Nature: Isotopic constraint on the twentieth-century increase in tropospheric ozone

 

Rice University researchers and collaborators used ice cores, like the one shown here from Antarctica, in combination with atmospheric chemistry models to establish an upper limit for the increase in ozone levels in the lower atmosphere since 1850. (Photo by Jeff Fitlow/Rice University)

Isotopic constraint on the twentieth-century increase in tropospheric ozone

Laurence Y. Yeung, Lee. T. Murray, Patricia Martinerie, Emmanuel Witrant, Huanting Hu, Asmita Banerjee, Anaïs Orsi & Jérôme Chappellaz

Nature 570 (2019) 224-227

Abstract

Tropospheric ozone (O3) is a key component of air pollution and an important anthropogenic greenhouse gas. During the twentieth century, the proliferation of the internal combustion engine, rapid industrialization and land-use change led to a global-scale increase in O3 concentrations; however, the magnitude of this increase is uncertain. Atmospheric chemistry models typically predict an increase in the tropospheric O3 burden of between 25 and 50 per cent since 1900, whereas direct measurements made in the late nineteenth century indicate that surface O3 mixing ratios increased by up to 300 per cent over that time period. However, the accuracy and diagnostic power of these measurements remains controversial. Here we use a record of the clumped-isotope composition of molecular oxygen (18O18O in O2) trapped in polar firn and ice from 1590 to 2016 ad, as well as atmospheric chemistry model simulations, to constrain changes in tropospheric O3 concentrations. We find that during the second half of the twentieth century, the proportion of 18O18O in O2 decreased by 0.03 ± 0.02 parts per thousand (95 per cent confidence interval) below its 1590–1958 ad mean, which implies that tropospheric O3 increased by less than 40 per cent during that time. These results corroborate model predictions of global-scale increases in surface pollution and vegetative stress caused by increasing anthropogenic emissions of O3 precursors. We also estimate that the radiative forcing of tropospheric O3 since 1850 ad is probably less than +0.4 watts per square metre, consistent with results from recent climate modelling studies.

DOI: 10.1038/s41586-019-1277-1

 

 

GRL: Evaluating indices of blocking anticyclones in terms of their linear relations with surface hot extremes

Evaluating Indices of Blocking Anticyclones in Terms of Their Linear Relations With Surface Hot Extremes

RevPalBot: Sedimentary organic matter characterization of the Whitehill shales (Karoo Basin, South Africa): An integrated quantitative approach using FE-EPMA and LA-ICP-MS

Sedimentary organic matter characterization of the Whitehill shales (Karoo Basin, South Africa): An integrated quantitative approach using FE-EPMA and LA-ICP-MS

Gelu Costin, Annette E. Götz, Katrin Ruckwied

Permian black shales of South Africa’s Karoo Basin have been classified as potential unconventional gas resources, the Whitehill Formation of the southern basin parts being the main target for future shale gas exploration and production. Here, we present a novel approach of SOM characterization integrating routine palynofacies analysis and high-resolution BSE imaging, modal mineral analysis, quantitative carbon analysis, EPMA trace element analysis, WDS mapping of carbon particles and carbon peak shift method using field emission microprobe as well as LA-ICP-MS trace element analysis. Black shales of the Whitehill Formation intersected in two deep boreholes of the southern Karoo Basin were studied: (1) massive carbonaceous siltstone (borehole KZF-1, southwestern basin) and (2) laminated shale made of alternating silty and clayey–silty laminae (borehole KWV-1, southeastern basin). Palynofacies indicates an outer shelf setting in the southwestern part (KZF-1) and a stratified basin in the southeastern part (KWV-1). The two shale types reveal striking differences in mineralogy, major and trace element concentrations, and shape and texture of organic particles. The EPMA trace element data of SOM show that the concentration of the elements in the organic particles does not correlate with the chemistry of the black shale when SOM is thermally untransformed. However, in the case of thermally affected and mobilized SOM, the organic acid-rich fluids liberated during the transformation and deformation of SOM can differentially dissolve chemical elements from the surrounding minerals. The LA-ICP-MS data show that the Whitehill shales are enriched in S, Mn, Mo, Ag, Cd, Re, Bi and U relative to the standardmarinemudMAG-1. Shales from the southeastern basin (KWV-1) are strongly depleted in Cu, Zn, As and Ba. The granulometric and mineralogical properties of the shales suggest an outer shelf setting with low-energy sediment fallout and sediment reworking, consistent with the paleoenvironmental interpretation based on palynofacies analysis. Different burial depths are inferred from different organic textures: KZF-1 shales show organic particles which preserve organic textures, randomly distributed throughout the siltstone, whereas KWV-1 shales show deformed, thermally transformed and mobilized organic particles, located in between distinct laminae.WDS quantitative element mapping allows for identifying various carbon concentrations within the organic particles. This variability of carbon at grain scale may indicate different carbon–hydrogen molecule speciations. The observed peak shift of carbon in EPMA on different textural types of SOM allowed a correlation between the peak shift and the degree of SOM maturation, indicating that this method potentially works as a vitrinite reflectance proxy. Ultimately, the evaluation of the diagenetic conditions of organic-rich shales from different parts of the basin based on textural criteria of the detrital material and organic particles combined with the carbon peak shift in different organic particles contributes to the assessment of the hydrocarbon potential.

Costin, G., Götz, A. E. & Ruckwied, K. (2019). Reviw of Palaeobotany and Palynology Sedimentary organic matter characterization of the Whitehill shales ( Karoo Basin , South Africa ): An integrated quantitative approach using. Review of Palaeobotany and Palynology. Elsevier B.V. 268, 29–42. DOI:  /10.1016/j.revpalbo.2019.05.008

 

Chem. Geol.: Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts – Implications for sulfur cycle in subduction zones.

Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts- Implications for sulfur cycle in subduction zones

Proteek Chowdhury, Rajdeep Dasgupta

To add to our understanding of sulfur cycle in subduction zones in general and constrain the effects of slab-released sulfate (SO42−) on the magma genesis and sulfur transport from sub-arc mantle to the arc volcanoes in particular we carried out an experimental study. High pressure-temperature, piston-cylinder experiments were carried out in Au-Pd capsules at 0.5–3 GPa and 1050–1325 °C to investigate (a) the effect of variable sulfur concentration from 0 to ~2 wt%, dissolved as SO42− on the stability field of a primary arc basalt with ~4 wt% H2O and determine (b) the sulfur content at anhydrite saturation (SCAS) of hydrous mafic magmas. Speciation of sulfur in the silicate melt was confirmed to be SO42− by S Kα X-ray peak position using electron microprobe. S-free hydrous clinopyroxene liquidus at 2 GPa is ~25 °C hotter than the hydrous clinopyroxene liquidus with ~0.1 wt% S in the liquidus melt as SO42− and the liquidus depression with further S-enrichment to anhydrite saturation (~2 wt% S) can be fitted by a power function ΔT(°C) = 26.52(±3.48)(Smelt in wt%)0.24(±0.06). Anhydrite-saturated experiments show that SCAS increases with increasing temperature and CaO content of melt and decreases with increasing SiO2 content of the melt. Previous SCAS models based mostly on lower PT experiments and/or on silicic melt compositions can’t capture our new experimental SCAS data. A new SCAS parameterization was developed using previous and our new experimental data. Calculations using our new parameterization and assuming 200–500 ppm S in the arc mantle show that <10% hydrous melting of mantle wedge would exhaust anhydrite, if present. Therefore, anhydrite even if present is expected to be exhausted by partial melting in the mantle wedge and parental basaltic melt will extract similar amount of S via mantle melting irrespective of the presence of sulfide or sulfate at subsolidus conditions. The S content, as dissolved SO42−, of hydrous arc basalts produced by 10–30% melting will be 500–4000 ppm, which is comparable to the melt inclusion S contents from various arcs. The sulfate undersaturated basalts may assimilate crustal sulfate and lead to high observed SO2 flux at the arcs, known as the “excess S”.

 

Chowdhury, P. and Dasgupta, R., 2019. Effect of sulfate on the basaltic liquidus and Sulfur Concentration at Anhydrite Saturation (SCAS) of hydrous basalts–Implications for sulfur cycle in subduction zones. Chemical Geologyhttps://doi.org/10.1016/j.chemgeo.2019.05.020

PNAS: Climate models can correctly simulate the continuum of global-average temperature variability

Feng Zhu, Julien Emile-Geay, Nicholas P. McKay, Gregory J. Hakim, Deborah Khider, Toby R. Ault, Eric J. Steig, Sylvia Dee, and James W. Kirchner
Proc. Natl. Acad. Sci. USA 116 (2019) 8728-8733.

DOI: 10.1073/pnas.1809959116

Significance

Climate models are foundational to formulations of climate policy and must successfully reproduce key features of the climate system. The temporal spectrum of observed global surface temperature is one such critical benchmark. This spectrum is known to obey scaling laws connecting astronomical forcings, from orbital to annual scales. We provide evidence that the current hierarchy of climate models is capable of reproducing the increase in variance in global-mean temperature at low frequencies. We suggest that successful climate predictions at decadal-to-centennial horizons hinge critically on the accuracy of initial and boundary conditions, particularly for the deep ocean state.

Abstract

Climate records exhibit scaling behavior with large exponents, resulting in larger fluctuations at longer timescales. It is unclear whether climate models are capable of simulating these fluctuations, which draws into question their ability to simulate such variability in the coming decades and centuries. Using the latest simulations and data syntheses, we find agreement for spectra derived from observations and models on timescales ranging from interannual to multimillennial. Our results confirm the existence of a scaling break between orbital and annual peaks, occurring around millennial periodicities. That both simple and comprehensive ocean–atmosphere models can reproduce these features suggests that long-range persistence is a consequence of the oceanic integration of both gradual and abrupt climate forcings. This result implies that Holocene low-frequency variability is partly a consequence of the climate system’s integrated memory of orbital forcing. We conclude that climate models appear to contain the essential physics to correctly simulate the spectral continuum of global-mean temperature; however, regional discrepancies remain unresolved. A critical element of successfully simulating suborbital climate variability involves, we hypothesize, initial conditions of the deep ocean state that are consistent with observations of the recent past.

QSR: Evidence of Ice Age humans in eastern Beringia suggests early migration to North America

Richard S. Vachula, Yongsong Huang, William M. Longo, Sylvia G. Dee, William C. Daniels, and James M. Russell

Quant. Sci. Rev. 205 (2019) 35-44.

DOI: 10.1016/j.quascirev.2018.12.003

 

Abstract

Our understanding of the timing and pathway of human arrival to the Americas remains an important and polarizing topic of debate in archaeology and anthropology. Traditional consensus, supported by archaeological and paleoenvironmental data, favors a ‘swift peopling’ of the Americas from Asia via the Bering Land Bridge during the last Glacial termination. More recent genetic data and archaeological finds have challenged this view, proposing the ‘Beringian standstill hypothesis’ (BSH), wherein a population of proto-Americans migrated from Asia during, or even prior to the Last Glacial Maximum (LGM) and lived in Beringia for thousands of years before their eventual spread across the American continents. Using a sediment archive from Lake E5 (68.641667° N, 149.457706° W), located on Alaska’s North Slope, we present new data supporting the BSH and shedding new light on the environmental impact of these early American populations. Fecal biomarkers support human presence in the environs of the lake, and our data demonstrate elevated biomass burning in this region during the last Glacial. Elevated burning defies the expectation that natural fires would be less frequent in the Arctic during the last Glacial, thereby suggesting human ignition as the likely culprit. Our data shed new light on the pathway and timing of human migration to the Americas and demonstrate the possibility of the sustainable coexistence of humans and the Ice Age megafauna in Beringia prior to their extinction.

JAS: Quantifying the Annular Mode Dynamics in an Idealized Atmosphere

Pedram Hassanzadeh and Zhiming Kuang

J. Atmos. Sci. 76 (2019) 1107-1124.

DOI: 10.1175/JAS-D-18-0268.1

Abstract

The linear response function (LRF) of an idealized GCM, the dry dynamical core with Held–Suarez physics, is used to accurately compute how eddy momentum and heat fluxes change in response to the zonal wind and temperature anomalies of the annular mode at the quasi-steady limit. Using these results and knowing the parameterizations of surface friction and thermal radiation in Held–Suarez physics, the contribution of each physical process (meridional and vertical eddy fluxes, surface friction, thermal radiation, and meridional advection) to the annular mode dynamics is quantified. Examining the quasigeostrophic potential vorticity balance, it is shown that the eddy feedback is positive and increases the persistence of the annular mode by a factor of more than 2. Furthermore, how eddy fluxes change in response to only the barotropic component of the annular mode, that is, vertically averaged zonal wind (and no temperature) anomaly, is also calculated similarly. The response of eddy fluxes to the barotropic-only component of the annular mode is found to be drastically different from the response to the full (i.e., barotropic + baroclinic) annular mode anomaly. In the former, the eddy generation is significantly suppressed, leading to a negative eddy feedback that decreases the persistence of the annular mode by nearly a factor of 3. These results suggest that the baroclinic component of the annular mode anomaly, that is, the increased low-level baroclinicity, is essential for the persistence of the annular mode, consistent with the baroclinic mechanism but not the barotropic mechanism proposed in the previous studies.

G-Cubed: Bubble Coalescence and Percolation Threshold in Expanding Rhyolitic Magma

Thomas Giachetti, Helge M. Gonnermann, James E. Gardner, Alain Burgisser, Sahand Hajimirza, Tobias C. Earley, Nathan Truong, and Pamela Toledo

Geochem. Geophys. Geosyst. 20 (2019) 1054-1074.

DOI: 10.1029/2018GC008006

Abstract

Coalescence during bubble nucleation and growth in crystal‐free rhyolitic melt was experimentally investigated, and the percolation threshold, defined as the porosity at which the vesicular melt first becomes permeable, was estimated. Experiments with bubble number densities between 1014 and 1015 m−3 were compared to four suites of rhyolitic Plinian pumices, which have approximately equal bubble number densities. At the same total porosity, Plinian samples have a higher percentage of coalesced bubbles compared to their experimental counterparts. Percolation modeling of the experimental samples indicates that all of them are impermeable and have percolation thresholds of approximately 80–90%, irrespective of their porosity. Percolation modeling of the Plinian pumices, all of which have been shown to be permeable, gives a percolation threshold of approximately 60%. The experimental samples fall on a distinct trend in terms of connected versus total porosity relative to the Plinian samples, which also have a greater melt‐bubble structural complexity. The same holds true for experimental samples of lower bubble number densities. We interpret the comparatively higher coalescence within the Plinian samples to be a consequence of shear deformation of the erupting magma, together with an inherently greater structural complexity resulting from a more complex nucleation process.

JGR-Solid Earth: Predicting Homogeneous Bubble Nucleation in Rhyolite

Sahand Hajimirza, Helge M. Gonnermann, James E. Gardner, and Thomas Giachetti
J. Geophys. Res.-Solid Earth 124 (2019) 2395-2416.
Abstract
Bubble nucleation is the critical first step during magma degassing. The resultant number density of bubbles provides a record of nucleation kinetics and underlying eruptive conditions. The rate of bubble nucleation is strongly dependent on the surface free energy associated with nucleus formation, making the use of bubble number density for the interpretation of eruptive conditions contingent upon a sound understanding of surface tension. Based on a suite of nucleation experiments with up to >1016 bubbles per unit volume of melt, and using numerical simulations of bubble nucleation and growth during each experiment, we provide a new formulation for surface tension during homogeneous nucleation of H2O bubbles in rhyolitic melt. It is based on the Tolman correction with a Tolman length of δ = 0.32 nm, which implies an increase in surface tension of bubbles with decreasing nucleus size. Our model results indicate that experiments encompass two distinct nucleation regimes, distinguishable by the ratio of the characteristic diffusion time of water, τdiff, to the decompression time, td. Experiments with >1013 m−3 bubbles are characterized by τdiff/td≪ 1, wherein the nucleation rate predominantly depends on the interplay between decompression and diffusion rates. Nucleation occurs over a short time interval with nucleation rate peaks at high values. For experiments with comparatively low bubble number density the average distance between adjacent bubbles and the diffusion timescale is large. Consequently, τdiff/td≫ 1 and nucleation is nearly unaffected by diffusion and independent of decompression rate, with bubbles nucleating at an approximately constant rate until the sample is quenched.

Nature Geoscience: Seismic velocity reduction and accelerated recovery due to earthquakes on the Longmenshan fault

Shunping Pei, Fenglin Niu, Yehuda Ben-Zion, Quan Sun, Yanbin Liu, Xiaotian Xue, Jinrong Su, and Zhigang Shao

Nature Geosci. 12 (2019) 387-392.

DOI: 10.1038/s41561-019-0347-1

Abstract

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength.