Molecular Alteration of Marine Dissolved Organic Matter under Experimental Hydrothermal Conditions
Jeffrey Alistair Hawkes1,2, Christian Tobias Hansen3, Tobias Goldhammer4, Wolfgang Bach4,5 and Thorsten Dittmar1, (1)University of Oldenburg, Research Group for Marine Geochemistry (ICBM-MPI Bridging Group), Oldenburg, Germany, (2)Uppsala University, Analytical Chemistry, Chemistry-BMC, Uppsala, Sweden, (3)University of Oldenburg, ICBM, Oldenburg, Germany, (4)MARUM Center for Marine Environmental Research, University Bremen, Germany, (5)University of Bremen, Department of Geosciences, Bremen, Germany
Abstract:
Marine dissolved organic matter (DOM) is a large (660 Pg) pool of reduced carbon that is subject to thermal alteration in hydrothermal systems and sedimentary basins. In natural hydrothermal systems, DOM is almost completely removed, but the mechanism, kinetics and temperature dependence of this removal have not been studied to date. We investigated molecular-level changes to DOM that was solid-phase extracted (SPE-DOM) from the deep ocean of the North Pacific Ocean. This complex molecular mixture was experimentally exposed to temperatures between 100-380 °C over the course of two weeks in artificial seawater, and was then characterized on a molecular level via ultrahigh-resolution mass spectrometry (FTICRMS & Orbitrap). Almost 93% of SPE-DOM was removed by the treatment at 380 °C, and this removal was accompanied by a consistent pattern of SPE-DOM alteration across the temperatures studied, which can likely be extrapolated down to temperatures around 68 °C. Higher molecular weight and more oxygen rich compounds were preferentially degraded, suggesting that decarboxylation and dehydration of carboxylic acid and alcohol groups are the most rapid degradation mechanisms. Nitrogen containing compounds followed the same overall trends as those containing just C, H and O up to 300 °C. Above this temperature, the most highly degraded samples contained very little of the original character of marine DOM, instead being mainly composed of very low intensity N- and S- containing molecules with a high H:C ratio (>1.5). Our experiments were conducted without a sedimentary or mineral phase, and demonstrate that profound molecular alteration and almost complete removal of marine SPE-DOM requires nothing more than heating in a seawater matrix.
