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

spatial variability of watermass conditions within the european epicontinental seaway during the early jurassic pliensbachian toarcian in order to constrain spatial variability in watermass conditions within the european epicontinental seaway prior to during and after the toarcian oceanic anoxic event carbon 13cbel 13ccarb and oxygen 18obel 18ocarb isotope records were obtained from three sections in the grands causses basin southern france these data were then compared with similar records along a north south transect across the european epicontinental seaway as the conclusions reached here strongly depend on the reliability of belemnite calcites as archives of palaeoceanographic changes an attempt was made to improve the understanding of isotope signals recorded in belemnite calcite intra rostral carbon and oxygen isotope data from six belemnite specimens belonging to the genus passaloteuthis were collected intra rostral carbon isotopes are influenced by vital effects whereas oxygen isotopes reflect relative changes in temperature and salinity palaeotemperatures calculated from 18obel isotope records from the grands causses basin confirm relatively low temperatures throughout the late pliensbachian similar cool water conditions have previously been shown in germany england spain and portugal a temperature increase of up to 6 c is observed across the pliensbachian toarcian boundary a pronounced negative shift of at least 3 vienna pee dee belemnite is recorded in bulk carbonate carbon during the lower harpoceras serpentinum zone typical of the toarcian oceanic anoxic event before and after the toarcian oceanic anoxic event a good correlation between 13ccarb and 13cbel exists indicating well ventilated bottom waters and normal marine conditions instead data for the toarcian oceanic anoxic event indicate the development of a strong north south gradient in salinity stratification and surface water productivity for the western tethyan realm this study thus lends further support to a pronounced regional overprint on carbon and oxygen isotope records in epicontinental seaways

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

toarcian carbon isotope shifts and nutrient changes from the northern margin of gondwana high atlas morocco jurassic palaeoenvironmental implications the early toarcian is marked by a global perturbation of the carbon cycle and major marine biological changes these coincide with a general decrease in calcium carbonate production and an increase in organic carbon burial and culminate in the so called toarcian oceanic anoxic event it is believed that the environmental crisis was triggered by the activity of the karoo ferrar large igneous province in order to further document the early toarcian palaeoenvironmental perturbations carbon isotope total organic matter calcareous nannofossils and phosphorus content of the amellago section in the high atlas rift basin of morocco were investigated this section is extremely expanded compared to the well studied european sections its position along the northern margin of the gondwana continent is of critical importance because it enables an assessment of changes of river nutrient input into the western tethyan realm the carbon isotope curve shows two negative excursions of equal thickness and amplitude at the pliensbachian toarcian boundary and at the transition from the polymorphum to the levisoni zone this confirms the supra regional nature of these shifts and highlights the possible condensation of the first boundary shift in european sections phosphorus content is used to trace palaeo nutrient changes and shows that the two negative carbon isotope shifts are associated with increased nutrient levels confirming that these episodes are related to enhanced continental weathering probably due to elevated greenhouse gases in the atmosphere in the high atlas basin the increase in nutrient levels at the pliensbachian toarcian boundary is moreover likely to be the main factor responsible for the coeval demise of the saharan carbonate platform a middle toarcian event centered on the boundary between the bifrons and gradata zones characterized by a positive carbon isotope excursion and nutrient level rise is documented in the amellago section

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

numerical paleoceanographic study of the early jurassic transcontinental laurasian seaway the forces governing marine circulation of a meridional transcontinental seaway is explored with the princeton ocean model the jurassic laurasian seaway which connected the low latitude tethys ocean with the arctic sea is modeled quantitatively the global ocean is found to have a profound influence on seaway dynamics a north south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow when the tethys waters were the denser water the net seaway flow was southward and conversely it was northward for denser arctic waters marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by boreal faunal groups and reduced salinities several times in the jurassic the model results suggest that this can be explained by southward flowing seaway currents which may have been related to an oceanic thermohaline circulation where no northern high latitude deep convection occurred

the carbonate signal and calcareous nannofossil distribution in an upper jurassic section balingen tieringen late oxfordian southern germany total abundance assemblage composition and the stratigraphic distribution of the six most abundant species of calcareous nannofossil were analysed in the upper oxfordian succession of balingen tieringen western swabian alb south germany this locality represents a deep shelf less than 200 m environment that is strongly influenced by its proximity to the jura shallow platform nannofossil abundance negatively correlates with carbonate content as well as with accumulation rate calcareous nannofossils thus did not produce the bulk of carbonates and their abundance was essentially controlled by the import of allochthonous carbonate mud gradual increase in carbonate content in the balingen tieringen succession can be related to an intensified carbonate mud export from the jura shallow platform to the swabian alb deep shelf during the late oxfordian changes in the nannofossil assemblage composition along the profile are interpreted as primary although a partial control by diagenesis on the assemblage can not be completely excluded the coccoliths lotharingius hauffii watznaueria britannica cyclagelosphaera margereli watznaueria barnesae watznaueria manivitae and the incertae sedis schizosphaerella spp display their highest relative abundance one after the other in a stratigraphic order this nannofossil succession parallels the gradual increase in allochthonous carbonate mud accumulation on the deep shelf as well as the gradual decrease in the intensity of siliciclastic input recorded on the jura shallow platform this suggests a climatic control by humidity on shallow platform carbonate production and export basinwards and on nutrient availability in deep shelf surface waters that itself controlled the nannofossil assemblage composition therefore the succession of highest relative abundance of l hauffii w britannica c margereli w barnesae w manivitae and schizosphaerella spp can represent a trophic preference continuum from more eutrophic to oligotrophic conditions

chemostratigraphy of the posidonia black shale sw germany ii assessment of extent and persistence of photic zone anoxia using aryl isoprenoid distributions aquatic depositional environments where anoxic conditions extend from bottom waters into the photic zone occurred frequently during the geologic past periods of photic zone anoxia pza can be recognized by the presence of specific lipids produced exclusively by the green sulphur bacteria chlorobiaceae that are preserved in the sedimentary record chlorobiaceae perform anoxygenic photosynthesis that requires light penetration into h2s saturated waters sediment samples integrate paleoenvironmental conditions over time intervals ranging from at best a few decades to more commonly several millennia photic zone anoxia recognized in paleoenvironmental analyses is thus defined as episodic with no further interpretation on the duration and persistence of the anoxic events we provide here a high resolution multiproxy chemostratigraphy study for the toarcian posidonia black shale from the dotternhausen section in sw germany in order to assess duration and seasonal fluctuation of photic zone anoxia based on the relative abundance of derivatives of chlorobiaceae lipids we assess the variability in the degree and persistence of photic zone anoxia using an aryl isoprenoid ratio air obtained by calculating the proportion of the short chain c13 17 versus the intermediate chain c18 22 aryl isoprenoids higher relative abundance of the short chain analogues is interpreted as indicating more intensive aerobic degradation of aryl isoprenoids air varies between values of 0 5 indicative of persistent pza phases and 3 0 for short termed episodic pza events absolute concentration of aryl isoprenoids is negatively correlated with the air which is in agreement with a progressive diagenetic breakdown of aryl isoprenoids the air decreases are validated using independent palecological and geochemical redox indicators for the posidonia shale in sw germany d 2004 elsevier b v all rights reserved

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

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

the toarcian radiolarian event in bedded cherts from southwestern japan abstract a drastic faunal turnover of radiolarians in the early toarcian termed the toarcian radiolarian event tre recognized in bedded chert sequences of southwestern japan is marked by the first co appearance of the genus praeparvicingula and mesosaturnalis hexagonus and by high diversity among multicyrtids the event resulted in the replacement of latest triassic through earliest jurassic taxa by late jurassic to early cretaceous taxa at tre levels in bedded cherts in southwestern japan the size of radiolarians decreased drastically reworked conodont fossils appeared and anoxic sediments black cherts with fes2 nodules were deposited the lithologic and faunal changes suggest that global marine environmental changes including the occurrence of anoxia caused the tre

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

palaeoenvironmental control on sponge microbialite reefs and contemporaneous deep shelf marl limestone deposition late oxfordian southern germany abstract the late oxfordian bimammatum zone deep shelf deposits of southern germany are characterised by abundant reefs that laterally pass into marl limestone alternations the bioconstructions are made from three main components sponges microbialites and allochthonous sediment a detailed morphological analysis of sponge microbialite reefs in the plettenberg section swabian alb southern germany is combined with investigation of calcareous nannofossil assemblages in laterally correspondent marl limestone alternations in order to trace the evolution of palaeoenvironmental conditions during reef development fluctuations of caco3 content in deep shelf deposits provides evidence of a succession of small scale lithological units generally formed by five to six marl limestone couplets within a small scale lithological unit a close link is observed between growth patterns of sponge microbialite reefs and the changes in the nannofossil assemblage composition phases of expansion of the build ups correspond to carbonate deposits characterised by a relative dominance of large oligotrophic nannoplankton species large watznaueria britannica watznaueria manivitiae and schizosphaerella spp marly intervals reveal a high relative abundance of small sized taxa mainly lotharingius hauffii small w britannica and subordinated biscutum dorsetensis and bioconstructions show a net reduction in size or a total demise this suggests that the same palaeoenvironmental parameters controlled at the same time marl versus limestone deposition reef growth phases and calcareous nannofossil assemblage composition successive reef growth phases recognized within a bioherm are correlated to the limestone hemi couplets each reef growth phase is composed of several stacked elementary sequences comprising the following succession i sponge ii microbialites and iii carbonate mud in turn the microbialitic crust is formed of three successive microbial layers 1 a fine crust of dense to clotted micrite 2 a centimetre scale to multicentimetric columnar layer of clotted to peloidal micrite 3 a centimetre scale stromatolitic crust the passage from an initial dense microbial layer with numerous microencrusters to a final stromatolitic crust is interpreted as resulting from an increase of the microbial growth rate mainly driven by changes in the accumulation rate deep shelf deposits resulted from changes in platform carbonate production and export basinwards controlled both by sea level and climate changes climate controlled the intensity of carbonate production in shallow platform environments and export towards the deep shelf as well as the trophic conditions of the deep shelf marine waters low trophic conditions favoured platform carbonate production and export basinwards and a large development of the sponge microbialite reefs in the deep shelf however a high carbonate accumulation rate in the deep shelf was also commonly responsible for the demise of sponge microbialite reefs during unfavourable and higher trophic conditions during marl deposition reduced development and frequent demise of the sponge microbialite reefs is observed

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

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

a blast of gas in the latest paleocene simulating first order effects of massive dissociation of oceanic methane hydrate carbonate and organic matter deposited during the latest paleocene thermal maximum is characterized by a remarkable 2 5 excursion in delta 13c that occurred over approximately 10 4 yr and returned to near initial values in an exponential pattern over approximately 2 x 10 5 yr it has been hypothesized that this excursion signifies transfer of 1 4 to 2 8 x 10 18 g of ch4 from oceanic hydrates to the combined ocean atmosphere inorganic carbon reservoir a scenario with 1 12 x 10 18 g of ch4 is numerically simulated here within the framework of the present day global carbon cycle to test the plausibility of the hypothesis we find that 1 the delta 13c of the deep ocean shallow ocean and atmosphere decreases by 2 3 over 10 4 yr and returns to initial values in an exponential pattern over approximately 2 x 10 5 yr 2 the depth of the lysocline shoals by up to 400 m over 10 4 yr and this rise is most pronounced in one ocean region and 3 global surface temperature increases by approximately 2 degrees c over 10 4 yr and returns to initial values over approximately 2 x 10 6 yr the first effect is quantitatively consistent with the geologic record the latter two effects are qualitatively consistent with observations thus significant ch4 release from oceanic hydrates is a plausible explanation for observed carbon cycle perturbations during the thermal maximum this conclusion is of broad interest because the flux of ch4 invoked during the maximum is of similar magnitude to that released to the atmosphere from present day anthropogenic ch4 sources

fluctuations in the trophic resource continuum a factor in global diversity cycles the trophic resource continuum trc in euphotic zones of the world s oceans is the spectrum of conditions from the richest runoff and upwelling areas to the most nutrient deficient subtropical seas that spectral range has likely varied through geologic history as more efficient phytoplankton taxa evolved as oceanic circulation rates changed and in response to oceanic mixing events reduced rates of oceanic turnover should produce regionally intensified eutrophy and oligotrophy that is trc expansion because oceanic waters as a whole are relatively nutrient rich increased rates of oceanic turnover should prevent extremely eutrophic and extremely oligotrophic conditions from developing causing trc contraction global biotic diversities in the oceans are strongly influenced by communities in oligotrophic environments nutrient deficient waters are more stable and more transparent than richer waters permitting higher degrees of specialization sparse nutrient supplies necessitate smaller phytoplankton and longer more complex food chains preservation potential of oligotrophic biotas is also greater because calcareous taxa are more prevalent and because calcium carbonate is less apt to be bioeroded or dissolved fluctuations in the trc should profoundly affect diversity of habitats within the euphotic zone of the oceans the evolutionary response to a trc expansion diversification would occur in a geologic time frame of millions of years while evolutionary responses to a trc contraction including extinction could occur in a biological time frame of generations a simple model indicates that a reduction in the trophic resource continuum from expansion during a major marine transgression down to modern levels could result in loss of 40 of the euphotic habitats how fluctuations in the trc control diversity of euphotic habitats in the oceans provides an hypothesis by which to interpret global taxonomic diversity cycles as well as taxonomically selective extinctions changes in species dominance and some morphologic trends

the response of marine phytoplankton and sedimentary organic matter to the early toarcian lower jurassic oceanic anoxic event in northern england abstract early toarcian organic rich sediments reflecting the lower jurassic oceanic anoxic event were investigated in the brown moor borehole north yorkshire northern england integrated micropalaeontological calcareous nannofossils and dinoflagellate cysts and geochemical rock eval pyrolysis analyses reveal a sequence of changes mainly driven by palaeoecological shifts these changes mainly involve the composition source and preservation rate of sedimentary organic matter as well as algal population dynamics a sequence of successive disappearances of individual species during the early toarcian dactylioceras semicelatum and harpoceras exaratum ammonite subzones affect the composition of the phytoplankton assemblages which become gradually dominated by opportunistic taxa this range top sequence is followed by the temporary disappearance of both calcareous nannofossils and dinoflagellate cysts the disappearance event which are replaced by tasmanites an increase in type ii kerogen marks this interval these modifications were related to a gradual water stratification and to the development of a stable pycnocline these factors controlled the development of an oxygen minimum zone within the water column and the distribution of nutrients in the surface waters the disappearance event is coincident with the maximum extent of the oxygen minimum zone a repopulation event and an increase in type iii kerogen mark the end of the phytoplankton crisis in the upper part of the h exaratum ammonite subzone preadapted survivors e g biscutum dubium biscutum finchii and crepidolithus crassus mainly characterise the repopulated calcareous nannofossil assemblages whereas a significant turnover affected the dinoflagellate cyst community the re established phytoplankton populations are a response to the gradual restoration of mixed marine waters characterised by meso eutrophic conditions at the surface and by sustained nutrient regeneration the integrated calcareous nannofossil and dinoflagellate cyst biostratigraphy provided a reliable biochronological framework for the brown moor borehole succession allowing correlation with lower toarcian anoxic strata in central italy the integrated micropalaeontological approach indicates some diachroneity of lower jurassic ammonite zones in the boreal and tethyan realms

early jurassic mass extinction a global long term event the end pliensbachian extinction event 187 ma has been interpreted either as one of 10 global periodically recurring mass extinctions of the past 250 m y or as a minor localized european event elevated levels of family extinction spanned five ammonite zones during the late pliensbachian and the early toarcian an interval of 7 5 m y and were distributed unequally in the boreal tethyan and austral realms detailed sampling of invertebrate macrofaunas through complete expanded sequences in northwest europe shows that most species extinctions occurred in the early toarcian following a regional anoxic event the early jurassic mass extinction event took place over a long time scale and it was global in extent

secular environmental precursors to early toarcian jurassic extreme climate changes the early toarcian oceanic anoxic event t oae about 183 myr ago was a global event of environmental and carbon cycle perturbations which deeply affected both marine biota and carbonate production nevertheless the long term environmental conditions prevailing prior to the main phase of marine extinction and carbonate production crisis remain poorly understood here we present a similar to 8 myr long record of early pliensbachian middle toarcian environmental changes from the lusitanian basin portugal in order to address the long term paleoclimatic evolution that ultimately led to carbonate production and biotic crises during the t oae paleotemperature estimates derived from the oxygen isotope compositions of well reserved brachiopod shells from two different sections reveal a pronounced similar to 5 degrees c cooling in the late pliensbachian margaritatus spinatum ammonite zones boundary this cooling event is followed by a marked similar to 7 10 degrees c seawater warming in the early toarcian that after a second cooling event in the mid polymorphum zone culminates during the t oae calcium carbonate caco3 contents the amount of nannofossil calcite and the mean size of the major pelagic carbonate producer schizosphaerella all largely covary with paleotemperatures indicating a coupling between climatic conditions and both pelagic and neritic caco3 production furthermore the cooling and warming episodes coincided with major marine regressions and transgressions respectively suggesting that the growth and decay of ice caps may have exerted a strong control on sea level fluctuations throughout the studied time interval this revised chronology of environmental changes shows important similarities with neogene and paleozoic episodes of deglacial black shale formation and thus prompts the reevaluation of ice sheet dynamics as a possible agent of mesozoic events of extinction and organic rich sedimentation c 2010 elsevier b v all rights reserved

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

toarcian anoxic event in europe an organic geochemical study abstract a total of 48 sediment samples comprising both black shales and associated organic poor interbeds were examined from toarcian organic rich sections throughout europe despite being contemporaneous deposits of a single oceanic anoxic event oae the molecular organic geochemical characteristics particularly the steroidal hydrocarbons of the black shales are highly variable this suggests different depositional environments in different regions although the sedimentary organic matter is dominantly of marine origin in all areas environmental conditions apparently varied considerably arguing against a simple oxygen minimum layer model for the accumulation of organic rich sediments furthermore molecular evidence of hypersaline conditions at two sites suggests that salinity stratification may have been an important factor for black shale deposition in some areas a more complex model for the widespread deposition of organic rich sediments during the toarcian oae is presented in the light of results arising from the present organic geochemical investigation

correlation between isotope records in marine and continental carbon reservoirs near the palaeocene eocene boundary changes in the isotope content of the large marine carbon reservoir can force shifts in that of the smaller carbon pools in the atmosphere and on land the carbon isotope compositions of marine carbonate sediments from the late palaeocene vary considerably exhibiting a sudden decrease close to the palaeocene eocene boundary which coincides with deep sea benthic extinctions1 and with changes in ocean circulation here we report that these fluctuations in the marine carbon isotope record are closely tracked by the terrestrial records provided by palaeosol carbonates and mammalian tooth enamel in using palaeosol carbonates to reconstruct the co2 content of the ancient atmosphere2 isotope shifts of this sort will have to be taken into account the sharp decrease in 13c 12c ratios in the late palaeocene provides a datum for precise correlation of marine and continental records and suggests that abrupt climate warming at this time may have played an important role in the evolution of land mammals

palaeoenvironmental significance of toarcian black shales and event deposits from southern beaujolais france new sedimentological biostratigraphical and geochemical data recording the toarcian oceanicanoxicevent t oae arereportedfromamarginalmarinesuccessioninsouthernbeaujolais france the serpentinum and bifrons ammonite zones record black shales with high 1 10 wt total organic carbon contents toc and dysoxia tolerant benthic fauna typical of the schistes carton facies well documented in contemporaneous nearby basins the base of the serpentinum ammonite zone however differs from coeval strata of most adjacent basinal series in that it presents several massive storm beds particularly enriched in juvenile ammonites and the dysoxia tolerant miniaturized gastropod coelodiscus this storm dominated interval records a marked negative 5 carbonate and organic carbon isotope excursion being time equivalent with that recording storm and mass flow deposits in sections of the lusitanian basin portugal pointing to the existence of a major tempestite turbidite event over tropical areas during the t oae although several explanations remain possible at present we favour climatically induced changes in platform morphology and storm activity as the main drivers of these sedimentological features in addition we show that recent weathering most probably due to infiltration of o2 rich meteoric water resulted in the preferential removal of 12 c enriched organic carbon dramatic toc loss and total destruction of the lamination of the black shale sequence over most of the studied exposure these latter observations imply that extreme caution should be applied when interpreting the palaeoenvironmental significance of sediments lacking toc enrichment and lamination from outcrops with limited surface exposures

arctic late paleocene early eocene paleoenvironments with special emphasis on the paleocene eocene thermal maximum lomonosov ridge integrated ocean drilling program expedition 302 we reconstruct the latest paleocene and early eocene 57 50 ma environmental trends in the arctic ocean and focus on the paleocene eocene thermal maximum petm 55 ma using strata recovered from the lomonosov ridge by the integrated ocean drilling program expedition 302 the lomonosov ridge was still partially subaerial during the latest paleocene and earliest eocene and gradually subsided during the early eocene organic dinoflagellate cyst dinocyst assemblages point to brackish and productive surface waters throughout the latest paleocene and early eocene dinocyst assemblages are cosmopolitan during this time interval suggesting warm conditions which is corroborated by tex86 reconstructed temperatures of 15 18 c inorganic geochemistry generally reflects reducing conditions within the sediment and euxinic conditions during the upper lower eocene spectral analysis reveals that the cyclicity recorded in x ray fluorescence scanning fe data from close to eocene thermal maximum 2 53 ma presence confirmed by dinocyst stratigraphy is related to precession within the lower part of the petm proxy records indicate enhanced weathering runoff anoxia and productivity along with sea level rise on the basis of total organic carbon content and variations in sediment accumulation rates excess organic carbon burial in the arctic ocean appears to have contributed significantly to the sequestration of injected carbon during the petm

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

early jurassic schizosphaerellid crisis in cantabria spain implications for calcification rates and phytoplankton evolution across the toarcian oceanic anoxic event 1 the toarcian oceanic anoxic event 183 myr ago represents a global perturbation marked by increasing organic carbon burial and a general decrease in calcium carbonate production likely triggered by elevated carbon dioxide levels in the atmosphere here we present quantitative analyses of calcareous nannofossil diversity and abundance from the castillo de pedroso section in cantabria northern spain we compare these data with geochemical data c and o isotopes obtained from biogenic and bulk carbonate records in order to highlight the response of calcareous phytoplankton to major climatic and paleoceanographic changes the pliensbachian toarcian boundary is characterized by an abrupt decrease in abundance of schizosphaerella punctulata the most important lithogenic contributor to hemi pelagic carbonates in the early jurassic the early toarcian nannofloral assemblages show an increase in abundance of mitrolithus jansae and small sized r selected taxa and a progressive decrease in s punctulata percentages the deep dwellers m jansae and s punctulata experienced a major crisis slightly prior to the deposition of the toarcian black shales that are characterized by high abundances of eutrophic taxa such as lotharingius spp and biscutum spp the return of s punctulata associated with lower percentages of eutrophic taxa was observed just above the toarcian black shales the toarcian episode reveals that high co2 levels and increasing primary productivity probably triggered a shift in abundance from highly calcified nannoliths such as s punctulata and m jansae to small sized r selected coccoliths that overall record a biocalcification crisis at the onset and during the toarcian episode

comparative study of lahars generated by the 1961 and 1971 eruptions of calbuco and villarrica volcanoes southern andes of chile abstract the villarrica and calbuco volcanoes of the andean southern volcanic zone are two of the most active volcanoes in chile and have erupted several times in the xx century the 1961 eruption at calbuco volcano generated lahars on the north east and southern flanks while the 1971 eruption at villarrica volcano generated lahars in almost all the drainages towards the north west and south of the volcano the deposits from these eruptions in the voipir and chaillupen river villarrica and the tepu river calbuco are studied the 1971 lahar deposits on villarrica volcano show a great number of internal structures such as lamination lenses grading of larger clasts and a great abundance of large floating blocks on top of the deposits the granulometry can be unimodal or bimodal with less than 5 by weight of silt clay material sem images reveal a great variety of forms and compositions of clasts the 1961 lahar deposits on calbuco volcano have a scarce number of internal structures steeper margins and features of hot emplacement such as semi carbonized vegetal rests segregation pipes and a more consolidated matrix the granulometry usually is bimodal with great quantities of silt clay material 10 by weight sem images show a uniformity of composition and forms of clasts differences on deposits reveal different dynamics on both lahars the villarrica lahar was generated by sudden melt of ice and snow during the paroxysmal phase of the 1971 eruption when a high fountain of lava was formed the melted water flowed down on the flanks of the volcano and incorporated sediments to become transition flows highly energetic and were emplaced incrementally dilution of the flows occurs when the lahars reached unconfined and flatter areas in cases where the lahar flow found large water streams dilution is enhanced the calbuco lahars were generated by the dilution of block and ash pyroclastic flows by flowing over the ice or snow or by entering active rivers transforming to debris flows the differences on dynamics of both flows show the importance to understand initiation processes of lahars in order to make better hazard assessment due to laharics flows

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

benthic foraminiferal assemblages record major environmental perturbations during the late pliensbachian early toarcian interval in the peniche gssp portugal abstract this is the first ecostratigraphic and palaeoecological analysis of the benthic foraminiferal assemblages defined as morphogroups in the pliensbachian toarcian transition and the toarcian oceanic anoxic event t oae of the peniche section the toarcian global boundary stratotype section and point foraminiferal assemblages from the uppermost pliensbachian emaciatum zone reveal the presence of a stable benthic environment with good oxygen and availability of trophic resources as evidenced by the high diversity of specialists the pliensbachian toarcian transition reveals high foraminifera abundance mainly specialists although it is considered a slightly condensed interval the base of the levisoni zone shows dramatic changes in life style of foraminifera increase of epifauna and potentially deep infauna an increase in opportunistic forms and a decrease in diversity these changes suggest unfavourable conditions related to food availability diminution and depleted oxygenation corresponding to the beginning of the 13c negative excursion palaeoenvironmental conditions become severe in the lower to middle levisoni zone where there is a barren interval for foraminifera coincident with the highest values of total organic carbon after this interval foraminiferal assemblages experienced a recovery with increasing diversity and occurrence of specialists consistent with a well oxygenated environment and good food availability these changes observed in benthic foraminiferal fauna from the peniche section through the uppermost pliensbachian lower toarcian are comparable to what was reported previously on ostracods brachiopods and calcareous nannofossils foraminiferal assemblages from peniche section are comparable to other tethyan spain italy north africa and nw european sections england from the same stratigraphic interval with the suborder lagenina dominating the assemblages

shelf and open ocean calcareous phytoplankton assemblages across the paleocene eocene thermal maximum implications for global productivity gradients abrupt global warming and profound perturbation of the carbon cycle during the paleocene eocene thermal maximum petm ca 55 ma have been linked to a massive release of carbon into the ocean atmosphere system increased phytoplankton productivity has been invoked to cause subsequent co2 drawdown cooling and environmental recovery however interpretations of geochemical and biotic data differ on when and where this increased productivity occurred here we present high resolution nannofossil assemblage data from a shelf section the u s geological survey usgs drill hole at wilson lake new jersey and an open ocean location ocean drilling program odp site 1209 paleoequatorial pacific these data combined with published biotic records indicate a transient steepening of shelf offshelf trophic gradients across the petm onset and peak with a decrease in open ocean productivity coeval with increased nutrient availability in shelf areas productivity levels recovered in the open ocean during the later stages of the event which coupled with intensified continental weathering rates may have played an important role in carbon sequestration and co2 drawdown

reversed deep sea carbonate ion basin gradient during paleocene eocene thermal maximum 1 the paleocene eocene thermal maximum petm 55 ma ago was marked by widespread caco3 dissolution in deep sea sediments a process that has been attributed to massive release of carbon into the ocean atmosphere system the pattern of carbonate dissolution is key to reconstructing changes in deep sea carbonate chemistry and ultimately the rate magnitude and location of carbon input here we show that during the petm the deep sea undersaturation was not homogeneous among the different ocean basins application of a sediment model to a suite of data records from different sites and ocean basins shows that a globally uniform decrease in deep sea carbonate ion concentration co32 is inconsistent with the data rather we demonstrate that deep sea co32 increased from the atlantic through the southern ocean into the pacific our results show that the petm deep sea co32 basin gradient during dissolution was reversed relative to the modern

nannofossil assemblage fluctuations during the paleocene eocene thermal maximum at sites 213 indian ocean and 401 north atlantic ocean palaeoceanographic implications the paleocene eocene thermal maximum petm was an abrupt global warming event at f55 ma this event had profound effects on the biosphere quantitative investigations of calcareous nannofossil assemblages were performed across the petm at deep sea drilling project site 213 in the tropical indian ocean and site 401 in the bay of biscay north atlantic ocean at a temperate latitude calcareous nannofossil assemblages from both sites show major changes before during and after the petm fluctuations at site 213 are affected by selective dissolution of the least robust taxa by contrast assemblages in the lower part of the petm at site 401 show less diagenetic alteration and display high abundances of genera such as discoaster fasciculithus and ericsonia these taxa reflect relatively warm and probably oligotrophic surface waters abundances of the genus chiasmolithus an indicator of relatively cool and eutrophic waters decreased dramatically the upper portion of the petm is characterized by an increase in the abundance of toweius spp and zygrhablithus bijugatus interpreted as a return to cooler and more eutrophic conditions the elimination of fasciculithus was probably due to competition with the holococcolith species z bijugatus which occupied the same ecological niche the occurrences of long armed and asymmetrical discoaster species such as discoaster araneus discoaster anartios and discoaster okadai and rhomboaster species such as rhomboaster calcitrapa and rhomboaster cuspis are characteristic of the petm interval d 2004 elsevier b v all rights reserved

unconformities related to the jurassic cretaceous synrift post rift transition of the northern north sea in the jurassic cretaceous north sea basin the synrift sequence is separated from the post rift sequence by the base cretaceous or late cimmerian unconformity the unconformity covers almost the entire basin has a distinct character in seismic reflection data and wireline logs and hence is easily identified and correlated making it the most important marker horizon in the area the unconformity displays great local complexity in many localities and great variability on a regional scale from one locality to another thus the unconformity is classified as a nonconformity a disconformity and an angular unconformity we suggest that these variations basically reflect different structural position within the basin so that the short wavelength variation is dominated by local structural development e g the rotational history of a fault block whereas the long wavelength variation reflects basin scale tectonic thermal and isostatic processes the merging of this unconformity with younger erosional surfaces its complex configuration and polychronous character makes the general term base cretaceous unconformity inadequate thus the term northern north sea unconformity complex is used here

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

evidence of surface water oligotrophy during the paleocene eocene thermal maximum nannofossil assemblage data from ocean drilling program site 690 maud rise weddell sea 1 nannoplankton assemblages at ocean drilling program site 690 maud rise weddell sea experienced an abrupt and dramatic transformation at the onset of the paleocene eocene thermal maximum petm at 55 m y the major assemblage shift suggests a change from colder more productive surface waters to warmer more oligotrophic conditions significant restructuring of assemblages during the later part of the petm indicates that nannoplankton communities were not stable and that surface water conditions changed although they remained warm and oligotrophic combined with benthic foraminiferal assemblage data nannoplankton assemblage results suggest increased sequestration of nutrients in shelf environments and starvation of the open ocean although the petm was a short lived event it appears to have had long term effects on nannoplankton leading to the extinction of fasciculithus a dominant paleocene genus

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

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

the palynological response to the toarcian oceanic anoxic event early jurassic at peniche lusitanian basin western portugal abstract the uppermost pliensbachian and lower toarcian lower jurassic succession exposed at peniche in the central part of the lusitanian basin western portugal was examined for palynomorphs the 45 samples span the emaciaticeras emaciatum dactylioceras polymorphum and hildaites levisoni ammonite biozones abs and the succession includes the global boundary stratotype section and point gssp for the toarcian stage and the toarcian oceanic anoxic event t oae a low diversity dinoflagellate cyst flora typical of the sub boreal realm was recovered from the emaciaticeras emaciatum and dactylioceras polymorphum abs the dominant element is the cold water species luehndea spinosa which is an index for the pliensbachian to earliest toarcian and is thought to have migrated from the more northerly boreal realm prior to the t oae dinoflagellates thrived in the lusitanian basin except during a brief warm period in the earliest toarcian despite the latter the recovery from this event was relatively rapid and was characterised by a return to relatively cool temperatures the hildaites levisoni ab at peniche represents the t oae and the overlying strata and is characterised by a profound reduction in dinoflagellate cyst relative abundances this dinoflagellate cyst blackout and the associated rise of prasinophytes reflects significant environmental stress such as marine anoxia elevated temperatures and reduced salinity with the former two probably being most important the low proportions of dinoflagellate cysts following the t oae indicates a protracted recovery phase from the bottom and water column anoxia developed throughout the lusitanian basin

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

the paleocene eocene carbon isotope excursion in higher plant organic matter differential fractionation of angiosperms and conifers in the arctic abstract a study of upper paleocene lower eocene p e sediments deposited on the lomonosov ridge in the central arctic ocean reveals relatively high abundances of terrestrial biomarkers these include dehydroabietane and simonellite derived from conifers gymnosperms and a tetra aromatic triterpenoid derived from angiosperms the relative percentage of the angiosperm biomarker of the summed angiosperm conifer biomarkers was increased at the end of the paleocene eocene thermal maximum petm different when observed with pollen counts which showed a relative decrease in angiosperm pollen stable carbon isotopic analysis of these biomarkers shows that the negative carbon isotope excursion cie during the petm amounts to 3 for both conifer biomarkers dehydroabietane and simonellite comparable to the magnitude of the cie inferred from marine carbonates but significantly lower than the 4 5 of the terrestrial c29 n alkane m pagani n pedentchouk m huber a sluijs s schouten h brinkhuis j s sinninghe damste g r dickens and the iodp expedition 302 expedition scientists 2006 arctic s hydrology during global warming at the paleocene eocene thermal maximum nature 442 671 675 which is a compound sourced by both conifers and angiosperms conspicuously the angiosperm sourced aromatic triterpane shows a much larger cie of 6 and suggests that angiosperms increased in their carbon isotopic fractionation during the petm our results thus indicate that the 4 5 c29 n alkane cie reported previously represents the average cie of conifers and angiosperms at this site and suggest that the large and variable cie observed in terrestrial records may be partly explained by the variable contributions of conifers and angiosperms the differential response in isotopic fractionation of angiosperms and conifers points to different physiological responses of these vegetation types to the rise in temperature humidity and greenhouse gases during the petm

abundance and size changes in the calcareous nannofossil schizosphaerella relation to sea level the carbonate factory and palaeoenvironmental change from the sinemurian to earliest toarcian of the paris basin abstract abundance and size changes in the calcareous nannofossil schizosphaerella are investigated throughout the upper sinemurian to lowermost toarcian of the sancerre couy core paris basin measurements from 116 samples are compared to 487 and 411 measurements of caco3 content and total organic carbon toc content respectively our results confirm that schizosphaerella was better adapted to proximal areas than coccoliths as expressed by the stepwise rise in abundance of schizosphaerella followed later on by the rise in abundance of coccoliths during the major transgression of the sinemurian the new results show that changes in the size of schizosphaerella were mostly a response to early jurassic temperature variations coldhouse greenhouse changes large average sizes of schizosphaerella correspond mostly to proximal cool environmental conditions of the late sinemurian medium average sizes correspond to more distal conditions and cool surface waters whereas small average sizes are associated with warm episodes these results suggest that the size response of schizosphaerella to the early toarcian oceanic anoxic event and to preceding environmental perturbations of the early jurassic was probably not a physiological response to lower calcification but rather represent changes in abundances of different ecophenotypes or sub species related to climate change the contribution of schizosphaerella carbonate caco3schizo to the total carbonate bulk caco3 in the paris basin was minor

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

spatial and temporal distribution of calcareous nannofossils along a proximal distal transect in the lower jurassic of the umbria marche basin central italy abstract three upper pliensbachian lower toarcian successions of the umbria marche basin were analysed for sedimentology and calcareous nannofossil content across a 120 km long transect from proximal to distal areas with respect to the shallow latium abruzzi platform the uppermost part of the corniola unit uppermost pliensbachian and the marne di monte serrone formation p p lower toarcian polymorphum to levisoni ammonite zones that contains organic matter rich levels were studied in the three localities the analysis of the sedimentary structures in the studied successions indicates that the basin was a shallow water system above storm wave base during the late pliensbachian early toarcian the stratigraphic evolution of the sedimentary structures recorded in the studied localities shows that the upper pliensbachian corresponded to a regressive sedimentary interval whereas a complete transgressive regressive relative sea level cycle occurred during the lower toarcian polymorphum zone a further transgression occurred in the lowermost levisoni zone lower toarcian changes from carbonate dominated to marl dominated sediments form decimetre to metre scale lithological units that can be correlated from succession to succession and may have resulted from short term sea level and or palaeoenvironmental variations the mean duration of these lithological units suggests that they formed in tune with the first orbital eccentricity cycle 100 ka the analysis of calcareous nannofossil quantities abundance per mm2 of the smear slide percentage mean abundance shows both spatial and temporal changes in the community structure schizosphaerella spp and crepidolithus crassus were more abundant in proximity to the shallow latium abruzzi platform these two taxa were likely affiliated to oligotrophic conditions in marine surface waters explaining why they were generally more abundant in carbonate rich intervals than in marls mitrolithus jansae is interpreted as being a deep dwelling species which was more abundant during rising sea level in the earliest toarcian species of the genera lotharingius biscutum and calyculus were more abundant when higher trophic conditions occurred and were thus more common during periods of marl deposition the stratigraphic distribution of calcareous nannoplankton and its interpretation in terms of palaeoecological affinities allow three main phases in the environmental evolution during the early toarcian anoxic event to be recognised the first phase corresponds to a long term transgression and high nannoplankton abundance in distal areas mainly lotharingius biscutum and calyculus species suggests high productivity that may have resulted from upwelling the beginning of accumulation and preservation of organic matter in the sediments occurred during this phase the scarcity of nannoplankton during the second phase may indicate oligotrophic conditions in surface waters possibly due to water stratification during the highest sea level stand a high abundance of schizosphaerella spp and c crassus is recorded in proximal areas probably resulting from sporadic nutrient input from continental or shallow platform areas the maximum of organic matter preservation is recorded during this phase during the third phase relative sea level fall and efficient water mixing by frequent storm events led to enhanced nannoplankton productivity in proximal and distal areas and to a decrease in organic matter accumulation in the umbria marche basin

new insights in the pattern and timing of the early jurassic calcareous nannofossil crisis abstract the toarcian calcareous nannofossils crisis associated with the early toarcian oceanic anoxic event t oae in the early jurassic period is thought to represent one of the most important biocalcification crises during the mesozoic occurring simultaneously with profound disturbance of the carbon cycle however the causes are still under debate particularly with regard to the pattern timing of the biocalcification crisis relative roles of intrinsic and extrinsic processes as drivers of the crisis and also causal mechanisms of the t oae in this study a new quantification of toarcian calcareous nannofossil abundance and size is presented for the sancerre borehole paris basin france beyond the recognition of a severe biocalcification crisis defined by the major drop in abundance and the reduction in size of the most important pelagic carbonate producer schizosphaerella punctulata for the first time this study proposes an insight into the pace and timing of the nannoplankton crisis at sancerre the carbonate production of the lower toarcian sediments previously attributed to obliquity forcing of climate allows estimating a duration of 210 kyr for the biocalcification crisis and of 120 kyr for the shift towards lower carbon isotope values the onset of the biocalcification crisis marked by a fertility event lasted 60 kyr and the calcium carbonate values remained low for 150 kyr the subsequent recovery of carbonate and nannoplankton lasted 60 kyr and 550 kyr respectively additionally a link between the biocalcification crisis the seawater palaeotemperature and the carbon isotope steps can be demonstrated this covariance provides compelling evidence of fundamental change in the response of the climatic warming and the carbon cycle systems triggering the biocalcification crisis these observations indicate that the biocalcification crisis can be regarded as a direct or indirect consequence of a global warming moreover a deficiency of the biological pump is proposed here as a complementary causal mechanism for explaining the negative carbon isotope excursion

hydrothermal venting of greenhouse gases triggering early jurassic global warming abstract the climate change in the toarcian early jurassic was characterized by a major perturbation of the global carbon cycle the event lasted for approximately 200 000 years and was manifested by a global warming of 6 c anoxic conditions in the oceans and extinction of marine species the triggering mechanisms for the perturbation and environmental change are however strongly debated here we present evidence for a rapid formation and transport of greenhouse gases from the deep sedimentary reservoirs in the karoo basin south africa magmatic sills were emplaced during the initial stages of formation of the early jurassic karoo large igneous province and had a profound influence on the fate of light elements in the organic rich sedimentary host rocks total organic carbon contents and vitrinite reflectivity data from contact aureoles around the sills show that organic carbon was lost from the country rocks during heating we present data from a new type of geological structures termed breccia pipes rooted in the aureoles within the shale of the western karoo basin the breccia pipes are cylindrical structures up to 150 meters in diameter and are mainly comprised of brecciated and baked black shale thousands of breccia pipes were formed due to gas pressure build up during metamorphism of the shales resulting in venting of greenhouse gases to the toarcian atmosphere mass balance calculations constrained by new aureole data show that up to 1800 gt of co2 was formed from organic material in the western karoo basin about 15 times this amount of co2 27 400 gt may have formed in the entire basin during the intrusive event u pb dating of zircons from a sill related to many of the pipes demonstrates that the magma was emplaced 182 5 0 4 million years ago this supports a causal relationship between the intrusive volcanism the gas venting and the toarcian global warming

toarcian extreme warmth led to tropical cyclone intensification abstract models predict that tropical cyclone intensity should increase on a warming planet observing this relationship remains however a difficult task since no clear trend is yet emerging from the anthropogenic record the geological past offers the opportunity to study this relationship by looking at episodes of extreme warmth such as the toarcian oceanic anoxic event t oae early jurassic ca 181 ma in this study we document an increase in the occurrence of storm related deposits during the onset of the t oae in the tide dominated high atlas basin of morocco associated with a drastic deepening of the mean storm weather wave base the palaeolatitude of the high atlas basin 18 north during the early jurassic rules out winter storms as the driving mechanism behind the formation of these deposits and points to a dramatic intensification of tropical cyclone intensity during the t oae global warming these new results combined with the previously reported ubiquitous appearance of storm deposits during the t oae in tropical seas of the western tethyan area support the concept that the globally averaged intensity of tropical cyclones will increase in the coming century due to the anthropogenic climate change

strontium isotope profile of the early toarcian jurassic oceanic anoxic event the duration of ammonite biozones and belemnite palaeotemperatures we profile 87sr 86sr 13c 18o sr ca mg ca and na ca in belemnites through pliensbachian and toarcian strata on the yorkshire coast uk which include the early jurassic oceanic anoxic event the 87sr 86sr profile shows that the relative duration of ammonite subzones differ by a factor of up to 30 the lower jurassic exaratum subzone is 30 times longer than the clevelandicum subzone because the exaratum subzone in yorkshire which contains the anoxic event is condensed by a factor of between 6 5 and 12 2 times relative to adjacent strata using our 87sr 86sr profile the resolution in correlation and dating attainable in the interval is between 1 5 m and 15 m of section and better than 0 25 myr in parts of the sequence this stratigraphic resolution equals that attainable with ammonites a new age model is provided for late pliensbachian and early toarcian time that is based on the 87sr 86sr profile through the sequence the sr ca mg ca na ca and 18o of belemnite carbonate covary suggesting that elemental ratios may be useful for palaeotemperature measurement our 13cbelemnite data splits into three the previously reported positive isotope excursion to 6 5 in the early toarcian we speculate that the excursion s resulted from addition to surface waters of isotopically heavy co2 via ebullition of methanogenic co2 from the sediment during early burial of organic rich 10 toc sediments

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

further evidence for the development of photic zone euxinic conditions during mesozoic oceanic anoxic events periods in earth history characterized by extensive organic carbon deposition so called oceanic anoxic events oaes are the subject of considerable scrutiny insight into the extent of anoxic conditions in the water column has been provided by the reported occurrence of derivatives of isorenieratene a carotenoid found only in green sulphur bacteria and thus indicative of photic zone euxinia i e a water column containing free h 2 s in north atlantic deep sea sediments deposited during the cenomanian turonian boundary oae2 bonarelli event in this study we have used the distributions of chlorophyll and bacteriochlorophyll degradation products maleimides 1 h pyrrole 2 5 diones and high molecular weight porphyrins to examine further the cenomanian turonian boundary oae2 as well as other oaes of the early toarcian posidonienschiefer event and early aptian selli event in particular methyl isobutyl me i bu maleimide on structural grounds appears to be diagnostic of green sulphur bacteria this compound occurs in five of seven examined marls that record the early toarcian oae marche umbria italy further expanding the geographical range of toarcian sediments where evidence for photic zone euxinic conditions has been found me i bu maleimide occurs in three of six black shales spanning the livello selli marche umbria italy the type locality for the aptian oae1a providing the first evidence for photic zone euxinic conditions during this event with respect to the cenomanian turonian oae maleimide evidence for photic zone euxinic conditions was found in all the north atlantic sites investigated including those characterized by relatively organic lean sediments several tethyan sites and one site off western australia these data indicate that euxinic conditions were common in the water column during mesozoic oceanic anoxic events

the pacific south american modes and tropical convection during the southern hemisphere winter abstract atmospheric circulation features and convection patterns associated with two leading low frequency modes in the southern hemisphere sh are examined in multiyear global reanalyses produced by ncep ncar and nasa dao the two leading modes referred to as the pacific south american psa modes are represented by the first two eof patterns the two patterns are in quadrature with each other and are dominated by wavenumber 3 in midlatitudes with large amplitudes in the pacific south american sector in the pacific anomalies in the subtropics and in the midlatitudes are opposite in phase taken together the two psa modes represent the intraseasonal oscillation in the sh with periods of roughly 40 days the evolution of the psa modes shows a coherent eastward propagation a composite analysis was conducted to study the evolution of tropical convection and the corresponding circulation changes associated with the psa modes outgoing longwave radiation olr anomaly composites during the mature phase

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

calibrating the magnitude of the toarcian carbon cycle perturbation sedimentary rocks recording the toarcian oceanic anoxic event t oae are marked by a negative carbon isotope excursion cie reaching up to 7 and ranking among the largest known in the phanerozoic these records suggest a dramatic perturbation of the carbon cycle that has been linked to the transfer of juvenile carbon from the endogenic to the exogenic reservoirs nevertheless the magnitude of the toarcian cie varies significantly from one substrate to another hence complicating mass balance evaluation of the potential driving mechanisms here we show using high resolution paired records of 13ctotal organic carbon toc and hydrogen index from the denkingen core and dotternhausen quarry germany that the amplitude of the negative cie in marine organic matter is considerably exaggerated by changes in organic matter sourcing our corrected 3 4 cie implies a constant carbon isotope fractionation between organic and carbonate carbon across the t oae and may point to a prominent role of cyanobacteria as organic matter producers during the early jurassic these results also suggest that the difference between inorganic and organic carbon isotope values in marine strata constitutes a poor proxy of pco2 levels across the t oae moreover this corrected 3 4 cie together with evidence for 5 c seawater warming across the t oae point to the input of 25 000 gt c with much more 13c enriched signature than previously modeled our results thus allow reconciling inorganic and organic 13c records of the t oae and have important implications for the evaluation of their causal mechanisms

pliensbachian early jurassic calcareous nannofossils from the peniche section lusitanian basin portugal a clue for palaeoenvironmental reconstructions abstract quantitative analysis of early jurassic calcareous nannofossil assemblages from the peniche section in portugal have been performed in order to interpret palaeoenvironmental changes occurring in the lusitanian basin during the late pliensbachian davoei margaritatus and spinatum ammonite zones nannofossil data are compared to already published carbon and oxygen stable isotope data organic matter content wt total organic carbon toc and biomarker analysis a significant change in calcareous nannofossil assemblages and species diversity at the transition between the margaritatus and spinatum ammonite zones matches with the pattern shown by geochemical data this suggests that a profound change in environmental conditions occurred at that time in the davoei and margaritatus ammonite zones in a context of general sea level rise the lusitanian basin was characterized by water column stratification that favoured the sedimentation and preservation of organic matter biomarkers and oxygen isotope trends suggest that stratification of water masses occurred because of low salinity in surface waters the shallowest part of the water column characterized by oligotrophic conditions was inhabited by the probable calcareous dinocyst schizosphaerella spp while the deep dweller crepidolithus crassus flourished in the lower photic zone layers in the spinatum ammonite zone a regressive trend occurred and a salinity increase is inferred on the basis of oxygen isotope values water masses were probably less stratified at that time the upper photic zone nannofossil assemblages were still dominated by schizosphaerella spp whilst in the deep photic zone mitrolithus jansae a mediterranean taxon replaced c crassus a taxon with nw european affinity this pattern may indicate a change in palaeoceanographic conditions related to surface current circulation the sea level fall occurring during the spinatum ammonite zone may have resulted in the partial isolation of the lusitanian basin from the nw europe basins because of the creation of a threshold alternatively tectonic drowning of north african carbonate platforms in the late pliensbachian may have promoted better exchanges of nannoplankton between the lusitanian basin and the mediterranean tethys via the creation of new seaway connections

phytoplankton response and geochemical evidence of the lower toarcian relative sea level rise in the umbria marche basin central italy abstract quantitative and qualitative analyses have been carried out on the lower toarcian organic rich lithologies of the umbria marche domain marne di monte serrone formation with the aim to emphasize the potential of organic and geochemical parameters as palaeoenvironmental indices the organic matter and phytoplankton data suggest that the palaeoenvironmental evolution of this area was controlled by a third order cyclicity induced by climatic eustatic fluctuations during the early toarcian sea level rise two different phases have been recognized the phase of facies retrogradation referable to the transgressive system tract lying in the early and middle part of the d tenuicostatum zone this phase was characterized by a relative increase of basinal caco 3 a better fertility within the photic zone and a decrease of sea bottom oxygenation and of the micronutrients in surface waters the maximum flooding event at the top of the retrogradation phase corresponds with the maximum of palaeoenvironmental distality sedimentary starvation and total organic carbon toc values the phase of facies progradation corresponding to the high stand system tract occurred in the uppermost portion of the d tenuicostatum zone this phase induced a more efficient water circulation producing a re oxygenation at the sea bottom a better nutrient recycling and a reflourishing of nannoplakton the caco 3 in the basin was mainly extrabasinal because the carbonate platforms started producing and exporting the carbonate mud basinwards again this approach confirms that eustatic fluctuations strongly influenced the variations of phytoplankton total abundance and species diversity and the distribution of organic matter in the sediments it also regards a significant tool for the investigation of lithologies where the lack of significant sedimentary structures does not give detailed palaeoenvironmental information

paleotemperature variations of early jurassic seawater recorded in geochemical trends of belemnites from the basque cantabrian basin northern spain abstract here we present new o isotope mg ca and sr ca data of early jurassic belemnites from the basque cantabrian basin of northern spain a succession deposited in a periodically anoxic restricted hemipelagic basin nearly 200 pliensbachian lower toarcian belemnites were collected and examined using optical cathodoluminescence and chemical techniques to constrain the effects of potential diagenetic alteration after a precise diagenetic screening of samples 176 belemnites were selected to provide records of changes in seawater geochemistry and temperature the records show a fairly good correlation of 18o with mg ca ratios r 0 65 and a weak correlation of 18o with sr ca ratios r 0 34 the oxygen isotope data if interpreted in terms of paleotemperature have revealed a period of relatively uniform and warm seawater temperatures during the early pliensbachian followed by a sharp temperature drop during the late pliensbachian earliest toarcian the onset of this cooler period occurred within the protogrammoceras celebratum subzone and the coolest seawater temperatures were reached at the end pliensbachian pleuroceras spinatum zone this was followed by a rapid seawater warming during the harpoceras serpentinus zone of the early toarcian in addition the data set provides new information regarding paleoceanographic conditions for anoxic events in a critical region of connection between the boreal and tethys realms it is the background to relatively high 18o values that prevailed during the anoxic episodes concerning the impact of the early toarcian anoxic event in northern spain the detected 18o and mg ca excursions reached their minimum and maximum respectively during the lower h serpentinus zone while the anoxic black shale interval developed earlier during the upper dactylioceras tenuicostatum lowermost h serpentinus zones

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