HR: 16:15h
AN: B52F-02    [PDF]
TI: Carbon-Isotopic Analysis of Individual Pigments by HPLC-Moving Wire-IRMS
AU: * Sessions, A L
EM: als@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences Mail Code 100-23, Pasadena, CA 91125 United States
AU: Keely, B J
EM: bjk1@york.ac.uk
AF: University of York, Department of Chemistry Heslington, York, YO10 5DD United Kingdom
AU: Hayes, J M
EM: jhayes@whoi.edu
AF: Woods Hole Oceanographic Institution, Department of Geology and Geophysics MS#4, Woods Hole, MA 02543 United States
AB: We have developed a method for directly analyzing the carbon isotope ratios of individual pigments, including chlorophyll (chl) and its derivatives, by coupling a high-performance liquid chromatograph (HPLC) to an isotope-ratio mass spectrometer (IRMS) via a novel `moving-wire' interface. Pigments were separated on a reversed-phase C18 column, using a binary gradient modified from Airs et al. (2001, J. Chrom. A 917, 167-177). The HPLC effluent was dried onto a continuously-spooling nickel wire, and the involatile sample residue was combusted to CO$_{2}$ and transferred to the IRMS for isotopic analysis. Replicate analyses of a standard solution yield precision for $delta^{13}$C of better than 0.2$\permil$ for injections containing ~5 $\mu$g of chl-a. A five-fold improvement in sensitivity should be attainable using capillary HPLC to further reduce solvent volumes. The biomarker potential of tetrapyrrole pigments, combined with the geochemical information recorded by isotopic compositions, makes this combination a potent tool for biogeochemical studies. As a demonstration, we analyzed chlorophyll degradation products in sediments from a lake and a salina. First, compounds derived from bacteriochlorophylls (bchl)-c and -d were extracted from sediment cores taken at Kirisjes Lake (Larsmann Hills, Antarctica). These pigments are products of green sulfur bacteria and indicate the presence of an anoxic photic zone. The $\delta^{13}$C values of bchl-related compounds are near -25$\permil$. Using published fractionations for {\it Chlorobium} species to extrapolate, dissolved CO$_{2}$ in Kirisjes Lake probably had a $\delta^{13}$C value of -12 to -21$\permil$ and was strongly influenced by the recycling of organic carbon, possibly including methane. Second, compounds derived from chl-a, bchl-c, and bchl-d were isolated from sediments taken below a living microbial mat in the hypersaline les Salines de la Trinital (South Catalonia, Spain). The sediments contain visible remnants of past microbial mats and pigment distributions that indicate the major primary producers have been cyanobacteria and green bacteria. Components derived from chl-{\it a} have $\delta^{13}$C values of about -9$\permil$, and are derived mainly from cyanobacteria that lived near the surface of the mat. We estimate that dissolved CO$_{2}$ used by those cyanobacteria had $\delta^{13}$C near +11$\permil$. Such a strongly enriched value would likely result from the drawdown of dissolved CO$_{2}$ by vigorous photosynthesis. In contrast, components derived from bchl-c and bchl-d have $\delta^{13}$C values ranging from -15 to -19$\permil$, indicating dissolved CO$_{2}$ with $\delta^{13}$C of -2 to -12$\permil$ and again suggesting a strong role for recycled organic carbon. These differing estimates of CO$_{2}$ isotopic composition likely reflect the different microhabitats of cyanobacteria (aerobic mat surface) versus green sulfur bacteria (lower, anoxic levels) and can be used to infer the biogeochemical structure of the mat community.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 4802 Anoxic environments
DE: 4808 Chemical tracers
DE: 4894 Instruments and techniques
SC: Biogeosciences [B]
MN: 2003 Fall Meeting