Environmental effects of Deccan volcanism across the Cretaceous–Tertiary transition in Meghalaya, India

Earth and Planetary Science Letters 310 (2011) 272–285

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Environmental effects of Deccan volcanism across the Cretaceous–Tertiary transition in Meghalaya, India B. Gertsch a,⁎, G. Keller b, T. Adatte c, R. Garg d, V. Prasad d, Z. Berner e, D. Fleitmann f a

Earth, Atmospheric and Planetary Science Department, Massachussetts Institute of Technology, Cambridge MA 02139, USA Department of Geosciences, Princeton University, Princeton NJ 08544, USA Institut de Géologie et Paléontology, Université de Lausanne, Anthropole, CH-1015 Lausanne, Switzerland d Marine Micropalaeontology Group, Birbal Sahni Institute of Palaeobotany, Lucknow 226007, India e Institute for Mineralogy & Geochemistry, Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany f Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland b c

a r t i c l e

i n f o

Article history: Received 22 February 2011 Received in revised form 24 July 2011 Accepted 10 August 2011 Available online 29 September 2011 Editor: G. Henderson Keywords: Deccan volcanism Iridium Meghalaya KT boundary mass extinction

a b s t r a c t The Um Sohryngkew section of Meghalaya, NE India, located 800–1000 km from the Deccan volcanic province, is one of the most complete Cretaceous–Tertiary boundary (KTB) transitions worldwide with all defining and supporting criteria present: mass extinction of planktic foraminifera, first appearance of Danian species, δ13C shift, Ir anomaly (12 ppb) and KTB red layer. The geochemical signature of the KTB layer indicates not only an extraterrestrial signal (Ni and all Platinum Group Elements (PGEs)) of a second impact that postdates Chicxulub, but also a significant component resulting from condensed sedimentation (P), redox fluctuations (As, Co, Fe, Pb, Zn, and to a lesser extent Ni and Cu) and volcanism. From the late Maastrichtian C29r into the early Danian, a humid climate prevailed (kaolinite: 40–60%, detrital minerals: 50–80%). During the latest Maastrichtian, periodic acid rains (carbonate dissolution; CIA index: 70–80) associated with pulsed Deccan eruptions and strong continental weathering resulted in mesotrophic waters. The resulting super-stressed environmental conditions led to the demise of nearly all planktic foraminiferal species and blooms (N 95%) of the disaster opportunist Guembelitria cretacea. These data reveal that detrimental marine conditions prevailed surrounding the Deccan volcanic province during the main phase of eruptions in C29r below the KTB. Ultimately these environmental conditions led to regionally early extinctions followed by global extinctions at the KTB. © 2011 Elsevier B.V. All rights reserved.

1. Introduction For the past 30 years, the Cretaceous–Tertiary boundary (KTB) mass extinction has been attributed to an extraterrestrial impact based mainly on the presence of a global Ir enrichment in a thin KTB red clay layer (Alvarez et al., 1980), which was subsequently attributed to the Chicxulub impact crater on Yucatan that distributed impact glass spherule ejecta near the KTB in Central and North America (e.g., Pope et al., 1991; Schulte et al., 2010; Smit et al., 1996). This theory and its corollary interpretations have remained controversial because of contradictory evidence (Keller, 2010), including the discovery of the oldest impact spherule layer in late Maastrichtian sediments in NE Mexico and Texas that indicates a pre-KT age for the Chicxulub impact (e.g., Keller et al., 2003, 2007, 2009a). Also for the past 30 years, Deccan volcanism has been advocated as potential cause for the KTB catastrophe (e.g., Courtillot et al., 1986, 1988; Duncan and Pyle, 1988; MacLean, 1985). But this hypothesis

⁎ Corresponding author. E-mail address: [email protected] (B. Gertsch). 0012-821X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2011.08.015

was considered unlikely because volcanism was generally believed to have occurred over about one million years prior to the mass extinction leaving sufficient time for recovery between eruptions. More recently, major studies of the Deccan Volcanic Province (DVP) have greatly improved understanding of the age and tempo of eruptions, revealing three major phases: initial phase-1 in C30n at ~67.4 Ma, the main phase-2 in C29r just before the KTB, and the last phase-3 in the early Danian (base C29n). Phase-2 is the most critical period of Deccan volcanism as it accounts for ~80% of the entire 3500 m thick Deccan lava pile, and erupted in rapid pulses over a short interval in C29r just prior to the KTB mass extinction (Chenet et al., 2007, 2008, 2009; Keller et al., 2008, 2009b,c). In another interpretation, Hooper et al. (2010) suggest that although the bulk of the major eruptions started in C29r, it continued into C29n. New data from ten deep wells in the Krishna-Godavari Basin support Chenet et al.'s model of two separate volcanic phases with the major phase-2 in C29R below the KTB and Phase-3 in C29N (Keller et al., 2011). It cannot be ruled out that minor volcanic eruptions continued in C29R above the KTB, although these would have been locally more restricted. Phase-2 created the world's longest lava flows, spanning N1500 km across India and into the Gulf of Bengal (Keller et al., 2011;

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Self et al., 2008). These lava flows ended at or near the KTB mass extinction (Keller et al., 2008), as revealed in intertrappean sediments overlying phase-2 basalt flows in Rajahmundry quarries (Andra Pradesh) and Jhilmili (Madhya Pradesh, Keller et al., 2009a,b). These studies strongly suggest that the biotic and environmental effects of Deccan volcanism at KTB time may have been vastly underrated. This report investigates the climatic and environmental consequences of Deccan phase-2 volcanism in the most complete KTB marine sequence known from India and comparable to the most complete sequences worldwide (e.g., Tunisia, Texas, Spain). The section is exposed along the Um Sohryngkew River in Meghalaya, NE India, and is about 800–1000 km from the main Deccan volcanic province (Fig. 1). A thin red clay layer enriched in Ir and other Platinum Group Elements (PGEs) marks the KTB (Bhandari et al., 1993, 1994; Garg et al., 2006; Pandey, 1990). Our investigations are based on the same sequence studied by these workers and employ a multi-proxy approach that includes: 1) biostratigraphy to provide high-resolution age control and evaluation of the biotic effects of Deccan volcanism; 2) carbon isotope stratigraphy as independent marker of the KTB; 3) sedimentology, microfacies analysis and bulk rock mineralogy to identify environmental changes; 4) clay mineralogy to infer paleoclimatic conditions, and comparison with data from other sites in India; 5) platinum group, trace and major elements geochemistry to evaluate evidence for an impact at the KTB; and 6) major and trace element geochemistry to identify a causal-relationship between Deccan volcanic activity and periods of high-stress conditions in marine environments. 2. Methods The Um Sohryngkew section was examined in the field for lithological changes, burrows and shell layers, which were described,

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measured and photographed (by RG and VP). A total of 143 samples were collected at an average of 50 cm, except for the KT transition where samples were taken at 10 cm intervals. In the laboratory, samples were processed for foraminiferal extraction using standard methods (Keller et al., 1995). Carbon and oxygen isotope measurements were carried out on powdered bulk rock samples at the stable isotope laboratories at the University of Bern, Switzerland, using an Optima (Micromass, UK) ratio mass spectrometer equipped with an online carbonate preparation line (Multi Carb) with separate vials for each sample and a VG Prism II ratio mass spectrometer, respectively. The results were calibrated to the PDB scale with standard errors of 0.05‰ for δ 13C. Major and trace elements were analyzed at the Geological Institute of the University of Lausanne, Switzerland, by XRF spectroscopy with a PANalytical PW2400 with a RX tube (Rh anode). PGEs were analyzed at the Institute for Mineralogy and Geochemistry, University of Karlsruhe, by isotope dilution HR-ICP-MS (Axiom, VG Elemental) after pre-concentration and matrix reduction by Ni-fire assay (Gertsch et al., 2011). Bulk rock and clay mineral assemblages were analyzed by X-ray diffraction (Xtra ARL Diffractometer) at the Geological Institute of the University of Lausanne, Switzerland, based on procedures described by Adatte et al. (1996). The semi-quantification of bulk-rock mineralogy is based on XRD patterns of random powder samples by using external standards with an error margin between 5 and 10% for the phyllosilicates and 5% for grain minerals. Clay mineral analysis follows the methods described by Adatte et al. (1996). The intensities of the identified minerals are measured for a semi-quantitative estimate of the proportion of clay minerals, which is therefore given in relative percent without correction factors, because of the small error margin (b5%).

3. Geological context and lithology

Fig. 1. A) Geographic map of India with position of Meghalaya and the Deccan Volcanic Province. B) Map of the Meghalaya area with the location of the Um Sohryngkew section near the village of Therria.

The Meghalaya area is located in northeastern India, north of Bangladesh, and characterized by the Shillong Plateau, which includes Garo, Khasi, Jaintia and Mikir hills (Fig. 1). The Shillong Plateau is tectonically related to the formation of Himalaya and corresponds to an uplifted Precambrian massif of the peninsular India shield formation with up to 6 km of Cretaceous through Miocene marine to continental sedimentary rocks that unconformably overlie the basement along the eastern, western and southern limbs (Alam et al., 2003; Clark and Bilham, 2008; Das Gupta and Biswas, 2000; Ghosh et al., 2005; Rao et al., 2008; Reimann, 1993; Rowley, 1996). The Um Sohryngkew section lies on the southern side of the Shillong Plateau near the village of Therria along the Um Sohryngkew river. The sedimentary record shows uninterrupted marine shelf sedimentation from the Campanian to the Eocene during the formation of a gulf on the northeastern edge of greater India due to rifting along the Indo-Burmese orogen (Acharyya and Lahiri, 1991; Banerji, 1981; Krishnan, 1968; Nagappa, 1959; Reimann, 1993). Sediments consist mainly of thick sandstone layers, marls, shale and carbonates characteristic of coastal, estuarine and nearshore environments (Banerji, 1981; Krishnan, 1968; Nagappa, 1959). For this study, investigations focused only on the Maastichtian– Danian interval of the Um Sohryngkew section. Sediments in the lower part of the section (0–14.26 m) consist of bioturbated clayey marls (0–8 m), silty sandy shales (8–10 m), clayey marls (10–14.26 m), and three 10–40 cm thick sandstone beds (2.4 m, 4.5 m, 13.3 m; Fig. 2). The KTB (14.26–14.28 m) is marked by a 2 cm thick rust-red-colored sandysilty layer with abundant subangular quartz in a red-brown matrix enriched in Ir and other PGEs, but devoid of calcite and microfossils (Figs. 2, 3A, B). In the lower Danian, silty, sandy shale with a 10 cm thick bioturbated sandstone layer (15.3–15.4 m) is followed by shales, clayey marls, marls, and marly limestones (15.5–30 m).

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Fig. 2. Litholog of the Um Sohryngkew section and illustrations: A) Outcrop of the KT transition with Maastrichtian gray marls topped by the KT red layer and Danian shales; B) close-up of the KT boundary. Thin section micrographs of the KT red layer show abundant sub-angular quartz crystals (gray) in a brown matrix.

4. Biostratigraphy Previous micropaleontological studies of the Maastrichtian to Paleocene Um Sohryngkew section of Meghalaya based on planktic foraminifera failed to identify the KTB red layer and PGE anomalies, possibly because different intervals or sections were collected, or sampling intervals were too large (e.g., Mukhopadhyay, 2007). In contrast, nannofossil and dinoflagellate studies (Garg et al., 2006; Nandi, 1990) were based on sample collections of the same intervals where PGE anomalies were identified (Bhandari et al., 1993, 1994; Pandey, 1990). This report is based on planktic foraminifera from the same section and samples previously reported by Garg et al. (2006), and represents the first documentation of the mass extinction across the KTB transition of the Um Sohryngkew section. Age and biozones are based on the combined planktic foraminiferal zonal scheme of Keller et al. (1995, 2002), and Li and Keller (1998). 4.1. Late Maastrichtian The latest Maastrichtian nannofossils zone Micula prinsii marks the 4 m below the KTB (Garg et al., 2006), which correspond to planktic foraminiferal zones CF2 and CF1 (Keller et al., 2009d). Planktic foraminiferal assemblages in this interval are dominated by Guembelitria blooms (N95%) that characterize zone CF1 and CF2 in shallow-water environments globally (see reviews in Keller and Abramovich, 2009; Pardo and Keller, 2008). The remaining assemblage consists of rare and

sporadic occurrences of heterohelicids, planoglobulinids, pseudoguembelinids, racemiguembelinids, globotruncanids and rugoglobigerinids (Fig. 3). However, in the 0.6 m below the KTB only rare, dwarfed and stress-resistant species are present (e.g., heterohelicids and guembelitrids). The exclusion of all subsurface dwellers suggests a shallower inner neritic environment. Enhanced carbonate dissolution in this interval may be linked to Deccan volcanism. In the Micula murus zone only rare planktic foraminifera are preserved in the predominantly sandy-silty shales, clayey marls and sandstones.

4.2. KT Boundary and early Danian The Um Sohryngkew section contains the most complete KTB transition in India, which can be correlated to the El Kef stratotype section and point (GSSP) in Tunisia (Cowie et al., 1989; Keller et al., 1995; Remane et al., 1999). As at El Kef, the KTB is identified by the mass extinction of planktic foraminifera followed by the first appearances of Danian species in zones P0 and P1a (e.g., Parvularugoglobigerina extensa, P. eugubina, Woodringina hornerstownensis, Globoconusa daubjergensis; Keller et al., 1995, 2002; Molina et al., 2006). Also present are the same three KTB-supporting criteria, the negative δ 13C shift, the Ir anomaly and other PGEs in a thin red layer. The δ 13C excursion at the Um Sohryngkew section shows the same trend as in the complete and expanded KTB sequences in Tunisia and Texas (Keller et al., 2002, 2009a).

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Fig. 3. A) Planktic foraminifera, δ13C record, platinum group elements (PGEs) ans sea-level across the KT transition. The KT boundary (red line) is defined by the disappearance of Cretaceous species, the δ13C shift and PGE enrichments in the red layer. Note the latest Maastrichtian planktic foraminiferal assemblages are dominated (N 95%) by the disaster opportunist Guembelitria cretacea. B) Zoom on platinum-group elements at Um Sohryngkew across the KT transition, which show peak values in the KT red layer and background contents in the late Maastrichtian and early Danian.

The mass extinction pattern differs from the deep-sea record by its lower diversity (24 species as compared with over 30 in comparable shallow environments, Keller and Abramovich, 2009), the rare, sporadic pre-KTB species occurrences in the 4 m below the KTB, and blooms (N95%) of the disaster opportunist Guembelitria cretacea (Fig. 3). These faunal assemblages reflect super-stressed environmental conditions at the time of Deccan phase-2 eruptions in C29r below the KTB, coincident with M. prinsii and CF1 zones (Keller et al., 2011). Such high Guembelitria blooms are best known from the aftermath of the mass extinction in the earliest Danian, but they have also been observed below the KTB (zone

CF1) in sections throughout the Tethys, such as Bulgaria, Israel, Sinai, Egypt, Texas and in the volcanically active Ninetyeast Ridge (reviews in Keller and Abramovich, 2009; Pardo and Keller, 2008). In the earliest Danian at Um Sohryngkew, normal low diversity assemblages evolved with the first index species, P. extensa and P. eugubina present at 10 cm and 20 cm, respectively, above the KTB red layer, marking zones P0 and P1a (Fig. 3). The first appearances of Parasubbotina pseudobulloides and Subbotina triloculinoides at 1.15 m above the KTB red layer mark the subdivision of zone P1a into subzones P1a(1) and P1a(2). The top of subzone P1a(2) is defined by

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the last appearance (LA) of P. eugubina at 19.8 m (Fig. 3). The presence of these early Danian biozones indicates a relatively continuous and high rate of sediment accumulation.

5. Carbon isotopes In the Um Sohryngkew section, δ 13C was measured on selected bulk-rock samples that contain N10% carbonate. The δ13C values are generally very negative (−1 to −7.5‰) but their global trends are comparable to former studies (Barrera and Keller, 1990; Keller and Lindinger, 1989; Stueben et al., 2002; Zachos et al., 1989). Diagenetic influence on the δ13C values in Um Sohryngkew is evaluated by oxygen isotope values (−8.39 to −5.64‰) and the coefficients of correlations between δ 13C values and calcite percentages (R2 = 0.19), and δ13C and δ18O (R2 = 0.62) respectively. Very low negative δ18O values suggest strong diagenesis (dissolution–precipitation processes), but their effects on the δ13C trends across the KTB are limited in Um Sohryngkew as indicated by low coefficients of correlation between δ13C values and calcite percentages, and δ13C and δ18O respectively. In the late Maastrichtian, negative values vary between − 2 and −3‰ (11.75–13.75 m, Fig. 3). No data is recoverable in the 50 cm below the KTB due to strong carbonate dissolution. However, just below the KTB red layer δ 13C values show a drop to −4‰ followed by rapid decrease to −7.25‰, forming a trough (14.35–14.70 m) in zones P0 and P1a(1). The return to pre-KTB δ 13C values of −2‰ is observed in zone P1a(2) (16.25–19.75 m; Fig. 3). In the Um Sohrynkew section, carbonate is very low (b20%) and the dissolution and re-precipitation processes in tropical environments, such as precipitation from waters enriched in dissolved inorganic carbon (DIC) with low δ 13C due to oxidation of organic matter, may explain the very negative δ 13C values (Tucker and Wright, 1990).

6. Mineralogy: results 6.1. Bulk-rock Quartz and phyllosilicates are the dominant minerals in the Um Sohryngkew section, whereas calcite, K-feldspars and plagioclases are intermittently abundant (Fig. 4). Unquantified minerals record important values in the lower part of section below the KTB and consist of poorly crystallized phyllosilicates, organic matter, phosphate minerals and Fe-hydroxide/-oxide minerals. In upper Maastrichtian sediments, mineralogical assemblages are dominated by detrital minerals, such as quartz, phyllosilicates, K-feldspars and plagioclases, whereas calcite and ankerite are rare or absent (0–25%; Fig. 4). The silty red KTB layer consists mainly of detrital components, such as quartz (21%), plagioclase (18%), K-feldspars (8.5%) and phyllosilicates (14%), together with high goethite (19.8%) and low calcite (10%). In lower Danian (P1a zone) sediments, calcite rapidly increases and dominates (25–40%), whereas phyllosilicates and unquantified minerals decrease. At the P1a/P1b boundary, calcite content drops to 10–20% and remains low, whereas quartz, K-feldspars, phyllosilicates, and plagioclases contents increase slightly (Fig. 4). 6.2. Clay minerals Clay assemblages (fraction b2 μm) in the Um Sohryngkew section are composed of smectite, kaolinite, chlorite, illite–smectite (I–S) mixed layer and illite (Fig. 4). The basal part (0–13.4 m) is dominated by illite (20–60%) and kaolinite (20–45%). Smectite, chlorite and I–S mixed layer show low values (0–20%) with scattered peaks. The KT transition (13.4–14.4 m) is marked by a gradual increase in kaolinite (50%) and decrease in illite (10%). Smectite gradually increases (15%), whereas chlorite (0–10%) and I–S mixed layer (2–20%) remain low

Fig. 4. Bulk-rock and clay mineralogy data of the Um Sohryngkew section. A three-points moving average is plotted for all minerals, except goethite, ankerite, chlorite, I/S mixed layers and kaolinite/illite ratios. Unquantified minerals refer to organic matter and poorly crystallized minerals. Bulk-rock assemblages dominated by detrital minerals (quartz, phyllosilicates, plagioclases and K-feldspars) and clay assemblages dominated by kaolinite suggest high continental runoff caused by humid and warm climates in Meghalaya.

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and constant. The KTB red layer (14.26–14.28 m) is composed of illite (51%), kaolinite (40%) and chlorite (9%). In the upper part of the Um Sohryngkew section (14.4–30 m) kaolinite (40–50%) and illite (40%) dominate. Smectite and I–S mixed layers gradually decrease (b5%), whereas chlorite remains low with scattered peaks and no significant trend (0–25%). The kaolinite/illite index reflects steady values throughout the sequence, except for a gradual increase across the KTB followed by a maximum in the lower part of subzone P1a(1) followed by an abrupt return to background ratios. 7. Major, trace and platinum group element geochemistry Major elements (MEs) across the upper Maastrichtian–lower Danian at the Um Sohryngkew section are grouped based on similar trends (Fig. 5). The largest group includes Al, K, Mg, Na, Si and Ti, which show relatively stable high concentrations in the lower part of the section, followed by a gradual pre-KTB decrease (K, Mg, Ti), or abrupt KTB drop (Al, Na, Si). All reach minimum values at the KTB and constant low values in the lower Danian. Ca shows an inverse trend, with low concentrations in the upper Maastrichtian M. prinsii zone, a sharp increase just above the KTB, and constant high values in the lower Danian. In contrast, Fe and P show stable low concentrations throughout the section, interrupted only by peak values at the KTB (Fe = 214,782 ppm (21.5 wt.%), P = 3582 ppm (0.36 wt.%); Fig. 5). Trace elements (TEs) were normalized with Al based on Van der Weijden (2002) for two reasons: 1) The detrital fraction composed of quartz, phyllosilicates, Na-plagioclase and K-feldspars is dominant (60–90%), whereas calcite content is usually low (0–40%) (Fig. 4). 2) Al shows the second lowest coefficient of variations (Table A, see supplementary material). Trace element trends normalized to Al reveal important changes across the KTB (Fig. 6). Nearly constant ratios are observed in As, Co, Cr, Ni, Pb, V and Zn during the upper

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Maastrichtian and lower Danian but with major positive peaks at the KTB. Cu ratios remain relatively high with no enrichment at the KTB. In contrast, Mn, Sr, U, and Zr show low Al-normalized values in the upper Maastrichtian relative to the lower Danian, but differ in their response to the KTB event (Fig. 6, Table 1). PGEs were analyzed across the KTB transition to quantify Ir, Pd, Pt, Rh and Ru concentrations (Fig. 3A, B). All PGEs show similar patterns with low concentrations in the upper Maastrichtian and lower Danian, and peak values in the KTB red layer (Fig. 3A, B). Below the KTB red layer, Ir, Rh and Ru concentrations are stable with 0.1, 0.15, and b0.3 ppb, respectively; Pd concentrations are at or below the detection limit (b0.9 ppb), and Pt values range from less than 2 ppb to 2.84 ppb. All PGEs peak within the KTB red layer (Ir: 11.79 ppb, Pd: 73.86 ppb, Pt: 86.48 ppb, Rh: 93.44, Ru: 108.24 ppb). In the basal Danian, all PGE concentrations return to low background values. Ir, Pt, Rh and Ru values are gradually decreasing and Pt decreases rapidly to form a trough (4.16 ppb) followed by a slight increase.

7.1. Element geochemistry At the Um Sohryngkew section, the detrital fraction dominates (60–90%) across the KTB transition and is composed of phyllosilicates, quartz, plagioclases and K-feldspars, whereas calcite content remains low but variable (5–40%; Fig. 4). Nevertheless, there are variations prior to, at and after the KTB (Fig. 4). For example, very low calcite (0–20%) and very high detritus (80–90%) below and at the KTB change to higher calcite (20–40%) and lower detritus (60–80%) in the basal Danian. ME trends are similar to detritus, except for Ca content, which increases in the lower Danian. MEs show a sharp drop at the KTB followed by constant low values in the lower Danian zone P1a(1) with constant detrital input, including Al, K, Mg, Na, Si, and Ti (Fig. 5).

Fig. 5. Major elements across the KT transition in the Um Sohrynkew section are separated into three groups with similar trends: (A) High late Maastrichtian contents followed by lower values in the early Danian; (B) Low late Maastrichtian contents followed by higher values in the early Danian; (C) Constant late Maastrichtian and the early Danian concentrations, but peak values at the KT boundary.

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Fig. 6. Trace elements at Um Sohrynkew are grouped into two categories based on trends: (A) Constant late Maastrichtian and the early Danian concentrations with peak values at the KT boundary, except for Cu; (B) Low late Maastrichtian contents followed by a gradual increase to higher values in the early Danian. Dashed line represents the Al-normalized ratio for each element in the average shale.

ME and TE trends can be assessed by calculating enrichment factors (EFME = Y/Yaverage shale, where Y is a specific major element; EFTE = (X/Al) / (X/Al)average shale, where X is a specific trace element) using average shale values of Wedepohl (1971). Fig. 8A and B shows this calculation for the upper Maastrichtian (pre-KTB), lower Danian (post-KTB), the Maastrichtian–Danian combined, and separately for the KTB red layer. Average EFME and EFTE calculated for the entire interval are generally close to 1, which indicates a major and trace element composition nearly similar to average shale (Fig. 7A, B). Exceptions are Mg, Na, Cu, and Mn, which are slightly depleted relative to average shale, whereas Ca, U, and Zr, are enriched relative

to average shale (Fig. 7A, B). For pre-KTB and post-KTB intervals, most average EFsME and EFsTE remain relatively close to the averages calculated for the entire interval (Fig. 7A, B), except for small differences in the average EFTE between pre-KTB and post-KTB sediments (Fig. 7A, B). The KTB red layer composition in MEs and TEs differs significantly from upper Maastrichtian and lower Danian marls and shales (Fig. 7A, B). MEs record enrichments in Ca, Fe and P, whereas all other major elements are slightly depleted (Fig. 7A). TE enrichment factors display a distinct composition relative to average shale with important enrichments in As, Co, Cr, Ni, Pb, and Zn, whereas all other TEs show

Table 1 Major and trace element abundances measured in red clay (Chester, 2000), chondrite (Anders and Grevesse, 1989), crust (Wedepohl, 1995), deccan basalts (Crocket and Paul, 2004, 2008) and shale (Wedepohl, 1971). As [ppm]

Ba [ppm]

Ce [ppm]

Co [ppm]

Cr [ppm]

Cu [ppm]

Ga [ppm]

Hf [ppm]

La [ppm]

Mn [ppm]

Nb [ppm]

Nd [ppm]

Red clay Chondrite Crust Deccan basalts

162 2 – –

371 2 584 122

0 1 60 32

184 502 24 –

897 2660 126 135

10 126 25 214

18 10 15 –

0 0 5 4

30 0 30 –

331 1990 774 1549

17 0 19 10

0 0 – –

Ni [ppm]

Pb [ppm]

Rb [ppm]

S [ppm]

Sc [ppm]

Sr [ppm]

Th [ppm]

U [ppm]

V [ppm]

Y [ppm]

Zn [ppm]

Zr [ppm]

Red clay Chondrite Crust Deccan basalts

2135 11,000 56 90

131 2 15 –

46 2 78 8

13,407 62,500 – 46

48 6 16 36

131 8 333 218

25 0 9 2

9 0 2 0

110 57 98 –

35 2 24 31

435 312 65 109

225 4 203 106

Red clay Chondrite Crust Deccan basalts Shale

Al [ppm]

Ca [ppm]

Fe [ppm]

K [ppm]

Mg [ppm]

Na [ppm]

P [ppm]

Si [ppm]

Ti [ppm]

53,263 8680 79,913 71,499 88,381

39,234 9280 39,308 72,899 15,723

214,782 190,400 43,923 100,017 48,348

15,475 558 19,924 3902 29,885

4539 98,900 22,319 37,581 15,684

3108 5000 23,739 17,508 11,870

3583 1220 786 1047 698

158,196 106,400 287,539 227,553 275,383

3126 436 4076 13,485 4675

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Fig. 7. (A) and (B) Calculated enrichment factor (EF) relative to average shale for major and trace elements measured in the Um Sohryngkew section. EF are calculated for pre-KTB and post KTB data, separately and together, and for the KT red layer; (C) Calculated enrichment factor (EF) relative to chondrite for some major, trace and platinum group elements measured in the KTB red layer at Um Sphryngkew compared to EFs of platinum group elements of Stevns Klint, Denmark; (D) Normalized PGEs relative to continental flood basalt for several types of rocks (e.g. flood basalts, meteorites) and KTB layers of Um Sohryngkew, Stevns Klint and Caravaca.

EFsTE close to 1. These enrichments correlate with large peaks in Ir, Pd, Pt, Rh and Rh and confirm the unique geochemical composition of the KTB red layer (Figs. 3A, B, 7A, B). 8. Geochemical proxies

value for average shales ranges from 70 to 75 (Nesbitt and Young, 1982). In the Um Sohryngkew section, CIA, PIA and CIW yield similar trends with relatively constant values (60) during the upper Maastrichtian, followed by a gradual increase (70–80) in the uppermost Maastrichtian (13.5–14.26 m, Fig. 8). A sharp decrease in all indices marks the KTB, followed by steady low values (30–40, Fig. 8).

8.1. Weathering indices 8.2. Volcanism proxies Chemical weathering indices were calculated based on major element concentrations, such as Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW), all of which are commonly used to characterize weathering profiles and the extent of weathering (Price and Velbel, 2003). For example, CIA values for fresh basalt (30–45) and granite (45–55) are very low (Nesbitt and Young, 1982). Illite, montmorillonites and beidellites, which are formed under contrasting dry and seasonal climates, show CIA values of 75–85. Kaolinite, a clay mineral produced under constant humid conditions, yields highest CIA values N90. CIA

The influence of Deccan volcanism can be determined based on several volcanism proxies, including Na/K, K/(Fe + Mg), Ca/Na and Mg/Na ratios (Dessert et al., 2003; Sageman and Lyons, 2003). Na/K and K/(Fe + Mg) ratios reveal the balance between detrital and volcanogenic input and are interpreted to reflect the increase or decrease of riverine siliciclastic flux relative to background volcanic input (Sageman and Lyons, 2003). In Meghalaya, Na/K ratio shows mostly steady low values in the upper Maastrichtian, a gradual increase prior to the KTB, and constant higher values in the lower Danian, whereas K/(Fe + Mg) ratio

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Fig. 8. Summary of all proxies used in this study (weathering, hydrothermal activity, volcanism vs detritism, basalt weathering, cosmic input) based on major and trace element geochemistry.

shows the opposite trend (Fig. 8). Compared to Sageman and Lyons (2003), both proxies record values indicative of a predominantly detrital influence across the KTB transition in the Um Sohryngkew area (0.1 b Na/K b 0.3; 0.2 b K/(Fe + Mg)b 0.8; Fig. 8). Ca/Na and Mg/Na ratios recorded in basaltic river waters show remarkably high values (Dessert et al., 2003), which may have significantly affected the geochemical signal of near-shore environments during the erosion of Deccan basalt traps. However, in the Um Sohryngkew section, these ratios show opposite trends that render the use of these ratios invalid as geochemical proxies for subaerial Deccan volcanism in marine sediments (Fig. 8). This indicates that Meghalaya was not part of the drainage domain of the Deccan volcanic province (Dessert et al., 2003).

8.3. Hydrothermal activity proxies Hydrothermal proxies consist mainly of Al/(Al + Fe + Mn), together with single elements, such as Pb, Zn, Cu and Co, (Chester, 2000; Pujol et al., 2006). In the Um Sohryngkew section, hydrothermal proxies record steady high values (0.6–0.7) in the upper Maastrichtian and lower Danian marls and shale, which indicates that hydrothermal influence was absent (Fig. 8). The Al/(Al + Fe + Mn) ratios show a negative peak that is linked to the presence of goethite at the KTB, and not related to a potential hydrothermal influence. These trends are corroborated by single elements, which record constant values in the Maastrichtian and Danian, but peak only in Pb and Zn at the KTB (Fig. 6).

8.4. Extraterrestrial proxies Extraterrestrial impact(s) and dust input(s) were evaluated based on the Ni/Cu ratio and PGEs. The Ni/Cu ratio has been used as extraterrestrial proxy due to the similar Cu contents in chondrite and continental crust, and also because of the large concentrations of Ni (10,624 ppm) present in C1-chondrite in comparison to continental crust (47 ppm) (Munsel et al., 2011). In the Um Sohryngkew section, Ni/Cu ratios peak at the KTB (213.5) and suggest an extraterrestrial source, but are constant during the upper Maastrichtian (2) and lower Danian (4). A slight difference between Maastrichtian and Danian Ni/Cu ratios likely results from lower oxygen conditions in the early Danian, rather than from increased input of extraterrestrial dust because neither element is enriched relative to average shale (Fig. 7B). PGEs, including Ir, Pd, Pt, Ru and Rh, are common proxies used to detect extraterrestrial impact(s) due to their rare occurrence on Earth (Wedepohl, 1991, 1995). In the Um Sohryngkew section, a marked peak in all PGEs occurs at the KTB, but no significant enrichment in PGEs is recorded during the late Maastrichtian and early Danian (Fig. 3). 9. Discussion 9.1. Depositional environment: planktic foraminifera In the Um Sohryngkew section, planktic foraminiferal biostratigraphy reveals high and relatively continuous sedimentation during the upper Maastrichtian and lower Danian (Fig. 3). However, in the

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uppermost 2 m of the Maastrichtian species assemblages reflect super-stress condition, intermittent strong carbonate dissolution (e.g., poor preservation, broken or fragmented shells), sporadic species occurrences, variable abundances, dwarfing of species and

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Guembelitria blooms. Guembelitria blooms are best known from the aftermath of the KTB mass extinction when they thrived worldwide to the exclusion of other species and are therefore known as disaster opportunists. But similar blooms are also known from the latest

Fig. 9. A) Summary of major results in the Um Sohryngkew section. Note the paleoclimatic conditions show decreasing intensities in “mock aridity” from central to eastern India, related to Deccan volcanism. Note also the good correlation between the main Phase-2 of Deccan volcanism, high chemical weathering indices and disaster/opportunist Guembelitria blooms during the terminal Maastrichtian, which suggest a close link between Deccan volcanism and high-stress environmental conditions. B) Model of the possible feedbacks and environmental consequences resulting from the Deccan main phase-2 during the terminal Maastrichtian.

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Maastrichtian zone CF1, where they correlate with periods of intense continental runoff and submarine volcanic activity (reviews in Keller and Abramovich, 2009; Pardo and Keller, 2008). The Guembelitria blooms in Meghalaya correlate well with low amounts of productivity sensitive elements, such as P, Cu, and Ni, and high weathering indices during the latest Maastrichtian when Deccan volcanic activity reached its maximum in phase-2 (Figs. 5, 6, 8). Strong weathering indices indicate intense chemical weathering in nearby continental areas of the Um Sohryngkew section and are likely due to acid rains resulting from SO2 emissions from Deccan volcanism (Self et al., 2006). The acidic waters lead to super-stress conditions in the Meghalaya area inhibiting CaCO3, production, which favored blooms of the small thin-shelled surface dwellers Guembelitria blooms and explains the intermittent strong dissolution of planktic foraminiferal assemblages (Fig. 9A, B). Similar Guembelitria blooms indicating highstress conditions are observed in the latest Maastrichtian and early Danian throughout the Tethys and into the South Atlantic (e.g., Egypt, Israel, Tunisia, Bulgaria, Texas, Argentina) (Abramovich and Keller, 2002; Adatte et al., 2002; Keller, 2002; Keller et al., 2007). 9.2. Sea-level: lithology and mineralogy Combined with lithologies and foraminiferal assemblages, bulkrock mineralogy is an excellent environmental proxy to infer fluctuations in sea-level and associated erosion and continental runoff based on the distribution of calcite and detrital minerals (quartz, phyllosilicates, plagioclases and K-feldspars; Adatte et al., 2002). High calcite content generally indicates deeper environments, diminished erosion and low continental runoff, whereas high detritus input suggests shallower environments and high continental runoff. In Meghalaya, detrital minerals in bulk-rock assemblages are dominant and calcite is low but fluctuating in the upper Maastrichtian marls and shale, which suggests deposition in a shallow water environment (b100 m depth) with high terrigenous influx. For example, the thin sand layer at 13.3 m (0.66 m below the KT boundary) coincides with a marked decrease in species richness of planktic foraminifera and the absence of deeper dwelling species, which indicates a drop in sea level to inner neritic depth (Fig. 3A). Enhanced carbonate dissolution between this sandstone layer and the KTB may be due to the sandy, shallow water environment and/or acid rain linked to the main phase of Deccan volcanism in C29r (Chenet et al., 2007). Above the sandstone layer, sea level gradually increased and reached a maximum at the KTB clay layer. Similar sea-level changes have been observed in KTB sections from Israel, Egypt, Tunisia, Texas, and Mexico (Adatte et al., 2002; Keller et al., 2003, 2007). Across the KT transition and in the lower Danian high detrital input continues into a relatively shallow though deepening marine environment. The sandstone layer marks a small sea level drop and possibly short hiatus, as indicated by the abrupt increase in Danian species coincident with the P1a(1)/P1a(2) subzone boundary (Fig. 3). A hiatus at this interval has been documented from many lower Danian sequences (Keller et al., 2003). A deepening and more open marine environments dominated by carbonate sedimentation prevailed during the lower Danian (Figs. 3 and 4). 9.3. Climate proxies: clay and bulk-rock mineralogy Clay minerals are byproducts resulting from the interplay between climate, continental morphology, tectonic activity and sea-level variations, and therefore can be used as climatic and environmental proxies (Chamley, 1989). In the Um Sohryngkew section, clay assemblages are dominated by kaolinite, which is formed in tropical soils under constant warm and humid conditions, and illite, which is a byproduct of tectonic uplift and physical weathering (Chamley, 1989; Robert and Chamley, 1990). Illite is poorly crystallized, which

indicates significant chemical weathering caused by hydrolysis. Illite/Smectite (I/S) mixed layers are poorly ordered (R = 0) and contain 25–50% of expandable layers. Smectite is a “smectoid”, an I/S mixed layers with around 80–90% of expandable layers. High I/S mixed layers and low smectite contents suggest a diagenetic overprint linked to high burial depths (N3 km, Reimann, 1993) that resulted in the transformation of smectite into I/S mixed layers (Chamley, 1989). Smectites and I/S mixed layer are therefore not reliable paleoclimatic proxies. The paleoclimate of Meghalaya is characterized by predominantly humid conditions and strong chemical weathering, as indicated by high kaolinite, poorly crystallized illite and high kaolinite/illite ratios (Fig. 4). The dominant detrital minerals (Fig. 6), high weathering indices and palynological data (Nandi, 1990) support this general pattern (Fig. 8). All of these indices suggest that predominant chemical weathering in combination with physical weathering under humid conditions prevailed across the KTB in near-shore environments of Meghalaya. High kaolinite and poorly crystallized illite contents linked to increasingly humid conditions across the KTB transition are not restricted to Meghalaya but are encountered worldwide from low to middle latitudes, except in areas close to India (Abramovich et al., 2002; Adatte et al., 2002, 2005; Keller et al., 1998; Madhavaraju et al., 2002; Pardo et al., 1999; Robert and Chamley, 1990). Terrestrial and marine sections from central and eastern India close to the DVP show dramatically different compositions of clay mineral assemblages with high smectite and absent kaolinite, which reveals predominantly arid to semi-arid conditions with seasonal wet and dry cycles (Keller et al., 2008, 2009c). Most studies relate the global increase in kaolinite and poorly crystallized illite input into the oceans to the short warm event of the latest Maastrichtian (zone CF1), which generated wetter conditions, more rainfall and intensified continental runoff probably linked to Deccan volcanism and its gas emissions (see reference above). Local aridity close to the DVP is interpreted as a result of “mock aridity” (e.g. volcanically induced xeric conditions and extreme geochemical alkalinity in the context of a regionally more humid climate) induced by Deccan volcanism (Harris and Van Couvering, 1995; Khadkikar et al., 1999). In this context, paleoclimatic information gathered from the Um Sohryngkew section reveals that the “mock aridity” zone caused by Deccan volcanism across the KTB transition is restricted to the Deccan volcanic province with gradually decreasing intensities from central India to the rim of the DVP (Fig. 9A, B). 9.4. KTB red layer geochemistry and potential origins On a global basis, the KTB red clay layer shows peak concentrations in PGEs (Ir, Rh, Ru, Pd, Pt) accompanied by enrichments in several major (Fe, P) and trace (As, Ba, Co, Cr, Cu, Ni, Sb, Sc, Th, U, V, Zn) elements, which are all postulated to originate from a single extraterrestrial impact based mainly on the Ir and siderophile elements (e.g. Co, Ni) (Alvarez et al., 1980; Bhandari et al., 1993, 1994; Joyce Evans et al., 1993; Martinez-Ruiz et al., 2006). In the Um Sohryngkew section, the KTB red layer has high silt content (Fig. 4) with peak concentrations in major, trace and platinum group elements, except for Cu (Figs. 3, 5, 6). Maximum values in PGEs, specific TEs (e.g. Cr) and the Ni/Cu ratio suggest an extraterrestrial origin (Figs. 5, 6, 8; Table 1), as previously observed by Bhandari et al. (1993, 1994). Normalized PGEs relative to chondrite, which are commonly used to identify extraterrestrial signals at the KTB, show a fairly flat line but all values fall in the “extraterrestrial origin” field defined by Kramar et al. (2001) and are similar to values of the KTB red clay layer at Stevns Klint, Denmark (Fig. 7C). Based on the observations that the Chicxulub impact predates the KTB (Keller et al., 2003, 2007, 2009a), the Ir and PGE anomalies at the KTB in Meghalaya and worldwide is likely from another large impact for which no impact crater is known to date.

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Nevertheless, the impact hypothesis can only partly account for the PGE and TE enrichments at the KTB. Based on major and trace element concentrations of chondrite, an extraterrestrial impact cannot explain the peaks in As, Pb, U, and V, or the low Mn concentrations observed in the KTB red layer at Um Sohryngkew (Table 1). Enrichment factors based on average chondrite composition (EFchondrite = (X/Fe) / (X/Fe)chondrite, where X is a specific major, trace or platinum group element), are very low for most trace elements (Cr, Ni) and high for P (Fig. 8C), which does not support an extraterrestrial origin. Thus, elemental enrichments in the KTB red clay layer clearly show that an extraterrestrial impact alone cannot be the sole source of this geochemical signature. Several studies have proposed intense phase-2 Deccan Trap volcanism as alternative cause for the KTB mass extinction and as alternative source for the trace and platinum group element enrichments in the KTB red layer (Courtillot et al., 1986; MacLean, 1985; Toutain and Meyer, 1989; Zoller et al., 1983). In Meghalaya, the volcanic proxies used in this study to investigate the volcanic influence in the KTB show no causal link (Fig. 8, Table 1), though single element concentrations cannot rule out volcanism as direct (e.g. triggered by a sudden eruption) or indirect (e.g. intense basalt weathering) source, as hypothesized by Bhandari et al. (1993, 1994). At Um Sohryngkew, normalized PGEs relative to continental flood basalts show high and fluctuating values for the KTB red clay layer, but these results do not correlate with normalized PGEs for the KTB at Stevns Klint and Caravaca (Fig. 7D). These contrasting results suggest that further investigations are needed to evaluate the influence of Deccan volcanism on the KTB geochemistry. Hydrothermal activity is a frequently overlooked or underestimated factor in KTB enrichments, and its proxy (Al/[Al + Mn + Fe]) indicates a significant influence in the KTB red layer (Fig. 8). However, several problems remain: 1) Neither hydrothermal activity nor submarine volcanism is known to be prevalent at the KTB and Deccan volcanism was predominantly continental with limited interaction with marine environments (Keller et al., 2008; Self et al., 2008). 2) Environmental perturbations, such as lower seawater pH and acids rains, due to lava–seawater interaction could be locally restricted (Edmonds and Gerlach, 2006). And 3) sediments affected by hydrothermal activity are generally enriched in both Fe and Mn (Chester, 2000), not only Fe as in the Um Sohryngkew section (Fig. 5). In the Um Sohryngkew section, goethite (FeO(OH)) is the main mineral in the KTB layer and results from late diagenetic alteration of pyrite after deposition, as observed globally (e.g. Elles, Tunisia), except for rare localities, where a hematite-rich layer occurs at the KTB (Adatte et al., 2005; Pardo et al., 1999). Very high enrichments in some TEs (As, Cr, Co, Cu, Ni), high Fe contents, and highly significant correlation rate (R 2 = 0.8–1) of As, Cr, Co, Cu, Ni, U, V and Zn with Fe (Table B, see supplementary material) support the primary precipitation of pyrite under sulfidic redox conditions in the KTB red layer during deposition and compaction of sediments (Gavrilov, 2010; Pujol et al., 2006; Schmitz, 1985, 1992). However, enrichment factors of the redox-sensitive proxies U and V are not high enough to confirm sulfidic conditions (Fig. 7; Algeo and Maynard, 2004; Brumsack, 2006; Tribovillard et al., 2006). Although present-day weathering may have partly leached redox-sensitive trace elements, environmental conditions are not drastically reducing, but only dysoxic, by the time of the deposition of the red clay layer to trap abundant Fe, S and most pyritelinked TEs at or below the sediment–water interface. Reduced sedimentation rates across the KTB, which are known to concentrate MEs, TEs and PGEs (Bruns et al., 1997; Donovan et al., 1988), are likely significant factors in the KTB geochemistry, as observed by extremely large PGE and P concentrations in Meghalaya. These results suggest a strongly condensed sedimentation in the KTB red layer linked to a rapid sea-level rise culminating in the maximum flooding surface globally observed at or near the KTB (Fig. 3; Adatte et al., 2002, 2005; Donovan et al., 1988).

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9.5. Depositional scenario across the KT boundary During the late Maastichtian C29r (zone CF1) Deccan volcanism reached its maximum (phase-2) accumulating 80% of the entire 3500 m thick Deccan lava pile with some mega-flows spanning over 1500 km across India and out into the Bay of Bengal (Chenet et al., 2007, 2008; Keller et al., 2008; Self et al., 2008). Volcanic phase-2 ended at or near the KT mass extinction as evident from planktic foraminifera in intertrappean sediments in deep wells of the KrishnaGodavari Basin (Keller et al., 2011), as well as shallow sequences from Rajahmundry, Andhra Pradesh and Central India (Jhilmili, Madhya Pradesh, Keller et al., 2008, 2009b,c). In Meghalaya to the northeast, the late Maastrichtian at the Um Sohryngkew section was deposited in a shallow near-shore sea (b100 m) about 800–1000 km from the Deccan volcanic province. In this coastal area of India, climate change due to Deccan volcanism resulted in humid conditions that contrasted with the semi-arid conditions and “mock aridity” that dominated the center of the Indian continent (Fig. 9A, B). Abundant precipitation, high continental runoff and high weathering resulted in a major influx of detritus (quartz, K-Feldspars, plagioclases), which led to increasingly turbid and mesotrophic waters. These high-stress environmental conditions were amplified by periodic acid rains associated with Deccan pulses, which increased chemical weathering (Fig. 9A, B) and led to the exclusion of most planktic foraminifera and blooms of the disaster opportunist Guembelitria. Similar Guembelitria blooms correlate with the main phase-2 of Deccan volcanism in C29r below the KTB in shallow water sequences worldwide (Keller and Abramovich, 2009; Pardo and Keller, 2008). Prior to the KTB mass extinction increasing volcanic intensity and SO2 release led to acid rains that raised the pH of seawater and inhibited or reduced carbonate precipitation and production leading to the early disappearance of many planktic foraminiferal species. Well prior to the KTB mass extinction these conditions favored the survival of small species leading to dwarfism, and particularly small thin-walled species that resulted in the observed Guembelitria blooms. In the Um Sohryngkew section, the mass extinction coincides with a silty red layer, a major Ir anomaly and the negative δ 13C shift that marks the productivity crash due to the mass extinction. Danian species evolved shortly thereafter as also observed globally (Figs. 3, 9A, B). At the KTB red layer, the origin of PGEs and trace element enrichments (e.g., As, Co, Cr, Ni, Pb, U and Zn, Figs. 3, 5 and 6) indicate an extraterrestrial source from a second major impact postdating Chicxulub (Keller et al., 2003, 2007, 2009a), but not as sole origin. Other enrichment processes, including fluctuating redox conditions and condensed sedimentation likely contributed to the KTB geochemical signature. During the early Danian marine productivity gradually recovered (upper part of zone P1a) and diversity slightly increased, but species remained very small, indicating continued high-stress conditions (Fig. 3). In NE India humid conditions and abundant precipitation lead to steady detrital input into the ocean, whereas on the Indian continent semi-arid to arid (“mock aridity”) conditions prevailed (Fig. 9A). 10. Conclusions • The Um Sohryngkew section of Meghalaya, India, represents one of the most continuous Cretaceous–Tertiary boundary (KTB) sequences in India that correlates globally with the GSSP section at El Kef, Tunisia, and yields critical information related to the main phase-2 of Deccan volcanism during the latest Maastrichtian C29r. • Sediment deposition occurred in a shallow-water environment (b100 m) dominated by high continental runoff due to subtropical humid conditions with abundant precipitation, which contrasts

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with the local semi-aridity induced by Deccan volcanism in central India. • Super-stress conditions prevailed during the latest Maastrichtian (M. prinsii, CF1 zones) as indicated by G. cretacea blooms (N95%), which are likely due to the combination of mesotrophic conditions and acid rains linked to Deccan volcanism. • The KTB silty red layer is enriched in major elements (Fe, P), trace elements (As, Co, Cr, Ni, Pb, Zn) and platinum group elements (Ir, Ru, Rh, Pt, Pd), which are comparable to other major KTB localities. An extraterrestial origin is supported in part, but condensed sedimentation, fluctuating redox conditions at the time of deposition, and secondary redox fluctuations likely account for the relatively high enrichments observed at Um Sohryngkew. Supplementary materials related to this article can be found online at doi:10.1016/j.epsl.2011.08.015. Acknowledgements We thank Mike Widdowson and three anonymous reviewers for insightful comments. The material of this study is based upon work supported by the US National Science Foundation through the Continental Dynamics Program and Sedimentary Geology Program under NSF Grants EAR-0207407, EAR-0447171, EAR-0750664 and EAR 1026271 (GK). We thank Tiffany Monnier for sample preparation for XRF analysis and Jean-Claude Lavanchy (University of Lausanne) for XRF measurements. A special thank you to André Villard (University of Neuchâtel) for thin section preparation. References Abramovich, S., Keller, G., 2002. High stress late Maastrichtian paleoenvironment: inference from planktonic foraminifera in Tunisia. Palaeogeography, Palaeoclimatology, Palaeoecology 178, 145–164. Acharyya, S.K., Lahiri, T.C., 1991. Cretaceous palaegeography of the Indian subcontinent: a review. Cretac. Res. 12, 3–26. Adatte, T., Stinnesbeck, W., Keller, G., 1996. Lithostratigraphic and mineralogic correlations of near K/T boundary sediments northeastern Mexico: implications for origin and nature of deposition. The Cretaceous–Tertiary Event and Other Catastrophes in Earth History, Boulder, Colorado. Geol. Soc. Am. Spec. Pap. 307, 211–226. Adatte, T., Keller, G., Stinnesbeck, W., 2002. Late Cretaceous to early Paleocene climate and sea-level fluctuations: the Tunisian record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 178, 165–196. Adatte, T., Keller, G., Stueben, D., Harting, M., Kramar, U., Stinnesbeck, W., Abramovich, S., Benjamini, C., 2005. Late Maastrichtian and K/T paleoenvironment of the eastern Tethys (Israel): mineralogy, trace and platinum group elements, biostratigraphy and faunal turnovers. Bull. Soc. Geol. Fr. 176, 37–55. Alam, M., Alam, M.M., Curray, J.R., Luftar Rahman Chowdhury, M., Royhan Gani, M., 2003. An overview of the sedimentary geology of the Bengal Basin in relation to the regional tectonic framework and basin-fill history. Sediment. Geol. 15, 179–208. Algeo, T.J., Maynard, J.B., 2004. Trace-element behavior and redox facies in core shales of Upper Pennsylvanian Kansas-type cyclothems. Chem. Geol. 206, 289–318. Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial Cause for the Cretaceous–Tertiary boundary. Science 208, 1095–1108. Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197–214. Banerji, R.K., 1981. Cretaceous–Eocene sedimentation, tectonism and biofacies in the Bengal basin, India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 34, 57–85. Barrera, E., Keller, G., 1990. Foraminiferal stable isotope evidence for gradual decrease of marine productivity and Cretaceous species survivorship in the earliest Danian. Paleoceanography 5, 867–890. Bhandari, N., Shukla, P.N., Cini Castognoli, G., 1993. Geochemistry of some K/T sections in India. Palaeogeogr. Palaeoclimatol. Palaeoecol. 104, 199–211. Bhandari, N., Gupta, M., Panday, J., Shukla, P.N., 1994. Chemical profiles in K/T boundary section of Meghalaya, India: cometary, asteroidal or volcanic. Chem. Geol. 113, 45–60. Brumsack, H.-J., 2006. The trace metal content of recent organic carbon-rich sediments: implications for Cretaceous black shale formation. Palaeogeogr. Palaeoclimatol. Palaeoecol. 232, 344–361. Bruns, P., Rakoczy, H., Pernicka, E., Dullo, W.-C., 1997. Slow sedimentation and Ir anomaly at the Cretaceous/Tertiary boundary. Geol. Rundsch. 86, 168–177. Chamley, H., 1989. Clay Sedimentology. Springer Verlag, Berlin. Chenet, A.-L., Quidelleur, X., Fluteau, F., Courtillot, V., 2007. 40 K/40Ar dating of the main Deccan large igneous province: further evidence of KTB age and short duration. Earth Planet. Sci. Lett. 263, 1–15.

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