New Chronostratigraphic Constraints on the Lower Jurassic Pliensbachian–Toarcian Boundary at Chacay Melehue (Neuquén Basin, Argentina)

The Pliensbachian–Toarcian boundary interval is characterized by a ~3 ‰ negative carbon-isotope excursion (CIE) in organic and inorganic marine and terrestrial archives from sections in Europe, such as Peniche (Portugal) and Hawsker Bottoms, Yorkshire (UK). A new high-resolution organic-carbon isotope record, illustrating the same chemostratigraphic feature, is presented from the Southern Hemisphere Arroyo Chacay Melehue section, Chos Malal, Argentina, corroborating the global signicance of this disturbance to the carbon cycle. The negative carbon-isotope excursion, mercury and organic-matter enrichment is accompanied by high-resolution ammonite and nannofossil biostratigraphy together with U-Pb CA-ID-TIMS geochronology derived from intercalated volcanic ash beds. A new age of ~183.71 ± 0.40/-0.51 Ma for the Pliensbachian–Toarcian boundary, and 182.77 +0.11/-0.21 for the tenuicostatum– serpentinum zonal boundary, is assigned based on high-precision U-Pb zircon geochronology and a Bayesian Markov chain Monte Carlo (MCMC) stratigraphic age model.


Introduction
The Early Jurassic Pliensbachian-Toarcian (Pl-To; ~184 Ma) carbon-isotope excursion (CIE) is marked by a -3‰ shift in δ 13 C in both bulk-rock carbonate and organic carbon 1,2 . It is associated in time with a second-order extinction event affecting ammonites, belemnites, gastropods, and many other benthic and pelagic groups, that effectively de nes the stage boundary [3][4][5] . This event precedes the onset of the Early Toarcian Oceanic Anoxic Event (T-OAE) and its associated CIEs, appears relatively short-lived (~50-200 kyr 6 and has been linked to the beginning of activity in the Karoo and Ferrar Large Igneous Provinces (LIP), recording an initial release of volcanogenic CO 2 and other gases [7][8][9] .
The Pl-To event has been studied in the Tethyan and northwest European realms 1,6,10, as well as Canada, Chile, and Japan 7,11−13 . The age of the boundary is presently computed based on a combination of cyclostratigraphy and U-Pb geochronology and is estimated to be 184.2 Ma with an age of 183.2 ± 0.1 for the top of the tenuicostatum Zone 14 . Other U-Pb ages that help to constrain this boundary age include dates from Argentina 15,16 , the USA 17 and Canada 18-20 , although these lack strong biostratigraphic control, speci cally with respect to the European ammonite zones. Dateable stratigraphic sections that can be bio-and chemostratigraphically correlated to marine sections elsewhere, speci cally to the GSSP in Peniche, Portugal 5, are essential to precisely date the base of the Toarcian. An improved age model for the Pl-To event offers greater insight into the driving mechanism of the observed environmental phenomena and the relationship with emplacement of the Karoo-Ferrar Large Igneous Province.
Here, we present a new high-resolution carbon-isotope chemostratigraphy and biostratigraphy that is calibrated using U-Pb ID-TIMS zircon dates. A new age-depth model for the Lower Jurassic (Pliensbachian-Toarcian) Chacay Melehue stream section in Neuquén Province, Argentina, is also presented, which constrains the age of both the Pliensbachian-Toarcian boundary and the onset of the negative carbon-isotope excursion in the earliest Toarcian. Using this new geochronology and biostratigraphy, combined with correlations to the GSSP and other well-de ned sections, we explore the relationship between the Pliensbachian-Toarcian event and Karoo-Ferrar volcanism.

Palaeogeography And Tectonic Setting Of The Neuquén Basin
The Neuquén Basin is located on the eastern side of the Andes in west-central Argentina and central Chile, between 32° and 41° S (Fig. 1). The depositional area was a roughly triangular, north-southoriented back-arc basin and foreland, now containing more than 6 km of Triassic to Cenozoic sediments in its most central part 21 . The basin had a complicated tectonic history associated with the break-up of Gondwana, subduction of the proto-Paci c plate and the development of the Andean magmatic arc 22 . Sediments were laid down in several depositional cycles representing deposition from the time of prerifting through to foreland-basin development 23 . The sediments studied here form part of the marine Cuyo or Cuyano Group (Lower to Middle Jurassic). The deposition of the Cuyo Group was favoured by marine transgression during subsidence in the post-rift phase of basin development 22 . Sediments entered the Neuquén Basin from two main source areas: the Chilean Coastal Cordillera that supplied immature volcaniclastic material, and cratonic areas to the south and northeast from which more mineralogically mature sediment was derived [24][25][26] .

Chacay Melehue Stratigraphy And Depositional Setting
The Arroyo Chacay Melehue stratigraphic section presented here is located at S37°15'18.15", W70°30'26.55" (Fig 1) and comprises more than ~1200 m of sediment spanning the latest Pliensbachian to Oxfordian interval 27,28 . At the base of the section are epiclastic and pyroclastic deposits of the La Primavera Formation, which are thought to have been derived from an andesitic strato-volcano complex, referred to as the Chilean Coastal Cordillera, on the western side of the Neuquén embayment during the latest Triassic-Early Jurassic 29,30 (Fig. 1).
Previous studies of sedimentary units at Chacay Melehue suggest that the section was deposited in a marginal marine to offshore environment, recording transgressive-regressive cycles of sedimentation within the Neuquén Basin 31 . Tuffaceous beds present throughout the section are typically ning upwards and inferred to be largely ne-grained turbidites, redepositing previously laid down ash beds. The presence of discrete volcaniclastic beds at the bottom of the section, and the presence of volcaniclastic material in the sandstone beds throughout the section, indicates that the section was proximal to a volcanic arc situated to the west 32 (Fig 2.). Up-section, coarser grained material decreases in relative abundance, suggesting that either the grain size from the source area changed or that the basin experienced a relative sea-level rise, increasing the distance between source and depocentre at Chacay Melehue. A deepening environment is also suggested by the presence of dark-coloured shale units with organic enrichment stratigraphically above 11 m in the section, suggesting deposition in an oxygendepleted environment (Fig. 2).
The presence of two distinct, slumped deposits (~14.5-17 m, Fig. 2) may suggest increased weathering and local sediment overloading at Pl-To boundary time, possibly due to an enhanced hydrological cycle 33 . Percival et al. 34 and Xu et al. 35 have previously suggested enhanced continental weathering during the Pl-To boundary interval and T-OAE based on excursions in Os 187 /Os 188 , as well as evidence of centimetre-scale gravity-ow deposits from the T-OAE interval in the Mochras core, Cardigan Bay, Wales. Many other records of the T-OAE/CIE also show similar evidence for an enhanced hydrological cycle and increased weathering and erosion during this event following Karoo-Ferrar volcanism 11,36 .

Geochronological And Biostratigraphic Constraints At Chacay Melehue
Ammonites and other fossils were sampled wherever found in situ, and tuffaceous samples were collected throughout the section (full details of horizons and determinations are given in the supplementary data).
Biostratigraphic determination of the Chacay Melehue section con rms the presence of deposits of Late Pliensbachian through earliest Toarcian age (Fig. 2). This section was previously studied for geochronology 37  At the Global Stratotype Section and Point (GSSP) for the base of the Toarcian at Peniche (Portugal), the LO of Lotharingius barozii is in sediments of the uppermost emaciatum ammonite Zone, ~50 cm below the base of the Toarcian Stage. Furthermore, the Pliensbachian-Toarcian boundary at this locality is marked by the lowest occurrence of Dactylioceras (Eodactylites) simplex, which is considered to allow global correlation of this level, thereby providing strong support for the proposition that the geochronology in this part of the Chacay Melehue section constrains the age of the boundary. CM-ASH-5 did not yield an interpretable age and was largely comprised of inherited or reworked zircons. Leanza et al. 37 also sampled and analyzed two ash beds in the Chacay Melehue locality using U-Pb ID-TIMS: one of the ashes, at ~24 m in the section, yielded an age of 185.7± 0.40 Ma: this bed is located biostratigraphically above the Pliensbachian-Toarcian boundary, is cross-bedded, and has a very wide array of zircon ages within the zircon population. Consequently, it appears likely that the bed is largely made up of reworked volcaniclastic material, despite the tightly clustered age ranges of the youngest zircons that contribute to this precise date, but probably do not give an accurate depositional age. A second ash bed was dated by Leanza et al. 37 , which produced an age of 182.3 ± 0.4 Ma; its exact stratigraphic position within the succession is, however, unknown with respect to our measured section.
Field photographs in Leanza et al. 37 could not be matched to the outcrop at the times of our eld investigations.
To improve constraints on the age of the Pliensbachian-Toarcian boundary and the age of the lower Toarcian tenuicostatum-hoelderi boundary we used a Bayesian Markov chain Monte Carlo (MCMC) model in which stratigraphic superposition is imposed on U-Pb zircon dates 48 . The result is an agedepth model incorporating dates from all beds above and below each sample to produce an internally consistent age model (Fig. 3. B & C.). This model allowed us to extrapolate ages at speci c depths, assuming relatively constant sedimentation rates of the deposits between the ash beds that provide the geochronological constraints (Fig 3C). To determine the age of the Pliensbachian-Toarcian boundary, we assessed the stratigraphic position of the boundary to be at 11.08 m in the section, concurrent with the LO of Dactylioceras (Eodactylites), and interpolated the age to be 183.71 +0.40/-0.51 Ma (Fig. 3 C). A similar exercise was performed for the tenuicostatum-hoelderi boundary (concurrent with the tenuicostatum-serpentinum zone boundary in NW Europe), using the LO of Harpoceras serpentinum (Schlotheim), Cleviceras exaratum (Young & Bird) and Hildaites cf. murleyi (Moxon) (LO 21.66 m). Thus, at 21.66 m in the section an age of 182.77 +0.11/ -0.21 Ma was interpolated from the model (Fig. 3C).
The age-depth model coupled with biostratigraphy provides a new more precise age for two of the major events in the earliest Toarcian as well as a new age for the Pliensbachian-Toarcian boundary.
The Pliensbachian-toarcian Boundary Carbon-isotope Excursion Total organic carbon (TOC) concentrations across the studied stratigraphic interval range from values of 0-1% in the uppermost Pliensbachian disciforme Zone (0 to 11m, Fig. 2), to values of 1.5-4% in the tenuicostatum Zone (11 to 22 m), and values of 0.5-1% higher up in the section. As the TOC content increases up through the tenuicostatum Zone, the δ 13 C TOC record shows a marked negative shift, initiated at ~13 m in the studied section (Fig. 3), and with values gradually falling from a background of ~-27.5‰, to -30.1‰ at ~15 m (Fig. 3). The δ 13 C TOC values above ~16 m in the section shows a gradual positive shift, returning to ~-26.5‰ at ~18 m. Subsequently, from ~18-30 m in the section, δ 13 C TOC values are relatively stable, oscillating by 1-2‰ around an average value of -27‰ (Fig. 2). In the upper part of the studied section, above a poorly exposed stratigraphic interval, δ 13 C TOC values are signi cantly more negative, averaging around ~-29‰ and falling as low as -29.8‰; this shift to lower values coincides with a gradual increase to relatively more elevated TOC values of up to ~2% in this uppermost part of the section. Tmax°C values range from 296 to 506°C throughout the section, HI values range from 3 to 23 mg HC/gTOC, and S2/S3 <1, suggesting that organic matter in the section is made up of higher plant material and/or hydrogen-poor organic constituents that have been oxidized and/or suffered thermal maturation 45 .
The carbon-isotope pro le of Chacay Melehue can be chemostratigraphically correlated to other biostratigraphically well-constrained sections, speci cally to the base-Toarcian GSSP in Peniche, Portugal 47 (Fig. 3). The δ 13 C signatures of Chacay Melehue (bulk organic carbon) and Peniche (bulk carbonate) show a remarkably similar ~2‰ negative carbon-isotope excursion across the Pl-To boundary.
In the Chacay Melehue section, sedimentary mercury [Hg] concentrations are 300-700 ppb in the lowest 5 m of the section with values decreasing to 20-50 ppb through the sediments displaying the negative excursion in the section (~10 to 20 m; Fig. 4). Hg/TOC values show a small increase at the Pl-To transition, against a falling trend and, at around 23 m in the studied succession, with values of up to 0.23 ppm/wt%, are followed upwards by reduced values of ~0.05 ppm/weight% (Fig. 4). Hg/TOC values strongly increase up to 0.67 ppm/weight % towards the top of the studied succession, coinciding also with increasing TOC values and decreasing δ 13 C TOC values (Fig. 4), possibly representing the onset of the T-OAE negative CIE. The observed trend in the Hg/TOC pro le at Chacay Melehue is similar in shape and order of magnitude to other records, such as at Mochras (Cardigan Bay Basin, UK) and Peniche (Lusitanian Basin, Portugal; Fig. 4; 50 ).

Age Implications Of Chacay Melehue Chemo-, Chrono-And Biostratigraphy For The Pliensbachian-toarcian Boundary And T-oae
Integrated global correlation of the Chacay Melehue data with other successions well documented by ammonite biostratigraphy, chemostratigraphy, magnetostratigraphy and/or geochronology (Fig. 4), support a link between the Pl-To boundary event and the onset of Karoo LIP activity (Fig. 5). This relationship is further supported by the stratigraphic distribution of elemental mercury in the section, inferred to have been volcanogenically derived and transported through the atmosphere before nal deposition in marine sediments. The signature of elemental mercury observed in the Chacay Melehue section also supports the case for initial volcanism at the Pl-To transition, followed by a period of volcanic quiescence and then a second larger pulse of volcanic mercury release, presumably from the onset of Ferrar volcanism and continued Karoo volcanism (Fig. 5) leading up to the T-OAE negative CIE.
The onset of environmental perturbations at the Pl-To boundary likely resulted in global warming, oceanic anoxia, intensi ed weathering, and a calci cation crisis, in a similar manner to, and setting the stage for, the larger perturbations recorded during the Toarcian Oceanic Anoxic Event that had its focus in the serpentinum Zone (= ~falcifererum Zone = ~hoelderi Zone). Caruthers et al. 7,52 have also suggested that the pulsed extinction events across the Pl-To boundary and throughout the Early Toarcian appear to have been associated with peaks of intrusive volcanism in Karoo and Ferrar and silicic volcanism in Chon Aike (Fig. 1, 5); however, these igneous provinces are chemically distinct, and the likely environmental impact of each is very different. For example, the Karoo LIP was emplaced relatively rapidly and intruded into Permian organic-rich sediments 53,54 (Fig. 5), whereas Chon Aike, which is a Silicic Large Igneous province, was emplaced over a longer period and likely did not result in rapid hydrothermal venting of greenhouse gases, but more gradual gaseous release over a relatively long period from ~160-190 Ma 55 .
Mass-transport deposits (slumps) developed synchronously with the Pl-To negative carbon-isotope excursion, in a similar style to that observed at numerous locations for the T-OAE 33-36, support the inference of increased weathering and sediment supply to (unstable) slopes due to a globally accelerated hydrological cycle.
The chemostratigraphy from Chacay Melehue strengthens the case for the global nature of the previously observed Pl-To negative carbon-isotope excursion and disturbance to the carbon cycle. The ~3‰ negative excursion in δ 13 C TOC values closely follows the stratigraphically lowest occurrence of Dactylioceras (Eodactylites) cf. simplex (Fucini) in the section, a taxon closely allied to the principal marker for the base Toarcian GSSP at Peniche, Portugal 47 (Fig. 4).
In addition, the Chacay Melehue section provides new constraints for the age of the Pl-To boundary at 183.71 +0.40/-0.51 Ma, as well as for the tenuicostatum-serpentinum zonal boundary at ~182.77 +0.11/-0.21 Ma, with the latter occurring stratigraphically close to the onset of the negative carbonisotope excursion associated with the T-OAE. Using the carbon isotopes, modelled and measured radiometric ages and the biostratigraphy we can estimate the duration of the Pliensbachian-Toarcian event to be around 0.535 +0.28/-0.30 Ma.
These dates and zonal durations are consistent with recent astrochronological estimates for the ages of this boundary 49 , which suggests a million-year duration for the earliest Toarcian tenuicostatum (or concurrent polymorphum) Zone 6,49,56,57 . Furthermore, astrochronological constraints on the duration of the Pl-To negative CIE suggest a duration of ~200 kyr 6,58 , which agrees with the geochronological constraints on the duration of this event, as illustrated here.