Isotopic and Stoichiometric Insights Into the Biogeochemistry of Particulate Organic Matter in the Canadian Arctic Archipelago and Baffin Bay

Abstract Marine particulate organic matter (POM) is a complex geochemical mixture comprising both living and detrital particles with a variety of sources, ages, and cycling rates. In the Arctic, the main source of POM is marine plankton biomass with variable contributions from resuspended sediments, sea‐ice algae, glaciers, and rivers. Stable (δ 13 C, δ 15 N) and radiocarbon (as Δ 14 C) isotopic measurements, together with elemental stoichiometry (C:N ratios), provide insight into POM sources, cycling, diagenetic state, and ecophysiological processes. Climate change alters water column stratification, nutrient delivery, primary production, and terrestrial C fluxes that directly impact Arctic POM biogeochemistry. Here we report POM δ 13 C, δ 15 N, Δ 14 C values and C:N ratios from samples collected from the subsurface chlorophyll‐ a maximum throughout the Canadian Arctic Archipelago (CAA), together with previous data from Baffin Bay, and quantify relative contributions of organic matter (OM) from allochthonous sources such as rivers and sediment resuspension in the CAA. Terrestrial and sedimentary OM contributions ranged from 1%–79% and 1%–24%, respectively, with high terrestrial contributions nearest the Mackenzie River, as well as smaller CAA rivers. Low POM Δ 14 C values and higher sedimentary OM contribution estimates reveal where tidal currents across shallow CAA straits resuspend aged sedimentary OM to the upper water column. We find below‐Redfield C:N ratios across the CAA, likely indicative of a nutrient‐limited community dominated by picophytoplankton. POM isotopic and stoichiometric data reveal a biogeochemical mosaic within the Canadian Arctic POM—from highly productive autotrophic regions with high C:N ratios to oligotrophic picoplankton‐dominated ecosystems with low C:N ratios.