Geochemistry,chronology and stratigraphy of Neogene tuffs of the Central Andean region

Tuff layers are vital stratigraphic tools that allow correlations to be made between widely dispersed exposures. Despite their widespread occurrence in the central Andes, tuffs from both natural exposures and sedimentary cores extracted from the region's extensive salars (salt pans) are relatively unstudied. Here we lay the foundation for a tephrostratigraphic framework in the central Andes (14–28°S) by chemically and morphologically characterizing ash shards, and in some cases dating 36 Neogene distal tuffs. These tuffs occur in lacustrine and alluvial deposits from the southern Bolivian Altiplano and adjacent Atacama Desert. All tuffs are calc-akaline rhyolites, consistent with their setting in the Central Andean Volcanic Zone. Five of the older tuffs were ⁴⁰Ar/³⁹Ar dated and yield an age range of 6.63–0.75 Ma. Organic material associated with tuffs deposited into paleolake sediments, paleowetland deposits, or urine-encrusted rodent middens provide constraints on the age of several Late Pleistocene and Holocene tuffs.These tuffs provide key stratigraphic markers and ages for lake cycles and archeological sites on the Bolivian Altiplano and for assessing rates of surficial processes and archeology in both the Atacama and Altiplano. While modern climate, and consequently questions about geomorphic processes and climate change, differs in the hyperarid Atacama and the semi-arid Altiplano, the most extensive air-fall tuffs covered both regions, placing the Atacama and the Bolivian Altiplano in the same tephrostratigraphic province. For example, the Escara B tuff (～1.85 Ma), can be securely identified in both the Altiplano and Atacama. On the Altiplano, dates from the Escara B and E tuffs securely establish the age of the Escara Formation—representing the oldest expansive lake documented on the Bolivian Altiplano. By contrast, the presence of the Escara B tuff below ～6 m of alluvial sediment at the Blanco Encalado site in the Atacama desert yields information about sedimentation rates in this hyperarid region. Indeed, most tuffs from the Atacama Desert are older than 600,000 years, even though they occur within fluvial terraces immediately adjacent to the alluvial fans that are still active. Most of these geomorphic surfaces in the Atacama also possess well-developed saline soils that, when combined with the radiometric ages of the distal tuffs, suggest slow rates of geomorphic change and exceptional landscape stability for this area during the Quaternary.In contrast, younger tuffs are more abundant in the more recent lake records of the Altiplano. The Chita tuff was deposited at ～15,650 cal yr B.P., during the regressive phase of the region's deepest late Quaternary lake cycle—the “Tauca lake cycle”—which spanned 18.1–14.1 cal yr B.P. Two Holocene tuffs, the Sajsi tuff and the Cruzani Cocha tuff, are widespread. The Sajsi tuff was deposited just before 1700 cal yr B.P., whereas the Cruzani Cocha tuff appears to be mid-Holocene in age and shows some chemical affinities to a Holocene tuff (202B) deposited between 4420 and 5460 cal yr B.P. in a urine-encased rodent midden in the Atacama Desert.     

Energy balance and evaporation loss of an agricultural reservoir in a semi‐arid climate (south‐eastern Spain)

A typical agricultural water reservoir (AWR) of 2400 m² area and 5 m depth, located in a semi‐arid area (southern Spain), was surveyed on a daily basis for 1 year. The annual evaporation flux was 102·7 W m^?2, equivalent to an evaporated water depth of 1310 mm year^?1. The heat storage rate G exhibited a clear annual cycle with a peak gain in April (G ～ 45 W m^?2) and a peak loss in November (G ～ 40 W m^?2), leading to a marked annual hysteretic trend when evaporation (λE) was related to net radiation (R_n). λE was strongly correlated with the available energy A, representing 91% of the annual AWR energy loss. The sensible heat flux H accounted for the remaining 9%, leading to an annual Bowen ratio in the order of 0·10. The equilibrium and advective evaporation terms of the Penman formula represented 76 and 24%, respectively, of the total evaporation, corresponding to a annual value of the Priestley–Taylor (P–T) coefficient (α) of 1·32. The P–T coefficient presented a clear seasonal pattern, with a minimum of 1·23 (July) and a maximum of 1·65 (December), indicating that, during periods of limited available energy, AWR evaporation increased above the potential evaporation as a result of the advection process. Overall, the results stressed that accurate prediction of monthly evaporation by means of the P–T formula requires accounting for both the annual cycle of storage and the advective component. Some alternative approaches to estimating R_n, G and α are proposed and discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

<Emphasis Type="Italic">R/S</Emphasis> analysis of the <Emphasis Type="Italic">Dst</Emphasis> index

The paper addresses estimation of the Hurst exponent for time series of the hourly values of the Dst index for the period from 1957 to 2011. It is found that the Hurst exponent is 0.79–0.94 for yearly intervals and 0.8–1.0 for monthly intervals. Based on R/S graphs, the Dst cycles are identified; they range from 3–4 months to 2.2 years and from 8.5 to 22 years in length. It is shown that a Dst time series can be quite satisfactorily described by an α-stable Levy process.     

Heightened sensitivity of a poorly buffered high arctic lake to late-Holocene climatic change

A diatom-based paleolimnological investigation was conducted on late Holocene sediments from a poorly buffered lake, informally named “Rock Basin Lake”, on Ellesmere Island, Arctic Canada. The fossil diatom record is unlike any other obtained thus far from high arctic regions, exhibiting dynamic assemblage shifts over the entire 3300 yr sedimentary record. Multiple proxies (i.e., diatoms, pH reconstructions, biogenic silica, C/N ratios, total organic carbon) appear to sensitively track rapid limnological changes, which are associated with distinct climate intervals as inferred from other regional proxy records. The highly responsive nature of the diatom assemblages in Rock Basin Lake, relative to those recorded from nearby alkaline sites, appears to be related to this lake's limited ability to buffer changes in pH. The dynamic species responses suggest that the diatoms in Rock Basin Lake are faithfully tracking climatic changes, and that low-alkalinity lakes may provide the most sensitive diatom-based paleolimnological records from high arctic regions.     

Desertification of Mediterranean rocky reefs caused by date-mussel, Lithophaga lithophaga (Mollusca: Bivalvia), fishery: effects on adult and juvenile abundance of a temperate fish

Macroalgal beds in temperate rocky habitats provide shelter and food for many adult reef fishes and their juvenile stages. In the Mediterranean Sea, the fishery of the endolithic date-mussel Lithophaga lithophaga (which involves dismantling of rocky substrates inhabited by these bivalves) may cause formation of barrens in shallow rocky reefs. Preliminary data collected in SW Apulia (SE Italy) show that rocky reefs impacted by this destructive fishery display different distribution patterns of adult Coris julis (a common labrid fish in the Mediterranean basin), and lower abundance of juveniles. The ecological implications of date-mussel fishery for dynamics of fish populations and rocky-reef ecosystem functioning (e.g., nursery role) are discussed.     

Probabilistic Microzonation of Urban Territories: A Case of Tashkent City

—?The problem of accounting for local soil effect on earthquake ground motion is especially urgent when assessing seismic hazard – recent needs of earthquake engineering require local site effects to be included into hazard maps. However, most recent works do not consider the variety of soil conditions or are performed for generalized site categories, such as “hard rock,”“soft soil” or “alluvium.” A technique of seismic hazard calculations on the basis of the Fourier Amplitude Spectra recently developed by the authors allows us to create hazard maps involving the influence of local soil conditions using soil/bedrock spectral ratios. Probabilistic microzoning maps may be constructed showing macroseismic intensity, peak ground acceleration, response and design spectra for various return periods (probability of exceedance), that allow optimization of engineering decisions. An application of this approach is presented which focused on the probabilistic microzoning of the Tashkent City.     

A method for constructing high-order differential boundary conditions for solving external boundary value problems in geoelectromagnetism

A general approach to the construction of differential boundary conditions for vector fields satisfying the Helmholtz equation is proposed on the basis of the field expansion in multipole series and the application of annihilating operators to them. The resulting differential constraints can be used as boundary conditions in solving external boundary value problems. Examples of their application to the solution of forward geoelectric problems in three-dimensionally inhomogeneous media are examined. Their use at a finite distance from the source of an anomaly is shown to yield more accurate results than those obtained under the assumption that the anomalous field at this distance vanishes. Another effect of their application is a substantial decrease in the dimensions of the modeling domain and therefore in the time required to solve the forward problem. The “safe” distance for using the Dirichlet-type boundary conditions is estimated.