The CO<sub>2</sub> system in the Mediterranean Sea: a basin wide perspective

Abstract

Abstract. The Mediterranean Sea (MedSea) is considered a "laboratory basin" being an ocean in miniature, suffering dramatic changes in its oceanographic and biogeochemical conditions derived from natural and anthropogenic forces. Moreover, the MedSea is prone to absorb and store anthropogenic carbon due to the particular CO 2 chemistry and the active overturning circulation. Despite this, water column CO 2 measurements covering the whole basin are scarce. This work aims to be a base-line for future studies about the CO 2 system space-time variability in the MedSea combining historic and modern CO 2 cruises in the whole area. Here we provide an extensive vertical and longitudinal description of the CO 2 system variables (total alkalinity -TA, dissolved inorganic carbon -DIC and pH) along an East-West transect and across the Sardinia-Sicily passage in the MedSea from two oceanographic cruises conducted in 2011 measuring CO 2 variables in a coordinated fashion, the RV Meteor M84/3 and the RV Urania EuroFleets 11, respectively. In this sense, we provide full-depth and length CO 2 distributions across the MedSea, and property-property plots showing in each sub-basin post-Eastern Mediterranean Transient (EMT) situation with regard to TA, DIC and pH.The over-determined CO 2 system in 2011 allowed performing the first internal consistency analysis for the particularly warm, high salinity and alkalinity MedSea waters. The CO 2 constants by Mehrbach et al. (1973) refitted by Dickson and Millero (1987) are recommended. The sensitivity of the CO 2 system to the atmospheric CO 2 increase, DIC and/or TA changes is evaluated by means of the Revelle and buffer factors.

The role of ocean acidification in <i>Emiliania huxleyi</i> coccolith thinning in the Mediterranean Sea

“…The Mediterranean Sea is known to be highly supersaturated with respect to carbonate throughout the entire basin from the surface to depth (Álvarez et al, 2014;Schneider et al, 2010Schneider et al, , 2007. At the same time, it serves as a sink for anthropogenic CO 2 , which is absorbed in the surface layers and transported into the deeper parts of the basin through mixing (Ait-Ameur and Goyet, 2006;Bethoux et al, 2005).…”

“…Coccolith calcification is expressed by coccolith weight and increases under conditions that are favorable for coccolithophore calcification. Therefore it is an indicator for the calcification rate of the coccolithophore cell (Bach et al, 2012). A global study on the variations of coccolith weight over the natural range of CO 2 concentrations in the world's oceans has shown that coccolith weight decreases with increasing CO 2 and this effect can also be traced back in sedimentary archives of the last glaciation under natural CO 2 variations (Beaufort et al, 2011).…”

Measuring pH variability using an experimental sensor on an underwater glider

“…In contrast, the pH in the neighbouring North Atlantic Ocean decreased by just 0.0017 a −1 associated with an increase in c(DIC) of around 1.4 µmol kg −1 a −1 and a temperature rise of 0.01 • C a −1 (Bates et al, 2012). The greater potential of the Mediterranean Sea to store anthropogenic carbon can be explained by its higher alkalinity, warmer temperatures, and thus lower Revelle factor (Álvarez et al, 2014;Touratier and Goyet, 2011) when compared with other oceans, such as the North Atlantic.…”

“…The c(DIC) and A T were greatest at depths below 250 m with lower values seen closer to the surface (Fig. 5a-b), which is typical of the northwest Mediterranean Sea (Copin-Montégut and Bégovic, 2002;Álvarez et al, 2014). The higher values of A T and c(DIC) at depth and in the east support the notion that this is LIW, as this water mass has previously been identified as having an A T of around 2590 µmol kg −1 and a c(DIC) of roughly 2330 µmol kg −1 (Álvarez et al, 2014), coinciding with the warmer saltier waters.…”

“…Temperature and salinity were obtained from the Sea-Bird CTD sensor on the ship rosette sampler, and the seawater equilibrium constants presented by Mehrbach et al (1973) were used as refitted by Dickson and Millero (1987), which has been recommended by previous studies (e.g. the CARINA data synthesis project) for the Mediterranean Sea (Álvarez et al, 2014;Key et al, 2010). The sulfate constant described by Dickson (1990) and the parameterisation of total borate presented by Uppström (1974) were used.…”