45 / 2023-07-24 15:08:04

Re-examining the impact of photochemistry on terrestrial organic matter in global biogeochemical cycles

global carbon cycle, photochemistry, terrestrial DOM

Photochemistry of dissolved organic matter (DOM) is well-known as a highly impactful chemistry process of DOM, being a significant sink for carbon. Based on the current global carbon cycle models, at the maximum, only half of the terrestrial DOM transported to the ocean is completely mineralized to inorganic carbon in the river and estuarine zones. However, the other half of terrestrial DOM has not been well identified in seawater and sediments by lignin biomarkers. Results from our ongoing study indicate that a significant portion of terrestrial DOM been molecularly rearranged, potentially by reactive oxygen species during photochemical decomposition, to form molecularly unrecognizable organic molecules such as humic substances. These previously unknown organic compounds are now partly revealed by advanced NMR and ultrahigh resolution mass spectrometry. If these molecularly rearranged organic molecules are bio-refractory, they can make a significant contribution to the river, estuarine, and oceanic DOM and particulate materials. Until now, marine sedimentary organic matter is thought to mainly originate from autochthonously-produced organic materials deposited in anoxic to sub-oxic environments, although it is arguable that autochthonous materials are more bio-labile than humic substances to be preserved in sediments. The current study provides a new understanding of the pathway in which the large pool of terrestrial DOM becomes disseminated among the large carbon reservoirs and will impact the general understanding of how carbon, nitrogen, and iron are cycled on both short and long-time scales.