Evolution, environment, and isotope systematics of a glauconite deposit on the modern sea floor along the southeastern United States continental margin

Glauconite grains are common in surficial carbonate sand deposits on the upper Florida-Hatteras Slope. Multiple lines of evidence suggest the glauconite varies in age from Miocene to Quaternary. Some of the youngest glauconite formed inside the Quaternary foraminifer Globorotalia truncatulinoides. All of the carbonate hosting glauconite is between {dollar}sim{dollar}20,000 yrs B.P. and 2.3 Ma. Rb-Sr dating of the sequential stages of evolution of glauconite grains in several samples shows a "younging" trend where the model dates decrease from an average of approximately 13 Ma to 4 Ma with progressive maturation. A detrital clay component is lost and the grains go from light to medium green, but the trend eventually reverses and the darkest grains have model dates of {dollar}sim{dollar}5 Ma, perhaps due to a mixture of modern grains ({dollar}sim{dollar}0 Ma) and grains reworked from Miocene outcrops ({dollar}sim{dollar}13 Ma). The detrital clay component affects the dates of glauconite even in sediments that are overwhelmingly carbonate.; Glauconite appears to be forming (or did form) at sites close to Miocene out/subcrops. Glauconite may form where iron-rich groundwater discharges into seawater which is high in potassium. On the Florida-Hatteras Slope, this appears to be where Miocene rocks crop out on or just below the surface. The discharge of groundwater may have occurred during and prior to the Pleistocene, but fresh water has not been measured in modern marine sediments on the Florida-Hatteras Slope.