In Search of Iridium & the Smoking Gun

Is this the smoking gun?

Or this? (Western Ghats, Deccan Large Igneous Province; source)

Probably you’ve heard about the meteorite that killed off the dinosaurs, but just in case you haven't … it hit the Earth (Yucatan Peninsula) 65 million years ago, sending massive amounts of debris into the atmosphere. Major climate change ensued, so quickly and dramatically that many species (not just dinosaurs) were driven to extinction. The ejected debris included iridium from the meteorite itself—the “DNA” that “solved” the crime. It rained down on Earth forming a thin layer now exposed at sites around the world—the iridium layer or anomaly. What a terrific story! So much of drama!! And it solves what used to be a hugely challenging and vexing puzzle.

Or maybe not. When I visited the iridium site at Raton, New Mexico, I already knew the story wasn’t so simple; probably many readers do as well. But the meteorite-as-dino-killer story lives on, understandably. The mind-boggling horror is irresistible: “NIGHT OF THE DINOSAURS … the end of the Dinosaur Age on planet Earth”!!!

“Iridium Layer marks End of Dinosaur Age on Planet Earth” is not a B-movie title; it’s the lead on a faded interpretive sign.

However, it's possible that the smoking gun is not the famous iridium layer (1) but rather extensive thick basalt flows in west-central India—the Deccan Large Igneous Province (Deccan Traps). Covering more than 500,000 sq km, it’s one of the largest LIPs in the world (Mukherjee et al. 2016). Like the Yucatan meteorite crash, these eruptions took place about 65 million years ago, roughly concurrent with dinosaur extinction. Volcanism on that scale likely ejected enough material (and possibly iridium) to cause significant climate change, perhaps leading to dino demise. Even more intriguing, the Yucatan meteorite impact may have caused the massive volcanism (see Sources below for more details).

There also are complicating factors regarding the scale and timing of extinctions. For example, some dinosaurs already were in decline, some survived (the birds), and not all life forms were affected. Maybe climate-changing meteorite-caused massive volcanism exacerbated challenges already faced by species in decline. Or maybe there’s another surprising puzzle piece waiting to be discovered! In any case, the iridium layer is worth a visit … nothing wrong with a little mystery in the drama.

“What minor evils might arise from the contact [impact] were points of elaborate question. The learned spoke of slight geological disturbances; of probable alterations in climate, and consequently in vegetation …” Edgar Allen Poe, 1839 (2)

The iridium site at Raton, New Mexico is just north of town, a short distance up a winding paved and gravel road passable to cars. In addition to the famous anomaly, there’s a picnic table and great views.

Town of Raton below Raton Mesa—note basalt cap on horizon.

The iridium layer is exposed on a short steep bank next to the IRIDIUM LAYER sign. I say “exposed” rather than “visible” because I wasn’t sure I found the actual layer. Neither of my guidebooks offered specific guidance. The interpretive sign was much more helpful, though some words were illegible:

… a thin clay-like layer—just above the level of the IRIDIUM LAYER sign … 8 inches beneath the coal layer … weathers to a fine white powder. This layer consists of melted rock (glass since altered to clay) blown out of the impact crater (asteroid). High concentration of iridium and shocked minerals … suggesting “not of this planet” (3)

According to the interpretive sign, the iridium anomaly lies in the narrow layer between the grayer rocks from late Cretaceous times, the “final period of the dinosaurs,” and the early Tertiary tan rocks above, from the “Era of Mammals” (the early part of the Tertiary is now called Paleogene). Is that a minor fault offsetting the layer mid photo?

Back at home, I searched Google images for help. Sure enough, I’m not the only one who has had trouble finding the iridium layer. One amateur geologist who visited the well-known site near Trinidad, Colorado (about 20 miles from Raton) went so far as to have backscatter scanning electron microscopy (BSEM) and chemical analyses done on what he thought was clay from the iridium layer, only to learn there was no iridium. As he explained, the distinctive clay layer (kaolinite) he sampled marks the Cretaceous-Tertiary (Paleogene) boundary, but “the iridium I’ve since learned isn’t actually concentrated in the clay layer itself but in the 2 layers directly above it (red arrows in photo): that is the impact layer (smectite - blue arrow in photo), and the 2-inch coal layer directly above that.

He also noted that kaolinite “is thought to result from the altering of volcanic ash beds in acidic coal swamps, but in this case it’s the result of a doomsday shroud of impact material interacting with a coal swamp.” But do we know? Maybe it’s altered volcanic ash after all.

Source (used for personal, educational purposes).

“I was at the right place and was able to identify the boundary layer, I just didn’t have all the facts. But at least I’ve learned something from my mistake, so it turns out not to be such a bad thing. And now you’ve learned something, too.” anonymous amateur geologist on scienceBuzz

Notes

(1) Whatever the cause, the K/T boundary at the Raton site marks environmental change, for it's defined by the disappearance of Proteacidites pollen. At Sugarite State Park nearby, a spike of fern spores occurs just above this boundary, and has been interpreted as “opportunistic fern species replacing the normal plant community that was devastated by the extinction event.” (Paul Bauer, p 262 in Price 2010)

(2) Poe was referring to a comet that destroyed life on Earth, as described by one of the dead. See The Conversation of Eiros and Charmion.

(3) Actually, iridium is of this planet, especially “in molten rock deep within.” High concentrations of iridium could be evidence of large-scale volcanism (source).

Sources

Cowen, R. No date. The K-T extinction. UC Berkeley. http://www.ucmp.berkeley.edu/education/events/cowen1b.html

Muehlberger, WR, Muehlberger, SJ, and Price, LG. 2005. High Plains of northeastern New Mexico, a guide to geology and culture. NM Bureau of Geology and Mineral Resources.

Mukherjee, S, et al. 2016. Tectonics of the Deccan large igneous province: an introduction. Geological Society, London, Special Publications 445: 1-9. http://sp.lyellcollection.org/content/445/1/1

PBS.org. No date. What killed the dinosaurs? http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/volcanism.html

Price, LG (ed). 2010. The geology of northern New Mexico’s parks, monuments, and public lands. Socorro: New Mexico Bureau of Geology and Mineral Resources.

Sanders, R. 2015. Did dinosaur-killing asteroid trigger largest lava flows on Earth? Berkeley News. http://news.berkeley.edu/2015/04/30/did-dinosaur-killing-asteroid-trigger-largest-lava-flows-on-earth/