Although schemes exist for detecting time-averaging, many are limited to Quaternary fossils or depend on an unreliable taphonomic clock. Here we present a detection method free from these limitations that utilizes oxygen isotope (d¹⁸O) variation within sclerochronologically determined years in bivalve mollusks.

In principle, clams that grew in the same place at the same time will have identical annual d¹⁸O profiles. To evaluate this assumption we collected four sets of bivalve mollusks (Chione cortezi) from the northern Gulf of California: (1) two clams living in the same place at the same time; (2) two living in different places at the same time; (3) two from the same place that lived at different times; and (4) two that lived in different places at different times. For each specimen we analyzed d¹⁸O values in the second or third year of growth. Using daily growth increments we assigned d¹⁸O samples to specific fortnights (+/- one fortnight). We devised three metrics to compare annual d¹⁸O profiles: annual isotopic amplitude (AIA), absolute isotopic values (AIV; max and min d¹⁸O values), and number of "non-synchronous enrichment events" (NEE). Enrichment events are defined as a set of three d¹⁸O samples where the first is followed by a second with a more positive value and is in turn followed by a third with a more negative value. Enrichment events result from either protracted cold periods and/or evaporative isotopic enrichment of the ambient water. Clams that grew in the same place at the same time have identical annual isotopic profiles. Clams living in different places at the same time have similar AIA and AIV but one or more NEEs. Clams that grew in the same place at different times or in different places at different times can have different AIA, AIV, and one or more NEEs.

While this method cannot detect the absence of time-averaging (clams living in different places or at different times could have identical annual isotopic profiles), it may be useful for detecting time-averaging in obrution deposits containing suspected census assemblages.

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