Photochemical processes drive thermal responses of dissolved organic matter in the dark ocean

How dissolved organic matter (DOM) responds to climate warming is critical for understanding its effectiveness as a natural climate solution. Here, we use a highly resolved dataset of 821 DOM samples covering the surface waters to the deep Atlantic, Southern, and Pacific oceans to examine molecular-level responses to warming water temperatures, i.e. their thermal responses. In general, the strength and diversity of thermal responses among individual molecules both decline towards the deep waters, but they show decreasing and increasing trends with more recalcitrant molecules in concentration, respectively. Their contrasting trends concur with the more important role of photochemical processes in explaining the diversity of thermal responses than the strength. By projecting global ocean thermal responses from 1950-2020, we predict increases in the diversity are unexpectedly largest at deeper depths (> 1,000 m). Such increases could elevate recalcitrant deep-ocean carbon sink by approximately 10 Tg C yr ^-1 which accounts for > 5% of the carbon flux survived to the deep ocean. Our findings highlight the importance of photochemical legacies in driving DOM thermal responses and further help predict the future oceanic carbon sink under global change.