The Habitat of the Nascent Chicxulub Crater

Abstract

An expanded sedimentary section provides an opportunity to elucidate conditions in the nascent Chicxulub crater during the hours to millennia after the Cretaceous-Paleogene (K-Pg) boundary impact. The sediments were deposited by tsunami followed by seiche waves as energy in the crater declined, culminating in a thin hemipelagic marlstone unit that contains atmospheric fallout. Seiche deposits are predominantly composed of calcite formed by decarbonation of the target limestone during impact followed by carbonation in the water column. Temperatures recorded by clumped isotopes of these carbonates are in excess of 70°C, with heat likely derived from the central impact melt pool. Yet, despite the turbidity and heat, waters within the nascent crater basin soon became a viable habitat for a remarkably diverse cross section of the food chain. The earliest seiche layers deposited with days or weeks of the impact contain earliest Danian nannoplankton and dinocyst survivors. The hemipelagic marlstone representing the subsequent years to a few millennia contains a nearly monogeneric calcareous dinoflagellate resting cyst assemblage suggesting deteriorating environmental conditions, with one interpretation involving low light levels in the impact aftermath. At the same horizon, microbial fossils indicate a thriving bacterial community and unique phosphatic fossils including appendages of pelagic crustaceans, coprolites and bacteria-tunneled fish bone, suggesting that this rapid recovery of the base of the food chain may have supported the survival of larger, higher trophic-level organisms. The extraordinarily diverse fossil assemblage indicates that the crater was a unique habitat in the immediate impact aftermath, possibly as a result of heat and nutrients supplied by hydrothermal activity.

Whump, Slosh, Slosh, Slosh—Filling the Crater That Did in the Dinosaurs

“…Next, the marl that contains most of the meteoric iridium is unlikely to have formed within a few decades as proposed by Bralower et al (2020). Accumulation rates in the deep sea show that Danian microfossil-rich mud accumulates very slowly, about~0.5 cm/kyr (Herbert & D'Hondt, 1990;Mukhopadhyay et al, 2001).…”

“…The first open ocean sediments, deposited on top of the impact debris in the transitional unit, carry most of the meteoritic material (signified by a small increase in meteoric iridium) and increased concentrations of He 3 signifying reduced sedimentation rates (Lowery et al, 2018). These sediments also include the first abundant Danian species, including dinoflagellates that may record red-tide-like blooms in the new crater lake (Bralower et al, 2020). Bralower et al interpret the meteoric iridium signal to reflect air-fall of meteoric dust within a few decades of the impact event.…”

“…Over the next couple of minutes, the transient crater collapsed to form an impact structure about 200 km in diameter (Gulick et al, 2019). Bralower et al (2020) describe the contents of the crater and its post-impact history.…”

“…Thereafter, the ocean, which had been blasted out of the way, rushed back into the crater, carrying more impact debris and material swept off the nearby land into the hole to form a thick layer of reworked suevite (Gulick et al, 2019) and the first part of a limestone called the "transitional unit" (Bralower et al, 2020;Gulick et al, 2019). These sediments were laid down by strong currents, submarine landslides and "seiche waves"-the sloshing of water laden with mud and rock in the crater-a product of the seismic ringing of the giant impact earthquake (Figure 1).…”

“…These sediments were laid down by strong currents, submarine landslides and "seiche waves"-the sloshing of water laden with mud and rock in the crater-a product of the seismic ringing of the giant impact earthquake (Figure 1). Bralower et al show that some of the sediment was deposited a temperature upwards of 88°C through some combination of direct precipitation of carbonates from vaporized limestone and hydrothermal alteration of the sediments by hot springs above the pool of impact melt in the center of the crater (Bralower et al, 2020).…”