Isotopic evidence for biogenic molecular hydrogen production in the AtlanticOcean

[摘要] Oceans are a net source of molecular hydrogen (H₂) to the atmosphere.The production of marine H₂ is assumed to be mainly biological byN₂ fixation, but photochemical pathways are also discussed. We presentmeasurements of mole fraction and isotopic composition of dissolved andatmospheric H₂ from the southern and northern Atlantic between 2008 and2010. In total almost 400 samples were taken during 5 cruises along atransect between Punta Arenas (Chile) and Bremerhaven (Germany), as well asat the coast of Mauritania.

The isotopic source signatures of dissolved H₂ extracted from surfacewater are highly deuterium-depleted and correlate negatively withtemperature, showing δD values of (−629 ± 54) ‰ forwater temperatures at (27 ± 3) °C and(−249 ± 88) ‰ below (19 ± 1) °C. The resultsfor warmer water masses are consistent with the biological production of H₂.This is the first time that marine H₂ excess has been directlyattributed to biological production by isotope measurements. However, theisotope values obtained in the colder water masses indicate that besidepossible biological production, a significant different source should beconsidered.

The atmospheric measurements show distinct differences between bothhemispheres as well as between seasons. Results from the global chemistrytransport model TM5 reproduce the measured H₂ mole fractions andisotopic composition well. The climatological global oceanic emissions fromthe GEMS database are in line with our data and previously published fluxcalculations. The good agreement between measurements and model resultsdemonstrates that both the magnitude and the isotopic signature of the maincomponents of the marine H₂ cycle are in general adequately representedin current atmospheric models despite a proposed source different frombiological production or a substantial underestimation of nitrogen fixationby several authors.

[学科分类] 地球化学与岩石