Organic thermometry for chondritic parent bodies

A unique spectroscopic feature has been identified in a study of twenty-five different samples of meteoritic insoluble organic matter (IOM) spanning multiple chemical classes, groups, and petrologic types, using carbon X-ray Absorption Near Edge Structure (XANES) spectroscopy. The intensity of this feature, a 1s − σ exciton, appears to provide a precise measure of parent body metamorphism. The intensity of this exciton is also shown to correlate well with a large negative paramagnetic shift observed through solid state ¹³C NMR. Experiments reveal that upon heating primitive IOM is transformed into material that is indistinguishable from that in thermally processed chondrites, including the development of the 1s − σ exciton. A thermo-kinetic expression is derived from the experimental data that allows the intensity of the 1s − σ exciton to be used to estimated the effective temperature integrated over time. A good correlation is observed between the intensity of the 1s − σ exciton and previously published microRaman spectral data. These data provide a self-consistent organic derived temperature scale for the purpose of calibrating Raman based thermometric expressions.     

THE UNUSUAL STORMS OF FEBRUARY 1992 IN SOUTHERN CALIFORNIA

During February 1992, a series of relatively warm storms passed eastward across southern California, yielding intense precipitation that triggered widespread mass movement, flooding, property damage, and loss of life. These storms were triggered by an intense low pressure system (976 mb) off northern California which deepened as its eastward progress was initially blocked by a high pressure ridge (1040 mb) across western North America. Between February 10 and 13, large areas of Ventura and Los Angeles counties experienced cumulative precipitation of 200–400 mm with intensities reaching 40–50 mm hr^-1. Mass movement, mainly as soil slips that transformed downslope into debris flows, occurred where cumulative precipitation exceeded 300 mm and when sustained intensities exceeded 25 mm hr^-1. Stream response was rapid, particularly in urban areas where impermeable surfaces and storm drains fed concrete stream channels. The canalized upper Los Angeles River and Arroyo Simi exceeded all previous discharges for over 43 and 36 years of record, respectively. Other streams, from the large Santa Clara River to modest Malibu Creek, yielded recurrence intervals for the peak discharge of between 8 and 24 years, but the rapidity of flooding everywhere was remarkable. Whereas main trunk streams, canalized or not, responded predictably, the storm series emphasized the problems of poorly controlled development of potentially unstable hillsides and floodable lowlands and indicated a need to reassess the assumptions upon which such development is permitted. [Key words: climatology, cyclonic storm, geomorphology, mass movement, flooding, California].     

ACCELERATED EROSION ON THE NORTH CAROLINA COASTAL PLAIN

The Atlantic Coastal Plain of the United States has long been considered to have negligible soil erosion owing to low relief and permeable soils. Several recent studies point to a need for reassessment. Post-settlement erosion on the North Carolina Coastal Plain was estimated at the regional scale based on soil-profile truncation of a representative upland soil (Norfolk series). Apparent post-settlement soil-profile truncation ranges from 2.5 to 30 cm (mean 14.5 cm), implying a minimum regional average rate of 0.55 mm/yr since the region was settled in the 1700s. This amounts to a conservative estimated average rate of 9.3 t/ha/yr, a rate of the same general magnitude as estimated erosion rates in the North Carolina Piedmont, long recognized as a problem erosion area. Results imply accelerated water erosion despite the low slopes and permeable soils, and suggest the possibility of significant eolian erosion in the region. This rapid erosion in an area noted for its geological stability is a testament to the importance of vegetation cover and human agency in landscape evolution in the Coastal Plain. [Key words: accelerated erosion, soil profile, geomorphology, coastal plain, North Carolina.]     

DDT in endangered Galapagos sea lions (Zalophus wollebaeki)

We characterize for the first time the presence of DDT and its metabolites in tropical Galapagos sea lions (Zalophus wolleabeki). ∑DDT concentrations in Galapagos sea lion pups sampled in 2005 and 2008 ranged from 16 to 3070 μg/kg lipid. Concentrations of ∑DDT in pups in 2008 averaged 525 μg/kg lipid and were 1.9 times higher than that (281 μg/kg lipid) detected in pups in 2005. These concentrations are lower than those reported in many pinnipeds elsewhere, comparable to those in Hawaiian monk seals, and higher than those in southern elephant seals. The health risk characterization showed that 1% of the male pups exceeded the p,p′-DDE toxic effect concentration associated with anti-androgenic effects reported in rats. The findings provide preliminary guidance on the relationship between DDT use and ecological impacts, serving as a reference point against which possible future impact of tropical DDT use can be assessed.     

Twentieth century simulation of the southern hemisphere climate in coupled models. Part II: sea ice conditions and variability

We examine the representation of the mean state and interannual variability of Antarctic sea ice in six simulations of the twentieth century from coupled models participating in the Intergovernmental Panel on Climate Change fourth assessment report. The simulations exhibit a largely seasonal southern hemisphere ice cover, as observed. There is a considerable scatter in the monthly simulated climatological ice extent among different models, but no consistent bias when compared to observations. The scatter in maximum winter ice extent among different models is correlated to the strength of the climatological zonal winds suggesting that wind forced ice transport is responsible for much of this scatter. Observations show that the leading mode of southern hemisphere ice variability exhibits a dipole structure with anomalies of one sign in the Atlantic sector associated with anomalies of the opposite sign in the Pacific sector. The observed ice anomalies also exhibit eastward propagation with the Antarctic circumpolar current, as part of the documented Antarctic circumpolar wave phenomenon. Many of the models do simulate dipole-like behavior in sea ice anomalies as the leading mode of ice variability, but there is a large discrepancy in the eastward propagation of these anomalies among the different models. Consistent with observations, the simulated Antarctic dipole-like variations in the ice cover are led by sea-level pressure anomalies in the Amundsen/ Bellingshausen Sea. These are associated, to different degrees in different models, with both the southern annular mode and the El Nino-Southern Oscillation (ENSO). There are indications that the magnitude of the influence of ENSO on the southern hemisphere ice cover is related to the strength of ENSO events simulated by the different models.     

The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: Evidence from iron-rich pyroxene

Abstract— FeO-rich (Fs^6–34) pyroxene lacking cathodoluminescence (CL), hereafter black pyroxene, is a major constituent of some of the chondrules and fragments in unequilibrated (type 3) enstatite chondrites (UECs). It contains structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite with red CL, associated with mm-sized blebs of low-Ni, Fe-metal and, in some cases, silica. These occurrences represent clear evidence of pyroxene reduction. The black pyroxene is nearly always rimmed by minor element (Cr, Mn, V)-poor enstatite having a blue CL. More commonly, red and blue enstatites, unassociated with black pyroxene, occur as larger grains in chondrules and fragments, and these constitute the major silicate phases in UECs. The REE abundance patterns of the black pyroxene are LREE-depleted. The blue enstatite rims, however, have a near-flat to LREE-enriched pattern, ～0.5–4x chondritic. The petrologic and trace element data indicate that the black pyroxene is from an earlier generation of chondrules that formed in a nebular region that was more oxidizing than that of the enstatite chondrites. Following solidification, these chondrules experienced a more reducing nebular environment and underwent reduction. Some, perhaps most, of the red enstatite that is common throughout the UECs may be the product of solid-state reduction of black pyroxene. The blue enstatite rims grew onto the surfaces of the black pyroxene and red enstatite as a result of condensation from a nebular gas. The evolutionary history of some of the enstatite and chondrules in enstatite chondrites can be expressed in a four-stage model that includes: Stage 1. Formation of chondrules in an oxidizing nebular environment Stage 2. Solid-state reduction of the more oxidized chondrules and fragments to red enstatite in a more reducing nebular environment Stage 3. Formation of blue enstatite rims on the black pyroxene as well as on the red enstatite. Stage 4. Reprocessing, by various degrees of melting, of many of the earlier-formed materials.     

Petrology and geochemistry of Patuxent Range 91501, a clast‐poor impact melt from the L‐chondrite parent body and Lewis Cliff 88663, an L7 chondrite

Abstract— We have performed petrologic and geochemical studies of Patuxent Range (PAT) 91501 and Lewis Cliff (LEW) 88663. PAT 91501, originally classified as an L7 chondrite, is rather a unique, near total impact melt from the L‐chondrite parent body. Lewis Cliff 88663 was originally classified as an “achondrite (?)”, but we find that it is a very weakly shocked L7 chondrite. PAT 91501 is an unshocked, homogeneous, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine, low‐Ca pyroxene, augite and chrome‐rich spinels with interstitial albitic plagioclase and minor silica‐alumina‐alkali‐rich glass. Only ～10% relic chondritic material is present. Olivine grains are homogeneous (Fa_25.2–26.8). Low‐Ca pyroxene (Wo_1.9–7.2En_71.9–78.2Fs_19.9–20.9) and augite (Wo_29.8–39.0En_49.2–55.3Fs_11.8–14.9) display a strong linear TiO₂‐Al₂O₃ correlation resulting from igneous fractionation. Plagioclase is variable in composition; Or_3.0–7.7Ab_79.8–84.1An_8.2–17.2.‐Chrome‐rich spinels are variable in composition and zoned from Cr‐rich cores to Ti‐Al‐rich rims. Some have evolved compositions with up to 7.9 wt% TiO₂. PAT 91501 bulk silicate has an L‐chondrite lithophile element composition except for depletions in Zn and Br. Siderophile and chalcophile elements are highly depleted due to sequestration in centimeter‐size metal‐troilite nodules. The minerals in LEW 88663 are more uniform in composition than those in PAT 91501. Olivine grains have low CaO and Cr₂O₃ contents similar to those in L5–6 chondrites. Pyroxenes have high TiO₂ contents with only a diffuse TiO₂‐Al₂O₃ correlation. Low‐Ca pyroxenes are less calcic (Wo_1.6–3.1En_76.5–77.0Fs_20.4–21.4), while augites (Wo_39.5–45.6En_46.8–51.1Fs_7.6–9.4) and plagioclases (Or_2.6–5.7Ab_74.1–83.1An_11.2–23.3) are more calcic. Spinels are homogeneous and compositionally similar to those in L6 chondrites. LEW 88663 has an L‐chondrite bulk composition for lithophile elements, and only slight depletions in siderophile and chalcophile elements that are plausibly due to weathering and/or sample heterogeneity.     

Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift,Kenya

In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 10⁴ km³ during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.     

Boron in chert and Precambrian siliceous iron formations

In order to assess the importance of siliceous sediments as a sink for oceanic B and to determine the effect of diagenesis on the mobilization of B, samples were analysed from chert nodules, bedded cherts, and siliceous banded iron formations from a variety of sedimentary environments and geologic ages. Boron analyses on bulk samples were made by prompt gamma neutron activation analysis. The distribution of B in rocks was mapped using α-track methods.Nodular Phanerozoic cherts typically contain 50–150 ppm B, and bedded cherts somewhat less. The B is initially concentrated in tests of silica-secreting organisms, but some is lost in early diagenesis as silica progressively recrystallises to quartz.Banded iron formation silica of Archean and Proterozoic age usually contains < 2 ppm B. This conforms with the view that such silica is not of biogenic origin but, since many iron formations are undoubtedly of marine origin, raises the question whether Precambrian oceans were impoverished in B. Analyses of Precambrian marine argillaceous sediments, averaging 70 ppm B, do not resolve this question.