Multiple phyla, one time resolution? Time averaging in benthic foraminifera, mollusk, echinoid, crustacean and otolith fossil assemblages

Time averaging – mixing of remains of organisms separated by decades, centuries or millennia in a single sedimentary layer – affects not only paleoecological or geochronological inferences, but also the resolution of geochemical proxy records extracted from their biomineralized hardparts. Taxonomic differences in intrinsic skeletal durability are expected to produce temporal mismatch between co-occurring species, but the importance of this effect is difficult to assess due to lack of direct estimates of time averaging for many higher taxa. Moreover, burial below the taphonomic active zone and early diagenetic processes may alleviate taxonomic differences in disintegration rates in subsurface sediments.

Here, we compare time averaging across five phyla of major carbonate producers co-occurring in a sediment core from the northern Adriatic shelf. We dated individual bivalve shells, foraminiferal tests, tests and isolated plates of irregular and regular echinoids, crab claws and fish otoliths. In spite of different skeletal architecture, mineralogy and life habit, all taxa showed very similar time averaging varying from ~1800 to ~3600 yrs (interquartile age ranges). Thus, remains of echinoids and crustaceans – two groups with multi-elemental skeletons assumed to have low preservation potential – can still undergo extensive age mixing comparable to that of the co-occurring mollusk shells. Moreover, just like macrofossil assemblages, benthic foraminifera from shallow shelf settings can be time-averaged over 10³ yrs, which may limit their utility as high-resolution environmental proxies. In spite of similar time averaging of the studied taxa, their median ages differed by up to ~3700 yrs, reflecting species-specific timing of seafloor colonization during the Holocene transgression.

Our results suggest that taxa of different skeletal durability can have comparable temporal resolution due to sequestration of fragile remains below the taphonomic active zone. However, significant age offsets between co-occurring species can complicate paleoecological analyses of multi-taxic fossil assemblages and make both age models and geochemical proxy records sensitive to taxon choice.