Conservation of dissolved organic matter molecular composition during mixing of the deep water masses of the northeast Atlantic Ocean

Characterizing the composition of marine dissolved organic matter (DOM) is important for gaining insight into its role in oceanic biogeochemical cycles. Using Fourier transform ion cyclotron resonance mass spectrometry, we analyzed the molecular composition of solid phase extracted (SPE) DOM from the northeast Atlantic to investigate the specificity of the DOM pool of the individual major water masses of the North Atlantic. All 272 measured samples from depths ranging from 87 to 5609 m and latitudes from 24°N to 68°N shared 96% similarity (on a Bray–Curtis scale) in their DOM composition. Small variations between subsurface and deep samples and among latitudinal groupings were identified, but overall, water mass specific SPE-DOM composition was not apparent. A strong correlation between a calculated degradation index and water mass age indicates variability in portions of the DOM pool, and ocean-scale differences were observed between the North Atlantic and deep North Pacific. However, within the deep northeast Atlantic, conservative mixing primarily drives the molecular composition of SPE-DOM.