicuous ichnodiversity/oxygenation crisis in Mochras. Note the pivotal position of the Cardigan Bay Strait (nowCardigan Bay Basin – CBB) in the Laurasian Seaway between Boreal Sea and Peri-Tethys domainPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 4 of 34

et al. (2016); here we follow convention and use romantype to denote chonozones (see discussion of distinction
between biozones and chronozones in Jurassic stratigraphy in Hesselbo et al. 2020a).
For numerical cyclicity (spectral) analysis, occurrencesof trace fossils Phycosiphon morphotypes 2, 3, Thalassinoides, Schaubcylindrichnus, Planolites and Trichichnus(Figs. 2a, b, 5, 7 and 8), as well as intervals with preserved
lamination and undulated bedding (see below) were used
to compile discrete binary time series. Ranges of trace
fossils, undulated bedding and lamination were digitizedby visual sampling with an even step of 10 cm. Resultant
time series consist of approximately 2550 binary digits
each (Additional file 1).
To allow comparison between the ichnological recordand existing δ13CTOC, TOC content, Ca content, Zr/
Rb, Ti/Al, Si/Al and gamma ray records, curves basedon smoothed data are juxtaposed (Fig. 3). Ichnologicalrecords were converted to curves by applying a smoothing spline algorithm (De Boor 2001) directly to binary
time series with smoothing factor values in the 8–9
range, in each case experimentally adjusted to obtain an
optimal degree of smoothing.
Geochemical data taken from Xu et al. (2018a, supplementary data sheet) and gamma ray data from Woodland (1971) were subjected to analogous smoothing, withsmoothing factor values in the 4–6 range.
The runs test was conducted to confirm statisticallythat the trace fossils, undulated bedding and lamination
occur in a non-random order. This non-parametric test
evaluates the null hypothesis of the two states being distributed randomly in a binary time series (Hammer andHarper 2006). For the present study, the adopted hypotheses are:H0: distribution of occurrences is random;
H1: distribution of occurrences is non-random.
Runs are defined as uninterrupted sequences of one
state, in the present case either an occurrence (N1)or non-occurrence (N2). The null hypothesis can berejected if the observed number of runs (R) and the number of runs expected for the random time series ( R̄) are

statistically different. PAST 4.0 software (Hammer et al.
2001) was used to compute the test statistic (Z-score).
The null hypothesis was rejected at a significance level of
5% if an absolute value of computed Z-score was greaterthan the critical value of 1.96 taken from a standard normal table (Additional file 2).
A time series, both binary and continuous, can be seen
as a sum of component constant-frequency waves. Mathematical transform operations serve as tools to extract
the frequencies of component waves from time series. A
range of well-established numerical methods that derivefrom the Fourier transform are applied in the spectral
analysis of continuous time series. However, they are notsuited for analysis of binary time series (Weedon 2003).
Instead, the Walsh transform provides a way to conduct power-spectral analysis of binary data. The Walshtransform outputs the decomposed signal as a functionof sequency, which is half of the average number of zerocrossings per unit of space (or time). From the point ofview of results interpretation, the Walsh sequency canbe equated with commonly recognized concept of frequency. Conceptual basis of the Walsh method and its
applicability in cyclostratigraphy have been described by
Negi and Tiwari (1984), Weedon (1989), van Echelpoel(1994) and Maiti and Tiwari (2012). According to Tiwari(1987) and Negi et al. (1993) the Walsh analysis not only
outputs the peaks related to main orbital periodicities,
but their component periodicities and beat periodicities
as well.
Binary time series were subjected to analysis using theWalsh transform module implemented in PAST 4.0 software (Hammer et al. 2001). To find the optimal balance
between the signal-to-noise ratio and spectral resolution, terminations of binary data series were adjusted bytrial and error. Obtained power-spectra display peaksin the stratigraphic thickness domain (number of cycles
per one metre, Fig. 9). Conversion of peak values fromthe thickness domain into the time domain was basedon two alternative assumed average sedimentation rate
value of 4.6 cm/kyr (upper number in the Fig. 9) and(See figure on next page.)
Fig. 2 Investigated part of the Mochras Borehole showing stratigraphy (stages, ammonite zones, and subzones), lithology, ichnology
and geochemistry of the Toarcian section. a Tenuicostatum and Serpentinum zones. b Bifrons–Aalensis zones. O, other, rarely occurring trace
fossils: Ar, Arenicolites; Ast, Asterosoma; Ch, Chondrites; Lo, Lockeia; Mon, Monocraterion; Pol, cf. Polykladichnus; Rh, Rhizocorallium; Sip, Siphonichnus;
Sk, Skolithos. Trichichnus (filamentous mat-forming, sulfide-oxidizing bacteria and “pyritized tubes”) represent traces of unidentified life activity/
disputable origin. Ub—micrite/mudstone of ‘massive’ appearance, with undulated or cuboid parting. Geochemistry: δ13CTOC—carbon isotope
curve (after Xu et al. 2018a); XRF-derived calcium record (after Xu et al. 2018a). Cyclicity: thin dashed lines—4th order (c. 20 kyr) cycles; solid
gray lines—3rd order (c. 100 kyr) cycles; red lines—2nd order (c. 405 kyr) cycles; grey bar on the right—1st order (c. 2.5 Myr) cycles expressedin bottom oxygenation (shaded) and ichnology. Foraminiferal biotic crisis after Reolid et al. (2019). Red bars in the metric depth column—positionof the sections presented in Fig. 4. Biostratigraphy from Page in Copestake and Johnson (2014) and McArthur et al. (2016) as summarised
in Additional file 3Pieńkowski et al. Progress in Earth and Planetary Science

770
775

Ar
Sk.
Ch
Ch
Ch

780

~2.4 Myr

790
795
ss
s

800
805

s

s

RhLo

810

isorenieratane
s
s
s

815
820

Lo

830

Tenuicostatum

s

s
s

835

beginning-ve CIE

s
s
s
s
s
s
s
s

840
845
850
855

Pol Ch

Sk

s
s
s
s
s
s
s

860
865

870

Pl/To
-ve CIE

Sk
Ar Sk

875
880

d

885

d
dn

Rh
Sk

890

d

Ast

cl
mmd
claystone

mmd marl/mudstone
wst

wackstone/silty mudstone

pfs

packstone/very fine sandstonegsdgrainstone/sandstone
CaCO3 cement concentrations

dn

CaCO3 diagenetic nodules

Fig. 2 (See legend on previous page.)~2.4 Myr

1 234

12
Ph Th S Te Z Pl Pa O Tr Ub L

wst
pfs
gs

Spinatum

isorenieratane

Ast
Sk

825

Pliensbachianend -ve CIE

Rh
Ch

Foraminiferal biotic crisis (Reolid et al., 2019)

Toarcian

Serpentinum
Falciferum
Exaratum785

clPage 5 of 34

CaCO3 decline

Bifrons(a)(2024) 11:18

sδ CTOC[‰]
13

CaCO3

-31 -29 -27 -25 0

20

40 60%Ph Phycosiphon incertum,
morphotypes Ph1, Ph2, Ph3, Ph4
Th Thalassinoides
inferred hiatuses
S Schaubcylindrichnus
Te Teichichnus
condensed sections
Z Zoophycos
Pl Planolites
Ub undulated bedding
O other trace
floral remains
fossils
L lamination: 1 – continuous, Pa Palaeophycus
large-scale cross/inclined bedding
2 – broken
Tr Trichichnus
siderite/other iron compounds
siderite bands
parallel/low-angle/ripple
lamination (current indicators)
sharp boundary/erosion
benthic fauna

dehydration structures

conglomerates/mud clastsPieńkowski et al. Progress in Earth and Planetary Science

Paleogene

Pseudoradiosa
Aalensis
Levesquei Pseudorad.

(b)(2024) 11:18

Page 6 of 34

m
602
605

truncation>0.3 Myr

610
615

Sk

620

PolAr
Sk
Ast

625
630

Ar
PolSk

635

~2.4 Myr640

ToarcianThouarsense Dispansum

645
650

Sk

655

Sip

660
665
Sip
Ch

670

Pol
Ar

675
680
685

Sk

~2.4 Myr

690

Variabilis

695

s

Sk.Sk

700
705

Pol
Pol
Ch
Sk
PolSk

710
715
720

Ar
SkSk

725

Sk

d

730

Sk

735

Mon
SkSk

Bifrons740~2.4 Myr

Sk

745
SkSk.Sip

750
755

Rh

760
765

wst
pfs
gs

cl
mmd

Fig. 2 continued

δ13CTOC[‰]

Ar

1 234

1 2

O Tr Ub L
Ph Th S Te Z Pl Pa

CaCO3-31 -29 -27 -25 0

20

40 60%

730
740
750
760
770

undulated bedding

790
800
810
820

Tenuicostat.

830
840
850
860

Levesquei Pseudo.

m
600

1.50

TOC

2.00

2.50

δ CTOC[‰]

4.50

3.50 -31.50-30.00-28.50

5

5.25

Ca

15

20

des
Th
T
h lassin
hala
i oid

10

-27.00-25.50-24.00 70

65

60

gamma ray
55

50

45

40

35

30

630
640
650

690
710
730
740
750
760
770
780
790
800
810

Schaubcy
yli
cy
lind
drrichnus

700

Falc.

3.75

620

680

Exaratum

Si/Al

13

3.00

670

Com. Fibul. Crass.

Bifrons

1.00

3.00

610

720

Serpentinum

0.50

660

Variabilis

Thouars. Dis. Pseudoradiosa Aal.

870

2.25

Th
h lassin
hala
i oid
des
lind
Schaubcycyylidrrichnus T

Falc.
Exaratum

Serpentinum

780

0.08 1.50

TrrichichnusT

BifronsCom. Fibul. Crass.

720

0.07

undulated bedding

710

Pllanoli
es
lite

690

Ph
hy
yc
cosip
iphon
p
type Ph
h3

680
700

0.06

undulated bedding

670

Ti/Al

Th
T
h lassin
hala
i oid
des

650

0.05

es
Pllanoli
lite

640

0.04

undulated bedding

630

3.0

Page 7 of 34ThT
h lassin
hala
i oid
des

620

2.5

Pllanoli
lite
es

610

Zr/Rb

2.0

(2024) 11:18

Ph
hy
yc
iphon
p
type Ph
h2
cosip

1.5

Th
T
h lassin
hala
i oid
des

Levesquei Pseudo.

m
600

660

Variabilis

Thouars. Dis. Pseudoradiosa Aal.

Pieńkowski et al. Progress in Earth and Planetary Science

820

Tenuicostat.

830
840
850
860
870

Fig. 3 Smoothing spline-smoothed curves reflecting selected ichnotaxa abundance against the background of geochemical and gamma ray
records from the Mochras borehole. Contains British Geological Survey materials © UKRI 2024

Othertrace Dominating bottom
conditions
fossils

p. fs.g.s.

Light-gray packstones
and grainstones/
sandstones, bioturbatedwith Thalassinoides,
Phycosiphon (Ph1, Ph2
and Ph4)

Sk Stronger currents;
Ar well-oxygenated
Ast bottom

5 w.-st.

Gray wackstones siltstones, bioturbated with Sk Moderate currents;
Tr oxygenatedPhycosiphon (Ph2);
Schaubcylindrichnus,
Ch bottom
Zoophycos, Teichichnus

4

Pol
Gray mudstones,
Ch Weak currents;
subordinate wackstones/
oxygenatedsiltstones, bioturbated with Tr bottom
Phycosiphon (Ph3)Sk

m-mdw.-st.

Pinstripe gray mudstone3

m-mdw.-st.

2 m-mdlaminated with wackstones
and siltstones, lamina often
discontinuous, small scale
ripples, rare Phycosiphon(Ph3)
Gray ‘massive’ mudstones,
with udulated or cuboid
parting; homogenization ofTr
the sediment by meiofaunabioturbation; rare
Phycosiphon (Ph3)

Weak currents;
dysaerobic bottom

4

C5

5

C4

6

C3

4

C5

5

C4

6

C3

5

C2

f.u.

Strong currents;
erosion, depositionof coarse grains

Conglomerate, no trace
fossils

Cyclesbottom cut-out cycle

6

Lithology and basic ichnofabrics

f.u.

7

Page 8 of 34

c.u.

Lithofacies
no.

(2024) 11:18

full cyclePieńkowski et al. Progress in Earth and Planetary Science

4
Suspension fallout;
dysaerobic bottom

3

C1

Suspension fallout,intermittent weakcurrents; anaerobic,
intermittent
dysaerobic bottom

Dark gray, planar laminatedmudstones; bioturbation
structures not observed

Suspension fallout;anaerobic bottom

1

5

C2

4
m-md

C1
1 –2

top cut-out cycle

m-md

Dark gray mudstones,
broken planar lamination;rare bioturbation structures(meiofaunal and rare
Phycosiphon (Ph3)

c.u.

1–2

Fig. 4 Seven main lithofacies (1–7) and ichnofabrics in the Mochras profile; to the right—main types of bigradational contourite drift sedimentary
cycles built of main lithofacies—full cycle, top cut-out cycle, bottom cut-out cycle (referred to Stow et al. 2002; Stow and Faugeres 2008). Contourite
cycle divisions reflect variations in current velocity. C1–C5—“Full” contourite cycle. For lithology and trace fossil abbreviation see Fig. 2a, b

4.1 cm/kyr (lower number in the Fig. 9). Consequently,
time values corresponding to peak values were calculated
as a proportion of 21,700 and 24,400 yr per one metre,
respectively.

The most reliable sedimentation rates can be obtained
for the Tenuicostatum Zone (~ Polymorphum Zone),
where existing radioisotopic (Sell et al. 2014) and astrochronological (Huang and Hesselbo 2014; Rocha et al.

Pieńkowski et al. Progress in Earth and Planetary Science(a) 775.79 m(2024) 11:18(b) 772.03 m

Page 9 of 34(c) 755.85 mc.u(d) 754.86 m

Ph3

mmd

Ph2

wst

wst

f.u

f.u

Th

Te

mmd/wstgs

Th

Th-Ph4

Ph2

pfs

Th

Tr
Tr

Pl

Ph1

L2

Ph2

Ph3

c.u

Ph2

Tr

Th
S

Th-Ph4
mmd

wst

wst

772.97 m

wst

Th

c.u

Tr
776.81 m

wst

pfs wst-pfs

Tr

Th

Tr

Ub

Tr

mmd

Th

wst

Tr

wst

f.u

Th

mmd

Th

Ph3
pfs

Ph2

Th
pfs

mmd/wstc.u

f.u

Th-Ph4L2
Ph3756.82 m

755.85 m

Fig. 5 Sections of the Mochras core showing common types of lithology, ichnology, and cyclicity of sedimentation. a 775.79–776.81 m—
bigradational (symmetrical) cycle, with gradually coarsening upward (cu) and gradually fining-upward (fu) phases, cut from the top
by a conspicuous erosional surface (red bar), capped by sandstone and fining-upward succession. b 772.03–772.97 m—full bigradational(symmetrical) cycle, with gradually coarsening upward (cu) and gradually fining-upward (fu) phases. A and B—no primary sedimentary structuresare visible, due to pervasive bioturbation. c, d 754.86–756.82 m—bigradational, top-cut cycle, some current ripples can be seen in the middle
part; the cycle is followed by mudstone, in places with lamination, passing into the coarsening-upward phase of the next cycle. Characteristic
bigradational (“symmetrical”) cycles with bioturbated coarser parts result from variations in current velocity and are referred to as contourite cycles(Stow et al. 2002; Stow and Faugeres 2008—see also Fig. 4)

2016; Huang 2018) constraints give duration of ~ 0.8
Myr (although, assuming inferred hiatuses, this duration
could be extended to ~ 1 Myr—Ruebsam and Al-Husseini2020). With 33.8 m of sediments representing this timein Mochras, an average sedimentation rate of 4.1 cm/kyr
is calculated. On the other hand, the sedimentation ratein the Pliensbachian section in Mochras (representing
a similar depositional setting and stable sedimentationPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18(a)Page 10 of 34

(b)
Ph3

(c)

(d)

(e)

Ph2
S
Fig. 6 Some sedimentary features of the investigated section. a Rip-up clasts pointing to the high energy of currents and erosion, 746.7 m. b
Conglomerate resting on an erosional surface. Most pebbles are composed of local material (in some Phycosiphon incertum morphotype 3, Ph3,
is visible), usually cemented by calcium carbonate. Shell debris is also common, 627.4 m. c Conglomerate—pebbles are composed of both local
and exotic material (e.g., a larger chert pebble in the bottom-left), 627.3 m. d Erosional surface with coarse sandstone/grainstone, 630.9 m. e
Mudstone–wackestone with erosional (channelized) surface and cross-bedded sandstone above. S, Schaubcylindrichnus; Ph2, Phycosiphon incertum
morphotype 2, 683.3 mrate) was estimated at about 4.6 cm/kyr (Ruhl et al. 2016;Pieńkowski et al. 2021). Therefore, alternative time values
were calculated for peaks in the spectra assuming thisvalue. For comparison, the conversion was also tested
with 3.3 and 2.8 cm/kyr sedimentation rates, which could
be representative for more condensed sections with lessstable sedimentation or more frequent erosional surfaces.
However, the results based on these two lower alternative
sedimentation rates were discarded as less meaningful.

3 Results
3.1 Lithology and sedimentary structures

The succession primarily comprises siliciclastic andcalcareous deposits: mudstone/micrite, siltstone/
wackestone and sandstone/grainstone with varying
proportions of calcareous and siliceous grains of silt
to fine-sand size. The siliciclastic input in coarser siltysandy lithofacies is more pronounced in the lower partof the section with a C­ aCO3 content typically between
10 and 30%, while the mudstone contains a greater
siliciclastic component and locally (805–840 mbs) is
carbonate-free (Fig. 2a; Xu et al. 2018a). Macrofossils

are mainly represented by pelagic cephalopods (Xuet al. 2018a; Ullmann et al. 2021). Crinoids are muchless common in the Toarcian than in the Pliensbachiansection (Pieńkowski et al. 2021) and are most likely
redeposited from shallower settings due to their buoyancy (Savarese et al. 1997). Benthic shelly organisms
are rare, and they are represented by in situ burrowing
bivalves found in a few horizons (Fig. 7c, d). Benthic
foraminifers are also present, and their assemblages areimportant to determine bottom life conditions (Copestake and Johnson 2014; Reolid et al. 2019). Driftedflora occurs locally, mostly in the Tenuicostatum Zone
and the Exaratum Subzone, with some occurrences also
in the Variabilis Zone. Low-angle cross-lamination iscommon, particularly in the Exaratum, mid-Bifrons,
mid-Variabilis, Thouarsense, and mid-Pseudoradiosazones. Siderite (usually dispersed, in places forming
bands and nodules, oxidised to iron oxide) is common
in the Tenuicostatum Zone and Exaratum Subzone, as
noted by Xu et al. (2018a). Dewatering/compactional
cracks are observed only in a few horizons (in the
Bifrons and Variabilis zones).

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 11 of 34

(a)

(b)

Ph2

Ph3

Ph3

Ph2

lam.

(c)

(e)

Th

Th

Ph2

b.b
Th

Th

Ph2

(d) b.b

Th

Ph4

Pl

Th

Th

(f)

b.b

(g)
Ph2

(h)

Pl

Th

Th

(i)

(j)
Th

Ph2

(k)

Tr
Te

Th

Tr

Ch

Fig. 7 Sedimentological and ichnological features. a Fluctuations of bottom currents: lamination (lam.) in the bottom (stagnation, no trace
fossils), followed by altered periods of more stagnant/more dynamic (arrowed) current conditions. Changing oxygenation and nutrient delivery
is reflected in different morphotypes of Phycosiphon incertum (Ph2, Ph3): small Ph3 (more stagnant conditions) and larger Ph2 (more dynamic
conditions), depth 824.0 m. b Enlarged framed fragment of A. c Ichnofabric showing succession (tiering) of the earliest/shallowest Phycosiphon
incertum morphotype 2 (Ph2), followed by Thalassinoides (Th) and bivalve burrow (b.b.), 603.6 m. d Cross-bedded (low-angle), very fine sandstone
with mudstone intercalations. Bivalve resting trace ?Lockeia (b.b.), 818.67 m. e Thalassinoides burrow systems (Th) in siltstone/sandstone, in places
infilled by Phycosiphon incertum morphotype 4 (Ph4)—arrowed. Numerous Phycosiphon incertum morphotype 2 (Ph2) in the background, 823.35 m.
f Thalassinoides galleries of tubular burrows (Th, deeper tier), Phycosiphon incertum morphotype 2 (Ph2, shallower tier) in the background, 726.1 m.
(g) Trichichnus—pyritized horizontal and oblique, unbranched or branched, thread-like, cylindrical structure produced by bacteria consortium,
712.6 m. h Planolites (Pl—thinner, randomly oriented tubes) and Thalassinoides (Th—larger burrows), 782.7 m. i Post-depositional colonizationof a sandstone/grainstone bed with Phycosiphon incertum morphotype 2 (Ph2, earlier stage—shallower tier) and Thalassinoides isp. (Th, later stage—
deeper tier), 821.6 m. j Teichichnus (Te, shallow tier) and pyritized, aggregated Trichichnus cylindrical vertical structures (Tr, deeper tier), 850.5 m. k
Chondrites (Ch, deep tier) and shallow tier Thalassinoides (Th) and Planolites (Pl), 669.5 m. Scale = 1 cm

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

(b)

(a)

Page 12 of 34

(c)
S

(e)

S

(d)
Z

(f)
Ph1

Z

Ph3

(g)

Rh

Rh

Z

(i)

Ph3

Th

Ph3

Ph2

Th
Th
Sk
Ph3

(h)
Ph3

Ph2
Fig. 8 Other ichnological features. a Phycosiphon incertum morphotype 2—spreiten burrows visible in horizontal section. This level contains
isorenieratane (Xu et al. 2018a, b); in that case, this biomarker does not indicate anoxic bottom conditions, 811.60 m. b Palaeophycus—cylindrical,
pyritized structure with conspicuous lining, 785.2 m. c Schaubcylindrichnus (S)—cylindrical, lined structures flattened by compaction, 702.5 m.
d Mottled mudstone (“undulated bedding”), sediment homogenized by meiofauna activity. This level contains isorenieratane (Xu et al. 2018a,b); in that case, this biomarker does not indicate permanent anoxic bottom conditions, 819.2 m. e Zoophycos galleries (Z) composed of fan-like,spreiten structures, 782.6 m. f Rhizocorallium (Rh), Thalassinoides (Th) and Phycosiphon incertum morphotypes 1 (Ph1) and 3 (Ph3), 801.8 m. g
Chondrites, 803.1 m. h Lamination in claystone/mudstone, in places broken by bioturbation (Phycosiphon incertum morphotype 2, Ph2). In the lower
and upper part “pinstripe” broken lamination with silty lamina and incipient ripples indicate recurrent activity of weak currents. Despite the presenceof isorenieratane in this interval (Xu et al. 2018a, b), current activity and the presence of bioturbation do not support for anoxic conditions
on the bottom surface. Depth 811.60 m. i Stiff ground and uneven omission surface with Thalassinoides (Th) and Skolithos (Sk); Phycosiphon incertum
morphotypes 2 (Ph2) and 3 (Ph3) were left in soft sediment. Depth 730.0 m. Scale = 1 cm

Pieńkowski et al. Progress in Earth and Planetary Science

217 108 72
244 122 81
9(a)104117

Phycosiphon type Ph2
Phycosiphon type Ph3
33.4
29.8

relative power

108
121

6
5

Page 13 of 34

periods [kyr]54 43.4 36.2 31.0 27.1 24.1 21.7 19.7 18.1 16.7 15.5 14.5 217 108 72
61 48.8 40.7 34.9 30.5 27.1 24.4 22.2 20.3 18.8 17.4 16.3 244 122 8187(2024) 11:18

36.2
40.7

26.8
30.1
25.9
29

4

9
8
7

19.9
22.4
21.3
24

5

2

2

1

1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
sequency [cycles / metre]
periods [kyr]relative power

7
6
5
4

66
81 74.2
91

39.9 35.6
44.9 40

Planolites
Trichichnus

19.6
22
22.2
25
24
27

17.7
19.9
18
20.2

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
sequency [cycles / metre]
periods [kyr]54 43.4 36.2 31.0 27.1 24.1 21.7 19.7 18.1 16.7 15.5 14.5 217 108 72
61 48.8 40.7 34.9 30.5 27.1 24.4 22.2 20.3 18.8 17.4 16.3 244 122 81

155
174

ecc. harmonic?

8(c)Thalassinoides
Schaubcylindrichnus

4
3

9

115
129

6

3

217 108 72
244 122 81(b)periods [kyr]54 43.4 36.2 31.0 27.1 24.1 21.7 19.7 18.1 16.7 15.5 14.5
61 48.8 40.7 34.9 30.5 27.1 24.4 22.2 20.3 18.8 17.4 16.3

9(d)

54 43.4 36.2 31.0 27.1 24.1 21.7 19.7 18.1 16.7 15.5 14.5
61 48.8 40.7 34.9 30.5 27.1 24.4 22.2 20.3 18.8 17.4 16.3

undulated bedding
lamination

8

17.5
19.7

111
125
42.6
47.9

25.7
28.9

7
6
5
4

3

3

2

2

1

1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
sequency [cycles / metre]

132 75148 8435.3
39.7 29.6
33.3

19.9 19.1 17
22.4 21.5 19.1

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
sequency [cycles / metre]Fig. 9 Overlapping Walsh power-spectra of the time series based on: a Phycosiphon morphotypes Ph2 and Ph3. b Thalassinoides
and Schaubcylindrichnus. c Planolites and Trichichnus. d Undulated bedding and lamination. Spectra are scaled true to analysis results (not rescaled).
Calculated time values assigned to peaks and upper abscissa axes based on two alternative sedimentation rates 4.6 cm/kyr (upper number)and 4.1 cm/kyr (lower number)

As in the Pliensbachian section, the same six main
lithofacies types (Pieńkowski et al. 2021; their Fig. 8)
are identified in the Toarcian section of Mochras: (1)Dark grey, planar-laminated siliceous or calcareous
micritic mudstone and siliceous claystone with continuous (without bioturbation) and broken lamination(with few bioturbation structures); (2) Massive, poorly
bedded mudstone/carbonaceous micritic mudstone,
in some cases showing centimetre- to decimetre-scale
banding with faint, slightly undulated (undulated bedding), or cuboid parting, which is connected to homogenization by meiofaunal bioturbation of sub-millimetre
size (Figs. 4, 5 and 8d); discrete trace fossils are relatively rare; (3) Pinstripe carbonaceous mudstone thatis laminated with wackestone-siltstone, in which planar laminae are often discontinuous and lenticular towavy, planar parallel, or low angle cross-lamination,
starved ripples are present, and bioturbation structures
can obliterate these in mottled mudstone-wackestone(Figs. 4 and 5); (4) Interlaminated, heterolithic siltstone or wackestone with mudstone, usually bioturbated, with only local sedimentary structures; thislithofacies shows irregular arrangement of mudstone,
wackestone-siltstone or grainstone-sandy siltstone inpockets, lenses and streaks, and less commonly, a rapid
alternation of thin irregular layers of these three lithologies (Figs. 4 and 5); (5) Packstone/siltstone to veryfine sandstone, with primary sedimentary structures—
usually obliterated by bioturbation—including parallel
bedding and small-scale cross-bedding (Figs. 4 and 5);(6) Sandstone-grainstone, usually bioturbated, with primary structures (parallel bedding and cross-bedding)observed in some horizons (Fig. 4). Additionally, theconglomerate lithofacies (7) is present—it is too rare
to play an important role in basic models of cycles, but
the presence of conglomerate beds (composed both of
local and exotic material—Fig. 6a–c) is important as anindicator of intermittent high-energy currents. Also ofPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 14 of 34

note (compared to Pliensbachian) is a general rarity oflamination.
Contacts between these different lithofacies may beeither gradational, yielding indistinct bedding (Figs. 4 and5), or sharper due to the erosive action of relatively strong
bottom currents, thereby yielding more distinct bedding (Figs. 4–6). Compared to the Pliensbachian section(Pieńkowski et al. 2021) of note are much more common
sharp, erosional boundaries in the Mochras Toarcian section, associated with coarse-grained sediments (including conglomerates, either with clay pebbles derived fromlocal compacted mudstone or of exotic lithology suchas limestone and chert), pointing to high-velocity currents, erosion/non-sedimentation and hiatuses (Fig. 6).
In places, hiatal levels are marked by induration of thesediment surface occurring as semi-consolidated/firm
muddy/silty substrates with sharp-walled, unlined,
uncompacted passively filled burrows (Fig. 7i).
Lithofacies types 1–7 and related ichnofabric in the
Toarcian of Mochras are not arranged randomly, but
appear in a cyclical order (Figs. 2a, b, 4 and 5). Fully
developed, usually 0.5–3 m thick, bigradational grading
sequences (some ‘couplets’ of previous authors) commonly begin from mudstone lithofacies type 2 (representing mixed layer, mottled by burrowing activity of themeiofauna), or mudstone lithofacies type 1 (laminated),
gradually passing into lithofacies type 3—pinstripe mudstones with ripple-drift cross laminated siltstones, or
lithofacies 4—bioturbated mudstone-wackestone, followed usually by bioturbated wackestone-siltstone andpackstone-grainstone with traces of stronger currents(Fig. 5). Above this coarsening-upward part is a finingupward suite, developed in reverse order, although lesscomplete; usually only lithofacies types 6–5–4 are present. This fining-upward phase of the sequence is usually thinner than the coarsening-upward phase. In somecases, the whole sequence is incomplete; either top or
bottom parts can be missing (top cut-out cycles or bottom cut-out cycles—Fig. 5). These successions are likethose observed in the Pliensbachian part of the section (Pieńkowski et al. 2021). The main difference is themore common occurrences of erosion surfaces in the
Toarcian section, locally with lithofacies type 7 and hiatuses (Fig. 6). Of note are also the markedly finer sediments occurring of earliest Toarcian age (Tenuicostatum
Zone, and ealiest Exaratum Subzone) and again in theThouarsense-Dispansum zonal transition (Figs. 2, 3).

geometry (Wetzel and Uchman 2012). Below, trace fossils
are briefly described. The trace fossils show significant
similarities to those described in Pieńkowski et al. (2021)
from the Pliensbachian. Their synopsis is presented inTable 1.
Phycosiphon incertum with its four morphotypes (Ph1,
Ph2, Ph3, Ph4) is the most abundant trace fossil in the
Mochras Toarcian. The exception is the Tenuicostatum
Zone (and to a lesser extent also the Thouarsense-Dispansum zonal transition), characterised by Phycosiphon
scarcity and dominance of Trichichnus. Previous ichnological analysis of contourites has revealed changes inichnofabric attributes over relatively short lateral distances, including trace fossil composition, cross-cutting
relationships, or ichnofabric index (Hüneke and Stow
2008; Wetzel et al. 2008; Wetzel and Uchman 2012;
Rodríguez-Tovar et al. 2016, 2017; Rodriguez-Tovar
and Molina 2018). Recently, Dorador et al. (2019) andPieńkowski et al. (2021) have indicated that the distanceto the bottom current ‘core’ exerts a tangible and widespread influence on specific macro-benthic tracemaker
communities in contourite deposits. This parameter itself
reflects other bottom current features, such as hydrodynamic energy, grain size, nutrient transport, etc. Specifically, strong domination of Phycosiphon is unique for the
Pliensbachian-Toarcian contourite deposits in Mochras.
Phycosiphon, common in turbidites and hyperpycnites(e.g., Wetzel and Uchman 2012; Knaust et al. 2014), is
also reported from contourite deposits (Baldwin and
McCave 1999; Wetzel et al. 2008) but as a rare, subordinate trace fossil. Lack of Phycosiphon in other contourite deposits may be more apparent than real: in previous
papers diffuse Phycosiphon burrows may simply havebeen described as “bioturbation”. In particular, this ichnotaxon could be difficult to recognize in recent unconsolidated contourite deposits. Other deposit-feeding feeding
traces, e.g., Rhizocorallium, are rare.
Schaubcylindrichnus isp. is common through the late
Bifrons and Variabilis zone strata. In the Toarcian ofMochras, Teichichnus isp. is much less numerous than in
the Pliensbachian (Pieńkowski et al. 2021). Teichichnus
and Schaubcylindrichnus do not usually occur together.
Thalassinoides crosscuts Phycosiphon of the morphotypes Ph1, Ph2 and Ph3, but is cut by Ph4, Schaubcylindrichnus and Teichichnus. Trichichnus (relatively rare in
Pliensbachian in Mochras, see Pieńkowski et al. 2021) isparticularly common in the Tenuicostatum Zone, whichis associated with declining abundance in Phycosiphon
and low ichnodiversity. Trichichnus is also abundant in
the mid-Falciferum Subzone, in the Thouarsense-Dispansum zone transition, and (less abundantly) in the midPseudoradiosa Zone. The presence of Trichichnus usually
points to oxygen-depleted conditions, but interpretation

3.2 Trace fossils, ichnofabrics

Trace fossils exhibit sharp outlines and possess a characteristic geometry that allows their classification interms of ichnotaxonomy, while bioturbation structures
have less distinct outlines and do not display a recurrent

In the vertical section, oval spots, may be in contact, 14–18 mm across, filledwith slightly coarser material than the surrounding rock. They are interpreted
as cross-sections of vertical to inclined elongated bulb-like structures tapering at both ends and spreading out from a common vertical or inclined shaft
Inclined, branched bars, or circular to oval spots, 0.2–1.5 mm wide, darker
than the surrounding deposits. They are cross sections of downward spreading branched tunnels
Vertical or steeply oblique, slightly curved, wedge-like structure, 10–12 mm
high and up to 3.5 mm wide, widening upward, descending from a mudstone lamina, and penetrating through a siltstone lamina. The filling is composed of irregular stripes/patches of mudstone and siltstone
In the vertical plane, a triangular structure, c. 10 mm wide, 10 mm high,which vertex is oriented downward
Variable oriented, repetitive narrow U-shaped lobes, 1–15 mm wide, whichin cross section show dark, single or paired centres (some show fish-hookshapes) and light mantles composed of coarser sediment. Commonly,
in a diffused appearance due to soft-soupy host sediment. The largemorphotype Ph1 is 8–15 mm long and 2–3 mm wide, the medium, mostcommon Ph2 is 3–8 mm long and 0.3–2 mm wide, the small Ph3 is up of
2–3 mm long and 0.1–0.3 mm wide, usually horizontally-oriented, and Ph4
is distinguished by its occurrence in isolated patches, preferentially in fillingsof Thalassinoides or occasionally in Teichichnus

Sub-horizontal cylindrical, lined, passively filled burrow, 2–4 mm wide, which Burrow of variable filter-, deposit-feeding, carnivorous or omnivorous invershow traces of collapsing
tebrates, mostly polychaetes in a wide range of facies (Pemberton and Frey
1982; Jensen 1997; Knaust 2017)
A horizontal, unlined, cylindrical tunnel, actively filled with sedimentthat usually differs from the surrounding. It is elliptical, sub-circularin cross sections, 2–3 mm in diameter

Vertical to subvertical branched, unlined cylinders, which are c. 1 mm wide,
Polykladichnus is a domichnion mostly in marginal marine and continental
crowded in clusters, which are up to 20 mm wide and up to 20 mm high. The deposits (Schlirf and Uchman 2005). Burrows of this pattern are produced
branches diverge in the middle-upper part at acute angle
by some nereidid polychaetes (Wang et al. 2019)
In the vertical section: a dumbbell structure, about 60 mm wide, with lenslike, swelled terminations, which are 15 mm wide, and the thin interconnecting bar, which is 1.4–2 mm thick. In horizontal section: spreite structures,
5 cm thick with swelling at the end. The lens-like terminations are interpreted
as a cross-section of the marginal tunnel

Asterosoma isp.

Chondrites isp. (Figs. 7k and 8g)

Lockeia isp. (Fig. 7d)?Monocraterion isp.

Phycosiphon incertum Fischer-Ooster
1858 (Figs. 7a–c, e, f, i, 8a, f and i)

Palaeophycus isp. (Fig. 8b)

Planolites isp. (Fig. 7h, k)

cf. Polykladichnus isp.

Rhizocorallium isp. (Fig. 8f )Rhizocorallium is produced by suspension feeders (only short oblique, retrusive
forms) or deposit feeders, mostly crustaceans (Fürsich 1974; Schlirf 2000)
or “worms” (Knaust 2013), mainly in in shallow marine and marginal marine
deposits (e.g., Farrow 1966; Hakes 1976) and rarely in deep-sea (Uchman 1992)
or even in non-marine deposits (Fürsich and Mayr 1981)

Locomotion and deposit feeding, actively filled burrow of variable “worms”
in many marine and non-marine facies (e.g., Pemberton and Frey 1982; Fillion
and Pickerill 1990, and references therein). According to Locklair and Savrda(1998), it is an open burrow that was filled with overlying sediment

Deposit-feeding trace of a small unknown vermiform organisms (Wetzel
and Bromley 1994; Naruse and Nifuku 2008; Bednarz and McIlroy 2009;
Wetzel 2010; Izumi 2014) in a wide-range of facies, mostly fine-grained lower
shoreface and deeper, mainly siliciclastic facies (Goldring et al. 1991; Savrda
et al. 2001; Pemberton et al. 2012; Callow et al. 2013). Its tracemaker colonizedfreshly deposited turbidites with a lot of food and well-oxygenated porewaters (Wetzel and Uchman 2001). The four morphotypes correspond to these
distinguished by Rodríguez-Tovar et al. (2014)

A domichnion of suspension feeders (Westergård 1931; Hallam and Swett
1966; Pacześna 1996; Jensen 1997; Schlirf and Uchman 2005; Stachacz 2016)

A bivalve resting trace, usually preserved as an amygdaloidal hypichnial
mound (Schlirf et al. 2001; Mikuláš et al. 2013)

A feeding deep-tier chemichnion of unknown “worms”, which can burrow
below the redox boundary (Fu 1991; Uchman 1999; Wetzel 2008), mostly
in offshore and deeper, rarely in nearshore restricted environments (e.g.,
Knaust 2017; Baucon et al. 2020)

A selective-feeding burrow of a worm (Pemberton et al. 2001), which occurs
mostly in siliciclastic, rarely carbonate, typically in various shallow-marine
environments, especially in the upper lower shoreface (Pemberton et al. 2001;
Bromley and Uchman 2003; Pervesler and Uchman 2004)

A dwelling and feeding burrow of suspension-feeding annelids (e.g., Hakes
1976) or small crustaceans (Goldring 1962). It occurs mostly shallow-marine
settings (Crimes 1977), especially in storm beds (Frey and Goldring 1992)

U-shaped, tilted tube, 1.5–4 mm in dimeter. The width is increasing
towards the top of the limbs. The trace fossil is 22 mm wide and up to
13 mm deep

Arenicolites isp.

Interpretations and the basic references

Description

Ichnotaxon

Table 1 Trace fossils of the Toarcian deposits in the Mochras corePieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18
Page 15 of 34

A steeply inclined, simple tube, c. 1.5 mm thick, which shows a darker core
and thick lighter mantle. In the upper part, it may show swelling

Isolated, vertical, or steeply inclined, simple, unlined cylinder, c. 1–2 mm wide Dwelling and feeding burrows of annelids or phoronids (Alpert 1974), mostly
and at least 15 mm long. The tube is lined with mudin shallow-water-high energy settings, but may also occur in deep-sea deposits (e.g., Wetzel 2007)
A vertical, blade-like elongate spreite structure, 15–25 mm wide,
up to 60 mm in the vertical plane. Specimens with zigzag margins in verticalcross section resemble Teichichnus zigzag Frey and Bromley 1985, and specimens showing only a few spreiten laminae are considered as an incipient
Teichichnus

Siphonichnus isp.

Skolithos isp.

Teichichnus isp. (Fig. 7j)

Traces of filamentous mat-forming sulfide-oxidizing bacteria (mostly
Thioploca-related taxa,), which are able to provide an electron exchange
between oxic and suboxic/anoxic sediment layers (Kędzierski et al. 2015; seealso Kjeldsen et al. 2019). Trichichnus occurs mostly in fine-grained sedimentsof shallow water (e.g., Frey 1970), and deep-sea environments (e.g., Kennedy
1975; Wetzel 1981, 1983; Kotlarczyk and Uchman 2012)

A vertical to oblique, occasionally horizontal, straight or slightly winding,thread-like, unbranched or branched cylindrical structure, up to 1 mm
in diameter

A few mm thick, planar to oblique structures. The characteristic of Zoophycos spreiten structure is not observed in Mochras, but this is commonly
the case in some fine-grained deposits (Voigt and Häntzschel 1956). The
trace fossil is distributed rather evenly through the whole section, but veryrare in the tenuicostatum—lower exaratum interval. Usually, only one, rarely
two–three whorls appear

Trichichnus isp. (Fig. 7g and j)

Zoophycos (Fig. 8e)

Fodinichnion of unknown producers, possibly sipunculids (Wetzel and Werner
1981), polychaete annelids, arthropods (Ekdale and Lewis 1991), or echiuran
worms (Kotake 1992), with controversial feeding strategy (e.g., Bromley 1991;
Locklair and Savrda 1998; MacEachern and Burton 2000; Bromley and Hanken
2003; Löwemark et al. 2006). Since the Mesozoic, Zoophycos has shown a tendency to occur in deeper environments than in the Paleozoic, from below theshelf to abyssal depths (Zhang et al. 2015)

Domichnion and fodinichnion of mostly scavenging and deposit-feeder crustaceans (Fürsich 1973; Frey et al. 1978, 1984; Ekdale 1992; Bromley 1996; Schlirf
2000), in variable, presumably shallow marine environments (Frey et al. 1984;
Mángano and Buatois 1991; Pemberton et al. 2001), but also in the deep-sea
facies (Uchman 1995, 1998; Uchman and Tchoumatchenco 2003; Wetzel et al.
2007)

Thalassinoides isp. (Figs. 7c, e, f, h, I, i, 8f, i) In the vertical section: elliptical spots up to 5–20 mm wide, or short barsof comparable size, which colour differs from the surrounding rock. In
the horizontal section: branched, cylindrical burrow networks. They areusually associated with coarser (packstones-grainstones), somewhat stiffer
sediments. The filling is preferentially burrowed with Phycosiphon (Ph4)A feeding structure that resulted from the upward or downward displacementof a causative tube, mainly in fine-grained fully marine and brackish deposits
Knaust (2017)

A dwelling trace of suspension feeding bivalves (Stanistreet et al. 1980; Gingras et al. 2008; Dashtgard 2011) or a pascichnion of bivalves (Knaust 2015),
common in shallow-marine and marginal-marine deposits (Pollard 1988)
often related to salinity fluctuations and freshwater influx (Knaust 2015). Rarein deep-sea deposits (Krobicki et al. 2006)

Solitary funnel feeder and dwelling structure produced by enteropneusts,
maldanid polychaetes, or synaptid holothurians (Nara 2006; Löwemark
and Nara 2013)

Single, horizontal to oblique, simple tubes, ~ 2–5 mm in diameter, or rarely
a bunch of subparallel tubes, displaying a white, calcareous/silty wall. In
the Mochars core, trace fossil occurs in mudstones and wackestones, mainlyin the upper Bifrons and Variabilis zones

Schaubcylindrichnus isp. (Fig. 8c)

Interpretations and the basic references

Description

Ichnotaxon

Table 1 (continued)Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18
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Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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must be compatible with occurrences of other trace fossils, representing shallower tiers and/or more oxygenatedconditions. The abundance and diversity of other trace
fossils disaffirms dysoxic conditions in this case.
Among other trace fossils, of note are occurrences ofdwelling structures of filter-feeding organisms, building vertical structures stabilized by a mucus lining (Skolithos, ?Monocraterion, Arenicolites, Siphonichnus, cf.
Polykladichnus). With respect to behavioural groups, it
should be noted that the general abundance of suspension feeders in the Toarcian section of Mochras, althoughmore frequent than in the Pliensbachian (Pieńkowski
et al. 2021) is still low, probably because their filter apparatus can easily be plugged when the mineral suspension
concentration is high (Pieńkowski 1985; Thistle et al.
1991).
All described trace fossils are important components ofichnofabrics (ichnofabric is understood as an overall texture and structure formed by bioturbation or bioerosion;see Bromley and Ekdale 1986). Constituents of ichnofabrics may be attributed to several controls and attributes,
in particular to the tiering patterns and the manner ofcolonization (Ausich and Bottjer 1982; Taylor et al. 2003).
The complete (“ideal’) tiering pattern in the Mochras
Toarcian section can be summarized as follows. The shallowest tier consists first of an indistinct mottling that is
produced in the mixed layer by bioturbation in water-saturated soupy sediment near the sea floor, followed next
by the shallow tier of Phycosiphon (Ph1, Ph2, Ph3) showing a patchy distribution. The middle tier is occupied byThalassinoides, and slightly deeper Schaubcylindrichnus,
Planolites, rare Teichichnus, and yet deeper another generation of Phycosiphon (Ph4), reworked Thalassinoides
and rarely Teichichnus. The yet deeper tier is characterized by Teichichnus and Zoophycos, and the deepest one
is occupied by Trichichnus. Rare traces (cf. Polykladichnus, Skolithos, Arenicolites, Siphonichnus, Rhizocorallium) usually occupy a middle tier, below Phycosiphon(Ph1, Ph2, Ph3). On the other hand, Chondrites occupies
a deep tier. However, such a complete tiering patternoccurs rarely, and the most common tiering pattern islimited to shallowest tiers: Phycosiphon (Ph1, Ph2, Ph3),
Thalassinoides (middle tier), with Schaubcylindrichnus
slightly deeper, and Trichichnus occurring at the deepest
level.

the Cleveland Basin, some 300 km to the NE of Cardigan
Bay, is represented by organic-rich (black) shale up to late
Bifrons Zone age, known as the Mulgrave Shale (formerly
the Jet Rock), similar to the Schistes Cartons of France
and Posidonia Shale of Germany (Jenkyns 1988; Atkinson
et al. 2022). These strata were deposited in a restricted
shelf basin (Powell 2010) that was episodically affectedby storm processes (e.g., Kemp et al. 2018). Higher levels
within the Cleveland Basin Toarcian are clearly depositedin a storm-dominated shoreface setting (e.g., Hesselbo
and King 2019).
Based on sediment transport processes, the actionof two different types of deep-water currents and theirdepositional products have been recognized in the
Mochras section, namely contourite drift facies produced by along-slope contour currents (see Pieńkowski
et al. 2021 for the Pliensbachian part of the section) andturbidite deposits created predominantly by downslope
sediment density flows (Xu et al. 2018a—Exaratum Subzone, Serpentinum Zone, of the Toarcian section). Xu
et al. (2018a) illustrated few cm-scale sharp-based bedsin the T-OAE interval described as turbitites. However,fining upwards of these beds is not gradual, with coarser
intercalations sharply separated not only from below, butalso from above without continuous transition; therefore,
these beds may alternatively represent repeated intensifications of along-slope currents. It is also possible that
sediments of distal turbidites could occasionally havebeen “pirated” by contourite currents (Gong et al. 2017;
Rodríguez-Tovar et al. 2019a, b), and such a situation
could also have occurred in Mochras.
Apart from this T-OAE interval, the Toarcian section
in Mochras does not reveal any features of typical turbidites. The silty-sandy beds and laminae in the studiedsection show no systematic vertical grading or stacking
of structures, such as those recognized in the Bouma(1962) intervals. The Mochras section overall records
continuous sedimentation with structures characteristic of current traction, controlled by fluctuation of their
speed; therefore, deposition by turbiditity currents was
unlikely. In addition, turbidites are primarily bioturbatedfrom the top (e.g., Uchman and Wetzel 2011) and theuppermost layers of a turbidite exhibit total bioturbation, which decreases with depth as lamination and crossbedding appear. In contrast, contourites typically exhibit
more continuous and uniform bioturbation, whichappears throughout the entire contourite bed; this situation is typical for both the Pliensbachian and Toarcianof Mochras. However, the variability of ichnofabric pattern in contourites may depart from this typical picture(Rodríguez-Tovar et al. 2019a, b, 2022).
Interpretation of a wide spectrum of bathymetry andoccurrences of contourite currents is recognized, i.e.

4 Discussion
4.1 Sedimentary environment, ichnodiversity, cyclicity
of sedimentation
4.1.1 Sedimentary environment

The lithofacies in the Toarcian at Mochras contrast withthose in other settings for the UK Jurassic (as is also thecase for the Pliensbachian). For example, the Toarcian inPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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everywhere the sea floor had a considerable relief, commonly of more than a hundred metres amplitude, comprising moats, drifts, mounds and channels (e.g., Bein
and Weiler 1976; Surlyk and Lykke-Andersen 2007;
Esmerode et al. 2008; Van Rooij et al. 2010; Rebesco
et al. 2013), although the definition of Faugeres and Stow
(2008) is also applied, according to which the term contourite should be generally used for sediment accumulated in relatively deep water (markedly deeper than c.
200 m-deep shelf platform) and deposited or significantly
reworked by stable currents.
The deposits in Mochras can be classified as mixed
siliciclastic-calcareous/biogenic contourites, the mostfrequent contourite facies in the modern oceans(Fagueres and Mulders 2011). In less bioturbated sectionsof Mochras, primary sedimentary structures are visible
and are mostly represented by current structures such asplanar parallel lamination, low angle cross-lamination,
starved ripples, in places erosive bases, and gradational
normal grading bed transitions (Fig. 5a), like cases studied by Shanmugam (2000) and Knapp et al. (2017). Stagnant conditions or very slow currents allowed verticalsettling of the suspended particles from the nepheloid
layer (Ewing and Thorndike 1965) producing mottled
mudstone (Fig. 8d), or laminated mudstone-claystone(Figs. 7a and 8h), where the grain-supported laminae areinterpreted to be the result of very weak contour currents that winnowed out clay-sized sediment (e.g., Shanmugam 2000).
Higher bottom current velocities led to deposition ofsilt or sand layers with planar or low-angle cross-bedding,
produced by bedload transport (Figs. 5c, d and 7d) andperiodically these currents were strong enough to erode
older sediments and carry coarse-grained sediments,
including clay intraclasts and pebbles (Fig. 6). Comparedto the Pliensbachian (Pieńkowski et al. 2021), visible
plant remains are more frequent (see also Ullmann et al.
2021) and indicate more phytodetrital pulses in connection with fluvial discharges in the hinterland, which possibly could have led to acidification (Müller et al. 2020)
and intermittently reduced salinity of sea water (Deraand Donnadieu 2012), both adding (along with the Toarcian rise in p
­ CO2; see Hermoso et al. 2009) to collapsesof the carbonate factory (Bodin et al. 2023), suggested
particularly for the Tenuicostatum Zone to Exaratum
Subzone interval (Fig. 2a). Relatively weaker bioturbation
can be attributed to oxygen depletion or high sedimentation rate (Stow and Fagueres 2008); assuming rather
stable sedimentation rate, weaker bioturbation is here
largely attributed to decreasing circulation and oxygenation. One should bear in mind that strong bottom currents (Fig. 6) also do not favour preservation of biogenic
structures (Tucholke et al. 1985).

Fully developed bigradational intervals in the Mochras
section (Fig. 5) correspond to a standard contourite
sequence (Faugeres et al. 1984; Stow and Holbrook 1984;
Stow et al. 2002; Huneke and Stow 2008; RodríguezTovar and Hernandez-Molina 2018), a continuation of
the situation for the Pliensbachian (Pieńkowski et al.
2021). The standard contourite model was enhanced byintroduction of interval divisions (C1 to C5; Stow et al.
2002; Stow and Faugères 2008, their Fig. 13.9) and recognition of variations within partial contourite sequences(Rebesco et al. 2014; Shanmugam 2017), corresponding to observations in the Mochras profile (Figs. 4 and
5) where most frequent vertical sequences consist oflithofacies types 1/2–3–4–5–6 (in coarsening-upward
order) and subsequent fining-upward couplet composedof 5–4 lithofacies types. The fining-upward phase of the
fully developed cycle is usually thinner and incomplete.
In places, these sequences are interrupted by erosionalsurfaces and coarse sediments, including conglomerates (lithofacies type 7; Fig. 6). Periods of strong currents
marked by erosional surfaces and coarser material, pointing to intermittent non-deposition/erosional periods,
appear first in the late Falciferum Subzone of the Serpentinum Zone, and occur mostly in the late Bifrons Zone,
Variabilis-Thouarsense transition, and the late Levesquei
Subzone (Fig. 2a, b). In such cases the bigradational grading cycles are often incomplete, lacking their upper parts(top-cut-out cycles; see Fig. 5a). The depositional rate at
Mochras during the Toarcian remained generally stable
and relatively high, but the sections mentioned aboveshow condensation (Figs. 2a, b, 5 and 6).
4.1.2 Ichnodiversity

Occurrences of trace fossils also show a cyclic character, more or less corresponding with lithological changes
and sedimentary cycles. At one end of a spectrum is thedomination of opportunistic, r-selected Phycosiphon, diffused shape of trace fossils, indications of soft-bottom
conditions (Fig. 7f ), mostly simple tiering, and scarcity of highly specialized K-selected forms, all indicating a generally stressful environment (e.g., Ekdale 1985),
which was probably caused by high sedimentation rate
associated with unstable substrate and intermittent benthic food availability, interrupted by transient oxygendepleted, more stagnant conditions.
The other end of the spectrum is characterised byhigher substrate stability (stiffer conditions), lower sedimentation rate, continuous delivery of suspended nutrients by currents (often adsorbed onto suspended clay
minerals – Mayer 1994; Thistle et al. 1985), and usually fairly good oxygenation of the bottom, leading tomore complex tiering under equilibria (Taylor et al.
2003). A high degree of bioturbation is characteristicPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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for contourites due to additional food supply (the vertical particle flux is supplemented by lateral current-carried supply) and faunal abundance (Suess 1980; Wetzel
et al. 2008). In agreement with Caswell and Frid (2017),
changes in community composition are usually linked
to local redox conditions, whereas changes in populations of r-selected opportunists are driven by primary
productivity.

as Skolithos, Arenicolites, Monocraterion, Siphonichnus,
Rhizocorallium and Chondrites at its boundaries, pointing to more stable conditions. Both 2nd and 3rd ordercycle boundaries show sharp-walled, uncompacted, passively filled burrows that point to condensation/non-sedimentation (Fig. 8i).
Erosional surfaces, commonly associated with coarsegrained sediments, always indicate sedimentation breaks
and play an important role in the interpretation of theboundaries of the cycles. Usually, boundaries of the 2nd
or occasionally 3rd order cycle are placed at these surfaces. High velocity contourite currents are known togenerate erosional surfaces and contourite lag facies,
resulting from winnowing, reworking processes and hiatuses (Faugerez and Mulder 2011). In Mochras, the lag
facies contain a large range of grain sizes and compositions, forming irregular centimetre-thick to metre-thickbeds of poorly sorted sediments, mainly coarse sands,
gravels, and pebbles or rip-up clasts (Fig. 6).
The highest, 1st order, cycles are composed of six 2ndorder cycles. The beginning of the Toarcian sequence of
these cycles is anchored at the Pliensbachian-Toarcian
boundary, where the succession of four previous 1storder cycles of the latest Sinemurian to Pliensbachianage ends (Pieńkowski et al. 2021). These 1st order cycles
in the Toarcian differ from those observed in the Pliensbachian, mainly because a regular, short- and long-term
cyclicity of ­CaCO3 content (Ruhl et al. 2016; Pieńkowski
et al. 2021) disappears in the Toarcian, making the­CaCO3 content generally of less use in reconstructing
the cyclicity. Reduced C­ aCO3 content can be attributed
to environmental change that occurred at the beginning of the Toarcian. In a similar fashion to the Pliensbachian 1st order cycles, the greatest ichnodiversity
tends to appear in their middle parts. Higher ichnodiversity usually also coincides with condensation (Fig. 2a, b).
This interpreted ~ 2.4 Myr cyclicity retains its regularity
throughout the entire Pliensbachian–Toarcian section of
Mochras, although Charbonnier et al. (2023) postulated
much shorter cyclicity (~ 1.6 Myr) for the Sancerre corein Paris Basin—see discussion below.
Because generally the whole Toarcian in Mochras isdominated by Phycosiphon, any lack of this morphotype
is noteworthy, as it indicates an exceptionally unfavourable environment, most likely associated with stagnationand oxygen depletion. Of particular note is the finegrained interval between 823 and 868 mbs, mostly Tenuicostatum Zone, dominated by mudstone and claystone,
with significantly reduced C­aCO3 content, frequentsiderite occurrences (Xu et al. 2018a) and a marked
foraminiferal biotic crisis (Reolid et al. 2019). Here, Phycosiphon is reduced in frequency, and other trace fossils
are very scarce, while Trichichnus and undulated bedding

4.1.3 Cyclicity of sedimentation and spectral analysis
of ichnological data

Four orders of cycle duration are determined based on
visual inspection of the sedimentary succession. A 4thorder cyclicity is interpreted as a superposition of precession (c. 20 kyr) and obliquity terms (c. 38 kyr, Waltham2015). Precession and obliquity cycles are hardly distinguishable from each other in direct observation, but are
identified discretely by spectral analysis. The 3rd and2nd order cycles are identified with short eccentricity (c.
100 kyr) and long eccentricity (c. 405 kyr), respectively(Kent et al. 2018). A 1st-order cyclicity is attributed to
eccentricity modulation of the c. 2.4 Myr period, whichis caused by secular resonance between Earth and Mars
(Hinnov 2000; Laskar et al. 2004, 2011).
As described previously for the Pliensbachian(Pieńkowski et al. 2021), the distinction of cycles in the
Mochras Toarcian is based mainly on grain size and ichnological features and their lower boundaries are placedin the fine-grained, mottled, or laminated mudstone(lithofacies 1 and 2; see Fig. 4). Both the fully developed(bigradational), and the incomplete lithological coupletswith their ichnological content, constitute the 4th ordercycles, basic ‘building blocks’ of the hierarchical order ofcycles in Mochras (Figs. 2a, b and 5). Commonly, in thesecycles, meiofauna mottling (undulated bedding) appears
first, followed by colonization of Phycosiphon (Ph3) as thefirst recognizable trace fossil, then Ph2 and other trace
fossils return. These 4th order cycles are arranged inhigher hierarchical successions (3rd order cycles), usuallycontaining four, locally five, 4th order cycles. As every
3rd order cycle commences with the 4th order cycle, the
3rd order cycles start from meiofauna mottling and Ph3.
Schaubcylindrichnus, occasionally accompanied by Planolites, tends to appear at the tops of these cycles. In manycases Zoophycos can be found in the middle, and occasionally K-selected (equilibrichnia) forms, such as Skolithos, Arenicolites and Siphonichnus (dwelling/suspension
feeders’ structures), bivalve resting tracks Lockeia, larger
deposit feeder structures Rhizocorallium, and Chondrites(chemichnion), appear at boundaries of these cycles.
Next, 2nd order cycles are composed of four 3rd ordercycles. Sixteen out of 24 of the 2nd order cycles show
occurrences of K-selected (equilibrichnia) forms (suchPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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are prolific. Only for the middle of this interval (~ 838
to ~ 853 mbs) can some intermittent improvement ofbottom life conditions be inferred, indicated by less continuous occurrences of Trichichnus, brief re-appearancesof Phycosiphon, and presence of cf. Polykladichnus andChondrites (Fig. 2a). This slight recovery is also observedin foraminiferal assemblages (Reolid et al. 2019).
Less severe ichnological crises are noted in theThouarsense-Dispansum zones with Trichichnus occurrences and diminished ichnodiversity. Here, Phycosiphon scarcity is also observed although, compared tothe earliest Toarcian, less pronounced. Relatively abundant Trichichnus occurrences in the mid-Pseudoradiosa
Zone are accompanied by appearances of other diversified trace fossils; thus, this section can be regarded as
not affected by oxygen depletion. The Bifrons and Variabilis zones generally represent well-oxygenated intervals, shown by common occurrences of Thalassinoides,
Schaubcylindrichnus, Teichichnus and other trace fossils, including domichnia; only over depths 710–720 mbs
is a slight decrease in oxygenation inferred, marked bydecreased ichnodiversity and presence of Trichichnus.
Smoothed curves of particular ichnotaxa occurrences
juxtaposed with smoothed Zr/Rb, Ti/Al, Si/Al, Ca, TOC,
δ13CTOC, and gamma ray records reveal a high degree ofsimilarity (Fig. 3). The relationship of particular trace fossil occurrence curves to each proxy record is, to someextent, disparate, and the parallelism of respective curves
varies through the succession. Nonetheless, the outlines
of several curve crests are visually coherent, and thephase difference is insignificant given the timescale. Most
notably, the T-OAE-negative CIE has coincident expressions in the records of all analyzed ichnotaxa. The Zr/Rb
record, a proxy for silt/clay ratio, displays resemblance
to the highest number of ichnotaxa occurrences (five;Fig. 3), and the Thalassinoides occurrence curve resembles the highest number of proxy records (five). The bestvisual coherence appears to occur between the curvesof the following: Zr/Rb—Phycosiphon type 3, Thalassinoides and Planolites; Si/Al—undulated bedding; Cacontent—Thalassinoides and undulated bedding; TOC
content—Schaubcylindrichnus.
Interpretation of cyclicity and resulting calculation of time based on these cycles attributed to certain
orbital cycles becomes more complex in sections witherosional surfaces, associated with inferred hiatuses
in sedimentation, resulting in less regular sedimentation rate and cyclicity. It is reasonable to interpret thatmuch of the lost time is represented at the erosional/
hiatal surfaces. It is difficult to say how long lasting
were time intervals of non-sedimentation, or how deep
was erosion of previously deposited sediments. In suchcases, the 1st order (~ 2.4 Myr) cycles can provide sometime frameworks, allowing for approximate adjustmentof the 2nd and 3rd order cycles to the ~ 2.4 Myr duration of 1st order cycles. This is particularly importantin the Toarcian above the Serpentinum Zone, wheremost erosional/hiatal surfaces with coarser sediments
occur, usually concentrated in the middle parts of the
1st order cycles (Fig. 2a, b).
For numerical cyclicity (spectral) analysis only frequently occurring trace fossils can be used. Phycosiphon
incertum is by far the most common trace fossil in the
Mochras section. As it is represented by four distinct
morphotypes (Ph1, Ph2, Ph3, Ph4), they are treated separately for analysis of cyclicity (in the Toarcian Ph2 andPh3 were used). Thalassinoides, Schaubcylindrichnus,
Planolites, and Trichichnus are also frequent enough to
be used for analysis of cyclicity as well. Other ichnotaxa
are not frequent enough for spectral analysis, although
their cyclic appearances yield useful information concerning a more general hierarchy of cycles. Additionally,lamination and undulated bedding can be used for spectral analysis.
The runs test results indicate the non-random distribution of either trace fossil, lamination and undulated bedding occurrences in the studied sedimentary succession(Additional file 2), demonstrating the advisability of timeseries analysis. Absolute values of all computed Z-scoresare within the critical region of > 1.96, which allows rejection of the null hypothesis of the data randomness at the
95% confidence level.
The peak values of the Walsh power-spectra (Fig. 9)
correlate to the calculation-based expected duration of
the orbitally forced periodicities (Table 2). The resultant spectral peaks (Fig. 9) have estimated time values
assigned as explained in Sect. 2. Component frequencies
of precession, obliquity and short eccentricity, as well
as beat frequencies, are present in the resultant spectra
besides the main terms (Tiwari 1987; Negi et al. 1993).
The longer the periodicity, the more prone it is to be
blurred by instantaneous changes in the sedimentation
rate. Besides, shorter-term periodicities can be recordedwith higher precision than these longer-term, as a larger
number of them are contained within the studied timeseries.
Either one or more spectral peaks in the sequency
range of circa 0.9–1.2 cycles/m are reflected by most of
the analysed time series, with the exception of Trichichnus (Table 2). These prevalent, albeit relatively low-powerpeaks are interpreted as precessional terms corresponding to the visually determined 4th order cycles. Schaubcylindrichnus occurrences reflect the most (three)component terms in the range of precession, which inthis case are characterised by spectral power on par tothe other periodicities.

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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Table 2 Spectral peak values recognized in the present study against the background of calculated present-day component orbital
periodicities
Phycosiphon
morphotype
Ph2 [kyr]104–117

Phycosiphon
morphotype
Ph3 [kyr]Thalassinoides[kyr]

108–121

Schaubcylindrichnus[kyr]

115–129

Planolites[kyr]

Trichichnus[kyr]

Lamination[kyr]

155–174

132–148

Undulated
bedding[kyr]

111–125

81–91

75–84
66–74.2

36.2–40.7

25.9–29

39.9–44.9

26.8–30.1

21.3–24

19.9–22.4

24–27

35.6–40

42.6–47.9

25.7–28.9

22.2–25
18–20.2

19.6–22
17.7–19.9

173

Obliquity
AM

112

Eccentricity bands

95

59–64 Combined
effect40–42 Obliquity
bands
35.3–39.7

29.8–33.4

Calculated
orbital periods[kyr]29.6–33.3
19.1–21.5

17.5–19.7

17–19.1

37.5

Main
obliquity
term(Toarcian)

29–30 Obliquity
bands
19.9–22.4

22–24 Precessional
19
bands
16–17

Present-day component orbital periodicities after Berger (1977); main obliquity term for Toarcian taken from Waltham’s (2015) Milankovitch Calculator; obliquity
amplitude modulation (AM) after Hinnov (2000)

Periodicities in the range of either main obliquity term
(Phycosiphon Ph3 and undulated bedding) calculated
for the Toarcian (Waltham 2015), or theoretical component obliquity terms (remaining time series) are distinct
in all Walsh spectra, even though obliquity is not readily distinguished visually in the sedimentary succession.
Periodicity of c. 28 kyr is observed in the occurrencesof Phycosiphon (Ph2 and Ph3) and Planolites, which canprobably be identified with the shortest component termof obliquity (Tiwari 1987; Laskar et al. 2011). Presumably, the peak near the 0.31 cycles/m value displayed by
Schaubcylindrichnus occurrences (Fig. 9) can be associated with the combined effect of obliquity and precession, as proposed by Berger (1977).
Peaks interpreted as short eccentricity (3rd order)
terms are the most intensely (high-power) represented
in the spectrograms (Fig. 9). Periodicities in the range ofc. 105–120 kyr recorded by Phycosiphon Ph2 and Ph3,
Schaubcylindrichnus and Planolites (Fig. 9) can be identified as the longer component term (c. 112 kyr) of shorteccentricity cycles, presumably reflecting the 3rd ordercycles distinguished in the sedimentary and ichnologicalrecord (Fig. 2a, b). In addition, ~ 90 kyr periodicity displayed by Thalassinoides can be interpreted as the shortest eccentricity component term (~ 95 kyr).
Trichichnus occurrences record also the periodicityof the ~ 170 kyr term, which likely can reflect a cyclicity
of c. 150–170 kyr, also present in the Pliensbachian ofMochras, and there preliminarly regarded as non-astronomical (Pieńkowski et al. 2021). However, this cyclicity
could be associated with the amplitude modulation (AM)of the obliquity (Hinnov 2000). Such AM 173 kyr periodicity was hitherto detected in the sedimentary record by
the means of time series analysis by Laurin et al. (2015),
Boulila et al. (2018), Vahlenkamp et al. (2018) and Huang
et al. (2021).
The peak clusters present in the leftmost parts of the
Planolites, Thalassinoides and Trichichnus spectrograms
may be associated with 2nd order ~ 405 kyr cycles (registered by recurrent appearances of Skolithos, Siphonichnus, Arenicolites, and Chondrites, which were notincluded in the spectral analysis due to their low frequency). However, this remains a presumption due tospectral resolution of spectra below the sequency valuesof 0.1 cycles/m being inadequate to confidently interpretthese peaks in the time domain.
The most complete representation of orbital periodicities is reflected by both Phycosiphon morphotypes,
Thalassinoides, Schaubcylindrichnus and Planolites(Fig. 9) which clearly display the frequencies of the shorteccentricity terms (~ 95 and 112 kyr), obliquity terms(~ 29 and ~ 40 kyr) and precessional terms (~ 17–25
kyr). Spectra of both Phycosiphon morphotypes show agood congruence, especially considering an interpreted
longer component term of short eccentricity. In the caseof Phycosiphon Ph2, it can be supposed that evidentPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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periodicities associated with either 4th or 3rd ordercycles are derivative of periodic short non-occurrencesof this pervasively occurring trace fossil. Interestingly,
Trichichnus occurrences seem solely to reflect periodicities related to obliquity and its AM frequency.
Longer-term ‘grand cycles’ which are not included inspectral analysis, are characterized by a gradually growing and subsequently falling Phycosiphon frequency andgeneral ichnodiversity. Judging by the duration of ~ 100
kyr and ~ 405 kyr eccentricity cycles, these longer-term
eccentricity “grand cycles” would be of duration c. 2.4
Myr, corresponding to the period of around 2.4 Myr
eccentricity modulation related with Mesozoic greenhouse sequences, caused by Earth–Mars secular resonance (Hinnov 2000; Laskar et al. 2004, 2011; Martinez
and Dera 2015). Whereas individual repetitions of the∼2.4-Myr cycle range from 2.0 to 2.9 Myr in the Cenozoic (Pälike et al. 2006), cyclostratigraphic studies suggestthat this period fluctuated between maximal values ranging from ∼1.3–3.3 Myr during the Mesozoic (Ikeda andTada 2013, 2020). Indeed, the periodicity of grand eccentricity cycles prior to 50 Ma remains uncertain. Charbonier et al. (2023) postulated a chaotic behaviour of the
Solar System and ~ 1.6 Myr Earth-Mars resonance cyclicity (instead of ~ 2.4 Myr) for the Early Jurassic, based on
the Sancerre-Couy core (Paris Basin). However, Wu et al(2022), while also suggesting transient chaotic behaviour of the Solar System in the Late Cretaceous of theSongliao Basin (China), revealed evidence for two chaotic
secular resonance transitions in the orbital motions ofEarth and Mars, involving the orbital eccentricity modulations of 2.4 Myr and 1.2 Myr cycling, without indicationof ~ 1.6 Myr cycling. We support the ~ 2.4 beat (Martinez
and Dera 2015), because the observed general ichnological cycles, which are built on well-established hierarchyof sub-cycles, are much less compatible with the ~ 1.6
Myr duration of grand orbital cycles of Charbonier et al.
(2023) than with the ~ 2.4 Myr cycles (Fig. 2a, b).
The existence of yet longer cyclicity, ~ 7–8 Myr eccentricity (Martinez and Dera 2015; Pieńkowski et al. 2021;
Charbonier et al. 2023) is supported, based on two possible such cycles in the combined Pliensbachian-Toarcian
section.

with severe stagnation of circulation through the Laurasian Seaway (and the Cardigan Bay Strait) at that time
(Van Schootbrugge et al. 2019). Thermohaline circulation driven by density differences between water masses
due to variations in water temperature and salinity is
still regarded as the major driving force of contourites(McCave 2008; Faugeres and Mulder 2011; Rebesco et al.
2014). The biogenic input from shallower zones, likely
carbonate platforms, was less marked than in Pliensbachian, and was replaced by a stronger siliciclastic
input, which also carried more floral debris and possibly connected to acidification (Xu et al. 2018a, b; Müller
et al. 2020)—although δ44/40Ca and δ88/86Sr records do
not support acidification (Li et al. 2021). Bottom currents carried in suspension a considerable amount of particulate organic matter, supplying food to deep-marine
benthic organisms (Thistle et al. 1985). As for the Pliensbachian, it is proposed here that the deep-water circulation in the elongated, NE-SW trending Cardigan Basin(Fig. 1) was forced by enhanced bottom-water circulation, i.e. cooler and denser waters flowing with changing velocity from the Boreal Sea, around the Shetland
Platform-Scottish Landmass island, towards the south,
to the Peri-Tethys/proto-Atlantic, approximately parallelto the bathymetric contours of the margin of the Welsh
Platform (Fig. 1).
Interpretations of circulation (Bjerrum et al. 2001;
Dera et al. 2009; Dera and Donnadieu 2012; Ruvalcaba
Baroni et al. 2018) point to the predominant southward flow from the Arctic into the Tethys through theLaurasian Seaway during the Early Jurassic. The Cardigan Bay Strait, linking cooler and shallower waters of
the Boreal Sea with warmer and deeper waters of PeriTethys (Fig. 1), would then have been situated to sustain
a continuous (persisting for up to millions of years andover large areas—Shanmugam 2008, 2017) thermohalinedriven contour current circulation between these twomarine realms (Pieńkowski et al. 2021). These invigorated flow conditions were punctuated by times of sluggish circulation or stagnanation characterised by loss ofichnodiversity and Phycosiphon crises (usually also bymass Trichichnus occurrences), as well as appearance of
undulated bedding. These intervals occur in the Tenuicostatum Zone through to earliest Exaratum Subzone age
strata and, to a lesser extent, in the latest Thouarsense
Zone to Dispansum Zone.
This sluggish circulation is associated with climate
warming under high atmospheric p­ CO2, when the Laurasian Seaway was influenced by strong clockwise circulation in the Tethys, which brought warm saline waters
onto European shelves and then diminished the effectsof flow from the north through the Laurasian Seaway(Ruvalcaba-Baroni et al. 2018). In the same time, loss of

4.2 Toarcian deep sea current circulation

Oxygen deficiency and resulting decline in ichnodiversity
can be attributed to the diminishing current intensity,
but also to other related or independent causes, including high organic productivity and sea level. The earliest Toarcian crisis that had begun already in the latest
Pliensbachian was exceptional in its severity (Xu et al.
2018a; Reolid et al. 2019; Pieńkowski et al. 2021; Bodin
et al. 2023). Most likely, the benthic crisis was associatedPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 23 of 34

polar sea ice and stronger high-latitude continental runoff rates could further result in a weakening of thermohaline circulation, and bottom water oxygen depletion also
in deep marine settings (Dera and Donnadieu 2012; Vande Schootbrugge et al. 2019).
For the Pliensbachian-Toarcian boundary interval, this
scenario is applicable, as a warming pulse is postulated(associated with initial stage of the Karoo magmatism;
Dera et al. 2009; Pieńkowski et al. 2016; Ruebsam et al.
2020). However, for the remaining part of the Tenuicostatum Zone many authors (e.g., Brandt 1986; McArthur
et al. 2000; Guex et al. 2001; Suan et al. 2010; Dera et al.
2011; Korte and Hesselbo 2011; Krencker et al. 2014;
Korte et al. 2015; Ruebsam et al. 2020; Ullmann et al.
2020) suggest overall persistent cool conditions, whichshould (according to the thermohaline-oceanic circulation mechanism) cause enhanced circulation—which
is not observed in Mochras, where stagnation dominates. Only in a short interval between ~ 838 and ~ 853
mbs, is there a slight improvement of bottom conditions(confirmed also by foraminiferal assemblage; see Reolid
et al. 2019), which could be associated with somewhat
increased current intensity, possibly caused by a transientcooling, but this signal is weak.
Concerning the circulation problem, even more
intriguing is the To-CIE interval, well known to be associated with the most severe carbon-cycle disruption, high­pCO2 and extreme greenhouse conditions, which shouldcause even stronger diminution of southward flow andextreme sea-floor current stagnation at that time; instead,shortly after the beginning of the To-CIE (in our calculation ~ 200 kyr), the deep-sea circulation significantly
accelerated. Only at the beginning of To-CIE did sluggish circulation and oxygen depletion persist, which isindicated by the continued Phycosiphon crisis and common occurrences of Trichichnus. For the remaining partof the To-CIE event, certainly from its climax (~ 820mbs), gradually improving circulation can be interpreted.
This is recorded by common current-generated structures (pin-stripe current lamination, occasionally ripplecross lamination), some sharp boundaries and coarser
sediments, and gradual return of more diversified trace
fossils, such as Phycosiphon, Thalassinoides, Schaubcylindrichnus, Planolites, Lockeia, Asterosoma, Skolithos andRhizocorallium, and concomitant demise of Trichichnus(Fig. 2a). The recovery is also visible in the foraminiferal
assemblage (Reolid et al. 2019).
It is noteworthy that anoxia was suggested by somegeochemical indices to be coincident with this ‘recovery’ phase of the To-CIE: Xu et al. (2018a) reported
trace amounts of the biomarker isorenieratane (withslightly elevated gammacerane indices) at two horizons (811.66 mbs and 819.10 mbs) at and just above

the peak of the To-CIE. Isorenieratane is a pigment ofphotosynthetic green sulphur bacteria, Chlorobiaceae,that is regarded as an indication for photic zone euxinia in the water column (Koopmans et al. 1996). We
investigated these horizons in detail, finding that they
contain bioturbated mudstones, either by meiofauna(Fig. 8e), or more distinctly by Phycosiphon incertum
type 2 (Fig. 8h). This means that anoxic to euxinic conditions in the photic zone would not necessarily haveaffected the seafloor in Mochras, which was continuously oxygenated by deep-sea bottom currents. Thisobservation is of more general significance, suggestingthat geochemical indices (even if correctly pointing tocertain conditions in a shallow-water column) may notalways indicate euxinic bottom conditions, because
deep-sea currents could deliver enough oxygen tosustain life there, even if the shallow-photic zone wasanoxic. However, a scenario of bottom re-oxygenation(to the degree required by trace makers) and overprinting by bioturbation of sea floor that had been previously affected by anoxia is also possible—although thiswould still point to the short and transient nature of
any oxygen crisis and, given the water depth and muddy
productive nature of the water column, the sea-floor isunlikely to have been in the euphotic zone.
Observed lithological and ichnological fluctuations in
the Toarcian are thus associated with alternating periods of vigorous currents and more stagnant conditions
on various timescales, while nutrient availability seems
to have been relatively stable and sufficient all the time
(as shown by the TOC content; see Xu et al. 2018a) andplayed a subordinate role in setting ichnological trends.
Hierarchical cyclic successions observed both in lithology
and ichnology indicate that orbital forcing was still themain controlling mechanism. However, at the beginningof the Toarcian, the lithological and ichnological appearance of these cycles became different, which can belinked to non-orbital, intrinsic Earth mechanisms leading to prolonged ichnological crisis in the Tenuicostatum
Zone. More generally, it is also observed that comparedto the Pliensbachian, the energy of contour currents in
the Toarcian (after the Tenuicostatum Zone) was overall higher, temporarily producing marked erosional surfaces and carrying coarser sediment, which collectively
resulted also in less regular thicknesses of sedimentary
packages corresponding to individual ~ 100 and ~ 405 kyrcycles (although the 1st order cycles retained approximately stable thicknesses, which point to a generally
stable average sedimentation rate in a longer period oftime). Although there is no clear correlation between
the organic matter carbon isotopes and observed higher
energy of currents, there is a return to generally lighter
values of δ13C from the middle part of the Bifrons Zone.

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 24 of 34

Of note also is the repetition of the two fine-grainedintervals characterized by particularly sluggish circulation, impoverished ichnodiversity (including the Phycosiphon crises), and abundance of Trichichnus: the lower
one of latest Spinatum Zone to Tenuicostatum Zone age,and the upper one belonging to the Thouarsense and Dispansum zones. These intervals are separated by ~ 7.5 Myr
in Mochras. A similar mega-cycle was observed in the
Pliensbachian section (Pieńkowski et al. 2021), and comparable long cycles have also been hypothesised by Martinez and Dera (2015) and Charbonnier et al. (2023) andtentatively identified with a very long eccentricity term.

the resultant cyclicity of 2.4–2.5 Myr observed in
Mochras.
The 2.4 Myr cyclicity in the present work was directly
observed in the core by visual scrutiny of the variability of sedimentological and ichnological parameters. Inturn, long-period cyclicities were detected by Charbonier et al. (2023) based solely on the numerical spectral decomposition of magnetic susceptibility time series
from the Sancerre-Couy borehole, Paris Basin, France.
Such spectral techniques reveal component frequencies of basic orbital periodicities that directy affect the
sedimentary record. Importantly, Pliensbachian grandorbital cycles in Mochras (Pieńkowski et al. 2021), put
together on an interpreted 405 kyr cyclicity based on­CaCO3 cyles (Ruhl et al. 2016), clay minerals (Deconinck
et al. 2019), δ13C (Storm et al. 2020) and ichnology-sedimentology (Pieńkowski et al. 2021) also show prevalence(despite some discrepancies) of ~ 2.4–2.5 Myr cyclicity. This arrangement, based on direct ichnological andsedimentological observation, persists into the Toarcian at Mochras. Therefore, we favour effective influence of ~ 2.4–2.5 Myr cyclicity on sedimentary processes
in Mochras, because adoption of the ~ 1.6 Myr cyclicity
postulated by Charbonier et al. (2023) is less comparable
to observed long-term sedimentary and paleoecologicaltrends observed both for the Pliensbachian (Pieńkowski
et al. 2021) and the Toarcian (this study). The possibilityof blurring and distortion of orbital cyclicity by autocyclic depositional processes, or tectonic processes, particularly in shallower epicontinental environments suchas at Sancerre, should not be totally ruled out in that
context.
As shown by our data from the Toarcian of Mochras(Fig. 2a, b), the effects of orbital forcing were no doubt
also influenced by the palaeoceanographic, palaeoenvironmental and tectonic factors. The picture obtained differs from surrounding epicontinental seas, showing also
more complicated mechanisms of oceanographic processes, as a result of mutual influence of orbital forcing
and non-orbital changes related to large-scale volcanism
and climatic/environmental disturbances. The mechanism linking sluggish deep-sea circulation with climate
warming, proposed by Dera and Donnadieu (2012),
Ruvalcaba Baroni et al. (2018) and Van Schootbrugge
et al. (2019), suggested for the Sinemurian–Pliensbachiansection (Pieńkowski et al. 2021), appears to be still valid
for most of the Toarcian in Mochras, but fails to explainchanges that occurred in the earliest Toarcian (Tenuicostatum–Exaratum interval). Severity and continuity of thelatest Pliensbachian to earliest Toarcian stagnation andbenthic crisis probably occurred during transition fromsupposed icehouse to greenhouse conditions (Dera andDonnadieu 2012; Krencker et al. 2014; Bodin et al. 2023)

4.3 Climate changes and carbon cycle disturbances:
impact on deep‑sea circulation and orbital forcing

Two intrinsic Earth events should be considered carefullyin terms of their influence on the Toarcian hemipelagic
sedimentation in Mochras. The first was the Pliensbachian-Toarcian (Pl-To) boundary event, characterized
by a CIE that is globally documented and linked to theinitial phase of volcanism in the Karoo Province; the second event with profound impact on the marine and continental environment was the To-CIE (also known as theJenkyns Event), dated to the latest Tenuicostatum Zone
to Exaratum Subzone of the Serpentinum Zone, linked tolarge-scale eruptions in the Karoo-Ferrar basaltic province and associated with a rapid increase in atmospheric­pCO2 levels and average atmospheric and marine temperatures (see e.g., Hesselbo et al. 2000, 2007; Littler et el.
2010; Hesselbo and Pieńkowski 2011; Bodin et al. 2016,
2023; Pieńkowski et al. 2016; Fantasia et al. 2018; Xu et al.
2018a; Ruebsam et al. 2020; Al-Suwaidi et al. 2022; Ruhl
et al. 2022).
It is important to understand how these major environmental changes, indicated by disturbances in the carbon
cycle, affected the deep-sea circulation. For a long time(since the Sinemurian) this circulation was controlled ina stable way by orbital forcing, expressed in cyclic fluctuations of southward flowing thermohaline contour currents in the Cardigan Bay Basin, particularly expressed inlong-term trends governed by grand eccentricity (~ 2.4
Myr) cycles (Pieńkowski et al. 2021). Notably, Charbonnier et al. (2023) postulated shortened long-period
cyclicities of ~ 1.6 and ~ 3.4 Myr in Early Jurassic orbital
periods, possibly reflecting the chaotic orbital motion of
the inner planets (likely corresponding to the Cenozoic
2.4 Myr and 4.7 Myr eccentricity terms). This is incompatible with our interpretation even if the 7–8 Myr very
long eccentricity cycle and the Toarcian duration postulated both by Charbonier et al. (2023) and by us are inagreement. Possibly, in Mochras we would have to deal
with the mutual modulation of the two cyclicities indicated by Charbonier et al. (2023), giving approximatelyPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 25 of 34

and from sea-level lowstand to marked sea-level rise(Pieńkowski 2004; Haq 2017; Ruebsam et al. 2020; Bodin
et al. 2023).
Stagnant conditions at the Pliensbachian–Toarcian
boundary did not end at the beginning of the Tenuicostatum Zone, but continued through the whole zone
(short-lived, slight improvement of oxygenation at thedepth interval between 838.5 mbs and 847.0 mbs is too
weak to be considered as significant break in the general
stagnation). If the Tenuicostatum Zone was a cool periodof time, then, according to the cooling/faster circulation
paradigm, it would have resulted in a faster circulation
throughout the entire zone. Instead, the Pliensbachian–Toarcian boundary event occurred at the beginning of
long-lasting stagnation in Mochras. The sea-level rise
postulated for this time interval (Pieńkowski 2004; Haq
2017; Ruebsam et al. 2020; Bodin et al. 2023) could have
had some impact on deep-sea circulation, but it is uncertain in which direction. Sea-level rise would have led tolower energy of sedimentary processes and stagnation oncontinental shelves (due to the raised wave base), but it isunclear what would have happened in the case of much
deeper water in the Cardigan Bay Basin. The influenceof salinity drop in the early Toarcian epicontinental sea
in UK (Remirez and Algeo 2020; Hesselbo et al. 2020b)
could be considered as well, because density gradient is
one of the main driving forces of deep-sea currents. By
analogy with the recent Mediterranean and Atlantic
waters (e.g., Bethoux et al. 1999), peri-Tethyan waters
should have been more saline (due to evaporation) thanwaters in the Laurasian Seaway or Boreal Sea, where
salinity could be diminished by freshwater influx fromrivers and intermittent ice melt. However, the exact influence of the salinity gradient (which is elusive itself ) on
current intensity in the area studied remains uncertain.
The prolonged stagnation during the Tenuicostatum
Zone could have resulted from coincidence of several
overlapping causes, such as a warming phase of orbital
forcing connected with a possible ~ 2.4 Myr grand eccentricity minimum, amplified by a concomitant warming
effect, caused by initial Karoo volcanism emissions andconsequent increase in ­pCO2. Due to the mutual feedback effect and inertia of deep-sea oceanographic processes, this stagnation would last for a long time (~ 800kyr). Stagnation of deep-sea circulation in the Laurasianseaway may challenge interpretations of continued cold
conditions during the Tenuicostatum Zone, although
the data from Yorkshire are unambiguous on this point(Korte et al. 2015, their Fig. 2). On the other hand, theCleveland Basin could be a local anomaly, as some otherreports (e.g., Ruebsam et al. 2020, their Fig. 5) claimthat during the Tenuicostatum (= Polymorphum) Zone
the Earth’s climate system shifted between contrastingclimatic conditions, particularly at the beginning and theend of this zone.
Extreme global warming occurred during the following Exaratum Subzone of the Serpentinum Zone, withbenthic sea water temperature suggested to increase
from ~  + 3 to 5 °C in mid-latitude Laurasian Seaway locations (see discussions in McArthur et al. 2000; Pálfy andSmith 2000; Gómez et al. 2008; Suan et al. 2010; Dera andDonnadieu 2012; Ullmann et al. 2020), or even as muchas + 10 °C suggested for some sea-surface temperatures(Ruebsam et al. 2020). Such temperature increases, jointlywith the demise of polar sea ice and stronger high-latitude continental runoff rates—which could have freshened the Arctic surface seawaters—would be expected tocause a general thermohaline circulation collapse (Deraand Donnadieu 2012). However, this is not evident in
Mochras—instead, the Exaratum Subzone extreme global
warming ended the previously dominating stagnation,
counterarguing the previously supposed mechanism
linking stagnation with warming. Likely, this time theglobal warming was so strong, that its consequences fordeep-sea circulation were opposite—instead of sluggish
circulation caused by northward currents only neutralizing dominating southward flow, the Tethyan northward
flow became strong enough to prevail over the southward currents. As a consequence, the reversed deep-sea
circulation became faster in the Cardigan Bay Basin. In
particular, Dera and Donnadieu (2012) in their General
Circulation Models (GCM) simulations found that forsignificantly higher ­pCO2 levels (> 1600 ppm), the depthof mixed layer appears deeper in the European basins.
This suggests a good ventilation of the water column during the To-CIE, which is obviously not the case in semiclosed, epicontinental seas in western Europe. However,since this simulation was based on a homogeneous epicontinental bathymetry of 200 m, even more significant
results could be expected with a deeper bathymetry of
the Cardigan Bay Basin. Moreover, it is worth notingthat for other periods such as the Late Permian, GCM
simulations show that massive rises in atmospheric C­ O2
concentrations may drive more vigorous and more symmetrical deep-sea circulation under warmer climates(Winguth et al. 2002; Winguth and Maier-Reimer 2005).
According to Dera and Donnadieu (2012), the Exaratum Subzone thermal anomaly was probably related toa strengthening of warm equatorial Tethyan westward
currents, drifting along the northern Gondwanan margins. Furthermore, the drift of these currents throughwesternmost areas and their subsequent clockwise rotation due to southward directed boreal flows is consistent
with neodymium isotope data (Dera et al. 2009), as wellas ammonite and nannofossil migration routes evidenced
for the Early Toarcian (Reggiani et al. 2010; Dera et al.

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Page 26 of 342011). It is also likely that more vigorous NW Tethyan
currents would account for major disruptions in faunal
provincialism and northward expansion of marine Mediterranean faunas at the beginning of the Toarcian (Macchioni and Cecca 2002; Vörös 2002; Arias and Whatley2005; Dera et al. 2011).
During the Tenuicostatum Zone to Exaratum Subzone period of time, the sedimentation rate in Mochrasremained relatively stable and was still paced by precession (~ 20 kyr) and short- and long-eccentricity cycles(~ 100 and 405 kyr), which are all well expressed (Fig. 2a).
Circulation decelerated just after the To-CIE event, inthe earliest Falciferum Subzone (785–795 mbs), which isreflected by impoverished ichnodiversity and lithological features. From this time on, Toarcian circulation evidently became generally faster, particularly with regard
to the middle parts of 1st order cycles, where morecondensed sections and more frequent erosional/omission surfaces occur. The observations confirm that thewarming/stagnation versus cooling/enhanced circulation
mechanism returned after the To-CIE disturbances.
Importantly, existing data on sea-water temperature fluctuations in the Toarcian (McArthur et al. 2000;
Gómez et al. 2008; Suan et al. 2010; Krencker et al. 2014;
Ruebsam et al. 2020; Ullmann et al. 2020) support ourconclusions regarding a large-scale correlation between
sea-water temperature and inferred fluctuations of deepsea current in the Cardigan Bay Basin. From the late Falciferum Subzone onwards, oxygenation of the sea floor
generally improved; circulation intensity and average
oxygenation achieved highest levels in the late Bifrons
and Variabilis to earliest Thouarsense zones, as shownby sedimentological and ichnological features (Fig. 2b).
This is likely associated with a long-term cooling trend,
extending from the latest Falciferum Subzone to theThouarsense Zone, with more intensified cooling in the
Variabilis Zone. The latter was interrupted by a brief
warming in the middle part of this zone (McArthur et al.
2000; Krencker et al. 2014). This brief warming seems tobe also reflected in a slightly deteriorated ichnodiversity
in Mochras (707–713 mbs), at the same time marking aboundary of ~ 2.4 Myr cycle.
The latest Thouarsense–Dispansum time interval in
Mochras shows the next significant deterioration ofichnodiversity/oxygenation. This is likely connected toa warming trend that intensified in Dispansum Zone
(Gómez et al. 2008), exactly when we are dealing withthe conspicuous ichnodiversity/oxygenation crisis in
Mochras. A weaker warming in the Pseudoradiosa Zone
also corresponds to a slight deterioration of ichnodiversity around 610–615 mbs. The repetitive correlations
between Toarcian sea-water temperature reconstructions elsewhere and contourite current intensity in

the Cardigan Bay Basin point to a strong relationshipbetween these variables, linking deep-sea circulation to
major climate changes.
Toarcian sea-level changes are sometimes suspected asa possible factor in observed oceanographic processes.
According to Ayranci et al. (2018), high-stand and transgressive-system tracts in Devonian contourites are represented by dominantly massive mudstone lithofacies, lessintense bioturbation, and higher TOC values. Rising relative sea-level trends are confirmed for the Tenuicostatum
Zone and also early Serpentinum, Bifrons and early
Variabilis zones (Sellwood 1972; Graciansky et al. 1998;Pieńkowski 2004; Hesselbo 2008; Rocha et al. 2016; Barthet al. 2018; Haq 2017; Ruebsam and Al-Husseini 2021),
but their relation to observed ichnological and sedimentological features in the Mochras profile is uncertain(Fig. 2a, b). Assuming a deep-sea setting of the Cardigan Bay Basin, major oceanographic/climate changes
would have had a stronger impact than eustatic sea-levelchanges with amplitudes of only tens of metres (Haq
2017). Generally, a higher sea level might have enhanced
the exchange of waters between adjacent basins and thecirculation of currents, but the impact of eustatic (or tectonically-induced) sea level on deep-sea currents or the
sediments entrained within them remains uncertain.
In addition to the beginning of the Toarcian (as discussed above), the relations between sedimentary(orbital) cycles and δ13C fluctuations are not as clear
as they were in older Early Jurassic stages at Mochras(Storm et al. 2020). Some of the minor negative excursions at depths of 750, 719, 697, 675, 667 and 630 mbs
seem to be related with more stagnant conditions (shortlived minor Phycosiphon crises, appearances of lamination and Trichichnus), but this relation requires more
systematic study and a much higher resolution C-isotope
curve. This is particularly important in the upper partof the profile, where the variations in δ13C values are
subdued.
4.4 Duration of the Toarcian and ammonite zones basedon ichnological astrochronology

The Toarcian section of the Mochras core represents themost expanded and highest resolution cyclostratigraphic
dataset for this stage to date (even if much of the Aalensis Zone is eroded), and provides the most reliable basis
for an astrochronological time scale, reflecting Milankovitch forcing, predominantly at precession/obliquity andthe short- and long-eccentricity periodicities (Fig. 2a, b).
The new results allow an estimation of the duration of the
Toarcian stage at a minimum of 9.4 Myr (without most of
the Aalensis Zone). Results were obtained from ichnological and sedimentological signals by counting successive
4th, 3rd and 2nd order cycles (interpreted respectively asPieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

Table 3 Duration of Toarcian biochronozones
Aalensis> 0.1 Myr

Pseudoradiosa1.4 ± 0.02 Myr

Dispansum

0.3 ± 0.07 Myr

Thouarsense

0.8 ± 0.1 Myr

Variabilis

2.4 ± 0.1 Myr

Bifrons

2.1 ± 0.5 Myr

Serpentinum

1.4 ± 0.4 Myr

Tenuicostatum

0.9 ± 0.1 Myr

TOARCIAN> 9.4 Myr

Note that most of the Aalensis Zone is missing in the Mochras section. All
durations are minima and the uncertainties relate only to precision of theplacement of the zonal boundaries in the core based on ammonite distributions
recorded in Additional files 3–5

precession/obliquity, short eccentricity, long eccentricity). Uncertainty values of zone durations are estimatedbased on depth-domain uncertainties of zonal boundaries (Additional file 3). For each zone, summed uncertainties of the base and top boundary are treated as apercentage of nominal zone thickness and proportionally converted to time domain. In case of the Dispansum
and Pseudoradiosa zones the time-domain uncertainties
are represented by spurious digits and are therefore negligible. Astronomical durations for the ammonite chronozones (based mainly on ~ 405 kyr, 2nd order cycles)
are: Tenuicostatum Zone = 0.9 ± 0.1 Myr, Serpentinum
Zone = 1.4 ± 0.4 Myr, Bifrons Zone = 2.1 ± 0.5 Myr, Variabilis Zone = 2.4 ± 0.1 Myr, Thouarsense Zone = 0.8 ± 0.1
Myr, Dispansum Zone = 0.3 ± 0.07 Myr, Pseudoradiosa
Zone = 1.4 ± 0.02 Myr. The Aalensis Zone (truncated at itstop) is preserved probably representing no more than 0.5
Myr of deposition. The obtained durations of successive
ammonite zones (Table 3) are slightly longer compared tothose shown in GTS2020 (Hesselbo et al. 2020a).

5 Conclusions
1. An integrated ichnological-sedimentological studyof the Toarcian siliciclastic-calcareous contourite
deposits in Mochras helps facilitate understanding ofcontourite deposition processes over a long period ofgeological time and its ichnodiversity. Together with
the Pliensbachian part of the section (Pieńkowski
et al. 2021), the current study comprises an expanded
ichnological and sedimentary record of contourite
deposits, adding new information on the sedimentary dynamics, variability of oceanographic history,
and basin interconnectivity.
2. Ichnological signals point to a common and strongly
repetitive mechanism driving the observed fluctuations in benthic conditions, which is indicated byPage 27 of 34

spectral analysis and distinction of four hierarchicalorders of cycles attributed to orbital forcing.
3. Long-term climate change controlled the circulationof contour-parallel bottom currents in the Cardigan
Bay Basin and orbital forcing has set the timing for
cyclic ichnological records, with the amplifying feedback of climate-related palaeoceanographic variations of thermohaline-driven contour currents circulating between the Boreal ocean and Peri-Tethys,
through the Cardigan Bay Strait. There is continuityof ~ 2.4 Myr, rather than ~ 1.6 Myr cyclicity expressedin deep-sea circulation and the probable occurrenceof yet longer, ~ 7–8 Myr (~ 7.5) periodicity which may
or may not be under an orbital control. The mutual
modulation of the two cyclicities (~ 3.4 and ~ 1.6
Myr) indicated by Charbonier et al. (2023), giving
approximately the resultant cyclicity of 2.4–2.5 Myr
observed in Mochras would be a plausible explanation, if the hypothesis of chaotic behaviour of the
Solar System in Mesozoic is confirmed.
4. In more temperate climates, the formation of highdensity waters was associated with low temperaturesat higher latitudes and sinking of cooler and denser
water masses, which then flowed south. However,in the early Toarcian this secular, extrinsic paradigm
has been modified due to intrinsic Earth processes
related to large-scale volcanism and higher temperatures, which led to intense evaporation and increasein salinity in the middle latitudes. This hypotheticallybegan to play a dominant role in circulation, stopping
or reversing the directions of marine currents, which
resulted in a temporal disturbance of a long-term
orbitally-controlled mechanism, but without eliminating the influence of orbital forcing.
5. The most severe oxygen crisis (dysoxic, but notanoxic), evidenced by trace fossils (Phycosiphon scarcity, mass appearances of Trichichnus) commenced
shortly before the Pliensbachian-Toarcian boundaryand ended at the early stage of the Toarcian negative
carbon isotope excursion (To-CIE), which is in contrast to the general pattern of the T-OAE anoxic phenomenon in relatively shallow marine settings and its
synchroneity with the To-CIE. The extreme climate
warming, coeval to To-CIE, is therefore hypothesised
to have caused significant changes in deep marine
circulation, reversing its direction and improving
oxygenation of the deep-sea floor.
6. Ichnological results, both macroscopic observations
and spectral analysis, confirm the cycles that appear
in the Mochras visual descriptions, and a new minimum estimate of duration of the Toarcian (9.4 Myr)and constituent ammonite zones is proposed.

Pieńkowski et al. Progress in Earth and Planetary Science(2024) 11:18

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7. Contourites of the Cardigan Bay Basin provide information on inter-ocean circulation, benthic conditions, and climate changes, which can be extractedusing sedimentological and ichnological methods.
The Cardigan Bay Strait played an important role in
the Early Jurassic (at least Pliensbachian and Toarcian) marine circulation as a major link between thenorthern and southern part of the Laurasian Seaway,
and in general between the Boreal and peri-Tethys
oceanic domains.
8. Ichnological records in continuous hemipelagic successions seem to be a sensitive and reliable basis forthe recognition of cyclic orbital forcing of climaticand palaeoceanographic conditions. Smoothedcurves based on specific ichnotaxa occurrence timeseries bear resemblance to geochemical and geophysical proxy records, indicating concurrent response to
the changing environmental conditions.

Funding
The research is financed by the National Science Centre, Poland, from theprogramme Opus 13, Grant Agreement No. 2017/25/B/ST10/02235 and theinternal Polish Geological Institute Grant No. 62.9012.2016.00.0. This is a contribution to the ICDP and NERC JET Project: SPH and KNP were supported fromthe Natural Environment Research Council (NERC), Grant No. NE/N018508/1.
For the purpose of open access, the author has applied a ‘Creative Commons
Attribution (CC BY) licence to any Author Accepted Manuscript version arisingfrom this submission.

Abbreviations
To-CIE	Toarcian negative carbon isotope excursion
T-OAE	Toarcian oceanic anoxic event

Supplementary Information

Availability of data and materials
All data generated during this study are included in the paper and its Additional files. Downhole log data from Mochras are available via the UK Onshore
Geophysical Library (UKOGL) website https://​ukogl.​org.​uk/.

Declarations
Competing interests
The authors declare that they have no known competing financial interests
or personal relationships that could have appeared to influence the work
reported in this paper.
Author details
1
Polish Geological Institute–National Research Institute, Rakowiecka 4,00‑975 Warsaw, Poland. 2 Faculty of Geography and Geology, Jagiellonian
University, Gronostajowa 3a, 30‑087 Kraków, Poland. 3 Department of Earthand Environmental Sciences, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK.
Received: 16 June 2023 Accepted: 17 February 2024

The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00612-3.
Additional file 1. Matrix of binary time series used for spectral analysis.
Additional file 2. Description of time series and the runs test methodology and results.
Additional file 3. Toarcian ammonite biozones for the Llanbedr (Mochras
Farm) borehole, compiled from KN Page in Copestake and Johnson (2014)
and McArthur et al. (2016); refer to Additional files 4 and 5 for details.
Additional file 4. Biostratigraphically significant ammonite fossils fromthe Lower Toarcian of the Llanbedr (Mochras Farm) borehole, Wales, andproposed correlations to Yorkshire coast outcrops; all fossil identifications
by KN Page.
Additional file 5. Biostratigraphically significant ammonite fossils fromthe Upper Toarcian of the Llanbedr (Mochras Farm) borehole, Wales; allfossil identifications by KN Page.
Acknowledgements
The National Geological Repository of the British Geological Survey, Keyworth,
Nottingham, UK, is acknowledged for sharing the Mochras drill core for investigations. There staff are warmly thanked for longstanding support of this project. Two anonymous reviewers provided corrections and helpul comments.
Author contributions
GP: conceptualization, data curation, formal analysis, investigation, writing—original draft (lead); AU: conceptualization, data curation, formal analysis,
investigation, funding acquisition (lead), project administration (lead fromthe National Science Centre, Poland side), writing—review and editing; KN:
numerical analyses (lead), writing—review and editing (partial); KNP: refinement of ammonite biostratigraphy; SPH: conceptualization, review and editing, funding acquisition (partial), formal analysis, project administration (leadfrom the ICDP and NERC project JET sides).

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