CATHODOLUMINESCENCE OF PELAGIC LIMESTONES

Little has been published on the cathodoluminescence (CL) of pelagic carbonates. We have looked at numerous pelagic limestones from the Jurassic-to-Eocene Umbria-Marche stratigraphic sequence of the Italian Umbria-Marche Apennines using CL. Typically the micritic coccolith matrix of these carbonates is bright orange in CL; planktic forams are CL-dark.

Based on the CL-dark character of most shallow-marine fossils, we would have expected the forams and the coccoliths of pelagic limestones, both of which lived in the photic zone, to have been CL-dark. The bright orange CL of the matrix was a surprise and requires explanation. Our first hypothesis was that the matrix acquired its CL color by recrystallization during replacement of marine by non-marine pore waters after the emergence of the Apennines. Such a wholesale chemical replacement would call into question the value of chemical and isotopic data measured on the pelagic limestones. However, we then observed that many samples of unlithified nannofossil ooze from DSDP/ODP sites also have dull to bright orange CL. We therefore favor the hypothesis that the CL color of the matrix of the Apennine limestones is primary.

The nanofossils may have acquired their CL color by incorporation of Mn2+ ions into the surface layer of the calcite as it descended through the oceanic oxygen minimum zone. Oceanic waters typically have a pH of 7.5-8.5. At this acidity, in the oxidizing surface waters, Mn occurs mostly as Mn4+, which is insoluble and therefore unavailable for incorporation in growing calcite crystals. In the oxygen minimum zone, Mn occurs as Mn2+ which is soluble and thus available. The quencher, Fe, occurs as insoluble Fe3+ under both conditions. The coarser crystals of the forams would incorporate less Mn2+ during descent through the oxygen minimum zone.

Whatever the origin of the CL color in the matrix, its contrast with the CL-dark forams aids identification of the latter by making visible subtle features unrecognizable in thin section.