| Summary: | 2nd International Ocean Research Conference, One planet one ocean, 17-21 November 2014, Barcelona, Spain 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). The resulting Cant oceanic invasion affects the chemical balances of the CO2 system in seawater and translates into a decrease of surface ocean pH and a lowering of the calcium carbonate (CaCO3) saturation states. The magnitude of these changes has important implications for many calcifying organisms. Establishing the rate at which they occur is important to study the fate of marine biota, but also to understand the future diminishing capacity of the oceans to uptake atmospheric CO2. However, in situ measurements documenting the evolution of ocean pH and CaCO3 saturation over time are still limited, while they are the only way to fully constrain the extent and implications of acidification. Here we present acidification observations and the associated changes in CaCO3 saturation between 1992 and 2011 along a repeat transect at 24.5°N in the North Atlantic, together with an estimation of the anthropogenic and non-anthropogenic contributions to that signal. Interestingly, the greater pH changes, as large as 0.05 units, were found in the upper ocean below the seasonal thermocline and not in the surface. These translated into changes of -0.3 and -0.4 units for aragonite and calcite saturation states, respectively. The deep ocean showed slight acidification rates in the westernmost flank. At the surface our results are consistent with those reported in fixed Time Series Stations in the North Atlantic Subtropical Gyre Peer Reviewed
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