# Calcium isotopes support spatial redox gradients on the Tethys European margin across the Triassic-Jurassic boundary

Ashley N. Prow-Fleischer, Zunli Lu, Clara L. Blättler, Tianchen He, Pulkit Singh, Preston Cosslett Kemeny, Jordan P. Todes, Alexandre Pohl, Tripti Bhattacharya, Bas van de Schootbrugge, et al.

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► To cite this version:

Ashley N. Prow-Fleischer, Zunli Lu, Clara L. Blättler, Tianchen He, Pulkit Singh, et al. Calcium isotopes support spatial redox gradients on the Tethys European margin across the Triassic-Jurassic boundary. Chemical Geology, 2025, 673, pp.122530. 10.1016/j.chemgeo.2024.122530. hal-04839572

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# Calcium isotopes support spatial redox gradients on the Tethys European margin across the Triassic-Jurassic boundary

Ashley N. Prow-Fleischer<sup>a,*</sup>, Zunli Lu<sup>a</sup>, Clara L. Blättler<sup>b</sup>, Tianchen He<sup>c</sup>, Pulkit Singh<sup>d</sup>, Preston Cosslett Kemeny<sup>b</sup>, Jordan P. Todes<sup>b</sup>, Alexandre Pohl<sup>e</sup>, Tripti Bhattacharya<sup>a</sup>, Bas van de Schootbrugge<sup>f</sup>, Paul B. Wignall<sup>g</sup>, Simona Todaro<sup>h</sup>, Jonathan L. Payne<sup>d</sup>

## ARTICLE INFO

Editor: Dr. Christian France-Lanord

### Keywords:

End Triassic mass extinction Calcium isotope ratios Diagenesis

Carbonate-based redox proxies Geochemical tracers

## ABSTRACT

The end-Triassic mass extinction was among the most severe biotic crises of the Phanerozoic. It has been linked with the global expansion of marine anoxia, and the prolongation of these conditions within epeiric seas has been proposed as a cause for the suppression of biodiversity during the early Jurassic Hettangian Stage. Testing this interpretation is complicated by spatially heterogeneous patterns of local marine redox conditions within the western Tethys European Epicontinental Shelf. In this study, we assess the redox state within this region by focusing on two carbonate successions in Italy, a peritidal platform at Mount Sparagio, Sicily, and an offshore ramp deposit at Val Adrara in the Southern Alps. Based on previously published $I/\text{Ca}$ ratios, these locations

record distinct local background redox conditions, with Val Adrara showing a notably lower pre-extinction oxygen saturation state than Mount Sparagio. Here, we measure $\delta^{13}\text{C}$ and $\delta^{18}\text{O}$ at Mount Sparagio and $\delta^{44}\text{Ca}$ and trace element ratios at both sites to identify the roles of mineralogical and diagenetic effects on the pres-ervation of primary redox signals. A numerical framework of multiple elemental (Sr, Mg, Mn, I) and isotopic ($\delta^{13}\text{C}$, $\delta^{18}\text{O}$, $\delta^{44}\text{Ca}$, $\delta^{238}\text{U}$, and $\delta^{34}\text{S}_{\text{CAS}}$) ratios was constructed to recognize modes of carbonate diagenesis and source-mixing in the data. While diagenesis is impossible to completely rule out, our state-of-the-art approach provides robust evidence against common forms of diagenetic alteration as the main drivers of the overall paleoredox proxy trends.

Where the redox signals are largely preserved, we interpret differences in pre-extinction $I/\text{Ca}$ between the two sites to reflect distinct local oxygenation states. Drawing from published Community Earth System Model simulations, we propose that ocean circulation and hydrological regime could have been important drivers of spatial heterogeneity in paleo-redox conditions across the European Epicontinental Shelf.

## Introduction

The end-Triassic mass extinction (~201.5 Ma) was among the most severe biotic crises of the Phanerozoic, resulting in the loss of nearly 20 % of marine animal families (Bambach, 2006; Kiessling et al., 2007; Ogg et al., 2016). Losses were substantial among many groups, especially scleractinian corals, which lost up to 96 % of their genera (Hautmann,

2004). The disappearance of characteristic Triassic fauna closely coincides with an abrupt negative carbon-isotope ($\delta^{13}\text{C}$) excursion in both organic matter and carbonates of up to 5 ‰ (Hesselbo et al., 2002; van de Schootbrugge et al., 2008; Bachan et al., 2012; Bachan and Payne, 2016; Wignall and Atkinson, 2020). The negative $\delta^{13}\text{C}_{\text{org}}$ and $\delta^{13}\text{C}_{\text{carb}}$

excursions are contemporaneous with the emplacement of the Central Atlantic Magmatic Province with highly depleted carbon hypothesized

to originate from either flood basalts or methane hydrate destabilization (Bachan et al., 2012; Hesselbo et al., 2002; Ruhl et al., 2011). This rapid injection was closely followed by enhanced organic carbon burial leading to a prolonged positive excursion into the early Jurassic (van de Schootbrugge et al., 2008; Bachan et

<footnote><sup>a</sup> Department of Earth and Environmental Sciences, Syracuse University, New York, USA</footnote>

<footnote><sup>b</sup> Department of the Geophysical Sciences, University of Chicago, IL, USA</footnote>

<footnote><sup>c</sup> College of Oceanography, Hohai University, Nanjing, China</footnote>

<footnote><sup>d</sup> Department of Earth and Planetary Sciences, Stanford University, California, USA</footnote>

<footnote><sup>e</sup> Biogéosciences, UMR 6282 CNRS, Université de Bourgogne, Dijon, France <sup>t</sup>
Department of Earth Sciences, Utrecht University, Utrecht, the Netherlands <sup>g</sup> School of Earth and Environment, University of Leeds, Leeds, UK</footnote>

<footnote><sup>h</sup> Department of Earth and Marine Sciences, University of Palermo, Palermo, Italy</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: asprow@syr.edu (A.N. Prow-Fleischer).</footnote>

<footnote>https://doi.org/10.1016/j.chemgeo.2024.122530</footnote>

<footnote>Received 15 February 2024; Received in revised form 25 November 2024; Accepted 29 November 2024</footnote>

<footnote>Available online 5 December 2024</footnote>

<footnote>0009-2541/© 2024 Published by Elsevier B.V.</footnote>
al., 2012). Recovery of marine communities was spatially variable, being notably rapid in nearshore communities in NW Europe (Atkinson and Wignall, 2019) but skeletal abundance remained suppressed until the early Sinemurian (~199.4 Ma) in Tethyan carbonates (Kiessling et al., 2007; Singh et al., 2023).

Enrichment of redox-sensitive elements and the restructuring of microbial communities support widespread marine anoxia as a kill mechanism (van de Schootbrugge et al., 2008; Wignall and Atkinson, 2020; He et al., 2020; Beith et al., 2021; Onoue et al., 2022). Under-standing the extent and degree of deoxygenation in the shallow ocean (<100 m), where most organisms live, is essential to assessing the ecological impacts of the event. The western Tethys European Epicon- tinental Shelf (EES) displays a gradient in redox state with greater anoxic development between pre- and during-extinction intervals in the northern region compared to the eastern margin (Fig. 1c). While fewer studies have examined the redox patterns on the ocean margin, shallow water redox proxies suggest heterogeneous spatial patterns in redox development thus raising questions about whether some have been diagenetically altered or if they record original spatial variation.

In this study, we focus on two locations: Mount Sparagio in Sicily and

Val Adrara in the Lombardy Basin of the southern Italian Alps (Fig. 1b).

The peritidal carbonate succession at Mount Sparagio, in the south-eastern region of the EES, records an abrupt positive shift in carbonate- associate sulfur isotope ($\delta^{34}\text{S}_{\text{CAS}}$) ratios coincident with the extinction

interval, indicating a rapid global drawdown of isotopically heavy sulfur into euxinic sediments (Fig. 2d; He et al., 2020). This $\delta^{34}\text{S}_{\text{CAS}}$ signature roughly coincides with a negative excursion in carbonate-phase $\delta^{238}\text{U}$,

another indicator of global anoxic bottom water extent, recorded in the shallow carbonate ramp deposits at the Val Adrara (Fig. 3d; Jost et al., 2017). I/Ca ratios, a proxy for local to regional subsurface hypoxia, have been measured at both sites (He et al., 2022; Singh et al., 2023). The relative sensitivities of each proxy to alteration, in addition to both sites having I/Ca measurements, allow for a direct comparison of the diagenetic factors affecting the geochemical signals at these sites.

While both Val Adrara and Mount Sparagio record signals of enhanced global extent of anoxic settings during the positive $\delta^{13}\text{C}$

excursion, the local seawater conditions, as indicated by I/Ca (or 1/(Ca Mg)), are distinct (He et al., 2022; Singh et al., 2023). Background I/Ca values prior to the positive excursion at Mount Sparagio are elevated relative to Val Adrara (Figs. 2 and 3). During the excursion, I/Ca values decreased to near zero at Val Adrara.

At Mount Sparagio, the highly variable background I/Ca values only reduce to between 1 and 2 µmol/mol, higher still than the pre-excursion values at Val Adrara. We explore whether the discrepancy in local expressions of the I/Ca signal along the

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

**Fig. 1.** Paleogeographical context of EES during the Late Triassic Rhaetian stage (~205 Ma). (a) Paleogeographic reconstruction of Pangaea, highlighting the location of the EES (box) and approximate location of modern-day Italy. (b) Study sites on map of modern Italy modified from Bachan et al. (2012). (c) Map of the northwestern Tethys epeiric sea. Arrows indicate proposed circulation pathways accounting for speculative bathymetry and emerged landmasses, and arrow colour distinguishes between cool water (blue) and warm water (red). The thickness of the arrow indicates the current strength. Dotted lines are proposed flow paths modified from Ruvalcaba Baroni et al. (2018) with considerations based on CESM 1.2.2 simulations for the Triassic-Jurassic and Rhaetian configurations (Scotese, 2014). Modern-day geographic coordinates were rotated to paleo-coordinates using the R package Chronosphere (version 0.6.1) with the Scotese PALEOMAP reconstruction (Kocsis and Raja, 2023). The study sites (1 and 2) are in red text whereas other studied localities in the region are in black text (3–9). IB: Iberian Mountains, AM: Amorican, BM: Bohemian Massif, CSH: Corsican-Sardinian High. 1: Mount Sparagio, Italy (Todaro et al., 2018), 2. Val Adrara, Italy (van de Schootbrugge et al., 2008), 3. Kuhjoch, Austria (Li et al., 2022b), 4. Rosswinkel, Luxembourg (Richoz et al., 2012), 5. Mingolsheim, Germany (Luo et al., 2018), 6. Mariental, Germany (Richoz et al., 2012), 7. Filixkirk, United Kingdom (Beith et al., 2021), 8. St. Audrie's, United Kingdom (Li et al., 2022b), 9. Doniford Bay, United Kingdom (Paris et al., 2010). See Table S1, supplementary materials for literature summary of redox conditions at these sites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Chemo-stratigraphy and redox trends at Mount Sparagio (modern coordinates: 38°3′44.18″ N, 12°43′9.19″ E). The grey bar indicates the extinction interval. Vertical purple and red lines indicate range of observations for megalodontoids bivalve and microfossil *T. parvoesiculifera*. Solid lines trending with (a) carbon- and (b) oxygen isotope ratios are loess-smoothing regressions. (c) I/Ca values showing pre- (3.5 µmol/mol), during- (1.4 µmol/mol) and post-extinction (1.6 µmol/mol) interval averages. (d) published $\delta^{34}\text{S}_{\text{CAS}}$ (He et al., 2022), (e) $\delta^{238}\text{U}$ (He et al., 2020), and (f) new $\delta^{44}\text{Ca}$ data with lines connecting points to highlight trends. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

northern and southern EES margins reflects diagenetic overprinting, differences in original mineralogy, or contrasting primary redox conditions.

Calcium isotope ratios ($^{44}\text{Ca}/^{40}\text{Ca}$, commonly reported as $\delta^{44}\text{Ca}$) are a versatile tool for constraining post-depositional processes, such as dolomitization or recrystallization (Fantle et al., 2020), and for finger-printing the effects of different primary carbonate minerals (Higgins et al., 2018). At low-temperature, non-hydrothermal conditions, kinetic isotope effects are believed to dominate, with important rate-dependent fractionation of Ca isotopes into different calcium-bearing mineral phases (Tang et al., 2008). Under slow precipitation rates, such as may occur during authigenic or diagenetic carbonate precipitation, no appreciable fractionation may be observed (Fantle and DePaolo, 2007). Thus, $\delta^{44}\text{Ca}$ can be used to track the diagenetic history of carbonates. In addition, calcite and aragonite exhibit different fractionations (Gussone et al., 2005), which makes $\delta^{44}\text{Ca}$ a potentially useful indicator of primary carbonate mineralogy.

Here, we use $\delta^{44}\text{Ca}$ to identify modes of diagenesis (fluid vs. sediment buffered), mineralogical shifts and carbonate-source mixing as factors controlling the preservational state of existing carbonate-based redox proxy data ($\delta^{238}\text{U}$, $\delta^{34}\text{S}_{\text{CAS}}$, I/Ca). We first investigate the presence of co-variability of $\delta^{44}\text{Ca}$ with other elemental components (Sr/Ca, Mg/Ca, Mn/Sr) considered diagnostic of different diagenetic and source-mixing regimes. These analyses allow us to test the robustness of the redox gradient in the EES margin. Last, we discuss potential drivers of the spatial redox pattern in the EES based on simulations conducted with the Community Earth System model (CESM 1.2.2) by Li et al. (2022a).

## Methods and materials

### Overview of carbonate-based redox proxies

The variations in seawater $\delta^{34}\text{S}$ and $\delta^{238}\text{U}$ compositions are controlled by the burial fluxes under reducing conditions and the size of the seawater reservoir (Bottrell and Newton, 2006; Chen et al., 2021). Microbial sulfate reduction occurring under anoxic conditions imparts a large negative fractionation in the available sulfate pool by preferential incorporation of the lighter $^{32}\text{S}$ isotope into sulfide minerals and organic matter produced by these microbes, thus leaving the remaining seawater sulfate pool enriched in $^{34}\text{S}$ (Kampschulte and Strauss, 2004; Sim et al., 2011).

The reduced form of uranium ($U^{4+}$) is less soluble than the oxidized form ($U^{6+}$). The preferential removal of $^{238}\text{U}$ in anoxic settings leads to its depletion in the seawater pool (Zhang et al., 2020). Consequently, the isotopic signatures of both $\delta^{238}\text{U}$ and $\delta^{34}\text{S}_{\text{CAS}}$ recorded in carbonate

respectively decrease and increase in response to expanding seafloor anoxia. The magnitudes of their isotopic offsets from seawater values are often considered to reflect the global extent of anoxia (Fike et al., 2015; Zhang et al., 2020). Iodate ($IO_3^−$), the oxidized form of iodine, can be incorporated into the
calcite lattice whereas the reduced form iodide (I⁻) is excluded (Lu et al., 2020). Iodate is rapidly reduced under low-oxygen conditions, so low I/Ca (or I/(Ca Mg)) likely reflects carbon-ate accumulation beneath dysoxic-anoxic water masses (Lu et al., 2020).

## Study sites and geological setting

Approximately centered at 30°N paleolatitude, the EES was a warm, semi-restricted collection of silled basins and carbonate platforms during the early Mesozoic. Fragmentation of Pangaea permitted limited

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Chemostratigraphic trends at Val Adrara (modern coordinates: 45°43′29.33″N 9° 57′32.29″E). The grey band indicates the extinction interval. Blue vertical bars indicate occurrences of benthic organisms. (a) δ¹³Cₚcarb and (b) δ¹⁸O (van de Schootbrugge et al., 2008) with loess-smoothing highlighting points that deviate from overall trends. (c) I/Ca with pre- (0.36 µmol/mol), during- (0.08 µmol/mol) and post- (0.23 µmol/mol) extinction averages (Singh et al., 2023). Error bars for (d) δ²³U (Jost et al., 2017) and (e) new δ⁴⁴Ca are 2σ. New calcium isotope measurements (circles) are connected by a line, whereas Jost et al.'s (2016) (triangles) are not connected to show isotopic offset between datasets. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

water exchange between the Boreal, Tethys, and the Panthalassa Ocean to the west (Fig. 1), although the precise timing of connection and amount of exchange in the Late Triassic are poorly constrained (Dore, 1991; Sha, 2002; Korte et al., 2015). Italy was mostly submerged in the southeastern region of the EES during the Triassic-Jurassic boundary interval, but intermittent subaerial exposure is evident in both Sicilian and Tuscan sections (Santantonio and Carminati, 2011).

## Stratigraphy of Mount Sparagio

The Ionian region of the southwestern Tethys margin (~15–20°N) was characterized by a wide carbonate shelf with extensive reef rimmed lagoons the interiors of which were transitional to evaporitic (sabkha-like) environments (Iannace and Frisia, 1994; Muttoni et al., 2015; Todaro et al., 2022). Shallow water carbonate deposition continued until the early Hettangian when the platform drowned. The Rhaetian to lower Hettangian succession at Mount Sparagio was deposited in a peritidal setting with intermittent subaerial exposure (Todaro et al., 2018). The studied section at Mount Sparagio is subdivided into three informal units A, B, and C, on the basis of microfossils association (Todaro et al., 2017, 2018). Units A (below the studied section) and B are assigned to Rhaetian age based on the co-occurrence of mega-lodontoids bivalves and the benthic foraminiferan *Triasina hantkeni*. Unit B records a reduction of the megalodontoids assemblage both in size and diversity while the micro-benthic community survives as in Unit A (Todaro et al., 2023). Units A and B are characterized by the stacking of shallowing-upward cycles capped by red paleosols. There is an approximately 10 m-thick barren interval capping unit B, concurrent with the onset of anoxic signals at Mount Sparagio (He et al., 2020; He et al., 2022). Unit C is dated to the early Hettangian due to the absence of *T. hantkeni* and the occurrence of *Thaumatoporella parvovesiculifera* microfossils (*incertae sedis*), which originated in the Upper Triassic.

(Schlagintweit, 2013; Todaro et al., 2018). Unit C was deposited in the same peritidal setting and paleosols are represented by green-marly horizons (Todaro et al., 2017).

Todaro et al. (2018) measured a ~3.5 ‰ negative excursion in the δ¹³Cₚcarb recorded at Mount Sparagio that was coincident with the disappearance of megalodontoids bivalves in Unit B and was followed by a protracted positive excursion continuing through Unit C. We reevaluated δ¹³Cₚcarb and δ¹⁸O due to some uncertainties in the correlation between the two study sites by using different samples (Fig. 2a, b) that also span and additional ~50 m higher in the section. In addition, a moderate positive correlation (R² 0.57) between δ¹³Cₚcarb and δ¹⁸O at Mount Sparagio was not viewed as evidence for meteoric diagenesis by Todaro et al. (2018), but rather a consequence of the covariation of both isotopic systems in response to warming. He et al. (2020) reported two consecutive >10 ‰ positive δ³⁴SₚCAS shifts coinciding with the mega-lodonitoids
extinction (Fig. 2d). He et al. (2022) measured a stepwise

decrease in I/Ca at the onset of the extinction interval suggesting a decrease in localized oxygen saturation state but not the development of fully anoxic conditions (Fig. 2c). Together these trends were attributed to global expansion of oxygen minimum zones impinging on shelf areas increasing pyrite and organic matter burial; whereas peritidal environments like Mount Sparagio may have remained oxic such that the low I/ Ca reflects regional depletion of the iodate pool into underlying anoxic water masses. However, while the fluctuations in pre-excursion I/Ca values were proposed to be due to periodic shallowing, diagenesis could not be excluded as a potential factor for the lower-than-average pre-excursion values.

## Stratigraphy of Val Adrara

The Alpine-Tethyan realm is dominated by platform to ramp depositional environments with locally dolomitized lithofacies (Jadoul et al.,

1992). The sedimentary sequences at Val Adrara, in the Southern Italian Alps Lombardy Basin, record deposition on a passive-margin carbonate ramp (Santantonio and Carminati, 2011) and are assigned Rhaetian through Hettangian ages based on palynological associations and carbon-isotope stratigraphy (Fig. 3; Galli et al., 2007; van de Schootbrugge et al., 2008; Bachan et al., 2012). The lowermost Rhaetian-age Zu Limestone consists of an alternation between thinly bedded mulluscan-rich wackestones and meter-thick mulluscan-brachiopod-rich packstone within a coralline framework representative of shallow- ing upward cycles (McRoberts, 1994). The open marine fossil assemblages and microfacies suggest an unrestricted lagoonal to inner shelf environment. The Zu Limestone is sharply overlain by the Malanotte Formation, which is devoid of fossils and is interpreted as the extinction interval (Fig. 3; Jadoul and Galli, 2008; Bachan et al., 2012). The lower

~3 m of the Malanotte Formation consists of thinly bedded micritic

limestone (marl), which grades into carbonate-rich mudstones and wackestones (Jadoul and Galli, 2008). A 5.0 ‰ negative excursion in δ¹³Cₚcarb coincides with the extinction interval and occurs entirely within

the lower silty-marl portion of the Malanotte Formation (van de Schootbrugge et al., 2008; Bachan et al., 2012). However, neither site has been absolutely dated, so the timing and duration of this interval is poorly constrained (Bachan et al., 2012; Todaro et al., 2018). Therefore, in both stratigraphic columns, the extinction interval is not assigned to either the Rhaetian or Hettangian and is broadly referred to as the Triassic-Jurassic boundary interval (Figs. 2 and 3). The negative excursion at Val Adrara is followed by a small ~1.5 ‰ positive excursion terminating at the top of the Malanotte Formation (Bachan et al., 2012). Jost et al. (2017) reported a negative uranium isotope excursion

reaching a minimum of ~0.63 ‰ and coincidentally returning to baseline with δ¹³C in the overlying section. The negative δ²³U excursion in the underlying Zu limestone (Fig. 3d) was regarded as being due to either a local effect or alteration because a similar shift was not observed in the nearby Italcementi section.

The overlying Albenza Formation is a > 100 m sequence of thinly bedded (0.5 to 2 m) cycles of cross-stratified oolitic shoal and restricted lagoonal peloidal grainstones facies interbedded with cm-scale dolo-micrite layers (McRoberts, 1994; Bachan et al., 2012). There are no microfacies evidence of supratidal conditions. Instead, the unusual abundance of ooids and marine cements on a progradational ramp-style margin may reflect variation in carbonate saturation state (Jadoul and Galli, 2008).

Another pulse of volcanism is inferred from a second 4.0

‰ negative δ¹³Cₚcarb excursion at the base of the Albenza Formation, and the ~100 m protracted positive carbon-isotope excursion terminates at the top of the Albenza Formation (Bachan et al., 2012; Bachan and Payne, 2016). Evidence of expanded global anoxia commences during this positive excursion and may have suppressed faunal recovery in this region (Singh et al., 2023). The Albenza Formation is sparsely fossiliferous with skeletal grains only becoming abundant again at the Albenza-Sedrina contact (Singh et al., 2023). The overlying Sedrina Formation marks the transition back to open ocean conditions consisting of inter-bedded peloidal packstone and brachiopod wackestone at the base that grades into a thinly bedded cherty limestone with abundant sponge spicules (McRoberts, 1994; van de Schootbrugge et al., 2008; Singh et al., 2023). The upper Sedrina Formation is interpreted to have been deposited below storm-wave base following rapid subsidence due to the onset of extensional activity in the Hettangian (Santantonio and Carminati, 2011).

van de Schootbrugge et al. (2008) measured organic and inorganic

δ¹³Cₚcarb and δ¹⁸O at Val Adrara and noted that some light C-isotope points deviate from the overall trend (Fig. 3a) are also characterized by light O-isotope values, especially during the negative excursion in the lower Malanotte and Albenza Formations. But in the absence of a correlation between δ¹³Cₚcarb and δ¹⁸O and because δ¹³Cₚcarb smoothly crosses lithofacies these may be interpreted as an original signal. Jost

et al. (2016) measured δ⁴⁴Ca through the lower 100 m of Vala Adrara which records a ~0.8 ‰ negative excursion (Fig. 3e). Coupled

carbon-calcium isotope cycle box modeling estimated that 20 % of this excursion may be attributed to ocean acidification while 80 % was due to an increased proportion of aragonite deposition, consistent with higher Sr concentration in the basal Malanotte Formation (Fig. 5h). Jost et al. (2017) measured δ²³U spanning most of the section and reported a negative excursion coincident with the negative δ¹³C excursion and

persisting through the protracted positive excursion (Fig. 3d). The magnitude of the δ²³U excursion is consistent with a 40 to 100 increase in global anoxic extent. Singh et al. (2023) measured I/Ca at Val

Adrara and found that values averaged 0.36 µmol/mol before the extinction and dropped near zero with high variability during the positive excursion. Because there is no correlation with lithology or other elemental ratios, this was interpreted to reflect fluctuating water column anoxia. The lack of statistically significant correlations between isotopic and elemental ratios examined in each study has been suggested to indicate little modification by meteoric or burial diagenesis at Val Adrara. But van de Schootbrugge et al. (2008) noted the implicit uncertainties when using cut-off values as a sole determination of diagenetic alteration.

## Sample preparation and analysis

Calcium-isotope data on different powered samples from the upper Zu Limestone and Malanotte Formation were previously reported by Jost et al. (2016). These data bracket the negative carbon isotope

excursion, but only include the rising limb of the subsequent positive excursion (Fig. 3). We report bulk carbonate δ⁴⁴Ca and trace elemental (Sr, Mn, Mg) concentrations data that span the full stratigraphic range for both sites (Figs. 2–5). We also measure carbon- and oxygen-isotope compositions for new samples from Mount Sparagio (n = 52).

## Carbon and oxygen isotope measurements

For samples from the Mount Sparagio section, about 10 mg of carbonate powder at different stratigraphic heights than Todaro et al. (2018), were drilled from
the cleaned rock slab using a hand drill. Powder was reacted with 100 % H₃PO₄ at 70 °C to extract CO₂ for ¹³C/¹²C and ¹⁸O/¹⁶O measurements. We used the Chinese national

standard, an Ordovician carbonate from a site near Beijing (reference number GBW04405, δ¹³C 0.57 0.03 ‰ VPDB; δ¹⁸O 8.49 0.13 ‰ VPDB) to monitor the accuracy. These measurements were performed using a Finnigan MAT 253 mass spectrometer at the Nanjing Institute of Geology and Palaeontology.

Calcium isotope ratios analysis

Measurement of carbonate δ⁴⁴Ca values at the University of Chicago

closely followed the method of Razionale et al. (2022). Briefly, carbonate samples were dissolved in buffered acetic acid at a ratio of ~1 mg/mL and Ca²⁺ was chromatographically isolated using a Dionex ICS-

6000 with variable strength methyl-sulfonic acid (MSA) eluent. Samples were matrix matched at 1 ppm Ca²⁺ in 2 % nitric acid for sample- standard bracketing and each post-chromatography eluent was

measured twice by Neptune XT as 90 integration cycles monitoring masses 42, 43, 43.5, and 44. Outlier cycles exceeding 3σ for the ⁴⁴Ca/⁴²Ca, ⁴⁴Ca/⁴³Ca, or ⁴³Ca/⁴²Ca intensity ratios were removed, as were entire sets of cycles when the ⁴⁴Ca/⁴²Ca ratio of successive bracketing standards changed by >0.2 ‰. Following a correction for strontium interference and a run-specific concentration effect, values of δ⁴⁴/⁴²Ca were corrected to NIST 915b and scaled to δ⁴⁴/⁴⁰Ca ratios using a kinetic mass law (Young et al., 2002) and assuming no radiogenic ⁴⁰Ca-excess. Samples are reported relative to seawater by setting the δ⁴⁴/⁴⁰Ca value of 915b equal to 1.15 ‰. Measurements were only considered successful if the two final δ⁴⁴/⁴⁰Ca determinations on the same post-column solution were within 0.2 ‰. Accuracy and precision were confirmed through measurement of δ⁴⁴/⁴⁰Ca values in Princeton University Aragonite (PUA), measured as -1.42 ± 0.17 ‰ (2σ, n = 30), and of San Salvador Sand (SSS), measured as -1.31 ± 0.15 ‰ (2σ, n =

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Plot of δ¹³C versus δ¹⁸O at Mount Sparagio (closed squares) and Val Adrara (open circles) colored by lithology. Regions of isotope ranges for different types of primary and diagenetic carbonate from Hudson (1975).</figcaption></figure>

28). The precision of 915b measurements ( 0.16 ‰, 2σ, n 40) agreed with these reference materials; furthermore, for samples chromatographically isolated at least twice (36/49), two times the mean standard deviation in replicate δ⁴⁴/⁴⁰Ca values was 0.15 ‰. Precision for each sample is reported as the largest of either replicate measurements or the mean reproducibility of 915b, PUA, and SSS (±0.16 ‰, 2σ).

Trace element concentrations

A suite of minor- and trace-element abundances were measured using a Thermo Scientific iCAP RQ inductively-coupled plasma mass spectrometer (ICP-MS) at the University of Chicago, following a standard geochemical protocol as described in (Bryant et al., 2022). In brief, small aliquots of the dissolved samples were subsampled, adjusted to approximately 10 ppm Ca in 2 % HNO₃ prior to analysis to mitigate matrix effects, and spiked with an internal standard solution containing Sc, Y, and In. At the beginning of each run, a calibration curve was generated for each element to calculate elemental concentrations.

correlation of R² 0.604 in the new data (Fig. 4).

At Val Adrara prior to the onset of the positive carbon-isotope excursion, δ⁴⁴Ca composition steadily decreases from 1.0 ‰ in the upper Zu Limestone to a minimum of 1.65 ‰ in the Malanotte Formation in our new dataset (Fig. 3, Table S3). Calcium isotope ratios were measured for two samples at the same stratigraphic heights at Val Adrara as Jost et al. (2016) with one at 20 m in the Zu Limestone and the other at 67.2 m in the Malanotte Formations. Both have a systematic offset of about 0.14 ‰ with these new data being more positive. A shift
toward lower δ<sup>44</sup>Ca is observed in the Malanotte Formation in both datasets (Fig. 3). During the positive δ<sup>13</sup>C excursion in the Albenza Formation, δ<sup>44</sup>Ca values
are highly variable between 1.72 ‰ and

1.20 ‰. A gradual trend toward more positive δ<sup>44</sup>Ca values, ranging

from 1.12 ‰ to 0.98 ‰, occurs at the lithological change from oolitic grainstone to fossiliferous packstone in the upper Albenza Formation.

variations in δ<sup>13</sup>C and δ<sup>18</sup>O values of calcite from the end of the

4

Calibration standards, calibration blanks, and duplicate samples were
enriched relative to Val Adrara. The δ Ca values begin to fluctuate

analyzed intermittently throughout each run for quality control. V, Cr, Mn, and Fe ratios are reported from analytical runs in Kinetic Energy Discrimination (KED) mode, while Sr, U, and Mg ratios are reported from analytical runs in Standard (STD) mode. Data are reported as molar ratios to Ca Mg. In addition, Mg/(Mg Ca) ratios were independently measured via ion chromatography during chromatographic separation of calcium, as outlined above.

**Results**

The newly measured δ¹³C values at Mount Sparagio show similar trends compared to Todaro et al. (2018). As no-repeat analyses were
performed, there is only a slight apparent positive offset (< 0.2 ‰; Fig. S1) in the new data. The covariation in δ¹³C and δ¹⁸O reported by
Todaro et al. (2018) of R² = 0.572 is corroborated by a slightly higher

rapidly with a spread from 1.10 ‰ to 0.40 ‰ at the same time as δ³⁴S_CAS increases and I/(Ca Mg) decreases. The carbon isotopic composition of Unit B at Mount Sparagio is highly variable, with δ¹³C values ranging from 3.15 ‰ to 2.46 ‰ (Table S2). A positive trend emerges approximately 50 m below the inferred extinction interval at

the top of Unit B and continues through the end of the record, peaking at a value of 3.04 ‰. A corresponding positive trend is also observed in the δ¹⁸O record.

Minor- and trace-element concentrations at Mount Sparagio are relatively uniform throughout except for the Triassic-Jurassic boundary interval where Mg, Sr, Mn, U, and V concentrations, all normalized to (Ca + Mg), increase by almost an order of magnitude then decrease to pre-excursion values (Fig. 5). There is a step decrease in Cr/(Ca + Mg) from an average of ~5.5 µmol/mol to 2.5 µmol/mol at the Triassic- Jurassic boundary. At Val Adrara, Mg/(Ca + Mg) ratios remain lower
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Log-scaled (a, b) minor- and (c, d, e, f) redox-sensitive trace-elemental molar ratios normalized to Ca + Mg content at Mount Sparagio and Val Adrara (g-l).</figcaption></figure>

than 0.1 mol/mol throughout with only one value at ~175 m in the Albenza Formation that is elevated in both Mg/(Ca Mg) (~0.4 mol/mol) and Mn/(Ca Mg) (~0.3 mmol/mol) (Fig. 5). Redox sensitive trace elements Mn, V, and Cr decrease at the Triassic-Jurassic boundary interval in the Malanotte Formation. V and Cr/(Ca Mg) remain low for the lower 100 m of the Albenza Formation and then begin trending toward pre-excursion values. Mn/(Ca Mg) remains low until the Sedrina Formation. Sr/(Ca Mg) are overall higher than at Mount Sparagio (~1 mmol/mol) with some elevated values (~3 mmol/mol) during the Malanotte Formation. There is a significant positive correlation (Pearson *r* 0.72, *p* < 0.001) between Mn/Sr and I/Ca ratios at Val Adrara (Fig. S2) and low to moderate positive correlation at Mount Sparagio (*r* = 0.35, *p* < 0.01).
Here we integrate published geochemical datasets with newly measured δ<sup>44</sup>Ca values to constrain the impact of sedimentary processes, mineralogy, and diagenesis on these three (δ<sup>238</sup>U, δ<sup>34</sup>S<sub>CAS</sub>, I/Ca)redox

proxy signals. By building a framework of geochemical tracers with differential responses and sensitivities to diagenetic regimes (Table 1), potential influences of lithification processes on primary redox signals can be teased apart. In terms of I/Ca, we aim to investigate (1) whether the deoxygenation trend observed at both sites is the result of diagenetic loss of carbonate-associated iodate; and (2) whether the overall low I/Ca at Val Adrara could be the result of more severe diagenesis compared to Mount Sparagio.

## Review of diagenetic redox proxies

Table 1 summarizes the predicted directional changes in isotopic and elemental compositions of a primary carbonate initially formed under normal-marine conditions undergoing one of two modes of diagenesis (sediment- and fluid-buffered) and one of four fluid compositions (meteoric, seawater, evaporitic brine, and reducing pore fluid). The table is structured in this manner to highlight contrasting susceptibilities to alteration under different diagenetic environments of non-stoichiometric redox-sensitive elements compared to that of major constituents. Covariation between two or more paleo-proxies can elucidate the complex post-depositional history of the sediment and final delity of the original signal (Fantle et al., 2020).

## Assessing possible diagenetic influence on redox proxies

At Mount Sparagio, there is little covariation between δ<sup>44</sup>Ca and δ<sup>34</sup>S<sub>CAS</sub> or I/Ca (Fig. 6c, d, and e). The δ<sup>44</sup>Ca values at Mount Sparagio are relatively high and a few points are elevated with respect to Mg/Ca (> 0.5 mol/mol) or Mn/Sr (> 2.0 mol/mol), especially during the

### Table 1

Summary of the expected response of tracers under different conditions.

excursion, but there are no systematic correlations to indicate that dolomitization or reducing fluids significantly impacted the redox signals. Two out of five samples during the δ<sup>34</sup>S<sub>CAS</sub> excursion interval show elevated Mn/Sr and Mg/Ca, and slightly higher δ<sup>44</sup>Ca. These two δ<sup>34</sup>S<sub>CAS</sub> values may have been post-depositionally elevated by reducing pore fluids, whereas other samples from the δ<sup>34</sup>S<sub>CAS</sub> excursion appear to be

better preserved. At Val Adrara, the low δ<sup>238</sup>U values during the excursion, indicative of expanded global anoxia, have low Mg/Ca, Mn/Sr, and δ<sup>44</sup>Ca (Fig. 6b and 4f). Neither the new dataset nor that of Jost et al. (2016) reveals any covariation between δ<sup>238</sup>U and δ<sup>44</sup>Ca, suggesting minimal diagenetic alteration of the δ<sup>238</sup>U signal. While the δ<sup>44</sup>Ca data reported by Jost et al. (2016) in the lower part of Val Adrara is offset slightly from the new data and appears to have different trends, we attribute this to differences in sampling resolution where Jost et al.'s (2016) higher resolution reveals more variation than our single point that spans the TJ boundary interval. But because the two datasets map onto the same δ<sup>238</sup>U and δ<sup>44</sup>Ca spaces pre- and post-extinction we don't interpret the difference to be substantial.

The decreases in I/Ca from the background level into the excursion interval at both sites are not associated with any systematic increase in Mg/Ca, Mn/Sr, or δ<sup>44</sup>Ca (Fig. 6c,f). Only one value at each site during the excursion interval is considerably elevated enough in either Mn/Sr or Mg/Ca to suggest potential alteration.

Because there is no process currently known to elevate I/Ca (Hardisty et al., 2017), the highly variable I/Ca values at Mount Sparagio prior to the excursion may be due to either (1) increased oxygenation during shallowing upward cycles that increased I/Ca, as suggested by He et al. (2022), or (2) the low δ<sup>44</sup>Ca values may indicate that I/Ca was reduced from high (> 4 µmol/mol) primary values by meteoric fluid-buffered diagenesis. There is a moderate positive correlation between Mn/Sr and I/Ca at both sites, suggesting limited, but plausible, influence of meteoric fluid advection, which could lower I/Ca if the fluids were reducing (Fig. S1). At Mount Sparagio, the distinct separation of the lower fossiliferous limestone units from the excursion interval (marly mudstone) in δ<sup>13</sup>C versus δ<sup>18</sup>O space (Fig. 4) and the geochemical overlap of only a few samples in the lower unit with that of freshwater cements does not support the hypothesis (2) that I/Ca variability is due to loss from meteoric fluids. This is corroborated by the cross plot of δ<sup>13</sup>C versus δ<sup>44</sup>Ca where few samples fall within the window of meteoric fluid diagenesis (Fig. 7).

At Val Adrara, there is a trend toward higher Mn/Sr values with greater seawater-buffered diagenesis indicated by increasing δ<sup>44</sup>Ca, but these data also have higher I/Ca values. There is no evident relationship between the scatter in Mn/Sr vs. I/Ca data and fluid-buffered diagenesis as indicated by δ<sup>44</sup>Ca. Thus, the large pre-excursion variability at Mount Sparagio could be robust and fluid-buffered diagenesis may not have affected the redox signal. However, the lack of co-variation between these geochemical tracers is not grounds for the exclusion of a diagenetic

<table><thead><tr><th rowspan="2">Process / Elemental ratios</th><th colspan="4">Non-redox sensitive</th><th colspan="4">Redox sensitive</th></tr><tr><th>Major isotopes</th><th colspan="3">Minor-element ratios</th><th colspan="3">Trace isotopes</th><th colspan="2">Minor/Trace-element ratios</th></tr></thead><tbody><tr><td>Style</td><td>Environment</td><td>δ<sup>13</sup>C</td><td>δ<sup>18</sup>O</td><td>δ<sup>44</sup>Ca</td><td>Mg/Ca</td><td>Sr/Ca</td><td>δ<sup>238</sup>U</td><td>δ<sup>34</sup>S<sub>CAS</sub></td><td>I/Ca</td><td>Mn/Sr</td></tr><tr><td rowspan="4">Fluid- buffered</td><td>Meteoric (oxic)</td><td>↓</td><td>↓↓</td><td>- or ↓</td><td>↓↓</td><td>↓</td><td>↑</td><td>-</td><td>- or ↓</td><td>↑</td></tr><tr><td>Seawater (oxic)</td><td>-</td><td>-</td><td>↑</td><td>-</td><td>↓</td><td>-</td><td>-</td><td>-</td><td>↑</td></tr><tr><td>Reducing pore fluids</td><td>↓</td><td>-</td><td>-</td><td>-</td><td>-</td><td>↑↑</td><td>↑</td><td>↓</td><td>↑↑</td></tr><tr><td>Dolomitization (evaporitic brine)</td><td>-</td><td>↑</td><td>↑↑</td><td>↑↑</td><td>↓↓</td><td>↓</td><td>-</td><td>↓</td><td>↑</td></tr><tr><td>Sediment- Buffered</td><td>Closed System</td><td>-</td><td>↓</td><td>-</td><td>-</td><td>↓</td><td>-</td><td>↑</td><td>↓</td><td>↑</td></tr></tbody></table>

Symbol key: Up arrow (↑) indicates isotopically heavier or more positive. Down arrow (↓) indicates isotopic lighter or more negative. The number of arrows indicates the sensitivity of the proxy to that process/mode of diagenesis. A dash (-) indicates that the proxy is essentially unaffected by the process. Reducing pore fluids can originate from either meteoric or seawater.

Sources: δ<sup>13</sup>C and δ<sup>18</sup>O (Oehlert and Swart, 2014; Ahm et al., 2018; Swart and Oehlert, 2018; Reis et al., 2019), δ<sup>44</sup>Ca (Fantle and Higgins, 2014; Kimmig and Holmden, 2017; Higgins et al., 2018); Mg/Ca (Higgins and Schrag, 2006; Stewart et al., 2015); Sr/Ca (Stewart et al., 2015; Ahm et al., 2018; Higgins et al., 2018); δ<sup>238</sup>U (Romaniello et al., 2013; Chen et al., 2018; Lau
<figure><img src="image_8.png" /></figure>

**Fig. 6.** Cross-plots of δ⁴¹Ca against carbonate-based redox proxies. The top row (panels a-c) is colour-coded by the Mg/Ca ratio, indicative of the degree of dolomitization, where a maximum value of 1.0 mol/mol on the colour scale is stoichiometric dolomite. The bottom row (panels d-f) is colored by Mn/Sr ratio, indicative of reducing pore fluids, where a higher value, up to 2 mol/mol, suggests a greater degree of anoxic development. Panels a and d comprise of δ³⁴S_CAS data from Mount Sparagio only, panels b and e are δ²³U data from Val Adrara only, and panels c and f contain I/Ca data from both sites. The dotted boxes encompass the approximate range of values during the positive carbon excursion. The solid grey boxes encompass the approximate range of background (pre- and post-excursion) values. The small schematic within each panel (its placement not indicative of starting composition) illustrates the expected trajectory of geochemical evolution for that proxy.

due to dolomitization (brown dotted arrow, top only), increasing local or global anoxic conditions (red dotted arrow), reducing pore fluids (blue dotted arrow, bottom only) and recrystallization from primary carbonate under fluid-buffered conditions (black solid arrow). The magnitude of the arrow does not indicate the relative sensitivity of the proxy to that process. Note that while the simplified schematics are linear, the trajectories of geochemical evolution from the primary signal are often curvilinear due to complex exchange processes and variable fluid-rock ratios over time, which affects the degree of fractionation in isotopic systems. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

influence on the redox signal; therefore, we continue the investigation with another approach.

## Carbonate mineralogy

We can further disentangle the diagenetic history and identify the original carbonate source materials of these sites by plotting δ⁴¹Ca against other minor elemental (Sr/Ca, Mg/Ca; Fig. 8) and isotopic ratios (δ¹³C, δ¹⁸O; Fig. 7). Approximate end-member ratios for different carbonate mineralogies are given in Table 2.

Samples from Val Adrara largely fall within the Sr/Ca vs. Mg/Ca space for aragonite and high-magnesium calcite (HMC; Fig. 8a) whereas samples from Mount Sparagio have very low Sr/Ca (<1 mmol/mol) and high δ⁴¹Ca throughout.

regardless of lithology, indicative of recrystallization under fluid-buffered conditions (Gussone et al., 2020). At Val Adrara, the marl limestone of the Malanotte Formation is elevated in Sr/Ca ratios relative to other lithologies. The two samples with Sr/Ca greater than 1 mmol/mol at Mount Sparagio also belong to the marl layer within the Triassic.

Jurassic boundary interval (heights 203 m and 209 m; Fig. 2). Jost et al. (2016) recognized a large shift in Sr concentration within the Malanotte Formation, coincident with the negative excursion in δ⁴¹Ca (Fig. 3).

Increases in Sr/Ca ratios are observed at both sites in the new datasets (Fig. 5).

Here we evaluate the covariation in Sr content and δ⁴¹Ca using a simple linear mixing model between aragonite and calcite endmembers, and a plausible recrystallization pathway starting with 100% aragonite.

(Fig. 8b). Using a mass-balance approach, Jost et al. (2016) concluded that up to 80% in the negative δ⁴¹Ca shift at Val Adrara in the Malanotte Formation could be attributed to an increase in aragonite in the original

mineralogy. A greater contribution of aragonite to the original sediment could result from (1) migration of carbonate facies and mixing between deeper-water calcite and shallow-water aragonite sediments and/or (2) a shift in primary production favoring organisms that precipitate aragonitic skeletons (Farkaš et al., 2016). At both Val Adrara and Mount Sparagio, the thin marl layer at the Triassic-Jurassic boundary interval occurs just before an inferred eustatic deepening indicated by a transition to carbonate mud-dominated lithology. Therefore, eustatic changes are unlikely to have increased the proportion of aragonite. Pelagic
cal- cifers did not become abundant enough to be major sediment producers until the Middle Jurassic (Suzuki and Oba, 2015) and are, therefore, not a likely contributor to the change in aragonite observed at Val Adrara. Benthic organisms with aragonitic skeletons may be, on average, more susceptible to ocean acidification and rising carbonate compensation depth than their calcite-producing counterparts (Hautmann et al., 2008); however, only a modest relationship between skeletal miner- alysis and extinction risk has been observed at the Triassic-Jurassic

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Covariation between bulk δ¹³Ccarb and δ⁴Ca at Val Adrara (solid square) and Sparagio (open circles) colored by δ¹⁸O value (Higgins et al., 2018). Dotted boxes are the approximate ranges isotopic composition of Triassic carbonate sources based on the estimated deviation from modern seawater value (grey bar) (Farkas et al., 2007). Regression lines are given for data from Val Adrara (black, solid) and Mount Sparagio (blue, dotted). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** (a) Sr/Ca and Mg/Ca for Val Adrara (closed shapes) and Mount Sparagio (open shapes) colored by lithology on log-log scale. End-member carbonate mineralogy (e.g., dolomite, aragonite) labels placed in the approximate ranges of their compositions. LMC low-magnesium calcite and HMC high-magnesium calcite. (b) Cross-plot of Sr content and δ⁴Ca with a linear mixing model (red solid line) (Farkaš et al., 2017) between aragonite and calcite endmembers in Table 2 relative to Triassic seawater composition of ~0.4 ‰ (Farkas et al., 2007). In the model, aragonite has a Sr concentration of 8000 ppm and δ⁴Ca of ~1.8 ‰. Primary calcite has a Sr concentration of 500 ppm and δ⁴Ca of ~1.5 ‰. Dotted blue line is a possible trajectory for fluid-buffered neomorphism of 100 % aragonite with marine water of Triassic composition based on a model by Lau et al. (2017). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

boundary (Kiessling et al., 2007). Abundant megalodontoids bivalves at Mount Sparagio (Todaro et al., 2018) could have contributed to the slightly higher Sr/Ca values before their extinction (Fig. 5b). Petro- graphic analysis on thin sections at Val Adrara revealed an increase in
abundance of aragonitic calcareous algae within the Malanotte Formation (Singh et al., 2023). A shift in environmental conditions favoring the growth of calcareous algae that disaggregated in fine-grained aragonitic needles could explain both the proportional increase in

**Table 2**

Approximate end-member mineralogical element and isotopic compositions of carbonate precipitated from normal marine conditions.

effects did not completely overprint the primary redox signals pre-and during excursion and that there was a true difference in local conditions between the sections.

<table><thead><tr><th></th><th>Aragonite</th><th>High-Mg Calcite</th></tr></thead><tbody><tr><td>Low-Mg Calcite</td><td></td><td></td></tr><tr><td>Dolomite</td><td>Calcite Cement</td><td></td></tr></tbody></table>

Potential Drivers of contrasting local redox conditions on the Tethys

Sr/Ca (mmol/mol)
Mg/Ca

(mmol/mol) Modern δ<sup>44</sup>Ca (SW, ‰)

<table><tr><td>7–10</td><td>1.5–4</td><td>1–1.5</td><td>&lt;1.0</td><td>&lt;1.0</td></tr><tr><td>2–6</td><td>6–50</td><td>2–4</td><td>100–1000</td><td>0.5–15</td></tr><tr><td>-1.38</td><td>-1.13</td><td>-0.8</td><td>-0.68</td><td>-0.42</td></tr></table>

margin

I/Ca data for Val Adrara and Mount Sparagio suggest the existence of a redox gradient in the EES at the Triassic-Jurassic boundary, with dissolved oxygen decreasing northwards. A literature compilation for

Sources: Sr/Ca and Mg/Ca (Cicero and Lohmann, 2001; Holcomb et al., 2009; Gussone et al., 2016), average δ<sup>44</sup>Ca for modern carbonate platform aragonite sediment and high-Mg calcite from Higgins et al. (2018), low-Mg calcite from (Blättler et al., 2012; Marriott et al., 2004), calcite cement from Erhardt et al. (2020), and dolomite from Ahm et al. (2018).

aragonite at Val Adrara and the texture of the sediment. At Mount Sparagio, two points elevated in Sr/Ca ratio and low in Mg/Ca (Figs. 4, 7a) close to the Triassic-Jurassic boundary suggests this increase in aragonite deposition also occurred here but a closer inspection at Mount Sparagio would be needed to explain the regional extent of this layer.

The mixing of sediments from different carbonate sources (e.g. platform, pelagic, and meteoric cement) can be recognized by a linear covariation between δ<sup>13</sup>C and δ<sup>44</sup>Ca (Ahm et al., 2018; Higgins et al., 2018). At Val Adrara, there is a moderate covariation (Pearson's r

0.70, *p*-value 0.005) between δ<sup>13</sup>C and δ<sup>44</sup>Ca that suggests some mixing between sources, if not a reflection of open-system neo-morphism, but the values stay in the region of marine deposition (Fig. 6). In contrast, there is no correlation in δ<sup>13</sup>C and δ<sup>44</sup>Ca values for samples from Mount Sparagio (Pearson's *r* 0.08, *p*-value 0.56) and

most points fall near the zone of Triassic seawater value (Farkas et al., 2007). There is an apparent non-linear covariation of Sr and δ<sup>44</sup>Ca at Val Adrara (Fig. 8b). Neomorphism of aragonite in seawater fluid-buffered

settings results in loss of Sr that is more rapid than diagenetic reset-ting of δ<sup>44</sup>Ca. None of the values at Val Adrara fall on the mixing curve but the marlstones with higher Sr content generally track the trajectory

of recrystallization from an original sediment with high aragonite content. In contrast, the carbonates at Mount Sparagio are inferred to have been originally calcite that was influenced by marine fluids. High δ<sup>44</sup>Ca

and low Sr content are also common in marine cements (Cicero and Lohmann, 2001; Gussone et al., 2020), which may still capture primary seawater trace metal composition (Erhardt et al., 2020).

The partition coefficient of iodate into the calcium carbonate lattice varies with its polymorphs, incorporating more readily into calcite compared to aragonite, and decreasing with increased dolomite stoichiometry (Feng and Redfern, 2018; Hashim et al., 2022). There is not a substantial amount of stoichiometric (1:1 Mg/Ca) dolomite at either site even in the oolitic dolomite section of the Sedrina Formation at Val Adrara (Figs. 5, 6c, 7a). At Mount Sparagio, the step change to lowered average I/Ca values (1.4 µmol/mol) during the extinction in the basal

part of the TJ boundary unit (Fig. 2c) is coincident with a negative shift in δ<sup>44</sup>Ca (Fig. 2e) and an increase in Mg/(Ca Mg) ratios (Fig. 5a). This

indicates that dolomitization may have been responsible for lowering I/Ca of these few meters compared to the rest of the post-extinction interval average (1.6 µmol/mol). Because calcite has a greater partition coefficient than aragonite, a conversion from calcite to aragonite could exclude iodate. However, this is

unlikely to occur. Because we have evidence for aragonite neomorphism at Val Adrara the recrystallization process itself may exclude a little iodate, but only a statistically significant loss observed between pre- and post-extinction average I/Ca is expected to occur only under reducing conditions (Hardisty et al., 2017). It is impossible to fully separate the potential impacts of aragonite neomorphism at Val Adrara or fluid-buffered calcite diagenesis and primary pre-excursion

redox variations at Mount Sparagio on the variability of I/Ca values. However, we consider it likely that diagenetic the redox conditions before and after extinction for several sites on the

EES confirms that this gradient was of regional significance (Fig. 1c). To investigate the mechanisms leading to differing regional conditions in ocean anoxia, we analyzed the output of the 200 Ma time-slice from the Community Earth System Model (CESM) version 1.2.2 simulation conducted by Li et al. (2022a). While this simulation does not resolve the carbon cycle perturbation nor provides oceanic dissolved oxygen concentrations, the background temperature, precipitation, and surface wind fields may shed light on regional atmospheric and oceanic circulation patterns that may have primed the northern and southern EES for the contrasting palaeoceanographic conditions during the extinction interval.

There are notable differences in the hydroclimatic conditions between the two study sites on the EES (Fig. 9) with Val Adrara being estimated to have received significantly more precipitation (up to 120 mm month<sup>-1</sup>) compared to Mount Sparagio (60–70 mm month<sup>-1</sup>) (Fig. 9a). This result is consistent with conceptual models for the regional circulation patterns throughout the Jurassic period which propose that increased atmospheric CO<sub>2</sub> enhanced the hydrologic cycle at northern latitudes, leading to more freshwater runoff from Laurasia into the restricted northern basins (van de Schootbrugge et al., 2020; Onoue et al., 2022). Density stratification due to an overlying low-salinity water mass may have established shallow-ocean dysoxia at Val Adrara. Deep-water anoxia spilling/upwelling into this shallow location may be an alternative scenario to be tested in future modeling work.
Regional ocean circulation may have reinforced the redox gradient on the EES. Ocean-atmosphere general circulation models simulating this Boreal-Tethys gradient during the Toarcian Ocean Anoxic Event (T- OAE, ~183 Ma) suggest that a limb from an equatorial gyre may have produced strong currents on the southern margin permitting greater vertical mixing compared to the north (Ruvalcaba Baroni et al., 2018). These currents may have been weakened due to rough bathymetry and restriction within the northern EES. Annual wind stress results from CESM simulations for the Triassic-Jurassic boundary indicate that Mount Sparagio was in the path of high oncoming easterly wind bands (Fig. 9b), whereas Val Adrara was positioned in a zone of relatively low wind stress. Based on these patterns of wind stress, regional geography, and anoxic development within the EES water circulation pathways are proposed to explain the spatial pattern of oxygen depletion at the Triassic-Jurassic boundary (Fig. 1c).

## Conclusions

This study reports carbonate Ca isotope ratios from two Italian sec- tions, Mount Sparagio and Val Adrara, that record the end-Triassic mass extinction. Geochemical data are used to assess the role of diagenetic alteration and carbonate mineralogy on primary redox signals. The lack of covariation between diagenetic indicators ($\delta^{44}$Ca, Mg/Ca, Mn/Sr) and redox proxies (I/Ca, $\delta^{34}$S$_{CAS}$, and $\delta^{238}$U) suggests that some of the pri-

mary signal is likely preserved and the inferred differences in local redox conditions between the two sites are legitimate, although diagenesis of inferred primary aragonite and calcite phases also contributes to geochemical variation. Earth system model reconstructions of the Triassic-Jurassic boundary suggest that the two sites were situated in different hydrological regimes and circulatory patterns could have primed the northern site, Val Adrara, for more reducing conditions.

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Climatic conditions on the EES: (a) annual precipitation and, (b) air temperature and wind stress field (vectors) (b) simulated with CESM1.2.2 for the late Triassic (200 Ma time slice; Li et al., 2022a). See Fig. 1 for corresponding site descriptions and symbology. Positioning of coordinates are at slightly higher latitudes compared to Fig. 1 due to northwestward plate movement between reconstructions (a roughly 5 million year time difference).</figcaption></figure>

Looking forward, a coupled atmospheric-ocean model integrating biogeochemical tracers and bathymetry may offer a better appraisal of the potential drivers of the redox gradient. Paired with an assessment of pre- and post-extinction patterns at the sites, we may gain an improved understanding of how the physical environment interacted with marine life during the end-Triassic mass extinction.

## CRediT authorship contribution statement

**Ashley N. Prow-Fleischer:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Formal analysis, Data cura- tion, Conceptualization. **Zunli Lu:** Writing – review & editing, Super- vision, Project administration, Investigation, Funding acquisition, Formal analysis, Conceptualization. **Clara L. Blättler:** Writing – review & editing, Validation, Resources, Investigation. **Tianchen He:** Writing – review & editing, Methodology. **Pulkit Singh:** Writing – review & editing, Methodology. **Preston Cosslett Kemeny:** Writing – review & editing, Funding acquisition. **Jordan P. Todes:** Resources, Methodol- ogy. **Alexandre Pohl:** Writing – review & editing, Formal analysis. **Tripti Bhattacharya:** Writing – review & editing, Software, Methodol- ogy. **Bas van de Schootbrugge:** Writing – review & editing. **Paul B. Wignall:** Writing – review & editing. **Simona Todaro:** Writing – review & editing, Resources. **Jonathan L. Payne:** Writing – review & editing, Supervision, Resources, Project administration, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal re- lationships which may be considered as potential competing interests:

Zunli Lu reports financial support was provided by National Science Foundation. Jonathan L. Payne reports financial support was provided by National Science Foundation. If there are other authors, they declare that they have no known competing financial in- terests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This study was supported by NSF Frontier Research in Earth Sciences grant # 2121445 to Z.L., grant # 2121392 to J.L.P., and #2120406 to C. L.B. We thank Andy Ridgwell and Chadlin Ostrander for constructive

feedback on the manuscript, and Zonglin Yang for help with data analysis. Alex Dunhill, Simona Todaro and Pietro de Stefano are thanked for assistance with field sampling at the Monte Sparagio site. PCK was supported during this work by the TC Chamberlin Fellowship at the University of Chicago and by an NSF EAR postdoctoral fellowship (#2204376). AP acknowledges the support of the French Agence Natio- nale de la Recherche (ANR) under references ANR-22-
CE01-0003 (project ECO-BOOST) and ANR-23-CE01-0003 (project CYCLO-SED) and of the programme TelluS of the Institut National des Sciences de l'Univers, CNRS (project ROSETTA). This is a contribution of UMR 6282 Biogéosciences Team 'SESDS'.

# Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.chemgeo.2024.122530.

## Data availability

Newly generated $\delta^{44}$Ca, minor- and trace-element, and carbon isotope data can be currently viewed in the Supplementary section. All new data will be archived in the open repository Pangaea. Readers are directed to He et al. (2022) and Singh et al. (2023) for previously published I/Ca data, van de Schootbrugge et al. (2008) for $\delta^{13}$C and $\delta^{18}$O for the Val Adrara site, He et al. (2020) for $\delta^{34}$S$_{CAS}$, and Jost et al. (2016, 2017)) for previously published $\delta^{44}$Ca and $\delta^{238}$U for Val Adrara. Community Earth System Model 1.2.2 simulations are freely available from Li et al. (2022a).

## References

Ahm, A.S.C., Bjerrum, C.J., Blättler, C.L., Swart, P.K., Higgins, J.A., 2018. Quantifying early marine diagenesis in shallow-water carbonate sediments. Geochim.

Cosmochim. Acta 236, 140–159. https://doi.org/10.1016/j.gca.2018.02.042. Atkinson, J.W., Wignall, P.B., 2019. How quick was marine recovery after the end-Triassic mass extinction and what role did anoxia play? Palaeogeogr. Palaeoclimatol. Palaeoecol. 528, 99–119. https://doi.org/10.1016/j.palaeo.2019.05.011.

Bachan, A., Payne, J.L., 2016. Modelling the impact of pulsed CAMP volcanism on $\text{CO}_2$ and $\delta^{13}$C across the Triassic-Jurassic transition. Geol. Mag. 153 (2), 252–270. https://doi.org/10.1017/S0016756815000126.

Bachan, A., van de Schootbrugge, B., Fiebig, J., McRoberts, C.A., Ciarapica, G., Payne, J.

L., 2012. Carbon cycle dynamics following the end-Triassic mass extinction:

constraints from paired $\delta^{13}\text{C}_{\text{carb}}$ and $\delta^{13}\text{C}_{\text{org}}$ records. Geochim. Geophys. Geosyst. 13 (9), 1–24. https://doi.org/10.1029/2012GC004150.

Bambach, R.K., 2006. Phanerozoic biodiversity mass extinctions. Annu. Rev. Earth Planet. Sci. 34, 127–155. https://doi.org/10.1146/annurev.earth.33.092203.122654.

Beith, S.J., Fox, C.P., Marshall, J.E.A., Whiteside, J.H., 2021. Recurring photic zone euxinia in the northwest Tethys impinged end-Triassic extinction recovery. Palaeogeogr. Palaeoclimatol. Palaeoecol. 584 (May), 110680. https://doi.org/10.1016/j.palaeo.2021.110680.

Blättler, C.L., Henderson, G.M., Jenkyns, H.C., 2012. Explaining the Phanerozoic Ca isotope history of seawater. Geology 40 (9), 843–846. https://doi.org/10.1130/G33191.1.

Bottrell, S.H., Newton, R.J., 2006. Reconstruction of changes in global sulfur cycling from marine sulfate isotopes. Earth Sci. Rev. 75 (1–4), 59–83. https://doi.org/10.1016/j.earscirev.2005.10.004.

Bryant, R.N., Present, T.M., Ahm, A.S.C., McClelland, H.L.O., Razionale, D., Blättler, C.L., 2022. Early diagenetic constraints on Permian seawater chemistry from the Capitan Reef. Geochim. Cosmochim. Acta 328, 1–18. https://doi.org/10.1016/j.gca.2022.04.027.

Chen, X., Romaniello, S.J., Herrmann, A.D., Hardisty, D., Gill, B.C., Anbar, A.D., 2018. Diagenetic Effects on Uranium Isotope Fractionation in Carbonate Sediments from the Bahamas. Geochim. Cosmochim. Acta 237, 294–311. https://doi.org/10.1016/j.gca.2018.07.026.

Chen, X., Tissot, F.L.H., Jansen, M.F., Bekker, A., Liu, C.X., Nie, N.X., 2021. The Uranium Isotopic Record of Shales and Carbonates through Geologic Time.

Cicero, A.D., Lohmann, K.C., 2001. Sr/Mg variation during rock-water interaction: Implications for secular changes in the elemental chemistry of ancient seawater. Geochim. Cosmochim. Acta 65 (5), 741–761. https://doi.org/10.1016/S0016-7037(00)00594-9.

Dore, A., 1991. The structural foundation and evolution of Mesozoic seaways between Europe and the Arctic. Palaeogeogr. Palaeoclimatol. Palaeoecol. 87, 441–492.

Erhardt, A.M., Turchyn, A.V., Dickson, J.A.D., Sadekov, A.Y., Taylor, P.D., Wilson, M.A., Schrag, D.P., 2020. Chemical composition of carbonate hardground cements as reconstructive tools for Phanerozoic Pore fluids. Geochim. Geophys. Geosyst. 21(3), 1–20. https://doi.org/10.1029/2019GC008448.

Fante, S.M., DePaolo, D.J., 2007. Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: the $\text{Ca}^{2+}$ (aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments. Geochim. Cosmochim. Acta 71 (10), 2524–2546. https://doi.org/10.1016/j.gca.2007.03.006.

Fante, S.M., Higgins, J., 2014. The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates: implications for the geochemical cycles of Ca and Mg. Geochim. Cosmochim. Acta 142, 458–481. https://doi.org/10.1016/j.gca.2014.07.025.

Fante, S.M., Barnes, B.D., Lau, K.V., 2020. The role of diagenesis in shaping the geochemistry of the marine carbonate record. Annu. Rev. Earth Planet. Sci. 48, 549–583. https://doi.org/10.1146/annurev-earth-073019-060021.

Farkas, J., Bohm, F., Wallmann, K., Blenkinsop, J., Eisenhauer, A., van Geldern, R., Munnecke, A., Voigt, S., Veizer, J., 2007. Calcium isotope record of Phanerozoic oceans: implications for chemical evolution of seawater and its causative mechanisms. Geochim. Cosmochim. Acta 71, 5117–5134. https://doi.org/10.1016/j.gca.2007.09.004.

Farkas, J., Frýda, J., Holmden, C., 2016. Calcium isotope constraints on the marine carbon cycle and $\text{CaCO}_3$ deposition during the late Silurian (Ludfordian) positive

$\delta^{13}$C excursion. Earth Planet. Sci. Lett. 451, 31–40. https://doi.org/10.1016/j.epsl.2016.06.038.

Farkas, J., Frýda, J., Holmden, C., 2017. Corrigendum to “Calcium isotope constraints on the marine carbon cycle and $\text{CaCO}_3$ deposition during the late Silurian (Ludfordian) positive $\delta^{13}$C excursion” [Earth Planet. Sci. Lett. 451 (2016) 31–40]. Earth Planet. Sci. Lett. 469, 170–171. https://doi.org/10.1016/j.epsl.2017.04.013.

Feng, X., Redfern, S.A.T., 2018. Iodine in calcite, aragonite and vaterite $\text{CaCO}_3$: Insights from first-principles calculations and implications for the I/Ca geochemical proxy. Geochim. Cosmochim. Acta 236, 351–360. https://doi.org/10.1016/j.gca.2018.02.017.

Fike, D.A., Bradley, A.S., Rose, C.V., 2015. Rethinking the ancient sulfur cycle. Annu. Rev. Earth Planet. Sci. 43, 593–622. https://doi.org/10.1146/annurev-earth-060313-054802.

Galli, M.T., Jadoul, F., Bernasconi, S.M., Cirilli, S., Weissert, H., 2007. Stratigraphy and palaeoenvironmental analysis of the Triassic-Jurassic transition in the western Southern Alps (Northern Italy). Palaeogeogr. Palaeoclimatol. Palaeoecol. 244, 52–70. https://doi.org/10.1016/j.palaeo.2006.06.023.

Gill, B.C., Lyons, T.W., Frank, T.D., 2008. Behavior of carbonate-associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy. Geochim. Cosmochim. Acta 72 (19), 4699–4711. https://doi.org/10.1016/j.gca.2008.07.001.

Gussone, N., Böhm, F., Eisenhauer, A., Dietzel, M., Heuser, A., Teichert, B.M.A., Reitner, J., Wörheide, G., Dullo, W.C., 2005. Calcium isotope fractionation in calcite and aragonite. Geochim. Cosmochim. Acta 69 (18), 4485–4494. https://doi.org/10.1016/j.gca.2005.06.003.

Gussone, N., Filipsson, H.L., Kuhnert, H., 2016. Mg/Ca, Sr/Ca and Ca isotope ratios in benthonic foraminifers related to test structure, mineralogy and environmental controls. Geochim. Cosmochim. Acta 173, 142–159. https://doi.org/10.1016/j.gca.2015.10.018.

Gussone, N., Ahm, A.S.C., Lau, K.V., Bradbury, H.J., 2020. Calcium isotope data: potential and limitations. Chem. Geol. 544 (April), 119601. https://doi.org/10.1016/j.chemgeo.2020.119601.

Hardisty, D.S., Lu, Z., Bekker, A., Diamond, C.W., Gill, B.C., Jiang, G., Kah, L.C., et al., 2017. Perspectives on proterozoic surface ocean redox from iodine contents in

ancient and recent carbonate. Earth Planet. Sci. Lett. 463, 159–170. https://doi.org/10.1016/j.epsl.2017.01.032.

Hardisty, D.S., Lu, Z., Planavsky, N.J., Bekker, A., Philippot, P., Zhou, X., Lyons, T.W., 2014. An Iodine Record of Paleoproterozoic Surface Ocean Oxygenation, pp. 619–622. https://doi.org/10.1130/G35439.1.

Hashim, M.S., Burke, J.E., Hardisty, D.S., Kaczmarek, S.E., 2022. Iodine incorporation into dolomite: experimental constraints and implications for the iodine redox proxy and Proterozoic Ocean. Geochim. Cosmochim. Acta 338, 365–381. https://doi.org/10.1016/j.gca.2022.10.027.

Hautmann, M., 2004. Effect of end-Triassic $\text{CO}_2$ maximum on carbonate sedimentation and marine mass extinction. Facies 50 (2), 257–261. https://doi.org/10.1007/s10347-004-0020-y.

Hautmann, M., Benton, M.J., Tomášových, A., 2008. Catastrophic Ocean acidification at the Triassic-Jurassic boundary. Neues Jahr. Geol. Palaontol. Abhandl. 249 (1), 119–127. https://doi.org/10.1127/0077-7749/2008/0249-0119.

He, T., Corso, J.D., Newton, R.J., Wignall, P.B., Mills, B.J.W., Todaro, S., Stefano, P.D., Turner, E.C., Jamieson, R.A., Randazzo, V., Rigo, M., Jones, R.E., Dunhill, A.M., 2020. An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction. Sci. Adv. 6 (37). https://doi.org/10.1126/sciadv.abb6704.

He, T., Newton, R.J., Wignall, P.B., Reid, S., Dal Corso, J., Takahashi, S., Wu, H., Todaro, S., Di Stefano, P., Randazzo, V., Rigo, M., Dunhill, A.M., 2022. Shallow Ocean oxygen decline during the end-Triassic mass extinction. Glob. Planet. Chang. 210, 103770. https://doi.org/10.1016/j.gloplacha.2022.103770.

Hesselbo, S.P., Robinson, S.A., Surlyk, F., Piasecki, S., 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30 (3), 251–254. https://doi.org/10.1130/0091-7613(2002)030<0251>TAMEAT>2.0.CO;2.

Higgins, J.A., Schrag, D.P., 2006. Beyond methane: towards a theory for the Paleocene–Eocene thermal Maximum. Earth Planet. Sci. Lett. 245 (3–4), 523–537. https://doi.org/10.1016/j.epsl.2006.03.009.

Higgins, J.A., Blättler, C.L., Lundstrom, E.A., Santiago-Ramos, D.P., Akhtar, A.A., Grüger A.M.S., Bialik, O., Holmden, C., Bradbury, H., Murray, S.T., Swart, P.K., 2018. Mineralogy, early marine diagenesis, and the chemistry of shallow-water carbonate sediments. Geochim. Cosmochim. Acta 220, 512–534. https://doi.org/10.1016/j.gca.2017.09.046.

Holcomb, M., Cohen, A.L., Gabitov, R.I., Hutter, J.L., 2009. Compositional and morphological features of aragonite precipitated experimentally from seawater and biogenically by corals. Geochim. Cosmochim. Acta 73 (14), 4166–4179. https://doi.org/10.1016/j.gca.2009.04.015.

Hudson, J.D., 1975. Carbon isotopes and limestone cement. Geology 3, 19. https://doi.org/10.1130/0091-7613(1975)3<19:CIALC>2.0.CO;2.
Jannace, A., Frisia, S., 1994. Changing dolomitization styles from Norian to Rhaetian in the southern Tethys realm. Spec. Publs Int. Ass. Sediment 21, 75-89.

Jacobsen, S.B., Kaufman, A.J., 1999. The Sr and O isotopic evolution of Neoproterozoic seawater. Chem. Geol. 161 (1), 37-57. https://doi.org/10.1016/S0009-2541(99)00080-7.

Jadoul, F., Galli, M.T., 2008. The Hettangian shallow water carbonates after the Triassic/Jurassic boundarylimestones. Geochem. Geophys. Geosyst. 17, 1312-1338. https://doi.org/10.1002/2015GC06205Received.

Jadoul, F., Berra, F., Frisia, S., 1992. Stratigraphic and paleogeographic evolution of a carbonate platform in an extensional tectonic regime: the example of the Dolomia Principale in Lombardia (Italy). Riv. Ital. Paleontol. Stratig. 114 (3), 453-470. https://doi.org/10.13101/2039-4942/911.

Jost, A.B., Bachan, A., van de Schootbrugge, B., Brown, S.T., DePaolo, D.J., Payne, J.L., 2016. Additive effects of acidification and mineralogy on calcium isotopes in Triassic/Jurassic boundarylimestones. Geochem. Geophys. Geosyst. 17, 1312-1338. https://doi.org/10.1002/2015GC06205Received.

Jost, A.B., Bachan, A., van de Schootbrugge, B., Lau, K.V., Weaver, K.L., Maher, K., Payne, J.L., 2017. Uranium isotope evidence for an expansion of marine anoxia during the end-Triassic extinction. Geochem. Geophys. Geosyst. 18 (8), 3093-3108. https://doi.org/10.1002/2017GC006941.

Kampschulte, A., Strauss, H., 2004. The sulfur isotopic evolution of Phanerozoic seawater based on the analysis of structurally substituted sulfate in carbonates. Chem. Geol. 204, 255-286. https://doi.org/10.1016/j.chemgeo.2003.11.013.

Kiessling, W., Aberhan, M., Brenneis, B., Wagner, P.J., 2007. Extinction trajectories of benthic organisms across the Triassic-Jurassic boundary. Palaeogeogr. Palaeoclimatol. Palaeoecol. 244 (1-4), 201-222. https://doi.org/10.1016/j.paleo.2006.06.029.

Kimmig, S.R., Holmden, C., 2017. Multi-proxy geochemical evidence for primary aragonite precipitation in a tropical-shelf 'calcite sea' during the Hmantian glaciation ScienceDirect Multi-proxy geochemical evidence for primary aragonite precipitation in a tropical-shelf 'calcite sea'. Geochim. Cosmochim. Acta. https://doi.org/10.1016/j.gca.2017.03.010.

Kocsis, A.T., Raja, N.B., 2023. chronosphere: Evolving Earth System Variables (Computer software). https://rdr.io.github/adamkocsis/chronosphere/FREADME.md.

Korte, C., Hesselbo, S.P., Ullmann, C.V., Dietl, G., Ruhl, M., Schweigert, G., Thibault, N., 2015. Jurassic climate mode governed by ocean gateway. Nat. Commun. 6, 1-7. https://doi.org/10.1038/ncomms10015.

Lau, K.V., Hardisty, D.S., 2022. Modeling the impacts of diagenesis on carbonate paleoredox proxies. Geochim. Cosmochim. Acta 337, 123-139. https://doi.org/10.1016/j.gca.2022.09.021.

Lau, K.V., Maher, K., Brown, S.T., Jost, A.B., Altmair, D., DePaolo, D.J., Eisenhauer, A., Kelley, B.M., Lehrmann, D.J., Paytan, A., Yu, M., Silva-Tamayo, J.C., Payne, J.L., 2017. The influence of seawater carbonate chemistry, mineralogy, and diagenesis on calcium isotope variations in Lower-Middle Triassic carbonate rocks. Chem. Geol. 471 (May), 13-37. https://doi.org/10.1016/j.chemgeo.2017.09.006.

Li, X., Hu, Y., Guo, J., Lan, J., Lin, Q., Bao, X., Yuan, S., Wei, M., Li, Z., Man, K., Yin, Z., Han, J., Zhang, J., Zhu, C., Zhao, Z., Liu, Y., Yang, J., Nie, J., 2022a. A high-resolution climate simulation dataset for the past 540 million years. Sci. Data 9 (1), 1-10. https://doi.org/10.1038/s41597-022-01490-4.

Li, J., Song, H., Tian, L., Bond, D.P.G., Song, H., Du, Y., Rohr, C., Chu, D., Wignall, P.B., Tong, J., 2022b. Dynamic Ocean redox conditions during the end-Triassic mass extinction: evidence from pyrite framboids. Glob. Planet. Chang. 218, 103981. https://doi.org/10.1016/j.gloplacha.2022.103981.

Lohmann, C.K., 1988. Geochemical patterns of meteoric diagenetic systems and their application to studies of paleokarst. In: Choquestte, P.W., James, N. (Eds.), Paleokarst. Springer-Verlag, pp. 58-80.

Lu, Z., Lu, W., Rickaby, R.E.M., Thomas, E., 2020. Cambridge elements. In: EarthHistory of Oxygen and the iprOxy. Cambridge University Press. https://doi.org/10.1017/9781108688604.

Luo, G., Richoz, S., van de Schootbrugge, B., Algeo, T.J., Xie, S., Ono, S., Summons, R.E., 2018. Multiple sulfur-isotopic evidence for a shallowly stratified ocean following the Triassic-Jurassic boundary mass extinction. Geochim. Cosmochim. Acta 231, 73-87.

Santantonio, M., Carminati, E., 2011. Jurassic rifting evolution of the Apennines and Southern Alps (Italy): Parallels and differences. Bull. Geol. Soc. Am. 123 (3-4), 468-484. https://doi.org/10.1130/B30104.1.

Schlagintweit, F., 2013. Thaumatoporella ladders unraveled. Studia Universitatis Babes-Bolyai. Geologia 58 (1), 5-9. https://doi.org/10.5038/1937-8602.58.1.1.

Scotese, C.R., 2014. Atlas of Jurassic Paleogeographic Maps. In ArcGIS Vol. 4 (The Jurassic and Triassic, Maps 32-42, Mollweide Projection, PALEOMAP Project).

Sha, J., 2002. Hispanic Corridor formed as early as Hettangian: on the basis of bivalve fossils. Chin. Sci. Bull. 47 (5), 1-4.

Sim, S.M., Bosak, T., Ono, S., 2011. Large sulfur isotope fractionation does not require disproportionation. Science 333, 17-20.

Singh, P., Lu, W., Lu, Z., Jost, A.B., Lau, K., Bachan, A., Schootbrugge, B.V.D., Payne, J.L., 2023. Reduction in animal abundance and oxygen availability during and after the end- Triassic mass extinction. Geobiology 2022, 175-192. https://doi.org/10.1111/gbi.12533.

Stewart, J.A., Gutjahr, M., Pearce, F., Swart, P.K., Foster, G.L., 2015. Boron during https://doi.org/10.1016/j.gca.2018.04.015.

11

Marriott, C.S., Henderson, G.M., Belshaw, N.S., Tudhope, A.W., 2004. Temperature dependence of δ²¹Li, δ²⁴Ca and Li/Ca during growth of calcium carbonate. Earth Planet. Sci. Lett. 222 (2), 615-624. https://doi.org/10.1016/j.epsl.2004.02.031.

McRoberts, C.A., 1994. The Triassic-Jurassic ecosignature transition in the Lombardian Alps, Italy. Palaeogeogr. Palaeoclimatol. Palaeoecol. 110 (1-2), meteoric diagenesis and its potential implications for Marinoan snowball Earth δ B-pH excursions. Geology 43 (7), 627-630. https://doi.org/10.1130/G36652.1.

Suzuki, N., Ona, M., 2015. Oldest fossil records of marine protists and the geologic history toward the establishment of the Modern-Type Marine Protist World. In: Ohtsuka, S., Suzuki, T., Horiguchi, T., Suzuki, N., Not, F. (Eds.), Marine Protists. Springer Japan, pp. 359-394. https://doi.org/10.1007/978-4-431-55130-0_15. 145-166. https://doi.org/10.1016/0031-0182(94)90117-4.

Muttoni, G., Tartarotti, P., Chiari, M., Marieni, C., Rodelli, D., Dallanave, E., Kirscher, U., 2015. Palaeolatitudes of late Triassic radiolarian clerts to Argolis, Greece: insights on the paleogeography of the western Tethys. Palaeogeogr. Palaeoclimatol. Palaeoecol. 417, 476-490. https://doi.org/10.1016/j.palaeo.2014.10.010.

Oehlert, A.M., Swart, P.K., 2014. Interpreting carbonate and organic carbon isotope covariance in the sedimentary record. Nat. Commun. 5, 1-7. https://doi.org/10.1038/ncomms5672.

Ogg, G.J., Ogg, G.M., Gradstein, F., 2016. Jurassic. In: A Concise Geologic Time Scale.

Elsevier B.V. https://doi.org/10.1016/B3978-0-444-59467-9.90001-2.

Onoue, T., Michalik, J., Shirozu, H., Yamashita, M., Yamashita, K., Kusaka, S., Soda, K., 2022. Extreme continental weathering in the northwestern Tethys during the end- Triassic mass extinction. Palaeogeogr. Palaeoclimatol. Palaeoecol. 594 (January), 110934. https://doi.org/10.1016/j.palaeo.2022.110934.

Paris, G., Beaumont, V., Bartolini, A., Clémence, M.E., Garden, S., Page, K., 2010.

Nitrogen isotope record of a perturbed paleoecosystem in the aftermath of the end- Triassic crisis, Doniford section, SW England. Geochem. Geophys. Geosyst. 11 (8), 1-15. https://doi.org/10.1029/2010GC003161.

Razionale, D., Bryant, R.N., Blättler, C.L., 2022. Adapting automated instrumentation for high-throughput calcium isotope measurements by multi-collector inductively coupled plasma mass spectrometry. Rapid Commun. Mass Spectrom. 36 (7), 1-13. https://doi.org/10.1002/rcm.9249.

Reis, A., Erhardt, A.M., McGlue, M.M., Waite, L., 2019. Evaluating the effects of diagenesis on the δ¹³C and δ¹⁹O compositions of carbonates in a mud-rich depositional environment: a case study from the Midland Basin, USA. Chem. Geol. 524, 196-212. https://doi.org/10.1016/j.chemgeo.2019.06.021.

Richoz, S., van de Schootbrugge, B., Pross, J., Pittmann, W., Quan, T.M., Lindström, S., Heunisch, C., Fiebig, J., Maquil, R., Schouten, S., Hauzenberger, C.A., Wignall, P.B., 2012. Hydrogen sulphide poisoning of shallow seas following the end-Triassic extinction. Nat. Geosci. 5 (9), 662-667. https://doi.org/10.1038/ngeo1539.

Romaniello, S.J., Herrmann, A.D., Anbar, A.D., 2013. Uranium concentrations and 238U/235U isotope ratios in modern carbonates from the Bahamas: assessing a novel paleoredox proxy. Chem. Geol. 362, 305-316. https://doi.org/10.1016/j.chemgeo.2013.02.002.

Ruhl, M., Bonis, N.R., Reichart, G.-J., Damsté, J.S.S., Kürschner, W.M., 2011.

Atmospheric carbon injection linked to end-triassic mass extinction. Science 333, 430-434. https://doi.org/10.1126/science.1204255.

Ruvalcaba Baroni, I., Pohl, A., van Helmond, N.A.G.M., Papadomanolaki, N.M., Coe, A. L., Cohen, A.S., van de Schootbrugge, B., Donnadieu, Y., Slomp, C.P., 2018. Ocean Circulation in the Toarcian (early Jurassic): a Key Control on Deoxygenation and Carbon Burial on the European Shelf. Paleogeogr. Paleoclimatol. 33 (9), 994-1012. https://doi.org/10.1029/2018PA003394.

Swart, P.K., Oehlert, A.M., 2018. Revised interpretations of stable C and O patterns in carbonate rocks resulting from meteoric diagenesis. Sediment. Geol. 364, 14-23.

https://doi.org/10.1016/j.sedgeo.2017.12.005.

Tang, J., Dietzel, M., Böhm, F., Köhler, S.J., Eisenhauer, A., 2008. Sr²⁺/Ca²⁺ and

⁴⁴Ca/⁴⁰Ca fractionation during inorganic calcite formation: II. Ca isotopes. Geochim. Cosmochim. Acta 72 (15), 3733-3745. https://doi.org/10.1016/j.gca.2008.05.033.

Todaro, S., Di Stefano, P., Zarcone, G., Randazzo, V., 2017. Facies stacking and extinctions across the Triassic-Jurassic boundary in a peritidal succession from western Sicily. Facies 63 (3), 20. https://doi.org/10.3389/feart.2022.875466.

Todaro, S., Rigo, M., Randazzo, V., Di Stefano, P., 2018. The end-Triassic mass extinction: a new correlation between extinction events and δ13C fluctuations from a Triassic-Jurassic peritidal succession in western Sicily. Sediment. Geol. 368, 105-113. https://doi.org/10.1016/j.sedgeo.2018.03.008.

Todaro, S., Agosta, F., Parrino, N., Cavalcante, F., Di Stefano, P., Giarrusso, R., Pepe, F., Renda, P., Tondi, E., 2022. Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy. Mar. Pet. Geol. 135, 105400. https://doi.org/10.1016/j.marpetgeo.2021.105400.

Todaro, S., Rigo, M., Tackett, L., Di Stefano, P., 2023. Evidence of a biodiversity crisis documented on a peritidal carbonate succession from western Tethys (Sicily): new data on the End Triassic Mass Extinction. Ital. J. Geosci. 142 (1), 122-130. https://doi.org/10.3301/IJG.2023.04.

van de Schootbrugge, B., Houben, A.J.P., Ercan, F.E.Z., Verreussel, R., Kerstolt, S., Janssen, N.M.M.B., Nikitenko, B., Suan, G., 2020. Enhanced arctic-tethys connectivity ended the toarcan oceanic anoxic event in NW Europe. Geol. Mag. 157 (10), 101593-101611. https://doi.org/10.1017/S0016756819001262.

van de Schootbrugge, B., Payne, J.L., Tomasovych, A., Pross, J., Fiebig, J., Benbrahim, M., Fölmi, K.B., Quan, T.M., 2008. Carbon cycle perturbation and stabilization in the wake of the Triassic-Jurassic boundary mass-extinction event. Geochim. Geophys. Geosyst. 9 (4). https://doi.org/10.1029/2007GC001914.

Wignall, P.B., Atkinson, J.W., 2020. A two-phase end-Triassic mass extinction. Earth Sci. Rev. 108, 203282. https://doi.org/10.1016/j.earscirev.2020.103282.

Young, E.D., Galy, A., Nagahara, H., 2002. Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance. Geochim. Cosmochim. Acta 66 (6), 1095-1104. https://doi.org/10.1016/S0016-7037(01)00832-8.

Zhang, F., Lenton, T.M., del Rey, Å, Romaniello, S.J., Chen, X., Planavsky, N.J.,

Clarkson, M.O., Dahl, T.W., Lau, K.V., Wang, W., Li, Z., Zhao, M., Isson, T., Algeo, T. J., Anbar, A.D., 2020. Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: a critical review. Geochim. Cosmochim. Acta 287, 27-49. https://doi.org/10.1016/j.gca.2020.05.011.