Stable seafloor conditions, sea level and food supply during the latest Maastrichtian at Brazos River, Texas

Abstract

Abstract The Cretaceous/Paleogene (K/Pg) boundary extinction event is inextricably linked to the Chicxulub impact. Environmental changes like sea level changes and changes in food supply prior to or after the impact could however have contributed to the biotic turnover. In this study, benthic foraminifera from two cores from the Brazos River area in Texas are quantitatively analyzed in order to reconstruct environmental changes across the K/Pg boundary. Cluster analysis as well as non-metric multidimensional scaling reveals three distinct benthic foraminiferal assemblages: an upper Maastrichtian assemblage, a post-impact assemblage, and a lower Danian assemblage. The rather uniform upper Maastrichtian assemblage at Brazos River, composed of e.g. Clavulinoides midwayensis, Gavelinella dumblei, Cibicidoides harperi, Gyroidinoides aequilateralis and Bulimina kickapooensis, indicates a stable paleodepth of about 75–100 m up to the K/Pg boundary. Food supply indicators such as percentage Buliminaceae and foraminiferal numbers suggest a relatively low and stable food supply during this interval. Environmental disruption caused by the Chicxulub impact led to the settlement of a typical pioneer assemblage, characterized by endobenthic morphotypes such as Ammobaculites spp. and Pseudouvigerina naheolensis. During the early Danian, the typical Midway-type benthic foraminiferal shelf fauna (e.g. Alabamina midwayensis, Anomalinoides acutus, Gyroidinoides subangulatus, Praebulimina carseyae and Cibicidoides alleni) developed, indicating a paleodepth of 75–100 m. No clear water depth changes can be derived from the benthic foraminiferal record during this interval, although the sedimentary record suggests a shallowing followed by a transgressive pulse in Zone P1a. Food supply indicators suggest a lower food supply in the early Danian than during the upper Maastrichtian. Because of the stability of the late Maastrichtian environment we conclude that environmental disruption caused by the impact was most likely the principal cause of the benthic foraminiferal turnover across the K/Pg boundary observed at Brazos River.