In the analysis of sediments, C/N ratios are a proxy for
paleoclimate research, having different uses whether the
sediment cores are terrestrial-based or marine-based. Carbon-to-nitrogen ratios indicate the degree of nitrogen limitation of plants and other organisms. They can identify whether molecules found in the sediment under study come from land-based or algal plants. Further, they can distinguish between different land-based plants, depending on the type of photosynthesis they undergo. Therefore, the C/N ratio serves as a tool for understanding the sources of sedimentary organic matter, which can lead to information about the ecology, climate, and ocean circulation at different times in Earth's history. Vascular plants from terrestrial sources tend to have C/N ratios greater than 20. The lack of
cellulose, which has a chemical formula of (C6H10O5)n, and greater amount of proteins in algae versus vascular plants causes this significant difference in the C/N ratio.
Instruments Examples of devices that can be used to measure this ratio are the
CHN analyzer and the continuous-flow isotope ratio
mass spectrometer (CF-IRMS). However, for more practical applications, desired C/N ratios can be achieved by blending commonly used substrates of known C/N content, which are readily available and easy to use.
By sediment type Marine Organic matter that is deposited in marine sediments contains a key indicator as to its source and the processes it underwent before reaching the floor as well as after deposition, its carbon to nitrogen ratio. The microbial communities utilizing the sinking organic carbon as an energy source, are partial to nitrogen-rich compounds because much of these bacteria are nitrogen-limited and much prefer it over carbon. As a result, the carbon-to-nitrogen ratio of sinking organic carbon in the deep ocean is elevated compared to fresh surface ocean organic matter that has not been degraded. An exponential increase in C/N ratios is observed with increasing water depth—with C/N ratios reaching ten at intermediate water depths of about 1000 meters and up to 15 in the deep ocean (deeper than about 2500 meters) . This elevated C/N signature is preserved in the sediment until another form of diagenesis, post-depositional diagenesis, alters its C/N signature once again. Lastly, ammonia, the product of the second reduction reaction, which reduces nitrate and produces nitrogen gas and ammonia, is readily adsorbed on clay mineral surfaces and protected from bacteria. This has been proposed to explain lower-than-expected C/N signatures of organic carbon in sediments undergoing post-depositional diagenesis. Though wood from living trees around lakes have consistently higher C/N ratios than wood buried in sediment, the change in elemental composition is not large enough to remove the vascular versus non-vascular plant signals due to the refractory nature of terrestrial organic matter. Results of studies that show abrupt shifts in algal dominance and vascular dominance often lead to conclusions about the state of the lake during these distinct periods of isotopic signatures. Times in which algal signals dominate lakes suggest a deep-water lake, while times in which vascular plant signals dominate lakes suggest the lake is shallow, dry, or marshy. Using the C/N ratio in conjunction with other sediment observations, such as physical variations, D/H isotopic analyses of fatty acids and alkanes, and δ13C analyses on similar biomarkers can lead to further regional climate interpretations that describe the more significant phenomena at play. == Soil ==