Patricia Zunino

Deep and intermediate layer warming in the western Mediterranean: Water mass changes and heaving

[1] Temperature and salinity have increased in the Western Mediterranean Deep Water during the second half of the 20th century, but surprisingly, the two water masses contributing to its formation (Atlantic Water and Levantine Intermediate Water) show no clear warming trends, at least for the same period of time. Previous studies concerned with long term TS property changes in the Western Mediterranean had dealt with changes on isobaric surfaces. In this study we decompose these changes into their two natural contributions: Changes on isopycnal surfaces and changes induced by heaving (vertical displacements of density surfaces). This decomposition shows that LIW temperature and salinity have increased when changes on density surfaces are analyzed. The upward movement of deep layers has induced cooling on isobaric levels masking LIW temperature trends and underestimating deep layers temperature trends.

Transports and budgets of anthropogenic CO2 in the tropical North Atlantic in 1992–1993 and 2010–2011

The meridional transport of anthropogenic CO2 (Cant) in the tropical North Atlantic (TNA) is investigated using data from transoceanic sections along 7.5°N and 24.5°N, carried out in the early 1990s and 2010s. The net Cant transport across both sections is northward. At 7.5°N, this transport increased from 315 ± 47 kmol s−1 in 1993 to 493 ± 51 kmol s−1 in 2010; similarly, across 24.5°N it grew from 530 ± 46 kmol s−1 in 1992 to 662 ± 49 kmol s−1 in 2011. These changes result from modifications in the intermediate and deep circulation patterns, as well as from Cant increase within the thermocline waters. In deep waters, lateral advection causes a net Cant input of 112 ± 60 kmol s−1 (234 ± 65 kmol s−1) in 1992–1993 (2010–2011); within these deep waters, the storage rate of Cant is not statistically different from the net Cant input, 139 ± 21 kmol s−1 (188 ± 21 kmol s−1) in 1992–1993 (2010–2011). The Cant increase in deep waters is due to the large injection of Cant across the 24.5°N by the Deep Western Boundary Current and the northward recirculation of North Atlantic Deep Water along 7.5°N. In contrast, a large net Cant output in the upper layer is caused by the Florida Current. Despite this net Cant output, the Cant accumulates at a rate of 215 ± 24 kmol s−1 (291 ± 24 kmol s−1) referenced to year 1993 (2010). From the two Cant budgets, we infer a Cant air-sea flux of 0.23 ± 0.02 Pg yr−1in the TNA, much larger than previous estimates.

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

The subpolar North Atlantic (SPNA) is important in the global carbon cycle because of the deep water ventilation processes that lead to both high uptake of atmospheric CO2 and large inventories of anthropogenic CO2 (C-ant). Thus, it is crucial to understand its response to increasing anthropogenic pressures. In this work, the budgets of dissolved inorganic carbon (DIC), C-ant and natural DIC (DICnat) in the eastern SPNA in the 2000s, are jointly analyzed using in situ data. The DICnat budget is found to be in steady state, confirming a long-standing hypothesis from in situ data for the first time. The biological activity is driving the uptake of natural CO2 from the atmosphere. The C-ant increase in the ocean is solely responsible of the DIC storage rate which is explained by advection of C-ant from the subtropics (65%) and C-ant air-sea flux (35%). These results demonstrate that the C-ant is accumulating in the SPNA without affecting the natural carbon cycle.

Introduction to the French GEOTRACES North Atlantic Transect (GA01): GEOVIDE cruise

The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special_issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue. 1 Scientific context and objectives Understanding the distribution, sources, and sinks of trace elements and isotopes (TEIs) will improve our ability to understand past and present marine environments. Some TEIs are toxic (e.g. Hg), while others are essential micronutrients involved in many metabolic processes of marine organisms (e.g. Fe, Mn). The availability of TEIs therefore constrains the ocean carbon cycle and affects a range of other biogeochemical processes in the Earth system, whilst responding to and influencing global change (de Baar et al., 2005; Blain et al., 2007; Boyd et al., 2007; Pollard et al., 2007). Moreover, TEI interactions with the marine food web strongly depend on their physical (particulate/dissolved/colloidal/soluble) and chemical (organic and redox) forms. In addition, some TEIs are diagnostic in allowing the quantification of specific mechanisms in the marine environment that are challenging to measure directly. A few examples include (i) atmospheric deposition (e.g. 210Pb, Al, Mn, Th isotopes, 7Be; Baker et al., 2016; Hsieh et al., 2011; Measures and Brown, 1996); (ii) mixing rates of deep waters or shelf-to-open ocean (e.g. 231Pa/230Th,114C, Ra isotopes, 129I, 236U; van Beek et al., 2008; Casacuberta et al., 2016; Key et al., 2004); (iii) boundary exchange processes (e.g. εNd, Jeandel et al., 2011; Lacan and Jeandel, 2001, 2005); and (iv) downward flux of organic carbon and/or remineralization in deep waters (e.g. 234Th/238U, 210Pb/210Po, Baxs; Buesseler et al., 2004; Dehairs et al., 1997; Roca-Martí et al., 2016). In such settings, TEIs provide chemical constraints and allow the estimation of fluxes which was not possible before the development of their analyses. Finally, paleoceanographers are wholly dependent on the development of tracers, many of which are based on TEIs used as proxies, in order to reconstruct past environmental conditions (e.g. ocean productivity, patterns and rates of ocean circulation, ecosystem structures, ocean anoxia; Henderson, 2002). Such reconstruction efforts are essential to assess the processes involved in regulating the global climate system, and possible future climate change variability. Despite all these major implications, the distribution, sources, sinks, and internal cycling of TEIs in the oceans are still largely unknown due to the lack of appropriate clean sampling approaches and insufficient sensitivity and selectivity of the analytical measurement techniques until recently. This last point has improved very quickly as significant improvements in the instrumental techniques now allow the measurements of concentrations, speciation (physical and chemical forms), and isotopic compositions for most of the elements of the periodic table which have been identified either as relevant tracers or key nutrients in the marine environment. These recent advances provide the marine geochemistry community with a significant opportunity to make subBiogeosciences, 15, 7097–7109, 2018 www.biogeosciences.net/15/7097/2018/ G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) 7099 Figure 1. Schematic diagram of the mean large-scale circulation adapted from Daniault et al. (2016) and Zunino et al. (2017). Bathymetry is plotted in color with color changes at 100 and 1000 m and every 1000 m below 1000 m. Black dots represent the Short station, yellow stars the Large ones, orange stars the XLarge ones, and red stars the Super ones. The main water masses are indicated: Denmark Strait Overflow Water (DSOW), Iceland–Scotland Overflow Water (ISOW), Labrador Sea Water (LSW), Mediterranean Water (MW), and lower North East Atlantic Deep Water (LNEADW). stantial contributions to a better understanding of the marine environment. In this general context, the aim of the international GEOTRACES programme is to characterize TEI distributions on a global scale, consisting of ocean sections, and regional process studies, using a multi-proxy approach. The GEOVIDE section is the French contribution to this global survey in the North Atlantic Ocean along the OVIDE section and in the Labrador Sea (Fig. 1) and complements a range of other international cruises in the North Atlantic. GEOVIDE leans on the knowledge gained by the OVIDE project during which the Portugal–Greenland section has been carried out biennially since 2002, gathering physical and biogeochemical data from the surface to the bottom (Mercier et al., 2015; Pérez et al., 2018). Rationale for the GEOVIDE section i. The North Atlantic Ocean plays a key role in mediating the climate of the Earth. It represents a key region of the Meridional Overturning Circulation (MOC) and a major sink of anthropogenic carbon (Cant) (Pérez et al., 2013; Sabine et al., 2004; Seager et al., 2002). Since 2002, the OVIDE project has contributed to the observation of both the circulation and water mass properties of the North Atlantic Ocean. Despite the importance of the MOC on global climate, it is still challenging to assess its strength within a reasonable uncertainty (Kanzow et al., 2010; Lherminier et al., 2010). The MOC strength estimated from in situ measurements on OVIDE cruises has thus helped to validate a time series for the amplitude of the MOC (based on altimetry and ARGO float array data) that exhibits a drop of 2.5± 1.4 Sv (95 % confidence interval) between 1993 and 2010 (Mercier et al., 2015), consistent with other modelling studies (Xu et al., 2013). This time series, along with the in situ data, shows a recovery of the MOC amplitude in 2014 at a value similar to those of the mid1990s, confirming the importance of the decadal variability in the subpolar gyre. During OVIDE, the contributions of the most relevant currents, water masses, and biogeochemical provinces were localized and quantified. This knowledge was crucial for the establishment of the best strategy to sample TEIs in this specific region. In addition to the OVIDE section, the Labrador Sea section offered a unique opportunity to complement the MOC estimate, to analyse the propagation of anomalies in temperature and salinity (Reverdin et al., 1994), and to study the distribution of TEIs along the boundary current of the subpolar gyre, coupling both observations and modelling. Moreover, recent results provided evidence that CO2 uptake in the North Atlantic was reduced by the weakening of the MOC (Pérez et al., 2013). The most significant finding of this study was that the uptake of Cant occurred almost exclusively in the subtropical gyre, while natural CO2 uptake dominated in the subpolar gyre. In light of these new results, one issue to be addressed was the coupling between the Cant and the transport of water, with the aim to understand how the changes in the ventilation and in the circulation of water masses affect the Cant uptake and its storage capacity in the various identified provinces (Fröb et al., 2018). Finally, as the subpolar North Atlantic forms the starting point for the global ocean conveyor belt, it is of particular interest to investigate how TEIs are transferred to the deep ocean through both ventilation and particle sinking, and how deep convection processes impact the TEI distributions in this key region. ii. A better assessment of the factors that control organic production and export of carbon in the productive North Atlantic Ocean together with a better understanding of the role played by TEIs in these processes is research priorities. Pronounced phytoplankton blooms occur in the North Atlantic in spring in response to upwelling and water column destratification (Bury et al., 2001; Henson et al., 2009; Savidge et al., 1995). Such www.biogeosciences.net/15/7097/2018/ Biogeosciences, 15, 7097–7109, 2018 7100 G. Sarthou et al.: French GEOTRACES North Atlantic Transect (GA01) blooms are known to trigger substantial export of fastsinking particles (Lampitt, 1985), and can represent a major removal mechanism for particulate organic carbon, macronutrients, and TEIs to the deep ocean. iii. In the North Atlantic, TEI distributions are influenced by a variety of sources including, most importantly, the atmosphere and the margins (Iberian, Greenland, and Labrador margins). 1. Atmosphere. Atmospheric inputs (e.g. mineral dust, anthropogenic emission aerosols) are an important source of TEIs to the North Atlantic Ocean due to the combined effects of anthropogenic emissions from industrial/agricultural sources and mineral dust mobilized from the arid regions of North Africa (Duce et al., 2008; Jickells et al., 2005). Model and satellite data for the GEOVIDE section suggested that an approximately 10fold decrease in the atmospheric concentrations of mineral dust was expected from south to north (Mahowald et al., 2005). As there had been relatively few aerosol TEI studies in the northern North Atlantic compared to the tropical and subtropical North Atlantic prior to GEOVIDE, constraining atmospheric deposition fluxes to this region had been identified as a research priority (de Leeuw et al., 2014). During the GEOVIDE campaign, a multi-proxy approach (e.g. aerosol trace element concentrations, dissolved and particulate Al and Mn, seawater 210Pb, Fe, Nd, and Th isotopes, 7Be) was taken to achieve the objective of better constraining the atmospheric deposition fluxes of key trace elements. 2. Margins. The continental shelves can act

Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean

Since the Industrial Revolution, the North Atlantic Ocean has been accumulating anthropogenic carbon dioxide (CO2) and experiencing ocean acidification, that is, an increase in the concentration of hydrogen ions (a reduction in pH) and a reduction in the concentration of carbonate ions. The latter causes the ‘aragonite saturation horizon’—below which waters are undersaturated with respect to a particular calcium carbonate, aragonite—to move to shallower depths (to shoal), exposing corals to corrosive waters. Here we use a database analysis to show that the present rate of supply of acidified waters to the deep Atlantic could cause the aragonite saturation horizon to shoal by 1,000–1,700 metres in the subpolar North Atlantic within the next three decades. We find that, during 1991–2016, a decrease in the concentration of carbonate ions in the Irminger Sea caused the aragonite saturation horizon to shoal by about 10–15 metres per year, and the volume of aragonite-saturated waters to reduce concomitantly. Our determination of the transport of the excess of carbonate over aragonite saturation (xc[CO32−])—an indicator of the availability of aragonite to organisms—by the Atlantic meridional overturning circulation shows that the present-day transport of carbonate ions towards the deep ocean is about 44 per cent lower than it was in preindustrial times. We infer that a doubling of atmospheric anthropogenic CO2 levels—which could occur within three decades according to a ‘business-as-usual scenario’ for climate change—could reduce the transport of xc[CO32−] by 64–79 per cent of that in preindustrial times, which could severely endanger cold-water coral habitats. The Atlantic meridional overturning circulation would also export this acidified deep water southwards, spreading corrosive waters to the world ocean.