Organic Matter In Marine Sediments Research Articles

Sorptive preservation of labile organic matter in marine sediments

ORGANIC matter preserved in marine sediments provides a molecular record of marine biological processes1, accounts for approximately 20% of all carbon burial2 and plays a key role in balancing the long-term flux of oxygen to the atmosphere3. Only recently has it been appreciated that more than 90% of the organic matter preserved in most marine sediments is intimately associated with mineral surfaces4. Little is known, however, of the effect that sorption to mineral surfaces might have in controlling either the lability or quantity of organic matter in the marine sedimentary record. The preserved organic material could be either intrinsically stable, or stabilized through interactions with mineral matrices. We show here that sorption of organic matter to mineral surfaces in marine sediments stabilizes the component molecules, slowing remineralization rates by up to five orders of magnitude. Sorptive protection can therefore account for the enigmatic preservation of intrinsically labile molecules such as amino acids and simple sugars in marine deposits5,6 and links the preservation of organic carbon in marine sediments to the deposition of mineral surfaces.

Terrestrial organic carbon contributions to sediments on the Washington margin

Elemental and stable carbon isotopic compositions and biomarker concentrations were determined in sediments from the Columbia River basin and the Washington margin in order to evaluate geochemical approaches for quantifying terrestrial organic matter in marine sediments. The biomarkers include: an homologous series of long-chain n-alkanes derived from the surface waxes of higher plants; phenolic and hydroxyalkanoic compounds produced by CuO oxidation of two major vascular plant biopolymers, lignin and cutin. All marine sediments, including samples collected from the most remote sites in Cascadia Basin, showed organic geochemical evidence for the presence of terrestrial organic carbon. Using endmember values for the various biomarkers determined empirically by two independent means, we estimate that the terrestrial contribution to the Washington margin is ~ 60% for shelf sediments, ~ 30% for slope sediments, and decreases further to ≤15% in basin sediments. Results from the same geochemical measurements made with depth in gravity core 6705-7 from Cascadia Seachannel suggest that our approach to assess terrestrial organic carbon contributions to contemporary deposits on the Washington margin can be applied to the study of sediments depositing in this region since the last glacial period.

The Carbon Cycle: A Well-Known Tale is Full of Holes

This article discusses a recent Dahlem conference on the role of nonliving organic matter in the Earth's carbon cycle. Some points discussed are the huge amounts of saturated hydrocarbon cell wall material residue in the oceans, the biodegradability of charcoal, the anaerobic biodegradation of organic matter in marine sediments, limits on ocean productivity, and the nonliving organic matter in the soil, about twice as much as in the atmosphere and three times as much as in living plants.

On the equivalence of the power and reactive continuum models of organic matter diagenesis

The decomposition of organic matter in marine sediments is usually described by models in which it is assumed that organic matter is composed of a small number of reactive types (a discrete finite distribution), and that each type follows first-order kinetics. Two diagenetic models adopting a more physically realistic assumption that organic matter is composed of an infinite number of reactive types (a continuous distribution) have recently been developed: the reactive continuum model and the power model. The reactive continuum model, as it has been applied, is based on a gamma distribution of organic matter reactive types and can generate an apparent reaction order greater than one. The power model is based on the premise that the reactivity of organic matter gradually decreases with time; thus, the apparent rate constant becomes a continuously varying parameter. It is shown in this note that the reactive continuum model, based on a gamma distribution, is mathematically equivalent to the power model for simple closed-system decay.

The relationship between sedimentary organic carbon isotopic composition and organic biomarker compound concentration

A study of organic biomarker compounds which could serve as tracers of terrigenous and marine sedimentary organic matter sources was performed on samples from a 208.7 m hydraulic piston core hole (DSDP Hole 619) from the hemipelagic Pigmy Basin in the northern Gulf of Mexico. Organic carbon-normalized concentrations of total long chain (C 37–C 39) alkenones and some individual C 27–C 29 desmethyl sterols were determined to be useful proportional indicators (tracers) of preserved marine and terrigenous organic carbon, respectively. The alkenones, whose only known source is marine phytoplankton of the class Prymnesiophyceae, generally occurred in higher concentrations in interglacial isotope stages 1 and 5a-b than in the intervening glacial stages. Sterols (C 27-C 29), apparently of a dominantly terrigenous origin, occurred in lower concentrations during interglacial stages than in glacial stages. Tracers of both terrigenous and marine organic matter appear to be affected by the differential diagenetic alteration of the biomarker/C org ratios, as indicated by a simple, first-order kinetic model. The lack of any desmethylor 4a-methylsterol which is linearly related to the proportion of marine sedimentary organic matter (as scaled by δ 13C org) indicates that either 1. (1) sedimentary diagenesis has obscured the biomarker/C org vs. δ 13C org record 2. (2) phytoplanktonic assemblage changes caused variations in the biomarker/C org ratio of the primary input. Preferential preservation of terrigenous sterols may result in a biased sedimentary record of sterol input which could be misinterpreted as indicating solely terrigenous sterol sources. A simple model which characterizes the effects of sedimentary diagenesis on the relationship between C org-normalized biomarker ratios and δ 13C org demonstrates the potential problems of long-term, differential-diagenetic skewing on those tracer records.

Lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the Gulf of California: Comment and Reply

The role of bottom-water oxygenation in the degradation and preservation of marine sedimentary organic matter is the subject of controversy. Two opposing theories have been proposed to explain the typically higher organic carbon concentrations found in laminated, fully anoxic sediments and sedimentary rocks, when compared to their bioturbated, more oxygenated counterparts. One theory holds that preservation of organic matter is enhanced in laminated sediments because of slower net rates of organic carbon decomposition under fully anoxic conditions (e.g., Cranfield, 1989, 1992). Alternatively, it has been suggested that organic-rich laminated sediments reflect high levels of primary productivity in the surface waters and, consequently, increased proportions of organic matter in the particulate flux deposited at the water-sediment interface, rather than enhanced preservation in the sediments (e.g., Pedersen and Calvert, 1990). Calvert et al (1992) compared data on modern surface sediments of the Guaymas Basin accumulating insides as well as outside the zone where the oxygen minimum impinges on the sea floor. The choice of sediments deposited in close proximity minimizes the effects related to variations in the nature and mass flux or organic matter reaching the water-sediment interface. The data were collected on the first 2-5 cm of gravity cores. With themore » high sediment-accumulation rates in the Guaymas Basin (see Calvert et al., 1992), organic matter in the sediments analyzed has been subjected to early diagenetic processing for only short periods of time, at most 10-20 years. Calvert et al. reported the absence of systematic variations of the total-organic-carbon concentrations and Rock-Eval hydrogen and oxygen indices with bottom-water oxygen concentrations. This led them to conclude that bottom-water oxygen levels exert no major influence on the concentration and composition (in terms of the H/C and O/C ratios) of organic matter in the sediments. 10 refs.« less

A kinetic model for microbic organic-matter decomposition in marine sediments

Abstract The microbial decomposition of organic matter in marine sediments has been described using a rate law which is linear in the concentration of the metabolizable organic matter, entirely independent of the biomass concentration, and displays a Monod-like dependence on the concentration of the oxidant species being utilized. This paper develops a quasi-reaction mechanism to explain these observed dependencies. The Monod dependence on the oxidant concentration and the biomass independence can both be traced back to the preponderance of occupied sites on the organic matter and pseudo-steady state concentrations of occupied sites during most of the decay process. In the framework of the proposed reaction scheme, the linearity of the rate with the organic matter concentration is possible only if there is a linear proportionality between surface area and mass of organic matter. The rate law described above divides natural organic matter into a finite collection of reactive types. This paper also reformulates this rate law model in terms of a continuum of reactive types as proposed by Boudreau and Ruddick (Am. J. Sci. 291, 507–538, 1991). It is shown that this continuum model can produce a better fit to kinetic rate data than the discrete model.

A kinetic model for microbic organic-matter decomposition in marine sediments

The microbial decomposition of organic matter in marine sediments has been described using a rate law which is linear in the concentration of the metabolizable organic matter, entirely independent of the biomass concentration, and displays a Monod-like dependence on the concentration of the oxidant species being utilized. This paper develops a quasi-reaction mechanism to explain these observed dependencies. The Monod dependence on the oxidant concentration and the biomass independence can both be traced back to the preponderance of occupied sites on the organic matter and pseudo-steady state concentrations of occupied sites during most of the decay process. In the framework of the proposed reaction scheme, the linearity of the rate with the organic matter concentration is possible only if there is a linear proportionality between surface area and mass of organic matter. The rate law described above divides natural organic matter into a finite collection of reactive types. This paper also reformulates this rate law model in terms of a continuum of reactive types as proposed by Boudreau and Ruddick (Am. J. Sci. 291, 507–538, 1991). It is shown that this continuum model can produce a better fit to kinetic rate data than the discrete model.

Occurrence of particle‐bound polysulfides and significance of their reaction with organic matters in marine sediments

Inorganic polysulfides (Sn2−, where n > 1) contribute to the incorporation of sulfur into organic matter in reducing sediments, especially when conditions exist for partial oxidation of sulfide. Inorganic polysulfides have been assumed previously to exist only in the aqueous phase. Our results suggest that a significant fraction of inorganic polysulfides is bound to the particulate‐phase of sediments and can react with carbon‐carbon unsaturated bonds in organic matter, thereby attaching it to the particulate phase through polysulfide linkages. This mechanism may affect the bioavailability of the reactive organic matter in sediments and hence may play an important role in the preservation of organic matter in anoxic marine sediments.

Early diagenesis of organic matter in marine sediments: progress and perplexity

Early diagenesis is the combination of biological, chemical, and physical processes which change the quantity and composition of the organic matter in the upper several hundred meters of marine sediments. During early diagenesis, from 30% to more than 99% of the organic matter deposited on the sediment surface is remineralized by sediment organisms. Rates of organic matter oxidation or oxidant consumption have now been measured in a variety of sediments and range over at least three orders of magnitude, from 20 mmol C m −2 year −1 to 60 mol C m −2 year −1. High rates of organic matter oxidation are associated with high rates of sediment accumulation, organic matter influx to the sediment surface, and organic matter burial. Some patterns in the decomposition rates of different organic substances have emerged. Small, dissolved molecules (e.g. acetate) are remineralized much more rapidly than the biopolymers in fresh algal detritus, which in turn break down faster than refractory macromolecules such as lignins and melanoidins. Much of the organic matter in sediments consists of macromolecules of uncertain origin and composition which resist decomposition. However, the specific compound classes which have been measured in sediments (e.g. hydrolyzable amino acids and carbohydrates, fatty acids and hydrocarbons) often decompose as slowly as TOC (total organic carbon) in the upper 1 m of coastal sediments. In many sediments, a substantial fraction of the organic matter deposited on the sediment surface is not remineralized during early diagenesis. This preservation of organic matter in sediments remains partly a puzzle. One key process could be the incorporation of organic compounds into refractory geomacromolecules or humic substances. However, detailed information on the composition and structure of geomacromolecules and concrete evidence for a mechanism of geomacromolecule formation remain elusive. An alternative hypothesis is that much of the organic matter preserved in sediments is refractory when deposited and survives diagenesis with little change.

Evaluation of the Toxicity of Organic Matter in Marine Sediments

Evaluation of the Toxicity of Organic Matter in Marine Sediments

Sources of organic matter to surficial sediments from the Scotian Shelf and Slope, Canada

On the basis of chemical analyses of sediments sampled by grab and by box corer on the Scotian Shelf and Slope off Nova Scotia, Canada, an attempt is made to assign to source the organic matter (OM) found in surface and near-surface sediments. Concentrations of organic carbon and nitrogen were higher in Shelf basins and the Laurentian Channel than on the Shelf banks and Slope. Organic carbon/nitrogen (C/N) ratios within the basins and Channel imply OM is predominantly marine in origin. There is no progressive decrease in C/N across the shelf and down the slope that could be interpreted as due to declining terrestrial influence offshore, but a decrease with distance west of Cabot Strait implicates the Gulf of St Lawrence as a major source of terrigenous OM to the Shelf east of 62°W. The distribution of less-polar organic fractions ( n-alkanes, isoprenoids, fatty acids, sterols) in the Gulf indicates that there is OM of both marine and terrestrial origin in surface unconsolidated sediments with a dominance of the terrigenous component. This study of a well-characterized continental margin illustrates that existing estimates of the mass and composition of organic sedimentation world-wide are far from being robust. The distribution of OM in marine sediments is complex and it is premature to generalize about the proportion of terrigenous to marine OM preserved in oceanic sediments.

Low organic carbon accumulation rates in Black Sea sediments

THE Black Sea, the world's largest anoxic marine basin, is frequently used as a modern analogue for the formation of organic-rich sediments and carbonaceous rocks1–3, on the widely held assumption that anoxic conditions promote the preferential preservation of organic matter in sediments. Data for testing this hypothesis have so far been equivocal4–7, but here we use radiocarbon ages obtained using accelerator mass spectrometry for the organic fraction of recent Black Sea sediments to estimate the organic carbon accumulation rates. These range from 0.69 to 2.09 g C m−2 yr−1 and are significantly lower than earlier estimates based on varve counting6. Depending on the value taken for the rate of primary production in the Black Sea4,8, between 0.7 and 2.1% of the organic carbon is preserved in the bottom sediments. When compared with carbon accumulation rates in equivalent oxygenated environments9, these results indicate that the modern Black Sea is not a site of anomalously high organic carbon accumulation. This suggests that anoxic conditions in the water column may not be a prerequisite for the preservation of organic matter in marine sediments, and that models of the origin of carbonaceous facies in the geological record may therefore need to be modified.

A simple rate model for organic matter decomposition in marine sediments

A model is presented for the decomposition of organic matter in marine sediments. The model considers the organic matter as a whole. In this model the first-order rate parameters (k) gradually decrease with time (t) according to logk = −0.95 logt − 0.81 (N= 140; r = 0.987). This equation is found to be valid over eight orders of magnitude and is based on organic carbon versus depth profiles of well-dated cores and laboratory experiments. The predicted continuous decrease in the reactivity of organic matter is consistent with multi-G models and in situ measurements of sulphate reduction.Although all organic carbon versus depth profiles follow the same reactivity decrease with time, their reactivity at the sediment-water interface (i.e., “apparent initial age”) is different. Apparent initial ages calculated on the basis of various experiments and field measurements show consistent trends.

C-Isotope stratigraphy, a monitor of paleoenvironmental change: A case study from the early cretaceous

Today's disturbance of the global carbon cycle induced by anthropogenic processes has raised new interest in the history of the global carbon cycle and its relationship to climate and other geochemical cycles. Carbon-isotope stratigraphy proves to be most useful as a monitor of the history of the carbon-cycle during the last 200 million years. In the introductory paragraphs of this review the mode of functioning of the global carbon-cycle is summarized and the connection between carbon-cycle and carbon isotope geochemistry is documented. A case study on the disturbance of the global carbon cycle during the Aptian-Albian is presented. The disturbance of the carbon cycle lasting up to millions of years is recorded in the carbon-isotope stratigraphy of pelagic sediments. It is superimposed on high frequency sedimentological cycles, related to climate and oceanographic cycles of 20, 40 or 100 ky duration. The data reviewed suggest that the change in the global carbon system was linked to a global acceleration of geochemical cycles triggered by a long-term change in atmospheric CO2 controlled by the rate of sea-floor formation and by volcanic activity. Increased accumulation rates of terrestrial material and terrestrial organic matter in marine sediments may be used as an indicator of an intensified hydrological cycling resulting in higher water-discharge rates. An intensification of the Aptian-Albian water cycle is further reflected in continental sediments monitoring a period of elevated humidity. An increase in water discharge rates should have affected the transfer rate of dissolved nutrients from continents to oceans. Elevated concentrations of phosphorus may have led to an increase in Aptian-Albian oceanic productivity enhancing the transfer of marine organic matter from the oceanic into the sedimentary reservoir. Increased productivity, increased bulk sedimentation rates and poorly oxygenated deep-water led to increased preservation of marine and terrestrial organic matter in marine sediments. The accelerated output of marine organic carbon from the oceanic reservoir is ultimately registered in the positive carbon-isotope excursion of the marine carbonate carbon-isotope stratigraphy.

Measurement of volatile fatty acids in pore water from marine sediments by HPLC

Lower molecular weight volatile fatty acids are important intermediates in the anaerobic degradation of organic matter in marine sediments. The analysis of these compounds at the low in situ concentrations, however, still presents difficulties. A new derivatization procedure for the analysis of these compounds, coupled with high-performance liquid chromatography, has been modified for the analysis of volatile fatty acids in marine pore water to cover a linear calibration range from 0·5 μ m to 10 m m. The modifications resulted in the detection of concentrations 40 times lower than in the original method and in good recoveries of fatty acid standards added to pore water (mean 101%). This modified procedure was then used to analyse fatty acids in pore water from sediments along a gradient of organic enrichment. The relative ratios of the individual acids were 1:0·1:0·02:0·01:0·02:0·01 for acetate, propionate, n-butyrate 2-methylbutyrate, iso-valerate and n-valerate. There was a tendency for the concentration of the more reduced acids to increase (propionate and n-butyrate) as organic enrichment increased. Several fatty acids were found that have not commonly been reported in marine pore water. These include 2-methylbutyrate, which is a specific anaerobic degradation product of iso-leucine, which, to the authors' knowledge, has not been previously found in marine pore water.

87:6405 Accumulation dynamics of dispersed organic matter in marine sediments and variations in displacement velocities of crustal plates: Aparin, V.P. and V.Ya. Trotsyuk, 1986. Int. Geol. Rev., 28(7):846–849

87:6405 Accumulation dynamics of dispersed organic matter in marine sediments and variations in displacement velocities of crustal plates: Aparin, V.P. and V.Ya. Trotsyuk, 1986. Int. Geol. Rev., 28(7):846–849

Oceanographic controls on the accumulation of organic matter in marine sediments

Summary The concentration of organic matter in marine deposits depends on the relative rates of accumulation of the various sedimentary components and the ease with which these components are preserved after burial. The supply of organic matter to any area of sea floor is controlled by the primary production rate in the surface ocean and the depth through which particulate material must settle. The accumulation of organic matter in sediments depends on the primary setting flux and on the bulk sedimentation rate, more carbon surviving decomposition where it is buried rapidly. The preferential preservation of organic matter under anoxic conditions, a widely-accepted explanation for the formation of sapropels, black shales and petroleum source beds, is considered to be of secondary importance in governing the accumulation of carbon in marine deposits. The modern sediments of the Black Sea, a widely-used analogue of the environment of black shale formation, do not have exceptionally high carbon contents, although an earlier sapropel is known to have formed during the change from an oxic lake to an anoxic marine basin. Likewise, the available information on modern anoxic fjords shows that they do not have sediments containing more carbon than their oxic counterparts. And finally, the inverse correlation between the carbon content of slope sediments and the oxygen content of the near-bottom waters in areas where the oxygen minimum intersects the sea floor is shown to be one of coincidence and understandable on the basis of the relationship between sedimentary carbon levels and other sediment properties. The supply of organic matter to marine deposits, controlled by the primary production rate, and the bulk sedimentation rate appear to be the most important factors which determine the organic content of sediments. Recent work on the location of former upwelling centres suggests that variations in palaeoproductivity may be responsible for the formation of many organic-rich rocks and may provide a better basis for predicting the location of petroleum source beds in the geological record.

Preservation of reactive organic matter in marine sediments

It is generally assumed that the reactivity of organic matter and the amount preserved in sedimentary deposits necessarily increases with total sedimentation rate. In some environments, such as deltas, where supply of unreactive terrigenous debris may vary independently of reactive organic matter input, the amount of reactive organic material preserved can in fact theoretically correlate either directly or inversely with sedimentation rate. The amount preserved can be shown quantitatively by transport-reaction models to depend on (1) the relative importance of electron acceptor concentration in the overlying water, (2) advection during sedimentation, (3) dilution by sedimentation, (4) solute diffusion, (5) initial flux of organic matter, and (6) the magnitudes of reaction rate constants. Both single reaction rate constant and multiple reaction rate constant models suggest that, at steady state, maximum preservation with respect to a given oxidant occurs when D s k = w 2 , where D s = whole sediment diffusion coefficient of the electron acceptor, k = first-order rate constant of the dominant organic fraction, and w = sedimentation rate. This is the likely basis for the reported correlation between average reactivity, k, of carbon in a deposit, percent carbon preservation, and w 1.5–w 2. Because of the variety of factors which determine these relationships, such correlations are probably valid only within specific classes of depositional environments.

The nature and distribution of organic matter in the surface sediments of world oceans and seas

Existing data on the distribution of organic carbon and nitrogen in marine sediments have been analyzed in order to better understand the physical and chemical processes involved in this aspect of the global carbon cycle. Maps of the global distribution of organic carbon and nitrogen in the sediments of world oceans and seas are presented. Correlation analyses of the available information dealing with the distribution of marine sedimentary organic matter has revealed that in terms of bulk parameters (%C O rg and %N Kjeldahl), there is an apparent accumulation of organic matter on the continental slope (water column depth 200–2000 m). Specific interactions between clays and organic matter, although indicated in laboratory experiments, have not been detected by these analyses.