the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Mochras Borehole, Cardigan Bay Basin, NW Wales
- 1WildFIRE Lab, Global Systems Institute, University of Exeter, Exeter, EX4 4PS, UK
- 2Camborne School of Mines, Department of Earth and Environmental Science, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
- 3Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
- 4Biogéosciences, UMR 6282 CNRS, Université de Bourgogne/Franche-Comté, 21000 Dijon, France
- 5Core Scanning Facility, British Geological Survey, Keyworth, NG12 5GG, UK
- 1WildFIRE Lab, Global Systems Institute, University of Exeter, Exeter, EX4 4PS, UK
- 2Camborne School of Mines, Department of Earth and Environmental Science, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
- 3Environment and Sustainability Institute, University of Exeter, Penryn Campus, Penryn, TR10 9FE, UK
- 4Biogéosciences, UMR 6282 CNRS, Université de Bourgogne/Franche-Comté, 21000 Dijon, France
- 5Core Scanning Facility, British Geological Survey, Keyworth, NG12 5GG, UK
Abstract. The Late Pliensbachian Event (LPE), in the Early Jurassic, is associated with a perturbation in the global carbon cycle (positive carbon isotope excursion (CIE) of ~ 2 ‰), cooling of ~5 °C, and the deposition of widespread regressive facies. Cooling during the Late Pliensbachian has been linked to enhanced organic matter burial and/or disruption of thermohaline ocean circulation due to North Sea doming. Orbital forcing had a strong influence on the Pliensbachian environments and recent studies show that the terrestrial realm and the marine realm in and around the Cardigan Bay Basin were strongly influenced by orbital climate forcing. In the present study we build on the previously published data for long eccentricity cycle E459 ± 1 and extend the palaeoenvironmental record to include E458 ± 1. We explore the environmental and depositional changes on orbital time scales for the Mochras core during the onset of the LPE. Clay mineralogy, XRF elemental analysis, isotope ratio mass spectrometry, and palynology are combined to resolve systematic changes in erosion, weathering, fire, grain size and riverine influx. Our results indicate distinctively different environments before and after the onset of the LPE positive CIE, and show increased physical erosion relative to chemical weathering. We also identify 5 swings in the climate, in tandem with the 405 kyr eccentricity minima and maxima. Eccentricity maxima are linked to precessionally repeated occurrences of a semi-arid, monsoonal climate with high fire activity and relatively coarser fraction of terrestrial runoff. In contrast, 405 kyr minima in the Mochras core are linked to a more persistent, annually wet climate, low fire activity, and relatively finer grained deposits across multiple precession cycles. The onset of the LPE +ve CIE did not impact the expression of the 405 kyr in the proxy records, however, during the second pulse of lighter carbon (12C) enrichment, the clay minerals record a change from dominant chemical weathering to dominant physical erosion.
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Teuntje P. Hollaar et al.
Status: open (until 20 Feb 2023)
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RC1: 'Comment on cp-2022-87', Stephane Bodin, 21 Dec 2022
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The authors present a high-resolution geochemical, palynological, and clay mineral assemblage dataset from the Upper Pliensbachian of the Mochras core. This dataset is used to discuss the factors at the origin of the Late Pliensbachian cooling event. Overall, this study brings new and interesting data and discussion about an event that could have seen the development of polar glaciation during the Mesozoic, and I therefore recommend it for publication after moderate revisions. Those suggested revisions are linked to my two main comments about this manuscript; Firstly, the structure of the discussion; Secondly, the claim of an earliest onset of the North Sea doming compared to a Late Toarcian – Early Aalenian onset as generally described in the literature
- The Chapter 1.5.2 is rather long and confusing as the main guideline of the discussion is not straightforward. I would recommend to separate the discussion about the origin of the clay mineral and their assemblage variation in a separated subchapter that should appear at the beginning of the discussion. Further subdivision of this chapter might also help its readability.
- In the literature, the onset of the North Sea Doming is said to occur around the Toarcian-Aalenian transition (e.g., Underhill & Partington, 1993). The claim of an earlier onset of this tectonic event can be tracked to the discussion part of Korte et al. (2015) as a putative explanation for the Late Pliensbachian cooling in light of the Aalenian cooling example. These authors were using the argument of regressive facies in the upper Pliensbachian of the North Sea region to substantiate their proposal of an earlier onset. However, Late Pliensbachian regression is a worldwide phenomenon observed in far away regions such as North Africa, the Sverdrup Basin in Canada, the Neuquen Basin in Argentina, or the Arabian Plate. Using this regressive trend as an argumentation for an earlier start of the North Sea Doming is therefore not warranted. I would suggest to tone this hypothesis down throughout the manuscript and clearly emphasise its limitation. This applies notably for the abstract, as well as the part in lines 492–494, where sentences such as “An early phase of regional tectonic updoming of the North Sea disrupted the circulation in the N-S Laurasian Seaway (including the Viking Corridor) and therefore diminished the connectivity between western Tethys and the Boreal realm…” give the impression that this putative early updoming phase is a well-established fact.
In general, I actually don’t think that there is a need to evoke such tectonic phase. The global Late Pliensbachian sea-level low stand might on its own account for poor connectivity between western Tethys and the Boreal realm without having to invoke a regional uplift in the North Sea. Hence, instead of using “North Sea doming”, I would for instance adopt a more neutral position and favour terms like “shallowing in the North Sea and Viking corridor”.
Below, I also list some line specific comments. Overall, I enjoyed reading this manuscript and I’m happy to see that Mesozoic cooling events are also of interests for other research groups.
Stéphane Bodin
Specific comments:
Line 35: Problem in the numeration of the chapter. The Introduction should by numbered under “1” and “1.1”? Same remark for all the main chapter of this manuscript.
Line 44: You could here make reference to Bodin et al. (2023) which further confirms the temporal correlation between the LPE and a long-term sea-level low stand.
Adding a figure (maybe in Figure 1) showing the Upper Pliensbachian of the Mochras core and its d13C record, and highlighting in it the high-resolution studied part would help to better contextualize the here-presented data.
Line 336: Shouldn’t that be a reference to Fig 4 instead of Fig 2 as written?
Lines 357–359: “pelagic settings in the Tethys region often received abiotic fine grained carbonate […] via carbonate producing organisms (such as coccolithophores in zooplankton pellets)”. Calcareous nannoplankton production was very limited during the Jurassic and likely not at the origin of limestone-marl alternations. These latter are best explained by the cyclic export of shallow marine carbonates, as deduced from the disappearance of limestone-marl alternations during time of neritic carbonate factory collapse (e.g. Krencker et al., 2020)
Lines 388–392: “The grainsize changes inferred here reflect two overall coarsening upwards sequences (Fig. 3 and 4). These sequences may reflect changes in clastic transport due to changes in the proximity to the shore/siliciclastic source, changes in runoff due to a changing hydrological cycle, or accelerated bottom currents with greater carrying capacity of coarser sediments”. Could this grainsize change also reflect weathering intensity, with parent rocks being weathered down toward finer grain size during more intense weathering periods as indicated by high K/I and low S/I, and vice versa?
Lines 399–417: This discussion about the interpretation of clay mineral assemblages change should appear earlier in the text as the K/I and S/I ratio are already used in chapter 1.5.1.
Lines 514–515: “siliciclastic versus clay content”. Change “siliciclastic” for “silt” as the clay in the core are also siliciclastics.
Lines 540–541: “These regressive facies may have been caused by an early phase of North Sea doming”. As already stated earlier, this regressive trend is seen on a global scale and can therefore not be considered as a footprint of the North Sea Doming.
References cited:
Bodin, S., Fantasia, A., Krencker, F.-N., Nebsbjerg, B., Christiansen, L., Andrieu, S., 2023. More gaps than record! A new look at the Pliensbachian/Toarcian boundary event guided by coupled chemo-sequence stratigraphy. Palaeogeography, Palaeoclimatology, Palaeoecology 610, 111344.
Krencker, F.-N., Fantasia, A., Danisch, J., Martindale, R., Kabiri, L., El Ouali, M., Bodin, S., 2020. Two-phased collapse of the shallow-water carbonate factory during the late Pliensbachian–Toarcian driven by changing climate and enhanced continental weathering in the Northwestern Gondwana Margin. Earth-Science Reviews 208, 103254.
Underhill, J.R., Partington, M.A., 1993. Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence. Geological Society, London, Petroleum Geology Conference series 4, 337–345.
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RC2: 'Comment on cp-2022-87', Wolfgang Ruebsam, 08 Feb 2023
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Review for Hollaar et al. - Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Mochras Borehole, Cardigan Bay Basin, NW Wales.
The work by Hollaar et al. presented detailed data on late Pliensbachian strata from the Mochras Core. Data provide detailed insights into environmental (oceanographic and continental weathering) during a period of major environmental change. The paper is well written and logically structures. Interpretation of the data are sound and proof and supported by the data. I basically agree with the interpretations by the authors.
However, I’d like to point out one issue that might require some attention. Much of the oceanographic changes reconstructed for late Pliensbachian times is linked to updoming in the North Sea area. Updoming can explain the regional development of a regressive facies and to major changes in current systems across the shallow shelf sea. According to my understanding and what have read in the works by Underhill and coauthors, North Sea dominig occurred in the late Toarcian to early Aalenian (e.g., Underhill and Partington, 1993; GSL – Petr.Geol. Conf. 4, 337-345). Korte et al. (2015) argued that dominig in the late Toarcian was one parameter controlling shelf currents and heat transport across the shallow shelf. Are you sure that the same model can be applied to the late Pliensbachian? To my best knowledge, the works quoted by Korte et al. (2015) provide no evidence for doming during the late Pliensbachian. Maybe the authors can discuss this issue more detailed.
Some specific comments below. Hope you consider the comments constructive and helpful.
Best regards,
Wolfgang Ruebsam
Lines 36-37: Fully agree that marked climate changes occurred throughout the Early Jurassic. However, the view of an overall warm and high pCO2 Early Jurassic word has been challenged by several works. It is more likely that Jurassic climate shifted between cold and warm phases, including icehouse periods (e.g., Dera et al., 2011; Korte and Hesselbo, 2011; Krencker et al., 2019; Ruebsam and Schwark, 2021). The work by McElwain et al. (2005) provides stomata-based pCO2 estimates for the early Toarcian only. This work provides no information on CO2 levels in the Pliensbachian. This work further attests to contrasting CO2 levels in the early Toarcian. Thus, quoting to McElwain et al. (2005) and saying that the Early Jurassic was an overall warm and high pCO2 world is not correct.
Lines 60-63: Updoming in the North Sea region will have impacted current systems at the northwestern West Tethys shelf. This area was a very sallow seaway (shelf area) and I’m not convinced that changes in water circulation at this shallow shelf will have impacted global ocean circulation. Changes in the thermohaline circulation may have occurred in relation to global-scale tectonic changes (continent configuration). The work by Bjerrum et al. (2001) further attests to the presence of southwards-directed current system throughout the Viking Corridor. Thus, there no proof that warm Tethyan current transported warm water massed towards polar latitudes via this narrow seaway. The current system indicated in figure 1 (red arrow) is speculative. On the contrary, there is robust evidence (e.g., 18O data; modeling) that a southwards-directed Arctic current system transported low-saline cooler water masses to the northwestern West Tethys shelf via the Viking Corridor (Bjerrum et al., 2001; Dera and Donnadieu, 2012).
Method part - lines136 and following: Detailed data on the Mochras Core geochemistry have been published previously: Ruhl et al., 2016 – XRF; Strom et al., 2020 – TOC, H, 13Corg; can the author please clarify if all data presented here (TOC, 13Corg, XRF) have been newly generated in this study? If data were taken from previous studies, those works must be quoted.
Figure 4: This figure nicely defines the late Pliensbachian +veCIE. I think most (or all?) of the data shown here have been generated in previous works and are not part of this study. Thus, this figure should be shown in the introduction part and not in the results. Showing the figure in the introduction may give the reader a good impression of the late Pliensbachian events, as recorded in the sediments of the Mochras Core.
Figure 3: Figure 3 should be shown after figure 4, as the Pliensbachian events are defined in the latter. Moreover, figure 3 shows a lot of data interpretation, which should not be part of the results section.
Lines 355-372: Here changes in the composition and nature of the sedimentary organic matter are described on the basis of palymnological data. These data could be compared with HI/OI data that were presented in Storm et al. (2020). Integration of Rock Eval (or HAWK) data may allow assessing changes in the preservation of marine organic matter.
Lines 355-372: Could changes in grain size (and Zr/Rb, Si/Al) also be affected by sea-level variations? 4thorder sea-level cycles have been related to long-eccentricity forcing. Could be interesting to explore this aspect.
Lines 448-449: Tmax values of 421-434°C indicate that the sediments (and the organic matter) may have reached the early oil window and experienced a burial temperature of at least 60°C. Thus, the thermal maturity/diagenesis should be classified as weak-moderate.
Line 492 and following: Korte et al (2015) explained that North Sea doming and uplift occurred in the late Toarcian (relevant works were quoted in Korte et al.). Is there any data that support an early updoming in the Pliensbachian?
The late Pliensbachian record a long-term sea-level lowstand (e.g., Haq, 2018). A low eustatic sea level will have narrowed small ocean gateways (such as the Viking Corridor) and thereby impacted current systems and faunal realm. This aspect should be added to the discussion. In lines 501 and following it is explained how a sea level highstand in the Bifrons Zone could have terminated anoxia in the European Basin System. This indicates that eustatic sea level changes strongly impacted oceanographic conditions at this shallow shelf sea.
Lines 494-495: As pointed out before, the current directions in at the northwestern Tethys shelf remains debated. There is strong evidence for a southwards directed Arctic current and (to my best knowledge) not strong evidence for a northward directed current through the Viking corridor (e.g., Bjerrum et al., 2001; Dera and Donnadieu, 2012).
Lines 510-512: Correct, but s is highly speculative if the Hispanic Corridor efficiently connected the Panthalassic and the Tethys Ocean and thereby impacted global ocean circulation pattern during the Pliensbachian-Toarcian.
Lines 540-541: As mentioned earlier, the late Pliensbachian records a global eustatic sea level lowstand. It is unlikely that the doming was the major factor causing the development of a regressive facies.
Teuntje P. Hollaar et al.
Data sets
Terrestrial palaeo-environmental proxy data of the Upper Pliensbachian, Mochras Borehole sediments, deposited in the Cardigan Bay Basin, Wales Teuntje Hollaar https://doi.org/10.5285/d6b7c567-49f0-44c7-a94c-e82fa17ff98e
Llanbedr (Mochras Farm) Core Scanning Dataset Magret Damaschke, Simon Wylde, Mengjie Jiang, Teuntje Hollaar, Clemens Vinzenz Ullmann https://doi.org/10.5285/c09e9908-6a21-43a8-bc5a-944f9eb8b97e
Teuntje P. Hollaar et al.
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