Calcitization of aragonitic bryozoans in Cenozoic tropical carbonates from East Kalimantan, Indonesia

Aragonite is commonly lost during the diagenesis of carbonate rocks, producing a significant bias in the fossil record. However, taphonomic loss of aragonitic biota can be nullified when skeletal aragonite is replaced by calcite. Here we report calcification of the originally aragonitic skeletons of two cheilostome bryozoan taxa—Reussirella sp. and Reptadeonella toddi—in muddy reefs from the Miocene of East Kalimantan. The calcitic composition of these Miocene fossils is shown using Raman spectroscopy, which enabled precise in situ analysis of skeletal walls without contamination from sediment or chamber-filling cement. Compared with Recent and fossil relatives preserved in original acicular aragonite, the calcitized bryozoans have skeletons with blocky microstructures, undulose extinction patterns, and small patches of highly birefringent acicular crystals that may be aragonite relics. Calcitization of the bryozoans as well as scleractinian corals in the East Kalimantan Miocene reefs was probably made possible because of the high input of fine-grained clastic sediments that partially sealed the buried skeletons, although other factors must be invoked to explain the lack of calcitized molluscs in the same reefs. A general conclusion from this study is that not all bryozoans preserved in deposits from which aragonitic molluscs have been lost through diagenetic dissolution possessed original skeletons of calcite. The potential for selective loss of molluscs but not bryozoans and corals must be taken into account during biofacies analysis.