Targeted Isolation of Refractory and Semi-Labile DOM for Detailed Molecular Study: First Results from the Central North Pacific Subtropical Gyre.

Past attempts to isolate marine DOM have focused on optimizing C recoveries, with the primary goal of capturing the entire DOM pool. However, no method has consistently isolated more than 60-70% of total DOC across all depths and water masses. Furthermore, all isolations suffer from varying degrees of compositional selectivity, which fractionates recovered material. Substantial research has shown that both reactivity and ¹⁴C age of DOM is strongly linked to molecular weight. Higher molecular weight (HMW) material is semi-labile (SL) and younger, while lower molecular weight (LMW) material has old ¹⁴C ages at all depths. The HMW-DOM represents actively cycled material, transporting OC and nutrients via advection and remineralization, while LMW-DOM represents refractory DOM (RDOM), the OC storage reservoir of the deep sea. Therefore, total DOM composition may be far less useful to studying DOM processes than these two end members individually.

Here we describe a new approach to large-volume DOM isolation for detailed molecular level study, focusing on independent isolations of age/ reactivity fractions. We used a combination of ultrafiltration and solid phase extraction to selectively isolate SLDOM and RDOM respectively from surface to deep sea in the Central Pacific. Radiocarbon analysis revealed substantial differences in the Δ¹⁴C age range of the two fractions confirming our basic hypothesis based on a simple MW partition. There were significant and consistent offsets in C/N ratio, δ¹⁵N and δ¹³C values between the two fractions at all depths; however, within fractions, these values had little depth-related variation. Average δ¹³C values were the most similar, with a small but consistent offset between fractions. SLDOM had a significantly lower average C/N ratio than the RDOM material at all depths, and the δ¹⁵N values of the two fractions were also significantly different, with heavier values in the RDOM fraction. Overall, the δ¹³C values suggest a similar oligotrophic ocean surface C source for DOM at all depths, while C/N and δ¹⁵N offsets indicate either strong δ¹⁵N fractionation of labile N-containing compounds, or independent sources for SLDOM and RDOM pools. Together, these data suggest the need for targeted compound-specific ¹⁴C measurements to further resolve RDOM and SLDOM sources and cycling.