Lithic fragments,glasses and chondrules from Luna 16 fines

Bulk compositions of igneous and microbreccia lithic fragments, glasses, and chondrules from Luna 16 fines as well as compositions of minerals in basaltic lithic fragments were determined with the electron microprobe. Igneous lithic fragments and glasses are divided into two groups, the anorthositic-noritic-troctolitic (hereafter referred to as ANT) and basaltic groups. Chondrules are always of ANT composition and microbreccia lithic fragments are divided into groups 1 and 2. The conclusions reached may be summarized as follows: (1) Luna 16 fines are more similar in composition to Apollo 11 than to Apollo 12 and 14 materials (e.g. Apollo 11 igneous lithic fragments and glasses fall into similar ANT and basaltic groups; abundant norites in Luna 16 and Apollo 11 are not KREEP as in Apollo 12 and 14; Luna 16 basaltic lithic fragments may represent high-K and low-K suites as is the case for Apollo 11; rare colorless to greenish, FeO-rich and TiO₂-poor glasses were found in both Apollo 11 and Luna 16; Luna 16 spinels are similar to Apollo 11 spinels but unlike those from Apollo 12). (2) No difference was noted in the composition of lithic fragments, glasses and chondrules from Luna 16 core tube layers A and D. (3) Microbreccia lithic fragments of group 1 originated locally by mixing of high proportions of basaltic with small proportions of ANT materials. (4) Glasses are the compositional analogs to the lithic fragments and not to the microbreccias; most glasses were produced directly from igneous rocks. (5) Glasses show partial loss of Na and K due to vaporization in the vitrification process. (6) Luna 16 chondrules have ANT but not basaltic composition. It is suggested that either liquid droplets of ANT composition are more apt to nucleate from the supercooled state; or basaltic droplets have largely been formed in small and ANT droplets in large impact events (in the latter case, probability for homogeneous and inhomogeneous nucleation is larger. (7) No evidence for ferric iron and water-bearing minerals was found. (8) Occurrence of a great variety of igneous rocks in Luna 16 samples (anorthosite, noritic anorthosite, anorthositic norite, olivine norite, troctolite, and basalt) confirm our earlier conclusion that large-scale melting or partial melting to considerable depth and extensive igneous differentiation must have occurred on the moon.     

Thinking About Global Warming

Attitudes toward global warming are influenced by various heuristics, which may distort policy away from what is optimal for the well-being of people. These possible distortions, or biases, include: a focus on harms that we cause, as opposed to those that we can remedy more easily; a feeling that those who cause a problem should fix it; a desire to undo a problem rather than compensate for its presence; parochial concern with one’s own group (nation); and neglect of risks that are not available. Although most of these biases tend to make us attend relatively too much to global warming, other biases, such as wishful thinking, cause us to attend too little. I discuss these possible effects and illustrate some of them with an experiment conducted on the World Wide Web.     

The influence of human activity in the Arctic on climate and climate impacts

Human activities in the Arctic are often mentioned as recipients of climate-change impacts. In this paper we consider the more complicated but more likely possibility that human activities themselves can interact with climate or environmental change in ways that either mitigate or exacerbate the human impacts. Although human activities in the Arctic are generally assumed to be modest, our analysis suggests that those activities may have larger influences on the arctic system than previously thought. Moreover, human influences could increase substantially in the near future. First, we illustrate how past human activities in the Arctic have combined with climatic variations to alter biophysical systems upon which fisheries and livestock depend. Second, we describe how current and future human activities could precipitate or affect the timing of major transitions in the arctic system. Past and future analyses both point to ways in which human activities in the Arctic can substantially influence the trajectory of arctic system change.     

Influence of vegetation change on watershed hydrology: implications for paleoclimatic interpretation of lacustrine δ18O records

Stratigraphic shifts in the oxygen isotopic (¹⁸O) and trace element (Mg and Sr) composition of biogenic carbonate from tropical lake sediment cores are often interpreted as a proxy record of the changing relation between evaporation and precipitation (E/P). Holocene ¹⁸O and Mg and Sr records from Lakes Salpetén and Petén Itzá, Guatemala were apparently affected by drainage basin vegetation changes that influenced watershed hydrology, thereby confounding paleoclimatic interpretations. Oxygen isotope values and trace element concentrations in the two lowland lakes were greatest between ~ 9000 and 6800 ¹⁴C-yr BP, suggesting relatively high E/P, but pollen data indicate moist conditions and extensive forest cover in the early Holocene. The discrepancy between pollen- and geochemically-inferred climate conditions may be reconciled if the high early Holocene ¹⁸O and trace element values were controlled principally by low surface runoff and groundwater flow to the lake, rather than high E/P. Dense forest cover in the early Holocene would have increased evapotranspiration and soil moisture storage, thereby reducing delivery of meteoric water to the lakes. Carbonate ¹⁸O and Mg and Sr decreased between 7200 and 3500 ¹⁴C-yr BP in Lake Salpetén and between 6800 and 5000 ¹⁴C-yr BP in Lake Petén Itzá. This decline coincided with palynologically documented forest loss that may have led to increased surface and groundwater flow to the lakes. In Lake Salpetén, minimum ¹⁸O values (i.e., high lake levels) occurred between 3500 and 1800 ¹⁴C-yr BP. Relatively high lake levels were confirmed by ¹⁴C-dated aquatic gastropods from subaerial soil profiles ~ 1.0–7.5 m above present lake stage. High lake levels were a consequence of lower E/P and/or greater surface runoff and groundwater inflow caused by human-induced deforestation.     

Organic carbon inputs,common ions and degassing: rethinking mixing dissolution in coastal eogenetic carbonate aquifers

Caves deliver freshwater from coastal carbonate landscapes to estuaries but how these caves form and grow remains poorly understood. Models suggest fresh and salt water mixing drives dissolution in eogenetic limestone, but have rarely been validated through sampling of mixing waters. Here we assess controls on carbonate mineral saturation states using new and legacy geochemical data that were collected in vertical profiles through three cenotes and one borehole in the Yucatan Peninsula. Results suggest saturation states are primarily controlled by carbon fluxes rather than mixing. Undersaturation predicted by mixing models that rely on idealized end members is diminished or eliminated when end members are collected from above and below actual mixing zones. Undersaturation due to mixing is limited by CO₂ degassing from fresh water in karst windows, which results in calcite supersaturation. With respect to saline groundwater, controls on capacity for mixing dissolution were more varied. Oxidation of organic carbon increased pCO₂ of saline groundwater in caves (pCO₂ = 10^–2.06 to 10^–0.96 atm) relative to matrix porosity (10^–2.39 atm) and local seawater (10^–3.12 atm). The impact of increased pCO₂ on saturation state, however, depended on the geochemical composition of the saline water and the magnitude of organic carbon oxidation. Carbonate undersaturation due to mixing was limited where gypsum dissolution (Cenote Angelita) or sulfate reduction (Cenote Calica) increased concentrations of common ions (Ca²⁺ or HCO₃^?, respectively). Maximum undersaturation was found to occur in mixtures including saline water that had ion concentrations and ratios similar to seawater, but with moderately elevated pCO₂ (Cenote Eden). Undersaturation, however, was dominated by the initial undersaturation of the saline end member, mixing was irrelevant. Our results add to a growing body of literature that suggests oxidation of organic carbon, and not mixing dissolution, is the dominant control on cave formation and enlargement in coastal eogenetic karst aquifers. Copyright © 2016 John Wiley & Sons, Ltd.     

Cobble cam: grain‐size measurements of sand to boulder from digital photographs and autocorrelation analyses

A new application of the autocorrelation grain size analysis technique for mixed to coarse sediment settings has been investigated. Photographs of sand‐ to boulder‐sized sediment along the Elwha River delta beach were taken from approximately 1·2 m above the ground surface, and detailed grain size measurements were made from 32 of these sites for calibration and validation. Digital photographs were found to provide accurate estimates of the long and intermediate axes of the surface sediment (r² > 0·98), but poor estimates of the short axes (r² = 0·68), suggesting that these short axes were naturally oriented in the vertical dimension. The autocorrelation method was successfully applied resulting in total irreducible error of 14% over a range of mean grain sizes of 1 to 200 mm. Compared with reported edge and object‐detection results, it is noted that the autocorrelation method presented here has lower error and can be applied to a much broader range of mean grain sizes without altering the physical set‐up of the camera (～200‐fold versus ～6‐fold). The approach is considerably less sensitive to lighting conditions than object‐detection methods, although autocorrelation estimates do improve when measures are taken to shade sediments from direct sunlight. The effects of wet and dry conditions are also evaluated and discussed. The technique provides an estimate of grain size sorting from the easily calculated autocorrelation standard error, which is correlated with the graphical standard deviation at an r² of 0·69. The technique is transferable to other sites when calibrated with linear corrections based on photo‐based measurements, as shown by excellent grain‐size analysis results (r² = 0·97, irreducible error = 16%) from samples from the mixed grain size beaches of Kachemak Bay, Alaska. Thus, a method has been developed to measure mean grain size and sorting properties of coarse sediments. Copyright © 2009 John Wiley & Sons, Ltd.     

The marginal Wealden (non-marine Lower Cretaceous) between Wiltshire and Buckinghamshire,England

Non-marine Lower Cretaceous beds of Wealden aspect have long been known from the northern margin of the Wessex–Weald Basin, between Wiltshire and the south Midlands. Termed the Whitchurch Sands Formation, these badly exposed and generally poorly fossiliferous strata appear to represent interdigitating alluvial and brackish-marine units separated by significant sedimentary breaks. Geological Conservation Review sites within the Whitchurch Sands are described and interpreted for their chronostratigraphic, palaeoenvironmental and palaeoclimatic significance.