Uranium oxidation and probable subaerial weathering of phosphatized limestone from the Pourtales Terrace*

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Abstract
Phosphatized limestones from the Pourtales Terrace, Straits of Florida, have undergone fresh‐water diagenesis, as shown by several selective petrologic features such as dissolution of aragonitic skeletal material and preservation and recrystallization of Mg‐calcite. This interpretation is supported by the fact that greater than 94% of the uranium in these samples is in the U(VI) oxidation state.On the deeper Pourtales Escarpment, phosphatized limestones show no alteration features indicative of fresh‐water diagenesis. The uranium in these samples is a mixture of U(IV) and U(VI) with oxidation‐state ratios similar to other sea‐floor phosphorites.Our interpretation of these data is that uranium in the terrace samples was oxidized when that area was emergent. The karsted surface of the terrace, therefore, is a result of subaerial exposure of this portion of the Floridan platform. The escarpment samples were never above sea level, and so more closely retain their original uranium oxidation‐state ratios.