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.

Abstract: Photozoan and heterozoan carbonate systems differ in their biotic assemblages and depositional facies distribution. These, in turn, control their response to relative sea-level change, which affects stratigraphic architecture. Understanding the controls over the occurrence of different carbonate types is important when interpreting the fossil record. The differentiation of carbonate systems today into photozoan and heterozoan assemblages is directly dependent on the environmental requirements of the biocalcifiers active in the modern world. Because biocalcification mechanisms and environmental requirements have changed through time, it becomes increasingly difficult to apply this differentiation to older carbonate systems. This paper reviews general controls over the distribution of biotic assemblages in the modern world, investigating the major limiting factors affecting carbonate assemblages. Selected examples from icehouse and greenhouse time intervals are also discussed to highlight the effects of major limiting factors in the geological past. Icehouse times are characterized by stronger temperature and nutrient gradients, with environments spanning a larger spread of possible conditions. Times of climatic changeover are recorded by broad community shifts from the photozoan to heterozoan–photozoan transition. Greenhouse times, conversely, are characterized by gentler temperature gradients and biota suggesting that mesotrophic conditions were more widespread. This translates to a higher chance to find an expanded occurrence of heterozoan–photozoan transitional settings. Carbonate systems, with their unique biological and geochemical characterization, have a still largely unexplored potential to provide a record of Earth history events that have no modern analogues. However, it is critical to be fully aware of the factors and processes operating at different timescales that have an effect on biogenic assemblages and carbonate systems. The use of the terms photozoan and heterozoan must be used very critically going back in time. Rather than simply applying one model, the intrinsic complexities of carbonate systems require a detailed understanding of how and where sediment is produced, and eventually transported and deposited, before reliable paleoenvironmental scenarios can be constructed.