RECALIBRATION OF PLANKTIC FORAMINIFERAL BIOZONE P0 WITH 3He DATA IMPLIES LIGHTNING-FAST SPECIATION AFTER THE K/PG BOUNDARY

The immediate aftermath of the Cretaceous/Paleogene (K/Pg) mass extinction in the marine realm is delimited in planktic foraminiferal biostratigraphy as Zone P0, defined as the interval between the extinction of Cretaceous species and the first appearance of the new Paleocene taxon Parvularugoglobigerina eugubina in the aftermath of the Chicxulub impact. Zone P0 includes the initial recovery of marine productivity, including bacteria and phytoplankton, and the first originations of new species. Constraining the age of this interval is crucial for understanding how quickly this initial recovery and diversification occurred. However, the duration of Zone P0 is calibrated from paleomagnetic reversal datums based on the flawed assumption of constant sedimentation rates across the K/Pg boundary. These magnetostratigraphic calibrations provide estimates from 6 to 40 kyr for the duration of P0, with 30 kyr currently canonized in both the Geologic Time Scale and the Cenozoic Planktonic Foraminiferal Biozonation of Wade et al. (2011, Earth Science Reviews) as the duration of P0. Here, we provide a new calibration for this interval using published records of ³He, a cosmic-dust derived isotope already used to estimate the duration of the K/Pg boundary clay (which lies within P0). Using ³He accumulation rates from six boundary sections with both planktic foraminiferal biostratigraphy and ³He data, we find a duration of Zone P0 of between 3.8 and 11.1 kyr, with an average of 7.0 kyr, significantly shorter than the currently defined duration of 30 kyr. This recalibration suggests that new planktic foraminiferal species which began to appear 50% of the way through P0 evolved within 3.5 kyr after the extinction, an order of magnitude faster than previously documented rapid speciation events of extant and fossil taxa in other groups, which range from 10-20 kyr or longer. Our results demonstrate how quickly evolution can occur in the aftermath of drastic change, and help bridge the gap between “ecological evolution” observable on human timescales and the evolution of new morphospecies in the fossil record.