CO2-forced Late Miocene cooling and ecosystem reorganizations in East Asia.

In parallel with pronounced cooling in the oceans, vast areas of the continents experienced enhanced aridification and restructuring of vegetation and animal communities during the Late Miocene. Debate continues over whether pCO2-induced global cooling was the primary driver of this climate and ecosystem upheaval on land. Here we present an 8 to 5 Ma land surface temperatures (LST) record from East Asia derived from paleosol carbonate clumped isotopes and integrated with climate model simulations. The LST cooled by ~7 °C between 7.5 and 5.7 Ma, followed by rapid warming across the Miocene-Pliocene transition (5.5 to 5 Ma). These changes occurred synchronously with variations in alkenone and Mg/Ca-based sea surface temperatures and with hydroclimate and ecosystem shifts in East Asia, highlighting a global climate forcing mechanism. Our modeling experiments additionally demonstrate that pCO2-forced cooling would have altered moisture transfer and pathways and driven extensive aridification in East Asia. We, thus, conclude that the East Asian hydroclimate and ecosystem shift was primarily controlled by pCO2-forced global cooling between 8 and 5 Ma.

Rapid and precise measurement of carbonate clumped isotopes using laser spectroscopy.

Carbonate clumped isotope abundance is an important paleothermometer, but measurement is difficult, slow, and subject to cardinal mass (m/z) interferences using isotope ratio mass spectrometry (IRMS). Here, we describe an optical spectroscopic measurement of carbonate clumped isotopes. We have adapted a tunable infrared laser differential absorption spectrometer (TILDAS) system to measure the abundances of four CO2 isotopologues used for clumped isotope thermometry. TILDAS achieves the same precision (0.01‰ SE) as IRMS measurements rapidly (∼50 min per carbonate analysis) and using small samples (<2 mg of calcite), without making assumptions about 17O abundance in the sample. A temperature calibration based on 406 analyses of CO2 produced by digestion of 51 synthetic carbonates equilibrated at 6° to 1100°C is consistent with results for natural carbonates and previous calibrations. Our system results were indistinguishable from IRMS systems after replicating the InterCarb interlaboratory calibration. Measurement by TILDAS could change the landscape for clumped isotope analysis.

Rapid and Precise Analysis of Carbon Dioxide Clumped Isotopic Composition by Tunable Infrared Laser Differential Spectroscopy.

The high precision measurement of doubly substituted ("clumped") isotopologues in CO2 is a topic of significant interest in isotope geochemistry. Here we describe the performance of a new isotope ratio laser spectrometer using tunable infrared laser differential absorption spectroscopy (TILDAS). The TILDAS instrument has two continuous-wave lasers to simultaneously measure the four isotopologues involved in the 12C16O2 + 13C16O18O ⇆ 13C16O2 + 12C16O18O exchange reaction. CO2 samples are trapped in a low volume (∼250 mL) optical multipass cell with a path length of 36 m. Each sample is compared to a reference gas, and clumped isotopologue precision of 0.01‰ (SE) is achieved within 20 min for 15 µmol samples. Similar precision is also achieved for bulk isotopic composition. The degree of rare isotope clumping in excess of strictly random distribution (Δ16O13C18O) measured by this TILDAS instrument varies linearly with theoretically calculated values and shows a very weak dependence on bulk isotopic composition.

Choice of 17 O correction affects clumped isotope (Δ47 ) values of CO2 measured with mass spectrometry.

RATIONALE: The clumped isotope composition of CO2 (Δ47 ) derived from carbonate is widely used as a paleotemperature proxy with broad applications in geoscience. Its accuracy is, however, limited by inter-laboratory discrepancies of reference materials and disagreement among carbonate geothermometer calibrations. Here we show how the correction for the abundance of 17 O influences these discrepancies. METHODS: We used CO2 -H2 O equilibration at known temperatures and phosphoric acid digested carbonates to generate CO2 samples with a wide range in 13 C and 18 O compositions. All samples were purified using an offline vacuum line. We used a Thermo MAT 253 isotope ratio mass spectrometer with a Faraday collector array for m/z 44-49, to measure δ13 C, δ18 O, δ47 , and Δ47 values. Data were compiled using a traditional correction ('Santrock') for the 17 O interference in m/z 45 as well as a more recently proposed correction ('Brand') for 17 O interference. Two reference frames using CO2 with distinct 13 C compositions were constructed to simulate an inter-laboratory comparison. RESULTS: The traditional Santrock 17 O correction leads to a simulated Δ47 inter-laboratory comparison offset of 0.06 ‰, and a 0.1 ‰ Δ47 range in CO2 -H2 O 23°C equilibrations that is dependent on the 13 C composition. The more recent Brand 17 O correction removes these discrepancies. The traditional 17 O correction yields distinct temperature-Δ47 calibration curves for synthetic carbonates precipitated using different methods to degas CO2 , while the more recent 17 O correction collapses all calibration data onto a single curve. CONCLUSIONS: The 17 O correction strategy employed by CO2 and carbonate clumped-isotope researchers can have a large effect on the accuracy of Δ47 values. Use of the traditional 17 O correction may have caused errors in published studies as large as 0.1 ‰ and may account for Δ47 differences among laboratories and disagreement among previously published carbonate clumped isotope thermometry calibrations. Copyright © 2016 John Wiley & Sons, Ltd.