Multi-geochemical Proxies On The Reconstruction Of Early Triassic Paleoceanographic Environments, Chaohu Area, Anhui, South China

Of the’Big Five’ mass extinctions in marine and terrestrial ecosystem took place during the Phanerozoic Eon, the third one (the Latest Permian mass extinction, LPME) was thought to be the largest disaster with extreme loss of biodiversity and species. Rebuilt of both ecosystems was delayed in the whole Early Triassic and didn’t recover to the pre-crisis level until Middle Triassic. After extensive efforts by lots of researchers in the past few years, it was confirmed that contemporaneous ocean was staying in some kind of abnormal condition. Chaohu was located in relatively deep sedimentary environment belonged to the Lower Yangtze area, forming a suit of fine grained detrital mud rocks and/or limestone at that time. As a candidate of the Induan-Olenekian boundary (IOB) stratotype in the Tethyan region, the Lower Triassic of several sections from Chaohu area were intensively researched. Based on these previous studies in this area, the present work aimed to:(1) investigate in-situ trace element and isotopic compositions of the traditional paleontology material-conodont using some newly developed techniques, (2) show some more light on marine paleo-environment and chemically evolutionary process according to those proxies, providing new evidence on paleo-environemt associated with LPME and the following delayed recovery in the Mesozoic time, and (3) offer more trustworthy geochemical materials in the competition of the Global Stratotype Section and Point (GSSP) for IOB.Conodonts were hard tissues of some extinct marine animals, their rapid evolution, ubiquity and richness in many marine strata made them one of the most used index fossil in the work of division and correlation from Cambrian to Triassic sections. They are composed mainly of carbonate fluorapatite and particularly attractive as potential proxies of paleo-environmental reconstruction due to their biostratigraphic importance and relatively resistant to diagenesis. Their micro-element, oxygen isotope, strontium isotope and neodymium isotope compositions are paid much attention in past decades. However, the small size (～0.1 to 0.5 mm, generally) of conodont elements as the most limited factor for further analysis has impelled previous researchers to digest plenty of individuals using acid. Although the measured results are relatively stable with high precision, the apparent disadvantage is that it is needed a large number of samples because single or few elements are far from the minimum requirement by analytical instrument. Recent advances in microanalytical techniques, such as laser ablation or ion microprobe, can provide the opportunity to analyze specific histologies within single conodont elements, with both high sensitivity and high spatial resolution. This study has taken this approach to choose properly clean sites avoiding those easily contaminated component, and determine the chemical systematics of conodont tissues.Before further work on regaining trace element compositions of ancient seawater, we should identify modern processes responsible for REEs fractionation because of the potential utility of REEs as paleo-environmental and paleoceanographic proxies. Thus, the first part of the present dissertation reviews existing literature on the diagenetic fluxes of REEs in marine sediments and porewaters in order to systematize existing knowledge on this subject. REEs undergo significant redistribution among sediment phases during both early and late diagenesis as a consequence of adsorption and desorption processes. Remobilization of REEs commonly leads to inter-elemental fractionation, variously leading to enrichment or depletion of the light, middle, or heavy REE fractions. REE remobilization can be facilitated by redox changes, e.g., through reductive dissolution of host phases in suboxic and anoxic porewaters. Characteristic REE distribution patterns develop through these processes:(1) a’flat distribution’signifying predominantly terrigenous siliciclastic influence, (2) a’middle-REE bulge’probably due to adsorption of light and heavy REEs to Mn-and Fe-oxyhydroxides, respectively, and (3)’heavy-REE enrichment’ indicative of hydrogenous (seawater) influence. Cerium is redox sensitive elements and its anomalies can be used as proxies of redox status in ambient conditions, and europium is potentially an indicator of terrigenous, eolian and hydrothermal influences.Based on the features of REEs in modern seawater and correlated sediment pore water, this work investigate comprehensively the REE compositions of the Early Triassic conodont and bulk rocks from west Pingdingshan section (WPDS) in Chaohu area. The profiles of ∑REE, Ce/Ce*, Eu/Eu* and ∑REE/Th for conodonts exhibit peak values around IOB, while La/Sm and La/Yb ratios increase to peak values in the earliest Spathian, concurrent with a decline in Sm/Yb ratios indicating a pronounced decline in MREE enrichment at that time. For most proxies, the whole-rock samples yield different patterns of REE variation. Whole-rock ∑REE and Ce/Ce* vary modestly through most of the section before decreasing sharply in the earliest Spathian. Eu/Eu* ratios peak around the IOB and SSB and then also decline in the earliest Spathian. The La/Sm, La/Yb and Sm/Yb profiles show complex patterns that are difficult to generalize, although increasing La/Sm and decreasing Sm/Yb ratios in the earliest Spathian document a decrease in MREE enrichment similar to that of the conodont samples. Whole-rock samples exhibit only limited ∑REE/Th variation. All conodont samples from WPDS exhibit a pronounced’MREE bulge’ pattern, a phenomenon documented by previous studies on conodont REEs. The author considered that oceanographic environment was totally different from modern oceans in Early Triassic, with more developed anoxia, stagnant circulation, weakened primary productivity and bioturbation, which may cause different signatures characterized by MREEs rather than HREEs enrichment and then well preserved in conodonts. Alternatively, those conodont and other authigenic minerals with ’MREEs bulge’ record an altered signatures during diagenesis processes, that may be associated with Fe-/Mn-oxyhydroxides adsorbing LREEs and HREEs, segregating MREEs in porewaters.Diagenesis processes could significantly affect REEs content recorded in conodont, generating different distributions from seawater. It is essential to evaluate the extent of diagenetic influence on the REE compositions of both conodonts and whole-rock samples from the WPDS study section. (1) One method of testing detrital siliciclastic influence is by examining the relationship of ∑REE and Th or Al concentrations. Conodont samples are generally enriched in both ∑REE and Th relative to whole-rock samples and show a strong positive relationship between ∑REE and Th, indicating that REEs probably derived from the detrital siliciclastic fraction. (2) Another approach to evaluating detrital siliciclastic influence is based on Y/Ho ratios, which differ between sediments with detritally sourced REEs (-25-30) versus hydrogenously sourced REEs (60). At WPDS, Y/Ho ratios average 30.4±4.7 for conodont samples and 32.5±2.1 for whole-rock samples, demonstrating the pervasive influence of detrital siliciclastics on measured REE compositions. (3) A crossplot of Y/Ho versus ∑REE/Th provides evidence of two distinct REE components in the conodont samples. The first ("detrital") component has Y/Ho (-28) and ∑REE/Th (～15) similar to that of both the whole-rock samples. The second component, which is equivalent to the "primary" component, has slightly higher Y/Ho (～35), much higher ∑REE/Th (>1000). Most conodont samples plot along a trend between these two endmember compositions, with the detrital component exerting relatively greater influence than the primary component. (4) A crossplot of Ce/Ce* and Pr/Pr* shows negative covaration for whole-rocks but positive covariation for conodont samples, while a crossplot of Eu/Eu* and Gd/Gd* exhibit pronounced negative correlation for both conodont and whole-rock samples. The Ce-Pr and Eu-Gd anomaly patterns are largely the product of diagenetic influences on the REE compositions of both conodonts and whole-rocks, rather than the ambient water column where they formed.Oxygen isotope preserved in conodonts is a potentially useful proxy for surface seawater paleo-temperature. Compared with those published results during every substage from Late Permian to Early Triassic, microanalytically in-situ oxygen isotopic composition of conodonts and associated paleo-temperature in this article show systematic deviation, but similar trends:(1) A persistent warming from Changhsingian to Griesbachian, with a magnitude of more than 15℃; (2) the temperature stayed around 42℃ during Dienerian, that is no cooling event assumed by others; (3) it increased in the whole Smithian and peaked around 45℃ prior to the Smithian-Spathian boundary which was documented as Late Simthian thermal maximum, (4) which was followed by a cooling event during most of the Spathian stage. Comparable in-situ measurements relative to acid solution results indicate the feasibility for microanalytical technique. Moreover, it would be better because the less requirement of conodont samples and relative simpler preparation procedures. No discrepancy of oxygen isotope exists between different conodont crown issues.According to former researches on the strontium isotope compositions of Late Permian and Early Triassic seawater, a persistent increase didn’t stop at a peak around 0.708 until Anisian in Middle Triassic. The 87Sr/86Sr ratios of Late Permian conodonts from Meishan section yielded comparable values relative to that of published carbonates samples. However.the strontium isotope of Early Triassic conodont from Chaohu area is much higher than that of carbonates, which is interpreted as a result of terrigenous input. Both of the study sections are located at the northwest to detrital deposit area,which could bring plenty of siliciclastic materials by continental weathering. Elevated weathering rates and corresponding terrigenous input into oceans resulted in increasing seawater 87Sr/86Sr during Early Triassic and higher 87Sr/86Sr values than that of Late Permian recorded by conodonts. Studies on in-situ 87Sr/86Sr compositions of Meishan’s conodont, which may well preserve that of paleo-seawater, indicate no apparent differences regardless of species and histologies. Extensive substitution between Sr and Ca in conodont elements would be responsible for nearly uniform strontium contents and its isotope composition.Widespread oceanic anoxia has been considered as one of the contributors to LPME and the followed prolonged recovery in Early Triassic. According to plenty of geochemical proxies on paleoceanographic environment at that time, although there is considerable evidence for expanded oceanic anoxia, recent studies have yielded spatially and temporally complex and sometimes inconsistent redox changes over the world. Black shales or argillaceous rocks are chosen to measure the molybdenum isotope and associated trace elements in this work in order to provide some new and credible evidences for contemporaneous oceanic redox condition. Paired δ9895Mo values, U and Mo contents show that (1) δ98/95 Mo increased from Latest Permian to Earliest Triassic with that of pre-PTB and post-PTB are 0.91‰ and 1.28%o, respectively; associated δ13C negative excursion and peak values of Mo content, it indicates that anoxic condition was expanded during that period; however, (2) low δ98/95 Mo values with authigenic Mo enrichment and presence of microcrystal pyrite around IOB, SSB and Late Spathian may be a consequence of terrigenous influence, manganese hydroxides and recurrent redox condition, which caused fractionation of δ98/95 Mo to lower values. The present work indicates that δ98/95 Mo is a potential proxy for paleoceanographic redox changes, it is essential to evaluate the δ98/95 Mo data combined with other proxies and observation in field.Taking advantage of laser ablation and secondary ion microprobe, this work measured in-situ trace element, strontium isotope and oxygen isotope compositions of conodont from Chaohu area and found that REEs and 87Sr/86Sr of them were contaminated by terrigenous input and diagenesis, yet comparable evolutionary profiles between δ18O and associated surface seawater paleo-temperatures and those measurements published by others. The results indicate that each geochemical proxy has its own limits and geological significance and that it is workable for in-situ microanalysis on conodont and other trace fossils. Meanwhile, it is critical for careful sample screen, preparation and interpretation when we use those geochemical proxies for reconstruction of paleoceanographic environment. It should be used with caution before evaluating diagenesis, terrigenous influence and other possible processes. As an exploratory research on in-situ microanalysis for conodont, this work gathered much experience for further work in the future.