Sequestration of Organic Matter in Marine Mud: Flume Experiments, Biogeochemistry, Mass Physical Properties, and 3D Modeling
Published:December 01, 2012
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Richard Bennett, Juergen Schieber, Arndt Schimmelmann, Matthew Hulbert, Kenneth Curry, Jessica Douglas, Andrew Head, Ann Curry, 2012. "Sequestration of Organic Matter in Marine Mud: Flume Experiments, Biogeochemistry, Mass Physical Properties, and 3D Modeling", New Understanding of the Petroleum Systems of Continental Margins of the World, Norman C. Rosen, Paul Weimer, Sylvia Maria Coutes dos Anjos, Sverre Henrickson, Edmundo Marques, Mike Mayall, Richard Fillon, Tony D’Agostino, Art Saller, Kurt Campion, Tim Huang, Rick Sarg, Fred Schroeder
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Organo-clay fabric and physico-chemistry of marine mud play important roles in early sediment diagenesis including the development of mass physical properties, consolidation behavior, and sequestration of organic matter (OM) in sediments over geologic time. Transmission electron microscopy (TEM) images of nano- and microfabric reveal that organic matter is sequestered following enzymatic digestion despite the pervasive openness of pore-fluid pathways observed in 3D rotated images. The locations of sequestered organic matter correspond to those predicted by modeling of the potential energy of interaction. Initial flume experiments on high porosity clay-mineral-rich mud deposited under dynamic flow and static (vertical settlement) conditions demonstrate differences in clay fabric and the distribution of organic matter (we define the term organo-clay fabric as the contiguous association and arrangement of organic matter and clay domains). These differences in organo-clay fabric impact the preservation-degradation mechanisms and dynamics during depositional and burial processes. Organo-clay fabric and physico-chemical modeling of potential energy fields coupled with direct observations of organo-clay fabric, three-dimensional (3-D) clay fabric reconstructions, sediment static and dynamic properties, and controlled flume experiments are providing new insight into the developmental history of sedimentary sequences, nano- to macroscale environmental processes, and diagenesis from unconsolidated mud to shale.
Marine continental margin fine-grained sediments are major depositional sinks for organic matter. These deposits, rich in organic matter, are the precursor of source rocks of oil and gas and are recognized not only as the progenitor of fossil fuels but also as major active carbon reservoirs on geological time scales. This paper focuses on two of four major hypotheses important in the sequestration of organic matter in marine mud commonly composed predominately of clay minerals containing high percentages of illite, smectite, chlorite, and mixed-layer clays (Ransom et al., 1998). The four hypotheses receiving great attention by researchers in recent years regarding organic matter protection in clay mineral-rich mud are: (A) chemical alteration of organic matter against biotic degradation (recalcitrance) (Hedges and Keil, 1995); (B) lack of available oxidants (Hedges et al., 1999); (C) physical protection by isolation (i.e., total encapsulation) or by extreme tortuosity (Mayer et al., 2004; Rothman and Forney, 2007; Middelburg and Meysman, 2007); and (D) physico-chemical protection of organic matter against enzymatic degradation by direct association with clay mineral domains (Keil and Hedges, 1993; Curry et al., 2007; Bennett et al., 2012). The literature is extensive with papers addressing hypotheses A – D and reflects considerable earlier research results that were achieved by indirect methods and techniques. Our present research addresses the two hypotheses (C) physical protection and (D) physico-chemical protection, and it provides direct transmission electron microscope (TEM) observations of the organo-clay fabric (organic matter and clay fabric). The fabric comprises three types or modes of domain particle contact among platelike clay particles (phyllosilicates). These modes are termed edge-to-face (EF), edge-to-edge (EE), and face-to-face (FF) fabric (Fig. 1). A clay domain is the parallel alignment (face-to-face) of crystals of fixed lattice clay minerals such as illite and they are commonly somewhat offset, (Moon, 1972; Bennett and Hulbert, 1986). The edges and faces of clay particles carry electrostatic charges that significantly impact their behavior when in aqueous environments. These fabric characteristics are a central focus for our physico-chemical modeling of organic matter sequestration as predicted by their potential energy fields (Fig. 2) and associated electrostatic and dynamic fields of organic matter-clay domain interaction, the “building blocks” of finegrained sediment.
The objective of the flume experimental research was to understand the role of organic matter in the development of clay fabric and potential organic matter sequestration in aggregates, floccules, and related fabric characteristics under the different conditions of marine continental margin transport and deposition i.e., dynamic (current ~20 cm/s) versus static (still water). Different percentages of total organic carbon (TOC) ranging from nearly zero percent to 7.2% (percent dry mass corrected for salt content) were measured in sub-samples from the basal layers to the deposits at the sediment-water interface, respectively.
The two depositional modes of clay rich suspensions having different percentages of OM are classified here as dynamic and static (Schieber et al., 2011). In the flume experiments, “dynamic” mode implies that clay-organic matter floccules pass through the basal shear layer of the flow and travel as a bedload. As sufficient concentrations of floccules or aggregates (non-generic term) accumulate, they form migrating ripples and a mud bed accretes. The “static” mode implies that clays and organic matter are deposited vertically under the influence of gravity, may flocculate in suspension, and settle at the bottom of a vertical tube free from direct current-driven forces (laminar and turbulent flow).
Microscope (TEM) images shown here were representative of samples with the lowest levels (weight percent dry mass) of organic matter present in the sediment deposits (dynamic and static) during the initial deposition in one of the flume experiments (Fig. 3). The total deposition in the dynamic deposit was a ~8.5-cm deep sedimentary column, whereas the static-settled deposit measured only ~7.5 cm in total thickness. The initial basal layer of the dynamic deposit had a TOC concentration of 0.31% (mass corrected for salt content) and was slightly lower in water content at 66.7% than the static basal deposit with 69.7% at 6.9 cm sub-bottom. The static basal deposit had a TOC concentration of 0.22% and was thus slightly lower in organic matter than the basal dynamic deposit. Differences were observed in the TOC and water content in the static and dynamic deposits as organic matter was added to the flume during the experiment. The deposit at the sediment-water interface in the static environment was characterized by a TOC concentration of 3.0% and water content of 82.4% (percent total mass) whereas the deposit at the sediment water interface in the dynamic deposit was characterized by a TOC of 7.2% and a water content of 92.0%. Both static and dynamic sedimentary deposits were handled and subsampled using the same techniques in order to minimize the introduction of differential changes in the sediment properties. The observed differences in TOC and water content between the static and dynamic deposits are primarily a result of the processes and mechanisms that drive organo-clay fabric development under different environmental forcing factors during transport and deposition. We discuss these briefly and show a few examples.
The dynamically driven process in a flow regime resulted in deposits comprising organic matter-rich clay macroaggregates frequently containing what appeared to be microaggregates. Fabric is characterized by varying degrees of edge-to-edge and edge-to-face signatures (Fig. 3A). In contrast, the sample representing a static quiescent environment shows clay deposits characterized by a relatively uniform distribution of clay sediment loosely associated with organic matter forming small floccules (microaggregates). Fabric is characterized by some edge-to-face signatures (Fig. 3B). A detailed description of the fabric characteristics is in the caption of Figure 3.
The second experimental approach was the use of the TEM to examine hundreds of photomicrographic images of nano- and microfabrics subsampled from several different fine-grained sediments including not only the Indiana University flume deposits, but also laboratory-prepared chitin-rich clay muds, polychaete fecal pellets, and marine muds from the Gulf of Mexico continental margin. The TEM investigations required extensive laboratory preparation and processing of sediment samples to preserve the structural integrity of the organo-clay fabric with staining and tagging to make the location of organic matter visible. Standard TEM stains of lead citrate and uranyl acetate (Hayat, 1993) were used to enhance contrast of organic matter. Silver proteinate staining was used for high resolution visualization of chitin locations in the matrix in some samples (Curry et al., 2007). Details of the techniques can be found in earlier published works (Bennett et al., 1977, 1981, 1989; Baerwald et al., 1991; Curry 2007, 2009). As part of the TEM experimental approach, techniques were developed for serial sectioning of embedded clay samples to create photomicrographic mosaics from which 3-D images were reconstructed to provide visualization of clay nano- and microfabrics for quantitative and qualitative analysis using ImagePro Plus 7.0 graphics software (Douglas et al., 2011). The 3-D clay fabric images reveal details of the aggregates, porosimetry, the fabric signatures, and the morphology at various scales.
Natural samples show the generally close association of organic matter with clay surfaces and especially in close proximity with clay domain contacts as predicted by the potential energy model. Two natural samples can be contrasted here (Fig. 4). Figure 4A shows sediment from the Gulf of Mexico taken near the site of the Deep-Water Horizon oil spill (Head et al., 2012). Oil itself is electron transparent and no special staining has been used to visualize the oil. However, organic matter of recent biological origin can be seen throughout the sample. Figure 4B shows a 3.5 μm2 section of a polychaete fecal pellet (ca. 0.5 mm diameter). Polychaete worms are responsible for bioturbation of massive volumes of marine mud deposits and results in mixing and on-going modification of the initial depositional fabrics in surficial sediment. The remolded mixture of clay and organic matter results in close association of clay surfaces and interstices with organic matter. A 3-D reconstruction of electron micrographs taken from serial sections of the fecal pellet sample shows the clay-organic matter association (organo-clay fabric) from four differently tilted angles (Figs. 5A–D) and a short video is provided (Fig. 5E). Our ability to rotate such images and take various quantitative measurements gives us a powerful technique for the understanding of clay-organic matter dynamics and early diagenetic processes.
A laboratory consolidated model of a smectite-illite clay with chitin added (10% dry-weight) as representative of marine organic matter was subjected to in situ enzymatic digestion using chitinase and was compared to an undigested sample with respect to the distribution of chitin that remained (Fig. 6; Curry et al., 2007).
Our theoretical approach to interpreting samples with respect to organic matter sequestration and protection is based on the expansion of an earlier 2-D model (Bennett and Hulbert, 1986) that described the potential energy fields at the interstices of clay domains (e.g., at edge-to-face contacts). In seawater, very near electrostatically charged clay surfaces, hydrated ions of opposing charge, are concentrated near the clay domain surface and form a so-called electrical double layer. Organic matter contains chemical functional moieties that can hold charges; for example, negative charges on carboxyl groups and positive charges on amino groups. Electrostatic attraction between opposing charges of organic matter and clay surfaces contribute to the strength of adsorption, and thus to the organic-inorganic interaction. As developed in a recent 3-D model (Bennett et al., 2012) for negatively charged organic matter, a charged or polar particle approaching the clay particle or a clay fabric signature will experience a potential energy field determined by the combination of the negative charge of the clay particle(s), electrostatic charges (hydrated ions) of the seawater, van der Waals attraction, and Born repulsion. When multiple clay particles are aggregated or are in close contact the potential energy at each location (point) in the field of the 3-D assemblage is determined by the effects of all the surrounding particles in the vicinity of the given point in space. The expanded potential energy model is used in this investigation to predict organic matter sequestration sites in the fabric.
The general expression of the potential energy relationship is:
We note here that by convention, repulsive interactions are given a positive sign and attractive ones a negative sign. If the charges of both the clay faces and the approaching particles are the same (negative) and the distance between the clay face and the approaching particle is symbolized by d, then the form of this equation is:
where the coefficients for each term are indicated by k.
The electrostatic interaction decreases exponentially as the distance of separation increases. London-van der Waals attraction between planar particles (and those, as organic matter is modeled here, are of sufficiently large radius of curvature to be treated as planar) is inversely proportional to the square of the distance of separation. Born repulsion is modeled as inversely proportional to the distance of separation raised to the 12th power. Additional details of the equation are found in Bennett et al. (2012) where details are provided that describe the potential energy wells at edge-to-face and face-to-face fabric positions and also on clay domain faces.
Organic matter protection initially begins as it attaches to clay domains in the water column and at the seafloor sediment; this attachment not only hastens organic matter removal from regions of greatest degra-dative biological activity but also provides a small degree of physical protection because the mineral phase impedes attack from some directions. As the organic matter and mineral complexes continue to be modified by hydrodynamic and consolidation stresses, some of the organic matter is more permanently protected from degradation by physical protection; that is, by becoming encased within clay domains arranged so that the pathways attacking microbes or exogenous enzymes must travel either become very long and tortuous or are blocked entirely.
In addition, some of the organic matter may be located at, or migrate to, locations where it has physico-chemical protection associated with the interaction energy between the organic matter and the clay domains. Our model predicts moderate attraction for the organic matter in the vicinity of the face of the clay particle or domain and stronger attraction at an edge-to-face contact and, in an offset face-to-face contact, where one particle or domain terminates and the other continues (Figs. 1 and 2; Bennett et al., 2012).
Our research focus addresses two important hypotheses of organic matter sequestration (protection) that are characteristic of clay mineral-rich marine mud: (1) physical protection and (2) physico-chemical protection. The processes and mechanisms that drive organic matter protection are important during sediment transport, deposition, and especially during early diagenesis near the sediment/water interface. Marine mud deposits are globally abundant and contribute massive stratigraphic sequences and complexity to the marine continental margin deposits and geological record. Our analytical techniques include direct TEM observations of organic matter/clay fabric interrelationships at nano-to micrometer scales. TEM observations provide the underpinning to assess not only the clay fabric morphology of aggregates that trap organic matter (physical protection) during sediment transport and deposition but also provide a reliable foundation for theoretical physico-chemical modeling of organic matter sequestration (physico-chemical protection) as predicted by potential energy fields associated with specific clay fabrics and associated electrostatic and dynamic fields of organic matter/clay domain interaction.
Flume experiments simulating static and dynamic deposits indicate that static depositional process of early flocculation and aggregation of clay minerals and organic matter results in a different clay fabric than is observed in turbulent (dynamic) processes when both the clay mineral types and initial mass ratios of organic matter-to-clay are initially comparable (same mineralogy and TOC in water column (Schieber et al., 2011).
Compound macroaggregates made up of smaller aggregates may entrap large amounts of organic matter during sediment transport in turbulent and laminar flow. These sizable compound aggregates form a deposit having relatively strong physico-chemical bonding of clay domains as evidenced by the relatively large number of edge-to-edge signatures (Fig. 3A). The macroaggregates appear as tightly bound units of clay/ organic matter suggesting enhanced physical protection of organic matter against enzymatic access and digestion. The combination of macro- and microaggregates is interpreted as interspersing with the clay to a greater extent than the static-formed fabric with its predominate microaggregate structure characterized edge-to-face fabric. This enhanced interspersion is attributed to the greater energy of the system that comes from turbulence as sediment and organic matter aggregate. The formation of compound aggregates presents greater available surface area for organic matter, which our model suggests will afford superior physico-chemical protection to organic matter via adsorption. The efficient interspersion of clay into high concentrations of organic matter especially at high porosity may initially have the effect of enhancing the post-depositional organic matter degradation rate by increasing the surface area of organic matter exposed to degrading enzymes despite some physical protection from the clay mineral phase but at low organic matter concentrations ( ≤ 2 wt% TOC, Bennett et al., 2004) we expect the turbulent-formed clay fabric to afford enhanced physico-chemical protection to organic matter over static-formed fabric.
Our two natural samples are both from the Gulf of Mexico. One is from the vicinity of the Deep-Water Horizon oil spill (Fig. 4A) and the other is a fecal pellet from a polychaete (Fig. 4B). The Deep-Water Horizon sample shows a modest concentration of organic matter that is largely associated with clay mineral surfaces and located at clay fabric sites as predicted by our deep energy well theory. Liu et al. (2012) have characterized biological degradation of oil from the spill in sediment near the wellhead as “slow and light,” consistent with both the recalcitrance of many of its components and our model of organic matter physico-chemical protection against enzymatic digestion. Organic matter also can be seen encapsulated by fabric which suggests some physical protection against enzymatic digestion, although the degree of encapsulation cannot be known from a 2-D image.
By contrast, organic matter observed in the 2-D fecal pellet images features a high concentration of partially digested organic matter (plant and animal fragments, Fig. 4B). The 3-D reconstruction of the fecal pellet clay fabric allows an assessment of the degree of organic matter encapsulation in clay, demonstrates a high degree of compaction compared with the Deep-Water Horizon sample, and thus highlights the physical protection of organic matter against enzymatic digestion afforded by the clay fabric. Small pore throats envisioned in the fabric surrounding the organic matter aggregates would physically exclude or diminish the access of enzymes. The fabric of the fecal pellet is denser than any of our other samples, so the organic matter is necessarily found close to clay surfaces where we expect physico-chemical protection of OM against enzymatic activity.
During the initial contacts between organic matter and clay particles, there is no apparent preferential location of the organic matter at the different clay fabric signatures: edge-to-face, bridging edge-to-edge, edges of face-to-face fabric locations, and clay domain faces (extensive surface areas, Fig. 6A). As the organic matter is degraded (Fig. 6B) during early sediment diagenesis the experimental observations confirm that organic matter is retained at high potential energy model sites (Bennett et al., 2012) and will remain at these predicted sites protected from enzymatic digestion over geological time scales (Curry et al., 2007).
We note that the amount of organic matter (determined by TOC and total organic matter measurements) in clay sediment and observed in open pores, pore networks, and at specific fabric sites, will be a function of not only the potential energy fields but also depends on the percentage of common clay minerals per unit volume (i.e., increasing amount of clay minerals with increasing wet bulk density, Mg/m3, Bennett et al., 1985), the amount of initial organic matter loading of clay minerals during transport and deposition, the redox state including availability of oxygen and other oxidizers, the post-depositional biogeochemical processes, the physical protection of organic matter to enzymatic activity by boundary conditions, and complexity of the porosimetry (e.g., tortuosity), and availability of enzymes spatially and temporally.
This research was funded initially by a National Science Foundation Grant Number SGER-0438079 (Small-Grant for Exploratory Research) followed by current NSF Grants OCE-0824569 and OCE-0930879 (USM) and NSF Grants OCE-0824566 and OCE-0930685 (SEAPROBE, Inc.). The flume experiments were supported by NSF Grant OCE-0930829 (Indiana University). The authors gratefully acknowledge the opportunity provided by NSF to pursue this research. The authors appreciate the efforts of Dr. Patricia Biesiot, USM, who contributed to various phases of the early initial study of marine polychaete pellets. The authors appreciate the efforts of Dr. Charlotte Brunner, USM, who contributed the two Deep Water Horizon cores. The authors and institutions supporting this research do not endorse any products, including software, mentioned in this report.
Figures & Tables
New Understanding of the Petroleum Systems of Continental Margins of the World
- Atlantic Ocean
- carbon sequestration
- clastic sediments
- continental margin sedimentation
- Deepwater Horizon oil spill
- electron microscopy data
- electrostatic properties
- fecal pellets
- fluid dynamics
- flume studies
- Gulf of Mexico
- marine environment
- marine sediments
- North Atlantic
- oil spills
- organic carbon
- organic compounds
- physical properties
- source rocks
- TEM data
- three-dimensional models