Using Otolith Microchemistry as a Proxy for the Environmental Effects of Metal Mining

Assessing the effects of metal mining in an environment relies on monitoring physical, chemical, and biological parameters over extended time periods. However, detecting change is difficult in areas where baseline data is sparse or non-existent. Biominerals, which may archive chemical information, are becoming important tools for monitoring environmental change. In particular, otoliths have been referred to as continuous recorders of exposure to the environment. Otoliths are calcified structures in the inner ear of teleost fish, composed of layers of aragonite deposited continuously throughout the lifetime of the fish in a protein matrix. Both the inorganic and organic portions have the capacity to incorporate a wide range of trace elements, the amounts of which can be influenced by periodic changes in concentrations in the environment. Otoliths can serve as recorders of exposure to trace elements owing to their metabolic stability, their continuous growth throughout the life of the fish, and their annular structure that provides a corresponding time scale.

This study examined geochemical signatures in annular growth zones of otoliths collected from areas impacted by metal mining using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Otoliths taken from water bodies adjacent to Cu, Pb, and Zn mining contain single peaks of those three metals that are interpreted to indicate when the fish came into contact with mine tailings. Fish stocked in a closed open-pit Ni-Cu-Cr mine contain constant levels of these base metals throughout their life history, reflecting the closed habitat in which they live. Otoliths from fish captured near and downstream from a rare element pegmatite mine contain signatures of Li, Cs, and elevated Rb, whereas those from lakes distant to or upstream from the pegmatite do not have such concentrations of those elements. The results of these studies indicate that otolith microchemistry can provide useful information in evaluating the extent of mining activity effects in an environment, as well as provide information on fish movements into and out of affected areas.