Mass, nutrient and oxygen budgets for the northeastern Atlantic Ocean

Abstract

Abstract. The northeast Atlantic is a key horizontal and vertical crossroads region for the meridional overturning circulation, but basic nutrient and oxygen fluxes are still poorly constrained by observations in the region. A surface to bottom northeast Atlantic Ocean budget for mass, nutrients (nitrate and phosphate) and oxygen is determined using an optimization method based on three surveys of the OVIDE transect (from Greenland to Portugal) completed with the World Ocean Atlas 2009. Budgets are derived for two communicating boxes representing the northeastern European basin (NEEB) and the Irminger Sea.For the NEEB (Irminger) box, it is found that 30 % of the mass import (export) across the OVIDE section reach (originate from) the Nordic Seas, while 70 % are redistributed between both boxes through the Reykjanes Ridge (9.3 ± 0.7 × 10 9 kg s −1 ).Net biological source/sink terms of nitrate point to both the Irminger and NEEB boxes as net organic matter production sites (consuming nitrate at a rate of −7.8 ± 6.5 kmol s −1 and −8.4 ± 6.6 kmol s −1 , respectively). Using a standard Redfield ratio of C : N = 106 : 16, nitrate consumption rates indicate that about 40 TgC yr −1 of carbon is fixed by organic matter production between the OVIDE transect and the Greenland-Scotland Ridge. Nutrient fluxes also induce a net biological production of oxygen of 73 ± 60 kmol s −1 and 79 ± 62 kmol s −1 in the Irminger and NEEB boxes, which points to the region as being autotrophic.The abiotic air-sea oxygen flux leads to an oceanic oxygen uptake in the two regions (264±66 kmol s −1 in the north and 443±70 kmol s −1 in the south). The abiotic flux is partitioned into a mixing and a thermal component. It is found that the Irminger Sea oceanic oxygen uptake is driven by an air-sea heat flux cooling increasing the ocean surface oxygen solubility. Over the northeastern European basin the mixing component is about half the thermal flux, presumably because of the oxygen minimum in the subtropical thermocline.

Dissolved Organic Nitrogen Production and Export by Meridional Overturning in the Eastern Subpolar North Atlantic

“…The net nitrate input into the eSPNA must be balanced by an equivalent net biological consumption (Figure 1c), reinforcing the observation of a net full-depth autotrophic balance in the eSPNA, in good agreement with previous studies in the area (Mazé et al, 2012;Zunino et al, 2015). Our budgets also show that the water column-integrated rates of net uptake of nitrate (7.5 ± 6.6 TgN/year) and net production of DON (7.1 ± 2.6 TgN/year) are roughly equivalent within error bars, such that the net nitrate consumption could be counterbalanced by net DON production.…”

“…One must consider to which extent the biogeochemical rates are representative of the mean state of the eSPNA, as both currents and nutrient distributions can suffer significant interannual fluctuations (Mazé et al, 2012;Mercier et al, 2013). One must consider to which extent the biogeochemical rates are representative of the mean state of the eSPNA, as both currents and nutrient distributions can suffer significant interannual fluctuations (Mazé et al, 2012;Mercier et al, 2013).…”

“…One must consider to which extent the biogeochemical rates are representative of the mean state of the eSPNA, as both currents and nutrient distributions can suffer significant interannual fluctuations (Mazé et al, 2012;Mercier et al, 2013). The combined impact of the interannual variability of circulation and nutrient concentrations was assessed in Mazé et al (2012). During the OVIDE 2002 cruise, the amplitude of the MOC and these currents were very close to the climatological values of Mercier et al (2013) and, thus, representative of the mean state of the system.…”

Dissolved inorganic carbon budgets in the eastern subpolar North Atlantic in the 2000s from in situ data

“…The biological consumption/production of DIC inferred from the other terms of the DIC budget (Table ) gives −0.043 ± 0.039 Pg C yr −1 . This is in agreement with the estimate of Maze et al [] obtained from nutrient and oxygen budgets for the same region: −0.040 ± 0.023 Pg C yr −1 . Note that their value comes from the sum of their estimates of biological consumption in their Irminger and northeastern European Basin boxes, −0.019 ± 0.016 Pg C yr −1 and −0.021 ± 0.016 Pg C yr −1 , respectively.…”

Inorganic carbon and water masses in the Irminger Sea since 1991

“…6), probably due to a decline in preformed DIC values, i.e. a loss of solubility as a consequence of the positive surface temperature trends in the Irminger Sea from 1991-2015 (Stendardo and Gruber, 2012;Maze et al, 2012). The long-term warming trends in the surface ocean and thus the decreasing O 2 solubility leads to a deoxygenation of −0.8 ± 0.3 mol m −2 yr −1 over the 24-year period, which is consistent with the suggested loss in preformed DIC.…”