This is a repository copy of Calibration of the oxygen and clumped isotope thermometers for (proto-)dolomite based on synthetic and natural carbonates.

9The application of clumped isotopes ( 47 ) in carbonate minerals as a sensitive temperature proxy in 10 paleo-environments depends on a well-constrained clumped isotope fractionation for the necessary 11 step of phosphoric acid digestion of the carbonate mineral to produce CO 2 . Published estimates for 12 clumped isotope fractionations vary, and the effect of different carbonate mineralogies is still under 13 debate. Differences in the sample preparation design and sample digestion temperatures are 14 potential sources for varying acid fractionations and could be a source for discrepant  47 -15 temperature calibrations observed in different laboratories. To evaluate the clumped isotope acid 16 fractionation at 70 °C and simultaneously account for a potential cation effect we analyzed a set of 17 eight carbonate minerals (calcite, aragonite, dolomite and magnesite) that were driven to a 18 stochastic isotope distribution by heating them to temperatures of 1000 °C. Our study reveals 19 significant cation-and mineral-specific differences for the  47 acid fractionation of carbonate 20 minerals digested at 70 °C or 100 °C. The  47 acid fractionation at 70 °C for calcite is 0.197±0.002 ‰, 21 for aragonite 0.172±0.003 ‰, whereas dolomite has a significantly larger acid fractionation of 22 0.226±0.002 ‰. For magnesite digested at 100 °C we observed a  47 acid fractionation of 23 0.218±0.020 ‰. Projected to an acid digestion at 25 °C, our acid fractionation for calcite of 0.260 ‰ 24 is statistically indistinguishable from existing studies. We further show that the  47 of the calcite 25 standards ETH-1 and ETH-2 of 0.265 ‰ and 0.267 ‰, respectively, are in the range of the 26 determined acid fractionation projected to 25 °C suggesting that they have an identical and near 27 stochastic isotope distribution. The observed differences in the  47 acid fractionation between calcite 28 and aragonite ( 47 = -0.025 ‰) and between calcite and dolomite ( 47 = -0.029 ‰) does not 29 correlate with the phosphoric acid fractionation of oxygen isotopes, but rather depends on the radius 30 of the cation as well as on the mineral structure. Our results reveal that the acid fractionation of 31 dolomite at 70 °C is significantly distinct from the one of calcite, but at 90 °C the two are within error 32 © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ with M being the metal cation of the corresponding carbonate mineral (e.g. calcite and aragonite 52 (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), magnesite (MgCO 3 ), siderite (FeCO 3 ), witherite (BaCO 3 )). Having 53 attained equilibrium this exchange reaction (Eq. 1) is thermodynamically controlled and produces an 54 excess in the doubly substituted isotopologue relative to a stochastic isotope distribution (0.5 ‰ at 0 55 °C), which decreases with increasing temperature and approaches a stochastic isotope composition 56 at 1000 °C (Schauble et al., 2006). Ghosh et al. (2006a) demonstrated the feasibility...

Orbitally forced cyclic variations in sedimentary sequences provide evidence for short-term fluctuations of Earth climate and a tool for high-resolution timescale calibration. We here present stratigraphic and geochronological evidence for precession-forcing in Middle Triassic hemipelagic limestones of the Buchenstein Formation (Dolomites, northern Italy). High-resolution stratigraphy of several correlative sections of the Buchenstein Formation documents a coherent cycle pattern. Isotope dilution thermal ionization mass spectrometry zircon U–Pb geochronology of tuffs bracketing the cyclic interval reveals an average cycle duration of 18.5 ± 2.1 kyr, consistent with a shorter climatic precession cycle in the Middle Triassic compared with today. This suggests a predominantly precession-controlled climate in this low-latitude setting of the western Tethys and allows high-precision calibration of the Anisian–Ladinian boundary interval. From integrating cyclostratigraphic and U–Pb geochronological constraints, our best estimate for the age of the Anisian–Ladinian boundary is 241.464 ± 0.064/0.097/0.28 Ma. We also provide precise estimates for lithostratigraphic boundaries, biostratigraphic markers and magnetic reversals within the boundary interval. Stratigraphic intervals with elevated sedimentation rate record a sub-Milankovitch signal that may be equivalent to patterns in adjacent carbonate platforms such as the Latemar platform. The origin of this sub-Milankovitch signal remains unknown but highlights the potential to investigate shorter-term climatic variations in Mesozoic sedimentary sequences. Supplementary material: Isotope dilution thermal ionization mass spectrometry U–Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables as well as the results of the Bayesian age modelling are available at https://doi.org/10.6084/m9.figshare.c.3861817 .

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Middle Triassic magmatism in the Southern Alps (northern Italy) consists of widespread volcanoclastic deposits, basaltic lava flows and intrusive complexes. Despite their importance in understanding the geodynamic evolution of the westernmost Tethys, the timing of magmatic activity and the links between the different igneous products remain poorly understood. We present a comprehensive high-precision zircon U-Pb geochronology dataset for the major intrusive complexes and several volcanic ash layers and integrate this with a high-resolution stratigraphic framework of Middle Triassic volcanosedimentary successions. The main interval of Middle Triassic magmatism lasted at least 5.07 ± 0.06 myr. Magmatic activity started with silicic eruptions between 242.653 ± 0.036 and 238.646 ± 0.037 Ma, followed by a <1 myr eruptive interval of voluminous basaltic lava flows. Coeval mafic to intermediate intrusions dated at 238.190 ± 0.055 to 238.075 ± 0.087 Ma may represent feeder and subvolcanic complexes related to the basalt flows. The youngest products are silicic tuffs from latest Ladinian to early Carnian sequences dated at 237.680 ± 0.047 and 237.579 ± 0.042 Ma. Complemented by zircon trace element data, our high-resolution temporal framework places tight constraints on the link between silicic and mafic igneous products in a complex geodynamic setting. Supplementary material: Isotope dilution thermal ionization mass spectrometry U-Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables, sample coordinates, supplementary geochemical data, cathodoluminescence images of isotope dilution thermal ionization mass spectrometry dated zircons and supplementary field documentation are available at