ABSTRACT The latest Cretaceous (Maastrichtian) through earliest Paleogene (Danian) interval was a time marked by one of the five major mass extinctions in Earth’s history. The synthesis of published data permits the temporal correlation of the Cretaceous-Paleogene boundary crisis with two major geological events: (1) the Chicxulub impact, discovered in the Yucatán Peninsula (Mexico), and (2) eruption of the Deccan Traps large igneous province, located on the west-central Indian plateau. In this study, environmental and biological consequences from the Chicxulub impact and emplacement of the Deccan continental flood basalts were explored using a climate-carbon-biodiversity coupled model called the ECO-GEOCLIM model. The novelty of this study was investigation into the ways in which abiotic factors (temperature, pH, and calcite saturation state) acted on various marine organisms to determine the primary productivity and biodiversity changes in response to a drastic environmental change. Results showed that the combination of Deccan volcanism with a 10-km-diameter impactor would lead to global warming (3.5 °C) caused by rising carbon dioxide (CO 2 ) concentration (+470 ppmv), interrupted by a succession of short-term cooling events, provided by a “shielding effect” due to the formation of sulfate aerosols. The consequences related to these climate changes were the decrease of the surface ocean pH by 0.2 (from 8.0 to 7.8), while the deep ocean pH dropped by 0.4 (from 7.8 to 7.4). Without requiring any additional perturbations, these environmental disturbances led to a drastic decrease of the biomass of calcifying species and their biodiversity by ~80%, while the biodiversity of noncalcifying species was reduced by ~60%. We also suggest that the short-lived acidification caused by the Chicxulub impact, when combined with eruption of the Deccan Traps, may explain the severity of the extinction among pelagic calcifying species.

The highest stages of the stratigraphic range of the planktonic foraminiferal Rotalipora cushmani were investigated in a 313-k.y.-long interval of the classical Tethyan Bottaccione section (Gubbio, Italy), the type locality of the C org- rich Bonarelli Level, which is the sedimentary expression of the worldwide latest Cenomanian oceanic anoxic event 2 (OAE 2).The disappearance of R. cushmani is associated with the major turnover of marine microfauna and microflora that involves both planktonic and benthic foraminifera, and calcareous nannofossils, slightly before the onset of OAE 2, which, according to current available data, was triggered by a massive pulse of submarine mafic volcanism accompanying the initial emplacement of the Caribbean large igneous province (CLIP). This pulse of volcanic activity probably turned the climate in a strengthened greenhouse mode, accelerating continental weathering and increasing nutrient supply in oceanic surface waters via river runoff and triggering higher fertility in the global ocean. Our investigation shows that the marine biotic turnover started ~55 k.y. before the onset of OAE 2 and is closely coeval with the first major episode, as recorded by the unradiogenic trend in 187 Os/ 188 Os, of the ongoing magmatic activity of the CLIP, which produced increasing p CO 2 , ocean dissolution and/or acidification with a severe carbonate crisis and fertilization through enormous quantities of biolimiting metals. The marine microfauna and microflora reacted rapidly to new conditions of higher p CO 2 and fertility by undergoing marked changes following three main steps. We evaluate this pattern and postulate that the first pulse of volcanogenic CO 2 from the CLIP emplacement (ca. 94.2 or 94.6 Ma) played a fundamental role in the marine biotic turnover recorded shortly before the onset of OAE 2 and notably in the local or regional disappearance of R. cushmani in the central-western Tethys.

The recent discovery of the direct link between Deccan volcanism and the end-Cretaceous mass extinction also links volcanism to the late Maastrichtian rapid global warming, high environmental stress, and the delayed recovery in the early Danian. In comparison, three decades of research on the Chicxulub impact have failed to account for long-term climatic and environmental changes or prove a coincidence with the mass extinction. A review of Deccan volcanism and the best age estimate for the Chicxulub impact provides a new perspective on the causes for the end-Cretaceous mass extinction and supports an integrated Deccan-Chicxulub scenario. This scenario takes into consideration climate warming and cooling, sea-level changes, erosion, weathering, ocean acidification, high-stress environments with opportunistic species blooms, the mass extinction, and delayed postextinction recovery. The crisis began in C29r (upper CF2 to lower CF1) with rapid global warming of 4 °C in the oceans and 8 °C on land, commonly attributed to Deccan phase 2 eruptions. The Chicxulub impact occurred during this warm event (about 100–150 k.y. before the mass extinction) based on the stratigraphically oldest impact spherule layer in NE Mexico, Texas, and Yucatan crater core Yaxcopoil-1. It likely exacerbated climate warming and may have intensified Deccan eruptions. The reworked spherule layers at the base of the sandstone complex in NE Mexico and Texas were deposited in the upper half of CF1, ~50–80 k.y. before the Cretaceous-Tertiary (K-T) boundary. This sandstone complex, commonly interpreted as impact tsunami deposits of K-T boundary age, was deposited during climate cooling, low sea level, and intensified currents, leading to erosion of nearshore areas (including Chicxulub impact spherules), transport, and redeposition via submarine channels into deeper waters. Renewed climate warming during the last ~50 k.y. of the Maastrichtian correlates with at least four rapid, massive volcanic eruptions known as the longest lava flows on Earth that ended with the mass extinction, probably due to runaway effects. The kill mechanism was likely ocean acidification resulting in the carbonate crisis commonly considered to be the primary cause for four of the five Phanerozoic mass extinctions.