Florida's Trail Ridge formed from Karst Isostacy?

[Squirrel Girl]

I can't find an on-line link (yet) but Earth magazine has a short article in the Sept 2010 issue on what might have caused Trail Ridge to be elevated. The conclusion is that is do to isostatic rebound. Not from deglaciation, as IR is usually related to, but due to unweighting of the terrain due to karstification!!!! I'm dubious, but, well, maybe. The article mentions that there may be more rock removal due where it's not seen (below the water table) relative to other karst regions where people think of surface, air-filled caves. But then, they talked about karstification due to mixed water solution. I didn't think FL caves formed from that process. Jerry?

[jason]

The original karst-driven uplift hypothesis was presented by Opdyke and others (1984). Florida has paleoshorelines that are at elevations that are too high to be explained by glaciation-induced fluctuations in sea level. Since Florida is too far from the terminus of the Laurentide Ice Sheet to have been affected by ice loading/unloading of the crust and Florida is tectonically stable, finding an explanation for these shorelines has been problematic. Opdyke investigated the potential role of karstification in driving uplift by basically adding up all the calcium that was being exported from Florida springs, calculating how much limestone that corresponded to and then solving for how much and how fast the earth surface would rise based on removing that material by forming caves. Basically, since most of the dissolution is occurring below the surface, the ground elevation will raise in response to the change in density of the limestone below. It was a pretty neat idea. The hypothesis has since been revisited by Adams and others (2010) using a computer model which is getting a lot press for some reason (incidentally, Opdyke and Adams are both professors at UF - though Opdyke is retired). Adams computer model suggested that uplift was happening a lot faster than Opdyke's calculations (0.047 mm/yr vs. 0.24 mm/yr).

As for mixing.... (and I haven't seen the article you're referring to) - in Florida, most people invoke mixing to explain cave formation either at or below the water table. Most people think of cave formation by mixing corrosion as a fresh water - salt water mixing phenomenon - but the mixing of any two water masses that are equilibrated with limestone at different pCO2s can potentially dissolve limestone. Personally, I think the role of mixing in cave formation (in Florida as well as coastal caves with haloclines) is greatly exaggerated.

[Squirrel Girl]

Most people think of cave formation by mixing corrosion as a fresh water - salt water mixing phenomenon - but the mixing of any two water masses that are equilibrated with limestone at different pCO2s can potentially dissolve limestone.

Yeah. Most people like me.

The article, quoted a geologist from Northern Ireland (Michael Simms) who is a karst researcher who said "given that the mixing of fresh- and saltwater below the water table could enhance dissolution of the limestone." Maybe he's a karst expert who doesn't know a lot about Florida in particular?

[jason]

The Adams article scaled karstification rates to precipitation based on a reformulation of an equation in White's karst hydrologeology text book that related basin-wide dissolution rates to stream discharge. I don't recall any mention of freshwater-seawater mixing in either the Adams or the Opdyke paper...

There are several caves and submarine springs along the Gulf Coast that have haloclines... But generally, most of the caves in Florida were never subjected to the mixing of fresh and salt water.