To date, most compound-specific radiocarbon analyses have been performed on lipids extracted from marine sediments. Measurements from organic-poor environments such as the water column typically are more difficult to achieve because of the correspondingly smaller yields of identifiable pure compounds. Recently we were able to utilize the large-volume pumping capacity available at the Natural Energy Laboratory of Hawaii (NELHA) to obtain sufficient biomass directly from the water column for subsequent extraction and CSRA. To our knowledge this represents the first CSRA data set obtained from suspended particulate matter obtained from the water column. The oligotrophic open ocean represents one of the most biomass-poor regions of the planet, yet by volume this region comprises the vast majority of the ocean. As such, the metabolic processes mediated by microbes in this environment potentially are fundamental to the control of the global cycles of carbon and energy. Our data address the carbon sources to microbes, both bacteria and archaea, in the surface ocean and in the mesopelagic zone at 600m depth. The data are consistent with a predominantly heterotrophic metabolism in the bacteria and a predominantly autotrophic metabolism in the archaea.
Here we will discuss the specific limitations imposed by the small sample sizes yielded by this study. The purified lipid samples contained between 2.8 – 224 micrograms of carbon; nine of the samples contained < 20 micrograms of carbon, the threshold usually considered permissible for CSRA. Analysis of the smallest samples was done at the Keck Carbon Cycle AMS Laboratory at the University of California, Irvine. The results showed that the “ultra-small” samples were limited not by AMS measurement but rather by the incorporation of processing blanks at various stages throughout the separation procedure. The uncertainties in the size and C-14 content of these blanks were the ultimate limiting factors. Full propagation of error was required to evaluate the data. We determined that the errors associated with purification and combustion of the samples and the errors associated with AMS statistics were equal (or were limited by the AMS) when samples contained > 20 micrograms of carbon; below 20 micrograms of carbon, the error was governed by the contributions from laboratory processing and combustion blanks. This suggests that AMS capability now has exceeded separation science: the ultimate limitation on CSRA applications is the procedure used to purify and combust individual compounds.
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