Chondrule heritage and thermal histories from trace element and oxygen isotope analyses of chondrules and amoeboid olivine aggregates

We report combined oxygen isotope and mineral‐scale trace element analyses of amoeboid olivine aggregates (AOA) and chondrules in ungrouped carbonaceous chondrite, Northwest Africa 5958. The trace element geochemistry of olivine in AOA, for the first time measured by LA‐ICP‐MS, is consistent with a condensation origin, although the shallow slope of its rare earth element (REE) pattern is yet to be physically explained. Ferromagnesian silicates in type I chondrules resemble those in other carbonaceous chondrites both geochemically and isotopically, and we find a correlation between ¹⁶O enrichment and many incompatible elements in olivine. The variation in incompatible element concentrations may relate to varying amounts of olivine crystallization during a subisothermal stage of chondrule‐forming events, the duration of which may be anticorrelated with the local solid/gas ratio if this was the determinant of oxygen isotopic ratios as proposed recently. While aqueous alteration has depleted many chondrule mesostases in REE, some chondrules show recognizable subdued group II‐like patterns supporting the idea that the immediate precursors of chondrules were nebular condensates.     

A morphometric analysis of gullies scoured by post-fire progressively bulked debris flows in southwest Montana, USA

In the fall of 2001, an intense thunderstorm in southwest Montana triggered many debris flows in the burned area of Sleeping Child Creek. In most instances, the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor. The presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows, a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow. In this contribution, we present a morphometric analysis of six of the gullies to better understand this relatively understudied process. We find that the locations of the rill heads and gully heads conform to slope-area thresholds that are characteristic of erosion by overland flow. Our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area, thus lending support to an assumption used in a recently proposed debris flow incision law. Finally, the debris flow fans have been relatively unaltered since deposition, suggesting that the valley may be currently aggrading while the hillslopes are being denuded.     

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to-para abundance ratios (OPRs) of cometary ammonia were determined from the NH₂ emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28±0.07 (nuclear spin temperature: Tspin=24±2 K) and 1.26±0.08 (Tspin=25±2 K), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH₂ emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH₂ on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 (Tspin>17 K) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.     

Optimized Computation of Transfer Coefficients in Surface Layer with Different Momentum and Heat Roughness Lengths

A new method for computing the surface transfer coefficients is proposed, based on state-of-the-art empirical flux-profile relationships. The influence of the roughness length ratio is first demonstrated with the classical iterative calculation method. Then a non-iterative algorithm is developed, taking into account the difference between momentum and heat roughness lengths.The new method is validated by comparison with the reference iterative computation. The large gain-in computer processing time (CPU) time gain for the calculation of surface fluxes in Eulerian grid models is finally assessed.     

Late Glacial and Holocene chironomid assemblages in Lac Long Inférieur (southern France, 2090 m): palaeoenvironmental and palaeoclimatic implications

Changes in lake water temperature and trophic states were inferred using chironomid fossil assemblages from Lac Long Inférieur (Southern Alps, France). In the Late Glacial, a colder period, possibly analogous to the Younger Dryas, is characterised by a peak in Micropsectra, a cold stenothermic taxon. The increase in temperatures during the Late Glacial interstadial is indicated by a decrease in the percentages of cold stenothermic taxa (Tanytarsus lugens/Corynocera oliveri grp.) and by an increase in taxa linked to the development of vegetation in the littoral zone. The beginning of the Holocene is marked by the presence of taxa adapted to warmer and more eutrophic waters. During the Holocene, the progressive warming of the climate and increase in lake trophic status were indicated by the increase of eutrophic and warmer water indicators. An increase in tributary inflow into Lac Long Inférieur was also inferred by the increase in rheophilous taxa, reflecting increased snowmelt. During the Subatlantic, the composition of the chironomid spectra suggests a re-cooling of the climate and/or a decrease in lake trophic status.     

Assessment of Geochemical Anomaly Uncertainty Through Geostatistical Simulation and Singularity Analysis

Natural Resources Research - Geochemical anomalies are commonly separated into different geochemical anomaly levels based on one or more thresholds. However, this practice may cause some important...     

An Improved Data-Driven Multiple Criteria Decision-Making Procedure for Spatial Modeling of Mineral Prospectivity: Adaption of Prediction–Area Plot and Logistic Functions

Natural Resources Research - Assigning realistic weights to targeting criteria in order to synthesize various geo-spatial datasets is one of the most important challenging tasks for mineral...     

The beam attenuation to chlorophyll ratio: an optical index of phytoplankton physiology in the surface ocean?

The particulate beam attenuation coefficient (c_p) is proportional to the concentration of suspended particles in a size domain overlapping that of the phytoplankton assemblage. c_p is largely insensitive to changes in intracellular chlorophyll concentration, which varies with growth irradiance (a process termed ‘photoacclimation’). Earlier studies have shown that the ratio of c_p:chlorophyll (i.e., c_p^*) exhibits depth-dependent changes that are consistent with photoacclimation. Similar relationships may likewise be expected in the horizontal and temporal dimensions, reflecting changes in mixing depth, incident irradiance, and light attenuation. A link between c_p^* and more robust photoadaptive variables has never been explicitly tested in the field. Here we use five historical field data sets to directly compare spatial and temporal variability in c_p^* with two independent indices of photoacclimation: the light-saturated, chlorophyll-normalized photosynthetic rate, P^b_opt, and the light-saturation index, E_k. For the variety of oceanographic conditions considered, a first-order correlation emerged between c_p^* and P^b_opt or E_k. These simple empirical results suggest that a relationship exists between a bio-optical variable that can potentially be retrieved remotely (c_p^*) and physiological variables crucial for estimating primary productivity in the sea.