ALBERT

Description: The narrow region of soil around roots, the so-called rhizosphere, defers in its hydraulic properties from the bulk soil. The rhizosphere hydraulic properties primarily depend on the drying and wetting rate of mucilage, a polymeric gel exuded by plant roots. Under equilibrium conditions mucilage increases the water holding capacity. Upon drying mucilage turns hydrophobic and makes the rhizosphere temporarily water repellent. There are several models of root water uptake, from analytical models of water flow to a single root to complex numerical models that consider the root architecture. Most of these models, however, do not account for the specific hydraulic properties of the rhizosphere. Here we describe a single-root model that includes the altered hydraulic properties of the rhizosphere due to mucilage exudation. We use the model to reproduce existing experiments reporting unexpected and puzzling hysteresis in the rhizosphere, which could not be explained under the assumption of homogeneous hydraulic properties. In our model the hydraulic properties depend on the concentration of mucilage. This enables a continuous transition from the bulk soil to the root surface. We assumed that: (a) mucilage increases the water holding capacity in equilibrium conditions, (b) hydrophobicity, swelling and shrinking of mucilage cause a non-equilibrium relation between water content and water potential and (c) mucilage reduces the mobility of water molecules in the liquid phase resulting in a lower hydraulic conductivity at a given water content. Our model reproduces well the experiments and suggests that mucilage softens drought stress in plants during severe drying events. This article is protected by copyright. All rights reserved.

Description: Abstract The occurrence of high‐pressure (HP) garnet‐bearing metamorphic rocks in the southern Barberton granitoid–greenstone terrane (BGGT), South Africa, has been proposed as a key indicator of the onset of modern plate tectonics at ca. 3.2 Ga. Here, we report new zircon/titanite U–Pb ages of garnet‐bearing HP metamorphic rocks and associated granitoids in the BGGT that argue against such an interpretation. The results show that HP metamorphism occurred synchronously with granitoid magmatism in the Stolzburg domain, the supposed subducted plate, during two episodes at 3.4 and 3.2 Ga, and that these two episodes of magmatism occurred on both sides of the assumed suture zone, a feature that cannot easily be explained by the subduction model. In contrast, the coupled magmatism and metamorphism probably resulted from partial convective overturn (PCO), another viable mechanism for production and differentiation of continental crust during early Archean.

Description: The flow of water from soil to plant roots is controlled by the properties of the narrow region of soil close to the roots, the rhizosphere. In particular, the hydraulic properties of the rhizosphere are altered by mucilage, a polymeric gel exuded by the roots. In this paper we present experimental results and a conceptual model of water flow in unsaturated soils mixed with mucilage. A central hypothesis of the model is that the different drying/wetting rate of mucilage compared to the bulk soil results in non-equilibrium relations between water content and water potential in the rhizosphere. We coupled this non-equilibrium relation with the Richards equation and obtained a constitutive equation for water flow in soil and mucilage. To test the model assumptions, we measured the water retention curve and the saturated hydraulic conductivity of sandy soil mixed with mucilage from chia seeds. Additionally, we used neutron radiography to image water content in a layer of soil mixed with mucilage during drying and wetting cycles. The radiographs demonstrated the occurrence of non-equilibrium water dynamics in the soil-mucilage mixture. The experiments were simulated by numerically solving the non-equilibrium model. Our study provides conceptual and experimental evidences that mucilage has a strong impact on soil water dynamics. During drying, mucilage maintains a greater soil water content for an extended time, while during irrigation it delays the soil re-wetting. We postulate that mucilage exudation by roots attenuates plant water stress by modulating water content dynamics in the rhizosphere.

Description: A section of the orogenic middle crust (Orlica-Śnieżnik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D 1 ), a subsequent subvertical foliation S2 (D 2 ), and a late subhorizontal axial planar cleavage S3 (D 3 ). The synfom was intruded by pre-, syn- and post-D 2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I 1−2 garnet−staurolite growth, syn-D 2 staurolite breakdown to garnet−biotite−sillimanite/andalusite, I 2−3 cordierite blastesis and late-D 3 chlorite growth. Garnet porphyroblasts show a linear Mn−Ca decrease from the core to the inner rim, a zone of alternating Ca−Y- and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca−Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P−T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from ~4 kbar/485 °C to ~4.75 kbar/575 °C during I 1−2 . This was followed by a progression to 4−5 kbar/580−625 °C and a subsequent pressure decrease to 3−4 kbar during D 2 . Pressure decrease below 3 kbar is ascribed to I 2−3 , whereas cooling below ~500 °C occurred during D 3 . In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. Since Lu-rich garnet rims dominate the Lu−Hf budget, the 344±3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I 1−2 . For the dated sample of amphibole−biotite granitoid sheet, a Pb−Pb single zircon evaporation age of 353±1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D 1 , and a Carboniferous burial-exhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D 2 . In the light of published ages, a succession of telescoping stages with time spans decreasing from c . 10 to 2−3 Ma is proposed. The initially long period of tectonic quiescence (I 1−2 phase, c .10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P−T−D histories within the orogenic wedge. An elevated gradient of ~30 °C km −1 and assumed high heating rates of ~20 °C Ma −1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2−3 Ma, D 2 phase) in the crust require the activity of deformation phases. This article is protected by copyright. All rights reserved.

Description: Here we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS-determined mean U-Pb radiogenic isotopic ratios of 0.084743 ± 0.000027 for 206 Pb/ 238 U and 0.67676 ± 0.00023 for 207 Pb/ 235 U (weighted means, 2 s uncertainties). Its 206 Pb/ 238 U age of 524.36 ± 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The δ 18 O value (determined by laser fluorination) is 8.26 ± 0.06‰ VSMOW (2 s ), and the mean 176 Hf/ 177 Hf ratio (determined by solution ICP-MS) is 0.282396 ± 0.000004 (2 s ). The SIMS-determined δ 7 Li value is -0.6 ± 0.9‰ (2 s ), with a mean mass fraction of 1.0 ± 0.1 μg g −1 Li (2 s ). Zircon M127 contains ~ 923 μg g −1 U. The moderate degree of radiation damage corresponds well with the time-integrated self-irradiation dose of 1.82 × 10 18 alpha events per gram. This observation, and the (U-Th)/He age of 426 ± 7 Ma (2 s ), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U-Pb ages by means of SIMS analysis in combination with hafnium and stable-isotope (oxygen and potentially also lithium) analysis. This article is protected by copyright. All rights reserved.

Description: Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical and zircon U–Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mts. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U–Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian–Ordovician and culminating at ca . 500 Ma. We propose that the Ordovician succession forms an ‘Altai sedimentary wedge’, the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (early Cambrian) to the development of the ‘Ikh-Mongol Magmatic Arc System’ and the giant ‘Altai sedimentary wedge’ above a north-dipping subduction zone (Late Cambrian–Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro–Ordovician geodynamic evolution of the Mongolian collage system.

Description: Decadal predictions by Earth system models aim to capture the state and phase of the climate several years in advance. Atmosphere-ocean interaction plays an important role for such climate forecasts. While short-term weather forecasts represent an initial value problem and long-term climate projections represent a boundary condition problem, the decadal climate prediction falls in-between these two timescales. In recent years, more precise initialization techniques of coupled Earth system models and increased ensemble sizes have improved decadal predictions. However, climate models in general start losing the initialized signal and its predictive skill from one forecast year to the next. Here we show that the climate prediction skill of an Earth system model can be improved by a shift of the ocean state towards the ensemble mean of its individual members at seasonal intervals. We found that this procedure, called ensemble dispersion filter, results in more accurate results than the standard decadal prediction. Global mean and regional temperature, precipitation, and winter cyclone predictions show an increased skill up to 5 years ahead. Furthermore, the novel technique outperforms predictions with larger ensembles and higher resolution. Our results demonstrate how decadal climate predictions benefit from ocean ensemble dispersion filtering towards the ensemble mean.