Rates of cave dissolution: a moot point?
In an earlier post, I discussed the use of speleothems (such as stalagmites) in paleoclimatology, concluding that since we can use multiple independent methods to estimate the age and growth rate of secondary formations, the thousands of speleothems older than ~5,000 years already contradict the global flood hypothesis. I stand by this evidence, and if you are also convinced by it, then it may seem pointless to discuss how long it takes to dissolve caves in the first place. On the other hand, evidence from radiometric dating seems abstract to some, and difficult to understand. I want to add a more tangible approach, therefore, to my argument.
Steve Austin’s cave dissolution model regarding Kentucky limestone
In 1980, Austin published an ICR Acts and Facts article entitled “Origin of Limestone Caves”, in which he argued that given the average rainfall in Kentucky, 59 cubic meters of limestone bedrock could be dissolved each year per square kilometer of land. To put that in perspective, 59 cubic meters is roughly equal to a room 13x13x13 feet in size (i.e. a large bedroom with a high ceiling), and a square kilometer encompasses about 8×8 residential blocks. Since caves dissolve preferentially along flow conduits, each city block might be underlain by a room-sized cavity after only 64 years. After ~4,000 years, the cave system would be equivalent in volume to two Wal-Mart Supercenters (236,000 cubic meters).
Dr. Austin believes that his calculations should alarm conventional geologists who reject his young-Earth timeline—and he is right, seemingly. Geologist Greg Neyman responded, however, that the residents of Kentucky, rather than ‘uniformitarian geologists’, should be concerned by these numbers, “since according to this creation science model, Kentucky would be so full of holes as to be unlivable.” Greg is also correct.
Despite Greg’s critique, young-Earth creationists continue to cite Austin’s cave dissolution model without question (e.g. here). Conversely, geologists continue to assign long ages (tens to hundreds of thousands of years) to cave formation without any reference to Austin’s proposed rate of dissolution. Why the lack of communication?
The problem with Dr. Austin’s model is that he estimates cave formation to occur faster than the facts permit, but still too slowly to account for the world’s large cave systems (like Mammoth Caves in Kentucky, on which he focuses). Even if we grant such rapid dissolution of limestone, it is still difficult to explain how the 500+ kilometers of passages found in Mammoth Caves could have formed since the Flood. Moreover, many of the world’s large cave systems are filled with secondary formations (speleothems) that require themselves many years to form (see Appendix). Dr. Austin and others have thus addressed the problem by treating it in parts, and apparently with the expectation that nobody will recognize one plus one is actually larger than…one.
Fuzzy number crunching: how fast precisely?
For those not yet convinced, I want to examined Dr. Austin’s calculations in detail. In short, he determines the amount of calcite that can be dissolved annually by 1) estimating the carbonate concentration of groundwater based on calcium concentration; 2) estimating the volume of groundwater that passes through the bedrock based on measured rainfall per square kilometer. Multiplying (1), a mass per volume, by (2), a volume, yields the mass of calcite dissolved each year per kilometer. Seems straightforward, right? Too simple! That is, until one verifies the assumptions that go into such a calculation.
First, Austin’s model assumes that “about 1.0 meter of the 1.22 meters of mean annual rainfall go into the aquifer [i.e. groundwater]”. He even prefaces the assumption with “it is reasonable to assume”, but why? What makes this reasonable? Anyone reading this has access to Google Maps or a similar program that offers a bird’s-eye view of Kentucky. What do you see? My map appears pretty green, because Kentucky is blanketed with trees—healthy ones at that. All of these trees require water, and any water taken up by trees does not infiltrate the bedrock. The process by which trees and other vegetation take up water from the soil is called transpiration. Precipitation can also return to the hydrologic cycle through evaporation. Collectively, these routes are referred to as evapotranspiration, and the Kentucky Climate Center reports a mean annual evapotranspiration rate of 32.77 inches for Kentucky.
To estimate the amount of rainfall that actually infiltrates the bedrock, we simply deduct 32.77 inches (0.83 meters) from the total rainfall amount: 48 inches (1.22 meters). The difference is 15.26 inches (0.39 meters), so Austin’s model is already off by a factor of ~2.5. Within the article, Austin speculates that Kentucky may have received significantly more rain in the past, thereby soliciting credibility for his initial calculation. There is no direct evidence, however, of higher rainfall for Kentucky in the past 5,000 years—certainly not approaching 2.5 times modern values. Using real climate data, Austin’s annually produced 59 cubic meter cave is thus reduced to 23.6 cubic meters (the equivalent of dropping the ceiling in our bedroom to only 5 feet). [Note: the evapotranspiration figure here is higher than I should have used; see comments for further discussion]
Secondly, Austin’s model assumes that about 100% of the calcium in groundwater is derived from calcite dissolution. He argues to this point by noting that rainwater contains negligible amounts of calcium and magnesium, so it must all be derived from the ground. On this point, he is correct—calcium in rainwater accounts for much less than 1% that dissolved in groundwater. So what is the source of the remaining ~48.9 milligrams per liter of calcium?
Before water can infiltrate into the bedrock, it must pass through the active soil horizons (with the exception of water that falls very near disappearing streams). Soil horizons—particularly in forested ecosystems—are heavily enriched in calcium, because they contain the decayed litter of tree leaves and twigs. Trees and other vegetation actively take up calcium from the weathered bedrock through their roots, thereby enriching the uppermost soil horizons in calcium and other nutrients by several orders of magnitude.
Since calcite dissolves rather quickly in even slightly acidic rainwater, much of the 49 mg/L of calcium in groundwater is derived from the uppermost soil horizons, and not subsurface caverns. Recent work in monitoring calcium isotopes at spring outlets and along carbonate aquifers confirms this phenomenon, since 44Ca is heavily depleted in the O/A horizons and shifts the isotopic composition of groundwater negative (more so during the wet months).
Carbon isotopes in cave deposits also confirm that a bulk of the dissolved carbonate material is derived from the soil rather than the surrounding bedrock. The δ values of most soils is between -25‰ and -15‰, while limestone bedrock is close to 0‰ (give or take). Carbon isotope values of speleothems are commonly between -10‰ and -2‰, reflecting a mixing value between the soil and bedrock signatures.
If the calcium in Austin’s equation is derived largely from the soil and not the expanding cave, then his estimate is wrong by nearly an order of magnitude. Austin is far from explaining the presence of large caverns—certainly in humid climates like Kentucky or Southeast Asia, but more so in arid climates like the American Southwest or Israel/Turkey, which he neglects to mention.
Cave dissolution occurs neither as rapidly nor as simply as Austin proposes. Soil activity, groundwater chemistry, and the presence of joints and faults in the bedrock play a significant role. Austin’s young-Earth model can benefit from none of these, however, since 1) rich soils could not have been present immediately after the flood, 2) pore waters would have been saturated in carbonate, and 3) joints and faults cannot form to serve as flow conduits in unconsolidated sediment (i.e. soft sediment that hasn’t yet been cemented together). More recent estimates suggest that cave conduits typically widen by less than a centimeter every ten years—a far cry from Austin’s 59-meter-long tunnel.
Along with speleothem formation, the problem of cave dissolution remains an immovable stumbling block to the young-Earth creationist, who must propose that both processes can complete in a few thousands of years. The massive, decorated caverns across the world may stand as testament to the beauty of God’s creation, but they strongly preclude the notion of a recent, global flood. Our time is better spent, I believe, reconciling these observations with scripture to better understand both.
Good post, John. You introduced some information that I didn't cover in “deceptive tactic #6” in my presentation “Are the Young Earth Creation Ministries Shooting Straight With Us” at http://www.slideshare.net/TimH/are-the-creation-ministries-shooting-straight-with-us-part-1. While I think Austin's use of an encyclopedia to obtain the amount of water entering the aquifer was rather sloppy scholarship, I think the actual amount is probably closer to Austin's number than yours. Evapotranspiration (ET) numbers are derived from in-depth studies of small watersheds (on the order of a few acres or less) where the moisture input and plant cover are relatively uniform. In the Mammoth Cave area, most of the water enters the cave system from sinkholes at the surface (e.g., see top photo on this website: http://www.karstwaters.org/educationlinks/karst.htm. I doubt if the ET numbers calculated for small study watersheds in Kentucky took into account the loss of water into sinkholes.
Actually, I think the loss of surface runoff into sinkholes was the biggest “hole” in Austin's analysis. In a 1971 article in the Journal of Hydrology by Evan T. Slusher and William B. White, the authors found that most water passing through a cave network enters during major rainfall events, when there is no chance to dissolve calcium and magnesium to the concentrations identified by Austin. (This J. of Hydrology article came out 1 year prior to the J. of Hydrology article Austin used to obtain his average Ca and Mg concentrations — too bad he didn't use it!) During heavy rainfall events, we see water gushing from the outlets of cave systems along side rivers, indicating a pretty rapid throughput. I think this plus “assuming present rates and conditions” were the big flaws in Austin's analysis.
Thanks, Tim, for keeping me honest. 🙂 I subtly tried to avoid talking about direct infiltration through sinkholes and other conduits because it makes any quantitative discussion too complicated (esp. without more detailed geographic data from the region). But you are right. Presumably, much of the water that actually carves out the cave enters in this manner.
On the other hand, sinkholes only cover a small percentage of the landscape, but Austin envisaged 82% of the total precipitation entering the cave system as slightly acidic, highly undersaturated rainwater. I don't mean to speculate, but I can't imagine how 82% of annual precipitation could bypass soil/vadose zones altogether. Either way, this question is worth following up in recent literature, which may have more detailed figures on cave flowthrough.
Lastly, before 'sinkhole infiltration' is a viable factor, sinkholes themselves must form, and this occurs by much slower infiltration through soil, regolith, joints, etc. When all steps are considered, I believe one of the steepest challenges—and most easily understood—to the Creation Museum lies in its own backyard.
Thanks again for the feedback!
Good points. My understanding is that a cave network starts in weak points in the limestone, and grows from there (so much for “assuming present rates and conditions!). This happens at multiple levels – not just one as many YECs envision. One look at all the different levels shown in a 3-D map of Mammoth Cave kind of blows apart the idea that it formed through the simple mechanism Austin described.
I've been told that the hydrology of karst areas is a nightmare for National Weather Service hydrologists who have to forecast rivers in the Mammoth Cave and similar areas. The exact route the water lost in sinkholes takes underground isn't always known, introducing uncertainty when you have to predict which part of rivers will flood when local heavy rainfall occurs.
I drove out to Sinking Creek in Kentucky to see the location where the data was taken that Austin used. I ran into the owner of the land immediately to the south of where the terminating sinkhole is for the creek, and he took me to where it was located. The cave entrance looked like it was only a few feet high. He said when they get a real good storm (e.g., 3″ plus), the water backs up in the sinkhole like it would in a slow draining bath tub and takes several hours to empty out.
I think your analysis of calcium flux is flawed. Yes, much of the calcium is leached from the soil or vegetation, but since that calcium is ultimately derived from the bedrock, it's legitimate to exclude it only if there's a huge non steady state amount locked away in soil and vegetation somehow. If the landscape is steady state then the dissolved calcium is all derived from the bedrock.
Thanks for your feedback, Steve. Calcium cycling is a complicated topic, which I have admittedly simplified here. But I am relying on several recent papers that have tried to quantify Ca fluxes in forested ecosystems (e.g. Holmden and Belanger, 2010 and references therein). Ca loss to groundwater from soil, even in forested ecosystems with silicate bedrock, is pretty substantial and should be considered in any models.
But your objection is rather about the ultimate source of Ca in soil. In the long term (steady-state models), as much as 50% or more of Ca lost to groundwater is ultimately derived from wet and dry atmospheric deposition (the actual percentage depends on the geography/climate of the region). Wind-blown dust and dissolved Ca in rain is small in terms of concentration, but constitutes a large external source to the system. Therefore, one cannot say that all dissolved calcium is derived from the bedrock.
Since the Ca loss to groundwater is greater than atmospheric deposition, Ca tends to decrease in soils over time. Conversely, the loss of exchangeable Ca in soils over time verifies that soil-derived Ca is an important source to groundwater.
Steve, I apologize that I did not give you enough credit in that response. You are absolutely right in pointing out that much of the Ca lost to groundwater is ultimately derived from the bedrock (regardless of the relative contribution from dust/rain). I should have been more clear in the article and in my response. Thanks again for your comment.
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