Seasonal calibration of the end‑cretaceous Chicxulub impact event (2021)

Though the year of the Chicxulub asteriod impact is dated to plus or minus 43,000 years, astonishingly the time of year can be dated to within a few weeks of late spring/early summer.

This is largely thanks to a remarkable set of fossils and deposits in North Dakota that record the actual impact.

[from the paper:] " Impact-triggered tsunami deposits have been reported in nearshore-marine and continental shelf deposits around the Gulf of Mexico and the Caribbean, < 1000 km from the Chicxulub crater, and evidence of seismically induced surges ~ 3000 km from the crater were recently documented at the Tanis (North Dakota, USA) KPg mass-death assemblage. Extreme, long-term global climatic shifts including a prolonged multi-year dark post-impact winter, resulting from infusion of CO2 and SO2 admixed with soot and atmospheric dust, are considered the primary agents of critical terrestrial and marine ecological collapse. [...] The Tanis Event-deposit preceded the iridium-rich dust-sized fallout and was emplaced exclusively within the period of coarse ejecta accretion, which began in the study region ~ 13 min after impact and lasted for ~ 1 to 2 hours. The massive water surge, possibly originating from the nearby WIS, entombed the remains of autochthonous freshwater fish, turtles, reptiles, dinosaurs, and plants, mixed with allochthonous marine organisms including fish, ammonites, dinoflagellates, foraminifera, and marine reptiles, all intimately associated with impact ejecta emplaced via primary deposition.

The sediment package is capped and temporally constrained by an iridium-rich clay layer (tonstein) that typifies the KPg boundary in the Western Interior. [...] Additional seasonally aligned insect activity at Tanis is represented through mayflies. The annual periodicity of synchronized adult mayfly emergence is constrained to a very short time span, and therefore is a reliable tool for temporal constraint. Body fossils of multiple adult mayflies occur as compression fossils in the fine-grained silt in upper Unit 2 and uppermost Unit 1 of the Event-deposit. Mayfly burrow casts, some of which preserve the remains of larval mayflies, have also been found excavated into the wood of large (~ 20–30 cm diameter) tree trunks and aligned in dense subparallel groupings that follow the grain of the wood (presumably following the path of least resistance). The burrows, preserved in full relief as matrix infillings, occur exclusively in wood that had previously died, i.e. exhibited no attached branches with leaves or other evidence that they were fresh at the time of burial. Extant mayflies exhibit highly constrained annual behaviour, beginning their life cycle during the Spring spawning period, at which time eggs are deposited in a freshwater environment. Larvae, living on the bottom or in U-shaped burrows excavated into soft substrate, mature over a period of months to years. The final adult moult and emergence occur en masse during a very short (< several week) time span that typically occurs in the latest Spring and Summer, between April and July 50–52 . Adults live subaerially only for a period of hours to days before dying in large groups. The adult mayfly body fossils at Tanis therefore indicate that the depositional event occurred during a short time span in late Spring or Summer, after the appearance-window for adult mayflies, and prior to a complete disintegration of their very delicate bodies. "