Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean Lesley A. Salt, S. M. A. C. Heuven, M. E. Claus, E. M. Jones, H. J. W. Baar To cite this version: Lesley A. Salt, S. M. A. C. Heuven, M. E. Claus, E. M. Jones, H. J. W. Baar. Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean. Biogeosciences, European Geosciences Union, 2015, 12 (5), pp.1387-1401. 10.5194/bg-12-1387-2015. hal-01251672 HAL Id: hal-01251672 https://hal.archives-ouvertes.fr/hal-01251672 Submitted on 6 Jan 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Biogeosciences, 12, 1387–1401, 2015 www.biogeosciences.net/12/1387/2015/ doi:10.5194/bg-12-1387-2015 © Author(s) 2015. CC Attribution 3.0 License. Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean L. A. Salt1,*, S. M. A. C. van Heuven2,**, M. E. Claus3, E. M. Jones4, and H. J. W. de Baar1,3 1Royal Netherlands Institute for Sea Research, Landsdiep 4, 1797 SZ, Texel, the Netherlands 2Centre for Isotope Research, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, the Netherlands 3Department of Ocean Ecosystems, University of Groningen, Nijenborgh 7, 9747 AG, Groningen, the Netherlands 4Alfred Wegener Institute for Polar and Marine Research, 120161, 27515, Bremerhaven, Germany *now at: CNRS, UMR7144, Equipe Chimie Marine, Station Biologique de Roscoff, Place Georges Teissier, 29680 Roscoff, France **now at: Alfred Wegner Institute, Climate Sciences Department, Postfach 120161, 27515 Bremerhaven, Germany Correspondence to: L. A. Salt ([email protected]) Received: 20 March 2014 – Published in Biogeosciences Discuss.: 12 May 2014 Revised: 16 January 2015 – Accepted: 19 January 2015 – Published: 5 March 2015 Abstract. Observations along the southwestern Atlantic 1 Introduction WOCE A17 line made during the Dutch GEOTRACES- NL programme (2010–2011) were compared with histor- The Atlantic Ocean contains the largest store of anthro- ical data from 1994 to quantify the changes in the an- pogenic carbon (Cant/ of all the world’s oceans, accounting thropogenic component of the total pool of dissolved inor- for approximately 38 % of the total Cant inventory (Sabine et ganic carbon (1Cant/. Application of the extended multi- al., 2004). Within the Atlantic, the North Atlantic has been linear regression (eMLR) method shows that the 1Cant from found to be responsible for the majority of the uptake of 1994 to 2011 has largely remained confined to the upper Cant, due to the formation of North Atlantic Deep Water 1000 dbar. The greatest changes occur in the upper 200 dbar (NADW; Lee et al., 2003; Sabine et al., 2004). However, a in the Subantarctic Zone (SAZ), where a maximum in- recent Atlantic Basin inventory analysis indicates that in the crease of 37 µmol kg−1 is found. South Atlantic Central Wa- past decade the South Atlantic has been more effective at ter (SACW) experienced the highest rate of increase in Cant, sequestering Cant (Wanninkhof et al., 2010) than the North at 0.99 ± 0.14 µmol kg−1 yr−1, resulting in a maximum rate Atlantic. These authors calculated a rate of increase in the − of decrease in pH of 0.0016 yr−1. The highest rates of acidi- North Atlantic inventory of 1.9 Pg C decade 1, whereas the −1 fication relative to 1Cant, however, were found in Subantarc- South Atlantic inventory grew at a rate of 3.0 Pg C decade . tic Mode Water (SAMW) and Antarctic Intermediate Water Calculations by Ríos et al. (2012) indicate that the south- (AAIW). The low buffering capacity of SAMW and AAIW western Atlantic Ocean dominates the South Atlantic sink −1 combined with their relatively high rates of Cant increase of Cant, with a storage rate of 0.25 ± 0.035 Pg C decade . of 0.53 ± 0.11 and 0.36 ± 0.06 µmol kg−1 yr−1, respectively, Quantifying the exact rate of increase in anthropogenic car- has lead to rapid acidification in the SAZ, and will continue bon in ocean waters is inherently problematic due to the to do so whilst simultaneously reducing the chemical buffer- highly variable nature of dissolved inorganic carbon (DIC) ing capacity of this significant CO2 sink. within the ocean and the relatively small fraction of total DIC that the anthropogenic component represents (∼ 3 %; Ríos et al., 2010). In the past decade, a number of methods for calcu- lating the increase in Cant (1Cant/ between reoccupation of 0 ocean transects have been developed (TrOCA, φCT, eMLR). Despite the differing approaches and assumptions, there is overall coherence in the determinations of the anthropogenic Published by Copernicus Publications on behalf of the European Geosciences Union. 1388 L. A. Salt et al.: Rapid acidification of mode and intermediate waters component of inorganic carbon in the Atlantic Ocean (Lee et ical effects. The greater sensitivity of some water masses to al., 2003; Vázquez-Rodríguez et al., 2009a; Peng and Wan- acidification has been well documented by González-Dávila ninkhof, 2010; Wanninkhof et al., 2010). et al. (2011) through the application of the buffering fac- The southwestern Atlantic has been occupied several times tors described by Egleston et al. (2010). González-Dávila et over the past 20 years, and several techniques to determine al. (2011) highlighted waters originating at high latitudes as Cant have been applied to the WOCE ’94 A17 transect by particularly sensitive to increases in the concentration of dis- Ríos et al. (2010). These methods included 1C* (Gruber solved CO2 ([CO2 (aq)]), in particular Antarctic Intermediate et al., 1996), TrOCA (Tracer combining Oxygen, inorganic Water (AAIW) and upper Circumpolar Deep Water (uCDW) ◦ Carbon and total Alkalinity; Touratier et al., 2007), φCT due to low ratios of total alkalinity (AT/ to DIC. (Vázquez-Rodríguez et al., 2009a), and TTD (transit time A number of the biological consequences of ocean acidi- distributions; Waugh et al., 2006) and showed general con- fication are related to the changes in carbonate, and thus cal- formity in the distribution of Cant. The presence of the west- cium carbonate (CaCO3/, ion concentration. Carbonate ions ern boundary current in the South Atlantic Ocean means that are used by marine calcifying organisms to form both vari- the Cant signal penetrates deeper and is larger in the west- eties of calcium carbonate: aragonite (e.g. by pteropods) and ern half of the basin compared to the eastern half (Wan- calcite (e.g. by coccolithophores and foraminifera). Arag- ninkhof et al., 2010; Ríos et al., 2010; Vázquez-Rodríguez onite is the less metastable form of CaCO3 resulting in a et al., 2009a). Similarly, Murata et al. (2008) show that the saturation horizon (Ar D1) approximately 2 km shallower Cant signal in Subantarctic Mode Water (SAMW) can be than that of calcite in the South Atlantic Ocean, below which −1 ◦ ∼ 7 µmol kg higher west of 15 W compared to the east. depth the CaCO3 present will be in dissolved form. A number Mode and intermediate water formation constitute a major of experiments have observed shell dissolution in pteropods pathway of Cant into the South Atlantic Ocean interior (Mc- incubated at elevated partial pressure of CO2 (pCO2/ (Orr et Neil et al., 2001; Sabine et al., 2004). The SAMW is formed al., 2005; Lischka et al., 2011) associated with a lowering of in the Subantarctic Zone (SAZ), between the Subtropical the aragonite saturation state. Recently similar results have Front (STF) and Subantarctic Front (SAF), where a calcu- been observed in situ in the Southern Ocean (Bednaršek et −1 lated anthropogenic CO2 uptake of 0.07–0.08 PgC yr oc- al., 2012), indicating that species are already being affected curs (Sabine et al., 1999; McNeil et al., 2001). A total CO2 by Cant accumulation. Organisms that use aragonite are thus −1 2− sink of 1.1 Pg C yr was calculated by McNeil et al. (2007) much more vulnerable to decreases in [CO3 ] driven from for the SAZ, making it the largest CO2 sink in the Southern the surface increase in [CO2]. Ocean and a significant sink for anthropogenic atmospheric This study examines the increase in Cant in the southwest- CO2. ern Atlantic Ocean between two occupations of the WOCE The increase in DIC that results from the uptake of an- A17 line, which took place in 1994 and 2010/2011. We cal- thropogenic CO2 from the atmosphere leads to increasing culate the changes in Cant (1Cant/ in the different water proton, bicarbonate ion and carbon dioxide concentrations masses and subsequently examine the pH changes driven C − ([H ], [HCO3 ], [CO2]) and decreasing carbonate concen- by the invasion of anthropogenic carbon between WOCE 2− ‘94 A17 and GEOTRACES-NL (2010/2011). These results trations ([CO3 ]), a process referred to as ocean acidifica- tion. Sabine et al. (2004) state that approximately 50 % of are furthermore put into context with regard to the differing the total amount of Cant in the world’s oceans resides in the buffering capacities of individual water masses. upper 400 m. The associated decrease in pH has been cal- culated as 0.1 pH units in the surface ocean relative to pre- 2 Data industrial times (Orr et al., 2005) and is ongoing.