Carbon export from the upper water column of the polar oceans by using natural radionuclides

The Arctic and Southern Oceans account together for about 20 to 35% of the global oceanic uptake of carbon dioxide (CO2) from the atmosphere. This is partly caused by the biological pump, which transforms CO2 into organic matter in surface waters and pumps a fraction of it to the deep ocean. A major challenge to polar research is to predict how climate change will affect the marine ecosystem functioning and the carbon uptake at high latitudes. The limited baseline data on ecosystem dynamics in the Arctic and Southern Oceans, the rapid impacts of climate change on polar marine systems, together with their relevance within the global carbon cycle, have motivated this thesis. This work represents an attempt to contribute to the knowledge of the particle and carbon export fluxes driven by the biological pump and the processes that control these fluxes in the upper water column of the polar oceans. The two pairs of radionuclides Th-234/U-238 and Po-210/Pb-210 have been used in three studies in combination with other techniques that provide complementary information on ecologically relevant parameters and particle export. First, the export of particulate organic carbon (POC) was quantified during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean in summer 2012. The POC fluxes at 100 m were high, averaging 26 ± 15 mmol C m-2 d-1, which are comparable to other studies of the later stages of blooms in the Southern Ocean. However, the export efficiency of the bloom was generally low: only <20% of the daily net primary production (NPP) reached 100 m, presumably due to an active recycling of carbon and nutrients in surface waters. In contrast, the transfer efficiency of POC measured between 100 and 300 m was high (~60%), likely as a consequence of the direct sinking of diatoms. Second, carbon export fluxes in the central Arctic were assessed for the first time by the joint application of the Th-234/U-238 and Po-210/Pb-210 pairs during the record sea-ice minimum in 2012. The Th-234/U-238 proxy revealed that POC fluxes at the base of the euphotic zone were very low (2 ± 2 mmol C m-2 d-1) during August and September, when prasinophytes would have contributed significantly to the fluxes. On the other hand, the Po-210/Pb-210 proxy indicated that particle fluxes were higher before July/August than in the late summer, with maximum fluxes under heavy sea-ice conditions associated with a massive export of sea-ice algal aggregates composed of diatoms. More than 30% of the annual NPP was exported from the euphotic zone, showing a high export efficiency of the biological pump in the central Arctic. Last, particle dynamics and particle export in the water column of the Arctic Ocean were studied by analysing the distribution of Pb-210 and Po-210 on a pan-Arctic scale in summer 2007. Substantial deficits of Po-210 (>50% with respect to Pb-210) were detected from surface waters to 200 m on the shelves, but also, and more surprisingly, in the basins. The Po-210 deficits were particularly pronounced in the Makarov Basin, where the annual NPP was higher than in the Eurasian Basin by a factor of 3 and was dominated by diatoms. This observation, together with the results from 2012, indicates that diatoms could have a more important role in production and export of organic carbon in the central Arctic than is currently thought.