Organic Matter

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Organic matter is defined as the organic compounds and particles produced by organisms that are incorporated and deposited in sedimentary, marine, and aquatic environments. Organic compounds are compounds that contain carbon and hydrogen, but can also include oxygen, nitrogen, phosphorus, sulfur, etc. Organic matter can be as simple as a methane (CH4) or as complex as kerogen with several aromatic rings incorporating oxygen nitrogen and sulfur. Organic matter is preserved in the geologic record when the deposition occurs in an anoxic environment or when sedimentation rate is high enough to bury organic matter before degradation.

Compound Specific Organic Matter

n-Alkanes

The n-Alkane is a group of organic compounds that utilize single bonds to create a chain of carbon. Terrestrial and aquatic photosynthetic organisms utilize n-alkanes to protect themselves from the extremes of their environment. Organisms will make carbon chains of varying lengths as they see fit to deal with these extremes. Since different organisms will synthesize different n-alkanes, quantitative analysis of chain length abundance can be used to determine major organic matter source. Long chain length n-alkanes of C27, C29, and C31 are sourced from higher order plants and trees [6]. Mid-chain lengths of C21, C23, and C25 are all associated with non-emergent macrophytes typically associated with lakes [6]. Short chain length n-alkanes such as C17 are associated with algae and photosynthetic bacteria [6]. Since alkanes lack functional groups, they are resistant to degradation by bacteria and other organisms that exist in the water column and sediment. This resistant makes them ideal biomarkers for comprehensive studies.


Alkanoic Acids

The alkanoic acid, also referred to as Fatty acid, is a group of organic compounds that consists of a carbon chain with a carboxylic group attached to the end member. Similar to n-alkanes, fatty acids are sourced from different organisms ranging from the terrestrial to the aquatic. Saturated Fatty acids with a carbon chain of C16-C18 are sourced from algae and plankton [1]. Fatty acids also sourced from algae and plankton are poly and monounsaturated fatty acids C18:2, C18:3, and C16:1ω7 [1]. Long chained saturated fatty acids with a chain length greater than >C24 are sourced from higher order plants and macrophytes [1]. Bacterial inputs are seen with odd numbered, monounsaturated, and branched fatty acids with chain lengths C15, C17, C18:1ω7, C18:1ω9, and iso and anteiso C15 [1]. Alkanoic acids are very sensitive to diagenesis and alterations. Alkanoic acids have been shown to be degraded and altered twice as fast as n-alkanes in oxic conditions [1]. The carboxylic functional group attached to organic acids makes them liable thus susceptible to degradation and alteration by microbial communities [4]. Although alkanoic acids can degrade, they are usefully in the same way as alkanes. Different organisms make them in different chain lengths and isomers making them great for identifying source. They are also typically seen in high concentrations making them ideal for biomarker analyses.


Kerogen

Kerogen is a complex organic macromolecule that is formed from diagenetic processes acting on organic matter. Kerogen is the insoluble fraction of organic matter found in lithified sediment [3]. Kerogen is categorized into three different types based upon maturity and source. Type I is defined as a low maturity state sourced from lacustrine environments where the major input comes from algal and bacterial lipids [3]. Type II is associated with planktonic inputs into either a marine or lacustrine system [3]. Type III at low maturity has a major input from higher order plants when terrestrial organic matter is deposited into either a marine or lacustrine environment [3]. The different types of kerogens can also be identified based upon their H/C and O/C ratios [3]. This can be a useful tool when the history of thermal maturity is unknown. When the H/C ratio is high, the kerogen has more alkyl groups rather than aromatic groups. This higher ratio would mean that the kerogen has a lower maturity (Type I or II). Lowered H/C ratio indicates that there is a greater number of double carbon bonds which are directly related to aromaticity. Increased aromaticity correlates to increased thermal maturity. O/C ratios in kerogen typically are related to the presence of ketones, esters, and ethers in the macrostructure. Increasing the ratio means higher presence of these functional groups. These functional groups are typically connections between aromatic clusters.

Particulate Organic Matter

Particulate organic matter (POM) is defined as organic matter that is larger than 0.5 μm [11]. POM can be sourced from both marine/aquatic organisms and terrestrial debris from higher order plants. The analysis of POM is referred to as palynofacies analysis and is conducted by filtration of the POM followed by analysis via light microscopes [9]. This type of analysis is typically used since it is able to observe both plant material and membranes and organs of plankton and algae. Analysis of POM can also be useful to understand thermal maturity when chemical analysis cannot be used.


Marine and Aquatic Sources

POM that is sourced from marine environments are inferred as marine snow. The marine snow can contain; microalgal cells, bacteria, organic detritus, and particles of coccoliths, diatom skeletons, and clay particles coated in organic matter [11]. In nonmarine environments POM is seen as preserved clusters of aquatic organisms. POM created from these environments are typically amorphous organic matter (AOM). AOM are particulates that appear structureless at the microscopic level [8]. The main optical properties are organs and some recognizable organisms of bacterial, algal, and plankton [9]. AOM can be degraded by thermal maturation and biodegradation. AOM, sourced from aquatic/marine organisms, contain more alkyl groups than POM originating from other sources.


Terrestrial Sources

POM that is sourced from terrestrial environments consists of debris from higher order plants. These particulates have structures conducive to the higher order plants they are sourced from. POM sourced from terrestrial environments are referred to as phytoclasts [9]. Phytoclast are broken down into two main groups preserved fragments and transformed particles. Preserved fragments consist of cuticle and membrane (CM), translucent litho-cellulosic fragments (TLC), and altered litho-cellulosic fragments (ALC) [9]. These preserved fragments are the least altered by thermal maturity or biodegradation. Transformed particles consists of amorphous particles (AP), gelified particles (GP), and opaque particles (OP) [9]. Transformed particles are the most altered with the opaque particles having been the most matured. Phytoclasts, being sourced from higher order plants, contain the highest abundance of aromatic hydrocarbons. This is due to the presence of lignin in the plant material.


Use in Identifying Thermal Maturity and Source

Characteristics and composition of different POM are used to identify the thermal maturity and major contributing organic matter source. Amorphous organic matter is typically sourced from marine and aquatic organisms. This type of organic matter has high concentrations of aliphatic hydrocarbons. The presence of unmodified AOM seen as either a Type I or Type II thermal maturity [5]. As the AOM becomes opague or begins to degrade the thermal maturity increases. Terrestrial inputs are typically associated with higher thermal maturities. This is due to their association with aromatic hydrocarbons. The occurrence of gellified particles or amorphous particles are often interpreted as Type III thermal maturity [5]. To identify different sources, a ratio between different POM can be used isolate changes in organic inputs. This ratio consists of the concentration of amorphous organic matter (AOM) verses that of the terrestrial POM (preserved fragments, transformed particles, etc.). A ratio of these two particulates can determine when sea/lake level change or how the surrounding environment has changed (fluvial, climate, etc.) [9].

Geomorphology of Organic Matter Preserving Environments

Environments that preserve organic matter are typical either anoxic marine/aquatic environments or places with rapid sedimentation. Deep marine environments can be cut off from ocean circulation or deep-water current by uplift or subsidence. Both of these tectonic events create a zone that is protected from oxygen rich water which degrade organic matter. Lacustrine environments are formed from glacial or tectonic activity which produce large depression on the surface. When the lake is filled, the water column tends to stratify leaving an anoxic zone at the bottom of the water column. Places with high sedimentation rates can preserve organic matter by allowing for the burial of organics before degradation can occur. Deltas are capable of transporting and burying large amounts of organic matter. Deltas can either feed into a lacustrine environment or into a marine environment. Both of these systems are aided by the presence of a fluvial system depositing sediment and organic matter.

Modern Analogues

Lakes

Lakes can have varying amounts of organic matter preserved in their sediments [6]. Depending on location, lakes can have large inputs of organic matter from both terrestrial and aquatic organisms. Though there can be large inputs, majority of the organic matter introduced into a lake is removed. Around 85% of organic carbon is oxidized before exiting the epilimnion section of the water column [7]. The presence of bioturbation can also limit the amount of carbon that makes it to preservation. Bioturbators are able to reintroduce deposited organic matter to the water column where oxidation can continue [7]. Organic matter is preservation is due in part by an anoxic bottom caused by water column stratification and rapid sedimentation. Lakes are frequently used in paleoclimate studies. The organic matter concentration can vary greatly, but will always preserve the environmental changes experienced by the lake and the surrounding area [6]. Organic compounds such as alkanes and alkanoic acids produced by organisms within the lake can be used to understand aridity in the area. Since the organic compounds are synthesized using the carbon and hydrogen available, any evaporation of the lake water is recorded as an isotopic signal in the organic compound.


Wetlands

Wetlands play a vital role in the global carbon cycle and function as one of the most significant carbon sinks [2]. Wetlands are known to sequester large quantities of carbon every year due to high rates of primary production and anaerobic conditions that slow down organic matter recycling [10]. This sequestration occurs as vertical accumulations of organic-rich sediments [3]. Wetlands take up approximately 6-8% of freshwater surfaces but constitute an estimated one-third of the global organic soil pool [2]. The cycling of organic matter takes place as production, deposition, preservations, and recycling in wetland environments. Typically, the anaerobic conditions slow the release and recycling of OM, but changes in water chemistry can increase the rate of release and recycling.

References

  1. Arnold, T. E.; Brenner, M.; Kenney, W. F.; Bianchi, T. S. Recent Trophic State Changes of Selected Florida Lakes Inferred from Bulk Sediment Geochemical Variables and Biomarkers. Journal of Paleolimnology 2019, 62 (4), 409–423.
  2. Bernal, B.; Mitsch, W. J. Comparing Carbon Sequestration in Temperate Freshwater Wetland Communities. Global Change Biology 2012, 18 (5), 1636–1647. https://doi.org/10.1111/j.1365-2486.2011.02619.x.
  3. Cao, X.; Yang, J.; Mao, J. Characterization of Kerogen Using Solid-State Nuclear Magnetic Resonance Spectroscopy: A Review. International journal of coal geology 2013, 108, 83–90.
  4. Dodla, S. K.; Wang, J. J.; DeLaune, R. D. Characterization of Labile Organic Carbon in Coastal Wetland Soils of the Mississippi River Deltaic Plain: Relationships to Carbon Functionalities. Science of the total environment 2012, 435, 151–158.
  5. Emmings, J. F.; Hennissen, J. A.; Stephenson, M. H.; Poulton, S. W.; Vane, C. H.; Davies, S. J.; Leng, M. J.; Lamb, A.; Moss-Hayes, V. Controls on Amorphous Organic Matter Type and Sulphurization in a Mississippian Black Shale. Review of Palaeobotany and Palynology 2019, 268, 1–18.
  6. Meyers, P. A. Applications of Organic Geochemistry to Paleolimnological Reconstructions: A Summary of Examples from the Laurentian Great Lakes. Organic geochemistry 2003, 34 (2), 261–289.
  7. Meyers, P. A.; Ishiwatari, R. Lacustrine Organic Geochemistry—an Overview of Indicators of Organic Matter Sources and Diagenesis in Lake Sediments. Organic geochemistry 1993, 20 (7), 867–900.
  8. Pacton, M.; Gorin, G. E.; Vasconcelos, C. Amorphous Organic Matter—Experimental Data on Formation and the Role of Microbes. Review of Palaeobotany and Palynology 2011, 166 (3–4), 253–267.
  9. Sebag, D.; Copard, Y.; Di-Giovanni, C.; Durand, A.; Laignel, B.; Ogier, S.; Lallier-Vergès, E. Palynofacies as Useful Tool to Study Origins and Transfers of Particulate Organic Matter in Recent Terrestrial Environments: Synopsis and Prospects. Earth-Science Reviews 2006, 79 (3–4), 241–259.
  10. Steinmuller, H. E.; Dittmer, K. M.; White, J. R.; Chambers, L. G. Understanding the Fate of Soil Organic Matter in Submerging Coastal Wetland Soils: A Microcosm Approach. Geoderma 2019, 337, 1267–1277.
  11. Volkman, J. K.; Tanoue, E. Chemical and Biological Studies of Particulate Organic Matter in the Ocean. Journal of oceanography 2002, 58 (2), 265–279.