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Geochronology, 3, 181–198, 2021 https://doi.org/10.5194/gchron-3-181-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. An evaluation of Deccan Traps eruption rates using geochronologic data Blair Schoene1, Michael P. Eddy2, C. Brenhin Keller3, and Kyle M. Samperton4 1Department of Geosciences, Princeton University, Princeton, NJ 08544, USA 2Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN 47907, USA 3Department of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA 4Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA Correspondence: B. Schoene ([email protected]) and Brenhin Keller ([email protected]) Received: 14 April 2020 – Discussion started: 5 May 2020 Revised: 22 February 2021 – Accepted: 23 February 2021 – Published: 16 April 2021 Abstract. Recent attempts to establish the eruptive history in constant eruption rates with relatively large uncertainties of the Deccan Traps large igneous province have used both through the duration of the Deccan Traps eruptions and pro- U−Pb (Schoene et al., 2019) and 40Ar=39Ar (Sprain et al., vides no support for (or evidence against) the pulses identi- 2019) geochronology. Both of these studies report dates with fied by the U−Pb data, (2) the stratigraphic positions of the high precision and unprecedented coverage for a large ig- Chicxulub impact using the 40Ar=39Ar and U−Pb datasets do neous province and agree that the main phase of eruptions be- not agree within their uncertainties, and (3) neither dataset gan near the C30n–C29r magnetic reversal and waned shortly supports the notion of an increase in eruption rate as a re- after the C29r–C29n reversal, totaling ∼ 700–800 kyr dura- sult of the Chicxulub impact. We then discuss the importance tion. These datasets can be analyzed in finer detail to deter- of systematic uncertainties between the dating methods that mine eruption rates, which are critical for connecting vol- challenge direct comparisons between them, and we high- canism, associated volatile emissions, and any potential ef- light the geologic uncertainties, such as regional stratigraphic fects on the Earth’s climate before and after the Cretaceous– correlations, that need to be tested to ensure the accuracy of Paleogene boundary (KPB). It is our observation that the eruption models. While the production of precise and accu- community has frequently misinterpreted how the eruption rate geochronologic data is of course essential to studies of rates derived from these two datasets vary across the KPB. Earth history, our analysis underscores that the accuracy of The U−Pb dataset of Schoene et al. (2019) was interpreted a final result is also critically dependent on how such data by those authors to indicate four major eruptive pulses be- are interpreted and presented to the broader community of fore and after the KPB. The 40Ar=39Ar dataset did not iden- geoscientists. tify such pulses and has been largely interpreted by the com- munity to indicate an increase in eruption rates coincident with the Chicxulub impact (Renne et al., 2015; Richards et al., 2015). Although the overall agreement in eruption du- 1 Introduction ration is an achievement for geochronology, it is important to clarify the limitations in comparing the two datasets and There is increasing recognition that volcanic activity can im- to highlight paths toward achieving higher-resolution erup- pact global climate on both human and geologic timescales. tion models for the Deccan Traps and for other large igneous This relationship is apparent from historical explosive erup- provinces. Here, we generate chronostratigraphic models for tions (Minnis et al., 1993; Robock, 2000) and inferred for both datasets using the same statistical techniques and show larger, effusive eruptions through the Phanerozoic (Ernst and that the two datasets agree very well. More specifically, we Youbi, 2017; Self et al., 2014). Mafic large igneous provinces infer that (1) age modeling of the 40Ar=39Ar dataset results (LIPs) have been correlated with brief hyperthermal climate episodes such as the Paleocene–Eocene Thermal Maximum Published by Copernicus Publications on behalf of the European Geosciences Union. 182 B. Schoene et al.: An evaluation of Deccan Traps eruption rates using geochronologic data (PETM), as well as several mass extinctions (Bond and Wig- studies attempted to use their respective datasets to calculate nall, 2014). The reasons for such disastrous climate and eruption rates by estimating the volume of erupted basalts ecosystem responses remain a focus of debate among Earth as a function of time. The original plots used to illustrate historians. Critical to this discussion are precise chronologies the eruption rates, however, can be easily interpreted to show of LIP eruptions, particularly since they have never been ob- that the two geochronological datasets disagree significantly served in recorded human history. Advances in geochrono- (Fig. 2). Schoene et al. (2019) use the U−Pb dataset to ar- logical techniques and applications over the last 2 decades gue that the Deccan Traps erupted in four distinct pulses have evolved to show that LIPs erupt > 105 km3, usually in separated by relative lulls in volcanism that lasted up to less than a million years, as opposed to tens of millions as 100 kyr or more. Sprain et al. (2019) plot the 40Ar=39Ar previously thought (Burgess and Bowring, 2015; Kasbohm dataset in a way that gives the impression that there was et al., 2020; Davies et al., 2017; Svensen et al., 2012). How- a large increase in eruption rate associated with the Chicx- ever, large uncertainties remain regarding the rates of extru- ulub impact, though this was not the intent of the authors sive versus intrusive magmatism, as well as the flux of vol- (Sprain, 2020). This was a key message sent by the asso- canic versus non-eruptive volatiles, such as CO2 and SO2, ciated “News and Views” piece in the same issue of Sci- that are thought to drive climate change (Black and Manga, ence (Burgess, 2019), and the notion that the U−Pb and 2017; Burgess et al., 2017; Ganino and Arndt, 2009; Self 40Ar=39Ar datasets disagreed substantially has been propa- et al., 2014; Svensen et al., 2004). gated by subsequent discussion and news coverage on Sci- The Deccan Traps, India, is the youngest LIP that is enceMag.org (Kerr and Ward, 2019; Voosen, 2019). Authors temporally associated with a mass extinction, spanning the of subsequent papers (Henehan et al., 2019; Hull et al., 2020; Cretaceous–Paleogene Boundary (KPB) (Fig. 1; Courtillot Linzmeier et al., 2020; Milligan et al., 2019; Montanari and et al., 1988; McLean, 1985). This extinction is also famously Coccioni, 2019; Sepúlveda et al., 2019) also seem to con- associated with collision of the Chicxulub bolide off the clude that the datasets do not agree on the eruption rates southern Mexican coast (Alvarez et al., 1980; Hildebrand of the Deccan Traps and/or that the dataset of Sprain et al. et al., 1991; Smit and Hertogen, 1980), and thus it has been (2019) suggests an inflection in eruption rates of Deccan debated whether or not the Deccan Traps played a role in Traps at the KPB. the extinction (Hull et al., 2020; Keller et al., 2008; Schulte Throughout this paper, we assume that the individual erup- et al., 2010). Furthermore, the temporal coincidence of the tion ages for all samples from each study are accurate as re- two potentially Earth-changing events has led to specula- ported, and while both methods bring uncertainties to this tion about whether the Chicxulub impactor could have had assumption, this permits us to simply discuss how the data an influence on eruption rates in the Deccan Traps (Byrnes in each study were used to determine the eruptive history of and Karlstrom, 2018; Rampino and Caldeira, 1992; Richards the Deccan Traps. In doing so, we show that the conclusion et al., 2015). Impacts and extinction aside, the Deccan Traps that the eruption rates derived from the datasets of Schoene provide an ideal setting in which to investigate the rates of et al. (2019) and Sprain et al. (2019) disagree is incorrect and LIP volcanism within a stratigraphic context because they are that in fact they agree quite well. This confusion has arisen relatively young and contain a well-exposed, accessible, and in part because Fig. 4 in Sprain et al. (2019) that purports well-studied stratigraphy (Fig. 1; Beane et al., 1986; Chenet to plot eruptive flux does not have units of flux or rate and et al., 2009, 2008; Kale et al., 2020; Mitchell and Widdow- is therefore misleading. We apply the same analysis to both son, 1991; Renne et al., 2015; Schoene et al., 2015; Subbarao geochronological datasets, using units of volumetric erup- et al., 2000). tion rate. The results are used to argue that the two datasets Two geochronological datasets appeared in the same issue largely agree at their respective levels of precision, and that of Science in 2019, both with the aim of establishing erup- the lower-precision 40Ar=39Ar dataset does not support or re- tion rates of the Deccan Traps and comparing their eruption fute the model of pulsed eruptions established by the U−Pb history to the climatic and biologic events associated with the dataset. Adequately testing the pulsed eruption model will mass extinction and the timing of the Chicxulub impact. One require higher precision 40Ar=39Ar data, more U−Pb data, paper (Sprain et al., 2019) uses 40Ar=39Ar geochronology and/or an exploration of the stratigraphic correlations used of plagioclase from erupted basalts and the other (Schoene in each study. Furthermore, it is important to remind read- et al., 2019) uses U−Pb geochronology on zircon from in- ers that any eruption rate model is completely dependent on terbeds between basalt flows that are thought to contain ash the model of eruptive volumes, which comes with large and fall deposits.

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