Bathyal and abyssal ecological changes indicate increased seasonal organic carbon fluxes following glacial retreat

As the Arctic warms and ice cover recedes, forecasts predict increases in organic carbon flux to bathyal and abyssal depths due to increased productivity. Here we use marine sediment records from the Gulf of Alaska during the last deglacial as an analog for ecosystem responses to ice loss and test whether glacial retreat resulted in ecological changes consistent with increased organic matter fluxes. We combine analyses of benthic foraminiferal assemblages and geochemical proxies (δ¹³C, δ¹⁸O, redox sensitive metals) from sediment cores retrieved at 692 and 3,667 m water depth in a constrained multivariate ordination to analytically separate faunal variation due to environmental changes related to deglaciation, changes in seafloor oxygenation, and unrelated differences between the sites. We find that opportunistic benthic foraminiferal species increase in abundance at both slope and abyssal settings across the glacial-interglacial transition, suggesting an increase in seasonally pulsed organic matter export to the seafloor as deep as 3,667 m. In addition, we show benthic foraminifera associated with low-oxygen conditions independently increase in abundance during the deglacial at both depths. Contemporaneous increases in benthic foraminiferal diversity, assemblage evenness, and density suggest the increased organic carbon flux and decreased oxygenation positively affected protist communities, however these conditions would negatively affect the metazoan faunas. Thus, glacial retreat was associated with increased organic carbon flux to bathyal and abyssal depths in this past ice-proximal region and the change was significant enough to affect the ecological function and biodiversity of ecosystems as deep as the abyss. These paleoecological data are important for constraining our expectations of how future Arctic ecosystems will respond to changing climates.