Acidification observations in a transoceanic line of the North Atlantic Ocean

Abstract

Ocean acidification is directly related to increasing atmospheric CO2 levels due to human activities and the active role of the global ocean as a sink for this anthropogenic CO2 (Cant). Once in solution, Cant affects the chemical balances of the CO2 system in seawater and translates in a decrease in ocean pH. In addition to the more classical studies on the estimation of the anthropogenic carbon account in the oceans, establishing the rate at which ocean acidification is taking place is of crucial importance when studying how the carbon cycle will evolve under a climate change scenario. The magnitude of this change is critical, for instance, to determine the fate of many marine organisms, particularly those that produce CaCO3 and are also keystone in ecosystems and/or food webs (pteropods, coccolithophorids, foraminifera and corals). Oxidation of organic matter, which is mediated by microorganisms, also lowers pH by adding CO2 into seawater. Past reported pH changes of seawater and future pH projections proposed from Ocean General Circulation Models are important to better understand temporal variability of pH. However, in situ measurements documenting the evolution of ocean pH over time are still limited, while they are the only practical tool to fully constrain the extent of acidification. Here we present acidification observations between 1992 and 2011 along a repeat transoceanic line at 24.5°N in the North Atlantic Ocean, together with anthropogenic and non-anthropogenic contributions to that signal. pH changes were found to be essentially zero below 1000 dbar while in the upper ocean, significant pH decreases as large as 0.05 units were observed throughout the first 800 dbar. In general, anthropogenic and non-anthropogenic contributions to this change were estimated to be of similar magnitude. However, the anthropogenic weight was found to be significantly enhanced in the westernmost flank, probably related to the Deep Western Boundary Current influence. In the surface, our observed pH changes are consistent with those reported at BATS and ESTOC Time Series, which are located at both sides of the North Atlantic Subtropical Gyre