The calcium-carbonate content of shallow-water surficial sands off Sapelo Island typically is less than 12% and in many places is less than 8%. However, an arcuate belt of carbonate-rich sand, concave landward, lies off the entrances of Altamaha and Doboy Sounds. Everywhere within this lobe the calcium-carbonate content exceeds 12% and, toward its southern end, is greater than 28%. Subsurface samples (20- to 25-cm core depths) generally reflect the same areal distribution but tend to be slightly higher in carbonate content, and locally the subsurface concentrations lie somewhat more landward. These occurrences of calcium carbonate evidently represent an equilibrium between rates of in situ production of carbonate by organisms, wave and current reworking of nearshore sediments, and to a lesser degree, rates of supply of other detritus. At water depths of less than 6 m, sands within the lobe contain relatively low amounts of carbonate despite high in situ organic productivity, because of sediment reworking by tidal and wave-induced bottom currents. Farther offshore, at water depths greater than 6 m, waves are less effective in redistributing sediment; sands in this sector of the lobe are enriched in carbonate because of high rates of in situ production combined with relatively minor removal of shell material.

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