The hypothesis of a large asteroid impact playing a role in dinosaurs extinction was strengthened by the discovery of a global iridium anomaly in the geologic record, at the Cretaceous–Paleogene (K-Pg) boundary.

Given we still have several unconfirmed impact craters, for example, the 21 km diameter São Miguel do Tapuio structure centered at 5°37.6′S, 41°23.3′W in Brazil(1), close to my city:

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Wouldn't a systematic search for other iridium layers in the geologic record help prove/disprove many structures like this as produced by a impact event? Or even know about impact events that were previously unknown, because the resulting craters were completely eroded and became unrecognizable? Global petroleum exploration surely left a lot of rock plugs that could be searched for such layers. Yet the only iridium anomaly I see people writing about is the famous K-Pg one. So, how many of these do we know? Or has nobody ever bothered seeking them?


(1). Crósta, A. P., et al. “Impact Cratering: The South American Record—Part 2”. Geochemistry, vol. 79, no 2, maio de 2019, p. 191–220. ScienceDirect, https://doi.org/10.1016/j.chemer.2018.09.002.

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    $\begingroup$ This would be better answered at the Earth Science SE. $\endgroup$ Commented Aug 26, 2021 at 14:11
  • $\begingroup$ @Jean-MariePrival Now that there is a answer, I don't think is worth to erase the question and re-ask elsewhere. Anyway, if the moderators think it would be better to migrate the question, I'm OK with it. $\endgroup$
    – ksousa
    Commented Aug 27, 2021 at 12:34

1 Answer 1


There are other iridium anomalies, particularly the Devonian-Carboniferous boundary (D-C boundary), about 350 Ma ago. The K-Pg boundary is only 66 Ma old.

Just because there is an iridium anomaly doesn't mean it was due to a meteorite impact. One of the differences between the K-Pg boundary and the D-C boundary is the D-C boundary lacks shocked quartz, which is evidence of a meteorite impact. The iridium anomaly at the D-C boundary is thought to be the result of paleo-redox changes.

At or very close to the D-C boundary, there exists a geographically widespread black-shale interval, and Ir abundances reach anomalous maxima of 0.148 ppb (Montagne Noire, France), 0.138 ppb (Alberta, Canada) 0.140 ppb (Carnic Alps, Austria), 0.156 ppb (Guangxi, China), 0.258 ppb (Guizhou, China), and 0.250 ppb (Oklahoma). The discovery of global D-C Ir anomalies argues for an impact-extinction model. However, nonchondritic ratios of Ir to other important elements and a lack of physical evidence (shocked quartz, microtektites) do not support such a scenario. The fact that all Ir abundance maxima are at sharp redox boundaries in these sections leads us to conclude that the Ir anomalies likely resulted from a sudden change in paleo-redox conditions during deposition and/or early diagenesis.

Additionally, concerning the D-C boundary in Western Australia (second reference & third reference),

the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium Frutexites; iridium, platinum, iron, manganese, cobalt, arsenic, antimony, and cerium are preferentially concentrated in filaments of this organism, with concentrations ranging from two to five times that of the matrix. It is possible that Frutexites extracted these elements directly from seawater, without the need for their derivation from an extraterrestrial source.

Edit: 28 August 2021

When considering meteorite/bolide impacts on Earth one needs to consider when the impact occurred and what has happened to the Earth since then.

The Vredefort Crater in South Africa is thought to have been created by the impact from an asteroid during the Hadean Eon, some 4 billion years ago. Also, the Sudbury Basin in Canada is thought to have been created by a bolide impact during the Paleoprotezoic Era some 1.849 billion years ago.

Iridium is a very dense metal, with a density of 22.56 g/cm3 at room temperature and 19 g/cm3 when liquid.

It is thought that the total amount of iridium in the planet Earth is much higher than that observed in crustal rocks, but as with other platinum-group metals, the high density and tendency of iridium to bond with iron caused most iridium to descend below the crust when the planet was young and still molten.

So any iridium that may have been present on the Earth's surface during its early history is most likely now locked in rocks below the Crust.

The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. Native platinum used by ancient Ethiopians[34] and by South American cultures always contained a small amount of the other platinum group metals, including iridium. Platinum reached Europe as platina ("silverette"), found in the 17th century by the Spanish conquerors in a region today known as the department of Chocó in Colombia.

Iridium is one of the nine least abundant stable elements in Earth's crust, having an average mass fraction of 0.001 ppm in crustal rock; platinum is 10 times more abundant, gold is 40 times more abundant, and silver and mercury are 80 times more abundant.

Within Earth's crust, iridium is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below), impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in the Bushveld igneous complex in South Africa, (near the largest known impact crater, the Vredefort crater) though the large copper–nickel deposits near Norilsk in Russia, and the Sudbury Basin (also an impact crater) in Canada are also significant sources of iridium.

As for Norilsk, the Popigai crater is 880 km from Norilsk. It was created 35 million years ago. It created an iridium enriched layer that has been detected on land in the Massignano section near Ancona in Italy and in marine sediments around the world.

So such layers exist, they're just not well known.

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    $\begingroup$ Also there is a local iridium anomaly here: en.wikipedia.org/wiki/Eltanin_impact $\endgroup$
    – Connor Garcia
    Commented Aug 26, 2021 at 17:50
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    $\begingroup$ @ConnorGarcia: Yo've made me aware of something I didn't know. Thanks $\endgroup$
    – Fred
    Commented Aug 26, 2021 at 18:01
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    $\begingroup$ Speculated [Eltanin impact tsunami](www.geosci.usyd.edu.au/users/prey/Teaching/Geos-2111GIS/Ref/Ward-TsunamicImpact.pdf). $\endgroup$
    – Fred
    Commented Aug 26, 2021 at 18:05
  • $\begingroup$ Thank you. I'm still puzzled about this small number of 3 (K-Pg + D-C +Eltanin), and one isn't even impact-related. In the last few hundred million years probably we had dozens of sizable impacts. I guess there's lots of undiscovered such anomalies, either global or at least local. Shouldn't more of them be already discovered by now? $\endgroup$
    – ksousa
    Commented Aug 27, 2021 at 12:44
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    $\begingroup$ @ksousa: See my latest edit. $\endgroup$
    – Fred
    Commented Aug 27, 2021 at 16:48

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