carbon mineralization and oxygen dynamics in sediments with deep oxygen penetration lake superior to understand carbon and oxygen dynamics in sediments with deep oxygen penetration we investigated eight locations 160 318 m depth throughout lake superior despite the 2 4 weight percent organic carbon content oxygen penetrated into the sediment by 3 5 to 12 cm at all locations such deep penetration is explained by low sedimentation rates 0 01 0 04 cm yr 21 high solubility of oxygen in freshwater and a shallow 2 cm bioturbation zone in response mainly to oxygen variations in the bottom waters the sediment oxygen penetration varied seasonally by as much as several centimeters suggesting that temporal variability in deeply oxygenated sediments may be greater than previously acknowledged the oxygen uptake rates 4 4 7 7 mmol m 22 d 21 average 6 1 mmol m 22 d 21 and carbon mineralization efficiency 90 of deposited carbon were similar to those in marine hemipelagic and pelagic sediments of comparable sedimentation rates the reactivity of organic carbon was found to decrease with age similarly to the power law documented in marine environments the burial flux of carbon into the deep sediment 0 7 mmol m 22 d 21 was 2 5 of the previously estimated primary production maximum volume specific carbon degradation rates were 0 3 1 5mmol cm 23 d 21 bioturbation coefficient near the sediment surface was 3 8 cm 2 yr 21 these results indicate that carbon cycling in large freshwater systems conforms

methane flux and origin in the othrys ophiolite hyperalkaline springs greece abstract the occurrence and origin of methane ch4 generated by serpentinization of ultramafic rocks is of current timely interest in planetary geology astrobiology and energy resource exploration as it may contribute in particular to decipher the source of methane on mars the origin of life and the potential of abiotic hydrocarbon synthesis methane of dominant abiotic origin in serpentinized peridotites on continents ophiolites or igneous intrusions has been documented so far with complete carbon and hydrogen isotope composition in six countries in the philippines turkey oman new zealand japan and italy we report the discovery of two new sites in greece at archani and ekkara located in the othrys ophiolite massif portable sensors based on fourier transform infrared spectrometry ftir and tunable diode laser absorption spectroscopy tdlas allowed to realize that out of 21 ophiolitic springs methane is released only by four hyperalkaline ph from 10 7 to 11 3 and calcium hydroxide ca oh type waters all other 17 springs with ph

impact of suboxia on sinking particulate organic carbon enhanced carbon flux and preferential degradation of amino acids via denitrification fluxes of particulate organic carbon poc through the oxygen deficient waters in the eastern tropical north pacific were found to be relatively less attenuated with depth than elsewhere in the eastern north pacific the attenuation coefficient b for the flux was found to be 0 40 versus the composite value of 0 86 determined by martin et al 1987 to examine this further sinking poc was collected using sediment traps and allowed to degrade in oxic and suboxic experiments using a kinetic model it was found that degradation proceeded at similar rates roughly 0 8 day 1 under oxic and suboxic conditions but a greater fraction of bulk poc was resistant to degradation in the suboxic experiments 61 vs 23 amino acids accounted for 37 of poc collected at 75m but following degradation the value dropped to 17 and 16 in the oxic and suboxic experiments respectively poc collected from 500m was 10 amino acids the non aa component of poc collected at 75m was not degraded under suboxic conditions while under oxic conditions it was these results suggest that microbes degrading oc under suboxic conditions via denitrification preferentially utilize nitrogen rich amino acids this preferential degradation of amino acids suggests that 9 more nitrogen may be lost via water column denitrification than is accounted for when a more redfieldian stoichiometry for poc is assumed

abiotic methane in continental ultramafic rock systems towards a genetic model abstract methane ch4 often in substantial quantities is reported for numerous surface manifestations seeps springs and aquifers in continental serpentinized ultramafic rocks in ophiolites peridotite massifs and intrusions frequently this gas is considered to have a dominant abiotic origin with variable though generally minor components of biotic gas thermogenic or microbial abiotic ch4 production through low temperature fischer tropsch type reactions ftt between a carbon c compound and h2 is endorsed by most of scholars but direct derivation from olivine hydration serpentinization magmatic sources and fluid inclusions have also been suggested here we review the application of ftt in geology discussing the appropriateness of several c compounds co2 co formic acid formate or elemental c as ch4 precursors and of aqueous versus gas phase reactions we examine published gas geochemical and flux data that provide clear constraints on the methane origins in the analysis we add new isotopic and gas seepage data acquired in surface gas manifestations at acquasanta in the voltri ophiolite genova italy multiple lines of evidence including a stable isotope compositions of ch4 co2 and helium b radiocarbon free ch4 c isotopic disequilibrium between ch4 and h2o d low temperatures of ch4 production based on clumped isotopes and heat flow data and e methane seepage forms and intensities suggest that ch4 is not formed directly in water or from magmatic sources or fluid inclusions rather all the lines of evidence taken together are compatible with the hypothesis of low temperature

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

methane in serpentinized ultramafic rocks in mainland portugal we report a new case of methane ch4 of apparent abiotic origin in continental serpentinized ultramafic rocks multiple analytical techniques on site and in the laboratory revealed methane and ethane degassing from hyperalkaline ph 11 ca2 oh mineral waters in boreholes drilled in the alter do chao igneous intrusion at cabeco de vide in mainland portugal the c and h isotopic composition of ch4 13c 20 2h 283 suggests a dominant abiotic origin although minor thermogenic contributions cannot be excluded similarly low methane to ethane ratios suggest a predominantly non microbial source consistent with previous microbiological data showing the lack of methanogenic archaea in these waters heavier hydrocarbons co2 and h2 are below detection limits this case study confirms that ch4 from serpentinized ultramafic rocks can be transported by hyperalkaline fluids linked to deep circulation of meteoric waters maximum depth of cabeco de vide serpentinized rocks is less than 1 km and present temperatures are likely lower than 50 c serpentinization and related gas formation may have occurred at any time during thermal evolution of the igneous intrusion so gas formation temperature cannot be easily determined this case is an opportunity to test thermometry provided by ch4 isotopologue analyses the existence of methane in continental serpentinized igneous rocks is more widespread than previously thought and petroleum systems with similar serpentinized ultramafics in reservoir rocks may have traces of the observed 13c enriched ch4

elemental and major biochemical changes across an oxidation front in a relict turbidite an oxygen effect abstract the elemental and major biochemical compositions of the relict f turbidite sampled in two cores from the madeira abyssal plain were determined this fine grained distal sequence occurs at ca 9 m core depth and includes a surficial oxidized horizon defined by a distinct color change oxygen diffused downward through sediments above this interface and in ca 10 kyr destroyed 80 of the organic substances that below the front had survived degradation in the presence of porewater sulfate for ca 140 kyr these deposits provide an opportunity to establish the extent and selectivity of oxic sedimentary degradation under natural conditions without the usual complications of bioturbation and varying sources or sedimentation rates in both cores a sample from the upper oxidized layer was compared to two samples from the underlying unoxidized layers the unoxidized sequences of both turbidities contained 0 93 1 02 wt organic carbon oc and 0 10 0 11 wt total nitrogen tn approximately 20 of the initial oc and 40 of the initial tn remained in the oxidized horizons with a consequent decrease in atomic c n ratio from ca 11 to ca 5 all samples gave very low yields of lignin phenols and comparable oc normalized yields of total aldoses and amino acids and indicated predominantly marine organic matter om and nonselective oxic degradation of these biochemical classes compositions of individual aldoses and amino acids generally were also similar in surface and deep sediments except that the oxidized horizons yielded markedly elevated 3 5x percentages of nonprotein amino acids this study clearly demonstrates that prolonged exposure to oz can lead to organic matter alteration which is far more extensive than that obtained with sulfate alone in comparison to early diagenesis however alteration of the measured biochemicals was largely nonselective such oxidation reactions could control the distribution and composition of organic matter in slowly accumulating continental rise and deep ocean environments

organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source we compared the burial efficiency of organic carbon buried oc deposited oc in a diverse set of 27 different sediments from 11 lakes focusing on the potential effects of organic matter source oxygen exposure and protective sorption of oc onto mineral surfaces average oc burial efficiency was high mean 48 and it was particularly high in sediments receiving high input of allochthonous organic matter mean 67 further oc burial efficiency was strongly negatively related to the oxygen exposure time again particularly so in sediments receiving high allochthonous loads on the other hand oc burial efficiency was not related to the mineral surface area which was used as a proxy of the sorption capacity of the mineral phase for oc the high oc burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes as well as to a strong dependence of oc burial efficiency on oxygen exposure time this study demonstrates that the carbon sink in lake sediments alters the oc export from the continents to the sea and that the fate of oc in lake sediments burial vs mineralization to carbon dioxide and methane is highly sensitive to environmental conditions

trace element distributions within ordinary chondrite chondrules implications for chondrule formation conditions and precursors the concentrations of thirty six lithophile elements in the major silicate phases of chondrules have been determined by ion microprobe two types of porphyritic chondrule olivines normal and incompatible rich have been identified the incompatible rich olivines which have forsterite contents that are greater than fo98 appear to have formed under highly reducing conditions the olivine and low ca pyroxene grains in porphyritic chondrules have relatively unfractionated l chondrite normalized ree abundances compared to expectations from equilibrium distribution coefficients this is consistent with porphyritic chondrules having formed at cooling rates of the order of 1000 c h or faster the alkali metal na k rb and cs to al ratios in chondrule glasses vary by two orders of magnitude the lower values possibly being the result of volatile loss from chondrules no correlation was found between alkali metal depletions in chondrule glasses and enrichments in the surrounding rims but any such correlation may have been destroyed by the diffusive redistribution of these elements in rims that is likely to occur even under mild metamorphic conditions the refractory lithophiles with the exception of mg are all highly incompatible in silicate melts and are as a result concentrated in the chondrule glasses the refractory lithophiles in the glass are in most cases unfractionated from one another suggesting that neither vapor phase nor crystal liquid fractionation played a major role in the formation of chondrules or their precursors based largely on the observation that the refractory lithophiles are concentrated in the chondrule glasses it is suggested that previously reported correlations between these elements in bulk chondrule analyses are due to chondrules themselves as opposed to refractory condensates having been their own immediate precursors a simple monte carlo simulation using the compositions of the major chondrule silicate phases reported here supports this hypothesis it is argued that the upper limit of about one observed in the atomic na al ratios of chondule glasses is not due to an albitic precursor but simply reflects that for na to be incorporated into the structure of the melt it must take part in the coupled substitution naalo2 sio2

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

a high toc shale in a low productivity world the late mesoproterozoic arctic bay formation nunavut abstract the latest mesoproterozoic arctic bay formation borden basin nunavut canada is up to 1130 m thick and contains a significant proportion of unusually organic rich black shale up to 12 3 wt total organic carbon insofar as increased biological productivity is related to organic matter burial this organic rich succession is seemingly incongruent with the low biological productivity world hypothesised for much of the proterozoic to better understand the conditions leading to development of this organic rich unit we explore the redox geochemistry of the arctic bay formation using a multi proxy approach nitrogen isotopes iron speciation total organic carbon total sulphur and trace metal abundances redox proxy data support a stratified water column with oxic surface waters underlain by intermittently euxinic waters which are in turn underlain by persistently ferruginous deeper waters the highly alkaline restricted marine basin in which the arctic bay formation was deposited may have allowed for rapid sequestration of highly reactive iron in carbonate minerals resulting in an excess of sulphur that resulted in sulphurisation of organic matter estimates for organic matter burial rates during deposition of the arctic bay formation suggest that they were perhaps 5 6 times mid proterozoic average values although there are permissible scenarios in which it was extremely productive underscoring that such organic rich sedimentary rocks could be produced in a low productivity world

fluid inclusion and stable isotopic constraints on fluid sources and evolution of the luojiahe cu deposit in the southern margin of the north china craton abstract the luojiahe cu deposit in the zhongtiaoshan region is located in the southern margin of the north china craton the orebodies are hosted in the mafic volcanic sedimentary sequences of the metamorphosed greenschist facies neoarchean songjiashan group the luojiahe cu mineralization can be divided into the primary volcanogenic massive sulfide vms mineralization stage stage i banded or stockwork ores and the subsequent metamorphic remobilization stage stage ii coarse vein ores three types of quartz selected for fluid inclusion fi studies were collected from the stage i banded q1 and stockwork q2 ores and stage ii coarse vein q3 ores four types of fis were identified 1 liquid rich fis l type 2 pure vapor and vapor rich fis v type 3 daughter mineral bearing fis s type and 4 ch4 h2o fis c type systematical microthermometric and h o isotopic studies show that the stage i ore forming fluids consist predominantly of high salinity evolved seawater 125 220 c 23 9 27 9 wt nacl equiv and some magmatic hydrothermal fluids 249 339 c 34 5 42 2 wt nacl equiv the two fluid end members are represented by the l type fis in q1 and the s and v type fis in q2 the temperature and salinity variation trends of the l type fis in q1 indicate a mixing process between the hot evolved seawater and cold seawater at stage i furthermore the v and s type fi coexistence in q2 and their microthermometric data suggest that fluid unmixing has occurred in original magmatic fluids at stage i in contrast the stage ii ore forming fluids consist of ch4 rich metamorphic fluids 192 350 c 10 6 43 2 wt nacl equiv carbon isotopic analysis of the stage ii calcite 4 58 to 10 83 and graphite 32 01 to 39 16 in the ore hosting chlorite schist indicates that the metamorphic ore forming fluids had exchanged carbon isotope with graphite the generation of ch4 may have resulted from the interaction between h2o released by metamorphic devolatilization and graphite the continuous consumption of h2o in the hydrothermal fluid system may have increased the fluid salinity and triggered fluid unmixing in the ch4 nacl h2o system in addition the vms metallogenic environment is generally favorable for microbial communities it is considered that the graphite at luojiahe may have been derived from sedimentary organic matter formed in seafloor hydrothermal vent systems as also supported by carbon isotopic data we propose that at stage i the main mineralization may have been resulted from 1 fluid mixing of hot evolved seawater and cold seawater in the near surface environment and 2 fluid unmixing caused by the percolation of magmatic fluids into syn volcanic faults forming the stockwork ores at stage ii the interaction between h2o and graphite may have resulted in the reduction of ore forming fluids and cu precipitation and fluid unmixing in the ch4 nacl h2o system may have further promoted the cu mineralization

significant role of organic sulfur in supporting sedimentary sulfate reduction in low sulfate environments abstract dissimilatory sulfate reduction dsr is a major carbon mineralization pathway in aquatic sediments soils and groundwater which regulates the production of hydrogen sulfide and the mobilization rates of biologically important elements such as phosphorus and mercury it has been widely assumed that water column sulfate is the main sulfur source to fuel this reaction in sediments while this assumption may be justified in high sulfate environments such as modern seawater we argue that in low sulfate environments mineralization of organic sulfur compounds can be an important source of sulfate using a reaction transport model we investigate the production of sulfate from sulfur containing organic matter for a range of environments the results show that in low sulfate environments 50 of sulfate reduction in well oxygenated systems porewater sulfate profiles often exhibit sub interface peaks so that sulfate fluxes are directed out of the sediment our measurements in lake superior the world s largest lake corroborate this conclusion offshore sediments act as sources rather than sinks of sulfate for the water column and sediment dsr is supported entirely by the in sediment production of sulfate sulfate reduction rates are correlated to the depth of oxygen penetration and strongly regulated by the supply of reactive organic matter rate co regulation by sulfate availability becomes appreciable below 500 m level the results indicate the need to consider the mineralization of organic sulfur in the biogeochemical cycling in low sulfate environments including several of the world s largest freshwater bodies deep subsurface and possibly the sulfate poor oceans of the early earth

have olivine will gas serpentinization and the abiogenic production of methane on mars 1 spatial variability of methane ch4 on mars suggests the presence of localized subsurface sources here we show that olivine hydration in the martian regolith and crust may be a major ch4 source which contributed significantly to the warming of early mars methane production is kinetically and thermodynamically favored during low t aqueous alteration of olivine rich rocks sustained release of ch4 on present day mars may come through the breakdown of ancient ch4 hydrates and from springs driven by geothermal heat

origin of methane in serpentinite hosted hydrothermal systems the ch4 h2 h2o hydrogen isotope systematics of the hakuba happo hot spring abstract serpentinite hosted hydrothermal systems have attracted considerable attention as sites of abiotic organic synthesis and as habitats for the earliest microbial communities here we report a systematic isotopic study of a new serpentinite hosted system the hakuba happo hot spring in the shiroumadake area japan 36 42 n 137 48 e we collected water directly from the hot spring from two drilling wells more than 500 m deep all water samples were strongly alkaline ph 10 and rich in h2 201 664 mol l and ch4 124 201 mol l despite the relatively low temperatures 50 60 c thermodynamic calculations suggest that the h2 was likely derived from serpentinization reactions hydrogen isotope compositions for happo 1 happo 3 were found to be as follows d h 2 700 710 d ch 4 210 300 and d h 2 o 85 84 the carbon isotope compositions of methane from happo 1 and 3 were found to be c 13 34 5 and 33 9 respectively the ch4 h2 h2o hydrogen isotope systematics indicate that at least two different mechanisms were responsible for methane formation happo 1 has a similar hydrogen isotope compositions to other serpentinite hosted systems reported previously the elevated d ch 4 with respect to the equilibrium relationship suggests that the hydrogen of the happo 1 methane was not sourced from molecular hydrogen but was derived directly from water this implies that the methane may not have been produced via the fischer tropsch type ftt synthesis but possibly by the hydration of olivine conversely the depleted d ch 4 with respect to the equilibrium relationship in happo 3 suggests the incorporation of biological methane based on a comparison of the hydrogen isotope systematics of our results with those of other serpentinite hosted hydrothermal systems we suggest that abiotic ch4 production directly from h2o without mediation by h2 may be more common in serpentinite hosted systems hydration of olivine may play a more significant role in abiotic methane production than previously thought

hydrogen isotope fractionation in land based serpentinization systems comment on origin of methane in serpentinite hosted hydrothermal systems the ch4 h2 h2o hydrogen isotope systematics of the hakuba happo hot spring by suda et al earth planet sci lett 386 2014 112 125

mechanism for burgess shale type preservation exceptionally preserved fossil biotas of the burgess shale and a handful of other similar cambrian deposits provide rare but critical insights into the early diversification of animals the extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft bodied fossil assemblages has remained enigmatic since walcott s initial discovery in 1909 here we demonstrate the mechanism of burgess shale type preservation using sedimentologic and geochemical data from the chengjiang burgess shale and five other principal burgess shale type deposits sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation low sulfate concentrations in the global ocean and low oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability subsequently rapid entombment of fossils in fine grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments a permeability barrier provided by bed capping cements that were emplaced at the seafloor is a feature that is shared among burgess shale type deposits and resulted from the unusually high alkalinity of cambrian oceans thus burgess shale type preservation of soft bodied fossil assemblages worldwide was promoted by unique aspects of early paleozoic seawater chemistry that strongly impacted sediment diagenesis providing a fundamentally unique record of the immediate aftermath of the cambrian explosion

organic carbon burial efficiencies in sediments the power law of mineralization revisited the reactivity of marine organic carbon toward oxidation has been described as a simple power law function that decreases with carbon age and is applicable from fresh phytoplankton to nearly kerogen we demonstrate that a correction to this law that accounts for a slowing in carbon mineralization under anoxic conditions makes the law consistent with observed carbon burial efficiencies which are known to be higher in anoxic environments our analyses of data from freshwater lakes reveal that the reactivity of autochthonous organic carbon follows similar trends in freshwater and marine environments the similarity suggests that the power law may be applicable to environments with diverse water chemistries such as the anoxic or low sulfate oceans of the geologic past using this power law we calculate the theoretical burial efficiencies for organic carbon under a range of conditions including those in anoxic ancient oceans the calculations suggest that in abyssal sediments the buried fraction of deposited organic carbon may have been 6 40 times higher in precambrian anoxic oceans than today whereas for coastal areas the difference was much smaller oxygenation of the deep ocean then would have caused a large scale decrease in organic carbon burial with a corresponding negative feedback on rates of atmospheric oxygen accumulation

effect of sea level on organic carbon preservation in the okinawa trough over the last 91 kyr abstract the burial of marine organic carbon primarily synthesized by marine autotrophic organisms into sediment removes co2 from the ocean atmosphere system however linkages between export productivity and organic carbon burial and the controlling factors remained ambiguous especially for marginal seas in this study we measured redox sensitive trace metal concentrations and planktic foraminifer fauna assemblages for a sediment core collected from the middle okinawa trough spanning the past 91 kyr through compilation with other published data we found that the export productivity was decoupled from organic carbon burial variability of manganese and total sulfur contents suggested that both bottom water and surface sediment had experienced significant redox change in the last glacial cycle by using the ratio of marine organic carbon to reactive phosphorus i e ocmarine preact we reconstructed organic carbon burial efficiency which showed relatively higher values during the last glacial and low values during the middle late holocene after synthesizing our new and reported data we conclude that the middle okinawa trough bottom water redox state was controlled by deep water ventilation induced by sea level change and related kuroshio current intrusion instead of by the oxygen consumption owing to local export productivity

effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments in order to study the effect of oxygen on degradation rate of bulk sedimentary organic matter we developed a new incubation method in contrast to most previous experiments 1 all of the sediment undergoing decomposition was maintained under oxygenated or oxygen free conditions and 2 organic matter of varying lability was studied the production of co2 during incubations of sediment in glass test tubes corrected for dissolution precipitation of calcium carbonates was used as a measure of degradation rates the laboratory experiments using surficial and buried continental shelf and slope sediment from the open skagerrak northeastern north sea demonstrated that the effect of oxygen on the degradation rate of sedimentary organic matter is a function of the lability of the decomposing material fresh material was degraded with little difference in rates in the presence or absence of oxygen whereas old material was decomposed significantly up to 3 6 times faster with oxygen than without oxygen an implication of these findings is that bioturbation by exposing old buried material to oxygen may enhance integrated organic carbon oxidation in marine sediments this constitutes a previously unexplored mechanism by which faunal reworking may stimulate carbon degradation the anoxic decomposition rates of organic material buried at 20 cm depth in sediment were the lowest measured we found however that the extent of oxidation of this buried old sediment was considerably larger than that of surficial sediment under oxygenated conditions which indicated that the oxic anoxic oxic redox transitions deposition under oxic conditions burial under anoxia and reexposure to oxygen promoted degradation our results therefore also suggest that the extent of long term decomposition of sedimentary organic material is smaller under oxygenated or anoxic conditions alone than when the material is exposed to the repeated activities of both oxic and anoxic microorganisms