The brachiopod-bearing outcrops are located in the Crevillente and Reclot mountains in Alicante province, SE Spain (Fig. 2A), which are integrated in the Eastern Subbetic Domain of the External Betic Zone. The Mesozoic sedimentary rocks of this area represent part of the South-Iberian Palaeomargin, located in the westernmost Tethys Ocean. During most of the Jurassic and Early Cretaceous, the Subbetic Domain was characterized by marine sedimentation with environments ranging from shallow carbonate platforms to pelagic troughs and swells (Azéma et al., 1979; Vera et al., 2004). In the Eastern Subbetic, the Lower Jurassic succession is typified by the Gavilán and Zegrí formations (e.g. García-Hernández et al., 1979; Ruiz-Ortiz et al., 2004; Nieto et al., 2008) (Fig. 2B), which have been presented in the composite section herein arranged (Fig. 2C).

In the studied outcrops, the upper Sinemurian part of the Gavilán Formation consists of micritic and pseudo-oolithic whitish wackestone beds, showing lateral and upward transitions to oolithic grainstone/packstone levels with intraclasts and peloids. They were deposited in a proximal platform environment. The top of this lithostratigraphical unit shows intense extensional fissures. These were a consequence of an initial pre-rifting stage (Vera, 1988, 1998; Molina et al., 1999) interpreted as the first tectonic pulses and related to the drowning of the westernmost Tethyan platforms in the framework of the Central Atlantic Ocean opening (Ruiz-Ortiz et al., 2004).

The upper member of the Gavilán Formation (upper Pliensbachian) overlies these shallow-water platform facies. It consists of red crinoidal grainstone beds with abundant glauconite. Brachiopods, benthic foraminifera, peloids, and intraclasts are also common. These layers often show irregular tops with condensed pavements interpreted as omission surfaces or hardgrounds with ammonoids, belemnites, and brachiopods.

The onset of the marly sedimentation is represented by the Zegrí Formation, which dominates throughout the early-middle Toarcian in the basin, suggesting a pelagic depositional environment. This formation is made up by alternating yellowish to greenish marl/marly mudstone beds with calcarenites interspersed at the base of the formation, showing towards the top a continuous increase in the marly content.

In the lowermost deposits (samples EFC3 to CC1.0), the analysed brachiopod shells predominantly belong to Calcirhynchia plicatissima (Table 1, Fig. 6), which shows a continuous record in these sediments. The acme of this species, together with that of Prionorhynchia regia and Gibbirhynchia curviceps, plus the first occurrence of multicostate zeilleriids and the genus Securina, typify the Assemblage 1 of brachiopods in the easternmost Subbetic (Baeza-Carratalá, 2013). This assemblage is assigned to the Sinemurian-Pliensbachian transition (Raricostatum-Aenigmaticum zones).

In the middle part of the Lower Jurassic succession, the analysed shells are among the prevailing taxa of Assemblage 2 (Baeza-Carratalá, 2013). Thus, several species of Prionorhynchia and Cirpa are selected for geochemical analyses (Table 1), since they occur profusely in the upper member of the Gavilán Formation. In these deposits, spiriferinides acquire great diversity and abundance as well, and they were selected for analyses only when multicostate rhynchonellides were not available. Data from ammonite faunas in these sediments range in age from the lower (Demonense Zone) up to the upper (Algovianum Zone) Pliensbachian (Braga, 1983; Iñesta, 1988; Tent-Manclús, 2006).

The biochronological control of brachiopods prior to and after the Jenkyns Event is more accurate if the Subbetic record is correlated with that of the nearby Iberian basin, where an extensive succession of ammonites allows for a precise calibration. Thus, Lobothyris arcta is recorded prior to the Jenkyns Event in the Emaciatum-Polymorphum zones (García Joral et al., 2011, Baeza-Carratalá, 2013) together with several spiriferinid (Calyptoria vulgata) and athyridide representatives (the so-called Koninckinid fauna; Fig. 6), which became extinct in the earliest Serpentinum Zone as an effect of the Jenkyns Event (Ager, 1987; Vörös, 2002; Comas-Rengifo et al., 2006; Baeza-Carratalá et al., 2015, 2018a; Vörös et al., 2016, 2019). After this event, Soaresirhynchia bouchardi led the repopulation interval in many Western Tethyan basins (Fig. 6), constituting monospecific assemblages in the Elegantulum Subzone of the Serpentinum Zone (García Joral & Goy, 2000; García Joral et al., 2011; Baeza-Carratalá et al., 2011, 2017; Comas-Rengifo et al., 2013, 2015). Therefore, Soaresirhynchia bouchardi was selected for geochemical analyses of this stratigraphic post-crisis interval (Table 1).

Finally, Telothyris pyrenaica is selected (Table 1) as representative for the lower-middle Toarcian, Serpentinum-lowermost Bifrons zones (García Joral & Goy, 1984, 2000; García Joral et al., 2011; Baeza-Carratalá et al., 2016c), signifying the re-establishment of the background conditions after the extinction event.

Assessing possible disparities due to different vital effects in the incorporation of the trace elements in the shell of the most representative genera analyzed, i.e. Cirpa, Calcirhynchia, and Prionorhynchia, is important to discard possible interferences in the geochemical signals. Seemingly, these taxa are structurally influenced by similar factors with minor variations: external characters are practically comparable in outline, folding, ribbing multicostate pattern, and beak features, all of them showing minute pedicle foramen pointing to rhizo-pedunculate shells. Regarding the internal architecture, representatives of the Wellerellidae (Cirpa and Calcirhynchia) are typified by well-developed hamiform crural developments whereas Prionorhynchia shows a simpler raduliform crura. On the other hand, these taxa show differences in the microstructural pattern of the secondary layer of the shell. In Cirpa and Calcirhynchia microstructure is characterized by coarse calcite fibres forming the eurinoid pattern whereas Prionorhynchia shows thinner and anvil-like calcite fibres typifying the leptinoid pattern (Fig. 4). According to Simon et al. (2018) the eurinoid type seemingly requires more metabolic energy and oxygenated habitats to construct coarser fibres than the leptinoid pattern. Thus, a theoretical approach could relate these differences among taxa to adaptive strategies or to dissimilar metabolism and feeding. However, these genera are recorded together virtually in the same ecospace and with similar body size, as inhabitants of the shallow platforms in the background conditions of the Sinemurian-Pliensbachian transition, and later in their Pliensbachian heyday under hemipelagic conditions (Baeza-Carratalá, 2013).

Regarding the incorporation of trace elements in these taxa, there is not a great difference in the content of trace elements among the most analysed genera Cirpa, Calcirhynchia, and Prionorhynchia (Figs. 12, 13). The axis PC1 reveals a strong interaction between Mg, Al, Ti, Fe, Co, Rb, Cd, Ba, Pb, and REEs which could be related to continental influx (e.g. Chester et al., 1977; Pye, 1987; Tribovillard et al., 2006; Chester & Jickells, 2012; Zaky et al., 2016) whereas PC2 and PC3 are related to redox sensitive elements (e.g. Calvert & Pedersen, 1997; Tribovillard et al., 2006).

The most common elements in the shells (excluding Ca) are Mg, Al, and Fe (Fig. 7). In marine environments these elements may be related to detrital inputs from emerged lands, mainly in the case of Al and Fe (Fig. 11). Brachiopod shells consist of low-Mg calcite, which explains the abundance of Mg in the shells compared to other elements (Fig. 7). Fe is included in the biological cycling (Bruland et al., 1991; Morel & Price, 2003; Smrzka et al., 2019), and is complexed in the particulate organic matter that is consumed by brachiopods. Iron is a micronutrient, and present high reactivity with oxygen as Fe²⁺ and low solubility as Fe³⁺ (and would not be incorporated into shell lattice except during diagenesis). Fe²⁺ facilitates electron transport in chloroplasts, mitochondria and bacteria. The Fe²⁺ can enter in the brachiopod metabolism through feeding and incorporate via coprecipitation because present octahedral coordination as Ca in calcite. The Sr is the following most abundant element in the studied shells, independently of the eurinoid or leptinoid microstructure pattern (Fig. 7). Sr and Mg replace Ca via adsorption of cations within the lattice with an initial surface uptake (Comans & Middelburg, 1987; Stipp & Hochella, 1991; Smrzka et al., 2019).

In the peri-Iberian platforms system, recent analysis performed in different macroinvertebrate taxa by Ullmann et al. (2020) provides a very comprehensive geochemical database for the upper Pliensbachian-lower Toarcian brachiopods derived from two stratigraphical sections from the Iberian Range (Spain) and the Lusitanian Basin (Portugal). The brachiopod taxa from the Subbetic, herein analysed, shows lower values in Sr and Mg if compared with their Portuguese or Central Iberian counterparts analysed by Ullmann et al. (2020). It would be attributable to the different environmental conditions and ecological setting prevailing in the different Iberian palaeomargins (Rodríguez-Tovar & Uchman, 2011; Rodríguez-Tovar & Reolid, 2013; Reolid et al., 2015, 2018). The depositional environment for the mainly Mediterranean Cirpa briseis (Pliensbachian) in the Subbetic consists of epioceanic and well-oxygenated platforms typified by crinoidal grainstone beds with abundant brachiopods, benthic foraminifera, peloids, and intraclasts. In addition, specimens of the Lusitanian Basin constitute a considerably younger occurrence than in the Subbetic area, since C. briseis is recorded in the Pliensbachian (Lavinianum-Emaciatum zones). On the other hand, the dataset of the Portuguese Cirpa fallax (lower Toarcian) collected by Ullmann et al. (2020) is the inhabitant of a low-energy, distal homoclinal ramp typified by hemipelagic sequences and organic matter rich facies (Duarte, 2007) where alternation of marlstone and argillaceous limestone beds prevailed (Duarte, 2007; Comas-Rengifo et al., 2013; Ullmann et al., 2020). Moreover, C. fallax is recorded in the Toarcian (Semicelatum Subzone, Polymorphum Zone) coevally with the representative components of the koninckinid fauna from the NW-European palaeobioprovince (Alméras et al., 1988, 1996; Alméras & Elmi, 1993, Comas-Rengifo et al., 2013), which is substantially different to the koninckinid fauna from the Mediterranean palaeobioprovince (Baeza-Carratalá et al., 2015). Therefore, differences in the Sr and Mg content between brachiopods from different Iberian palaeomargins may be related to different age, faunas and environmental conditions.

Following with the most abundant trace elements, Cr, Ni, and Zn, included in the PC2 axis (Fig. 11), usually present content < 60 ppm (Fig. 7). The concentration of these elements is slightly higher in Calcirhynchia shells (Fig. 12 and 13). Ni and Zn are micronutrients. Nickel constitutes a micronutrient required by phytoplankton growth in the photic zone (Calvert & Pedersen, 1993; Dupont et al., 2010) and, under oxic conditions, is dissolved as Ni²⁺ (Fig. 14). Zinc is required for biological cycling by eukaryotes and this element exhibits a nutrient-type profile in modern oceans with surface water depletion (Zhao et al., 2014). Under oxic conditions Zn²⁺ appears as dissolved phase (Fig. 14).

The next most abundant elements in the brachiopod shells are Ti and Ba (< 10 ppm, Fig. 6), which are included in the PC1 axis related to continental influx (Figs. 11, 14). Among the elements with minority content (commonly < 5 ppm), the Cu, Pb, V, Co, Mo, Cd, U, and REE are remarkable (Fig. 7), most of them considered redox sensitive elements. Molybdenum species in oxic seawater is molybdate (MoO₄ ^2-) that is reduced and subsequently enriched in sediments in close association with dissolved sulfides (e.g. Helz et al., 2011) (Fig. 14). In oxygen depleted conditions, molybdenum is incorporated into pyrite. In oxic waters, molybdenum concentrations are controlled by co-precipitation and adsorption onto Fe and Mn (oxy)hydroxides (Algeo & Rowe, 2012). However, the correlation coefficient indicates a similar way of incorporation in the shell for Zn and Mo (R² = 0.64) in the brachiopod shells (Fig. 13), showing Zn a nutrient-type profile in marine environments (Zhao et al., 2014).

Copper and vanadium are also essential nutrients for marine plankton as well (Smrzka et al., 2019) and they are constituents of numerous enzymes (Nalewajko et al., 1995; Moore et al., 1996) being as a dissolved phase under oxic conditions (CuCO₃, VO₂(OH)₃ ^2- and H₂VO₄ ^-) (Fig. 14). Vanadium (PC3) and molybdenum (PC2) show a high affinity for hydrogen sulfide and are co-enriched in anoxic sediments (e.g. Tribovillard et al., 2006; Reolid et al., 2012), but V is commonly present in the organic matter and absent in pyrite (Algeo & Maynard, 2004) (Fig. 14). On the other hand, Co, Cr, and REE in sediments are derived from terrigenous detrital input (Fig. 14), but Cr appears in the PCA with a higher interaction with redox sensitive elements (PC2 axis, Fig. 11).

With respect to the Cd (PC1 axis), Tribovillard et al. (2006) stated that Cd, in contrast to Ni, Cu, and Zn (PC2 axis), presents one coordination state (Cd (II)) in the water column and sediments and has a nutrient-like role and, consequently, it implies a relatively short residence time in the water column. Cadmium is present as a dissolved phase (CdCl⁺) in the seawater, showing photic zone depletion and deep-water enrichment. It is primarily transported to sea bottom via CaCO₃ particles and associated to organic carbon particles. The phytoplankton and organic matter present high content of Cd compared with terrigenous material, thus making Cd a useful tracer for primary productivity (Gendron et al., 1996; Piper & Dean, 2002).

With respect to the REE, the main sources to the modern oceans are river inputs and atmospheric dust (Greaves et al., 1994; Holser, 1997). Because the Al content in marine sedimentary environments is controlled by continental influx (Calvert & Pedersen, 1993), the presence of Al in the shells has been plotted versus other elements (Fig. 10). Some elements present a high correlation coefficient with respect to Al, which is the case for REE (R²= 0.90) and Ba (R²=0.63), and lower correlation for Fe (R²=0.39). Other elements, such as Cr, Zn, and Ni have a very low correlation index (< 0.20, Fig. 12) which is congruent with the PCA (Fig. 11) where they are grouped along the PC3 axis group (redox sensitive elements). However, this approach takes into account the behaviour of the trace elements in the seawater using data of concentration in the brachiopod shells. But we cannot discard other factors which can influence the co-precipitation and absorption processes of these elements in the calcite structure.

In the Eastern Subbetic domain, the brachiopod fauna shows a progressive diversification from the Sinemurian to the late Pliensbachian-early Toarcian interval, punctuated by several critical episodes that conditioned the diversity dynamics of this group in the basin. The homogeneous platform system, established in the Western Tethys during the earliest Jurassic, subsequently drowned related to the Central Atlantic Ocean opening (Bernoulli & Jenkyns, 1974; Winterer & Bosellini, 1981). After an earlier diversification stage probably ongoing from the early Sinemurian (Baeza-Carratalá et al., 2018b), a speciation burst and radiation event during the Sinemurian-Pliensbachian transition (Raricostatum-Aenigmaticum zones) led to a bloom in brachiopod diversity from the late Sinemurian onwards. This peak of prosperity was probably related to an incipient pre-rifting stage of the westernmost Tethyan platform system. In the Sinemurian, the initial tectonic pulses of disintegration of the South-Iberian Palaeomargin (Vera, 1988, 1998; Molina et al., 1999) (Fig. 15) played an important role diversifying the ecological niches and facilitating the establishment and renewal of brachiopod communities (Baeza-Carratalá, 2013). This event of speciation in the Sinemurian-Pliensbachian transition concurred with the clear disconnection of the Mediterranean/Euro-Boreal brachiopod bioprovinces in the Betic Domain, since the sudden increase of the bathymetry makes achievable the diversification not only for brachiopod communities but also for necto-planktic fauna (e.g. Sandoval et al., 2001). In other Tethyan basins this initial disintegration phase was also detected around the Sinemurian-Pliensbachian boundary, triggering greater diversification of depositional conditions in the late Sinemurian, which resulted in a great increase in diversity of the brachiopod fauna (cf. Hallam, 1981; Delance & Laurin, 1983).

The definitive extensional collapse of the platform system is evidenced in the South-Iberian Palaeomargin by faulting and blocks tilting (Rey, 1998; Tent-Manclús, 2006) (Fig. 13). The transition to more pelagic facies and the progressive establishment of epioceanic environments during the Pliensbachian produced changes in the environmental and depositional conditions. This also entailed alteration of the benthic biota, greatly influenced by substrate. A virtually complete renewal episode from the Sinemurian-Pliensbachian transition onwards meant the extinction of all endemic Sinemurian taxa together with several index-groups in the basin such as multicostate zeilleriids (Baeza-Carratalá, 2011, 2013; Baeza-Carratalá & García Joral, 2012) and the replacement of the prolific Prionorhynchia regia and Calcirhynchia plicatissima stocks by new Pliensbachian multicostate rhynchonellides (Prionorhynchia spp., Cirpa spp.), herein selected for the analysis (Table 1, Fig. 6A). Haq (2018) proposed a maximum flooding episode in the Sinemurian/Pliensbachian transition, just between the JSi5 and JPl1 events. Thus, the replacement of the Pliensbachian assemblages of the Mediterranean bioprovince, better adapted to epioceanic environments, was produced by the continuous deepening of the basin around the Sinemurian/Pliensbachian boundary.

The late Pliensbachian interval coincides with the great speciation phase of the benthic biota in the Western Tethys as a whole (cf. Hallam, 1981). The decrease in the faulting activity allowed the irregularities of the sea floor to be smoothed out enabling the spreading of brachiopods. With the settlement of polyspecific and very prolific communities, with profusion of genera such as Prionorhynchia, Cirpa, and Liospiriferina, a maximum in biodiversity and abundance was reached in the Pliensbachian-Toarcian transition (Figs. 6B, C), due to the favourable environments and the arrival of taxa from closer Euro-Boreal basins (e.g. Lobothyris arcta). A similar significant turnover in the Pliensbachian/Toarcian boundary was also noticed in ammonoids (Sandoval et al., 2001, 2012) linked to a relative drop in the sea level (JPl8 regressive event in Haq (2018); Fig. 1), prior to the onset of the Toarcian transgression. This interval is also partly concurring with a global cooling episode during the late Pliensbachian (Suan et al., 2010; Korte & Hesselbo, 2011; Korte et al., 2015; Gómez et al., 2016; Bougeault et al., 2017; De Lena et al., 2019; Storm et al., 2020).

The early Toarcian mass extinction event (Fig. 6) was preceded by several biotic changes, especially noticeable in the brachiopod assemblages. The progressive warming of the seawater and sea-level rise from the latest Pliensbachian onwards (Danise et al., 2013) led to several brachiopod clades to perform global adaptive strategies also noticed in the Subbetic brachiopod occurrences. During the pre-extinction interval, assemblages suffered a replacement by taxa better adapted to warmer conditions, with L. arcta, C. vulgata, and Pseudogibbirhynchia? moorei as foremost representatives (cf. García Joral et al., 2011, Baeza-Carratalá, 2013; Baeza-Carratalá et al., 2015, 2018a). On the other hand, koninckinid fauna (with Koninckella and Koninckodonta recorded together with the usually associated Nannirhynchia and Pseudokingena species) occurs in the Subbetic area prior to the sudden increase in temperature reached in the Jenkyns Event. In this biotic crisis, the worldwide global extinction of orders Athyridida and Spiriferinida (Vörös, 2002; Comas-Rengifo et al., 2006; García Joral et al., 2011; Baeza-Carratalá et al., 2015; Vörös et al., 2016, 2019) is recorded in the Subbetic as well. In this sense, one of the last spiriferinid representatives (Calyptoria vulgata) suddenly appears in the Subbetic just prior to the Jenkyns Event, as a result of its long-term migration from the Arab-Madagascan margins to the epicontinental peri-Iberian ones triggered by increasing temperatures (Baeza-Carratalá et al., 2018a).

The lowermost Serpentinum hyperthermal event coinciding with the biotic crisis (Reolid et al., 2021) led to the total demise of brachiopod fauna in the basin (Fig. 15). Rodrigues et al. (2019), also from outcrops of the Subbetic, proposed a decreased ¹³C fractionation during photosynthesis in C3 plants and arid environments from the Iberian Palaeomargin. Based on differences in the magnitude of the CIE recorded in land plants and marine substrates in the External Subbetic, Ruebsam et al. (2020) infer that the early Toarcian warming was paralleled by an increase in atmospheric CO₂ levels from ~500 ppmv to ~1000 ppmv. According to Rodrigues et al. (2019) and Ruebsam et al. (2020) the climatic belts in the South-Iberian Palaeomargin displaced to the north, turning more arid in this area.

After the Jenkyns Event, the repopulation began with Soaresirhynchia bouchardi as opportunistic taxa (García Joral & Goy, 2000; Gahr, 2005; García Joral et al., 2011; Baeza-Carratalá et al., 2017) probably related to its alleged low metabolic rate (Ullmann et al., 2020). The subsequent post-Jenkyns Event recovery during the Serpentinum-Bifrons zones was due to the stabilization of temperatures, which facilitated the settlement of the so-called “Spanish Bioprovince” of brachiopods (García Joral & Goy, 1984, 2000; Alméras & Fauré, 1990; García Joral et al., 2011), with the herein analysed Telothyris gr. pyrenaica, characterizing the background conditions and the well-oxygenated environments.

The increase in concentration of the analyzed trace elements in brachiopod shells could be mainly related to: oxygenation degree, external inputs and productivity:

These possibilities are analyzed below.

The first option, enrichment in redox sensitive elements related to oxygen depleted conditions is herein discarded. In fact, macrobenthic organisms including brachiopods, disappear in suboxic and anoxic conditions but these conditions did not occur in the studied area. In addition, the prolific brachiopod-bearing Sinemurian to Pliensbachian facies in the Subbetic area consist of oolithic grainstone to packstone and crinoidal grainstone sediments, thus confirming current activity and oxygenation. The growth of brachiopod calcitic shells, like other marine organisms with mineral skeletons, is produced by epithelial secreting cells at the mantle margin that obtain the Ca²⁺ and CO₃ ^2- through metabolism (breathing and feeding). For this reason, increasing values of redox sensitive elements in the sediments detected in other Toarcian outcrops of the Iberian margins (Rodríguez-Tovar & Reolid, 2013; Reolid et al., 2014) and related to precipitation under oxygen depleted conditions do not necessarily reach the metabolism of benthic organisms with the same intensity.

The biological cycling mostly affects to Mo, Cu, Ni, Zn, Cd, Co, Fe, and V (Bruland et al., 1991; Morel & Price, 2003; Smrzka et al., 2019), which are complexed in the particulate organic matter that is consumed by brachiopods, such as U typically related to organic matter (McManus et al., 2005). Therefore, the active incorporation via co-precipitation during the growth of shells is facilitated for Co, Zn, and Fe that can enter in the metabolism through feeding and are ions with the octahedral coordination, the same as Ca in calcite (Fig. 12). Other elements usually present in calcitic shells such as Mg, Ba, Sr, and U (Delaney & Boyle, 1982; Russell et al., 1994; Thébault et al., 2009; Benito & Reolid, 2012; Pérez-Huerta et al., 2013; Keul et al., 2013) have higher coordination number and substitute Ca via adsorption of cations within the lattice with an initial rapid surface uptake, followed by slower removal from solution (Comans & Middelburg, 1987; Stipp & Hochella, 1991). Live phytoplankton incorporate Ba by metabolic uptake or adsorption (Tribovillard et al., 2006) and many studies have examined the link between primary productivity and biogenic Ba abundance (e.g. McManus et al., 1999; Jeandel et al., 2000; Prakash Babu et al., 2002; Reolid et al., 2012).

Special attention is paid on the Mg concentrations (Fig. 7). This is a particularly important element in skeletal calcite because it varies with temperature, growth rate, and taxa (Buesing & Carison, 1992; see discussions on Pérez-Huerta et al., 2014; and Clark et al., 2016), as seen in the extant articulate brachiopods Terebratulina unguicula and Terebratalia transversa, where Mg increases as a response to higher growth rate and productivity. Experimental works have confirmed an increasing incorporation rate of other trace elements such as U into calcite with increasing growth rate of calcite crystals (Weremeichik et al., 2017). Moreover, the U abundance in sediments usually shows a good correlation with the organic matter rain rate in the water column (McManus et al., 2005). However, U content in the analyzed shells is very low (Figs. 7, 9). Cadmium presents a high affinity for adsorption onto calcite (Boyle, 1981; Papadopoulos & Rowell, 1989; Tesoriero & Pankow, 1996). Cd²⁺ and Ca²⁺ have similar chemical behavior and Cd²⁺ can replace Ca²⁺, forming surface complexes of CdCO₃ (otavite) that co-precipitate with calcite (Papadopoulos & Rowell, 1989). The affinity series for divalent metals with respect to calcite incorporation is Cd>Zn>Co>Ni>Ba (Reeder, 1996; Smrzka et al., 2019), but this does not correspond to the abundance of these elements in the analyzed shells (Fig. 6) since the concentrations of these elements in seawater are variable.

Therefore, the increasing values of trace elements in brachiopod calcitic shells are related to their increased content in seawater as dissolved phase and/or high primary productivity, the last one also favoured by high content of the dissolved phase of some trace elements (Figs. 14, 15). The high phytoplankton productivity implicates increasing food resources for brachiopods that feed on phytoplankton such as diatoms and dinoflagellates (Fürsich & Hurst, 1974; Suchanek & Levinton, 1974; Stanton & Nelson, 1980; McCammon, 1981; James et al., 1992) and favored abundance of other trophic resources composing the seston as particulate organic matter, organic colloids and bacteria (Levinton & Suchanek, 1972; Suchanek & Levinton, 1974; Steele-Petrovic, 1979; James et al., 1992) and dissolved organic matter (dissolved carbohydrates and aminoacids; James et al., 1992) (Fig. 14). As well-known, brachiopods may be active/passive suspension feeders. There are experimental evidences on uptake of dissolved nutrients for brachiopods (McCammon & Reynolds, 1976; Doherty, 1981; Rosenberg et al., 1988; Tkachuck et al., 1989). The lophophore and mantle can directly assimilate dissolved nutrients (McCammon & Reynolds, 1976; Doherty, 1981; see discussion in McCammon, 1981; Rosenberg et al., 1988; Tkachuck et al., 1989) and surely related trace elements that work as micronutrients.

From bottom to top, the first excursion in the geochemical signal recorded in the composite stratigraphical section occurred between the samples EFC2 and BOL1 in correspondence with the Sinemurian-Pliensbachian transition (Raricostatum-Aenigmaticum zones), where distribution of most of the trace elements (Mo, Ni, Cr, Ba, Pb, Ti, Al, Fe, Cd, and REE) marks a positive excursion in the Calcirhynchia shells (Figs. 8, 9). This excursion is mainly based in one data point corresponding to a well-preserved specimen of Calcirhynchia, but it is necessary to be cautious.

The diversity dynamics (burst in brachiopod species) and recent taphonomic assessment of these deposits (Baeza-Carratalá et al., 2014) provide a refined palaeoenvironmental setting, locating these shell beds in a shallow-water well-oxygenated bottom frequently affected by storms. In this context, the concentration values suddenly increase for most of the trace elements in this crucial timespan, decreasing upwards in most of the Pliensbachian samples to lower contents. The variations in content of trace elements in the shells are related to palaeotectonic events and climatic changes with incidence on the input of these elements to the sea from continental runoff and volcanic sources resulting on increased concentration in the seawater and primary productivity (Figs. 14, 15).

The geochemical perturbations in this timespan are concurrent with the initial tectonic pulses of rifting and drowning of the platform system developed in the South-Iberian Palaeomargin (Vera, 1988, 1998; Molina et al., 1999; Ruiz-Ortiz et al., 2004) (Fig. 15), which diversified the ecological niches leading to a renewal of brachiopod communities (Baeza-Carratalá, 2013). The abundance of the analysed trace elements in the calcite shells can be controlled by the transition to hemipelagic conditions and the change of current pattern favouring upwelling and increased productivity. The Cd content in brachiopod shells achieves the maximum values in a punctual excursion just concurring with the Sinemurian-Pliensbachian transition. Cadmium is a micronutrient and could favour an increase in productivity that may be in good accordance with the increasing in brachiopod diversity occurred in the Sinemurian-Pliensbachian transition (Figs. 6, 9).

The increased tectonic activity related to fragmentation of the South-Iberian Palaeomargin (García Hernández et al., 1989; Vera, 2001) and the opening of Hispanic Corridor (Aberhan, 2002; Korte et al., 2015; Schöllhorn et al., 2020; Storm et al., 2020) would have favoured the sediment runoff due to uplift of emerged reliefs (Figs. 14, 15). We cannot discard alternative sources for the trace elements analysed related to hydrothermal sources, submarine volcanic outgases or eruptions (e.g. Tribovillard et al., 2006) for example in the case of Cd, Zn, Cu, and Pb (Chester & Jickells, 2012). Sinemurian alkaline submarine volcanism in the Subbetic area has been reported (Comas et al., 1986; García-Hernández et al., 1989; Portugal et al., 1995; Olóriz et al., 2002), which would be connected to the extensional palaeotectonic event of the first phase of the Atlantic Ocean opening that favoured the fragmentation of the western Tethys palaeomargins (Ruiz-Ortiz et al., 2004; Reolid et al., 2018) and the reactivation of the Central Atlantic Magmatic Province (Ruhl et al., 2016; Storm et al., 2020) (Fig. 15). Recent studies place the first clear evidences of volcanic activity and even the emplacement of the oceanic crust just in the late Sinemurian-earliest Pliensbachian, coinciding with the initial drifting stage (Sahabi et al., 2004; Klingelhoefer et al., 2009). Reolid et al. (2013) also linked volcanic activity with a higher tectonic activity episode in the Median Subbetic (Comas et al., 1986; Reolid & Abad, 2014). Increased values of Zn, Cd, Cu, and Pb in the seawater for this interval could be related with these mechanisms due to their hydrothermal/volcanic origin in seawater (Chester & Jickells, 2012).

In addition to the palaeotectonic event, the palaeoclimatic changes had an important relevance around the Sinemurian-Pliensbachian boundary. A global warming episode, mainly developed in the late Sinemurian, has been recorded in the westernmost Tethyan basins, followed by a decrease in palaeotemperature in the early Pliensbachian (e.g. Hesselbo et al., 2000; Korte & Hesselbo, 2011; Gómez et al., 2016). The increased values of Mg/Ca ratio is congruent with that global warming event that partly coincides with a perturbation in the oxygen and carbon cycles (excursions of the δ¹³C and δ¹⁸O) and remarkably with the trace elements herein analysed. The increased temperature and humidity reported for the Sinemurian-Pliensbachian transition (Korte & Hesselbo, 2011; Schöllhorn et al., 2020) could facilitate the increase of continental weathering (Deconinck et al., 2003) and the subsequent input of trace elements to the sea, where the concentration of the dissolved phases and of those incorporated to the biological cycling increased.

This timespan is typified by enrichment in most of the trace elements, especially in those related to be proxies of continental influx (PC1, Fig. 9) as Ba, Ti, Al, Fe, Pb, and REEs (Fig. 8). Therefore, the global warming event in the latest Sinemurian could have been the responsible for an increase in trace elements content in brachiopod shells by adsorption on the shell surface or co-precipitation via incorporation of these elements to the metabolism after feeding of phytoplankton, bacteria, organic detritus, colloidal material, and dissolved nutrients (i.e. trophic resources according to Rosenberg et al., 1988; Tkachuck et al., 1989; James et al., 1992). Thus, weathering of the continental crust is the ultimate source of REE in the water reservoirs, whereas riverine discharge and aeolian deposition are the primary fluxes (Zaky et al., 2016, and references therein).

The brachiopod diversity peak and turnover episode, with the demise of endemic Sinemurian fauna (e.g. Cincta peiroi, Praesphaeroidothyris cisnerosi; Baeza-Carratalá, 2013) and their replacement by deeper Mediterranean Pliensbachian fauna, correlates with a regressive-transgressive cycle (Hallam, 1988; O’Dogherty et al., 2000; Sandoval et al., 2001, 2012; Schöllhorn et al., 2020).

This interpretation is congruent with the absence of black shales and oxygen depleted conditions in the studied area during the Sinemurian-Pliensbachian boundary that would limit the brachiopod survival as well as favour the co-precipitation of some trace elements with iron sulfides (Mo, Ni, and Cu) and fixation to buried organic matter (U, Cd, Zn, and V) but not an increase in their content in the brachiopod shells. Oxygen-depletion should be therefore reasonably excluded to explain trace elements variations in this timespan.

The return to the standard conditions in the Pliensbachian shows a very slight decreasing trend in the trace elements analyzed subsequently (CC1.1 to CC2.2 samples). After that, the next main perturbation imprints on the trace elements are located in the uppermost Pliensbachian-lowermost Toarcian (Emaciatum-Polymorphum zones), with several pulses showing positive excursions from CC2.2 to Z2B samples and a peak in Z2B, just prior to the extinction boundary. If we compare this interval with the δ¹³C and δ¹⁸O curves (Gómez & Goy, 2011; García Joral et al., 2011; Gómez et al., 2016; Ruebsam et al., 2020;) (Fig. 1), a high correlation between general trends of trace element perturbations and the C and O cycling oscillations can be deduced. Their occurrence in the pre-extinction interval of the Jenkyns Event really suggests a multi-phased stage in this biotic crisis (Figs. 4, 15), which started earlier in the latest Pliensbachian as deduced by several authors (cf. Little, 1996; Macchioni & Cecca, 2002; Wignall & Bond, 2008; Dera et al., 2010; Caruthers et al., 2013; Arias, 2013; Rita et al., 2016; Baeza-Carratalá et al., 2017). In the analyzed samples, the onset of these perturbations is coincident with the first record of koninckinid fauna in the Subbetic (CC 2.2 sample onwards) which was considered as precursor signal of the Jenkyns Event (Baeza-Carratalá et al., 2015, 2017).

The maximum positive excursion in trace elements is reached in the sample Z2B, just in the base of the marls of the Polymorphum Zone (lower Toarcian, Figs. 8, 9). An anoxic event is a recurrent factor used to explain the mass extinction occurring in the early Toarcian (e.g. Jenkyns, 1988; Bassoullet & Baudin, 1994; Harries & Little, 1999; Pálfy & Smith, 2000; Wignall et al., 2005; Mailliot et al., 2006; Ruebsam et al., 2020). However, in the Subbetic area, neither sedimentary evidence (black shales) nor TOC content points to the anoxia as the primary factor for the biotic crisis. Maximum TOC values from the Subbetic area in the basal Serpentinum Zone are around 0.4-0.9 wt.% (for the biotic crisis. Maximum TOC values from the Subbetic area in the basal Serpentinum Zone are around 0.4-0.9 wt.% (Reolid et al., 2013, 2014; Rodríguez-Tovar & Reolid, 2013; Rodrigues et al., 2019; Ruebsam et al., 2020) instead of the 5-15 wt.% present in the NW-European basins (e.g. Röhl et al., 2001; Bucefalo-Palliani et al., 2002; Mailliot et al., 2006; McArthur et al., 2008; Baroni et al., 2018; Thibault et al., 2018; Fantasia et al., 2019a). Moreover, under oxygen depleted conditions most of the trace elements would be incorporated to the bottom as sulfides (Fe, Mo, Ni, Cu, Pb) and complexed in the particulate organic matter (U, Mo, Cu, Cd, Zn, Co, Ni, V), instead in dissolved phase in the sea water. On the contrary, under oxic conditions, Zn, Ni, and Cu are strongly complexed by dissolved organic matter. Moreover, Zn, Mo, Cr, Cu, and Ni are related in the PC2 axis as redox sensitive elements, usually dissolved in the seawater under oxic conditions (Tribovillard et al., 2006).

Indeed, the maximum brachiopod diversity is reached just prior to the timing of this biotic crisis, dated in the westernmost Tethys in the lowermost Serpentinum Zone. Thus, in the Subbetic Domain, we can invoke from the biotic signals an alternative triggering mechanism as a primary responsible factor in this biotic crisis, such as an increasing temperature gradient. Firstly, koninckinid beds early appear in the basin just in the levels showing several trace elements perturbations (sample CC 2.2 upwards). This brachiopod clade (Athyridida) became extinct during the Jenkyns Event (Vörös, 2002; Baeza-Carratalá et al., 2015; Vörös et al., 2016) and their record in the Subbetic area (South-Iberian Palaeomargin), as results of their adaptive strategies, was considered as precursor signal of the Jenkyns Event (Baeza-Carratalá et al., 2015). On the other hand, the sudden occurrence in the basin of Calyptoria vulgata and Lobothyris arcta (Baeza-Carratalá, 2013; Baeza-Carratalá et al., 2018a), typical of warmer regions (García Joral et al., 2011; Baeza-Carratalá et al., 2018a), suggests a thermal maximum around the Z2B sample, in the Polymorphum Zone, just prior to their total extinction, not surpassing the hyperthermal event of the Serpentinum Zone.

Many authors have proposed a global warming related to the negative CIE of the Jenkyns Event and the correlative T-OAE (e.g. Suan et al., 2010; Korte & Hesselbo, 2011; Korte et al., 2015; Them et al., 2017; Ruebsam et al., 2019; Baghli et al., 2020; Reolid et al., 2020) (Figs. 1, 15). The Mg/Ca values at the beginning of the lower Toarcian increase respect to the Pliensbachian and confirm the global warming related with the Jenkyns Event. The enhanced weathering related to this climate change (Montero-Serrano et al., 2015; Fu et al., 2017; Them et al., 2017) drove to the subsequent detrital and nutrient input to the marine basin (Rodríguez-Tovar & Reolid, 2013; Danise et al., 2015; Fantasia et al., 2019b; Kemp et al., 2019) and the increase as dissolved phase where oxygen depleted conditions did not develop. This increased primary productivity also favoured the labile organic matter (particulate, colloidal and dissolved organic matter) for suspension-feeders such as brachiopods, with the subsequent increase of some trace metals in the shells (Fe, Cr, Zn, Ni, Mo, and Cu).

After reaching the maximum concentration values in the Jenkyns Event interval, the content of most trace elements slowly decreases progressively towards the top of the stratigraphical succession, although showing higher values than prior the Jenkyns Event. Thus, the repopulation interval just starts with the record of the opportunistic Soaresirhynchia bouchardi (Z2C1 sample) in the Serpentinum Zone (usually recorded in the Elegantulum Subzone) where the trace elements values start their decreasing trend. The reestablishment of the background conditions is reached from this sample upwards (Z2C1- Z2C2), in coincidence with the recovery of the brachiopod diversity led by the “Spanish Bioprovince” of brachiopods (García Joral & Goy, 1984, 2000; Alméras & Fauré, 1990; García Joral et al., 2011).

Some trace elements such as Ti, Fe, and REE, considered as reliable proxies of possible continental influx (Figs. 11, 14) (Zaky et al., 2016; Smrzka et al., 2019, and references therein), reach their highest values in the brachiopod shells during the Jenkyns Event interval and just in the onset of the repopulation interval (Z2C1 sample). Other typical elements such as Al, Ba, and Pb show a short increase delayed respect to the peak of Ti, Fe, and REE (Fig. 8).

Summarizing, in the South-Iberian Palaeomargin redox fluctuations in the seawater does not appear to have been a primary cause for oscillation of trace elements concentration in brachiopod shells. In fact, the record of benthic fauna is quite continuous, and this would not be the case in an anoxic environment. Other factors related to the global change induced by the Jenkyns Event (global warming, enhanced weathering, and subsequent changes in primary productivity) had impact on both assemblage structure and shell composition (with increasing content on some trace elements). In this sense, seawater temperature must be invoked to play a decisive role in this mass extinction event, as it is deduced by the biotic signals correlated with the palaeotemperatures deduced for the peri-Iberian platform system (García Joral et al., 2011; Sandoval et al., 2012, Gómez et al., 2016; Rosales et al., 2018; Ullmann et al., 2020).