Decadal acidification in the water masses of the Atlantic Ocean

Abstract

Global ocean acidification is caused primarily by the ocean's uptake of CO 2 as a consequence of increasing atmospheric CO 2 levels. We present observations of the oceanic decrease in pH at the basin scale (50°S-36°N) for the Atlantic Ocean over two decades . Changes in pH associated with the uptake of anthropogenic CO 2 (ΔpHCant) and with variations caused by biological activity and ocean circulation (ΔpHNat) are evaluated for different water masses. Output from an Institut Pierre Simon Laplace climate model is used to place the results into a longer-term perspective and to elucidate the mechanisms responsible for pH change. The largest decreases in pH (ΔpH) were observed in central, mode, and intermediate waters, T he uptake of anthropogenic CO 2 (Cant) by the ocean has lowered seawater pH since preindustrial times. This largescale and long-term change is referred to as "ocean acidification," a process that has led to changes in seawater carbonate chemistry (1-3) with impacts on the chemical speciation of seawater and biogeochemical cycles. A predominant effect is the decrease of carbonate ions in seawater that impacts calcareous marine organisms (4-6). The uptake of Cant is the main cause for the gradual reduction of seawater pH, but biological, physical, and chemical "natural" changes in the ocean, such as changes in the oxidation of organic matter, impact pH as well (7,8).Several studies have focused on ocean acidification in the last decade. Model-based studies have examined pH changes on a global scale (2, 9), and observation-based studies have focused on time-series stations (10-13) and specific regions such as the North Pacific (7, 14) and North Atlantic (8). However, investigations of basin-wide pH changes throughout the water column from direct measurements are sparse (15), in large part because of a dearth of quality referenced pH measurements.In this study, we present the first (to our knowledge) measurement-based changes in pH along meridional lines in the Atlantic from 50°S-36°N using observations from three cruises: OACES/ CO 2 (1993), CITHER-II (1994), and FICARAM-XV (2013) (Fig. 1A and SI Appendix, Table S1). We investigate the total change in pH and refer to it as acidification. The change in pH is separated into anthropogenic (ΔpHCant) and natural (ΔpHNat) components and is evaluated for the water masses along the section over two decades . Both components are related to Cant uptake and to processes such as the remineralization of organic matter and changes in water mass. The observed pH changes are compared with outputs from an Institut Pierre Simon Laplace (IPSL) climate model to place the observations into context and to elucidate the mechanisms controlling anthropogenic and natural pH changes, which are clearly discerned in climate models. ResultsMore than 4,000 pH measurements referenced to the seawater scale (SWS) at 25°C in the Atlantic Ocean (50°S-36°N) were examined to estimate the changes in oceanic pH between 1993 and 2013. We used pH measurements referenced to the SWS at 25°C thr...

Biogeochemical sensor performance in the SOCCOM profiling float array

“…The lowest pH values correspond to greater depths, where acidification signals are weaker. Offsets for sensor pH measurements near 1000 m are about 0.01 in pH, consistent with observed acidification rates in deep, Southern Ocean waters [ Ríos et al ., ]. The slope of 0.93 for sensor pH data versus GLODAPv2 (Table ) then results because the lowest pH values observed by the floats have been shifted the least by acidification and the highest values near the surface have been shifted the most.…”

Attributing seasonal pH variability in surface ocean waters to governing factors

“…The invasion of anthropogenic carbon dioxide (CO 2 ) from the atmosphere into the ocean has not only attenuated accumulation of CO 2 in the atmosphere and thus global warming but also caused ocean acidification [ Kleypas et al ., ]. Long‐term observations show that ocean pH declines on average about 0.002 units per year in open ocean settings [ Bates et al ., ; Lauvset et al ., ; Ríos et al ., ], consistent with theoretical calculations. Model simulations indicate that ocean pH has already declined by 0.1 units since the start of the industrial time and that surface water pH will decline further by 0.3 to 0.5 units depending on location and emission scenarios [ Orr , ; Bopp et al ., ].…”

Rapid anthropogenic changes in CO2 and pH in the Atlantic Ocean: 2003–2014

“…The overall rate for the GOSHIP‐CLIVAR era was −0.0021 ± 0.0007 yr −1 , −0.0013 ± 0.0008 for the CLIVAR‐WOCE/CLIVAR‐SAVE era, and −0.0018 ± 0.0004 yr −1 for the entire GOSHIP‐WOCE/GOSHIP‐SAVE era. These are in agreement with other studies in the Pacific and Atlantic which range from −0.0014 to −0.0022 yr −1 [ Bates , ; González‐Dávila et al ., ; Byrne et al ., ; Waters et al ., ; McGrath et al ., ; Vázquez‐Rodríguez et al ., ; Lauvset and Gruber , ; Ríos et al ., ; Williams et al ., ]. This is also in agreement with the value −0.0018 yr −1 expected based on thermodynamics and the rate of increase of CO 2 in the atmosphere [ Waters et al ., ].…”