Abstract. ---- This study explores time-averaging (temporal mixing) at very high sampling resolution: that of adjacent shells collected from the same stratum. Nine samples of the bivalve Chione fluctifraga were collected from four Holocene cheniers (beach ridges) on the Colorado Delta (Gulf of California) and 165 shells were dated using radiocarbon-calibrated amino-acid racemization (D-alloisoleucine/L-isoleucine). The age range of shells within samples averages 661 years and, in seven out of nine samples, exceeds 500 years. The sample standard deviation ranges from 73 to 294 years and averages 203 years. Thus, even within-sample estimates of time-averaging indicate extensive temporal mixing in bioclastic deposits. No matter how carefully collected, data from shell beds may not be suitable for studying processes on time-scales shorter than hundreds to thousands of years. Comparison of our data with the estimates obtained from other cheniers at coarser sampling resolutions, indicates that pooling of samples drastically increases time-averaging in paleontological data. Time-averaging is homogeneous among strata within cheniers, but varies among cheniers. Thus, deposits of seemingly identical origin may vary in their temporal resolution -- apparently comparable shell beds may differ in paleontological patterns (e.g., species diversity) due to cryptic variation in time-averaging. Age-distributions of dated shells indicate that, at 50 year resolution, the samples provide a continuous and uniform record for the entire interval. The incompleteness observed in the samples can easily be simulated by sampling a 100%-complete, uniform record. The mean sample completeness of the actual samples (63.6%) is very close to that predicted by the simulations (67.3%). Shell beds can record the optimal type of time-averaging, where paleobiological data are a time-weighted average of the faunal composition from the spectrum of environments that existed during the entire interval of time. Coordinated stasis may reflect a long-term averaging of taxa from a similar spectrum of environments, and not necessarily ecological locking. Also, within the range of radiocarbon dating, shell beds can provide a 100%-complete, high resolution record.
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