Late Pleistocene paleoclimates of North America as inferred from stable isotope studies of speleothems

Some speleothems (CaCO₃ cave deposits) can be demonstrated to have been formed in oxygen isotopic equilibrium with their parent seepage waters and thus a record of relative fluctuations in depositional temperature can be obtained from the δ¹⁸O variations in successive growth layers of such deposits. These temperature fluctuations reflect variations in the average annual air temperature at the surface above the cave, and therefore permit inference of past continental climate changes. Equilibrium deposits have been obtained from caves in San Luis Potosi, Bermuda, Kentucky, West Virginia, Iowa, and Alberta, ranging in age from 200,000 years BP to the present, as determined by ${}^{230}\text{Th}{}^{234}\text{>U}$ dating of the speleothems. The δ¹⁸O time curves for the six sites show the following synchronous climatic fluctuations: warm periods from 190 to 165 and from 120 to 100 Ka, at 60 and 10 Ka, and cold periods from 95 to 65 and from 55 to 20 Ka. The periods of thermal maxima correspond in time to the interglacial periods of the marine foraminiferal isotopic and faunal temperature records and to periods of high sea stand as observed from radiometric dating of raised coral reefs. Maxima and minima in insolation appear to be synchronous with this record as well.     

Gibbs energy of aluminous minerals

The discrepancy between the tabulated Gibbs Energies of Formation for Al₂SiO₅ and corundum relative to muscovite and kaolinite is considered to lie principally with the latter two minerals. New values for heat of formation of gibbsite [Gbs] will affect the tabulated H _f ⁰ , G _f(298,1) ⁰ for the other aluminous minerals which are referred to gibbsite as calorimetric aluminum reference. Gibbs Energy Difference Functions, calculated from phase equilibria in the system CaO-Al₂O₃-SiO₂-(H₂O-CO₂), can be used to estimate consistent H _f ⁰ , G _f(298,1) ⁰ values for aluminous minerals. A self consistent data set is presented referred to G _f(298,1) ⁰ [Corundum]=–378.08 kcal mol^–1. Two independent values for G _f(298,1) ⁰ [Anorthite]=–961.52 and –960.29 kcal, from a recalculation of the H _f ⁰ [Anor] based upon the revised H _f(298,1) ⁰ [Gbs]=–309.325 kcal mol^–1 and from measurement of silica activity on the anorthite-saturated part of the CaO-Al₂O₃-SiO₂ liquidus, respectively, are considered to show the magnitude of the discrepancy and are used in the calculations.     

Is upper-mantle phosphorus contained in sodic garnet?

Garnets crystallized experimentally from within the anhydrous melting ranges of an olivine tholeiite, a tholeiitic andesite and an augite leucitite at pressures between 18 and 45 kbars contain up to 0.4% Na₂O and 0.6% P₂O₅. The Na and P are thought to form a substitution couple, replacing Ca and Si in the garnet structure; representing limited solid solution between grossular (Ca₃Al₂Si₃O₁₂) and the phosphate Na₃Al₂P₃O₁₂. This substitution is enhanced by increasing pressure and by falling temperature (increasing degree of crystallization) at constant pressure.Current knowledge of the crystalline site of P in the upper mantle is hampered by lack of data on the stability of apatite and other phosphates at appropriate pressures and temperatures. If all samples of garnetiferous upper mantle brought to the surface by magmatic processes have been depleted to some extent by previous escape of a partial-melt fraction, P₂O₅ concentrations below 0.1% in their garnets could nevertheless signify that this phase was the sole predepletion host for P in the upper mantle, at the depths from which such inclusions are derived. If garnet and apatite are the principal minerals containing P in the upper mantle, it may be possible to use covariances between P and rare-earth elements in mafic liquids to detect which of these phases was the dominant host for P at the site of magma genesis. This approach confirms the widely-held opinion that strongly alkalic mafic magmas are products of upper-mantle partial fusion in the presence of residual garnet. It also leads to a contrasting proposal that mid-ocean ridge basalts may be generated by upper-mantle partial fusion at comparatively small depths, in the presence of residual apatite.     

Palaeomagnetic secular variation studies of Finnish lake sediment and the carriers of remanence

Palaeomagnetic and mineral magnetism measurements have been carried out on two cores from Lake Vuokonjarvi in Finnish Karelia. The sediment probably covers 5000 years of continuous deposition at a mean sedimentation rate of about 0.8 mm/yr.The magnetic declination exhibits fluctuations of similar amplitude(～20°) and character to those recorded in northern England and northern Ireland. Magnetic inclination variations are of higher amplitude(～15°) than those found in Britain. Matching the palaeomagnetic patterns with the dated British master curves permits an estimate of the rate of deposition of the Finnish sediments, which is suggested to be more reliable than estimates from radiocarbon dating of the Vuokonjarvi sediment.The stable natural remanence is shown to be carried by fine-grained magnetite and titanomagnetite grains and to have grown by post-depositional alignment during a period of the order of 100 years. Laboratory dehydration of the sediment results in loss of around 40% of the stable natural remanence. Such behaviour is also found in lake sediments from central and southern Europe and should be considered in interpreting palaeomagnetic data from dried out lake sections and ocean cores.     

The question of climatic stability in America before 1900

Belief that regional climates had significantly changed in historic times, possibly as a result of human activities, is firmly established in the Western tradition. Europe itself was believed to have undergone such changes, especially a warming trend, since ancient times. It was also widely believed that the European colonization of North America would duplicate many of the changes that had occurred in Europe, including the climatic changes that were supposed to have taken place. In the New World, it was at first believed that climatic change was occurring, as a result of human settlement and land use changes, but these views gave way to the idea of climatic stability. The first extensive compilations of reliable North American precipitation and temperature data in the latter part of the nineteenth century seemed to support the notion of stable climates. Ideas relating to macroclimatic change and stability that were entertained in America before 1900, mainly between 1770 and 1870, are examined.     

A note on point process analysis of Chinese earthquake data

In the analysis of Chinese earthquake history byLee andBrillinger (1979), the autointensity function of the earthquake sequence is estimated, based on a parametric model for the catalog inclusion probabilities. We show that an estimate of the autointensity function with essentially the same features can be obtained by smoothing the data to yield a nonparametric estimate of the inclusion probabilities. In addition, however, we illustrate that a somewhat less smooth estimate would lead to an autointensity function with little structure. The shape of the estimated autointensity function is hence dependent on the assumed smoothness of the inclusion probabilities. This is an issue which cannot be resolved statistically.     

Low-frequency shear and structural relaxation in rhyolite melt

The frequency-dependence of the shear viscosity and modulus of rhyolite melt has been determined over a 10–14 log₁₀ Pa s viscosity range and a 0.03–63 rad s^–1 angular frequency range. The frequency-dependent viscosity determined at high frequencies is 5 orders of magnitude lower than the Newtonian viscosity. At lower frequencies, a frequency-independent viscosity identical with the Newtonian viscosity is observed. The measured shear modulus increases from zero to 30.5 ± 2.5 GPa with increasing frequency. The viscoelastic regime consists of a maximum in viscous loss centered on the Max-well relaxation time. The width and height of the loss modulus as a function of frequency is inconsistent with a single relaxation-time. The frequency-dependent shear modulus is best described by a distribution of relaxation-times with a sharp cutoff at times slightly longer than the Maxwell relaxation time, and a long tail at shorter times extending up to 5 orders of magnitude less than the Maxwell relaxation time. This distribution of relaxation-times is in contrast with the single-relaxation-time behavior observed in low viscosity silicate melts.