Sedimentology of the Middle Marker (3.4 Ga), Onverwacht Group,Transvaal, South Africa

The Middle Marker is a thin (3–6 m) sedimentary unit at the base of the Hooggenoeg Formation in the 3.4 Ga old Onverwacht Group, Barberton Mountain Land, South Africa. The original sediments consisted largely of current-deposited volcaniclastic detritus now represented by green to buff-colored silicified volcaniclastic rock and fine-grained gray chert. Black chert, possibly formed by the silicification of a non-volcaniclastic precursor, makes up a significant part of the unit. The Middle Marker is underlain and overlain by mafic and commonly pillowed volcanic flowrock. Although the original sediment has been replaced by and/or recrystallized to a microquartz, chlorite, sericite, carbonate and iron oxide mosaic under lower greenschist-grade metamorphism, sedimentary textures and structures are remarkably well preserved. Textural pseudomorphs indicate the primary volcaniclastic sediment consisted of a mixture of crystal, vitric and lithic debris. Middle Marker sediments were deposited as a prograding, cone-flanking volcaniclastic sedimentary platform in a relatively-shallow and locally current/wave-influenced subaqueous sedimentary environment. Available paleocurrent data indicate a largely bimodal, orthogonal distribution pattern which is quite similar to both ancient and modern shallow marine/shelf systems. Diagnostic evidence for tidal activity is lacking. As felsic volcanic activity waned, an extensive airfall blanket of fine-grained volcanic ash and dust was deposited in a low-energy subaqueous environment. The sedimentary cycle was terminated with a renewal of submarine mafic volcanism. Middle Marker volcaniclastic sediments accumulated in an anorogenic basin removed or isolated from the influence of continental igneous and metamorphic terranes. Although compositionally dominated by a volcanic source, Middle Marker sediments owe their final texture and sedimentary structures to subaqueous sedimentary rather than volcanogenic processes.     

Development of a stochastic weather generator for the sub-polar North Atlantic

The article presents an approach for creating a computationally efficient stochastic weather generator. In this work the method is tested by the stochastic simulation of sea level pressure over the sub-polar North Atlantic. The weather generator includes a hidden Markov model, which propagates regional circulation patterns identified by a self organising map analysis, conditioned on the state of large-scale interannual weather regimes. The remaining residual effects are propagated by a regression model with added noise components. The regression step is performed by one of two methods, a linear model or artificial neural networks and the performance of these two methods is assessed and compared. The resulting simulations express the range of the major regional patterns of atmospheric variability and typical time scales. The long term aims of this work are to provide ensembles of atmospheric data for applied regional studies and to develop tools applicable in down-scaling large-scale ocean and atmospheric simulations.     

Evaluation of nearshore wave models in steep reef environments

To provide coastal engineers and scientists with a quantitative evaluation of nearshore numerical wave models in reef environments, we review and compare three commonly used models with detailed laboratory observations. These models are the following: (1) SWASH (Simulating WAves till SHore) (Zijlema et al. 2011), a phase-resolving nonlinear shallow-water wave model with added nonhydrostatic terms; (2) SWAN (Simulating WAve Nearshore) (Booij et al. 1999), a phase-averaged spectral wave model; and (3) XBeach (Roelvink et al. 2009), a coupled phase-averaged spectral wave model (applied to modeling sea-swell waves) and a nonlinear shallow-water model (applied to modeling infragravity waves). A quantitative assessment was made of each model’s ability to predict sea-swell (SS) wave height, infragravity (IG) wave height, wave spectra, and wave setup ( \( \overline{\eta} \) ) at five locations across the laboratory fringing reef profile of Demirbilek et al. (2007). Simulations were performed with the “recommended” empirical coefficients as documented for each model, and then the key wave-breaking parameter for each model (α in SWASH and γ in both SWAN and XBeach) was optimized to most accurately reproduce the observations. SWASH, SWAN, and XBeach were found to be capable of predicting SS wave height variations across the steep fringing reef profile with reasonable accuracy using the default coefficients. Nevertheless, tuning of the key wave-breaking parameter improved the accuracy of each model’s predictions. SWASH and XBeach were also able to predict IG wave height and spectral transformation. Although SWAN was capable of modeling the SS wave height, in its current form, it was not capable of modeling the spectral transformation into lower frequencies, as evident in the underprediction of the low-frequency waves.     

Is it possible to limit global warming to no more than 1.5°C?

This study explores the feasibility of limiting increases in global temperature to 1.5°C above pre-industrial levels. A probabilistic simple climate model is used to identify emissions paths that offer at least a 50% chance of achieving this goal. We conclude that it is more likely than not that warming would exceed 1.5°C, at least temporarily, under plausible mitigation scenarios. We have identified three criteria of emissions paths that could meet the 1.5°C goal with a temporary overshoot of no more than 50 years: early and strong reductions in emissions, with global emissions peaking in 2015 and falling to at most 44–48 GtCO₂e in 2020; rapid reductions in annual global emissions after 2020 (of at least 3–4% per year); very low annual global emissions by 2100 (less than 2–4 GtCO₂e) and falling to zero (or below) in the 22nd century. The feasibility of these characteristics is uncertain. We conclude that the proposed date of review of the 1.5°C goal, set at 2015, may be too late to achieve the necessary scaling up of emissions cuts to achieve this goal.     

A New Interlaboratory Characterisation of Silicon,Rare Earth Elements and Twenty‐Two Other Trace Element Concentrations in the Natural River Water Certified Reference Material SLRS‐6 (NRC‐CNRC)

The natural river water reference material SLRS‐6 (NRC‐CNRC) is the newest batch of a quality control material routinely used in many international environmental laboratories. This work presents a nine‐laboratory compilation of measurements of major and trace element concentrations and their related uncertainties, unavailable in the NRC‐CNRC certificate (B, Cs, Li, Ga, Ge, Hf, Nb, P, Rb, Rh, Re, S, Sc, Se, Si, Sn, Th, Ti, Tl, W, Y, Y, Zr and REEs). Measurements were mostly made using inductively coupled plasma‐mass spectrometry. The results are compared with equivalent data for the last batch of the material, SLRS‐5, measured simultaneously with SLRS‐6 in this study. In general, very low concentrations, close to the quantification limits, were found in the new batch. The Sr isotopic ratio is also reported.     

Earliest human remains in Eurasia: New 40Ar/39Ar dating of the Dmanisi hominid-bearing levels,Georgia

Several hominid remains have been discovered in the open-air site of Dmanisi (Georgia), the oldest prehistoric site in Eurasia. Two major arguments prove that this site is close in age to the Plio-Pleistocene boundary: a Villafranchian fauna and the morphological characteristics of hominid remains recently ascribed to Homo georgicus. Direct dating of the lower hominid-bearing level was carried out on volcanic glass and minerals using the ⁴⁰Ar/³⁹Ar method. The concordant results from two different sampled locations allow the determination of the age of the earliest human presence in Eurasia. This radioisotopic result strengthens the argument that the first dispersal of hominids outside Africa occurred at least 1.8 Ma ago.     

The effect of clouds on aerosol growth in the rural atmosphere

Measurements of accumulation mode aerosol in the atmospheric boundary layer under cloudy and cloud-free conditions, and in the lower free troposphere under cloud-free conditions, were conducted over the rural northwest of England. Normalised size distributions in the cloud-free boundary layer (CFBL) and the cloud-free free troposphere (CFFT) exhibited almost identical spectral similarities with both size distributions possessing a concentration peak mode-radius of ≈0.05 μm or less. By comparison, aerosol distributions observed in cloudy air exhibited a distinctive log-normal distribution with mode-radii occurring at ≈0.1 μm concomitant with a local minimum at ≈0.05 μm. The consistent and noticeable difference in spectral features observed between cloudy and cloud-free conditions suggest that a greater amount of gas-to-particle conversion occurs on cloudy days, presumably through in-cloud aqueous phase oxidation processes, leading to larger sized accumulation mode particles. Apart from the distinct difference between cloudy and cloud-free aerosol spectra on cloudy days, aerosol concentration and mass were observed to be significantly enhanced above that of the ambient background in the vicinity of clouds. Volatility analysis during one case of cloud processing indicated an increase in the relative contribution of aerosol mass volatile at temperatures characteristic of sulphuric acid, along with a smaller fraction of more volatile material (possibly nitric acid and/or organic aerosol). Growth-law analysis of possible growth mechanisms point to aqueous phase oxidation of aerosol precursors in cloud droplets as being the only feasible mechanism capable of producing the observed growth. The effect of cloud processing is to alter the cloud condensation nuclei (CCN) supersaturation spectrum in a manner which increases the availability of CCN at lower cloud supersaturations.