The effect of erosion on palaeoclimatic and topographic corrections to heat flow

The increase of surface temperature following a recent period of glaciation lowers the heat flow at shallow depths. At greater depths, increase of heat flow above the steady-state value might be expected, reflecting lowered surface temperature during the glacial period itself. Thus a plot of the climatic perturbation to heat flow against depth should have a sigmate form. The fact that measurable negative and positive deviation in heat flow (measurable amplitude of the sigmate variation) has not been observed in some deep boreholes may be explained by taking into account the erosion which accompanies ice ages. Quite small amounts of erosion during the period of lowered surface temperature result in perturbations to heat flow which are nearly constant, or decrease smoothly with depth, damping the sigmate variation and rendering it undetectable. Measured heat flow in this situation is not strongly dependent on depth but is nevertheless significantly different from the steady-state value.Ignoring the effect of erosion on topographic corrections in regions which have been subjected to valley glaciation may result in large errors in heat flow estimates, particularly for shallow measurements, such as in lakes.     

Hyperbolic echo zones in the eastern Atlantic and the structure of the southern Madeira Rise

The Madeira Rise is a 450 km northeast-trending structural-sedimentary feature which lies west of the Madeira Islands. Its northern half is controlled by a basement ridge, but its southern section consists of an apparent current-controlled sediment deposit. Its maximum sediment thickness is about 1 km over a relatively level basement. There are two reflectors which can be traced within the sediment pile. A shallow reflector (R1) may mark the termination of a rapid constructional phase of the drift. Sediment cores taken on the southern Madeira Rise have recovered brown marls and chalks with sedimentation rates of only about 1.5 cm/1000 years over the last 225,000 years.A striking zone of hyperbolic reflectors mapped around the flank of the southern Madeira Rise in the 4000–4800 m depth range may be the expression of bedforms created by contour-following currents. Another zone of hyperbolic echoes is found on the continental rise at about 24–26°N in a similar depth range. Trend determinations in this area suggest that the bedforms which give rise to the hyperbolae are oriented north-northeast and may be similar to the abyssal furrows discovered by DEEP-TOW observations on the Blake-Bahama Outer Ridge.The bottom photographs available from both hyperbolic echo zones show a tranquil bottom. This suggests that the hyperbolic bedforms are relict.     

Characterization of sedimentary humic matter by alkaline hydrolysis

Humic matter fractions from modern sediments of Lake Huron and Lake Michigan have been compared. Large yields of saccharinic acids from alkaline hydrolysis suggest that these fractions contain large portions of carbohydrate materials. Evidence for contributions of aquatic lipid (C-16 fatty acids) and of liginin (phenolic acids) to these sediments is also present in the hydrolysis products. Qualitative differences among fulvic acid, humic acid and humin from the same lake are minor, suggesting common (or similar) organic sources for these fractions. The lability of sedimentary humic matter to alkaline hydrolysis is inversely related to its degree of exposure to oxidative weathering. Lability may also be related to diagenetic state as fulvic acids generally yield greater quantities of hydrolysis components than humic acids which in turn yield more than humin.     

Regional climate model simulations of daily maximum and minimum near-surface temperatures across Europe compared with observed station data 1961–1990

Simulations of the present-day temperature climate in Europe obtained with the dynamic regional climate model HadRM3P from the Hadley Centre are evaluated. Observed daily temperature maxima (T _x) and minima (T_n) for the 1961–1990 period at 185 stations are compared with their nearest corresponding HadRM3P grid-box data. The model generally performs well over the UK and elsewhere between latitudes 50 and 55°N, with biases mostly within ±0.5 K. In other areas coherent regions with seasonal biases up to more than ±5 K exist. In some areas, biases in climatological averages are associated with even larger errors (up to more than ±15 K) in the upper/lower extreme temperature range. Both areas with systematically overestimated and underestimated intra-seasonal daily temperature variances exist, but overestimation dominates. Too hot summer T _x south of about 45°N are associated with drying soils in the model. This problem may occur further north in future integrations with a greenhouse-gas induced warming. Given the existence of errors in the simulations of the present-day climate, we recommend that results from future scenario integrations are treated with care.     

The Neoproterozoic Fiq glaciation and its aftermath, Huqf supergroup of Oman

The <1.5‐km thick Fiq Member of the Ghadir Manqil Formation, Huqf Supergroup, Oman, contains a succession of Marinoan‐age glacially and non‐glacially influenced deposits overlain by a transgressive, ¹³C‐depleted, deep‐water dolostone (Hadash Formation) that deepens up into the marine shales and siltstones of the Masirah Bay Formation. The Fiq Member and Hadash–Masirah Bay Formations are well exposed in the core of the Jebel Akhdar of northern Oman and provide a valuable insight into the processes operating during a Neoproterozoic glacial epoch and its aftermath. The Fiq Member comprises seven stratigraphic units (F1–F7) of proximal and distal glacimarine, non‐glacial sediment gravity flow, and non‐glacial shallow marine facies associations. These units can be correlated over almost the entire Neoproterozoic outcrop belt (ca. 80 km) of the Jebel Akhdar. Four units contain glacimarine rainout diamictites, commonly at the top of cycles beneath strong lithofacies dislocations suggesting flooding. The units are thought to have been generated by combined glacio‐isostatic and glacio‐eustatic forcing caused by changing volumes of terrestrial glacier ice. The lateral persistence and thickness of massive diamictite units increase upwards in the stratigraphy, the youngest (F7) diamictite being abruptly overlain by the Hadash Formation. Correlation of lithofacies associations across the rift basin and palaeocurrents indicate that siliciclastic sediment and glacially entrained debris were derived from both basin margins. Open‐water conditions existed during interglacials, attested to by the presence of wave‐rippled sandstones in the western part of the basin. The Hadash carbonate also exhibits variations between east and west, showing that despite an overall deep‐water depositional setting, rift margin and intrabasinal structure continued to exert a control on facies development during the post‐glacial aftermath. Onlap of basin margins continued through the deposition of the Masirah Bay Formation. The sedimentology and stratigraphy of the Fiq Member and Hadash–Masirah Bay Formations have a number of implications for the Snowball Earth hypothesis. The overall stratigraphic evolution of the Fiq Member suggests a dynamic, temperate/polythermal style of glaciation, perhaps nucleated on uplifted continental or rift margin topography, with marine‐terminating glaciers. Some transgressions coupled to deglaciations within the Fiq glacial epoch were accompanied by minor deposition of carbonate. However, final deglaciation triggered the deposition of a <8‐m thick, deep‐water dolomite contaminated with siliciclastics, with a lithofacies assemblage still reflecting the underlying bathymetric template, followed by relatively deep marine shales and siltstones. The preservation of relatively deep marine Masirah Bay sediments above the Fiq basin margin suggests either tectonic collapse of the rift shoulder or, more likely, rapid eustatic rise accompanying deglaciation.     

Mechanisms of weathering of meteorites recovered from hot and cold deserts and the formation of phyllosilicates

Petrographic, mineralogical and chemical analysis of naturally weathered equilibrated ordinary chondrites collected from ‘ hot’ deserts and Antarctica has revealed striking similarities and also pronounced differences in weathering between the two environments. Terrestrial weathering in all meteorites studied is dominated by oxidation and hydration of Fe,Ni metal, producing Fe-oxides and oxyhydroxides that have partially replaced the metal grains and have also occluded primary intergranular pores to form veins. Troilite weathers readily in ‘ hot’ desert environments but undergoes very little alteration under Antarctic conditions. Most of the primary porosity of ordinary chondrites has been occluded by the time that ∼ 15 to 25% of the initial Fe⁰ and Fe²⁺ has been oxidised to Fe³⁺ in both environments. Results from modelling the volume changes upon alteration of primary minerals to a range of weathering products demonstrates that the primary porosity of most meteorites is sufficient to accommodate weathering products. Dilation of primary pores and brecciation, which has been observed in parts of some meteorites, will only occur if the meteorite is especially metal-rich, or has a low primary porosity. These weathering products are absent from recent falls but have formed in a fall after ∼ 100 yr of museum storage.Cl-bearing akaganéite and hibbingite are common weathering products in Antarctic finds but occur in abundance in only one ‘ hot’ desert meteorite, Daraj 014. The majority of Fe-rich weathering products in meteorites from both environments contain low, but variable concentrations of Si, Mg and Ca. In most meteorites a proportion of these elements are inferred to be present as a very finely crystalline mineral with a ∼ 1.0-nm lattice fringe spacing; where seen within intragranular fractures this mineral has a topotactic relationship with olivine and orthopyroxene. In the heavily-weathered Antarctic finds ALHA 78045 and 77002, Si is concentrated in cronstedtite, a Fe-rich phyllosilicate. An unidentified hydrous Si-Fe-Ni-Mg mineral or gel has also partially replaced taenite in ALHA 78045. In addition to Fe-rich weathering products, ‘ hot’ desert meteorites contain sulphates, Ca-carbonate and silica, whereas such minerals are largely absent from Antarctic finds. The abundance of silicate weathering products in Antarctic meteorites is unexpected and indicates that olivine and pyroxene undergo significant chemical weathering in these environments. As preterrestrial cronstedtite is abundant in CM2 carbonaceous chondrites, the Antarctic environment may be a powerful analog for aqueous alteration in the asteroidal parent bodies of primitive meteorites.     

Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896

DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa_17.5±2.1, [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (δ17O = +2.55, δ18O = +3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (∼10 vol%), typically in the form of corroded lithic fragments <100 μm in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of ∼80-100 GPa. The relatively young 3.704 ± 0.035 Ga 40Ar-39Ar crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history.Based on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of ∼1630°C under relatively slow cooling rates (∼10°C h⁻¹) to ∼1300°C, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes’ Law indicates a settling velocity >1 m h ⁻¹ during the first few hours of crystallization on asteroidal bodies of >25 km radius.The ∼3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid.The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in IIE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Δ17O = 0.77 ± 0.04) should be extended to IIEs to verify this possible petrogenetic link.     

On the Behavior of the Stable Boundary Layer and the Role of Initial Conditions

Previous studies of the stable atmospheric boundary layer using techniques of nonlinear dynamical systems (MCNIDER et al., 1995) have shown that the equations support multiple solutions in certain parameter spaces. When geostrophic speed is used as a bifurcation parameter, two stable equilibria are found—a warm solution corresponding to the high-wind regime where the surface layer of the atmosphere stays coupled to the outer layer, and a cold solution corresponding to the low-wind, decoupled case. Between the stable equilibria is an unstable region where multiple solutions exist. The bifurcation diagram is a classic S shape with the foldback region showing the multiple solutions. These studies were carried out using a simple two-layer model of the atmosphere with a fairly complete surface energy budget. This allowed the dynamical analysis to be carried out on a coupled set of four ordinary differential equations. The present paper extends this work by examining additional bifurcation parameters and, more importantly, analyzing a set of partial differential equations with full vertical dependence. Simple mathematical representations of classical problems in dynamical analysis often exhibit interesting behavior, such as multiple solutions, that is not retained in the behavior of more complete representations. In the present case the S-shaped bifurcation diagram remains with only slight variations from the two-layer model. For the parameter space in the foldback region, the evolution of the boundary layer may be dramatically affected by the initial conditions at sunset. An eigenvalue analysis carried out to determine whether the system might support pure limit-cycle behavior showed that purely complex eigenvalues are not found. Thus, any cyclic behavior is likely to be transient.