No abstract

The quantitative role of the Atlantic Meridional Overturning Circulation (AMOC) in dissolved organic carbon (DOC) export is evaluated by combining DOC measurements with observed water mass transports. In the eastern subpolar North Atlantic, both upper and lower limbs of the AMOC transport high-DOC waters. Deep water formation that connects the two limbs of the AMOC results in a high downward export of non-refractory DOC (197 Tg-C·yr−1). Subsequent remineralization in the lower limb of the AMOC, between subpolar and subtropical latitudes, consumes 72% of the DOC exported by the whole Atlantic Ocean. The contribution of DOC to the carbon sequestration in the North Atlantic Ocean (62 Tg-C·yr−1) is considerable and represents almost a third of the atmospheric CO2 uptake in the region.

The eastern subpolar North Atlantic (eSPNA) is a key region in the Atlantic Meridional Overturning Circulation (AMOC), playing an important role in biogeochemical cycles and climate regulation. Quantitative basin-scale biogeochemical budgets are still scarce despite the current need of establishing baselines of knowledge in a changing ocean. The physico-chemical data from the eight repetitions of the OVIDE section (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016) are an unique opportunity to develop a novel evaluation of biogeochemical budgets in the eSPNA by combining robust and well established decadal mean mass transports with carbon variables, oxygen and inorganic nutrients in a full-depth inverse box model. The net balance between the carbon fixation and the respiration throughout the whole water column shows that the eSPNA is an important dissolved inorganic carbon (DIC) sink area where 119 ± 43 kmol·s-1 and 49 ± 31 kmol·s-1 of organic and inorganic carbon, respectively, are currently exported. The uptake due to mixed layer depth oxygenation of 807 ± 114 kmol·s-1 of oxygen from the atmosphere and its subsequent southward export are responsible for deep Atlantic Ocean oxygenation. Deep water formation processes connect the northward upper limb with the southward lower limb of the AMOC leading to tracer export to the deep ocean. With regard to the net macronutrient budgets, all element consumptions are balanced within uncertainties. The results presented here for carbon export and oxygen uptake are in agreement with the upper range of previous observations based on different methods. The findings of this integrated budget driven by circulation in the highly dynamic region of the eSPNA can be taken as a reference in future biogeochemical evaluations of the North Atlantic. Highlights► A large DIC sink due the biological pump is quantified in the eSPNA. ► The eSPNA is an autotrophic area of DIC sink (168±53 kmol·s -1 ) and oxygen uptake. ► Whole-water column biogeochemical budgets are based on 14 years of observations. ► Budgets of elements C, O, N, P and Si are consistently integrated and optimized.

No abstract