the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

persistent late pleistocene holocene seasonal upwelling and varves off the coast of california abstract seasonal production of siliceous phytoplankton and a seasonal flux of terrigenous clastic material produced varved sediments along the continental slope off california during the late pleistocene to mid holocene light colored sediment within laminations and in sediment trap samples was deposited during summer upwelling and contains an abundance of the diatoms skeletonema costatum and chaetoceros spp resting spores dark colored sediment deposited in the fall and winter contains abundant thalassiosira pacifica and has more terrigenous material distribution of diatoms in varves shows that seasonal upwelling has persisted along the california coast and has remained strongly seasonal since the late pleistocene

geochemical evidence for iron mediated anaerobic oxidation of methane anaerobic oxidation of methane aom by sulfate has been recognized as a critical process to maintain this greenhouse gas stability by limiting methane flux to the atmosphere we show geochemical evidence for aom in deep lake sediments and demonstrate that aom is likely driven by iron fe reduction pore water profiles from lake kinneret sea of galilee israel show that this sink for methane is located below the 20 cm depth in the sediment which is well below the depths at which nitrate and sulfate are completely exhausted as well as below the zone of methanogenesis iron dependant aom was verified by fe iii amended mesocosm studies using intact sediment cores and native iron oxides were detectable throughout the sediments because anaerobic fe iii respiration is thermodynamically more favorable than both sulfate dependent methanotrophy and methanogenesis its occurrence below the zone of methane production supports the idea that reduction of sedimentary iron oxides is kinetically or biologically limited similar conditions are likely to prevail in other incompletely pyritized aquatic sediments indicating that aom with fe iii is an important global sink for methane

high resolution sedimentology and micropalaeontology of laminated diatomaceous sediments from the eastern equatorial pacific ocean leg 138 scanning electron microscope sem based analyses of the laminated diatom oozes encountered during leg 138 reveal three major laminae types the first lamina type is composed of multiple layers of 20 m thick diatom mats which form laminae dominated by assemblages of the pennate diatom thalassiothrix longissima more than one variety subspecies of t longissima occurs within these laminae referred to as the t longissima group the second lamina type is composed of a mixed assemblage of several species of diatoms centric and pennate varieties calcareous nannofossils and subordinate quantities of radiolarians silicoflagellates and foraminifers the third lamina type is dominated by an assemblage of nannofossils and minor amounts of those fossil components mentioned above this last form of lamination is compositionally similar to the background sediment type foraminifernannofossil ooze f no two lamina associations occur within the laminated intervals the first comprises alternations of t longissima group and mixed assemblage laminae average thickness is 6 mm and the second is composed of t longissima and nannofossil rich laminae average thickness is 3 5 mm the arrangement of laminae probably originates from the deposition of multiple layers of 20 m thick mats from one mat flux episode the much thinner nannofossil rich laminae are interpreted to represent periods of more normal deposition between mat flux episodes the occurrence of several varieties subspecies of t longissima within individual mat layers is consistent with observations of rhizosolenia diatom mats in the modern world ocean

a 20 year varve record of siliceous phytoplankton variability in the central gulf of california abstract year to year variability in the structure of the preserved siliceous phytoplankton assemblage is reconstructed from a box core retrieved from the varved sediments along the guaymas slope of the gulf of california a 20 year record from 1953 through 1972 is obtained from interpretation of the varve stratigraphy supported by 210 pb and 228 th dating ecological time series derived from microfossil analysis show a strong coherence with interannual sea level anomalies indicating a response to large scale climatic variability associated with the el nino southern oscillation phenomenon el nino periods are generally marked by increases in preserved abundances of the total siliceous assemblage and in particular by greater numbers of individuals within species whose distribution is limited to tropical and subtropical waters

seasonal changes in sulfur biogeochemistry of a dilute dimictic arctic lake implications for paired sulfur isotope records from ancient oceans abstract calibration of ancient marine sulfate levels has been done largely using experimental studies of the kinetic isotope effect associated with microbial sulfate reduction and work from modern ocean basins or high sulfate lakes that are largely restricted and meromictic the sulfur isotope record of sulfates and sulfides from sedimentary sequences have been used to reconstruct sulfate levels throughout the geologic record and indicate that sulfate concentrations were low relative to modern oceans 28 mm for most of earth s history despite the higher potential for modern low sulfate systems to be a better analog for ancient oceans there are few sulfur isotope studies that have been carried out in these environments here we present 34s systematics of a modern low sulfate 330 600 m euxinic lake on the ice free margin of southwestern greenland we find large isotope fractionations 20 between water column sulfate and sulfides with this fractionation increasing from 23 9 during open water conditions to 42 0 under annual ice cover while these large kinetic isotope effects associated with microbial sulfate reduction esr are expressed in the water column of this lake the underlying sedimentary sulfides preserve a notably smaller range of 34s values 13 0 26 6 geochemical modeling of our data suggests that the 34s of water column sulfate and sulfides along with sedimentary sulfides are primarily controlled by esr and a reservoir effect established under strong thermal stratification during open water conditions under ice covered conditions esr appears to be the dominant control on 34s values and when combining both seasonal data sets esr increases as a function of sulfate levels in the lake sulfur isotopic data presented here highlight the complexity of seasonal biogeochemical cycling of sulfur in low sulfate systems and how system openness affects 34s fractionations in modern euxinic depositional environments specifically a reservoir effect is only documented in the water column 34s data under open water conditions however a reservoir effect is apparent from sedimentary sulfides under both ice cover and open water conditions due to time averaging pyrite 34s values our results add to the growing body of work on modern low sulfate systems to help further constrain the use of ancient 34s records for paleoenvironmental reconstructions

a review of geological evidence for ancient el nino activity in peru geological evidence geomorphological sedimentological paleontological and archeological for the occurrence of el nino events in peru during the quaternary has been reevaluated new data concerning thermally anomalous molluscan assemblages in holocene deposits of north central peru are briefly described as are new data regarding the distribution of warm and cold water mollusks in late pliocene and early pleistocene deposits of northwestern peru no conclusive evidence of prehistoric el nino events was found in the new data or the reviewed literature most references to el nino events are best characterized as descriptions of el nino like conditions these conditions may accurately represent events or only mimic events past studies lack evidence of two necessary criteria for proof of an el nino event a demonstrably short lived event and the advection of warm coastal marine waters into the southeastern pacific ocean the glaciological record of precipitation in the andes when combined with historical accounts of el nino phenomena on the coast is the sole example of a geological data set that meets both criteria

kimmeridge clay sedimentation a new cyclic model abstract the kimmeridgian upper jurassic fine clastic sequences of the boulonnais northern france and dorset southern england represent oxygen controlled environments five facies types with increasingly deteriorating living conditions ranging from normal shelf environments to submarine lifeless desert have been distinguished the widespread occurrence of this facies in the late jurassic north atlantic shelf sea and the palaeogeographic situation require a large scale model to explain the kimmeridge clay sedimentation anoxic conditions are generally explained by one of three models none of which applies directly to the kimmeridge clay a new model derived from the overall palaeogeographic and palaeoclimatic situation is therefore proposed in this model water mass stratification and anoxic bottom conditions refer to a high latitude monsoonal type wind circulation causing a water current and counter current system in the north atlantic shelf sea and the arctic ocean variations in the amount of oxygen in the bottom waters allow recognition of four different cycles with periods ranging from seasonal to 3 107 years they influenced the counter current system and the resulting anoxic conditions

biogeochemical and 16s rrna gene sequence evidence supports a novel mode of anaerobic methanotrophy in permanently ice covered lake fryxell antarctica lake fryxell mcmurdo dry valleys antarctica contains a constantly cold water column and perennial ice cover although carbon and sulfur cycling in this amictic lake have been studied previously a paired investigation of 16s rrna gene based microbial diversity and geochemistry of lake fryxell is lacking here we used a combination of radiotracer based rate measurements geochemical measurements and molecular microbial community analysis to investigate the anaerobic oxidation of methane aom and associated processes in lake fryxell the results show that while aom and sulfate reduction appear coupled in the upper regions of the anoxic water column in deep anoxic waters where aom rates are highest sulfate is unlikely to be the electron acceptor for aom despite significant rates of aom in these waters no putative aom associated archaea or bacteria were observed due to a lack of documented aom electron acceptors and putative anmes we suggest novel modes of aom dominate in this extreme environment first the notable abundance of the bacterial genus dehalococcoides suggests that reductive dehalogenation could fuel aom further taxa of the candidate phylum op9 the atribacteria and the bathyarchaeota formerly known as the miscellaneous crenarchaeotal group both commonly observed at cold methane seeps globally may mediate aom possibly using humic acids as electron shuttles in lake fryxell

el nino southern oscillation variability from the late cretaceous marca shale of california changes in the possible behavior of el nino southern oscillation enso with global warming have provoked interest in records of enso from past greenhouse climate states the latest cretaceous laminated marca shale of california permits a seasonal scale reconstruction of water column flux events and hence interannual paleoclimate variability the annual flux cycle resembles that of the modern gulf of california with diatoms characteristic of spring upwelling blooms followed by silt and clay and is consistent with the existence of a paleo north american monsoon that brought input of terrigenous sediment during summer storms and precipitation runoff variation is also indicated in the extent of water column oxygenation by differences in lamina preservation time series analysis of interannual variability in terrigenous sediment and diatom flux and in the degree of bioturbation indicates strong periodicities in the quasi biennial 2 1 2 8 yr and low frequency 4 1 6 3 yr bands both characteristic of enso forcing as well as decadal frequencies this evidence for robust late cretaceous enso variability does not support the theory of a permanent el nino in the sense of a continual el nino like state in periods of warmer climate

biogeochemical signatures and microbial activity of different cold seep habitats along the gulf of mexico deep slope abstract microorganisms and the processes they mediate serve as the metabolic foundation of cold seeps we characterized a suite of biogeochemical constituents and quantified rates of two key microbial processes sulfate reduction sr and anaerobic oxidation of methane aom to assess variability between habitats at water depths exceeding 1000 m in the northern gulf of mexico rates of sr were highest in sediments beneath microbial mats lower in brine influenced and oil influenced sediments and lowest in animal habitats sediments collected near tubeworms had the highest sr rates for animal habitats rates of aom generally were low but higher rates were associated with brine influenced oil influenced tubeworm and urchin inhabited sediments rates of both sr and aom were orders of magnitude lower at deep slope sites compared to upper slope sites examined previously as observed at upper slope sites sr and aom rates were often loosely coupled at one site aom rates exceeded sr rates suggesting that an alternate electron acceptor for aom is possible extremely depleted 13c values in methane illustrated the broad significance of biogenic methane production at deep slope sites brine influenced habitats were characterized by extremely high concentrations of ammonium and dissolved organic carbon serving as important focused sources of these chemicals to adjacent environments

perchlorate and chlorate biogeochemistry in ice covered lakes of the mcmurdo dry valleys antarctica abstract we measured chlorate clo 3 and perchlorate clo 4 concentrations in ice covered lakes of the mcmurdo dry valleys mdvs of antarctica to evaluate their role in the ecology and geochemical evolution of the lakes clo 3 and clo 4 are present throughout the mdv lakes streams and other surface water bodies clo 3 and clo 4 originate in the atmosphere and are transported to the lakes by surface inflow of glacier melt that has been differentially impacted by interaction with soils and aeolian matter concentrations of clo 3 and clo 4 in the lakes and between lakes vary based on both total evaporative concentration as well as biological activity within each lake all of the lakes except the east lobe of lake bonney support biological reduction of clo 3 and clo 4 either in the anoxic bottom waters or sediment the younger less saline lakes miers and hoare have surface clo 3 and clo 4 concentrations and ratios of clo 3 cl and clo 4 cl similar to source streams while lake fryxell has concentrations similar to input streams but much lower clo 3 cl and clo 4 cl ratios reflecting the influence of a large cl source in bottom sediments clo 3 and clo 4 in lake bonney are the highest of all the lakes reflecting the lake s greater age and higher concentration of cl clo 4 appears to be stable in the east lobe and its concentration is highly correlated with cl concentration suggesting that some clo 4 at depth is a remnant of the initial seawater that formed lake bonney clo 3 and clo 4 concentrations provide a simple and sensitive means to evaluate microbial activity in these lakes due to their relatively low concentrations and lack of biological sources unlike no 3 no 2 and so 4 2

revised ages for laminated sediment and a holocene marker diatom from the northern california continental slope abstract conventional and accelerator mass spectrometry 14 c ages indicate that laminated sediment in three cores from the northern california continental slope near 38 n and 39 n were deposited between 42 000 and 25 000 yr b p this revises and refines our previous estimates that laminated sediment accumulated during the late pleistocene to early holocene j v gardner and e hemphill haley 1986 geology 14 691 694 preservation of laminated sediment on the upper slope in this area suggests a period of intense coastal upwelling high primary productivity and resultant depletion of oxygen in bottomwaters preceding the onset of global glacial conditions the transition from pleistocene to holocene conditions and the establishment of a modern climatic regime driven by the california current included the incursion of the subtropical diatom pseudoeunotia doliola p doliola is common in sediment younger than about 10 000 yr and thus is a reliable marker species for identifying holocene deposits off northern california

distribution of silicoflagellates in plankton and core top samples from the gulf of california plankton and surface sediment samples from the gulf of california were examined to determine the present geographic distribution of silicoflagellate species in this area variations in the species composition of the silicoflagellate assemblage were found to be related to water mass distributions eight species were identified in these samples octactis pulchra is associated with high levels of primary productivity in the surface waters and is found in greatest abundance in the central gulf of california dictyocha messanensis dominates the silicoflagellate assemblage in stations outside the gulf of california and increases in relative abundance with decreasing amounts of octactis pulchra dictyocha calida dictyocha sp a and dictyocha sp b are associated with equatorial waters and have the highest relative abundance near the mouth of the gulf dictyocha epiodon and distephanus speculum are associated with cold california current water and dictyocha epiodon is present in minor abundance in gulf samples dictyocha sp 2 has a patchy distribution with low relative abundance

varve formation in the gulf of california insights from time series sediment trap sampling and remote sensing the varved sediments that accumulate in the central gulf of california guaymas and carmen basins provide a record that allows for the resolution of annual to decadal scale climate variability in this region time series sediment trapping combined with remotely sensed observations of sea surface temperature avhrr and color czcs have been used to examine the mechanism of varve formation in the central gulf sst and surface pigment concentration records for both the mainland and baja sides of the gulf display similar seasonal trends high temperatures and low pigment concentrations occur synchronously on both sides of the central gulf from june through to october in association with the 1991 92 el nino warm temperatures persist in the central gulf until december low ssts typically occur from december through may with high pigment concentrations marking the period from november through april the summer early fall june october is a time of high terrigenous and biogenic sediment fluxes during this time of year the total flux is dominated by eolian transported lithogenic material and dark laminae are deposited during this period during el nino years high terrigenous fluxes may extend into winter light laminae represent deposition from november through may during which time biogenic material dominates the total sediment flux high opal fluxes beginning in november are attributed to seasonal cooling of surface water and upward mixing of nutrients true upwelling conditions do not develop in the gulf until late winter spring during this time the total flux is still dominated by opal but the magnitude of the flux is reduced this may be an artifact of phytoplankton grazing and not lower production rates

weak coupling between sulfate reduction and the anaerobic oxidation of methane in methane rich seafloor sediments during ex situ incubation abstract we investigated coupling between sulfate reduction sr and anaerobic oxidation of methane aom by quantifying pore water geochemical profiles determining rates of microbial processes and examining microbial community structure at two sites within mississippi canyon lease block 118 mc118 in the northern gulf of mexico sediments from the northwest seep contained high concentrations of methane while sediments from the southwest seep contained methane gaseous n alkanes and liquid hydrocarbons and had abundant surficial accumulations of gas hydrate volumetric 21 5 mol cm 3 day 1 and integrated 1429 mmol m 2 day 1 rates of sr at mc118 in ex situ incubations are the highest reported thus far for seafloor environments aom rates were small in comparison with volumetric rates ranging from 0 1 to 12 6 nmol cm 3 day 1 diffusion cannot adequately supply the sulfate required to support these high sr rates so additional mechanisms possibly biological sulfide oxidation and or downward advection play important roles in supplying sulfate at these sites the microbial communities at mc118 included sulfate reducing bacteria phylogenetically associated with desulfobacterium anilini which is capable of complex hydrocarbon degradation despite low aom rates the majority of archaea identified were phylogenetically related to previously described methane oxidizing archaea to evaluate whether weak coupling between sr and aom occurs in habitats lacking the complex hydrocarbon milieu present at mc118 we compiled available sr and aom rates and found that the global median ratio of sr to aom was 10 7 1 rather than the expected 1 1 the global median integrated aom rate was used to refine global estimates for aom rates at cold seeps these new estimates are only 5 of the previous estimate

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

lamina scale analysis of sedimentary components in cretaceous black shales by chemical compositional mapping implications for paleoenvironmental changes during the oceanic anoxic events abstract relative abundances and distribution patterns of mineral components in finely laminated livello bonarelli lb black shale from the cenomanian turonian boundary cretaceous were investigated by 10 m resolution compositional mapping using an electron probe microanalyzer sedimentary components such as biogenic silica si bio calcium phosphate calcium carbonate and pyrite were reconstructed by image processing of the compositional maps total organic carbon c org content was also determined at a sampling interval of 1 5 mm the depth profiles of these sedimentary components covering 60 of the total 104 cm lb sequence indicate the highly variable nature of their deposition especially the c org contents vary widely ranging from org are clearly discernible in the lb and correspond to dark colored intervals repeated occurrences of c org rich and si bio rich intervals are one of the most prominent features in the lb these features could be explained partly by dilution of c org by si bio but largely by the presence of two distinct paleoenvironmental settings one involves predominant deposition of organic matter and the other is dominated by si bio deposition the thickness of individual intervals varies widely mostly ranging from 1 5 to 35 mm representing oceanic changes on the order of 10 3 10 4 years furthermore we found 100 m scale laminations of fine grained pyrite aluminosilicate and calcium carbonate which when superimposed on the alternation of c org rich and si bio rich intervals suggests 10 2 year scale fluctuations in oceanic environments

diatoms in the gulf of california seasonal flux patterns and the sediment record for the last 15 000 years flux and taxonomic composition of the diatom assemblage was determined for biweekly sediment traps in the central gulf of california over a 2 year period fluxes and relative abundance of taxa are not significantly different in traps from the eastern side of the guaymas basin and the western side of the carmen basin indicating that patterns of diatom production are uniform across the central gulf at all seasons of the year the annual maximum in diatom flux occurred during the winter season of both years either as a sustained event over many weeks 1990 or as a series of short blooms 1991 1992 of the most abundant taxa only hyalochaete chaetoceros species showed a clear seasonal cycle with a flux maximum in the spring of both years factor analysis defines five assemblages representing 1 summer and fall conditions of surface stratification and nutrient limitation 2 winter bloom triggered by onset of strong northwesterly winds 3 late spring to summer production probably related to sporadic events during periods of weak southeasterly winds 4 spring 1991 inferred to reflect the distal edge of a coastal upwelling phase 5 lateral advection of resuspended sediments from the continental shelf possibly resulting from trapped coastal waves during summer hurricanes and from storms during the 1991 1992 el nino southern oscillation enso the diatom data support the inference that subtropical pacific water penetrates deeper into the gulf during enso events in order to compare the sediment trap record to a sediment core a second factor analysis was done using only the taxa which are preserved in the sediment and omitting the most abundant taxa which showed little downcore variation this produces five factors representing 1 summer fall low production 2 early winter bloom 3 late winter spring upwelling 4 enso conditions and 5 a late spring summer assemblage of uncertain significance variation of species characterisic of each factor was examined in piston core atlantis ii 125 8 56jpc taken from the oxygen minimum zone in guaymas basin correlation to nearby deep sea drilling project hole 480 indicates that the core penetrates to about 15 ka with laminated sediments in the holocene 0 10 ka and late deglacial 11 13 ka and massive nonlaminated sediments around younger dryas time 10 11 ka and before 13 ka downcore variations in the diatom assemblage do not correspond directly to presence of laminations implying that the processes controlling bottom water oxygen content are not determined by conditions of surface water production presence of calcareous nannofossils downcore correlates inversely with two diatom assemblages enso and winter bloom indicators and not with occurrence of laminations suggesting that occurrence of calcareous nannofossils in the sediment is at least partly a function of surface water production prior to 12 ka production was lower than at present during all seasons implying weaker winds throughout the year subtropical waters extended to the midgulf as they do today during enso events between 11 and 12 ka conditions were similar to those of the present with relatively high winter and spring production resulting from northwesterly winds enso events were still common although less so than during the preceding period younger dryas time was one of transition with intermediate conditions a slackening of winter spring northwesterly winds and reduction of total annual production and increased occurrence of enso conditions in the early to middle holocene winter northwesterlies increased to a maximum resulting in very high production but apparently did not persist into spring subtropical waters mostly disappeared from the gulf in the late holocene northwesterly winds have supported high production during both winter and spring seasons

biogenic and abiogenic components of laminated hemipelagic sediments in the central gulf of california abstract alternating light and dark submillimeter to millimeter thick layered sediments rhythmites are found along the slopes of the central and southern gulf of california where a strong oxygen minimum zone intersects the sediment water interface two short intervals of distinct alternations were sampled lamina by lamina from two cores in the central gulf both cores are from the slope of the guaymas basin one core is from the mainland side the other core is from the baja side each lamina sample was analyzed for its opal micro floral assemblage textural character trace metal and bulk chemical cu mn fe ni zn ca k al sio2 and c org content and mineralogy on both sides of the gulf the light laminae contain more diatom frustules up to 90 and less terrigenous and organic material than the dark laminae the sediment is predominantly silt sized the dark laminae contain more terrigenous material which is primarily clay size on both sides of the gulf the light laminae apparently represent sedimentation during the dry winter season of northwesterly winds whereas the dark laminae represent the wet summer season of southeasterly winds