by ggpab » Feb 22, 2008 8:46 pm
Wow - Great question.
As Teresa mentions, there is only limited potential for underwater calcite deposition and is highly unlikely in passages with high flow rates. For the sake of arguments lets only consider speleothem calcite formed above the water table, which are all the pretty stalagmites and stalactites, and that are now simply found below the water table. ie - they have been drowned.
Water levels in coastal aquifers is a function of three boundary conditions - 1. Speleogenesis and the existence of high flow path rates to efficiently drain the aquifer, 2. Climate which controls how much water there is in the aquifer. Since these are coastal aquifers this also means how high the water table is above the saline water. 3. Sea level.
1. Speleogenesis. Most (but not all) of the caves in the Yucatan are relatively very shallow with speleogenesis linked to 'mixing corrosion' where the infiltrating rain water hits the water table and also importantly at the fresh-saline halocline mixing zone. This is a very efficient chemical drill for making caves. The halocline in the Yucatan is shallow, being only ~10 m at the coast, and slopping deeper to ~ 100 m in the middle of the peninsula. Still quite shallow.
Speleogenesis in Florida has played differently for a variety of reasons, which I would argue are related to structural geology and minerology, the Florida rock having lower primary porosity and permeability (the small holes in the rock and how well interconnected they are), and also that the Florida rock isn't nearly as pure as that of the Yucatan which only has 5% or less insoluble residue after you dissolve the rock. In short, in Florida you have dirt whereas in the Yucatan it is very limited. The overall effect though is that volume of fresh water in Florida is very much larger than that in the Yucatan. The depth of the halocline is very deep in Florida, being 300 m or more. I have been hunting for direct observations of the depth of the fresh-saline halocline in Florida for over five years now, I am resigned to the likely fact that not even the USGS or the Florida Geological Survey have mapped out the depth of the fresh-saline interface inland from the coast, with most 'data' based on computer modeling of where it should be. The big flow caves in Florida punch through the fresh water part of the aquifer driven by probably much more subtle geochemical drives compared to the Yucatan and are often at some significant depth being 30 - 100 m below the water table. In the Yucatan, the principal depth of cave development are more like water table, 5, 10, 20 m.
2. Lets ignore climate for now.
3. Sea level. Given the much deeper average depth of passages in Florida, and that alot of what has been explored is some distance inland >>10 km from the coast, then even if you look at the minimum sea level at - 120 m lower than present over the Quaternary (the last 1.65 million years) you still have flooded caves. Ergo speleothem deposition to be reflooded. Most likely that most of the Florida caves were continuously flooded.
What you think of as the flooded Yucatan caves are all located 10 km or less from the Caribbean coast. There is alot of it, but is all squished in a thin strip. If you drop sea level to -120 m during the last glacial then you get almost a near direct drop in the water table to that level as well in these caves. Lots of speleothem deposition everywhere.
However the lower sea level is, the more deeply into 'glacial' periods you are, and the mores likely the region is to be arid with not enough water percolating into the ground to create speleothem. Also, the major factor in dissolving calcite is the CO2 in the rooting zone (NOT carbonic acid in the atmosphere), and so it is cold, dry, and with limited solid CO2 since the vegetation is all stressed. This can reach a tipping point shutting down speleothem formation. Now you can see why deep speleothem are rare - the cave has to be in the right conditions and just the right location to be drained AND climate and vegetation still have to be adequate for calcite deposition to not have been shut down.
I have heard of some deep speleothem from Florida, but I would love to know more. I suspect that they may be in caves very close to the coast first of all, and second of all I am wondering if they are true speleothem or other sedimentary forms that have been mistaken for them. If there is deep speleothem in Florida (ie > 50 m) then it is going to pose some real challenges in getting any information from them because they may well be older than the Quaternary, and dating calcite older than ~500 000 years old is - well - still experimental.
Water table caves which in Florida do have speleothem? That is a whole different story.
Anyway - my 2 + 2 cents worth.
Bye bye.
Trish