Pleistocene/Holocene sands up to several meters thick, which contain 5% to 40% phosphate grains, occur on the continental shelf of Onslow Bay, North Carolina. Altered foraminiferal specimens, 98% of which belong to the genus Quinqueloculina, exhibit gradational surface discoloration (white to dark yellow-brown) that progresses from late to early-formed chambers. The percentage of extensively altered specimens varies directly with phosphate concentration in the sand fraction. Microprobe analyses of polished sections from completely discolored specimens indicate that alteration involves a decrease in %CaO and concomitant enrichment in %FeO and %P₂O₅. Degree of alteration diminishes from the outside to the inside of exterior-facing chamber walls (mean v lues are: 70 to 78 to 82% CaO; 18 to 11 to 7% FeO; 0.8 to 0.5 to 0.4% P₂O₅). Interior chamber walls are less altered (mean values are: 84% CaO, 6% FeO, 0.3% P₂O₅). On a CaO-FeO-P₂O₅ diagram the compositional changes through successive chambers of a single specimen parallel those from unaltered through altered specimens. The chemical compositions of completely discolored specimens fall on a proposed alteration trend between unaltered calcareous specimens and chamber fillings. Chamber fillings contain 0.9% CaO, 49% FeO, 12% MgO, and 1.6% P₂O₅; they are generally black. Relative concentrations of CaO-FeO-MgO plot within the compositional range of siderite and magnesite. Constant MgO values (7.5%) in alte ed foraminiferal tests demonstrate that initial diagenesis involves conversion to high-magnesium calcite. Subsequent alteration is largely ferruginization and minor phophatization of the test and the diagenetic materials forming within the chambers.

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