Testing commonly used X‐ray fluorescence core scanning‐based proxies for organic‐rich lake sediments and peat

X‐ray fluorescence (XRF) core scanning has become widely available for geological studies during the last decade. The data obtained from XRF core scanning, however, may be strongly influenced by the amount of organic matter, water content, density and porosity of the sediment matrix. In this study we discuss the usefulness of XRF core scanning to distinguish different kinds of organic‐rich sediments and peat based on examples from tropical Lakes Kumphawapi and Nong Leng Sai in Thailand. We examined how sedimentary factors influence XRF core scanning analyses by comparing elemental and scattering ratios to lithological changes and quantitative LOI, TOC, biogenic silica (BSi) and grain‐size values. Our comparison suggests that the (inc/coh) scattering ratio is of limited use as an indicator for variations in LOI and TOC in peaty gyttja or peat. In Lake Kumphawapi's sediments, Si/Ti ratios reflect clastic input associated with grain‐size variations rather than BSi contents. The Ti‐normalized ratios of Si, Zr, Sr, K and Rb are linked to mineral input and associated grain‐size variations. We conclude that XRF core scanning of organic‐rich tropical lake sediments and peat is useful to infer palaeoenvironmental conditions. However, XRF core scanning data does not stand‐alone and needs to be underpinned by additional proxies.     

Recent sedimentary histories of shallow lakes in the guatemalan savannas

Shallow basins in the savannas of Peten, Guatemala filled with water after 305±55 BP (calibrated age+1430–1660 AD). Aguadas Chimaj and Chilonche possess dilute waters and iron-rich, clayey sediments that are poor in Ca and Mg, reflecting the highly weathered nature of riparian soils. Low ²¹⁰Pb flux rates to Chimaj (0.085 pCi cm^-2 yr^-1) and Chilonche (0.134 pCi cm^-2 yr^-1) are attributed to low ²²²Rn emission rates from the nearby Caribbean Sea. Mean sediment accumulation rates in Chimaj and Chilonche for the past 150 years are 0.015 g cm^-2 yr^-1 and 0.047 g cm^-2 yr^-1 respectively. Forest expansion after 305 BP is documented in pollen profiles from the small aguadas and larger Lake Oquevix. Regional reforestation postdates the 9th century Classic Maya collapse and coincides with indigenous depopulation that was a consequence of European intrusion that began in the early 1500s. The timing of forest regrowth indicates the importance of historical anthropogenic factors in controlling Peten's vegetation. Nevertheless, other sedimentological lines of evidence (e.g. lithology, algal remains and charcoal particles) suggest that changing climate and/or local hydrology may have played a role in the reforestation process.     

The evolution of a quasar population based on a simple source model

The cosmological evolution of a population of quasars and AGNs is considered. The applied source model provides that the star collisions in the central stellar system should be the principal mechanism of supply of the gas falling on to the black hole. The continuity equation for the assumed population is solved for the zero birth function as well as for its special type. The solutions obtained depend on the distribution of two parameters characterizing a single source: the radius of stellar core and the number of stars in the system. All these solutions allow to describe practically whole evolutionary track of the considered source population, and to relate the evolution to some features of a single source.     

Modeling of PCB concentrations in water and biota (Mytilus edulis) in New Bedford Harbor, Massachusetts

The water column concentration and bioaccumulation of the polychlorinated biphenyl (PCB) congener CB052 was modeled in New Bedford Harbor, Massachusetts, using site-specific hydrodynamics and loading information. Equilibrium partitioning theory was used to estimate interstitial water CB052 concentrations from sediment concentrations in New Bedford Harbor and Buzzards Bay, Massachusetts. The rate of CB052 vertical flux from the interstitial water to the overlying water column was calculated by multiplying the vertical concentration gradient at the sedimentwater interface by a flux coefficient. The vertical flux coefficient and the flux rate from model-generated water-column concentrations were calculated using an interative procedure. Movement of CB052 within New Bedford Harbor was simulated using calibrated two-dimensional, vertically-integrated, finite element hydrodynamic and transport models. Quasi-steady-state water column concentrations and a field-derived bioconcentration factor were used to predict the expected concentration of CB052 in blue mussels (Mytilus edulis) at two stations in New Bedford Harbor. The model was used to predict the effects of two remedial scenarios (i.e., reducing average sediment total PCB concentrations to 50 ppm or 10 ppm) on concentrations of CB052 in water and blue mussel tissue. Based on the model results, the CB052 concentration in blue mussels would be reduced by 33–53% for the 50 ppm option and by 67–84% for the 10 ppm option.     

Biogeochemistry of iron in an acidic lake

In this paper, the fate of iron in Lake Cristallina, an acidic lake in the Alps of Switzerland, is discussed. A simple conceptual model is developed in order to explain the observed diel variation in dissolved iron(II) concentration. Biotite weathering provides reduced iron that is oxidized and subsequently precipitated in the lake. The amorphous Fe(III)hydroxide (FeOOH xH₂O), found in the sediments of Lake Cristallina, is an Fe(II) oxidation product. This oxygenation reaction is most probably catalyzed by bacteria surfaces, as indicated by the relatively high estimated oxidation rate compared to the oxidation rate of the homogeneous oxidation of inorganic Fe(II) species at the ambient pH of Lake Cristallina (pH 5.4 at 4 °C) and by the scanning electron micrograph pictures. Under the influence of light, these amorphous iron(III)hydroxide phases are reductively dissolved. The net concentration of Fe(II) reflects the balance of the reductive dissolution and the oxidation/precipitation reactions and tends to parallel the light intensity, leading to a diurnal variation in the Fe(II) concentration. The rate of the photochemical reductive dissolution of Lake Cristallina iron(III)hydroxides is greatly enhanced in situ and in the laboratory by addition of oxalate to the lake water.     

Moonquake predetermination and tides

A pattern in moonquakes, which correlates with the monthly tidal cycle, also correlates with a phase shifted pattern of a 7-month tidal cycle. The lead of approximately 2 months in moonquake occurrence can be explained if local tidal forces are combined with a moonquake-driving force. This force, assumed to result from the 6-yr physical libration in latitude, would cause N-S sliding of an outer layer across a solid layer within a decoupled core in a lunar model. During 1969–1971, the sliding would be southward with a progression of monthly maxima in the combined forces. Where these forces control moonquakes, the reversal in direction of the 6-year cyclic force in early 1972 should cause a minimum in moonquake activity. Decreases toward such a minimum did occur simultaneously in the similar progressions of monthly moonquakes and maxima in the combined forces at the most active hypocenter. Repetition of the 1966–1971 force pattern in 1975–1977 should produce a corresponding repetition of the moonquake pattern. In 1973, the yet to be determined pattern in moonquakes could correspond to the pattern of either an entirely cyclic force, or a combination of the cyclic force and an additional secular force. The physical libration in latitude controls moonquakes in both cases. If moonquakes are driven by an entirely cyclic force, the use of a simple lunar model could lead to a better understanding of moonquake causes and of possible analogs in earthquake control.     

The effect of the earth's bow shock and magnetosheath on the interaction of a discontinuity in the solar wind with the magnetosphere

A theoretical model is proposed for the interaction of a plane discontinuity in the solar wind with the magnetosphere. The presence of the bow shock and magnetosheath are taken into account, the calculation being based on the Spreiter et al. (1966) gas-dynamic model for a solar wind Mach Number M = 5. The model proposed predicts the manner in which the shape of the interplanetary discontinuity is distorted in its passage through the magnetosheath; it is found that the point of first impact with the magnetopause makes an angle of 56° with the Sun-Earth line for relatively quiet solar wind conditions.     

Determining the cooling history of in situ lower oceanic crust—Atlantis Bank, SW Indian Ridge

The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (∼850°C) drilled in Hole 735B is estimated at 11.93±0.14 Ma, based on U-Pb analyses of zircon by ion probe. This crystallization age is older than the ‘crustal age’ from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949-11.935 Ma). ⁴⁰Ar/³⁹Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42±0.21 Ma), implying cooling rates of >800°C/m.y. over the first 500,00 years to temperatures below ∼330-400°C. Fission-track ages on zircon (mean 9.35±1.2 Ma) and apatite reveal less rapid cooling to <110°C by ∼7 Ma, some 4-5 m.y. off axis.Comprehensive thermochronometric data from the structurally intact block of gabbro between ∼700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range ∼900-330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T≤280°C) are not consistent with these predictions and record anomalously high temperatures for crust >700 m below sea floor at 8-10 Ma (i.e. 2-4 m.y. off axis). We offer two hypotheses for this thermal anomaly:

(i)

Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from ∼19.5 to 7.5 Ma.

(ii)

Late detachment faulting, which led to significant crustal denudation (2.5-3 km removed), further from the ridge axis than conventionally thought.