New age 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 significance of this disturbance to the carbon cycle. The negative carbon-isotope excursion, mercury and organic-matter enrichment are 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 ~ 183.73 + 0.35/− 0.50 Ma for the Pliensbachian–Toarcian boundary, and 182.77 + 0.11/− 0.15 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.

www.nature.com/scientificreports/ 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 and Ferrar Large Igneous Provinces.
Here, we present a new high-resolution carbon-isotope chemostratigraphy and biostratigraphy that is calibrated using U-Pb ID-TIMS zircon dates for the Lower Jurassic (Pliensbachian-Toarcian) Chacay Melehue stream section in Neuquén Province, Argentina. A new age-depth model for this section 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-defined sections, we explore the relationship between the Pliensbachian-Toarcian event and Karoo and Ferrar LIP activity.

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 north-south-oriented back-arc basin and foreland, now containing more than 6 km of Triassic to Cenozoic sediments in its most central part 29 . The basin had a complicated tectonic history associated with the break-up of Gondwana, subduction of the proto-Pacific Plate and the development of the Andean magmatic arc 33 . Sediments were laid down in several depositional cycles representing deposition from the time of pre-rifting through to foreland-basin development 28 . The strata studied here form part of the marine Cuyo 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 33 . 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 [34][35][36] .

Chacay Melehue stratigraphy and depositional setting
The Arroyo Chacay Melehue stratigraphic section presented here is located at S37°15′ 18.15ʺ, W70°30′ 26.55ʺ ( Fig. 1) and comprises more than ~ 1200 m of sediment spanning the latest Pliensbachian to Oxfordian interval 37,38 . 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 39,40 (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 41 . Tuffaceous beds present throughout the section are typically fining upwards and inferred to be largely fine-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, indicates that the section was proximal to a volcanic arc situated to the west 42 (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 oxygen-depleted 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 43 . Percival et al. 44 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-flow deposits from the T-OAE interval in the Mochras core, Cardigan Bay Basin, UK. 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, coinciding with and/or following Karoo and Ferrar volcanism 12,46 .

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 confirms the presence of deposits of Late Pliensbachian through earliest Toarcian age (Fig. 2). This section was previously studied for geochronology 15,47 . Sample 2296R collected at 17.34 m in the Chacay Melehue section (see supplementary Fig. 1), and located in the tenuicostatum zone ~ 6 m above the Pliensbachian-Toarcian boundary, was analysed by Riccardi & Kamo 15 . This sample has a mean 206 Pb/ 238 U age of 183.11 ± 0.12 and a Bayesian eruption age estimate 48 of 182.82 ± 0.28 Ma.
Here, we have analysed 4 additional samples, CM-ASH-1, 3, 5 and 6, from within the same section. Data are corrected to the EARTHTIME tracer ET535, based on U-Pb CA-ID-TIMS analyses of individually abraded zircon crystals (see supplementary information section for details on the methodology).
CM-ASH-1, at 8.69 m, has an estimated maximum depositional age of 184.10 ± 0.54 Ma and sits in the latest Pliensbachian disciforme Andean ammonite zone, equivalent to the latest margaritatus-spinatum northwest European ammonite zones [50][51][52] . The bivalve Kolymonectes weaveri Damborenea is also present here from 0.   [23][24][25][26][27] , and the Neuquén Basin reconstruction (after Vicente 28 ), and (B) the interior seaway, and depositional tracts within the basin (adapted and modified after [29][30][31][32]  Leanza et al. 47 also sampled and analyzed two ash beds in the Chacay Melehue locality using U-Pb CA-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. 47 , which produced an age of 182.3 ± 0.4 Ma; its exact stratigraphic position within the succession is, however, To improve constraints on the age of the Pliensbachian-Toarcian boundary and the age of the lower Toarcian tenuicostatum-hoelderi boundary we used Chron.jl 49 , which is a model framework that allows the interpretation of mineral age spectra in a stratigraphic context. Chron.jl 48 uses a Bayesian Markov chain Monte Carlo (MCMC) model in which stratigraphic superposition is imposed on U-Pb zircon dates 49 . The result is an age-depth model incorporating dates from all beds above and below each sample to produce an internally consistent age (Fig. 3B,C.). This model allowed us to extrapolate ages at specific 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 FO of Dactylioceras (Eodactylites), and interpolated the age to be 183.73 + 0.35/− 0.50 Ma (Fig. 3). A similar exercise was performed for the tenuicostatum-hoelderi zone boundary  (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 11 m, 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 to 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 significantly 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. T max °C values range from 296 to 506 °C throughout the section, Hydrogen Index values range from 3 to 23 mg HC/gTOC, and S2/S3 < 1 (S2 = mg hydrocarbons/ g rock, S3 = mg CO 2 / rock; RockEval data are available in supplementary data file), 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 59 .
The carbon-isotope profile of Chacay Melehue can be chemostratigraphically correlated to other biostratigraphically well-constrained sections, specifically to the base-Toarcian GSSP in Peniche, Portugal 6 (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. Additionally, the combined chemo-, chrono-and biostratigraphic framework from Chacay Meleue is here also compared and correlated with other stratigraphically well-constrained sections such as from the Mochras borehole, Cardigan Bay Basin, UK 45,60,61 and Almonacid de la Cuba, Teruel Basin, Spain 51 (Fig. 4, Supp. Fig. 2). In

Age implications of Chacay Melehue chemo-, chrono-and biostratigraphy for the Pliensbachian-Toarcian boundary and T-OAE
The onset of environmental perturbations at the Pl-To boundary likely resulted in global warming, oceanic anoxia, intensified weathering, and a calcification 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. 8,64 suggested that the long-term environmental change that resulted in pulsed extinction events in the Pliensbachian-Toarcian appear to have been associated with the onset and peaks of intrusive magmatism in Karoo, Ferrar and silicic volcanism in Chon Aike (Figs. 1, 5); however, these igneous provinces are chemically distinct, and resulted in different environmental impacts. For example, the Karoo LIP was emplaced relatively rapidly and intruded into Permian organic-rich sediments 65-68 (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 69 . 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 6 ( Fig. 4).
In addition, the Chacay Melehue section provides new constraints for the age of the Pl-To boundary at ~ 183.73 + 0.35/− 0.50 Ma, as well as for the tenuicostatum-serpentinum zonal boundary at ~ 182.77 + 0.11/− 0.15 Ma, with the latter occurring stratigraphically close to the onset of the negative carbonisotope excursion associated with the T-OAE.
These dates and zonal durations are consistent with recent astrochronological estimates for the ages of this boundary 60 , which suggest a million-year duration for the earliest Toarcian tenuicostatum (or concurrent polymorphum) Zone 7,60,70-73 . Furthermore, astrochronological constraints on the duration of the Pl-To negative CIE Figure 5. Distribution of 206 Pb/ 238 U absolute ages from the Karoo and Ferrar Large Igneous Provinces, and dates from this study on a numerical timescale, relative to the carbon-isotope data (2-pt moving average) from the Mochras Borehole (Cardigan Bay Basin, UK 45,60 ), spanning the Pliensbachian-Toarcian transition. The numerical timescale is obtained using age tie-points for the ammonite zone boundaries based on geochronological constraints from this study, and linear interpolation in between. Major carbon-cycle perturbations are also indicated. Magnetic polarity scale adapted from Hesselbo et al. 1 7,72 , which agrees with the geochronological constraints on the duration of this event, as illustrated here. Integrated global correlation of the Chacay Melehue data with other successions well documented by ammonite biostratigraphy, chemostratigraphy, magnetostratigraphy and/or geochronology (Fig. 4), demonstrate that the Pl-To boundary event may be tied to the onset of LIP activity in Karoo but pre-dates the peak of substantial magmatism in Karoo and Ferrar by ~ 400 kyr (Fig. 5). This relationship between the Pl-To boundary event and the onset of Karoo magmatic activity is further supported by the increase in elemental mercury in the Chacay Melehue section, and correlative records (Fig. 4), inferred to have been volcanogenically derived and transported through the atmosphere before final deposition in marine sediments.