Problems with Accurate Carbon Measurements in Marine Sediments and Particulate Matter in Seawater: A New Approach

The determination of carbon concentrations in marine sediments and particulate matter is hindered by the difficulty in analytically partitioning the total carbon between organic carbon and carbonate carbon phases. Attempts to "selectively" remove organic carbon by burning the sample at $500^\circC$, or carbonate carbon by acidifying with HCl, usually affect the carbon of the remaining phase as well. A new approach permits direct measurement of both organic and carbonate carbon in a single sediment sample. The carbonate carbon concentration is determined from the $CO_2$ evolved during phosphoric acid treatment. Subsequently, a concentrated dichromate/sulfuric acid solution is added to the remaining sediment-acid mixture, and the $CO_2$ liberated from the oxidation of organic carbon is measured. Total carbon is calculated as the sum of organic and carbonate carbon. A LECO carbon analyzer is modified so that the amount of $CO_2$ evolved can be measured by the instrument's thermal conductivity detector. In addition, total carbon content is determined on another subsample using the LECO dry combustion furnace. This provides a check on the values determined by the $H_3PO_4/dichromate$ technique. Any other commercially available instrument relying on combustion furnace and thermal conductivity or infrared absorption for $CO_2$ detection could be converted in an analogous manner.