The formation and alteration of the Renazzo‐like carbonaceous chondrites III: Toward understanding the genesis of ferromagnesian chondrules

To better understand the formation conditions of ferromagnesian chondrules from the Renazzo‐like carbonaceous (CR) chondrites, a systematic study of 210 chondrules from 15 CR chondrites was conducted. The texture and composition of silicate and opaque minerals from each observed FeO‐rich (type II) chondrule, and a representative number of FeO‐poor (type I) chondrules, were studied to build a substantial and self‐consistent data set. The average abundances and standard deviations of Cr₂O₃ in FeO‐rich olivine phenocrysts are consistent with previous work that the CR chondrites are among the least thermally altered samples from the early solar system. Type II chondrules from the CR chondrites formed under highly variable conditions (e.g., precursor composition, redox conditions, cooling rate), with each chondrule recording a distinct igneous history. The opaque minerals within type II chondrules are consistent with formation during chondrule melting and cooling, starting as S‐ and Ni‐rich liquids at 988–1350 °C, then cooling to form monosulfide solid solution (mss) that crystallized around olivine/pyroxene phenocrysts. During cooling, Fe,Ni‐metal crystallized from the S‐ and Ni‐rich liquid, and upon further cooling mss decomposed into pentlandite and pyrrhotite, with pentlandite exsolving from mss at 400–600 °C. The composition, texture, and inferred formation temperature of pentlandite within chondrules studied here is inconsistent with formation via aqueous alteration. However, some opaque minerals (Fe,Ni‐metal versus magnetite and panethite) present in type II chondrules are a proxy for the degree of whole‐rock aqueous alteration. The texture and composition of sulfide‐bearing opaque minerals in Graves Nunataks 06100 and Grosvenor Mountains 03116 suggest that they are the most thermally altered CR chondrites.     

Simulations of precipitation using NRCM and comparisons with satellite observations and CAM: annual cycle

The accurate representation of rainfall in models of global climate has been a challenging task for climate modelers owing to its small space and time scales. Quantifying this variability is important for comparing simulations of atmospheric behavior with real time observations. In this regard, this paper compares both the statistical and dynamically forced aspects of precipitation variability simulated by the high-resolution (36?km) Nested Regional Climate Model (NRCM), with satellite observations from the Tropical Rainfall Measuring Mission (TRMM) 3B42 dataset and simulations from the Community Atmosphere Model (CAM) at T85 spatial resolution. Six years of rainfall rate data (2000?C2005) from within the Tropics (30°S?C30°N) have been used in the analysis and results are presented in terms of long-term mean rain rates, amplitude and phase of the annual cycle and seasonal mean maps of precipitation. Our primary focus is on characterizing the annual cycle of rainfall over four land regions of the Tropics namely, the Indian Monsoon, the Amazon, Tropical Africa and the North American monsoon. The lower tropospheric circulation patterns are analyzed in both the observations and the models to identify possible causes for biases in the simulated precipitation. The 6-year mean precipitation simulated by both models show substantial biases throughout the global Tropics with NRCM/CAM systematically underestimating/overestimating rainfall almost everywhere. The seasonal march of rainfall across the equator, following the motion of the sun, is clearly seen in the harmonic vector maps. The timing of peak rainfall (phase) produced by NRCM is in closer agreement with the observations compared to CAM. However like the long-time mean, the magnitude of seasonal mean rainfall is greatly underestimated by NRCM throughout the Tropical land mass. Some of these regional biases can be attributed to erroneous circulation and moisture surpluses/deficits in the lower troposphere in both models. Overall, the results seem to indicate that employing a higher spatial resolution (36?km) does not significantly improve simulation of precipitation. We speculate that a combination of several physics parameterizations and lack of model tuning gives rise to the observed differences between NRCM and the observations.     

Evaluation of sewage source and fate on southeast Florida coastal reefs

Water, sponge and coral samples were collected from stations impacted by a variety of pollution sources and screened for human enteric viruses as conservative markers for human sewage. While human enteroviruses and adenoviruses were not detected, noroviruses (NoV; human genogroups I and II) were detected in 31% of samples (especially in sponge tissue). Stations near inlets were the only ones to show multiple sample types positive for NoV. Fecal indicator bacteria and enteric viruses were further evaluated at multiple inlet stations on an outgoing tide. Greatest indicator concentrations and highest prevalence of viruses were found at the mouth of the inlet and offshore in the inlet plume. Results suggest that inlets moving large volumes of water into the coastal zone with tides may be an important source of fecal contaminants. Efforts to reduce run-off or unintended release of water into the Intracoastal Waterway may lower contaminants entering sensitive coastal areas.     

Application of entropy analysis of in situ droplet-size spectra in evaluation of oil chemical dispersion efficacy

In situ droplet-size distributions were measured using a laser in situ scattering and transmissiometry (LISST-100X) particle size analyzer during the evaluation of natural and chemical dispersion efficiency of crude oils under different wave and current conditions. An entropy grouping of the in situ dispersed oil droplet-size spectra has classified the multi-modal droplet-size distributions into different groups based on similar droplet-size spectra characteristics within groups and distinction between groups. A generalized linear logistic regression model was fitted to analyze the effects of a number of factors and their interactions on the grouping of oil droplet-size spectra. The grouped results corresponded to the oil dispersion efficiency at different levels. This new method for droplet-size distribution data analysis can have significant implication in field evaluation of natural and chemical dispersion efficiency of oil.     

Oxygen isotope evidence of Little Ice Age aridity on the Caribbean slope of the Cordillera Central, Dominican Republic

Climate change during the so-called Little Ice Age (LIA) of the 15th to 19th centuries was once thought to be limited to the high northern latitudes, but increasing evidence reflects significant climate change in the tropics. One of the hypothesized features of LIA climate in the low latitudes is a more southerly mean annual position of the Intertropical Convergence Zone (ITCZ), which produced more arid conditions through much of the northern tropics. High-resolution stable oxygen isotope data and other sedimentary evidence from Laguna de Felipe, located on the Caribbean slope of the Cordillera Central of the Dominican Republic, support the hypothesis that the mean annual position of the ITCZ was displaced significantly southward during much of the LIA. Placed within the context of regional paleoclimate and paleoceanographic records, and reconstructions of global LIA climate, this shift in mean annual ITCZ position appears to have been induced by lower solar insolation and internal dynamical responses of the global climate system. Our results from Hispaniola further emphasize the global nature of LIA climate change and the sensitivity of circum-Caribbean climate conditions to what are hypothesized to be relatively small variations in global energy budgets.     

Reactive iron delivery to the Gulf of Alaska via a Kenai eddy

Mesoscale anticyclonic eddies in the Gulf of Alaska are an important mechanism for cross-shelf exchange of high iron, low nitrate coastal waters and low iron, high nitrate offshore waters. A Kenai eddy was sampled in September 2007, 8 months after formation. The subsurface eddy core layer contained reactive iron concentrations more than eight times greater than waters at the same depths outside the eddy. The subsurface core of the Kenai eddy (25.4≤σ_θ≤25.8) is suggested to be seasonally important as these waters can be brought to the surface with storm events and deep winter mixing. The deeper core layer (25.8≤σ_θ≤27.0) is suggested to be a source of iron to HNLC waters on a longer timescale, due to isopycnal mixing and eventual eddy relaxation. The subsurface and deeper core layers are important reservoirs of iron that can promote and sustain primary productivity over the lifetime of the Kenai eddy. In addition, dissolved and leachable particulate manganese are shown to be excellent tracers of eddy surface and subsurface waters, respectively.     

How well was the Sun observed during the Maunder Minimum?

In this paper we examine how well the Sun and sunspots were observed during the Maunder Minimum from 1645 to 1715. Recent research has given us the dates of observations by Hevelius, Picard, La Hire, Flamsteed, and about 70 other observers. These specific observations allow a lower estimate of the fraction of the time the Sun was observed to be deduced. It is found that 52.7% of the days have recorded observations. There are additional 12 observers who provide general statements that no sunspots were observed during specific years or intervals despite diligent efforts. Taking these statements to mean, unrealistically, that every day during these intervals was observed, gives an upper estimate of 98% of the days. If the general statements are relaxed by assuming that 100 ± 50 days per year were actually observed by these diligent observers, than our best estimate is that 68%±7% of the days during the Maunder Minimum were observed. In short, this supports the view that the Maunder Minimum existed and was not an artifact of few observations. Some sunspots are probably still missed in modern compilations, but the existence of a prolonged sunspot minimum would not be threatened by their discovery in future research. Additional support for intense scrutiny of the Sun comes from a report of a white-light flare in 1705 and from the numerous reports of new sunspots entering the disk of the Sun.     

The effects of in situ light reduction on the growth of two subtropical seagrasses,Thalassia testudinum and Halodule wrightii

The effects of in situ light reductions on two species of subtropical seagrasses, Thalassia testudirum (reduced to 14% and 10% of surface irradiance; SI) and Halodule wrightii (reduced to 16% and 13% SI) were examined over a 10-mo period (October 1992-September 1993) in relation to leaf elongation rates, sediment pore-water ammonium, and blade chlorophyll concentrations. No significant changes in pore-water ammonium levels were noted among treatments with time, but blade chlorophyll concentrations in both species were higher in plants exposed to the darkest treatments (10% and 13% SI) relative to controls exposed to 50% SI. In all treatments, blade chlorophyll concentrations were highest and chlorophyll a:b ratios lowest during the warner months, coincident with higher water temperatures. Leaf elongation rates in T. testudinum plants decreased relative to unshaded controls after 1 mo of treatment in autumn, but no significant differences in leaf elongation were noted among treatments for H. wrightii in late autumn or winter when very low growth rates (<0.1 cm shoot^?1 d^?1) were recorded. There were no differences between treatments during the spring growth period for T. testudinum (no data are available for H. wrightii), suggesting that growth (ca. 1 cm shoot^?1 d^?1) was probably not related to available light but was supported by belowground reserves. After 10 mo of treatment, all H. wrightii plants at 13% SI (1,600 mol m^?2 yr^?1) and 16% SI (2,000 mol m^?2 yr^?1) disappeared from experimental plots; similarly, no T. testudinum plants exposed to 10% SI (1,300 mol m^?2 yr^?1) remained, although 4% of the plants at 14% SI (1,800 mol m^?2 yr^?1) survived nearly 12 mo of reduced irradiance. In neither species were leaf elongation rates, which showed little change among treatments, a reliable indicator of the underwater light environment.