Isotopic composition of lead and strontium in lavas and coarse-grained blocks from Ascension Island,South Atlantic—an addendum

Natural marokite (CaMn₂O₄) has been studied at high pressures and temperatures using a diamond-anvil press coupled with laser heating in the pressure range 100–250 kbar. A mixture of marokite, CaMnO₃ (perovskite) and MnO (rocksalt) has been observed in all runs in the above pressure range by X-ray diffraction study of the quenched samples. It was interpreted that marokite disproportionates into the mixture CaMnO₃ (perovskite) + MnO (rocksalt) at pressures below 100 kbar. A general comparison of the molar volume for all known compounds having the marokite-related structures (including CaFe₂O₄ and CaTi₂O₄) with those for a mixture of perovskite plus rocksalt structures suggested that the mixture is more stable than the marokite-related structures at high pressures, as confirmed by the present experimental result. The CaFe₂O₄-modification of common nepheline (NaAlSiO₄) is also suggested to be unstable relative to the component oxides of α-NaAlO₂ + SiO₂ (stishovite) at high pressures.     

Characteristics of southern California atmospheric rivers

Theoretical and Applied Climatology - Atmospheric rivers (ARs) are channels of high water vapor flux that transport moisture from low to higher latitudes on synoptic timescales. In areas of...     

Probabilistic projections of transient climate change

This paper describes a Bayesian methodology for prediction of multivariate probability distribution functions (PDFs) for transient regional climate change. The approach is based upon PDFs for the equilibrium response to doubled carbon dioxide, derived from a comprehensive sampling of uncertainties in modelling of surface and atmospheric processes, and constrained by multiannual mean observations of recent climate. These PDFs are sampled and scaled by global mean temperature predicted by a Simple Climate Model (SCM), in order to emulate corresponding transient responses. The sampled projections are then reweighted, based upon the likelihood that they correctly replicate observed historical changes in surface temperature, and combined to provide PDFs for 20 year averages of regional temperature and precipitation changes to the end of the twenty-first century, for the A1B emissions scenario. The PDFs also account for modelling uncertainties associated with aerosol forcing, ocean heat uptake and the terrestrial carbon cycle, sampled using SCM configurations calibrated to the response of perturbed physics ensembles generated using the Hadley Centre climate model HadCM3, and other international climate model simulations. Weighting the projections using observational metrics of recent mean climate is found to be as effective at constraining the future transient response as metrics based on historical trends. The spread in global temperature response due to modelling uncertainty in the carbon cycle feedbacks is determined to be about 65–80 % of the spread arising from uncertainties in modelling atmospheric, oceanic and aerosol processes of the climate system. Early twenty-first century aerosol forcing is found to be extremely unlikely to be less than ?1.7 W m^?2. Our technique provides a rigorous and formal method of combining several lines of evidence used in the previous IPCC expert assessment of the Transient Climate Response. The 10th, 50th and 90th percentiles of our observationally constrained PDF for the Transient Climate Response are 1.6, 2.0 and 2.4 °C respectively, compared with the 10–90 % range of 1.0–3.0 °C assessed by the IPCC.     

Salinity and Temperature Regimes in Eastern Alaskan Beaufort Sea Lagoons in Relation to Source Water Contributions

Shallow estuarine lagoons characterize >70 % of the eastern Alaskan Beaufort Sea coastline and, like temperate and tropical lagoons, support diverse and productive biological communities. These lagoons experience large variations in temperature (?2 to 14 °C) and salinity (0 to >45) throughout the year. Unlike lower latitude coastal systems, transitions between seasons are physically extreme and event-driven. On Arctic coastlines, a brief summer open-water period is followed by a 9-month ice-covered period that concludes with a late-spring sea ice breakup and intense freshwater run-off. From 2011 to 2014, we examined interannual variations in water column physical structure (temperature, salinity, and δ¹⁸O) in five lagoons that differ with respect to their degree of exchange with adjacent marine waters and magnitude of freshwater inputs. Temperature, salinity, and source water composition (calculated using a salinity and δ¹⁸O mixing model) were variable in space and time. During sea ice breakup in June, water column δ¹⁸O and salinity measurements showed that low salinity waters originated from meteoric inputs (50–80 %; which include river inputs and direct precipitation) and sea ice melt (18–51 %). Following breakup, polar marine waters became prevalent within a mixed water column over the summer open-water period within all five lagoons (26–63 %). At the peak of ice-cover extent and thickness in April, marine water sources dominated (75–87 %) and hypersaline conditions developed in some lagoons. Seasonal runoff dynamics and differences in lagoon geomorphology (i.e., connectivity to the Beaufort Sea) are considered key potential drivers of observed salinity and source water variations.     

The formation of peridotitic suite inclusions in diamonds

Olivine, orthopyroxene and garnet grains belonging to the peridotitic suite of mineral inclusions in natural diamonds typically show compositions poorer in Ca and Al and richer in Mg and Cr than the same minerals in peridotite nodules in kimberlite. Other features suggest the crystallisation of diamonds from magmas of kimberlitic affinities, and it is suggested that the genesis of peridotitic suite diamonds is linked with that of a CO₂-bearing magma. It is shown that the generation of kimberlitic magma from common garnet-peridotite (with 5 wt.% clinopyroxene) in the presence of CO₂ may rapidly remove by melting all Ca-rich solid phases (clinopyroxene and/or carbonate). Further melting may form liquids in equilibrium with olivine, orthopyroxene, and garnet with the distinctive compositions of the diamond inclusions. The amount of melting and CO₂ necessary for the loss of clinopyroxene (and/or carbonate) are estimated at approximately 5.0 wt.% and 0.5 wt.% respectively.     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994