ABSTRACT The last major mass extinctions in Earth history (e.g., end-Guadalupian, end-Permian, end-Triassic, and end-Cretaceous) are all correlated closely in time with the main-phase eruptions of major flood basalt provinces (Emeishan, Siberian, Central Atlantic Magmatic Province, and Deccan Traps, respectively). The causal relationship between flood volcanism and mass extinction is not clear, but likely involves the climate effects of outgassed volatile species such as CO 2 , SO 2 , Cl, F, etc., from some combination of magma and country rocks. In a surprising “coincidence,” the end-Cretaceous (K-Pg boundary) micro-faunal extinction also corresponds precisely in time to what may have been the largest meteor impact of the past billion years of Earth history, the Chicxulub crater at 66.05 Ma. The Deccan Traps eruptions were under way well before K-Pg/Chicxulub time and are most likely the result of the mantle plume “head” that initiated the presently active Reunion hotspot track—thus the Deccan Traps were clearly not generated, fundamentally, by the impact. However, recent high-precision 40 Ar/ 39 Ar geochronology indicates that conspicuous changes in basalt geochemistry, lava flow morphology, emplacement mode, and a possible 50% increase in eruption rate at the Lonavala/Wai subgroup transition in the Deccan Traps lava group corresponded, within radioisotopic age precision, to the K-Pg boundary and the Chicxulub impact. This has led to the testable hypothesis that the M w ~11 seismic disturbance of the Chicxulub impact may have affected the Deccan eruptions. Here we review a broad landscape of evidence regarding Deccan volcanism and its relation to the K-Pg boundary and attempt to define what we see as the most important questions than can and should be answered by further research to better understand both the onshore and largely unknown offshore components of Deccan-related volcanism, and what their climate and environmental impacts at K-Pg time may have been.

The interval spanning the uppermost Hell Creek Formation to the overlying lowermost Fort Union Formation in north-central Montana encompasses a marked paleoenvironmental change (associated with the formational contact), the Chicxulub impact event, and the Cretaceous-Paleogene boundary. We have examined the record of this transition at the Hell Creek Formation lectostratotype to determine the placement of these events using a series of lithological, geochemical, palynological, and 40 Ar/ 39 Ar geochronological analyses. The claystone derived from the Chicxulub impact is identified based on lithological criteria, enrichment of iridium and osmium, and osmium isotope ratios. The impact claystone also contains a Cyathidites fern spike. The first continuous lignite horizon in the section immediately overlies this claystone and represents the Hell Creek–Fort Union formational contact. A tuff ~3 m above the impact layer is dated to 66.024 ± 0.059 Ma. Given this evidence, at the lectostratotype the Cretaceous-Paleogene boundary is coincident with the impact claystone and therefore with the formational contact. Due to poor preservation and apparent reworking of palynomorphs surrounding the formational contact, the Cretaceous-Paleogene boundary is difficult to identify based on biostratigraphically significant taxa. The presence of marine dinoflagellates is suggestive of reworking of older marine sediments during the deposition of the Cretaceous-Paleogene boundary interval.

Abstract A recent calibration of the 40 Ar/ 39 Ar geochronometer is based on an optimization analysis of 40 K activity data, isotopic data for the Fish Canyon sanidine (FCs) standard, and pairs of 40 Ar/ 39 Ar+ 238 U– 206 Pb data from selected samples meeting well-documented quality criteria. Inclusion of 238 U– 206 Pb data in the calibration incorporates the precisely known 238 U decay constant. Thus, this calibration is inherently consistent with the U–Pb chronometer. Initial presentation of the calibration included an inappropriate datum and should be eschewed in preference to a revision. Compared with previous calibrations, including those focusing mainly on the age of a standard (e.g. astronomical calibrations), the optimization calibration provides superior accuracy in the sense of propagated age uncertainty, particularly for ages much older than the FCs. Recent literature reveals that the optimization-based calibration has been misused and misrepresented in some cases; discussion of these cases clarifies the correct usage of the approach. Apparent conflict between 40 Ar/ 39 Ar and 238 U– 206 Pb ages for a Quaternary tuff do not appear to be a result of error in one of the three parameters determined by the optimization approach for the 40 Ar/ 39 Ar system. The optimization approach easily accommodates new constraints, but rigorous quality control is needed to maintain accuracy.

Abstract A review of geochronological data underlying the geological timescale for the Triassic yields a significantly different timescale calibration than that published in the most recent compilation (Geologic TimeScale 2004). This is partly due to the availability of new radio–isotopic data, but mostly because strict selection criteria are applied and complications arising from biases (both systematic and random) are accounted for in this contribution. The ages for the base and the top of the Triassic are constrained by U–Pb ages to 252.3 and 201.5 Ma, respectively. These dates also constrain the ages of major extinction events at the Permian–Triassic and Triassic–Jurassic boundaries, and are statistically indistinguishable from ages obtained for the Siberian Traps and volcanic products from the Central Atlantic Magmatic Province, respectively, suggesting a causal link. Ages for these continental volcanics, however, are mostly from the K–Ar ( 40 Ar/ 39 Ar) system, which requires accounting and correcting for a systematic bias of c . 1% between U–Pb and 40 Ar/ 39 Ar isotopic ages (the 40 Ar/ 39 Ar ages being younger). Robust age constraints also exist for the Induan–Olenekian boundary (251.2 Ma) and the Early–Middle Triassic (Olenekian–Anisian) boundary (247.2 Ma), resulting in a surprisingly short duration of the Early Triassic, which has implications for the timing of biotic recovery and major changes in ocean chemistry during this time. Furthermore, the Anisian–Ladinian boundary is constrained to 242.0 Ma by new U–Pb and 40 Ar/ 39 Ar ages. Radio–isotopic ages for the Late Triassic are scarce, and the only reliable and biostratigraphically-controlled age is from an upper Carnian tuff dated to 230.9 Ma, yielding a duration of more than 35 Ma for the Late Triassic. All of these ages are from U–Pb analyses applied to zircons with uncertainties at the permil level or better. The resulting compilation can only serve as a guideline and must be considered a snapshot, resolving some of the issues mainly associated with inaccurate and misinterpreted data in previous publications. However, further advances will require revision of some of the data presented here.