As a contribution to the Regional Carbon Cycle Assessment and Processes phase 2 (RECCAP2) project, we present synthesized estimates of Arctic Ocean sea-air CO2 fluxes and their uncertainties from 8 surface ocean pCO2-observation products, 18 ocean biogeochemical hindcast and data assimilation models and 6 atmospheric inversions. For the period of 1985−2018, the Arctic Ocean was a net sink of CO2 of 116 ± 4 TgC yr−1 in the pCO2 products and 92 ± 30 TgC yr−1 in the models. The CO2 uptake peaks in late summer and early autumn, and is low in winter when sea ice inhibits sea-air fluxes. The long-term mean CO2 uptake in the Arctic Ocean is primarily caused by steady-state fluxes of natural carbon (70 ± 15 %), and enhanced by the atmospheric CO2 increase (19 ± 5 %) and climate change (11 ± 18 %). The annual mean CO2 uptake increased from 1985 to 2018 at a rate of 31 ±13 TgC yr−1dec-1 in the pCO2 products and 10 ± 4 TgC yr−1dec-1 in the models. Moreover, 77 ± 38 % of the trend in the net CO2 uptake over time is caused by climate change, primarily due to rapid sea ice loss in recent years. Both, the mean CO2 uptake and the trend, is substantially weaker in the atmospheric inversions. Uncertainties across all estimates are large, in the pCO2 products because of scarcity of observations and in the models because of missing processes.

Changes in the lower-trophic marine ecosystem and biogeochemical cycles were considered with respect to the warming Arctic Ocean. The time series monitoring of the hydrography and marine settling particles supplies a large amount of information regarding the marine lower-trophic ecosystem in with respect to the changing Arctic Ocean. To investigate the interannual relationship between the settling particles and hydrographic conditions of the western Arctic Ocean, bottom-tethered sediment trap moorings were deployed at the Northwind Abyssal Plain (NAP) and at the Chukchi Abyssal Plain from October 2010 to September 2014 to the east and west of the Chukchi Borderland. The settling particles at both stations contained a significant amount of lithogenic matter, with biogenic particles being a lateral advection of shelf materials from shelf to basin. The many peaks of settling particle fluxes at Station NAP in 2013–2014 corresponded to the hydrographic events of passing oceanic eddies over the station, in addition to seasonal biological production. Additionally, abundant gelatinous matter was found in some summer samples. The settling flux data of particulate organic matter and the mole ratio and stable isotope ratios of particulate organic carbon and nitrogen in the settling particles reflected the changes in the composition of trapped plankton assemblages under various hydrographic conditions over the course of the four years of the study.