authigenic uranium relationship to oxygen penetration depth and organic carbon rain we have measured u in benthic incubation chambers sediment pore waters and in sediments along the california continental margin sedimentary u uptake rates based on a combination of sediment pore water profiles and benthic incubation chambers generally agree with those predicted from sediment accumulation rate data this agreement supports the view that most of the continental margin sedimentary u is delivered by diffusion across the seawater sediment boundary the average rate of authigenic u accumulation for all the sites examined here is 0 2 nmol cm 2 y 1 which is consistent with published global estimates of sedimentary u uptake in addition the accumulation rate of u in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate these relationships highlight the potential utility for the u accumulation rate as a proxy for these processes

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

early diagenesis in sediments from danish coastal waters microbial activity and mn fe s geochemistry abstract depth distributions of bacterial respiration of o2 no3 and so42 were compared with geochemical data for mn fe and s in coastal sediments from water depths between 26 and 520 m as water depth increased the zone of so42 respiration was found deeper in the sediment and was eventually separated from the surface located activity of o2 and no2 respiration at the deepest station additional so42 reduction activity was observed in small detrital aggregates on the sediment surface dissolved mn2 and fe2 appeared between the o2 plus no3 containing surface layer and the h2s plus fes containing sediment below this was a result of mn and fe reductions coupled to either the oxidation of sulfide or the mineralization of organic matter tracer experiments showed that both fes fes2 and s0 were important radiolabelled products of sulfate respiration in this intermediate zone in the same zone the overall degradation of organic matter seemed to be underestimated by the assay of so2 4 respiration and additional mineralization by mn and fe reductions was likely

anaerobic microbial biogeochemistry in sediments from two basins in the gulf of maine evidence for iron and manganese reduction abstract rates of sulphate reduction denitrification and glucose turnover were determined in surface sediments collected from jeffreys and wilkinson basins in the gulf of maine these data were compared to porewater profiles of iron and manganese three distinct anaerobic biogeochemical situations were evident 1 sulphate reduction was dominant and active near the sediment water interface deeper portions of jeffreys basin 2 denitrification was active in the upper few centimetres and was underlain by rapid sulphate reduction slope of jeffreys basin 3 denitrification was dominant wilkinson basin an apparent iron and manganese reduction region was evident in all cores since glucose turnover was active and both denitrification and sulphate reduction were low within this metal reduction zone it is possible that microbial metal reduction is a significant process within the basin sediments high concentrations of dissolved iron occurred within sediments deeper than those supporting active sulphate reduction sufficient iron was remobilized during iron reduction to not only precipitate all of the sulphide generated during sulphate reduction but to also allow for dissolved iron to diffuse into deeper sediments therefore large vertical regions containing dissolved iron are not necessarily regions of iron reduction and iron reduction is restricted to a narrow sediment layer

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

trace metal enrichments in lake tanganyika sediments controls on trace metal burial in lacustrine systems abstract we investigate the distributions of several key diagenetic reactants c s fe and redox sensitive trace metals mo cd re u in sediments from lake tanganyika east africa this study includes modern sediments from a chemocline transect which spans oxygenated shallow waters to sulfidic conditions at depth as well as ancient sediments from a longer core 2 m taken at 900 m water depth modern sediments from depths spanning 70 335 m are generally characterized by increasing enrichments of c s mo cd and u with increasing water depth but static fe distributions it appears that the sedimentary enrichments of these elements are to varying degrees influenced by a combination of organic carbon cycling and sulfur cycling these modern lake characteristics contrast with a period of high total organic carbon corg total sulfur stot and trace metal concentrations observed in the 900 m core a period which follows the most recent deglaciation 18 11 ky this interval is followed abruptly by an interval 11 6 ky that is characterized by lower c s u and mo consistent with other work we suspect that the low concentrations of s mo and u may indicate a period of intense lake mixing during which time the lake may have been less productive and less reducing as compared to the present an alternative but not mutually exclusive hypothesis is that changes in the lake s chemical inventory driven by significant hydrological changes could be influencing the distribution of sedimentary trace elements through time

vanadium geochemistry along groundwater flow paths in contrasting aquifers of the united states carrizo sand texas and oasis valley nevada aquifers abstract vanadium v concentrations were measured along the flow paths in two contrasting aquifer systems to study how changing solution composition and redox conditions affect v cycling and transport in groundwater flow systems the oasis valley groundwater system in nevada is an unconfined system where groundwaters generally remain oxic along the flow path whereas the carrizo sand aquifer is a deep confined coastal plain aquifer in texas that contains well defined fe iii and so 4 2 reduction zones along the flow path speciation modeling predicts that in groundwaters from the oasis valley v chiefly occurs as the vanadate oxyanion e g h 2 vo 4 hvo 4 2 which is mobile along the entire flow path because of vanadate s solubility in oxic and alkaline waters furthermore the data indicate that v concentrations increase in groundwaters along the flow path suggesting that mineral dissolution and or desorption reactions progressively contribute v to groundwaters of the oasis valley system geochemical modeling is consistent with dissolution of mafic phenocrysts that are common in the local tuffaceous aquifer rocks as a probable source of v to oasis valley groundwaters within the carrizo sand aquifer groundwater v concentrations decrease with flow beyond the recharge zone and into the confined portion of the aquifer thereafter total v concentrations remain low and relatively constant along the remainder of the studied flow path exhibiting no systematic variations across the fe iii and so 4 2 reduction zones groundwaters from the carrizo sand aquifer are generally reducing and as such geochemical modeling suggests that both v v and v iv species occur in carrizo sand groundwaters and furthermore that the proportion of v iv species increases concurrent with the decrease in total v concentrations along the flow path results from sequential extraction experiments on carrizo sand aquifer sediment samples suggest that v adsorption onto fe iii mn iv oxides oxyhydroxides acts to decrease the aqueous v concentrations in carrizo sand groundwater furthermore geochemical modeling is consistent with complexation of v iv with dissolved organic matter dom within the carrizo sand aquifer groundwaters which owing to the low dom concentrations appears to buffer v at low but relatively constant concentrations within the confined and reducing carrizo sand groundwaters these results support previous reports of v iv dom binding and imply that aquifer material maintains an essential role in not only determining the availability of v to enter aqueous solution but also in influencing partitioning between solid and aqueous phases

enzymatic iron and uranium reduction by sulfate reducing bacteria abstract the potential for sulfate reducing bacteria srb to enzymatically reduce fe iii and u vi was investigated five species of desulfovibrio as well as desulfobacterium autotrophicum and desulfobulbus propionicus reduced fe iii chelated with nitrilotriacetic acid as well as insoluble fe iii oxide fe iii oxide reduction resulted in the accumulation of magnetite and siderite desulfobacter postgatei reduced the chelated fe iii but not fe iii oxide desulfobacter curvatus desulfomonile tiedjei and desulfotomaculum acetoxidans did not reduce fe iii only desulfovibrio species reduced u vi u vi reduction resulted in the precipitation of uraninite none of the srb that reduced fe iii or u vi appeared to conserve enough energy to support growth from this reaction however desulfovibrio desulfuricans metabolized h2 down to lower concentrations with fe iii or u vi as the electron acceptor than with sulfate suggesting that these metals may be preferred electron acceptors at the low h2 concentrations present in most marine sediments molybdate did not inhibit fe iii reduction by d desulfuricans this indicates that the inability of molybdate to inhibit fe iii reduction in marine sediments does not rule out the possibility that srb are important catalysts for fe iii reduction the results demonstrate that although srb were previously considered to reduce fe iii and u vi indirectly through the production of sulfide they may also directly reduce fe iii and u vi through enzymatic mechanisms these findings as well as our recent discovery that the so reducing microorganism desulfuromonas acetoxidans can reduce fe iii demonstrate that there are close links between the microbial sulfur iron and uranium cycles in anaerobic marine sediments

the geochemistry of uranium and thorium in coastal marine sediments and sediment pore waters pore water profiles of uranium and thorium isotopes in the muddy sediments of buzzards bay ma permit an assessment of the effect of diagenetic redox reactions on the geochemical behavior of these elements uranium shows a pronounced minimum pore water concentration 1 2 dpm kg near the sediment water interface 0 3 cm which coincides with the pore water fe maximum u concentrations increase with depth to a broad maximum which is greater than the overlying seawater value then decrease laboratory sediment tank experiments maintained without macrofauna also show the near interface minimum and increase and sediment incubation experiments show removal of u from the pore water to a constant value by about 40 days the 234u238u activity ratio is equal to the seawater value in all samples oxidation state separations of pore water u in hcl solution using anion exchange demonstrate that u is present in the pore water as u vi the coincidence of the pore water uranium and fe maximum is consistent with the hypothesis of reduction of u vi to u iv and removal from solution at about the same depth or pe as fe is reduced increases in pore water u with depth appear to be related to release of authigenic seawater u vi to solution possibly as alkalinity and ph increase and organic matter with which the u is associated is oxidized lower concentrations at depth in the sediment column may be linked to increases of effective u reducing agents like hydrogen sulfide in the pore water or to the activities of macrofauna in contrast pore water profiles of the long lived th isotopes 232th and 230th show low activities 02 dpm kg and relatively little change with depth although values are greater than those in the overlying water solid phase activities are similarly constant with depth and the distribution may be controlled by a sorption equilibrium between pore water and the solid phase the short lived isotope 234th shows greatest pore water activities in the upper 5 cm and is present at virtually zero activities elsewhere in the sediment column the upper few centimeters of the sediment generally contain excess 234th scavenged from the overlying water column and distributed by bioturbation as well as pore water fe and mn profiles which show maxima indicating reduction and dissolution of fe and mn oxyhydroxides the pore water 234th profile can be explained by release of 234th from the solid phase as femn oxide surface coatings are reduced and dissolved

high resolution reconstruction of a holocene coastal sequence nw germany using inorganic geochemical data and diatom inventories abstract holocene deposits of the nw german coastal plain consist of many different lithological facies for example tidal flats brackish water sediments and peat beds the effects of the holocene sea level rise on palaeoenvironmental conditions of this coastal sequence were studied by inorganic geochemical methods in conjunction with diatom analyses three hundred samples from two parallel sediment cores which cover the entire nw german holocene were taken at high resolution and were examined for major and minor elements and bulk parameters selected samples were analysed for redox sensitive trace elements and ree distribution reactive iron and bulk sediment 34 s 32 s ratios chemical parameters for example si al and ca sr ratios and total organic carbon toc contents correlate with depositional factors such as wave energy and lithofacies changes which allow a detailed reconstruction of the palaeoenvironment diatom analyses reveal information about changes between marine brackish and limnic conditions and serve to reconstruct palaeosalinity early diagenetic effects are evident in the toc rich intervals most peat layers are affected by sulphate reduction and resulting pyrite formation as well as by enrichments in redox sensitive trace elements the highest enrichments are seen for as mo re and u indicating a distinct seawater influence s isotope ratios of peat samples are compatible with pyrite formation under both open and closed system conditions depending on exposition to seawater the inorganic geochemical and diatom data suggest limnic conditions at the base of the sequence and repeated changes towards marine conditions within the overlying clastic units on the other hand data obtained from the clastic units yield evidence of a recurrent succession from open to restricted marine brackish lagoonal and finally fen environments clastic sediments overlying peat layers correlate with the increase of marine derived geochemical signatures and pelagic diatoms attest transgressive overlaps the analyses suggest that major controls on the palaeoenvironments were i climate related oscillations of the coast line and ii the morphology of the coastal region allowing marine incursions even into distal semi terrestrial lowlands

paleozoic mesozoic crayfish from antarctica earliest evidence of freshwater decapod crustaceans discovery of an early permian claw from antarctica extends the fossil record of crayfish by 65 m y and demonstrates that decapod crustaceans had radiated into freshwater habitats by the late paleozoic burrows in lower triassic rocks of antarctica are among the oldest apparently constructed by crayfish their morphology is similar to modern crayfish burrows and this demonstrates that burrowing behavior was established early in the evolution of this group the new discoveries show that the earliest permian crayfish were distributed in high paleolatitudes of southernmost pangea where they lived in freshwater lakes fed by glacial meltwater modern crayfish habitat used as a guide to crayfish temperature tolerance indicates that summer temperatures of streams and lakes near the south pole that supported the crayfish probably reached 10 20 c during permian triassic interglacial intervals

uranium enrichment in metalliferous sediments from the mid atlantic ridge abstract previous studies of metal rich sediments at mid ocean ridge spreading centres have suggested that u fe ratios are relatively constant from site to site a suite of cores from the mid atlantic ridge has been analysed for u and fe content and most samples exhibit constant u fe ratios sulphide bearing parts of one core however contain excess u compared with that observed in other metalliferous sediments but apparently only during oxidative alteration of pyrite phases fission track registration has demonstrated that this excess u is intimately associated with sulphide grains in the sediment scanning electron microscope examination indicates that this u enrichment is limited to very small areas within the oxidation layer and is associated with an enhanced p concentration ion probe analyses allow semi quantitative analyses of various elements through a u rich grain the observed distribution of u coupled with its specific enrichment suggests that microbial mediation might be controlling this enrichment u fe ratios in fully oxidised tag sediments are constant and approximately an order of magnitude higher than in young plume particulates this discrepancy is attributed to a second type of post depositional u enrichment producing an homogeneous distribution of u throughout the sediment

records of trace metals in sediments from the oregon shelf and slope investigating the occurrence of hypoxia over the past several thousand years article i nfo hypoxic b62 mol kg or 1 43 ml l o2 to anoxic conditions have been repeatedly observed over the last 10 years on the oregon shelf while similar conditions are absent in historical records from 1950 to 1999 this study seeks to identify whether similar instances of decadal length hypoxia anoxia have occurred in the oregon coastal zone prior to recorded history and to shed light on potential causes for these events we have measured redox sensitive metals uranium vanadium andmolybdenumconcentrationprofilesin7coresacrossthecoastal affected region and in 3 cores from deeper water sites results indicate regional variability in redox conditions through time the northern sites show no metal enrichment throughout the cores while the southern sites show strong metal enrichment at the base of the cores indicative of previous hypoxic anoxic conditions the southernsitesindicateprogressionintimetowardlesshypoxic anoxicburial inconflictwithrecenthydrograph

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

suboxic trace metal geochemistry in the eastern tropical north pacific abstract we analyzed al ti fe mn cu ba cd u mo v and re in water column settling particulate and sediment 0 to 22 cm samples from the intense oxygen minimum zone omz of the eastern tropical north pacific near mazatlan mexico the goal was to determine how the geochemistry of these elements was influenced by suboxic water column conditions and whether the sediments have a unique suboxic geochemical signature the water column was characterized by a mn maximum reaching 8 nmol kg 1 at 400 m concentrations of cu ba cd mo re u and v were unaffected by the low o2 conditions and were comparable to those of the open ocean sinking particles were composed of lithogenic particles of detrital origin and nonlithogenic particles of biogenic origin al ti and fe were mostly at least 79 lithogenic about 75 of the mn was nonlithogenic significant amounts at least 58 of cu ba cd and mo were nonlithogenic sediment geochemistry varied across the continental shelf and slope cadmium u and re have prominent maxima centered at 310 m with 12 3 ppm 10 9 ppm and 68 3 ppb respectively at the core top high values of mo averaging 6 8 ppm and v averaging 90 ppm are seen in omz surface sediment additional down core enrichment occurs for all redox sensitive elements in the top 10 cm for u mo v and re surface sediments are a poor indicator of metal enrichment comparison of the nonlithogenic composition of sediments with sinking particles suggests that direct input of plankton material enriched in metals makes a significant contribution to the total composition especially for cd u and mo we evaluated re mo and cd u ratios as tracers for redox environments rhenium and mo concentrations and re mo ratios do not lead to consistent conclusions concurrent enrichments of re and mo are an indicator of an anoxic depositional environment in contrast high re mo ratios are an indicator of suboxic conditions cadmium is enriched in surface sediments while u has considerable down core enrichment the concentrations of cd and u and the cd u ratio do not follow patterns predicted from thermodynamics though the water column is suboxic these four redox sensitive elements indicate that the sediments are anoxic the implication for paleostudies is that a trace metal sediment signature that indicates anoxic conditions is not necessarily attributable to an anoxic water column

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

mineralogical and geochemical evidence for biogenic and petroleum related uranium mineralization in the qianjiadian deposit ne china abstract the sandstone hosted qianjiadian uranium deposit hosted in the upper cretaceous yaojia formation in the qianjiadian area songliao basin ne china has been studied for over twenty years however there exists debate on whether mineralization fluid is low temperature groundwater in favor of biogenic mineralization or diabase related magmatic hydrothermal fluid for this deposit and other adjacent geologically linked sandstone hosted uranium deposits baixingtu baolongshan and huitianzhao deposits in the north china this study provided new data from petrographic and geochemical analyses geochronology of uranium minerals the empa chemical dating method and petroleum hydrocarbon biomarkers in host sandstones two types of host sandstone were recognized in calcareous sandstones pitchblende coexists with colloform pyrite and poikilitic calcite cement while quartz and feldspars were extensively corroded in non calcareous sandstones coffinite coexists with colloform pyrite and only feldspars were slightly corroded this suggests that pitchblende formed in high ph ph 9 fluid while the ph of ore forming fluid for coffinite was lower ph 7 9 besides empa chemical dating revealed two stages of uranium mineralization stage a occurred in 43 28 ma with pitchblende as the only uranium mineral species stage b occurred in 19 3 ma with the formation of both pitchblende and coffinite the uranium mineralization was biogenic under low temperature groundwater condition and thus not from a diabase related mafic magmatic hydrothermal fluid based on the following lines of evidence i some coffinite occurs as phosphorus rich microorganism like microspherules structure ii pyrite aggregates were generated from bacterial sulfate reduction as indicated by the 34s values as low as 41 4 iii calcite cement in calcareous sandstone contains only single phase aqueous inclusions and have 13c values as low as 11 2 indicating that part of the carbon was derived from organic matter oxidation iv the organic matter oxidation is further supported by hydrocarbons extracted from petroleum inclusions within calcite cement showing occurrence of c26 c31 17 21 25 norhopanes typically resulting from heavy biodegradation

evidence of uranium biomineralization in sandstone hosted roll front uranium deposits northwestern china we show evidence that the primary uranium minerals uraninite and coffinite from high grade ore samples u3o8 0 3 in the wuyiyi wuyier and wuyisan sandstone hosted roll front uranium deposits xinjiang northwestern china were biogenically precipitated and psuedomorphically replace fungi and bacteria uranium vi which was the sole electron acceptor was likely to have been enzymically reduced post mortem accumulation of uranium may have also occurred through physio chemical interaction between uranium and negatively charged cellular sites and inorganic adsorption or precipitation reactions these results suggest that microorganisms may have played a key role in formation of the sandstone or roll type uranium deposits which are among the most economically significant uranium deposits in the world

microscopy study of biologically mediated alteration of natural mid oceanic ridge basalts and magnetic implications 1 microbial communities have been shown over the last few years to be a significant component of the subseafloor crustal environment however their role in the low temperature alteration of seafloor basalts remains an open question among the diversity of microorganisms that can contribute to oceanic rock weathering sulfate reducing bacteria have been suggested based on sulfur isotope studies to be major actors previous laboratory experiments conducted on basalt samples from the juan de fuca ridge incubated with model sulfate reducing bacteria for several months showed that bacterial activity can play a role in the decrease of seafloor magnetic signal in this paper we characterized alteration features at the nanoscale in one of these basalt samples in order to better understand the mechanisms of the magnetic signal decrease for that purpose we used a combination of focused ion beam milling transmission electron microscopy and scanning transmission x ray microscopy fossilized microbial cells and phyllosilicates were evidenced at the surface of the sample within the sample alteration rims mostly composed of fe and s and measuring 100 300 nm in thickness were observed around titanomagnetites crystals that bear most of the magnetic signal in contrast these features were not observed on noninoculated control samples this study offers a detailed view of the specific mineral assemblages formed in the presence of model sulfate reducing bacteria that can be looked for in the oceanic crust these observations contribute to understand the potential role of microbes in the alteration of the oceanic crust