sedimentary dynamics of the kimmeridge clay tempests and earthquakes storms are suggested to be an important influence upon the formation of epicontinental black shales controlling both their distribution and facies types a variety of storm produced event beds are described from the kimmeridge clay of dorset these include horizons of graded rip up clasts silt laminae thin graded mud horizons and shell pavements biostratinomy and population dynamics indicate that the pavements represent brief benthic colonization events in a predominantly anaerobic dysaerobic environment differential near surface lithification has also affected the sediments leading to the formation of flow phenomena these include loop structures microfolding and brecciation earthquake induced fissuring also occurs at a number of horizons the kimmeridge clay shows well developed cyclically alternating organic rich shales and mudstones on a decimetre scale a feedback mechanism of storm induced benthic oxygenation and temperature stratified inhibition of storm mixing may account for the abrupt nature of the cyclicity subsidence and sedimentation rate were probably important factors controlling the thickness and organic richness of the sediments whilst the influence of storms was important in controlling the distribution and facies types of these important source rocks the model proposed for the kimmeridge clay may be of general application to many epicontinental black shales in the geological record

lack of organic matter accumulation on the upwelling influenced somalia margin in a glacial interglacial transition abstract monsoon induced upwelling has been operating off somalia for at least the last 160 000 years making the nw indian ocean one of the most productive oceanic areas in the world we have studied the sediments recovered during the indusom cruise of the r v marion dufresne from three different environments off this margin a core from the highly productive surface waters core md85 674 a core where the oxygen minimum intersects the slope core md85 664 and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production core md85 668 though the first two environments are a priori favourable to organic matter om accumulation no marine om enrichment can be observed in the cores to understand this anomaly emphasis is placed upon the transition from isotopic stage 6 to stage 5 because the end of the glacial stage 6 is characterised by intensified upwelling and primary productivity in this region whereas stage 5 experienced a marked decrease in productivity various palaeoenvironmental markers such as om v ni cu zn ba and p have been used to identify periods of high productivity and or bottom water oxygen depletion the results suggest that 1 the lack of marine om in two settings favourable to organic carbon storage may be due to the nature of primary productivity coccolithophorid production would not be prone to om accumulation in contrast to the contribution of om by diatoms or particularly naked phytoplankton dinoflagellates or other peridinians 2 ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of ba as a proxy indicator 3 the period of intensified upwelling in stage 6 as well as the stage 6 5 transition left no geochemical imprint on the sediments this means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments in other words the somalia upwelling does not participate to the so called biological pump and has no positive effect upon long term carbon storage

lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the gulf of california a test of the hypothesis that hydrogen rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the guaymas basin in the gulf of california the laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin above and below the depth of the oxygen minimum the sediments are homogenized by a burrowing infauna the two sediment types have very similar bulk chemical and mineral compositions and the organic matter is overwhelmingly marine the total organic carbon contents and rock eval parameters hydrogen and oxygen indices of the two sediment types are not significantly different whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin this maximum lies deeper than the core of the oxygen minimum thus both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters

mobility of arsenic and trace element inventories in sediment cores from masuda city southwestern japan the vertical distribution of arsenic and other trace and major elements has been studied in four sediment cores from masuda city nagashima and okite in the shimane prefecture of southwestern japan the sediment cores were also subjected to leaching techniques and 14c dating the stratigraphic sequences in the cores consist of silt and sandy silt at top passing downward into gray to black clays elevated values of as pb zn cu ni cr and v are present in several horizons while abundances of these elements tend to be higher in the black and gray clays probably due to adsorption onto clay sediments higher concentrations of fe and total sulfur ts occur in black clays the correlations of the trace metals with iron suggest their adsorption onto fe oxy hydroxides whereas correlations with sulfur in some cores indicate that they were precipitated as fe sulfides age determinations suggest that clay horizons at 5 m depth were deposited at around 5 000 and 6 000 years bp 14c ages during the transgressive phase of sea level change the results of the leaching techniques in the core samples show that higher amounts of as were extracted with deionized water even at neutral ph as can be released from sediments to groundwater and therefore groundwater pollution is a concern in masuda city and the surrounding area

organic carbon and sedimentation rate further evidence for anoxic deep water conditions in the cenomanian turonian atlantic ocean abstract the relationship between organic carbon content and sedimentation rate based on data derived from recent to miocene sediments deposited under oxic and anoxic sea water conditions is used to characterize the different depositional environments these data are compared with data of cenomanian turonian dsdp sediments from the atlantic ocean the results may support deposition in anoxic environments due to extremely reduced deep water circulation as the main cause for the formation of cenomanian turonian black shales in the north atlantic rather than increased supply of organic matter due to higher oceanic productivity

vertical geochemical variations and arsenic mobilization in the shallow alluvial aquifers of the chapai nawabganj district northwestern bangladesh implication of siderite precipitation core sediments from three disturbed boreholes jor ghat and raj and two undisturbed boreholes dw1 and dw2 were collected in the study area of the chapai nawabganj district of northwestern bangladesh for geochemical analyses in the study area groundwater samples from fourteen as contained private wells and five nested piezometers at both the dw1 and dw2 boreholes were also collected and analyzed the groundwater arsenic concentrations in the uppermost aquifer 10 40 m of depth range from 3 to 315 g l mean 47 73 73 41 g l while the arsenic content in sediments range from 2 to 14 mg kg mean 4 36 3 34 mg kg an environmental scanning electron microscope esem with an energy dispersive x ray spectrometer was used to investigate the presence of major and trace elements in the sediments groundwaters in the study area are generally the ca hco3 type with high concentrations of as but low levels of fe mn no3 and so 4 2 the concentrations of as fe mn decrease with depth in the groundwater showing vertical geochemical variations in the study area statistical analysis clearly shows that as is closely associated with fe and mn in the sediments of the jor core r 0 87 p 0 05 for fe and r 0 78 p 0 05 for mn and ghat core r 0 95 p 0 05 for fe and r 0 93 p 0 05 for mn while as is not correlated with fe and mn in groundwater the comparatively low fe and mn concentrations in some groundwater and the esem image revealed that siderite precipitated as a secondary mineral on the surface of the sediment particles the correlations along with results of sequential extraction experiments indicated that reductive dissolution of feooh and mnooh represents a mechanism for releasing arsenic into the groundwater

storm induced concentration of sulfurized marine origin organic matter as a possible mechanism in the formation of petroleum source rock abstract black shales though laminated are not systematically synonymous with quiet conditions of deposition a number of papers report about black shales yielding sedimentary structures echoing relatively high hydrodynamic conditions here we examine two sedimentary sequences pertaining to the late jurassic argiles de ch tillon formation of the boulonnais area northernmost france each of them including a meter thick black shale horizon the two laminated black shales contain sulfurized i e organic s rich organic matter reaching a maximum of 9 however some differences set the two black shales apart the lower one was deposited under calm suboxic to anoxic bottom water conditions the classic way the upper black shale was deposited under oxic agitated bottom water conditions not compatible with stable sulfidic conditions required for organic matter sulfurization the upper black shale experienced hydrodynamically induced concentration of sulfurized recalcitrant organic matter of marine origin the sulfurized organic matter could be preserved and quantitatively accumulated owing to its non putrescible nature leading to the formation of a potential hydrocarbon source rock the association of pyrite inclusions with sulfurized organic matter probably modified the hydrodynamic behavior of organic particles the storm induced remobilization and concentration of sulfurized organic matter implies that the sulfurization factory operated in proximal shallow environments such as estuaries mud flats or mangrove environments such a model is in agreement with findings from modern coastal shelf environments notably mangroves and fully transposable to many other sedimentary situations of any geological time period

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

paleodepositional conditions in the orca basin as inferred from organic matter and trace metal contents abstract the orca basin located in the gulf of mexico is a highly stratified basin that collects sedimentary particles of clastic origin supplied by the mississippi river and of biogenic origin marine surface productivity the strong pycnocline induces anoxic bottom conditions that are expected to be favorable to organic matter om accumulation and prone to enrichment of sediments in redox sensitive trace metals here we report on om and trace metal contents in the upper 750 cm below sea floor cmbsf of core md02 2552 mostly holocene and deposited under permanently stratified bottom conditions the organic content is dominated by marine derived amorphous om contrary to expectations sedimentary om in the basin is highly degraded probably due to a long residence time of organic particles at the basin s basin s halocline at 2240 m also unexpected is the limited sedimentary enrichment of redox sensitive trace metals we suggest that the non enrichment in u and v could be explained by the so called basin reservoir effect that is limited metal availability because of low rates of water renewal in a strongly restricted subpycnoclinal water mass the non enrichment in cu could have the same cause plus the fact that the relatively degraded om may not readily form organometallic complexes however a marked enrichment is observed for mo and a moderate one for ni and cr which could be accounted for by sporadic presence of h 2 s in bottom waters such occurrences of h 2 s would not be sufficient to promote a detectable fes to fes 2 conversion but they would induce the capture of mo in the form of thiomolybdate and accessorily ni cr as ni cro 4 complexes

the inorganic geochemistry of cretaceous black shales dsdp leg 41 in comparison to modern upwelling sediments from the gulf of california

organic matter characteristics of cenomanian turonian source rocks implications for petroleum and gas exploration onshore senegal one hundred and fifteen samples of cenomanian turonian sediments from the senegal margin basin believed to be the main source rocks in the area have been characterised to determine their organic facies and their oil generating potential the samples have been subjected to detailed organic petrographic studies palynofacies maceral analysis vitrinite reflectance measurements combined with rock eval and leco determinations total organic carbon values comprised between 0 3 and 3 5 indicate poor to moderate primary bioproductivity and or unfavourable organic matter om preservation rather high c n ratio values supported by the presence of well preserved ligno cellulosic debris and of collotelinite suggest a notable contribution of terrestrial om this is supported by hydrogen and oxygen index values that testify to type iii om s c ratio values lower than 0 36 together with the presence of abundant reworked organic particles indicate that the studied sediments were deposited in a transitional and mixing zone between a coastal area with well oxygenated waters and a fully marine environment with more or less pronounced anoxia the rather low marine productivity and poor om preservation are here strongly reinforced by a very marked dilution of the om by mineral inputs in addition to these combined factors indicating that the om is mainly of the gas prone type a moderate maturity is shown by tmax and vitrinite reflectance values ranging from 435 to 445 c and 0 4 to 0 9 respectively the reservoir facies consisting of sandstones interbedded with shales are characteristic of prograding fluvio deltaic formations developed during maastrichtian regression faults and rollover structures most probably constitute the main migration pathways and traps for hydrocarbons respectively overall the characteristics of the studied formations highlight the importance of local factors in om accumulation even during the so called global cenomanian turonian anoxic event

oscillating redox conditions in the vocontian basin se france during oceanic anoxic event 2 oae 2 abstract the cenomanian turonian boundary 94 ma was marked by an episode of widespread organic carbon burial largely associated with low oxygen concentrations in bottom oceanic waters named oceanic anoxic event 2 oae 2 in the vocontian basin se france the pont d issole section exposes a 22 m interval called niveau thomel the local sedimentary expression of the oae 2 characterized by levels of laminated dark shales with marine derived organic matter enrichments up to 2 5 wt an episodic disruption in total organic carbon toc enrichment is observed in bioturbated limestones within the niveau thomel reflecting bottom water reoxygenation associated to the plenus cold event oscillations in redox conditions during the oae 2 are evidenced by variations in the degree of pyritisation of the sediments reflecting suboxic to anoxic conditions associated with organic rich intervals these variations are additionally highlighted by enrichments in redox sensitive trace elements mo v zn in sediments containing at least 1 5 wt toc which may reflect a threshold between suboxic and anoxic conditions at pont d issole section low mn and fe concentrations in 0 3 wt toc samples mark a large o2 depletion in pore waters phosphorus released from the sediments under deoxygenated conditions could have contributed to sustain the primary productivity in the basin promoting further o2 consumption a large negative excursion in the sulfur isotopic composition of pyrite 34spy with minimum values down to 40 suggests that microbial sulfate reduction was non sulfate limited and was characterized by pyrite precipitation in an open system with regard to the marine sulfate reservoir this feature is probably linked to a higher sulfate availability within the vocontian basin due to important large igneous province outgassing at the onset of the oae 2

chemostratigraphy of the jurassic system applications limitations and implications for palaeoceanography current chemostratigraphical studies of the jurassic system primarily involve the use of one sedimentary component marine organic carbon one divalent transition metal substituted in carbonate manganese and two isotopic tracers strontium isotope ratios 87 sr 86 sr and carbon isotope ratios 13 c carb and 13 c org in carbonate and in organic matter other parameters such as mg ca and sr ca ratios in calcite oxygen isotope ratios 18 o in carbonate sulphur isotope ratios 34 s in carbonate hosted sulphate nitrogen isotope ratios 15 n org in organic matter osmium isotope ratios 187 os 188 os in black shales and neodymium isotope ratios 143 nd 144 nd in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors stratigraphical patterns in total organic carbon toc can be of either local or regional significance depending on the lateral extent of the former nutrient rich and productive water mass divalent manganese follows a similar pattern being concentrated most probably as a very early diagenetic phase only in oxygen depleted waters that typically underlie zones of elevated organic productivity shifts in mg ca and sr ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes strontium isotope ratios from pristine skeletal calcite provide a global signal 13 c values from carbonates with minimal diagenetic overprint potentially do the same although small spatial differences in palaeo water mass composition may have been locally significant oxygen isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful because they reflect palaeotemperature the evaporation precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase nitrogen isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis neodymium isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems preliminary data on sulphur isotope ratios in carbonates and osmium isotope ratios in organic rich shales both potentially offering global correlation indicate that these tracers may be valuable although the records at present are not sufficiently well established to allow high resolution regional correlation in all cases biostratigraphically well dated reference sections against which the relevant geochemical data have been calibrated are required in the first instance to date studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern boreal and southern tethyan europe carbon isotope stratigraphy has been undertaken primarily but not exclusively on bulk pelagic sediments from the alpine mediterranean or tethyan domain and strontium isotope stratigraphy has been undertaken largely on calcitic skeletal material belemnites and oysters from northern and southern europe in many sections including those containing ammonites multi parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means strontium isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record

a climate change event detected in visean oil shales from devon island arctic canada abstract important depositional characteristics indicative of climate change has been discovered in 50 m sedimentary succession consisting of oil shales with a marlstone and carbonate matrix of the visean shales from the lower carboniferous emma fiord formation on devon island canada s arctic archipelago older carboniferous tournaisian shales are however absent from devon island but elsewhere such as big marsh nova scotia tournaisian shales show no comparable evidence for climate change the techniques used include reflected light microscopy rockeval analyses instrumental neutron activation analysis inaa for elemental concentration indeed visean shales from the lower part the emma fiord formation from devon island have a marlstone matrix with abundant algal matter the matrix also has low carbonate 32 and total organic carbon toc content between 33 3 and 55 6 moreover the toc show a positive correlation with authigenic uranium and mo which suggests a high productivity due to the high rain deeper water lacustrine depositional environment other visean oil shales from the emma fiord formation on devon island have somewhat different characteristics from those with marlstone matrix indeed they have a high 75 carbonate content with calcified algal matter and low toc 1 14 17 4 a very low correlation between toc and authigenic u and mo most importantly very high oil generation 98 413 l t potential of marlstones as compared to low yield of carbonate mudstones 4 1 17 0 l t shows a dramatic climatic shift to low precipitation and much dryer conditions such a drastic climatic condition has not been detected for the tournaisian oil shales

occurrence and origin of ultralaminar structures in amorphous kerogens of various source rocks and oil shales abstract forty low maturity type i ii kerogens from source rocks and oil shales ranging in age from infra cambrian to miocene were examined by transmission electron microscopy tem when previously observed by light microscopy and or uv fluorescence microscopy the above samples like a very large number of kerogens seemed homogeneous and amorphous in fact tem revealed the presence of ultralaminar structures in 22 of these amorphous kerogens the abundance of ultralaminae depends on the considered sample and some of the tested kerogens are nearly exclusively composed of ultralamina accumulations based on thickness three main groups of ultralaminae can be identified parallel studies on extant algae indicated that some of the above ultralaminae likely derived from the resistant outer walls of microalgae the fossilization of such outer walls should occur via the selective preservation pathway and result in the formation of highly aliphatic ultralaminar kerogens the lack of ultralaminae in infra cambrian kerogens is consistent with a cyanobacterial origin

geochemical disturbance and paleoenvironmental changes during the early toarcian in nw europe the early toarcian oceanic anoxic event oae is associated with a rapid and severe environmental change whose causes are still a matter of debate a new paleoenvironmental interpretation is proposed based on a multidisciplinary dataset chemical mineralogical and paleontological obtained from a borehole drilled by andra french national radioactive waste management agency in the lower toarcian of the eastern paris basin france during the earlymost toarcian tenuicostatumzone the presence ofwell oxygenated sea water is reflected by an abundant and diverse fauna alongwith paleoredox proxies u al mo al and v al typical of oxidizing conditions associated with predominant mechanical weathering on the continent at the base of the serpentinum zone an important paleoenvironmental change is marked by an initial phase of organic matter enrichment 12 toc during the negative shift of 13c carbon isotope excursion cie euxinic anoxic conditions indicated by cobalt enrichment and the development of framboidal pyrite lead to the disappearance of the benthic and pelagic fauna and alloworganic matter to be preserved in the sediments at this time the lowconcentration of paleoproductivity proxies cd p ni and zn suggests weak chemical weathering on the continent due to a rather arid climate low concentrations of sr and coccoliths are due to an input of fresh water probably coming from the arctic through the laurasian seaway thus the isolation of the paris basin from the tethyan realm and the influx of freshwater favour haline stratification and the preservation of organic matter following the cie oxygen concentration gradually increases up to amaximumflooding surface dated as coevalwith the serpentinum zone elegantulum subzone paleoproductivity proxies indicate a weak primary productivity the top of the lower toarcian is characterized by a second enrichment in organic matter 12 toc due to an important input of nutrients this enhanced primary production generates euxinic anoxic conditions in the sediment and in part of the water column leading to a high enrichment in paleoredox proxies mo u v co and causing the sudden disappearance of the benthic fauna and a progressive decline of the pelagic fauna the metal enrichment is related to intense chemical weathering on the continent due to a more humid climate

local to global controls on the deposition of organic rich muds across the late jurassic laurasian seaway muds deposited in large scale epicontinental seaways provide deep insights into palaeoclimates biogeochemical cycles sedimentation processes and organic carbon burial during exceptionally warm periods throughout the phanerozoic temporal changes can be monitored at single locations but the key larger scale oceanographical and related biogeochemical processes are likely to be more clearly revealed by comparisons between individual sub basins within seaways here we compare inorganic geochemical records from the jurassic upper pectinatites wheatleyensis to lower pectinatites pectinatus ammonite zones of the swanworth quarry 1 core from the wessex basin dorset uk to time equivalent records from the ebberston 87 core in the cleveland basin yorkshire uk 400 km apart our synthesis shows that while the dorset sediments were deposited in an energetically more dynamic setting than the yorkshire sediments the overarching climatic and oceanographical processes responsible for variations in organic carbon enrichment and sedimentation were similar intervals of coeval organic carbon rich sedimentation occurred in both basins and a particulate shuttle was intermittently active in both basins consistent with recent climate simulations we conclude that tropical climate conditions associated with enhanced nutrient supply were key drivers of sedimentation between the jurassic wessex and cleveland basins

tracing bottom water redox conditions during deposition of lower and upper jurassic organic rich sedimentary rocks in the lusitanian basin portugal insights from inorganic geochemistry abstract the lower jurassic agua de madeiros am fm and vale das fontes vf fm formations and the upper jurassic cabacos formation cab fm are considered the most important units with petroleum source rock potential in the lusitanian basin and the likely sources for its widespread hydrocarbon occurrences nonetheless the bottom water redox conditions during their deposition remain debatable to resolve this controversy we investigated whole rock inorganic geochemical data of the most significant organic rich levels of the am vf and cab fms cropping out at basin scale significant variations in toc ts and redox sensitive elements contents and their ratios are evident within and between the organic rich levels of the studied units a positive covariation between primary productivity proxies toc enrichment and paleo redox proxies for the lower jurassic units clearly suggest an interdependence between enhanced primary productivity organic matter accumulation and bottom water redox conditions for the am fm the paleo redox proxies indicate that the black shales deposition took place beneath bottom waters that ranged from suboxic to strongly euxinic conditions with a clear predominance of the latter a toc threshold of 6 7 wt clearly separates samples deposited under suboxic to anoxic conditions from those deposited under true euxinic conditions and is interpreted as the minimum amount of organic matter om accumulation within sediments required to sustain h2s production at rates that outpaced its consumption through precipitation of sulphides and or om sulphurization allowing the diffusion of h2s to the bottom waters for the vale das fontes fm the paleo redox proxies suggest a prevalence of suboxic to anoxic bottom waters despite all black shales displaying toc contents greater than the above threshold the less reducing conditions relative to the am fm euxinic black shales are attributed to i a larger proportion of continental om ii lower reactivity of the om reaching the sediment water interface decreasing oxygen consumption and iii higher availability of reactive iron increasing the buffer effect to h2s produced through om remineralization by sulphate reducing bacteria both units display mo ef vs u ef covariation patterns that deviate from the field of sediments deposited under unrestricted open marine conditions towards higher mo enrichment relative to that of u indicating an accelerated transport of aqueous mo to the sediments consistent with the operation of a metal oxyhydroxide particulate shuttle linked to mn and fe redox cycling within the water column in a weakly restricted basin the mo cot regression line slope indicates a moderate degree of bottom water mass restriction with mo aq concentration of 60 70 of the present day seawater value and bottom water renewal times of 10 20 years akin to the present day cariaco basin the redox conditions of the cab fm organic rich levels are dependent on the location of the depositional environment in the basin with deposition taking place beneath oxygenated bottom waters at cabo mondego and under suboxic conditions at vale de ventos and pedrogao the latter denoting a larger degree of oxygen depletion than the former in this context the overall potential for om preservation should increase from cabo mondego to vale de ventos and pedrogao

annual layering in the upper jurassic kimmeridge clay formation uk quantified using an ultra high resolution sem edx investigation abstract this paper reports the results of an ultra high resolution investigation of the geochemistry of the late jurassic kimmeridge clay formation kcf using scanning electron microscope energy dispersive x ray analysis sem edx sub millimetre banding is revealed in which each band approximately 112 m thick comprises a couplet of coccolith rich and coccolith poor sediment major element analysis reveals that regularly spaced peaks in the relative amount of calcium are associated with coccolith rich bands it is argued that each couplet represents an annual layer the calcium peaks corresponding to an annual bloom of coccolithophorids there is also a broad correlation between the calcium aluminium ratio and the ratios of sodium aluminium phosphorus aluminium and potassium aluminium to calcium aluminium ratios we tentatively suggest that there may be a link here sodium phosphorous and potassium today being key nutrients for coccolith productivity aluminium is taken as representing the background terrigenous clay fraction if each banded couplet is indeed annual and if the thickness of such bands were to be representative of the kcf as a whole then the post compactional thickness is broadly in agreement with sedimentation rates of 100 m per year calculated by oschmann 1990 this study confirms the view that the kcf in southern england accumulated in a predominantly anoxic environment that favoured the preservation of organic material within the bottom sediment bundles of bands clustering in groups of about 8 10 are tentatively related to short term changes in insolation possibly sun spot cycles

redox productivity and paleotectonic studies in the southern tethyan margin of northern tunisia abstract in northern tunisia the cenomanian turonian sediments are characterized by organic rich material these deposits are commonly attributed to the bahloul formation and record the worldwide oae2 event in general the bahloul formation is related to paleogeographic and tectonic events these results suggest that multiple causes including primary productivity sea level changes climatically driven organic carbon burial and structural framework this formation is composed of three differentiated units the first unit u1 is composed of laminated limestones with small intercalations of marls caco 3 varies between 48 and 83 the second unit u2 is characterized by very thin beds of marls argillaceous and laminated limestones caco 3 ranging from 57 to 85 lastly the third unit u3 is represented by a set of thick black shales with some levels of marls isotopic 13 c carb study shows a large positive excursion in the bahloul formation it is characterized by three peaks that exceed 3 first and the second peaks are identified at the lower unit u1 with isotopic values recording 3 2 and 3 3 respectively third peak is recorded in the middle of the unit u2 and it attains 3 2 this section records important toc values reaching up to 3 84 in unit u1 with average values exceeding 3 the rock eval data shows a marine organic matter with kerogen type ii 429 max 435 c which can generate oils a significant enrichment of major and trace elements are also recorded which took place during the deposition of the bahloul formation in general high levels of p cu and ni indicate a high surface primary productivity which led to the accumulation of large quantities of organic matter on the sea floor

trace and rare earth element ree characteristics of mudstones from eocene pinghu formation and oligocene huagang formation in xihu sag east china sea basin implications for provenance depositional conditions and paleoclimate abstract the interpretation of depositional conditions provenance and paleoclimate provide important information to understand hydrocarbon source rock formation and distribution the concentrations of trace elements and rare earth elements rees of fifty mudstone samples from eocene pinghu formation and oligocene huagang formation in xihu sag east china sea basin were determined by inductively coupled plasma mass spectrometry icp ms to test the depositional conditions provenance and paleoclimate evidences from the visual comparisons of the simple bivariate plots diagrams and statistical comparisons of the mann whitney rank sum test suggest that there may be a common source for the pinghu formation and huagang formation mudstones sedimentary provenance from bivariate plots diagrams of hf versus la th and la sc versus co th shows that pinghu formation mudstones are mainly composed of mixed felsic basic source and acidic source whereas huagang formation is mainly acidic source additionally the ree vs la yb discrimination diagram shows that the property of the provenance for the pinghu formation mudstones consists sedimentary rock alkaline basalt and a small amount of continent tholeiitic basalts whereas that of the huagang formation is mainly alkaline basalt and sedimentary rock discriminant diagrams consisting of la th sc and th co zr 10 suggest that the mudstones of two formations were deposited in continental island arc and active continental margin tectonic setting redox proxies v ni v v ni and ce indicate that most of the mudstones of two formations were deposited in a relatively dysoxic anoxic condition the cool and humid climate represented by sr cu and rb sr ratios were prevailing in xihu sag during the eocene and oligocene period and the oligocene was cooler than eocene moreover five long term base level cycles siii1 siii5 and four sequence boundaries sb2 sb5 were recognized in the pinghu formation and huagang formation based on the previous research we find that the abnormal surfaces of geochemical parameters such as v ni sr ba u ti v ti ti and ree to some extent agree with sequence boundaries sb2 sb3 sb4 and sb5 in mudstones additionally the increment of ti and ree concentrations and the decrease of sr ba v ni u ti and v ti ratios above the sb4 indicates that terrigenous clast grew significantly and the sea level dropped after late eocene which is related with the late eocene tectonic movement

petrographic and geochemical study of organic matter in surficial laminated sediments from an upwelling system mejillones del sur bay northern chile a study of recent laminated sediments accumulated in the oxygen minimum zone of mejillones bay northern chile shows that the organic matter is autochthonous and deposited under varying oceanographic conditions sedimentary units dominated by light laminations have low values of total organic carbon total nitrogen and sulphur and chaotoceros content coupled with high values of yellow amorphous organic matter these were deposited under conditions of low primary productivity favoring the recycling of the major part of metabolizable organic matter in the water column and the accumulation only of non metabolizable organic matter in the bottom sediments in contrast a unit rich in dark laminations with high values of total organic carbon total nitrogen and sulphur shows high values of brown and black aom and chaetoceros content this sedimentary unit reflects increased flocculation of organic particles during a period of high productivity and fast transit of organic material through the column water thus diminishing the recycling of metabolizable organic matter and its accumulation with non metabolizable organic matter in the sediments finally a unit characterized by presence of both light and dark laminations was formed by abrupt alternations of the two oceanographic states described previously

comprehensive evolution of a petroleum system in absolute time the example of brynhild norwegian north sea abstract petroleum systems are inherently complex comprising multiple components and processes which variably interact in space and time to form petroleum accumulations improved understanding of these complex systems requires a holistic approach of integrating diverse geological geophysical and organic and inorganic geochemical data here we present an applied and comprehensive re os geochronological and geochemical study on the brynhild petroleum system from the norwegian north sea essential components of this system include shale source rocks of the mandal and farsund formations reservoir sandstones of the ula formation and their bitumen extracts and produced brynhild crude oil rock eval trace metal and re os data reveal the excellent oil generation potential of the shaley source rocks high contents of organic matter containing large amounts of type i kerogens and constrain their depositional conditions anoxic bottom waters in a high productivity regime driven by enhanced supply of land derived nutrients seven re os shale ages between 146 and 134 ma are consistent with existing volgian ryazanian age estimates and constrain the initial 187os 188os of the shales to 0 52 in the lowermost farsund formation and 0 75 in the uppermost mandal formation solvent extracted reservoir rocks of the ula formation contain insoluble organic matter with type iii type iv kerogens likely supplied by erosion of older a precise 179 ma re os age with unrealistically low initial 187os 188os coal bearing sediments or organic rich shales the brynhild crude oil has relatively low 187os 188os and a generation age of 44 ma based on an asphaltene only re os isochron maltenes from the brynhild crude oil are isotopically affected by interaction with organic rich source rocks that represent the seal for the brynhild oil field compared with the crude oil bitumen from ula formation sandstones have contrasting re os properties high 187os 188os ratios and young generation ages between 8 and 0 ma based on asphaltene bitumen maltene re os triplets asphaltene only re os regressions and re os model ages for components with extremely high 187re 188os the 44 ma and 8 0 ma re os ages agree well with regional burial models that indicate oil generation from 44 ma to present day a genetic link between crude oil and bitumen is indicated by their similar alkane patterns v and ni contents and v ni and pristane phytane ratios modeling of 187os 188os for individual organic components through time shows that shales of the mandal formation developed 187os 188os ratios that match those of the crude oil and bitumen at 44 ma and 8 ma respectively and therefore confirm the source for both hydrocarbon phases we suggest that bitumen formed during in reservoir mixing of a 44 ma old early mature asphaltene poor oil brynhild crude oil with an 8 ma old asphaltene rich oil

proliferation of shallow water radiolarians coinciding with enhanced oceanic productivity in reducing conditions during the middle permian south china evidence from the gufeng formation of western hubei province the middle permian is a critical period when a possible cooling regime prevailed and massive deep water cherty sediments were deposited in the south china craton an integration of radiolarian paleoecologic and geochemical studies from the middle permian gufeng cherty succession of western hubei province south china tests the relationships among oceanic productivity redox conditions and radiolarian responses as well as origin of the cherts a total of 21 species and six undetermined species in 10 radiolarian genera are identified from the gufeng formation in the northern margin basin of the yangtze platform of the south china craton three radiolarian pseudoalbaillella globosa follicucullus monacanthus and follicucullus scholasticus zones were established and constrained the gufeng formation as roadian to middle capitanian in age the gufeng cherts embrace a biologic and hydrothermal origin in the northern marginal basin and southern margin basin the nanpanjiang basin of the south china craton respectively the high oceanic primary productivity in surface oceans resulted in the widespread reducing conditions in the bottom water column such high oceanic productivity event slightly predated the middle permian kamura cooling event of the panthalassian ocean both the elevated continent weathering due to pre eruptive crustal uplift of the emeishan flood volcanism and active oceanic upwelling may have stimulated high oceanic productivity and facilitated the deposition of the gufeng cherts all radiolarians bloomed in relatively oxic conditions only shallow water radiolarians mainly spherical forms proliferated in the reducing conditions which also coincided with an enhanced oceanic productivity the abundance of spherical radiolarians therefore is an ideal proxy indicating oceanic primary productivity

dynamic evolution of the ediacaran ocean across the doushantuo formation south china abstract oceanic redox conditions and atmospheric oxygenation likely played a crucial role in the evolution of macroscopic multicellular eukaryotes during the early ediacaran period however the oxidation mechanism and magnitude of the ediacaran ocean atmosphere system remain controversial to constrain the oceanic redox conditions and the contemporaneous oxygenation of the atmosphere we present a comprehensive investigation of redox sensitive elements e g mo and u fe speciation and s isotopes of pyrite from the platform slope and basin sections across the doushantuo formation in south china our results suggest a redox stratified ediacaran ocean with a fluctuating chemocline from the slope to platform location across the doushantuo formation in particular euxinic intermittently euxinic conditions developed not only at the platform and slope but also in the deep basin furthermore these euxinic conditions indicate that high sulfate concentrations may have accumulated not only at the ending of the ediacaran doushantuo formation but also at the middle and beginning thus these results suggest that the extensive euxinic conditions associated with the continuous oceanic sulfate input were in response to progressive oxygenation of the atmosphere during the early ediacaran period integrated with previously published results if the dissolved organic carbon doc reservoir existed different mechanisms may be responsible for the oxidation of deep ocean in each part of the studied sections across the doushantuo formation one mechanism is oxidation by sulfate through a bacterial sulfate reduction bsr process under anoxic conditions another mechanism is oxidation by dissolved free oxygen under oxic suboxic conditions finally a dynamic evolution model of the ediacaran ocean atmosphere system across the doushantuo formation south china was suggested

enhancement of accumulation and anoxic degradation of organic matter controlled by cyclic productivity a model abstract the aim of the study is to determine the relative influences of the genetic sources the export efficiency and the anaerobic degradation of organic carbon as factors controlling short term cyclic organic rich sedimentation in areas of high marine productivity a cyclic organic sequence of the kimmeridge clay formation of yorkshire u k was studied as an example of ancient sedimentation this formation outcropping in the cleveland basin near marton is known as a lateral equivalent of the major oil source rock unit of the north sea on the basis of petrographic and geochemical results a new model is proposed to explain the short term cyclic sedimentation of organic carbon under continuously anoxic bottom conditions the model assumes that the cyclicity is primarily controlled by variations of mineral free phytoplanktonic productivity such variations are thought to induce changes in carbon recycling in the photic zone so that the export efficiency of the metabolizable organic matter is modified a high sulphate reduction intensity relative to the organic matter flux is observed for the highest productivity periods which also reflects an enhancement of export efficiency as the redox conditions did not change significantly such variations in sulphate reduction intensity must be attributed not only to quantitative variations in the export productivity but also to its qualitative variations with more metabolizable organic matter reaching the anoxic domain during high productivity periods biosedimentary processes depending on productivity such as aggregation are thought to play an important role in reducing the transit time of metabolizable organic matter to the sediment finally the model implies that the amplitude of export productivity variations probably were larger than that of toc variations recorded in sediment which are mainly due to biologically refractory organic matter from plankton

vanadium accumulation in carbonaceous rocks a review of geochemical controls during deposition and diagenesis abstract published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks reduction adsorption and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon dissolved vanadate v v species predominate in oxic seawater and are reduced to vanadyl ion v iv by organic compounds or h 2 s vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle the large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff vanadium rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate rich units a correspondence between vanadium content and organically bound sulfur is consistent with high activities of h 2 s during sediment deposition excess h 2 s exited the sediment into bottom waters and favored reduction of dissolved v v to v iv or possibly v iii the stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic phosphate concentrated to more reducing euxinic conditions that favor vanadium accumulation during diagenesis some vanadium organic complexes migrate with petroleum out of carbonaceous rocks but significant amounts of vanadium are retained in refractory organic matter or clay minerals as carbon in the rock evolves toward graphite during metamorphism vanadium is incorporated into silicate minerals

organic rich biolaminated facies from a kimmeridgian lagoonal environment in the french southern jura mountains a way of estimating accumulation rate variations abstract during kimmeridgian times the present day southern jura was a carbonate platform in the palaeolagoon at the village of orbagnoux organic matter was deposited giving birth to calcareous laminites with organic carbon reaching 12 three facies may be distinguished two types of laminites are encountered one made of parallel laminae and the other made of undulating laminae the laminites alternate with massive limestone beds in which no internal structure may be seen the lithology sedimentology and the faunal floral and organic content of laminites and of massive limestones was thoroughly investigated at different scales using various methods field observation transmitted light and scanning electron microscopy rock eval pyrolysis biomarkers infrared study of kerogen palynofacies observation it is suggested that the different types of laminites parallel or undulating may reflect variations in the accumulation rates linked with the carbonate and organic matter productions undulating laminae would correspond to periods of lack of sedimentation whereas parallel laminae would reflect alternations of periods of sedimentation and periods of non sedimentation these variations might in turn reflect climatic changes affecting the emerged lands supplying the lagoon with detrital products organic or inorganic the accumulation and preservation of organic matter were made possible by the development of microbial mats building oxygen tight barriers at the water sediment interface these barriers separated overlying oxygenated water from anaerobic sediments these phenomena may explain one way in which potential hydrocarbon source rocks can form under a poorly oxygenated water column

a paleoceanographic approach to the kimmeridge clay formation the upper jurassic lower cretaceous kimmeridge clay formation kcf is northern europe s premier source rock and can be understood using a new but relatively simple oceanographic model this explains or accommodates most current observations about the kcf and its depositional environment and draws upon paleogeographic paleoclimatic geochemical sedimentological and paleontological evidence

sedimentary corg p ratios paleocean ventilation and phanerozoic atmospheric po2 the corg p ratios of organic rich facies tocn1 exhibit considerable variation through the phanerozoic from b10 1 in the permian phosphoria formation to n1000 1 in some devonian black shales relative to the composition of phytoplankton c p 106 1 which provide the bulk of organic c and p to organic rich marine sediments the range of phanerozoic sedimentary corg p ratios largely reflects the influence of benthic redox conditions on diagenetic pathways of c and p remobilization enhanced preservationoforganiccatlowerredoxpotentials combinedwithenhancedsedimentaryretentionofremineralizedorganicpathigher redox potentials results in a strong relationship between benthic redox conditions and sedimentary corg p ratios the strong secular coherence of the phanerozoic corg p record i e limited variation within narrow time slices suggests that these ratios are controlled primarily by factors influencing benthic redox conditions on a global scale such as atmospheric po2 inversion and scaling of the phanerozoic corg p record yields a new atmospheric po2 model that has strong similarities to existing models based on elemental and isotopic mass balances yet differs in some potentially significant respects in particular the new model suggests that atmospheric po2 wasmarkedlylowerduringtheearlytomiddlepaleozoicthanpreviouslyinferred duringmostofthedevonian atmospherico2levels mayhavebeensufficientlylow b13 astopreventthesustainedcombustionofplantmaterial thisfeatureofthemodelmayaccount for ananomalous gapinthefossilcharcoalrecordduringthedevonian theredox dependentburialofpintheoceanresults inpositive and negative feedback mechanisms that tend to stabilize atmospheric oxygen concentrations on geologic timescales temporal variations in the burial of p relative to c suggest that the strength of these feedback mechanisms varied over the phanerozoic 2007 elsevier b v all rights reserved

new insights into the origin of kabbaldurga charnockites karnataka south india abstract a detailed study of the structure and petrology of the rocks bordering the kabbaldurga type charnockites provides important constraints on the origin of these charnockites the structural elements register three phases of deformation and show a uniform pattern in the larger area a pattern consistent with the regional structure of the precambrian of southern karnataka in the kabbaldurga area however some of the earlier structures are poorly preserved yet there are vestiges of early folds described by banded layered charnockites as in the neighbouring kodamballi area and a consistent development of dilatant structures which can be related to the kinematics of deformation in the larger terrain at kabbaldurga the pegmatitic charnockites occur as veins of diverse orientation but they rarely follow the shear generated structures the metamorphic reactions invoked by previous workers to explain in situ transformation of gneiss to charnockite were based on chemical similarity of some close pairs but the petrographic and chemical variations in the pegmatitic charnockites and the peninsular gneisses at kabbaldurga quarry are compelling features which cannot be explained by the hypothesis of in situ transformation we have argued on the basis of rock and mineral chemistry that derivation of the pegmatitic charnockites by dehydration melting in metabasites offers a better explanation pressure temperature values at least 850 to 900 c 7 kbar obtained by us for the granulites of this area viewed against the results of experimental dehydration melting in basic rocks with hornblende and or biotite provide strong support for this model in the field leucosomes within the basic granulites of kabbaldurga are not uncommon the compositions of the pegmatitic charnockites tonalitic and granitic match those of the melts produced in experiments further the pattern of variation in the composition of hornblende and plagioclase in the basic granulites of the kabbaldurga area is compatible with extraction of melts this alternative model for the origin of the kabbaldurga charnockites is petrologically feasible and does not require either in situ transformation or structurally controlled growth which incidentally are not ubiquitous at kabbaldurga

a review on the huoqiu banded iron formation bif southeast margin of the north china craton genesis of iron deposits and implications for exploration abstract the huoqiu iron ore field in northwest anhui province is located in the north china craton ncc as a large banded iron formation bif iron ore field ore bodies occur in a middle high grade of neoarchean metamorphic formation forming a banded silicon iron series from north to south the main ore bodies can be divided into two sub belts from bottom to upper layers i e the a b ore belt consisting of leptynite schist magnetite quartz formation and the d ore belt consisting of schist marble hematite quartz formation based on a dataset from geological settings geophysical and geochemical exploration ore forming conditions and structural analysis of the iron deposit we discuss structural types sedimentary environments deep tectonic and ore controlling factors as well as characteristics and distribution of this colossal bif ore field in the huoqiu region using la icp ms techniques we obtained the oldest u pb age of ca 2 7 ga for plagioclase amphibolite as its original rock and 1 8 ga for magmatic granite in the huoqiu group the hf isotopes of zircon were also determined resulting in the oldest hf model age of 3 5 ga geochemical data indicate that the protolithes of amphibolites belong to a series of subalkaline rocks with enrichments of large ion lithophile elements and depletions of high field strength elements which are typical volcanic arc rocks the amphibolites have low k 2 o concentrations with low ratios of ti v 22 7 to 25 9 averaging 24 5 similar to island arc tholeiite this suggests that the iron deposit and bif are of the superior type in the huoqiu region

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

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member

environmental controls on jurassic marine ecosystems during global warming the fossil record has the potential to provide valuable insights into species response to past climate change if paleontological data are combined with appropriate proxies of environmental change here we use a novel multivariate approach that combines a suite of geochemical proxies with high resolution quantitative macroinvertebrate fossil data to study responses to early toarcian warming ca 183 ma we show that benthic and nektonic ecosystems became decoupled during warming and were driven by different environmental variables benthic turnover was mostly driven by variations in seawater dissolved oxygen concentration whereas turnover among the nektonic cephalopods was primarily controlled by variations in weathering nutrient runoff and primary productivity although rapid warming has been invoked as the main trigger of this event the paleotemperature proxy was a poor predictor of marine community dynamics and abiotic factors indirectly linked to temperature were more important given that similar environmental changes characterize other episodes of global warming and are impacting present day marine communities we predict that similar ecological responses occurred during other past events and are a probable outcome of current changes

timing and tectonic setting of the sijiaying banded iron deposit in the eastern hebei province north china craton constraints from geochemistry and sims zircon u pb dating the north china craton ncc one of the oldest continental blocks in asia has a complicated evolutionary history with the age of the old crust up to 3 8 ga and records the important geological events of the earth the sijiaying bif the largest banded iron formation located in eastern hebei province in the east part of ncc records an important thermo tectonic event previous studies focused on the geological description but the timing and tectonic setting have been unclear so far the sijiaying bif is hosted in neoarchean metamorphic rocks which includes biotite leptynite hornblende plagioclase gneiss and biotite plagioclase gneiss using major element contents and ratios of the host rocks the protoliths of the hornblende plagioclase gneiss and biotite plagioclase gneiss are shown to be dacite whereas those of the biotite leptynites are sedimentary rocks based on geology and geochemistry of the host rocks we infer that the sijiaying bif is an algoma type deposit sims zircon u pb dating shows igneous zircons from the biotite plagioclase gneiss and hornblende plagioclase gneiss have u pb ages of 2535 8 ma and 2543 14 ma respectively zircons from the biotite leptynite have an age of 2537 13 ma we infer that the ages of 2543 2535 ma represent the time of the sijiaying bif paas normalized rey profiles of the ore samples are characterized by lree depletion and hree enrichment positive la and eu anomalies and an average y ho weight ratio of 32 indicating a mixture of submarine hydrothermal fluids and deep seawater ree data was combined with the 18o analyzed by previous researcher of the ore from individual magnetite bands to infer that the sijiaying bif precipitated from hydrothermal fluids discharging on the sea floor the hornblende plagioclase gneiss and biotite plagioclase gneiss that are enriched in lile and lree and depleted in high field strength elements hfse nb ta and ti have geochemical signatures that are similar to those of island arc volcanic rocks the biotite leptynite shows the characteristics of continental island arc based on the geology and geochemistry of associated wall rocks and ore samples we suggest that the sijiaying bif formed in a back arc basin in neoarchean

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

anoxia and fluctuating climate recorded from the devonian carboniferous black shales tethys himalaya india a multi proxy approach due to inaccessibility and tough conditions high altitude low oxygen region very less information is available about the indian tethys himalayan sequences particularly geochemical data are scarce an attempt is made in the present study to evaluate the sediment geochemistry and paleo redox conditions of devonian carboniferous black shales from the spiti region tethys himalaya using multi proxy approach besides black shales this paper also provides the first in depth study of the geochemistry of phosphatic nodules hosted by the black shales that will allow primary and secondary processes to be understood geochemical investigations particularly redox sensitive trace element ratios for e g v v ni 0 5 ni co 4 3 7 2 v cr 2 and positive ce anomalies suggest deposition of these black shales under sub oxic to anoxic marine conditions thus recording an ocean anoxic event in the region these ocean anoxic events of devonian carboniferous spiti black shales which possess distinctive geochemical signatures can very well be correlated with global events such as hangenberg events and the lower alum shale event the span of the chemical index of alteration cia values 54 78 variation of 4si parameter 82 3 and c values 0 1 0 6 recorded in the black shales strongly indicate varying degrees of weathering conditions in the source area which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in devonian carboniferous period in the spiti region the distinct geochemical characteristics such as y sr and rare earth element ree enrichments particularly middle ree enrichments with a hat shaped pattern with high p2o5 contents up to 14 wt distinguish the nodules from the host rocks black shales sem scanning electron microscope results supplemented by xrd x ray diffraction studies of the nodules show the dominance of calcium silica and phosphatic phases the diversity of clay minerals significant variations in mineralogical composition and lack of any continuous vertical trend in the nodules suggest a relatively low impact on the primary environmental signal by burial diagenesis

major elements trace hydrocarbon sources in over mature petroleum systems insights from the sinian sichuan basin china abstract identifying hydrocarbon sources in over mature petroleum systems is important and challenging here we present a pilot study using inorganic geochemistry to trace hydrocarbon sources in the giant natural gas accumulations within the sinian neoproterozoic dengying formation in the sichuan basin sw china we investigated the major elements ti al fe mn mg ca na k and p as tracers of hydrocarbon sources for the first time our results show that the over mature solid bitumen in the reservoirs contains parts per million amounts of major elements in addition to carbon i e organic matter na and k in the bitumen occur mainly in the v ni porphyrin form al fe and mn are present in the fe porphyrin form the behavior of p is relatively complex and it may reside mainly in a non porphyrin organic form the other elements ti mg and ca may be present as emulsified invisible nano minerals the mn fe ratio and ternary 10mn fe p diagram can be used for hydrocarbon source rock correlations based on the proxies it is suggested that the lower cambrian qiongzhusi formation was the main source of hydrocarbons in the dengying formation our data suggest that inorganic geochemistry is an effective method for the study of over mature hydrocarbons and complements traditional organic geochemical approaches

pyritization of trace metals in anoxic marine sediments the pyritization of reactive trace elements in different anoxic marine sediments was investigated to determine the importance of factors such as h2s reactive fe pyrite content and salinity in controlling this process the areas studied included anoxic sulfidic sediments baffin bay a hypersaline coastal lagoon from texas and green canyon a hemipelagic oil seepage area anoxic nonsulfidic sediments with high sedimentation rates atchafalaya bay mississippi delta system hemipelagic anoxic nonsulfidic sediments with low sedimentation rates gulf of mexico shelf and slope and orca basin an euxinic hypersaline basin and organic rich marsh sediments atchafalaya bay results indicate that the degree of trace metal pyritization dtmp of all trace metals except cd increased with increasing degree of pyritization dop irrespective of the type of sedimentary environment involved however for as hg and mo the dtmpdop values were generally above the 1 1 ratio line whereas the transition metals co cu mn and ni displayed a close to linear increase in dtmp with dop and moderate incorporation into pyrite chromium and the class b metals pb and zn were also gradually incorporated into the pyrite phase but without reaching the dtmp levels exhibited by the transition metals cadmium was not incorporated to a significant extent these results are consistent with the chemical attributes of these different classes of trace elements availability of dissolved trace metals and organic matter content are apparently important factors controlling the incorporation of co cr cu and ni into pyrite in anoxicsulfidic fe poor and h2s rich environments

assessment of metal concentrations in lake sediments of southwest japan based on sediment quality guidelines geochemical analysis of the bottom sediments of lakes banryoko onbe hamahara and kijima in shimane prefecture of southwest japan was carried out to determine their metal concentrations and to assess the potential for ecological harm by comparison with sediment quality guidelines the work conducted includes water quality measurement and analyses of trace and major elements and rare earth element ree of sediments results showed that water quality of the lakes contrasts slightly between their upper and lower parts average abundances of as pb zn cu ni and cr in banryoko sediments were 27 33 90 27 25 and 46 ppm respectively compared to 31 52 175 44 44 and 75 ppm at onbe 11 26 96 13 13 and 35 ppm at hamahara and 24 43 193 31 12 and 30 ppm at kijima these concentrations exceeded the lowest effect level that has moderate impact on aquatic organisms as proposed by the new york state department of environmental conservation pb and cu abundances are comparable to the coastal ocean sediment database threshold while as and zn exceed this value indicating the concentrations of these metals are potentially toxic increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments anthropogenic sources and surface soil erosion the ree patterns of sediments in the study areas are broadly comparable to the average upper continental crust but show some contrast between lakes due to differing source litho type significant positive correlations between fe2o3 and as pb zn and cu were found in the sediments suggesting these metals may be adsorbed on fe oxides in the lake sediments

distribution of arsenic and other trace elements in the holocene sediments of the meghna river delta bangladesh geochemical study of the holocene sediments of the meghna river delta chandpur bangladesh was conducted to investigate the distribution of arsenic and related trace and major elements the work carried out includes analyses of core sediments and provenance study by rare earth element ree analysis results showed that the cores pass downward from silty clays and clays into fine to medium sands the uppermost 3 m of the core sediments are oxidized average oxidation reduction potential orp 230 mv and the orp values gradually become negative with depths 45 to 170 mv indicating anoxic conditions prevail in the meghna sediments the ree patterns of all lithotypes in the study areas are similar and are comparable to the average upper continental crust arsenic and other trace elements pb zn cu ni and cr have greater concentrations in the silts and clays compared to those in the sands positive correlation between as and fe was found in the sediments indicating as may be adsorbed on fe oxides in aquifer sediments

sequence stratigraphical interpretation of organic facies variations in marine siliciclastic systems general principles and application to the onshore kimmeridge clay formation uk

geochemical study of arsenic and other trace elements in groundwater and sediments of the old brahmaputra river plain bangladesh the geochemical study of groundwaters and core sediments from the old brahmaputra plain of bangladesh was conducted to investigate the distribution of arsenic and related trace elements groundwaters from tube wells are characterized by ph of 6 4 7 4 dissolved oxygen do of 0 8 1 8 mg l ca contents of 5 50 mg l and fe contents of 0 2 12 9 mg l arsenic concentrations ranged from 8 to 251 g l with an average value of 63 g l a strong positive correlation exists between as and fe r2 0 802 p 0 001 concentrations in groundwater the stratigraphic sequences in the cores consist of yellowish silty clays at top passing downward into grayish to yellowish clays and sands the uppermost 3 m and lower parts from 13 to 31 m of the core sediments are oxidized average oxidation reduction potential orp 170 and 220 mv respectively and the orp values gradually become negative from 3 to 13 m depths 35 to 180 mv indicating that anoxic conditions prevail in the shallow aquifers of the brahmaputra plain age determinations suggest that clay horizons at 10 m depth were deposited at around 2 000 and 5 000 years bp 14c ages during the transgressive phase of sea level change elevated concentrations of as pb zn cu ni cr and v are present in the silts and clays probably due to adsorption onto clay particles significant concentrations of as occur in black peat and peaty sediments at depths between 9 and 13 m a strong positive correlation between as and fe was found in the sediments indicating as may be adsorbed onto fe oxides in aquifer sediments

chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae origin of ultralaminae abstract new structures termed ultralaminae were recently observed by transmission electron microscopy usually in high amounts in a number of kerogens from oil shales and source rocks morphological similarities were noted between ultralaminae and the thin ca 15 nm resistant outer walls composed of non hydrolyzable macromolecules algaenans commonly occurring in extant chlorophyceae especially in the cosmopolitan genus scenedesmus identification of the pyrolysis products of s quadricauda algaenan showed i a highly aliphatic structure based on a macromolecular network of long up to c32 polymethylenic chains probably cross linked by ether bridges and ii a close correlation based on the formation of n alkylnitriles between this algaenan and two ultralaminar kerogens the rundle oil shale mainly composed of ultralamina accumulations and the green river shale ultralaminae dispersed within an amorphous matrix these fossil ultralaminae therefore likely originated from the selective preservation of the thin algaenan containing outer walls of scenedesmus and or of other chlorophyceae containing outer walls of a similar morphology and composition relative distributions of n alkylnitriles and of n alkanoic acids in the pyrolysates of s quadricauda algaenan and of the rundle oil shale indicated that nitriles are not derived from secondary reactions of carboxylic acids but originate from preexisting nitrogen functions likely amides previous evidence of kerogen formation via selective preservation of algaenans was restricted to rather uncommon kerogens the present results added to ultralamina common occurrence and abundance point to a wide involvement and to a large contribution of the selective preservation of algaenan containing thin outer walls of chlorophyceae in the formation of kerogens in a number of lacustrine source rocks and oil shales all the available information suggest that the three way correlation selective preservation of algaenans from thin resistant chlorophycean outer walls formation of fossil ultralaminae presence of n alkylnitriles with a typical distribution in the pyrolysates of ultralaminar kerogens observed in this study might be of a general character

multiple controls on the preservation of organic matter in devonian mississippian marine black shales geochemical and petrographic evidence geochemical and petrographic analysis of an interval of black shales between the huron shale member of the new albany shale upper devonian famennian and the sunbury shale lower mississippian tournaisian suggests that multiple controls influenced the accumulation of organic matter stratigraphic trends were determined following detailed analysis of a core from the central appalachian basin d6 montgomery county ky specifically c s fe relationships mo v v ni ni co and v cr were used to assess paleo redox conditions during sediment accumulation the role of clastic input was evaluated using ti al k al and si al and c p was used as a proxy for paleoproductivity the terrestrial to marine maceral ratio t m vitrinite plus inertinite alginite plus bituminite was used to evaluate changes in om source c s fe relationships and trace element ratios suggest anoxic conditions prevailed during deposition of much of the upper part of this interval however bottom water conditions were intermittently dysoxic during accumulation of the huron shale member and possibly dysoxic to marginally oxic during accumulation of the lowermost huron shale member productivity appears to have been an important factor in the accumulation of corg and high c p ratios are consistent with a productivity anoxia feedback mechanism in addition nutrients from increased terrestrial weathering may have contributed to enhanced productivity variations in sediment influx may also have influenced corg concentrations through sediment starvation one of the more significant observations in this work is an increase in terrestrial om inertinite and vitrinite up section which may reflect vegetative changes occurring on land during this time interval most of this increase is due to inertinite much of which appears to be fusinitic in origin and hence may be fossil charcoal this latter observation has implications for the expansion of land plants into drier niches and for late devonian atmospheric oxygen levels introduction of more resistant terrestrial om to anoxic basins may have been an important factor in the preservation of high carbon 10 intervals especially in the cleveland member of the new albany shale upper devonian famennian and the sunbury shale lower mississippian tournaisian

carbon sulfur iron systematics of the uppermost deep water sediments of the black sea box cores recovered during leg 4 of the 1988 r v knorr black sea oceanographic expedition from deep water regions of the basin were dominated by coccolith rich microlaminated unit 1 sediment and muddy gray turbidite layers both organic carbon oc and pyrite sulfur values for unit 1 display narrow ranges with mean concentrations of 5 3 1 1 1 wt and 1 3 0 3 wt respectively unit 1 is not enriched in pyrite s relative to sediments deposited under oxygenated bottom waters normal marine sediments with comparable oc concentrations carbon sulfur relationships evaluated on a calcium carbonate free basis to avoid spurious correlations resulting from dilution effects demonstrate that oc and pyrite s are essentially decoupled these observations combined with the persistence of elevated pore water sulfide to depth and a strong correlation between pyrite s and the detrital fe component argue strongly for limitation of pyrite formation in unit 1 by the availability of reactive fe unit 1 fe limitation is further indicated by degree of pyritization dop studies a measure of the extent to which the original potentially reactive fe has been transformed to pyrite these studies show sulfidation of reactive fe ranging from 57 to 78 with dop values independent of oc concentration unit 1 dop profiles suggest that the majority of the pyrite is formed in the sulfidic water column and or very close to the sediment water interface pyrite s concentrations of unit 1 when compared with the particulate reduced sulfur fluxes measured in time series sediment traps are compatible with predominantly water column pyrite formation because of the limitations in the supply of reactive fe associated with the comparatively high supply of oc the microlaminated sediment is characterized by c s ratios greater than those typical of holocene oxically deposited sediments the turbidite muds of the deep basin display high reduced s values relative to holocene normal marine sediments in samples with low oc concentrations low c s ratios this reflects pyrite formation under anoxic sulfidic bottom water conditions in a probable upper slope source region as well as during transport and final deposition intermediate dop values for the turbidites in part a product of their rapid rate of deposition reveal that fe limitation is ultimately not a factor and that further pyrite should form during burial however the very rapid rate at which unit 1 pyrite forms suggests fundamental differences between the overall reactivities of the fe phases associated with the microlaminated and turbiditic sediments the signature of water column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high dop values of unit 1 and the comparatively high levels of s associated with the low concentrations of oc of the turbidite muds this agrees with similar conclusions based on studies of ancient sedimentary rocks

geochemical characteristics of the silurian shales from the central taurides southern turkey organic matter accumulation preservation and depositional environment modeling abstract the silurian age organic rich and organic lean shales located within aydincik gulnar and silifke districts of mersin province in central taurides southern turkey is considered as an unconventional resources these organic rich and lean shales are controlled by the similar geological tectonic and sedimentary processes as the passive margin basin has acted as the provenance region throughout the deposition of these shales geochemical data of major and trace elements obtained for a total of 39 outcrop samples were interpreted to determine the depositional model water column productivity amount of organic matter and preservation conditions total organic carbon toc values of these organic rich and lean shales range from 0 56 to 4 06 wt and 0 01 to 0 38 wt several redox indicators such as v systematics v sc v ni as well as ratios of ni co th u and mo mn showed that the organic rich shales were deposited in suboxic to anoxic environments as opposed to oxic dysoxic conditions for organic lean ones however organic matter enrichment is not restrained by water column productivity as indicated by a lack of correlation between toc and productivity indexes p ti and ba al but rather probably controlled by dysoxic anoxic water column environment in addition a detrital material input cannot be ignored for its influence on organic matter enrichment in the shale deposition the increase in the clastic fluxes probably resulted from the deposition in the extended shelf with coastal upwelling a lack of correlation between k2o al2o3 sio2 and toc contents among these shales imply that clay minerals may not influence organic matter preservation according to w la n w yb n and ree values a high sedimentation rate is determined during the shale deposition which would result in a dilution of organic matter furthermore slight marine influences with sulphur limitation and low salinity semi arid warm to humid warm climatic conditions are widespread according to total sulphur ts sr ba sr cu chemical index of alteration cia c value and related discrimination diagrams in the end a depositional model of the central taurides shales is established to display an excellent preservation condition as the major controlling factor for organic matter enrichment in the silurian shales

present environment of dam lake sambe southwestern japan a geochemical study of bottom sediments geochemical analyses of lakebed and core sediments from lake sambe on the outskirts of oda city in shimane prefecture in southwestern japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements the lake water showed a stratified condition with respect to dissolved o2 and as fe and mn concentrations in the bottom layers which increased in the summer the chemical composition of the sediments as measured by x ray fluorescence included major and trace elements p ca sc ti v cr fe ni cu zn as sr zr pb and th and total sulfur ts elevated values of as zn v fe p and ts were present in several layers of the upper cores from 0 to 5 cm and other surface sediments increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments fine grained organic rich sediments and post depositional diagenetic remobilization moreover correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto fe oxy hydroxides whereas correlations with sulfur indicate that they were precipitated as fe sulfides the average abundances of as pb zn and cu exceeded the lowest effect level and interim sediment quality guideline values that the new york state department of environmental conservation and the canadian council of ministers of the environment determined to have moderate impact on aquatic organisms in addition concentrations of as and zn exceeded the coastal ocean sediment database threshold value indicating potentially toxic levels therefore the presence of trace metals in the lake sediments may result in adverse effects on biota health

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

formation of syngenetic and early diagenetic iron minerals in the late archean mt mcrae shale hamersley basin australia new insights on the patterns controls and paleoenvironmental implications of authigenic mineral formation a section through the late archean mt mcrae shale comprising in ascending order a lower shale interval lsi a banded iron formation bif an upper shale usi and a carbonate c1 has been analyzed for total fe and al contents and authigenic fe present as carbonate oxide sulfide and silicate phases the authigenic mineralogy is controlled by the episodic addition of fe from hydrothermal activity and removal of fe by sulfide relative to rates of clastic sedimentation the lsi and bif have mean fet al values of 2 and 5 respectively that record iron enrichment from hydrothermal sources iron was precipitated primarily as siderite accompanied by fe rich chlorite from anoxic bottom waters rich in dissolved fe pyrite formation was probably limited by the availability of sulfate which was present at low concentrations and became rapidly depleted the usi has generally lower fet al values 0 6 1 3 similar to those found in paleozoic shales with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle this interval contains authigenic fe predominantly as pyrite where high values for dop 0 8 indicate the existence of a water column that became rich in dissolved sulfide euxinic when sulfate concentrations increased due to a transient or secular increase in ocean atmosphere oxygenation high concentrations of dissolved sulfide maintained low concentrations of dissolved fe which allowed only minor amounts of fe to be precipitated as carbonates and silicates the usi also has elevated concentrations of organic matter that most probably reflect increased productivity and likely limited euxinia to midportions of the water column on the basin margin the carbonate c1 represents a basinal chemistry where sulfide has been removed and fet al values are 1 indicating that hydrothermal activity again produced dissolved fe rich bottom waters detailed iron speciation of the mt mcrae shale can be used to recognize spatial and temporal variations in iron and sulfur inputs to the late archean hamersley basin just prior to the paleoproterozoic rise in atmospheric oxygenation and our refined methods have relevance to all fe rich deposits

paleo ocean redox environments of the upper ordovician wufeng and the first member in lower silurian longmaxi formations in the jiaoshiba area sichuan basin a study of paleo ocean redox environments is important for understanding the deposition of black shale and has practical implications for shale gas exploration here we selected a total of 52 shal

geological and geochemical examination of arsenic contamination in groundwater in the holocene terai basin nepal geological and geochemical study has been carried out to investigate arsenic contamination in groundwater in nawalparasi the western terai district of nepal the work carried out includes analyses of core sediments provenance study by rare earth elements analyses 14c dating and water analyses results showed that distribution of the major and trace elements are not homogeneous in different grain size sediments geochemical characteristics and sediment assemblages of the arsenic contaminated nawalparasi and uncontaminated bhairahawa area have been compared geochemical compositions of sediments from both the areas are similar however water chemistry and sedimentary facies vary significantly extraction test of sediment samples showed significant leaching of arsenic and iron chemical reduction and contribution from organic matter could be a plausible explanation for the arsenic release in groundwater from the terai sediments

orbitally induced limestone marlstone rhythms in the albian cenomanian cismon section venetian region northern italy sedimentology calcareous and siliceous plankton distribution elemental and isotope geochemistry abstract a multidisciplinary study of the upper albian cenomanian portion of the cismon section venetian region northern italy was undertaken in order to characterize the cyclic alternations of carbonate rich and carbonate poor layers and to investigate their possible origin and cyclic patterns limestone semicouplets are characterized by abundant radiolarians and micarbs micron sized calcitic fragments common planktonic foraminifera strong bioturbation good oxygenation as expressed by the mn and v v ni parameters high si al ratio low k al in the absence of pyrite and organic matter the marlstone semicouplets are on the contrary frequently laminated rich in pyrite and organic matter with common fish remains in the absence of radiolarians the mn and v v ni parameters indicate that the marlstone deposition occurred in dysoxic to anoxic conditions which allowed the preservation of organic matter the low si al ratio and high k al ratio the latter related to abundant feldspars indicate that marlstones received a substantial contribution from a terrigenous source the abundance of biogenic silica suggests that during limestone deposition surface waters were rather fertile due to an efficient recycling of nutrients from deeper waters the enhanced fertility was coupled with an efficient current system at the welloxygenated sea floor which prevented the organic matter from accumulating during marlstone deposition surface waters were less fertile than in the limestone semicouplets but still within the mesotrophic spectrum as indicated by the presence of fertility index species among calcareous nannofossils whereas the bottom waters were oxygen depleted to accommodate fertility at the surface with stagnation at the bottom the scenario implies an enhanced stratification caused by an increase in freshwater runoff monitored by 18o depleted values in the meantime the water discharge supplied the nutrients to the surface waters necessary for the calcareous nannofossils to proliferate the cyclic organization of limestone marlstone couplets as revealed by power spectral analysis seems to represent climatic cycles the alternating conditions are 1 efficient mixing highly fertile surface and near surface waters and a well oxygenated seafloor to 2 enhanced water runoff or decreased evaporation or both and oxygen depleted bottom waters we suggest that this deterministic sedimentary cyclicity could be explained by periodic orbital climatic cycles in tune with the milankovitch cycles within this scenario the latest cenomanian bonarelli level is an exceptional event marked by a large positive 13c spike it is characterized by high to very high fertility conditions in surface waters caused by increased upwelling that engendered high nutrient levels in near surface waters the enhanced production of organic matter was in excess for the oxygen available at the bottom floor thus allowing the organic matter to accumulate in the sediments

the geochemistry of dsdp sediments from site 223 indian ocean abstract detailed major and trace element chemistry is presented for 41 sediment samples from dsdp site 223 borehole cores a marked change in chemical and mineralogical character is shown at the end of the early miocene epoch which relates to tectonic events and associated changes in sedimentary regime enrichment in the contents of such elements as mg cr and ni compared with average values for fine grained sediments occurs throughout the sequence and is particularly marked in the upper group of samples a basic ultrabasic provenance is suggested the oman ophiolites leaching with combined acid reducing agent indicated typical lithogenous character ordering for the elements and emphasised the enrichment of mg cr ni and li cu zn pb fe and ti over values for near shore muds and terrigenous material factor analysis on the bulk chemical data identifies the main lithogenous and biogenous components subdividing the latter it separates the upper and lower group of chemically dissimilar sediments and delineates a mn hydroxide phase it also shows the essentially independent roles of na ba and p

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

a molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the kimmeridge clay facies palaeoenvironmental implications abstract structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite a bituminous shale and an oil shale of the kimmeridge clay formation kcf in dorset were studied in order to gain insight into i the type and extent of water column anoxia and ii changes in the concentration and isotopic composition of dissolved inorganic carbon dic in the palaeowater column the samples studied fit into the curve of increasing 13 c of the kerogen 13 c toc with increasing toc reported by huc et al 1992 their hypothesis that the positive correlation between toc and 13 c toc is the result of differing degrees of organic matter om mineralisation in the water column was tested by measuring the 13 c values of primary production markers these 13 c values were found to differ on average by only 1 among the samples implying that differences in the extent of om mineralisation cannot fully account for the 3 difference in 13 c toc the extractable om in the oil shale differs from that in the other sediments due to both differences in maturity and differences in the planktonic community these differences however are not likely to have significantly influenced 13 c toc either all three sediments contain abundant derivatives of isorenieratene indicating that periodically euxinia was extending into the photic zone the sediments are rich in organic sulfur as revealed by the abundant sulfur compounds in the pyrolysates the prominence of c 1 c 3 alkylated thiophenes over n alkanes and n alkenes is most pronounced in the pyrolysate of the sediment richest in toc this suggests that sulfurisation of om may have played an important role in determining the toc 13 c toc relationship reported by huc et al 1992

geochemistry of eastern mediterranean sediments primary sediment composition and diagenetic alterations abstract two cores recovered in the eastern mediterranean were analysed for major minor and trace elements the primary chemical composition of the sediment is different at each location probably because the lithological sources and the relative biogenic contributions differ carbonates are important for the concentration of ca mg and sr whereas aluminosilicates determine the concentration of si al k li y and be and to a lesser extent that of fe cr ti mg zn and zr in sapropels organic carbon and sulphur seem to be closely related bromine mo p fe v cu zn co ni and cr are closely associated with organic and sulphidic compounds the concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation followed by a gradual transition back to normal conditions the primary composition is overprinted by diagenetic processes sulphate reducing conditions occurred during and just after sapropel deposition a progressive oxidation front mechanism which became active after sapropel deposition is responsible for additional major geochemical changes this diagenetic phenomenon has strong implications for the chemistry of fe mn ni co zn cu cr v u as and sb

diagenesis of the kimmeridge clay formation onshore uk study of sedimentology carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the kimmeridge clay formation indicates strong lateral variations in the depositional and early diagenetic processes reflecting the basin and swell topography of the depositional basin the mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate rich with thin poorly developed oil shales they were deposited under dysaerobic conditions with consequent poor organic matter preservation early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions the shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite smectites are present sediments from other locations comprise black organic rich mudstones the thick beds and good organic matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment these indurated mudstones contain a few shell fragments and much pyrite indicating intense reaction in the sulphate reduction zone carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites the much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite smectites and the loss of kaolinite

seawater redox variations during the deposition of the kimmeridge clay formation united kingdom upper jurassic evidence from molybdenum isotopes and trace metal ratios 1 the kimmeridge clay formation kcf and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the mesozoic in this study we use the molybdenum mo stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the kcf facies with lower organic carbon contents toc 1 7 wt were deposited under mildly reducing suboxic conditions while organic rich facies toc 7 wt accumulated under more strongly reducing anoxic or euxinic local conditions trace metal abundances are closely linked to toc content suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the kcf and may have been related to orbitally controlled climate changes long term variations in 98 95mo are associated with the formation of organic rich intervals and are related to third order fluctuations in relative sea level differences in the mean 98 95mo composition of the organic rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the pectinatites wheatleyensis and lower pectinatites hudlestoni zones than during the deposition of the upper pectinatites hudlestoni and pectinatites pectinatus zones the global extent of reducing conditions during the kimmerigidan was greater than today but was less widespread than during the toarcian early jurassic oceanic anoxic event this study also demonstrates that the mo isotope system in jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present day sedimentary environments

trace elements as palaeoenvironmental markers in strongly mature hydrocarbon source rocks the cretaceous la luna formation of venezuela abstract the late cretaceous la luna formation is the main hydrocarbon source rock formation in venezuela in the lake maracaibo area the formation shows a steep maturation gradient affecting sedimentary organic matter om and clay minerals related to local burial history and thrusting events the influence of the gradient is examined through geochemical organic and inorganic and mineralogical investigations carried out on samples from five cored boreholes in the maracaibo basin the analytical procedure comprises rock eval pyrolysis vitrinite reflectance measurement palynofacies observation major and trace element content determination performed on bulk rock samples or after a sequential leach protocol dop calculation and the examination and analysis of clay mineral assemblages the maturation gradient seems to have had no effect upon the distribution of the trace elements referred to as proxy indicators for palaeodepositional conditions of om rich sediments ba p v mo ni co cu cd cr and mn consequently trace elements appear to be reliable palaeoenvironmental markers they indicate that during the la luna fm deposition the water column must have had variable conditions with high primary production near the surface and the correlative development of euxinic conditions at the bottom this provides further arguments for an upwelling system affecting the northern platform of south america during late cretaceous time the relative scarcity of reactive iron in the depositional environment must have taken an active part in a process involving sulphur and acting on the environmental conditions as well as on om vulcanisation this process favours aliphatic om storage and may be relevant for many other om rich facies or hydrocarbon source rocks

are the kimmeridge clay deposits affected by burn down events palynological and geochemical studies on a 1 metre long section from the upper kimmeridge clay formation dorset uk abstract two independent analytical approaches palynology and inorganic geochemistry were applied to identify potential oxygen burn down events in the late jurassic kimmeridge clay formation kcf the kcf interval of the rotunda ammonite zone spanning 121 82 122 72 m depth was sampled from the swanworth quarry 1 borehole dorset uk at 2 5 5 0 cm resolution samples were analysed for total organic carbon toc concentrations of elements that are known to be productivity and or nutrient related e g cu p detrital e g al ti zr and redox sensitive sulphide forming e g v mo fe mn s and palynofacies components including analysis of organic walled dinoflagellate cysts dinocysts on a species level the toc contents generally exceed 2 wt with a maximum of 8 8 wt at 122 37 cm depth and elevated values in the central part of the investigated interval this interval of relatively higher toc values correlates well with the maximum recovery of marine palynomorphs and low al values suggesting that the toc is primarily of marine organic matter om changes in v al mo al fe al mn al and s patterns at 122 37 m depth mark a shift from anoxic conditions in the lower part of the studied interval to more oxic conditions in its upper part such a shift could explain the relatively high toc and marine palynomorph concentrations in the lower part of the studied interval as a result of better preservation and the subsequent decrease as an effect of a post depositional burn down i e om oxidation as the amount of marine palynomorphs and toc content diminishes from the middle part of the section upwards species specific changes in dinocyst assemblages can be observed in particular concentrations of circulodinium spp cyclonephelium spp sirmiodinium grossi senoniasphaera jurassica and systematophora spp decrease rapidly in comparison to other species such as glossodinium dimorphum and cribroperidinium sp 1 which may suggest selective degradation of dinocysts due to oxidation we suggest that post depositional oxygenation of bottom and pore waters within the sediment was most probably the cause for decreasing toc values and reduced recovery of marine palynomorphs towards the top of the studied interval in comparison to high toc and marine palynomorph values in the central part of the studied interval due to anoxic conditions

environmental changes at the frasnian famennian boundary in central morocco northern gondwana integrated rock magnetic and geochemical studies rock magnetic magnetic susceptibility and hysteresis parameters and geochemical analyses major and trace elements were carried out on whole rock samples of two frasnian famennian boundary sections anajdam and bou ounebdou in the central morocco western meseta during the frasnian the decreasing trend of the magnetic susceptibility signal mainly carried by low coercivity magnetite grains indicates a gradual reduction of detrital influx this decrease in detrital input parallels a frasnian long term sea level rise in the late frasnian kellwasser horizons that are classically considered to represent highstand deposits the magnetic signal exhibits the lowest intensities in connection with maximum diamagnetic contribution of the carbonate fraction with respect to geochemical data the two black carbonate rich kellwasser horizons are characterized by noticeable positive anomalies of bottom water dysoxic proxies and of marine primary productivity markers our data thus suggest that in central morocco the late frasnian marine environments were marked by a relatively important biogenic productivity favouring the onset of oxygen depleted conditions during periods of maximum transgression on the continental platforms

comparative study by infrared spectroscopy of the organic matter of phosphate rich oulad abdoun basin and black shale timahdit basin series morocco abstract fourier transform infrared spectroscopy has been used to get quantitative information on the oxidation state of the organic matter of stable residues extracted from strata of the phosphate rich basin of oulad abdoun and from oil shales of the timahdit basin morocco throughout the respective filling of both basins the redox conditions have been more oxidizing in the top part of the sediments in oulad abdoun basin than in the timahdit basin this is interpreted as the result of the sedictural evolution of the basin oulad abdoun basin having evolved in stable platform conditions while the timahdit basin evolved in subsiding ones the phosphate rich beds contain the more oxidized organic stable residues sedimentological evidence shows that the oxidation was associated reworking processes these processes are responsible for the progressive concentration of the phosphatic grains and prove to be necessary for the p 2 o 5 content to become higher than 10 this value being the highest encountered in the sediment within which the phosphatic grains form

organic facies variation in the late kimmeridgian of the boulonnais area northernmost france recent studies of the upper kimmeridgian of the boulonnais area northernmost france have provided a sequence stratigraphical framework that is here used to help interpret variations in sedimentary organic matter om content and composition in response to fluctuations in relative sea level the organic facies are characterised using a combination of palynofacies bulk organic geochemistry toc rock eval pyrolysis and sulphur data and the particle size distribution of the total palynological residues the organic facies show a good correlation with the sequence stratigraphy exhibiting peak toc hi total s and sorg values in the lithofacies deposited around the two mfs the palynofacies in these intervals is characterised by a high content of fluorescent orange marine aom there is a strong correlation between the orange aom and the total and organic sulphur contents similar positive correlations between orange aom and organic sulphur have previously been observed in the distal organic rich sediments of the kimmeridge clay formation of dorset lateral time equivalents of the boulonnais facies but there the sulphur contents are lower indicating that organic sulphur content of orange aom is not fixed suggestive of preservation related parallel but not intrinsically related trends the s data can be used to estimate an apparent burial efficiency and hence the proportion of the primary productivity preserved this allows a rough estimate of palaeoproductivity model calculations suggest that for a sulphide retention of 30 70 and uncompacted sedimentation rate estimates in the range 5 10 cm ka the mean palaeoproductivity was in the range 52 175 gc m2 a up to 6 of which was preserved thus if the preservation is high as is the case for mfs the palaeoproductivity does not need to be above average for shelf waters to produce a given toc

enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in mesozoic limestones and shales there have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition among them molybdenum mo is frequently cited as an effective proxy for sediments and sedimentary rocks recently helz et al helz g r miller c v charnock j m mosselmans j l w pattrick r a d garner c d vaughan d j 1996 mechanisms of molybdenum removal from the sea and its concentration in black shales exafs evidences geochim cosmochim acta 60 3631 3642 and adelson et al adelson j m helz g r miller c v 2001 reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments geochim cosmochim acta 65 237 252 suggested that mo does not behave conservatively in the water column when h2s reaches a threshold concentration above this concentration a switch operates and mo is scavenged by forming bonds with metal rich notably iron particles sulfur rich organic molecules and pyrite in this paper mo trapping by sulfur rich organic matter om in ancient marine deposits is emphasized the following mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized om the akkuyu formation turkey the calcaires d orbagnoux france and kimmeridge clay uk and its timeequivalent in boulonnais france the kashpir oil shales russia and the la luna formation venezuela the sulfur rich om is identified by either measured organic s abundance or kerogen microscope observation our results show that mo is systematically more enriched relative to the other redox sensitive sulfide forming elements studied u v ni cu zn cr and mo enrichment is positively correlated with the amount of sulfurized om but not with pyrite abundance these results illuminate the role played by sulfurized om in geologic scale mo capture and retention but they also underline the role played by reactive iron significant om sulfurization is only possible when reactive iron is limited nevertheless pyrite formation though limited could act as an initial mo trap prior to mo uptake by om that is sulfurized after the pyritization step in future paleoenvironmental reconstructions attention must be paid to this enhanced mo enrichment in the presence of sulfurized organic matter in such cases the use of mo could lead to overestimation of the reducing conditions of the depositional environment

diagenetic pyritisation under eastern mediterranean sapropels caused by downward sulphide diffusion recurrent organic rich layers sapropels in eastern mediterranean sediments are enriched in corg fe and s sulphur and fe are enriched in a zone immediately below the sapropels whereas corg is not 34s values of bulk sediments and simple mass balance calculations indicate that so42 reduction has taken place in an open system with all hs formed at or close to the sediment surface formation of pyrite in the sapropel was fe limited and consequently excess hs was able to migrate downwards downward sulphidisation this resulted in the formation of pyrite below the sapropel by reaction of this hs with solid phase ferric iron and fe2 diffusing upwards from underlying sediments the fe2 source probably includes fe hydr oxide layers formed at former oxidation fronts above previously deposited and buried sapropels this downward sulphidisation mechanism allows accumulation of twice as much s in alternating organic rich anoxic organic poor suboxic sediments compared to what is preserved in organic rich anoxic sediments

organic matter and trace element rich sapropels and black shales a geochemical comparison a distinct pliocene eastern mediterranean sapropel i 282 recovered from three ocean drilling program odp leg 160 sites has been investigated for its organic and inorganic composition this sapropel is characterized by high organic carbon corg and trace element contents and the presence of isorenieratene derivatives the latter suggests that the base of the photic zone was sulphidic during formation of the sapropel combined with evidence of bottom water anoxia preservation of laminae high redox sensitive trace element contents and the abundance and isotopic composition of pyrite this leads to the tentative conclusion that almost the entire water column may have been anoxic this anoxia resulted from high productivity and not from stagnation because an approximation of the trace element budget during sapropel formation shows that water exchange with the western mediterranean is needed entire water column anoxia has been suggested earlier for several black shales with regard to the depositional environment and the corg content however only the cenomanian turonian boundary event ctbe black shales appear to be comparable to this sapropel the proposed trace element removal mechanism of scavenging and co precipitation in an anoxic water column is thought to be similar for both types of deposits the ultimate trace element source for the sapropel however is seawater whereas it is hydrothermal and fluvial input for ctbe black shales because they have a larger temporal and spatial distribution nonetheless the corg rich eastern mediterranean pliocene sapropel discussed here may be considered to be a younger analogue of ctbe black shales

geochemical study of organic matter rich cycles from the kimmeridge clay formation of yorkshire uk productivity versus anoxia abstract in this contribution we study two meter scale cycles from the kimmeridge clay formation cored near marton yorkshire which shows cyclic organic matter om distribution our aim is to try to understand the factors responsile for om accumulation the first cycle called lower cycle shows a total organic carbon toc content between 1 and 10 whereas the second called upper cycle shows a toc content varying between 5 and 35 the geochemical composition major elements and trace elements the organic geochemistry toc hi palynofacies and mineralogy of the sediments have been determined in both cycles the cyclicity is expressed through variations in the nature and in the relative abundance of the various types of organic matter constituents furthermore dilution effects by inorganic components of the sediment cannot account for the toc cyclicity for lower cycle the mo v and u content is low and little variable as is the intensity of the oxidation which om suffered from this indicates that variations in phytoplanktonic productivity may be held responsible for the cyclicity in steady and mildly reducing redox conditions in the upper cycle cyclicity also appears to depend on productivity but variations in the concentration of mo v and u and in the oxidation state of the om suggest the environment was temporarily more reducing it is proposed that larger amounts of h 2 s were released into marine bottom waters as a result of initial om decomposition forced the oxic anoxic boundary to rise in the water column and thus favoured om storage the main driving force for variations in the om concentration was the productivity of organic matter walled phytoplankton redox conditions of the depositional environment could have had a positive action but only by acting as a positive feedback effect

geochemical paleoredox indicators in devonian mississippian black shales central appalachian basin usa the degree of anoxia that existed during accumulation of devonian mississippian black shales in central kentucky has been debated widely in this study geochemical data were used to elucidate paleodepositional environments for these units using analyses from ten cores collected from the outcrop belt in central kentucky sulfur carbon and selected trace element relationships were compared the sunbury shale tournasian lower mississippian and the cleveland and huron members of the ohio new albany shale fammenian upper devonian show different relationships between carbon sulfur and trace elements c s fe relationships for the sunbury suggest anoxic possibly even euxinic conditions prevailed during sediment accumulation analysis of c s fe data suggests that the cleveland may have accumulated under anoxic conditions with intermittent dysoxia whereas the huron member may have accumulated under a relatively wide range of conditions from anoxic to dysoxic to possibly oxic these three units exhibit different degrees of trace element enrichment with the approximate order of enrichment relative to an average shale being mo pb zn v ni cu cr co the sunbury shows the highest levels of enrichment followed by the cleveland with the huron showing only slight enrichment for most of these trace elements differing degrees of enrichment may reflect differences in depositional environment during accumulation high mo concentrations in the sunbury generally 200 550 ppm may infer euxinic conditions prevailed during sediment accumulation the cleveland and the huron both show considerably lower mo contents averaging around 100 ppm geochemical ratios including ni co v cr and v v ni also indicate variable paleoredox conditions for these shales based on previously established thresholds v cr and ni co ratios infer at least anoxic conditions during accumulation of the sunbury anoxic to dysoxic conditions during cleveland accumulation and dysoxic to oxic conditions for the huron v v ni ratios tend to indicate consistently lower oxygen regimes than do other paleoredox indicators and this discrepancy is greatest for the huron member it is suggested that thresholds established for paleoredox indicators in previous studies should not be applied strictly but that relative differences in these indicators collectively can infer variations in the degree of anoxia in addition relationships between corg and redox elements may help elucidate the role of anoxia in om accumulation it may be concluded from this study that the devonian mississippian black shales of central kentucky accumulated under variable bottom water conditions at least anoxic conditions prevailed during accumulation of much of the sunbury shale and the upper part of the cleveland member and possibly euxinic conditions for the sunbury bottom water conditions may have been intermittently anoxic and dysoxic during deposition of the lower cleveland during accumulation of the huron member it is likely that conditions ranged from anoxic to dysoxic to marginally oxic possibly being close to normal marine conditions at times during accumulation of the lowermost huron sediments

globally enhanced mercury deposition during the end pliensbachian extinction and toarcian oae a link to the karoo ferrar large igneous province the mesozoic era featured emplacement of a number of large igneous provinces lips formed by the outpouring of millions of cubic kilometres of basaltic magma the radiometric ages of several mesozoic lips coincide with the dates of oceanic anoxic events oaes as a result of these coincidences a causal link has been suggested but never conclusively proven this study explores the use of mercury as a possible direct link between the karoo ferrar lip and the coeval toarcian oae t oae mercury is emitted to the atmosphere as a trace constituent of volcanic gas and may be distributed globally before being deposited in sediments modern marine deposits show a strong linear correlation between mercury and organic matter content results presented here indicate departures from such a simple linear relationship in sediments deposited during the t oae and also during the pliensbachian toarcian transition an event that saw elevated benthic extinctions and carbon cycle perturbations prior to the t oae a number of depositional settings illustrate an increased mercury concentration in sediments that record one or both events suggesting a rise in the depositional flux of this element complications to this relationship may arise from very organic rich sediments potentially overprinting any hg toc signal whereas environments preserving negligible organic matter may leave no record of mercury deposition however the global distribution of coevally elevated hg rich levels suggests enhanced atmospheric mercury availability during the early toarcian potentially aided by the apparent affinity of hg for terrestrial organic matter although the relative importance of aquatic vs terrestrial fixation of hg in governing these enrichments remains uncertain a perturbation in atmospheric hg is most easily explained by enhanced volcanic output it is suggested that extrusive igneous activity caused increased mercury flux to the early toarcian sedimentary realm supporting the potential of this element as a proxy for ancient volcanism this interpretation is consistent with a relationship between lip formation and a perturbed carbon cycle during the pliensbachian toarcian transition and t oae the recording of these two distinct hg excursions may also indicate that the karoo ferrar lip released volatiles in temporally distinct episodes due either to multiple phases of magmatic emplacement or sporadic release of thermogenic gaseous products from intrusion of igneous material into volatile rich lithologies

interplay between sedimentary organic matter and dissolved oxygen availability in a coastal zone of the humboldt current system mejillones bay northern chile abstract geochemical and petrographical analyses were done in 26 surficial sediment samples from mejillones bay northern chile to investigate the processes that participate in the preservation of sedimentary organic matter annual variability of chlorophyll a and dic 13 c measured in surface waters of three different parts of mejillones bay did not show any significant spatial variability in biological productivity on the other hand bottom dissolved oxygen showed strong changes associated with water depth and season surface sediment composition was statistically analyzed displaying organic facies which are in agreement with the principal zones of bottom oxygenation annual mean condition identified in this bay in the oxic coastal zone 1 ml l 1 the total carbon tc total nitrogen tn total organic carbon toc hydrogen index hi and oxygen index oi were 1 3 0 1 0 4 355 8 mg hc g toc and 237 6 mg co 2 g toc respectively these results and the presence of almost exclusively dispersed amorphous organic matter suggest an environment where most metabolizable organic matter is degraded within the water column and or in the sediment water interface in the microxic hemipelagic zone 1 located deeper than 65 m the tc tn toc hi and oi were 10 5 1 0 6 6 529 7 mg hc g toc and 103 1 mg co 2 g toc respectively these values and the presence of abundant agglomerated amorphous organic matter suggest a better preservation of metabolizable and non metabolizable compounds we can conclude that biological productivity and dissolved oxygen exert a similar influence on the sedimentary organic matter but the characteristics and quantity of organic material preserved in bottom sediments of this bay are controlled principally by the availability of dissolved oxygen at the sediment water interface according to this conclusion we suggest that the sedimentary record of mejillones bay must be interpreted in paleoceanographic terms primarily in relation to the temporal variability of dissolved oxygen more than biological productivity the oi seems to be a good proxy of paleoxygenation in this coastal system

basinal restriction black shales re os dating and the early toarcian jurassic oceanic anoxic event 1 profiles of mo total organic carbon toc through the lower toarcian black shales of the cleveland basin yorkshire united kingdom and the posidonia shale of germany and switzerland reveal water mass restriction during the interval from late tenuicostatum zone times to early bifrons zone times times which include that of the putative early toarcian oceanic anoxic event the degree of restriction is revealed by crossplots of mo and toc concentrations for the cleveland basin which define two linear arrays with regression slopes ppm of 0 5 and 17 the slope of 0 5 applies to sediment from the upper semicelatum and exaratum subzones this value which is one tenth of that for modern sediments from the black sea mo toc regression slope 4 5 reveals that water mass restriction during this interval was around 10 times more severe than in the modern black sea the renewal frequency of the water mass was between 4 and 40 ka the mo toc regression slope of 17 applies to the overlying falciferum and commune subzones the value shows that restriction in this interval was less severe and that the renewal frequency of the water mass was between 10 and 130 years the more restricted of the two intervals has been termed the early toarcian oceanic anoxic event but is shown to be an event caused by basin restriction local to nw europe crossplots of re os and mo against toc show similar trends of increasing element concentration with increase in toc but with differing slopes together with modeling of 187 os 188 os and d 98 mo the element toc trends show that drawdown of re os and mo was essentially complete during upper semicelatum and exaratum subzone times mo toc regression slope of 0 5 drawdown sensitized the restricted water mass to isotopic change forced by freshwater mixing so that continental inputs of re os and mo via a low salinity surface layer created isotopic excursions of up to 1 3 in d 98 mo and up to 0 6 for 187 os 188 os restriction thereby compromises attempts to date toarcian black shales and possibly all black shales using re os chronology and introduces a confounding influence in the attempts to use d 98 mo and initial 187 os 188 os for palaeo oceanographic interpretation

geochemical characteristics redox conditions and organic matter accumulation of marine oil shale from the changliang mountain area northern tibet china abstract the organic rich changliang mountain oil shale located in the north qiangtang depression northern tibet is considered to be excellent mineral resource in china the changliang mountain oil shale reported here was deposited in a tidal flat lagoon environment and is characterized by black thin layered oil shales intercalated with dark gray marls here we present geochemical data from the changliang mountain oil shale profile in order to investigate the mechanism of organic matter om accumulation and to establish the formation model for the marine oil shale deposition total organic carbon toc values range from 2 96 to 23 47 in the oil shale samples while the marl samples contains low toc contents ranging from 0 06 to 0 21 in the organic rich oil shale sediments many redox indicators including mo al ratios v cr ratios th u ratios ni co ratios and the relationship of mo to toc suggest a deposition under dysoxic to anoxic environments subsequently the bottom water evolved into an oxic water body when the organic poor marls were deposited however the negative and or weak relationship between toc and productivity indices p ti and ba al indicates that om accumulation was not controlled mainly by primary productivity but dysoxic anoxic bottom water environment a stratified water column may be initiated by the supply of fresh water from the continent nearby combining with warm and humid climate which is beneficial to the reproduction of marine organisms the death and bury of these organisms could lead to the formation of dysoxic anoxic bottom waters and enhance the preservation of om in this study a preservation model of the changliang mountain oil shale was established the model implies that excellent preservation is the major controlling factor for om enrichment in the oil shale layer in addition factors such as mixed deposition with clay minerals and detrital matter input cannot be ignored for their influence on om enrichment