Marine Science Faculty Publications


Geothermal Springs of the West Florida Continental Shelf: Evidence for Dolomitization and Radionuclide Enrichment

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On the sea bed of the West Florida continental shelf about 45 km SSW of Ft. Myers, Florida, an 85-km2 area has been discovered in which six thermal springs discharge warm, chemically altered seawater from vents and seepage zones. The spring water apparently originates in the subsurface ocean around the Florida Platform and penetrates the highly porous strata of the platform about 500–1000 meters below sea level. It percolates toward the interior of the platform and is geothermally heated to about 40°C en route. Then it rises along more vertical flow channels and is discharged in warm submarine springs.

Beneath the platform, several chemical processes alter the percolating seawater. One process seems to be a secondary dolomitization of the limestone of the platform because, in the discharging seawater, magnesium is lower by 2.7 mmole/kg and calcium higher by 3.6 mmole/kg than in normal seawater with the same chlorinity. Other reactions within the sediments of the platform enrich the spring effluents 1000-fold in226Ra, 10,000-fold in222Rn, and 90-fold in228Ra compared to the seawater surrounding the platform. Thus, the springs may be important sources of radionuclides for the Gulf of Mexico. The percolating seawater also loses all of its oxygen and nitrate to reduction processes, loses most of its phosphate and 40% of its238U, and roughly quadruples its silica content.

Coastal carbonate platforms are fairly common geological features. Thus, processes like those beneath the West Florida Shelf may function on a world-wide basis to play an important role in the diagenesis of carbonate sediments.

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Earth and Planetary Science Letters, v. 52, issue 2, p. 345-354