ARCTIC BIVALVE SHELL ASSEMBLAGES AS HIGH TEMPORAL- AND SPATIAL-RESOLUTION ARCHIVES OF ECOLOGICAL REGIME CHANGE

Ecosystem monitoring of Alaskan shelves since 1980 has revealed that the boundary between the Arctic and Subarctic regimes shifted northward toward the Chukchi Sea between 1998 and 2001, with the reduction of the sea ice-influenced bottom water pool in the Bering Sea. The front of this ‘regime change’ is a region of high-benthic productivity south of St. Lawrence Island (SLIP), supporting large populations of benthivore birds and mammals. Five biomonitoring stations reveal a gradient in seafloor productivity and bottom water chemistry, creating an opportunity to test the spatial and temporal resolution of high-latitude molluscan shell assemblages as archives of historical ecological change. We expect that Alaskan dead shell assemblages can capture very recent shifts in biologic gradients due to the combined effects of abundant inputs of dead shells, high-benthic productivity, and high-rates of postmortem loss in cold, undersaturated overlying water. Using dead shell assemblages sieved from SLIP stations in 2014-2018 and environmental and live-mollusk data from same stations 1990-2018, we find that bivalve shell assemblages most resemble community composition from before the regime change. They are enriched in the obligate deposit-feeder Nuculana as opposed to facultative deposit-feeding Macoma and detect known station-level differences in faunal trends known to have occurred within the last two decades. Age-dating of ~100 bivalve shells from these stations using AMS-calibrated amino-acid racemization indicates that all were <1600 years old with a median age of 50 years. These maximum shell ages are an order of magnitude lower than encountered on shelves from lower latitudes having comparable sedimentation rates (0.2 cm/yr), indicating an apparent failure of long-term sequestration of shells. The memory of these Alaskan assemblages is relatively short, providing a strong signal of biological change during the last few decades. With marine regime change suspected throughout the Arctic, molluscan death assemblages, typically discarded during benthic monitoring, can become a powerful means of acquiring information at newly transitioning regions, even under the taphonomically challenging conditions of very cold, highly productive, well-oxygenated waters.