The refractory component of DOM is expected to be chemically reduced and condensed, containing few heteroatoms. Here we isolate hydrophobic compounds from seawater, with chemical characteristics similar to those described above, in an attempt to determine if they represent the long-lived component of DOM. During three cruises in 2003 and 2004 within the Southern California Bight (SCB), solid phase extracts from several sites were fractionated on a hydrophobic resin using organic solvents. The resulting 5 fractions from each sample site were then chemically (1H-NMR) and isotopically (13C and 14C) characterized. Hydrophobic fractions contained primarily aliphatic NMR resonances while semi-polar fractions contained more heteroatom-substituted carbons and were chemically similar to ultrafiltered DOM. NMR data show that the fractionation technique isolated similar classes of compounds for each fraction regardless of sample site. This suggests that the material isolated with this technique has a similar chemical structure throughout the SCB. The most polar fractions extracted by this method have very similar Δ14C signatures and are more modern than hydrophobic fractions. In addition to being older, the hydrophobic fractions show a more variable Δ14C distribution. Based on this data we find that, for compounds isolated by this method, chemical structure is closely correlated with 14C content. We also investigated seasonal Δ14C variations in the context of total organic carbon (TOC) concentrations. The TOC concentrations increase in the surface waters from the spring to the fall indicating an accumulation of fresh DOM over this time period. Interestingly, the most polar fraction does not show the most significant temporal change in Δ14C. However, we are able to identify a fraction that does accumulate “fresh” compounds during the fall. Our data shows that DOM in surface waters is composed of discrete chemical pools each with unique Δ14C signatures, which cycle on different time scales.
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