Energy-®ltering transmission electron microscopy (EFTEM)and electron energy-loss spectroscopy (EELS) investigation of

clay±organic matter aggregates in aquatic sediments

Yoko Furukawa

Naval Research Laboratory, Sea¯oor Sciences Branch, Stennis Space Center, MS 39529, USA

Abstract

High resolution (<10 nm) transmission electron microscopy (TEM), energy-®ltering TEM (EFTEM), and electronenergy loss spectroscopy (EELS) have been used for the direct microstructural imaging and analysis of clay-organic

matter aggregates in ®ne-grained aquatic sediments from Jourdan River Estuary, MS, USA. EFTEM and EELS allowrapid, high-resolution spatial mapping and analysis of light elements such as carbon. The study area sediments arecomprised of discrete organic matter masses and aggregates of clay plates. Clay aggregates often include organic mat-

ter. The comparison of clay aggregate images obtained by the TEM bright-®eld technique and the EFTEM carbonmapping technique shows that carbon within clay domains has spatial features that are in the same size scale as thefeatures of individual clay plates within the aggregates (<�20 nm). These intimate spatial associations suggest that

organic matter in clay aggregates is either closely attached to the surfaces of individual clay plates or structurallyincorporated into clay crystals. Organic matter within clay aggregates does not appear to exist as discrete or massivemasses that ®ll the pore spaces within the clay aggregates. These intimate associations, whether they are due to che-mical interaction or physical sequestration, should a�ect the reactivity of organic matter during early diagenesis. The

EELS spectra of clay aggregates show that organic matter always coexists with calcium, suggesting that Ca-containingsmectite, rather than Ca-poor clay minerals such as kaolinite or illite, is preferentially associated with organic matter inthe study area sediments. Further research is required to determine whether this is due to the sources and depositional

history or physico-chemical properties of di�erent clay minerals. Published by Elsevier Science Ltd.

Keywords: Energy-®ltering transmission electron microscopy; EFTEM; Electron energy-loss spectroscopy; EELS; Sediment micro-

fabric

1. Introduction

A large fraction of organic matter (OM) in ®ne-

grained siliciclastic aquatic sediments is believed to beassociated with clay mineral particles (e.g. Keil et al.,1994; Mayer, 1994a,b; Bergamaschi et al., 1997; Hedges

and Oades, 1997; Keil and Cowie, 1999). Direct obser-vations of aquatic sediments and their precursor marinesnow at micron and sub-micron scales using transmis-

sion electron microscopy techniques have revealed closespatial relationships between OM and clay particles andaggregates (Heissenberger et al., 1996; Leppart et al.,1996; Ransom et al., 1997; Lienemann et al., 1998).

Such close spatial associations are also observed in thelithi®ed counterparts (Boussa®r et al., 1995). On theother hand, associations at nanometer scales have been

inferred through characterization of bulk sediments aswell as size- and density-fractionated sediments. Keil etal. (1994) found that more than 90% of organic matter

within the sediment samples from the Washington con-tinental margin is inseparable from mineral phases byphysical means. Other studies have observed linear cor-

relations between bulk OM content and measuredavailable surface area in aquatic sediments from diversesedimentary environment (Keil et al., 1994; Mayer,

1994a,b; Hedges and Keil, 1995). Mayer (1994b) showedthrough N2 sorption experiments that small pores of 10nm or smaller account for most of the surface area in®ne-grained siliciclastic marine sediments.

These bulk characterizations have led to a hypothesisthat a certain fraction of sedimentary OM may be phy-sically protected from diagenetic remineralization and

0146-6380/00/$ - see front matter Published by Elsevier Science Ltd.

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Organic Geochemistry 31 (2000) 735±744

www.elsevier.nl/locate/orggeochem

preserved due to its microstructural location withinsedimentary clay domains (Hedges and Oades, 1997).Kubicki et al. (1997) suggested that clay±OM associa-tions primarily occur at the clay edges. At the same

time, pores of <10 nm diameter (i.e. ``mesopores'' inMayer, 1994b), are abundant at such clay edges (Mayer,1999). Such small pores are also commonly seen within

clay aggregates of marine sediments in which individualclay plates are aggregated in face-to-face, face-to-edge,or random arrangements (Bennett et al., 1991). Organic

molecules located within a mesopore may have lessinteraction with pore ¯uid constituents due to low per-meability, and as a result, may be more likely to be

preserved during early diagenesis than organic mole-cules existing outside a mesopore. The mesopores arealso physically too small to accommodate organic car-bon-hydrolyzing enzymes whereas they are large enough

to accommodate ordinary OM molecules (Mayer, 1999).This mesopore protection hypothesis has importantimplications for the mechanistic and quantitative treat-

ment of OM degradation kinetics. In early diageneticstudies, OM degradation rates would have to be quan-ti®ed not only in terms of the chemical reactivity of the

organic compounds but also in terms of the micro-structures. The spatial arrangement of clay plates andOM is also an important parameter in geotechnical stu-

dies because it a�ects the rheological properties of sedi-ments (Bennett et al., 1991).Despite the scienti®c importance and strong sugges-

tions from the indirect characterizations, the actual

nanometer-scale spatial relationship between clayaggregates and sedimentary OM has not been wellunderstood. This is primarily due to technical limita-

tions: transmission electron microscopy (TEM), the pri-mary tool for the microstructural studies of aquaticsediments, has not been able to routinely produce nan-

ometer-scale images of phases that are comprised ofonly light elements such as carbon. When heavy metalstains are employed in order to make organic carbon``visible'' to electron beams by creating bonding between

organic constituents and electron-dense elements, theyreduce the image resolution. Moreover, it is question-able whether or not a su�cient amount of stain solution

permeates into the mesopores, the potential sites for theclose associations between clay surfaces and OM. Thegreater part of the clay surface area is considered to be

contained in such small pore spaces (Mayer, 1994b).Consequently, the existing TEM studies of clay±OMassociations in aquatic sediments and precursor marine

snow have focused on microstructural features that aregenerally no less than 50 to 100 nm (Heissenberger et al.,1996; Ransom et al., 1997; Lienemann et al., 1998). Suchresolution is insu�cient for resolving features most rele-

vant to the mesopore protection hypothesis (i.e. <10 nm).Energy-®ltering TEM (EFTEM), a new frontier in

electron microscopy, allows the high resolution spatial

mapping of elements, including light elements such ascarbon. By using EFTEM, one can obtain high-resolu-tion images created by beam electrons that have beenthrough known amounts of energy loss. The minimum

amount of energy loss experienced by TEM beam elec-trons due to the excitation of inner shell electrons iselement speci®c (Egerton, 1986). Consequently, a spatial

distribution of a given element within the specimen canbe created by using all electrons that have experiencedthe amount of energy loss equivalent to the required

excitation energy for the element of interest. The theo-retical spatial resolution limit has been considered anddetermined to be well below a few nm (e.g. Krivanek et

al., 1995).Electron energy-loss spectroscopy (EELS), having

been utilized to study degradation products of terrestrialplants (Watteau et al., 1996), is another technique pre-

viously underutilised for the study of aquatic sediments.In this technique, the electron energy loss is directlyinvestigated using the two-dimensional plots of electron

energy vs. electron counts, rather than by forming two-dimensional spatial images. The plots can be used toqualitatively identify elements that coexist within a

given spatial area.This study investigates spatial arrangements and the

chemistry of the clay±OM association in aquatic sedi-

ment samples from the Jourdan River Estuary near thetown of Bay St. Louis, MS, USA. EFTEM is used inconjunctionwith high-resolution TEM in order to spatiallyresolve the intimate associations between OM and clay

mineral aggregates at scales relevant to the sizes of indivi-dual clay plates and associatedmesopores. EELS is used toobtain aggregate- and individual particle-scale chemistry

of the clay±OM association. Pore water and solid phasechemistry is also presented to allow us to discuss the sour-ces and fates of OM in the study site sediments.

2. Methods

Sediment samples were collected from Jourdan RiverEstuary test site as part of an ongoing study on thereactive transport of biogeochemical constituents during

early diagenesis of ®ne-grained siliciclastic sediments.The coring site (BSL6) is located in a marsh at themouth of Jourdan River in Bay of St. Louis (Fig. 1).

The actual coring took place within 2 m of an edge of agrassy patch and the water depth was 50 cm at the timeof sampling that was within an hour of low tide in July

1998. The test site was accessed by a pontoon boat, andwas sampled by the hand-coring method using two 15cm-diameter Plexiglas tubes and one 2.2�17.0�40 cmslab-shaped corer. For a comparison, selected geochem-

ical data from a nearby coring site (BSL4; see Fig. 1),which were sampled in the similar manner as the BSL6site, are also presented.

736 Y. Furukawa /Organic Geochemistry 31 (2000) 735±744

Sediments from one of the tube cores were sliced into1 cm-thick sections and centrifuged to yield pore watersamples within 6 h of coring. The pore water samples

were kept frozen in tightly capped high-densitypolyethylene bottles until the time of aqueous phaseanalysis (of chloride by AgNO3 titration, ammonia by

Fig. 1. Bay of St. Louis and its relation to Mississippi±Louisiana Gulf Coast. The sampling sites are shown. Areas indicated in dark

gray in the bottom map are generally covered with marsh vegetation.

Y. Furukawa /Organic Geochemistry 31 (2000) 735±744 737

spectrophotometry, and sulfate by liquid ion chroma-tography; Gieskes et al., 1991), which took place withintwo weeks. The solids, after discarding plant fragmentsvisible to the unassisted eyes, were kept frozen until the

time of solid phase analysis (of total organic carbonand total organic nitrogen by Carlo Elba CNS analy-zer, and bulk mineralogy by X-ray powder di�raction),

which took place within three months. The other tubecore was kept refrigerated until the time of 1 cm-reso-lution grain size (by sieving and pipette analysis; Folk,

1974) and porosity (by standard techniques; Lambe,1951) analyses, which took place within one week ofcoring.

Samples for the TEM investigations were taken assub-samples of the slab-type sediment core, which col-lected sediments from zero to 20 cm below the water±sediment interface. The slab cores were kept in a refrig-

erator until the time of sub-sampling. Within 24 h ofcoring, the slab was laid horizontally, opened, and asmall amount of sub-sample was taken using a mini-

corer (Lavoie et al., 1996) for the subsequent embed-ding, ultrathin-sectioning, and TEM study. The sub-sample was located at 9 cm below the water±sediment

interface where the sediment was free of visible grassroots and fragments. The color (anoxic black) and tex-

ture (®ne-grained, cohesive, and away from burrows)were also criteria in choosing the representative sub-sample. The grain size and geochemical analyses, shownin Figs. 2±4, indicate that the BSL6 sediments do not

exhibit any detectable abrupt change or boundarydowncore, and thus TEM samples may be taken fromany given depth to represent the continuum of the sedi-

mentary processes at the site.In order to spatially map the distribution of organic

carbon, the sub-sample was embedded in elemental sul-

fur rather than resin. Although resin is the conventionalmedium for embedding aquatic sediments, it is a carboncompound and overwhelms carbon energy loss signals

from organic carbon under EFTEM and EELS. Thesub-sample was air dried at room temperature, mixedwith 99.98% purity powdered elemental sulfur (AldrichChemical Company), heated to 130�C for approxi-

mately 20 min in an oven, and cooled to form anamorphous mass at room temperature. The resultingsmall block of sulfur-embedded sample particles was

ultramicrotomed using Leica Ultracut to the thicknessof 60 to 100 nm and the ultrathin-sections were moun-ted on 2000-mesh Cu grids. The extremely ®ne mesh

better supports the ultrathin-sectioned beam sensitivesulfur-embedded samples.

Fig. 2. Grain size distribution of BSL6 sediments shown for each 1 cm interval below the water±sediment interface. The grain size is

expressed in Phi.

738 Y. Furukawa /Organic Geochemistry 31 (2000) 735±744

Although essential for the carbon mapping, the ele-mental sulfur embedding medium employed in thisstudy is disadvantageous in terms of preservation ofsome of the microstructural features. The most rigorous

preparation of ultrathin-sections, which involves pore¯uid replacement by progressively concentrated alcoholsolutions and ultimately by resin, is considered to

embed marine sediments without microfabric distor-tions (Baerwald et al., 1991). Consequently, anothermini-core was taken from the same slab core at the same

depth and embedded in resin following the ¯uid repla-cement sequence. The microwave protocol (Lavoie etal., 2000) was employed for the ¯uid replacement

sequence and resin curing in order to hasten the process.The microfabric sample in resin block was ultrathin-sectioned using Leica Ultracut to the thickness of 70nm, mounted on ordinary 200 mesh Cu grids, and

stained using lead citrate and uranyl acetate solutions(Hayat, 1970).The ultrathin-sections were examined with a JEOL

3010 transmission electron microscope (TEM) at theNaval Research Laboratory, Stennis Space Center, MS,USA, operated at 300 kV in conjunction with Gatan

Image Filter (GIF 2000) which allows EELS and

EFTEM imaging. A comprehensive review of the theoryand application of EFTEM and EELS can be found inReimer (1995). A brief description pertinent to thisstudy follows. GIF allows energy selection (``®ltration'')

of beam electrons by bending the electron paths with amagnetic prism. The bending angle is a function ofenergy. When a TEM beam electron is transmitted

through a thin material, it may cause various excitationsby transferring (losing) energy to the material, and theminimum energy transfer (loss) required for one type of

excitation, ionization of inner shell electrons, is elementspeci®c. GIF takes advantage of this element speci®cenergy loss, selected by the magnetic prism, to map ele-

mental distributions by projecting images only usingelectrons that have lost certain amounts of energy,rather than using all transmitted electrons as in ordinarybright-®eld imaging. GIF can also create EELS spectra

by plotting the energy loss vs. number of electrons. Incase of carbon, the minimum element-speci®c energyloss (i.e. inner shell loss edge) is at 284 eV, and the

majority of energy loss occurs within several eV abovethat value, following which the incident of energy lossgradually decreases with energy. In this study, a 5 eV

wide energy selecting slit was set to transmit electronsthat have lost 285 to 290 eV in order to map the spatialdistribution of carbon. Some of the energy loss between

285 and 290 eV results from background, which includesenergy loss due to other elements whose inner shell lossedges occur much before 284 eV. In order to calculatethe background contribution and subtract from the

image created by the 285±290 eV electrons, two moreenergy-®ltered images are collected at 245±250 eV and265±270 eV. The background decay with energy is cal-

culated from these two images and extrapolated to 285±290 eV in order to subtract the background contribu-tions. The background modeling based on the power

law is carried out by the GIF 2000 software package.The typical imaging and spectroscopy protocol

employed during this study is described below. First, theultrathin-sections are scanned under the ordinary TEM

bright-®eld mode in order to determine the areas ofinterest for further high-resolution imaging. Once anarea is selected, a high-resolution bright-®eld image is

recorded using a CCD camera (Gatan 694 Slow-ScanCamera). Next, a quick electron energy loss spectrum(EELS) of the area is acquired in order to calibrate the

GIF energy selecting slit. Finally, three energy-®lteredimages of the area with the energy ranges speci®edabove are recorded using the same CCD camera. A

high-quality EELS spectrum is recorded for a 30 nm-diameter circular region that includes all spatial featuressuch as clay sheet faces, clay edges, and intra-aggregatepore spaces after an aggregate for analysis is randomly

selected. The EELS spectra show the smooth back-ground decay of the amount of electrons which experi-enced the energy loss as a function of the energy level.

Fig. 3. Total organic carbon (TOC, wt.%) and atomic ratio of

organic carbon and nitrogen (C/N), expressed as depth pro®les

below the water±sediment interface. Despite the close proximity

to grassy marsh, C/N signature in BSL6 suggests soil origin for

the OM. On the other hand, in BSL4, high TOC values are

correlated with elevated C/N, indicating the vertically hetero-

geneous in situ input of decaying plant matter.

Y. Furukawa /Organic Geochemistry 31 (2000) 735±744 739

The energy resolution of electron energy loss in theexperimental con®guration used in this study is con-sidered to be better than 2 eV. The bright-®eld imageswere taken at the exposure time of 0.01 to 0.1 s, whereas

the energy-®ltered images required much longer expo-sure, typically around 60 s, due to the small number ofelectrons that experience 245±295 eV energy loss.

Areas for the high-resolution bright ®eld imaging andenergy-®ltered imaging were selected to represent claydomains commonly observed in the study area samples.

Moreover, clay mineral particles that have been ultra-thin-sectioned perpendicular to their cleavage planeswere preferentially chosen because such orientation is

ideal for the observation of both types of clay mineralsurfaces, cleavage surfaces and sheet edges.The resin-mounted ultrathin-sections were also

observed using TEM in order to investigate the clay

aggregate structures and pore space geometry.

3. Sediment characterization results

X-ray powder di�raction of the study site sediments

reveals that illite, smectite, and kaolinite clays are pre-sent. Also present are quartz and plagioclase feldspar.No carbonate phase was detected. Based on the grain

size analysis, the site sediments are characterized as siltyor clayey mud (Fig. 2). Depth pro®les of the totalorganic carbon (TOC) and organic carbon to organicnitrogen atomic ratio (C/N) are shown in Fig. 3, for

both BSL6 and BSL4. The C/N values of BSL6 aregenerally at the high end of the range expected for soilOM, suggesting that the OM in the sediments is a mix-

ture of soil organic matter transported by Jourdan Riverand some in situ input of decaying marsh plant matter.Compared to BSL4, which shows a high TOC and

strong correlation between TOC and C/N, BSL6 hasreceived relatively small amount of in situ input ofmarsh plant matter. The pore water pro®les of ammonia

and sulfate (Fig. 4) indicate active anoxic remineraliza-tion of OM in the study area sediments.

4. TEM results and discussion

Bright-®eld images of six clay aggregates and their

corresponding carbon elemental maps by EFTEM areshown in Fig. 5. Note that only those aggregates thatyielded carbon signals are shown. Areas represented by

the carbon maps do not exactly match, and are some-times slightly smaller than, their bright ®eld counter-parts due to the inevitable small amount of specimen

Fig. 4. Depth pro®les of ammonia and sulfate normalized by chloride concentration, indicating the active anoxic remineralization of

OM.

740 Y. Furukawa /Organic Geochemistry 31 (2000) 735±744

drift during the acquisition of images. The elementalmaps can only be calculated from areas that are inclu-ded in all three consecutive energy-®ltered images.Side-by-side comparisons of the bright-®eld images

and carbon maps reveal the intimate spatial relationshipbetween individual clay plates and organic carbon evenat these scales of 20 nm or less. Organic carbon signals

often mimic the nm-scale features observed in thebright-®eld images, such as the orientation of platy clayparticles and pore spaces that occur between sheets

within the clay domains, suggesting that OM in theseaggregates is intimately associated with individual clayplates either by attaching to the clay surfaces or by

incorporating into the plate structure. None of the car-

bon maps represent organic matter as discrete masseslacking spatial features of 20 nm or less. The images donot yield quantitative information regarding the asso-ciation of OM with di�erent types of clay aggregate

features: one cannot quantitatively compare the amountof OM associated with cleavage surfaces on the exteriorof aggregates, clay sheet edges, and cleavage surfaces

located inside the aggregates from these images. How-ever, presence of the coinciding platy features in thecarbon maps and bright-®eld images indicates that

organic carbon in these aggregates is closely associatedwith the clay mineral plates of <20 nm.For example, Fig. 5(a) shows a pair of bright-®eld

and carbon map images of a portion of clay aggregate.

Fig. 5. Bright-®eld and energy-®ltering images of clay aggregates. In the EFTEM images, bright (white) areas represent where carbon

signals signi®cantly exceeded the background signals, whereas in the bright-®eld images dark areas represent where electron dense

materials such as clay minerals exist. Side-by-side comparisons of the bright-®eld and EFTEM images reveal the intimate spatial

associations between individual clay plates and OM. The <20 nm scale platy orientations seen in the bright ®eld images are also seen

in the EFTEM carbon maps, suggesting that the OM in these clay aggregates is intimately associated with either the surfaces or

interlayer spaces in individual clay plates. C, individual clay plates; J, junctions of edge-to-face associations of individual clay plates,

which are likely to accommodate mesopores; E, edges of clay sheets, which are likely to accommodate mesopores.

Y. Furukawa /Organic Geochemistry 31 (2000) 735±744 741

On the left in the bright-®eld image, dark areas representindividual clay plates whereas bright areas representeither the elemental sulfur matrix or holes in the ultra-thin section. The carbon map on the right, on the other

hand, shows areas from which carbon signals are detectedas bright areas and areas with little or no carbon signalas dark areas. Presumably the carbon signals come from

intrinsic organic carbon. A comparison of these imagesreveals that organic carbon is very closely associatedwith individual clay plates at the scale of 20 nm or less.

One would expect carbon signals to come from allparts of a clay aggregate if OM were to exist as massivebinding material. Instead, in these samples shown in

Fig. 5, carbon signals mimic the features of individualclay plates such as their orientations and plate thickness.The spatial resolution of these images is not su�cient todetermine whether OM in these samples is either closely

attached to the surfaces of individual clay plates orincorporated into plate structures. Likely mesoporelocations such as at the edge to face junctions of play

plates [indicated by arrows in Fig. 5(a)] do not seem tohave particularly large concentration of OM. On theother hand, other locations with possible concentration

of mesopores, plate edges [indicated by arrows in Fig.5(b) and (c)], seem to yield strong carbon signals.

EELS spectra obtained from 10 separate clay aggre-gates are shown in Fig. 6. Note that all high-qualityEELS spectra obtained during the course of investiga-tion are shown regardless of the presence or absence of

the carbon edge. The spectra indicate that not all clayaggregates contain organic carbon, and some aggregatescontain more carbon than others. These observations

also show that when the carbon edge (®lled arrow,begins at �284 eV and slowly tapers o� after reachingmaximum at �295 eV) is present, calcium edge (gray

arrow, at �350 eV) is also present except for one case(Fig. 6a). A possible explanation is that OM in the studyarea sediments is more likely to be associated with Ca-

rich smectite clays rather than Ca-poor phases such asillite and kaolinite. Whether this association is due tothe transport and depositional history, the chemicalproperties, or nature of mesopores in di�erent types of

clay minerals cannot be determined from the dataavailable at this point.Typical TEM images of the resin-embedded and

stained ultrathin-sections are represented in Fig. 7.Numerous micron- to submicron-sized clay aggregates(CA) and possible polysaccharide networks (P) can be

seen in the low resolution image (a) within the highlyopen (i.e. porous) microfabric. The dimensions of these

Fig. 6. EELS spectra of 12 separate clay aggregates. Appreciable carbon signals (black ®lled arrows) are almost always associated

with calcium signals (gray arrows), except for one occasion (a). This may be because OM in the study area sediments is preferentially

associated with Ca-smectite rather than Ca-poor phases such as illite and kaolinite.

742 Y. Furukawa /Organic Geochemistry 31 (2000) 735±744

clay aggregates and constituent individual clay platesare very similar to those observed during the EFTEM

imaging in elemental-sulfur embedded samples. Thegray areas represent pore spaces. The white patches areultrathin-sectioning artifacts due to sediment grainsbeing plucked out by the slicing knife. The high-resolu-

tion image of the interior of a clay aggregate shows theplaty stacking features that are also commonly seen inthe sulfur-embedded ultrathin-sections.

5. Conclusion

Earlier studies of mineral-OM association microfabricin marine sediments and marine snow (Heissenberger etal., 1996; Ransom et al., 1997; Lienemann et al., 1998)

have revealed the intimate micron to sub-micron scalespatial associations between mineral particles and OM.The present study further shows that the close spatial

associations are still intact even at scales of <20 nm.This is evident by the <20 nm features that appear inboth bright-®eld and carbon-map images, including the

coinciding orientation of platy carbon masses and clayplates. This must result from close microstructuralassociations between OM and individual clay plates at

those scales. OM does not exist as discrete masses largerthan the size of individual clay plates when it is asso-ciated with clay aggregates.Some previous studies have argued that pore spaces

smaller than 10 nm might be the preferred sites for thepreservation of organic carbon through early diagenesis(Mayer, 1994a,b; Hedges and Keil, 1995). Presumably,

such small pore spaces physically protect OM fromdirect interaction with carbon-hydrolyzing enzymes and

pore water constituents essential for OM remineraliza-tion. This argument has been indirectly supported bythe previous observation of bulk sediments, that show(1) a linear relationship between measured surface area

and OM content (Keil et al., 1994; Mayer, 1994a,b;Hedges and Keil, 1995), and (2) the greater part of thesurface area being accounted for by pore spaces smaller

than 10 nm (Mayer, 1994b). Whereas the present studydoes not yield direct evidence regarding this argument,such as the direct observation of OM residing within a

10 nm pore, it does provide a framework to support theassociation. The coinciding <20 nm features in the setsof carbon maps and bright-®eld images (Fig. 5) posi-tively indicate that OM does not exist as discrete masses

of size larger than the size of individual clay particle inthe c-axis direction (�50 nm or less) when it is associatedwith clay aggregates. Also, there seems to be a relatively

high concentration of OM at the edge of clay plates, wherea high concentration of mesopores is expected.The EELS results show that only certain kinds of clay

mineral aggregates may be associated with OM at ascale of <50 nm in the study area sediments. When Cedge is present, Ca edge is also present (Fig. 6). The

most likely explanation for this is that OM in clayaggregates is always associated with Ca-bearing mineralphases, i.e. smectite clays. It is not clear from the avail-able data if this inferred smectite±OM association is due

to the abundance of mesopores in smectite, transportand depositional history of smectite aggregates, or thechemical associations between smectite and OM.

Fig. 7. TEM images of resin-embedded ultrathin-sections. The ultrathin-sections have been stained with lead citrate and uranyl

acetate (Hayat, 1970). Numerous micron- to submicron-sized clay aggregates (CA) and possible polysaccharide networks (P) are seen

in the low-resolution image (a) within the highly open (i.e. porous) microfabric. The gray areas represent pore spaces, and white areas

are the ultrathin-sectioning artifacts, created by the grains being plucked out by the slicing knife. The high resolution image (b) shows

the interior of a clay aggregate, exhibiting the characteristic platy, face-to-face stacking of individual clay plates whose dimensions

along the c-axis are typically less than 50 nm. The 0.7±1 nm sheet lattices of clay minerals are not easily recognizable in the high-

resolution image because staining reduces the image resolution.

Y. Furukawa /Organic Geochemistry 31 (2000) 735±744 743

This study shows that EFTEM and EELS are appro-priate tools for the investigations of clay±OM associa-tions in aquatic sediments at the scales relevant to thephysical and chemical interactions between OM, clay

aggregates, and OM-hydrolyzing enzymes. Further stu-dies combining this technique with bulk sedimentarycharacterizations, size and density fractionation techni-

ques, and TEM-assisted mineralogical techniques areplanned to determine the controlling variables for thepreservation and remineralization of sedimentary OM.

Acknowledgements

Discussions with G. Cody and L. Mayer determinedthe course of this study. I thank R. Mang, A. Reed, andC. Vaughan for their assistance in collecting the ®eld

samples, M Tuel for C/N analysis, and A. Falster forXRD. J. Watkins assisted me in preparing the TEMsamples. T. Daulton provided support for the TEM

instruments. This study was funded by Dr. D. Lavoie asa sub-component to her NRL 6.1 Core Research, andby ONR 322GG (Dr. J. Kravitz, Program Manager).

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