Reconstruction of Middle and Late Triassic marine palaeotemperatures using oxygen isotopes of conodont apatite
Final Report Abstract
The goal of this study was to compile an oxygen isotope record measured on Middle and Late Triassic conodonts and to reconstruct the palaeotemperature history for the entire Triassic. Key questions were whether the recovery of the reef ecosystem in the Middle and Late Triassic was related to climate cooling and whether the Carnian humid episode was a global event and related to global warming. The main results can be summarized as follows: Due to the unexpected low conodont abundance in two Italian sections reported to have an abundant fauna, we explored the possibility to measure the oxygen isotope composition of individual conodonts by secondary ion mass spectrometry (SIMS). In cooperation with the newly installed SIMS laboratory at the GeoForschungszentum Potsdam, we first had to test appropriate standard materials (gem-quality apatites). All tested apatites (including Durango apatite - the generally used reference material) showed a relatively large inter- as well as intracrystal heterogeneity in δ18O and do not represent suitable standards for the calibration of SIMS δ18O analyses on biogenic apatite. Our data suggest that the different matrixes of standard materials and biogenic apatite result in a matrix effect influencing the measured oxygen isotope ratios. Thus, SIMS analyses on biogenic apatite will only become reliable once matrix effects can be corrected by using a set of apatite standard materials with a range in the trace element compositions. By studying the Carnian in South China, we showed that the carbon isotope excursion known especially from sections in the Western Tethys is also observed in the Eastern Tethys and that the Carnian Humid Episode represents a global event. Oxygen isotopes measured on conodont apatite indicate a double-pulse warming with initial warming of 4° C observed in the Julian 2 (late Early Carnian) followed by a cooling intermezzo and a second warming pulse of 7° C in the later Carnian (Tuvalian 2). The Carnian dry-wet transition coincides with the negative carbon isotope excursion and the 4° C climate warming. Climate warming is suggested to have been triggered by time-equivalent volcanism (Wrangelia Large Igneous Province) and to result in an intensification of the hydrological cycle, enhanced siliciclastic influx and a shutdown of the carbonate production (especially in the Western Tethys). Carbonate production in the studied deeper water location in South China declined as well at the Julian 1/2 transition with bivalve-holothurian wacksteones being overlain by manganiferous ammonoid wackestone and deep water black shales with intercalated Halobia wackestones. Triassic temperatures reconstructed from δ18O of conodont apatite indicate significant cooling from high temperatures recorded at the Smithian/Spathian transition (Early Triassic) to moderate temperatures in the Anisian (Middle Triassic) with Middle and Late Triassic temperatures being generally in the range of modern tropical temperatures. The currently available dataset indicates moderate warming in the Carnian (with superimposed temperature variations in conjunction with the Carnian Humid Episode), cooling in the early Norian and again warming during the later Norian). When comparing the temperature record with reef evolution, it becomes obvious that first metazoan reefs only occurred after significant climate cooling had occurred in the late Spathian to Early Anisian suggesting a causal relationship between reef re-establishment and climate evolution. Whether the Triassic reef climaxes in the early Carnian and late Norian Ladinian were influenced as well by temperature variations can only be answered when further data become available e.g. for the late Norian and early Rhaetian.
Publications
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(2017): Early Carnian conodont fauna at Yongyue, Zhenfeng area and its implication for Ladinian-Carnian subdivision in Guizhou, South China.- Palaeogeography, Palaeoclimatology, Palaeoecology
Zhang, Z., Sun, Y., Lai, X., Joachimski, M.M., Wignall, P.B.
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(2015): High amplitude redox changes in the late Early Triassic of South China and the Smithian/Spathian extinction.- Palaeogeograph. Palaeoclimat. Palaeoecol., 427,62-78
Sun Y., Wignall, P.B., Joachimski, M.M., Bond, D.P., Grasby, Sun, S.E., Yan, C., Wang, L., Chen, Y., Lai, X.
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(2016): Climate warming, euxinia and carbon isotope perturbations during the Carnian (Triassic) crisis.- Earth Planet. Sci. Lett., 444, 80100
Sun, Y., Wignall, P.B., Joachimski, M.M., Bond, D.P.G., Grasby, S.E., Lai, X.L., Wang, L.N., Zhang, Z.T., Sun, S.
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2016: Chemical and Oxygen Isotope Composition of Gem-quality Apatites: Implications for Oxygen Isotope Reference Materials for Secondary Ion Mass Spectrometry (SIMS).- Chem. Geol. 440, 164-178
Sun, Y., Wiedenbeck, M., Joachimski M.M., Beier, C., Kemner, F., Weinzierl, C.
