BRACHIOPOD VERSUS BIVALVE RADIOCARBON AGES: IMPLICATIONS FOR PHANEROZOIC TRENDS IN TIME-AVERAGING AND PRODUCTIVITY

Radiocarbon dating of rhynconellid brachiopod shells (Hemithiris) from subtidal shell gravels of Puget Sound – first reported here – and dates for terebratulids from Brazil (Carroll et al., 2003) indicate that, contrary to experimental evidence for rapid disintegration, articulate brachiopod shells can have time-averaged durabilities comparable to those of low-organic mollusk shells. In Puget Sound, 2 rhynchonellid shells, selected for their poor condition, yielded marginally older ages (903 and 974 C-14 years; 77 & 154 calibrated yrs BP once corrected for the marine reservoir effect) than 2 co-occurring poor-condition aragonitic bivalve shells (Clinocardium; 6 & 349 C-14 years; younger than local marine reservoir, i.e. 0 cal yrs BP; Tucson AMS). Maximum shell ages for terebratulids in tropical Brazil were several thousand years, comparable to ages of subtidal venerid bivalves in Caribbean Panama (Kidwell et al. 2005) and subtropical intertidal Gulf of California (Flessa et al. 1993). Although data are still scant, this suggests: (1) scales of time-averaging are lower in cool temperate than in (sub)tropical settings; (2) as already pointed out by Carroll et al., articulate brachiopod and bivalve assemblages may be more comparable taphonomically than previously supposed, contrary to Kidwell & Brenchley (1994), who suggested that scales of time-averaging might differ strongly if observed differences in the maximum thicknesses of their shellbeds were driven by differential durability; (3) recategorizing articulate brachiopods as “high durability” somewhat weakens Kidwell & Brenchley's argument for a secular increase in time-averaging; but (4) correspondingly strengthens their alternative hypothesis that the Phanerozoic increase in shellbed thickness reflects increasing productivity by shelly benthos, with low-productivity brachiopods being replaced by higher productivity bivalve clades – i.e., Bambach's (1993) favored driver of Seafood through Time.