Upwelling

So why can we find so many large fossil shark teeth in Florida?

Plenty of sharks lived during the Mid Miocene.

Why were there so many large sharks?

Abundance of prey.

Why was their an abundance of prey?

Lots of nutrients.

Why were there loads of nutrients?

Plenty of upwelling over submerged Central Florida.

A commonly overlooked important process for ocean biodiversity, upwelling brings water from the bottom of the Ocean to the top but brings nutrients along and phytoplankton use those nutrients to flourish. Zooplankton eat the phytoplankton and portions of that energy eventually makes its way up to the largest predators i.e. whales and sharks.

Even in today's oceans, about 25% of global fish caught come from 5 upwelling sites which occupy only 5% of the world's oceans.*

Upwelling is a complex process driven by wind. As surface water is moved around by winds, cooler (usually) nutrient rich water is drawn to the surface to replace it. But if upwelling still goes on worldwide, why was it stronger in the ancient past?

It must be mentioned during these major upwelling periods in Florida, central Florida was completely submerged, with only portions in the north exposed as dry land. According oi geo-chemical dating of strontium isotopes, three major sea level stands occured during ~23-22 MA, ~18-16 MA and ~13-7 MA (Hine). Florida was not always almost completely underwater during the Miocene, but for a great portion it was.

Upwelling around central Florida was not only enhanced, but persistent, unlike most other upwelling sites (seem to be seasonal). This was due to a process called Topographic steering. Basically the Loop Current, which nowadays only flows well offshore nowadays, flowed over the submerged Florida platform and had to flow around the Northern straits of Florida. This bending in the loop current, caused the persistent upwelling. Along with enhanced upwelling (which happened when there was maximum topographic steering- current being deflected from hitting sea floor highs and nutrient-rich bottom water replacing that, brought about massive amounts of marine biodiversity as well as phosphate deposits (where it is commercially mined in the Bone Valley region of Central Florida). A similar process occured in North Carolina's Miocene seas, which is why there is great fossil hunting there too.

Yes, basically the seas were being fertilized at massive rates and primary productivity soared. And as you can tell from this site &/or fossil hunting in Florida, plenty of sea life lived at that time.

But where did all of those nutrients on Florida's ocean floor come from? The Appalachian mountains were weathering (and still are) and via rivers, nutrient-rich sediments made their way to Florida (and built it up, burying the carbonate factory that Florida was during the Mesozoic). I'm sure the "why's" of that goes on, but that's beyond the scope of this site.

Eventually upwelling seemed to decline as sea levels lowered (due to global cooling) likely due to reduced atmospheric carbon dioxide from carbon being buried back into the oceans over time (from Prothero "After the Dino's" book). At least that seems to be the leading theory.

Sources:

* Jennings, S., Kaiser, M.J., Reynolds, J.D. (2001) "Marine Fisheries Ecology." Oxford: Blackwell Science Ltd. ISBN 0-632-05098-5

5 major modern upwelling sites are: the Canary Current (off Northwest Africa), the Benguela Current (off southern Africa), the California Current (off California and Oregon), the Humboldt Current (off Peru and Chile), and the Somali Current (off Somalia and Oman)- From wikipedia cited from Cury P, Bakun A, Crawford RJM, Jarre A, Quinones RA, Shannon LJ, Verheye HM. (2000). Small pelagics in upwelling systems: patterns of interaction and structural changes in “wasp-waist” ecosystems. ICES Journal of Marine Science, 57:603-618.

http://oceanexplorer.noaa.gov/facts/upwelling.html

Hine Geology of FL Book Pg. 170-173