Novel Method to Separate Marine Dissolved Organic Matter Based on Thermal Reactivity

Marine dissolved organic matter (DOM) is a vastly complex mixture of molecules with diverse chemistries, sources, and ages. Ramped pyrolysis/oxidation (RPO) is a novel method that allows unbiased analysis of the entire extracted sample. Here we used RPO to quantitatively oxidize solid phase extracted (SPE) DOM into CO₂, measure the oxidation reaction rate as a function temperature (thermogram), and collect the CO₂ in a contiguous series of fractions for subsequent carbon isotope analyses. The resulting thermal information, together with radiocarbon and stable carbon isotope measurements, yield unique information about how DOM differs across a range of depths (45-4000m) in the Eastern North Pacific. Samples from one of the sites, Station M (34°50’N, 123°00’W), were divided in half and subjected to acid hydrolysis to remove the more ‘modern’ fraction, dominated by amino acids and carbohydrates, prior to RPO. Hydrolysis made all thermograms more similar and decreased the radiocarbon age of the shallower samples, suggesting that what was left behind is the older, more degraded DOM. Stable carbon isotopes show a relationship with oxidation temperature, with lower temperature fractions being significantly more depleted in ¹³C. Radiocarbon data suggest a surprising homogeneity within samples, consistent across a range of bulk ages, though from surface to deep we see the expected age gradient and good correspondence with measurements of bulk DOC from similar depths. Differences in thermograms across a range of depths reveal variability within the DOM pool and allow us to make novel connections between chemical properties and isotopic composition.