HIGH RESOLUTION ISOTOPIC SAMPLING OF GROWTH BANDS OF A MODERN MERCENARIA CAMPECHENSIS SPECIMEN AND IMPLICATIONS FOR PALEOCLIMATIC ANALYSIS

Annual growth increments of bivalves show corresponding fluctuations in oxygen isotopic composition. Seasonal variations in bi-annual patterns due to environmental conditions are difficult to obtain in the slow growing adult portion of the hard shells. As such, high-resolution sampling during the adult period of shell growth is critical for paleoclimatic studies. Although excessive samples can become cost-prohibitive, coarsely resolved sampling strategies can limit the usefulness of the isotopic record generated. To test the effectiveness of coarse versus fine sampling resolution, a modern specimen of Mercenaria campechensis was collected from Miguel Bay in southwest Florida. This specimen is being used to establish a Modern calibration to better constrain the large-scale paleoclimatic reconstruction of the Pliocene and Pleistocene epochs in southern Florida.

This shell was initially sampled every 2 mm with a 1mm diameter drill bit, which constituted 32 samples over approximately 12 years of growth. The same valve of the M. campechensis specimen was then re-sampled using a computer aided tri-axial mounted drilling system, with a sampling resolution of 2.5 samples per mm, producing approximately 150 samples. The juvenile section of the shell (with the fastest extension time) was excluded. Both the coarse resolution and fine resolution samples were then analyzed for seasonal variations in δ¹³C and δ¹⁸O composition.

The isotopic composition of both dark translucent and light opaque banding is difficult to ascertain for the mature section of the shell because of slow growth rate relative to the juvenile regime. Translucent and opaque banding increments are as small as one millimeter per band in the adult regime. The high resolution sampling produced increased isotopic peak amplitudes, for both δ¹⁸O and δ¹³C, versus that which was found in the previous study. Additionally, these results confirm the correspondence between isotopic composition and bi-annual shell growth bands.Higher resolution methodology is pertinent to capturing the full amplitude of seasonal changes and corresponding variations.