SHELL-AGE EVIDENCE OF RECENT ECOLOGICAL CHANGES, LITTLE POSTMORTEM TRANSPORT, AND FLATTENING OF AGE-FREQUENCY DISTRIBUTIONS WITH TIME: INSIGHTS FROM AN URBAN SHELF
Drawing on death assemblages rescued from benthic surveys and biomonitoring, we have intensely dated three species via 14C-calibrated amino acid racemization: 235 Parvilucina tenuisculpta from 14 sites (uses chemosymbionts; especially abundant living near wastewater discharges), 278 Nuculana taphria from 17 sites (deposit feeder), and 50 Cyclocardia bailyi from 2 sites (suspension feeder). Although exact numbers vary slightly depending on how Asp and Glu racemization rates are modeled at our sites, Parvilucina yields the most strongly right-skewed AFD (median age 36 y, max age 8.2 ky) and this shape persists across the shelf: all sites continue to receive newly dead shells. N. taphria yields a right-skewed AFD in shallow water (med age 735 y, max 8.5 ky, <50 m) where it lives today but a two-tailed ~normal or flat AFD in deeper water, with few specimens <~5 ky (med age 14 ky, max 24 ky, 90 m): these shells are ghosts of populations that existed during lower sealevel. C. bailyi yields two-tailed AFDs in both shallow and deep water (median ages of 2.4 ky and 11.5 ky, max 5.9 and17.5 ky): populations migrated with sealevel rise but are rarely sampled alive now at any depth.
These spatial patterns emerge despite high shelf energy– AFDs are not homogenized, boding well for ecological inference from seafloor death assemblages elsewhere. However, the offshore flattening of the AFDs of shallow-water species suggests that, in the absence of catastrophic burial, this coarsening of temporal resolution will also be the fate of AFDs with progressive burial, because in both scenarios death assemblages become detached from living populations. We are now positioned to model the processes that generate these AFDs and the implications for the ecological fidelity of the permanent sedimentary record.