Biogeography of cretaceous/tertiary (k/t) planktic foraminifera

Abstract

As a result of the process‐level connections between biogeography, systematics, and ecology, analyses of the spatial distribution of organisms across major extinction events can be used to probe the paleobiological structure of those events and constrain the range of plausible explanatory models. Investigation of the biogeography of planktic foraminiferal faunas across the Cretaceous/Tertiary (K/T) boundary using a high‐resolution, taxonomically‐consistent, global database of more that 22 different boundary sections and deep‐sea cores reveals that this group did not undergo a stratigraphically instantaneous ‘mass extinction’ at the K/T boundary that is co‐incident with unambiguous independent evidence for a bolide impact. Rather, this event is better described as an extended or prolonged faunal turnover that was spread over a stratigraphic interval that, in some sections, spans several meters which, in turn, are estimated (using radiometric calibration) to represent as much 2–3 m.y. Beginning in the uppermost Maastrichtian Pseudotextularia deformis Zone, this K/T planktic foraminiferal faunal turnover proceeded in three taxonomically, ecologically, and geographically distinct waves. The first of these was entirely confined to the uppermost Maastrichtian and involved large, highly ornamented, deeper dwelling species that persisted in low relative abundances during this interval and were, for the most part, confined to the Tethyan faunal realm. Extinction of this fauna was completed by the uppermost P. deformis Zone‐prior to any independent evidence for bolide impact. The remaining Cretaceous planktic foraminiferal fauna was composed of smaller, more abundant, surface‐dwelling species that were almost evenly divided, in terms of species richness, into coiled (globigerine) and serial (heterohelicid) morphotypes. While this uppermost Maastrichtian planktic foraminiferal fauna exhibited marked patterns of spatial differentiation between low through middle and high latitude species assemblages, it apparently encountered and crossed the K/T boundary intact, with no disruption of either taxic or spatial structure. The final extinction of this K/T survivor fauna proceeded in two temporally and geographically distinct episodes. First, in most lower Danian Zones P0 and Pla, survivor species inhabiting low and middle latitude Tethyan habitats were progressively replaced by the evolving indigenous Danian planktic foraminiferal fauna. This replacement was accompanied by a decline in primary productivity with the Tethyan faunal province. High latitude boreal and austral survivor species remained relatively unaffected by this second extinction wave. These high latitude K/T survivor species, however, were decimated by the third wave of trans‐K/T extinctions in Zones Plb and Plc where they appear to have been ecologically replaced by indigenous high latitude Danian species. By the end of Zone Plc only three “Cretaceous”; species remained (all belonging to a single genus), two of which disappear from the fossil record in the overlying upper Danian Zone Pld. Correlation of these temporal and spatial patterns with independent evidence of changes in the physical and biotic structure of marine environments indicates that all three waves of trans‐K/T planktic foraminiferal extinctions were driven by a combination of environmental and biotic factors. This long‐term environmentally driven trans‐K/T extinction model also takes stratigraphic effects resulting from a coeval and rapid rise in eustatic sea level into explicit consideration. Hiatuses resulting from this transgression removed the critical trans‐K/T interval (upper uppermost Maastrichtian P. deformis Zone through Danian Zone Pla) in most low and middle latitude deep‐sea cores, thus juxtaposing faunas of very different age and giving the misleading appearance, both in terms of the fauna and associated chemical and isotopic indicators, of an abrupt change of the K/T boundary.