Present-day Measurements Research Articles

Quantitative estimates of the effect of climate change on dust storm activity in Australia during the Last Glacial Maximum

Quantitative estimates are made of the effect of climate change upon dust activity during the Last Glacial Maximum (LGM), which peaked 18,000 years ago, using present-day measurements of the occurrence of dust storms recalculated in the light of information on LGM climatic change. Dust activity was increased by 57% in northeast Australia and by 52% in the southeast, in relation to present day. Dust storm seasons were lengthened and dust paths were modified. Estimates based upon climate alone, without considering the increased supplies of sediment to dust source areas during the LGM, however, probably underestimate LGM dust activity.

From Newton's Moon to Einstein's Moon

Three centuries ago Isaac Newton and his contemporaries sought to understand the details of lunar motion in terms of Newton's theory of gravity. Today, our most precise confirmations of key aspects of Albert Einstein's relativistic theory of gravity are achieved by measuring features of the Moon's orbit to 1-centimeter precision using a technique known as lunar laser ranging (LLR). These present-day measurements address some of the same questions posed by Newton. Do Earth and the Moon (figure 1) fall toward the Sun at the same rate? What, in detail, produces the precession of the lunar orbit's major axis?

Isostatic postglacial rebound over Fennoscandia with a self-gravitating spherical visco-elastic Earth model

Accounting for isostasy in glaciological models has always been a necessity but these models mostly use very simple parameterizations (Le Meur and Huybrechts, 1996). The need for more realistic isostatic parameterizations rapidly became apparent, especially in the treatment of bedrock-sensitive issues such as the grounding-line migration (Huybrechts, 1990a, b). To this end, a rather sophisticated Earth model, avoiding most of the common assumptions, has been developed and is presented here. The two key groups of parameters, to which the model is most sensitive, are the Earth properties and the rheological law used for the mantle. The aim of this paper is first to justify the use of Maxwell rheology for the mantle and then to tune the most sensitive Earth parameter, namely the mantle viscosity, in order to match the numerous present-day uplift data over Fennoscandia. So, in the first instance, a short review of the different available rheologies is presented and discussed. The visco-elastic theory, as well as the mathematical background used in the present model, is also briefly sketched. Secondly, a glacial scenario over Fennoscandia served as an input for the model in a calibration test on the mantle-viscosity values. The degree of agreement of the model outputs with the present-day measurements gives a reference set of Green functions, to which one can reasonably refer when modelling the isostatic response over areas where such a control is not possible (Le Meur and Huybrechts, 1996). Finally, a closer look to the time-dependent surface displacements will confirm the ability for the model to reproduce correctly the main postglacial rebound characteristic features.

Isostatic postglacial rebound over Fennoscandia with a self-gravitating spherical visco-elastic Earth model

Accounting for isostasy in glaciological models has always been a necessity but these models mostly use very simple parameterizations (Le Meur and Huybrechts, 1996). The need for more realistic isostatic parameterizations rapidly became apparent, especially in the treatment of bedrock-sensitive issues such as the grounding-line migration (Huybrechts, 1990a, b). To this end, a rather sophisticated Earth model, avoiding most of the common assumptions, has been developed and is presented here. The two key groups of parameters, to which the model is most sensitive, are the Earth properties and the rheological law used for the mantle. The aim of this paper is first to justify the use of Maxwell rheology for the mantle and then to tune the most sensitive Earth parameter, namely the mantle viscosity, in order to match the numerous present-day uplift data over Fennoscandia. So, in the first instance, a short review of the different available rheologies is presented and discussed. The visco-elastic theory, as well as the mathematical background used in the present model, is also briefly sketched. Secondly, a glacial scenario over Fennoscandia served as an input for the model in a calibration test on the mantle-viscosity values. The degree of agreement of the model outputs with the present-day measurements gives a reference set of Green functions, to which one can reasonably refer when modelling the isostatic response over areas where such a control is not possible (Le Meur and Huybrechts, 1996). Finally, a closer look to the time-dependent surface displacements will confirm the ability for the model to reproduce correctly the main postglacial rebound characteristic features.

Historical data of surface ozone at tropical latitudes

During the last decades of the 19th century day-time and night-time data of surface ozone were collected by means of the Schönbein method at the coastal observatories of Rio de Janeiro, Brazil (23°S) and Manila, Philippines (14°N) and at the mountain observatory of Oaxaca de Juarez, Mexico (17°N, 1574 m a.s.l.). Taking into account the uncertainties of the method, daily readings have been converted to ppbv concentrations by a procedure already applied to other series of historical data. The calculated time-averaged values show that 1 century ago the ozone levels at tropical latitudes were considerably lower than at mid-latitudes of the northern and southern hemispheres, similar to the present-day measurements.

Thermobarometry of hydrothermal alteration in the Los Azufres geothermal system (Michoacan, Mexico): Significance of fluid-inclusion data

Fluid inclusions were analysed systematically in samples from different wells and depths of the Los Azufres (Mexico) active hydrothermal system. Fluids were studied either in authigenic minerals (primary inclusions) in quartz, calcite or anhydrite, or in magmatic minerals (secondary inclusions in microcracks). The fluids are aqueous and have low salinity, ∼1 eq wt.% NaCl. They were trapped in P- T conditions very close to that fixed by the liquid-vapor equilibrium curve. Fluid pressure is lower than the calculated hydrostatic one assuming a liquid-dominated system, because of the existence of a two-phase vapor-dominated zone at a depth between 500 and 1500 m. Temperature was estimated also by a good number of techniques. Present-day distribution of temperature is given by direct measurements in the wells by Kuster® equipment. Downhole temperature may be estimated using different dissolved species geothermometers and the chemical composition of the produced fluids. The present-day measurements above the downhole give in general lower estimates than those obtained by other means. However, agreement between downhole temperature, chemical geothermometer estimates and fluid-inclusion data is relatively good.

Evaluation of a Model of Long-Term Response of Catchments to Atmospheric Deposition of Sulfate

The delivery of acid rainfall to catchments raises questions about potential acidification of soils and surface waters. A prime concern in making postdictions and predictions about such effects is how catchments store and release sulfate, a major strong acid anion associated with acid rain. Processes considered to have an important role in sufate dynamics in a catchment are mineral weathering, sulfate adsorption in soils and biological uptake. All of these processes can have large time constants. Thus, the unavailability of long time series of accurate measurements makes model identification and parameter estimation problematic. We present a relatively simple model of sulfate dynamics for a catchment, use a regionalized sensitivity analysis procedure to select appropriate ranges of parameter values and examine prediction uncertainties due to errors in both the specification of historical deposition sequences and the model parameters. Results show that a linear combination of parameters explains almost all of the ability of the model to reproduce the distribution of present-day measurements of stream sulfate concentrations in a section of the Shenandoah National Park in Virginia. We also infer that uncertainties in inputs are less important than are variations in physical parameters to obtaining acceptable model results

Cumulative deformation in the barnes ice cap and implications for the development of foliation

In recent years, it has become apparent that the development of foliation and crystallographic fabric in glacier ice is strongly dependent upon cumulative deformation. However, because glaciers lack suitable strain markers and past velocity fields are unknown, the nature of the cumulative deformation is not well understood. Part of the Barnes Ice Cap, Baffin Island N.W.T., Canada has undergone little overall change in position in the past 2000 years, so a long-term steady-state velocity field can be estimated from present-day measurements in a section in which flow is essentially two-dimensional. By means of a numerical model, particle paths, isochrons, strain rates, and cumulative strains are computed for this section. There is no correspondence between the principal directions of strain rate and cumulative strain in most of the ice, the greatest divergence of about 45° being near the base of the glacier where the maximum cumulative extension direction, X, is sub-horizontal. All ice particles initially undergo a restricted amount of pure shear, and then pass gradually into a long domain of nearly simple shear. They return to a second short stage of nearly pure shear near the margin. The effects of the early stage of pure shear are never eliminated, but the cumulative strain is increasingly dominated by simple shear down-glacier. Cumulative strain magnitude, magnitude of strain rate, and age of the ice all increase rapidly with depth, and change much more slowly with distance down-glacier. We argue that foliation is formed during flow by modification of primary inhomogeneities in ice. The three most important inhomogeneities are probably sedimentary layering, crevasse infillings, and irregularities such as ice glands or lenses. During deformation, the first will behave as isochrons, the second will rotate as material planes, initially perpendicular to the isochrons, and the third will be stretched parallel to the X-direction. The numerical model indicates that the isochrons, planes representing crevasse infillings, X-direction, and particle paths are within 1° of mutual parallelism in the most distal ice. Thus a single subhorizontal foliation, dipping slightly more steeply up-glacier than the flow lines, and curving upwards near the margin would be produced in this case. This corresponds with observations.

Studies of fresh and fossil tracks in meteoritic hypersthene

From measurements of the total recordable lengths of fossil tracks and freshly-produced Fe ion tracks in meteoritic hypersthene, the abundance of Fe relative to all VH ions (20 ≦ Z ≦ 28) in the time-averaged cosmic-ray flux is found to be ≈ 0.35. After allowance is made for the effects of fragmentation of nuclei at a depth of ≈ 11 cm within the meteorite, this value is found to be in broad agreement with presentday measurements. These crystals have been irradiated with a number of heavy ions with a view to calibrating them for the identification of VVH cosmic rays by measurement of track-etch velocity, V T , as a function of residual range. A good fit to the calibration data is obtained by using a value for the constant K in the primary-ionization equation of 10.5 (±0.5). The relationship between V T and primary ionization J appears to be approximately linear.

Cosmic ray tracks in the Shalka meteorite

Track densities of heavy cosmic rays have been studied in hypersthene crystals taken from seven known depths below a fusion crust in the Shalka meteorite. By comparing these with a theoretical model, a preatmospheric radius of 20 ± 2cm has been estimated for the meteorite, assuming it to have been originally spherical. The fossil track-length distributions show a prominent peak in the region of 12–15 μm; and this has been attributed, on the basis of accelerated-ion calibration experiments, to cosmic ray Fe ions. The value (≥0.38 ± 0.04) of relative abundance of Fe ions to the total Fe-group ions (20 ≤ Z. ≤ 28) found in the cosmic ray flux averaged over the last ∼19 m.y. is shown to be broadly similar to those given by present-day measurements.

Astronomical alinements in Britain, Egypt and Peru

Photogrammetric air surveys have been made of Stonehenge, the Great Temple of Amon-Re, Karnak, and the desert lines near Nasca, Peru. The latter designs have been correlated in time with pottery of type Nasca 3 and 4. New astronomical alinements have been found for the trilithons and station stones at Stonehenge. The Amon-Re temple alined precisely, within the limits of present-day measurements, with the rising of the Sun at midwinter at the time of rebuilding of the structure by Tuthmosis III. No significant astronomical alinements were found for the desert lines. The possible significance of the British and Egyptian alinements are discussed together with the implications of the negative result in Peru. Some comments are made concerning the precision of the megalithic yard, and the geometries and stellar alinements of prehistoric British structures.