Initial results of coring at Prees, Cheshire Basin, UK (ICDP JET project): towards an integrated stratigraphy, timescale, and Earth system understanding for the Early Jurassic

This is the final version. Available on open access from Copernicus Publications via the DOI in this recordData availability: Full core scan data (https://doi.org/10.5285/91392f09-25d4-454c-aece-56bde0dbf3ba, BGS Core Scanning Facility, 2022) will be available after 1 November 2024 via the Natural Environment Research Council (NERC) National Geoscience Data Centre (https://webapps.bgs.ac.uk/services/ngdc/accessions/index.html#, last access: 12 October 2023). Downhole logging data (https://doi.org/10.5880/ICDP.5065.001​​​​​​​, Wonik, 2023) will be made available via the ICDP (https://www.icdp-online.org/projects/by-continent/europe/jet-uk/, last access: 12 October 2023). The JET Operational Report is published as Hesselbo et al. (2023); full information about the operational dataset, the logging dataset, data availability and the explanatory remarks is available on the ICPD-JET project website: https://www.icdp-online.org/projects/by-continent/europe/jet-uk/ (last access: 12 October 2023). A subset of data, additional biostratigraphic tables, and vector graphics files for Figs. 3–5 are included as the Supplement. Supplementary Data File 1 tabulates the corrected depth scale for Prees 2C. Supplementary Data File 2 summarizes the ammonite-based chronostratigraphy of the Prees 2 cores (ammonite identifications by Kevin N. Page). Supplementary Data File 3 summarizes the ammonite-based chronostratigraphy for the Hettangian to Early Pliensbachian of the Llanbedr (Mochras Farm) borehole (updated by Kevin N. Page). Supplementary Data File 4 tabulates the organic carbon-isotope ratios, TOC, and carbonate content of low-resolution samples taken at the Prees drill site; TOC and carbonate data are calculated using calibration based on portable XRF (Supplementary Data File 5) and a gas source isotope ratio mass spectrometer (Supplementary Data File 6). Supplementary Data File 5 tabulates portable XRF results for bulk rock powders of low-resolution samples taken at the Prees drill site; uncertainties stated in the table are given for the fit to the raw data and do not reflect the true reproducibility of the data. Empty fields indicate values under the detection limit. Sample SSK116001 acted as a repeat sample which was measured 70 times over the course of the data acquisition to determine the repeatability and drift of the instrument. LE stands for “light elements”. Supplementary Data File 6 tabulates gas source isotope ratio mass spectrometry (GS-IRMS) data (oxygen- and carbon-isotope ratios of carbonate as well as carbonate content calculated as calcite) for a set of 24 samples covering the entire core length and reflecting a representative spread of carbonate content. Comparison of GS-IRMS data with p-XRF data was used to create a calibration curve to calculate the carbonate (and TOC) content of all low-resolution samples. Supplementary Data File 7 tabulates pyrolysis data (Rock-Eval 6) for Prees 1 well cuttings and Wilkesley borehole samples. Supplementary Data File 8 contains vector graphics files (.svg) for Figs. 3–5.Drilling for the International Continental Scientific Drilling Program (ICDP) Early Jurassic Earth System and Timescale project (JET) was undertaken between October 2020 and January 2021. The drill site is situated in a small-scale synformal basin of the latest Triassic to Early Jurassic age that formed above the major Permian–Triassic half-graben system of the Cheshire Basin. The borehole is located to recover an expanded and complete succession to complement the legacy core from the Llanbedr (Mochras Farm) borehole drilled through 1967–1969 on the edge of the Cardigan Bay Basin, North Wales. The overall aim of the project is to construct an astronomically calibrated integrated timescale for the Early Jurassic and to provide insights into the operation of the Early Jurassic Earth system. Core of Quaternary age cover and Early Jurassic mudstone was obtained from two shallow partially cored geotechnical holes (Prees 2A to 32.2 m below surface (m b.s.) and Prees 2B to 37.0 m b.s.) together with Early Jurassic and Late Triassic mudstone from the principal hole, Prees 2C, which was cored from 32.92 to 651.32 m (corrected core depth scale). Core recovery was 99.7 % for Prees 2C. The ages of the recovered stratigraphy range from the Late Triassic (probably Rhaetian) to the Early Jurassic, Early Pliensbachian (Ibex Ammonoid Chronozone). All ammonoid chronozones have been identified for the drilled Early Jurassic strata. The full lithological succession comprises the Branscombe Mudstone and Blue Anchor formations of the Mercia Mudstone Group, the Westbury and Lilstock formations of the Penarth Group, and the Redcar Mudstone Formation of the Lias Group. A distinct interval of siltstone is recognized within the Late Sinemurian of the Redcar Mudstone Formation, and the name “Prees Siltstone Member” is proposed. Depositional environments range from playa lake in the Late Triassic to distal offshore marine in the Early Jurassic. Initial datasets compiled from the core include radiography, natural gamma ray, density, magnetic susceptibility, and X-ray fluorescence (XRF). A full suite of downhole logs was also run. Intervals of organic carbon enrichment occur in the Rhaetian (Late Triassic) Westbury Formation and in the earliest Hettangian and earliest Pliensbachian strata of the Redcar Mudstone Formation, where up to 4 % total organic carbon (TOC) is recorded. Other parts of the succession are generally organic-lean, containing less than 1 % TOC. Carbon-isotope values from bulk organic matter have also been determined, initially at a resolution of ∼ 1 m, and these provide the basis for detailed correlation between the Prees 2 succession and adjacent boreholes and Global Stratotype Section and Point (GSSP) outcrops. Multiple complementary studies are currently underway and preliminary results promise an astronomically calibrated biostratigraphy, magnetostratigraphy, and chemostratigraphy for the combined Prees and Mochras successions as well as insights into the dynamics of background processes and major palaeo-environmental changes.ICDPNatural Environment Research Council (NERC)German Research FoundationHungarian Scientific Research FundNational Science Centre, PolandPolish Geological Institut

Rhythmic Shifts in Fire Activity in Earth's Geological Past were Driven by Orbital Cycles

Fire is a key component of the Earth System and has shaped terrestrial ecosystems since the Late Silurian, shortly after the arrival of the first land plants. Present day anthropogenic warming has increased the fire weather season and the risk of extreme fire events in many regions of the world. The probability of fire will continue to rise with the future predicted warming. To understand long-term patterns and feedbacks in climate and fire activity we need to look at the geological record. Past periods of global warmth are all associated with enhanced wildfire, in a similar way to current global warming, and models exceeding the worst case scenario of 4 °C warming by the Intergovernmental Panel on Climate Change (IPPC). Fire activity has been reconstructed for some specific past events of global warmth accompanied by perturbations of the carbon cycle reflected in carbon-isotope records. However, it remains unknown how the fire regime and behaviour acted at times of no perturbation in the carbon-cycle in the geological past. Research on fire activity in the Quaternary (last 2.6 Myr) provides strong evidence that wildfires are driven by climate forcing on an orbital time scale. Fire activity has been shown to increase at times of enhanced seasonal contrast driven by precession and the amplitude modulator eccentricity in several regions. As in present day fire-prone areas, this creates a humid season that allows biomass to gather, followed by a dry season that lowers the fuel moisture levels to allow ignition and fire spread. These climate driven changes in fire activity are often linked to a change in fuel type, load and structure on an orbital time scale. In this thesis I present three reconstructions of wildfire activity from the Early Jurassic and examine the role of orbital forcing on these records: (i) one record spanning ~350,000 yr (350 kyr) at a time of no carbon-perturbation (background climate); (ii) one ~800,000 yr (800 kyr) record spanning the so-called Late Pliensbachian Event, and; (iii) a ~900,000 yr (900 kyr) record spanning the Sinemurian-Pliensbachian boundary. The first record serves to assess wildfire activity in geological time during a climatically relatively stable period, and the other two records examine if the wildfire regime is impacted by climatic cooling or warming, respectively. The study location is Mochras, Cardigan Bay Basin, NW Wales, UK. The Llanbedr (Mochras Farm) core constitutes an ideal archive for study of orbital forcing of wildfire, due to the well-established astrochronological framework and the high terrestrial organic content. Wildfire activity in the geological past is inferred from fossil charcoal abundance and two sets of charcoal counts are presented from a range of depositional environments and ages (Jurassic to Miocene) to assess the reproducibility of the fossil charcoal proxy. The reproducibility of charcoal counts between two researchers was significant in all depositional environments and in order to take small differences in charcoal counts into account an error bar of ~40 charcoal particles is suggested. A multi-proxy dataset, from the Late Pliensbachian, comprising charcoal counts, clay mineralogy, palynofacies (marine and terrestrial organic microfossils) and carbon mass spectrometry records, shows that fire activity in a ‘background’ climate during the Late Pliensbachian is strongly driven by ~20 kyr precession and modulated by 405 kyr eccentricity forcing. Also in the Early Jurassic, wildfire activity is greatest at times of high seasonal contrast in rainfall, where the rainy season allows biomass to build up and the subsequent dry season lowers the moisture status of the fuels and increases the ignitability. These climatically driven shifts in wildfire activity are potentially accelerated via orbital shifts in vegetation. Following, the Late Pliensbachian ‘background’ record is extended and spans two long eccentricity cycles. This record also covers the start of the Late Pliensbachian Event. Long eccentricity modulates changes in the hydrological cycle as inferred from clay mineralogy, grain-size inferred from elemental data (core-scan XRF), and microscopic charcoal abundance. The positive carbon-isotope excursion marking the onset of the Late Pliensbachian Event is associated with enhanced physical erosion relative to chemical weathering. Lastly, multi-proxy records of the Late Pliensbachian and the Sinemurian-Pliensbachian boundary are compared; both records comprising charcoal counts, clay mineralogy, palynofacies and mass spectrometry. I compare the fire record of the Late Pliensbachian associated with climatic cooling and the fire reconstruction from the Sinemurian-Pliensbachian boundary associated with climatic warming. Both fire records show a predominant control of the 100 kyr eccentricity cycle. Placing both fire records on the intermediate-productivity gradient indicates that both fire regimes were limited by moisture and not productivity (biomass). Thus, although these records likely represent different climatic backgrounds, fire activity was suppressed due to high fuel moisture levels and not low biomass abundance in the Mochras core. Overall, this thesis shows that fire activity in the Cardigan Bay region during the late Sinemurian and Pliensbachian was strongly driven by orbital forcing. Insolation driven changes in humidity at a precessional, short-, and long-eccentricity time scales led to fivefold increases and decreases in charcoal and inferred fire activity. Strong seasonality and intermediate levels of moisture and biomass productivity are linked to extremes in fire activity