T137 Early Career Investigators in Mineralogy and Crystallography

Monday, 23 September 2024
1:30 PM - 5:30 PM
207A

SIGNALS OF AUTUMN SEASONS TRAPPED IN FRESHWATER PEARL NACRE

Paper number: T137-8
Program: Technical Sessions
Time: 3:10 PM – 3:25 PM

Authors

Abstract

Biocarbonate minerals have become critical tools for climate science, preserving geochemical and isotopic signals from their environment as they grow layers over time. To better understand the mechanisms of how this process works and what variables are at play, we have targeted a modern system in cultured (farmed) freshwater pearls. The pearl in this study was grown over the course of four years in Kentucky Lake, TN, USA, which is a well-monitored lake system. Thus, for the first time, we were able to correlate micron-scale geochemical and mineralogical observations in a pearl to sub-monthly measurements of environmental variables in Kentucky Lake. In line with previous biocarbonate studies, our first observations revealed negative correlations between novel SIMS-based δ18O measurements (10 mm spots) and temperature in the lake, with peaks in δ18O denoting winter seasons along the nacre transect. These peaks correlated with higher dissolved oxygen levels in the lake. More surprisingly, cathodoluminescence (CL) imaging of the pearl uncovered hidden heterogeneity in the nacre caused by thin concentric layers of elevated manganese (Mn2+) that appear as bright green in CL, especially in layers that preceded the “winter” δ18O peak signals. We interpret these high manganese layers as formed during “autumn” seasons, caused by lake turnover events where manganese oxides (Mn4+) and other forms of manganese that had been resting at the bottom of the lake during stagnant, stratified seasons got mixed into the water column, reduced to Mn2+, and incorporated into the aragonite crystal structure of the growing nacre as a substitution for Ca2+. Raster maps of peak features in Raman spectra led us to further assess changes in the carbonate bonding environment of nacre aragonite across the pearl and helped to tease the origin of an additional blue CL signal likely caused by aragonite structural defects or shifts in nacre tablet orientation. Overall, the study shows how CL imaging coupled with other mineralogical techniques can lend a powerful new view into biominerals and the environmental signals they preserve.

Reference: Farfan, G. A., Bullock, E. S., Zhou, C., & Valley, J. W. (2023). Geochemical and mineralogical proxies beyond temperature: Autumn seasons trapped in freshwater nacre. Geochimica et Cosmochimica Acta, 355, 126-137.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-403589

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