Cholla Fields of New Mexico

On my trip down the Dry Cimarron River, across the High Plains, and then on to the Cimarron proper, I often saw vegetation brand new to me but apparently common in New Mexico. From a distance I thought it was grassland with heavy shrub cover, maybe mesquite. But soon the distinctive form of the “shrubs” was obvious: thickish stems, awkward branching, no leaves. These were cacti.

Dry Cimarron valley, land of cacti and clastic plugs (mid photo).

The cactus was unfamiliar, but easy to identify: Cylindropuntia imbricata, tree cholla. It’s large, three to seven feet tall, even 10 or 15 under favorable conditions. Often there’s a short “trunk” that soon branches. Spines are sparse, revealing green jointed stems. The yellow fruits stay on the plants through the off season. The most helpful features are the tubercles—bumps on the stems bearing specialized buds (areoles) that produce spines (more here). Cactus tubercles are usually described as “small rounded projections” but those of the tree cholla are large and elongate. The sparse spines make them easy to spot.

Yellow fruit from previous year still on plants in early May.

Stems are sometimes reddish in winter.

Prickly pear cactus (Opuntia polyacantha) has “small rounded” tubercles.

Distinctive elongate tubercles of the tree cholla.

Being a veg ecologist of the American West, I was suspicious of these cholla fields. Heavy shrub cover in grassland often indicates over-grazing. Livestock almost always prefer grass to shrubs, giving the shrubs a competitive advantage that can lead to dominance. How about tree chollas? Surely no cow or sheep would eat one if grass were available.

Grassland thick with tree chollas, Dry Cimarron Valley (with ghosts of travelers past).

Tree cholla grassland on the High Plains north of Clayton.

Tree chollas, just west of Cimarron.

Back at home, I looked for tree cholla vegetation types in the US National Vegetation Classification. I found a good match: Opuntia imbricata Ruderal Shrubland (Opuntia is a synonym for Cylindropuntia). My suspicions were confirmed:

“This anthropogenic (semi-natural) shrubland results from overgrazing of native grasslands. It occurs particularly in slightly moister settings along broad swales. Infestation of Opuntia imbricata [synonym for Cylindropuntia imbricata] is variable, and cover of Opuntia in this type ranges from 25% to nearly 100%. Grass cover and composition is highly variable, depending on site and grazing regime and history. This broadly defined community grades to grassland (often with some Opuntia cover) or to other grazing-induced shrubland communities, such as those dominated by Prosopis, Acacia, and/or Mimosa” [shrubs in the pea family] (NatureServe Explorer).

Now the inevitable counter-questions: What about bison (buffalo)? There was a time not all that long ago when bison numbering in the millions roamed the American West, including New Mexico. What did these grasslands look like then? How common was tree cholla? Did bison eat it as readily as grass? The answer: we don’t know and probably never will (add these to life’s persistent questions).

But these questions are irrelevant where tree cholla has been introduced. It’s now widely naturalized—in eastern and southern Europe, northern and southern Africa, southern South America, and eastern Australia, often to the detriment of grasslands. For example, in Queensland, it threatens native semi-arid grasslands, and has been listed as a priority environmental weed (source).

Ironically, while land managers try to find a way to eradicate tree cholla (e.g., fire, chaining, chemicals), gardeners are encouraging it. “C. imbricata is one of the best chollas for desert landscaping. … It is an attractive plant in desert gardens, and cultivation is quite easy” [for the same reason it’s so hard to eradicate in pastures]. “All you have to do is plant one of the ten-twelve-inch stems in the ground. Water it after it has a chance to put out roots, in a few weeks.” Saguaroland Bulletin v.30:no.1-10 (1976)

Summer flowers (NPS).

Tree cholla is not restricted to desert gardens. It has long been grown in Europe, though not without challenges:

“It is the commonest member of the group to which it belongs in European gardens, and was first introduced to cultivation in the earlier years of the nineteenth century. In spite of this, however, it is a plant whose flowers, which are very attractive but which will only expand under the influence of direct sunshine and unfortunately do not last long, are not often seen in the United Kingdom. Sir Edmund Loder informs us that his example at Leonardslee has but seldom flowered; when it did so in 1908 this happened during a time of very hot, sunny weather, and individual flowers only remained fully open during one afternoon, from about midday till sunset.” Curtis’s Botanical Magazine v. 135; 1909

Curtis's Botanical Magazine, 1909 (courtesy Biodiversity Heritage Library).

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/