Marine Carbon Cycle

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Deploying our McLane in-situ pump off the fantail of the R/V Sproul, about 200km west of San Diego. This is how we filter large volumes of water at depth to collect particulate organic matter.

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 isotopic tracers. These  include H and S isotope measurements of individual compounds and bulk organic materials, micro-scale C isotope analyses of particulates via the moving-wire device, and clumped-isotope analysis. All these different analyses help constrain the origins of organic matter in different ways.

This research constitutes one of the two main efforts that frequently get us out into the field (the other being work on ancient rocks) to collect marine particulates and sediments. Most recently, in 2014, we spent 6 weeks in the Santa Barbara Basin studying natural methane vents on the seafloor, and collecting sediments and porewaters for our work on incorporation of S into organic matter. If you are interested in going to sea as part of your research, this is a good area to pursue!

Recent papers on this subject:

Hansman RL and Sessions AL (2015) Measuring the in situ carbon isotopic composition of distinct marine plankton populations sorted by flow cytometry. Limnology and Oceanography: Methods, doi:10.1002/lom3.10073.

Bennett SA, Hansman RL, Sessions AL, Nakamura K, and Edwards KJ (2011) Tracing iron-fueled microbial carbon production within the hydrothermal plume at the Loihi seamount. Geochimica et Cosmochimica Acta 75, 5526-5539.

Redmond MC, Valentine DL, Sessions AL (2010) Identification of novel methane-, ethane-, and propane-oxidizing bacteria at marine hydrocarbon seeps identified by stable isotope probing. Applied and Environmental Microbiology 76, 6412-6422.

Sauer PE, Schimmelmann A, Sessions AL, and Topalov K (2009). Simplified batch equilibration for D/H determination of non-exchangeable hydrogen in solid organic material. Rapid Communications in Mass Spectrometry 23, 949-956.

Li C, Sessions AL, Kinnamen F, Valentine DL (2009). Hydrogen-isotopic variability in lipids from Santa Barbara Basin sediments. Geochimica et Cosmochimica Acta 73, 4803-4823.

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.

Eek KM, Sessions AL and Lies DP (2006). Carbon-isotopic analysis of microbial cells sorted by flow cytometry. Geobiology 5, 85-95.

Copyright 2015 by Alex Sessions. All rights reserved.