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Proc Natl Acad Sci U S A. Apr 13, 2010; 107(15): 6721–6725.
Published online Mar 22, 2010. doi:  10.1073/pnas.1001706107
PMCID: PMC2872409
Geology, Evolution

Compound-specific carbon isotopes from Earth’s largest flood basalt eruptions directly linked to the end-Triassic mass extinction

Abstract

A leading hypothesis explaining Phanerozoic mass extinctions and associated carbon isotopic anomalies is the emission of greenhouse, other gases, and aerosols caused by eruptions of continental flood basalt provinces. However, the necessary serial relationship between these eruptions, isotopic excursions, and extinctions has never been tested in geological sections preserving all three records. The end-Triassic extinction (ETE) at 201.4 Ma is among the largest of these extinctions and is tied to a large negative carbon isotope excursion, reflecting perturbations of the carbon cycle including a transient increase in CO2. The cause of the ETE has been inferred to be the eruption of the giant Central Atlantic magmatic province (CAMP). Here, we show that carbon isotopes of leaf wax derived lipids (n-alkanes), wood, and total organic carbon from two orbitally paced lacustrine sections interbedded with the CAMP in eastern North America show similar excursions to those seen in the mostly marine St. Audrie’s Bay section in England. Based on these results, the ETE began synchronously in marine and terrestrial environments slightly before the oldest basalts in eastern North America but simultaneous with the eruption of the oldest flows in Morocco, a CO2 super greenhouse, and marine biocalcification crisis. Because the temporal relationship between CAMP eruptions, mass extinction, and the carbon isotopic excursions are shown in the same place, this is the strongest case for a volcanic cause of a mass extinction to date.

Keywords: astrochronology, CO2, Jurassic, large igneous provinces, n-alkane

Plants record through photosynthetic pathways the atmospheric values of An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq1.jpg that in turn reflect the exchangeable surface oceanic carbon reservoir (1). One of the most direct known plant proxies is the An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq2.jpg measurements of n-C25n-C31 n-alkanes derived from leaf wax lipids of plant cuticles (2). We analyzed An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq3.jpg, the carbon isotopic composition of wood (An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq4.jpg), and total organic carbon (An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq5.jpg) from sediments from two overlapping lacustrine sections interbedded with the lavas of the Central Atlantic magmatic province (CAMP) (3) in eastern North America to obtain a carbon isotope record unambiguously tied to the eruptions and climate proxies to compare with and calibrate organic carbon and carbonate An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq6.jpg records from elsewhere (37). This allows direct determination of the relationship between one of the largest Phanerozoic mass extinctions, the end-Triassic extinction (ETE) (8), carbon isotopic and CO2 (9) excursions, the biocalcification crisis (10), and their proposed cause, the CAMP (3, 1113). Core and outcrop samples were obtained from the Newark (New York, New Jersey, and Pennsylvania) and Hartford (Connecticut and Massachusetts) rift basins at 19–20° N paleolatitude (14) in the tropical humid to arid transition of central Pangea (Fig. 1). A hierarchy of Milankovitch-forced lake level cycles permeate the sampled strata in these basins, and in conjunction with a high sediment accumulation rate and magnetostratigraphic and radioisotopic calibration (SI Text and Datasets S1S3), a 2.4 m.y. unbroken record of events surrounding the ETE can be placed into the high-precision (< 20 ky) Newark/Hartford basin astronomically calibrated geomagnetic polarity time scale (NBAGPTS) (1517) (Fig. 2). The combined section is registered to the marine record by high-resolution magnetostratigraphy (17, 18), high-precision U-Pb ages, and cyclostratigraphy (7, 19), and spans the late Rhaetian of the Late Triassic to the early Sinemurian of the Early Jurassic (Fig. 2). Our data indicate the n-alkanes from these strata preserve an original leaf wax signal (SI Text).

Fig. 1.
Map of Pangea at 201 Ma showing the distribution of the CAMP and the localities discussed in the text: 1, St. Audrie’s Bay; 2, Newark basin; 3, Hartford basin; 4, Kennecott Point; 5, Val Adrara; 6, Moroccan CAMP sections. Base map (orthographic ...
Fig. 2.
Compound-specific, wood, and total organic carbon isotopes from Newark and Hartford basin strata interbedded with the CAMP lava flows (diagonal hachures) compared to marine total organic carbon (An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq7.jpg) records from St. Audrie’s Bay (and East Quantoxhead), ...

Carbon Isotope Results

All three sets of carbon isotopic data from the Newark and Hartford basin sections show considerable variability seemingly tied in part to the lacustrine cyclicity (spanning approximately 9‰ in An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq9.jpg, approximately 7‰ in An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq10.jpg, and approximately 15‰ in An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq11.jpg) through the section, but show largely parallel trends around the ETE (Fig. 2). Specifically, there is an approximately 2–4‰ shift to 13C-depleted values exactly at the ETE and associated fern spike (16) as reflected in the Newark and Hartford basins. The duration of this distinct excursion in the Newark and Hartford basins is only about 20–40 ky. This is succeeded by a distinct 13C-enriched interval (by approximately 3–4‰) lasting another 20–40 ky, totaling about 80 ky in the negative-positive couplet. The An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq12.jpg values then return to more 13C-depleted values and oscillate through the rest of the record. Only the wood data seems to show any sort of distinct postcouplet pattern with a tendency to more 13C-depleted values in the CAMP interval. Our carbon isotopic record below the ETE is constrained to a few hundred thousand years by an absence of strata containing organic matter.

We compared this composite record to a dataset from Late Triassic–Early Jurassic age predominantly marine strata at St. Audrie’s Bay, England (Fig. 2). Previous studies of this section include a key analysis of An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq13.jpg that has framed the discussion about the cause of the ETE event (4) and more recent data that carries the isotopic record into the Sinemurian (7) (Fig. 2). This An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq14.jpg record was originally described as displaying an “initial” isotopic excursion enriched in 12C beginning at the onset of the ETE, and an upper, “main” 12C-enriched excursion coinciding with the first appearance datum of Jurassic ammonites (4, 20) (Fig. 2), but the data extending the section upward show conclusively that the main excursion does not exist but rather just reflects the base of a protracted interval of 12C enrichment (7).

Correlation between the eastern North American continental sections and the St. Audrie’s Bay marine record is afforded by independent non-carbon-isotopic means, specifically the base of the ETE below and the Hettangian–Sinemurian transition above (Fig. 2), with the interval between them being calibrated by the NBAGPTS, magneto- and cyclostratigraphy (7, 17), and radioisotopic ages (21) (SI Text). Given these constraints and any range of plausible accumulation rates for St. Audrie’s Bay, available carbon data can be compared to the eastern North American records described here.

The accuracy of the correlation between St. Audrie’s Bay and the Newark–Hartford section is corroborated by a comparison between the cyclostratigraphically defined duration of the Hettangian and radioisotopic dates from eastern North America and elsewhere. The duration of the Hettangian at St. Audrie’s Bay from the projected position of the first appearance of Psiloceras spelae to the position of the Hettangian–Sinemurian boundary at the global stratotype section and point (GSSP) is 1.8 m.y. (7). If the age of the projected position of P. spelae in the Newark–Hartford section is placed at 100 ky after the age of the oldest eastern North American basalt (i.e., 201.3 Ma) the cyclostratigraphic age of the Hettangian–Sinemurian boundary (based on the correlations to the GSSP in ref. 7) is 199.5 Ma, in complete agreement with the independent marine radioisotopic age (22), which again yields a duration of 1.8 m.y. (201.3 - 199.5 = 1.8).

Our An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq15.jpg measurements from St. Audrie’s Bay (SI Text) track for the most part variations in An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq16.jpg in the initial excursion (Fig. 4) and, like the An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq17.jpg (7), appear cyclical. However, the initial excursion is unusual because the C25 n-alkane is the most abundant and the most 13C-depleted in the samples (SI Text). Given that all of the St. Audrie’s samples share the same postdepositional history, this difference is most simply explained as a change in contribution of different organisms, a possibility suggested for the origin of the initial excursion at other localities (6). Thus, a change in community structure is likely responsible for at least part of the initial excursion as seen at the St. Audrie’s Bay section. This interval shows an unusual abundance of fern spores but no exceptional concentration of organic-walled aquatic organisms at St. Audrie’s Bay (20), suggesting a terrestrial ecosystem change. The duration of the initial excursion at St. Audrie’s Bay has been independently estimated at 20–40 ky (7) in agreement with our estimates based on the Newark–Hartford data.

Fig. 4.
Detailed comparison and correlation between the Newark and Hartford compound-specific carbon isotopes of the weighted mean odd C25–C31 n-alkanes (An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq18.jpg) from lacustrine rocks interbedded with the CAMP flood basalt flows (from Fig. 2) with environmentally ...

The initial excursion thus can be identified as beginning at the extinction level in the Newark basin possibly extending through the time of the deposition of the oldest CAMP lava flows. Critically, extinctions persist into strata postdating the initial excursion (e.g., conodonts) at St. Audrie’s Bay (20) and elsewhere, demonstrating that extinctions persisted a short time after the initial CAMP pulse.

Discussion

Both the combined Newark and Hartford An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq22.jpg and the St. Audrie’s Bay An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq23.jpg data exhibit oscillations in values younger and older than the initial excursion that are not directly connected with extinctions, but rather track the lacustrine cyclicity and exhibit coherent power around an approximately 100 ky periodicity (Fig. 3) (SI Text). An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq24.jpg and An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq25.jpg are also coherent around approximately 100 ky. The magnitude of these oscillations decreases after the positive excursion following the initial excursion, and we suggest they reflect climate-related ecological changes altering both the taxonomic composition of the plant communities supplying the waxes and the isotopic discrimination of the members of the community under highly stressed conditions. With only the two available fiducials (the ETE and the Sinemurian–Hettangian boundary), the St. Audrie’s An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq26.jpg and An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq27.jpg data also exhibit coherent approximately 100 ky periodicity using the NBAGPTS for temporal constraints. The thickness periodicities are also fully consistent with those seen in An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq28.jpg, percent total organic carbon, and percent carbonate described in ref. 7 (Fig 3), again very strongly suggesting that both the time scale and the correlation between the two sections are correct.

Fig. 3.
Comparison between power spectra of carbon isotopic data from the Newark and Hartford basins and the St. Audrie’s Bay section using the same time scale (NBAGPTS) based on the correlation to the base of the ETE and the Hettangian–Sinemurian ...

Although the isotopic excursion and ETE predate the onset of CAMP in the Newark and Hartford basins, the lava flows in these basins do not reflect the full temporal extent of the eruptions. The Culpeper basin of Virginia and Argana and Central High Atlas basins of Morocco demonstrate that the CAMP eruptions were more extensive than the three flow sequences preserved in the Newark and Hartford basins (23) (Fig. 4). In addition, some CAMP flows in Morocco are most likely slightly older than those recognized in eastern North America (24, 25) (SI Text). These older flows may be synchronous with the initiation of the ETE.

It is unlikely that the initial excursion seen at St. Audrie’s Bay and in eastern North America was generated by mantle CO2 alone, because of the similarity in atmospheric and mantle An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq34.jpg values (26). Hence, an injection of 12C carbon from CAMP-triggered methane clathrates (4, 9) dissociation or thermogenic methane (27) release from intrusive metamorphism (13) has been hypothesized. One alternative is that an intensification of the hydrological cycle, as a result of greater CO2-forced warming causing an increase in available moisture driving greater isotopic discrimination (28) caused a shift toward more 13C-depleted values in the plant wax n-alkanes. This is entirely consistent with the enhanced lacustrine cyclicity and n-alkane cyclicity in the Newark and Hartford data after the ETE. A concurrent shutoff of the biological pump would result in a homogenization of the water column caused by the extinction of zooplankton grazers (cf. K-T boundary in ref. 29) with surface waters becoming enriched in 12C. The short duration of the initial excursion is also incompatible with the modeled estimates of the recovery time from a methane clathrates dissociation event by over an order of magnitude (20–40 ky compared to 700–1000 ky from ref. 26). Other proposed killing mechanisms include CAMP-outgassed or metamorphism-related volatiles such as sulfuric acid (27) or halogens (13, 30); however, none of these models have yielded unique predictions testable in the sedimentary record. We also cannot exclude a role for a bolide impact as a killing mechanism, although evidence consisting mostly of modest Ir anomalies (16) is also compatible with a volcanic origin.

Carbonate An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq35.jpg records from St. Audrie’s Bay (31) track the An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq36.jpg and An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq37.jpg data, which in turn match the variations in the Newark and Hartford An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq38.jpg data that are orbitally paced. Because the St. Audrie’s Bay carbonate data are unlikely to have been affected by local community change without some process linking them via the exchangeable carbon reservoirs, there seems to have been a significant role for orbital cycles pacing the Earth’s exchangeable reservoirs as has been suggested for a number of other isotopic excursions (32). However, the behavior of n-alkanes in the initial excursion is qualitatively different from the other fluctuations, suggesting a different origin such as the effects of the initial influx of greenhouse gases and the extinctions themselves, the exact expression of which would be influenced by synergetic changes in the local conditions and local communities (e.g., ref. 6). The long-term maintenance of relatively 13C-depleted average values in the Jurassic for at least the length of the Hettangian and early Sinemurian are plausibly the result of nonorbital changes, such as a long-term change in the biological pump, perhaps related to the extinction itself (e.g., ref. 29).

Our data also allow isotopic, CO2 proxy, and floral data from Jamesonland, Greenland (9) to be compared to Newark and Hartford data and the CAMP (Fig. 4). The 13C-depleted interval in the Jamesonland An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq39.jpg data plausibly corresponds to a similar interval seen in the Newark and Hartford An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq40.jpg data and the interval of CAMP flows in eastern North America and Morocco. In contrast, concentrated floral extinctions are limited to the older CAMP flows. Given this correlation, the interval of elevated CO2 based on plant stomatal data corresponds nearly exactly to the CAMP episode, strongly supporting a direct role for the eruptions.

Osmium isotopic data from St. Audrie’s Bay (33) (Fig. 4) agree with this interpretation and suggest that the CAMP was supplying weathering products to the ocean very soon after the initiation of eruption shortly after the ETE. 187Os/188Os values shift about 0.4 from higher to lower values over the ETE, suggesting an input of unradiogenic Os of mantle (or extraterrestrial) origin or a reduction of continentally derived Os or both, but the fact that Os concentrations increase over the same interval (33) suggests the former, namely the weathering products of the CAMP.

Conclusions

With this correlation of isotopic excursions and marine stratigraphy to the CAMP, it is possible to refine the sequence of marine and continental biotic events in relation to the eruptions (Fig. 4). The onsets of marine and continental extinctions were simultaneous with each other and the initial excursion, with the input of CO2 leading to an oceanic biocalcification crisis (10). In the northern tropics, the continental extinction is extremely abrupt with the disappearance of about 50% of the palynoflora and a synchronous extinction among tetrapods (16). Subsequently, there was a regional fern spike followed by macrofossil first appearances of several fern taxa and cheirolepidaceous conifers with adaptations for environmental stress (34) coupled with a flood of the pollen species Classopollis meyeriana (23) [seen also in higher latitudes (35)]: These overlap the known approximately 300 ky eruptive maximum of CAMP (23). Footprint evidence indicates that theropod dinosaurs abruptly increased in size and relative abundance during this time (16). This is followed by a slow return to larger leaved cheirolepidaceous conifers and different species of Classopollis, comprising a late Hettangian floral recovery about 1 m.y. after the beginning of the initial excursion and directly following 13C-enriched interval.

Our n-alkane data also reveal changes in the contributing organisms that vary both temporally and geographically, as would be expected of a community response to a significant increase in temperature and/or humidity caused by enhanced CO2 forcing (36).

The negative An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq41.jpg excursions in the Newark, Hartford, and St. Audrie’s Bay documented by the n-alkane, wood, total organic carbon data, and the correlations to Jamesonland strongly suggest a massive input of 13C-depleted CO2 and/or other greenhouse gases coincident with the onset of CAMP. The very tight association between the onset of the CAMP and the abrupt extinctions in the tropics and subtropics, the more protracted extinctions well into the CAMP episode in the higher latitudes during the time of maximum eruptive rate, and the association of the elevated CO2 levels tied to the entire known CAMP episode strongly argues for a major causative role for CAMP-generated CO2 in the extinctions themselves. Our interpretation of the carbon isotopic data from these continental and marine sections provides strong direct evidence by direct superposition that the eruption of a giant flood basalt province could cause a climatic catastrophe resulting in a major mass extinction.

Methods

For n-alkane extraction, approximately 4–12 g of powdered sedimentary rock samples were extracted on a Dionex Accelerated Solvent Extraction system using three washes of dichloromethane (DCM) or 9[ratio]1 DCM/methanol to produce a total lipid extract (TLE). The TLE was saponified with 0.5N KOH/methanol, and then extracted with hexane. This neutral extract was dried with anhydrous Na2SO4 to remove traces of water, then eluted from a silica gel column (1 mL bed volume, silica gel 2% deactivated) into two fractions, first a hexane elutable and second a combined DCM and methanol elutable fraction. If required, the hexane elutable fraction was further fractionated into adduct [straight chain (C≥14) n-alkanes] and nonadduct fractions by urea adduction, or by sequestering the straight chain alkanes in zeolites, rinsing with hexane, and alkane recovery by dissolution of the zeolite with aqueous HF.

Compound-specific carbon isotopic measurements were determined by isotope ratio monitoring–gas chromatography/mass spectrometry using a Thermo DeltaVPlus MS coupled to an Agilent 6890 GC via a GCC-III combustion interface at Brown University. The δ13C values for individual compounds were determined based on introduction of reference CO2 gas pulses (previously and subsequently calibrated with a series of well-characterized standard materials), reported as means of duplicate runs (σ =  ± 0.3 to 0.6), and expressed in ‰ relative to the Pee Dee belemnite (PDB). The bulk organic matter of sediment wood was analyzed to determine the ratio of 13C/12C by mass spectrometry. The samples were cleaned with deionized water, air-dried, ground into a fine powder with a ceramic mortar and pestle, fumed with 37% HCl in a bell jar at 60 °C for 50 h (to remove recalcitrant carbonate), and dried above a plate of silica gel desiccant at 60 °C for at least 24 h. All samples were weighed into silver capsules, with mass determined by total organic-carbon (TOC) content, grouped according to TOC, and processed with an automated micro-Dumas combustion technique using a Europa ANCA system plumbed into a 20-20-NT continuous flow mass spectrometer system at Lamont-Doherty Earth Observatory. C isotope ratios were measured against National Institute of Standards and Technology and International Atomic Energy Agency standard reference materials and combusted in the same manner as the samples (glucosamine, An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq42.jpg, C = 20.50%; methionine, An external file that holds a picture, illustration, etc.
Object name is pnas.1001706107eq43.jpg, C = 40.25% (Cornell only), all versus the PDB (13C/12C = 11237.2 ± 60 × 10-6). Precision of the analytical system is 0.12‰ for C at the typical sample sizes (4 μm C) used here.

Time series analysis was performed using Analyseries 2.037 (37).

Supplementary Material

Supporting Information:

Acknowledgments.

We thank Daniel Montluçon, Carl Johnson, and Camly Tran for laboratory assistance. We are grateful to Micha Ruhl for permission to use the St. Audrie’s Bay δ13C curve from refs. 7 and 19. We also thank Michael Rampino and Randall Irmis for reviewing the manuscript. J.H.W. and P.E.O. acknowledge support from the US National Science Foundation (Grant EAR 0801138 to J.H.W. and Grant EAR 0753496 to P.E.O.).

Footnotes

The authors declare no conflict of interest.

See Commentary on page 6555.

This article contains supporting information online at www.pnas.org/cgi/content/full/1001706107/DCSupplemental.

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