Oxygen Supply Rate Research Articles

Orbitally-controlled oscillations in planktic communities and cyclic changes in western Mediterranean hydrography during the Messinian

We use quantitative analyses of the planktic foraminiferal assemblages and stable isotope analyses of the Sorbas section in the western Mediterranean to reconstruct the cyclical changes in surface and deep water hydrographic conditions during the Late Miocene prior to the Messinian salinity crisis. Oscillations in winter and summer temperatures linked to cyclical fluctuations in the hydrographic conditions and nutrient availability were the main mechanisms driving the cyclical changes in the planktic foraminiferal assemblages during the Messinian. The winter intensification and southward expansion of the northern cool and dry winds during precession maxima lowered sea surface temperatures (below≃14–15°C), favoring water convection and the upward mixing of nutrients with the consequent increase in the proportion of cold, eutrophic water foraminifera. The low summer temperatures (below 24°C) inhibited the growth of warm, oligotrophic water foraminifera, as occurs today in the Mediterranean. By contrast, at times of precession minima the low influence of the high latitude air masses caused winter temperatures to remain relatively high which, combined with the lower surface salinities, led to the formation of a permanent pycnocline. Winter conditions prevented the growth of cold, eutrophic water foraminifera, while high summer temperatures and the formation of a permanent pycnocline stimulated the growth of warm, oligotrophic water foraminifera. The prevalence of stable water stratification during winter due to the large density gradient between the surface and intermediate waters prevented deep water formation and slowed down the rates of oxygen supply to the bottom, resulting in the formation of the sapropels. A sharp decrease in δ 13C of benthic and planktic foraminifera that occurred between 6.8 and 6.7 Myr is related to an increase in the residence time of Mediterranean waters. Because similar changes have been observed in other regions of the Mediterranean, we conclude that this change records a significant reduction in the Atlantic–Mediterranean water exchange at that time. Mediterranean climatic amplification increased throughout the Messinian as the Atlantic–Mediterranean water exchange was progressively more limited and oscillations in the ratio of planktic/benthic foraminifera and of warm-oligotrophic/cold-eutrophic species are good records of this increasing amplification, which culminated at 5.95 Myr with the deposition of gypsum–pelitic layers that mark the onset of the Messinian salinity crisis.

Kinetics of phenol oxidation by Candida tropicalis: effects of oxygen supply rate and nutrients on phenol inhibition.

The kinetics of phenol degradation was estimated in a fed-batch reactor system. Effects of oxygen and nutrient excess or limitation as well as the presence of several essential ions on the phenol- and oxygen-specific uptake rates achieved simultaneously in a bioreactor were shown. Candida tropicalis was grown on phenol as the only carbon and energy source. Applying the best fit of polynomial function, the maximum specific uptake rates of phenol and oxygen, the critical concentrations of phenol, the half-saturation constants and inhibition constants were determined. Linear relationship between specific phenol uptake rate and the exogenous respiration rate was found regardless of the kind and presence of essential nutrients. At oxygen limitation both the phenol uptake rate and the cell affinity to phenol decreased more strongly compared with those under nutrient limitation. Oxygen in excess resulted in a significant increase of cell tolerance toward phenol. The presence of essential nutrients increased the specific phenol degradation rate and led to complete phenol oxidation.

The relation of double peaks, observed in quartz hydride atomizers, to the fate of free analyte atoms in the determination of arsenic and selenium by atomic absorption spectrometry

The mechanism at the origin of double peaks formation in quartz hydride atomizers were investigated by continuous flow hydride generation atomic absorption spectrometry. Arsenic and selenium were used as model analytes. The effect of atomization mode (flame-in-gas-shield (FIGS), miniature diffusion flame and double flame (DF)) and some experimental parameters as oxygen supply rate for microflame and the distance from atomization to free atoms detection point, were investigated on the shape of both analytical signals and calibration graphs. Rollover of calibration graphs and double peak formation are strictly related each to the other and could be observed only in FIGS atomizer mode under some particular conditions. A mechanism based on incomplete atomization of hydrides cannot explain the collected experimental evidences because the microflame of FIGS is able to produce quantitative atomization of large amount of hydrides even at supply rate of oxygen close to extinction threshold of microflame. The heterogeneous gas–solid reactions between finely dispersed particles, formed by free atom recombination, and the free atoms in the gaseous phase are at the origin of double peak formation.

Oxygen supply, body size, and metabolic rate at the beginning of mammalian life

To test the relationship between hypoxia/ischemia tolerance and metabolic rate in neonatal tissues, isolated unperfused hearts of neonatal, juvenile, and adult mice were studied by microcalorimetry and microrespirometry. Additionally, microslices of mouse hearts were prepared and studied in a microcalorimeter under different oxygenation conditions. Neonatal hearts had a slower hypoxic/ischemic decline in heat output than adult organs, correlated with a higher uptake of physically dissolved oxygen from the incubation solution. In the slice experiments, the neonatal samples were found to exhibit a higher metabolic activity which enables them to maintain, at low pO 2, a similar metabolic rate as the adult tissue at high pO 2. This corresponds to the fetal adaptation to low intrauterine oxygen tensions and might be a common basis for the elevated neonatal hypoxia/ischemia tolerance as well as for the postnatal increase in metabolic rate up to the level to be expected from body size.

A multi-layer model of retinal oxygen supply and consumption helps explain the muted rise in inner retinal Po2 during systemic hyperoxia

A multi-layer mathematical model of oxygen supply and consumption in the rat retina is described. The model takes advantage of the highly layered structure of the retina and the compartmentalisation of the available oxygen sources. The retina is divided into eight layers, each with a distinct oxygen consumption or supply rate. When applied to the available data from intraretinal oxygen measurements in the rat under normal physiological conditions, a close fit between the model and the data was achieved ( r 2=0.98+0.005, n=6). The model was then used to investigate recent evidence of oxygen regulating mechanisms in the rat retina during systemic hyperoxia. Fitting our model to the experimental data ( r 2=0.988+0.004, n=25) allowed the relative oxygen delivery or consumption of the key retinal layers to be determined. Two factors combine to produce the relative stability of inner retinal oxygen levels in hyperoxia. The retinal layer containing the outer plexiform layer/deep retinal capillaries, switches from a net source to a net consumer of oxygen, and the oxygen consumption of the outer region of the inner plexiform layer increases significantly. The model provides a useful tool for examining oxygen consumption and supply in all retinal layers, including for the first time, those layers within the normally perfused inner retina.

Optimization of L-phenylalanine production of Corynebacterium glutamicum under product feedback inhibition by elevated oxygen transfer rate.

Production feedback inhibition both on cell growth and on product formation of phenylalanine fermentation might be alleviated by elevated oxygen supply. Batch fermentations by a high phenylalanine producing strain Corynebacterium glutamicum CCRC 18335 at various initial phenylalanine concentrations (P(0)) ranging from 0 to 20 g/L and different oxygen transfer rate coefficients (K(L)a) ranging from 23 to 76 h(-1) were studied. The fermentation parameters with respect to P(0) were strongly dependent on K(L)a. Cell yield favored higher K(L)a and lower P(0). Product yield with respect to varying phenylalanine concentration was evaluated by the relative oxygen availability (ROA). The optimal ROA for phenylalanine formation was strongly dependent on the product concentration. While P(0) was low, the product inhibition was less significant and the maximum product yield occurred while ROA was at 0.5-0.6. While P(0) was high, the product inhibition was significant and the maximum product yield occurred while ROA was at 0.8-0.9. These results suggest that the product feedback inhibition of phenylalanine fermentation processes can be alleviated by a gradual increase in oxygen supply rate while the increasing product concentration is taken into account. The strategy is demonstrated in a fed-batch culture with elevated oxygen supply. The final phenylalanine concentration was 23.2 g/L, which was 45% better than that of the fed-batch fermentation without elevated oxygen supply. Likewise, the maximum productivity was improved by 42% at 0.37 g/(L x h).

A Mathematical Model of Electron Transfer Within the Mitochondrial Respiratory Cytochromes

A simple mathematical model of electron flow along the mitochondrial respiratory cytochrome assembly and the transfer of electrons to molecular oxygen is presented. First, an expression for the current–voltage relationship for a biological oxygen electrode is derived, and from this the relationship between oxygen consumption rate and oxygen partial pressure is determined. An independent relationship between mitochondrial oxygen partial pressure and oxygen supply rate is then derived. By eliminating oxygen partial pressure from these two expressions, we may obtain a relationship between oxygen supply rate and oxygen consumption rate. This model is then used to investigate the effects of tissue dysoxia, uncoupling of oxidative phosphorylation, increased cellular diffusional resistance and inhomogeneities in oxygen supply on oxygen consumption. It is concluded that each of the above contribute in varying degrees to the phenomenon of “pathological oxygen supply dependency”

Composition and Activity of Marine Alkane-Degrading Bacterial Communities in the Transition from Suboxic to Anoxic Conditions.

The impact of the oxygen supply rate (OSR) on the metabolic activity and on the composition of hexadecane-degrading bacterial communities in a quasi-anoxic milieu (nominal DOT = 0%) was studied in continuous cultures containing intertidal sediment. The dilution rate was kept constant at 0.035 h-1. The OSR was stepwise reduced from 3.5 mmol O2 L-1 h-1 to 0.06 mmol O2 L-1 h-1. Activity was determined by analyzing the respiration quotient (RQ) and the rates of hexadecane degradation (QHex), of hexadecane mineralization, and of protein production (PPR). The community composition and size were investigated by fluorescence in situ hybridization (FISH), by dilution plating (colony forming units or CFU), and by most probable number (MPN). The culture showed an aerobic hexadecane metabolism down to an OSR of 0.35 mmol O2 L-1 h-1. Below this OSR, anaerobic metabolism was initiated. The relationship among the RQ, PPR, QHex, and the OSR can be approximated by hyperbola (Michaelis-Menten kinetics). We suggest that the metabolic adaptation of the culture to low OSRs is due to regulation of protein expression and enzyme activity. Reducing the OSR resulted in minor but significant changes in the concentration of different physiological and phylogenetic groups. This means that, in addition to protein expression and activity regulation, the adaptation of the population to low OSRs is due to changes in the community composition.

Bioleaching of a chalcopyrite concentrate using an extremely thermophilic culture

The ability of an extreme thermophile to oxidise a concentrate comprising of chalcopyrite (66%) and pyrite (11%) is described. A batch test at 70°C showed that a copper extraction of >98% was possible. A series of continuous tests were carried out in a three-stage pilot plant, employing standard-design mechanically agitated and aerated tanks. The effect of residence time, feed grind and the mass transfer supply of oxygen and carbon dioxide on bioleach performance were evaluated. The results showed that overall Cu extractions of 95% could be obtained. However, compared to mesophilic and moderately thermophilic bioleaching bacteria, the extreme thermophiles appeared to be more sensitive to the solids concentration employed, which may also be related to the particle size of the feed solids. Levels of oxygen consumption approaching the maximum rates currently being employed in commercial-scale bioleach tanks treating refractory gold pyritic concentrates could be achieved. To maintain these high oxidation rates, it was important to ensure that the supply rate of oxygen and carbon dioxide to the reactors was sufficient. The high copper extractions obtained in these tests showed that a process treating chalcopyrite concentrates using extreme thermophiles has the potential for further development and assessment for commercial applications.

転炉を用いた脱りん脱炭連続処理プロセスの開発

The authors propose a new refining process, by which hot metal is dephosphorized and decarburized continuously in the same basic-oxygen furnace, with intermediate deslagging following dephosphorization. After decarburization, slag is kept inside the furnace and reused to dephosphorize the next charge so as to minimize the quantity of slag discharge. Experiments were made to clarify the properties of this process, using an 8-ton experimental converter. The results confirmed high productivity or a higher rate of dephosphorzation achievable with an increased rate of oxygen blowing and the addition of iron o?e. Because of the dephosphorization by low-basicity slag and the ho - recycling of decarburization-slag to dephosphorize the next charge, line consumption was relatively low. The proper conditions of this process, such as the slag composition for dephosphorization rate of oxygen supply, bottom agitation, and rate of intermediate deslagging, are also discussed, based on the experimental results.

Ceria-based catalysts for the recovery of elemental sulfur from SO 2-laden gas streams

The catalytic reduction of SO 2 to elemental sulfur by CO or CH 4 over ceria-based catalysts under demanding operating conditions is reported in this paper. Cu- or Ni-containing ceria catalysts have been shown before to be highly active and selective for SO 2 reduction by CO in dry gas streams or in the presence of low amounts of H 2O. In this work, the activity/selectivity of 10 at.%La-doped ceria, Ce(La)O x , and Cu- or Ni-containing Ce(La)O x for the (SO 2+CO) reaction was tested in gas streams containing 10–45 mol%H 2O at high space velocities (>80,000 h −1). The addition of 5 at.% (∼2.5 wt.%) copper or nickel significantly improved the low-temperature (<500°C) reactivity of Ce(La)O x . This was correlated with the improved reducibility of ceria in the metal-ceria catalysts and the ensuing low-temperature activity for the water-gas-shift (WGS) reaction. The combined reduction of SO 2 and NO by CO was also studied in this work over the same catalysts. In dilute gas mixtures containing 0.1–1.0 mol%SO 2 and NO, stoichiometric amount of CO and in the presence of 40%H 2O, the NO presence in the feed gas enhances both the SO 2 conversion and the elemental sulfur yield. At 550°C, in the presence of NO, SO 2 conversion and sulfur yield over the 5%Ni-Ce(La)O x catalyst were 0.94 and 0.77, respectively, the NO conversion to N 2 was complete, and the CO 2 produced was the sum of the SO 2 and NO reduction reactions by CO. Ceria-based materials are also active for SO 2 reduction by methane to elemental sulfur at temperatures higher than 550°C. The addition of Cu or Ni has a different effect on the sulfur selectivity of ceria under fuel-rich conditions. The Cu-ceria system is a complete oxidation catalyst to a much higher temperature than Ni-ceria. Over Ni-CeO 2, dissociation of methane begins at <550°C, and side reactions favor H 2S production over elemental sulfur. However, the 5 at.%Ni-Ce(La)O x catalyst showed remarkable resistance to carbon deposition, both in the SO 2-methane reaction as well as in partial oxidation of methane by O 2 to synthesis gas with a 2:1 H 2:CO ratio. This is attributed to the high dispersion of nickel in this catalyst and the fast rate of oxygen supply from ceria to the nickel interface. Catalysts were characterized by temperature-programmed-reduction, XPS and STEM/EDS.

Oxygen-ion conductivity and catalysis

Oxygen-ion conducting solid electrolyte electrochemical cells were prepared with both Fe 2O 3–Sb 2O 4 and Pt working electrodes. The working electrode served as both catalyst, for a heterogeneous catalytic reaction taking place in the electrode compartment, and electrode. With the Fe 2O 3–Sb 2O 4 electrode, the electrochemical cell was used as a potentiometric sensor to determine surface oxygen activities during the partial oxidation of propene to acrolein. The sensor was found to be selective to surface oxygen involved in the competing total oxidation reaction. With the Pt working electrode, the cell was operated in a closed-circuit mode with oxygen being electrochemically supplied and removed from the catalyst surface over which the oxidation of carbon monoxide was being performed. The large changes in reaction rate compared to the rate of electrochemical oxygen supply were explained by the electrochemical reaction having different selectivities, for the formation of surface oxygen species, from the purely surface reaction.

Effects of Oxygen Supply Methods on the Performance of a Sequencing Batch Reactor for High Ammonium Nitrification

High ammonium nitrification was studied in a sequencing batch reactor (SBR) with two different oxygen supply methods: controlled oxygen supply rates (dissolved oxygen [DO] in the reactor controlled between 2 and 3 mg/L) and uncontrolled oxygen supply rates (DO in the reactor uncontrolled and changed ambiently with oxygen use rate). Experimental results showed that oxygen supply methods have significant effects on SBR performance for high ammonium nitrification. With the controlled oxygen supply method, ammonium and nitrite oxidations took place simultaneously. With the uncontrolled oxygen supply method, ammonium oxidation proceeded smoothly during the low DO period of a cycle; however, nitrite oxidation was at first completely inhibited at low‐DO but recovered during the high DO period of the same cycle. Ammonium and nitrite were oxidized sequentially during a cycle in SBR. Ammonium oxidizers developed an ability to endure the fluctuation of DO, but nitrite oxidizers did not. Extensive starvation under high DO concentrations plays an important role in ammonium oxidation under low DO conditions.

Modeling concurrent flame spread over a thin solid in a low-speed flow tunnel

A three-dimensional model of concurrent flame spread over a thin solid in a low-speed flow tunnel in microgravity was formulated and numerically solved. In a parametric study varying the flow velocity, oxygen level, and tunnel and solid fuel widths, two distinctive types of flame behavior were noted. In high-oxygen-percentage and/or higher-speed flows, the flames were long and far away from the quenching limit. In such cases, three-dimensional effects were dominated by heat loss to the wall in the downstream portion of the flame. In low-oxygen and low-speed flows, the flames were short and in the region near the quenching limit. These near-limit flames were controlled by the oxygen supply rate. Oxygen-side diffusion in the cross-wind direction became a dominant mechanism exhibiting large effects on the narrow three-dimensional flames. A number of trend reversals on spread rates and extinction limits were discovered for these nearlimit flames. Aided by the detailed flame profiles obtained in the computations, an explanation for the reversal phenomena is offered in the paper.

Development of European bitterling in the gills of freshwater mussels

The development of bitterling embryos within the unique environment of a freshwater mussel's gills requires a departure from typical cyprinid embryological development. Eggs are large (2·6×1·7 mm), illustrating the low risk of predation, and elliptical; a response to unionid gill morphology and a way of increasing the transfer rates of respiratory and excretory products to and from the tissues. The yolk sac develops elongated lateral processes during early ontogeny; these secure the embryo into the host's interlamellar space. Once the larva is capable of movement (8·2 mm) the lateral processes are lost and the larva becomes less dependent of the host's gills for the provision of oxygen. Hatching (3·3 mm) and pigmentation of the blood (6·4 mm) occur relatively early; this may increase the rate of oxygen supply to the tissues. Pigmentation of the eyes and appearance of the melanophores occurs relatively late (7·4 mm and 7·9 mm, respectively); embryos are not required to detect or avoid predators. Bitterling larvae generally emerge from the host once the yolk sac has been consumed (10·5 mm); this may mark a change in respiratory and nutritional requirements.

Effect of the oxygen supply on pattern of growth and corrinoid and organic acid production of Propionibacterium shermanii

Propionibacterium shermanii CDB 10014 is able to grow even at high oxygen transfer rates (24.0 mmol O2 l−1 h−1), in contrast to reports in the specialised literature, where all Propionibacteria are considered oxygen-sensitive microorganisms. Propionic acid is the main product in anaerobiosis. The presence of oxygen in the system leads to an inhibition of propionic acid production while acetic acid formation is enhanced. At high oxygen supply rates no propionic acid is produced and acetic acid is the main product. Lactic acid is also produced in reasonable quantities (2.7 g l−1). The growth rate (μmax) is higher in anaerobiosis (0.19 h−1) than in aerobiosis (0.12–0.15 h−1). The cell yield is higher in aerobiosis (0.18–0.22 g g−1) than in anaerobiosis (0.14 g g−1) suggesting the oxidative metabolism of glucose by Propionibacterium shermanii CDB 10014. No corrinoid production was detected at oxygen transfer rates of more than 13.6 mmol l−1 h−1.

Nitrification and oxygen utilisation in a membrane aeration bioreactor

A laboratory-scale membrane aeration bioreactor (MABR) was operated with a nitrifying biofilm attached to the surface of oxygen-permeable hollow fibres. Pure oxygen was supplied to the biofilm via the lumen of the hollow fibres. The process achieved high rates of nitrogen removal (98%) and nitrification (83%) at a nitrogen loading rate of 1.2 kg NH 4–N m −3 d −1. A specific nitrification rate of 5.4 g NH 4–N m −2 d −1 and a nitrogen removal rate of 6.6 g NH 4–N m −2 d −1 were demonstrated. Nitrification was virtually unaffected by a reduction in the oxygen supply rate up to the point where the supply matched the oxygen requirement of the wastewater. Exceptionally high oxygen utilisation efficiencies were achieved. It is proposed that interfacial oxygen mass transfer contributes to the excellent performance of the MABR at low oxygen supply rates.

Process Simulation of Desulfurization from Pyrite by Thiobacillus ferrooxidans in a Packed-Bed Medium-Recirculating Bioreactor under Different Recirculation Rates

Effect of medium-recirculation rate on desulfurization of pyrite by Thiobacillus ferrooxidans in a packed-bed bioreactor was examined for developing an efficient on-site desulfurization process of low grade coal. Linear relationship between elution rate of SO42- and oxygen supply rate indicated that desulfurization rate was limited by dissolved oxygen (DO) supply which was dependent on the recirculation rate. Based on these results, a mathematical model consisting of DO limitation and product inhibition was constructed and satisfactorily simulated the performance in the on-site type desulfurization process.

Hypoxic renal tissue damage by endothelin-mediated arterial vasoconstriction during radioangiography in man.

In vivo, a high rate of renal blood flow provides a more than sufficient rate of oxygen supply to the kidney. Limitations in tissue oxygenation may occur under systemic circulatory alterations (i.e. cardiovascular failure, arterioscelorosis), respiratory insuffiency (i.e. lung diseases, hypoxia, extreme anemia, acclimation to high altitude), or during perfusion of isolated organs with blood substitutes (Klause, N., and Gronow, G., 1990; Klause, N., et al., 1990). Hypoxic limitation of renal function at a maintained systemic respiration and circulation is less well understood. One example is tissue hypoxia in the kidney due to a vasoconstrictory side effect of radiocontrast media (RCM). Recent animal experiments support evidence that in the pathophysiology of RCM-induced renal vasoconstriction the balance between production of endothelium derived nitric oxide (NO) and endothelin (ET) is disturbed, whereby NO does not seem to play an important role (Bagnis et al., 1997, Morcos et al., 1997).

Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most