Abstract
Modern shell accumulations on Padre Island, Texas Gulf Coast, are composed of pristine aragonite whereas laterally adjacent Pleistocene beachrock on the mainland shores of Laguna Madre are made up of the same taxa but display a range of mixtures of aragonite and diagenetic low-Mg calcite. Cements and pedogenic crusts associated with the beachrock are 100% low-Mg calcite. Stable isotopic values for shells composed of 0, 15, 20, 45, 75 and 90% calcite display high correlations between the δ18O values and mineralogy (R2 = 0.95) and δ13C values and mineralogy (R2 = 0.87). The highly correlated changes in the stable isotopic compositions and mineralogy of the skeletal material are also evident in the data from the analyses of the pure calcite cements and pedogenic crusts. Changes in stable isotopic composition of the shells are the result of the early diagenetic alteration of the original aragonitic composition to low Mg-calcite.
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