The case for human causes of increased atmospheric CH4 over the last 5000 years

We propose that humans significantly altered atmospheric CH₄ levels after 5000 years BP and that anthropogenic inputs just prior to the industrial revolution accounted for up to 25% of the CH₄ level of 725 ppb (parts per billion). We base this hypothesis on three arguments: (1) the 100 ppb increase in atmospheric CH₄ that occurred after 5000 years BP follows a pattern unprecedented in any prior orbitally driven change in the ice-core record; (2) non-anthropogenic explanations for this increase (expansion of boreal peat lands or tropical wetlands) are inconsistent with existing evidence; and (3) inefficient early rice farming is a quantitatively plausible means of producing anomalously large CH₄ inputs to the atmosphere prior to the industrial revolution. If the areas flooded for farming harbored abundant CH₄-producing weeds, disproportionately large amounts of CH₄ would have been produced in feeding relatively small pre-industrial populations.     

Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO₂), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl₂-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields '¹⁸O values from 8.3 to 14.5‰ (average=10.9ǃ.7‰ [1C], n=30), whereas '¹⁸O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0ǂ.9‰, n=12). Calculated '¹⁸O_H2O values for the dilute mineralizing fluid from B veins range from -2.6 to 0.2‰ (average=-0.8ǂ.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower '¹⁸O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from 'D_FI=-56 to -134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.     

Impact of historic land‐use change on sediment delivery to a Chesapeake Bay subestuarine delta

Historic land use in the Chesapeake Bay drainage basin induced large fluxes of fluvial sediment to subestuarine tributaries. Stratigraphic and palaeoecologic analyses of deltaic deposits may be used to infer changes on the landscape, but are not sufficient to quantify past sediment supply. When viewed as an inverse boundary‐value problem, reconstruction of the sediment supply function may be achieved by combining deltaic sedimentation chronologies with an equation governing delta progradation. We propose that the diffusion equation is appropriate for simulating delta progradation and obtaining the sediment supply function provided a suitable diffusion constant (D) can be determined. Three new methods for estimating D are presented for the case of estuarine deltas. When the inverse boundary‐value technique was applied to Otter Point Creek, a tidal freshwater delta at the head of Bush River in upper Chesapeake Bay, D values ranged from 3763 to 6199 m² a^?1. Delta growth simulations showed a 1740–1760 initial pulse, a 1760–1780 erosive/redistributive interval, a 1780–1920 growth period, and a 1920‐present erosive/redistributive era. Coupling of simulated delta elevations with an empirical plant habitat predictive equation allowed for comparison of predicted versus actual relative habitat areas. Also, the model yielded reconstructed watershed erosion rates and stream suspended sediment concentrations that could be useful for development of water quality regulations. Copyright © 2001 John Wiley & Sons, Ltd.     

The factors influencing the abrasion efficiency of saltating grains on a clay‐crusted playa

The entrainment and subsequent transport of PM₁₀ (particulate matter <10 µm) has become an important and challenging focus of research for both scientific and practical applications. Arid and semi‐arid environments are important sources for the atmospheric loading of PM₁₀, although the emission of this material is often limited by surface crusts. It has been suggested that the primary mechanisms through which PM₁₀ is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities. To examine the importance of saltation abrasion in the emission of PM₁₀, a series of field wind tunnel tests were conducted on a clay‐crusted surface near Desert Wells, Arizona. In a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate, although there can be considerable variation in this relationship. This paper more closely examines the source of the variability in the abrasion efficiency, the amount of PM₁₀ emitted by a given quantity of saltating grains. The abrasion efficiency was found to vary with the susceptibility of the surface to abrasion, the ability of the sand to abrade that surface and the availability of material with a caliper size <10 µm within the crust. Specifically, the results of the study show that the abrasion efficiency is related to the crust strength, the amount of surface disturbance and the velocity of the saltating grains. It is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models, and can make theoretical models too complex and impractical to be of use. Copyright­© 2001 John Wiley & Sons, Ltd.     

Stochastic processes of soil production and transport: erosion rates,topographic variation and cosmogenic nuclides in the Oregon Coast Range

Landscapes in areas of active uplift and erosion can only remain soil‐mantled if the local production of soil equals or exceeds the local erosion rate. The soil production rate varies with soil depth, hence local variation in soil depth may provide clues about spatial variation in erosion rates. If uplift and the consequent erosion rates are sufficiently uniform in space and time, then there will be tendency toward equilibrium landforms shaped by the erosional processes. Soil mantle thickness would adjust such that soil production matched the erosion. Previous work in the Oregon Coast Range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield. Here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported. We quantify how soil production varies with soil thickness in the southern Oregon Coast Range and explore further the issue of landscape equilibrium. Apparent soil production is determined to be an inverse exponential function of soil depth, with a maximum inferred production rate of 268 m Ma^?1 occurring under zero soil depth. This rate depends, however, on the degree of weathering of the underlying bedrock. The stochastic and large‐scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth; the spatial variation of soil depth neither supports nor rejects equilibrium morphology. Our observed catchment‐averaged erosion rate of 117 m Ma^?1 is, however, similar to that estimated for the region by others, and to soil production rates under thin and intermediate soils typical for the steep ridges. We suggest that portions of the Oregon Coast Range may be eroding at roughly the same rate, but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle. Copyright © 2001 John Wiley & Sons, Ltd.     

Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

In the arid zone of central Turkana, north-western Kenya, where soil salinity affects 15–20% of the rangelands, growth performances of trees planted in saline soil rehabilitation trials have not been evaluated. Tree-planting trials have emphasised exotic species over indigenous ones. However, advantages and disadvantages of promoting exotic tree species have not been examined. The current study was aimed at evaluating growth performance of seven exotic and nine indigenous tree species used in saline soil rehabilitation trials. The tree species were established from 6-month-old saplings using microcatchments (FT1) from 1988 through 1990 and pitting treatment (FT2) from 1989 through 1992. The soils in FT1 and FT2 treatments were moderately to highly saline. The exotic tree species produced greater cover and volume during the first year (FT1) but by the second year, production was not sustained due to greater mortality (FT1 & FT2). The indigenous species in general had higher survival rates. Relative growth rates (RGR) of exotic and indigenous species did not differ (FT1 & FT2). Tree mortality was negatively correlated with RGR for exotic species in FT1 but not for indigenous ones. However, changes in plant performance were not in response to salinity alone. Rather, water scarcity superimposed on soil salinity might have influenced plant growth performance. Greater water and salinity stress and subsequently greater mortality in exotic species provided a more convincing reason for promotion of indigenous tree species. In the future, knowledge of salinity distribution and selection of indigenous species to match this will be a better way of rehabilitating sites affected by soil salinity in the arid zone of central Turkana, north-western Kenya.     

Spatial Ecology of Pre –Euro‐American Fires in a Southern Rocky Mountain Subalpine Forest Landscape

Natural disturbances such as fires have been widely studied, but less is known about their spatial ecology than about other aspects of them. We reconstructed and mapped pre–Euro‐American fire history in a subalpine forest landscape in southeastern Wyoming, and analyzed the fires using GIS. Mean fire interval varies little with topography (elevation, aspect, slope) and is spatially autocorrelated at distances of at least 2 km. Fires often spread downslope, and spread more than expected from the north and south and less than expected from the west, under the influence of particular synoptic climatic conditions. The landscape of 1868 a.d., at the time of Euro‐American settlement, was strongly influenced by fires. However, it contained large patches of connected forest and few high‐contrast edges, unlike the modern landscape, which is fragmented by industrial forestry and roads. The spatial ecology of the natural fire regime may be a useful guide for management.     

C NMR, micro-FTIR and fluorescence spectra, and pyrolysis-gas chromatograms of coalified foliage of late Carboniferous medullosan seed ferns, Nova Scotia, Canada: Implications for coalification and chemotaxonomy

The cuticles and cuticle-free compressions of three Carboniferous medullosan seed-fern leaf species (Macroneuropteris scheuchzeri, Neuropteris ovata var. simonii and Alethopteris lesquereuxii) were analyzed by elemental, ¹³C nuclear magnetic resonance (NMR), micro-FTIR (Fourier transform infrared) and coal petrographic techniques. The 13C NMR spectra of the cuticle-free compressions and the associated whole coal (high volatile A/B bituminous coal rank) are generally similar and consist of a large aromatic carbon peak, a smaller aliphatic carbon peak and a shoulder on the aromatic peak, representing phenolic carbons. In contrast, the ¹³C NMR spectra of the cuticles from the same leaves have a predominant peak for aliphatic carbons and a much smaller aromatic carbon peak. This difference in aromaticity between the cuticles and the cuticle-free compressions is also reflected in the higher atomic H/C ratios of the cuticles. Micro-FTIR spectra of the cuticles show oxygenated functional groups (carboxyl and ketone) similar to those in modern cuticles but their most characteristic feature is very strong bands in the aliphatic stretching region. The cuticle-free compressions (mainly vitrinite), in turn, show the absence or significant reduction in oxygenated functional groups, reduction in aliphatic stretching bands and, usually, increased absorbance of aromatic out-of-plane deformation in the 700–900 cm⁻¹ region. Fluorescence spectra for the cuticles from all three species show a great similarity with a λ_max at 580–590 nm, probably reflecting a similardegree of coalification, which is consistent with the similar vitrinite reflectance (R_r) and H/C and O/C ratios of the cuticlefree compressions.These results indicate that leaf cuticle-free compressions, which were initially cellulose rich ( 90% cellulose and hemicellulose, < 10% lignin), can alter, during peatification and coalification, to a macromolecular structure similar to that of coalified wood (initially 50% cellulose and hemicellulose, 30%–50% lignin). Thus, a lignin-enriched structure is not a prerequisite for the formation of the macromolecular structure of vitrinite. In addition, the micro-FTIR spectra reveal the complexity of the molecular structure in coalified seed-fern leaves. The micro-FFIR data reveal some significant differences among the cuticles that may be of chemotaxonomic value. Clearly, a combination of macro- and micro-techniques offers a better basis for the interpretation of the molecular structure of pre-macerals and their alteration during peatification and coalification. Also, the data presented in this paper provide important new information that extends the data from morphological and cuticular taxonomic studies of some seed ferns. The data are encouraging preliminary advances in the chemotaxonomy of medullosan seed fern species.Pyrolysis-gas chromatography (PY-GC) data for the cuticles of three seed-fern leaves indicate distinct chemical signatures for the two neuropterid leaves as compared to the Alethopteris leaf. This perhaps indicates a chemotaxomic factor, or it could be related to the greater thickness of the cuticle of Alethopteris. Mass spectrometric data are needed to identify individual components in the PY-GC chromatograms.     

Palaeomagnetism of the monzonite porphyry from Milton,New South Wales

Summary Directions of natural remanent magnetization aftermagnetic cleaning of specimens from monzonite porphyry at Milton are given. Thermal and alternating magnetic field stability tests indicate that the resultant direction is that of the earth's magnetic field at the time of cooling. Comprison of the pole position calculated from this result with other pole positions from rocks of known age confirms the probable Permian age of the intrusion and the wide divergence of Australian from European and North American pole positions of this age.