Tracing Marine Organic Matter

Sunset over Santa Barbara Basin. The photo was taken during a research cruise in 2005 where we collected marine particulate organic matter for isotopic studies.

One of the important outstanding questions regarding the modern carbon cycle concerns the origins of particulate organic matter in the oceans. When scientists collect and analyze these organic particles from the surface oceans, they are able to identify more than 90% of the molecular components that make up the particles. These include lipids, amino acids, sugars, and other common components of marine phytoplankton. However, when they collect organic particles from deep in the oceans (several thousand meters), the situation changes markedly: less than half of the molecular components can be identified, with the majority of the material being polymeric, insoluble "stuff" of uncertain origins. It is affectionately termed MUC (Molecularly Uncharacterized Carbon). What is it, and where did it come from? Various hypotheses include chemical alteration and polymerization of surface-derived material, preferential preservation of rare components that are present at the surface, addition of new material from bacteria at depth, and adsorption of refractory compounds from the dissolved phase. A variety of different analytical techniques have been used to try to characterize this MUC, and in general each different technique has yielded a different answer. There is an inescapable analogy here to the old fable about the blind men examining different parts of an elephant, who each reach different conclusions about its nature.

In an effort to help constrain the origins of this MUC, we have been developing and applying new tools for the analysis of D/H ratios of nonexchangeable, carbon-bound hydrogen in bulk organic fractions. The basic procedure is as follows. First, different biochemical fractions (lipids, amino acids, sugars, nucleic acids, and acid-insoluble residue) are extracted and separated via traditional column chromatography. Next, the D/H ratio of nonexchangeable hydrogen in each fraction is measured by steam equilibrating the sample with waters of differing isotopic composition, following by conventional EA/P/IRMS analysis. This procedure (described here) yields multiple D/H ratios that can be mathematically combined to yield the fraction of non-exchangeable hydrogen and its D/H ratio.

The approach yields useful constraints on the origins of organic matter because different types of biochemical molecules can have very different D/H ratios. In general, lipids tend to be strongly depleted in D, sugars are strongly enriched, and amino acids lie somewhere in the middle. Thus we expect that the D/H ratio of MUC will give us clues as to the biochemical precursors that make up this polymeric material. Thus far we have mostly been working on refining the methodology, but some preliminary data on marine algae suggest that (at least in living organisms) the acid-insoluble fraction looks like a combination of all three types of biochemicals. This work represents the PhD thesis of Ashley Jones, and is a collaboration with Dr. Arndt Schimmelman at Indiana University.

Recent papers on this subject:

Bennett S.A., Hansman R.L., Sessions A.L., Nakamura K., and Edwards K.J. (in press) Tracing iron-fueled microbial carbon production within the hydrothermal plume at the Loihi seamount. Geochimica et Cosmochimica Acta.

Jones A., Sessions A.L., Campbell B.J., Li C., and Valentine D.L. (2008). D/H ratios of fatty acids from marine particulate organic matter in the California Borderland Basins. Organic Geochemistry, 39:5, 485-500.

Copyright 2011 by Alex Sessions. All rights reserved.