Fourier Transform Infrared Spectroscopy Microprobe Analysis of Calcium Carbonate Polymorph and Protein Distribution of Fish Earstones

Calcium carbonate polymorph distribution within fish otoliths (earstones), effects the distribution of trace elements. Currently, it is assumed that the polymorph distribution is homogenous along a single otolith. It is currently known that aragonite and calcite are predominantly found within the otolith. FTIR microprobe analysis was performed on fish otoliths using a Smiths Detection IlluminatIR with a diamond ATR objective. Sagittal otoliths of larval bass and adult bluegills were extracted, thin sections and prepared for analysis. Spectra (50μm spot) were collected at 5 regions within the otolith (core, 3 regions along the sulcus, and edge). Initially, resultant IR spectra were fed through a library search to determine polymorph distribution and existence of protein bands (e.g., Amide II and Amide III). These spectra were then compared by principle component analysis. Data show that core-to-edge polymorph differences are most pronounced in larval fish. In several cases in both adult and larval fish protein bands were identified and were found to be inhomogeneously distributed across the otolith. Given that the bioinorganic chemistry of minerals is often used as a proxy for paleoclimate or environmental reconstruction the variance in polymorph type and organic constituents and its relative influence on trace element content may have significant impact on the interpretation of bioinorganic proxy data.