Production of mud-grade carbonates by marine fish: Crystalline products and their sedimentary significance

Recent work has established that marine teleost (bony) fish represent a prolific source of mud grade, mainly high-Mg calcite, carbonate sediment by means of primary precipitation within the intestine. Previously documented crystalline products display a diverse array of morphologies, many unique in shallow tropical marine settings, and have a wide range of magnesium contents (from 18 to 39 mol% MgCO3). This study utilizes scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and liquid ion chromatography to provide a more extensive and expansive morphological, mineralogical and chemical characterization of the crystalline forms produced by a wider range of piscine functional groups (covering 21 different fish species common in The Bahamas). Several crystal morphologies not previously described in fish-derived carbonates are documented, and chemical composition is found to be more variable than previously reported: in addition to high-Mg calcites with >18 mol% MgCO3, high-Mg calcites with lower MgCO3 contents and low-Mg calcites are identified. From the expanded species range, MgCO3 content in fish-derived carbonates ranges from ca 0?5 to > 40 mol%, and particle length is in the range 0?1 to >50 mu m (typically <2 mu m for individual crystals). Amorphous Mg-carbonates (with detectable CaCO3 of <2 mol%) are also found to varying extent in the precipitates of many species. Dominant mineralogy and MgCO3 content varies with producing species and crystal morphology (itself a species-dependent variable). Given the very small grain size and often high MgCO3 contents of these carbonates, interesting questions arise about their preservation potential. Thus, the extent to which carbonates produced by different species may follow different post-excretion preservation pathways is considered.