In 2022, we published a paper in Science Advances documenting a new candidate impact crater offshore West Africa 1. Intriguingly, this potential underwater crater appeared to be the same age (66 million years old) as the famous Chicxulub Crater in Mexico, that led to one of the greatest extinction events in Earth history and inaugurated the start of the Cenozoic; the ‘age of the mammals’ 2. This led to widespread public interest in the paper, including articles in New Scientist and National Geographic.
This newly identified crater still had a cloud of uncertainty hanging over it though. The bar of evidence is very high for impact craters, and most experts will only accept physical evidence of extreme shock pressures – diagnostic deformation fabrics in quartz minerals for example – to confirm an impact origin. This is because of the hype that often surrounds the announcement of any candidate impact structure, based on sometimes ambiguous circular anomalies in various geophysical datasets. In our paper, we had two 2-dimensional seismic profiles that showed characteristics consistent with an impact structure, but we couldn’t be sure that this would be circular (or near circular) in plan view, or that the proposed ‘central uplift’ that is so characteristic of large impact structures, had the dimensions and architecture that we would expect.
After publication of our article, we discovered that a commercial seismic company, PGS (now TGS), had recently acquired a ‘speculative’ 3D seismic survey in the same area offshore Guinea (Fig. 1). They agreed to provide this data, given the scientific significance of the project, and in support of our International Ocean Discovery Program (IODP) drilling proposal to drill into and recover core from the crater. As it turned out, one of their scientists, William Powell (now a co-author) had also identified this as a probable impact structure.
Our new study published in Communications Earth & Environment 3 presents this new, state-of-the-art 3D data, revealing the architecture of the crater in exceptional detail and confirms (beyond reasonable doubt!) an impact origin for the crater. This is the first time that an impact structure has ever been imaged fully with high-resolution seismic data like this and it is a real treasure trove of information to help us to reconstruct how this crater formed and evolved.
First, the structures deep below the seafloor at the time of impact reveal important details about the direction of the impactor that formed the crater. We can now image the central uplift, where the rock is weakened by the impact process to the point where it temporarily flows like a fluid and moves vertically back up into the transient (initial, bowl-shaped) crater (Fig. 3B), in detail and confirm that its dimensions are what we would expect for a crater of this size. The outer margin of this uplift and the edge of the surrounding moat, display radial reverse faults, that show that the rocks below the crater were thrust to the west immediately after impact. This, together with some other distinct fault patterns, show that the asteroid or comet that formed the crater hit the seabed at a shallow angle from the east.
Circular faults at a shallower depth showed that seabed sediments across an area of over 400 km2, (the 23 km diameter crater ‘brim’) and to a depth of ~300 m, flowed horizontally in towards the evacuated crater floor, after the crater collapsed (Fig. 3D). But we also see evidence of extensive faulting across a much larger area – many thousands of square kilometres in extent – that likely formed because of intense seismic shaking (equivalent to a magnitude 7 earthquake) following the impact.
Finally, detailed maps of the seabed at the time the crater formed show that there are large, arcuate scars around the crater, that we think formed during the ‘resurge’ stage (Fig. 3E). Given that the water depth in this location at the end of the Cretaceous period was around 500 to 800 m deep, this means that the water that surged back into the crater (after the initial outward-propagating tsunamis), was sufficiently turbulent and chaotic to strip tens of metres of sediment off the seabed.
This is not the end of our research on the Nadir Crater, and other candidate research craters. There are other observations to be made from the seismic data, that can help us to inform new computer simulations of these kind of events. These allow us to test some of the assumptions about the fundamental physics of this process, which is of course very difficult to observe in reality and can’t be replicated physically in a laboratory. There has never been an impact of this size in human history, and our models rely entirely on ancient craters on Earth, which are generally incomplete, or images from planets and rocky moons elsewhere in the solar system. And we have also written a proposal to drill into this structure, below 900 m of water, and 300 m of rock, to obtain cores so that we can find out the exact age (and its potential relationship with Chicxulub), understand the extreme physical processes that occurred during impact, and the environmental consequences of this event. These data open the door to take a major step forward in our understanding of this fundamentally important, planet-altering process, and we look forward to continuing this research, together with a diverse group of scientists, over the coming years.
References:
1. Nicholson, U., Bray, V.J., Gulick, S.P. and Aduomahor, B., 2022. The Nadir crater offshore West Africa: A candidate Cretaceous-Paleogene impact structure. Science Advances, 8(33), p.eabn3096.
2. Gulick, S.P., Bralower, T.J., Ormö, J., Hall, B., Grice, K., Schaefer, B., Lyons, S., Freeman, K.H., Morgan, J.V., Artemieva, N. and Kaskes, P., 2019. The first day of the Cenozoic. Proceedings of the National Academy of Sciences, 116(39), pp.19342-19351.
3. Nicholson, U., Powell, W., Gulick, S. et al. 3D anatomy of the Cretaceous–Paleogene age Nadir Crater. Commun Earth Environ 5, 547 (2024). https://doi.org/10.1038/s43247-024-01700-4
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