MINERALOGY MATTERS: ASSESSING THE MINERALOGIC HETEROGENEITY OF OTOLITH PAIRS USING NEUTRON DIFFRACTION, RAMAN SPECTROSCOPY, AND X-RAY DIFFRACTION

Otoliths are accretionary structures in fish that are used for coordination and hearing. Otoliths, also known as “ear bones,” are typically composed of the calcium carbonate (CaCO₃) polymorphs aragonite, calcite, and vaterite. Because of their accretionary nature, otoliths frequently exhibit seasonal to annual growth rings that can be used to infer the age of fish, and, via chemical analysis of otoliths, migrational patterns and other sources of environmental change.

Environmental conditions are inferred from the concentration of trace elements in an otolith, but the incorporation of trace elements depends, in part, on the otolith’s mineralogical composition. By definition, the polymorphs aragonite, calcite, and vaterite have different crystal structures and thus incorporate elements differently. In most cases, trace element concentrations are normalized to a single calcium carbonate polymorph, which are inferred using traditional microscopy or assumed for specific species. However, neutron diffraction data acquired from otolith pairs suggests that mineralogy can vary on an individual level. Here, we show the distributions of aragonite, calcite, and vaterite in otolith pairs from Chinook salmon. Unlike traditional light microscopy or Raman spectroscopy, neutron diffraction lacks spatial resolution but is deeply penetrating and can be used to quantify the mineralogy of bulk samples. Additionally, both X-ray diffraction and Raman spectroscopy were performed on otolith pairs to supplement neutron diffraction data. Together, these data suggest otolith compositions should be assessed on an individual basis to ensure trace element concentrations are calculated accurately.