Ecosystem monitoring since 1980 has established that the boundary between the Arctic and the Subarctic has shifted northward on the Bering continental shelf between 1998 and 2001. However, such quantitative benthic data have been collected consistently only since the 1980’s, whereas additional long term data are available from natural history collections (NHCs) since 1865 and from locally accumulating dead-shell assemblages. By extracting and integrating insights from these diverse sources, I address the following questions: (1) Do dead-shell assemblages capture the shifting Subarctic-Arctic boundary in the Pacific Arctic ecosystem? (2) How long does biogenic carbon persist on the Pacific Arctic continental shelf? (3) What is history of bivalve family geographic distributions in the N Bering and Chukchi Seas over the last 150 years? In habitats where either Subarctic or Arctic conditions have persisted over the last 40 years, bivalve death assemblages agree closely with counterpart living communities in taxon and guild composition and are not subject to significant post-mortem bias. Significant live-dead discordance occurs only in areas with documented changes in community composition; there, dead assemblages are mixtures of shells from pre- and post-transition communities. This spatial pattern is robust in both numerical abundance- and biomass-based measures of community composition. Live-dead discordance can thus reliably differentiate between stable and rapidly changing habitats in cold, high-latitude settings, relevant to evaluating climate change. Overall, shells from Arctic death assemblages are young: all specimens of Nuculana are < 1600 years old with a median age of 50 years, and all Macoma shells are < 850 years old with a median age of about 30 years. These maximum shell ages are an order of magnitude lower than encountered at lower latitudes, while median shell ages are similar to those at lower latitudes. The lowest median shell ages and highest rates of shell loss are in the northern Bering Sea and the southeastern Chukchi Sea, which are consistent centers of high benthic oxygen demand and high seafloor biomass. These results indicate that, within the Arctic, shell preservation is more strongly correlated with benthic biological activity than with the temperature and chemistry of overlying water. Using occurrence data for seven families extracted from NHCs, long-stable bivalve communities underwent a significant reorganization between 1940 and 1960. The biogeographic boundaries recognized from NHC occurrence data do not differ significantly from those based on quantitative biomonitoring data during the window of 1970-2000 when both data types are available. These findings indicate that the ecologic baseline inferred from the first quantitative surveys in the 1970s captures a recently completed (1960s) reorganization of the benthic community, which coincided with the initial, 20th-century onset of seasonal sea ice retreat. In addition to these chapters the following supplementary documents are included: a copy of “Estimating fossil biomass from skeletal mass in marine invertebrates” published in Lethaia (Appendix, complete datasets of death assemblages (Appendix C_SWL14_FAMILY_Dead Collected_Bivalve, age dating calibrating models (Appendix, and Arctic NHCs (Appendix J_ARCTIC_NHC_Bivalve



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