Shell survival and time‐averaging in nearshore and shelf environments: estimates from the radiocarbon literature

Radiocarbon dates provide a means for estimating the time a shell may persist in active sedimentary environments and the actual temporal extent of time‐averaging in marine deposits. Information compiled from the published literature on the radiocarbon age of marine shells gave information on a total of 734 radiocarbon dates from 276 localities from nearshore (< 10 m depth) and shelf (> 10 m depth) habitats. The median age of 128 nearshore shells is 2,465 years; that of 158 shells from the shelf is 8,870 years. The distribution of shell ages in both nearshore and shelf environments is strongly skewed: most dates are in the 0–3,000 range, and the number of shells in older age‐classes falls off rapidly. The maximum age of a shell in an active sedimentary environment is a measure of time‐averaging, because it estimates the amount of time represented in the deposit. The median duration of time‐averaging in 63 nearshore deposits is 1,250 years; the median duration of time‐averaging in 129 shelf deposits is 9,190 years. Radiocarbon‐dated shells from fossil deposits confirm our estimate of time‐averaging in nearshore environments: the median difference between maximum and minimum ages in 49 inactive beach ridges is 1,390 years; the median difference in other, predominantly nearshore deposits, is 830 years. Greater shell survival and longer durations of time‐averaging in shelf settings may result from lower rates of sedimentation, lower rates of taphonomic destruction, greater rates of bioturbation, the history of post‐glacial sea level, sample bias, or some combination of these factors. Our results may estimate the actual magnitude of time‐averaging of shelly deposits forming under conditions of low net sediment accumulation. Such extensive time‐averaging may confound attempts at detailed paleoecological and paleoenvironmental reconstruction. □Time‐averaging, benthic assemblages, taphonomy, radiocarbon, paleoecology.