Geochemistry of Macrofossil, Bulk Rock, and Secondary Calcite in the Early Jurassic Strata of the Llanbedr (Mochras Farm) Drill Core, Cardigan Bay Basin, Wales, UK
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Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Accepted Manuscript Journal of the Geological Society Geochemistry of macrofossil, bulk rock, and secondary calcite in the Early Jurassic strata of the Llanbedr (Mochras Farm) drill core, Cardigan Bay Basin, Wales, UK. Clemens V. Ullmann, Dominika Szűcs, Mengjie Jiang, Alexander J.L. Hudson & Stephen P. Hesselbo DOI: https://doi.org/10.1144/jgs2021-018 To access the most recent version of this article, please click the DOI URL in the line above. When citing this article please include the above DOI. Received 5 February 2021 Revised 30 May 2021 Accepted 10 June 2021 © 2021 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Supplementary material at https://doi.org/10.6084/m9.figshare.c.5463508 Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. The manuscript will undergo copyediting, typesetting and correction before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Although reasonable efforts have been made to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record once published for full citation and copyright details, as permissions may be required. Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Geochemistry of macrofossil, bulk rock, and secondary calcite in the Early Jurassic strata of the Llanbedr (Mochras Farm) drill core, Cardigan Bay Basin, Wales, UK. Clemens V. Ullmann1,2,*, Dominika Szűcs2, Mengjie Jiang2, Alexander J.L. Hudson2, Stephen P. Hesselbo2. 1University of Oxford, Department of Earth Sciences, 3 South Parks Road, OX1 3AN, United Kingdom 2University of Exeter, Camborne School of Mines, Penryn Campus, Treliever Road, TR10 9FE, Penryn, Cornwall, United Kingdom *correspondence: [email protected]; ORCID 0000-0002-5865-7289 Dominika Szűcs: [email protected] Mengjie Jiang: [email protected] Alexander J.L. Hudson: [email protected] Stephen P. Hesselbo: [email protected] MANUSCRIPT Author contribution: CVU performed sampling and geochemical analyses for bulk rock, diagenetic phases and macrofossils. DS contributed information on ammonite occurrences and ammonite preservation, MJ studied petrography of the rocks using SEM, AJLH measured bulk rock samples using portable XRF. The study was designed and the manuscript drafted by CVU in collaboration with SPH and input from all co-authors. Funding information: This ACCEPTEDwork was financed by the UK Natural Environment Research Council (grant number NE/N018508/1). Data Access Statement: All data generated during this study are included in the supplementary information files for this article. Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Abstract The Llanbedr (Mochras Farm) core (Wales, United Kingdom) yielded a > 1,300 m long mudrock sequence which has excellent potential for establishing an integrated stratigraphic scheme for the entire Early Jurassic Epoch. Lithological variations in the core are predominantly driven by hierarchical changes in carbonate content which also dominate – or may impact upon – many geochemical and physical properties of the core. The bulk carbonate carbon isotope record displays systematic fluctuations the largest of which correspond to previously identified phases of environmental perturbation. The magnitudes of negative carbon isotope excursions in carbonate are inflated compared to equivalents previously described elsewhere due to diagenesis and concomitant loss of primary carbonate. The marine macrofossil record of Mochras reveals biological and isotopic patterns that are generally comparable to other UK basins. Potentially significant differences between Cleveland and Cardigan Bay Basin are observed in the Pliensbachian and Toarcian fossils. This different expression may be related to different habitat structure or palaeoceanographic and water depth differences between these basins. Minima in macrofossil δ18O values generally coincide with peaks in macrofossil wood abundance and sea-level lowstands inferred from sequence stratigraphic interpretation of other UK sections. This relationship points to a possibleMANUSCRIPT relative sea-level control on observed oxygen isotope records and sediment provenance. Supplementary material: Analytical data for the Mochras core pertaining to this contribution is available online at https://doi.org/10.6084/m9.figshare.c.5463508. Supplementary files comprise Supplementary Table 1: Carbonate content from gas source mass spectrometer Supplementary Table 2: Carbonate content from RockEval analysis Supplementary Table 3: Carbonate content from XRD analysis SupplementaryACCEPTED Table 4: Carbonate content from portable XRF analysis Supplementary Table 5: Compilation of carbonate content in 5 m intervals Supplementary Table 6: C and O isotope ratios and carbonate content of bulk carbonate Supplementary Table 7: C and O isotope and element/Ca ratios of diagenetic phases Supplementary Table 8: C and O isotope and element/Ca ratios of macrofossils Supplementary Table 9: Standard data for gas source mass spectrometer measurements Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Supplementary Table 10: Standard data for ICP OES measurements Supplementary Table 11: Compilation of carbonate C and O isotope data MANUSCRIPT ACCEPTED Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Integrated stratigraphic schemes for extended intervals of geologic time are of great importance for understanding palaeoenvironments. They permit a unified view of multiple proxies for palaeoenvironmental and stratigraphic conditions, avoiding correlation errors and age uncertainties unavoidable in compilations of composite stratigraphic records. Integrated stratigraphic schemes also allow for high accuracy correlation to other sedimentary successions, allowing for a global view of palaeoenvironmental conditions and their change through time. Such integrated stratigraphic schemes are particularly important for the Early Jurassic for which comparatively large uncertainties in the numerical ages of stage boundaries, durations of ammonite biozones, and problems with long-range correlations still exist (Hesselbo et al. 2020a). The Llanbedr (Mochras Farm) borehole was drilled onshore in the Cardigan Bay Basin (Wales, UK) in 1967-1969 (history summarised in Copestake and Johnson, 2014). Below a 600 m thick Cenozoic cover, a thick Mesozoic sequence, including 1,300 m of Early Jurassic sedimentary strata, was recovered (Wood and Woodland, 1968; Woodland, 1971). All boreal ammonite zones of the Early Jurassic barring the earliest Hettangian tilmanni Zone were shown to exist at the site (Woodland, 1971; Copestake and Johnson, 2014), indicating that breaks in sedimentation and faults, if present, did not cause major stratigraphic gaps. MANUSCRIPT An updated ammonite biostratigraphy and foraminifera biostratigraphy are already available for the core (Copestake and Johnson, 2014). Low-resolution carbonate and organic matter carbon isotope chemostratigraphy has been previously published (Jenkyns et al., 2002; Katz et al., 2005; van de Schootbrugge et al., 2005a). More recently, higher resolution organic matter carbon isotope chemostratigraphy based on 1760 samples (Xu et al., 2018a; Storm et al., 2020) and Toarcian magnetostratigraphy (Xu et al., 2018b) have been established. The clay fraction of the middle Sinemurian to upper Pliensbachian part of the core has also been studied and systematic changes of relativeACCEPTED clay abundance linked to shifting climate conditions (Deconinck et al., 2019, Munier et al., 2020). Furthermore, the Pliensbachian section of the core has been the subject of cyclostratigraphic interpretation from time series of major element content, whose cyclic character is governed principally by regular fluctuations in carbonate content (Ruhl et al., 2016). Downloaded from http://jgs.lyellcollection.org/ by guest on October 1, 2021 Despite its importance for cyclo-, bio- and chemostratigraphy, the calcite fraction of the Mochras core has hitherto not been studied petrographically or geochemically in any detail. Carbonate carbon isotope chemostratigraphy at Mochras is limited to a dataset of Katz et al. (2005) at c. 8 m resolution, and 71 analyses for the Toarcian and upper Pliensbachian presented by (Jenkyns et al., 2002). Geochemical assessments of clearly diagenetic calcite such as veins and slickensides, as well as of macrofossil calcite, are lacking. In order to better understand the nature of the carbonate of the Mochras core and environmental signatures encoded therein, a detailed study of the various carbonate phases present at Mochras was undertaken. We report here C and O isotope data and carbonate contents for 961 bulk rock samples, and C and O isotope as well as element/Ca ratios for 243 samples of diagenetic phases, and 335 samples of macrofossil calcite.