GIANT SESSILE BARNACLES AS KEYS TO UNLOCKING MARINE SHELF CONDITIONS DURING THE MIDDLE MIOCENE CLIMATIC TRANSITION, CALVERT CLIFFS, MARYLAND

Barnacles build a hard calcite shell that is used in stable isotopic analysis to document modern and fossil whale migrations. However, barnacles are also common encrusters of hard substrates and shelled invertebrates like mollusks in shelf environments. Because of their sessile lifestyle, barnacle calcite could capture a record of water mass characteristics for marine shelves, such as water temperature and productivity and also reveal physiologic features like growth and age patterns. Here, we examined the stable isotopic ecology of the giant balanid barnacle, Balanus concavus, from the Middle Miocene, Calvert Cliffs, Maryland, to determine if shelf barnacles provide a record of environmental conditions during the Middle Miocene Climatic Transition (MMCT), a time when the East Antarctic Ice Sheet was growing, cooling the Earth just after the Middle Miocene Climatic Optimum when climate was much warmer than today. We hypothesize that giant sessile barnacles can be used to track shelf environmental conditions for the MMCT, a significant climatic state change from warmer to cooler conditions during the Cenozoic Era. We also examined their growth in relation to productivity, temperature, and age during this time.

Two Balanus concavus growing together on the scallop, Chesapecten nefrens, were collected from Shattuck Zone 17, Calvert Cliffs, Maryland. Two carinal plates from each barnacle were drilled along a growth transect for δ¹⁸O and δ¹³C. Results revealed that the barnacles started growing in the fall and lived for ~ 3 yrs; they did not have constant growth rates as they grew faster in the first year than in later years. δ¹⁸O revealed seasonal climatic conditions during the MMCT in the western mid-Atlantic shelf region, yielding ~ 5 ºC temperature change between summer and winter seasons. δ¹³C was mostly coupled with δ¹⁸O, with generally higher productivity during cooler winter seasons. In the last year of barnacle growth, δ¹³C decoupled from δ¹⁸O, likely tied to a metabolic carbon signal, similar to mollusks as they age. Thus, fossil barnacles provide a model for examining high-resolution physiologic responses to water mass characteristics on continental shelves driven by climatic change.