The United Plates of Barnacles: Analyzing the Consistency of Oxygen and Carbon Stable Isotopes in Barnacle Plates for Paleoenvironmental Analysis

Session: Timestamped Biomineralized Structures in Coastal Environmental Monitoring and Cultural Research

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Abstract:

Barnacle shells are used in stable isotopic analysis to determine modern and fossil whale migrations.  However, they also live in shallow-to-deep sea environments, and little is known about their paleoenvironmental utility for those vast regions.  Further, barnacles grow several types of calcitic plates, from basal plates adhering to the substrate to plates surrounding the body, but it is unknown whether all plates record similar environmental conditions.  We examined the stable isotopic ecology of Balanus concavus from the Middle Miocene, Calvert Cliffs, Maryland, to determine if their different plates record a similar paleoenvironmental record.  Barnacle plates can disarticulate after death, and form much of their fossil record, so it would be important to know if the isotopic ecology of one plate represents the entire barnacle.  We hypothesize that all plates on Balanus concavus consistently record similar water mass characteristics; thus, any plate may be used to reconstruct paleoenvironmental information for fossil barnacles.

Two complete B. concavus growing together on the scallop, Chesapecten nefrens, and one specimen not associated with that group, were collected from Shattuck Zone 17 of the Calvert Cliffs.  Five different plates were examined from these barnacles (rostralmarginal, rostrum, carinal, radius, and basal) and transects were drilled across their calcitic plates to obtain powders for δ¹⁸O and δ¹³C stable isotopic analysis.  Results revealed several consistent biological and environmental patterns:  All three barnacles lived for 2+ years and began growing in the fall, with mortality occurring in late summer after the hottest conditions.  Both δ¹⁸O and δ¹³C were coupled, indicating that productivity generally increased with lower, cooler temperatures and decreased in higher, warmer temperatures.  Except for the radius, stable isotopic results from all other plates were not statistically significantly different from each other based on means and 95% confidence intervals.  The radii appear to grow only in winter, while the other plates grow throughout the year.  While most barnacle plates are united in their stable isotopic signature, the radius should not be used to analyze yearly seasonal patterns.  However, the radius does provide biological input into shell growth.  Therefore, any barnacle plates found in the fossil record, except for the radius, are sufficient to reconstruct multi-annual to sub-annual past water mass characteristics, revealing important living conditions for the evolution of this important group of marine arthropods.