TIME AND TAPHONOMY - QUANTITATIVE ESTIMATES OF TIME-AVERAGING AND STRATIGRAPHIC DISORDER IN A SHALLOW MARINE HABITAT

We examined the radiocarbon age, taphonomic condition and stratigraphi c position of shells of the venerid bivalve Chione spp. from the tidal flats of Bahia la Choya, Sonora, Mexico. Shells in Bahia la Choya are time-averaged. Thirty shells yielded radiocarbon dates from modern (A .D. 1950 or younger) to 3569 years before present. The median calendar age of inner flat shells is 483 years; the median age of tidal channe l shells is 427 years. We interpret such long shell survival to be the result of frequent shallow burial. Such burial retards bioerosion of shells. The taphonomic condition of shells varied with environment. Sh ells from the surface of the inner flats were better preserved than sh ells from the tidal channel. Shells are more likely to be physically w orn and biologically degraded in the waters of the channel than on the quieter and more frequently exposed inner tidal flat. Taphonomic cond ition is an unreliable indicator of a shell's time-since-death. Poorly -preserved shells on the inner flats tended to be old, but in general shell condition was much more variable than shell age. A shell's condi tion is more likely the result of its total residence time on the surf ace than its time-since-death (surface time plus burial time). Two com posite short (44 cm and 50 cm) cores revealed varying degrees of strat igraphic disorder (the departure from perfect correlation between rela tive stratigraphic position and relative age). One of eight shells in the inner flats core was disordered; four of nine shells in the tidal channel were disordered. The actual age range of surface shells approx imates the age range of shells in cores. Stratigraphic disorder is a c onsequence of both time-averaging and physical and biogenic mixing. Ti me-averaging controls the degree of precision possible in paleoecologi cal studies. Environmental changes and ecological phenomena occurring within a span of 3500 years would not be recognized in deposits like t hose of Bahia la Choya. Time-averaging and stratigraphic disorder also constrain the temporal resolution possible in microstratigraphic stud ies of evolution. The extent of time-averaging and stratigraphic disor der will dictate an appropriate sample interval. In order to prevent t emporal overlap between successive samples in deposits like Bahia la C hoya, sample spacing should not be less than approximately 0.5 m.