189-21 Carbonate Stable Isotopes of Bivalve Shell Fragments and Their Viability as a Seasonal Environmental Recorder

Session: Paleontology, Paleoecology/Taphonomy (Posters)

Presenting Author:

Authors:

Abstract:

Stable isotope profiles derived from fossil bivalves are useful in examining seasonality in paleotemperatures and/or water chemistry. By using a computerized micromill and serial sampling of the shell through ontogeny, we can analyze the oxygen (δ¹⁸O) and carbon (δ¹³C) isotope values to examine seasonality in environmental conditions while the organism was growing. This sclerochronological approach requires well-preserved specimens, both physically and geochemically. Many localities in the early Paleogene of Montana only preserve partial and fragmentary bivalve remains, yet these fragments are still geochemically pristine. These fragments are too fragile or small to serial sample using micromilling techniques, preventing use of these localities in our stratigraphic record of seasonality after the K-Pg Mass Extinction.

Here, we assess whether these sites can provide usable seasonality data, by testing whether fragmentary bivalve remains capture variability in isotope data. We sample bivalves of different genera with good physical and geochemical preservation from a Late Cretaceous locality in Montana. First, we used standard sclerochronological methods to serially sample (n=40) three specimens. We then cut the shell across the medial plane and crushed the non-micromilled side into flakes. We then randomly sampled the resulting material, choosing both flakes (n=20) and fine-powdered shell (n=20), and compared the δ¹⁸O and δ¹³C values with those from the serially sampled portion.

We found that the range of δ¹⁸O values from the fragmentary material did not exactly reproduce the same minimum and maximum as the serially sampled shells, however, the mean δ¹⁸O value of both methods is similar across all three specimens. For δ¹³C, only one specimen captured the same median and range of values via both methods. The other two specimens instead had a higher range of δ¹³C values in the fragmentary material, and in one, drastically changed the median value. We theorize that by smashing the specimen into finer powder, we overly-homogenized the shell material, and consequently made it less likely that δ¹⁸O maxima and minima are consistently captured, but preserved the mean δ¹⁸O value. The higher range in δ¹³C values in the fragmentary material may be driven by incorporation of multiple shells layers that are not normally sampled during surficial sclerochronology. Additional work is being performed to compare the results of samples from powder vs. discrete flakes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8193

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.