5.1.1: Organic matter

Figure \(5.1\), on the main page for section 5.1, refers to “complex” organic matter and “simple” organic matter (C_org). We can think of complex organic matter as the biomass of dead organisms. From these high molecular-weight compounds, relatively simple low molecular-weight organic compounds can be produced through respiration and fermentation reactions, as discussed below.

A wide range of organic compounds exist in natural environments. In part, this characteristic stems from the fact that organic matter is not simply composed of the element carbon. It includes carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur as major components and other elements as well. These elements are combined in many ways to make complex organic molecules such as carbohydrates, proteins, nucleic acids, fats, oils, and waxes. Secondly, different organisms produce different assortments of organic compounds. For example, one of the largest sources of complex organic matter in terrestrial environments is plant material consisting of rigid polymers such as cellulose and lignin (De Leeuw and Largeau, 1993). However, in marine environments, the bulk of biomass is produced by single-celled organisms that are rich in lipids and nitrogenous compounds (Burdige, 2007). Third, in addition to biologically produced compounds, organic compounds can also form abiotically (McDermott et al., 2015; Reeves and Fiebig, 2020). Thus, organic compounds form along diverse biotic and abiotic pathways.

Adding to this variability, organic compounds found in sedimentary environments have commonly undergone some amount of degradation. In marine environments, for example, most of the organic matter produced by organisms is rapidly consumed before it is able to settle on the seafloor (Hedges and Keil, 1995). As a second example, kerogen is solid-phase organic matter in sedimentary rocks that forms by biological and thermal degradation pathways. It is thought to be the most abundant form of organic matter on Earth, with a mass of about 15,000,000 gigatons in the subsurface (Falkowski et al., 2000). Degradation produces new compounds and can leave residual complex organic matter altered relative to its initial state.

For detailed information about organic matter composition, De Leeuw and Largeau (1993) provide a thorough review of macromolecular organic compounds and their role in kerogen, coal, and crude oil formation. Arndt et al. (2013) reviews organic matter in marine sediments. Lastly, Hedges et al. (1997) and Kandasamy and Nagender Nath (2016) consider inputs of terrestrial organic matter to the oceans.

---