Coupling Mineralogy and Oxygen Isotopes to Seasonal Environmental Shifts Recorded in Modern Freshwater Pearl Nacre From Kentucky Lake

Abstract

Biominerals that accrete shell or skeleton are commonly used as windows to past geochemical environments. Using biominerals as paleoproxies depends on the assumption that biominerals faithfully and predictably record environmental parameters, yet little has been done from a mineralogical perspective to understand how various environmental shifts impact shell and skeleton mineralogy and how mineralogy influences proxy calibrations. In this study, we correlate a suite of environmental data from the Kentucky Lake Long-Term Monitoring Program with mineralogical and chemical signatures of modern cultured pearl nacre. Six transects were measured across three pearls from nucleus to edge for oxygen isotope ratio (δ18O) using SIMS (10-μm spots) and for mineralogy via nacre tablet thickness using SEM and via carbonate bonding environments using Raman spectroscopy. After confirming a strong seasonal correlation between δ18O and temperature, we extend δ18O correlations to relative nacre OH/O ratios, lake dissolved oxygen, and lake light levels at 1 m depth. Although nacre tablet thicknesses are not clearly correlated with temperature, we observe correlative trends with lake conductivity and relative nacre OH/O ratios. For the Raman spectroscopy measurements, there was a modest inverse correlation between increasing carbonate ν1 and ν4 vibrational mode peak heights with δ18O and lake dissolved oxygen levels. Standard deviations in Raman T:L peak area ratios, as an estimation of angle spread of tablet crystallographic orientation, correlate with lake reduction potential. This approach of combining geochemical, isotopic, mineralogical, and environmental data lends a more holistic view to biomineralization in natural systems and to the decoding of environmental mineral signatures.