A global context for megadroughts in monsoon Asia during the past millennium

Proxy reconstructions of precipitation from central India, north-central China, and southern Vietnam reveal a series of monsoon droughts during the mid 14th–15th centuries that each lasted for several years to decades. These monsoon megadroughts have no analog during the instrumental period. They occurred in the context of widespread thermal and hydrologic climate anomalies marking the onset of the Little Ice Age (LIA) and appear to have played a major role in shaping significant regional societal changes at that time. New tree ring-width based reconstructions of monsoon variability suggest episodic and widespread reoccurrences of monsoon megadroughts continued throughout the LIA. Although the El-Niño Southern Oscillation (ENSO) plays an important role in monsoon variability, there is no conclusive evidence to suggest that these megadroughts were associated with anomalous sea surface temperature anomalies that were solely the result of ENSO-like variability in the tropical Pacific. Instead, the causative mechanisms of these megadroughts may reside in protracted changes in the synoptic-scale monsoon climatology of the Indian Ocean. Today, the intra-seasonal monsoon variability is dominated by ‘active’ and the ‘break’ spells – two distinct oscillatory modes of monsoon that have radically different synoptic scale circulation and precipitation patterns. We suggest that protracted locking of the monsoon into the “break-dominated” mode – a mode that favors reduced precipitation over the Indian sub-continent and SE Asia and enhanced precipitation over the equatorial Indian Ocean, may have caused these exceptional droughts. Impetus for periodic locking of the monsoon into this mode may have been provided by cooler temperatures at the extratropical latitudes in the Northern Hemisphere which forced the mean position of the Inter-Tropical Convergence Zone (ITCZ) further southward in the Indian Ocean.     

Clinoform deposition across a boundary between orogenic front and foredeep – an example from the Lower Cretaceous in Arctic Alaska

The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post‐tectonic strata in the Colville foreland basin. The formation includes basin‐floor fan, marine‐slope and fan‐delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge‐front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge‐front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load‐induced flexural subsidence of the foreland basin.     

Detrital zircon geochronology of Palaeozoic Novaya Zemlya – a key to understanding the basement of the Barents Shelf

The Novaya Zemlya fold‐and‐thrust‐belt is the northern continuation of the late Palaeozoic Uralide Orogen. Little is known about its deeper structure and the basement history of the adjacent Barents and Kara shelves. Based on geological evidence and detrital zircon analysis of 28 samples from the northeastern and stratigraphically deepest part of the archipelago, we demonstrate that Cambro‐Ordovician turbidite‐dominated deposition was almost exclusively sourced from rocks consolidated during the Timanian orogeny (Timanian basement). A profound change in provenance occurred near the end of the Ordovician. Over 90% of the zircons from Silurian and about 80% from Devonian strata have ages characteristic of the Sveconorwegian Orogen, implying uplift of these rocks in the vicinity of Novaya Zemlya. The presence of Sveconorwegian and Grenvillian rocks in the high Arctic suggests revision of recent reconstructions of the Rodinia supercontinent, its break‐up and subsequent Caledonian orogeny.     

The stratigraphical signature of the Anthropocene in England and its wider context

The Anthropocene deposits of England, here regarded as those formed after ～1950 CE, are now extensive, take various forms, and may be characterized and recognized by a number of stratigraphic signals, such as artificial radionuclides, pesticide residues, microplastics, enhanced fly ash levels, concrete fragments and a novel variety of ‘technofossils’ and neobiotic species. They include the uppermost parts of both ‘natural’ deposits such as the sediment layers formed in lakes and estuaries, and more directly human-made or human-influenced ones such as landfill deposits and the ‘artificial ground’ beneath urban areas and around major constructions. ‘Negative deposits’ include the worked areas of quarries and regions such as the English Fenland, where thick peat deposits have ablated to leave a strongly modified underlying landscape, and extend beneath into the subterranean realm as mine workings, metro systems and boreholes. The production of these is still rapidly increasing and evolving in character, while the early signs of global change, such as warming, sea level rise, and modifications to biotic assemblages, are beginning to further modify the emerging geology of this new phase of Earth history.     

Possible effects of pre-existing basement topography on thrust fault ramping

Finite-element models show that one way in which thrust ramps may arise is through the mechanical interaction between basement and overlying sediments. In the simplest case, shear coupling between a planar basement—sediment contact causes the differential stresses in the sediments to die out with depth and distance from the applied load. For such cases, curved thrust faults may result if the strength of the rock is exceeded. Basement topography may also affect the location and shape of ramps by acting as a stress concentrator, by producing a stress shadow and by changing principal stress orientations. Modeling suggests that whether or not these basement topographic features cause ramping will depend on the height and angularity of the feature as well as the rock types that overlie it.Under the assumption of linear elasticity and for given boundary conditions, the Poisson's ratio plays an important role in determining the orientation and magnitude of the principal stresses. Calculations using experimentally measured Poisson's ratios predict that the earliest maximum compressive stress directions should be nearly vertical in the more cratonward portions of thrust belts. However, the stress directions which are inferred to have occurred earliest in this part of thrust belts are nearly horizontal. This suggests that non-elastic or ductile processes have an effect on the propagation of thrust faults.     

Petrographic constraints on models of intergranular pressure solution in quartzose sandstones

Petrographic and SEM observations on 478 samples of six quartzose sandstones provide a data base that can be used to evaluate the role of intergranular pressure solution (IPS) in sandstone diagenesis and to constrain predictive models of the pressure solution process. SEM examination of grain contacts that have experienced pressure solution suggests that IPS occurs at the interior portions of contacts where the greatest stress is concentrated and that granulation of quartz grains at points of contact may contribute to the process. The chemical compaction fabrics that result from IPS suggest that the process is most commonly induced by effective lithostatic stress and that the resulting strain is uniaxial.Numerous geological variables influence IPS. Grain size exerts a fundamental influence, with finer grained samples experiencing more IPS than coarser grained samples. On both local and regional scales, IPS is inhibited by poor sorting, an abundance of ductile grains, the presence of “shallow” cement, slow rates of shallow burial, and overpressured conditions. In contrast, IPS is enhanced by the presence of illite grain coatings, increased maximum burial depth, rapid rates of shallow burial, longer times spent at great depths, higher temperatures, and high volumes and rates of fluid flow.Silica budgets indicate that some of the analyzed sandstones approximate mass balance whereas others have exported silica. Calculations of fluid flow requirements indicate that advanced stages of IPS are favored by high volume, high velocity fluid flow. Such flow can occur as a result of uplift of basin margins, which is typical of foreland and intractonic basins.     

Volatiles H2O,CO2, and Cl in a subduction related basalt

The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H₂O and CO₂ in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K₂O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO₂ from 51 to 54 wt.% SiO₂. The concentration of H₂O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H₂O in these inclusions. The gas saturation pressures for pure H₂O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H₂O)(Pa)=(SiO₂−48.5 wt.%) × 1.45 × 10⁷. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.     

A fast method for the solution of a system of simultaneous linear equations—A method adapted to a particular problem

Mathematical Geosciences - In mineral deposit evaluation, the estimation of block values can involve the solution of as many as 100,000 systems of linear equations. To minimize computing costs it...     

Changes in the content and composition of pedogenic iron oxyhydroxides in a chronosequence of soils in southern California

Studies of the pedogenic iron oxyhydroxides in suites of latest Holocene to middle Pleistocene soils formed on fluvial deposits of the transverse ranges, southern California, indicate that the content and composition of iron oxyhydroxide change in a systematic manner. Analysis of total secondary free iron oxides (dithionite extractable, Fe₂O₃d) and ferrihydrite (oxalate extractable, Fe₂O₃o) shows that (1) a single-logarithmic model (Y = a + b log X) or double logarithmic model (log Y = a + b log X), where Y is the total mass of pedogenic Fe oxides (g/cm²-soil column) and X is soil age, describes the rate of increase in Fe₂O₃d with time; (2) the Fe₂O₃d content correlates linearly with soil reddening and clay content; (3) the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio, which indicates the degree of Fe oxide crystallinity, is moderately high to very high (0.22–0.58) in middle Holocene to latest Pleistocene soils and progressively decreases to less than 0.10 in older soils; (4) the value of the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio also appears to be infuenced by climate; and (5) temporal changes in Fe oxide content and mineralogy are accompanied by related, systematic changes in clay mineralogy and organic matter content. These relationships are attributed to a soil environment that must initially favor ferrihydrite precipitation and/or organic matter-Fe complexation. Subsequent transformation to hematite causes increasingly intense reddening and a concomitant decrease in the $\text{Fe}{}_2\text{O}{}_3\text{o}\text{Fe}{}_2\text{O}{}_3\text{d}$ ratio. The results demonstrate that iron oxide analysis is useful for numerical age studies of noncalcic soils and shows potential as an indicator of paleoclimates.