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Mercury Evidence for Pulsed Volcanism During the End-Triassic Mass Extinction

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Mercury Evidence for Pulsed Volcanism During the End-Triassic Mass Extinction Mercury evidence for pulsed volcanism during the end-Triassic mass extinction Lawrence M. E. Percivala,1,2, Micha Ruhla, Stephen P. Hesselbob, Hugh C. Jenkynsa, Tamsin A. Mathera, and Jessica H. Whitesidec aDepartment of Earth Sciences, University of Oxford, Oxford OX1 3AN, United Kingdom; bCamborne School of Mines and Environment and Sustainability Institute, University of Exeter, Penryn TR10 9FE, United Kingdom; and cOcean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom Edited by Edward A. Boyle, Massachusetts Institute of Technology, Cambridge, MA, and approved May 19, 2017 (received for review April 3, 2017) The Central Atlantic Magmatic Province (CAMP) has long been records (Fig. 1A and refs. 12 and 14). The apparent episodic em- proposed as having a causal relationship with the end-Triassic placement of CAMP basalts over several hundred kiloyears, at least extinction event (∼201.5 Ma). In North America and northern in Morocco and North America, is a key feature of this LIP. Africa, CAMP is preserved as multiple basaltic units interbedded The oldest known CAMP basalts are the lower Moroccan unit with uppermost Triassic to lowermost Jurassic sediments. How- [termed the Lower Formation in the High Atlas and the Tasguint ever, it has been unclear whether this apparent pulsing was a local Formation in the Argana basin (17, 18)]. This unit is overlain by feature, or if pulses in the intensity of CAMP volcanism charac- two further Moroccan basalt units: the Middle and Upper for- terized the emplacement of the province as a whole. Here, six mations in the High Atlas [the former named the Alemzi For- geographically widespread Triassic–Jurassic records, representing mation in Argana (17, 18)]. For clarity, the High Atlas names are varied paleoenvironments, are analyzed for mercury (Hg) concen- used henceforth in this study. These three basalt groups are trations and Hg/total organic carbon (Hg/TOC) ratios. Volcanism is interbedded with sedimentary deposits. A fourth extrusive unit, a major source of mercury to the modern environment. Clear in- the Recurrent Formation, is locally preserved higher in the creases in Hg and Hg/TOC are observed at the end-Triassic extinc- Moroccan sequence, with much thicker sediments between it and tion horizon, confirming that a volcanically induced global Hg the Upper Formation (Fig. 2B and refs. 14, 17, and 19). These cycle perturbation occurred at that time. The established correla- four basalt units are defined and correlated on the basis of distinct tion between the extinction horizon and lowest CAMP basalts igneous geochemistry. Based on geochemical correlation with allows this sedimentary Hg excursion to be stratigraphically tied to a specific flood basalt unit, strengthening the case for volcanic North American CAMP units, which are temporally constrained by astrochronological and radioisotopic geochronology, the Hg as the driver of sedimentary Hg/TOC spikes. Additional Hg/TOC – peaks are also documented between the extinction horizon and Moroccan Lower Upper formations are thought to have erupted the Triassic–Jurassic boundary (separated by ∼200 ky), supporting in quick succession, with several hundred kiloyears then passing pulsatory intensity of CAMP volcanism across the entire province before the eruption of the Recurrent Formation (2, 14). At least three major CAMP units are documented in North and providing direct evidence for episodic volatile release during EARTH, ATMOSPHERIC, the initial stages of CAMP emplacement. Pulsatory volcanism, and America: the Orange Mountain, Preakness, and Hook Mountain AND PLANETARY SCIENCES associated perturbations in the ocean–atmosphere system, likely basalts in the Newark Basin (20), with time-equivalent basalts had profound implications for the rate and magnitude of the end- known from other North American basins. The Orange Moun- Triassic mass extinction and subsequent biotic recovery. tain Basalt overlies thin, usually lacustrine, sediments deposited above the extinction horizon, and is thought to have been mercury | end-Triassic extinction | Central Atlantic Magmatic Province Significance he end of the Triassic Period was marked by a major mass Textinction event (∼201.5 Ma; e.g., refs. 1 and 2), one of the The end of the Triassic Period (∼201.5 million years ago) wit- five largest environmental perturbations of the Phanerozoic Eon. nessed one of the largest mass extinctions of animal life known Significantly increased extinction rates of marine fauna, and major from Earth history. This extinction is suggested to have coin- turnovers in terrestrial vegetation and vertebrate groups, have been cided with and been caused by one of the largest known epi- well documented (e.g., refs. 3–8). The end-Triassic mass extinction sodes of volcanic activity in Earth’s history. This study examines predated the onset of the Jurassic by ∼100 ky to 200 ky, as defined mercury concentrations of sediments from around the world by the first occurrence of the Jurassic ammonite species Psiloceras that record this extinction. Mercury is emitted in gaseous form spelae (9). The sedimentary record of the extinction correlates with during volcanism, and subsequently deposited in sediments. alarge(upto6‰) negative excursion in organic carbon isotopes We find numerous pulsed elevations of mercury concentrations 13 in end-Triassic sediments. These peaks show that the mass ex- (δ Corg:Fig.1A), indicative of a severe carbon cycle perturbation coincident with the biotic crisis (e.g., refs. 10–13). Moroccan strata tinction coincided with large-scale, episodic, volcanism. Such that record this global carbon cycle perturbation are transected by episodic volcanism likely perturbed the global environment over the lowest documented flows of the Central Atlantic Magmatic a long period of time and strongly delayed ecological recovery. Province (CAMP). Consequently, the end-Triassic extinction has Author contributions: L.M.E.P. performed research; L.M.E.P., M.R., and J.H.W. analyzed been postulated as precisely coincident with the onset of known data; L.M.E.P., M.R., S.P.H., H.C.J., T.A.M., and J.H.W. wrote the paper; and M.R., S.P.H., CAMP volcanism (e.g., refs. 2, 12, 14, and 15). and J.H.W. provided rock samples. CAMP represents the most aerially expansive continental Large The authors declare no conflict of interest. Igneous Province (LIP) known on Earth, consisting of volumetri- This article is a PNAS Direct Submission. × 6 2 cally large-scale flood basalt sequences covering at least 7 10 km 1To whom correspondence should be addressed. Email: [email protected]. across four continents and both hemispheres (Fig. 2A and ref. 16). 2 Present address: Institute of Earth Sciences, Géopolis, University of Lausanne, CH-1015 In North America and Morocco, CAMP basalts are interbedded Lausanne, Switzerland. with continental sediments, which have precise temporal constraints This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. and are stratigraphically well correlated with marine sedimentary 1073/pnas.1705378114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1705378114 PNAS | July 25, 2017 | vol. 114 | no. 30 | 7929–7934 Downloaded by guest on September 26, 2021 AB Fig. 1. (A) Stratigraphic correlation of the end-Triassic extinction with the Moroccan Lower Formation CAMP basalt. Argana lithology, carbon isotope, and paleomagnetic data are from ref. 14. Newark lithology and pCO2 data are from ref. 22; paleomagnetic and astrochronological data are from ref. 48; carbon isotope data are from ref. 12; and trilete spore data are from ref. 49. St Audries Bay lithology and astrochronology are from refs. 42 and 50; biostratigraphy and carbon isotope data are from refs. 10 and 50; trilete spore data are from ref. 51; and paleomagnetic data are from refs. 52 and 53. Stratigraphic cor- relation of CAMP units between Argana, Newark, and St Audries Bay is based on refs. 12 and 42. Kuhjoch biostratigraphy, lithology, and carbon isotope data are from ref. 11. The end-Triassic extinction horizon (marked as ETE) and Triassic–Jurassic boundary (marked as TJB) are also shown. (B) Example of Hg/TOC dataset from this study (Kuhjoch, Fig. 3) is shown to stratigraphically correlate with the lowest CAMP basalt unit that intersects the end-Triassic extinction horizon at Argana. See SI Appendix, Fig. S2 for a full stratigraphic correlation of end-Triassic records. extruded 14 ky to 20 ky after that event (2, 14, 21). Radioisotopic emplacement (22). However, the effect of local processes, such as dating has demonstrated that the Preakness Basalt and Hook diagenesis, on this record cannot be ruled out. Reconstructions of Mountain Basalt were emplaced 270 ky and 620 ky later, re- pCO2 based on stomatal indices (albeit at low temporal resolu- spectively (2). The Newark Basin Orange Mountain and Hook tion) show no such pulsing during the Triassic–Jurassic transition Mountain basalts have been geochemically established as equiv- (6, 23). Nor is there yet evidence of episodic CO2 increases as- alent to the Moroccan Middle and Recurrent formations, re- sociated with the early Moroccan CAMP pulses (CAMP pulses 1, spectively (14); the Preakness Basalt has no known Moroccan 3, and 4 in Fig. 2B) that were extruded coincident with, and in the counterpart (2, 14). Consequently, North American and Moroccan immediate aftermath of, the extinction event. records suggest that CAMP was emplaced in at least three
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