records of carbon and sulfur cycling during the silurian ireviken event in gotland sweden abstract early silurian 431 ma carbonate rocks record a ca 4 5 positive excursion in the stable isotopic composition of carbonate carbon 13ccarb associated with this isotopic shift is a macroevolutionary turnover pulse known as the ireviken event the onset of this carbon isotope excursion is commonly associated with a shallowing upward facies transition that may have been accompanied by climatic change as indicated by a parallel positive shift 0 6 in the stable isotopic composition of carbonate oxygen 18ocarb however the relationships among carbon cycle perturbations faunal turnover and environmental changes remain enigmatic here we present a suite of new isotopic data across the ireviken event from multiple sections in gotland sweden these samples preserve no systematic change in 18ocarb but show positive excursions of equal magnitude in both carbonate 13ccarb and organic 13corg carbon in addition the data reveal a synchronous perturbation in sulfur isotope ratios manifest as a ca 7 positive excursion in carbonate associated sulfate 34scas and a ca 30 positive excursion in pyrite 34spyr the increase in 34spyr values is accompanied by a substantial concomitant increase in stratigraphic variability of 34spyr the relatively constant offset between the 13ccarb and 13corg excursions throughout the ireviken event could be attributed to increased organic carbon burial or possibly a change in the isotopic composition of co2 sources from weathering however a positive correlation between carbonate abundance and 13ccarb suggests that local to regional changes in dissolved inorganic carbon dic during the shallowing upward sequence may have been at least partly responsible for the observed excursion the positive excursion recorded in 34scas suggests a perturbation of sufficient magnitude and duration to have impacted the marine sulfate reservoir an inverse correlation between cas abundance and 34scas supports the notion of decreased sulfate concentrations at least locally consistent with a concomitant increase in pyrite burial a decrease in the offset between 34scas and 34spyr values during the ireviken event suggests a substantial reduction in the isotopic fractionations epyr expressed during microbial sulfur cycling and pyrite precipitation through this interval decreased epyr and the concomitant increase in stratigraphic variation in 34spyr are typical of isotope systematics observed in modern shallow water environments associated with increased closed system behavior and or oxidative sedimentary reworking during early sediment diagenesis while the isotopic trends associated with the ireviken event have been observed in multiple locations around the globe many sections display different magnitudes of isotopic change and moreover are typically associated with local facies changes due to the stratigraphic coherence of the carbon and sulfur isotopic and abundance records across the ireviken event and their relationship to changes in local depositional environment we surmise that these patterns more closely reflect biogeochemical processes related to deposition and lithification of sediment than global changes in carbon and sulfur burial fluxes

perspectives on proterozoic surface ocean redox from iodine contents in ancient and recent carbonate abstract the proterozoic eon hosted the emergence and initial recorded diversification of eukaryotes oxygen levels in the shallow marine settings critical to these events were lower than today s although how much lower is debated here we use concentrations of iodate the oxidized iodine species in shallow marine limestones and dolostones to generate the first comprehensive record of proterozoic near surface marine redox conditions the iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters to assess the validity of our approach neogene quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate iodine contents despite the potential for diagenetic loss maximum proterozoic carbonate iodine levels are elevated relative to those of the archean particularly during the lomagundi and shuram carbon isotope excursions of the paleo and neoproterozoic respectively for the shuram anomaly comparisons to neogene quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends the baseline low iodine levels in proterozoic carbonates relative to the phanerozoic are linked to a shallow oxic anoxic interface oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes however the diagnostically low iodine data from mid proterozoic shallow water carbonates relative to those of the bracketing time intervals are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans this redox instability may have challenged early eukaryotic diversification and expansion creating an evolutionary landscape unfavorable for the emergence of animals

quantifying the missing sink for global organic carbon burial during a cretaceous oceanic anoxic event abstract the cretaceous experienced numerous global and local climatic perturbations to the ocean atmosphere system especially during periods of known widespread organic carbon burial termed oceanic anoxic events oaes the cenomanian turonian boundary event 93 9 ma or oae 2 is the best documented and widespread organic carbon oc burial event in earth history with more than 170 sections published despite the substantial number of locations the majority is found within the proto atlantic ocean tethys ocean and epicontinental seaways it has been hypothesized that the pervasive burial of oc during oae 2 caused the observed positive carbon isotope excursion 2 to 7 average 3 the isotope excursion can help constrain the global burial of oc even for unstudied portions of the global ocean this approach can solve for missing oc sinks by comparing model estimates with the known distribution of oae 2 sediments and their oc contents specifically mapping the known spatial extent of oc burial in terms of mass accumulation rates mars and comparing those results with the prediction using a forward box model to derive the amount of oc burial to reproduce the globally observed positive carbon isotope excursion the available oc data from outcrop and drill core with reasonable extrapolation to analogous settings without data quantifies 13 of the total seafloor mostly from marginal marine and epicontinental epeiric settings however this extrapolation for oc burial plus using most appropriate mars to unknown portions of the seafloor fail to account for the amount of oc burial predicted for a 3 positive carbon isotope excursion this discrepancy remains even when considering additional sinks of organic carbon burial such as coal lacustrine environments authigenic carbonate and the loss of oc associated with hydrocarbon reservoirs this outcome points to a large reservoir of oc that is not currently constrained such as highly productive margins and or equatorial regions or a small but significant increase deep ocean oc burial another possibility is that the carbon fluxes are less than those used in the model which would require less oc burial to explain a 3 carbon isotope excursion

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

variability in sulfur isotope records of phanerozoic seawater sulfate the s of seawater sulfate reflects processes operating at the nexus of sulfur carbon and oxygen cycles however knowledge of past seawater sulfate s values must be derived from proxy materials that are impacted differently by depositional and post depositional processes we produced new timeseries estimates for the s value of seawater sulfate by combining 6710 published data from three sedimentary archives marine barite evaporites and carbonate associated sulfate with updated age constraints on the deposits robust features in multiple records capture temporal trends in the s value of seawater and its interplay with other phanerozoic geochemical and stratigraphic trends however high frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints the amount of noise quantified from the variograms of each record increases with age for all s proxies indicating that post depositional processes obscure detailed knowledge of seawater sulfate s s value deeper in time

an iodine record of paleoproterozoic surface ocean oxygenation constraining oxygen levels in the early precambrian surface ocean has been a longstanding goal but efforts have been challenged by the availability of suitable proxies here we present a novel approach iodine geochemistry which broadens our perspective by providing constraints on shallow carbonate dominated marine settings iodate io 3 persists exclusively in oxic waters and is the sole iodine species incorporated into carbonate minerals allowing iodine to calcium ratios i ca in shallow carbonates to be used as a paleoredox indicator our data from a series of mesoarchean through paleoproterozoic carbonates deposited under shallow marine conditions reveal a progressive surface ocean oxygenation in the early paleoproterozoic these data seem to indicate that a largely anoxic surface ocean extended throughout the archean until the great oxidation event goe at ca 2 4 ga implying that previous inferences of pre goe oxygen production may reflect oxygen oases transient oxidation events or oxygen levels below those required for io 3 accumulation the data suggest formation and persistence of io 3 and consequently surface ocean oxygen concentrations of at least 1 m during the goe following the initial rise of oxygen carbonate associated iodine in globally extensive carbonate units deposited during the lomagundi positive carbon isotope excursion at ca 2 22 2 1 ga suggests a widespread aerobic iodine cycle beyond that operating prior to the event synchronous with high relative rates of organic carbon burial and apparent expansion of oxidative conditions

upper ocean oxygenation dynamics from i ca ratios during the cenomanian turonian oae 2 global warming lowers the solubility of gases in the ocean and drives an enhanced hydrological cycle with increased nutrient loads delivered to the oceans leading to increases in organic production the degradation of which causes a further decrease in dissolved oxygen in extreme cases in the geological past this trajectory has led to catastrophic marine oxygen depletion during the so called oceanic anoxic events oaes how the water column oscillated between generally oxic conditions and local global anoxia remains a challenging question exacerbated by a lack of sensitive redox proxies especially for the suboxic window to address this problem we use bulk carbonate i ca to reconstruct subtle redox changes in the upper ocean water column at seven sites recording the cretaceous oae 2 in general i ca ratios were relatively low preceding and during the oae interval indicating deep suboxic or anoxic waters exchanging directly with near surface waters however individual sites display a wide range of initial values and excursions in i ca through the oae interval reflecting the importance of local controls and suggesting a high spatial variability in redox state both i ca and an earth system model suggest that the northeast proto atlantic had notably higher oxygen levels in the upper water column than the rest of the north atlantic indicating that anoxia was not global during oae 2 and that important regional differences in redox conditions existed a lack of correlation with calcium lithium and carbon isotope records suggests that neither enhanced global weathering nor carbon burial was a dominant control on the i ca proxy during oae 2

behavior of carbonate associated sulfate during meteoric diagenesis and implications for the sulfur isotope paleoproxy abstract although carbonate associated sulfate cas is used widely as a proxy for the sulfur isotope composition of ancient seawater little is known about the effects of diagenesis on retention of primary 34 s signals our case study of the key largo limestone pleistocene florida is the first systematic assessment of the impact of meteoric diagenesis on cas properties geochemical and petrographic data show that meteoric diagenesis has affected the exposed coralline facies to varying degrees yielding differences now expressed as sharp reaction fronts between primary and secondary carbonate minerals within individual coral heads specifically analyses across high resolution transects in the key largo limestone show that concentrations of strontium and sodium decrease across the recrystallization front from original aragonite to meteoric low magnesium calcite by factors of roughly 5 and 10 respectively predictably 18 o values decrease across these same fronts the 13 c relationships are more complex with the most depleted values observed in the latest formed calcite such trends likely reflect carbon isotope buffering capacity that decreased as reaction progressed as well as protracted development of soil profiles and the associated terrestrial biomass and thus depleted 13 c during sea level lowstand conversely 34 s values of cas vary within a narrow buffered range from 20 6 to 22 6 compared to 20 8 22 0 of coeval pleistocene seawater across the same mineralogical transition despite sulfate concentrations that drop in the diagenetic calcite by an average factor of 12 collectively these data point to robust preservation of primary 34 s for carbonates that have experienced intense meteoric diagenesis which is encouraging news for those using the isotopic composition of cas as a paleoceanographic proxy at the same time the vulnerability of cas concentrations to diagenetic resetting is clear

sulfur isotopes track the global extent and dynamics of euxinia during cretaceous oceanic anoxic event 2 the mesozoic era is characterized by numerous oceanic anoxic events oaes that are diagnostically expressed by widespread marine organic carbon burial and coeval carbon isotope excursions here we present coupled high resolution carbon and sulfur isotope data from four european oae 2 sections spanning the cenomanian turonian boundary that show roughly parallel positive excursions significantly however the interval of peak magnitude for carbon isotopes precedes that of sulfur isotopes with an estimated offset of a few hundred thousand years based on geochemical box modeling of organic carbon and pyrite burial the sulfur isotope excursion can be generated by transiently increasing the marine burial rate of pyrite precipitated under euxinic i e anoxic and sulfidic water column conditions to replicate the observed isotopic offset the model requires that enhanced levels of organic carbon and pyrite burial continued a few hundred thousand years after peak organic carbon burial but that their isotope records responded differently due to dramatically different residence times for dissolved inorganic carbon and sulfate in seawater the significant inference is that euxinia persisted post oae but with its global extent dwindling over this time period the model further suggests that only 5 of the global seafloor area was overlain by euxinic bottom waters during oae 2 although this figure is 30 greater than the small euxinic fraction present today 0 15 the result challenges previous suggestions that one of the best documented oaes was defined by globally pervasive euxinic deep waters our results place important controls instead on local conditions and point to the difficulty in sustaining whole ocean euxinia

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

ice sheet elevations from across track processing of airborne interferometric radar altimetry 1 interferometric radar altimeters ira s use dual receive antennas to overcome one of the spatial limitations of pulse limited altimeters in a conventional ira measurement the range and across track direction of a scatterer are determined using the phase difference between the antennas we demonstrate a method of determining multiple elevation points across a swath orthogonal to the instrument ground track in regions of steep terrain such as ice sheet margins we use data from an airborne ira a prototype of the cryosat 2 instrument and compare the results to simultaneous airborne laser scanner als observations this application results in a 75 fold increase in measurement density compared to conventional radar altimetry along a 2 5 km ground track the rms departure between the ira and als derived measurements was 1 67 m based on our result although our approach is limited to areas of relatively steep slope a 25 to 75 fold increase in elevation measurements could be achieved in coastal regions of antarctica and greenland with similar processing of cryosat 2 data

testing the limits of paleozoic chronostratigraphic correlation via high resolution 500 k y integrated conodont graptolite and carbon isotope δ13ccarb biochemostratigraphy across the llandovery wenlock silurian boundary is a unified phanerozoic time scale achievable the resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration by virtue of deep ocean cores and astrochronology the cenozoic and mesozoic time scales carry error bars of a few thousand years k y to a few hundred k y in contrast most of the paleozoic time scale carries error bars of plus or minus a few million years m y and chronostratigraphic control better than 1 m y is considered high resolution the general lack of paleozoic abyssal sediments and paucity of orbitally tuned paleozoic data series combined with the relative incompleteness of the paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the paleozoic time scale to the level achieved during the mesozoic and cenozoic was viewed as impractical impossible or both here we utilize integrated graptolite conodont and carbonate carbon isotope delta c 13 carb data from three paleocontinents baltica avalonia and laurentia to demonstrate chronostratigraphic control for upper llandovery through middle wenlock telychiansheinwoodian similar to 436 426 ma strata with a resolution of a few hundred k y the interval surrounding the base of the wenlock series can now be correlated globally with precision approaching 100 k y but some intervals e g uppermost telychian and upper shein woodian are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and or extinction or carbon isotopic features to delineate such small time slices although producing such resolution during the paleozoic presents an array of challenges unique to the era we have begun to demonstrate that erecting a paleozoic time scale comparable to that of younger eras is achievable less

empirical links between trace metal cycling and marine microbial ecology during a large perturbation to earth s carbon cycle abstract understanding the global redox state of the oceans and its cause and effect relationship with periods of widespread organic carbon deposition is vital to interpretations of earth s climatic and biotic feedbacks during periods of expanded oceanic oxygen deficiency here we present a compilation of new and published data from an organic rich locality within the proto north atlantic ocean during the cenomanian turonian boundary event that shows a dramatic drawdown of redox sensitive trace elements iron geochemistry independently suggests euxinic deposition i e anoxic and sulfidic bottom waters for the entire section thus confirming its potential as an archive of global marine metal inventories in particular depleted molybdenum mo and vanadium v concentrations effectively record the global expansion of euxinic and oxygen deficient but non sulfidic waters respectively the v drawdown precedes the oae fingerprinting an expansion of oxygen deficiency prior to an expansion of euxinia molybdenum drawdown in contrast is delayed with respect to v and coincides with the onset of oae2 parallel lipid biomarker analyses provide evidence for significant and progressive reorganization of marine microbial ecology during the oae in this region of the proto north atlantic with the smallest relative eukaryotic contributions to total primary production occurring during metal depleted intervals this relationship may be related to decreasing supplies of enzymatically important trace elements similarly box modeling suggests that oceanic drawdown of mo may have approached levels capable of affecting marine nitrogen fixation predictions of possible nitrogen stress on eukaryotic production locally and globally are consistent with the low observed levels of mo and a rise in 2 methylhopane index values during the peak of the oae at the same time the environmental challenge presented by low dissolved oxygen and euxinia coincides with increased turnover rates of radiolarian clades calcareous nanofossils and foraminifera suggesting that the temporal patterns of anoxia euxinia and associated nutrient limitation may have contributed to the fabric of oae2 related turnover

shallow water redox conditions from the permian triassic boundary microbialite the rare earth element and iodine geochemistry of carbonates from turkey and south china redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments but their interpretation may not always be straightforward to evaluate the inherent complexities rare earths and yttrium rey iodine and major element concentrations are determined in two carbonate sections spanning the permian triassic p tr transition in demirtas turkey and cili south china we use major oxides to identify non seawater rey sources such as siliciclastics fe oxides phosphates and diagenetic fluids additionally we employ y ho ratio la anomaly and light rare earth element depletion to identify which samples preserve a seawater like reysn distribution in contrast to past interpretations we find that the p tr boundary microbialites in both sections contain rey signatures indicative of deposition in an oxic environment these boundary microbialites have their base at the extinction horizon and are widespread within the tethyan region in the cili section the underlying permian limestone also preserves a seawater like rey signature with a negative ce anomaly this indicates that the water column was oxygenated both before and after the extinction event the permian limestone in the demirtas section does not preserve a seawater like reysn distribution so the absence of a ce anomaly cannot be used to distinguish prevailing redox conditions during deposition however in these samples we find the presence of a diverse permian benthic community sufficient to identify deposition in an oxic environment the geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in south china the microbialite has been proposed as a disaster facies in part resulting from the exclusion of grazers although anoxia is one of the suggested mechanisms for the exclusion of grazers we find that it is not supported by geochemical and biotic evidence

time rock model for siluro devonian continental shelf western united states the carbonate shelf shelf edge and outer shelf basin were the sites of deposition of a facies complex that shifted in response to transgressive regressive t r cycles in the silurian lochkovian three t r cycles are identified and seven are identified from mid lower devonian pragian to high upper devonian middle famennian the last of these produced a thick upward deepening sequence truncated above by a famennian lower kinderhookian hiatus due to foreland effects of the antler orogeny the middle and upper members of the pilot shale compose an eighth cycle which spans the devonian mississippian boundary on the carbonate shelf most of the short lived t r cycles resulted from rapid deepening and progradation of the carbonate shelf edge exceptions occurred during the late early devonian when the carbonate shelf edge was a ramp and during the taghanic and frasnian onlaps when transgressions are known to have been strong the dolomite front tended to coincide approximately with the carbonate shelf edge but it was situated cratonward during extensive onlaps and basinward during the pre middle devonian regression

middle palaeozoic extinction events faunal and isotopic data abstract at least 9 and possibly as many as 12 extinction events of global or near global impact can be discriminated in the mid palaeozoic earliest silurian to early carboniferous on the basis of brachiopod coral conodont and ammonoid data and the history of carbonate build ups isotopic data from whole rock samples are presented for three of these events based on australian carbonate sequences constrained by conodont data these data represent the initial phase of a more extensive investigation using c o s sr and nd isotopic signatures derived from conodonts articulate and inarticulate brachiopod shell and fish remains from numerous australian and european sequences the aim of the project is to identify isotopic responses to extinction events and address causes for these changes in limestone sections analysed so far variations in carbon isotope compositions on a whole rock scale are most marked at horizons that can be correlated with times of significant reduction in biomass and diversity this is despite the fact that the whole rocks are in fact multicomponent systems with respect to carbon in part arising from diagenetic rearrangements of carbon distribution within the scale of the whole rock thus the carbon isotope data do provide evidence on a whole rock scale for fundamental changes in the global carbon cycle that are correlative with extinction events it is however unlikely that the magnitude of the isotopic shift will be precisely documented from such whole rock analyses the oxygen isotope results are less obviously related to the times of carbon isotope excursion this contrast in apparent resilience to post depositional modification of pristine isotope compositions exemplifies the extreme caution needed in evaluating isotopic data from carbonates

graptoloid evolutionary rates track ordovician silurian global climate change graptoloid evolutionary dynamics show a marked contrast from the ordovician to the silurian subdued extinction and origination rates during the ordovician give way during the late katian to rates that were highly volatile and of higher mean value through the silurian reflecting the significantly shorter lifespan of silurian species these patterns are revealed in high resolution rate curves derived from the conop constrained optimization scaled and calibrated global composite sequence of 2094 graptoloid species the end ordovician mass depletion was driven primarily by an elevated extinction rate which lasted for c 1 2 ma with two main spikes during the hirnantian the early silurian recovery although initiated by a peak in origination rate was maintained by a complex interplay of origination and extinction rates with both rates rising and falling sharply the global 13 c curve echoes the graptoloid evolutionary rates pattern the prominent and well known positive isotope excursions during the late ordovician and silurian lie on or close to times of sharp decline in graptoloid species richness commonly associated with extinction rate spikes the graptoloid and isotope data point to a relatively steady marine environment in the ordovician with mainly background extinction rates changing during the katian to a more volatile climatic regime that prevailed through the silurian with several sharp extinction episodes triggered by environmental crises the correlation of graptoloid species diversity with isotopic ratios was positive in the ordovician and negative in the silurian suggesting different causal linkages throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate

iodine to calcium ratios in marine carbonate as a paleo redox proxy during oceanic anoxic events periods of globally distributed extreme oxygen depletion so called oceanic anoxic events oaes have been recognized in the mesozoic geological record and appear to be characteristic of episodes of extreme warmth here we explore the application of iodine calcium ratios i ca in marine carbonate as a new geochemical proxy to constrain seawater redox change and provide additional insights into the response of ocean chemistry to ancient climatic warming iodine has long been known as a redox sensitive and biophilic element mainly present as iodate and iodide in seawater iodate converting to iodide under anoxic conditions laboratory experiments growing calcite crystals from solutions spiked with iodate show that this is the ionic species incorporated into the carbonate structure likely substituting for the co 3 2 ion a fall in the i ca ratio measured in carbonates formed in shallow water by marine calcifiers during the early toarcian and cenomanian turonian oaes is interpreted both as a response to a decrease in the iodate iodide ratio in ocean waters and the drawdown of the global iodine inventory under conditions of accelerated organic matter burial the results suggest that i ca ratios in carbonates may be used to monitor seawater oxidation levels throughout earth history

evidence for local and global redox conditions at an early ordovician tremadocian mass extinction abstract profound changes in environmental conditions particularly atmospheric oxygen levels are thought to be important drivers of several major biotic events e g mass extinctions and diversifications the early paleozoic represents a key interval in the oxygenation of the ocean atmosphere system and evolution of the biosphere global proxies e g carbon 13c and sulfur 34s isotopes are used to diagnose potential changes in oxygenation and infer causes of environmental change and biotic turnover the cambrian ordovician contains several trilobite extinctions some are apparently local but others are globally correlative that are attributed to anoxia based on coeval positive 13c and 34s excursions these extinction and excursion events have yet to be coupled with more recently developed proxies thought to be more reflective of local redox conditions in the water column e g i ca to confirm whether these extinctions were associated with oxygen crises over a regional or global scale here we examine an early ordovician tremadocian stage extinction event previously interpreted to reflect a continuation of recurrent early paleozoic anoxic events that expanded into nearshore environments 13c 34s and i ca trends were measured from three sections in the great basin region to test whether i ca trends support the notion that anoxia was locally present in the water column along the laurentian margin evidence for anoxia is based on coincident but not always synchronous positive 13c and 34s excursions mainly from carbonate associated sulfate and less so from pyrite data a 30 extinction of standing generic diversity and near zero i ca values although evidence for local water column anoxia from the i ca proxy broadly agrees with intervals of global anoxia inferred from 13c and 34s trends a more complex picture is evident where spatially and temporally variable local trends are superimposed on time averaged global trends stratigraphic sections from the distal and deeper part of the basin shingle pass and meiklejohn peak preserve synchronous global 13c and 34s and water column i ca evidence for anoxia but not at the more proximal section ibex ut although geochemical and paleontological evidence point toward anoxia as the driver of this early ordovician extinction event differences between i ca and 13c 34s signals suggest regional variation in the timing extent and persistence of anoxia

dynamic sulfur and carbon cycling through the end ordovician extinction revealed by paired sulfate pyrite δ34s geochemical records of the end ordovician hirnantian stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur 34spyr organic carbon 13corg and carbonate carbon 13ccarb these isotope excursions coincide with the end ordovician glaciation and mass extinction a relative increase in pyrite burial fpyr attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna an increase in fpyr would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate 34sso4 34sso4 here we present new high resolution paired sulfur isotope data from carbonate associated sulfate 34scas and pyrite from the hirnantian stage of western anticosti island quebec canada these data document a positive 20 enrichment in 34spyr comparable in magnitude to previous reports but no parallel excursion in 34scas this pattern provides new constraints on the origin of the 34spyr excursion and the nature of carbon sulfur coupling through hirnantian time specifically these observations preclude enhanced pyrite burial as the cause of the hirnantian 34spyr excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated rather the global 34spyr excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling epyr the epyr record shows a strong inverse correlation with 13c suggesting a mechanistic link between carbon cycling and processes controlling epyr during the hirnantian changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation e g through syndepositional sediment reworking and or chemocline migration and resultant restricted exchange of porewater sulfate the magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates which are sensitive to the abundance type and lability of metabolically relevant substrates environmental change associated with the end ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments leading to increased microbial sulfate reduction rates and reduced epyr as such the hirnantian 34spyr excursion may be viewed as a dynamic biological response to global climate change highlighting the connections between the carbon and sulfur biogeochemical cycles

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

a weathering hypothesis for glaciation at high atmospheric pco2 during the late ordovician new paired carbonate and organic carbon isotope analyses from nevada usa together with a consideration of the effects of mountain building and ice sheet coverage of the continents on atmospheric pco2 lead to a new hypothesis for the cause of the late ordovician glaciation we suggest that the taconic orogeny which commenced in the late middle ordovician caused a long term decline in atmospheric pco2 through increased weatherability of silicate rocks ice sheet growth was triggered when pco2 decreased to a threshold of 10 present atmospheric level and proceeded by positive ice albedo feedback in the midst of glaciation atmospheric pco2 began to rise as continental silicate weathering rates declined in response to coverage of weathering terrains by ice sheets at first this enhanced greenhouse effect was overcompensated for by ice albedo effects ultimately however atmospheric pco2 reached a level which overwhelmed the cooling effects of ice albedo and the glaciation ended the isotope results can be interpreted to indicate that atmospheric pco2 rose during the glaciation consistent with other proxy information although alternative interpretations are possible the large positive carbonate isotope excursion observed in late ordovician rocks around the world is explained as the expected response to increased carbonate platform weathering during glacioeustatic sea level lowstand rather than as a response to increased organic carbon burial

persistent oceanic anoxia and elevated extinction rates separate the cambrian and ordovician radiations recurrent mass extinction events at biomere a biostratigraphic unit boundaries characterize the middle cambrian to early ordovician tremadocian time interval that is between the major cambrian and ordovician radiations of animal life a role for anoxia in maintaining elevated extinction rates in the late cambrian has been proposed based on coincidence of an extinction with positive excursions in 13ccarb and 34scas cas carbonate associated sulfate here we examine an early ordovician extinction event at the base of the north american stairsian stage upper tremadocian and demonstrate concurrent onset of positive excursions in 13c and 34s inferred to reflect enhanced organic matter burial under anoxic waters sea level rise may have brought anoxic waters onto the shelf to initiate extinctions the evidence for 13c excursions and elevated extinction rates appears to wane in the tremadocian consistent with progressive oxygenation of the oceans reaching a threshold that helped facilitate initial stages of the great ordovician biodiversification event

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

i ca evidence for upper ocean deoxygenation during the petm anthropogenic global warming affects marine ecosystems in complex ways and declining ocean oxygenation is a growing concern forecasting the geographical and bathymetric extent rate and intensity of future deoxygenation and its effects on oceanic biota however remains highly challenging because of the complex feedbacks in the earth ocean biota system information on past global warming events such as the paleocene eocene thermal maximum petm 55 5 ma a potential analog for present and future global warming may help in such forecasting documenting past ocean deoxygenation however is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column as yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones omzs we apply a novel proxy for paleoredox conditions the iodine to calcium ratio i ca in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans and compared our reconstruction with modeled oxygen levels the reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the atlantic indian oceans and possibly the pacific ocean as well during the petm due to vertical and potentially lateral expansion of omzs

patterns of local and global redox variability during the cenomanian turonian boundary event oceanic anoxic event 2 recorded in carbonates and shales from central italy careful evaluation of the local geochemical conditions in past marine settings can provide a window to the average redox state of the global ocean during episodes of extensive organic carbon deposition these comparisons aid in identifying the interplay between climate and biotic feedbacks contributing to and resulting from these events well documented examples are known from the mesozoic era which is characterized by episodes of widespread organic carbon deposition known as oceanic anoxic events this organic carbon burial typically leads to coeval positive carbon isotope excursions geochemical data are presented here for several palaeoredox proxies cr ti v mo zn mn fe speciation i ca and sulphur isotopes from a section exposed at furlo in the marche umbrian apennines of italy that spans the cenomanian turonian boundary here oceanic anoxic event 2 is represented by a ca 1 m thick radiolarian rich millimetre laminated organic rich shale known locally as the bonarelli level iron speciation data for thin organic rich intervals observed below the bonarelli level imply a local redox shift going into the oceanic anoxic event with ferruginous conditions i e anoxic with dissolved ferrous iron transiently developed prior to the event and euxinia i e anoxic and sulphidic bottom waters throughout the event itself pre oceanic anoxic event enrichments of elements sensitive to anoxic water columns were due to initial development of locally ferruginous bottom waters as a precursor to the event however the greater global expanse of dysoxic to euxinic conditions during the oceanic anoxic event greatly reduced redox sensitive trace metal concentrations in seawater pyrite sulfur isotopes document a positive shift leading into the oae carbonate i ca ratios were generally low suggesting locally reduced bottom water oxygen conditions preceding the event and relatively increased o2 concentrations post event combined the furlo geochemical data suggest a redox stratified water column with oxic surface waters and a shallow chemocline overlying locally ferruginous bottom waters preceding the event globally widespread euxinic bottom waters during the oceanic anoxic event followed by chemocline shallowing but sustained local redox stratification following the event this article is protected by copyright all rights reserved

gradients in the carbon isotopic composition of ordovician shallow water carbonates a potential pitfall in estimates of ancient co2 and o2 abstract the carbon isotopic composition of the global dissolved inorganic carbon dic reservoir is best estimated from open ocean pelagic carbonate sediments 13 c carb however this is not practical for most of geologic time because seafloor subduction has removed the pre jurassic record and these time periods may have lacked planktonic calcifying organisms and therefore shallow water carbonate platform or periplatform sediments are utilized shallow water deposits are susceptible to a wide range of post depositional alteration processes and syn depositional controls on 13 c carb that include carbonate mineralogy water mass restriction and a host of related variables e g ph temperature organic decomposition evaporation and co 2 solubility that can produce local gradients in dic the degree to which shallow water 13 c curves diverge from open marine deposits may be critical to understanding how well global carbon cycle isotope mass balance models can predict organic carbon burial rates but documentation of such divergence is often hindered by factors that limit chronostratigraphic correlation in restricted water masses e g endemic faunas here we integrate strontium isotope 87 sr 86 sr stratigraphy and biostratigraphy to compare 13 c curves in a case study along a depth transect in middle late ordovician carbonate platform settings the restricted tidal flat and more open marine deposits are offset by a maximum of 2 during sea level drop and 0 during highstand flooding of the platform global carbon cycle models such as geocarbsulf use published 13 c carb curves to drive organic carbon burial rates under the assumption that 13 c carb reflects a global seawater signal we show here the potential pitfalls of using a published 13 c carb curve that violates this global assumption for the 460 million year middle late ordovician time bin in geocarbsulf improper usage of our locally depleted 13 c curve to drive global organic carbon burial would result in erroneous atmospheric o 2 10 8 o 2 equal to about a 30 reduction from what it should be at 15 2 o 2 using global 13 c and co 2 400 ppm too high equal to about a 13 increase from what it should be at 2570 ppm co 2 with detailed sedimentologic analysis it may be possible to identify and exclude 13 c samples that record the influences of local carbon cycling from global carbon cycle models such as geocarbsulf

silurian δ13c stratigraphy a view from north america three positive shifts in 13 c are recognized in well dated carbonate successions in north america and can be confidently correlated with shifts in the baltic region by using conodont and graptolite zonations the 13 c excursions reach distinct peaks of 3 to 4 at the ends of the amorphognathoides and siluricus conodont zones and within the ludensis graptolite zone the three events are associated with the most significant extinction horizons in the silurian and occur near clastic carbonate transitions that mark eustatic or oceanographic changes the magnitudes of the two oldest 13 c peaks compare well with the baltic region however the end siluricus excursion reaches far greater values of as much as 11 in sweden likely reflecting local modification of the global seawater signature

thermo rheological structure of the northern margin of the south china sea structural and geodynamic implications abstract rheological properties of continental lithosphere are key controls on the behavior of continental deformation using thermal structure constrained by surface heat flow data and measured thermal properties of rocks the present study calculates different thermo rheological structure scenarios for the ocean continent transition oct at the northern margin of the south china sea using two different models a conventional model taking into account frictional sliding and power law creep and a model that additionally includes a high pressure brittle fracture mechanism two compositions of the lower part of the lithosphere are considered a soft case with felsic granulite lower crust and wet peridotite lithospheric mantle and a hard case with mafic granulite lower crust and dry peridotite lithospheric mantle the former scenario shows a major rheological change from a jelly sandwich to a christmas tree type of rheology from north to south along the margin this complex rheological structure explains lateral changes in earthquake distribution and geometries of extensional faults of the oct at the northern margin of the south china sea further our analyses indicate that the initial lithospheric rheology profile probably has only one ductile layer in the lower part of upper crust such an initial lithospheric rheology model predicts focused extension to form asymmetric margins which is the case for the scs

oxygenation as a driver of the great ordovician biodiversification event the largest radiation of phanerozoic marine animal life quadrupled genus level diversity towards the end of the ordovician period about 450 million years ago a leading hypothesis for this great ordovician biodiversification event is that cooling of the ordovician climate lowered sea surface temperatures into the thermal tolerance window of many animal groups such as corals a complementary role for oxygenation of subsurface environments has been inferred based on the increasing abundance of skeletal carbonate but direct constraints on atmospheric o2 levels remain elusive here we use high resolution paired bulk carbonate and organic carbon isotope records to determine the changes in isotopic fractionation between these phases throughout the ordovician radiation these results can be used to reconstruct atmospheric o2 levels based on the o2 dependent fractionation of carbon isotopes by photosynthesis we find a strong temporal link between the great ordovician biodiversification event and rising o2 concentrations a pattern that is corroborated by o2 models that use traditional carbon sulfur mass balance we conclude that that oxygen levels probably played an important role in regulating early palaeozoic biodiversity levels even after the cambrian explosion an increase in biodiversity 450 million years ago coincided with a rise in atmospheric oxygen concentrations suggests a geochemical analysis oxygen availability may have thus helped spur the radiation alongside climatic cooling

middle late ordovician darriwilian sandbian decoupling of global sulfur and carbon cycles isotopic evidence from eastern and southern laurentia abstract middle late ordovician sequences from the appalachian basin and arbuckle mountain regions of north america were analyzed for carbonate associated sulfate 34 s cas and pyrite 34 s pyr paired with carbonate 13 c carb and organic matter 13 c org chemostratigraphy two major negative drops in 34 s cas 12 excursions are recognized the older decline in 34 s cas occurs within the histiodella holodentata phragmodus polonicus conodont zones and the younger drop is within the cahabagnathus sweeti amorphognathus tvaerensis baltoniodus gerdae subzone zones these overall these negative shifts in 34 s cas have an antithetical relationship with positive shifts in 34 s pyr 10 and 13 c carb 2 recorded in the same successions the older negative 34 s cas shift is coincident with the widely documented mid darriwilian 13 c excursion mdice and the younger negative 34 s cas shift is coincident with another positive 13 c carb shift in the early sandbian geochemical modeling of these sulfur isotope shifts suggests that a decrease in the global rate of pyrite burial or isotope fractionation between seawater sulfate and sedimentary pyrite could account for these negative 34 s cas trends additionally a substantial increase in the weathering flux of pyrite to the global oceans could also explain these secular sulfur isotope trends while increased crustal weathering is broadly consistent with a sea level lowstand and the seawater 87 sr 86 sr isotope record of change in continental weathering in the late darriwilian stage of the ordovician geologic and geochemical proxy evidence do not support distinct pulses of continental weathering required to generate two separate negative shifts in 34 s cas these antithetical isotope trends may be best explained by changes in the marine redox state that significantly reduced microbially mediated pyrite burial and organic matter remineralization rates pulses of oceanic ventilation would have expanded habitable environments for marine organisms and thus is broadly consistent with major increases in biodiversification during this period of the ordovician

integrated δ13ccarb conodont and graptolite biochemostratigraphy of the silurian from the illinois basin and stratigraphic revision of the bainbridge group

a molybdenum isotope perspective on phanerozoic deoxygenation events the careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters a synthesis and analysis of data from two mesozoic ocean anoxic events and the palaeocene eocene thermal maximum applies these techniques to constrain past ocean deoxygenation

geochemical evidence for expansion of marine euxinia during an early silurian llandovery wenlock boundary mass extinction abstract repeated biotic crises have become the hallmark for the silurian with the three most significant marine turnover events being related to dramatic global environmental perturbations causal mechanisms linking these marine extinction events with positive carbon isotope 13 c excursions paleoceanographic change and climate remain poorly constrained here we examine the positive 13 c excursion across the llandovery wenlock boundary and the associated ireviken extinction event this positive 13 c excursion has been interpreted to reflect a major change in global oceanographic climate state and enhanced organic carbon burial new geochemical data from two paleocean basins have been analyzed to determine local and global redox conditions using carbon and sulfur isotopes 34 s and iodine i ca mg proxies the high resolution 13 c and 34 s data from both sections show positive excursions indicative of global perturbations to each of these elemental cycles with minimal temporal offsets between the two systems numerical box modeling of 13 c and 34 s data indicates that these isotopic shifts can be generated by significant increases in the burial of organic carbon and pyrite which are most likely due to enhanced burial under euxinic anoxic and sulfidic conditions independently i ca mg values point to locally anoxic bottom waters in the distal and deeper basinal setting in nevada before during and after the ireviken positive 13 c excursion i ca mg values in the proximal shelf setting in tennessee show relatively oxic waters during the onset of peak 13 c values after which bottom water oxygen concentrations dropped throughout the remainder of the excursion this multiproxy paleoredox dataset provides the first direct evidence for local and global expansion of reducing marine conditions coincident with the silurian biotic event and positive 13 c excursion integration of these geochemical data for local and global scale changes in marine redox conditions with the paleontological data and evidence for eustatic sea level rise points toward a shoaling of anoxic and or euxinic waters onto the shelf as a driver for the ireviken extinction event

interpreting carbon isotope excursions carbonates and organic matter abstract variations in the carbon isotopic compositions of marine carbonate and organic carbon provide a record of changes in the fraction of organic carbon buried through time and may provide clues to changes in rates of weathering and sources of organic carbon paired carbonate and organic carbon isotope determinations provide a possibility of interpreting not only changes in the global carbon cycle through time but changes in atmospheric pco2 as well interpretations of these types of data are typically rather qualitative a quantitative basis is required to develop a better understanding of changes in the carbon cycle for this purpose we employ a simple model of the global carbon cycle which is subjected to a number of different perturbations each lasting 500 ky i e much longer than the residence times of carbon and phosphorus in the ocean atmosphere system in addition to standard considerations of carbon mass and isotopic fluxes to the ocean atmosphere system from weathering and volcanism and fluxes of organic carbon and carbonate carbon to sediments the model incorporates sensitivity of the photosynthetic carbon isotope effect to changes in pco2 the inclusion of this parameter leads to unexpected carbon isotope responses to forcing that causes increased rates of organic carbon burial a series of simple to more complex simulations illustrates the significant effects of varying differences between the carbon isotopic composition of sedimented carbonate and organic carbon b with constant b a 50 increase in organic carbon burial produces a parallel increase in carbonate and organic carbon isotopic compositions however the same simulation with b responsive to pco2 changes produces an initial parallel 13 c increase but this is followed by an even greater 13 c enrichment in organic carbon because pco2 falls in response to increased organic carbon burial the counterintuitive overall result of the enhanced organic carbon burial event is that the carbonate carbon isotopic composition actually decreases because of the more substantial increase in 13 c org in addition we illustrate the effects on carbon isotopic compositions of the oceanic inorganic carbon reservoir and buried organic matter of a 50 increase in volcanic co2 outgassing a 50 increase in weathering rate with coupled phosphate and riverine carbon flux responses a 50 decrease in shale associated organic carbon weathering a 50 decrease in silicate weathering rate and the possible effects of the rise in abundance of c4 plants in the late miocene to recent we compare the model simulated carbon isotopic responses for some of these experiments to paired carbonate and organic carbon records to illustrate how these records might be interpreted in light of the model response

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events