Elemental analyses of total nitrogen and carbon (and sulfur) is performed to provide carbonate and organic carbon and to get some idea of the composition of the organic matter (i.e., to distinguish between marine and terrigenous sources, based on total organic carbon/total nitrogen [C/N] ratios).
The total nitrogen, carbon, and sulfur are determined using a CHNS analyzer, model NA 1500 from Carlo Erba Instruments. For the CHNS analysis, freeze-dried and crushed samples are weighed (5-10 mg) and mixed with an oxidizer (vanadium pentoxide [V2O5]) in a tin capsule, which is then combusted in a reactor at 1000°C. The sample and container melt, and the tin promotes a violent reaction (flash combustion) in a temporarily enriched oxygen atmosphere. The combustion products CO2, SO2, and NO2 are carried by a constant flow of carrier gas (helium) that passes through a glass column packed with an oxidation catalyst of tungsten trioxide (WO3) and a copper reducer, both kept at 1000°C. At this temperature, the nitrogen oxide is reduced to N2. The N2, CO2, and SO2 are then transported by the helium to, and separated by, a 2-m-long packed column (Poropak Q/S 50/80 mesh) and quantified with a TCD (set at 290°C.)
The chromatographic responses are calibrated against preanalyzed standards, and the CHNS elemental contents are reported in weight percent. Eager 200 software is used for running the equipment, storing the data, and for postrun analysis.
TOC is usually calculated by difference between the total carbon (TC) value determined by the Carlo Erba CHNS and the carbonate carbon (IC) determined with the coulometer Model 5030 as described below.
The coulometer provides an absolute determination of carbon dioxide (CO2), which is used to determine carbonate weight percent in sediment samples. Freeze-dried, crushed samples (~10 mg) are acidified (with 2-N HCl) in a glass tube to convert the carbonate to CO2. Air that has been scrubbed of CO2 carries the generated CO2 through another scrubber to remove any SO2 and then to the coulometer cell. The coulometer cell contains two compartments that are partitioned with a ceramic frit. The larger compartment is filled with monoethanolamine (ME), a colorimetric indicator, and contains a platinum cathode. The smaller compartment is filled with a proprietary solution (anode solution) and contains a silver anode. In the coulometer cell, the CO2 is absorbed, reacting with the ME to form a titrable acid that causes the color to fade. A photodetection cell monitors the change in the solution color. As the percent transmittance (T) increases, the titration current is activated to generate base at a proportional rate. When the solution returns to its original color (T = 29%), the current stops indicating and the titration ends. A carbon program developed using Labview software reads the count value from the red display on the coulometer at the end of the titration and provides the IC and CaCO3 (in weight percent) of the samples analyzed.
Organic C/N ratios can be used to help identify the origin of the organic matter in sediments. (Shipboard Scientific Party, 1998). C/N ratios of 5-8 indicate unaltered algal organic matter, whereas C/N ratios of 25-35 indicate fresh land-derived organic matter. (Emerson and Hedges, 1988; Meyers, 1994). Interpretation of C/N ratios needs to be done with caution. Low C/N values in sediment containing low organic carbon may be biased by the tendency of clay to absorb ammonium ions generated during the degradation of organic matter (Müller, 1977). Preferential loss of nitrogen-rich, proteinaceous matter can elevate the C/N ratios of algal organic matter during settling to the seafloor (Meyers, 1997).