# Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

Jianpeng Wang, Jonathan Redfern, Luc G Bulot, Kevin G Taylor

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Jianpeng Wang, Jonathan Redfern, Luc G Bulot, Kevin G Taylor. Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco. Journal of African Earth Sciences, 2023, 207, pp.105069. 10.1016/j.jafrearsci.2023.105069. hal-05269172

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# Controls on preservation of organic matter during the Cenomanian Ocean Anoxic Event II (OAE2) and Turonian global sea-level rise: Agadir Basin, Morocco

Jianpeng Wang<sup>a, *</sup>, Jonathan Redfern<sup>a</sup>, Luc G. Bulot<sup>b, a</sup>, Kevin G. Taylor<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Handling Editor: Dr Mohamed Mohamed G Abdelsalam

**Keywords:**

OAE2

Organic carbon

Stratigraphy

Depositional environment

Geochemistry

Morocco

## ABSTRACT

The Ocean Anoxic Event II (OAE2) was a significant global event associated with a positive carbon isotope excursion, occurring from the Late Cenomanian to the Early Turonian (C/T). It has been commonly associated with enhanced organic carbon preservation. Carbon isotopic analysis carried out over the C/T interval in the Agadir Basin, Morocco, integrated with previous biostratigraphic data, allows a refined correlation of the OAE2 interval. Detailed sedimentary facies analysis identifies ten lithofacies that record the transition from shallow marine environments in the Late Cenomanian to relatively deeper conditions in the Early Turonian. The variation in lithofacies can be correlated to relative sea level changes that show a correlation to the global eustatic curve.

The OAE2 interval comprises dark grey mudstones beds that display low total organic carbon (TOC) values. Trace element and facies analysis suggest dilution from high detrital influx, along with oxic water conditions and low productivity. OM-rich black mudstones are identified in post-OAE2 Early Turonian strata. Trace element analysis suggests this increase in organic matter accumulation was related to increased sea surface productivity and oxygen-depleted bottom water conditions, which facilitated organic matter preservation. Deposition of OM-rich black mudstones is widely reported during the global Early Turonian marine transgression, suggesting the very high sea level was a major control on organic matter generation and preservation.

## 1. Introduction

The Oceanic Anoxic Event II (OAE2), spanning the latest Cenomanian to earliest Turonian, is an interval characterised by globally-enhanced organic matter preservation due to anoxic marine palaeoenvironments (Schlanger and Jenkyns, 1976; Schlanger et al., 1987). It was associated with an extremely warm palaeoclimate and high global sea level (Jenkyns, 2003, 2010; Forster et al., 2007; Sames et al., 2016; Joo et al., 2020). The event is marked by a globally-correlatable positive δ¹³C excursion within carbonate strata, resulting from increased global organic matter burial, which preferentially removed δ¹²C from seawater (Arthur et al., 1988; Tsikos et al., 2004). Although this positive δ¹³C excursion has been extensively recorded in a full range of marine environments from continental shelf to pelagic marine (Jenkyns et al., 1994; Tsikos et al., 2004; Gale et al., 2005; Keller et al., 2008; Jarvis et al., 2011), anoxic conditions were not pervasive during this interval, especially in shallow marine environments (Gertsch et al., 2010b;

El-Sabbagh et al., 2011).

Mudstones and interbedded carbonates were deposited in the Agadir Basin, Morocco, along the Atlantic continental margin during the Late Cenomanian and Early Turonian. This provides an opportunity to characterise the nature of the OAE2 event in a shallow marine environment. Sedimentological, mineralogical and palaeontological studies have previously been undertaken on the Azazoul section of the Agadir Basin, which characterised the response to palaeoclimate and sea level change during the C/T interval (Gertsch et al., 2010b; Jati et al., 2010; Fonseca et al., 2020). However, the limited resolution of biostratigraphic evidence and δ¹³C and δ¹⁸O data in these previous studies made it difficult to confidently locate the C/T boundary in this succession. Previous workers did not recognise any organic-rich mudstones during the OAE2 interval, although OM-rich black mudstones were developed after the OAE2 interval during the Early Turonian interval (Gertsch et al., 2010b; Jati et al., 2010).

In this study, we present new data that allows for a more precise

<footnote><sup>a</sup> North Africa Research Group, Department of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK</footnote>

<footnote><sup>b</sup> Laboratoire Géosciences Océan (LGO-UMR6538) CNRS, Université de Brest, Université de Bretagne Sud, Place Nicolas Copernic, 29280, PLOUZANE, France</footnote>

<footnote>* Corresponding author.
E-mail address: jianpeng.wang@manchester.ac.uk (J. Wang).</footnote>
definition of the C/T boundary in the Agadir Basin. This is achieved through high-resolution carbon isotope analysis, which is integrated with the biostratigraphy data obtained by planktonic foraminifera analysis by Jati et al. (2010). Furthermore, combined sedimentological and inorganic geochemical (trace and major elements) analysis have allowed us to further refine the palaeoenvironmental interpretation during the C/T interval, leading to a better understanding of the response of this shallow marine setting to the OAE2. Additionally, we performed organic and inorganic geochemical, as well as petrographic analysis, to explore the processes controlling organic matter preservation during the OAE2 and post-OAE2 intervals, thereby determining the dominant influence on quality and distribution of organic-rich mudstone deposition in this basin.

## 2. Geological setting

The Agadir Basin is part of the Western High Atlas range. With a basement of Precambrian metamorphic rocks (Stets and Wurster, 1982), the basin was filled by Palaeozoic, Mesozoic and Cenozoic marine and continental sediments (Nouidar, 2002). The Hercynian Unconformity marks the base of the Mesozoic section, which is overlain by rifted continental red bed facies that characterise the Triassic succession, capped by basaltic flows associated with the opening of the Atlantic Ocean (Nouidar, 2002; Daoudi et al., 2008). During the Jurassic period, a significant marine transgression occurred, leading to deposition of a thick, shallow marine carbonate platform in the Agadir Basin (Duval-Arnould et al., 2021). Epicontinental paralic and marine sediments filled the Agadir Basin during the Early Cretaceous with in gulf embayment setting (Behrens and Siehl, 1982). The Late Cenomanian-Early Turonian marine transgression led to deposition of another extensive marine carbonate sequence in the Agadir Basin (Gertsch et al., 2010b; Jati et al., 2010).

The Azazoul beach section, located at GPS coordinates 30°33′14.821″N, 9°44′24.997″W, is found approximately 25 km to the northwest of Agadir (Fig. 1). The coastal section provides a 100-m thick

cliff exposure that dips at an angle of 10° to the south, spanning the Late Cenomanian to Early Turonian time interval. Lithologies observed in this section consists of various rock types, including limestone, marly limestone, marls, oyster beds, mudstones and black shales.

## 3. Database and methods

During the study of the Late Cenomanian to Early Turonian interval of the Azazoul section, a detailed logging was carried out at a centimetre-scale, and a total of 107 samples were collected for analysis. The samples were systematically analysed in respect to petrography, carbon isotopic stratigraphy, biostratigraphy, mineralogy, organic and inorganic geochemistry.

In total, 69 samples were made into polished thin sections, primarily for lithofacies and biostratigraphy analysis. Total organic and inorganic carbon was measured on chosen 64 samples using a Leco carbon analyser in the University of Manchester and Jilin University, ensuring accurate and consistent results for the carbon content of the samples.

Carbon isotope and oxygen isotope analysis were conducted on all samples at the University of Liverpool using an Elemental Analyser coupled to the Thermo Scientific Delta VAdvantage mass spectrometer, which was fitted with Conflo IV gas handling system. Small amounts of samples, typically a few grams, were carefully selected from the fresh samples collected in the field. Before the isotope analysis, all samples were underwent pre-treatment to remove reactive organic compounds that might present in the bulk limestones. The reported data are present as delta (δ) values with respect to the Vienna Pee Dee Belemnite (VPDB) carbon and oxygen isotope scales (via NBS 19, NBS 18). Analytical precision (1σ), based on replicate analysis of in-house quality control calcite, was estimated to be better than ±0.1‰ for both carbon and oxygen isotope values.

A set of 37 samples were selected for X-ray fluorescence analysis to obtain trace and major element (TM) data using an Axios Sequential X-ray Fluorescence Spectrometer at the University of Manchester. To minimize the dilution effects of silica, the trace elements obtained were

<figure><img src="image_4.png" /><figcaption>Fig. 1. Location of Azazoul section in the Agadir Basin, Morocco. Modified after (Hollard et al., 1985).</figcaption></figure>
normalized to aluminium (Turekian and Wedepohl, 1961; Wedepohl, 1971, 1995; Brumsack, 1989; Morford and Emerson, 1999). All the trace elements are displayed as Al-normalized values with units of 10⁻⁴.

Mineralogical quantification was carried out on all the samples using a Philips PW1730 and Bruker D8 Advance at the University of Manchester, to identify the mineral compositions of collected samples.

A total of 64 samples were analysed for Total Organic Carbon content

(TOC) from the Azazoul section, using a Lecco carbon analyser from the University of Manchester and Jilin University. Additionally, Rock-Eval measurements were conducted at Jilin University on 17 samples to identify the type and maturity of organic matter and to detect petroleum potential. 100 mg sample was placed in a vessel and progressively heated to 550 °C under an inert atmosphere (helium).

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Cenomanian-Turonian succession of the Azazoul Beach in the Agadir Basin, with associated lithofacies, carbon and oxygen isotope curves.</figcaption></figure>
# 4. Results

## 4.1. Lithofacies and depositional environment

Ten lithofacies were identified in the Azazoul section (Fig. 2 and Table 1). These lithofacies were interpreted to record deposition under intertidal to deeper subtidal marine environments. Based on the detailed lithofacies interpretation (Fig. 2), skeletal components, mineralogy and sedimentary structures (Table 1, Figs. 3 and 4), the depositional environments of the Cenomanian/Turonian strata in the Agadir Basin can be summarised below.

### 4.1.1. Late Cenomanian, R. Cushmani zone

The upper part of the R. cushmani zone was identified and comprises lithofacies LF1, LF2 and LF3 (Figs. 2 and 3). These lithofacies together forms a facies association that records a shallowing-upward sequence, from deep-subtidal to shallow-subtidal facies.

### 4.1.2. C/T transition, Whiteinella archaeocretacea zone

In the studied section, a total of six lithofacies have been identified (Figs. 2 and 4). The boundary between the R. cushmani and the W. archaeocretacea zone is marked as an erosional surface, characterised by a karst surface filled with shell fragments, overlain by a thin layer of grainstone (LF5) (Fig. 4 B1). Above the erosional surface, two beds of

dark grey clay-rich mudstone (LF4) were developed with a thickness of 0.6 m and 0.8 m respectively. Between the two mudstone beds is a package of 2.5 m coarse-grained limestone with associated lithofacies LF5, LF6 and LF7. A thick package of oyster beds (LF6) overlies the second mudstone layer. Overlaying the massive oyster beds are fine-grain limestone alternated with grey-dark grey mudstone (LF7) deposition. This is interpreted to record a sea level rise. It is followed by a 2 m thick dark grey mudstone (LF1), suggesting deposition in lower energy conditions, probably deeper water settings, and then LF7, which suggests shallowing upward. Thin beds of LF3 sediments dominate the upper part of this zone, capped by an erosional surface filled with bioclastic-rich conglomerate.

The lithofacies association suggests rapid environmental change between high-energy shallow subtidal facies and deep subtidal facies. The upper part is associated with a shallowing-upward succession from a dominantly deep subtidal environment.

### 4.1.3. Early Turonian, H. helvetica zone

The dominant lithofacies identified in the H. helvetica zone are yellowish fine-grain limestones and OM-rich black mudstones (Fig. 5). The bioclastic conglomerate were overlain a yellowish and reddish limestone-marly limestone (LF8), containing massive calcite nodules. Above this unit, there are several metres of partially laminated calcite-rich black shales with occasional nodules (LF9), followed by a 5 m

<table><caption>**Table 1**

Characteristics and interpretation of lithofacies recognised in the C/T succession of the Agadir Basin.</caption><thead><tr><th>Lithofacies</th><th>Sedimentological features</th><th>Mineralogy</th><th>Skeletal composition</th><th>Interpretation</th></tr></thead><tbody><tr><td>LF1: Silt- and clay-bearing, carbonate-rich mudrock</td><td>Dark grey mudstone intercalated with some thinly nodular wackestone layers (Fig. 3 A1). Presence of erosional surface (Fig. 3 A2)</td><td>Carbonate dominant (calcite 44.76 ± 15.77%), Ankerite 7.40 ± 5.90%), clay minerals (25.54 ± 11.11%), kaolinite and smectite), quartz (average 13.08 ± 4.68%), albite (average 7.28 ± 3.29%), Muscovite and minor pyrite (&lt;1%).</td><td>Oyster and bivalve fragments, and ostracods are locally presented. Planktonic and benthic foraminifera are commonly developed</td><td>Subtidal environment, oxic conditions, between the FWWB and SWB</td></tr><tr><td>LF2: Shelly wackestone/marly limestone bedding couplet</td><td>Thin wackestone beds interbedded with marly limestone beds (Fig. 3 B1).</td><td>Calcite dominant (81.55 ± 12.21%), ankerite (8.98 ± 4.85%), quartz (5.18 ± 2.64%), clay minerals (2.43 ± 3.06%) and Albite (1.49 ± 1.85%)</td><td>Mainly include shell fragments, together with minor foraminifera, gastropods, ostracods fragments (Fig. 3 B2)</td><td>Subtidal environment, oxic water conditions, between the FWWB and SWB</td></tr><tr><td>LF3: Bioturbated, shelly packstone</td><td>Alternated with thinly marly limestone beds (Fig. 3 C1), and topped by erosional surface (Fig. 3 C2)</td><td>Calcite dominant (89.56 ± 3.67%), minor ankerite (0.89 ± 1.16%), quartz (6.24 ± 3.10%), minor halite (&lt;1%) and clay minerals (&lt;1%)</td><td>Bivalves fragments dominant, and presence of minor gastropods, ostracods, and benthonic foraminifera (Fig. 3 C2)</td><td>Shallow subtidal environment, above the FWWB</td></tr><tr><td>LF4: Silt-bearing, clay rich black mudrocks</td><td>Black mudstone, discontinuous planar parallel lamination (Fig. 4 A1 and A2). Turbidites with some shelly fragments were recognised</td><td>Kaolinite-dominant clay minerals (33.74 ± 4.29%), calcite (16.99 ± 8.17%), quartz (16.92 ± 3.22%), muscovite (12.71 ± 10.08%) albite (10.66 ± 1.86%), and minor pyrite (1.54 ± 1.21%).</td><td>Planktonic foraminifera and benthonic foraminifera are both moderately developed, with minor oyster fragments.</td><td>Subtidal facies, oxic conditions, between the FWWB and SWB</td></tr><tr><td>LF5: Bivalve-rich grainstone/floatstone</td><td>Nodular beds, interbedded with thin marly limestone. Extensively bioturbated (Fig. 4 B1)</td><td>Calcite dominant (91.08 ± 4.81%), quartz (6.72 ± 3.61%), with minor clay minerals (1.94 ± 2.37%), and very few other minerals.</td><td>Composed of shell fragments (oyster dominated), echinoid spines, gastropod, ostracod, benthonic foraminifera (Fig. 4 B2)</td><td>Shallow subtidal, high energy, oxic conditions, above the FWWB</td></tr><tr><td>LF6: Oyster build-up</td><td>Massive bedding, composed entirely of shell fragments, bed up to 7 m thick (Fig. 4 B2)</td><td>Calcite dominant</td><td>Oysters, displaying with different size and shapes, mostly over 10 cm in diameter</td><td>Intertidal, oxic conditions, above the FWWB</td></tr><tr><td>LF7: Grey nodular wackestone</td><td>Grey nodular wackestone interbedded with dark grey mudstone, and presence of parallel waves (Fig. 4 C1)</td><td>Calcite dominant (88.59 ± 13.86%), ankerite (0.90 ± 1.19%), quartz (6.64 ± 11.95%), albite (0.96 ± 1.35%), minor muscovite (1.11 ± 1.27%) clay minerals (0.84 ± 2.34%)</td><td>Bivalves fragments and planktonic foraminifera are moderately present, minor benthonic foraminifera (Fig. 4 C2)</td><td>Subtidal facies, oxic water conditions, between the FWWB and SWB</td></tr><tr><td>LF8: Yellowish/ reddish wackestone</td><td>Fine grain, continuous parallel planar lamination (Fig. 5 A1). Cherts and calcite nodules are present</td><td>Calcite (87.34 ± 6.68%), ankerite (1.4 ± 3.3%), quartz (10.28 ± 6.11%), minor halite (1.18 ± 0.44%) and muscovite (2.07 ± 1.02%)</td><td>Planktonic foraminifera are highly developed (Fig. 5 A2) and absence of benthonic foraminifera.</td><td>Subtidal, offshore, quite water conditions, below the SWB</td></tr><tr><td>LF9: Carbonate rich black mudrock</td><td>Partially continuous parallel planar (Fig. 5 B1), with some calcite nodules present</td><td>Calcite (80.93 ± 7.85%), quartz (14.83 ± 6.46%), muscovite (2.34 ± 1.67%), minor ankerite (0.55 ± 0.76%) and halite (1.00 ± 0.86%)</td><td>High planktonic foraminifera content, some radiolarian, rare shell fragments (Fig. 5 B2)</td><td>Subtidal, offshore, anoxic water conditions, below the SWB</td></tr><tr><td>LF10: Carbonate/ quartz nodule rich black mudrock</td><td>Black, partially continuous parallel planar. Cherts and nodules highly developed (Fig. 5 C1)</td><td>Calcite (45.92 ± 10.74%), quartz (43.38 ± 12.72%), muscovite (5.96 ± 3.79%), ankerite (1.44 ± 0.98%), halite (1.26 ± 0.85%) and other minerals (pyrite, gypsum etc.)</td><td>Relatively high planktonic foraminifera content, crinoid fragments present (Fig. 5 C2)</td><td>Subtidal, offshore, anoxic water conditions, below the SWB</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Summary of the Lithofacies 1–3 in the Azazoul section</figcaption></figure>

LF1: (A1) Black mudrocks intercalated with some thinly limestone layers; (A2) Photomicrograph, illustrating the individual beds with an erosional surface at the bottom, each bed shows a normal grading texture; LF2: (B1) Illustrating the stacking patterns from wackestone to marly limestone; (B2) Photomicrograph, illustrating the microfossils components, including bivalve fragments, planktonic foraminifera and benthonic foraminifera; LF3: (C1) Outcrop photograph, illustrating the irregular and erosional karst contact between the R. cushmani zone and W. archaeocretacea zone; (C2) Photomicrograph, illustrating the microfossils components, including bivalve fragments and benthonic foraminifera.

succession of black mudstone beds (LF10), characterised by a large number of large calcite nodules (with diameters up to 50 cm) and cherts (diameter up to 1 m). Towards the top of this zone, the lithofacies shift to dark-grey to black mudstones with fewer nodules (LF9) alternating with some thinly bedded fine-grain limestone layers.

The lithofacies association suggests a short interval of shallowing occurred prior to a significant marine transgression, which led to deposition of relatively thick deep marine sediments. Although there are some occasional shallow-water lithofacies, the *H. helvetica* zone in the Agadir Basin is characterised by dominantly deeper-marine sediments with abundant beds of organic-rich fine-grained mudstone.

## 4.2. Bulk carbonate $\delta^{13}C$ and $\delta^{18}O$

The $\delta^{13}C$ value at the base of the *R. cushmani* zone is +1.74‰ increasing upward gradually to +3.14‰ towards the middle part of this stage. Subsequently, there is a decrease to +1.54‰ (Fig. 2) (Table 2). Moving into the lower *W. archaeocretacea* zone, the $\delta^{13}C$ values initially decrease to +1.30‰, and subsequently most values present a continuous value around +2‰. The middle *W. archaeocretacea* zone is characterised by the highest $\delta^{13}C$ values, with an average value of +3.45‰, peaking at

+3.93‰. Thereafter, $\delta^{13}C$ values drop dramatically, reaching a minimum value of -3.50‰ at the top of *W. archaeocretacea* zone. The $\delta^{13}C$ values in the *H. helvetica* zone oscillate between -2.38‰ and -2.54‰.

The oxygen isotope values in the Azazoul section exhibit different relationships with the carbon isotope values in different zones. The oxygen isotope values show a negative relationship to the carbon isotope values in the middle *W. archaeocretacea* zone and upper *H. helvetica* zone, while showing a positive relationship in the *R. cushmani* zone, lower *W. archaeocretacea* zone and lower *H. helvetica* zone. A cross-plot of $\delta^{13}C$ and $\delta^{18}O$ values illustrates that most of the data are distributed within an $\delta^{18}O$ range from -3.5‰ to -1.0 and $\delta^{13}C$ range from -1‰ to +4‰ (Fig. 6). The outlier-circled points with negative $\delta^{13}C$ values are predominantly from the *H. helvetica* zone (Fig. 2) and are interpreted to reflect diagenetic alternation.

## 4.3. Trace and major elements

Trace and major elements were analysed (Fig. 7 and Table 3), with the purpose to explore the palaeoenvironment changes controlling the distribution of different lithofacies spatially and temporally.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Summary of the lithofacies 4 to lithofacies 7 in the Azazoul section LF4: (A1) Black mudstone bed with a thickness of 80 cm; (A2) Photomicrograph, illustrating silt-bearing and clay-rich mudstone. LF5 and LF6: (B1) Illustrating grainstone-floatstone beds (LF5); (B2) Photomicrograph showing benthonic foraminifera, echinoid, and oyster fragments in a grainstone-floatstone bed; LF7: (C1) Grey nodular limestone interbedded with dark grey mudstone, individually bed rare thicker than 20 cm; (C2) Photomicrograph, illustrating a wackestone texture with some bivalve fragments and foraminifera.</figcaption></figure>

### 4.3.1. Detrital influx sensitive elements

The concentrations of aluminium (Al), titanium (Ti), thallium (Th), and zirconium (Zr) show extremely strong correlation with each other in the Azazoul section (Fig. 7), particularly evident in the ratios Al/Ti ($R^2$ = 0.97) and Al/Zr ($R^2$ = 0.96). The high concentrations of these elements in certain intervals of the *R. cushmani* and *W. archaeoretacea* zone are associated with abundant clay mineral content of LF1 and LF4. This suggests a significant terrigenous input, leading to the deposition of clay-rich mudstones in these intervals. All the other lithofacies developed in the *R. cushmani* and the *W. archaeoretacea* zones show minor concentrations of these elements. These suggests that these lithofacies experienced relatively lower terrigenous input. Low concentrations of these elements in the *H. helvetica* zone are consistent with the lower proportion of clay mineral content in LF8, L9, and LF10, indicating lower detrital influx during the deposition of organic-rich mudstone in this zone.

### 4.3.2. Palaeoproductivity-sensitive elements

Owing to the high detrital influx, it is possible to use the ratio of elements/Al to correct the possible dilution by organic matter and authigenic minerals (Calvert and Pedersen, 1993; Morford and Emerson, 1999), presenting results as Al-normalized elements values (Fig. 7 and

Table 3). These elemental concentrations are based on the comparison of element/Al ratios to those in a standard shale (Turekian and Wedepohl, 1961; Wedepohl, 1971, 1995; Calvert and Pedersen, 1993; Morford and Emerson, 1999; Van der Weijden, 2002).

Phosphorus (P), nickel (Ni), zinc (Zn), and copper (Cu), can act as micronutrients that are commonly found in high concentration, fixed in sediments associated with organic matter preservation in the sediments (Tribovillard et al., 2006). Thus, these elements can be reliable indicators of OM productivity. Most of these Al-normalized elements show comparable trends, with moderate to high correlations with each other ($0.18<R^2 > 0.90$) (Fig. 7 and Table 3).

These elements show a depleted concentrations in the *R. cushmani* and *W. archaeoretacea* zone, indicating a relatively lower level of productivity. However, throughout the *H. helvetica* zone, there is a moderate to significant enrichment in the concentration of these elements, suggesting moderate levels of productivity during this interval in the Agadir Basin.

### 4.3.3. Redox conditions sensitive elements

The Al normalized redox-sensitive elements, such as Vanadium, Molybdenum and Uranium, exhibit a similar trend to the productivity-sensitive elements in the Azazoul section (Fig. 7 and Table 3). These
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Summary of the lithofacies 6–9 in the Azazoul section</figcaption></figure>

LF8: (A1) Large calcite nodules intercalated within the yellowish and reddish fine-grain limestone-marly limestone layers; (A2) Photomicrograph, showing the presence of abundant planktonic foraminifera and some shell fragments; LF 9: (B1) Partially laminated calcite-rich black mudstone; (B2) Photomicrograph, planktonic foraminifera highly developed in the OM-rich mudstone. LF10: (C1) Abundant nodules within quartz-rich black mudstone; (C2) Photomicrograph, showing the presence of crinoids and planktonic foraminifera in the OM-rich black mudstone.

<table><caption>**Table 2**
Summary of the δ¹³C and δ¹⁸O values in the three zones of the Azazoul section.</caption><thead><tr><th>Isotope</th><th>Planktonic<br>foraminifera<br>zones</th><th>n</th><th>Mean<br>(10–3)</th><th>Min<br>(10–3)</th><th>Max<br>(10–3)</th><th>SD<br>(10–3)</th></tr></thead><tbody><tr><td rowspan="3">δ13CVPDB</td><td>H.helvetica</td><td>37</td><td>0.28</td><td>-2.38</td><td>2.54</td><td>1.08</td></tr><tr><td>W.<br>archaeocretacea</td><td>51</td><td>1.79</td><td>-3.5</td><td>3.93</td><td>1.24</td></tr><tr><td>R.cushmani</td><td>18</td><td>2.44</td><td>1.54</td><td>3.24</td><td>0.52</td></tr><tr><td rowspan="3">δ18OVPDB</td><td>H.helvetica</td><td>37</td><td>-2.80</td><td>-3.29</td><td>-1.91</td><td>0.34</td></tr><tr><td>W.<br>archaeocretacea</td><td>51</td><td>-1.85</td><td>-3.14</td><td>0.75</td><td>0.41</td></tr><tr><td>R.cushmani</td><td>18</td><td>-1.61</td><td>-2.21</td><td>-1.15</td><td>0.24</td></tr></tbody></table>

elements also present depleted concentrations in the basal beds of this section, followed by a significant increase in concentration in the upper beds.

These redox-sensitive elements display extremely low concentrations in the *R. cushmani* and *W. archaeocretacea* zones, indicating prevailing oxygenated water conditions in these intervals. These elements are significantly to extremely concentrated in most intervals of the *H. Helvetica* zone, although a few intervals are associated with moderate

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** δ. ¹³C and δ¹⁸O crossplot illustrating the correlations between carbon and oxygen isotope data in the Azazoul section.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Enrichment factors for proxies representing the clastic influx, redox and palaeoproductivity, as well as TOC values in the Azazoul section.</figcaption></figure>

concentration. The high concentrations provide strong evidence for the presence of anoxic bottom water during the deposition of organic-rich mudstones.

### 4.4. Total organic carbon and rock-eval

The TOC content in the sediments of the *R. cushmani* and *W. archaeocretacea* zones is relatively low, even in the black mudstones developed in LF1 and LF4, with TOC values ranging from 0.4% to 0.6% (Fig. 7). These mudstones are in an immature to early mature stage with the Tmax values ranging from 412 to 423 °C. Hydrogen Index (HI) values for these sediments are between 30 and 136 mg HC/g TOC, and the Oxygen Index (OI) values range from 88 to 100 mg CO2/g TOC. The dominant kerogen of organic matter in the sediments is Type III with minor Type II.

Lithofacies LF9 and LF10 in the upper *H. helvetica* interval are characterised by high TOC contents. The sediments with a total thickness of approximately 10 m have an average TOC value of 2.5 wt%. The maximum TOC value of 9.2 wt% is recorded in upper part of this interval. The Hydrogen Index (HI) values for these sediments range from 540 to 980 mg HC/g TOC, while the Oxygen Index (OI) values in this interval range from 39 to 1229 mg CO2/g TOC. The kerogen type of

organic matter shows a mixture of type I and type II. The Tmax values of these black shales are relatively low from 410 °C to 414 °C, showing an immature stage.

## 5. Discussion

### 5.1. Cenomanian/turonian stratigraphic framework

The lack of datable in the Agadir Basin has resulted in poor biostratigraphic resolution, making it challenging to establish a reliable C/T stratigraphy in the region. This limitation has hindered the discrimination between global and local influences on palaeoenvironmental perturbations in this basin. However, the δ¹³C signature of the marine carbonates provides a powerful and complementary tool for C/T boundary identification, allowing regional and global correlation despite the biostratigraphic uncertainties (Keller et al., 2001, 2004; Tsikos et al., 2004; Caron et al., 2006; Jarvis et al., 2011; Farouk et al., 2017; Falzoni et al., 2018; Abdelhady et al., 2021; Salhi et al., 2022). However, an uncertainty that needs to be considered and mitigated, is that δ¹³C signature is readily modified by interaction with diagenetic fluids. The cross-plot of δ¹³C and δ¹⁸O values (Fig. 6) shows a dominantly positive correlation, indicating some influence of diagenesis on
<table><caption>**Table 3**
Summary of major and trace elements in the Agadir Basin.</caption><thead><tr><th>Elements</th><th>Planktonic foraminifera zones</th><th>n</th><th>Min</th><th>Max</th><th>Mean</th><th>SD</th><th>Element/AlAs Wedepohl (1971)</th></tr></thead><tbody><tr><td rowspan="3">Al (%)</td><td>H.helvetica</td><td>13</td><td>0.33</td><td>4.11</td><td>1.54</td><td>0.98</td><td rowspan="3">8.84</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0.46</td><td>11.4</td><td>5.52</td><td>4.01</td></tr><tr><td>R.cushman</td><td>10</td><td>0.34</td><td>7.37</td><td>3.12</td><td>2.28</td></tr><tr><td rowspan="3">Ti/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>395</td><td>627</td><td>519</td><td>65.0</td><td rowspan="3">520</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>442</td><td>805</td><td>616</td><td>97.0</td></tr><tr><td>R.cushman</td><td>10</td><td>608</td><td>818</td><td>713</td><td>72</td></tr><tr><td rowspan="3">Zr/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>1.21</td><td>25.4</td><td>15</td><td>6.39</td><td rowspan="3">18.1</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0</td><td>22.0</td><td>14.0</td><td>6.90</td></tr><tr><td>R.cushman</td><td>10</td><td>15.4</td><td>21.1</td><td>18.5</td><td>1.64</td></tr><tr><td rowspan="3">Th/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>0</td><td>3.33</td><td>1.70</td><td>0.79</td><td rowspan="3">1.36</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0.75</td><td>6.07</td><td>1.88</td><td>1.52</td></tr><tr><td>R.cushman</td><td>10</td><td>0.96</td><td>6.95</td><td>2.79</td><td>2.32</td></tr><tr><td rowspan="3">P/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>347</td><td>4790</td><td>1886</td><td>1082</td><td rowspan="3">79</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>41.3</td><td>502</td><td>163</td><td>121</td></tr><tr><td>R.cushman</td><td>10</td><td>79.4</td><td>432</td><td>201</td><td>120</td></tr><tr><td rowspan="3">Ni/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>9.04</td><td>82.4</td><td>43.0</td><td>22.0</td><td rowspan="3">7.69</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>2.23</td><td>7.37</td><td>3.72</td><td>1.25</td></tr><tr><td>R.cushman</td><td>10</td><td>2.53</td><td>4.72</td><td>3.75</td><td>0.59</td></tr><tr><td rowspan="3">Zn/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>3.46</td><td>449</td><td>151</td><td>138</td><td rowspan="3">10.7</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>3.05</td><td>64</td><td>10.2</td><td>15</td></tr><tr><td>R.cushman</td><td>10</td><td>4.72</td><td>23.5</td><td>7.54</td><td>5.46</td></tr><tr><td rowspan="3">Cu/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>4.14</td><td>37.4</td><td>18.8</td><td>10.70</td><td rowspan="3">5.09</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0</td><td>7.59</td><td>1.61</td><td>2.01</td></tr><tr><td>R.cushman</td><td>10</td><td>0.81</td><td>2.53</td><td>1.30</td><td>0.57</td></tr><tr><td rowspan="3">V/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>32.6</td><td>730</td><td>200</td><td>185</td><td rowspan="3">14.7</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>4.8</td><td>19.8</td><td>10.1</td><td>5.73</td></tr><tr><td>R.cushman</td><td>10</td><td>1.07</td><td>16.7</td><td>8.86</td><td>5.73</td></tr><tr><td rowspan="3">Mo/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>1.72</td><td>15.3</td><td>8.41</td><td>4.03</td><td rowspan="3">2.9</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0.05</td><td>1.08</td><td>0.34</td><td>0.31</td></tr><tr><td>R.cushman</td><td>10</td><td>0.11</td><td>1.34</td><td>0.45</td><td>0.39</td></tr><tr><td rowspan="3">U/Al (10-4)</td><td>H.helvetica</td><td>13</td><td>1.83</td><td>24.3</td><td>10.4</td><td>6.75</td><td rowspan="3">0.42</td></tr><tr><td>W.archaeocretacea</td><td>15</td><td>0.46</td><td>10.8</td><td>3.69</td><td>4.22</td></tr><tr><td>R.cushman</td><td>10</td><td>0.81</td><td>16.0</td><td>4.82</td><td>5.09</td></tr></tbody></table>

δ¹³C values. However, despite this diagenetic influence, most of the δ¹³C values track published global changes well, exhibiting the characteristic positive excursion associated with enhanced organic matter preservation during the OAE2 and negative δ¹³C excursion after the OAE2 interval (Fig. 8) (Keller et al., 2004; Tsikos et al., 2004; Jarvis et al., 2011; Farouk et al., 2017; Kuhnt et al., 2017; Abdelhady et al., 2021; Salhi et al., 2022). Therefore, despite the potential diagenetic influence, these δ¹³C values in this study are likely to be close to the primary isotopic composition and can be reliably applied for regional and global correlation.

Three distinct peaks have been commonly identified in the C/T δ¹³C profiles, and are referred as A, B and C (Pearce et al., 2009; Jarvis et al., 2011) or I, II, III (Caron et al., 2006). Using the biostratigraphic framework presented in this study as an age control, enabling to recognise several related δ¹³C peaks during the C/T interval (Fig. 8). Due to the absence of ammonites in the Agadir Basin, which are typically used as the definitive age dating standard, the identification C/T boundary in this study is based on δ¹³C correlation with the published planktonic foraminifera zones provided by Jati et al. (2010).

The last occurrence of the *R. cushmani* was identified just below the erosional karst surface (Gertsch et al., 2010a; Jati et al., 2010), and no *R. cushmani* species was found in the overlying beds. Peak I of δ¹³C curves is missing in the studied upper *R. cushmani* interval. Based on the δ¹³C profiles in the lower *R. cushmani* zone studied by Gertsch et al. (2010) and Jati et al. (2010), an initial rapid increase in δ¹³C values was recognised in the *R. cushmani* interval. This suggests that Peak I might be absent in the studied interval, and the entire studied *R. cushmani* interval can be placed within the OAE2 interval. Jati et al. (2010) recognised the presence of heterohelix spike around this δ¹³C peak, which is a common

feature observed above Peak II in the Eastbourne and Pueblo sections (Keller et al., 2004, 2008; Tsikos et al., 2004; Caron et al., 2006; Keller, 2008). This suggests the maximum δ¹³C peak in the Azazoul section is likely to be coeval with Peak II (Fig. 8).

The first occurrence of *Helvetoglobotruncana helvetica* in the Azazoul section was recognised by Jati et al. (2010) at the bottom of yellowish-reddish limestone beds (at 55 m), below which is an erosional surface filled with bioclastic conglomerates. In spite of the possibility of a diachronous planktonic foraminiferal zone, Peak III is typically identified below the first occurrence of *H. helvetica*. In this case, the only salient peak between Peak II and the base of *H. helvetica* zone could be Peak III, if there is no significant hiatus present, and we tentatively place the C/T boundary above the Peak III based on correlation with the Eastbourne and Pueblo section (Fig. 8).

Previous studies suggest the onset of the OAE2 interval commonly starts in the *M. geslinianum/S. gracile* zone, characterised as a sharp increase in δ¹³C excursion, and ends above the C/T boundary in the *W. devonense* zone, with a decrease in δ¹³C values (Jarvis et al., 2011). Therefore, the OAE2 interval in the Agadir Basin is identified from the upper part of the Late Cenomanian to very lower part of the Early Turonian (Fig. 8).

## 5.2. Palaeoenvironments and sea level changes

The integration of trace elements data and lithofacies, including lithological changes, mineral compositions, and the presence of fossil assemblages, is crucial for understanding and identifying palaeoenvironmental conditions and evolution in the Agadir Basin. The detrital input and fluctuation of bottom water oxygen concentration
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Carbon isotope curves correlation of the Azazoul section (Morocco), Eastbourne section (UK) and Pueblo section (USA).</figcaption></figure>

from the Late Cenomanian to Early Turonian interval are analysed to assess the influences of the OAE2 and marine transgression on the C/T sediments in studied basin.

The U-EF vs. Mo-EF graph (Fig. 9), based on multi-parameter datasets, has been demonstrated to be a reliable tool for palaeoredox condition analysis in various palaeoceanographic systems (Algeo and Tribovillard, 2009; Tribovillard et al., 2012). It can also be used to in

interpret the degree of water mass restriction (Fig. 9). Both U and Mo show minor enrichment in oxic water conditions and moderate enrichment (EFs<10) in dysoxic water conditions, while anoxic/euxinic water conditions yield high enrichment (EFs>10) (Tribovillard et al., 2012). Tribovillard et al. (2012) defined the following relationship: Low Mo/U ratios (~0.3 × SW) in unrestricted marine systems; Intermediate Mo/U ratios (~1 × SW) in suboxic and anoxic conditions; High Mo/U ratios (~3 × SW) in strongly euxinic water conditions. However, it's important to note that these parameters are derived from present-day marine water values and are most suitable for unrestricted environments (Tribovillard et al., 2012).

Based this data and interpretations provided, the Late Cenomanian and the Early Turonian interval in the Azazoul section can be characterised as having mixed anoxic/dysoxic/oxic water conditions. The U-EF vs. Mo-EF graph indicates that there was intermittent presence of anoxic water conditions in the Late Cenomanian interval, but these conditions became more common in the Early Turonian interval, indicating a shift towards more reducing conditions (Fig. 10). These results are consistent with the Ni/Co-U/Th cross plot (Jones and Manning, 1994). Furthermore, the Mo-U covariation analysis suggests the Agadir Basin experienced open marine conditions throughout the C/T interval, in accordance with the other trace elements concentrations.

### 5.2.1. *R. Cushmani* zone

The data and interpretations provided in the study suggest that sediments in the *R. cushmani* zone of the Agadir Basin were deposited during the OAE2 interval. The absence of OM-rich black mudstone deposition supports inference of dominantly shallow water oxic conditions during this time. This is further evidenced by the low concentration

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** U-EF vs. Mo-EF for the C/T sediments in the Agadir Basin (Algeo and Tribovillard, 2009; Tribovillard et al., 2012).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Cross-plot of TE ratios as palaeoeddox proxies in the Agadir Basin, based on Jones and Manning (1994) and Hatch and Leventhal (1992).</figcaption></figure>

of redox-sensitive elements in the sediments (Fig. 7). The high detrital influx indicated by high Al concentration (>10 wt %) suggests increased weathering during the Late Cenomanian, likely related to global warming (Leckie et al., 2002; Kidder and Worsley, 2010; Jarvis et al.,

2011). The presence of high clay mineral content in the organic-poor mudstones (LF1, Table 2) at the base of studied R. cushmani zone supports the inference of a humid climate during this interval (Chamley, 1989; Bolle and Adatte, 2001), favouring for the significant weathering.

Is consistent with the increased weathering owing to the global warming during the Late Cenomanian. The overlying fine-grain limestone/marly limestone couplet has low detrital sensitive element concentrations and rare clay mineral content, suggesting a significantly decreased terrigenous input.

The lack of significant change in sea-level or bottom water conditions, as indicated by the faunal assemblages (which suggests a range between the FWWB and SWB) (Fig. 10), suggests the decreased clastic input in this interval is related to decreased weathering rather than rapid marine transgression. A cooling period associated with decreased weathering and re-oxygenation of sediments was recognised in the Eastbourne and Pont D'Issole C/T sections, during the uppermost R. cushmani interval (Fig. 11) (Jarvis et al., 2011), which is consistent with the findings in this study.

### 5.2.2. *W. archaeocretacea* zone

The extremely high detrital influx, indicated by high Al concentration (Al₂O₃ up to 21 wt %) (Fig. 7) and kaolinite content (>30%) (Table 2), suggests a significant continental weathering occurred during W. archaeocretacea zone (Fedo et al., 1995; Liu et al., 2020). This is in accordance with the significantly increased pCO₂ readings globally after the 'Plenus Cold Event' (Jarvis et al., 2011) (Fig. 11). Dominantly oxic water conditions are indicated by depleted concentration of redox-sensitive elements (Figs. 7 and 10) throughout the W. archaeocretacea interval. These conditions are unfavourable for the

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Correlation of the sections in this study with Pont D'Issole sections based on carbon isotope curves and biostratigraphy, to show the $\rho$CO₂ perturbation across the OAE2 interval globally.</figcaption></figure>
organic matter preservation (rarely higher than 0.5%) in this interval. These clay-rich but organic-poor mudstones are likely driven by a combination of increased detrital input and higher sea levels.

### 5.2.3. *H. helvetica* zone

The deposition of a yellowish limestone (LF8) is interpreted to record a rapid sea level fall during the basal part of the *H. helvetica* zone. This is terminated by an erosional surface. Subsequently, the presence of OM-rich black mudstone lithofacies (LF9 and LF10) suggests a significant sea level transgression. Trace elemental analysis further suggests predominantly anoxic water conditions during this interval, though with periods of oxic/dysoxic redox conditions intercalated occasionally (Fig. 10).

To conclude, dominantly oxic water conditions during the Late Cenomanian interval, equivalent with OAE2 interval, are associated high terrigenous influence, reflecting a globally warming paleoclimate. The transition into the Early Turonian is marked by a short marine regression, followed by a major marine transgression, which can be correlated with the global eustatic cycle (Haq, 2014) (Fig. 12).

The OM-rich black mudstone deposition is coeval with the global Early Turonian transgression (Friedrich et al., 2012; Jarvis et al., 2015) (Fig. 12). The dramatic palaeoenvironment change that occurred in the Agadir Basin from the Late Cenomanian to Early Turonian might suggest the palaeoenvironment change was not solely related to the global marine transgression, but might have also involved tectonic subsidence (Jati et al., 2010).

### 5.3. Controls of organic matter accumulation

The absence of OM-rich mudstones during the Late Cenomanian interval, associated with the OAE2, can be attributed to several factors. The low organic content during the OAE2 interval is likely a result of low primary productivity, prevailing oxic conditions and strong dilution (Fig. 7) owing to considerable terrigenous input. These factors together limited the OM accumulation and preservation. Some discrete intervals with low terrigenous input are still associated with low OM enrichment, suggesting that terrigenous dilution was not the only contributor to impede the organic matter preservation. In addition to terrigenous dilution, biogenic dilution is also a potential control for the OM-poor sediments (Tyson, 2001), particularly in shallow marine regimes like the Agadir Basin, where a strong dilution by non-hydrogen-rich biogenic components, such as shell fragments, could reduce overall TOC content. In these shallow marine environments with moderate to low primary production, strong biogenetic dilution, and poor preservation potential due to oxic bottom waters, organic carbon is less likely to accumulate.

The OM-rich black mudstones were developed in the Agadir Basin

during the Early Turonian, which coincides with the global sea level maximum (Haq, 2014) (Fig. 12). This is evident from the elevated concentrations of increase of OM productivity-sensitive (Ni, Zn and Cu) and redox-sensitive (Mo, V and U) element concentration during organic carbon deposition. The decrease in terrigenous input, indicated by low detrital-sensitive element concentrations, likely occurred as a result of rapid marine transgression due to sea level rise during the Turonian. Widespread anoxic conditions are indicated by high or extremely high redox-sensitive element concentration in the OM-rich black mudstones (Fig. 7), but this anoxia was not consistent and was sporadically interrupted by oxic/dysoxic water conditions indicated by lower TE concentrations. The latter is also demonstrated by the presence of discrete horizons with bioturbation (Fig. 5, B1). The correlation between organic matter enrichment and productivity is stronger than the correlation with oxygen deficiency, but the highest organic matter accumulation was not associated with the most reducing conditions or highest productivity (Fig. 7). The decrease in organic matter content in the middle interval, corresponding to a high content of siliceous lithologies (chert dominant) (Fig. 5, C1), suggests a high amount of biogenic material dilution occurred during this time.

The data suggests the highest OM concentrations in the Early Turonian interval were influenced by the combined action of relatively high productivity, low biogenic dilution, optimum preservation conditions during periods of oxygen deficiency and low detrital influx.

## 6. Conclusions

The location of the OAE2 and Cenomanian-Turonian boundary in the Agadir Basin is defined with greater precision, based on integration of previously published biostratigraphy (planktonic foraminifera) and new high-resolution $\delta^{13}$C data.

A total of ten lithofacies are recognised in the Azazoul section. The Late Cenomanian sediments record shallow marine environments, with progressive deepening into the Early Turonian strata Lower Turonian, the latter of which is attributed to the Early Turonian global sea level rise.

No organic matter-rich black mudstone was recognised in the Azazoul section during the OAE2 interval. Several dark grey mudstone beds deposited during the OAE2, but these exhibit low OM enrichment, related to low productivity, oxidation of OM and high biogenic and clastic dilution. Overall, the environment of deposition at the time of OAE2 is interpreted to be an oxic shallow marine water condition.

OM-rich black mudstones are developed in post-OAE2 Early Turonian strata, which OM productivity-sensitive (Ni, Zn and Cu) and redox-sensitive (Mo, V and U) element concentration indicate were associated with increased surface water productivity and oxygen-depleted bottom

<figure><img src="image_15.png" /><figcaption>Fig. 12. The position of OAE2 interval and black mudstones in global eustatic cycle.</figcaption></figure>
water conditions. The main control is suggested to be the global Early
Turonian marine transgression.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Data availability

No data was used for the research described in the article.

Acknowledgements

The authors gratefully acknowledge the sponsoring companies of
North Africa Research Group (NARG) and L'Office National des
Hydrocarbures et des Mines (ONHYM) for their financial and scientific
support. We thank Dr. Aude Duval-Arnould from the University of
Manchester for her help and support during the fieldwork, Dr. Alastair
Bewsher and Dr. Paul Lythgoe from the University of Manchester for the
XRF analysis, Mr. Steve Stockley from the University of Manchester for
the powder samples preparation, Dr. Stephen Crowley from the Uni-
versity of Liverpool for the stable carbon and oxygen isotope analysis,
and Dr. John Waters from the University of Manchester for the XRD
training. We also appreciate the help from Professor Cathy Hollis from
the University of Manchester and Professor Gregory Price from the
University of Plymouth for their critical suggestions.

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