Integrated Geochemistry Of Brachiopods And Conodonts

The geochemistry of marine carbonate rocks is a common method for the reconstruction of paleoclimate and paleoenvironment.The isotopic and elemental geochemical information has important significance to understand the secular trend in the paleoceanic oceans,the oceanic hypoxia events and their resulting biological crisis mechanisms,etc.However,the original seawater information of carbonate rocks is usually subject to diagenetic effects.On the contrary,some biological steleton or bone fossils,due to their composition and/or special structure,nature,etc.,have a certain resistance to diagenetic alteration and can reflect true or near true primitive ancient seawater information(paleotemperature,sea level change,redox conditions,etc.),which become an important medium for the reconstruction of ancient climate and environment.Among them,brachiopods and conodonts are one of the most commonly used materials for the study of the Paleozoic and Triassic ancient marine environments and climates.The former is the most common and abundant animal in the Paleozoic ocean,and is widely distributed in the marine strata of all lithofacies,which is one of the standard fossils of the important biostratigraphy of the marine clastic strata.The latter is also standard fossil group of the most important biological chronology strata,with extensively distribution in the Cambrian and Triassic carbonate deposits.Brachiopods are marine benthic invertebrates,divided into two categories-articulate class and inarticulate class.The former is marked by firm intertwist structure and two sides called as dorsal and ventral shell respectively,with the component of calcium carbonate and similar geochemical characteristics to carbonate rocks.Inarticulate brachiopods shells are mainly composed by chitin or a small amount of calcium.The calcareous shells of brachiopods are usually divided into inner,middle and outer layers.Among them,the inner and outer layers are susceptible to be influenced by the diagenetic effect as the granular structure,while the middle layer with fibrous structure is not easily accountable by diagenesis,thus,which is a good material for isotope geochemical analysis.The oxygen isotope content can be used to reconstructe paleoceanic temperature,salinity and continental glacier volume.The strontium isotope value are often regarded as an indicator of global tectonic activity,sea level changes or terrestrial weathering and other related crust-mantle material flow balance.Conodonts bones consist of calcium phosphate,belonging to the biogenic apatite.It is a special fossil,only part of the organ of the marine animals,with blurred paleontological classification.The conodonts have wide distribution,from Cambrian to the Triassic marine strata all over the world,with large quantities,small sizes,resistance to damage,many species that have short time distribution and wide geographical spread,which is the most important elements of biological zones division.Since the sulfate radicals in the phosphate are usually conserved and are not easily destroyed by diagenesis,the amount of trace and rare earth elements is most likely to reflect the geochemical information of the original seawater at that time.Therefore,the conodonts are highly appreciated to indicate the change of the input of marine matierals and the change of chemical composition of seawater,especially using the rare earth element proxies(e.g.Ce/Ce*,Eu/Eu*)to suggest the deposition source,marine redox condition or volcanic effect,and using oxygen isotope to reconstructe ancient seawater temperature.However,not every fossils or every parts of fossils are well preserved.For example,only the oxygen isotope value of innermost mature shell of brachiopod can restore the original composition of the seawater,that is,can be used to reconstruct the ancient sea water temperature.However,the oxygen isotope component of immature shell and outer layer,or internal-external transition layer is light.For brachiopods,the common methods for discriminating diagenetic alteration are scanning electron microscopy(SEM),cathodoluminescence(CL),trace element method,but each method has its limit.Of these,the SEM discriminant method of brachiopods is only a preliminary evaluation of the surface features and the crystal rupture pattern.Normally only a part of the representative sample can be discriminated by SEM technique as the heterogeneity of shell materials.The cathodoluminescence discriminant method mainly depends on the relative content of Fe2+ luminescent quencher and Mn2+ luminescent activator.The nonluminescence shells may be affected by the absorption of element Fe during diagenetic process,while and well-preserved shells may also show bright luminescent.The trace elements discriminant method is based on the compositon of modern seawater,but the difference between modern seawater and paleo-seawater is not yet known,which may be affected by fossil type,diagenetic background,fossil species and so on.The rare earth elements(REE)signal of the conodonts may also be affected by terrestrial debris or volcanic materials,hydrothermal underwater.The discriminant method of diagenetic alteration for conodonts is mainly Color Alteration Index(CAI)and various REE proxies,containing total rare earth content ?REE,Th content,middle rare earth enrichment degree MREE/MREE*,etc.,and the covariance of each index and Y/Ho.The CAI discrimination of the conodonts may be affected by species,and the chromatic aberration in the course of the contrast may vary from person to person.Therefore,here,based on the methods mentioned above,we assess from various aspects whether it's suitable for geochemical analysis to use these two types of biological fossil shells or bones,with three case studies-Late Devonian,Early Permian and Permian-Triassic boundary,indicating the change of paleoevironment and paleoclimate during three key periods via the geochemical analysis of biological fossils.The conclusions are as follows: The fibrous layer of most Late Devonian and Early Permian brachiopods shells arrange in a regular,with clear crystal shape and smooth surface,while under the cathode light,most shells show nonluminance or slight-luminance,indicating less diagenetic alteration.The contemporary coral fossils are strongly lumincance,suggesting that corals may be affected by diagenetic alteration.The criterion for Late Devonian brachiopods shells is Mn/Ca<0.5 mmol/mol and Sr/Ca>1.2 mmol/mol as well-preserved;while for Early Permian brachiopods shells,is Mn/Ca<0.2 mmol/mol(horizon below 12 m),Sr/Ca>1.4 mmol/mol(horizon above 12 m)as well-preserved.In comparison,the Early brachiopods shells are better than Late Devonian ones.The oxygen isotope of the Late Devonian brachiopods shells has a decreasing trend at the FF boundary,and then repeatedly fluctuates significantly.The fluctuation range of the whole section is-9.66 ‰ to-5.23 ‰ with an average temperature of 46±7 °C.After adjusting the value of ?18Ow,the average temperature is reduced to 35±6 ?.As the region locating in the middle-high latitudes,has unpossible higher marine temperature similar to low latitude region,which probably because that the profile is in the vicinity of the active island arc,influenced by the frequent volcanic eruption.Volcanic activities brought nutrients and the warm seawater conditions,making the benthic animals prosperous in this region when their families are withered in the outside region due to F-F extinction and hypoxia events.The trend of strontium isotope value,suggest a transgression at the Frasnian-Famennian boundary,followed by a regression in the early stage of the Famennian,then finally remain steadily variation.The various proxies of Early Permian brachiopods shells are all have larger volatility during the glacier rapidly ablation(ie,below horizon 12 m),and then remain relatively stable.The fluctuation range of oxygen isotope in the whole section is-0.13 ‰-1.11 ‰(7.1-12.0 ?)with the average of 0.34 ‰±0.32 ‰(10.1±1.3 ?).The signs of prosperity in Early Permian glacial-interglacial transition coincides with the elevated sea water temperature at the end of the large ice age.The significantly increased biological diversity in the transition of glacier rapid ablation-stable period,is corresponding to the smooth change of sea water temperature after the fast water mixture process in the glacial fast ablation period.The increasing-decreasing of 87Sr/86 Sr is possibly beacause that the relative humid climate condition during the early stage of glacier erosion lead to the elevated input of fluvial high radiogenic Sr.Accompanied by the degration of glacier,continental weathering rate decline.At the same time,the sea-level rise and mantle low-Sr composition incorporate into seawater,leading to the final decreasing marine 87Sr/86 Sr value.The analyzed Permian-Triassic conodonts exhibit REE patterns consistent with mixed hydrogenous(seawater)and lithogenous(siliciclastic)sources,with specimens from below the LPME horizon showing dominantly hydrogenous influences(Y/Ho >50)and those from above the LPME horizon showing dominantly lithogenous influences(Y/Ho <50).Locally elevated Ce/Ce* ratios and U concentrations in pre-LPME conodonts suggest an episodic shoaling of the oceanic chemocline,causing incursions of suboxic to anoxic seawaters onto the shallow Yangou carbonate platform.Chemical index of alteration(CIA)values increase sharply at the LPME,suggesting increased rates of continental erosion and a relative increase in chemical over physical weathering,possibly as a consequence of globally elevated temperatures.To sum up,as the unique geographical location(isolated shallow carbonate platform),Yangou section(Jiangxi Province,South China)is less influenced by terrigenous signature,where the conodonts from nearly clay free(Al <0.5 %)carbonate facies can potentially preserve original seawater REE signatures.