Significance Cyclic variations in Earth’s orbit drive periodic changes in the ocean–atmosphere system at a time scale of tens to hundreds of thousands of years. The Mochras δ13C TOC record illustrates the continued impact of long-eccentricity (405-ky) orbital forcing on the carbon cycle over at least ∼18 My of Early Jurassic time and emphasizes orbital forcing as a driving mechanism behind medium-amplitude δ13C fluctuations superimposed on larger-scale trends that are driven by other variables such as tectonically determined paleogeography and eruption of large igneous provinces. The dataset provides a framework for distinguishing between internal Earth processes and solar-system dynamics as the driving mechanism for Early Jurassic δ13C fluctuations and provides an astronomical time scale for the Sinemurian Stage.

Abstract Global perturbations to the Early Jurassic environment (∼201 to ∼174 Ma), notably during the Triassic–Jurassic transition and Toarcian Oceanic Anoxic Event, are well studied and largely associated with volcanogenic greenhouse gas emissions released by large igneous provinces. The long-term secular evolution, timing, and pacing of changes in the Early Jurassic carbon cycle that provide context for these events are thus far poorly understood due to a lack of continuous high-resolution δ13C data. Here we present a δ13C TOC record for the uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic), derived from a calcareous mudstone succession of the exceptionally expanded Llanbedr (Mochras Farm) borehole, Cardigan Bay Basin, Wales, United Kingdom. Combined with existing δ13C TOC data from the Toarcian, the compilation covers the entire Lower Jurassic. The dataset reproduces large-amplitude δ13C TOC excursions (>3‰) recognized elsewhere, at the Sinemurian–Pliensbachian transition and in the lower Toarcian serpentinum zone, as well as several previously identified medium-amplitude (∼0.5 to 2‰) shifts in the Hettangian to Pliensbachian interval. In addition, multiple hitherto undiscovered isotope shifts of comparable amplitude and stratigraphic extent are recorded, demonstrating that those similar features described earlier from stratigraphically more limited sections are nonunique in a long-term context. These shifts are identified as long-eccentricity (∼405-ky) orbital cycles. Orbital tuning of the δ13C TOC record provides the basis for an astrochronological duration estimate for the Pliensbachian and Sinemurian, giving implications for the duration of the Hettangian Stage. Overall the chemostratigraphy illustrates particular sensitivity of the marine carbon cycle to long-eccentricity orbital forcing.

Prominent carbon-isotope excursions (CIEs) are identified globally in strata from the Triassic–Jurassic boundary (∼201 Ma) and the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma), both of which are expressed in the δ13C values derived from various marine and terrestrial organic and inorganic materials (1⇓–3). These isotopic events express changes in the δ13C composition of the combined global exogenic carbon pool and are linked to the elevated release of isotopically light volcanic, and/or thermogenic, and/or biogenic carbon into the global ocean–atmosphere system (resulting in negative CIEs, e.g., refs. 4 and 5) and global increase in organic-carbon sequestration in marine and/or terrestrial environments (resulting in positive CIEs, e.g., refs. 6 and 7). Bracketed by these globally recognized distinct large-amplitude δ13C events (up to 7‰ in marine and terrestrial δ13C TOC records), numerous δ13C shifts of somewhat lesser magnitude have been identified in the Hettangian to Pliensbachian interval. Stratigraphically expanded shifts were recorded at the Sinemurian–Pliensbachian boundary (8⇓⇓⇓⇓⇓–14) and the upper Pliensbachian margaritatus and spinatum zones (10, 15, 16). Furthermore, multiple stratigraphically less extended short-term δ13C shifts of ∼0.5 to 2‰ magnitude have been recognized throughout the Hettangian (17⇓–19), in the Sinemurian (17, 20⇓⇓⇓–24), and Pliensbachian (10, 11, 16, 22, 25⇓⇓–28), where they are recorded as individual shifts or series of shifts within stratigraphically limited sections. Some of these short-term δ13C excursions have been shown to represent changes in the supraregional to global carbon cycle, marked by synchronous changes in δ13C in marine and terrestrial organic and inorganic substrates and recorded on a wide geographic extent (e.g., refs. 10, 16, 23, and 24). However, due to the previous lack of a continuous dataset capturing and contextualizing all isotopic shifts in a single record, there is no holistic understanding of the global nature, causal mechanisms, and the chronology and pacing of these CIEs. Therefore, these δ13C shifts have largely been interpreted as stand-alone events, linked to a release of 12C from as-yet-undefined sources, reduced organic productivity (leaving more 12C in the ocean–atmosphere system) and/or 13C-depleted carbon sequestration and orbitally forced environmental change affecting the carbon cycle on the scale of Milankovitch cyclicity (17, 20, 24, 25). Evidence for the latter is so far limited to the Hettangian to early Sinemurian and the early Toarcian, where high-resolution isotope records provide the basis for cyclostratigraphic analysis (17⇓–19, 29⇓–31).

The data illustrated herein provide a continuous and biostratigraphically well-defined δ13C TOC record from uppermost Rhaetian (Triassic) to Pliensbachian (Lower Jurassic) strata, with a resolution high enough to examine CIEs of varying magnitudes and temporal extent in their stratigraphic context, thereby enabling a distinction between orbital, tectonic, oceanographic, or volcanic forcing mechanisms of the carbon cycle over this time interval.

Geological Setting The Llanbedr (Mochras Farm) borehole (hereafter referred to as Mochras) cored the Lower Jurassic of the Cardigan Bay Basin (Wales, United Kingdom), an extensional structure related to the breakup of Pangaea (32). In the Early Jurassic, the basin was located at a midpaleolatitude in the Laurasian Seaway on the northwest fringes of the European shelf (Fig. 1 and refs. 33 and 34). The uppermost Pliensbachian and lower Toarcian strata are regarded as having been deposited in an unrestricted, open-marine setting (35). Fig. 1. Early Jurassic paleogeography showing the location of the Mochras borehole (red star) within the northern Eurasian Seaway (red rectangle). Reprinted from ref. 39. Copyright (2019) with permission from Elsevier. The recovered sedimentary succession at Mochras comprises 32.05 m of continental Upper Triassic (Rhaetian) deposits (1,938.83 to 1,906.78 m below surface, mbs), ∼1,305 m of Lower Jurassic Hettangian to Toarcian marine strata (1,906.78 to 601.83 mbs), and is unconformably overlain by Paleogene–Neogene sandstones and glaciogenic sediments (601.83 to 0 mbs, ref. 36). Ammonite biostratigraphy of the core was defined to a zonal and even subzonal level, and all ammonite zones of the Lower Jurassic have been identified with the exception of the lowermost Hettangian tilmanni zone (37, 38). Due to the lack of the base-Jurassic biostratigraphic marker Psiloceras spelae, the Triassic–Jurassic boundary in the Mochras borehole is placed at a lithological change from calcitic dolostone to calcareous mudstone at ∼1,906.78 mbs (36, 38). About 1.7 m of biostratigraphically undefined strata lying between the base Jurassic and the base of the planorbis zone are referred to as “pre-planorbis beds,” likely equivalent to the basal Jurassic tilmanni zone (38). The relative thinness of the pre-planorbis beds suggests a base-Jurassic hiatus at the sharp lithological change at ∼1,906.78 mbs. A calcite-veined interval in the mid-Sinemurian oxynotum zone may be marked by a fault which, if present at all, cuts out less than one ammonite subzone (36). A small hiatus may also be present at the level of intraformational conglomerate at 627.38 mbs (36) within the upper Toarcian pseudoradiosa zone, and a further unconformity is present at the top of the Lower Jurassic (at 601.85 mbs), where sediments of the uppermost Toarcian aalensis zone are overlain by Paleogene strata (7, 36). In all other respects, the Lower Jurassic succession appears to be stratigraphically complete. However, core preservation below ∼1,290 mbs is largely limited to reserve collection samples, each of which aggregate ∼1.4 m intervals of broken core, with consequential reduction of stratigraphic resolution (see Materials and Methods and SI Appendix). The Jurassic succession at Mochras is markedly expanded, with relatively uniform lithology compared to coeval strata elsewhere (38, 39). The strata primarily comprise calcareous mudstone, with varying silt and clay content, alternating with strongly bioturbated calcareous siltstone and silty limestone (36). Average Rock-Eval thermal maturation parameter (T max = ∼430 °C) and vitrinite reflectance (R o = 0.38 to 0.63) from previous studies indicate the presence of immature to early mature sedimentary organic matter (7, 22, 40). δ13C data from total organic carbon (δ13C TOC ) and carbonates (δ13C carb ) generated in previous studies suggest that the Mochras sedimentary archive records the long-term pattern of global carbon-cycle change (7, 14, 22, 41, 42).

Results The high-resolution δ13C TOC and Rock-Eval data from Mochras presented here for the uppermost Rhaetian to Pliensbachian are combined with published data for the Toarcian derived from the same core (ref. 7 and Fig. 2). The compiled δ13C TOC record illustrates significant long- and short-term fluctuations in δ13C TOC through the Lower Jurassic of the Mochras core. At a longer time scale, the record shows a long-term ∼5‰ positive shift in δ13C TOC from the lowermost Hettangian to upper Sinemurian. The Sinemurian–Pliensbachian boundary is characterized by a symmetrically shaped ∼5‰ negative long-term trend and subsequent “recovery” (upper oxynotum to upper ibex zones, ∼1,360 to ∼1,060 mbs), reaching the lowest values in the lower jamesoni zone. The mid-Pliensbachian interval presents a stable plateau in δ13C TOC , followed by the upper margaritatus zone (subnodosus and gibbosus subzones) where δ13C TOC values rise gradually and culminate in an abrupt ∼2‰ positive excursion in the upper margaritatus zone (∼930 to ∼926 mbs). The margaritatus–spinatum zone boundary is marked by a sharp ∼4‰ drop in organic carbon-isotope ratios, followed by a gradual positive shift throughout the spinatum zone. The Toarcian record comprises a lower Toarcian overarching positive CIE interrupted by the large negative CIE associated with the T-OAE, as described in previous studies (7, 14, 22, 41). All larger-scale trends in the δ13C TOC data are reproduced in δ13C wood presented for the upper Sinemurian to Toarcian, although the latter dataset shows a larger degree of variability (Fig. 2). Fig. 2. δ13C TOC , TOC, and CaCO 3 (calculated from total inorganic carbon), and HI data for the uppermost Rhaetian to Toarcian (ref. 7 and this study) at Mochras. Blue line = seven-point moving average. Black squares = samples taken from core slabs; white squares = samples taken from reserve bags (∼1.4-m intervals) of broken core. Orange circles = δ13C wood . Depth of the samples from reserve bags refers to midpoint of the sample interval. Ammonite biostratigraphy after refs. 37 and 38. Correlation of the data to paleoenvironmental, oceanic, and magmatic events: (A) 87Sr/86Sr (14, 79). Intervals marked by plateau phases and distinct increases are marked with arrows. (B) Paleotectonic events (gray) and T-OAE (red) (6, 51, 80). (C) Occurrence of dinoflagellate cysts in the Mochras core (22). (D) Paleotemperatures: orange = warming, blue = cooling (9, 49, 51, 81), red = short-lived hyperthermals (21, 24, 48). (E) Timing of magmatic events: Central Magmatic Province (based on compilation in ref. 39), Karoo-Ferrar (82). Key for ammonite subzone numbering (question marks indicate uncertainties): 1) planorbis, 2) johnstoni, 3) portlocki, 4) laqueus, 5) extranodosa, 6) complanata–depressa?, 7) conybeari, 8) rotiforme, 9) bucklandi–lyra, 10) scipionianum, 11) sauzeanum, 12) obtusum–stellare, 13) denotatus, 14) simpsoni, 15) oxynotum, 16) densinodulum–raricostatum, 17) macdonnelli–aplanatum, 18) taylori–polymorphus, 19) brevispina, 20) jamesoni, 21) masseanum?–valdani, 22) luridum, 23) maculatum, 24) capricornus, 25) figulinum, 26) stokesi, 27) subnodosus–gibbosus, 28) exaratum, 29) falciferum, 30) commune, 31) fibulatum, 32) crassum, 33) fascigerum, 34) fallaciosum, 35) levesquei, and 36) pseudoradiosa. At a decameter scale, the δ13C TOC record is characterized by consecutive alternating positive and negative shifts of ∼0.5 to 2‰ magnitude, superimposed on the observed long-term isotopic trends. These fluctuations in δ13C TOC , hereafter referred to as medium-amplitude shifts, are particularly well-defined in the Hettangian to uppermost Sinemurian (between 1,906.78 and 1,340 mbs) and the mid-Pliensbachian ibex to lower margaritatus zones (between 1,120 and 980 mbs). The individual shifts appear larger in magnitude and more stratigraphically extensive in the Hettangian and Sinemurian compared with those in the Pliensbachian. Superimposed on these medium-amplitude shifts, fluctuations in δ13C TOC of up to 2‰ on a meter to centimeter scale occur, with larger magnitudes in the upper Sinemurian and Pliensbachian likely being an artifact of differing sample resolution. The calcium carbonate (CaCO 3 ) content of the Mochras strata is highly variable (∼0.6 to 95%; Fig. 2). The long-term shifts in CaCO 3 appear to negatively correlate with the broad δ13C TOC trends, with the exception of the upper Pliensbachian and lower Toarcian successions. On a decameter scale, the CaCO 3 shows a clear fluctuation in the Hettangian and Sinemurian interval, but the pattern does not correspond to the medium-scale shifts in δ13C TOC. The relatively higher variability in CaCO 3 on a meter to decameter scale over the Sinemurian–Pliensbachian transition and the upper Pliensbachian interval is associated with the higher data resolution obtained in these intervals. The total organic carbon (TOC) content and hydrogen index (HI) values are generally low throughout the Hettangian to Pliensbachian of the Mochras core (Fig. 2). TOC and HI values in the Hettangian and most of the Sinemurian (∼1.5 wt % and ∼80 mg HC/g TOC on average, respectively) notably increase in the upper Sinemurian to lower Pliensbachian (2.6 wt %, up to 380 mg HC/g TOC, respectively) and are moderately elevated through the Pliensbachian (∼1.4 wt % and ∼170 mg HC/g TOC on average, respectively). The increase in both TOC and HI accompanies the down-going limb of the Sinemurian–Pliensbachian negative CIE, and some stratigraphic intervals with distinctly enhanced TOC and HI values also occur in the Hettangian and Sinemurian, coinciding with minimum values in δ13C TOC (for example in the angulata, bucklandi, and lower raricostatum zones). Similarly, the lowermost Jurassic sediments of the pre-planorbis beds and planorbis zone are also marked by distinctly elevated TOC and HI values (of 3.2 wt % and up to 660 mg HC/g TOC, respectively), coinciding with negative δ13C TOC values of −28‰ (Fig. 2). Overall, there is no clear correlation between TOC and δ13C TOC (SI Appendix, Fig. S5). Throughout the Toarcian, TOC values fluctuate between 0.5 and 1.5 wt %, with HI values of ∼100 mg HC/g TOC (7). Slightly higher TOC and HI values are recorded in the bifrons and variabilis zones, and elevated TOC and HI values (up to 2.5 wt % up to 339 mg HC/g TOC, respectively) are associated with the negative CIE interval in the serpentinum zone (7). Predominant components of sedimentary organic matter identified by maceral analysis are liptinites, most of which are represented by liptodetrinite (up to 96.7 vol %; SI Appendix, Fig. S7), a product of aerobic and mechanic degradation of liptinitic macerals (43). Markedly smaller but variable amounts of less-degraded liptinite macerals are algal in origin (alginite). Bituminite, also known as amorphous organic matter (AOM), which originates from anaerobic decomposition of algae and faunal plankton under anoxic conditions (44, 45), is primarily present in Pliensbachian samples (up to 24 vol %). Terrestrial organic matter comprising coal clasts, vitrinite, inertinite, sporinite, and cutinite accounts for variable relative amounts (3.3 to 58.8 vol %) of the total organic matter. Notably, the Pliensbachian samples contain a larger relative amount of terrestrially derived organic matter and bituminite compared to the Hettangian and Sinemurian samples. This stratigraphic trend is also reflected in the comparatively high abundance of macrofossil wood in the Pliensbachian and Toarcian part of the core, contrasting with very rare occurrences in the Hettangian and Sinemurian.

Conclusions The Mochras δ13C TOC data from the uppermost Rhaetian to Pliensbachian interval, combined with available data from the Toarcian from the same core, provides a continuous, biostratigraphically well-defined, high-resolution chemostratigraphic record for the Lower Jurassic. Beside large-scale CIEs (>3‰) known from isotopic records elsewhere, CIEs of generally smaller magnitude (0.5 to 2‰) occur throughout the Hettangian to Pliensbachian interval. These medium-amplitude CIEs, including shifts that have previously been recorded in stratigraphically shorter intervals, appear less singular in the context of a continuous record. Spectral and ASM analysis of the data reveals that these medium-amplitude CIEs are paced by long-eccentricity (405-ky) cycles, exemplifying the impact of orbital forcing on the ocean–atmosphere carbon reservoir. Orbital tuning of the isotope record provides a duration estimate of 8.8 My for the Pliensbachian and offers an estimate for the Sinemurian Stage (6.6 My). Combined with published biostratigraphically defined radioisotopic age constraints for the Triassic–Jurassic and Pliensbachian–Toarcian boundaries, the data presented herein suggest a duration for the Hettangian Stage of ∼2.3 ± 0.5 My.

Materials and Methods Preserved core slabs, bagged core fragments known as the “reserve collection,” and registered specimens of the Mochras drill core are housed at the British Geological Survey National Geological Repository at Keyworth, United Kingdom. For this study, bulk rock samples between 1,290 and 863.3 mbs were collected from well-preserved core slabs at a 30-cm to 60-cm resolution. Bulk-rock samples below the ∼1,290 mbs level were largely sampled from reserve collections, each of which aggregate ∼1.4 m intervals of broken core. A single sample was taken from each bag and referred to the depth of the midpoint of the sampled interval (reserve bag samples marked as white squares in Fig. 2). Macroscopic fossil plant material was extracted from reserve bags only. The sample resolution, ammonite and foraminiferal biostratigraphy, and a lithological log of the Mochras drill core are shown in SI Appendix, Fig. S1. Detailed information on laboratory procedures for bulk (total) organic carbon-isotope (δ13C TOC ) analyses (1323 samples), fossil plant matter carbon-isotope (δ13C wood ) analysis (95 samples), Rock-Eval pyrolysis (667 samples), organic petrography (14 samples from the Hettangian, Sinemurian, and upper Pliensbachian), and spectral, ASM, and time-series analysis are also given in SI Appendix. Data Availability Statement. All data discussed in the paper will be made available in the SI Appendix and Dataset S1.

Acknowledgments We acknowledge funding from Shell International Exploration & Production B.V., the Natural Environment Research Council (grant NE/N018508/1), and the International Continental Scientific Drilling Program. We thank the British Geological Survey, especially Scott Renshaw and Tracey Gallagher, for enabling access to the Mochras core and Steve Wyatt (Oxford University) for laboratory assistance. M.J.L. and J.B.R. publish with the approval of the Executive Director, British Geological Survey. This manuscript is a contribution to the Integrated Understanding of the Early Jurassic Earth System and Timescale (JET) project, IGCP 632 (International Union of Geological Sciences and United Nations Educational, Scientific and Cultural Organization, IUGS-UNESCO), “Continental Crises of the Jurassic: Major Extinction events and Environmental Changes within Lacustrine Ecosystems” and IGCP 655 (IUGS-UNESCO), “Toarcian Oceanic Anoxic Event: Impact on marine carbon cycle and ecosystems.”

Footnotes Author contributions: M.S.S., S.P.H., H.C.J., and M.R. designed research; M.S.S., S.P.H., H.C.J., M.R., W.X., J.B.R., and O.G. performed research; M.S.S., M.R., C.V.U., M.J.L., and O.G. analyzed data; and M.S.S., S.P.H., H.C.J., M.R., C.V.U., W.X., M.J.L., and J.B.R. wrote the paper.

The authors declare no competing interest.

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