2. Meteorite or No Meteorite? Iridium Anomalies

Iridium (Symbol, Ir) is a chemical element, atomic number 77 and looks like this:

It is an incredibly rare element that is the second densest around. That coupled with its tendency to bind to iron makes it a very low abundant element in the crust. Most of it sank into the mantle and core when the Earth was young. O dear. 

Its abundance in meteorites however is somewhat different. Chondritic meteorites and asteroids have Iridium concentrations of about 455 parts per billion (ppb) compared to typically 0.3ppb in the Earth’s crust. 

An Iridium anomaly or “spike” in the sedimentary layers across the the Cretaceous-Paleocene boundary 65 million years ago is thought to be a key indicator of a meteorite impact. The spike appears as a sudden and stark few parts per billion above the average 0.8ppb. The exact value varies across the Earth. This crude little graph below shows that it reached 6.5ppb. 

Spot the Spike

The Iridium anomaly at Zumaya, Spain, has been the source of a debated second such event, dated at 55Ma - the time of the PETM. The anomaly is reproducible but differs from the magnitude of the KT extinction by being noticeably less dramatic but most importantly coincides with a decrease in δ¹³C which, as previously mentioned, coincides with the PETM. 

So, is it a seal of approval for a meteorite impact? Well, we're still looking for the impact crater…