Early to Mid‐Holocene molluscan community dynamics in Billefjorden, central Svalbard: the interplay of local geomorphology and regional climate

The postglacial evolution of High Arctic coastal environments is recorded in the rich fossiliferous archives of raised marine terraces. This study employs a multi‐proxy approach combining molluscan community analysis, taphonomy and sedimentology to reconstruct the Early to Mid‐Holocene environmental dynamics of the Mimerdalen embayment in the Billefjorden area of central Svalbard. Our results indicate that local benthic succession was driven by a complex interplay of glacio‐isostatic rebound and fluctuating sediment supply rather than simple climatic warming. We identify a four‐stage evolutionary trajectory: (1) an initial glaciomarine, brackish‐water phase dominated by the pioneer taxa Mya truncata and Hiatella arctica ; (2) a transition to fully marine conditions that facilitated the expansion of thermophilous species ( Mytilus edulis and Arctica islandica ); (3) a return to restricted brackish conditions due to forced regression and basin shallowing; and (4) a transition towards modern open‐fjord equilibrium, as slower glacio‐isostatic uplift and Mid‐Holocene Neoglacial cooling led to the disappearance of the thermally demanding taxa. Although not directly radiocarbon dated, valves of Arctica islandica and Zirfaea crispata are stratigraphically associated with dated M. truncata , suggesting their presence as early as ~10.4 cal. ka BP. This potentially advances their known regional arrival by up to 600 and 300 years, respectively, with A. islandica persisting until ~7.7 cal. ka BP. Comparisons with the Kapp Ekholm reference section demonstrate that local geomorphological stability and ephemeral disturbances, such as hyperpycnal deltaic input, were critical drivers of biodiversity, enabling the establishment of thermally demanding species. Ultimately, our findings confirm that Early Holocene sea temperatures significantly exceeded modern values, while emphasizing the overwhelming importance of salinity and seabed stability in shaping molluscan ecological structure. This study underscores the crucial role of local environmental heterogeneity in modulating the biological response to regional Holocene climate forcing in the High Arctic.