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Research Paper |
1 Laboratoire d’Étude des Géo-Environnements Marins, Université de Perpignan, 52, Av. Paul Alduy, 66860 Perpignan, France, and 2 Institute of Geological Sciences, Polish Academy of Sciences, ul. Twarda 51/55, 00-818 Warsaw, Poland
* E-mail: wiewiora{at}twarda.pan.pl
(Received 25 April 2003; revised 10 October 2003)
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ABSTRACT |
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 TOP ABSTRACT MARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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KEYWORDS: glauconite, nontronite, chlorite, illite, Mediterranean, Gulf of Lions
Continuing previous work on diagenetic processes in marine green grains (Giresse et al., 1988; Giresse & Wiewióra, 2001; Wiewióra et al., 1996, 1999, 2001), this paper presents an investigation of Holocene grains collected on the shelf of the Gulf of Lions (northwestern Mediterranean). The initial substrate described here is distinct from those studied previously on continental shelves of tropical marine environments. The grains are linked to an initial clay assemblage of the mud matrix with two alpine minerals, chlorite and mica, as dominant components. Two sites were selected to compare the glauconitization process: one near the mouth of the Rhône River and another at a distance from it.
The aim of this study was to follow the general evolution of maturity indicated by the colour of the grains, taking into account the multi-mineralic material of the grain as the matrix in which they are found. A specific methodological approach was developed (Wiewióra et al., 2001; Giresse & Wiewióra, 2001) that considers the mineralogy and the chemistry at a fine scale. This approach avoids problems with the global methods generally used in the studies of green grains.
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MARINE SEDIMENTARY SETTING |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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) of the northwestern Mediterranean Sea (Millot, 1990). The Rhône River is the major source of particulate matter for the Gulf of Lions and provides 80–90% of the total solid discharge (Aloisi et al., 1979). On the shelf, superficial sediments are the product of the transgression that occurred after the Last Glacial Maximum. Got & Aloisi (1990) estimated from the sediment volume accumulated during the last sealevel rise that 70% of particle inputs were trapped on the shelf and 30% were transported to the slope and basin. The thickness of the Holocene deposits decreases progressively from 30 to 5 m offshore and towards the Roussillon coast. These deposits have a narrow littoral sand belt due to river input, large mud deposit centres in front of river mouths, and mid-shelf mud deposits at between 50 and 70 m of water depth. Then, from 90 to 200 m depth, there is an area with thin Holocene deposits or outcrops of older sands that accumulated during the last low sea-level stand or the beginning of the Holocene transgression. The two sites of this study are located in the latter area.
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The distribution of clay minerals was previously investigated in the shelf located off the Rhône River mouth (Chamley, 1971) and in the Gulf of Lions margin (Monaco & Mear, 1984; Courp & Monaco, 1990). Off the Rhône mouth, two alpine minerals (illite and chlorite) were the most common clay minerals found. Few kaolinites and smectites were found. In the southwest part of the Gulf of Lions, the smectite content is significant (20–30% and >30% of the <2 µm clay fraction in the Lacaze-Duthiers canyon). These high smectite contents reflect erosion of Pliocene accumulations of the Roussillon plain. They indicate the offshore extension of the water layer influenced by river inputs during the flood of the small regional rivers and are a good tracer of fine suspension transport and sedimentation on this margin.
Vertical profiles for total Fe are relatively uniform with an average value of 3–3.5 wt.% (7.5–10 wt.% Fe2O3) and do not present any clear redox-related enrichment. Most of the Fe content (~90 wt.%) is held in the structure of clay, and other layer silicate minerals. The Fe content associated with the reducible phase varies from 6 to 9 wt.% (Marin & Giresse, 2001).
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MATERIALS AND METHODS |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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). The fines-dominated sediment comprises the transgressive and high-stand systems tracts and shows an upward increase in terms of the presence of hemi-pelagic material. The base of the core is composed of indurate grey mud (280–250 cm) which has been eroded at the top as a consequence of the last low-stand emersion (Fig. 2). This mud is overlain by shelly sands (250–200 cm) of the near-shore deposition and dated by 14C at between 12,000 and 10,000 y (Gensous et al., 1993) and is where the green grains of the present study were sampled. Upwards, a beige mud (200–100 cm) is dated at 7750 y (Ausseil-Badie, 1978). The upper 1 m thick layer represents the high-sea level deposition. Secular sediment accumulation rates obtained with the 210Pb geochronological method were estimated to be 0.48 cm y–1 (Radakovitch, 1995).
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). This physiographic position could explain the more significant contrast in sand content, which was ~50% in the lowest levels (25–10 cm depth core) and decreased to ~7% in the uppermost sediment. This change implies that the conditions of sedimentation were significantly different. Gravity-driven flow during deposition of the lower part of the core was indicated by the abundance of Miliolidae, sandstone fragments and mainly green grains that are studied here. According to its short distance from the outer shelf, the movement of these particles is presumed to be very limited. By analogy with the similar sands deposited during the beginning of the latest transgression, the age of these sediments is estimated to be 12–10,000 y. The accumulation rate of the upper muddy layers (0.125 cm y–1) was less than in the core KL 04 (Radakovitch, 1995). So, the two types of glauconito-genesis studied here began approximately at the same period of the early Holocene. But, in spite of a gravity-induced shift in the case of core EC3, the green grain-rich deposits of KL04 were buried more rapidly by the subsequent deposition than those of EC3.
Samples of glaucony
As a first step, sediment samples of ~10 ml in size were taken from the horizons marked in Fig. 2. The basic sedimentologic analyses and separation of the green grains and faecal pellets were performed after sieving sediments with a 50 µm mesh (Giresse et al., 1988; Giresse & Wiewióra, 2001). This sand fraction was treated with 1 N acetic acid, in order to dissolve carbonates and to preserve, as far as possible, the original chemical composition of the green matter. After dissolution, the acid solution was removed rapidly and the grains were washed with distilled water. The grains chosen for analysis were cleaned of the muddy matrix infilling fissures by ultrasonic bath.
Green infillings and faecal pellets were largely distinguishable in the sand fraction with the binocular microscope and with Transmission Light Microscope (TLM). It appeared that this insoluble sand fraction consisted mostly (average >95%) of green clay infillings and faecal pellets of mud-eaters; quartz grains are nearly absent. Consequently, this fraction was considered as representative of the green-grain fraction. The content of this fraction varies considerably, but the proportions of grain colours (white, pale green, medium green, dark green) vary too.
Methods
Traditionally, most studies concerning green grains were based on bulk-sample analysis, but recently, micro-analysis studies demonstrated the polyphasic composition which can occur in a single grain (Amouric et al., 1995; Wiewióra et al., 1996, 1999, 2001). Different grains of the same sample may differ in their composition, but a fine mixture of phases may occur within the same grain. Determination of the chemical and mineralogical composition of green grains is a complex problem and was investigated using SEM/EDS and XRD methods.
SEM/EDS.
About 80 selected grains were isolated from the bulk sediment, thinly coated with Au and examined morphologically using an Hitachi S. 4500 scanning electron microscope (SEM), fitted with an X-ray energy dispersive (EDS) microprobe (Analyser Sigma Kevex). Quantitative chemical data were obtained by analyses of the outer part of the grain. After moderate crushing, the inner parts of the grains were observed and nano-structures were analysed. A beam-size of ~1 µm gives an interaction volume of ~8 µm3; spot analysis can therefore be regarded as the average composition of the volume. The EDS data were calculated on an H2O-free basis, using a pre-registered standards technique. Quantitative data were treated with a correction ZAF system (Atomic Number, Absorption, Fluorescence) and with PROZA software supplied by Hitachi, manufacturer of the microprobe. Weight percent error varies with element and sample, but is usually <0.5%.
Jackson’s (1969) method was applied on assemblages of a few grains from light green to dark green to wash out soluble free oxides or hydroxides of Fe. The grains were analysed using the same SEM/EDX technique.
X-ray diffraction.
Analyses of <2 µm and 2–20 µm sediment fractions were performed by X-ray diffraction (XRD) using the focusing reflection diffractometer of Compagnie Générale de Radiologie (CGR, France). Co-K
radiation was used and the aggregates were oriented by sedimentation. In the case of very small samples represented by the bulk of a few hand-picked green grains placed in the capillary of the Lindemann glass, the focusing transmission diffractometer equipped with the multi-channels position sensitive detector PSD 120°2
of Inel (France) was used.
The mineral composition of the mud matrix was studied using transmission and reflection XRD techniques. It was not feasible to study the bulk-sample composition because quartz was too abundant. To determine clay and other layered minerals, the <2 µm and 2–20 µm fractions were separated and deposited on glass slides. The 20–50 µm fraction was used to prepare transmission diffraction preparations in the capillary of the Lindemann glass.
The mineral compositions of grains extracted from the two sediment columns KL04 and EC3 were studied with the use of transmission diffractometer and capillary samples. A few grains, similar in habit and colour, were examined to approximate a single-grain mineral composition.
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RESULTS |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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) belongs to the younger mud and includes only some beige faecal pellets (stick shaped). The 220–230 cm layer contains mostly pale green or medium green infillings of chambers of various littoral benthonic foraminifers such as Elphidium crispum and Ammonia beccarii or internal moulds of the walls of sea urchins (Fig. 3a,b,c). Medium green and dark green are both commonly cracked but dark green infillings are not common. Ellipsoidal faecal pellets (Fig. 3g) are light grey or light beige and green ones are very rare or absent.
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), of gastropods (Fig. 3f) and perforating algae channels (Fig. 3d). As these infillings are frequently separated after dissolution of the test, the green clay may form free and rather fine green-grain assemblages with a modal size value in the range 100–150 µm. The two deposits studied show a complete sequence of colouring with white, pale green, medium green and dark green grains. Surface cracking affects all the dark green grains and also most medium green grains.
Observations by SEM show the development of small neo-formed crystallites on inherited supports, as large clay mineral packages, dodecahedral pyrite or quartz grains. Every type of grain contains neo-formed flakes. Their sizes, structure and density prove the successive advancements of the mineralogical processes (Fig. 4). The present sequence shows slightly extended analogies with those described in our previous studies (Wiewióra et al., 1996, 1999; Giresse et al., 2001) and in the literature (Odin, 1988). The neo-formed flakes are especially scarce in beige pellets of KL04 70–80 cm. They are scattered (Fig. 4a), but exist inside various small geodes of porous beige or light green grains. In the other sediment levels they are generally <1 µm long (Fig. 4b). Crystallites of second generation are in some occurrences characterized by longer flakes (~1.5–2.5 µm) that tend to overgrow the small original ones. Medium to dark green grains show a higher density of neo-formed flakes that are better outlined (honeycombed or foliated structures) and in some places are larger (~2–3 µm long) (Fig. 4c,d). Lastly, the dark green grains show a box-work-like assemblage with a very high density resulting in a marked decrease of the porosity observed under SEM.
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and plotted in Figs 5 and 6. These results were compared with the mud matrix (<50 µm), which is regarded as the original material in faecal pellets and infillings (Table 1, Figs 5 and 6).
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). Slightly higher SiO2 content in the mud matrix is considered a result of the solid discharge of the Rhône River.
In the 220–230 cm level, the increase of K2O, up to 4.2 wt.%, from faecal pellets to dark green grains is marked. The relatively high K2O content of the mud matrix suggests an inherited detrital illite component from the Rhône River mouth (Fig. 5).
In core EC3, the two levels studied have nearly the same trend (Fig. 6). The most spectacular process is the marked increase in Fe concentration from the mud matrix to the less evolved grains (pale green infillings) similar to KL04 220–230 cm. This Fe enrichment (x5–7) is not linked directly to the colour of the grain and it remains more or less constant during the subsequent mineralogical evolution. In grains washed free of oxides (Table 2), there is a clear increase in Fe from light green to dark green grains in all three levels (Fig. 7). This Fe2O3 increase was previously masked by free Fe oxides.
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). Qualitative mineral composition is similar in sediments from both depths. In the higher fraction (2–20 µm, Fig. 8a), the intensities of reflections of mica, chlorite and quartz are stronger than in the <2 µm fraction (Fig. 8b) indicating higher relative contents of these minerals. Traces of plagioclase are also observed. The fine fraction is richer in kaolinite and in smectite (Fig. 8c).
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). The smectite and kaolinite content is higher in shallower samples in both cores relative to coarser micas and coarser chlorite. One may also observe that the 2–20 µm fraction from the KL04 220–230 cm level is richest in mica and chlorite of all the studied samples. The coarsest fraction (20–50 µm) separated from the same sample and studied by the transmission XRD (TXRD), shows that chlorite is the major phyllosilicate mineral. Dioctahedral mica (muscovite) as well as trioctahedral mica (biotite) were identified by 060 reflections. Chlorite was determined as a one-layer triclinic polytype belonging to subfamily C (Weiss & 
urovi
, 1983; Wiewióra, 1996). This confirms its detrital origin. Amphibole was observed in the same KL04 220-230 cm fraction.
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) from beige faecal pellets from the younger layer of the KL04 mud sample (70–80 cm) shows only detrital minerals: quartz, chlorite, mica, plagioclase and K-feldspar. No neo-formed phases were detected in the sample. Rapid alimentation by the Rhône River did not give enough time for diagenetic evolution inside these faecal pellets in this sample.
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) and the occupancy by Fe of the octahedral sheet of the smectite mineral, determined from d(060) = 1.523 Å as equal to 1.8 Fe atoms per unit cell (Brigatti, 1985; Wiewióra et al., 1996; Gates et al., 2002) show that the major mineral inside the grains is nontronite (Fig. 11, left). Minor quantities of illite, chlorite, quartz and traces of plagioclase, and K-feldspar are also present. Elevated background (20–35°2 Co) indicates the presence of the amorphous material, probably Fe oxides (based on SEM/EDS). Most phases, except nontronite and trace amounts of quartz and Fe oxides, decrease in grains from light to medium to dark green colour. In dark green grains, goethite is present. Glycolation revealed that the grains consist of interstratified nontronite/glauconite (80% N) at Reichweite = 1 (Fig. 12, left) based on simulation by Newmod (Reynolds, 1985).
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, right) consists mostly of nontronite with octahedral Fe around 1.8 Fe per unit cell, as in the core KL04 along with traces of illite. Detrital minerals are: chlorite, plagioclase, K-feldspar and calcite and Mg calcite from biogenic debris. The medium-green grain of the same sample have nontronite in part transformed into mixed-layer N/G phase accompanied by the important disappearance of the detrital minerals indicating more evolved samples. These processes are much more advanced in the dark green grains where the major phase is mixed-layer glauconite-nontronite with d = 11 Å and goethite is present (Fig. 11c, right). The process of transformation of nontronite into glauconite-nontronite is illustrated in Fig. 12, right. With the growing colour appeared a mixed-layer 92% glauconite–8% nontronite phase with Reichweite = 3 based on simulation by Newmod (Reynolds, 1985).
Grains from the deeper horizon (20–24 cm) from the same core EC 3 show mineral compositions like grains from the previous horizon including G-N (8% N, R = 3) in the dark green grain. There is a striking difference in mineralogy from faecal pellets which are very rich in detrital minerals: quartz, illite (and muscovite), chlorite, feldspars and carbonates (inherited calcite and dolomite). Another striking difference is in the illite content, very high in faecal pellets (see high intensity of 020 reflection at 4.466 Å , Fig. 13) and hardly detected in the light green grain. In this last sample, only some quartz and traces of plagioclase persisted during the early stage of diagenesis.
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DISCUSSION |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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In core EC3, ellipsoidal faecal pellets or those with irregular shape show very light pigmentation. The green clay usually fills chambers of very varied organisms: littoral benthonic foraminifers as in KL04, internal moulds of the radiole or wall cavities of sea urchins of gastropods and perforating alga channels with a modal size value in the range 100–150 µm.
The two studied deposits of the EC3 core exhibit a complete sequence of colouring with light pale green, medium green and dark green grains. Size, structure and density of the neo-formed crystallites inside grains prove the successive advances of the mineralogical process. These neo-formed nano-structures are scarce in beige pellets of KL04 70–80 cm, but ones that are generally <1 µm long are found inside beige or light green grains of the other levels. Medium to dark green grains have a higher density of neo-formed flakes that show a better structure (honeycombed or foliated structures) and are larger (~2–3 µm long). The dark green grains show a high density box-work-like assemblage that results in lower porosity.
Iron concentration
Core KL04 has a higher Fe2O3 concentration in the faecal pellets compared to the mud matrix (x3–5). In the EC3 core, the higher Fe concentration in the less evolved grains (faecal pellets or pale green infillings) compared to mud matrix is like that from the KL04 220–230 cm. A large proportion of Fe is in an amorphous form. The occurrences of similar amorphous Fe accumulations have also been reported on a large scale on the West African inter-tropical shelf (Giresse et al., 1988) or on the deep-water bottoms of the Ivory Coast-Ghana Marginal Ridge (Giresse & Wiewióra, 2001; Wiewióra et al., 2001). This Fe2O3 increase is associated with a decrease in SiO2 and Al2O3 contents due to dissolution, i.e. of detrital minerals such as quartz and feldspars. The K2O content reflects concerted reactions of dissolution of the detrital micas and more or less gradual uptake into the layers of the glauconite mineral. Similarly, the Mg content is attributed to dissolution of chlorite and fixation of this element in glauconite.
Mineral composition from the mud matrix to the grain
The qualitative mineral composition is similar in both levels of KL04 sediments. In the coarser fraction (2–20 µm), mica and chlorite are in higher relative concentration. In the fine fraction (<2 µm) kaolinite and smectite are most abundant.
A similar qualitative composition exists in both fractions from 1-2 cm and 20-24 cm in the EC3 core. In the 20-50 µm fraction, chlorite is the major phyllosilicate. The chlorite is a one-layer triclinic polytype belonging to subfamily C (Weiss & urovi, 1983; Wiewióra, 1996), which identifies allochtonic character.
The beige, stick-shaped, faecal pellets from the younger layer of KL04 mud sample 70–80 cm, shows only minerals of detrital origin including quartz, chlorite, mica, plagioclase and K-feldspar. The minerals identified in the mud matrix were no doubt the substrate phases for processes that occurred inside grains. Residence at the water-sediment interface and rapid alimentation by the Rhô ne River did not give enough time for diagenetic evolution inside these faecal pellets. Inside grains from sample KL04 from 220–230 cm, the major mineral is nontronite. There are minor quantities of illite, chlorite and quartz, and traces of plagioclase and K-feldspar. Passing from light green to dark green grains, most phases disappear except nontronite/glauconite (80% N, R = 1), traces of quartz and Fe oxides recrystallized into goethite.
In light green grains from core EC3, from 1–2 cm and 20–34 cm levels, the major mineral is nontronite. The second clay mineral is illite. The detrital minerals are: chlorite, plagioclase, K-feldspar and calcite. Medium-green grains show nontronite in part transformed into a mixed-layer phase. This process is accompanied by an important disappearance of the detrital minerals, much more advanced in the dark green grains in which the major phase is mixed-layer glauconite-nontronite (8% N, R = 3). The process is, to a certain degree, similar to that described in another Mediterranean site, the north Aegean sill, by Robert and Odin (1975). In spite of the global mineralogical methods used, these authors demonstrated a possible mineral evolution towards glauconite.
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CONCLUSIONS |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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In comparing results with several similar studies applied to marine tropical environments (Odin, 1988; Giresse et al., 1988; Giresse & Wiewióra, 2001; Wiewióra et al., 1996, 1999, 2001), some distinguishing features of this temperate sea floor become evident. The tropical environment studies described the rapid disappearance of inherited 1:1 phyllosilicates assemblages. In this temperate environment, mica and chlorite disappear too, and quite quickly. The green-grain accumulation is less important, in accordance with a lower concentration of Fe in the deposit or moreover, with the significant dilution of this element induced by the carbonate component. In spite of this low Fe concentration in the mud matrix (
10 wt.% Fe2O3), the successive steps of the glauconitization are evident.
Primary minerals do not affect the fundamental glauconitization process. Glaucony evolution was comparable in samples with different illite and chlorite concentrations in the substrate. Thus, the differences in each phase of glauconitization developed are mainly related to the time of grain residence at the water-sediment interface, i.e. the distal EC3 site characterized by a low sedimentation rate is more favourable than KL04, the latter affected by more direct Rhône River fluxes. In sediments from the head of the Lacaze-Duthiers canyon (at a depth of 590 m), characterized by a higher proportion of the soil input and longer time of grain residence in the water sediment interface, the diagenesis shows more advanced stages. The increasing proportion of glauconite layers in the glauconite-nontronite increases to 92% in the dark green grains. The order of interlayering is very high as is shown by R = 3. The proportion of layers means that diagenetic evolution in this particular semi-confined environment ended with a largely mature phase of mixed-layer glauconite-nontronite. With a certain satisfaction, we announce the first clearly-demonstrated case of glauconite formation during diagenetic processes in the Mediterranean Sea.
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ACKNOWLEDGMENTS |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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niarski for technical assistance with the XRD work. |
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TOPABSTRACTMARINE SEDIMENTARY SETTINGMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCES |
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