Foraminiferal reproduction and preservation have been studied in Holocene tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Foraminiferal reproduction at Choya Bay tends to occur in discrete (~ a few weeks) seasonal pulses, which are then followed by periods of homogenization and dissolution of several months duration. Foraminiferal number (number of tests/gram sediment) increases northward across the flat primarily because of decreasing intensity of bioturbation and increasing total carbonate weight percent (shell content) of sediments.

Despite intensive dissolution of foraminiferal reproductive pulses, tests which appear to be relatively fresh are actually quite old (up to ~ 2000 years based on 14C dates). We hypothesize that after reproduction some tests survive dissolution because of rapid advection (burial) downward by conveyor belt deposit feeders (e.g., callianassid shrimp, polychaete worms) into a subsurface shell layer, where tests are preserved until exhumation much later by biological activity or storms. Thus, taphonomic grade (surface condition) of foraminiferal tests in these sediments is not an infallible indicator of shell age (time since death). The condition of the test surface is indicative of the residence time of the test at the sediment-water interface (“taphonomically active zone”) and not test age.