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- Anthropogenic CO2 and ocean acidification in Argentine Basin water masses over almost five decades of observationsPublication . Fontela, Marcos; Velo, AntĆ³n; Gilcoto, Miguel; PĆ©rez, Fiz F.The chemical conditions of the Argentine Basin (western South Atlantic Ocean) water masses are evaluated with measurements from eleven hydrographic cruises to detect and quantify anthropogenic and natural stressors in the ocean carbon system. The database covers almost half-century (1972-2019), a time-span where the mean annual atmospheric carbon dioxide concentration (CO2atm) increased from 325 to 408 ppm of volume (ppm). This increase of atmospheric CO2 (83 ppm, the 64% of the total anthropogenic signal in the atmosphere) leads to an increase in anthropogenic carbon (Cant) across all the water column and the consequent ocean acidification: a decrease in excess carbonate that is unequivocal in the upper (South Atlantic Central Water, SACW) and intermediate water masses (Sub Antarctic Mode Water, SAMW and Antarctic Intermediate Water, AAIW). For each additional ppm in CO2atm the water masses SACW, SAMW and AAIW lose excess carbonate at a rate of 0.39 Ā± 0.04, 0.47 Ā± 0.05 and 0.23 Ā± 0.03 Ī¼molĀ·kg-1Ā·ppm-1 respectively. Modal and intermediate water masses in the Argentine Basin are very sensitive to carbon increases due low buffering capacity. The large rate of AAIW acidification is the synergic effect of carbon uptake combined with deoxygenation and increased remineralization of organic matter. If CO2 emissions follows the path of business-as-usual emissions (SSP 5.85), SACW would become undersaturated with respect to aragonite at the end of the century. The undersaturation in AAIW is virtually unavoidable.
- North Atlantic western boundary currents are intense dissolved organic carbon streamsPublication . Fontela, Marcos; PƩrez, Fiz F.; Mercier, HerlƩ; Lherminier, PascaleIn the North Atlantic, there are two main western boundary currents related to the Atlantic Meridional Overturning Circulation (AMOC): the Gulf Stream flowing northward and the Deep Western Boundary Current (DWBC) flowing southward. Here we analyze data from the OVIDE section (GO-SHIP A25 Portugal-Greenland 40-60 degrees N) that crosses the DWBC and the northward extension of the Gulf Stream, the North Atlantic Current. We show that North Atlantic western boundary currents play a key role in the transport of dissolved organic matter, specifically dissolved organic carbon (DOC). Revisited transports and budgets of DOC with new available data identify the eastern Subpolar North Atlantic (eSPNA) as an important source of locally produced organic matter for the North Atlantic and a key region in the supply of bioavailable DOC to the deep ocean. The East Greenland Current, and its upstream source the East Reykjanes Ridge Current on the eastern flank of the mid-Atlantic ridge, are export pathways of bioavailable DOC toward subtropical latitudes. The fast overturning and subsequent remineralization of DOC produced in the autotrophic eSPNA explains up to 38% of the total oxygen consumption in the deep North Atlantic between the OVIDE section and 24 degrees N. Carbon budgets that do not take into account this organic remineralization process overestimates the natural uptake of carbon dioxide (CO2) from the atmosphere by one third. The inclusion of DOC transports in regional carbon budgets reconciles the estimates of CO2 uptake in the North Atlantic between model and observations.
- Contrasting drivers and trends of ocean acidification in the subarctic AtlanticPublication . PĆ©rez, Fiz F.; Olafsson, Jon; ĆlafsdĆ³ttir, Solveig R.; Fontela, Marcos; Takahashi, TaroThe processes of warming, anthropogenic CO2 (Canth) accumulation, decreasing pHT (increasing [H+]T; concentration in total scale) and calcium carbonate saturation in the subarctic zone of the North Atlantic are unequivocal in the time-series measurements of the Iceland (IS-TS, 1985ā2003) and Irminger Sea (IRM-TS, 1983ā2013) stations. Both stations show high rates of Canth accumulation with diferent rates of warming, salinifcation and stratifcation linked to regional circulation and dynamics. At the IS-TS, advected and stratifed waters of Arctic origin drive a strong increase in [H+]T, in the surface layer, which is nearly halved in the deep layer (44.7Ā± 3.6 and 25.5 Ā± 1.0 pmol Ākgā1 Āyrā1, respectively). In contrast, the weak stratifcation at the IRM-TS allows warming, salinifcation and Canth uptake to reach the deep layer. The acidifcation trends are even stronger in the deep layer than in the surface layer (44.2Ā± 1.0 pmol Ākgā1 Āyrā1 and 32.6 Ā± 3.4 pmol Ākgā1 Āyrā1 of [H+]T, respectively). The driver analysis detects that warming contributes up to 50% to the increase in [H+]T at the IRM-TS but has a small positive efect on calcium carbonate saturation. The Canth increase is the main driver of the observed acidifcation, but it is partially dampened by the northward advection of water with a relatively low natural CO2 content.
- Cold-water corals in the Subpolar North Atlantic Ocean exposed to aragonite undersaturation if the 2 Ā°C global warming target is not metPublication . GarcĆa-IbƔƱez, Maribel I.; Bates, Nicholas R.; Bakker, Dorothee C.E.; Fontela, Marcos; Velo, AntĆ³nThe net uptake of carbon dioxide (CO2) from the atmosphere is changing the oceanās chemical state. Such changes, commonly known as ocean acidification, include a reduction in pH and the carbonate ion concentration ([CO3 2ā ]), which in turn lowers oceanic saturation states for calcium carbonate (CaCO3) minerals. The values for aragonite (aragonite; one of the main CaCO3 minerals formed by marine calcifying organisms) influence the calcification rate and geographic distribution of cold-water corals (CWCs), important for biodiversity. Here, high-quality measurements, collected on thirteen cruises along the same track during 1991ā2018, are used to determine the long-term changes in Ī©aragonite in the Irminger and Iceland Basins of the North Atlantic Ocean, providing the first trends of Ī©aragonite in the deep waters of these basins. The entire water column of both basins showed significant negative aragonite trends between ā 0.0014 Ā± 0.0002 and ā 0.0052 Ā± 0.0007 per year. The decrease in aragonite in the intermediate waters, where nearly half of the CWC reefs of the study region are located, caused the aragonite isolines to rapidly migrate upwards at a rate between 6 and 34 m per year. The main driver of the decline in Ī©aragonite in the Irminger and Iceland Basins was the increase in anthropogenic CO2. But this was partially offset by increases in salinity (in Subpolar Mode Water), enhanced ventilation (in upper Labrador Sea Water), and increases in alkalinity (in classical Labrador Sea Water, cLSW; and overflow waters). We also found that water mass aging reinforced the aragonite decrease in cLSW. Based on these aragonite trends over the last three decades, we project that the entire water column of the Irminger and Iceland Basins will likely be undersaturated for aragonite when in equilibrium with an atmospheric mole fraction of CO2 (xCO2) of ~880 ppmv, corresponding to climate model projections for the end of the century based on the highest CO2 emission scenarios. However, intermediate waters will likely be aragonite undersaturated when in equilibrium with an atmospheric xCO2 exceeding ~630 ppmv, an xCO2 level slightly above that corresponding to 2 ā¦C global warming, thus exposing CWCs inhabiting the intermediate waters to undersaturation for aragonite.
- The Northeast Atlantic is running out of excess carbonate in the horizon of cold-water corals communitiesPublication . Fontela, Marcos; PĆ©rez, Fiz F.; Carracedo, Lidia I.; PadĆn, XosĆ© A.; Velo, AntĆ³n; GarcĆa-IbaƱez, Maribel I.; Lherminier, PascaleThe oceanic uptake of atmospheric carbon dioxide (CO2) emitted by human activities alters the seawater carbonate system. Here, the chemical status of the Northeast Atlantic is examined by means of a high-quality database of carbon variables based on the GO-SHIP A25 section (1997-2018). The increase of atmospheric CO2 leads to an increase in ocean anthropogenic carbon (Cant) and a decrease in carbonate that is unequivocal in the upper and mid-layers (0-2,500 m depth). In the mid-layer, the carbonate content in the Northeast Atlantic is maintained by the interplay between the northward spreading of recently conveyed Mediterranean Water with excess of carbonate and the arrival of subpolar-origin waters close to carbonate undersaturation. In this study we show a progression to undersaturation with respect to aragonite that could compromise the conservation of the habitats and ecosystem services developed by benthic marine calcifiers inhabiting that depth-range, such as the cold-water corals (CWC) communities. For each additional ppm in atmospheric pCO2 the waters surrounding CWC communities lose carbonate at a rate of - 0.17āĀ±ā0.02 Ī¼mol kg-1 ppm-1. The accomplishment of global climate policies to limit global warming below 1.5-2 ā will avoid the exhaustion of excess carbonate in the Northeast Atlantic.
- New insights into the eastern subpolar North Atlantic meridional overturning circulation from OVIDEPublication . Mercier, HerlĆ©; DesbruyĆØres, Damien; Lherminier, Pascale; Velo, AntĆ³n; Carracedo, Lidia; F. PĆ©rez, Fiz; Morente Fontela, MarcosThe Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth's climate. However, there are few long time series of observations of the AMOC, and the study of the mechanisms driving its variability depends mainly on numerical simulations. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyse the seasonal to decadal variability of the subpolar AMOC across the Greenland-Portugal OVIDE (Observatoire de la Variabilit & eacute; Interannuelle & agrave; D & Eacute;cennale) line since 1993. We decompose the MOC strength variability into a velocity-driven component due to circulation changes and a volume-driven component due to changes in the depth of the overturning maximum isopycnal. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs (linked to the seasonal cycle of density in the East Greenland Current) and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs.
- Puzzling out the ecological niche construction for nitrogen fixers in a coastal upwelling systemPublication . Morente Fontela, Marcos; FernĆ”ndez-RomĆ”n, Daniel; BroullĆ³n, Esperanza; Farnelid, Hanna; FernĆ”ndez-Carrera, Ana; MaraĆ±Ć³n, Emilio; MartĆnez-GarcĆa, Sandra; RodrĆguez-Ramos, Tamara; Varela, Marta M.; MouriƱo-Carballido, BeatrizDiazotrophs are a diverse group of microorganisms that can fertilize the ocean through biological nitrogen fixation (BNF). Due to the high energetic cost of this process, diazotrophy in nitrogen-replete regions remains enigmatic. We use multidisciplinary observations to propose a novel framework for the ecological niche construction of nitrogen fixers in the upwelling region off NW Iberia-one of the most productive coastal regions in Europe-characterized by weak and intermittent wind-driven upwelling and the presence of bays. The main diazotroph detected (UCYN-A2) was more abundant and active during summer and early autumn, coinciding with relatively high temperatures (>16 degrees C), low nitrogen:phosphorus ratios (N:P < 7.2), and a large contribution of ammonium (>75%) to the total dissolved inorganic nitrogen available. Furthermore, nutrient amendment experiments showed that BNF is detectable when phytoplankton biomass and productivity are nitrogen limited. Seasonally recurrent biogeochemical processes driven by hydrography create an ecological niche for nitrogen fixers in this system. During the spring-summer upwelling, nondiazotroph autotrophs consume nitrate and produce organic matter inside the bays. Thereafter, the combined effect of intense remineralization on the shelf and sustained positive circulation within the bays in late summer-early autumn, conveys enhanced ammonium content and excess phosphate into the warm surface layer. The low N:P ratio confers a competitive advantage to diazotrophs since they are not restricted by nitrogen supply. The new nitrogen supply mediated by BNF could extend the productivity period, and may be a key reason why upwelling bays are more productive than upwelled offshore waters.