Offsets and inputs of natural uranium isotopes in the Mediterranean Sea

High precision measurements of natural uranium isotopes in the Atlantic Ocean, Mediterranean Sea,
and Black Sea reveal isotopic makeups which differ significantly from the well-known oceanic
composition. In the Mediterranean, water masses are strongly differentiated to the extent that they
are able to be fingerprinted on the basis of δ²³⁴U. Mediterranean deep water masses show the
highest enrichment, with an offset with respect to oceanic δ²³⁴U values of just over 1 ‰. The Black
Sea shows an even higher offset of up to ~40 ‰.
This offset provides an opportunity to look into the as of yet largely unstudied uranium inputs to the
Mediterranean, in particular rivers and submarine groundwater discharge (SGD), which are thought
to play key roles in uranium input to the global ocean. A simple box model, incorporating the
Mediterranean and Black Sea data from this study is constructed to provide a first estimate of the U
concentration and δ²³⁴U signature of rivers and SGD necessary for this offset to arise. These
estimates are then compared with new measurements of various coastal and submarine springs from
along the French Mediterranean Coast as well as with existing riverine data exists to speculate on
which inputs may be most responsible for this offset.