DISTRIBUTION OF EQUILIBRIUM CALCITE IN BRACHIOPODS FROM THE SOUTHERN AUSTRALIAN SHELF

Brachiopod shell chemistry is commonly used to estimate paleoenvironmental marine conditions and seawater chemistry. Previous studies have found that brachiopod primary layer calcite (PLC) and the outer portion of secondary layer calcite (SLC) are out of equilibrium with ambient seawater. This study further investigates the distribution of non-equilibrium calcite by sampling bulk shell, PLC, and SLC from the underside of brachiopod shells to better understand if brachiopods precipitate their shells at equilibrium with ambient seawater and to help define proper sampling methods. Forty eight living and Holocene brachiopods from 4 different species in seafloor sediment from the southern Australian shelf were analyzed for δ¹⁸O and δ¹³C, including eleven Anakinetica (Magadina) cumingi specimens that were also analyzed for Mg, Sr, and Ca concentrations. The anterior-most SLC samples are enriched in Mg and Sr, similar to non-equilibrium outer SLC identified by other researchers. Both the outer SLC and anterior-most SLC from the underside of the shell are out of equilibrium due to kinetic effects associated with brachiopod growth style. Juvenile cells are added to the anterior edge during periods of shell lengthening where they rapidly precipitate calcite that is out of equilibrium due to kinetic fractionation. As the cells mature they begin to more slowly precipitate SLC in equilibrium with ambient seawater and continue to thicken the shell throughout the lifespan of the organism. The thickness of non-equilibrium calcite is variable. Not all elements reach equilibrium at the same shell position; Mg has a thicker zone of non-equilibrium calcite compared to Sr, δ¹⁸O, and δ¹³C in the same specimens whereas Sr has a thicker non-equilibrium zone than δ¹⁸O and δ¹³C, which do not show any evidence of non-equilibrium values within SLC. These data suggest that brachiopods precipitate a large portion of their shells in equilibrium with ambient seawater and the best sampling location for equilibrium shell calcite is SLC collected from the posterior portion on the underside of the ventral valve.