4.1 Sedimentary environment, ichnodiversity, cyclicity of sedimentation
4.1.1 Sedimentary environment
The lithofacies in the Toarcian at Mochras contrast with those of other settings of the UK Jurassic (as is also the case for the Pliensbachian). For example, the Toarcian in the Cleveland Basin, some 300 km to the NE of Cardigan Bay, is represented by organic-rich (black) shales up to the top of the bifrons Zone, 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 affected by storm processes (e.g. Kemp et al. 2018). Higher levels within the Cleveland Basin Toarcian are clearly deposited in a storm-dominated shoreface setting (e.g. Hesselbo and King 2019).
Based on sediment transport processes, the action of two different types of deep-water currents and their depositional 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) and turbidite currents created predominantly by downslope sediment density flows (Xu et al. 2018a – exaratum Subzone of the Toarcian section). Xu et al. (2018a) illustrated few cm-scale sharp-based beds in the T-OAE interval. However, fining upwards of these beds are not gradual, coarser intercalations are sharply separated not only from below, but also from above without continuous transitions, therefore these beds may also represent repeated intensifications of bottom currents. It is also possible that sediments of distal turbidites could occasionally have been “pirated” by contourite currents (Gong et al. 2017) 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 studied section 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 bioturbated from the top (e.g., Uchman and Wetzel 2011) and the uppermost layers of the turbidite exhibit total bioturbation, which decreases with depth as lamination and cross bedding appear. In contrast, contourites typically exhibit a more continuous and uniform bioturbation, which appears throughout the entire contourite bed; this such situation is typical for both the Pliensbachian and Toarcian of Mochras.
Similarly to the Pliensbachian section (Pieńkowski et al. 2021), interpretation of a wide spectrum of bathymetry and occurrences of contourite currents is adopted, i.e. 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 (1993) is also applied, according to which the term contourites should be generally used for sediments in relatively deep water (markedly deeper than c. 200 m-deep shelf platform) and deposited or significantly reworked by stable currents.
The contourites in Mochras can be classified as mixed siliciclastic-calcareous/biogenic contourites, the most frequent contourite facies in the modern oceans (Fagueres and Mulders 2011). In less bioturbated sections primary sedimentary structures are visible and they are mostly represented by current structures such as planar parallel lamination, low angle cross-lamination, starved ripples, in places erosive bases, and gradational normal grading bed transitions (Fig. 4a), similar to cases studied by Shanmugam (2000) and Knapp et al. (2017). Stagnant conditions or very slow currents allowed vertical settling of the suspended particles from the nepheloid layer (Ewing and Thorndike 1965), producing mottled mudstone (Fig. 7e) or laminated mudstone-claystone (Figs. 6a and 7h), where the grain-supported laminae are interpreted 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 of silt or sand layers with planar or low-angle cross-bedding, produced by bedload transport (Figs. 4c, d and 6d) and periodically these currents were strong enough to erode older sediments and carry coarse-grained sediments, including clay intraclasts and pebbles (Fig. 5). Compared to the Pliensbachian (Pieńkowski et al. 2021), visible plant remains are more frequent and indicate more phytodetrital pulses in connection with fluvial discharges in the hinterland, which possibly could have led to an acidification (Müller et al. 2020) and intermittently reduced salinity of sea water (Dera and Donnadieu 2012), both adding (along with the Toarcian rise in pCO2; see Hermoso et al. 2009) to collapses of carbonate factory (Bodin et al. 2022), observed particularly in the tenuicostatum Zone – exaratum Subzone interval (Fig. 2a). Relatively weaker bioturbation can be attributed to oxygen depletion or high sedimentation rate (Stow and Fagueres 2008), although assuming rather stable sedimentation rate, weaker bioturbation can be largely attributed to decreasing circulation/oxygenation. One should bear in mind that strong bottom currents (Fig. 5) also do not favour preservation of biogenic structures (Tucholke et al. 1985).
Fully developed bigradational intervals in the Mochras section (Fig. 4) correspond to a standard contourite sequence (Faugeres et al. 1984; Stow and Holbrook 1984; Stow et al. 2002; Huneke and Stow 2008; Rodríguez-Tovar and Hernandez-Molina 2018), a continuation of the situation for the Pliensbachian (Pieńkowski et al. 2021). The standard contourite model was enhanced by introduction 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. 3 and 4) where most frequent vertical sequences consist of lithofacies types 1/2-3-4-5-6 (in coarsening-upward order) and subsequent fining-upward couplet composed of 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 erosional surfaces and coarse sediments, including conglomerates (lithofacies type 7; Fig. 5). Periods of strong currents marked by erosional surfaces and coarser material, pointing to intermittent non-deposition/erosional periods, appear first in the upper falciferum Subzone and occur mostly in the upper bifrons Zone, variabilis-thouarsense transition and the upper levesquei Subzone (Fig. 2a and b). In such cases the bigradational grading cycles are often incomplete, lacking its upper parts (top-cut-out cycles; see Fig. 4a). Toarcian depositional rate at Mochras remained generally stable and relatively high, but the sections mentioned above show condensation (Figs. 2a, b, 4 and 5).
4.1.2 Ichnodiversity
Occurrences of trace fossils also show a cyclic character, more or less corresponding with lithological changes and sedimentary cycles. Domination of opportunistic, r-selected Phycosiphon, diffused shape of trace fossils, indices of soft bottom condition (Fig. 6f), mostly simple tiering, scarcity of highly specialized, K-selected forms, all indicate a generally high sedimentation rate associated with instability of substrate and benthic food availability, interrupted by interim oxygen-depleted, more stagnant conditions. The other end of the spectrum is characterised by higher substrate stability (stiffer conditions), diminishing 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 to a more complex tiering under equilibria (Taylor et al. 2003). A high degree of bioturbation is characteristic 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). It is in agreement with Caswell and Frid (2017) that changes in community composition are usually linked to local redox conditions, whereas changes in populations of r-selected opportunists are driven by primary productivity.
4.1.3 Cyclicity of sedimentation
As was previously the case (Pienkowski 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 placed in the fine-grained, mottled, or laminated mudstone (lithofacies 1 and 2; see Fig. 3). Both the fully developed (bigradational), and the incomplete lithological couplets with their ichnological content, constitute the 4th order cycles, basic “building blocks” of the hierarchical order of cycles in Mochras (Figs. 2a, b and 4). Commonly, in these cycles, meiofauna mottling (undulated bedding) appears first, followed by colonization of Phycosiphon (Ph3) as the first recognizable trace fossil, then Ph2 and other trace fossils return. These 4th order cycles are arranged in higher hierarchical successions (3rd order cycles), usually containing 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 many cases 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 order cycles. Sixteen out of 24 of the 2nd order cycles show occurrences of K-selected (equilibrichnia) forms (such as Skolithos, Arenicolites, Monocraterion, Siphonichnus, Rhizocorallium and Chondrites at its boundaries, pointing to more stable conditions. Both 2nd and 3rd order cycle boundaries show sharp-walled, uncompacted, passively filled burrows that point to condensation/non-sedimentation (Fig. 7i).
Erosional surfaces, commonly associated with coarse-grained sediments, always indicate sedimentation breaks and play an important role in the interpretation of the boundaries of the cycles. Usually boundaries of the 2nd or occasionally 3rd order cycle are placed at these surfaces. High-velocity contourite currents are known to generate erosional surfaces and contourite lag facies, resulting from winnowing, reworking processes and hiatuses (Faugerez and Mulder 2011). The lag contains a large range of grain sizes and compositions, forming irregular centimetre-thick to metre-thick beds of poorly sorted sediments, mainly coarse sands, gravels, and pebbles or rip-up clasts (Fig. 5).
The highest, 1st order, cycles are composed of six 2nd order cycles. The beginning of Toarcian sequence of these cycles is anchored at the Pliensbachian-Toarcian boundary, where the succession of four previous 1st order cycles of the latest Sinemurian to Pliensbachian age 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 the CaCO3 content generally of less use in reconstructing the cyclicity. Reduced CaCO3 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 core in Paris Basin – see discussion below.
In general, the whole Toarcian section in Mochras is dominated by Phycosiphon. Therefore, any lack of this morphotype is noteworthy, as it indicates an exceptionally unfavourable environment, most likely associated with stagnation and oxygen depletion. Of particular note is the fine-grained interval between 823 and 868 mbs, dominated by mudstone and claystone, with significantly reduced CaCO3 content, frequent siderite occurrences (Xu et al. 2018a) and a marked foraminiferal biotic crisis (Reolid et al. 2019). Here, a Phycosiphon crisis occurs and other trace fossils are very scarce, while Trichichnus and undulated bedding are prolific. Only in the middle of this interval some intermittent improvement of bottom life conditions can be inferred, indicated by less continuous occurrences of Trichichnus, brief re-appearances of Phycosiphon, and presence of cf. Polykladichnus and Chondrites (Fig. 2a). Interestingly, this slight recovery is also observed in foraminiferal assemblages (Reolid et al. 2019). Less severe ichnological crises are noted in the thouarsense-dispansum transition with Trichichnus occurrences and diminished ichnodiversity. Here, a Phycosiphon crisis can also be observed although, compared to the earliest Toarcian, of much smaller scale. 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 at depth 710–720 mbs is a slight decrease in oxygenation inferred, marked by decreased ichnodiversity and presence of Trichichnus.
Interpretation of cyclicity and resulting calculation of time based on these cycles attributed to certain orbital cycles becomes more complex in sections with erosional surfaces, associated with inferred hiatuses in sedimentation, resulting in less regular sedimentation rate and cyclicity. It is reasonable to interpret that much 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 such cases, the 1st order (~ 2.4 Myr) cycles can provide some time frameworks, allowing for approximate adjustment of the 2nd and 3rd order cycles to the ~ 2.4 Myr duration of 1st order cycles. This is particularly important in the Toarcian above the serpentinum Zone, where most erosional/hiatal surfaces with coarser sediments occur, usually concentrated in the middle parts of the 1st order cycles (Fig. 2a, b).
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 a number of 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. 2022). Most likely, the benthic crisis was associated 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, replaced by a stronger siliciclastic input, which also carried more floral debris and possibly connected to acidification (Xu et al. 2018; Müller et al. 2020) – although δ44/40Ca and δ88/86Sr records do not support acidification (Li et al. 2001). Bottom currents carried in suspension a considerable amount of particulate organic matter, supplying food to deep-marine benthic organisms (Thistle et al. 1985). In a similar manner 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 parallel to 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 the Laurasian Seaway during the Early Jurassic. The Cardigan Bay Strait, linking cooler and shallower waters of the Boreal Sea and Scottish-English archipelago with warmer and deeper waters of Peri-Tethys (Fig. 1), would then have been situated to sustain a continuous (persisting for up to millions of years and over large areas - Shanmugam 2008, 2017) thermohaline-driven contour current circulation between these two marine realms (see Pieńkowski et al. 2021). These invigorated flow conditions were punctuated by sluggish current circulation or stagnant conditions in time intervals characterised by loss of ichnodiversity and Phycosiphon crises (usually also by mass Trichichnus occurrences), as well as undulated bedding appearances. These intervals occur in the tenuicostatum Zone – lowermost exaratum Subzone interval and (to a lesser extent) in the latest thouarsense- dispansum zones.
This sluggish circulation is associated with climate warming under high atmospheric pCO2, 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 effects of flow from the north through the Laurasian Seaway (Ruvalcaba-Baroni et al. 2018). In the same time demises of polar sea ice and stronger high-latitude continental runoff rates could further result in a thermohaline circulation collapse and bottom oxygen depletion, also in deep oceanic settings. (Dera and Donnadieu, 2012; Van de 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. 2019; Ullmann et al. 2020) suggest cooler conditions, which should (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 838.5 m and 847.0 mbs, is there a slight improvement of bottom life conditions (confirmed also by foraminiferal assemblage; see Reolid et al. 2019), which could be associated with somewhat increased current intensity, possibly caused by a transient cooling, but this signal is weak.
Concerning the circulation dilemma, even more intriguing is the To-CIE interval, associated with the most severe carbon-cycle disruption, high pCO2 and extreme greenhouse conditions, which should cause even stronger blockage of southward flow and extreme sea current stagnation at that time – instead, shortly after the beginning of the To-CIE (in our calculation c. 200 kyr), the deep-sea circulation significantly accelerated. Only at the beginning of To-CIE did sluggish circulation and oxygen depletion still continue, which as indicated by the Phycosiphon crisis and common occurrences of Trichichnus. For the remaining part of the To-CIE event, certainly from its climax (c. 823 mbs), gradually improving circulation can be interpreted. This is recorded by common current structures (pin-stripe current lamination, occasionally ripple-cross lamination), some sharp boundaries and coarser sediments, and gradual return of more diversified trace fossils, such as Phycosiphon, Thalassinoides, Schaubcylindrichnus, Planolites, Lockeia, Asterosoma, Skolithos and Rhizocorallium, and concomitant demise of Trichichnus (Fig. 2a). The recovery is also visible in the foraminiferal assemblage (Reolid et al. 2019).
It is particularly noteworthy that an oxygen crisis was suggested by some geochemical indices. Xu et al. (2018a) reported trace amounts of the biomarker isorenieratane (with slightly elevated gammacerane indices) at two horizons (811.66 mbs and 819.10 mbs) within the part of the To-CIE with the lowest δ13CTOC values detected. Isorenieratane is a pigment of photosynthetic green sulphur bacteria Chlorobiaceae, which is regarded 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. 7e), or more distinctly by Phycosiphon incertum type 2 (Fig. 7h). It means that possible anoxic to euxinic conditions in the photic zone would not have affected the seafloor in Mochras, which was continuously oxygenated by deep-sea bottom currents. This observation is of more general significance, pointing that geochemical indices (even if correctly pointing to certain conditions in a shallow-water column) do not always indicate euxinic bottom conditions, because deep-sea currents could deliver enough oxygen to sustain life there, even if the shallow-photic zone was anoxic. 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 this would still point to very short and transient stages of an oxygen crisis.
Observed lithological and ichnological fluctuations in the Toarcian are thus associated with alternating periods of vigorous currents and more stagnant conditions, while nutrient availability seemed to be relatively stable and sufficient all the time (as shown by the TOC content; see Xu et al. 2018a) and played a subordinate role in setting ichnological trends. Hierarchical cyclic successions observed both in lithology and ichnology indicate that orbital forcing was still the main controlling mechanism. However, at the beginning of the Toarcian, the lithological and ichnological appearance of these cycles became different, which can be linked to non-orbital, intrinsic Earth mechanisms leading to prolonged ichnological crisis in the tenuicostatum Zone. More generally, one can also observe that compared to 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 kyr cycles (although the 1st order cycles retained approximately stable thicknesses, which point to a generally stable average sedimentation rate in a longer period of time). More dynamic deep-sea circulation is probably associated with warmer and more turbulent atmospheric conditions. Although there is no clear correlation between the carbon cycle and observed higher energy of currents, there is a return to generally lighter values of δ13C from the middle part of the bifrons Zone.
Of note is also the repetition of the two fine-grained intervals characterized by particularly sluggish circulation, impoverished ichnodiversity (including the Phycosiphon crises), abundance of Trichichnus: the lower one in the uppermost spinatum and tenuicostatum Zone and the upper one in the uppermost thouarsense – lower dispansum Zone. These intervals are separated by ca. 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) and identified with a very long eccentricity term.
4.3 Climate changes and carbon cycle disturbances - impact on deep-sea circulation and orbital forcing
Two intrinsic Earth events should be considered carefully in terms of their influence on the Toarcian hemipelagic sedimentation in Mochras. The first was the Pliensbachian-Toarcian (Pl-To) CIE event, which is globally documented and linked to the initial phase of volcanism in the Karoo Province; the second event with profound impact on the marine and continental environment was the To-CIE (Jenkyns Event), dated to the latest tenuicostatum Zone – exaratum Subzone, linked to large-scale eruptions in the Karoo-Ferrar basaltic province and associated with a rapid increase in atmospheric pCO2 levels and average atmospheric and marine temperatures (see e.g., Hesselbo et al. 2007; Littler at el. 2010; Hesselbo and Pieńkowski 2011; Bodin et al. 2016 2022; Pieńkowski et al. 2016; Fantasia et al. 2018; Xu et al. 2018a; Ruebsam et al. 2019).
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 in a stable way by orbital forcing, expressed in cyclic fluctuations of southward flowing thermohaline contour currents in the Cardigan Bay Basin, particularly expressed in long-term trends governed by grand eccentricity (~ 2.4 Myr) cycles (Pieńkowski et al. 2021).
Noticeably, 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, while the 7–8 Myr very long eccentricity cycle and the Toarcian duration postulated both by Charbonier et al. (2023) and by us fit together well. Possibly, in Mochras we would have to deal with the mutual modulation of the two cyclicities indicated by Charbonier et al. (2023), giving approximately the resultant cyclicity of 2.4–2.5 Myr observed in Mochras. 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. In turn, long-period cyclicities were detected by Charbonier et al. (2023) based solely on the numerical spectral decomposition of magnetic susceptibility time series. Such spectral techniques tend to display component frequencies behind the resultant frequency, being recorded by environmental factors. Importantly, Pliensbachian grand orbital cycles in Mochras (Pieńkowski et al. 2021), put together on an interpreted 405 kyr cyclicity based on CaCO3 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 and sedimentological observation, persists into the Toarcian at Mochras. Therefore we maintain effective influence of ~ 2.4–2.5 Myr cyclicity on sedimentary processes in Mochras, because adoption of ~ 1.6 Myr cyclicity postulated by Charbonier et al. (2023) is less comparable to observed long-term sedimentary and paleoecological trends observed in Mochras, both in Pliensbachian (Pieńkowski et al. 2021) and Toarcian (this study). The possibility of blurring of orbital cyclicity by autocyclic depositional processes, or tectonic processes, particularly in shallower epicontinental environments, should not be totally ruled out in that context.
As shown by our data from Toarcian of the Mochras core (Fig. 2a, b), orbital forcing was influenced by the environmental disturbances mentioned above. The picture obtained differs from surrounding epicontinental seas, showing also more complicated mechanism 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), observed in the Sinemurian-Pliensbachian section (Pieńkowski et al. 2021), appears to be still valid for most of the Toarcian in Mochras, but fails to explain changes that occurred in the earliest Toarcian (tenuicostatum-exaratum Zone). Severity and continuity of the latest Pliensbachian-earliest Toarcian stagnation/bottom life crisis probably occurred in a transitional context from supposed icehouse to greenhouse conditions (Dera and Donnadieu 2012; Krencker et al. 2014) and from sea-level lowstand to marked sea-level rise (Pieńkowski 2004; Hag 2020; Ruebsam et al. 2020; Bodin et al. 2022).
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 the depth 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 period of time, then, according to the cooling/faster circulation paradigm, it would have resulted in a faster circulation throughout the entire zone. Instead, the Pl-To event commenced long-lasting stagnation in Mochras. The sea-level rise postulated for this time interval (Pieńkowski 2004; Haq 2020; Ruebsam et al. 2021; Bodin et al. 2022) could have had some impact on deep-sea circulation, but it is uncertain in which direction. Sea-level rise often led to lower energy of sedimentary processes and stagnation on continental shelves (due to the raised wave base), but it is unclear what would have happened in the case of much deeper water in the Cardigan Bay Basin. The influence of salinity drop in the early Toarcian epicontinental sea in UK (suggested by Remirez and Algeo 2020, but see Hesselbo et al. 2020 for discussion) could be pondered as well, because density gradient is one of main driving forces of deep-sea currents. Generally, peri-Tethyan waters should be more saline (due to evaporation) than waters in the Laurasian Seaway or Boreal Sea, where salinity could be diminished by fresh water influx from rivers and intermittent ice melt. However, the exact influence of the salinity gradient (which is elusive itself) on the 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 and the resulting increase in pCO2. Due to the mutual feedback effect and inertia of deep-sea oceanographic processes, this stagnation would last for a long time (c. 800 kyr). Stagnation of deep-sea circulation in the Laurasian seaway may challenge interpretations of continuous cooling 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, the Cleveland Basin could be a local anomaly, as some other reports (e.g. Ruebsam et al. 2020, their Fig. 5) claim that during the tenuicostatum (= polymorphum) Zone the Earth’s climate system shifted between contrasting climatic conditions, particularly at the beginning and the end of this zone.
The following tenuicostatum Zone-exaratum Subzone extreme global warming event of (ca. +3–5 oC, see Pálfy and Smith 2000; Gómez et al. 2008; Suan et al. 2010, Ullmann et al. 2020, or ca. +10 oC, see McArthur et al. 2000; Dera and Donnadieu 2012; Ruebsam et al. 2020). The temperature change of + 4.5 oC, jointly with the demise of polar sea ice and stronger high-latitude continental runoff rates (which could have freshened the Arctic surface seawaters), was expected to cause a general thermohaline circulation collapse (Dera and Donnadieu 2012). However, this is not evident in Mochras – instead, tenuicostatum Zone-exaratum Subzone extreme global warming has ended previously dominating stagnation, overturning the previous mechanism linking stagnation with warming. Likely, this time the global warming was so strong, that its consequences for deep-sea circulation were opposite – instead of sluggish circulation, usually caused by northward currents only being able to neutralise dominating southward flow, the Tethyan northward flow became strong enough to prevail over the southward currents. As a consequence, the reversed deep-sea ocean circulation became faster in the Cardigan Bay Basin. In particular, Dera and Donnadieu (2012) in their GCM (General Circulation Models) simulations found that for significantly higher pCO2 levels (> 1600 ppm), the depth of mixed layer appears deeper in the European basins. This would suggest a good ventilation of the water column during the To-CIE, which is obviously not the case in semi-closed, 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 noting that for other periods such as the Late Permian, GCM simulations show that massive rises in atmospheric CO2 concentrations may drive more vigorous and more symmetrical deep-sea circulations under warmer climates (Winguth et al. 2002; Winguth and Maier-Reimer 2005).
According to Dera and Donnadieu (2012), the tenuicostatum Zone–exaratum Subzone thermal anomaly was probably related to a strengthening of warm equatorial Tethyan westward currents, drifting along the northern Gondwanan margins. Furthermore, the drift of these currents through westernmost areas and their subsequent clockwise rotation due to southward directed boreal flows is consistent with neodymium isotope data (Dera et al. 2009), as well as ammonite and nannofossil migration routes evidenced for the Early Toarcian (Reggiani et al. 2010; Dera et al. 2011). 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 2002rös 2002; Arias and Whatley 2005; Dera et al. 2011).
Interestingly, during the tenuicostatum Zone–exaratum Subzone period of time, the sedimentation rate in Mochras remained relatively stable and it was still paced by precession (~ 20 kyr) and short- and long-eccentricity cycles (~ 100 and 405 kyr), which are well expressed in the section (Fig. 2a). Circulation decelerated just after the To-CIE event, in the earliest falciferum Subzone (785–795 mbs), which is reflected by impoverished ichnodiversity and lithological features. From this time on, Toarcian circulation evidently beccame generally faster, particularly with regard to the middle parts of 1st order cycles, where more condensed sections and more frequent erosional/omission surfaces occur. The observations confirm that that the warming/stagnation-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 at al. 2014; Ruebsam et al. 2020; Ullmann et al. 2020) support our conclusions regarding a large-scale correlation between sea-water temperature and inferred fluctuations of deep-sea current circulation in the Cardigan Bay Basin. From the late falciferum Subzone on, oxygenation of the bottom generally improved – circulation intensity and average oxygenation achieved its highest level in the late bifrons and variabilis to earliest thouarsense zones, as shown by sedimentological and ichnological features (Fig. 2b). This is likely associated with a long-term cooling trend, extending from the latest falciferum Subzone to the thouarsense 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 to be also reflected in a slightly deteriorated ichnodiversity in Mochras (707–713 mbs), at the same time marking a boundary of ~ 2.4 Myr cycle. The latest thouarsense-dispansum time interval in Mochras shows the next significant deterioration of ichnodiversity/oxygenation, which is likely connected to a warming trend, intensified in dispansum Zone (Gómez et al. 2008), exactly when we are dealing with the 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 relationship between these variables, linking deep-sea circulation to major climate changes.
Toarcian sea-level changes are sometimes suspected as a possible factor in observed oceanographic processes. According to Ayranci et al. (2018), high-stand and transgressive-system tracts in the contourites are represented by dominantly massive mudstone lithofacies, less intense bioturbation, and higher TOC values. Rising relative sea-level trends were confirmed in tenuicostatum Zone and also lower serpentinum, bifrons and lower variabilis Zones (Sellwood 1972; Graciansky et al. 1998; Pieńkowski 2004; Hesselbo 2008; Rocha et al. 2016; Barth et al. 2018; Haq 2020; 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-level changes of an amplitude of only tens of metres (Haq 2020). Generally, a higher sea level might have enhanced the exchange of waters between adjacent basins and the circulation of currents, but the impact of eustatic (or tectonically-induced) sea level on deep-sea currents remains still 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 earlier Lower Jurassic stages in Mochras (Storm et al. 2020). Some of minor negative excursions at depths of 750, 719, 697, 675, 667 and 630 mbs seem to be related with more stagnant conditions (short-lived minor Phycosiphon crises, appearances of lamination and Trichichnus), but this relation would need more systematic study and much higher resolution C-isotope curve. This is particularly important in the upper part of the profile, where the contrast in δ13C values is low.
4.4 Ichnology and spectral analysis
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 and Ph3 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 (Supplementary 2), demonstrating the advisability of time series analysis. Absolute values of all computed Z-scores are 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. 8) correlate to the calculation-based expected duration of the orbitally-forced periodicities (Table 1). The resultant spectral peaks (Fig. 8) have estimated time values assigned as explained in Section 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 presumptively can be recorded with higher precision than these longer-term, as a larger number of them are contained within the studied time series.
Either one or more spectral peaks in the sequency range of circa 0.9–1.2 cycles/m are reflected by most of analysed time series, with an exception of Trichichnus (Table 1). These prevalent, albeit relatively low-power peaks 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 in this case are characterised by spectral power on par to the other periodicities.
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 occurrences of Phycosiphon (Ph2 and Ph3) and Planolites, which probably lies in the shortest obliquity term (Tiwari 1987; Laskar et al. 2011). Presumably, the peak near 0.31 cycles/m value displayed by Schaubcylindrichnus occurrences (Fig. 8) 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. 8). Periodicities in the range of c. 105–120 kyr recorded by Phycosiphon Ph2 and Ph3, Schaubcylindrichnus and Planolites (Fig. 8) can be identified as the longer component term (c. 112 kyr) of short eccentricity cycles, presumably reflecting the 3rd order cycles distinguished in the sedimentary and ichnological record (Figs. 2a, b). In addition, c. 90 kyr periodicity displayed by Thalassinoides can be interpreted as the shortest eccentricity component term (c. 95 kyr).
Trichichnus occurrences record also the periodicity of the c. 170 kyr term, which likely can reflect a cyclicity of c. 150–170 kyr, also present in the Pliensbachian of Mochras, and 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, Chondrites, which were not included in the spectral analysis due to their low frequency). However, this remains a presumption due to spectral resolution of part of spectra below the sequency values of 0.1 cycles/m being inadequate to confidently interpret these peaks in the time domain.
The most complete representation of orbital periodicities is reflected by both Phycosiphon morphotypes, Thalassinoides, Schaubcylindrichnus and Planolites (Fig. 8) that clearly display the frequencies of the short eccentricity terms (c. 95 and 112 kyr), obliquity terms (c.29 and c. 40 kyr) and precessional terms (c. 17–25 kyr). Spectra of both Phycosiphon morphotypes show a good congruence, especially considering an interpreted longer component term of short eccentricity. In the case of Phycosiphon Ph2, it can be supposed that evident periodicities associated with either 4th and 3rd order cycles are derivative of periodic short non-occurrences of this pervasively occurring trace fossil. Interestingly, Trichichnus occurrences solely reflect periodicities related to obliquity and its AM frequency.
Longer-term “grand cycles” which are not included in spectral analysis, are characterized by a gradually growing and subsequently falling Phycosiphon frequency and general 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 suggest that this period fluctuated between maximal values of values ranging from ∼1.5–2.9 Myr during the Mesozoic (Ikeda and Tada 2013). Indeed, 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 core (Paris Basin). However, Wu et al (2022), while also suggesting transient chaotic behaviour of the Solar System in the Late Cretaceous of the Songliao Basin (China), revealed evidence for two chaotic secular resonance transitions in the orbital motions of Earth and Mars, involving the orbital eccentricity modulations of 2.4 Myr and 1.2 Myr cycling, without indication of ~ 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 hierarchy of 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).
Interestingly, an existence of yet longer cyclicity, c. 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 Pliensbachian-Toarcian section.
4.5 Duration of the Toarcian and ammonite zones based on ichnological astrochronology
The Toarcian section of the Mochras core represents the most 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 and the short- and long-eccentricity periodicities (Fig. 2a, b). The new results allow a estimation of the duration of the Toarcian stage at about minimum 9.7 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 precession/obliquity, short eccentricity, long eccentricity). Astronomical durations for the ammonite biochronozones (based mainly on ~ 405 kyr, 2nd order cycles) are: tenuicostatum Zone = 0.9 Myr, serpentinum Zone = 1.7 Myr, bifrons Zone = 1.7 Myr, variabilis Zone = 2.3 Myr, thouarsense Zone = 0.9 Myr, dispansum Zone = 0.3 Myr, pseudoradiosa Zone = 1.8. The aalensis Zone (truncated from the top) is preserved only as a 2 m fragment of core, probably representing no more than 0.1 Myr. The obtained durations of successive ammonite zones (Table 2) are slightly longer compared to those shown in GTS (Hesselbo et al. 2020).