Formation Of Protodolomite Research Articles

High Mg/Ca Molar Ratios Promote Protodolomite Precipitation Induced by the Extreme Halophilic Bacterium Vibrio harveyi QPL2.

Bacterial activities have been demonstrated as critical for protodolomite precipitation in specific aqueous conditions, whereas the relationship between the various hydrochemical factors and bacterial activity has not been fully explored. In this study, biomineralization experiments were conducted using a newly isolated extreme halophilic bacterium from salina mud, Vibrio harveyi QPL2, under various Mg/Ca molar ratios (0, 3, 6, 10, and 12) and a salinity of 200‰. The mineral phases, elemental composition, morphology, and crystal lattice structure of the precipitates were analyzed by XRD, SEM, and HRTEM, respectively. The organic weight and functional groups in the biominerals were identified by TG-DSC, FTIR, and XPS analysis. The amounts of amino acids and polysaccharides in the EPS of QPL2 cultured at various Mg/Ca molar ratios were quantified by an amino acid analyzer and high-performance liquid chromatography. The results confirm that disordered stoichiometric protodolomite was successfully precipitated through the activities of bacteria in a medium with relatively high Mg/Ca molar ratios (10 and 12) but it was not identified in cultures with lower Mg/Ca molar ratios (0, 3, and 6). That bacterial activity is critical for protodolomite formation as shown by the significant bacterial relicts identified in the precipitated spherulite crystals, including pinhole structures, a mineral coating around cells, and high organic matter content within the crystals. It was also confirmed that the high Mg/Ca molar ratio affects the composition of the organic components in the bacterial EPS, leading to the precipitation of the protodolomite. Specifically, not only the total EPS amount, but also other facilitators including the acidic amino acids (Glu and Asp) and polysaccharides in the EPS, increased significantly under the high Mg/Ca molar ratios. Combined with previous studies, the present findings suggest a clear link between high Mg/Ca molar ratios and the formation of protodolomite through halophilic bacterial activity.

An Ecological Perspective on Dolomite Formation in Great Salt Lake, Utah

Protodolomite was detected in sediments from Great Salt Lake (GSL), Utah, U.S.A. that have no history of elevated temperature or pressure, conditions that are thought to promote dolomitization of sedimentary carbonates. Protodolomite was abundant in a non-oolitic sediment core from the South Arm (SA) of GSL at 26% salinity. Protodolomite was also abundant in a non-oolitic NA sediment hand sample yet was absent in a nearby oolitic sediment hand sample in locations that likely receive allochthonous nutrients. Protodolomite was not detected in benthic photosynthetic mats from the SA. However, the mats comprised aragonite with halite and minimal calcite; benthic photosynthetic mats do not form in the NA. To begin to identify potential controls on the formation of protodolomite in the SA and NA of GSL, the composition and abundance of 16S rRNA gene transcripts, carbon cycling activities, and porewater geochemistry of the sediment cores were characterized. Transcripts affiliated with a dominant halophilic, heterotrophic sulfate-reducing bacterium were detected in the uppermost sections of the SA core and their abundance was positively correlated with rates of acetate oxidation/assimilation and concentrations of sulfide. Differences in the quality of organic matter between the SA and NA cores, as indicated by carbon to nitrogen ratios, indicate fresh deposition of photosynthetic biomass at the SA sediment core site but not in the NA sediment core site. Sediment grains from the SA core exhibit micrometer-sized euhedral protodolomite crystals that were not detected in the NA core. Collectively, these observations suggest that deposition of photosynthetic biomass drives the development of a sharp, anoxic lens of heterotrophic sulfate reduction. Sulfide, in turn, may promote dehydration of Mg2+-water complexes and primary protodolomite nucleation and growth. The scarcity of dolomite in the NA sediment core may result from constraints imposed by a combination of extreme hypersalinity and depositional environment on phototrophs and sulfate reducers, their activities, and the thermodynamics of protodolomite formation.

Organomineralization of proto-dolomite by a phototrophic microbial mat extracellular polymeric substances: Control of crystal size and its implication for carbonate depositional systems

Many have postulated that a specific microbial metabolism or the presence of microbes or/and their extracellular polymeric substances (EPS) can lead to the formation of dolomite. Although now there is the consensus that dolomite can be formed in the presence of microorganisms, the exact nature of the involvement of microbes in the dolomite nucleation remains a matter of debate. The focus is now in understanding how microbial mats determine the mineralogy of dolomite. Here we report the effect of the EPS extracted from phototrophic microbial mat isolated from a sabkha in Qatar dominated by cyanobacteria (Lyngbya aestuarii) in the formation of dolomite precursors at 25 °C and 40 °C. Both the temperature and the presence of EPS impact the size and morphology of minerals, promoting spherulitic and dumbbell growth in sulfate free solutions. The formation of proto-dolomite was enhanced by the abundance of carboxylated molecules in EPS which controlled the polymorphism of carbonates. Our study emphasizes the dual importance of organic matter and temperature in dolomite formation and their impact on mineral morphology and chemical composition in sabkhas.

The use of principle component analysis and MALDI-TOF MS for the differentiation of mineral forming Virgibacillus and Bacillus species isolated from sabkhas.

Occurrence of mineral forming and other bacteria in mats is well demonstrated. However, their high diversity shown by ribotyping has not been explained, although it could explain the diversity of formed minerals. Common biomarkers as well as phylogenic relationships are useful tools for clustering the isolates and the prediction of their potential role in the natural niche. In this study, a combination of MALDI-TOF MS with PCA was shown to be a powerful tool to categorize 35 mineral forming bacterial isolates isolated from Dohat Faishakh sabkha, northwest of Qatar (23 from decaying mats and 12 from living ones). The 23 strains from decaying mats belong to the Virgibacillus genus as identified by ribotyping and are shown to be highly involved in the formation of protodolomite and a diversity of minerals. They were used as internal references for the categorization of sabkha bacteria. Combination of the isolation of bacteria on selective mineral forming media, their MALDI TOF MS protein profiling and PCA analysis established their relationship in a phylloproteomic dendrogram based on protein biomarkers including m/z 4905, 3265, 5240, 6430, 7765, and 9815. PCA analysis clustered the studied isolates into 3 major clusters, showing strong correspondence to the 3 phylloproteomic groups that were established by the dendrogram. Both clustering analysis means have evidently demonstrated a relationship between known Virgibacillus strains and other related bacteria based on profiling of their synthesized proteins. Thus, larger populations of bacteria in mats can be easily screened for their potential to exhibit certain activities, which is of ecological, environmental and biotechnological significance.

Precipitation of protodolomite facilitated by sulfate-reducing bacteria: The role of capsule extracellular polymeric substances

The origin of dolomite has long puzzled geologists. It has recently been documented that sulfate-reducing bacteria (SRB) are capable of catalyzing the formation of protodolomite, a previously proposed precursor of ordered sedimentary dolomite. However, the catalytic mechanism of SRB remains incompletely understood. This experimental study is aimed at probing the effect of capsule extracellular polymeric substances (capsule EPS) from SRB on the crystallization of protodolomite in vivo. The capsule EPS tested herein was isolated from a protodolomite-mediating SRB, Desulfotomaculum ruminis, and added into a solution wherein the degree of oversaturation was close to the growth medium of D. ruminis at stationary phase. The solid products were characterized with X-ray diffraction (XRD), Raman spectroscopy and, scanning and transmission electron microscopy (SEM and TEM). Our results indicated that aragonite emerged in the reactors without capsule EPS, while Ca-Mg carbonates (Mg-calcite and protodolomite) were produced in the systems amended with capsule EPS. The incorporation amount of Mg2+ in Ca-Mg carbonates was enhanced with the increasing concentration of capsule EPS. The predominant occurrence of protodolomite was found in the reactor with 140 mg/L capsule EPS. These resulting protodolomites were spherical in shape, and composed of numerous nano-particles. The catalytic influence of capsule EPS on the precipitation of protodolomite might be attributed to their strong Mg2+ binding capacity, potentially diminishing Mg-hydration, which is a potent inhibitor of protodolomite crystallization. The results of Fourier transformation infrared (FT-IR) spectra showed that Mg2+ was bonded with carboxyl and hydroxyl groups on capsule EPS. This inferred adsorption capacity of capsule EPS was also supported by new calculations of complexation chemistry between Mg-H2O complex and organic compounds present in capsule EPS.

Proto-dolomite formation in microbial consortia dominated by Halomonas strains.

Microbes can be found in hypersaline environments forming diverse populations with complex ecological interactions. Microbes in such environments were found to be involved in the formation of minerals including dolomite, a mineral of economic importance and whose origin has been long-debated. Various reports on in vitro experiments using pure cultures provided evidence for the microbial role in dolomite formation. However, culturing experiments have been limited in scope and do not fully address the possible interactions of the naturally occurring microbial communities; consequently, the ability of microbes as a community to form dolomite has been investigated in this study. Our experiments focused on examining the microbial composition by culturing aerobic heterotrophs from the top hypersaline sediments of Al-Khiran sabkha in Kuwait, a modern dolomite-forming environment. The objectives of this study were to assess the ability of two microbial consortia to form dolomite using enrichment culture experiments, mineralogy, and metagenomics. Proto-dolomite was formed by a microbial community dominated by Halomonas strains whereby degradation of the extracellular polymeric substances (EPS) was observed and the pH changed from 7.00 to 8.58. Conversely, proto-dolomite was not observed within a microbial community dominated by Clostridiisalibacter in which EPS continuously accumulated and the pH slightly changed from 7.00 to 7.29.

Sulfate resistance of carbonated ternary mortar blends: Portland cement, reactive MgO and supplementary cementitious materials

Abstract: Reactive MgO (rMgO) and supplementary cementitious materials are promising to formulate sustainable construction binders. This study compares the effect of GGBFS and FA on the properties of mortar systems containing both r-MgO and Portland cement (rMgO-PC) as binder. The experimental results reveal that the incorporation of 10% GGBFS or 10% FA increases the carbonation degree of the rMgO-PC system at rMgO replacement levels of 0%, 30%, 50% and 70%. The incorporation of 10% GGBFS or 10% FA reduces the expansion of mortars with the 0% and 30% rMgO replacement levels during the sulfate exposure. Sulfate exposure not only leads to the formation of ettringite and gypsum in the mortars with the 0% and 30% rMgO replacement levels, but also accelerates the formation of protodolomite in the 50% and 70% rMgO replacement levels. However, only the ettringite and gypsum were found to jeopardize the volume stability of carbonated mortars.

The catalytic role of planktonic aerobic heterotrophic bacteria in protodolomite formation: Results from Lake Jibuhulangtu Nuur, Inner Mongolia, China

Abstract Dolomite nucleation and subsequent crystallization are kinetically-controlled processes. Modern dolomite-forming environments provide clues to the trigger factors that facilitate dolomite formation under Earth surface conditions. It has been documented that certain types of benthic microorganisms promoted the precipitation of protodolomite from sediment pore waters. As protodolomite is thought to be a possible precursor of sedimentary ordered dolomite, microbial mediation has thus been suggested as one interpretation of the occurrence of dolomite in modern sediments. To date, however, it is still unclear whether planktonic microorganisms could directly initiate protodolomite crystallization in the upper water column of present dolomite depositing environments. In this study, we report on the occurrence of authigenic protodolomite in the upmost sediments of a high-sulfate, Chinese inland saline lake (Lake Jibuhulangtu Nuur). This lake was therefore considered to be a natural laboratory to test the catalytic effect of planktonic aerobic heterotrophic bacteria on protodolomite formation. Laboratory mineralization experiments were conducted in a liquid medium that mimicked the ion concentrations and pH condition of lake surface water. The incubation experiments showed that aragonite formed in the abiotic systems, while protodolomite predominantly occurred in the bioreactors using either an enrichment culture or pure isolates of aerobic heterotrophic and halophilic bacteria from lake water. The resulting microbially-induced protodolomite crystals displayed spherical morphology and had MgCO3 composition ranging from 42.7 mol% to 47.1 mol%. These protodolomite spherulites were formed by aggregation of randomly-distributed nano-crystals. Compared to synthetic abiotic protodolomite, microbially-induced protodolomite contained considerable amounts of organic matter, which might occur as intracrystalline inclusion or was located between nano-crystals of protodolomite spherulite. Our results support the emerging view that dissolved sulfate is not an inhibitor for the formation of low-temperature (proto-)dolomite. The presence of organic matter intimately associated with dolomite crystals may serve as a hallmark indicative of a biotically induced origin for some types of dolomite.

Precipitation of Ordered Dolomite via Simultaneous Dissolution of Calcite and Magnesite: New Experimental Insights into an Old Precipitation Enigma

In the present study, we demonstrate that ordered dolomite can be precipitated via simultaneous dissolution of calcite and magnesite under hydrothermal conditions (from 100 to 200 °C). The temperature and high-carbonate alkalinity have significantly copromoted the dolomite formation. For example, when high-purity water was initially used as interacting fluid, only a small proportion of disordered dolomite was identified at 200 °C from XRD patterns and FESEM observations. Conversely, a higher proportion of ordered dolomite, i.e., clear identification of superstructure ordering reflections in XRD patterns, was determined when high-carbonate alkalinity solution was initially used in our system at the same durations of reaction. For this latter case, the dolomite formation is favorable therefrom 100 °C and two kinetic steps were identified: (1) protodolomite formation after about 5 days of reaction, characterized by rounded submicrometric particles from FESEM observations and by the absence of superstructure ...

Distribution and origin of protodolomite from the late Miocene‐Pliocene Red Clay Formation, Chinese Loess Plateau

The Pliocene epoch is considered the most recent analog of modern warming because CO2levels were similar to the present. To explore the carbonate minerals formed in the warmer Pliocene epoch, we studied two continuous sections of the Red Clay Formation on the Chinese Loess Plateau (CLP) by X‐ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Stable Isotope Mass Spectrometry. The Red Clay Formation on the CLP exhibits diagnostic FTIR absorption features of calcite and protodolomite. This allowed quantification of the two carbonate minerals by the FTIR method. Using the FTIR method we estimate the average concentration of protodolomite in Bajiazui is 3.6% whereas the Duanjiapo section is 6.0%. Protodolomite occurrence is more consistent and the concentration is higher from ∼6.5–4.2 Ma B.P. and decreases markedly from 4.2–2.6 Ma B.P. Red Clay protodolomite is depleted in bothδ13CPDB and δ18OPDB, ranging from −4.1‰ to −10.4 and from −6.7‰ to −11.6, respectively, and has a slightly higher δ18O value than the calcites. SEM observations show that Red Clay protodolomite is composed of euhedral rhombic crystals that range from 1–20 μm in diameter, grow into the soil voids and coexist with authigenic calcite and palygorskite. These observations imply that the protodolomite grew in situ and is authigenic from pedogenesis. Dolomitization in the Red Clay sequence appears to be the result of overcoming kinetic barriers. We propose that in the Red Clay a warm climate with seasonal dry conditions leads to the formation of calcrete from soil pore waters thereby enriching the pore solutions with respect to Mg2+ and significantly increasing the Mg/Ca ratio bringing about the formation of protodolomite.

Lake Greenly, Eyre Peninsula, South Australia: sedimentology, palaeoclimatic and palaeohydrologic cycles

Abstract Lake Greenly is a continental playa located in a mid-latitude region (34°30′S), 33 m above mean sea-level on Eyre Peninsula, South Australia. The uppermost 3 m of central Lake Greenly sediments contain two complete cycles and one half cycle of carbonate-evaporite deposition. The sequence is represented by alternating units of protodolomite and fine-grained gypsarenite capped by 30 cm thick surficial low magnesian calcite mud which grades laterally into skeletal peloidal sand and beachrock near the lake margin. The favoured depositional model suggests that calcite was deposited at the sediment-water interface during “freshening” episodes marked by higher lake levels and lower salinities. At the same time individual flooding events deposited skeletal peloidal sand as thin interbeds which thicken and coarsen laterally towards the lake margin. The dolomite was probably deposited during periods of low levels and high brine concentrations caused by increased aridity and evaporation and represents a “dolomitization front”. During this stage water was drawn up by evaporative pumping with subsequent precipitation of gypsum and halite at the sediment-air interface. Although gypsum precipitation did not control dolomitization, it probably assisted the process by raising the Mg Ca ratio and reducing the number of competing ions until formation of protodolomite by early diagenetic replacement of calcite took place.

Geochemical behavior of transition metals during the formation of protodolomite in Minamidaitojima Island, Japan

The average transition-metal contents of twenty-nine “pure” carbonate rocks composed of protodolomite, low-Mg calcite, and mixtures of both minerals from Minamidaitojima Island are: Mn, 6.1; Fe(II), 12.8; Ni, 0.71; Cu, 0.59; Zn, 0.39; and Cd, 0.10 ppm. These extremely low contents probably correspond to the background concentrations of these metals in Quaternary carbonate rocks with little or no silicates and oxides. The positive correlations of the concentration of transition metals except for Cu with that of Fe(II) suggest that these metals might have acted together with Fe during the formation of protodolomite and low-Mg calcite in this island. The reduced form of Fe in the protodolomite means that the dolomitization of carbonates in this island had been taken place under reducing environments. This circumstance supports the idea that the protodolomite might have been formed by the action of concentrated seawaters of lagoon origin.

Synthetic studies of protodolomite from brine waters.

Synthetic studies of protodolomite were carried out for understanding the conditions of protodolomite formation in sedimentary environments. Protodolomite was tried to be precipitated from concentrated sea water (by evaporating sea water) by adding 0.4M-Na2CO3 solution drop by drop. The obtained minerals were amorphous carbonate, aragonite, Mg-poor calcite, Mg-rich calcite, protodolomite, huntite, monohydromagnesite, monohydrocalcite and trihydrocalcite, depending on the concentrations of calcium and magnesium ions in the brine water, the added amount of sodium carbonate and reaction time. Protodolomite seems to be formed diagenetically from metastable minerals given above. The concentration of carbonate ions, Mg2+/Ca2+ ratio and temperature in a parent solution have an important influence on the formation of calcite type carbonate minerals and the Mg contents of these minerals.

Carbonate/clay-mineral relationships and the origin of protodolomite in L-2 and L-3 carbonate reservoir rocks of the Bombay high field, India

This paper consists of two parts. The first deals with carbonate/clay-mineral relationships and the second with the origin of protodolomite in L-2 and L-3 carbonate reservoir rocks of the Bombay High Oil Field, India. Six carbonate/clay-mineral assemblages, viz. (1) calcite-protodolomite-chlorite-kaolinite, (2) calcite-protodolomite-montmorillonite-kaolinite, (3) calcite-montmorillonite-kaolinite, (4) calcite-protodolomite-montmorillonite, (5) calcite-protodolomite-vermiculite, and (6) calcite-montmorillonite-chlorite-kaolinite are recognized among the L-2 and L-3 carbonate reservoir rocks of the offshore Bombay High Oil Field, India. These form disequilibrium assemblages on a Al 2O 3 (kaolinite), CaO (calcite) and MgO (magnesite) facies diagram. Therefore, the clay minerals are terrigenous in origin and have at least contributed the magnesium needed for the formation of protodolomite. The protodolomite seen in L-2 and L-3 rocks is limpid, water-clear, non-zoned, and inclusion-free with an average grain diameter of 7–10 μm and on the X-ray chart shows a consistent (104) peak between 2.93 Å to 2.90 Å ‘ d’ spacing, reading 30–40 mole% MgCO 3 solid solution in calcite. By comparison with other types of dolomite, viz (1) replacement, (2) evaporite, (3) sabkha, and (4) schizohaline, the present protodolomite seems to be of biogenic origin and in all probability appears to be a new form of dolomite to be reported from an ancient subsurface carbonate formation.

Protodolomite in Daito-jima, Okinawa.

This report describes protodolomites found in Daito-jima to the east of the Okinawa Islands, Japan. To investigate the conditions for sedimentary protodolomite formation, the distribution and chemical composition of protodolomites were studied through field observations and laboratory analyses. From the thin section studies of carbonate rocks, it was observed that porous parts of coral were cemented by protodolomite crystals and that aragonite in hard tissue of coral was transformed into protodolomite. These observed facts may show that protodolomite was formed through the transformation from aragonite and magnesian calcite in magnesium rich solution. The distribution and chemical composition of protodolomite in Minami-daito-jima suggest that protodolomites may be formed in concentrated seawater.

Synthesis of Protodolomite, High-Magnesian Calcite, Hydromagnesite, and Nesquehonite from Calcium-Magnesium Bicarbonate Solutions by Freeze Drying: ABSTRACT

The synthesis of true cation-ordered dolomite at low temperature has been tried many times in the past without success. However, the natural formation of early diagenetic dolomite in various marine and nonmarine sedimentary environments is frequently observed when magnesium-calcium solutions of molar ratio greater than seven react with preexisting calcium carbonate. We have been able to synthesize protodolomite via an intermediate, amorphous Ca-Mg-carbonate phase from CaCO3 and MgCO3 solutions with a ratio of Mg/Ca = 1. Our experiments were as follows. Reagent grade CaCO3 and MgCO3 were dissolved (0.01 molar) in distilled water with the following Mg/Ca ratios: 0.2, 0.5, 0.8, 1.0, 1.7, 3.0, 5.3, 25. Freeze drying these solutions produced X-ray amorphous Ca-Mg-carbonate phases (gels). These substances were then moistened with distilled water and maintained in sealed tubes at 55°C for a week. A definite relation was observed between the final crystalline product and the Mg/Ca ratio of the initial solution. Solutions with a Mg/Ca ratio up to 0.8 formed only low-magnesian calcites with varying magnesium content. At a ratio Mg/Ca = 1, protodolomite was obtained within the composition range of Ca0.65 Mg0.35 to Ca0.54 Mg0.46. At a ratio Mg/Ca >1.7, high-magnesian calcite with a maximum of 19 mole MgCO3, as well as hydromagnesite and nesquehonite, developed. The synthesized protodolomite consisted of spherules (diameter 1 and 5 µm) and spherular aggregates. The X-ray patterns, differential-thermal-analysis curves, and the congruent solubility of the material clearly distinguish it from other low-temperature magnesian calcite and prove that protodolomite (according to its original meaning) was synthesized. There is some evidence of protodolomite formation resulting from this mechanism in nature. We have found protodolomite with a radiocarbon age of only a few decades in speleothems of the Eibengrotte (a small cave in West Germany), and C. C. von der Borch and J. B. Jones have described modern spherular dolomite from sediments of the Coorong area in South Australia. End_of_Article - Last_Page 450------------

Synthesis of protodolomite from aqueous solution at normal temperature and pressure.

To estimate the conditions of the protodolomite formation, laboratory syntheses from simple aqueous solutions have been carried out under normal temperature and pressure conditions. Protodolomites have been synthesized from CaCl2 + MgCl2 + (NaCl) + NaHCO3 solutions. This paper shows the concentrations of calcium and magnesium ions in the parent solution, which are required for the protodolomite formation. The synthesis experiment indicates that concentrated seawater is favorable to the protodolomite formation. That is, the high value of [Mg2+] / [Ca2+], and the high concentrations of sodium and chlorine ions in a parent solution are favorable to the protodolomite formation.

Pore-Water Chemistry and Early Diagenesis of Nearshore Marine Sediments: ABSTRACT

End_Page 801------------------------------Pore-water chemistry and mineralogy of carbonate and terrigenous sediments from Kaneohe Bay, Oahu, Hawaii, were analyzed to determine differences in pore-water compositions, in nature and extent of early diagenetic reactions, and in fluxes of constituents between these sediment types. Pore waters, extracted at in-situ temperatures and analyzed for pH and concentrations of Ca++, Mg++, K+, Na+, Cl-, SO=4, reduced sulfides, SiO2, NH+4, PO 4, NO-3,and Sr++, of 23 1-3 m gravity cores show chemical gradients of dissolved species with sediment-burial depth. The sediment pore waters are anaerobic, exhibiting an increase in reduced sulfides, H+, and alkalinity and a decrease of SO=4 with depth. Owing to reactions resulting in the formation of diagenetic ferrous sulfide, the pH's of terrigenous sediment pore waters are higher at an equivalent depth than those of carbonates, whereas reduced sulfides are lower and sulfate reduction is more rapid. Calcium and Mg++ are removed from pore waters with increasing depth as a result of (1) formation of protodolomite, or (2) precipitation of calcite and substitution of Mg++ for Fe++ in clay minerals resulting in formation of ferrous sulfide (Drever reaction). Dissolved SiO2 increases with depth by solution of siliceous plankton or amorphous aluminosilicates, whereas NH+4, PO^identity4, and NO-3 increase with depth because of bacterial oxidation of organic matter. Sodium, K+, and Cl- vary sympathetically, reflecting the original salinity of the pore waters. Lateral gradients of dissolved species in bay pore waters reflect the fact that the southern end of the bay is a more efficient trap for organic matter. These gradients imply a lateral component of flux of dissolved constituents. This study shows that (1) calculations of fluxes in and out of marine sediments must take into account variability of pore-water compositions and fluxes among sediment types, and (2) nutrient regeneration in pore waters can be a significant source of nutrients to overlying waters, whereas reduced sulfide fluxes may be significant enough to inhibit infaunal and epifaunal growth. End_of_Article - Last_Page 802------------