The stable isotopic signature of biologically produced molecular hydrogen (H2

Abstract. Biologically produced molecular hydrogen (H2) is characterised by a very strong depletion in deuterium. Al-though the biological source to the atmosphere is small com-pared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H2. Large uncertainties exist in the quantification of the individ-ual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of δD from the various H2 sources are scarce and for biologically produced H2 only very few measure-ments exist. Here the first systematic study of the isotopic composition of biologically produced H2 is presented. In a first set of ex-periments, we investigated δD of H2 produced in a bioga