The Suess Effect in the Eastern Mediterranean Basin 1988-2010

Nearly a third of the anthropogenic CO₂ (C_ant) added to the atmosphere is sequestered by the ocean's surface. Marine sequestering of CO₂ is not globally uniform, and the variability is particularly strong in marginal seas and coastal waters. Several studies estimated large C_ant inventory in the Mediterranean deep water, which act as source for intermediate depths in the Atlantic Ocean predicting a significant reduction in pH. Warm and alkaline upper water column of the Eastern Mediterranean (EM) are susceptible to absorb CO₂ from the atmosphere, which is transported to the deep water by active overturning circulation. δ¹³C of the Dissolved Inorganic Carbon (δ¹³C_DIC) provides a measure to quantify the penetration of isotopically light C_ant , however, knowledge on the distribution of δ¹³C_DIC, is currently lacking. We conducted a survey of δ¹³C_DIC and carbonate system properties during 2009-2010 along the Southeast-Med (SE-Med) water column and compare the δ¹³C_DIC results to the cruise conducted in 1988 [1]. The vertical profiles of δ¹³C_DIC during 2009-2010 differ significantly from the uniform 1988 profiles. Comparing the average profiles of 1988 and 2010 yields a δ¹³C_DIC temporal difference (Δ¹³C_DIC ‰ yr^-1), propagating to depth of about 700m. A Δ¹³C_DIC depletion rate of -0.13±0.03‰ and -0.11±0.03‰ decade^-1 was found for surface and intermediate waters (200-400m), respectively. The surface Δ¹³C_DIC change, representing only ~50% of the atmospheric change (-0.26‰ decade^-1 fully equilibrated mixed layer). A mean C_ant accumulation rate of 0.38±0.12mol C m^-2 yr^-1 for the upper 700 m over the last 20 years was estimated, based on the mean depth-integrated δ¹³C_DIC profile. We calculate the seasonal pCO₂ distribution in this region, and estimate a nearly annual super-saturation pCO₂, indicating relatively moderate CO₂ sink of the surface water.

[1] Pierre (1999) Marine Geol. 153, 41–55.