Millennial-scale changes in abundance of brachiopods in bathyal environments detected by postmortem age distributions in death assemblage (Bari Canyon, Adriatic Sea)

Abstract

Abstract Inferring the composition of pre-Anthropocene baseline communities on the basis of death assemblages (DAs) preserved in a surface mixed layer requires discriminating among recently-dead shells sourced by living populations and older shells from extirpated populations. Here, we assess the distribution of postmortem ages in the DA formed by the brachiopod Gryphus vitreus at 580 m depth in the Bari Canyon (Adriatic Sea), with no individuals collected alive. The Gryphus DA exhibits millennial time averaging (inter-quartile range = 1250 years) and two modes in abundance at 500 and 1750 years BP. As high abundance of species in time-averaged DAs can reflect passive accumulation of shells sourced by populations with low standing population density, we reconstruct changes in annual density on the basis of the abundance maxima detected in the distribution of postmortem ages and on the basis of estimates of per-specimen disintegration rate. We find that adults (>20 mm) achieved densities of at least 10–20 individuals/m 2 (assuming lifespan is 10 years), and the pulses in abundance were thus associated with a high population density in the past, followed by the decline over the last few centuries. We infer that bathyal populations were volatile during the Late Holocene, with brachiopods sensitive to siltation that was induced by temporal changes in sediment dispersal into the Bari Canyon due to deforestation and climatic changes.