The importance of organic matter (OM) in biogeochemical processes in seas and oceans was recognized long ago (e.g., [1, 4]). Numerous studies by renowned Russian scientists are devoted to the geology and geochemistry of petroleum hydrocarbon formation: V.I. Vernadsky, I.M. Gubkin, I.O. Brod, N.B. Vassoevich, V.V. Veber, N.A. Kudryavtsev, A. A. Trofimuk, A.E. Kontorovich, and their students and followers. In these studies, organogeochemical indicators are productively used.

It was realized that the information potential of lipids, hydrocarbons, and their derivatives—polysaccharides, lignin and other heteropolymers—which includes hundreds of indicators, is enormous, but has not been assessed. The sharp change in biosphere processes under the influence of natural and anthropogenic factors was the impetus for a systematic study of important biosphere changes based on an interdisciplinary approach to the Earth and life, physical, and information sciences. This led to the scientific understanding that biogeochemistry and its multidisciplinary methodology are the best tools for solving the fundamental problems of a rapidly changing biosphere.

The article summarizes and critically analyzes the available data on organogeochemical indicators and outlines the most promising areas of research thereof.

MATERIALS AND METHODS

Studies of organogeochemical indicators are based on data on suspended particulate matter (SPM), plankton, benthos, bottom sediments, and sea, river, and pore water. Historically, a number of methods stand out, from simple chemical, isotopic, optical, and histochemical to gas, thin-layer, liquid chromatography, chromatography-mass spectrometry (CMS), and pyrolysis. A description of methods for studying the molecular and isotopic composition and for the optical-luminescent study of OM is the subject of a separate article and is beyond the scope of this work.

RESULTS AND DISCUSSION

Table 1 presents clusters of organogeochemical indicators characterizing the genesis of OM in sediments, the setting of its formation, and the processes occurring during sedimentogenesis and diagenesis. The indicators were combined into clusters taking into account both the chemical and group composition of organic compounds and the methods of their identification and study. Judgments about the source of OM (aquatic and terrigenous components), the degree of its transformation in diagenesis, and reflection of environmental features were made on based on the ratios of chemical elements and their isotopes in OM, the predominant molecular weights, the ratios of structural monomers (lignin), the presence of aromatic structures in OM, etc.

Table 1.   Organogeochemical indicators of biogeochemical processes
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Cluster 1 includes indicators of elemental and group composition, morphoforms, and dispersion of OM; pyrolytic indices; petrochemical and histochemical characteristics; and maceral ratios. Carbon, its percentage and atomic ratios with nitrogen, phosphorus, sulfur and other indicators of chemical elemental composition have universal significance [39]. They are necessary to assess the generation, decomposition, and transformation of OM, the rates of sulfate reduction and deoxygenation, the occurrence of anoxia of waters, and the geology of gas hydrate formation in the ocean.

Table 2.   Importance of organogeochemical indicators in oceanology
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Determination of organic carbon (Corg) made it possible to quantitatively estimate the total OM content in bottom sediments and other products of sedimentogenesis based first on the classical Knop method, then on methods involving high-temperature catalytic combustion of samples (900°C) in an air stream. Based on many thousands of determinations and integration of more than 200 regional maps, a map of OM distribution in sediments of the World Ocean, the main and, in most cases, only source of chemical energy for biochemical processes, was presented (Fig. 1). Chemical-analytical determination of Corg in several thousand samples showed that the main pattern of OM accumulation in bottom sediments is the circumcontinental localization of OM masses and fluxes. This zoning correlates well and largely determines the distribution of benthos within the sea and ocean floors, its trophic groups, and types of nutrition (e.g., [15]).

Fig. 1.
figure 1

Distribution of Corg in upper layer of ocean sediments, % of dry sediment.

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The Corg content and many other indices of the first cluster were determined out in most samples obtained at all stages of deep-sea drilling (DSDP-1968–1983; ODP-1985–2003; IODP-2003–2013). This work is currently ongoing on various vessels. The distribution of organic and carbonate carbon in the sedimentary cover of the ocean was refined and distribution maps were compiled, taking into account the lithospheric plate movements over the past 100–150 Ma (e.g., [13]). It has been established that the accumulation of carbon on sea and ocean floors in the modern era and geological history has always been uneven and cyclical.

The C/S, C/N, C/H, and C/O ratios reflect the living conditions of biota in the water column and on the bottom, which become impossible for the life of macroforms when C/S is less than 1–1.5, when hydrotroilite appears, followed by free hydrogen sulfide. The ratio of humic (HA) and fulvic (FA) acids and kerogen reflect the origin, type of OM (humic, sapropelic, mixed) and the degree of its lithification, while the hydrogen (HI) and oxygen (OI) indices reflect the transformation of OM in the direction of oil and gas generation. Depolymerization of biomolecules and humification are typical processes of changes in the chemical structure of OM during sedimentogenesis and early diagenesis. The content of heteropolycondensed structures (HA, FA and their insoluble anhydride forms) increases by 10–20 times; proteinlike compounds, by 100–200 times; polysaccharides and lipids, by approximately 3–4 times. During the humification of SPM, the relative Corg content increases (carbonization) and the trophic value of OM decreases.

The study of seston OM, separation and filtration SPM sampled in the water column, and sediments of a thin surface layer in thin sections made it possible to observe the transformation of OM during sedimentogenesis under a microscope using dyes. It was possible to observe a subtle picture of the distribution and redox conditions of the transformation of microcomponents of OM, as well as the initial levels and degree of its gelification and humification. Several morphological and genetic types of suspended OM have been identified: aggregates of organomineral particles, fragments of plant and animal origin inhabited by microflora, flakes of “sea snow,” spores, pollen, and living planktonic algae, bacteria, protozoan remains, zooplankton larvae and eggs, fecal pellets, remains of chitinous shells, as well as dye-stained flakes of unknown origin. This is a special branch in the study of OM, which is as important as the currently widespread methods of molecular organic geochemistry (Fig. 2). The study of lithified microcomponents of OM (macerals) of bottom sediments in various areas of the ocean in terms of the content and ratio of alginite, vitrinite, liptinite, and inertinite have significantly augmented the pattern of the genesis and sources of terrigenous matter.

Fig. 2.
figure 2

Ratio of OM and mineral mass in bottom sediments and kerogen (Bobyleva and Romankevich, unpublished article).

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The total Corg content in the entire water mass in the World Ocean is estimated at 750–1000 × 1015 g [44]. Integration of multifarious data of the Corg content in the World Ocean has made it possible to estimate the masses of living, particulate, and dissolved OM in the ocean, which are expressed in quantities of 4, 50, and 1000 × 1015 g, respectively, and form a pyramid. Generalization of accumulated data (more than 5500 determinations) on the dissolved and particulate OM content in the Arctic Ocean [51] made it possible to calculate the distribution (mg L–1) and fluxes (g m–2 s–1) Corg at depths from 5 to 4000 m on six transects in the Arctic Basin and, in general, the flux into the Arctic Basin (Fig. 3). The existence of a large-scale imbalance in the OM synthesis–breakdown system in Russia’s Arctic seas was discovered. Here, powerful fluxes of thermal abrasion carbon are comparable to river flows and create unique accumulation depots of Corg [11, 12, 14, 23, 27, 50, 52].

Fig. 3.
figure 3

Vertical profiles of DOC fluxes (g m–2 s–1) over transects. (a) Fram Strait, (b) North Cape–Serkap, (c) Canadian archipelago, (d) Svalbard–Franz Josef Land, (e) Bering Strait, (f) Beaufort Gyre [51].

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Cluster 2, in addition to saturated high-molecular hydrocarbons, contains simple lipids, polar lipids, n-alkanes, sterol esters, fatty acid esters, triglycerides, fatty acids, fatty alcohols, and sterols. Their group composition and individual compounds make it possible to understand how the OM composition changes in the water column in the plankton–SPM–bottom sediment system, e.g., in terms of polarity, reactivity, and bioavailability. A sharp predominance of the dissolved fraction over the particulate fraction in the ocean was revealed. The presence of triglycerides, sterol esters, and fatty acid esters reflects the content of labile OM in seawater as a result of its intravital extracellular release and postmortem lysis of cells [2, 3]. During sedimentation, the group composition of lipids in POM changes as a result of OM decomposition, resynthesis of lipids from nonlipid OM, and their transition from solution to suspension. Types of bottom sediments are characterized by different compositions of simple lipids and their individual components (e.g., [10, 17]).

The study of the composition of lipid indicators by thin-layer and liquid and gas chromatography showed that during sedimentogenesis and early diagenesis, changes in the group composition of lipids in an oxidizing and reducing environment are directed towards the selective conservation of low-polarity compounds. The series of increasing stability of lipids in the ocean appears as follows: polar lipids–free fatty sterols, fatty alcohols–hydrocarbons. High-molecular-weight hydrocarbons are the most widely used group of compounds, for which gas chromatography and gas chromatography–mass spectrometry are well developed (n-alkanes C10–C40). Table 3 presents their most characteristic indicators and calculation formulas, reflecting the features of the transformation of OM in sedimentation. Calculation of the carbon index (CPI) is based on the predominance of high-molecular-weight even n-alkanes in marine vegetation, and high-molecular-weight odd n-alkanes in terrestrial vegetation. In sediments during sedimentogenesis and diagenesis, the CPI index approaches 1.0 as a result of transformation of OM, loss of functional groups, structural rearrangement, and the new formation of hydrocarbons and new double bonds. The oddity indices CPI, HCPI, and OCPI make it possible to estimate the ratio of terrigenous and hydrogenous OM, as well as the degree of its transformation by biota. The TAR index makes it possible to estimate the contribution of terrestrial vegetation to the OM composition. Paq reflects the ratio in the composition of suspended matter and sediments of higher terrestrial vegetation and coastal vegetation. Low values of the ratio of the sum of short-chain normal n-alkanes to long-chain ones (S/L index) reflect the dominance of terrigenous OM; the ratio of some isoprenoid n-alkanes (e.g., Pr/Ph) reflects redox conditions in the water column and sediments. The pristane/phytane ratio is widely used in coal and oil and gas geology to understand the conditions for the transformation of OM into oil and gas hydrocarbons, as well as at the peat stage of OM transformation. The distribution of n-alkanes without clear dominance in the homologous series of odd n-alkanes reflects the active bacterial transformation of OM. A significant content of waxes from higher plants is marked by n-alkanes C27–C33 or indicates supply of aeolian OM. Preferential consumption of short-chain n-alkanes by microbiota leads to a selective increase in long-chain n-alkanes.

Table 3.   Indicator characteristics of n-alkanes
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Cluster 3 combines high-molecular-weight alkanoic acids, long-chain alkyl diols, dialkyl glycerol tetraesters, waxes (esters of saturated carboxylic acids and higher alcohols), terpenes, terpenoids, and their transformation products, naphthenes, ketones, hopanes and hopenes.

High-molecular-weight acids. Alkane and alkyne high-molecular-weight acids C20 and C30 and their relationship to n-alkanes (HMWH-alkanoic acids (C20–C30)/HMWH-n-alkanes (C20–C32)) reflect the input of soil and river OM into bottom sediments. Their determination in sedimentary material carried by the Ob, Yenisei, Lena, Indigirka, and Kolyma rivers with simultaneous measurement of δ13C and Δ14C showed that the average age of OM in bottom sediments increases from east to west from 6400 BP (Kolyma River) to 11 400 BP (Ob River). This agrees well with data on climate change in this direction and the degree of degradation of OM [32].

Long-chain alkyl diols (LCD). In bottom sediments and SPM, C281.13 and C281.14, C301.13, C301.14 and C301.15 diols were found, which are synthesized by diatoms and other marine microalgae. Their synthesis correlates with the average annual temperature and pH of the environment and makes it possible to judge the temperature conditions for sediment formation of alluvial fans, coastal water upwelling, and alternating glacial and interglacial stages [33, 41]. Alcohols C321.15 are predominantly synthesized by freshwater algae.

Dialkyl glycerin tetraethers. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are part of the lipids of cell membranes of soil bacteria and are indicators of average annual temperatures (AAT) and pH of the environment during their formation. This allows them to be used for paleoreconstruction of the environment [29]. The BIT index reflects the ratio of brGDGTs, which are produced by bacteria in soils and rivers, to isoprenoid GDGT (crenarchaeol), most of which is produced by marine archaea [24, 25, 48]. For example, a study of the content of brGDGTs, lithological composition and OM composition in a sediment core from the Svyataya Anna Trough (depth 473 m; age Holocene, Upper Pleistocene, 13.3 ka) made it possible to calculate the BIT index and isomer ratio and reconstruct pH and MAT during brGDGTs synthesis [25]. The BIT index made it possible to determine the temperatures of glacial and interglacial stages (Dansgaard–Oeschger events) and factors determining the erosion of Amazon River sedimentary material, the ratio of river runoff and marine OM, and humidity–aridity of the climate. The BIT index is higher for cold stages; for warmer stages, it decreases [33].

Isoprenoid lipids. Highly branched isoprenoids (HBI), unsaturated alkenes (IP25 (C25:1), HBI-II (C25:2), HBI-III (C25:3)), and sterols (brassicasterol and dinosterol) are synthesized by ice diatoms at the ice–water interface in the marginal ice zone (MIZ). The possibility of their use as indicators of bioproductivity and ice conditions for various regions of the Arctic and Antarctic [19, 20, 36] indicates that the factors determining the production of these isoprenoid lipids (IP25 and others) remain undisclosed, but their distribution and IP25 ratio, HBI–II and HBI–III reflect ice-free conditions, seasonal ice, and permanent ice cover, respectively [49]. The complexity of interpreting data on the synthesis, distribution, and sedimentation of isoprenoid lipids requires a complex system of various methods for studying HBIs and a detailed characterization of the environment of formation and melting of ice fields. It should be noted that the pathways for HBI synthesis can be elucidated using biochemical methods (genomics). Isoprenoid dialkyl glycerol tetraesters GDGTs with 1–5 cyclopentane rings can serve as indicators of the average surface water temperature. These structures are named in organic geochemistry based on the type of carbon atoms in the molecule: TEX86 (TEX = (2r + 3r + 5r)GDGTs/(2r + 3r + 5r + 1r)GDGTs), where r is the number of cyclopentane rings in the molecule. Isoprenoid GTGTs are part of the cell membranes of marine archaea Crenarchaeota, the development of which correlates with temperature in the range 5–30°C. In the ocean, Archaea of Crenarchaeota constitute a significant part of the pelagic plankton and on average 20–30% of the nanobiota. Based on TEX86 calibration and T °C in bottom sediments, a relationship has been proposed linking these indicators (e.g., [35]).

Ketones (Alkenones). Long-chain n-ketones—saturated and unsaturated alkenones—are of great importance for determining the temperature of the environment. They were discovered in bottom sediments, then in cultures of coccolithophores, in which the ratio of saturated and unsaturated alkenones changes depending on temperature. Special indices were developed (\({\text{U}}_{{37}}^{{\text{k}}}\) = C37:2–C37:4/(C37:2 + C37:3 + C37:4) and \({\text{U}}_{{37}}^{{{\text{k}}{\kern 1pt} '}}\) = C37:2/(C37:2 + C37:3), which correlate well with the temperature of the surface layer of water and are used along with δ18O/δ16O as indicators of changes in paleotemperature, river runoff, and climate. The alkenone method for determining temperatures does not depend on the “light” composition of ice and is applicable to pelagic sediments. Determination of protium and deuterium in alkenones makes it possible to judge changes in salinity, and, together with the DNA of haptophyte algae (coccolithophores, etc.), to identify stages of climate change and glaciations in marine sediments [48].

Hopanes, terpenes, and terpenoids. The study of methane bubble flows in Laptev Sea sediments showed that biomarkers of methanotrophic microorganisms are a good indicator of its genesis. At “methane” stations, the average C30 content of hopenes in sediments was two times higher than in the background. The average C30 concentration of αβ-hopane (diploptene) and C32 αβ-hopane near methane seeps is 1.5 times higher compared to background stations [6]. The chirality and configuration of atoms in the series of triterpenoid hydrocarbons (hopane C31 and C32) are indicators of hydrocarbon sources and fluxes. In oils, the sources of hopanes are biohopanes with the R configuration. Steranes and hopanes retain the structure of their predecessors and are indicators of the input of petroleum hydrocarbons from seeps during methane bubble discharge.

Cluster 4 includes lignin and its structural monomers of the phenylpropane type, connected by ether and C–C bonds: aldehydes–p-hydroxybenzaldehyde, vanillin, syringaldehyde; ketones–p-hydroxyacetophenone, acetovanillone, acetosyringone; acids–p-hydroxybenzoic, vanillic, lilac, p-coumaric, ferulic, cinnamic are very effective organochemical indicators of terrigenous OM.

Lignin is widespread in coniferous and deciduous trees, shrubs, grasses, and mushrooms. It is resistant to decomposition and acts as the main supplier of cyclic compounds for humic substances. Lignin from coniferous trees is almost entirely composed of guaiacylpropane structures, while lignin from deciduous trees and grasses also contains syringylpropane and hydroxyphenylpropane structures. The main route of its entry into bottom sediments is river runoff and thermal abrasion. Lignin in the sea is synthesized in extremely small quantities by flowering plants and some macrophytes and has a specific composition represented by p-hydroxybenzaldehyde. Planktonic algae do not contain lignin.

Over the past 30 years, lignin and its molecular structures have been identified in many areas of the Pacific, Atlantic, and Indian oceans, in Arctic and Antarctic sediments, and in many rivers and estuaries. The determination of lignin phenols for the first time revealed p-hydroxybenzaldehyde, vanillic, syringic, and cinnamic structures in lichens of the Siberian Arctic, which distinguishes them from mosses and vascular plants [7]. The composition of phenolic monomers and their ratios (P/V, S/V, C/V) are good indicators of the transformation of OM during sedimentation, diagenesis, the formation of humic and sapropelic OM, input of anthropogenic lignin (wood processing, paper industry) [9], and coal formation processes [8]. The Ad/Al, S/V, C/V, and P/V indices reflect different degrees of OM conversion and microbiological activity. The thawing of permafrost rocks is recorded by an increase in the acidic forms of lilac and vanillic structures (the (Ad/Al)v and (Ad/Al)s indices increase). Determination of lignin in the Arctic seas showed that its content is significantly higher (up to 30% of OM) compared to shelf sediments of most areas of the World Ocean [9].

Cluster 5 contains polycyclic aromatic hydrocarbons (PAHs). They are common in soils, bottom sediments, water, and biota, and come from natural and anthropogenic sources [21]. The molecular composition of natural PAHs makes it possible to estimate the contribution of pyrogenic (forest fires) and petrogenic (natural oil seeps) sources of OM [21, 22, 54]. The share of naphthalene in the total PAH content reflects the contribution of terrigenous plant material [53]. The Fl/(Fl + Py) ratio is a sensitive indicator of the maturity and genesis of OM, the value of which increases with a general increase in the concentration of pyrogenic PAHs. The calculated index BaA/228 makes it possible to judge the anthropogenic load. For water areas of urbanized areas, the BaA/228 value in bottom sediments is usually higher than 0.35, which is associated with an increase in the proportion of pyrogenic PAHs (exhaust gases, burning of garbage, coal). Elevated (>0.1) An/178 values in sediments are a consequence of the appearance of pyrogenic components in PAHs, including those caused by anthropogenic load. The (Py + Fl)/(Ch + Ph) ratio also reflects the mutual contribution of petrogenic and pyrogenic hydrocarbons and decreases with the increasing contribution of petrogenic PAHs.

BIOGEOCHEMICAL PROCESSES

Bacterial degradation. Degradation and synthesis of OM are interrelated conjugate processes that govern the functioning of the carbon cycle. In the modern ocean, the leading role in the degradation of OM is played by bacteria and archaea. Bacterial degradation of initial OM significantly changes its composition, which affects all classes of organic compounds and leads, e.g., to an increase in the fraction of branched iso- and anteisoacids C15:0, C17:0, C18:1, high-molecular-weight fatty acids, characteristic hopanoids, brominated and chlorinated pyrrole derivatives [40]. Hydroxy fatty acids are used as indicators of the cell walls of Gram-negative bacteria. At the same time, bacteria do not synthesize or accumulate sterols. The latter are indicators of plant OM (dinosterol in dinoflagellates, β-cytosterols in terrestrial vascular plants) and ergosterol in fungi. Bacterial transformation of geopolymers such as fulvic and humic acids can occur with the release of various products such as CH4, CO2, \({\text{PO}}_{4}^{{3 - }}\), N2O2, and carbonates (e.g., [31]).

Reliable markers of dissolved Corg are D-amino acids of bacterial origin, not found in animals and plants, in contrast to L-amino acids. The percentage of dissolved OM produced by bacteria in the lower reaches of large Siberian rivers, calculated based on the abundance of D-amino acids, was highest during the spring flood and decreased during the rest of the year. During the spring flood, 31–42% of the DOC was of bacterial origin, while in winter/early spring, it was 21–33% [34]. Metagenomic studies of bacteria and archaea (to a lesser extent) using classical cultivation methods are complemented by studies by new approaches: amplification, sequencing of genes encoding ribosomal RNA (16SpRNA) characteristic of prokaryotes, analysis of genes encoding metabolic reactions of carbohydrates, lipids, methane, and many others compounds in the degradation–synthesis of OM.

Deoxygenation and anoxia. Reduction in oxygen content in waters of the modern ocean (deoxygenation), expansion of oxygen-free zones in water and bottom sediments (anoxia), sulfate reduction, leading to the formation and emission of hydrogen sulfide into the water column, as well as methanogenesis, increasing CH4 fluxes into the atmosphere are global problems solved from their biogeochemical study. At the heart of these problems are natural processes, the rates of which have increased dramatically as a result of human activities. The main reasons for the occurrence of anoxic conditions are warming of waters, their acidification, hindered water exchange due to high temperature, salinity, and density stratification gradients, oxygen absorption by nanobiota, and influx to the bottom and enzymatic decomposition of a large amount of OM [37].

Some organogeochemical indicators can be used to characterize OM transformations in the anoxic conditions of modern waterbodies. The C : N : P ratio in the biomass of phytoplankton, the main supplier of primary production in the ocean, averages 106 : 16 : 1 [42]. Under hypoxic and, moreover, anoxic conditions, the rate of OM mineralization is lower than under oxygenated conditions, and the C : N : P ratio differs significantly from that inherent in plankton due to the more rapid bacterial decomposition of compounds containing nitrogen and phosphorus. Under oxygen conditions, ammonium released as a result of degradation of OM is oxidized by oxygen by nitrifying bacteria, first into nitrites, then into nitrates necessary for photosynthesis. Bacteria of Planctomycetales were discovered in oxygen-deficient conditions; these bacteria directly oxidize ammonium to N2, using oxygen from nitrites and/or nitrates [26]:

$$\begin{gathered} {\text{NH}}_{4}^{ + } + {\text{NO}}_{2}^{ - } \to {{{\text{N}}}_{2}} + 2{{{\text{H}}}_{2}}{\text{O}}, \\ 5{\text{N}}{{{\text{H}}}_{3}} + 3{\text{HN}}{{{\text{O}}}_{3}} \to 4{{{\text{N}}}_{2}} + 9{{{\text{H}}}_{2}}{\text{O}}. \\ \end{gathered} $$

As a result, such environments are characterized by a nitrate/phosphate ratio (N : P < 16) significantly lower than the Redfield value.

Anoxic conditions based on Corg content and hydrogen sulfide derivatives (acid-soluble sulfides, pyrite, elemental, organic sulfur) in bottom sediments are divided into two types. The first includes reduced sediments containing Corg ~ 0.5–2% and the presence of a direct correlation Corg–Σ sulfides; the second includes highly reduced deposits, characterized by a Corg content of 2–3%, no correlation Corg–Σ sulfides (due to the consumption of all reactive Fe to bind H2S) and release of free H2S into ooze and bottom water [5]. In contrast to these two types, oxidized sediments are characterized by Corg < 0.5% and no H2S derivatives. Under anoxic conditions, most OM remains in sediments with a high sedimentation rate, including the products of its bacterial transformation and incomplete decomposition. Anaerobic degradation usually leads to a higher content of lipids and other hydrogen-enriched compounds than does aerobic degradation of OM. The role of anoxia in OM accumulation in sediments is not only to create conditions for the slow OM decomposition, but also to the development of chemoautotrophic bacteria and growth of anaerobic microbial biomass, which contains specific indicators of its origin.

Bacteria both degrade OM and resynthesize it anew. Long-chain n-alkanes and fatty acids (C25–C50) are poorly utilized by bacteria; the rate of degradation of long-chain fatty acids is six to seven times slower than short-chain fatty acids. When analyzing the molecular composition of n-alkanes, a good indicator of anoxic conditions is the ratio of isoprenoids synthesized by bacteria, i-C19/i-C20 (pristane/phytane). A ratio smaller than 0.5 indicates anoxic conditions; greater than 1.0, aerobic conditions. The metabolism of chemoautotrophic bacteria, like plants, is configured to preferentially utilize light isotopes, and, as a consequence, diagenetic transformations of OM lead to a shift in its isotopic composition. Bacteria primarily use labile organic compounds: proteins, free sugars, hydrolyzable polysaccharides, nucleic acids, polar lipids, and certain other heterocyclic compounds, e.g., chlorophyll, which contain nitrogen. During diagenesis, this leads to enrichment of OM in sediments with the isotopes 15N (δ15N increases) and a lighter isotopic composition of released inorganic nitrogen compounds [30]. A tendency towards variation in δ13C depends on the ratio of the fractions of marine and terrigenous OM in sediments, their initial isotopic composition, and the content of labile compounds in them; they can change sign during diagenesis [16].

The isotopic composition of the hopanoid diploptene synthesized by bacteria (17α(H),21β(H)-hop-22(29)-ene) can also be used as a bioindicator of anoxic conditions, δ13C ~ –31‰ is characteristic of nitrifying bacteria; the lighter composition δ13C ~ –39‰, of aerobic methanotrophic bacteria; the heaviest composition δ13C ~ –26‰ was observed in cyanobacteria [28]. It is suggested that the difference in the isotopic composition of diploptene reflects changes in the microbial structure of the community of bacteria living at the oxygen–anoxic boundary in the water column. Branched fatty acids (e.g., aC15:0, iC15:0, MeC16:0), found in bottom sediments, are formed by sulfate-reducing bacteria and indicate reducing conditions in the environment, while monounsaturated fatty acids (e.g., 16:1ω7c and 18:1ω7c) are generated by sulfide-oxidizing bacteria (Beggiatoa/Thioploca). Joint consideration of phospholipid fatty acids with isoprenoid quinones makes it possible to assess under what conditions (reductive or oxidative) diagenesis occurs [38].

Methanogenesis. The methane cycle in the World Ocean is a multifaceted process, spanning the water column, bottom sediments in seas on shelves and in the continental zone of the ocean. It is associated with the formation of hydrogen-sulfide, anoxic, dead zones in the ocean. Methane is mainly formed by methanogenic archaea, which are strict anaerobes and complete the process of decomposition of OM preserved and de novo synthesized by sulfate reducers. The aerobic genesis of methane consists of dimethylation of polysaccharide esters of methylphosphoric acid in the photic zone. Its genesis is associated with the metabolism of phytoplankton (e.g., [43].

There are few organogeochemical genomic indicators of bacteria and archaea and the isotopic composition of methane during generation and oxidation that allow us to judge its biogenic, thermocatalytic, or abiogenic origin. In the series biogenic–thermocatalytic–abiogenic methane its isotopic composition becomes heavier. The range of variation in this series δ13C is from –70 to –50‰ for biogenic (mainly methylotrophic) genesis and from –80 to –60‰ for bacterial methane in carbonate seeps; for thermogenic from –50 to –30‰; for abiogenic, it is heavier than –35‰. Variation in δD correlates with variation in δ13C and biogenic bacterial δD varies from –400 to –200‰; thermogenic from –250 to –150‰; abiogenic methane δD is apparently heavier, –100‰. These data were obtained in different ways, including calculation based on the ratio of methane and low-molecular-weight hydrocarbons C1–C3 [18, 45–47]. The difficulty in determining the genesis of methane based on the isotopy of carbon and hydrogen is that under natural conditions, we always observe a mixture of methane in different proportions of biogenic and thermogenic origin. Oxidation (methanotrophy) of methane also changes its composition in terms of δ13C and δD, which further complicates interpretation of methanogenesis.

CONCLUSIONS

The molecular composition of DOM and POM in seawater and OM buried in bottom sediments is enormous. The organogeochemical indicators presented in the study are not limited to five clusters. In addition, the composition of these clusters and new ones will be augmented with indicators, among which primarily will be biochemical and genomic research.