H. L. Vacher | USGS Science

USGS1997

Geology and hydrogeology of the Florida Keys

This chapter discusses the geology and hydrogeology of the Florida Keys, and focuses on the islands formed of Pleistocene limestone. These islands, which are crossed when driving from Miami to Key West, are typically regarded as "the Florida Keys." The outstanding and fragile character of ecosystems on and around the Florida Keys has prompted State and Federal efforts to protect and preserve the remaining public portions of the region. The Florida Keys were largely ignored during the sixteenth, seventeenth, and eighteenth centuries, although the waters just offshore provided a major shipping thoroughfare to and from the New World. The Florida Keys are now recognized as one of the great recreational and environmental resources of the United States. The islands are outposts of a laid-back, tropical resort culture that has as its foundation warmth and clear water. A significant part of the attraction is fishing, diving, and boating around the area's coral reefs, which the islands protect. But the reefs were not always so highly valued. The Florida Keys that have protected the reefs for millennia, may now be the source of the agents that may accomplish what Agassiz thought was beyond man's power a century ago.

Florida↗

USGS1976

Hydrogeochemistry of Bermuda: A case history of ground-water diagenesis of biocalcarenites

Bermuda is composed of relatively young skeletal limestones currently undergoing diagenesis by the ground water passing through them. The saturated zone consists of separate fresh-water bodies laterally surrounded and underlain by extensive brackish aureoles, in which the meteoric water is mixed with sea water. The meteoric water enters the aquifer after passing through the soil or through marshes (outcrops of the ground-water bodies), in each case causing an influx of CO 2 to the saturated zone. Examination of the ground-water chemistry enables mapping of (a) the extent of mixing of meteoric ground water and sea water; (b) P CO2 ; (c) the extent of saturation with calcite and aragonite; (d) concentration of Sr; and (e) the amount of calcium and magnesium derived from the limestones. It is concluded that three processes control the chemistry of Bermudian ground water: (a) generation of elevated CO 2 partial pressures in soils and marshes; (b) dissolution of metastable carbonate minerals (principally aragonite); and (c) mixing with sea water. Bermuda ground water apparently approaches a steady state of aragonite dissolution (at slight subsaturation) and concurrent precipitation of calcite cement. Large Sr/Ca ratios in the ground water indicate that the dissolution of aragonite is incongruent. Dissolution is most pronounced near the marshes where CO 2 content is highest. Mixing with sea water is not significant in controlling calcite saturation. Only small amounts of magnesium enter the ground water by incongruent dissolution of magnesium calcite, an apparently slow process on the time scale of passage of the ground water through the saturated zone. All of the waters are well undersaturated with respect to dolomite. It is estimated that the present rate of recrystallization of aragonite to calcite is about 0.32 cm 3 of aragonite to calcite per m 3 of the saturated zone per year. At the present rate of chemical weathering, 360 m 3 of the saturated zone is lost each year through solution and transported to the sea by ground water.

Geological Society of America Bulletin↗