A DATE WITH NUTTALLIA: BIOTIC INTERACTIONS, PALEOECOLOGICAL CHANGE, AND CHRONOLOGICAL COMPLICATIONS IN QUATERNARY PUGET SOUND

Sea level rise has pronounced and wide-ranging effects on shallow marine environments, with some evidence suggesting a resultant increase in parasitism pressure. While parasitic organisms have generally poor preservation potential, their biotic interactions can be recorded as traces in their hosts. One group, the Gymnophallidae (Digenea) infests bivalves as metacercarial larvae, leaving behind characteristic pits identifiable in the fossil record.

Transgressive deposits assigned to the Upper Pleistocene are known from Whidbey Island, Washington, where subtidally deposited glaciomarine sediments include laterally extensive shell beds. Subfossil material from these beds and modern material from nearby death assemblages were compared for community structure and parasitic trace prevalence. Compared to modern death assemblages, subfossil shell bed assemblages had higher gymnophallid infestation prevalence, largely concentrated in the littleneck clam Leukoma staminea (Veneridae). Community structure varied also between subfossil and modern assemblages, with the former dominated by bivalves associated with higher-energy environments.

Low precision radiocarbon dating of shell bed material returned ages of ~1000 ¹⁴C y BP, inconsistent with the Late Pleistocene published ages of the shell beds. Taken at face value, this age drastically alters the interpretation of the deposit, suggesting a possible recent death assemblage or Native American midden origin. In the case of the latter, apparent differences in community structure would reflect selective harvesting by humans rather than changes in environment.

Dates of the death assemblage material cluster at ~300 and ~900 ¹⁴C y BP, but the presence of the recently introduced purple mahogany clam Nuttallia obscurata (Psammobiidae) likely establishes the age as less than 50 y. We will conduct further low precision ¹⁴C dating of N. obscurata to test the roles of time-averaging, sample contamination, and misidentification of field relationships in producing these surprising age relationships.