the white stone band of the kimmeridge clay formation an integrated high resolution approach to understanding environmental change the kimmeridge clay is a jurassic mudrock succession that shows milankovitch band climatic cyclicity a key issue is to determine how the subtle changes that define this cyclicity result from climatic change using material from the natural environment research council rapid global geological events rgge kimmeridge drilling project boreholes the white stone band was investigated at the lamination scale using backscattered electron imagery and quantitative palynofacies fabric analysis shows the lamination to represent successive deposition of coccolith rich and organic matter rich layers individual laminae contain unsorted palynological debris with a consistent ratio of marine and terrestrial components such mixed organic matter input is interpreted as the result of storm transport linking water column processes to laminae deposition suggests seasonal input with a coccolith bloom followed by a more diverse assemblage including dinoflagellates and photosynthetic chlorobiacean bacteria as the photic zone extended into the euxinic water column organic matter export to the sea bed underwent minimal cycling through oxidation and subsequently became preserved through sulphurization with greatly increased sequestration of carbon this was significantly increased by late season storm driven mixing of euxinic water into the photic zone increased frequency of storm systems would therefore dilute the coccolith input to give an oil shale hence climatically induced changes in storm frequency would progressively vary the organic content of the sediment and generate the climate cycle signal

molybdenum isotope signatures in continental margin marine sediments abstract we present molybdenum isotope data for four sediment profiles from continental margin settings each profile has a distinctive average isotope composition ranging from 98 95 mo 0 5 to 1 3 relative to j and m laboratory standard this range lies between the modern ocean water value 2 3 and the values typical of mo adsorbed onto mn oxides 0 7 0 1 an important finding of this study is the apparent co variation between the mo isotope composition and the accumulation rate of authigenic mo under reducing conditions this relationship suggests that the chemical processes responsible for mo accumulation under reducing conditions produce an isotope signature in marine sediments in addition to the relationship between mo accumulation and the mo isotope signature there is also a relationship between these parameters and the rate of organic carbon oxidation and burial these relationships suggest that the mo isotope signature of reducing sediments may serve as a tracer for the cycling of organic carbon in continental margin sediments however additional data will be required to refine any such relationships

sequence stratigraphy of source rocks applied to the study of the kim meridgian tithonian in the north west european shelf dorset uk yorkshire uk and boulonnais france abstract in dorset and yorkshire england the sedimentation of kimmeridgianfrithonian age mutabilis to pallasioides zones shows ordered organic sequences the distribution of the organic matter content per cent total organic carbon petroleum potential hydrogen indices is not random but occurs as widespread primary organic rich cycles nested within each other these organic rich cycles with thicknesses of about 0 5 m show regular variations in organic carbon contents reaching a maximum value midway through the cycle they formed in response to widespread phenomena and can be correlated over long distances the cycles were deposited in marine environments mainly below the storm wave base they show little sedimentological evidence of sea level fluctuation either transgression or regression in the boulonnais france a more proximal setting the comparable strata show a greater lithological contrast and depositional environments corresponding to bathymetric changes are more easily determined a comparison of the distribution of organic matter through the sequence dated by ammonites in the three areas studied shows a good correlation between the distal dorset and yorkshire and the proximal boulonnais areas the main result of the biostratigraphic revision in the boulonnais france concerns the argiles de ch tillon which can be subdivided into two members these belong to the autissiodorensis zone autissiodorensis subzone and the gigas elegans zone respectively thus the kim meridgian tithonian boundary is located within this formation transgression does not appear to be the main factor determining the accumulation of autochthonous organic matter type ii not every major transgressive facies change seen in the kimmeridgian tithonian of the boulonnais is characterized by an enrichment in organic matter in the basinal area good source rocks with high petroleum potentials result from a conjunction between the physiography the transgression and the climate this last causes the variations in primary productivity which produce the organic rich cycles transgressions act mostly to reveal the climatic variations they are a necessary condition for the accumulation of the autochthonous organic matter but they are not sufficient by themselves as demonstrated by the transgressions in the autissiodorensis and elegans zones in the first instance the strata are depleted in organic matter in both the distal and proximal areas in the second instance type ii organic matter accumulated in both distal and proximal environments the vertical fluctuations in the total organic carbon contents the types of organic matter and the petroleum potentials of the organic rich cycles determine the oil potentials of transgressive sequences in sedimentary basins

evidence for annual records of phytoplankton productivity in the kimmeridge clay formation coccolith stone bands upper jurassic dorset uk the upper jurassic kimmeridge clay formation is a c org rich sequence of mudrocks that was deposited in a dysoxic basin which served unusually to preserve primary coccolithophorid proportions strikingly contrasting within this dark coloured formation are the white coccolith stone bands which contain pristinely preserved coccolithophorid assemblages preserving potentially primary coccolithophorid abundances with minimal diagenetic alteration using methodology described herein scanning electron microscopy sem observation of mum scale laminae within selected stone bands has revealed five distinctive types of microlamina type 1 being barren clay type 2 comprising coccoliths associated with varying amounts of clay or amorphous organic matter type 3 being purely coccolithic type 4 being of putative microbial origin and type 5 containing frequent calcispheres these are all described and illustrated here illustrated sequences of these are discussed with reference to an hypothetical model sequence based on modem records representing a kimmeridgian year of coccolithophorid productivity annual signals are thus interpreted based on the observations presented here and including summaries of the palynomorph records from the stone bands we comment on watznaueriacean palaeoecology and particularly the interpretation of watznaueria fossacincta barnesiae as an opportunistic growth maximising taxon c 2004 elsevier b v all rights reserved

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

rhenium and molybdenum enrichments in sediments as indicators of oxic suboxic and sulfidic conditions of deposition abstract the trace elements re and mo both behave conservatively in seawater yet are strongly enriched in reducing sediments their potential for authigenic enrichment above crustal concentrations is greater than for many other elements due to the high ratio of metal sw metal crust we present sedimentary re and mo data from box and multi cores spanning a range of redox conditions from well oxygenated sites to locations with substantial sulfide concentrations at the oxic sites re and mo as expected accumulate at concentrations close to their crustal abundances re shows substantial enrichment in suboxic absence of o 2 and h 2 s sediments of the sea of japan in sediments within the oxygen minimum of the pakistan margin as well as in sediments underlying the sulfide bearing waters of the black sea and saanich inlet re enrichment occurs in these cores just below the depths of fe and u reduction only in the sediments underlying the sulfidic waters of the black sea and saanich inlet is there substantial authigenic mo accumulation absence of re and mo enrichments in sediment trap samples from the sulfide bearing basins suggests that the addition of both metals occurs at or below the sediment water interface the re and mo concentration profiles are modeled to hindcast the removal depth of re and mo in the sediments which along with the re mo ratio determine whether oxic suboxic or sulfidic conditions were present in the water column or in the sediments in the past using this approach historical redox conditions can be inferred even in environments such as continental margins where a substantial lithogenic component can obscure authigenic enrichments of other metals

the early toarcian and cenomanian turonian anoxic events in europe comparisons and contrasts two intervals of mesozoic time are demonstrably of sufficient geological brevity to qualify readily for the term anoxic event the earlyfalciferum zone orexaratum subzone of the toarcian and thewhiteinella archaeocretacea interval zone that straddles the cenomanian turonian boundary both periods of time saw regional deposition of anomalously organic rich strata black shales accompanied by a positive 2 pdb carbon isotope excursion in coeval biogenic carbonates and significant faunal change the duration of both events was probably less than half a million years and both were preceded by regional erosion and possible upwelling coincident sea level rise and transgression is likely a european view of these events however spotlights some significant differences whereas thefalciferum zone organic matter is generally more concentrated and more hydrogen rich on the palaeo european shelf than on the tethyan continental margins the reverse holds true for the black shales formed during thewhiteinella archaeocretacea interval zone furthermore the carbon isotope composition offalciferum zone organic matter from both north european and tethyan sites is anomalously negative compared to that developed in thewhiteinella archaeocretacea interval zone differing planktonic organisms may have been involved and their possible diverse ecological requirements may go some way to explaining the nature and distribution of facies deposited during these two events

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

fault pattern and seismotectonic potential at the south western edge of the ionian subduction system southern italy new field and geophysical constraints abstract the south western edge of the calabrian arc in southern italy has been investigated throughout a joint analysis of field marine and geophysical data which provided constraints on the fault pattern and on the seismotectonic potential the study was focused on a poorly known sector of a larger belt of seismically active faults slicing across the ne corner of sicily the so called tindari fault system our data pointed out that the investigated area including the mainland and the ionian offshore is deformed by oblique faulting with a general nw se tectonic trend earthquake distribution and seismic profiles pointed out active deformation in the offshore while the mainland is characterized by the occurrence of a nw se oriented 20 km long structural belt however scarce seismicity has been recorded in the last 30 years alongside this tectonic structure accounting for a possible silent segment of the larger fault system tomographic images revealed that the moho discontinuity is deformed by a ne dipping lithospheric tectonic structure which has been here retained the main mode of deformation and responsible for coseismic displacement in the area as a whole field and geophysical data agree with a general nw se trend segmented pattern of recent active faults that have the potentiality of generating magnitude 6 5 7 earthquakes

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

the late palaeocene early eocene and toarcian early jurassic carbon isotope excursions a comparison of their time scales associated environmental changes causes and consequences although the earth9s environment is constantly changing there have been a few unusual episodes over the last c 200 ma when change was extreme in terms of its rapidity severity long lasting consequences and unpredictability the geochemical and biotic records for two of these episodes the palaeocene eocene thermal maximum and the toarcian oceanic anoxic event early jurassic possess many significant similarities each event was associated with a major carbon isotope excursion significant levels of biotic extinctions severe global warming an enhanced hydrological cycle and evidence for widespread seawater anoxia both carbon isotope excursions can be subdivided into distinct stages with broadly similar characteristics and durations based on a detailed comparison the palaeocene eocene thermal maximum may have been an incipient oceanic anoxic event the geochemical and biotic changes during these two events are most readily explained by the abrupt large scale dissociation of methane hydrate that followed a period of more gradual environmental change linked to the emplacement of a large igneous province carbon release rates at those times were of the same order of magnitude as the current anthropogenic release rate of carbon to the atmosphere indicating that ancient events such as these may usefully serve as analogues for present day environmental change

environmental cycles in the late jurassic northwest european epeiric basin interaction with atmospheric and hydrospheric circulations abstract despite the large size and the heterogeneity of the northwest european late jurassic kimmeridgian lower tithonian epeiric basin the sedimentary environment was rather uniform in general deposition of mudstones and the microlaminated organic rich sediments took place under low oxic to anoxic conditions detailed environmental analysis clearly points to a seasonal alternation of aerobic with anaerobic conditions in substrate and bottom water resulting in one year life cycles in the benthic environment the widespread uniformity of facies and the seasonal alternation in the oxygen budget of the benthic environments are best explained by a summer to winter change in the atmospheric circulation of the northern hemisphere this change strongly influenced the oceanographic situation in the northwestern european epeiric basin superposed on this one year cyclicity three more hierarchical levels of cycles are stratigraphically recorded they reflect a complex history of orbital forced and non orbital forced climatic changes and or eustatic sea level fluctuations in the late jurassic

interaction of a warm season frontal system with the coastal mountains of the western united states part i prefrontal onshore push coastal ridging and alongshore southerlies abstract observational analyses and numerical simulations are used to investigate the interaction between a warm season frontal system trough and the complex terrain of the western united states prior to frontal landfall synoptically driven offshore flow was associated with the northward development of a thermal trough and a coastal pressure ridge and associated southerly winds moved northward along the coasts of northern california oregon washington and southwestern british columbia this coastal pressure ridge coupled with the subsequent weakening of the offshore flow resulted in the onshore push of cool marine air approximately 24 h before frontal passage the onshore surge of cool marine air into warm dry continental air associated with the thermal trough produced a mesofront that moved over the coastal and cascade mountains diagnosis of a realistic simulation of the event shows that both solar heating and adiabatic warming from downslope flow over the rockies and the cascade mountains are i

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

contrasting watermass conditions during deposition of the whitby mudstone lower jurassic and kimmeridge clay upper jurassic formations uk abstract the whitby mudstone formation wmf and kimmeridge clay formation kcf are both jurassic organic rich black shale units containing well preserved marine organic matter despite such gross similarities they exhibit opposite trends in 13 c toc versus wt decarbonated total organic carbon toc r 0 76 for wmf and 0 71 for kcf the 13 c toc versus toc trend also varies geographically within both units only the kcf data exhibit a positive correlation between 13 c toc and 15 n org r 0 76 and the presence of aryl isoprenoids probable indicators of photic zone anoxia maturity differences are insufficient to explain such contrasts bulk and compound specific isotopic variations in the kcf are interpreted largely as a reflection of primary productivity during deposition rather than a dominantly diagenetic signal due to incorporation of sulphurised carbohydrate the contrasts between the formations and the geographic variation in coeval shelf sediments are explained by a model stressing the relative positions and stabilities of the chemocline pycnocline nutricline and euphotic zone and the effect these have on productivity co 2 recycling and bacterial floras

comment on molybdenum isotope evidence for widespread anoxia in mid proterozoic oceans arnold et al 1 1 reported mo isotope compositions of mid proterozoic black shale which were different from those of euxinic sediments in the black sea on the basis of a mass balance model calculation they claimed that the area of oxic sedimentation in the mid proterozoic oceans was

comparative behavior of authigenic re u and mo during reoxidation and subsequent long term burial in marine sediments the trace element re is enriched in mildly reducing suboxic sediments close to or slightly later in the redox sequence than u and before reduction of mo in more reducing sulfidic sediments this work investigates the behavior of authigenic re during oxidation and long term burial in the sediments of several turbidites from the madeira abyssal plain exposure to bottom water o2 clearly remobilizes authigenic re as found in previous work but reimmobilization occurs in reducing sediments below the oxidation front over a depth range of 0 5 3 m the large spread of the re burndown peak must be kinetically controlled perhaps influenced by the corg content once reimmobilized however the authigenic re signal remains immobile for at least 3 4 myr authigenic u in contrast consistently forms peaks 0 5 m broad whereas authigenic mo is lost on oxidation and does not appear to be refixed unless authigenic pyrite is formed these results imply that any paleoenvironmental significance of authigenic re will be compromised in low sedimentation rate environments 10 cm kyr where there is significant exposure of authigenic re to bottom water o2 however authigenic re records from rapidly accumulating continental margin sediments with much less exposure to bottom water o2 still offer considerable potential as high fidelity recorders of past reducing conditions

carbon isotope record of the early jurassic toarcian oceanic anoxic event from fossil wood and marine carbonate lusitanian basin portugal thetoarcianoceanicanoxicevent oae intheearlyjurassic 183maago wascharacterizedbywidespreadnear synchronous deposition of organic rich shales in marine settings as well as perturbations to several isotopic systems characteristically two positive carbon isotope excursions in a range of materials are separated by an abrupt negative shift carbon isotope profiles from toarcian fossil wood collected in england and denmark have previously been shown to exhibit this large drop 7 i n 13 c values interpreted as due to an injection of isotopically light co2 into the ocean atmosphere system however the global nature of this excursion has been challengedonthebasisofcarbon isotopedatafromnektonicmarinemolluscs belemnites whichexhibitheavierthanexpectedcarbonisotope values here we present new data principally from fossil wood and bulk carbonate collected at centimetre scale from a hemipelagic section at peniche coastal portugal this section is low in organic carbon average toc 0 5 and the samples should not have suffered significant diagenetic contamination by organic carbon of marine origin the carbon isotope profile based on wood shows two positive excursions separated by a large and abrupt negative excursion which parallels exactly the profile based on bulk carbonate samples from the same section albeit with approximately twice the amplitude 8 in wood versus 3 5 in carbonate these data indicate thatthe negative carbon isotope excursion affected the atmosphereand by implication the global ocean as well the difference in amplitude between terrestrial organic and marine carbonate curves can be explained by greater water availability in the terrestrial environment during the negative excursion for which there is independent evidence from marine osmiumisotope records and plausibly changes in atmospheric co2 content for which independent evidence is also available the peniche succession is also notable for the occurrence of re deposited sediments their lowest occurrence coincides with the base of the negative excursionandtheirhighestoccurrencecoincideswithitstop thus slopeinstabilityandsedimentsupplycouldhavebeenstronglylinked to the global environmental perturbation an association that may misleadingly simulate the effects of sea level fall

mo total organic carbon covariation in modern anoxic marine environments implications for analysis of paleoredox and paleohydrographic conditions 1 sedimentary molybdenum mo s has been widely used as a proxy for benthic redox potential owing to its generally strong enrichment in organic rich marine facies deposited under oxygen depleted conditions a detailed analysis of mo s total organic carbon toc covariation in modern anoxic marine environments and its relationship to ambient water chemistry suggests that 1 mo s while useful in distinguishing oxic from anoxic facies is not related in a simple manner to dissolved sulfide concentrations within euxinic environments and 2 patterns of mo s toc covariation can provide information about paleohydrographic conditions especially the degree of restriction of the subchemoclinal water mass and temporal changes thereof related to deepwater renewal these inferences are based on data from four anoxic silled basins the black sea framvaren fjord cariaco basin and saanich inlet and one upwelling zone the namibian shelf representing a spectrum of aqueous chemical conditions related to water mass restriction in the silled basin environments increasing restriction is correlated with a systematic decrease in mo s toc ratios from 45 5 for saanich inlet to 4 5 1 for the black sea this variation reflects control of mo s by mo aq which becomes depleted in stagnant basins through removal to the sediment without adequate resupply by deepwater renewal the basin reservoir effect the temporal dynamics of this process are revealed by high resolution chemostratigraphic data from framvaren fjord and cariaco basin sediment cores which exhibit long term trends toward lower mo s toc ratios following development of water column stratification and deepwater anoxia mo burial fluxes peak in weakly sulfidic environments such as saanich inlet owing to a combination of greater mo aq availability and enhanced mo transport to the sediment water interface via fe mn redox cycling and are lower in strongly sulfidic environments such as the black sea and framvaren fjord these observations demonstrate that at timescales associated with deepwater renewal in anoxic silled basins decreased sedimentary mo concentrations and burial fluxes are associated with lower benthic redox potentials i e more sulfidic conditions these conclusions apply only to anoxic marine environments exhibiting some degree of water mass restriction e g silled basins and are not valid for low oxygen facies in open marine settings such as continent margin upwelling systems

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

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

black shales on the basin margin a model based on examples from the upper jurassic of the boulonnais northern france abstract organic rich shales were deposited during two phases of relative sea level rise in the kimmeridgian late jurassic basin margin sections of the boulonnais n france pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic rich shale formation and th u ratios and degree of pyritisation of fe suggest severe oxygen restriction whilst palaeoecological and sedimentological evidence suggests that seafloor oxygenation events were frequent interbedded hummocky cross stratified sandstones indicate deposition above storm wave base correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen poor waters that contracted in lateral extent during parasequence formation the boulonnais black shales are similar to other varieties of transgressive black shale in that they are developed upon surfaces of marine starvation but they differ in that they pass down dip into better oxygenated facies they are therefore classified as transgressive nearshore tn black shales that do not record the feather edge of basinal conditions deposition is interpreted to have occurred beneath a small volume of sub thermocline bottom waters that were readily de oxygenated whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub thermocline water why such conditions should pertain only during initial stages of base level rise remains obscure

synchrony between early jurassic extinction oceanic anoxic event and the karoo ferrar flood basalt volcanism a well known second order mass extinction took place during the pliensbachian and toarcian stages of the early jurassic first recognized as a minor pliensbachian peak in the global extinction rate it has alternatively been interpreted as a regional response to the early toarcian oceanic anoxic event detailed studies established it as a global long term event spanning five successive ammonoid zones here we present a revised time scale based on highprecision u pb ages resolved to the zone level which suggests that elevated extinction rates were sustained for about 4 m y and peak extinction occurred at 183 ma recent isotopic dating of flood basalts from the southern gondwanan karoo and ferrar provinces documents a culmination in volcanic activity ca 183 ma the onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater 87 sr 86 sr curve the synchrony of voluminous flood basalt eruptions and biotic crises as already noted for three of the major mass extinctions permits a causal relationship which in this case may be mediated by widespread oceanic anoxia

osmium isotope evidence for the regulation of atmospheric co2 by continental weathering the long term stability of earth9s climate throughout the phanerozoic stands in marked contrast to the dramatic fluctuations that have taken place on time scales as short as a few years reflecting the high efficiency of longer term climate regulation through negative feedbacks a fundamental mechanism is thought to involve control of co 2 in the ocean atmosphere system through continental weathering although unambiguous high resolution data supporting this hypothesis have hitherto not been available organic rich mud rocks from yorkshire england which were deposited during the toarcian oceanic anoxic event ca 181 ma early jurassic contain evidence of an exceptionally large excursion in the 187 os 188 os ratio of contemporaneous seawater from 0 4 to 1 0 the most likely explanation for this excursion is that it resulted from a transient increase in global continental weathering rates of 400 800 the os isotope excursion coincided with a well documented global 13 c excursion of 6 that affected all the major biospheric reservoirs of the time higher mean global temperatures caused global chemical weathering rates to increase substantially while in turn chemical weathering was very effective in reducing the elevated levels of atmospheric co 2 and the high temperatures to preexcursion levels

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

integrated stratigraphy of the kimmeridge clay formation upper jurassic based on exposures and boreholes in south dorset uk for the purposes of a high resolution multi disciplinary study of the upper jurassic kimmeridge clay formation two boreholes were drilled at swanworth quarry and one at metherhills south dorset uk together the cores represent the first complete section through the entire formation close to the type section we present graphic logs that record the stratigraphy of the cores and outline the complementary geophysical and analytical data sets gamma ray magnetic sus ceptibility total organic carbon carbonate 13 c org of particular note are the new borehole data from the lowermost part of the formation which does not crop out in the type area detailed logs are available for download from the kimmeridge drilling project web site at http kimmeridge earth ox ac uk of further interest is a mid eudoxus zone positive shift in the 13c org record a feature that is also registered in tethyan carbonate successions suggesting that it is a regional event and may therefore be useful for correlation the lithostratigraphy of the cores has been precisely correlated with the nearby cliff section which has also been examined and re described magnetic susceptibility and spectral gamma ray measurements were made at a regular spacing through the succession and facili tate core to exposure correlation the strata of the exposure and core have been subdivided into four main mudrock lithological types a medium dark dark grey marl b medium dark dark grey greenish black shale c dark grey olive black laminated shale d greyish black brownish black mudstone the sections also contain subordinate amounts of siltstone limestone and dolostone comparison of the type section with the cores reveals slight lithological variation and notable thick ness differences between the coeval strata the proximity of the boreholes and different parts of the type section to the purbeck isle of wight disturbance is proposed as a likely control on the thickness changes

massive dissociation of gas hydrate during a jurassic oceanic anoxic event in the jurassic period the early toarcian oceanic anoxic event about 183 million years ago is associated with exceptionally high rates of organic carbon burial high palaeotemperatures and significant mass extinction heavy carbon isotope compositions in rocks and fossils of this age have been linked to the global burial of organic carbon which is isotopically light in contrast examples of light carbon isotope values from marine organic matter of early toarcian age have been explained principally in terms of localized upwelling of bottom water enriched in 12c versus 13c refs 1 2 5 6 here however we report carbon isotope analyses of fossil wood which demonstrate that isotopically light carbon dominated all the upper oceanic biospheric and atmospheric carbon reservoirs and that this occurred despite the enhanced burial of organic carbon we propose that as has been suggested for the late palaeocene thermal maximum some 55 million years ago the observed patterns were produced by voluminous and extremely rapid release of methane from gas hydrate contained in marine continental margin sediments

degree of pyritization of iron as a paleoenvironmental indicator of bottom water oxygenation abstract a new method is presented for recognizing the degree of bottom water oxygenation in organic carbon bearing rock the method uses the parameter degree of pyritization dop which is the ratio of pyritic fe to pyritic fe acid soluble fe sediments with ages ranging from devonian to cretaceous have been classified on paleoecological and sedimentological criteria as having been deposited in aerobic conditions deposited in fully oxygenated bottom waters restricted marine conditions deposited in waters with low oxygen concentrations or inhospitable bottom conditions where little or no oxygen is present and h2s may be continually or intermittently present aerobic dop

a high resolution biomarker study of different lithofacies of organic sulfur rich carbonate rocks of a kimmeridgian lagoon french southern jura the upper jurassic calcaires en plaquettes formation southern jura france which was deposited in a lagoonal setting reveals three lithofacies limestones with fine scale parallel laminations limestones with undulated stromatolite type laminations and massive limestones it was examined at the millimetre scale if the lithological differences are accompanied by differences in the composition of organic matter om and if microbial mats played a role in the preservation of om as suggested previously four types of laminae light parallel dark parallel light undulated dark undulated and a massive limestone sample were analysed the om in all facies is characterised by a high hydrogen index 870 970 and a variable 13c content 23 5 or to 28 1 the molecular composition of om appeared similar in all samples e g i extractable biomarkers are dominated by n alkane and hopanoid carbon skeletons predominantly in a sulfur bound form ii isorenieratene derivatives indicating anoxic conditions in the photic zone are present in relatively large amounts iii kerogen pyrolysates are dominated by organic sulfur compounds notwithstanding the overall similarity in om composition some significant differences were observed e g i low molecular weight sulfur bound hopanoids and steroids are relatively more abundant in toc lean light coloured samples than in toc rich dark coloured samples this is probably related to differences in the degree of intermolecular sulfur cross linking of the om ii the concentrations and distributions of several biomarkers in the light coloured undulated sample differ from those in the other samples these differences are attributed to the periodic occurrence of microbial mats our data indicate that om in the calcaires en plaquettes formation is predominantly composed of sulfur bound lipids formed at times that bottom waters were euxinic a periodic deepening of the oxic anoxic interface and of the light penetration depth to below the sediment surface allowed microbial mats to develop organic remains of the mats however are swamped by sulfur bound planktonic lipids

first record of the early toarcian oceanic anoxic event from the southern hemisphere neuquen basin argentina abstract the first record of the early toarcian oceanic anoxic event c 183 ma from the southern hemisphere is described from the neuquen basin argentina identified chemostratigraphically on the basis of a relative increase in marine organic carbon and a characteristic negative carbon isotope excursion 13 c org in bulk rock and fossil wood the negative excursion of 6 in bulk organic carbon falling to 31 3 crosses the boundary of the tenuicostatum hoelderi andean ammonite zones equivalent to the tenuicostatum falciferum serpentinum zones of europe these data indicate that the early toarcian oceanic anoxic event was a global phenomenon supplementary material a detailed stratigraphic log chemostratigraphic data and nannofossil data are available at http www geolsoc org uk sup18411

photic zone palaeoenvironments of the kimmeridge clay formation upper jurassic uk suggested by calcareous nannoplankton palaeoecology nannoplankton abundance data has revealed significant and systematic population fluxes through all representative lithologies and cycle types of the kimmeridge clay formation kcf and also through intervals in which lithology is visually homogeneous the majority of kcf samples yielded nannofossil assemblages of low species richness 1 to 10 and marked unevenness independent of preservational state all samples were dominated by coccoliths of one family the watznaueriaceae while assemblage unevenness is characteristic of nannofossil populations such low species richness is atypical particularly when compared with coeval assemblages of similar latitude such anomalously low nannofossil diversity may be explained as a response to nutrient rich euphotic environments we postulate that consistently high trophic conditions supported the eurytopic watznaueriaceans but excluded most normal open ocean taxa the switching of dominance within this group most likely reflects different adaptations within an r selected ecological strategy related to nutrient concentration lowest diversities were recorded in mudstone oil shale lithologies where watznaueria britannica is the dominant species suggesting adaptation to the highest nutrient concentrations watznaueria barnesiae fossacincta is particularly dominant in coccolith stone bands where diversity is slightly higher suggesting adaptation to lower high nutrient levels cyclagelosphaera margerelii appears to be the most extremely r selected species exploiting unusual very high trophic lowered sea level conditions that excluded even w britannica d 2005 elsevier b v all rights reserved

authigenic molybdenum isotope signatures in marine sediments we present new mo isotope data from the mexican continen tal margin that in conjunction with previous data allow us to propose a mechanistic description of the mo isotope system in ma rine sediments we hypothesize that there are unique environmen tally dependent mo isotope signatures recorded in marine sedi ments that reflect the mechanisms responsible for authigenic mo accumulation open ocean anoxic sites defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water exhibit a 98 95 mo isotope signature of 1 6 we believe this val ue reflects mo sulfide formation via diagenetic processes within sediments quantitative formation of mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value 2 3 as typified by samples from the highly sulfidic black sea in contrast to these reducing settings manganese oxide rich sediments have measured mo isotope values that are more negative relative to seawater than any other sediment samples analyzed to date similar to fe mn crusts 0 7 most measured mo isotope compositions of marine sediments from open ocean settings appear to reflect a mix ture of lithogenic mo 0 0 and the mo signature of a specific authigenic mo accumulation mechanism we therefore suggest that mo isotopes may record unique signatures that reflect the domi nant chemical mechanism for mo sequestration into sediments

a sequence stratigraphy of the kimmeridgian and bolonian stages late jurassic wessex weald basin southern england a sequence stratigraphic framework for the late jurassic kimmeridgian and bolonian stages of the wessex weald basin southern england is proposed based on the integration of sedimentological geophysical and geochemical data the nerc funded rapid global geological events rgge boreholes of swanworth quarry 1 and 2 and metherhills 1 are used as reference sections eleven complete depositional sequences and their component systems tracts are recognized within the kimmeridge clay formation bounded by 12 sequence boundaries km1 12 seventy four boreholes have been used in this study during the kimmeridgian a major transgression associated with tectonic movements led to a marked change in basin geometry major deepening of the sea during the late kimmeridgian eudoxus zone led to a change from unconformities of basinwide extent to those confined to the basin margins the progressive deepening of the basin towards a highstand in the mid bolonian wheatleyensis to pectinatus zones was associated with a broad scale change in sedimentary faunal and geochemical characters the layer cake basin architecture and thickening of the kimmeridge clay formation towards the basin centre suggests a dominantly aggradational system the partially enclosed nature of the kimmeridgian and bolonian seas generated basin dynamics more analogous to those of modern day meromictic lakes consequently sequence stratigraphic approaches derived from continental shelf margins are inappropriate in epicontinental basins of this type

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

sulfidity controls molybdenum isotope fractionation into euxinic sediments evidence from the modern black sea molybdenum mo isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry it is therefore fundamental to understand the processes controlling mo partitioning into modern marine environments this study identifies the availability of dissolved sulfide as the dominant control on overall mo removal from the water column in euxinic systems mo isotopic composition of surface sediments from different localities of the black sea demonstrates complete fixation of mo only below 400 m water depth above a critical concentration of 11 mol l 1 aqueous hydrogen sulfide in the bottom water the mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2 3 in contrast significant mo isotope fractionation into less euxinic sediments is evident at shallower depths in the black sea as well as in temporarily euxinic deeps of the baltic sea consistent with the observed lower maximum sulfide concentrations in the respective water columns therefore mo isotope signatures in the modern black sea constrain the processes responsible for global mo removal from the ocean by euxinic sediments furthermore models of past ocean anoxia reconstruction have to consider that the seawater mo isotopic composition is not per se archived in euxinic sediments

the isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans despite important biological and biogeochemical consequences of extensive ocean anoxic events their identification is controversial the marine isotope geochemistry of molybdenum can help quantify the past oxygenation state of the ocean if the riverine input of mo isotopes is known analysis of a set of rivers that account for 28 of global river runoff suggests more variable mo isotopic ratios in rivers that are also isotopically enriched in the heavy isotopes suggesting near total anoxia in the proterozoic ocean and during mesozoic ocean anoxic events despite its important biological and biogeochemical consequences1 2 3 the identification of extensive oceanic anoxia in the geological record is controversial in particular global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean or in tectonically isolated basins such as the modern black sea the marine isotope geochemistry of molybdenum mo can help quantify the past oxygenation state of the ocean4 5 6 7 8 9 10 11 12 13 14 because to first approximation under oxic conditions lighter isotopes of mo are preferentially removed to sediments whereas in euxinic conditions quantitative removal leads to no fractionation however the isotopic composition of the mo input from rivers the main contributor of mo to the oceans is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4 5 6 7 8 9 10 11 12 13 14 here we present an isotopic analysis of mo in a set of rivers that together account for 22 of the global riverine water discharge we find a broad range of variability in the mo isotopic composition of these rivers with almost all samples enriched in the heavy isotopes compared with continental rocks our data remove key uncertainties associated with the marine mo isotope budget14 and strongly suggest near total anoxia in the mid proterozoic ocean8 as well as during mesozoic ocean anoxic events13

variation in marine total organic carbon through the type kimmeridge clay formation late jurassic dorset uk bulk geochemical data from the nerc rapid global geological events project are used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type kimmeridge clay formation in southern dorset there is a symmetrical stratigraphic pattern in mean and modal total organic carbon toc values which increase from 1 2 at the top and base to a peak of 8 9 in the middle of the formation to remove distortion caused by the strong positive skewness of the toc values the mean toc per zone was recalculated for the first mode of the toc distribution only and combined pragmatically with pyrolysis estimates of the reactive marine toc fraction to derive marine toc values these also vary symmetrically from c 0 5 to peak values of c 5 0 the mean toc is negatively correlated with sedimentation rate indicating that dilution is a significant controlling variable mean palaeoproductivity was assessed using modern marine sediment relationships between carbon burial efficiency sedimentation rate derived from rgge cyclostratigraphic analyses the likely dissolved oxygen range and water depth best estimates of palaeoproductivity range from 40 to 150 g c m 2 a 1 low to moderate by comparison with present day shelves and are positively but nonlinearly correlated with mean 13 c toc values

molybdenum isotope fractionation during adsorption by manganese oxides the isotopic composition of mo 97 95mo in seawater is 2 heavier than mo in marine ferromanganese crusts and nodules barling et al earth planet sci lett 193 2001 447 457 siebert et al earth planet sci lett 211 2003 159 171 to explore this phenomenon we have conducted an experimental investigation into the mass dependent fractionation of mo isotopes during adsorption onto mn oxyhydroxide two series of experiments were carried out a time series in which adsorption proceeded for 2 96 h and a ph series in which ph varied from 6 5 to 8 5 the extent of mo adsorption by mn oxyhydroxides decreases with increasing ph a trend typical of anion adsorption and takes 48 h to reach steady state lighter mo isotopes are preferentially adsorbed experimentally determined fractionation factors soln mnox exhibit no systematic variation with either time or experimental ph the mean soln mnox for all experiments is 1 0018 0 0005 2 s d comparison of the mo isotopic data for experimental solutions and mo adsorbed to mn oxyhydroxide with predictions for closed system equilibrium and rayleigh fractionation models indicates that isotope fractionation occurs as a result of closed system equilibrium exchange between dissolved and adsorbed mo the isotopic offset between dissolved and adsorbed mo is comparable to that observed between mo in seawater and mo in ferromanganese nodules and crusts it is therefore likely that adsorption of mo to mn oxyhydroxides is a significant factor in the fractionation of mo isotopes in the oceans

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

molybdenum isotope evidence for global ocean anoxia coupled with perturbations to the carbon cycle during the early jurassic relatively brief periods of severe paleoenvironmental change during the jurassic and cretaceous were associated with the widespread accumulation of organic rich marine deposits termed oceanic anoxic events oaes these intervals involved abrupt global warming of 5 10 c higher rates of continental weathering elevated extinction rates and large scale perturbations to the global carbon cycle the major oaes also overlapped temporally the emplacement of large igneous provinces however despite being known as oaes the extent of seawater anoxia at those times is undefined and the causative processes remain unclear here we show how changes in seawater molybdenum isotope ratios a proxy for seawater anoxia during the toarcian early jurassic oae define the onset and expansion of oxygen deficient conditions our data also place constraints on the areal extent of marine anoxia during the event and demonstrate that anoxia expanded and contracted periodically broadly in line with precession driven changes in 13 corg despite their intermittent occurrence over geological history oaes have an important contemporary relevance because the magnitude and high rates of environmental change then were broadly similar to those occurring at the present day

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

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

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

astrochronology of the late jurassic kimmeridge clay dorset england and implications for earth system processes the late jurassic kimmeridge clay formation kcf is an economically important organic rich source rock of kimmeridgian early tithonian age the main rock types of the kcf in dorset uk include grey to black laminated shale marl coccolithic limestone and dolostone which occur with an obvious cyclicity at astronomical timescales in this study we examine two high resolution borehole records swanworth quarry i and metherhills 1 obtained as part of a rapid global geological events rgge sediment drilling project datasets examined were total organic carbon toc and borehole wall microconductivity by formation microscanner fms our intent is to assess the rhythmicity of the kcf with respect to the astronomical timescale and to discuss the results with respect to other key late jurassic geological processes power spectra of the untuned data reveal a hierarchy of cycles throughout the kcf with similar to 167 m similar to 40 m 9 1 m 3 8 m and 1 6 m wavelengths tuning the similar to 40 m cycles to the 405 kyr eccentricity cycle shows the presence of all the astronomical parameters eccentricity obliquity and precession index in particular similar to 100 kyr and 405 kyr eccentricity cycles are strongly expressed in both records the 405 kyr eccentricity cycle corresponds to relative sea level changes inferred from sequence stratigraphy intervals with elevated toc are associated with strong obliquity forcing the 405 kyr tuned duration of the lower kcf kimmeridgian stage is 3 47 myr and the upper kcf early part of the tithonian stage elegans to fittoni ammonite zones is 3 32 myr two other chronologies test the consistency of this age model by tuning similar to 8 10 m cycles to 100 kyr short eccentricity and similar to 3 5 m cycles to 36 kyr jurassic obliquity the obliquity tuned chronology resolves an accumulation history for the kcf with a variation that strongly resembles that of earth s orbital eccentricity predicted for 147 2 ma to 153 8 ma there is evidence for significant non deposition up to i million years in the lowermost kcf baylei mutabilis zones which would indicate a kimmeridgian oxfordian boundary age of 154 8 ma this absolute calibration allows assignment of precise numerical ages to zonal boundaries sequence surfaces and polarity chrons of the lower m sequence c 2009 elsevier b v all rights reserved

preservation of carbohydrates through sulfurization in a jurassic euxinic shelf sea examination of the blackstone band toc cycle in the kimmeridge clay formation uk a complete total organic carbon toc cycle in the upper jurassic kimmeridge clay formation kcf comprising the extremely toc rich 34 blackstone band was studied to investigate the controlling factors on toc accumulation compared with the under and overlying strata toc in the blackstone band was enriched by a factor of six and concomitantly the 13ctoc shows a 4 enrichment al normalized toc values indicated that the enhanced toc values were probably caused by increased toc accumulation and not by a decreased dilution with inorganic matter the amounts of short chain alkylated thiophenes and the sulfur rich unresolved complex mixture ucm in the kerogen pyrolysates as well as the toc al ratios correlated linearly with 13ctoc for sediments with toc al ratios 1 7 the alkylated thiophenes and sulfur rich ucm both originate from sulfurized carbohydrate carbon ccarb suggesting that the primary cause of the toc maximum is the enhanced contribution of ccarb to toc since carbohydrates can be substantially 13c enriched over lipids in biomass the enhanced contribution of ccarb explains the enriched 13ctoc values compound specific isotope data showed that primary productivity during deposition of the kcf toc cycle varied little while a two member isotopic mixing model showed that the preservation of ccarb relative to that of the lipid carbon may have increased by a factor of 10 in the blackstone band the enhanced preservation of ccarb was most likely caused by more frequent or longer lasting events of photic zone euxinia as revealed by the concentration of isorenieratene derivatives enhanced contributions of ccarb have also been observed in other kcf cycles suggesting that enhanced preservation of ccarb rather than an increase in primary production exerted direct control on the toc cycles of the kcf

late jurassic climate and its impact on carbon cycling abstract the climate of the late jurassic has been characterized by high atmospheric co2 levels and by a monsoonal rainfall pattern in this study we traced the evolution of the global carbon cycle through the late jurassic with the help of carbonate carbon isotope stratigraphy the oxfordian tithonian 13c curve is marked by one major positive carbon isotope excursion with an amplitude 13 c 1 0 middle to late oxfordian and a second minor positive excursion with an amplitude 13 c late kimmeridgian the early kimmeridgian and early tithonian 13c values fluctuate around 13 c 2 0 3 and contrast with the less positive 13c values of early oxfordian and late tithonian age 13 c 1 0 5 a comparison of the late jurassic carbonate carbon isotope curve with the occurrence of organic rich sediments suggests that not only fluctuations in organic carbon burial but also in carbonate carbon burial had an impact on the c isotope record the oxfordian c isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level however episodes of enhanced organic carbon burial during the kimmeridgian and early tithonian are not reflected by prominent spikes in the c isotope record favourable conditions for carbonate platform growth at a time of high global sealevel may have resulted in the stabilisation of the c org c carb burial ratio and hence maintained the 13c record at steady but relatively positive values the middle and late tithonian c isotope values drop below 13 c 1 5 a similar shift to less positive c isotope values was recognized in other c isotope records from the tethys and atlantic oceans and reflects a decrease in the c org c carb burial ratio possibly related to a reorganisation of the global climate system globally widespread marine black shale mature quartzose sandstone assemblages suggest that the relative efficiency of the late jurassic carbon pumps was controlled by weathering erosion and runoff causing widespread marine eutrophication eutrophication favoured the organic carbon pump but it diminished the carbonate platform growth potential the monsoonal rainfall distribution pattern may explain why late jurassic carbonate platforms experienced less severe growth crises than early cretaceous carbonate platforms

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

control of the preservation of organic matter by productivity and sulphate reduction in kimmeridgian shales from dorset uk abstract the kimmeridgian shales of dorset uk have a very wide range of total organic carbon toc content and hydrogen index hi these cyclic changes of organic carbon accumulation are usually interpreted in terms of biomass changes induced by variations in productivity the influence of such variations on the intensity of sulphate reduction and consequently on the preservation of organic carbon were studied the pyrite content does not reflect the full extent of sulphate reduction for sediments with a toc content greater than 10 when easily reducible iron is no longer available h 2 s is preferentially fixed as organic compounds in algal bodies a sulphate reduction index sri initial c org residual c org was calculated for each sediment on the basis of the sulphate reduction reaction stoichiometry and a drastic change in sulphate reduction intensity was seen for toc contents of about 10 above this value the sri is low and decreases with increasing toc content the sediments are rich in organic carbon composed of well laminated stacked algal bodies and were probably deposited at an oxygen depleted water sediment interface in this instance the quality and the amount of organic matter buried depends directly on the organic fluxes towards the redox interface it is assumed that the high relative input of organic matter which can be metabolized exceeds the availability of sulphate in the pore waters so that part of it escapes sulphate reduction and is probably degraded by other processes methanogenesis for toc contents less than 10 the sri is higher and variable the sediments show traces of burrowing and the depositional conditions are thought to be dysaerobic it is proposed that under such conditions bioturbation brings both sulphate and reducible iron into the sediment and that sulphate reduction mainly expressed by the presence of pyrite consumes all the available organic matter which can be metabolized the strong variability in the intensity of sulphate reduction depends on various parameters such as the genetic type of planktonic organic matter the method of transportation to the sediment and possibly the earlier oxic degradation during transportation

effects of an oceanic anoxic event on the stable carbon isotopic composition of early toarcian carbon the early toarcian transgression is marked by the occurrence of organic carbon rich shales in large parts of western europe and in other parts of the world as well based on the positive carbon isotope excursion of pelagic limestones in the middle to upper part of the falciferum zone in several of the tethyan sections the widespread occurrence of the early toarcian shales was explained by an oceanic anoxic event oae jenkyns 1988 jenkyns and clayton 1997 the rapid burial of large amounts of organic carbon which is rich in 12 c would have led to a relative enrichment in 13 c of the global carbon reservoir and hence to an increase in delta 13 c of the limestones in southwest germany and a number of other locations both organic and inorganic carbon display a negative isotope excursion in the upper part of the tenuicostatum zone and the lower part of the falciferum zone synchronous to black shale deposition and predating the positive isotopic excursion kuspert 1982 1983 it is presently unclear what the exact cause for this widespread negative isotopic excursion is jenkyns and clayton 1997 the objective of our study was to examine if the negative carbon isotope excursion of organic matter om in the southwest german toarcian shales can be attributed to compositional changes of the om or if the excursion is related to variations in delta 13 c sub co2 aq in the photic zone to this end we analyzed the molecular and stable carbon isotope composition of om in ten samples spanning the early toarcian toc 2 5 10 5 percent hi 350 700 biomarker distributions and kerogen pyrolysates differ only slightly among the samples and delta 13 c values of primary production markers follow closely those of associated carbonate and kerogen delta 13 c sub carb toc this strongly suggests that differences in delta 13 c toc are related to differences in delta 13 c of co 2 aq in the photic zone supporting kuspert9s model of advection of isotopically light organic matter derived co 2 from anoxic bottom waters evidence for euxinic conditions extending into the photic zone was found by the presence of derivatives of carotenoids such as isorenieratane from anoxygenic phototrophic sulfur bacteria their concentrations are extremely low in black shales from the tenuicostatum zone but are significant in the falciferum and bifrons zone with a maximum in the exaratum subzone suggesting that photic zone euxinia pze was a common feature of the early toarcian sea in southwest germany our data therefore support the kuspert model of advection of isotopically light organic matter derived co 2 from anoxic bottom waters to the upper part of the photic zone resulting in a negative isotope spike the enrichment in 13 c in the middle of the falciferum zone which followed the negative spike may be attributed to the burial of isotopically light organic matter through worldwide black shale deposition