Carbon isotope measurements reveal unexpected cycling of dissolved organic matter in the deep Mediterranean Sea

One of the most intriguing aspects of dissolved organic carbon (DOC) dynamics in the Mediterranean Sea (Med Sea) is that in the intermediate and deep waters, DOC concentrations are equal to the lowest values found in the deep Atlantic and Pacific (36-42 μM). The very low DOC values in the deep Med Sea were unexpected since the renewal time of deep waters in the basin is only 20-126 years. Over this short timescale, we expected just a very small, nearly undetectable, fraction of refractory DOC (RDOC) to be removed. The first DOC isotope data show that DOC in Med Sea deep water is more depleted in both Δ14C and δ13C than in the deep Atlantic Ocean, with an estimated age of 4500-5100 years. These data suggest that at least 10%, and up to 45%, of the Atlantic RDOC entering the Med Sea is removed and replaced by isotopically lighter DOC in less than 126 years. Potential allocthonous sources include: fossil methane and methane derived DOC seeps from sediments, anthropogenic combustion products and terrestrial organic matter delivered by the atmosphere, rivers, and groundwaters. Using current information, it is not possible to quantify the relative contribution of these potential sources. Based on estimated flux and isotopic value, atmospheric input of soluble organic carbon from soils or combustion products as well as DOC from groundwater are the most likely sources of allochthonous DOC. Our results suggest that DOC cycling in the deep Med Sea is dynamic and support the idea that a substantial fraction (up to 45%) of what has traditionally been defined as "refractory" DOC imported from the Atlantic Ocean, can be removed on temporal scales of <126 y, thereby opening intriguing questions about deep sea DOC cycling.