Aimed at resolving the impact versus volcanism debate about the causes of the K-Pg mass extinction, the International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) began drilling into the focus of the Chicxulub impact structure off the Yucatán Peninsula, Mexico in 2016. The project recovered 830 m of rock core, of which about 140 cm contained the boundary between tsunami deposits and the post-impact marine limestones of Danian Age (basal Palaeogene); as close as one can get to the moment when the asteroid hit the sea floor. That an impact close to the start of the Danian had taken place was first discovered from abnormally high concentrations of the platinum-group metal iridium (Ir), shocked mineral grains and glass spherules, among other anomalous materials, in 350 marine and terrestrial sections across the globe. If the Chicxulub crater contained similar features to these ‘smoking guns’ then the link might seem to be done and dusted. A report on the crucial few centimetres from the Chicxulub drill core shows this to be the case (Goderis, S. and 32 others 2021. Globally distributed iridium layer preserved within the Chicxulub impact structure. Science Advances, v. 9, article eabe3647; DOI: 10.1126/sciadv.abe3647).
Yet the boundary layer at Chicxulub could not have been emplaced at the instant of impact. The gigantic power involved would have flung debris outwards, including seawater as well as the rocks that were once at considerable depth below the seabed. Much in the manner of a stone falling into a pond molten crust would have rebounded from the initial strike to form an axial peak and a ringed basin. Likewise huge tsunamis would have rolled away from the impact, then to return and fill the new basin, perhaps several times. Some of the ejected debris would have reached low orbit in the form of pulverised rock and asteroid to remain there for a while before completely falling back to Earth. The core includes about 130 m of once partly molten debris (suevite) above more-or-less intact granitic basement. Only the top 3.5 m show signs of having been deposited in water; fine-grained, well-sorted and laminated suevite containing clasts of once molten material and even late-Cretaceous foraminifera tests, formed probably by the refilling of the impact basin during the backflow of tusunamis. A mere 3 cm of silt and clay just below marine limestones has yielded the characteristic high Ir and nickel concentrations. This Ir-rich layer also contains the earliest Palaeocene foraminifera.
Grains in the Ir-rich layer were the last to settle, the main question being ‘How long after the impact took place did that happen?’ Being very fine they are estimated to have fallen-out from suspension and circulation in the atmosphere over a period of up to a few decades. Coarser material below them would have taken no longer than a few weeks to years. Yet these estimates are based mainly on Stokes’ law governing particles of different sizes falling through a viscous fluid. Taking an empirical view based on actual rates of clay sedimentation in the ocean (~5 mm per thousand years) the Ir-rich layer may have been deposited over 6000 years. That is hardly the ‘instant of the impact’. But the timing does say something interesting about the return of life to the seas; in geological terms it was swift, if the forams are anything to go by. Since the tsunamis swept onto and drained the surrounding land masses a great deal of nutrient would have ended up in the sea awaiting organisms at the bottom of the food chain. Biomarker chemicals and trace fossils in the Ir-rich layer suggest thriving bacterial communities, with forams, crustacea and larval fish.
The authors conclude ‘The clear association of the Ir anomaly within the Chicxulub impact structure and the recorded biotic response confirms the direct relationship between the impact event and the K-Pg mass extinction’. Whether that is accepted by those geoscientists with their eyes on the Deccan Trap hypothesis is not so certain …
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