A magnetic investigation of a Late Quaternary loess/palaeosol record in Siberia

A Late Quaternary loess/palaeosol sequence at Kurtak in the Yenisey River valley, southern Siberia, has been studied magnetically. The 34  m section (340 samples) exhibits variations in magnetic susceptibility which can be correlated with oxygen isotope stages 1–7. A detailed sampling of stage 5 (a further 209 samples) permits the identification of substages 5a–e. The susceptibility variations themselves are in the opposite sense to that found in the classic sections of the Chinese Loess Plateau, but are in agreement with the 'wind-intensity' model which has been put forward to explain similar findings in loess sequences in Alaska. Published results for the susceptibility of magnetite imply volume fractions of ≈0.2 per cent in glacial stages 2 and 4, dropping to ≈0.05 per cent in stages 1, 3 and 5. These fluctuations match the aeolian flux variations observed in core V21–146 from the north Pacific. Even though the warmer intervals are characterized by lower susceptibilities, the observed frequency dependence of susceptibility indicates that new magnetic material is produced as a result of pedogenesis during interglacial and interstadial times.     

Warming at 18,000 yr B.P. in the Tropical Andes

Palynological, paleopedological, and glacial geomorphological evidence from the area of the high plain of Bogotá in the Colombian Eastern Cordillera indicates a significant climatic warming around 18,000¹⁴C yr B.P. Comparison of dated vegetation changes, pedogenic episodes, and glacier fluctuations provides the basis for defining the so-called “La Laguna Interstadial” that lasted from 19,500 to 17,000 yr B.P. During this interstadial period, mean annual temperatures in the tropical Andes were up to 4°C higher than during the preceding and following stadial periods, when full-glacial conditions prevailed and temperatures were up to 8°C colder than at present.     

Deep crustal extensional faulting in the Ivrea Zone of Northern Italy

The Ivrea Zone in Northern Italy consists of amphibolite and granulite facies metasedimentary, metabasic, and ultrabasic rocks, and is thought to represent a section through the deep continental crust. Detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones, which, after the effects of later low temperature faulting and folding have been removed, appears to accommodate extension of the sequence. The shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks, and locally show evidence for development under prograde conditions. These observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting. In this model the effects of shear heating and displacement of cool hanging-wall rocks against hotter footwall rocks are calculated.

These observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust, especially when basic rocks are the dominant components of the rock pile. A restored section through the extended lower crust as observed in the Ivrea Zone is presented, and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed.     

Correlation and interpretation of paleosols and loess across European Russia and Asia over the last interglacial-glacial cycle

Loess-paleosol sequences of the last interglacial-glacial cycle are correlated from European Russia to central Siberia and the Chinese Loess Plateau. During cold periods represented by marine oxygen isotope stages (OIS) 2 and 4, loess deposition dominated in the Russian Plain and the Loess Plateau. In central Siberia, loess deposition took place also, but five to seven thin, weakly developed paleosols are identified in both stages. OIS 3, in the Chinese Loess Plateau near Yangchang, consists of a loess bed that is flanked by two weakly developed paleosols. At Kurtak, Siberia, OIS 3 is represented by two distinct, stacked paleosols with no loess bed separating the paleosols. In the Russian Plain, OIS 3 consists of a single, possibly welded paleosol, representing upper and lower stage-3 climates. Brunisols and Chernozems dominate the profiles in China and Siberia, whereas Regosols, Luvisols, and Chernozems are evident in the northern and southern Russian Plain, respectively. OIS 5 is represented in China and the Russian Plain by pedo complexes in a series of welded soils, whereas in contrast, the Kurtak site consists of six paleosols with interbedded loess. The paleosols consist largely of Brunisols and Chernozems. Although the three areas examined have different climates, geographical settings, and loess source areas, they all had similar climate changes during the last interglacial-glacial cycle.     

Stratigraphy and geochemistry of a sequence of Quaternary palaeosols in the Lower Teleki Valley,Mount Kenya,East Africa — Implications for interregional correlations

The relative weathering of a sequence of five palaesols formed in different parent materials, including bedrock of phonolitic composition, is described and assessed. The three lower palaeosols (units 3–5) are considered to be of interglacial origin. The saprolite (unit 5), in phonolite, formed prior to deposition of the lower Brunhes-age till, is considered to be as old as the Brunhes Matuyama boundary (0.73 myr). The other two palaeosols in this group (units 3 and 4), derived from loess and till, respectively, formed over middle to late Brunhes time under palaeoclimates that were wetter (and presumably warmer), and of sufficient leaching power to move clay, as well as organically complexed extractable Fe and Al. The palaeosols in unit 3 are more complex than the lower ones. Their upper and lower horizons have about the same chemical composition as the lowest two palaeosols, while the middle horizons are similar to the upper two palaeosols (units 1 and 2). Radiocarbon dates for unit 3 are considered to be too young to account for the strong weathering in the upper and lower horizons. This interpretation is supported by high D/L ratios of aspartic acid. Overlying sediments, emplaced by episodic colluvial (unit 2) and slope wash (unit 1) processes, are less weathered; the lower colluvial unit in this group was emplaced during the late stade of the last glaciation. The surface sediments were emplaced more recently during the same interval and weathered mainly during the post-glacial period. Tentative correlations with core 82PCS18 from the Canary Basin show that upper unit 3 is probably equivalent to Isotope Stage 5e, lower unit 3 to Isotope Stage 7, unit 4 to Isotope Stage 8, and unit 5 to Isotope Stage 9. Units 1 and 2 are firmly correlated with Isotope Stage 2.     

Thermally-induced grain growth of calcite marbles on Naxos Island,Greece

The island of Naxos is composed of an elliptically shaped structural and thermal dome of Miocene age. Peak metamorphic temperatures within the central migmatite complex exceeded 700° C, decreasing to about 300° C at the most distant exposures on the island. Equigranular calcite marbles which outcrop together with metapelites and metabasites over the whole island show a systematic pattern of increasing grain-size towards the central migmatite complex, with a significant discontinuity in the pattern corresponding approximately with the 500° C isotherm. The microstructures and grain-size distributions in the marbles are consistent with normal grain-growth. The variation of grain-size with peak temperature attained can be explained equally well by the assumptions that (a) a maximum grainsize had developed, particularly at higher temperatures, or that (b) the grain-size had been frozen-in by a combination of cooling and coarsening, both of which combine to reduce the rate of grain-growth.The grain-size data do not impose strong constraints on the mechanism of transport of heat responsible for the metamorphism, whether by conduction or by advection, but the 500° C discontinuity indicates that the tectonothermal history of the migmatitic core and its envelope of metasediments were different.     

Desert margins near the Chinese Loess Plateau during the mid-Holocene and at the Last Glacial Maximum: a model–data intercomparison

Expansion and contraction of desert margins around the globe have been inferred from a variety of proxy data and have since been linked, particularly in northern China and in the sub-Sahel, to changes in freshwater flux, vegetation cover, sea surface temperatures and, perhaps most importantly, monsoon circulations. We present a direct comparison of results from numerical general circulation model experiments for the mid-Holocene and for the Last Glacial Maximum (LGM) with the climatic conditions that have been inferred from loess–paleosol sequences taken from the Chinese Loess Plateau.During the mid-Holocene in northern China, the northwestward migration of the southeast desert margin that has been suggested by grain size analysis is also expressed in the model results. There is a statistically significant wetting of the Plateau region, and increased soil moisture is a consequence of an enhanced summer monsoon whose latent heat release deepens the cyclonic Tibetan low and brings increased low-level convergence and precipitation to the area. North of the desert region, this circulation dries the soil through enhanced atmospheric subsidence, although the northern margin of the desert does not migrate significantly.Expansion of the desert margin toward the southeast at the LGM is small, but there is a statistically significant drying of the Plateau. The local hydrological cycle is reduced, and there is an increase in large-scale atmospheric subsidence over the region that is caused by the presence of the Fennoscandian ice sheet upwind. Model results therefore suggest that, in addition to local micro- and mesoclimatic conditions, regional effects, such as monsoon circulations and distal orography, are also important factors in determining the location of desert margins.     

On the structural age of the Rhoscolyn antiform,Anglesey, North Wales

In the Rhoscolyn area of Anglesey, the late Precambrian interbedded psammites and pelites of the Monian Supergroup are folded into a kilometre‐scale antiform, plunging about 25°NE and with an axial surface dipping about 40°NW. Numerous folds of up to a few tens of metres in wavelength are present on both limbs of this antiform. These smaller‐scale folds also plunge about 25°NE but clearly belong to two separate episodes of folding, and it has become a matter of longstanding controversy as to whether the larger antiform belongs to the first or second of these episodes. Close examination of the cleavage/bedding asymmetries from all the lithologies, however, shows that the large antiform is a second‐generation structure, and that on the gently dipping northwest limb, the sense of cleavage/bedding asymmetry of the earlier cleavage in the psammitic units has been almost uniformly and homogeneously reversed (so that it appears to be axial planar to the antiform), while in the pelitic units the sense of cleavage/bedding asymmetry of the earlier cleavage has been preserved. Many of the small‐scale complexities of the observed cleavage/bedding relationships may be explained by appealing to differences in the timing of the formation of buckling instabilities relative to this reorientation of the early cleavage in the psammites during the second deformation. A first‐order analysis of the finite strains from around the large‐scale antiform shows that the orientation of the first cleavage prior to the second deformation was steeply dipping to the southeast. The second deformation correlates with the southeast‐verging Caledonian deformation affecting the Monian and Ordovician units elsewhere in northwest Anglesey, while the northwest‐verging first deformation event, which is not present in the Ordovician rocks, must have occurred before they were deposited. Copyright © 2004 John Wiley & Sons, Ltd.     

On the mechanical properties of synthetic kaolinite/quartz fault gouge

The mechanical properties and microstructures of mixed kaolinite/ quartz fault gouge have been studied by means of triaxial tests, wherein a 1 mm thickness of fault gouge was sheared between rigid, sintered alumina sliders. Test conditions ranged up to 200 MPa confining pressure, 175 MPa pore water pressure and temperatures to 600°C. Constant displacement rate tests were carried out at 5×10^-4 mm s^-1, and the stress relaxation technique was used to access a wide range of lower displacement rates down to 10^-10 mm s^-1. The effects of different ratios of clay: quartz were studied, and some experiments were cafried out using crushed Tennessee sandstone gouge and pre-faulted cylinders of Tennessee sandstone. Deformation-induced microstructures were studied by optical, scanning and transmission electron microscopy, and mineral chemistry changes were identified using EDS analysis in the STEM. The most important factor determining the strength of these gouges is effective confining pressure, and the effective stress law was obeyed at all test conditions. Increasing the proportion of quartz in the gouge slightly increases the strength, as does increase of temperature. Under all test conditions the gouges deformed by strain-hardening stable sliding. At 2 mm displacement the friction coefficient is similar to that for rock and rock sliding (μ= 0.85). Over wide ranges of strain-rate and temperature the strength of these gouges is little affected if the effective pressure is high, but strength drops rapidly with decreasing strain-rate if effective pressure is low (or if pore water pressure is high). Reloading of the gouge after stress relaxation at high temperatures demonstrated hardening arising from hydrothermal cementation during stress relaxation, which led to sudden rupture of the cemented gouge. This phenomenon may be responsible for cyclic rupture of natural faults. New mineral phases were produced in these experiments only at 600°C. At 400°C, anticipated production of pyrophyllite from kaolinite plus quartz was not found, but recrystallization of kaolinite is believed to have occurred and evidence of healing of cracks in quartz was seen. Microstructural studies revealed the ubiquitous development of P and R₁ foliations and other features, which are identical to those found in natural clay-bearing fault zones. Microstructural evolution is associated with the strain-hardening, and apparent hardening is strongly associated with the stress path during loading.     

Data‐driven modelling of shortwave radiation transfer to snow through boreal birch and conifer canopies

Hydrological and land surface models require simple but accurate methods to predict the solar radiation transmitted through vegetation to snow, backed up by direct comparisons to data. Twenty shortwave pyranometers were deployed in forest plots of varying canopy structures and densities in sparse birch forest near Abisko, Sweden, in spring 2011 and mixed conifer forest near Sodankylä, Finland, in spring 2012. Above‐canopy global and diffuse shortwave irradiances were also measured. These data were used to test a model that uses hemispherical photographs to explicitly estimate both diffuse radiation and direct beam transmission, as well as two models that apply bulk canopy parameters and versions of Beers Law. All three models predict canopy shortwave transmission similarly well for leafless birch forest, but for conifers, the bulk methods perform poorly. In addition, an existing model of multiple reflections between canopy and snow was found to be suitable for birch, but not conifers. A new bulk approach based on empirical relationships with hemisphere‐averaged sky view fraction showed improved performance for both sites; this suggests benefits of avoiding the use of plant area index calculated from optical methods, which can introduce errors. Furthermore, tests using common empirical diffuse radiation models were shown to underestimate shortwave transmission by up to 7% relative to using the data, suggesting that new diffuse models are required for high latitudes. Copyright © 2013 John Wiley & Sons, Ltd.