Abstract. The accelerated rate of increase in the atmospheric carbon dioxide (CO2) and the substantial fraction of anthropogenic CO2 emissions absorbed by the oceans are affecting the anthropocenic properties of seawater. Long-term time series are a powerful tool for investigating any change in ocean bio-geochemistry and its effects on the carbon cycle. We have evaluated the ESTOC (European Station for Time series in the Ocean at the Canary islands) observations of measured pH (total scale at 25 °C) and total alkalinity plus computed total dissolved inorganic carbon CO2 concentration (CT) from 1995 to 2004 for surface and deep waters, by following all changes in response to increasing atmospheric carbon dioxide. The experimental values for the partial surface pressure of CO2 from 1995 to 2008 were also taken into consideration. The data were treated to better understand the fundamental processes controlling vertical distributions in the Eastern North Atlantic Ocean and the accumulation of anthropogenic CO2, CANT. CT at constant salinity, NCT, increased at a rate of 1 μmol kg−1 yr−1 in the first 200 m, linked to an fCO2 increase of 1.7±0.7 μatm yr−1 in both the atmosphere and the ocean. Consequently, the ESTOC site has also become more acidic, −0.0018±0.0003 units yr−1 over the first 100 m, whereas the carbonate ion concentrations and CaCO3 saturation states have also decreased over time. The rate of change is to be observed over the first 1000 m, where at 300, 600, and 1000 m the NCT increases at a rate of 0.69, 0.61 and 0.48 μmol kg−1 yr−1, respectively. The vertical distribution of the carbonate system variables are affected by the water mass structure and, to a different extent, controlled by the production/decomposition of organic matter, the formation/dissolution of carbonates, and differences in their respective pre-formed values. At 3000 m, 30% of the inorganic carbon production is related to the dissolution of calcium carbonate, with a total of 35% at the bottom. The total column inventory of anthropogenic CO2 for the decade was 66±3 mol m−2. A model fitting indicated that the column inventory of CANT increased from 61.7 mol m−2 in the year 1994 to 70.2 mol m−2 in 2004. The ESTOC site is presented by way of a reference site to follow CANT changes in the North Atlantic Sub-tropical gyre.
Sequestration of atmospheric carbon dioxide as inorganic carbon in the unsaturated zone under semi-arid forests
