controls on mo isotope fractionations in a mn rich anoxic marine sediment gullmar fjord sweden abstract molybdenum isotopes have emerged as an important tool for determining the spatial extent of past ocean redox conditions however our understanding of mo cycling and associated isotopic fractionations in modern and ancient continental margin sediments is limited restricting attempts to fully utilize the potential of mo isotopes this is particularly the case with regard to the relative significance of mo isotope fractionations associated with biogeochemical mn and fe oxide cycling under conditions of low dissolved sulfide to rectify this we have investigated sediments from gullmar fjord sweden which contain high concentrations of mn oxides and exhibit a strong redox zonation between mn and fe reduction a combination of dissolved and particulate mo isotope measurements with porewater and solid phase mn and fe characterization reveals that the main influence on porewater 98 mo compositions is exerted by adsorption and desorption processes during reductive dissolution of both mn and fe oxyhydr oxide minerals in areas enriched in mn oxides distinctively low solid phase 98 mo values are observed 1 0 and 0 4 which occur due to the large mo isotope fractionation associated with mo adsorption to mn oxides however where mn oxide concentrations are lower authigenic 98 mo is largely controlled by fe oxyhydr oxide minerals resulting in more positive 98 mo isotopic compositions 0 5 and 2 our results suggest that for modern anoxic marine sediments where deposition occurs beneath an oxic to suboxic water column 98 mo is dominantly controlled by the relative availability of mn and fe oxides and the production of dissolved sulfide

is the central arctic ocean a sediment starved basin abstract numerous short sediment cores have been retrieved from the central arctic ocean many of which have been assigned sedimentation rates on the order of mm ka implying that the arctic basin was starved of sediments during plio pleistocene times a review of both shorter term sedimentation rates through analysis of available sediment core data and longer term sedimentation rates through estimates of total sediment thickness and bedrock age suggests that cm ka scale rates are pervasive in the central arctic ocean this is not surprising considering the physiographic setting of the arctic ocean being a small land locked basin since its initial opening during early cretaceous times we thus conclude that the central arctic ocean has not been a sediment starved basin either during plio pleistocene times or during pre pliocene times rigorous chronstratigraphic analysis permits correlation of sediment cores over a distance of 2600 km from the northwestern amerasia basin to the northwestern eurasia basin via the lomonosov ridge using paleomagnetic biostratigraphic and cyclostratigraphic data

fluvial and hydrothermal input of manganese into the arctic ocean abstract a total of 773 samples were analysed for dissolved manganese mn in the arctic ocean aboard r v polarstern during expedition ark xxii 2 from 28 july until 07 october 2007 from tromso norway to bremerhaven concentrations of mn were elevated in the surface layer with concentrations of up to 6 nm over the deep basins and over 20 nm in the laptev sea the general distribution of mn through the water column is consistent with previous studies but there are differences in the absolute concentrations that are most likely related to differences in sample area sampling and filtration the elevated concentrations of mn in the surface layer are related to fresh water input this was visible in the strong negative correlations observed between dissolved mn and salinity the correlation between mn and salinity and the correlation between mn and the quasi conservative trace water mass tracer po4 showed fluvial and melt water input and the pacific and atlantic origin of the surface waters a large portion of the mn delivered by the arctic rivers is removed in the shelf seas and does not pass into the central basins most likely a benthic flux is at the origin of the elevated concentrations of mn near the sediments in the barents and kara seas these elevated concentrations of mn apparently affected the deep basins as well as maxima in the concentrations of mn were observed that corresponded with lowered transmission over the continental slope a maximum in the concentration of mn in the deep basin corresponded with anomalies in light transmission potential temperature and dissolved iron confirming the hydrothermal origin the hydrothermal plume was observed throughout the nansen basin and over the deep gakkel ridge around 2500 m depth and a smaller plume was observed around 3200 m the concentration of mn at the mn maximum around 2500 m depth decreased exponentially consistent with a first order scavenging model the concentrations of mn were extremely low in the deep makarov basin 0 05 nm and slightly higher in the eurasian basin 0 1 nm outside the influence of the hydrothermal activity

iceberg hosted nanoparticulate fe in the southern ocean mineralogy origin dissolution kinetics and source of bioavailable fe abstract sediments from icebergs and glaciers contain nanopartculate fe iii oxyhydroxides including ferrihydrite which form in aqueous oxic micro environments where fe ii bearing rock minerals oxidise and high degrees of supersaturation are promoted by freezing and thawing an ascorbic acid extraction dissolves only labile fe present in fresh loosely aggregated ferrihydrite that is directly or indirectly bioavailable glacial and iceberg sediments contain ferrihydrite aggregates that provide 0 04 to 0 17 fe soluble in ascorbic acid rather larger than the concentrations in a limited suite of atmospheric dusts the dissolution behaviour of labile fe from glacial and iceberg sediments by ascorbic acid is controlled by the access of reactant or removal of solute through micropores to or from active sites in the interior of ferrihydrite aggregates a first order kinetic model is presented to examine the rates at which bioavailable fe can be supplied by melting icebergs in the weddell sea the model utilizes rate constants from the literature for the processes which solubilise fe from nanoparticulate ferrihydrite dissolution photochemical reduction and grazing and the processes that remove fe nanoparticulates sinking scavenging and incorporation in faecal material and render them less reactive transformation aging model results demonstrate that icebergs can supply bioavailable fe to the weddell sea by the dissolution of nanoparticulate ferrihydrite despite loss removal of nanoparticles by sinking aging transformation scavenging and incorporation into faecal pellets at rates that are comparable to atmospheric dust dissolution enhanced by photochemical reduction and grazing provides the most rapid rates of bioavailable fe production

traces in the dark sedimentary processes and facies gradients in the upper shale member of the upper devonian lower mississippian bakken formation williston basin north dakota u s a abstract black organic rich rocks of the upper shale member of the upper devonian lower mississippian bakken formation a world class petroleum source rock in the williston basin of the united states and canada contain a diverse suite of mudstone lithofacies that were deposited in distinct facies belts the succession consists of three discrete facies associations fas these comprise 1 siliceous mudstones 2 quartz and carbonate bearing laminated mudstones and 3 macrofossil debris bearing massive mudstones these fas were deposited in three facies belts that reflect proximal to distal relationships in this mudstone system the macrofossil debris bearing massive mudstones fa 3 occur in the proximal facies belt and contain erosion surfaces some with overlying conodont and phosphate lithoclast lag deposits mudstones with abundant millimeter scale siltstone laminae showing irregular lateral thickness changes and shell debris in the medial facies belt quartz and carbonate bearing laminated mudstones dominate exhibiting sub millimeter thick siltstone layers with variable lateral thicknesses and localized mudstone ripples in the distal siliceous mudstone facies belt radiolarites radiolarian bearing mudstones and quartz and carbonate bearing laminated mudstones dominate overall total organic carbon toc contents range between about 3 and 10 wt with a general proximal to distal decrease in toc content abundant evidence of bioturbation exists in all fas and the lithological and toc variations are paralleled by changes in burrowing style and trace fossil abundance while two horizontal traces and two types of fecal strings are recognized in the proximal facies belt only a single horizontal trace fossil and one type of fecal string characterize mudstones in the distal facies belt radiolarites intercalated into the most distal mudstones are devoid of traces and fecal strings bedload transport processes likely caused by storm induced turbidity currents were active across all facies belts suspended sediment settling from near the ocean surface however most likely played a role in the deposition of some of the mudstones and was probably responsible for deposition of the radiolarites the distribution pattern of high toc sediments in proximal and lower toc deposits in some distal facies is interpreted as a function of higher accumulation rates during radiolarian depositional events leading to a decrease in suspension derived organic carbon in radiolarite laminae the presence of burrows in all fas and nearly all facies in the upper bakken shale member indicates that dysoxic conditions prevailed during its deposition this study shows that in intracratonic high toc mudstone successions such as the upper bakken shale member bed load processes most likely dominated sedimentation and conditions promoted a thriving infaunal benthic community as such deposition of the upper bakken shale member through dynamic processes in an overall dysoxic environment represents an alternative to conventional anoxic depositional models for world class source rocks

mid to late pleistocene ice drift in the western arctic ocean evidence for a different circulation in the past the provenance of ice rafted debris ird in four arctic sediment cores implies that icebergs from the northwestern laurentide ice sheets drifted across the western arctic ocean along the 180 0 meridian toward fram strait during mid to late pleistocene deglaciations within the last 700 000 years this iceberg drift was different from the present day beaufort gyre circulation and resembled a dislocated transpolar drift tpd sea ice mainly followed the iceberg trajectories but also frequently drifted from the russian shelves eastward into the amerasian basin

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

osmotic regulation and the evolution of brackish and fresh water faunas present day brackish water areas contain a variety of animal species with differing abilities to adapt to dilute media and many studies on the mechanisms of adaptation to dilute media have been made using these species however many of these areas are unstable in composition small in volume and geologically transitory and thus hardly present a likely avenue for large scale evolutionary processes many of these areas are relatively poor in numbers of species and this can be interpreted as indicating that speciation has not proceeded to equilibrium large scale and geologically long lived brackish water areas resulting from continental drift such as the sarmatic sea may have provided the main types of situation in which the evolution of brackish and fresh water faunas has occurred and from where species have become dispersed into present day localized niches

organism sediment interactions in shale hydrocarbon reservoir facies three dimensional reconstruction of complex ichnofabric geometries and pore networks abstract the lithological and mineralogical characteristics of mudstones and siltstones and their stress strain behavior at the meter to nanometer scale can play a critical role in the exploitation of unconventional shale reservoirs shale fabrics that result from bioturbation can produce extensive interconnected networks of biologically redistributed sediment grains within reservoir mudstone facies the presence of biologically generated heterogeneities may substantially affect reservoir stimulation and thus production from shale facies this study presents volumetric evaluation of ichnofabrics dominated by phycosiphon like and aff chondrites and provides insights into the impact of trace fossils on the rheological and petrophysical characteristics of mudstones it is calculated that in addition to creating significant volumes of silty clay poor zones of enhanced porosity and permeability trace fossils create interpenetrating frameworks of brittle material that reduce communication distances from the low permeability matrix to the higher permeability silt rich burrows reducing communication distances to less than 1 cm increases the potential for diffusive transport of hydrocarbon molecules from the tight matrix to the wellbore connected volumes this is because shale ichnofabrics create abundant fracture prone planes of weakness and increase the surface area of the interface between the hydrocarbon rich matrix and porous burrow fills thereby promoting fluid exchange understanding of the three dimensional characteristics of ichnofabrics may form the basis of future modeling of fracture spacing and complexity that is critical to shale gas reservoir characterization

input composition and potential impact of terrigenous material from free drifting icebergs in the weddell sea abstract the hypothesis that fe fertilization of the southern ocean could account for at least part of the glacial interglacial difference in atmospheric co2 has lead to a search for the fe source iron fertilization in the open ocean can occur naturally through increased dust input cross shelf export upwelling of fe replete deep water and release of terrigenous fe from icebergs however the relative importance of these mechanisms in terms of the flux and bioavailability of the fe is the source of much interest and debate an unambiguous tracer of terrigenous material excess 224ra was used to show that free drifting icebergs are a significant fe source to surface waters in the southern ocean activity of 224ra half life 3 7 days was measured in the weddell sea in surface waters surrounding three free drifting icebergs and in iceberg alley inventories of excess 224ra indicate a local terrigenous input 1 3 orders of magnitude greater than estimates of aeolian dust fluxes to the southern ocean burdens of fine terrigenous material associated with current rates of iceberg ejection are estimated to be on the order of 90 106 tons per year for antarctica yielding a total fe input of 3 106 tons per year chemical and mineralogical composition of the terrigenous material confirms that 0 04 0 4 exists as labile fe in ferrihydrite that is potentially bioavailable on the order of 4 40 104 tons of per year these results suggest that free drifting icebergs can contribute significantly to atmospheric co2 drawdown though fe fertilization in the southern ocean

temporal variations in dissolved and suspended iron and manganese in the kalix river northern sweden abstract particulate and dissolved fe and mn were measured once a week from may to september 1982 in the kalix river north sweden during winter baseflow the dissolved fe concentration was 450 g l 1 early snow melt discharge increased the concentration slightly but during springflood maximum the concentration decreased to a minimum value of 60 g l 1 in mid july particulate matter in the kalix river is dominated by fe which ranges between 14 and 38 of the ashed suspended load 70 80 of the total fe load being retained by 0 45 m filters approximately 10 of particulate fe can be accounted for by a detrital phase the particulate fe al ratio showed a mean value of 6 5 which is more than ten times the ratio in mean world river particulate fe showed a peak during early snowmelt and decreased during maximum discharge similar to dissolved fe dissolved mn concentrations ranged from 1 1 to 53 4 m l 1 with a mean value of 11 2 12 4 g l 1 whereas particulate mn ranged from 3 1 ti 18 2 g l 1 with a mean concentration of 10 6 4 9 g l 1 during may and june most mn was transported in the dissolved phase whereas in july and august the particulate fraction dominated there was a drastic increase in non detrital particulate mn with a concomitant decrease in the dissolved fraction in early july with peak values in mid july and early august the decrease in dissolved mn and increase of particulate mn is probably the consequence of two processes namely addition of non detrital particulate mn to the river possibly from lakes and transformation of dissolved mn to a particulate phase within the river a combination of increased ph temperature and particulate mn triggered the precipitation of dissolved mn although the significance of each factor can not be evaluated from this study

hydrographic conditions of the devono carboniferous north american seaway inferred from sedimentary mo toc relationships abstract deepwater anoxia developed widely within a series of silled intracratonic basins in north america during the middle devonian to early carboniferous but the hydrographic factors contributing to this development are virtually unknown the present study demonstrates that the degree of enrichment of strongly hydrogenous elements such as molybdenum mo in the organic rich shales that accumulated in these basins has the potential to provide information about paleohydrographic conditions most of the study units 49 of 55 exhibit significant positive covariation between elemental mo and toc concentrations yet there is substantial variation in the statistical parameters that define these relationships specifically the regression lines characterizing mo toc covariation have slopes m ranging from 2 to 65 10 4 although a majority of the study units yield m values between 10 and 25 10 4 the parameter m exhibits systematic stratigraphic variation within individual basins and geographic variation among basins suggesting that mo toc relationships were controlled by slowly evolving watermass characteristics of the devono carboniferous north american seaway a recent study of modern anoxic silled basins algeo t j lyons t w 2006 mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions paleoceanography 21 pa1016 doi 10 1029 2004pa001112 demonstrated that m is closely linked to the degree of restriction of the subpycnoclinal watermass as reflected in aqueous mo concentrations mo aq and deepwater renewal times dw these relationships reflect control of sedimentary mo enrichment by both the amount of sedimentary organic matter i e host phase availability and the concentration of aqueous mo i e source ion availability when applied to black shales of the devono carboniferous north american seaway these relationships permit both qualitative and quantitative inferences regarding the degree of restriction of deepwaters in cratonic basins and the evolution of hydrographic conditions through time black shales associated with early stages of sea level rise commonly exhibit low m values 1000 yr higher sea level stands resulted in more open marine conditions and enhanced deepwater renewal leading to deposition of black shales with m values up to 65 equivalent to mo aq 70 of the normal seawater concentration and dw

composition oxygen isotope geochemistry and origin of smectite in the metalliferous sediments of the bauer deep southeast pacific the sediments of the bauer deep an open ocean basin situated on the northwest nazca plate in the southeast pacific constitute a regional metalliferous deposit dominated by authigenic smectite two 2 metre long cores from the bauer deep were examined to investigate the nature and origin of the smectite infra red and mossbauer spectroscopy and wet chemical analysis libo2 fusion of isolated smectite indicate the mineral is a mg rich al rich nontronite oxygen isotopic compositions for isolated smectite are uniform and translate to a non hydrothermal temperature of formation of about 3 c sem observations show an abundance of well preserved biogenic opal in surface and near surface sediment but postburial dissolution and transformation of this phase to smectite is evident at depth smectite formation is the result of interaction between iron oxyhydroxide ponded in the bauer deep following a hydrothermal origin at the adjacent east pacific rise and biogenic opal a reaction mechanism is proposed regional factors control smectite formation in particular formation is inhibited in areas of caco3 accumulation topographic elevations but favoured in areas of oxyhydroxide and opal ponding topographic depressions

manganese rich brown layers in arctic ocean sediments composition formation mechanisms and diagenetic overprint we present inorganic geochemical analyses of pore waters and sediments of two late quaternary sediment cores from the western arctic ocean southern mendeleev ridge rv polarstern expedition ark xxiii 3 focussing on the composition and origin of distinct brown colored mn rich sediment layers carbonate enrichments occur in association with these layers as peaks in ca al mg al sr al and sr mg suggesting enhanced input of both ice rafted and biogenic carbonate for the first time we show that the mn rich layers layers are also consistently enriched in the scavenged trace metals co cu mo and ni distinct bioturbation patterns specifically well defined brown burrows into the underlying sediments suggest these metal enrichments formed close to the sediment water interface the geochemical signature of these metal and carbonate rich layers most probably documents formation under warmer climate conditions with an intensified continental hydrological cycle and only seasonal sea ice cover both rivers and sea ice delivered trace metals to the arctic ocean while enhanced seasonal productivity exported reactive organic matter to the sea floor the coeval deposition of organic matter mn oxyhydr oxides and trace metals triggered intense diagenetic mn cycling at the sediment water interface these processes resulted in the formation of mn and trace metal enrichments and the degradation of labile organic matter with the onset of cooler conditions reduced riverine runoff and or a solid sea ice cover terminated the input of riverine trace metal and fresh organic matter resulting in deposition of grayish yellowish metal poor sediments oxygen depletion of arctic bottom waters under these cooler conditions is not supported by our data and did not cause the sedimentary mn distribution while the original composition and texture of the brown layers resulted from specific climatic conditions and corresponding diagenetic processes pore water data show that diagenetic mn redistribution is still affecting the organic poor deeper sediments given persistent steady state conditions purely authigenic mn rich brown layers may form while others may be partly or completely dissolved the degree of diagenetic mn redistribution largely depends on the depositional environment the mn and organic matter availability and apparently affected the co mo ratios of mn rich layers thus brown arctic layers are not necessarily synchronous features and should not be correlated across the arctic ocean without additional age control

recent change in organic carbon flux to arctic ocean deep basins evidence from acid volatile sulfide manganese and rhenium discord in sediments evidence of recent large scale change in redox conditions in arctic ocean basin sediments is found in profiles of solid phase acid volatile sulfide avs mn and re avs occurs at 2 5 5 5 cm in sediment cores collected from all major arctic basins implying that presently there is a sufficient supply of organic carbon oc to deplete o 2 and form sulfide re which is mobile under oxic conditions but precipitates under suboxic or anoxic conditions is not found enriched in the sediments suggesting that avs is a recent product of enhanced oc fluxes estimated diffusion rates suggest that avs production is recent and that a change from oxic to anoxic diagenesis has taken place within the past 50 y the most likely origin for such widespread change is the ice climate and we infer that reduction in ice cover over the past several decades has led to enhanced oc fluxes to the sea floor

preservation potential of sedimentary event layers preservation potential of sedimentary event layers e g ash layers or tempestites is a function of net sedimentation rate and biogenous mixing rate previous studies that have used an inverse peclet number to scale the problem in terms of advection or diffusion are in error due to the improper inclusion of the surface mixed layer thickness as the sole length scale herein focus is on time scales if the transit time i e time required to advect the signal through the biologically active zone is less than the dissipation time i e time required to destroy the signal then some fraction of the original signal is preserved a simple quantitative model based on the diffusion analogy of sediment mixing estimates the actual percentage of preserved signal under various combinations of sedimentation rate mixing rate and event layer thickness

geochemistry of surficial and ice rafted sediments from the laptev sea siberia the laptev sea as a part of the world s widest continental shelves surrounding the arctic ocean is a key area for understanding the land ocean interaction in high latitude regions with a yearly freshwater input of 511 km3 the lena river one of the eight major world rivers has an influencing control over the environment of this arctic marginal sea which is ice covered during most of the year in this paper the first measurements are presented of the major and trace element distribution within the 20 m grain size fraction of surficial sediments and of particulate matter in new and young ice from the laptev sea siberian arctic the concentration and distribution of major and trace elements have been determined in 51 surficial sediment samples covering the whole laptev sea shelf south of the 50 m isobath thirty one samples of particulate matter in newly formed ice were taken during the freeze up period in 1995 median concentration levels of heavy metals in surficial sediments ni 46 g g 1 cu 26 g g 1 zn 111 g g 1 and pb 21 g g 1 are within the concentration range of marine unpolluted sediments also the sediment laden ice showed no indication of anthropogenic perturbation of the trace metal inventory spatial distribution patterns of heavy metals are mainly determined by variation of mineral composition and different fluvial sediment sources in the eastern and western laptev sea the laptev sea shelf is cut by five north south trending submarine valleys enrichment of manganese in the oxic surficial sediment layer within these valleys and the occurrence of small ferromanganese nodules are caused by a high input of dissolved and particulate mn from the lena river and a strong diagenetic cycling of mn on the laptev sea shelf evidence is given that the content of as in surficial sediments is also strongly affected by diagenetic cycling this causes surficial sediment as concentrations of more than 100 g g 1 enrichment of mn and as was mainly observed in the submarine valleys distant from the major river mouth the sediment dilution due to higher sedimentation rates near the river outlets and a strong sediment re working by ice gouging in the shallow areas are the principle causes for the absence of diagenetic accumulation in this region during the formation of new ice scavenging of riverine suspended matter and resuspension of sediments followed by an incorporation of particles into the ice are important processes for the river shelf ocean transport of particulate trace elements it could be shown that particle loaded ice posseses the same geochemical signatures as the suspended matter and surficial seafloor sediments in the area of ice origin this has also strong implications for the contaminant transport on the arctic shelves and allows to use the geochemical signature of sea ice for the reconstruction of ice drift pattern within the arctic ocean

reconstructing the palaeoecosystem and palaeodepositional environment within the upper devonian lower mississippian bakken formation a biomarker approach abstract biomarkers have been used extensively to correlate oil families oil source rock relationships and source inputs and to help identify the palaeodepositional environment for a given geological sedimentary unit despite their application in characterising the bakken formation shales a number of biomarkers still appear unable to provide us a consistent understanding regarding heterogeneity in source organic matter and palaeoenvironment here we infer the palaeodepositional environment within the bakken formation shales using a biomarker suite and biomarker depth profiles we also use the occurrence and abundance of biomarkers to define organic facies and to make inferences regarding biotic interactions drawn from modern analogues the variations and co occurrence of biomarkers is explained by the existence of a proposed complex algal bacterium bacteriovore palaeoecosystem this approach not only helps explain the depth variation in occurrence and concentration of biomarkers and the molecular composition of associated organic matter but also advances the use of biomarker analysis and biomarker associations to determine a complex environment sensitive palaeoecosystem

climate controlled variability of iron deposition in the central arctic ocean southern mendeleev ridge over the last 130 000 years abstract distinct brown coloured mn rich layers are common features in quaternary sediments of the central arctic ocean but their paleoenvironmental significance is disputed glacial interglacial changes in bottom water ventilation variable riverine input and diagenetic processes have all been suggested to cause the cyclicity in optical and geochemical parameters in these deposits to better assess the nature and origin of these sediment intervals and their implications for nutrient cycles we applied bulk geochemical analyses and a sequential fe extraction scheme to a sediment record covering the last 130 ka on the southern mendeleev ridge all brown layers in this core are enriched in mn but also in fe oxyhydr oxides e g ferrihydrite lepidocrocite goethite that contribute to the characteristic sediment colour phosphorus and several trace metals as co cu mo ni are also enriched indicating that both mn and fe oxyhydr oxides adsorbed p and trace metals from the water column the metal enrichments are most probably caused by a combination of enhanced mn and fe input from arctic rivers and or the erosion of shelf deposits coupled with early diagenetic processes leading to precipitation of authigenic mn and fe oxyhydr oxides close to the sediment water interface both primary supply and diagenetic recycling of metals should have been enhanced by a more intense hydrological cycle under warmer climate conditions combined with seasonal increases in productivity and reactive organic matter export during ice free conditions in the central arctic ocean consequently the studied metal oxyhydr oxide rich layers were formed under interglacial interstadial climatic conditions in contrast sheet silicate bound fe e g chlorite a second major fe pool in these sediments 15 35 of total fe is enriched between the oxide rich layers thus the delivery of fe rich sheet silicates from the east siberian hinterland or the bering sea and subsequent deposition on the mendeleev ridge was enhanced by more intense physical weathering during glacial stadial periods these contrasts in fe speciation between the brown layers and the adjacent sediments document the direct response of biogeochemical element cycles to glacial interglacial variations in river input weathering regime and sea level in the arctic ocean

integrating depositional models ichnology and sequence stratigraphy in reservoir characterization the middle member of the devonian carboniferous bakken formation of subsurface southeastern saskatchewan revisited the upper devonian lower carboniferous bakken formation is a widespread siliciclastic unit in the subsurface of the williston basin that is subdivided into three members lower and upper organic rich shale members and a dolomitic silty and sandy middle member although the unit has become one of the most active oil plays in north america and numerous sedimentologic studies have been made no consensus about the depositional environments of the middle member has been achieved previous studies suggested several depositional and sequence stratigraphic scenarios including lowstand offshore shoreface normal regressive offshore shoreface incised estuary and falling stage shoreface complexes for the middle member we propose a new depositional and sequence stratigraphic model and compare it with some previous interpretations this new model includes a basal transgressive systems tract tst embracing shelf deposits a highstand systems tract comprising shelf to lower shoreface environments and an upper tst encompassing a brackish water embayment complex and offshore to shelf settings petrophysical characterization of the sedimentary facies reveals that bay mouth cross stratified fine grained sandstone flaser bedded very fine grained sandstone formed in wave dominated tidal flats offshore transition highly bioturbated interbedded very fine grained sandstone and siltstone and tidal flat very fine grained sandstone with common mud drapes possess the best reservoir qualities recognition of a restricted embayment within the bakken middle member has major implications for both exploration and production embayment facies with good reservoir quality constitute good oil prospects in localized areas whereas fully marine facies may represent good oil prospects of more regional extent

sequence stratigraphy and ichnology of early cretaceous reservoirs gadvan formation in southwestern iran abstract the siliciclastic gadvan formation from abadan plain southwestern iran is highly bioturbated and allows relationships between changes in ichnocoenoses within a depositional system to be documented and placed in a high resolution sequence stratigraphic framework relying on the sedimentary and ichnological characteristics the siliciclastic succession is divided into two facies associations a wave dominated offshore shoreface complex and a tide river influenced delta the first includes facies that have been deposited in shelf offshore upper offshore lower shoreface and upper middle shoreface environments the latter includes facies that have been deposited in prodelta and delta front integrated ichnologic and sedimentologic studies of the gadvan formation allow distinction between prodelta and delta front and open marine deposits with the identification of maximum flooding and ravinement surfaces as bounding surfaces of the stratal units detailed analysis on systematic changes in the stacking pattern cycle thickness cycle type and facies proportion are made eight ichnocoenoses could be differentiated in the studied sections the positions of the ichnocoenoses within genetically related stratal units genetically related ichnocoenoses indicate three large scale cycles ds1 to ds3 from oldest to youngest the cyclical nature of the gadvan formation is attributed to low amplitude eustasy in greenhouse conditions formed under interaction of eustatic high frequency cycles and longer term tectonically driven sea level variations during the long term transgressive sea level trend of the early cretaceous stratigraphic architectural style of sequences ds1 to ds3 which includes scarce evidence of lowstand deposits partial or total truncation of the hst and predominance of thick transgressive deposits is remarkably similar to long term transgressive sea level trend of the early cretaceous across the arabian plate this study suggests a more relatively seaward position of the siliciclastic successions of the gadvan formation of abadan plain than the mesopotamian basin upper zubair formation equivalent in western iraq and kuwait which would be concordant with the prevailing view of an easterly prograding coastline across the arabian plate this study reveals important sedimentological and ichnological features and permits the development of predictive models for the paleoenvironmental and sequence stratigraphical significance of trace fossil assemblages that can be readily compared or translated to analogous depositional systems worldwide the ichnological analysis is based on cores and can be especially applied to evaluate the applicability of current ichnological models to the study of cretaceous reservoirs of western iraq kuwait and western saudi arabia

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

origin of apparent magnetic excursions in deep sea sediments from mendeleev alpha ridge arctic ocean 1 arctic deep sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere and the lack of tight age control usually provides excessive freedom in the labeling of arctic excursions fortuitous variations in sedimentation rate have been invoked to explain the amplified excursions alternating field demagnetization of natural remanent magnetization nrm of sediment cores 08jpc 10jpc 11jpc and 13jpc recovered by the healy oden trans arctic expedition in august 2005 hotrax05 to the mendeleev alpha ridge yields apparent magnetic excursions in sediments deposited in the brunhes chron thermal demagnetization of the nrm however implies multiple magnetization components with negative inclination components usually unblocked below 350 c analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another magnetic experiments conducted under high and low temperatures show features that are characteristic for titano magnetite and titanomaghemite presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for x ray energy dispersive spectroscopy eds and by high resolution x ray diffraction xrd it is unlikely that anomalously thick intervals of negative inclination in these brunhes aged sediments are caused by unusual behavior of the magnetic field in the arctic area we therefore attribute low and negative nrm inclinations in these cores to partially self reversed chemical remanent magnetizations apparently carried by titanomaghemite and acquired during the oxidation of detrital titano magnetite grains the high ti contents and high oxidation states indicated by eds and xrd data provide the conditions required for partial self reversal by ionic reordering during diagenetic maghemitization and this process appears to have affected all hotrax05 cores collected from the mendeleev alpha ridge

ichnofauna record cryptic marine incursions onto a coastal floodplain at a key lower mississippian tetrapod site ichnofossils from a lower mississippian tournaisian coastal plain succession with a rich vertebrate fauna including early tetrapods record repeated short lived marine interactions that influenced floodplain lake development and ecosystems the ichnofauna contrasts with the more typical scoyenia ichnofacies of other devonian carboniferous tetrapod sites in the tournaisian ballagan formation of the scottish borders chondrites is identified in 128 horizons within a 500 m succession and is associated with lesser occurrences of phycosiphoniform burrows diplocraterion rhizocorallium and zoophycos chondrites and phycosiphoniform burrows commonly occur within dolostones interpreted as part of a saline hypersaline lake facies association containing a non marine to marginal marine macrofauna of bivalves ostracods spirorbis serpula and sarcopterygian fish marine scolecodonts are reported from 18 horizons of diverse lithology four of which co occur with chondrites the single colonisation single tier high ichnofabric index and thin beds mean 10 cm thick that characterise chondrites horizons indicate 1 adverse environmental conditions 2 rapid colonisation of the sediment and 3 short lived bioturbation episodes other ichnotaxa identified are monocraterion and asterosoma which occur exclusively within the overbank or fluvial facies associations and are here interpreted to have lived in freshwater conditions the chondrites trace makers are interpreted as having originated in shallow marine waters and were transported into floodplain lakes by storm surge events although a few examples suggest more significant transgressions ichnofossils are much more common than marine body fossils or scolecodonts and thus record cryptic marine incursions that might otherwise remain unidentified this study contributes to our understanding of the complex mosaic of environments present when terrestrial tetrapods first appeared

characteristic trace fossil associations in oxygen poor sedimentary environments the sedimentologic record of oxygen poor depositional environments typically includes trace fossils produced by deposit feeding organisms we propose an oxygen controlled trace fossil model in which increasing oxygen concentration of interstitial water in sediment parallels a transition from fodinichnia dominated through pascichnia dominated to domichnia dominated trace fossil associations in certain places such as parts of the late paleozoic oquirrh basin in utah this model provides a reasonable alternative to the more traditional depth controlled model of trace fossil distribution

paleoecology of the marine endobenthos endobenthic animals which reside within the sea bottom include stationary suspension feeders mobile deposit feeders and both stationary and mobile carnivores their activities especially with regard to dwelling feeding walking crawling and resting nesting are recorded as trace fossils abundance diversity and density of some kinds of trace fossils allow interpretation of the population strategies of the trace makers in terms of opportunistic r selected and equilibrium k selected strategies opportunistic ichnotaxa tend to be faciesbreaking traces which are highly localized in low diversity high density trace fossil associations in rocks representing environmental extremes e g variable salinities harsh temperatures low oxygen levels or shifting substrates equilibrium ichnotaxa usually are restricted to particular sedimentary facies and are characteristic of high diversity low dominance trace fossil associations in sediments reflecting stable predictable environmental conditions the most important environmental factors influencing the composition of trace fossil assemblages in marine settings are bathymetry substrate oxygen and hydrodynamic energy the four factors are closely interrelated because as water depth increases there is a general decrease in sediment grain size and hydrodynamic energy of the depositional environment as depth below the water sediment interface increases the firmness of the sediment due to compaction and dewatering increases and the oxygen content of interstitial waters drops drastically marine ichnofacies are largely substrate controlled soupgrounds are water saturated incompetent substrates typified by highly compressed and usually unidentifiable burrows softgrounds commonly contain numerous distinctive burrows and are zoned bathymetrically by the skolithos cruziana zoophycos and nereites ichnofacies firmgrounds are characterized by stiff compacted sediments in which traces of the glossifungites ichnofacies are excavated hardgrounds are cemented substrates in which bioerosion traces of the trypanites ichnofacies are bored woodgrounds are woody materials that have been exposed to the sea and bored by bivalves which produce characteristic traces of the teredolites ichnofacies tiering of endobenthic communities is common and is related to substrate preference of the burrowers and oxygen stratification of interstitial waters

conodonts of bakken formation devonian and mississippian williston basin north dakota abstract the bakken formation is a thin maximum 145 ft 45 m clastic unit in the subsurface of williston basin in the united states and canada the formation consists of three informal members which display an onlapping relationship a lower radioactive pyritiferous noncalcareous black shale is overlain by a light to dark gray dolomitic siltstone to calcareous sandstone and an upper black shale similar in lithology to the lower shale the finely laminated organic rich black shales of the bakken formation were deposited in an anoxic marine environment that was apparently offshore sediment starved and below wave base the middle member contains bedding features and fossil evidence indicative of a slightly dysaerobic current influenced marine environment the upper and ower black shales are considered important source rocks for hydrocarbons in the williston basin the bakken is similar in lithologic character and stratigraphic position to other black shale units deposited on the north american craton during the late devonian and early mississippian the bakken was initially considered entirely mississippian in age paleontologic study of regional physical equivalents and analysis of the macrofauna in saskatchewan has suggested that the bakken is actually both devonian and mississippian conodonts were obtained from cores of the bakken in an effort to determine the age of the formation in north dakota and to assess the oil generation potential nearly 700 conodonts have been recovered but are unevenly distributed within the bakken formation a majority was obtained from thin approximately 0 5 cm fossil rich beds within the upper shale conodonts from the top of the upper shale reveal a mississippian kinderhookian age and are here assigned to the lower siphonodella end page 1341 crenulata zone only rare fragmentary conodonts have been found in the middle member conodont evidence from the middle of the lower shale suggests a late devonian famennian age upper polygnathus styriacus zone for this member conodont color has been established as a geothermometer in carbonate rocks color alteration indices of conodonts from the bakken range from 1 5 to approximately 2 5 and indicate a pattern of increasing temperature with depth these results suggest possible hydrocarbon generation from shallower depths than has been reported previously for the bakken the lack of agreement in interpreted hydrocarbon generation depths may be due to among other things the clastic nature of the bakken formation end of article last page 1342

phosphorus distribution c n p ratios and δ13coc in arctic temperate and tropical coastal sediments tools for characterizing bulk sedimentary organic matter abstract onshore offshore trends in phosphorus p organic carbon oc and total nitrogen tn concentration p distribution elemental organic c n p ratios and stable carbon isotopic composition of oc 13 c oc of surficial sediments are presented from three river dominated coastal regimes the mackenzie river beaufort shelf in the canadian arctic the mississippi delta and louisiana shelf in the temperate gulf of mexico and the tropical amazon shelf these parameters measured in surficial sediments from the three sites are used to assess changes in the importance of terrestrial and marine organic matter sources to sediments as a function of distance from the locus of riverine discharge trends in elemental ratio data from the arctic transect and a portion of the gulf of mexico transects can be explained in terms of a two end member mixture of terrestrial and marine phytodetritus in the arctic transect covariation of organic c p ratios with 13 c oc is consistent with a two end member mixture of terrestrial and marine organic matter a similar relationship between c p ratios and 13 c oc is not evident in gulf of mexico or amazon shelf sediments at these sites the two end member model fails to explain the data adequately the most striking feature of the onshore offshore trends in elemental oc op and oc tn ratios is the occurrence of low ratios in sediments from deep water stations in the gulf of mexico and all stations on the amazon shelf resulting from p and n enrichments exceeding the redfield ratio for marine plankton three explanations are discussed to account for these low ratios 1 the dominance of refractory op and on compounds in the residuum of degraded organic matter 2 differential sorption of op and n compounds regardless of lability on the surfaces of the fine grained sediments characteristic of these sites and 3 a dominance of bacterial biomass or components derived from bacterial biomass elemental ratios and 13 c oc signatures indicate that differential sorption may be most important in the deep water gulf of mexico sites in contrast elemental ratios and 13 c oc on the amazon shelf are most consistent with a sedimentary organic matter pool dominated by bacterial biomass or derivative substances

manganese and color cycles in arctic ocean sediments constrain pleistocene chronology sequential variations in manganese mn content and color of deepsea sediments retrieved from the lomonosov ridge 87 n in the central arctic ocean apparently mimic low latitude 18 o glacial interglacial cyclicity thereby providing stratigraphic information that together with biostratigraphic data permit the construction of a detailed chronological model correlation of this mn and color chronology to established apparent brunhes age estimates of geomagnetic excursions reveals a remarkable fit between these two independently derived time scales the mn and color cycles probably provide paleoenvironmental information about material fluxes in the arctic ocean over the past 1 m y we suggest that the primary source for the observed manganese variations in our sediment core is northern siberia which has extensive peat bogs and boreal forests these siberian source areas could operate in an off and on mode tuned to pleistocene glacial and interglacial periods contrasts in ventilation of arctic ocean waters during interglacial glacial cycles probably could also enhance the observed mn and color variability

late devonian and early mississippian bakken and exshaw black shale source rocks western canada sedimentary basin a sequence stratigraphic interpretation the late devonian and early mississippian bakken and exshaw formations are a continuum of regionally correlated organic rich up to 35 total organic carbon black shale source rocks covering much of the western canada sedimentary basin the bakken formation is composed of 1 a black mudstone lower member 2 a gray mudstone sandstone middle member and 3 a black mudstone upper member the exshaw formation beneath the alberta plains and in exposures in the foothills and front ranges of the rocky mountains is composed of 1 a lower black shale member and 2 an upper siltstone member the basal black shale unit of the lower mississippian banff formation overlying the exshaw formation is a second organic rich interval these black shales are regionally significant hydrocarbon source rocks and local reservoirs the middle bakken member is a locally important reservoir rock with substantial economic potential the bakken and exshaw formations and the basal banff black shale are divisible into three systems tracts 1 a transgressive systems tract 2 a lowstand systems tract and 3 a second transgressive systems tract lodgepole and banff formation carbonates overlying the bakken and exshaw formations are part of a highstand systems tract a sequence boundary occurs between the lower and middle bakken members the conformable equivalent of this sequence boundary is within the exshaw black shale member variations in the internal composition of these systems tracts imply that two depocenters 1 the williston basin and 2 the prophet trough and the western margin of the north american craton were affected differently by relative sea level rise and fall during bakken and exshaw deposition because of differences in water depth and sediment accommodation spatial and temporal changes in black shale and gray mudstone sandstone as highlighted by this sequence stratigraphic interpretation may have significant impacts on source rock potential and hydrocarbon reservoir size location and quality

ichnology of a late devonian early carboniferous low energy seaway the bakken formation of subsurface saskatchewan canada assessing paleoenvironmental controls and biotic responses abstract in spite of the high interest on the late devonian early carboniferous bakken formation as one of the most important oil plays in north america and the numerous studies carried out in this unit no ichnologic analysis has been presented yet based on conventional sedimentologic data previous studies of the bakken interpreted the unit as formed entirely under open marine conditions however integration of ichnology has been key to the identification of not only open marine but also brackish water marginal marine conditions based on the geometry of the sedimentary bodies and the sequence stratigraphic framework the brackish water interval is interpreted as an embayment with limited or intermittent connection to the open sea salinity oxygen content and storms are regarded as the most important environmental parameters that controlled the distribution and nature of the trace fossils in the bakken formation sparse bioturbation relatively low ichnodiversity and the impoverished cruziana ichnofacies characterize the marginal marine deposits suggesting stressful brackish water conditions marine deposits from the middle member in contrast are highly bioturbated with a moderate ichnodiversity and the distal cruziana ichnofacies that flourished during fully marine well oxygenated conditions there is only a slight difference between the total ichnodiversity of the open marine interval ten ichnogenera and that of the brackish water interval seven ichnogenera ichnodiversity should not be considered at face value and isolated from other aspects when evaluating stress levels in order to detect a brackish water signal lack of bioturbation in the black shale of the lower and upper members resulted from anoxic conditions the passage from anoxic lower member to well oxygenated conditions middle member was gradational as is revealed by the occurrence of oxygen deficient trace fossils at the top of the lower member finally the contrasting styles in tempestite preservation in upper offshore deposits from the lower and upper open marine intervals is attributed to variations in the intensity and frequency of storm during deposition of the highstand and transgressive deposits

production and preservation of organic matter during deposition of the bakken formation late devonian and early mississippian williston basin abstract the organic rich up to 35 toc lower and upper black mudstone members of the bakken formation late devonian and early mississippian accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic palaeoceanographic and palaeoclimatic conditions during bakken deposition the semi isolated williston basin was located close to the equator 5 to 10 n in a zone of prevailing east to west winds a pattern of estuarine like marine circulation between the williston basin and open ocean conditions at the western craton edge of north america is proposed surface water discharged from the basin was replaced by nutrient rich deeper water sourced from the equatorial undercurrent in the pacific basin undercurrent flow into the williston basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition in response to increased productivity that led to a high rate of organic sedimentation and quiescent deep water conditions due primarily to the geographic isolation of the basin and the restriction of estuarine like circulation to the upper water column anoxic bottom water developed on the basin floor as sea level rose during lower and upper black mudstone deposition anoxic bottom water expanded across the basin enhancing organic matter preservation conditions in the williston basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic rich mud deposition these results have potentially important implications for the accumulation of similarly organic rich deposits across the north american interior during late devonian and early mississippian time

quantitative ichnology of modern pelagic deposits in the abyssal atlantic abstract quantitative sedimentologic aspects of bioturbation were investigated in a series of deep sea box cores which were collected over a broad latitudinal range 35 n to 30 s and bathymetric range 1 4 to 5 7 km in the central atlantic ocean all cores were thoroughly bioturbated and numerous generations of burrowing could be discerned in many instances preservation of individually distinct burrows ranged from poor to excellent and eight ichnogenera plus several unnamed biogenic structures were recognized the distribution of calcium carbonate organic carbon selected species of planktic foraminifera and sediment grain size were examined in several cores these measurements indicated that the surficial mixed layer which ranged from 3 to 10 cm thick was statistically more homogeneous than the underlying transition layer this relationship is seen most clearly with respect to lateral and vertical variations in the calcium carbonate content of the cores bioturbation of abyssal pelagic deposits appears to be a two phase process consisting of a homogenization of the upper few centimeters i e the mixed layer by shallow burrowing meiofauna and b heterogeneous mixing of the underlying strata i e the transition layer by relatively deep burrowers the heterogeneously mixed transition layer becomes the preserved trace fossil record under normal circumstances the homogenized mixed layer is not preserved

lateral variability in shallow marine ichnofabrics implications for the ichnofabric analysis method ichnological analysis commonly undertaken both in the field and in core studies is generally carried out in vertical logged successions this work documents the results of a detailed ichnofabric study of two highly bioturbated sandstone beds separated by a mudstone within the thick lower jurassic succession of the yorkshire coast uk the two sandstone beds contain relicts of hummocky cross stratification and rare bioclasts of bivalves and belemnites bioturbation of the beds is intense and the diversity of trace fossils is considerable trace fossils present include teichichnus rectus teichichnus isp a thalassinoides isp palaeophycus heberti diplocraterion parallelum rhizocorallium rosselia socialis schaubcylindrichnus isp chondrites isp planolites isp and phycosiphon incertum ichnofabrics were studied at 5 m intervals parallel to the modern cliff line which is parallel to the inferred jurassic palaeo coastline the results demonstrate that there is considerable patchiness to ichnofabric even in a comparatively simple depositional setting such as a storm influenced shelf this observation is in line with studies of the modern sea floor the implications of patchiness in ichnofabric are broad ranging but most importantly suggest that caution is needed in the use of ichnofabric analysis for intra regional correlations

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

changes in arctic ocean paleoproductivity and hydrography during the last 145 kyr the benthic foraminiferal record the benthic foraminiferal record of two sediment cores at 900 and 2500 m water depth in the marginal arctic ocean reflects fluctuations in paleoproductivity ice sheet extent and atlantic water inflow highest paleoproductivity is observed in interglacial periods and at the termination of interstadials to stadials within oxygen isotope substages ois 6 3 5 3 5 1 3 1 and termination ia stable and high paleoproductivity at the ice edge during terminations is indicated by the melonis zaandami benthic foraminiferal association periods of glacial ice sheet advance or retreat from the core site are dominated by benthic foraminiferal associations related to seasonal organic carbon flux temperate saline atlantic water entered the arctic ocean within ois 6 3 5 5 and the holocene coincident paleoproductivity maxima are related to extended seasonal ice retreat during glacial periods of stagnating atlantic water advection unusual low paleoproductivity values are observed in the arctic ocean indicating increasing ice coverage deep water was poorly ventilated during these periods and the deeper site became exposed to corrosive bottom waters

the lost devonian sequence sequence stratigraphy of the middle bakken member and the importance of clastic dykes in the lower bakken member shale north dakota usa abstract the late devonian to early mississippian bakken formation in the williston basin of north dakota usa shows a tri partite subdivision a middle mixed carbonate siliciclastic member is sandwiched in between two black siliciclastic mudstones the lower and upper bakken member shales however the transition from the lower shale member to the middle member does not represent a gradual coarsening but contains in places several millimeter to centimeter thick siliciclastic mudstones and carbonates that consist of three facies 1 a glauconitic carbonate rich siliciclastic mudstone 2 a carbonate mud to wackestone and 3 an echinoderm wacke to packstone with shell fragments these three facies are present in many all of the cores close and directly in the basin center in mountrail county north dakota at least one of these three facies is present in all 23 cores included in this study this thin carbonate unit at the transition between the lower and the middle bakken members is interpreted as representing the remnants of the transgressive systems tract it is assumed that relative sea level fell before deposition of the middle bakken member establishing a proximal coarse grained to distal fine grained depositional transect that successively migrated into the basin during the subsequent transgression the siliciclastic input was low to absent and the entire sedimentary system switched to depositing carbonates the proximal to distal transect during this time showed coarse grained packstones and grainstones close to the shoreline and a fining outwards towards the distal parts of the basin this transgression also eroded what remained of the regressive and most of the subsequent transgressive sediments leaving only the thin carbonate layer behind evidence for the regression even though no sediment is directly preserved along the lower to middle bakken member contact comes from the fill of clastic dykes that cut through the lower bakken member shale the fill of the clastic dykes is partly siliciclastic and partly carbonate and not similar to any of the surrounding sediment this indicates that these dykes must have originated before the middle bakken member was deposited yet the overlying sediment must have been carbonate at some point and siliciclastic another time as it is not present anymore this sediment must have been entirely removed by erosion the here presented model suggests that the bakken formation reflects two entire sea level oscillations the first encompasses the lower bakken member shale and the siliciclastic regressive portion of the lowstand only preserved as infill of the clastic dykes the subsequent transgression deposited the carbonates now blanketing the lower to middle bakken member transition and the highstand and subsequent regression plus lowstand are represented by the middle bakken member the transgressive surface and therewith the onset of the topmost bakken transgression is marked by the transition from the middle to the upper bakken shale member

animal substrate interactions in an early miocene wave dominated tropical delta delineating environmental stresses and depositional dynamics tacata field eastern venezuela abstract the miocene tacata reservoirs of the eastern venezuela basin contain deposits that accumulated as a result of the progradation of a wave dominated delta complex in a tropical setting integration of ichnologic and sedimentologic data helps to detect different stress factors allowing distinction between deltaic and associated along strike strandplain shoreface successions fluvial discharge was repeatedly interrupted by periods of storm wave reworking and suspension fallout leaving diagnostic ichnologic signatures in the deposits repeated storms rank among the most important controlling factors on the front of the tacata deltas lateral especially on the updrift side to the river mouths wave activity caused nearly continuous accretion of the associated strandplains evidence of tidal influence is essentially restricted to distributary channel and particularly interdistributary bay deposits although elements of the skolithos ichnofacies e g skolithos are present they tend to occur in subordinate numbers as a result of water turbidity distributary channel deposits are sparsely bioturbated but the presence of certain ichnotaxa e g ophiomorpha allows distinction from freshwater fluvial channels ichnofabrics from interdistributary bay deposits display all the characteristics expected from brackish water settings e g low ichnodiversity presence of monospecific suites and are also typically sparsely bioturbated teichichnus and planolites commonly associated with synaeresis cracks are dominant ichnotaxa in these fine grained low energy protected brackish water settings in contrast delta front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water settings such as chondrites and phycosiphon preservation of very thin storm layers is probably more common in wave dominated deltas than in strandplain systems because deltaic stresses preclude the establishment of an abundant infauna that otherwise would have completely reworked such thin sandstone layers amalgamated thick bedded storm deposits are typically unburrowed or contain deep vertical ophiomorpha scolicia ichnofabrics seem to be restricted to the fully marine parts of strandplain deposits and are therefore useful to detect possible delta asymmetry along depositional strike or vertical changes in degree of deltaic influence

first data on trace metal level and behaviour in two major arctic river estuarine systems ob and yenisey and in the adjacent kara sea russia dissolved and colloidal trace element concentrations cd cu fe ni and pb were determined in the ob and yenisey river estuary systems and in the adjacent kara sea in september 1993 the data show a natural low concentration level of dissolved 0 4 m trace metals cd cu fe ni and pb in the two rivers and in the kara sea as compared to world unpolluted rivers and the central arctic ocean suggesting that the region studied is pristine with respect to trace metals the pathway of trace metals transported from rivers to the ocean seems to be complicated and largely influenced by biogeochemical processes taking place in the estuarine mixing zone colloidal material 104 daltons 0 4 m in addition to its significant contribution to the so called dissolved fraction has been shown to play a fundamental role in determining the behaviour of both conservative and non conservative trace metals during estuarine mixing hence colloids may control to a large extent the fate of dissolved trace metals as well as their net input from the rivers to the kara sea

coastal erosion vs riverine sediment discharge in the arctic shelf seas this article presents a comparison of sediment input by rivers and by coastal erosion into both the laptev sea and the canadian beaufort sea cbs new data on coastal erosion in the laptev sea which are based on field measurements and remote sensing information and existing data on coastal erosion in the cbs as well as riverine sediment discharge into both the laptev sea and the cbs are included strong regional differences in the percentages of coastal erosion and riverine sediment supply are observed the cbs is dominated by the riverine sediment discharge 64 45 106 t a 1 mainly of the mackenzie river which is the largest single source of sediments in the arctic riverine sediment discharge into the laptev sea amounts to 24 10 106 t a 1 more than 70 of which are related to the lena river in comparison with the cbs the laptev sea coast on average delivers approximately twice as much sediment mass per kilometer a result of higher erosion rates due to higher cliffs and seasonal ice melting in the laptev sea sediment input by coastal erosion 58 4 106 t a 1 is therefore more important than in the cbs and the ratio between riverine and coastal sediment input amounts to 0 4 coastal erosion supplying 5 6 106 t a 1 is less significant for the sediment budget of the cbs where riverine sediment discharge exceeds coastal sediment input by a factor of ca 10

paleoenvironmental implications of cenozoic sediments from the central arctic ocean iodp expedition 302 using inorganic geochemistry 1 integrated ocean drilling program iodp expedition 302 arctic coring expedition acex recovered a unique sediment record from the central arctic ocean revealing that this region underwent major environmental fluctuations since the late cretaceous major and trace element composition of 1 300 samples were determined using x ray fluorescence xrf the results show significant compositional variability of the sediments with depth that can be attributed to changes in a provenance and pathways of detrital material b paleoenvironmental conditions and depositional processes and c diagenetic overprint of the primary record in addition to existing lithological units we introduce new geochemical units for a more process related approach interpreting the acex record in detail via the geochemical signature of siberian flood basalts we are able to reconstruct the discontinuous rifting and deepening of the central lomonosov ridge during the paleogene accompanied by changing current regimes and the onset of sea ice eocene biosiliceous sedimentation took place in a relatively shallow setting under predominantly anoxic bottom water conditions causing a positive anoxia productivity feedback although water column stratification was repeatedly interrupted by ventilation events anoxic to sulfidic conditions were even more extreme after biosilica production ceased and significant amounts of pyrite were deposited on the lomonosov ridge especially in organic matter rich paleogene deposits diagenetic processes obscured the paleoenvironmental signals fundamental environmental changes occurred in the middle eocene but geochemical and micropaleontological proxies point not to the identical sediment depth after approximately 26 ma of non deposition or erosion the middle miocene record shows the transition to dominantly oxic bottom water conditions although suboxic diagenesis seemingly affected these deposits

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

petroleum potential of the middle member bakken formation williston basin abstract the bakken formation is a widespread clastic unit between the mississippian madison group and the upper devonian big valley torquay or three forks formations the formation typically consists of three members a lower and upper organic rich shale and a middle calcareous sandstone and siltstone the three members of the bakken formation exhibit an onlapping relationship and converge and thin toward the margins or marginal shelf areas of the williston basin in manitoba the lower member is absent except for waskada field several correlative lithofacies have been recognized in the middle member the lowermost unit of the middle member is a regressive deposit the upper portions of the middle member show transgressive and regressive cycles and intertidal deposits interfinger with their distal equivalents conventional bakken production is restricted to areas where it is a thermally mature source rock the greater part of bakken production in recent years has been taken from the shale of the upper and lower members although there are outlying fields which produce from a sandstone within the middle member this suggests that there has been a migration of bakken oil from the central portion of the basin the sandstone unit may provide a conduit for migration