Description: Stable isotopes provide a valuable perspective on the timing of elevation change of the Tibetan Plateau. We begin our paper by looking in depth at isotopic patterns in modern Tibet. We show that the {delta}18O value of surface waters decreases systematically up the Himalayan front in central Nepal by about -2.8{per thousand}/km, in agreement with the patterns documented and modeled by previous research. On the Tibetan plateau itself there is no apparent correlation between elevation and the {delta}18O value of flowing surface waters. Both surface waters and soil carbonates display a northward increase in {delta}18O values, of about 1.5{per thousand}/{degrees} north of the Himalayan crest, even though elevation increases modestly. The isotopic increase with latitude reduces the isotope-elevation gradient for water in the northernmost plateau to -1 to -2{per thousand}/km. Carbonates in both soils and lakes form at higher temperatures than assumed by previous studies on the plateau. Temperature estimates from clumped-isotope ({Delta}47) analyses of modern soil carbonates significantly exceed mean annual air T and modeled maximum summer soil temperatures by 15.8{+/-}2.8{degrees} and 9.7{+/-}2.5 {degrees}C, respectively. Similarly elevated temperatures best account for the {delta}18O values observed in modern soil and lake carbonates. We recalculated paleoelevations from previous studies on the plateau using both higher formation temperatures and latitude-corrected isotopic values. With one notable exception, our revised model produces paleoelevation estimates very close to previous estimates. The exception is the reconstruction from late Eocene age deposits at Xoh Xil, for which we calculate elevations that are higher and much closer to the current elevation than previously reconstructed. Therefore, there is no evidence for northward progression through time of Tibetan elevation change. Instead, the available--but admittedly very scanty--evidence suggests that much of Tibet attained its modern elevation by the mid-Eocene. A truly robust test of the various geodynamic models of uplift await expansion and replication of isotopic records all across Tibet, especially in the center and north and for 〉15 Ma.

Description: The {delta}13C values of pedogenic (soil-formed) calcite preserved in the sedimentary record have been used to estimate atmospheric pCO2 using the paleosol calcite paleobarometer. A fundamental assumption for applying this paleobarometer is that atmospheric CO2 concentrations have a direct influence on the measured pedogenic calcite {delta}13C values as a result of open-system exchange between atmospheric and soil-respired CO2. Here we address the timing of calcite precipitation in relation to the soil saturation state and soil-atmosphere connectivity in a modern Vertisol (smectitic, clay-rich soil, seasonally saturated) in Brazoria County, Texas, U.S.A. Luminescent phases of calcite growth, under cathodoluminescence microscopy, have more negative {delta}13C values ({delta}13C = -11.1{per thousand} VPDB {+/-} 0.78 1{sigma}) than the non-luminescent phases ({delta}13C = -2.53{per thousand} VPDB {+/-} 1.41 1{sigma}). The luminescent phase of calcite formed during the water-saturated portion of the year, thereby minimizing the incorporation of atmospheric CO2, and negating its use for pCO2 estimations. The non-luminescent phase formed during the well-drained portion of the year when atmospheric CO2 mixed with soil-respired CO2 and is therefore useful for pCO2 estimation. From these results we present a model to independently test the saturation state of a paleosol at the time of pedogenic carbonate precipitation. Finally, we calculate soil-respired CO2 concentrations that are an order of magnitude lower than those that are typically assumed in the soil-carbonate paleobarometer equation.

Description: The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO 2 ) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO 2 beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO 2 record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO 2 thresholds in biological and cryosphere evolution. Editor’s summary The concentration of atmospheric carbon dioxide is a fundamental driver of climate, but its value is difficult to determine for times older than the roughly 800,000 years for which ice core records are available. The Cenozoic Carbon dioxide Proxy Integration Project (CenCO2PIP) Consortium assessed a comprehensive collection of proxy determinations to define the atmospheric carbon dioxide record for the past 66 million years. This synthesis provides the most complete record yet available and will help to better establish the role of carbon dioxide in climate, biological, and cryosphere evolution. — H. Jesse Smith

Description: Soil pore space CO2, O2, and apparent respiratory quotient (ARQ) data are from experimental soil wetting experiments. These laboratory wetting experiments were designed to test how calcite dissolution and precipitation can obscure the CO2 produced by soil respiration in response to rainfall events. Natural soil samples were collected from the University of Texas Stengl Lost Pines Biological Station (Latitude: 30.096°; Longitude -97.171°, Elevation: 145 m a.s.l.). Collected soil was sieved and filled to 25 cm depth into two separate tubs (49 cm long x 34 cm wide x 29 cm high) in the laboratory. Approximately 20 weight percent calcite was mixed into the treatment soil, and the other soil was left untreated. Soil gas samples were collected every four hours from a perforated horizontal gas well installed at 20 cm depth using an automated manifold system. Soil gas samples were dried and then the O2 and CO2 concentrations were measured using a Sable Systems (Las Vegas, Nevada, USA) Field Metabolic System. Temperature drift in the raw O2 data was corrected for by normalizing ambient air O2 measurements to 20.95%. W ARQ values were calculated as the relative difference in soil gas concentrations between the soil and ambient air corrected by a constant that accounts for faster diffusion of O2 through soil pore spaces (ARQ = -0.76*ΔCO2/ΔO2). Rain events were simulated by adding the equivalent of 2 cm and 1 cm of water at the beginning of the experiment and 9 days later.

Description: Pedogenic carbonate carbon isotope data, occluded organic matter carbon isotope data, and bulk magnetic susceptibility data are from the Jiaxian section of the Chinese Loess Plateau. Samples were collected from the field in July, 2017. Between 1 and 16 discrete pedogenic carbonate nodules were collected from 121 different depths over a 58-meter section. Separate bulk loess samples were collected every 10 cm for magnetic susceptibility analyses. 697 individual nodules were drilled and their carbonate δ13C values were measured at the University of Texas Stable Isotope Laboratory. Between 1 and 16 nodules were analyzed for each respective depth. When enough material was available (668 nodules), splits of the samples were acidified and rinsed before being analyzed for the δ13C values of occluded organic matter at the University of Texas Stable Isotope Laboratory. 17 and 109 replicate analyses are also included for carbonate and occluded organic matter, respectively. Magnetic susceptibility measurements were conducted at Nanjing University. Magnetic susceptibility data were used to correlate to previously published age models for the Jiaxian section that were based on paleomagnetic reversals.

Description: Production-Diffusion modeling results using a Monte Carlo approach to simulate carbon isotope excursions in organic matter and paleosol carbonate during the Paleocene-Eocene Thermal Maximum. These modeling experiments were designed to test the effects soil methane oxidation and increased soil respiration rates can have on the carbon isotope values of pedogenic carbonate. The results from four modeling experiments are included. Three methane oxidation experiments that simulate a large methane release over 100, 1000, and 10000 years, respectively, and one soil respiration experiment.