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Sunday, May 15, 2011

How old is Carlsbad Cavern (Guadalupe Mountains, New Mexico)?

The Guadalupe Mountains of New Mexico and Texas are home to more than 300 caves, including those of Carlsbad Caverns National Park. If you are not familiar with the geology of the region, the National Park Service has already published a number of brochures describing the intricate, and well decorated cave system (I would recommend starting with this PDF on the development of the caves).

Many visitors and researchers alike have wanted to know, how old are these caves? In the last post, I described the most common method of dating speleothems: uranium-thorium (U-Th) disequilibrium dating. Some of the younger speleothems at Carlsbad Caverns have been dated using the U-Th method (e.g. Polyak et al., 2004; Brook et al., 2006), and cover the past 12,500 years and 164,000 years, respectively. Forty-six U-Th ages were analyzed in the latter case, and were used to model highly variable stalagmite growth (0–70 mm/kyr) and climate over the last two glacial cycles.

But this only address part of the question, because it tells us when precipitation of speleothems began, and not when the caverns were actually carved out. Unfortunately, it is much more difficult to date the removal of something in geology than its appearance (e.g. erosion of the Grand Canyon vs. the sediments being eroded).

The curious case of Carlsbad Cavern: sulfuric acid dissolution

Several researchers in the region devised a novel solution to this question (Polyak et al., 1998). As it turns out, some of the larger caves of the Guadalupe Mountains were dissolved with the help of sulfuric acid (as opposed to just carbonic acid). The unique dissolution process left its mark in the form of sulfate minerals, such as alunite, that formed residues on the cave walls, and in small cavities. Alunite is a potassium-bearing mineral, which means that it can be dated using the 40Ar/39Ar method. Since alunite forms as a byproduct of dissolution, the model age should reflect the time of cave dissolution.

Polyak et al. (1998) obtained 15 ages from the purest alunite samples (determined by XRF), representing 5 different caves in the region. Model ages ranged from 3.89–12.26 million years (precision better than 3%) and were reproducible across multiple rooms from each cave. Moreover, clay minerals from the Permian bedrock were dated by the same method, and estimated to be 278±3 Ma. Several clay-rich samples of alunite, with unusually high K/Ca ratios, yielded anomalously old ages (~30 Ma), as expected. Thus contamination could be ruled out in the primary data set by analyzing for clay content and elemental ratios (K/Ca).

40Ar/39Ar model ages, tectonic uplift of the Guadalupe Mountains, and the age of Carlsbad Cavern

Model ages from each cave were also plotted against elevation, revealing a strong correlation. This result corroborates the current understanding of cave dissolution, which is thought to occur from groundwater interaction near the water table. As the mountains were uplifted, the water table dropped, and so caves were carved out at lower and lower elevations. In other words, the oldest caves are now found at the highest elevation, and the youngest caves are found much lower.

Carlsbad Cavern, currently at ~1,100 meters above sea level, was carved out about 4 million years ago, according to alunite model ages. Speleothems would have begun long after, however, and some are still forming today.

Sulfuric acid dissolution: mechanism of rapid cave formation in a young Earth?

Back in 1998, young-Earth creationist Michael Oard tried to work the results of Polyak et al. (1998) in his favor (original article here; responding to YEC-critic Art Strahler). Mr. Oard suggested that since sulfuric acid is a much stronger acid, it could have formed caves rapidly during or after the Flood, allowing more time for speleothem formation (~4,500 years versus...4,000 years?). Currently, some 10% of the world's caves are thought to have formed by sulfuric-acid dissolution, but Mr. Oard posits that number might be larger, and the evidence has since washed away.

Greg Neyman (Answers in Creation) has already responded to the article here, showing that Mr. Oard's optimism is hardly warranted, so I will address the remaining errors here.

1) Syn-Flood vs. Post-Flood: Mr. Oard suggests that cave dissolution might have occurred during the Flood, contra his critic that deemed caves as "post-Flood" features:

"...cave formation is not necessarily a post-Flood phenomenon as Strahler thought. It could have formed anytime after the limestone was first deposited in the Flood, since hydrothermal water would be expected to begin moving through the limestone soon after deposition."

This point is hardly worth discussing, since it only moves the possible age of the cave back by one year at most. Nonetheless, I'll mention that evidence of hydrothermal fluids is common in limestone bedrock (e.g. Tritlla et al., 2001). Hydrothermal fluids typically move through fractures in the bedrock and deposit calcite veins in their path. The calcite is a mixture of dissolved bedrock and CO2 from thermally altered organic matter. Hydrothermal fluids also contain trace elements, like strontium, that are incorporated into the recrystallized calcite. Overall, hydrothermal activity is very easy to detect in carbonates, because it shifts the chemistry on every level: 87Sr/86Sr ratios drop, along with δ18O and δ13C values. Mr. Oard's hypothesis can thus be tested, but I suspect that most of his readers will rest on his 'just-so' story.

2) Biogenic sulfur: Mr. Oard contradicts himself after he confuses the origin of sulfuric acid in the Polyak et al. (1998) study.

"The sulfuric acid is formed by the oxidation of hydrogen sulfide in hydrothermal water...The 34S/32S ratio indicates the hydrogen sulfide is biogenic."

Polyak et al. (1998) mention sulfur input from hydrothermal fluids as a factor for some caves, but not in the case of Carlsbad Cavern. The significance of isotopically light sulfur is that the sulfuric acid was ultimately sourced from decaying organic matter—not H2S in hydrothermal fluids. Hill (1990), cited by Mr. Oard, linked the biogenic sulfur signal to hydrocarbons (oil) in the underlying strata. In other words, sulfur-bearing oil was oxidized in the subsurface to produce small quantities of H2S, and that H2S was oxidized to sulfuric acid (H2SO4) as it was carried through the groundwater to the site of cave dissolution.

Now, I do not highlight this mistake for the sake of trivial amendment. The fact that sulfuric acid responsible for carving out Carlsbad Cavern was a byproduct of oil degradation raises a serious challenge to Mr. Oard's young-Earth timeline, for it requires that sedimentary organic matter had already matured to oil by the time Carlsbad Cavern was forming. But outside of controlled, high-temperature and high-pressure laboratory conditions, oil does not mature overnight! At the current rock temperature beneath Carlsbad Cavern, the process would have taken many thousands to millions of years. Thus Mr. Oard's assertion that cave dissolution might have taken place during the Flood is entirely contrary to the facts.

In summary, Mr. Oard's timelines does not allow enough time 1) for oil to have matured; 2) for oil to have chemically degraded; 3) for sulfuric acid to be transported to the site of dissolution, let alone dissolve the massive caverns; 4) for the water table to drop substantially, creating a vadose zone environment; and 5) for decorative speleothems (some the size of trees!) to have precipitated.

3) Geochronological mishap: Since my focus here is on the age of Carlsbad Cavern, I will conclude with Mr. Oard's misunderstanding of the available geochronological data. Since Mr. Oard must reject all radiometric dates from cave samples—though he does not explain why, scientifically, we should—he ends the article by blankly asserting that the available data is contradictory:

"It is of further interest that the dating of alunite resulted in significantly older dates for...caves in the Guadaloupe Mountains. The new dates range from 4 to 12 million years (Ma)...Previously, the cavern was dated at 1.2–0.75 Ma, or as much as 3 Ma based on the timing of mountain uplift. The younger dates were not only based on field evidence, but also on paleomagnetic, uranium-series, and electron-spin-resonance dating...This does not give one much confidence in dating methods." (emphasis added)

If you also read my last post, then Mr. Oard's error might seem obvious. Polyak et al. (1998) did not introduce 'new dates' for the cave, as though to correct available ones. Rather, the various studies were dating entirely different events.

Paleomagnetic, U-series, and electron-spin-resonance methods are applied to speleothems or sediments within the caves. The 40Ar/39Ar ages of Polyak et al. (1998) were applied to alunite formed during cave dissolution. Obviously, speleothems and cave sediments cannot form until the cave has actually been carved out, so we would expect these dates to be younger than those for the alunite. Despite the confidence in Mr. Oard's sarcastic assessment, it remains a non sequitur.

Conclusion

The available geochronological data are thus perfectly consistent with conventional understanding of Carlsbad Cavern's geological history. Uplift of the Guadalupe Mountains began some time in the early Cenozoic. In the mid-Miocene, H2S was introduced to the groundwater, was oxidized to sulfuric acid, and began dissolving caverns near the water table. The water table dropped slowly over the rest of the Miocene, and into the Pliocene, carving out Carlsbad Cavern around 4 million years ago. Since that time, continued fall of the water table created a vadose zone within the cavern, allowing for the precipitation of speleothems (as early as 1.2 Ma or more), and that process continues today.


References Cited:
Brook, G.A., Ellwood, B.B., Railsback, L.B., Cowart, J.B., 2006, A 164 ka record of environmental change in the American Southwest from a Carlsbad Cavern speleothem: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 237, p. 483–507.

Hill, C.A., 1990, Sulfuric acid speleogenesis of Carlsbad Cavern and its relationship to hydrocarbons, Delaware Basin, New Mexico and Texas: American Association of Petroleum Geologists Bulletin, v. 74, p. 1685–1694.

Polyak, V., McIntosh, W.C., Güven, N., Provencio, P., 1998, Age and Origin of Carlsbad Cavern and Related Caves from 40Ar/39Ar of Alunite: Science, v. 279, p. 1919–1921.

Polyak, V., Rasmussen, J.B.T., Asmeron, Y., 2004, Prolonged wet period in the southwestern United States through the Younger Dryas: Geology, v. 32, p. 5–8.

Tritlla, J., Cardellach, E., Sharp, Z.D., 2001, Origin of vein hydrothermal carbonates in triassic limestones of the Espadán Ranges (Iberian Chain, E Spain): Chemical Geology, v. 172, p. 291–305.

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