Condensed shellbeds record drastic ecological shifts during the late Holocene; A multiproxy study from Galway Bay, Western Ireland.

Shallow-marine shellfish communities in coastal regions are increasingly facing threats posed by ongoing environmental change, largely as a result of anthropogenic activities. The near-time geohistorical record offers opportunities to identify how these communities have responded in the past to episodes of environmental change that may have been triggered by similar ecological stressors, albeit of non-anthropogenic origin, thus providing longer term context  to understanding the ecological health of modern ecosystems. 

Shellbeds, high-density accumulations of shell remains, in a sedimentary sequence often represent perturbations of the “usual” environmental conditions under which sediments accumulated. Shellbeds can vary in thickness, spatial geometry, species composition, internal structure, and fidelity of shell preservation reflecting the complex processes that are often involved in their formation, for example via sedimentological processes including storm events and current winnowing. Alternatively, shellbeds can form as in situ biogenic accumulations. Identifying the processes responsible for the formation of shellbeds offers insight into the local ecological, physical, and environmental conditions at the time of their formation.

The results of a study of Holocene, (~9-3kyr) shellbeds that occur in the subsurface of Galway Bay, Western Ireland are presented. X-ray imaging of the cores indicates the spatial extent of the shellbeds extends across most of Galway Bay, ~450km2. Regional-scale observations suggest a variation in the shellbed biofacies from easterly bivalve-dominated shellbeds to westerly near mono-specific Turritella-dominated shellbeds. A chronostratigraphic framework using radiocarbon dating reveals the bivalve-dominated shellbeds predate the Turritella-dominated shellbeds by ~1500 years, with formation of the Turritella-dominated shellbeds occurring ~7-4kyr. Vertical stacking of the latter on the former locally is consistent with this and is attributed as a response to a Holocene transgression in Galway Bay.  

The bivalve-dominated shellbeds are similar in composition and taphonomy to extant populations identifiable from samples currently collected from Galway Bay.  Various taphonomic criteria identify the Turritella-dominated shellbeds as in situ communities, contemporaneous across Galway Bay, for which no modern analogue has been identified in the bay.  These near monospecific populations represent a  widespread event lasting ~3000 years, that developed rapidly and simultaneously across Galway Bay for a sustained period before terminating abruptly. This event was followed by re-establishment of the bivalve-dominated communities typical of nearshore modern Galway Bay.

Holocene in age Turritella-dominated shellbeds in NE Europe have been attributed as a response to the 8.2kyr event. Their being younger excludes this origin for the Galway Bay Turritella shellbeds suggesting they are an ecological response to a different episode of environmental change. Formation of these Turritella shellbeds is attributed to a set of environmental conditions that favoured development of an opportunistic community during the very Late Holocene transgression of Galway Bay.  

Future lipid biomarker and trace element analysis will elucidate these subtle changes in environmental conditions under which the shellbeds formed . This will confirm the potential of shellbeds as an indicator of the sensitivity of these shallow-marine ecosystems to environmental variations, and a predictor of the future for these shell communities as their ecosystems continue to come under increasing environmental stress from anthropogenic activities.