Holocene evolution of a coastal lagoon, Lake of Tunis, Tunisia

Historical records demonstrate that the Lake of Tunis, Tunisia, was an open bay that became separated from the Mediterranean by an accreting barrier spit, forming a lagoon. Closure of the lagoon was caused by increased sedimentation as a consequence of Roman deforestation of the Medjerda River drainage basin to the north and subsequent accelerated longshore transport. The separation of the lagoon from the Mediterranean was completed in the early 1500s. At present, the 48 km² lagoon averages less than 1 m in depth and reaches eutrophic conditions in the late summer. Consistent with the historical records, the lagoon sedimentary column shows three distinct successive environments of deposition: (1) an arid continental environment; (2) an open marine bay; and (3) the present brackish to hypersaline lagoon. These depositional environments are represented by the lower grey layer, which is less than 0-5 m thick, the middle olive-grey layer, which varies from 1 to 5 m in thickness, and the upper black layer, which is 1 m thick. All of the strata are predominantly silt plus clay, but usually contain at least 10% sand. The lower grey layer consists of pitted quartz sand, with very few abraded, broken molluscan fragments and benthic foraminifera with thick tests. An arid, subaerial depositional environment of latest Pleistocene time best explains these sediment and fossil assemblages. In the middle olive-grey layer, coral, coralline algae, open marine graeses and the dominance of foraminifera over ostracods (expressed as a low percentage of ostracods/ostracods plus foraminifera) attest to a depositional environment of an open marine bay. A sharp increase in the percentage of ostracods/ostracods plus foraminifera and organic carbon from the middle olive-grey layer to the upper black layer signifies a major change in depositional environment. The predominance of ostracods over foraminifera, abundance of gastropods characteristic of eutrophic conditions, high organic carbon content and absence of macro-fossils characteristic of open marine conditions clearly indicate that the upper black layer was deposited in a brackish to hypersaline, eutrophic lagoon which has persisted to the present. Seismic records indicate a karstic bedrock surface underlying the lagoon. The surface is marked by considerable relief, and shows a linear depression which may represent an early sixteenth century ship canal. Heavy metal analyses of total samples in five cores demonstrate that Mn and Fe vary randomly, and are apparently derived from natural sources only. Cd, Cr, Pb, Cu and Zn are typically highest in the uppermost unit, which reflects levels of human contamination since closure of the lagoon.     

EXAMINATION OF SOUNDING CURVE EXTRAPOLATION USED BY THE OFFSET WENNER SYSTEM1

The Offset Wenner resistivity sounding system provides for the extrapolation of the Wenner resistivity curve. The extrapolation technique was applied to data measured in the Solomon Islands and it is shown to be unreliable. An accurate method of predicting the reliability of extrapolation using measured resistances could not be found.     

Atmospheric Effects of the Emerging Mainland Chinese Transportation System at and Beyond the Regional Scale

Local surface travel needs in the Peoples Republic of China (mainlandChina) have traditionally been met largely by nonpolluting bicycles. A majorautomobile manufacturing/importing effort has begun in the country over thelast decade, and planning documents indicate that the Chinese may strive toacquire more than 100 million vehicles early in the next century. By analogywith large automotive fleets already existing in the western world, bothregional and global scale pollution effects are to be expected from theincrease. The present work adopts the latest projections of Chinese automobilemanufacture and performs some quantitative assessments of the extent ofpollution generation.Focus for the investigation is placed upon the oxidant ozone. Emissions of theprecursor species nitrogen oxides and volatile organics are constructed basedon data for the current automotive sector in the eastern portion of the UnitedStates. Ozone production is first estimated from measured values forcontinental/oceanic scale yields relative to precursor oxidation. Theestimates are then corroborated through idealized two dimensional modeling ofthe photochemistry taking place in springtime air flow off the Asian land massand toward the Pacific Ocean. The projected fleet sizes could increase coastaland remote oceanic ozone concentrations by tens of parts per billion (ppb)in the lower troposphere. Influences on the tropospheric aerosol system andon the major greenhouse gas carbon dioxide are treated peripherally. Nitrogenoxides created during the vehicular internal combustion process willcontribute to nitrate pollution levels measured in the open Pacific. Thepotential for soot and fugitive dust increases should be considered as theautomotive infrastructure develops. Since the emerging Chinese automotivetransportation system will represent a substantial addition to the globalfleet and all the carbon in gasoline is eventually oxidized completely, asignificant rise in global carbon dioxide inputs will ensue as well.Some policy issues are treated preliminary. The assumption is made thatalterations to regional oxidant/aerosol systems and to terrestrial climate areconceivable. The likelihood that the Chinese can achieve the latest vehiclefleet goals is discussed, from the points of view of new production, positivepollution feedbacks from a growing automobile industry, and known petroleumreserves. Vehicular fuel and maintenance options lying before the Chinese areoutlines and compared. To provide some perspective on the magnitude of theenvironmental changes associated with an Asian automotive buildup, recentestimates of the effects of future air traffic over the Pacific Rim aredescribed.     

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Abstract— We propose that mesosiderites formed when a 200–400 km diameter asteroid with a molten core was disrupted by a 50–150 km diameter projectile. To test whether impacts can excavate core iron and mix it with crustal material, we used a low‐resolution, smoothed‐particle hydrodynamics computer simulation. For 50–300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near‐catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to ～10 vol% of mesosiderite‐like material. The paucity of olivine in mesosiderites and the lack of metal‐poor or troilite‐rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine‐poor because mantle material was preferentially excluded from the metal‐rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 °C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 °C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at ～1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine‐rich (A‐type) and metal‐rich asteroids (M‐type) are known. Some S‐type asteroids may be scrambled differentiated bodies.     

Aptian综合年代地层事件:定量地层学的运用

以往发表的年代地层数据都是以Ma形式出现,以关键物种的首现面和末现面来界定的。一般而言,这些年龄是在用某种方法确定的某两个层的年龄基础上通过插值获得的,很大程度上仍然是估计值,而不是可以检验的科学结论。我们需要的是一个可以评价的、剖面重现的、标本重现的而且插值方法是经过严格检验的数据库。图形对比技术正是一个有用的方法,它可以综合众多剖面的分散分布的事件,并可以验证这些事件的顺序和年代。图形对比技术是一个定量的、非统计的方法,可以判别两个剖面之间存在的同时代关系。物种时限和非生物事件投点到X/Y图解上,时限可以通过对比线综合到单个的时间尺度上。其他剖面的数据通过重复以上过程把每个剖面的数据综合进来。获得的时限通过评价有这些物种出现的图形解而得到验证。Aptian阶(约124~112 Ma)时期环境发生巨大变化,用来确定Aptian阶的许多关键生物事件和年代地层事件并不都出现在同一个剖面。因此,这些事件的相对年龄始终是不确定的,也不是相关的。两个事件被提出来用于界定Aptian 阶的底界,包括菊石Deshayesites tuarkyricus 和磁极性带CM0。然而这两个事件标准地层剖面相隔600 km。图形对比显示这两次事件出现在7 万年间隔内。通过这一方法,来自23 条Barremian Aptian Albian     

Mineralogy,petrography, bulk chemical,iodine-xenon,and oxygen-isotopic compositions of dark inclusions in the reduced CV3 chondrite Efremovka

Abstract— We studied the petrography, mineralogy, bulk chemical, I-Xe, and O-isotopic compositions of three dark inclusions (E39, E53, and E80) in the reduced CV3 chondrite Efremovka. They consist of chondrules, calcium-aluminum-rich inclusions (CAIs), and fine-grained matrix. Primary minerals in chondrules and CAIs are pseudomorphed to various degrees by a mixture largely composed of abundant (>95%), fine-grained (>0.2 μm) fayalitic olivine (Fa_35–42) and minor amounts of chlorite, poorly-crystalline Si-Al-rich material, and chromite; chondrule and CAI shapes and textures are well-preserved. Secondary Ca-rich minerals (Ti-andradite, kirschsteinite, Fe-diopside) are common in chondrule pseudomorphs and matrices in E39 and E80. The degree of replacement increases from E53 to E39 to E80. Fayalitic olivines are heavily strained and contain abundant voids similar to those in incompletely dehydrated phyllosilicates in metamorphosed CM and CI chondrites. Opaque nodules in chondrules consist of Ni- and Co-rich taenite, Co-rich kamacite, and wairauite; sulfides are rare; magnetite is absent. Bulk O-isotopic compositions of E39 and E53 plot in the field of aqueously altered CM chondrites, close to the terrestrial fractionation line; the more heavily altered E39 is isotopically heavier than the less altered E53. The apparent I-Xe age of E53 is 5.4 Ma earlier than Bjurböle and 5.7 ± 2.0 Ma earlier than E39. The I-Xe data are consistent with the most heavily altered dark inclusion, E39 having experienced either longer or later alteration than E53. Bulk lithophile elements in E39 and E53 most closely match those of CO chondrites, except that Ca is depleted and K and As are enriched. Both inclusions are depleted in Se by factors of 3–5 compared to mean CO, CV, CR, or CK chondrites. Zinc in E39 is lower than the mean of any carbonaceous chondrite groups, but in E53 Zn is similar to the means in CO, CV, and CK chondrites. The Efremovka dark inclusions experienced various degrees of aqueous alteration, followed by low degree thermal metamorphism in an asteroidal environment. These processes resulted in preferential oxidation of Fe from opaque nodules and formation of Ni- and Co-rich metal, metasomatic alteration of primary minerals in chondrules and CAIs, and the formation of fayalitic olivine and secondary Ca-Fe-rich minerals. Based on the observed similarities of the alteration mineralization in the Efremovka and Allende dark inclusions, we infer that the latter may have experienced similar alteration processes.