The oceanic water column has not been a traditional object for geochemical research, and dissolved organic matter (DOM) used to be an exotic topic at geochemical conferences. This is no longer the case as the DOM pool is now recognized as paralleling the sedimentary record as an information-rich set of tracers. These molecules carry the signatures of their source and subsequent journey through the environment. Most of the photosynthetic production in the ocean is consumed by bacteria. Bacteria can only assimilate dissolved molecules, and DOM is therefore the main mediator for the flux of energy in the ocean. A minor fraction of DOM, however, escapes microbial decomposition. This fraction of DOM has accumulated to one of the largest organic carbon pools on Earth. DOM contains as much carbon as all living biomass on the continents and oceans combined and a similar amount of carbon as atmospheric CO2. Due to its great size, even minor changes in the DOM pool will impact global biogeochemical cycles and the heat budget of the Earth. The accumulation of DOM in the ocean over several millennia is enigmatic and contradicts established paradigms in geochemistry for organic matter stabilization. This chapter focuses on major concepts, the fundamental challenges, and the future directions of this fast-growing field of research. © 2014 Elsevier Ltd. All rights reserved.
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