Downward particle export and sequestration fluxes in the oligotrophic Atlantic Ocean

Abstract

Every year, the Biological Carbon Pump (BCP) transfers 5-15 Gt of fixed atmospheric CO2 from the surface ocean into its interior, where CO2 is stored over the time scales of thousands of years. Little is known about the functioning of the BCP in the oligotrophic areas, which cover about 60%
of the ocean’s surface, and where relatively low carbon export and sequestration would significantly contribute to the global carbon budget. Gaining more understanding of the factors controlling the BCP in these vast and currently expanding ocean provinces is important for understanding and predicting the global carbon cycle better. This thesis investigates the BCP in the sub-tropical Atlantic oligotrophic gyres.

234Th:238U disequilibria were used to investigate exports from the surface waters of the subtropical Atlantic during GEOTRACES cruise, which studied di-nitrogen (N2) fixation. 238U activity measured in the area was significantly lower than that predicted by the routinely used global 238U salinity relationship. Lower 238U activity was caused by the remote 238U removal processes. If not accounted for, 234Th and 234Th-derived exports in the study area would have been over-estimated by 20-80%. Therefore, precise estimations of both 238U and 234Th are important, especially in the low-productive areas, where 234Th:238U disequilibria are small, and where large uncertainties can
complicate the interpretation of particle dynamics and export. Further, derived from the validated 234Th:238U disequilibria, export fluxes of particulate organic carbon (POC) and nitrogen (PON) were assessed in relation to N2 fixation. Sinking diazotroph biomass contributed <1.5% to the total PON export, and it appeared to be mostly recycled within the euphotic zone. N2 fixation was a significant new N source in the area, but not the dominant one. Overall, upward physical processes were large, and regulated primary and export production in the area, as suggested by the isotopic signature and N:P stoichiometry of the export production, and a strong correlation upward deep-water nitrate flux with the observed POC and PON exports. The consecutive 2-year time-series of sediment trap samples from 3000 m depth were used to evaluate differences in POC sequestration in the cores of the North and South Atlantic ultraoligotrophic gyres, subjected to contrasting atmospheric dust input. The POC sequestration in the
Northern gyre, where dust input is large, was stronger and more efficient than that in the Southern gyre, where dust deposition is low. Surface observations from satellite and AMT transects, and modelled dust deposition data were used to evaluate the control over these differences. At both sites, POC flux at depth was strongly decoupled from the surface production. However, POC sequestration in the Northern gyre was persistently the strongest and the most efficient during the periods of high dust deposition in the summer. This strong and efficient POC flux to depth partly resulted from a shift in dominant POC ballasting phase from biogenic (carbonate+opal) to lithogenic as a result of dust input. Also, high input of dust-derived nutrient iron (Fe) during summer stratification possibly increased phytoplankton biomass and triggered N2 fixation. No significant enhancement in satellite-derived production was observed. Therefore, large and efficient POC sequestration in the Northern gyre was driven by both lithogenic ballasting and change in phytoplankton structure in response to elevated dust/Fe input.

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