Variability of river discharge and Atlantic-water inflow at the Laptev Sea continental margin during the past 15,000 years: implications from maceral and biomarker records

In order to reconstruct the depositional environment from the Laptev Sea continental slope and shelf during the past ~15,000 years BP maceral analysis was carried out on two sediment cores (PS2458-4, PS2725-5) and compared with organic-geochemical parameters. During the transition from the Last Glacial to the Holocene the environment of the Laptev Sea shelf was controlled by the post-glacial sea level rise, variations in river discharge, surface-water productivity, and Atlantic-water inflow along the Eurasian continental margin. Based on our results, we identify the following significant changes of the environment: (a) at approximately 13,500 years BP the first step of deglaciation (Termination 1a) is documented by the deposition of marine and fresh-water organic matter; (b) at approximately 10,400 years BP the first post-glacial influence of Atlantic-water inflow along the Eastern Laptev Sea continental margin is indicated by an increase in marine organic matter; (c) at the beginning of the Holocene an increased fluvial supply is documented by an increase in fresh-water alginite; and (d) since ~9500-8000 years BP modern marine conditions are established at the Laptev Sea continental margin as documented in increased amounts of marine macerals, biomarkers (dinosterol, brassicasterol, short-chain fatty acids), and dinoflagellate cysts.     

Magnetic equilibria resulting from a helically symmetric kink instability

Abstract

This paper explores magnetic equilibria which could result from the kink instability in a cylindrical magnetic flux tube. We examine a variety of cylindrical magnetic equilibria which are susceptible to the kink, and simulate its evolution in a frictional fluid. We assume that the evolution takes place under conditions of helical symmetry, so the problem becomes effectively two-dimensional. The initial cylindrical equilibrium field is specified in terms of its twist function k(r) = B _θ/(rB_z ) and for a variety of k(r) functions we calculate linear growth rates for the kink instability, assuming that it develops under helical symmetry with pitch τ. We find that the growth rate is sensitive to the value of τ.

We simulate nonlinear evolution of the kink using a Lagrangian frictional code which constrains the field to have helical symmetry of a given pitch τ. Ideal MHD is assumed and the plasma pressure is taken to be small in order to mimic conditions in the solar corona. In some cases the flux tube evolves to a new smooth helically symmetric equilibrium which involves a relatively small change in the maximum electric current. In other cases there is evidence of current-sheet formation.     

Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone

The western Hellenic subduction zone is characterized by a trenchward velocity of the upper plate. In the Ionian islands segment, complete seismic coupling is achieved, as is predicted by standard plate-tectonic models in which there is no slab pull force because the slab has broken off. The moderate local seismic moment rate relates to a shallow downdip limit for the seismogenic interface. This characteristic may be attributed to the ductility of the lower crust of the upper plate, which allows a décollement between the upper crust of the overriding plate and the subducting plate. Farther south, a deeper downdip limit of the seismogenic interface is indicated by thrust-faulting earthquakes, which persist much deeper in western Crete. A correspondingly larger downdip width of this seismogenic zone is consistent with the suggested larger maximum magnitude of earthquakes here. However, since the seismic moment release rate seems to be moderate in the Peloponnese and western Crete, like in in the Ionian islands, this seismically active interface cannot maintain complete seismic coupling across its larger downdip width. A cause may be the lateral addition of overweight to the part of the slab still attached in Crete, by the free fall of its part that has broken off from the surface further north. This increased slab pull reduces the compressive normal stress across the seismogenic interface and thus causes partial seismic coupling in its shallower part. However, the width of this part may provide an additional area contributing to slip in large earthquakes, which may nucleate deeper on stick-slip parts of the interface. Hints at anomalies in structure and seismicity, which need to be resolved, may relate to the present location of the edge of the tear in the slab.     

Predicting the Peak in World Oil Production

The US Department of Energy's Energy Information Administration (EIA) recently predicted that world oil production could continue to increase for more than three decades, based on the recent US Geological Survey (USGS) evaluation of world oil resources and a simple, transparent model. However, it can be shown that this model is not consistent with actual oil production records in many different regions, particularly that of the US, from which it was derived. A more careful application of the EIA model, using the same resource estimates, indicates that at best non-OPEC oil production can increase for less than two decades, and should begin to decline at the latest sometime between 2015 and 2020. OPEC at this point will completely control the world oil market and will need to meet increased demand as well as compensate for declining production of non-OPEC producers. OPEC could control the market even sooner than this, given its larger share of proven oil reserves, probable difficulties in transforming non-OPEC undiscovered reserves into proven reserves, and the converging interests of all oil producers as reserves are depleted. This has significant implications for the world economy and for US national security.     

Earthquake-induced burial of archaeological sites along the southern Washington coast about A.D. 1700

Although inhabited by thousands of people when first reached by Europeans, the Pacific coast of southern Washington has little recognized evidence of prehistoric human occupation. This apparent contradiction may be explained partly by geologic evidence for coastal submergence during prehistoric earthquakes on the Cascadia subduction zone. Recently discovered archaeological sites, exposed in the banks of two tidal streams, show evidence for earthquake-induced submergence and consequent burial by intertidal mud about A.D. 1700. We surmise that, because of prehistoric earthquakes, other archaeological sites may now lie hidden beneath the surfaces of modern tidelands. Such burial of archaeological sites raises questions about the estimation of prehistoric human population densities along coasts subject to earthquake-induced submergence. © 1996 John Wiley & Sons, Inc.     

Stability of model performance and parameter values on two catchments facing changes in climatic conditions

Abstract

Hydrological models are often used for studying the hydrological effects of climate change; however, the stability of model performance and parameter values under changing climate conditions has seldom been evaluated and compared. In this study, three widely-used rainfall–runoff models, namely the SimHYD model, the HBV model and the Xin’anjiang model, are evaluated on two catchments subject to changing climate conditions. Evaluation is carried out with respect to the stability in their performance and parameter values in different calibration periods. The results show that (a) stability of model performance and parameter values depends on model structure as well as the climate of catchments, and the models with higher performance scores are more stable in changing conditions; (b) all the tested models perform better on a humid catchment than on an arid catchment; (c) parameter values are also more stable on a humid catchment than on an arid catchment; and (d) the differences in stability among models are somewhat larger in terms of model efficiency than in model parameter values.     

A Cromerian Complex stalagmite from the Mendip Hills,England

In order to provide a better chronological constraint on a British Middle Pleistocene interglacial, a large stalagmite boss from the Mendip Hills was selected for palaeoclimate data using pollen analysis. Dating analyses by thermal ionisation mass spectrometry (TIMS) of uranium–thorium ratios and by magnetostratigraphy constrain the age of the sample to 450–780 ka. The isotopic consistency of the TIMS analyses, plus the presence of luminescence laminations, suggest that the sample has been preserved under closed-system conditions. Pollen assemblages have been recovered from the speleothems, despite the fact that the pH of calcite deposition is usually greater than 7. Furthermore the evidence presented here indicates that the pollen was probably transported by the speleothem feedwater, rather than entering the cave aerially. The pollen record contained within the stalagmite is interpreted as early–mid-interglacial but does not have clear Cromerian affinity. © 1997 John Wiley & Sons, Ltd.     

Anisotropy in the continental upper mantle: possible evidence from explosion seismology

Summary. Peculiarities of propagation in the upper mantle of western Europe are documented by profiles of stations recording body waves generated by explosions. Azimuthal variations of travel times and amplitudes of P waves and possible birefringence of S waves may be associated with an anisotropic layer at depths where the lithosphere-asthenosphere transition is supposed to be.