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Photomicrographs of vitrinite macerals in reflected light under oil immersion, depth 1583.1 m, (magnification 500X). (a) Plate showing the dominance of inertinite (here mostly semifusinite). Only four of the darker-grey particles are in fact vitrinite; (b) plate showing bright, angular fusinite and somewhat lighter semifusinite particles along the upper margin of the field. Darker grey vitrinite particles are present at the lower right-hand side.
Source publication
The shale-gas potential of mid-Carboniferous mudrocks in the Bowland-Hodder unit in the Cleveland Basin (Yorkshire, northern England) was investigated through the analysis of a cored section from the uppermost part of the unit in the Malton-4 well using a multidisciplinary approach. Black shales are interbedded with bioturbated and bedded sandstone...
Citations
... The same samples used for XRF analysis were also sub-sampled for pyrolysis analysis. All sub-samples were crushed, homogenized and sieved (250 µm) and analysed using a Rock-Eval(6) analyser (at the British Geological Survey) configured in standard mode (Behar et al. 2001;Słowakiewicz et al. 2015;Newell et al. 2016). Rock-Eval pyrolysis was prioritized over HAWK (Jarvie et al. 2015) and high-pressure water pyrolysis (Whitelaw et al. 2019) to more reliably compare results with previous Bowland sub-basin studies (e.g. ...
The Bowland sub-basin is a target for hydrocarbon exploration but to a large extent it remains unexplored. To determine the economic potential of the Bowland sub-basin, it is important to identify the oceanographic processes involved in the deposition of the Bowland Shale Formation in the Late Mississippian ( ca . 330 Ma). Palaeoceanographic processes are known to be a major control on the development of hydrocarbon source rocks. This study investigates core (Preese Hall-1 and Becconsall-1Z) materials from the Upper Bowland Shale, and compares to previously published data (outcrop Hind Clough), all from the Bowland sub-basin, Lancashire, UK. The sedimentology and geochemistry of this formation was determined via a multi-technique approach including x-ray fluorescence (XRF), sedimentology, gamma ray spectra, x-ray diffraction (XRD) and RockEval(6) TM pyrolysis. Key trace metal abundances and enrichment factors were used to assess sediment provenance and to determine the bottom water redox conditions during the deposition of the Upper Bowland Shale. Our results support interpretations that contemporaneous anoxia developed in bottom waters in at least three sites in the Bowland sub-basin. In a comparison with the Fort Worth Basin (Barnett Shale, USA), the Bowland sub-basin was apparently less restricted and deposited under a much higher mean sediment accumulation rate compared to the Fort Worth Basin. Knowledge from this study improves future resource estimates of the Bowland Shale Formation, and challenges the early assumptions that the Barnett Shale is an analogue to the Bowland Shale.
... mineral matrix effects, Tmax suppression); and 3) shale gas assessment specific methodologies and unified data processing/presentation approaches (Cornford et al., 1998;Dembicki, 2009;Espitalie & Joubert, 1987;Jarvie et al., 2007;Romero-Samiento, 2016). However, recent studies of the main unconventional hydrocarbon play in the UK, namely the Carboniferous Bowland Shale Formation, has shown fairly thin organic rich intervals interbedded with calcareous mudstones and turbidites (Hennissen et al., 2017, Andrews, 2013, Słowakiewicz et al., 2015, Könitzer et al., 2016, Waters et al., 2020, Emmings et al., 2019. Given this cyclical and laterally variable pattern of organic matter quantity and type there is a need for cost effective high-resolution geochemical screening method. ...
... mean 1.9, n=6) and the Craven Basin (range 1.76-3.72%, mean 2.3% (n=36) (Könitzer et al., 20015, Hough et al., 2014, Słowakiewicz et al., 2015. However, it should be borne in mind that not all organic matter has equal hydrocarbon generative potential. ...
... Tmax is defined as the temperature of the maximum rate of hydrocarbon generation measured on the S2 peak and is derived during the pyrolysis of non-volatile hydrocarbons contained within the sample (Behar et al., 2001). The oil-gas window of a shale is typically define by an approximate cut-off point 435°C, below which the kerogen is considered immature and commercially unproductive (Słowakiewicz et al., 2015). The HI is also affected by thermal maturity and tends to decrease with maturity. ...
Shale rock core from the Bowland Shale Formation, UK was analysed in the laboratory using Rock-Eval(6) pyrolysis and Fourier Transform Infrared Spectroscopy (FTIR). These methods are used to characterise the organic constituents of soil and rock. This research is a proof-of-concept study to investigate whether regression models developed using FTIR and Rock-Eval data for the same length of core can be used to estimate selected Rock-Eval parameters. The accuracy of the regression models was assessed using statistical methods, the results of which were used to choose preferred models for each Rock-Eval parameter. Models produced were shown to have an acceptable level of uncertainty for Total Organic Carbon, S1, S2 and S3 outputs which led us to conclude these are potentially suitable for estimating unknown down-core Rock-Eval parameter values. Conversely, the T max model had a higher variability in the cross-validation data above the acceptable level of uncertainty which could lead to erroneous estimates. Down-core interpolations of selected Rock-Eval parameters could be practically achieved by modelling FTIR data by maintaining standard sample frequencies for Rock-Eval while supplementing with higher frequencies for FTIR and chemometric analysis.
... Rock-Eval pyrolysis is an established technique for the assessment of free and structurally bound organic matter (OM) in source rocks during oil and gas exploration (Espitalié and Joubert, 1987;Tyson, 1995;Waters et al., 2019). Estimates of the amount of hydrogen (HI) and oxygen (OI) are compared to established reference samples and or published criteria to infer OM (kerogen)-type (Emmings et al., 2017;Hennissen et al., 2017;Slowakiewicz et al., 2015). Similarly, recent environmental studies have utilised variations in the profile of the bound hydrocarbons (S2), that broadly corresponds to strength and extent of intra and inter polymer bonding, to identify changes in OM that were used to understand hydrological-climate effects on peatland carbon, variable OM preservation in soils and historical oil spills in urban waterway sediments (Cooper et al., 2021;Girkin et al., 2018;Newell et al., 2016;Sebag et al., 2016;Thomas et al., 2019). ...
The UN Sustainable Development Goals highlight the myriad of socio-economic and environmental challenges occurring as a result of anthropogenic chemical pollution. Urban sediments from informal settlements (slums) on the Nairobi, Ngong and Mathare Rivers (n = 25), were evaluated for sediment quality. Microtox bioassay identified 8 sites as toxic, 9 as moderately toxic and 8 as non-toxic. Slum sediments were characterised by high total organic carbon and Rock-Eval pyrolysis revealed bound carbon from a mix of raw sewage and domestic refuse. Sediments from Kiambio, Kibera, Mathare and Kawangware slums contained high coprostanol at 55–298 μg/g and epicoprostanol at 3.2–21.7 μg/g confirming appreciable incorporation of untreated human faeces. Hormones, antianalgeiscs, antiinflamatories, antiepileptics and antibiotics most affected Mathare > Kiambio > Kibera > Mukuru > Kawangware slums. Carbamazepine, ibuprofen, diclofenac and acetaminophen concentrations are amongst the highest reported in Kenyan river sediments and were positively correlated with faecal steroids (sewage). Common persistent organic pollutants, such as organochlorine insecticides ΣDDT 1–59 μg/kg, mean 21.2 μg/kg, Σ¹⁶PAH 182–2218 μg/kg, mean 822 μg/kg and Σ³⁰ PCB 3.1–157.1 μg/kg, mean of 21.4 μg/kg were between probable effect likely and unlikely sediment quality guidelines (SQG). PAH source ratios and parent to alkyl-PAH distribution suggested vehicle exhaust, power stations (heavy oil), kerosene (cooking oil) and other pollution sources. Trace metal concentrations As, Cd, Cr, Hg and Ni were below SQG whereas Pb exceeded the SQG. This multi-contaminant characterisation of sediment quality in Nairobi supports the development and implementation of policies to improve urban infrastructure to protect ecological and human health. It demonstrates the need for environmental geochemists to engage in the science-policy interface associated with both global and national development frameworks, with particular reference to the Sustainable Development Goals, New Urban Agenda, and Kenya’s Vision 2030.
... During the Asbian period of the Carboniferous palaeomagnetic evidence suggests Britain was located at near-equatorial latitudes (Słowakiewicz et al., 2015). At this time there was likely to have been glacial eustasy with fluctuating sea levels (Gross et al., 2015) likely to have had a significant impact on sediment deposition. ...
... These platforms and basins were formed by a period of north-south regional extension in the early Carboniferous providing the beginnings of the shelf-type structural geology of the region. The area continued to be tectonically active over the period (Visean to Namurian) in which these shales were deposited, towards the late Carboniferous there was the start of a compressive regime which will have provided the necessary depth needed for the transformation of organic matter (Słowakiewicz et al., 2015). ...
... The total organic carbon content ranges from 0.37-2.45%, and is predominantly made up of Type III organic matter (humic), with some input from Type II organic matter (planktonic) (Słowakiewicz et al., 2015). This organic matter make-up is likely due to the transport of terrestrial material in a hemipelagic flow into the basins and across the carbonate platforms, where planktonic organic matter is seen this is as a result of organic matter in the water column of the basinal deposits. ...
Seal rocks, also called cap rocks, are a crucial and sometimes overlooked factor (due to not being the primary factor in exploration, having more of a role in the resource evaluation and development) in the evaluation of a potential gas accumulation, and is critical in downhole gasification (enhanced gas recovery) and storage of other gases. Shale rocks are the most common seal rock in conventional reservoirs; currently shales are providing an unconventional oil and gas source which can act as a potential buffer to the energy industry as it transitions towards renewable energies (which are still in their formative years) whilst there is a continued rise in demand for energy globally. Over the past ten years there has been a boom in shale gas production in the United States (Barsotti et al., 2016; Li et al., 2016; Yu et al., 2016), and it is anticipated that this boom may be repeated in the UK (Andrews, 2013). Downhole gasification (enhanced gas recovery) offers a potential way to produce from these “difficult-to-extract” (as a result of low permeability’s) reservoirs by using carbon dioxide as a displacement gas for methane. At the same time this carbon dioxide can be also be stored resulting in the environment being exposed to less greenhouse gas (Kim, Cho and Lee, 2017; D. Liu et al., 2019). However, it is erroneous to consider shales as a completely impermeable layer, and their ability to retain different fluids is variable (controlled by the capillary entry pressure and/or the permeability and the extent of diffusive loses) which could result in some/all of them being ineffective at retaining carbon dioxide. This is because shales are highly complex and anisotropic containing pores over several orders of magnitude. Typically they have a significantly low permeability and porosity, combined with structural and chemical heterogeneities of shales mean that physical processes are significantly impacted. Importantly the structure-transport relationship is complex resulting in processes such as hydrocarbon migration, methane extraction, gas storage, or carbon sequestration being poorly understand. This project proposes the development of several novel characterisation techniques and combinations of complementary techniques to characterise the multi-scale properties of shales in order to more accurately provide the information needed for secure decisions regarding gas production and storage. In this work mercury porosimetry, together with mercury thermoporometry, and computerised x-ray tomography (CXT) were performed on post-porosimetry samples containing entrapped mercury, to characterise the pore structure of cap-rocks. However limitations were identified where mercury was trapped in pores too large to sufficiently suppress the bulk melting point (thermoporometry) such that a separate melting peak formed. However, the combined use of mercury porosimetry and computerised x-ray tomography was effective at highlighting the location of trapped mercury, but was ineffective at providing quantitative results regarding the macroporosity of the sample. Further drawbacks of mercury porosimetry based analysis are the potential destruction of experimental material where further analysis cannot be carried out unless mercury forms part of the experimental technique (i.e. thermoporometry and computerised x-ray tomography as described above). Therefore, gas overcondensation, was proposed as an alternative technique as a bridge between micro-pore characterisation, below the limit of mercury detection, up to macro- pores which are undetected in conventional sorption experiments, with the additional benefit that the overcondensation method preserves experimental material. In previous work, gas sorption experiments typically consist of a boundary adsorption isotherm up to a restricted maximum pressure (e.g. up to 0.995 p/p0). Following this there is a pseudo-boundary desorption isotherm, which is merely a descending curve since complete pore- filling with liquid-like condensate was not achieved. As a result of this conventional gas sorption alone cannot prove the complete pore size range up to large macro-pores. Gas overcondensation experiments can be expanded with gas sorption scanning curves which have successfully revealed advanced condensation effects, allowing probing of the inter- relationship and spatial juxtaposition of multi-scale porosities. Gas overcondensation and scanning loops were successfully used for the Utica and Bowland samples to reveal where additional percolations knee develop that are characteristic of a particular pore size within the wider pore network (Utica). Work on the Bowland was able to determine that there are some large macro-pores shielded by pore necks of <4nm; complimentary adsorption calorimetry work was able to relate this shielding to pore necks by calculating the mass transfer and thermokinetic properties of the samples. Prior to the use of gas overcondensation mineralogy was assessed with the use of conventional gas sorption where results (Marcellus and Utica) showed an inverse relationship between carbonate and illite quantities (i.e. an increasing carbonate content was associated to a decreasing illite content). Utica surface areas demonstrated a strong correlation to illite quantity, whereas Marcellus surface areas demonstrated a weaker correlation to illite. For both samples there was good correlation to the total organic carbon. With the new information gained from gas overcondesation it has allowed for additional, and more advanced correlations to be made with other physical properties of shales such as the mineralogy. It was found that for the changeover period (Utica samples), from primarily clay to carbonaceous deposits, there was an associated growth in the disorder of the pore network over particular key length-scales. These length-scales were highlighted by percolation processes in the gas overcondensation and scanning curves. This peaking in disorder was also associated to a peak in total organic carbon content and the accessible porosity was shown to be dominated by the organic carbon phase. Following the identification of this trend with the use of gas overceondensation and mineralogy, numerical analysis techniques were used to replicate these findings with the use of the homotattic patch model. It was established that with the use of conventional gas sorption (nitrogen) isotherms and isotherms for the pure mineral phases of the sample good results can be generated indicating the associated quantity of each mineral to the sample.
... (2) Results on core samples, such as mineral composition analyzed with x-ray diffraction (XRD), providing quartz, clay minerals, dolomite, and calcite or accessory minerals like pyrite, halite, or anhydrite contents. Detailed quantitative mineral compositions were determined for 120 samples from drill core B. (3) Rock-Eval pyrolysis on core samples, providing useful parameters: total organic carbon (TOC) and oxygen index (OI) corresponding to oxygen content in OM, thus a potential proxy for oxidizing conditions (e.g., Anderson et al., 1981;Słowakiewicz et al., 2015). ...
... The MS logging was correlated with the results of Rock-Eval studies, such as TOC index and OI (Figures 5, 6; Figures S5, S6, supplementary material available as AAPG Datashare 131 at www.aapg.org /datashare), which may reflect the chemical conditions at the bottom of sedimentary basin (Słowakiewicz et al., 2015). Both indices, obtained as a result of OM pyrolysis, may help to estimate the quality of oxygenation conditions present during sediment deposition (e.g., Anderson et al., 1981;Langford and Blanc-Valleron, 1990;Peters et al., 2007). ...
... Both indices, obtained as a result of OM pyrolysis, may help to estimate the quality of oxygenation conditions present during sediment deposition (e.g., Anderson et al., 1981;Langford and Blanc-Valleron, 1990;Peters et al., 2007). In detail, the OI provides an estimate of the amount of oxygencontaining compounds, a parameter that increases upon aerobic biological decay and which reflects the early stage of chemical and depositional conditions of compaction and further cementation of analyzed rocks (Słowakiewicz et al., 2015). Note that the values of OI > 80 were excluded from interpretation because they coincided with very low (<0.5 wt. ...
... ish Geological Survey (BGS) ( Table 3). Pyrolysis was performed in standard mode (pyrolysis and oxidation as a serial process) 36 . Powdered rock samples (60 mg/dry wt) were heated isothermally at 300 °C (hold for 3 min) and then heated from 300 to 650 °C (hold for 3 min) at 25 °C/min in N 2 atmosphere. ...
Gas storage and recovery processes in shales critically depend on nano-scale porosity and chemical composition, but information about the nanoscale pore geometry and connectivity of kerogen, insoluble organic shale matter, is largely unavailable. Using adsorption microcalorimetry, we show that once strong adsorption sites within nanoscale network are taken, gas adsorption even at very low pressure is governed by pore width rather than chemical composition. A combination of focused ion beam with scanning electron microscopy and transmission electron microscopy reveal the nanoscale structure of kerogen includes not only the ubiquitous amorphous phase but also highly graphitized sheets, fiber- and onion-like structures creating nanoscale voids accessible for gas sorption. Nanoscale structures bridge the current gap between molecular size and macropore scale in existing models for kerogen, thus allowing accurate prediction of gas sorption, storage and diffusion properties in shales.
... The values of S1, S2, and S3 were measured by the integration of the respective peaks, with S1 representing in situ hydrocarbons, S2 representing the cracking of organic matter to hydrocarbons during heating, and S3 representing the organic matter analyzed as CO and CO 2 . Further details of this method are found in Słowakiewicz et al. (2014). ...
Quantification of risk to seal integrity in CCS, or gas extraction from hydraulic fracturing, is directly affected by the accessibility of organic pores within organic rich mudrocks. Knowledge of the host organic matter's mechanical properties, which are influenced by depositional environment and thermal maturity, are required to reduce operational risk. In this study we address the effect of both depositional environment and maturity on organic matter Young's modulus by means of Atomic Force Microscopy Quantitative ImagingTM, which is a nondestructive technique capable of nanomechanical measurements. Shales from varying marine depositional environments covering kerogen Types II (Barnett), IIS (Tarfaya), and II/III (Eagle Ford/ Bowland) are analyzed to capture variance in organic matter. The findings show organic matter has a Young's modulus ranging between 0.1 and 24 GPa. These marine shales have a bimodal distribution of Young's modulus to some degree, with peaks at between 3–10 and 19–24 GPa. These shales exhibit a trend with maturity, whereby Young's modulus values of <10 GPa are dominant in immature Tarfaya shale, becoming similar to the proportion of values above 15 GPa within the oil window, before the stiffer values dominate into the gas window. These peaks most likely represent soft heterogeneous aliphatic rich kerogen and stiff ordered aromatic rich kerogen, evidenced by the increase in the stiffer component with maturity and correlated with ¹³C NMR spectrocopy. These findings enable increased realism in microscale geomechanical fracture tip propagation models and may allow direct comparison between Young's modulus and Rock‐Eval parameters.
... We follow this scheme broadly, addressing RQ within this chapter and CQ within Chapter 4. Some authors have addressed either one or several of these properties specifically for the Bowland Shale (e.g. (Clarke et al., 2018(Clarke et al., , 2014aHennissen et al., 2017;Hough et al., 2014;Slowakiewicz et al., 2015)) but these studies have mainly been focused on the analysis of core samples, and a systematic examination of the stratigraphic variability in the properties within a single well has not been conducted. ...
... Where C is a TOC to Kerogen conversion factor. While the maturity of the Bowland Shale at PH-1 makes it difficult to infer original kerogen type (Clarke et al., 2018), it is generally believed to consist of a mixture of Types II (marine) and III (terrestrial) (Andrews, 2013;Clarke et al., 2018;Hennissen et al., 2017;Slowakiewicz et al., 2015). ...
The aim of this thesis is to assess the quality of shale intervals for gas production and to
consider where horizontal wells could be drilled with respect to geological faults. The
Bowland Shale, located in a region of NW England, is investigated where it is a potential
shale gas play. The shale is situated within a complex geological setting, which
necessitates an integrated analysis of fault patterns, reservoir and geomechanical
properties, and hydraulic fracture characteristics to realise these aims.
We ask to what extent the structural geology places limitations on where horizontal wells
can be drilled. Mapping of key stratigraphic surfaces and structures using a high-fidelity
3D seismic dataset allows the geological history to be reconstructed, and the detrimental
effects of Variscan compression realised. The results suggest that faulting does restrict
the available sites where horizontal wells can be drilled. Short, vertically stacked wells
may therefore be required to avoid major faults whilst ensuring gas can be extracted
commercially. We investigate if there is an opportunity for this in the Bowland Shale by
assessing both reservoir and geomechanical properties over the entire stratigraphic
section at a single well. From this, we find that the upper section exhibits particularly
good reservoir properties. These intervals of good reservoir properties also exhibit a
distinctive rock physics response that can be modelled and visualised using templates.
Within these rock types, three sections are highlighted that present the ideal combination
of good reservoir and completion properties. Importantly, they are also separated by
highly stressed intervals that may also mitigate risk of vertical fracture interference
between stacked wells.
However, to assess the risk of stimulated fractures propagating near major faults, further
study is required of the Bowland Shale’s typical hydraulic fracture characteristics. While
this is typically performed whilst stimulations are ongoing through analysis of
microseismic events; prior to stimulation, a model is required to attempt to predict
fracture geometries for a formation of specified rock properties and a specified treatment
schedule. Using a simulation model, we ask how far the fractures propagate laterally,
what their geometries are and if there is a risk of vertical interference between each
proposed landing zone.
It is determined that there is minimal risk of vertical fracture interference and potential
fracture barriers behave effectively. Within the pre-defined landing zones, the fractures
observed are relatively simple in geometry and propagate laterally over large distances
between 500 m and 1000 m. This latter observation does further limit the areas where
completion stages could be placed without risk of fractures interfering with the faults
mapped from seismic data. Bringing together the results of the mapping and hydraulic
fracture modelling, we can map a maximum of 13 well locations in the study area where
horizontals could be drilled, but some of these wells are very short (~ 500 m). There
remains excellent production potential in the area however, and by combing our results
with production rates using an analogue play, we estimate ~ 300 Bcf of gas production
could potentially be achieved.
... The Carboniferous interval in the Cleveland Basin is known only from a few boreholes drilled onshore for petroleum exploration (Malton 4, Kirby Misperton 1; Słowakiewicz et al., 2015), waste chemical disposal (Seal Sands near Hartlepool; Johnson et al., 2011;Andrews, 2013), and offshore for petroleum exploration in Quadrant 41 (Besly, 1998). The oldest strata penetrated belong to the Fell Sandstone Formation (Arundian = Viséan). ...
We report on the discovery of oil from the Boulby Mine and its likely productive source rock from Yorkshire in NE England, located to the west (<30 km) of the newly licensed petroleum exploration areas in the vicinity of the Mid-North Sea High. Oil samples from the mine, dripping out of halite in the roof, have likely been generated
from Zechstein Group Kirkham Abbey Formation (KAF) sapropelic carbonate rock as indicated by aliphatic and aromatic hydrocarbon biomarkers. Other potential source rocks of Carboniferous (Westphalian, Namurian, Vis´ean coals and mudrocks) and Jurassic (the Jet Rock, Bituminous Shales, Kimmeridge Clay Formation) age are ruled out on the basis of organic geochemical data. Boulby oil was generated in the peak-to-late oil-window and it is characterised by the high abundance of C32 and C34 homohopanes, slight even-over-odd predominance (EOP) of C20-25 n-alkanes indicating restricted carbonate-evaporite depositional conditions, and C29 ethyldiacholestane 20 S likely implying a clay-rich source rock. The structural framework and tectonic history of the Permian strata reveal the presence of several fault systems which served as conduits for migrating petroleum. Similar Zechstein-sourced oil is known from Poland and Germany, but the occurrence at Boulby is the first
positive identification of oil derived from Zechstein source rock in the North Sea area. The Boulby oil is reservoired in Zechstein 3 (Z3) Brotherton Formation dolomite and sealed by Z3 evaporite rocks. The proven oil occurrence at Boulby has significant implications in terms of reducing the risk of a lack of oil mature source rock for acreage offered in the neighbouring North Sea during the UK’s 30th and 31st licensing rounds.
... For the Cretaceous, the estimated sediment surface temperature used was 20°C (Holliday, 1999). These temperature estimates are consistent with the thermal history model presented by Słowakiewicz et al. (2015) that suggests that the maximum temperatures for the Lower Cretaceous succession reached ca. 40-50°C during the early Cenozoic. ...
In this study, we investigated Early Cretaceous (Valanginian, ca. 135 million years ago) climate from subtropical to boreal palaeolatitudes. Combined carbonate clumped isotope and oxygen isotope data derived from sub-arctic, boreal, and sub-tropical fossil belemnite rostra (Mollusca: Cephalopoda) provide new palaeotemperature estimates as well as a constraint on the oxygen isotope composition of seawater. Our belemnite data reveal balmy high-latitude marine temperatures (ca. 22 °C) and warm sub-tropical temperatures (ca. 31 °C). Supplementing our clumped isotope-based temperature estimates with published TEX86 data results in a conservative reconstruction of a latitudinal temperature gradient that is reduced compared to modern conditions. We find that modelling efforts are close to reproducing tropical temperatures when high pCO2 levels are considered. Warm polar temperatures imply, however, that data-model discrepancies remain. Early Cretaceous seawater oxygen isotope values show a modern profile and are much more positive (up to 1.5‰ SMOW) than typically assumed. Based on our findings, if the positive Cretaceous seawater δ¹⁸O values are not considered, carbonate δ¹⁸O thermometry would underestimate temperatures, most acute at middle and tropical latitudes.
