Atmospheric CO2 concentrations and δ13C over western and eastern Mediterranean basins during summer 2007

During summer 2007, two survey cruises covered large sections of both western and eastern Mediterranean basins. Atmospheric CO2 concentrations were measured with a Siemens Ultramat 5E analyser on both outward and return voyages, and 48 discrete air samples were collected with evacuated Pyrex flasks. The δ13C values of the CO2 from the flask air samples were measured in the laboratory following well-established procedures. CO2 concentrations in the western basin were characterised by marked variations, ranging from about 381 to about 394 ppmv. Peaks of up to 398 and 415 ppmv were occasionally recorded, in particular close to the Strait of Gibraltar. The δ13C values were very homogeneous, with a mean value of −9.70 ± 0.22‰, in contrast with the large δ13C fluctuations expected in this basin. The eastern basin, on average, showed far lower CO2 concentrations than those recorded in the western one. The overall range was between about 375 and 398 ppmv, but most values fell between 375 and 388 ppmv. The δ13C values of the discrete air samples were also very homogeneous, with a mean value of −9.37 ± 0.16‰, less negative than that of the western basin. The CO2 concentrations were slightly lower than those measured in December 2007 in the eastern Mediterranean in contrast with the hypothesis of heavy summer pollution in the Mediterranean basins. No quantitative relationship was found between CO2 concentrations and δ13C values. This discrepancy may be the effect of the large biospheric sink of CO2 on land and of the plant isotopic discrimination against 13C.

► Atmospheric CO2 concentrations were measured in both the Mediterranean basins.
► CO2 concentrations in the western basin were higher than those in the eastern one.
► δ13C(CO2) from the western basin are more negative than those from the eastern one.
► No quantitative relationship is apparent between CO2 concentrations and δ13C values.
► The biospheric sink of CO2 on land can be responsible for this discrepancy.