the posidonia shale lower toarcian of sw germany an oxygen depleted ecosystem controlled by sea level and palaeoclimate abstract the lower toarcian posidonia shale is famous for its excellently preserved fossils and its high amount of organic matter up to 16 both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions high resolution geochemical sedimentological and palaeoecological investigations of various sections of the posidonia shale in sw germany however indicate that oxygen availability was variable and ranged from short oxygenated periods to longer term anoxia the benthic macrofauna consists of nine fossil communities and was used in combination with geochemical data to reconstruct a time averaged oxygen curve anoxic conditions prevailed during the deposition of the toarcian black shales they were however punctuated by various short periods weeks to years with oxygenated bottom water conditions sedimentological e g distinctiveness of microlamination siliciclastic content and geochemical parameters e g organic matter content isotopic signatures 18o and 13c molecular redox parameters pristane phytane ratio arylisoprenoids exhibit a remarkable covariation and seem to be controlled by sea level fluctuations maximum oxygen depletion and an extreme negative shift of 13corg values 34 occurred during the early falciferum zone this is explained by the recycling of 12c enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia the subsequent transgression permitted enhanced water exchange with the tethyan ocean and caused improvement of living conditions at the end of the falciferum zone other important factors controlling the depositional environment are the overall palaeogeographic situation and climate the early jurassic is the latest period before break up of pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade and monsoon wind systems an estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti estuarine circulation and a negative water balance in the winter during the summer months a stratified water column with anoxic conditions below the halocline developed 18o data indicate low salinity in the surface water during the monsoon influenced summer high productivity was then located in the photic zone and the corresponding isotopically light 18o signal was fixed in the calcareous nannoplankton during the winter months a saline circulation system brought oxygen to the benthic environment favouring temporary benthic colonization especially during times of relative sea level high stand

the early toarcian anoxia a synchronous event in the western tethys an approach by quantitative biochronology unitary associations applied on calcareous nannofossils during the early toarcian major paleoenvironnemental and paleoceanographical changes occurred leading to an oceanic anoxic event oae and to a perturbation of the carbon isotope cycle although the standard biochronology of the lower jurassic is essentially based upon ammonites in recent years biostratigraphy based on calcareous nannofossils and dinoflagellate cysts is increasingly used to date jurassic rocks however the precise dating and correlation of the early toarcian oae and of the associated 13c anomaly in different settings of the western tethys are still partly problematic and it is still unclear whether these events are synchronous or not in order to allow more accurate correlations of the organic rich levels recorded in the lower toarcian oae this account proposes a new biozonation based on a quantitative biochronology approach the unitary associations ua applied to calcareous nannofossils this study represents the first attempt to apply theuamethod to jurassic nannofossils the study incorporates eighteen sections distributed across western tethys and ranging from the pliensbachian to aalenian comprising 1220 samples and 72 calcareous nannofossil taxa the biograph savary j guex j 1999 discrete biochronological scales and unitary associations description of the biograph computer program memoires de geologie de lausanne 34 282pp and ua graph copyright hammer o guex and savary 2002 softwares provide a discrete biochronological framework based upon multi taxa concurrent range zones in the different sections the optimized dataset generates nine uas using the co occurrences of 56 taxa these uas are grouped into six unitary association zones ua z which constitute a robust biostratigraphic synthesis of all the observed or deduced biostratigraphic relationships between the analysed taxa the ua zonation proposed here is compared to classic calcareous nannofossil biozonations which are commonly used for the southern and the northern sides of tethys the biostratigraphic resolution of the ua zones varies from one nannofossil subzone or part of it to several subzones and can be related to the pattern of calcareous nannoplankton originations and extinctions during the studied time interval the late pliensbachian early toarcian interval corresponding to the ua z ii represents a major step in the jurassic nannoplankton radiation the recognized ua zones are also compared to the carbon isotopic negative excursion and tocmaximum in five sections of central italy germany and england with the aim of providing a more reliable correlation tool for the early toarcian oae and of the associated isotopic anomaly between the southern and northern part of western tethys the results of this work show that the toc maximum and 13c negative excursion correspond to the upper part of the ua z ii i e ua 3 in the sections analysed this suggests that the early toarcian oae was a synchronous event within the western tethys

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

isorenieratane record in black shales from the paris basin france constraints on recycling of respired co2 as a mechanism for negative carbon isotope shifts during the toarcian oceanic anoxic event the prominent negative stable carbon isotope excursion in both carbonate and organic carbon recorded in organic rich sediments deposited during the toarcian oceanic anoxic event oae has commonly been explained by recycling of 13c depleted co2 the so called kuspert model more recently the massive release of 13c depleted methane or other forms of 13c depleted carbon was also proposed to account for the observed negative 13c excursions in organic carbon of terrigenous as well as of marine origin the occurrence of diagenetic products of the carotenoid isorenieratene isorenieratane and other aryl isoprenoids in toarcian black shales has been regarded as supporting evidence for the kuspert hypothesis as they point to strong stratification of the epicontinental seas a section of a drill core straddling the toarcian of the paris basin cirfontaineen ornois contained intact isorenieratane providing evidence that photosynthetic green sulphur bacteria were present at the time of deposition even prior to the oae however the isorenieratane abundances are very low in the section where the negative 13c excursion in organic carbon and phytane a chemical fossil derived from chlorophyll occurs the abundance of the isorenieratene derivatives increases once the 13c records have shifted to more positive values the 13c of isorenieratane generally circa 13 1 0 5 indicates that the respired co2 contribution at the chemocline was low and is thus not likely to be the main cause of the prominent up to 7 negative 13c shift recorded in toarcian organic carbon records

nannofacies analysis as a tool to reconstruct paleoenvironmental changes during the early toarcian anoxic event the black shales of lower toarcian schistes carton in france jet rock in england posidonia shale in germany are sediments formed by fine alternations of millimetric to sub millimetric clear and dark laminae a study of polished core surfaces dotternhausen core sw germany was carried out in sem in order to characterize the micro fabrics of different laminae and their content in nannofossils two main intervals were recognized where fabrics and nannofossil content are significantly different in the clay rich interval characterized by high wt toc and a large negative carbon isotope excursion both in carbonates and organic matter the dominant micro fabrics are well foliated or lumpy giving evidence of the absence or reduction of benthic life nannofossils are scarce and only concentrated in discrete laminae separated by large abiotic intervals assemblages are indicative of a deep nutricline but sporadic nutrient inputs occurred as demonstrated by fluctuation in abundance of specimens of the biscutaceae nutrients were probably delivered to the german basin via riverine influx different sized framboids of pyrite were recognized in this interval the smallest ones 2 m being probably precipitated in an anoxic and sulphidic water column the clay rich interval was likely deposited during low sea level in a restricted basin these conditions were followed by a sea level rise and better water mass circulation within the german basin sedimentation records an increase in carbonate deposition organic matter content decreased and carbon isotopes came back to pre excursion values micro fabrics became less markedly laminated pyrite framboids nearly disappeared and rich and diversified nannofossil assemblages are recorded sea water conditions were probably highly fluctuating as indicated by alternation of intervals dominated in turn by schizosphaerella spp flourishing under generally oligotrophic conditions where nutrients were temporarily available in surface waters due to storms and by crepidolithus spp a deep dweller the results of this work fit well with a silled basin model

the impact of the wind stress curl in the north atlantic on the atlantic inflow to the norwegian sea toward the arctic 1 the norwegian atlantic current nwac through the norwegian sea serves as a conduit of warm and saline atlantic water from the north atlantic to the arctic ocean an important factor for climate and ecology in this study we concentrate on how the north atlantic wind stress curl nawsc affects interannual variability on the major branch of the nwac the norwegian atlantic slope current nwasc based on wind stress data from the ncep reanalysis and estimated volume transport of the nwasc from current records during 1995 2003 in the svinoy section 62 n our analysis shows that the volume transport in the nwasc exhibits a maximum correlation of 0 88 with the zonally integrated nawsc at 55 n 15 months earlier our findings reveal the nawsc to be a major forcing for interannual variability of the nwasc in the range of 3 0 to 5 3 sv sv 106 m3 s 1 the 15 month time lag appears to be in accordance with a forced baroclinic rossby wave in response to the local ekman pumping changing the baroclinicity and strength of the north atlantic current nac after being converted to a nearly barotropic shelf edge current along the irish scottish shelf through interaction with bottom topography it appears as a barotropic response downstream in the svinoy section this result suggests the possibility of predicting conditions influenced by the nwasc more than a year in advance using the nawsc as a proxy

reply to comment on carbon isotope record of the early jurassic toarcian oceanic anoxic event from fossil wood and marine carbonate lusitanian basin portugal mcarthur s comment focuses on the origin of a negative carbon isotope excursion reported from terrestrial and marine organic matter and marine carbonate in early jurassic toarcian strata from a number of localities in northern and western europe he argues that the absence of a clear negative excursion in belemnite calcite falsifies the hypothesis that the observed negative excursion in other materials originates from a methane source we note that mcarthur telescopes the argument that we present in our paper we firstly interpret the new data as supporting the hypothesis that isotopically light carbon dominated the global ocean atmosphere biosphere carbon reservoirs during the early toarcian oceanic anoxic event oae and secondly conclude that this phenomenon could be explained by a massive input of carbon from a methane source as we discuss in the paper the belemnite c values could only be regarded as falsifying this hypothesis were it to be shown unambiguously that belemnite calcite more accurately recorded the isotopic characteristics of global carbon reservoir than all the other lithological constituents we have already noted the difficulty of accurately locating samples on extensive foreshore exposures where the dip is at a low angle and because belemnites are typically absent from strata displaying the peak of the negative excursion assignment of specimens to these levels needs to be documented with particular care moreover because belemnites must have been active swimmers inhabiting different geographic areas and different levels in the water column their carbon isotope ratios cannot always be relied upon asmonitors of a global ocean reservoir particularly at times of extreme ocean stratification the coccolith bearing bulk carbonate from peniche on the other hand is stratigraphically located with centimetric accuracy it cannot have been derived from actively swimming organisms and most likely records the isotopic characteristics of the surface waters of the lusitanian basin mcarthur is also mistaken in suggesting that the local absence of the negative excursion in datasets from some other european sections somehow proves that the excursion was not a global phenomenon a well known property of the stratigraphic record is that it is commonly extremely condensed or incomplete furthermore the earth and planetary science letters 259 2007 640 641 www elsevier com locate epsl

volcanism co2 and palaeoclimate a late jurassic early cretaceous carbon and oxygen isotope record a composite tethyan late jurassic early cretaceous carbon and oxygen isotope curve is presented c isotope data provide information on the evolution and perturbation of the global carbon cycle o isotope data are used as a palaeotemperature proxy in combination with palaeontological information the resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification positive c isotope anomalies in the valanginian and aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide a major warming pulse accompanies the aptian but not the valanginian c isotope event the observed change in early aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion both c isotope anomalies are accompanied by biocalcification crises that may have been triggered by p co 2 induced changes in climate and in surface water chemistry elevated nutrient levels in river influenced coastal waters and in upwelling regions further weakened marine calcification these conditions contrast with normal trophic conditions prevailing in the latest jurassic and favouring biocalcification the c and o isotope curves record a stable mode of carbon cycling and stable temperatures we conclude that biocalcification is mostly triggered and inhibited by co 2 conditions in the atmosphere ocean system

toarcian oceanic anoxic event an assessment of global causes using belemnite c isotope records from black shales markingthe toarcian oceanic anoxic event t oae the first explanation envisions recycling of dissolved inorganiccarbon dic with a light isotopic signature into the photic zone from the lower levels of a salinity stratifiedwater mass essentially requiring a regional paleoceanographic driver of the carbon cycle the second involvesthe rapid and massive dissociation of methane from gas hydrates that effectively renders the t oae a globalperturbation of the carbon cycle we present c isotope records from belemnites d

carbon isotope records of the early jurassic toarcian oceanic anoxic event from the valdorbia umbria marche apennines and monte mangart julian alps sections palaeoceanographic and stratigraphic implications the toarcian oceanic anoxic event ca 183 ma coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere this study is based on high resolution sampling of two stratigraphic successions located in valdorbia umbria marche apennines and monte mangart julian alps italy which represent expressions of the toarcian oceanic anoxic event in deep water pelagic sediments these successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations compared with coeval strata in northern europe generally 2 and low hydrogen indices on this basis they are similar to other toarcian black shales described from the tethyan region the positive and negative carbon isotope records from the two localities permit a high resolution correlation such that ammonite biostratigraphy information from valdorbia can be transferred to those parts of the monte mangart section that lack these fossils spectral analyses ofd 13 corg values and of caco3 percentages from the sedimentary records of both the valdorbia and monte mangart sections reveal a strong cyclic pattern best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon isotope excursion based on the carbon isotope curves obtained the high resolution correlation between the italian successions and a section in yorkshire northern europe confirms the supposition that the apparent mismatch between the dating of the toarcian oceanic anoxic event in the boreal and tethyan realms is an artefact of biostratigraphy

lower jurassic epicontinental carbonates and mudstones from england and wales chemostratigraphic signals and the early toarcian anoxic event sections through lower jurassic epicontinental carbonates from southern britain junction bed and equivalent show a positive carbon isotope excursion 13ccarbonate detectable in bulk rock in the falciferum zone of the lower toarcian isotopic data from organic matter in more clay rich sections from wales and north east england together with determinations on belemnite calcite indicate that highest 13c values are localized in the upper exaratum subzone of the falciferum zone levels of particular enrichment in organic carbon were developed in the early to mid exaratum subzone and hence pre date this 13c maximum these phenomena reflect the impact of the early toarcian oceanic anoxic event in the british area similar isotopic trends have been recorded in other toarcian sections from tethyan europe and are interpreted as reflecting the chemistry of sea water on the assumption of isotopic correlation between the english and tethyan sections the 13c maximum would be everywhere dated as latest exaratum subzone in terms of the north european ammonite scheme absolute oxygen isotope values in carbonates probably reflect both early diagenetic cementation and later temperature related burial diagenesis although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum zone subsequent climatic deterioration may have been triggered by drawdown of co2 related to regional excess carbon burial during the oceanic anoxic event using the positive 13c excursion as a correlative level in sections from different faunal provinces britain italy and spain implies that lower toarcian zonal stratigraphy is diachronous between northern and southern europe there is evidence for partitioning of water masses between the north european shelf and the tethyan continental margin during the early jurassic

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

observation and modeling of the suction pump effect during rapid dilatant slip 1 the temporary pressure drop observed during post peak dilatant slip in axi symmetric testing of a porous aeolian sandstone is modeled using a simple function of opening caused by sliding along the fault surface results suggest dilatancy of 0 02 mm accompanying frictional sliding less than 10 of the typical grain diameter of 0 25 0 50 mm the rapid void opening under nominally undrained conditions at constant upstream flow rate causes a strong suction pump effect a theoretical maximum pressure drop of 0 09 mpa or more consistent with the experimentally measured results can occur without a change in permeability properties

no evidence for thermogenic methane release in coal from the karoo ferrar large igneous province abstract the early jurassic toarcian oceanic anoxic event t oae and concurrent negative carbon isotope 13 c excursion have recently been attributed to either the release of methane ch 4 clathrates or thermogenic ch 4 gas associated with the karoo ferrar large igneous province lip into coals and organic rich shales 12 c enriched thermogenic ch 4 production associated with the karoo ferrar would result in residual material being 12 c depleted nearer the intrusions in this study geochemical analyses carbon isotopes volatile matter vm vitrinite reflectance r o are reported for two coal transects associated with dykes intruding the no 4l coal in the highveld coalfield karoo basin south africa vm decreases from over 35 to around 15 in one transect and the second transect shows a less pronounced decrease from 25 to 16 accompanying the decrease in vm content is an increase in r o from background levels of around 0 7 to over 4 adjacent to the dyke used as a palaeo geothermometer r o values indicate background temperatures of 100 c increasing to 300 c close to the contact despite changes in vm and r o there are no significant changes in 13 c certainly not of the magnitude that would be expected associated with large scale thermogenic ch 4 generation these and other gondwanan coals have low vitrinite and liptinite contents components more prone to ch 4 generation in part explaining the modest decreases in vm adjacent to the dykes this combined with the relatively narrow metamorphic aureole surrounding the intrusions and the likelihood that at least some of the volatiles generated by the intrusion were trapped as coalbed ch 4 or condensed as pyrolytic carbon suggests only limited ch 4 release in addition based on original estimates of moisture contents in these coals and the depth at time of intrusion 1 000 2 000 m the dykes would have lost most of their energy heating and evaporating water thus having very little remaining energy to generate thermogenic ch 4

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

the early toarcian early jurassic and the cenomanian turonian late cretaceous mass extinctions similarities and contrasts abstract the early toarcian eto and cenomanian turonian c t mass extinctions are comparable from a wide range of scales and perspectives from a broad standpoint their similarities include virtually identical extinction intensity at the familial and generic levels widespread basinal facies deposited during sea level highstands an overall greenhouse climate and anoxia as an important causal mechanism the high resolution macroinvertebrate data analyzed here consisting of stratigraphic ranges diversity and abundance point to smaller scale similarities the two events resulted in significant ecological disruption and in both cases the biotic responses were very similar taxa inhabiting the upper water column were unaffected by anoxia and included ammonites and in the eto belemnites in addition epifaunal taxa adapted to low oxygen conditions such as the buchiids posidoniids and inoceramids flourished in the post extinction environment during the survival interval as conditions ameliorated the biota became more diverse and gradually began to resemble pre extinction biotas furthermore the 13c curves predict the end of the survival interval and suggest that the period characterized by carbon isotope excursions represent disrupted environmental conditions this points to the potential application of 13c as a tool for determining the repopulation modes and timing for other mass extinctions

new oxygen isotope evidence for long term cretaceous climatic change in the southern hemisphere a new composite 18 o record generated from calcareous fine fraction and bulk sediments from the exmouth plateau details long term cretaceous climatic change at mid latitudes in the southern hemisphere assessment of new and previously published 18 o data indicates that a mid cretaceous global climatic optimum was achieved sometime between the time of the cenomanian turonian boundary and the middle turonian when surface ocean paleotemperatures were the highest of the past 115 m y periods of cooling and warming that reversed the general patterns were superimposed on long term aptian turonian warming and turonian maastrichtian cooling trends respectively extrapolation of southern hemisphere paleotemperature trends to maastrichtian paleotemperature data from a low latitude pacific guyot implies that maximum mid cretaceous low latitude paleotemperatures could have been in excess of 33 c

paleoceanographic changes of the late pliensbachian early toarcian interval a possible link to the genesis of an oceanic anoxic event abstract secular records of the elemental and isotopic composition of belemnite calcite were studied in pliensbachian and toarcian sections from the yorkshire coast uk and southern germany to investigate oceanographic change during an interval prior to and including the toarcian oceanic anoxic event oae records from southern germany are correlated to the uk stratigraphy using strontium isotope stratigraphy the geochemical trends measured from belemnite calcite are consistent between the two sections and are interpreted in terms of temperature and salinity of the northwest european epi continental sea the data suggest that a dramatic environmental change coincided with the toarcian oae belemnite mg ca sr ca and na ca ratios increase by a factor of between 1 7 and 2 coincident with a 3 negative shift in 18o from the mid tenuicostatum zone until the lower falciferum zone of the uk ammonite biostratigraphy a period of 0 6 0 7 myr taken at face value the mg ca and 18o data argue for an abrupt warming of 6 7 c and substantial freshening during this interval global warming accompanied by an accelerated hydrological cycle and increased runoff is proposed to explain these changes prior to these events data from lower in the yorkshire section suggest a possible cooling accompanied by a shift to more saline waters during the period from the upper pliensbachian margaritatus zone to the toarcian lower tenuicostatum zone this earlier event may also have been important in causing density stratification in the northwest european epi continental sea

the early toarcian anoxic event in the westernmost tethys subbetic paleogeographic and paleobiogeographic significance a positive delta 13 c excursion reveals an oceanic anoxic event in the subbetic corresponding to the westernmost end of the tethys during the early jurassic this positive delta 13 c excursion exists in five stratigraphic sections consisting of lower and middle toarcian pelagic sediments the positive delta 13 c excursion in the subbetic is located in the upper part of the early toarcian serpentinus approx falciferum biozone while in other tethyan regions alps apennines and dinarides it has been found in the lower part of the same biozone the most significant positive delta 13 c excursion detected in the subbetic is therefore about half a million years younger although other relative peaks had been detected this isotopic anomaly can be correlated with a maximum transgression that occurred earlier in the regions closer to the open sea of the tethys ocean other features of the anoxic event faunal crises and black shale deposits appear at different ages related t

tethyan carbonate carbon isotope stratigraphy across the jurassic cretaceous boundary an indicator of decelerated global carbon cycling the carbon isotope record in four pelagic carbonate sections from the southern alps northern italy across the jurassic cretaceous boundary has been correlated to biostratigraphy and magnetostratigraphy the carbon isotope curve from bulk carbonates shows a decrease from kimmeridgian to early tithonian cm24 cm22 values of 13c 2 07 0 14 to late tithonian and berriasian cm18 cm14 values of 13c 1 26 0 16 the change in the carbon isotope record coincides with changes in tethyan calcite and silica accumulation rates with a drop in the calcite compensation depth in the atlantic and tethys oceans and with changes in organic carbon burial along the eurasian margin of the tethys reduced surface water productivity due to diminished transfer rates of biolimiting elements into the atlantic and tethys oceans can explain these observations the decreased transfer rates of elements such as silica or phosphorus from continents into the oceans resulted from drier climatic conditions and decreased water runoff on continents bordering the tethys and atlantic oceans the proposed changes in tithonian berriasian ocean chemistry and paleoclimate suggest that variations in the global carbon cycle were coupled with changes in the global hydrological cycle and in associated material cycles

duration of the early toarcian carbon isotope excursion deduced from spectral analysis consequence for its possible causes abstract the marked 3 8 negative carbon isotope excursion associated with the early toarcian oceanic anoxic event oae 183 myr ago in the early jurassic period is thought to represent one of the most important perturbations of the c cycle in the last 200 myr however the origin of this excursion remains strongly debated primarily due to uncertainties in the estimation of its duration which ranges from 200 kyr to 1 myr here we present a new orbital calibration of the early toarcian carbon isotope excursion based on spectral analyses of two independent datasets generated from the sedimentary record of two hemipelagic sections from portugal peniche and sw germany dotternhausen in order to better constrain the timescale and hence the origin of this excursion these analyses reveal that orbital cycles exert a strong influence on both the calcium carbonate content in portugal and on the greyscale of black shales in germany which allow us to propose a duration of 1 9 myr for the early toarcian and of 900 kyr for the entire carbon isotope excursion the shift towards lower carbon isotope values lasted 150 kyr and carbon isotope values remained low for 450 kyr the subsequent increase of carbon isotope values lasted 300 kyr this calibration suggests that the sustained input of isotopically light carbon at the origin of the excursion occurred over 600 kyr and thus dismisses causal mechanisms implying relatively small source reservoirs such as the massive dissociation of methane hydrates in the light of our new cyclostratigraphic timescale the massive input of isotopically light carbon associated with the emplacement of the karoo ferrar basaltic province appears as the most likely cause of the toarcian global carbon isotope excursion we also show that the c isotope perturbation coincided with a transition from precession eccentricity dominated cycles to obliquity eccentricity dominated cycles suggesting that the oae was marked by a fundamental change in the response of the climate system which allowed the obliquity signal normally better recorded at high latitudes to be a dominant forcing factor of short term sedimentary cycles at tropical latitudes

numerical evaluation of mechanisms driving early jurassic changes in global carbon cycling the early jurassic early toarcian ca 183 ma carbon cycle perturbation is characterized by aabout 5 parts per thousand delta sup 13 c excursion in the exogenic carbon reservoirs a 1000 ppm rise in atmospheric co sub 2 and a 6 7 degrees warming two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light ch4 from 1 gondwanan coals by heating during the intrusive eruption of the karoo ferrar large igneous province lip or 2 the thermal dissociation of gas hydrates carbon cycle modeling indicates that the release of ch4 from gondwanan coals synchronous with the eruption of the karoo ferrar lip fails to reproduce the magnitude or timing of the co sub 2 and delta sup 13 c excursions however sensitivity analyses constrained by a marine cyclostratigraphically dated delta sup 13 c record indicate that both features of geologic record can be explained with the huge input of about 15 340 24 750 gt c over about 220 k y a result possibly pointing to the involvement of hydrothermal vent complexes in the karoo basin the simulated release of 6000 gt c from gas hydrates also reproduces aspects of the early toarcian rock record but the large mass involved raises fundamental questionsmore about its formation storage and release less

the norwegian atlantic current in the lofoten basin inferred from hydrological and tracer data 129i and its interaction with the norwegian coastal current 1 from three hydrological sections taken across the lofoten basin in may 2000 we estimated geostrophic transports of 7 2 sv sverdrup 106 m3 s 1 for the norwegian atlantic current nac and its division between northern 4 8 sv and eastern 2 4 sv branches from 129i 127i concentration ratio measurements and transport estimations we calculated 129i mass flux across the three sections it appears that in the lofoten basin a 129i tracer laden norwegian coastal current ncc is transporting northward 55 kg y of 129i assuming a volume flux of 0 7 sv b the estimated mass fluxes of 129i by the nac and the ncc are comparable c the total mass flux of 129i by the nac and ncc accounts only for about 1 3 of the 129i annual discharge 350 kg y from two reprocessing plants based in france and uk if these measurements are representative of annual mean it suggests an important transfer of 129i outside the nac ncc system

evidence for major environmental perturbation prior to and during the toarcian early jurassic oceanic anoxic event from the lusitanian basin portugal the timing and causal relationships between the pronounced negative c isotope excursion and paleoenvironmental perturbations associated with the toarcian oceanic anoxic event early jurassic remain unclear particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main c isotope anomaly here we present a new quantification of late pliensbachian early toarcian calcareous nannofossils abundance and size from the peniche reference section portugal together with o and c isotope records of well preserved brachiopod shells from the same section the brachiopod shell 13c curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions close to the pliensbachian toarcian boundary 2 and during the toarcian oceanic anoxic event 3 5 our results indicate that both c isotope negative excursions were characteristic of benthic and shallow water environments suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures probably as a result of increased co2 levels and enhanced greenhouse conditions co2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems as evidenced by synchronous drops of platform derived carbonate accumulation and drastic reductions in size 3 m of the main pelagic carbonate producer schizosphaerella we suggest that the early toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the karoo ferrar province

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

the response of two tethyan carbonate platforms to the early toarcian jurassic oceanic anoxic event environmental change and differential subsidence chemostratigraphic analyses 87 sr 86 sr d 13 ccarb of limestones from two jurassic platform carbonate sequences in italy trento and campania lucania platforms illustrate previously established trends found in pelagic sediments and skeletal carbonates from biostratigraphically well calibrated sections elsewhere in europe chemostratigraphic correlations between the platform carbonate successions and appropriate intervals from well dated reference sections allow the application of high resolution stratigraphy to these shallow water peritidal carbonates and furthermore elucidate the facies response to the early toarcian oceanic anoxic event oae lower jurassic toarcian levels of the western trento platform southern alps northern italy contain spiculitic cherts that appear where rising carbon isotope values characterize the onset of the oae a palaeoceanographic phenomenon interpreted as driven by increased nutrient levels in near surface waters there is a facies change to more clay rich facies at the level of the abrupt negative carbon isotope excursion also characteristic of the oae higher in the section the campania lucania platform southern apennines southern italy records a change to more clay rich facies where carbon isotope values begin to rise at the beginning of the oae but the negative excursion higher in the section occurs within oolitic facies although in both examples the early toarcian oae can be recognized by a change to more clay rich lithologies this facies development is diachronous and in neither case did the platform drown although the trento platform in the south west sector studied here was adversely affected by the oae it did not drown definitively until late aalenian time the campania lucania platform persisted throughout the jurassic and cretaceous differential subsidence rates which can be calculated using comparative chemostratigraphy are identified as a crucial factor in the divergent behaviour of these two carbonate platforms relatively fast in the case of the trento platform relatively slow in the case of the campania lucania platform it is proposed that where water depths remained as shallow as a few metres during the oae campania lucania platform dissolved oxygen levels remained high nutrient levels relatively low and conditions for carbonate secretion and precipitation remained relatively favourable whereas more poorly ventilated and or more nutrient rich waters trento platform adversely

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

structure and resilience of overwintering habitats of calanus finmarchicus in the eastern norwegian sea abstract very high concentrations of overwintering calanus finmarchicus were found in the eastern lofoten basin of the norwegian sea close to the shelf break in january 2001 2002 a coupled 3d hydrodynamic and ecological model was used to study the formation of this deep overwintering aggregation and its stability the ecological model includes nutrients phytoplankton and microzooplankton in addition to a stage structured model of c finmarchicus using a eulerian approach the model was initiated with an overwintering stock evenly distributed in the oceanic regions of the norwegian sea i e where depths 600 m spawning and development of the new generation take place in response to vertical mixing and phytoplankton development animals are assumed to begin their descent to overwintering depths of 700 1000 m as late stage vs model results show that in late summer high concentrations of animals were found at overwintering depths near the shelf break north of the north sea off the northeastern voring plateau and in the eastern lofoten basin along the slope of the barents sea shelf they remained there for months due to deep eddies and southward deep currents along the norwegian shelf the simulation experiments indicate that the combined effect of deep anticyclonic circulation and vertical migration behavior of the animals may explain the high concentrations of overwintering c finmarchicus found in field surveys in the eastern lofoten basin close to the shelf break

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

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

calcareous nannoplankton changes across the early toarcian oceanic anoxic event in the western tethys 1 calcareous nannoplankton were profoundly affected by environmental perturbations coincident with the early toarcian oceanic anoxic event t oae we quantify the abundance of nannofossils across the t oae at three locations in western europe where the event is marked by a prominent negative carbon isotope excursion cie data were treated by statistical analysis and the shannon diversity index was applied in order to describe nannofossil assemblage changes related to paleoenvironmental evolution in the basal toarcian before the t oae high proportions of taxa with an affinity for low temperature e g p liasicus t patulus bussonius occurred this observation is consistent with interpretations of published oxygen isotope records during the t oae the lowest abundance of nannofossils is observed but there is a peak of a coccolith calyculus from an organism that probably thrived in low saline surface waters depleted of nitrate at the end of the perturbation the lowest diversities of nannofossils occurred and assemblages are dominated by crepidolithus crassus a deep dweller this interval corresponded to progressive reoxygenation of deep water and the reoccupation of the deep photic zone by nannoplankton the highest abundance of nannofossils is recorded above the cie and testifies to the recovery of the entire nannoplankton community the t oae was widespread perhaps global and probably linked to major changes in the atmosphere and hydrosphere however the record of this event varies in the basins surrounding the western tethys suggesting regional imprints on the global signal the regional variability may attest to establishment of effective connections between the arctic and tethys oceans which allowed cool low saline water formed at high latitudes to stream toward the western tethys

nitrogen isotope evidence for water mass denitrification during the early toarcian jurassic oceanic anoxic event bulk sedimentary nitrogen isotope 15ntot data have been generated from lower jurassic black carbon rich shales in the british isles and northern italy deposited during the early toarcian oceanic anoxic event a pronounced positive 15ntot excursion through the exaratum subzone of the falciferum zone defined by characteristic ammonites in the british isles broadly correlates with a relative maximum in weight percent total organic carbon and in some sections with a negative 13corg excursion upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of 15ntot and parallels may be drawn with quaternary sediments of the arabian sea gulf of california and northwest mexican margin the development of early toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity approximately coincident phenomena include the following a relative climatic optimum realignment of major oceanic current systems and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event

phytoplankton evidence for the timing and correlation of palaeoceanographical changes during the early toarcian oceanic anoxic event early jurassic the causes and duration of the early toarcian anoxic event are controversial integration of data from calcareous and organic phytoplankton provides a biochronological framework that allows precise correlations across the western tethys in particular the carinolithus superbus nannofossil zone can be used to correlate the levels enriched in organic matter and the related 13 c negative excursion although a variable duration is given in the literature for this negative excursion it is likely that it lasted between 520 and 650 ka increased atmospheric p co 2 related to excess volcanic emissions magmatic activity in the karoo and ferrar provinces had an impact on climate and ocean chemistry and marked the inception of a biotic crisis affecting many organisms the beginning of the crisis within shallow carbonate platforms documented at southern latitudes predates the levels enriched in organic matter dinoflagellate cysts experienced a decrease in abundance in the c superbus zone until they temporarily disappeared the nannoplankton crisis was twofold a decrease in size and low calcified specimens are observed in addition to a drastic decrease in absolute abundance the increased atmospheric p co 2 as a result of the magmatic activity and temporarily amplified by transient methane release could have been the trigger for the biocalcification crisis which first affected the probably more reactive neritic system and eventually the nannoplankton community

primary and diagenetic isotopic signals in fossils and hemipelagic carbonates the lower jurassic of northern spain stable isotope and trace element analyses of 230 jurassic pliensbachian toarcian samples from northern spain have been performed to test the use of geochemical variations in fossils belemnites and brachiopods and whole rock hemipelagic carbonates as palaeoceanographic indicators although the succession analysed reinosa area westernmost basque cantabrian basin has been subject to severe thermal alteration during burial diagenesis the samples appear to be well preserved the degree of diagenetic alteration of the samples has been assessed through the application of integrated petrographic chemical and cathodoluminescence analyses it is demonstrated that brachiopods and whole rock carbonates although widely used for palaeoceanic studies do not retain their primary marine geochemical composition after burial diagenesis in contrast there is strong evidence that belemnite rostra preserve original isotopic values despite pervasive diagenesis of the host rock well preserved belemnite shells non luminescent to slightly luminescent typically show stable isotope values of 4ae3 to 0ae7 d 13 c 0ae7 to 3ae2 d 18 o and trace element contents of 950 l gg 1 sr and sr mn ratios 80 this study suggests that the degree to which diagenesis has affected the preservation of an original isotopic composition may differ for different low mg calcite fossil shells and hemipelagic bulk carbonates behaviour that should be considered when marine isotopic signatures from other ancient carbonate rocks are investigated multiple non luminescent contemporaneous belemnite samples passed the petrographic and geochemical tests to be considered as palaeoceanic recorders yet their d 13 c and d 18 o values exhibited moderate scatter such variability is likely to be related to the palaeoecological behaviour of belemnites and or highfrequency secular variations in sea water chemistry superimposed on the longterm isotopic trend a pronounced positive carbon isotope excursion up to 4ae3 is documented in the early toarcian serpentinus biozone which correlates with the toarcian d 13 c maximum reported in other european and tethyan regions

a review of the broad pattern of jurassic sea level changes and their possible causes in the light of current knowledge abstract an assessment is made of the current state of research into jurassic sea level changes based on new insights into depositional patterns in relation to these changes and new stratigraphic information from several widely separated regions across the world the bearing of this information on alternative models respectively involving sea level rise interrupted by stillstands or falls is then evaluated the gross pattern appears to be one of more or less gradual rise of sea level through the period interrupted by episodes of comparative stillstand rather than eustatic fall and several episodes of significant regression are shown to be the result of regional tectonics major episodes of eustatic rise took place in the early hettangian early sinemurian early pliensbachian early toarcian early and late bajocian middle callovian and late oxfordian to kimmeridgian a significant episode of rapid and very extensive regression possibly global took place at the end of the triassic but other regressive episodes in the late aalenian bathonian late oxfordian kimmeridgian and tithonian are clearly only regional in extent there is no evidence for glacioeustasy and most if not all of the regional or global changes recognised can readily be related to plate tectonics the events at the triassic jurassic boundary seem to be the result of mantle plume activity centred on the central atlantic region

expression of the early toarcian negative carbon isotope excursion in separated carbonate microfractions jurassic paris basin abstract the causes of the pronounced negative excursion in carbon isotope values that was recorded during the early toarcian oceanic anoxic event t oae are still under debate particularly with regard to the local versus global pattern of the excursion and the extent to which recorded signals are under a diagenetic control in this study we employ a novel microseparation technique in order to investigate the isotopic and mineralogical characteristics of different size fractions of the carbonate content from a toarcian section recovered from the sancerre couy borehole southern paris basin beyond the recognition of a 6 13 c excursion in the bulk carbonate content our data also demonstrate that biogenic particles such as coccoliths and inorganic grains precipitated as early diagenetic phases including dolomite both record the excursion with the same magnitude although several black shales occur through the paris basin toarcian section it is only that associated with the onset of the oae that coincides with a large negative carbon isotope excursion taken together these observations indicate that during this event the entire water column was characterized by homogeneous carbon isotope values such a pattern is incompatible with the idea that the negative excursion was generated simply through the upwelling of bottom waters enriched in re mineralized organic carbon cf the kuspert model since this would have required a strong vertical gradient in the water column additionally the paris basin data show that the decrease in carbonate 13 c values during the oae occurred in several discrete steps each of some 2 as has previously been found for organic carbon substrates in other european sections the stepped nature of the isotopic profile which is part of a stratigraphic signature previously ascribed to milankovitch forcing is compatible with regular pulsed input of light carbon into the whole atmosphere ocean system from a climatically sensitive source such as gas hydrate or from thermal methanogenesis of organic rich sediments in the karoo ferrar large igneous province contrasts in the amplitude of the negative carbon isotope excursion on a regional scale remain an important unexplained aspect of the toarcian record

going against the flow testing the hypothesis of pulsed axial glacier flow hypothesised lobe like flow of a temperate glacier in southeast iceland proposed from an analysis of ice surface crevassing patterns is appraised from both empirical and theoretical perspectives the hypothesis comprises the migration of individual lobes or pulses of ice through the glacier body with central lobes migrating more rapidly along a narrow central axial flow corridor our alternative hypothesis is that crevasse patterns at this glacier instead reflect simple surface ice responses to stresses caused by flow over uneven bed topography to substantiate our rejection of the lobe like pulsed axial flow hypothesis we provide a evidence for a prominent transverse foliation that exhibits no evidence of shear of the required magnitude to support the hypothesis and b an analysis of ice surface displacement obtained by feature tracking that shows a uniform flow field throughout the glacier tongue we argue that caution needs to be exercised when interpreting glacier flow solely from crevasse patterns and observations of minor displacements along near surface fractures and other features

palaeoceanography methane release in the early jurassic period arising from d b kemp a l coe a s cohen l schwark nature 437 396 399 2005 kemp et al reply dramatic global warming triggered by release of methane from clathrates has been postulated to have occurred during the early toarcian age in the early jurassic period1 kemp et al 2 claim that this methane was released at three points as recorded by three sharp excursions of 13corg of up to 3 magnitude but they discount another explanation for the excursions namely that some perhaps all of the rapid excursions could be a local signature of a euxinic basin caused by recycling of isotopically light carbon from the lower water column this idea has been proposed previously see ref 3 for example and is supported by the lack evidence for negative 13c excursions in coeval belemnite rostra4 kemp et al dismiss this alternative claiming that each abrupt shift would have required the recycling of about double the amount of organic carbon that is currently present in the modern ocean however their measurements are not from an ocean but from a restricted epicontinental seaway and so would not require whole ocean mixing to achieve the excursions

comment on carbon isotope record of the early jurassic toarcian oceanic anoxic event from fossil wood and marine carbonate lusitanian basin portugal by hesselbo s jenkyns h c duarte l v and oliveira l c v hesselbo et al 2006 hereinafter hjdo06 present stable isotopic profiles through lower toarcian sediments at peniche portugal and interpret them as evidence of a massive injection to the atmosphere of isotopically light co2 derived from methane oxidation the atmospheric inputs supposedly gave rise to negative isotopic excursions in all carbon reservoirs the paper follows others proposing that environmental change in early toarcian time was driven by the release to the atmosphere of methane either from marine clathrates hesselbo et al 2000 kemp et al 2005 or from organic rich shales that were baked by karoo ferrar intrusions mcelwain et al 2005 the methane hypothesis was tested by van de schootbrugge et al 2005 who highlighted the existence of sections that lacked a negative excursion in ac in one or more sample media amongst them is the section on the coast of yorkshire uk used to erect the methane hypothesis by hesselbo et al 2000 which lacks a negative excursion in the c of belemnite calcite mcarthur et al 2000 van de schootbrugge et al 2005 as the excursion is not seen in these sections it cannot represent a global event and so could not have resulted from methane release which would have influenced all reservoirs of carbon new data are now provided by hjdo06 for a section peniche

the early toarcian jurassic anoxic event stratigraphic sedimentary and geochemical evidence detailed paleogeographic studies in the southern alps suggest that an oxygen minimum model is appropriate for interpreting conditions on tethyan continental margins advection of manganese in this low oxygen layer may explain the local occurrence of carbonate ores of this element carbon sulfur and iron sulfur ratios although showing considerable scatter suggest that bottom waters at this time were locally euxinic containing free hydrogen sulfide this oceanic anoxic event was preceded by significant faunal turnover of ammonites in tethys and accompanied by widespread extinction of benthos in northern europe in response to the lateral spread of anoxic bottom waters during transgression similar changes may be recognized in other parts of the world other anoxic events may have taken place during the jurassic but documentation is as yet meagre furthermore models for such phenomena remain largely speculative although upwelling and increased planktonic productivity commencing in pre toarcian time are favored for the falciferum zone event documented here from author

early and mid jurassic molluscan biogeography and the establishment of the central atlantic seaway abstract a new analysis of lower and middle jurassic bivalve faunas of western north america and the southern andes in comparison to europe supported by evidence from other molluscs throws further light on the establishment of a central atlantic epicontinental seaway prior to the formation of a true oceanic strait between eastern north america and africa south america some restriction to free communication persisted until the early callovian but there was an episode of relatively free intermigration in the toarcian to early bajocian time interval which is associated with major rise of sea level one of the most significant consequences of this event was a spread into northwest europe of taxa which were previously confined to the pacific margins to occupy niches vacated as a result of an early toarcian extinction event

limited range of interspecific vital effects in coccolith stable isotopic records during the paleocene eocene thermal maximum 1 small but significant differences exist among stable carbon and oxygen isotopic excursions measured in coccolith dominated bulk carbonate and planktic foraminifera during the paleocene eocene thermal maximum petm one hypothesis suggests that the bulk carbonate isotopic record is compromised by changing nannofossil assemblages since modern nannofossils show a large 5 permil range of interspecific vital effects new techniques are employed here to separate different size fractions of coccoliths from petm sediments at odp site 690 for isotopic analysis removing a major portion of the variation in nannofossil assemblages isotopic compositions of coarse and fine coccolith fractions dominated by coccoliths of genus chiasmolithus and toweius respectively differ by less than 0 5 permil for both oxygen and carbon the near monogeneric toweius record closely parallels the main trends in the bulk carbonate isotope records including multiple steps in the negative carbon isotopic excursion suggesting that the trends in the bulk carbonate record are not artifacts of changing species assemblages because both coccolithophorids and symbiont bearing foraminifera like acarinina must inhabit the photic zone it is unlikely that the 103 year lags in isotope event onset between coccoliths and acarinina reflect true time transgressive invasion of isotopically depleted co2 into the water column the small range of vital effects among paleocene coccoliths is unlikely to result from diagenetic homogenization and instead may reflect more similar carbon acquisition strategies of paleocene coccolithophorid algae due to larger and or more similar cell sizes and higher atmospheric carbon dioxide the small range of vital effects suggests that bulk carbonate records are likely reliable for other early and pre cenozoic sediments where foraminifera are often scarce

pangaean climate during the early jurassic gcm simulations and the sedimentary record of paleoclimate results from new simulations of the early jurassic climate show that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m y the simulations conducted using the general circulation model gcm at the goddard institute for space studies include realistic representations of paleocontinental distribution topography epeiric seas and vegetation in order to facilitate comparisons between model results and paleoclimate data three major features of the simulated early jurassic climate include the following 1 a global warming compared to the present of 5 c to 10 c with temperature increases at high latitudes five times this global average average summer temperatures exceed 35 c in low latitude regions of western pangaea where eolian sandstones testify to the presence of vast deserts 2 simulated precipitation and evaporation patterns agree closely with the moisture distribution interpreted from evaporites and coal deposits high rainfall rates are associated primarily with monsoons that originate over the warm tethys ocean unlike the megamonsoons proposed in previous studies these systems are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography 3 decreases in planetary albedo occurring because of reductions in sea ice snow cover and low clouds and increases in atmospheric water vapor are the positive climate feed backs that amplify the global warming similar to other mesozoic climate simulations our model finds that large seasonal temperature fluctuations occurred over mid and high latitude continental interiors refuting paleoclimate evidence that suggests more equable conditions sensitivity experiments suggest that some combination of ocean heat transport increase high levels of co2 and improved modeling of ground hydrological schemes may lead to a better match with the geologic record we speculate also that the record itself is biased toward equable climatic conditions a suggestion that may be tested by comparing gcm results with more detailed phytogeographic analyses

factors affecting the diagenesis of quaternary sediments at odp leg 172 sites in western north atlantic evidence from pore water and sediment geochemistry abstract pore waters and quaternary sediments at odp leg 172 sites on the carolina slope cs site 1054 blake outer ridge bor sites 1057 and 1060 bahama outer ridge baor 1062 and bermuda rise br site 1063 were studied the sediments are mainly clayey and silty mud with intercalations of nannofossil rich and lutite rich beds towards the top sedimentation rate ranges from 4 2 cm ky at site 1054 to 23 cm ky at site 1060 average total organic carbon toc contents of the upper sediment units range from 0 40 to 0 58 at sites 1057 1060 1062 and 1063 and from 0 86 to 1 25 at site 1054 the organic matter at all the sites is mostly degraded except at site 1054 sediments at sites 1062 and 1063 have lower total sulfur ts 0 00 0 62 average 0 06 than those at the other sites site 1054 has the highest ts 0 8 1 4 and the lowest reactive iron 0 003 0 074 contents among all the sites the deep water sites 1062 and 1063 have concave down pore water sulfate concentration depth profiles relatively deep sulfate methane interfaces smis 65 and 38 meters below sea floor mbsf respectively and steep methane gradients at the base of the smi whereas the shallow water sites 1054 1057 and 1060 have near linear sulfate and relatively shallow methane gradients the near linear sulfate depth profiles and shallow smis 17 and 14 mbsf respectively at sites 1057 and 1060 are primarily due to high sulfate consumption by anaerobic methane oxidation amo at the smi concave down profiles and deep smis at sites 1062 and 1063 are the result of low rates of sulfate consumption through organic matter degradation due to a limitation of metabolizable organic matter site 1054 is unusual in having an 8 m thick oxic suboxic zone a 48 m deep smi and a linear sulfate depth profile despite the high organic matter content of sediments at this site the thick oxic suboxic zone is mainly due to intense bioturbation while the linear sulfate depth profile and the deep smi are likely caused by low sedimentation rate and by reactive iron limitation pore water magnesium calcium and strontium profiles indicate that carbonate precipitation has occurred near the smi this is confirmed by the presence of authigenic dolomite rhombs and nodules especially at and below smi at all sites

calcareous nannofossils and mesozoic oceanic anoxic events abstract twenty five years ago mesozoic oceanic anoxic events oaes were documented and formalised as intervals of widespread to global deposition of organic matter the toarcian early aptian oae1a and latest cenomanian oae2 oaes are truly global in nature commonly carbonate poor and typically represented by organic carbon rich black shales in some areas these oaes are also characterised by abundant radiolarian sands and silts they are associated with negative and positive excursions in the 87sr 86sr record in addition to large global carbon isotope anomalies in carbonate and or organic matter caused by a major perturbation of the global carbon budget increased rates of volcanism during the formation of the ontong java and manihiki and caribbean plates and the karoo ferrar traps are believed to have caused the geological responses associated with oae1a oae2 and the toarcian oae respectively excess volcanogenic co2 in the atmosphere most probably turned the climate into a greenhouse mode accelerating continental weathering and increasing nutrient content in oceanic surface waters via river run off higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes because mesozoic oaes are often represented by carbonate poor sediments quantitative studies of calcareous nannofossils have been applied to explore a the causes and effects of igneous tectonic events and climate changes relative to nannofloral increases and crises as well as b dissolution events and c diagenetic modifications characterization of calcareous nannofloras in oae intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis biological pump and biomineralisation carbonate pump that affect the organic and inorganic carbon cycle as well as adsorption of atmospheric co2 in the oceans types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events in selected sections it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pco2 by drastically reducing calcification as in the modern oceans during oaes the increase of nutrients and atmospheric co2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep photic zone nannoconids and schizosphaerellids similarly to the nannoconid crisis preceding deposition of the early aptian oae1a black shales a schizosphaerellid crisis is detected prior to the toarcian oae both oaes are further characterised by a rapid nannofloral speciation beginning approximately 1 5 myr before the oae but without extinctions global changes during the latest cenomanian oae 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions this event in fact was a time of extinctions followed by originations within calcareous nannofossils precise timing of the events before during and after oae1a oae2 and the toarcian oae indicates that they were intervals of enhanced oceanic productivity and that anoxia dysoxia post dated biotic changes

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