Extinctions associated with both volcanism and impact? 2.1. Cretaceous–Tertiary The K–T (65 Ma) mass extinction occurred at the same time as both flood basalt volcanism (Deccan Traps, India: Courtillot et al., 1986) and meteorite impact — a ~10-km bolide that left a worldwide iridium anomaly (Alvarez et al., 1980). The theory that the impact caused the K–T extinction gained credence with the discovery of a 65-m.y.-old impact crater, ~180 km in diameter, at Chicxulub, Mexico (Hildebrand et al., 1991, 1995). Although the relationship of the Chicxulub crater to the K–T boundary impact has been generally accepted for the last decade, there are recent reports that the Chicxulub crater may predate the K–T boundary by 300 kyr, and that another impactor may have been responsible for the extinction (Keller et al., 2002). The crater for this other proposed impactor is not known, but to explain the observed K–T iridium anomaly, it would be expected to be similar in size to the Chicxulub crater. Realistic dkill mechanismsT for both meteorite impact and flood basalt magmatism are difficult to prove, and thus the relative influences of these two events on the K–T extinction are still debated (e.g., Wignall, 2001). The fossil record does provide many clues, for example, the existence of disaster/opportunist planktonic foraminiferal assemblages in the Late Maastrichtian points to high stress conditions preceding the impact and the abrupt extinctions at the K–T boundary (Keller, 2003). These high stress conditions correlate temporally with periods of intense Deccan volcanism and were characterised by toxicity and low oxygen due to eutrophication. Stable isotope 301 studies demonstrate abrupt warming (~2–3 8C) of Late Maastrichtian intermediate ocean waters (Li and Keller, 1998), probably linked to increased atmospheric pCO2. This global warming, which may have been caused by the Deccan volcanism, would have increased weathering and runoff, providing increased flux of biolimiting elements into the oceans, a mechanism that would have contributed to the eutrophication and low oxygen. Hence, it appears that, in the case of the K–T extinction, there is good evidence for volcanically induced biotic stress that was compounded by a meteorite impact. 2.2. Triassic–Jurassic The eruption of the 200-m.y.-old Central Atlantic Magmatic Province (CAMP) happened at the same time, within analytical error, as marine Tr–J extinctions (Marzoli et al., 1999; Pálfy et al., 2000). Early searches for iridium anomalies (Orth et al., 1990) and shocked quartz were negative or ambiguous (Bice et al., 1992; Hallam, 1990), but recent results from the terrestrial Newark basin demonstrate a small iridium anomaly (maximum of 285 ppt: Olsen et al., 2002a). This compares to a value of 6300 ppt from the Gubbio K–T boundary section (Alvarez et al., 1980). Thus, any Tr–J impactor, if confirmed, is likely to be considerably smaller than the K–T meteorite. Moreover, there is some ambiguity about whether the Ir at the Tr–J boundary has an extra-terrestrial or terrestrial provenance. Olsen et al. (2002b) used the lack of correlation between Ir and elements such as Cs, Al, Cu and V to suggest that a volcanic origin for the Ir was unlikely. A different picture emerges when considering elements such as Ni, Cr and Ir, which may provide useful diagnostic tools for evaluating whether sedimentary rocks have volcaniclastic and/or extraterrestrial material incorporated within them (Kerr, 1998). In Fig. 2, Cr/Ir is plotted against Ni/Ir for a range of terrestrial rocks and chondritic meteorites. The range of Ni/Ir spans over four orders of magnitude, from chondritic meteorites with low Ni/Ir (generally b3104) to mid-ocean-ridge basalts that may have Ni/Ir exceeding 1000104. Plume-related basalts (e.g., Iceland, Réunion), large igneous provinces (e.g., Central Atlantic Magmatic Province, Ontong Java Plateau, Siberian Traps) and komatiites have 302 R.V. White, A.D. Saunders / Lithos 79 (2005) 299–316 Fig. 2. Cr/Ir vs. Ni/Ir abundance ratios in a range of terrestrial igneous rocks and chondritic meteorites, compared with sedimentary rocks spanning the K–T and Tr–J boundaries. Data sources: chondritic meteorites: Wasson and Kallemeyn (1988); Ontong Java Plateau (ODP Leg 192): Chazey and Neal (2004); Central Atlantic Magmatic Province (Western Newark Basin intrusives): Gottfried et al. (1991); Siberian Traps: Lightfoot et al. (1990) and Brügmann et al. (1993); komatiites: Brügmann et al. (1987) and Rehkämper et al. (1999); Reunion (ODP Leg 115): Fryer and Greenough (1992); Iceland: Rehkämper et al. (1999); MORB: Rehkämper et al. (1999). K–T boundary sedimentary rocks: point marked Alv from Alvarez et al. (1980); all others from Stüben et al. (2002) where delevated IrT samples belong to MU3 anomaly and have Ir of 0.5–1 ppb. Tr–J boundary sedimentary rocks are from Olsen et al. (2002a); delevated IrT are those samples with Ir of 0.08–0.29 ppb. dBackgroundT samples from both sections are included for comparison. intermediate Ni/Ir and Cr/Ir values (from ~10104 to ~500104). Sedimentary rocks from key sections such as the K–T boundary (Alvarez et al., 1980; Stüben et al., 2002) have Ni/Ir and Cr/Ir ratios that are lower than any known terrestrial igneous rocks (Fig. 2), which can only be explained by a significant extra-terrestrial component. In contrast, samples spanning the Tr–J boundary in the Newark Basin neither require nor rule out an extra-terrestrial component. The samples with elevated Ir do have lower Ni/Ir and Cr/Ir than the background samples, but all samples have Ni/Ir and Cr/Ir ratios that fall within the range of terrestrial basaltic rocks, including the nearby CAMP, and so it remains ambiguous whether the source of the high-Ir component is volcanic or extra-terrestrial. The fossil record at the Tr–J boundary demonstrates that many groups were in decline throughout the late Triassic (Tanner et al., 2004). Some groups appear to have been subject to only regional effects; for example, an abrupt crisis in terrestrial flora (McElwain et al., 1999) has not yet been recognised beyond the North Atlantic region (Hallam and Wignall, 1997). Difficulties in correlation between biostratigraphic and various radiometric dating methods mean that it is not yet clear whether the terrestrial and marine extinctions were synchronous (e.g., Pálfy et al., 2000). Palaeoclimate interpretations are ambiguous: a dramatic increase in atmospheric pCO2 inferred from stomatal density analysis (McElwain et al., 1999) appears to conflict with isotopic data from palaeosols (Tanner et al., 2004). A carbon isotope shift hints at the involvement of methane hydrates, but in general, the fossil record does not appear to tell a story of a single catastrophic event (Tanner et al., 2004). 2.3. Permo–Triassic The P–Tr extinction (~250 Ma) was contemporaneous with large scale volcanism of the Siberian Traps (Renne et al., 1995) and the adjacent West Siberian Basin (Reichow et al., 2002). A contemporaneous large bolide impact has been proposed, based on P–Tr boundary fullerenes containing trapped noble gases with isotopic ratios indicative of an extraterrestrial source (Becker et al., 2001). These results are controversial (Farley and Mukhopadhyay, 2001; Braun et al., 2001), and other claims for an P–Tr impact (Kaiho et al., 2001; Xu et al., 1985) have also been disputed (Koeberl et al., 2002; Zhou and Kyte, 1988). There has been a recent report of a possible endPermian impact structure, the Bedout High, located on the northwestern continental margin of Australia (Becker et al., 2004), but experts on shock metamorphism have not yet been convinced by the evidence presented (Kerr, 2004). Searches for indicators such as shocked quartz at the P–Tr boundary have turned up the dscentT of an impact (i.e., smaller and much less abundant shocked quartz grains than the K–T boundary: Retallack et al., 1998), and magnetic silicate aggregates interpreted as chondritic meteorite fragments and iron-rich metallic grains are associated with the P–Tr boundary in Antarctica (Basu et al., 2003). It seems, therefore, that an impact at the P–Tr boundary remains a possibility. Nonetheless, the size of such an impact remains poorly constrained. The suspected Antarctic chondritic meteorite fragments are not associated with significant Ir anomalies, which R.V. White, A.D. Saunders / Lithos 79 (2005) 299–316 Basu et al. (2003) note may be because the Ir is not concentrated in a thin layer, as it is at the K–T boundary.
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