Carbonate chemistry and acidification of the North East Atlantic

Supervisor: Eric Achterberg (National Oceanographic Centre, Southampton)
The increase in atmospheric CO2 concentrations, primarily driven by human resource consumption, is increasing the anthropogenic carbon (Cant) budget of the oceans, and affecting the carbonate system by decreasing the pH and the concentration of carbonate ions (CO32-) in seawater. The acidification process will affect speciation (e.g. metals and nutrients) and biogeochemical cycling, which will affect marine organisms and ecosystem functioning. The research cruise undertaken along the Extended Ellet Line (EEL) from Iceland to Scotland, has allowed us to continue a carbonate chemistry time series for this study region and involved the collection of high quality Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) water column samples with subsequent analysis with an overall analysis precision of 1.07 μmol/kg and 1.03 μmol/kg, respectively. In addition, high spatial resolution surface DIC measurements were undertaken. The analysis of complementary dissolved oxygen and nutrient data as well as ancillary data (potential temperature, salinity) has allowed the determination of the water mass structure across the section, allowing us to assess the DIC and TA signatures for the different water masses. Six different water masses were identified: Surface waters (SW), Sub-Polar Mode Water (SPMW), Intermediate Water (IW), Labrador Sea Water (LSW), Iceland-Scotland Overflow Water (ISOW) and Lower Deep Water (LWD). The analysis of DIC distributions highlighted the importance of biological control on surface and intermediate depth concentrations, as well as the accumulation of anthropogenic carbon in newly formed Labrador Sea mode waters. The highest DIC concentrations (up to 2178 μmol/kg) were found at IW and LSW due to a combination of biologically organic matter decomposition and Cant accumulation. The analysis of 4221 surface DIC samples along the cruise track has allowed the creation of an empirical algorithm to determine DIC surface concentrations in the North East Atlantic, using fluorescence, temperature and salinity underway measurements. The equation created (DICpre. = 433.15 + (-13.61 * Tº) + 51.75 * S + (-66.39 * Fluor.)) explains more than 79 % of the variability in DIC concentrations, therefore forms a robust and strong correlation which can be used to predict surface DIC in the area under investigation. The comparison of the obtained data with the previous year EEL cruises (2011 and 2012) as well as with the historical CARINA database (1993), has shown the trend in seawater acidification of the North East Atlantic. In the 20 year period surface waters pH decreased by 0.0614 units at -0.0031 units yr-1 while the pH across the whole water column has decreased by 0.024 units at a rate of -0.00121 units yr-1. Predictions for the year 2100, assuming constant acidification rates, decreased surface water pH by 0.33 units. The prediction probably underestimates pH decline since anthropogenic CO2 emission are increasing more rapidly. The continuity and extension of high spatial resolution sampling of carbonate chemistry parameters like the EEL is of key interest to help as input parameters in climate modelling and as baseline studies for assessing the actual state of the oceans and implement the corresponding regulations to reduce human impact on the oceans.