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. In the global oceans, freshly produced algae in the surface ocean typically have a carbon to nitrogen ratio of about 4 to 10. However, it has been observed that only 10% of this organic matter (algae) produced in the surface ocean sinks to the deep ocean without being degraded by bacteria in transit, and only about 1% is permanently buried in the sediment. An important process called sediment diagenesis accounts for the other 9% of organic carbon that sank to the deep ocean floor, but was not permanently buried, that is 9% of the total organic carbon produced is degraded in the deep ocean. The microbial communities utilizing the sinking organic carbon as an energy source, are partial to nitrogen-rich compounds because much of these bacterium 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 had not been degraded. An exponential increase in C/N ratios is observed with increasing water depth—with C/N ratios reaching 10 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. Post-depositional diagenesis occurs in organic-carbon-poor marine sediments where bacteria are able to oxidize organic matter in aerobic conditions as an energy source. The oxidation reaction proceeds as follows: CHO + HO → CO + 4H + 4e, with a standard free energy of –27.4 kJ mol (half reaction). Once all of the oxygen is used up, bacteria are able to carry out an anoxic sequence of chemical reactions as an energy source, all with negative ∆G°r values, with the reaction becoming less favorable as the chain of reactions proceeds.
The same principle described above explaining the preferential degradation of nitrogen-rich organic matter occurs within the sediments, as they are more labile and are in higher demand. This principle has been utilized in paleoceanographic studies in order to identify core sites that have not experienced much microbial activity, or contamination by terrestrial sources with much higher C/N ratios.
Lastly, ammonia, the product of the second reduction reaction, which reduces nitrate and produces nitrogen gas and ammonia, is easily adsorbed on clay mineral surfaces and protected from bacteria. This has been proposed as an explanation for lower than expected C/N signatures of organic carbon in sediments that have undergone post-depositional diagenesis.
Ammonium produced from the remineralisation of organic material, exists in elevated concentrations (1 - >14μM) within cohesive shelf sea sediments found in the Celtic Sea (depth: 1–30 cm). The depth of sediment exceeds 1m and would be a suitable study site to carry out paleolimnology experiments with C:N.