Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185 Comunicações Geológicas (2014) 101, Especial III, 1435-1437 IX CNG/2º CoGePLiP, Porto 2014 ISSN: 0873-948X; e-ISSN: 1647-581X

Deccan Volcanism: a main trigger of environmental changes leading to the K/Pg mass extinction? O vulcanismo do Deccan: maior responsável pelas mudanças ambientais que levaram à extinção em massa do K/Pg? T. Adatte1, A. Fantasia1, B. Samant2, D. Mohabey2, E. Font3, G. Keller4, H. Khozyem5, B. Gertsch1

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Abstract: Recent studies indicate that the bulk (80%) of Deccan trap eruptions occurred over a relatively short time interval in magnetic polarity C29r, whereas multi-proxy studies from central and south-eastern India place the Cretaceous-Paleogene (K/Pg) mass extinction near the end of this main phase of Deccan volcanism suggesting a cause-and-effect relationship. Beyond India multi-proxy studies also place the main Deccan phase in the uppermost Maastrichtian C29r below the K/Pg (planktic foraminiferal zones CF2-CF1), as indicated by a rapid shift in 187Os/188Os ratios in deep-sea sections from the Atlantic, Pacific and Indian Oceans, coincident with rapid climate warming, coeval increase in weathering, a significant decrease in bulk carbonate indicative of acidification due to volcanic SO2, and major biotic stress conditions expressed in species dwarfing and decreased abundance in calcareous microfossils (planktic foraminifera and nannofossils). Keywords: Mass extinction, Deccan volcanism. Resumo: Estudos recentes indicam que a maior parte (80%) das erupções das Trapas do Deccan ocorreram em um curto intervalo de tempo, durante o crono 29r, enquanto estudos multidisciplinares sobre a parte central e sul-leste da India colocam o limite K/Pg e a extinção em massa correspondente no fim desta fase principal do Deccan, o que sugere uma relação de causa a efeito. Além da India, os estudos multidisciplinares colocam a fase principal do Deccan no fim do Maestrichtiano, no crono 29r, abaixo do limite K/Pg (correspondente as biozonas dos foraminíferos planctónicos CF1-CF2), como sugerido pelos variações rápidas do 187Os/188Os das series marinhas profundas do Atlântico, Pacífico e Índico, coincidindo com o rápido aquecimento do clima, aumento da meteorização, diminuição significativa no volume de carbonato indicativa de acidificação devido ao SO2 vulcânica, e as principais condições de stress biótico expressos em espécie de nanismo e diminuição da abundância em microfósseis calcários (foraminíferos planctónicos e nanofósseis). Palavras-chave: Extinção em massa, Vulcanismo do Deccan. 1ISTE, Lausanne University, 1015 Lausanne, Switzerland. 2Department of Geology, Nagpur University, Nagpur 440 001, India. 3IDL-FCUL, Faculdade de Ciências de Lisboa, Lisboa, Portugal. 4Department of Geosciences, Princeton University, Princeton NJ 08540, USA. 5Geology Department, Faculty of Sciences, University of Aswan, Aswan, Egypt. *Corresponding author / Autor correspondente: thierry.adatte@unil.ch

Recent studies indicate that the bulk (80%) of Deccan trap eruptions occurred over a relatively short time interval in magnetic polarity C29r, whereas multi-proxy studies from central and south-eastern India place the Cretaceous-Paleogene (K/Pg) mass extinction near the end of this main phase of Deccan volcanism suggesting a cause-and-effect relationship. Beyond India multi-proxy studies also place the main Deccan phase in the uppermost Maastrichtian C29r below the K/Pg (planktic foraminiferal zones CF2-CF1), as indicated by a rapid shift in 187Os/188Os ratios in deep-sea sections from the Atlantic, Pacific and Indian Oceans, coincident with rapid climate warming, coeval increase in weathering, a significant decrease in bulk carbonate indicative of acidification due to volcanic SO2, and major biotic stress conditions expressed in species dwarfing and decreased abundance in calcareous microfossils (planktic foraminifera and nannofossils). These observations indicate that Deccan volcanism played a key role in increasing atmospheric CO2 and SO2 levels that resulted in global warming and acidified oceans, respectively, increasing biotic stress that predisposed faunas to eventual extinction at the K/Pg.

The nature and causes of mass extinctions in the geological past have remained topics of intense scientific debate for the past three decades. Central to this debate is the question of whether one, or several large bolide impacts, the eruption of large igneous provinces (LIP) or a combination of all them were the primary mechanisms driving the environmental changes that caused four of the five major extinction events recorded during the Phanerozoic.

The K/Pg is the only mass extinction associated with both impact (Chixculub) and flood basalts (Deccan Traps) and therefore an excellent case study to evaluate the potential causes and effects. Deccan eruptions were pulsating, with some gigantic megaflows 1500 km across India and with estimated volumes >10,000 km3 that may have erupted over very short time intervals. Deccan volcanism started in the late Maastrichtian (67.4 Ma) with

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the two main eruptive phases identified, in the latest Maastrichtian and the early Danian, respectively (Chenet et al., 2007). For comparison, the largest historical basalt eruption in 1783 in Iceland (Laki) ejected some 15 km3 of lava in about a year (Chenet et al., 2005). A single Deccan megaflow would have been equivalent to 667 Laki eruptions. The vast amount of carbon and sulphur dioxides injected into the atmosphere from just one Deccan megaflow probably was on the same order of magnitude as those estimated for the Chicxulub impact.

Model results predict that Deccan Traps emplacement was responsible for a strong increase in atmospheric pCO2 accompanied by rapid warming of 4°C (Dessert et al., 2001), followed by global cooling. During the warming phase, increased continental weathering of silicates associated with consumption of atmospheric CO2 likely resulted in a subsequent drawdown of greenhouse gases, leading to global cooling at the end of the Maastrichtian. Massive CO2 input together with massive release of SO2 may thus have triggered a global carbon crisis and the mass extinctions recorded in both the marine realm, as a result of ocean acidification, and the terrestrial areas due to acid rains (Fig. 1). Global stress conditions related to these climatic changes are well known and documented in planktic foraminifera by a marked diversity decrease, species dwarfing, dominance of opportunistic species and near disappearance of specialized species (e.g. Keller & Abramovich, 2009). Recent studies indicate that the bulk (80%) of Deccan trap eruptions (phase-2) occurred over a relatively short time interval in magnetic polarity C29r (Chenet et al., 2007). Multi-proxy studies from central and south-eastern India place the Cretaceous-Tertiary (KT) mass extinction near the end of this main phase of Deccan volcanism suggesting a potential cause-and-effect relationship (Keller et al., 2012).

Fig. 1. Flow chart for the model of massive Deccan volcanism as a main trigger of environmental changes leading to the K/Pg mass extinction. Fig. 1. Fluxograma do modelo do vulcanismo massivo do Deccan como principal responsável pelas mudanças ambientais que levaram à extinção em massa do K/Pg.

In the Rajhamundry area (Fig. 2), two Deccan basalt flows, known as the Rajahmundry traps, mark the largest lava flows extending for at least 1000 km across the Indian continent and into the Bay of Bengal (Keller et al., 2008). The sediments directly overlying the lower Rajahmundry trap contain the earliest Danian planktic foraminiferal assemblages of zones P0-P1a and mark the initial evolution in the aftermath of the K-T mass extinction. The

upper Rajahmundry trap was deposited during zone P1b corresponding to the lower part of magnetic polarity C29n. Data from infra- and intertrappean sediments drilled in the Krishna-Godavari Basin by India’s Oil and Natural Gas Corporation reveal swift and devastating effects on marine plankton in India. A 50% drop in diversity of planktic foraminifera preceded the first megaflow, another 50% drop thereafter, leaving just 7 to 8 survivor species. No recovery occurred between the next three mega-flows and the mass extinction was complete with the last phase-2 megaflow at the K/Pg.

Fig. 2. Map of India (Adatte & Keller, 2013) with locations of Rajhamundry and Jhilmili sections. Planktic foraminiferal biostratigraphy places the lower trap in the latest Maastrichtian with the uppermost lava flow near the K/Pg mass extinction; numerical ages from Cande & Kent (1995). Panorama view of Rajhamundry inter-trappean sequence (Balaji Quarry) showing upper and lower basalt trap flows. Fig. 2. Mapa da Índia (Adatte & Keller, 2013) com a localização das seções de Rajhamundry e de Jhilmili. A biostratigrafia dos foraminíferos planctónicos coloca a parte inferior das Trapas de Deccan no Maestrichtiano Superior, com a unidade de lava superior cerca da extinção em massa do K/Pg; idades numéricas de Cande & Kent (1995). Vista panorâmica da sequência sedimentares dos intertrappeanos de Rajhamundry (Balaji Pedreira) mostrando as unidades de lava inferior e superior.

In Central India a strong floral turnover is observed as a direct response to Deccan volcanic phase-2. In Lameta, in the sediments preceding the volcanic eruptions (infratrappean), a palynoflora of gymnosperms and angiosperms with a rich canopy of gymnosperms (Conifers and Podocarpaceae) and an understory of palms and herbs dominates (Samant & Mohabey, 2009). Immediately after the onset of Deccan phase-2, this floral association changed abruptly to a dominance of angiosperms and pteridophytes, at the expense of gymnosperms. In the overlying intertrappean sediments a sharp decrease in pollen and spores coupled with the appearance of fungi mark increasing stress conditions, apparently as a direct result of volcanic activity. The intertrappean sediments corresponding to Phase-2 (80% of Deccan basalt

Deccan Volcanism and K/Pg Mass Extinction 1437

emissions, latest Maastrichtian) are characterized by the highest Chemical Index of Alteration (CIA) values (Fig. 3). This can be explained by increased acid rains due to SO2 emissions rather than a global climatic shift, because clay minerals from the corresponding sediments do not reflect a significant climate change. The increased weathering is coeval with the sharp decline in pollen and an increase in fungal spores observed by Samant & Mohabey (2009) and corresponds to the main phase-2 of Deccan activity.

Fig. 3. Ti/Al ratio, Chemical Index of Alteration (CIA), basalt weathering intensity (K/Fe+Mg) in infra and intertrappean sediments from Nagpur, Central India, comparison with palynological data. Note that the highest CIA values are observed in intertrappean sediments located within flows from Phase 2 (Chron 29r, latest Maastrichtian-earliest Danian), which correspond to 80% of the Deccan lava emissions.

Fig. 3. Ti/Al, index de alteração química (CIA), intensidade da alteração dos basaltos (K/Fe+Mg) nos sedimentos infra- e inter-trappeanos de Nagpur, India Central, comparação com os dados palinológicos. Note-se que os valores mais elevados da CIA são observados em sedimentos inter-trappeanos localizados dentro de fluxos da Fase 2 (Chron 29r, Maestrichtiano Superior- Daniano Inferior), o que corresponde a 80% das emissões de lava.

Beyond India, multi-proxy studies also place the main Deccan phase in the uppermost Maastrichtian C29r below the K/Pg (planktic foraminiferal zones CF2-CF1 spanning 120ky and 160ky respectively), as indicated by a rapid shift in 187Os/188Os ratios in deep-sea sections from the Atlantic, Pacific and Indian Oceans (Robinson et al., 2009) , coincident with rapid climate warming, coeval increase in weathering, a significant decrease in bulk carbonate (indicative of acidification due to volcanic SO2), and major biotic stress conditions expressed in species dwarfing and decreased abundance in calcareous microfossils (planktic foraminifera and nannofossils). These observations indicate that Deccan volcanism played a key role in increasing atmospheric CO2 and SO2 levels that resulted in global warming and acidified oceans, leading to increased biotic stress and predisposing faunas to eventual extinction at the K/Pg.

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