Despite decades of research, the molecular compositions of marine particulate and dissolved organic matter (POM and DOM) remain uncertain. Detailed chromatographic analyses are able to identify up to 80% of the constituent molecules in POM from surface waters, but less than 20% of POM from particles after they have fallen into abyssal waters and sediments. Different analytical methods have also thus far produced disparate, and sometimes incompatible, results concerning factors affecting preservation or compositional changes during sinking. The composition of DOM is better constrained, but the primarily NMR-based studies have not been tested by independent methods. In this project, researchers from California Institute of Technology and from University of Illinois will apply existing methods for hydrogen-isotopic analysis to the study of marine POM and DOM in order to help constrain the biochemical composition of these materials. Hydrogen-isotopic analyses will place three new kinds of constraints on the molecular composition of these materials. The elemental ratio of hydrogen to carbon (H/C) in demineralized organic materials and selected biochemical fractions should allow distinction of fresh biomolecules (H/C > 1.5) versus kerogen derived from sediments and black carbon (H/C < 1.0). Second, the fraction of organic H that is susceptible to rapid isotopic exchange in water represents primarily O- and N-bound H, and is much higher in amino acids and carbohydrates (30-50%) than in lipids and hydrocarbons (<5%). Third, the isotopic composition of non-exchangeable H ( 8 Dn) in biomolecules differs substantially between lipids (typically Dn < -150% ), petroleum hydrocarbons (~-100%), and amino acids and carbohydrates (typically > -50%). The hydrogen data will be complemented by a suite of ancillary analyses such as GCMS, pyrolysis-GCMS, SEM, and quantification of organic sulfur. Samples will be examined from a variety of locations such as the San Pedro Basin (coastal CA), Station M (eastern abyssal Pacific), and other locations around the world. Samples will be acquired during a 5-day research cruise, as well as via collaborations and piggy-backing on other cruises. This research will measure, for the first time, D/H fractionations between biochemical classes in both marine phytoplankton and heterotrophic bacteria, and will examine fractionations accompanying the degradation of algal biomass, both potentially useful for carbon-cycle studies. The PIs will explore the use of exchangeability profiles (i.e., the fraction of exchangeable H as a function of time or temperature) to characterize the compactness and cross-linking of tertiary structure in organic macromolecules, a property that should also be useful to biochemists or polymer chemists.
Among the broader impacts is the transfer of H-isotopic analytical methods to marine applications. The researchers will also develop more automated and sensitive methods for measuring Dn using elemental analyzer technology, an achievement that will have important benefits for constructing high-resolution paleoclimate records based on organic D/H. The project will also provide a range of educational opportunities for undergraduate and graduate students at two major research universities. It will provide the basis for at least one Ph.D. dissertation and two undergraduate summer internships, and will provide an introduction to chemical oceanography for a high school volunteer.
