Fish Poop Establishes A Geologic Record

By collecting and analyzing a lot of fish excrement, scientists have determined that the aquatic vertebrates generate a large portion of the carbonate mud that deposits on the ocean floor in tropical waters (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1015895108). Marine sediment contains information about past ocean chemistry, and the origins of a large fraction of the material have long been a mystery. On the basis of studies carried out with bony fish collected in the Bahamas, Chris T. Perry of England’s Manchester Metropolitan University, Rod W. Wilson of England’s University of Exeter, and coworkers estimate that on average 14% of the carbonate mud produced annually in the archipelago’s waters comes from fish. The ocean dwellers produce carbonate by drinking seawater rich in calcium and magnesium ions, which then precipitate with bicarbonate (HCO₃ ^–) in their intestines. Using energy-dispersive X-ray microanalysis and scanning electron microscopy, the researchers found that different fish species excrete carbonate crystallites with widely varying magnesium content and different ellipsoid, spheroid, and dumbbell morphologies. The results, the team says, could help address “some of the major unknowns about the provenance of carbonate muds in the geologic record.”