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45608-AC2
Organic Matter Preservation in Marine Environments: Insights from a Study of Marine Dissolved Organic Matter - Composition, Age, and Source

Lihini Indira Aluwihare, University of California San Diego, Scripps Institution of Oceanography

Since the writing of this proposal we have had the opportunity to resample the California Current system, and sample both the Delaware River and Estuary, as well as the Eel River Basin. We applied the SPE method described in our original proposal (using a divinyl-benzene resin and six-step solvent elution scheme) to isolate several semi-polar and non-polar fractions of dissolved organic matter (DOM). In addition, we have expanded this isolation method to further separate Fraction B (the most “polar” and the largest of the fractions isolated by the SPE method) into attached lipids and polar material to better address the proposed questions: (1) is there a relationship between chemical composition and radiocarbon age of dissolved organic fractions? (2) is the average age of DOC a result of a broad DOC age continuum or a few discrete ages?

Using each fraction isolated as above, a 2 end-member mixing model was constructed based on proton NMR data and radiocarbon (D14C) data presented in the original proposal. The model results support the hypothesis that DOC consists primarily of two D14C fractions. These two fractions appear to be chemically distinct as well and can be classified as (A) a fraction dominated by alkyl functional groups (“lipids”) that is very depleted in 14C and (B) a 14C-enriched fraction that is extremely oxidized – C:O ~ 1. As such, these results address questions (1) and (2) and suggest (1) that there is a relationship between chemical composition and D14C, (2) that fractions of DOC examined thus far support the presence of discrete D14C groups rather than a broad D14C continuum. The modified method where Fraction B is further processed, using acid hydrolysis, to separate attached lipids from polar biochemicals provides more conclusive evidence in support of the hypothesis that lipid-like material in DOC is depleted in 14C relative to polar compounds. Attached lipids released by the acid hydrolysis of Fraction B are depleted in D14C relative to both the original Fraction B (by up to 200‰) and carbohydrates (by up to 350‰), which dominate the polar fraction. Lipids generated by the hydrolysis are currently being analyzed by GC-TOF MS to identify components that can be further separated for compound-specific D14C measurements. We were recently awarded an NSF-MRI grant to purchase our own instruments so we plan to focus on compound isolation during the upcoming year.

We have also begun to address the third question posed in our proposal - is the old component of DOC derived from terrestrial/riverine environments and if so, is it compositionally distinct? The isolation scheme discussed above was applied to samples collected in the Delaware River (salinity (PSU) = 0) and Estuary (salinity (PSU) = 22), as well as Eel River Basin. Results showed that POC and individual DOC fractions (lipid and sugars isolated from Fraction B) had distinct d13C and D14C signatures in the Delaware River. POC isotope values suggested export from terrestrial systems. Lipids and carbohydrates in DOC (Fraction B) were depleted in d13C relative to their marine counterparts, but they were relatively enriched in d13C relative to POC. We were unable to determine the D14C of POC; however, sugars and lipids were depleted in D14C relative to atmospheric CO2 and bulk Fraction B. At this stage we are unable to identify the source of these dissolved compounds; for example, are they produced in-situ during primary production in rivers (we would have expected a more enriched D14C signature if that was the case) or are these compounds being exported from a terrestrial reservoir that is distinct from POM? In the estuary two of the three carbohydrate fractions appear to be completely replaced as they carry a marine d13C signature and are significantly enriched in 14C (comparable to DIC). The attached lipid fraction also loses its riverine d13C signature and is much more depleted in 14C than its riverine counterpart. The D14C signature of estuarine lipids is reminiscent of values we observe for the lipid fraction in truly marine systems. Therefore, riverine lipids in Fraction B do not appear to be exported into the marine environment. However, the sugar fraction containing glucose retained a river-like d13C and D14C signature suggesting that riverine/terrestrial glucose was being exported into marine environments. A similar observation was made for glucose in the Eel River basin as well. Together, the preliminary riverine/coastal data suggest that some riverine/terrestrial/wetland DOC is exported to marine systems and that these compounds are often slightly depleted in D14C relative to autochthonous, fresh DOC (e.g. -40‰ for glucose versus +40‰ for other carbohydrates in the Delaware Estuary). In addition, while unique terrestrial components such as lignin may be exported into the marine environment, other more common components such as dissolved glucose (likely within a polymer such as cellulose) appear to be exported as well.

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