Breaking it Down: Mechanical Processes in the Weathering Engine

Elements ◽  
2019 ◽  
Vol 15 (4) ◽  
pp. 247-252 ◽  
Author(s):  
Suzanne P. Anderson
Keyword(s):  
Chemical Weathering ◽  
Break Rock ◽  
Slow Process ◽  
Near Surface ◽  
Mineral Alteration ◽  
Lens Growth ◽  
Mechanical Weathering ◽  
Long Timescales ◽  
Subcritical Cracking ◽  
Applied Stresses

The vast diversity of landscapes found on Earth results from interplay between processes that break rock down, produce mobile regolith, and transport materials away. Mechanical weathering is fundamental to shaping landscapes, yet it is perhaps less understood at a mechanistic level than chemical weathering. Ubiquitous microfractures in rock propagate and grow through a slow process known as subcritical cracking that operates at the low applied stresses common in the near-surface. Subcritical cracking is the most likely explanation for the mechanical processes associated with thermal stress, ice lens growth, mineral alteration, and root growth. The long timescales over which critical zone architectures develop require an understanding of slow processes, such as subcritical cracking.

Chemical Weathering on Venus

2019 ◽  
Author(s):  
Mikhail Zolotov
Keyword(s):  
Chemical Weathering ◽  
Uncertain Data ◽  
Chemical Kinetic ◽  
Gas Content ◽  
Atmospheric Gases ◽  
Emission Data ◽  
Near Surface ◽  
Fe Oxides ◽  
Basaltic Composition ◽  
Surface Materials

Chemical and phase compositions of the surface of Venus could reflect a history of gas–rock and fluid–rock interactions, recent and past climate changes, and a loss of water from the Earth’s sister planet. The concept of chemical weathering on Venus through gas–solid type reactions was established in the early 1960s after the discovery of the hot and dense CO2-rich atmosphere of the planet, inferred from Earth-based and Mariner 2 radio emission data. Initial models suggested carbonation, hydration, and oxidation of exposed igneous rocks and a control (buffering) of atmospheric gases by solid–gas type chemical equilibria in the near-surface rocks. Carbonates, phyllosilicates and Fe oxides were considered likely secondary minerals. From the late 1970s onward, measurements of trace gases in the sub-cloud atmosphere by the Pioneer Venus and Venera entry probes and by Earth-based infrared spectroscopy challenged the likelihood of hydration and carbonation. The atmospheric H2O gas content appeared to be low enough to allow the stable existence of H2O-bearing and a majority of OH-bearing minerals. The concentration of SO2 gas was too high to allow the stability of Ca-rich carbonates and silicates with respect to sulfatization to CaSO4. In the 1980s, the detection of an elevated bulk S content at the Venera and Vega landing sites suggested ongoing consumption of atmospheric SO2 to surface sulfates. The supposed composition of the near-surface atmosphere implied oxidation of ferrous minerals to Fe oxides, magnetite and hematite, consistent with the infrared reflectance of surface materials. The likelihood of sulfatization and oxidation has been illustrated in modeling experiments in simulated Venus’ conditions. The morphology of Venus’ surface suggests contact of atmospheric gases with hot surface materials of mainly basaltic composition during the several hundreds of millions years since a global volcanic/tectonic resurfacing. Some exposed materials could have reacted at higher and lower temperatures in a presence of diverse gases at different altitudinal, volcanic, impact, and atmospheric settings. On highly deformed tessera terrains, more ancient rocks of unknown composition may reflect interactions with putative water-rich atmospheres and even aqueous solutions. Geological formations rich in salt, carbonate, Fe oxide, or silica will indicate past aqueous processes. The apparent diversity of affected solids, surface temperatures, pressures, and gas/fluid compositions throughout Venus’ history implies multiple signs of chemical alterations that remain to be investigated. The current understanding of chemical weathering is limited by the uncertain composition of the deep atmosphere, by the lack of direct data on the phase and chemical composition of surface materials, and by the uncertain data on thermodynamics of minerals and their solid solutions. In preparation for further atmospheric entry probe and lander missions, rock alteration could be investigated through chemical kinetic experiments and calculations of solid-gas/fluid equilibria to constrain past and present processes.

Exploring the 36Cl/Cl input in arid environments: New insights gained by 81Kr groundwater dating in the Negev Desert, Israel

2021 ◽  
Author(s):  
Roi Ram ◽  
Roland Purtschert ◽  
Christof Vockenhuber ◽  
Reika Yokochi ◽  
Eilon M. Adar ◽  
...  
Keyword(s):  
Land Surface ◽  
Noble Gas ◽  
Climatic Conditions ◽  
Arid Environments ◽  
Near Surface ◽  
Spatial Differences ◽  
Practical Tool ◽  
Groundwater Dating ◽  
Long Timescales

<p>   <sup>36</sup>Cl and <sup>81</sup>Kr (half-lives of 301 and 229 kyr, respectively) are among a very few age tracers with dating capabilities in the 10<sup>4</sup>–10<sup>6</sup> yr timescale. Although widely applied since the 1980s in various hydrological studies, the <sup>36</sup>Cl/Cl system has been found complex as an effective dating tool. In contrast, <sup>81</sup>Kr has become a practical tool only recently and is considered to be an ideal dating tool due to the inert properties of the noble gas. In the present study, simultaneous measurements of both radioisotopes were used to assess the <sup>36</sup>Cl/Cl input ratios and the Cl<sup>-</sup> content for paleorecharge into the deep, transboundary Nubian Sandstone Aquifer (NSA) which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel).</p><p>   By means of <sup>81</sup>Kr data, reconstructed Cl<sup>-</sup> content of recharge that occurred during the late Pleistocene was found to be 300–400 mg/L with an initial <sup>36</sup>Cl/Cl ratio of 50 × 10<sup>-15</sup>. This latter value is in agreement with the <sup>36</sup>Cl/Cl ratio in recent local rainwater, indicating constancy over prolonged periods with possible variable climatic conditions. This similarity in values suggests a process that is rather insensitive to atmospheric <sup>36</sup>Cl fallout rates. Erosion and weathering of near-surface materials in the desert environment could dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. This near-surface Cl<sup>-</sup> reservoir integrates various sources and processes, including marine and terrestrial Cl<sup>-</sup>, cosmogenic <sup>36</sup>Cl fallout, and cosmogenic <sup>36</sup>Cl production in the shallow unsaturated zone, all of which are active over long timescales and accumulate on the land surface and in the epigene zone.  Spatial differences in the reconstructed initial <sup>36</sup>Cl/Cl ratio are attributed to differences in the mineral aerosol sources for specific recharge areas of the NSA. The results of this study highlight the potential of integrating <sup>81</sup>Kr age information in evaluating the initial <sup>36</sup>Cl/Cl and Cl<sup>-</sup> input, which is essential for the calibration of <sup>36</sup>Cl radioisotope as a long-term dating tool for a given basin.</p>

Mineralogy and origin of the upper Eastend and Whitemud Formations of south-central and southwestern Saskatchewan and southeastern Alberta

1969 ◽  
Vol 6 (2) ◽  
pp. 317-334 ◽  
Author(s):  
P. N. Byers
Keyword(s):  
Chemical Weathering ◽  
Upper Cretaceous ◽  
Volcanic Rocks ◽  
Source Area ◽  
Heavy Minerals ◽  
Major Constituent ◽  
Abrupt Decrease ◽  
South Central ◽  
Mechanical Weathering ◽  
Kaolinitic Clay

The Upper Cretaceous non-marine Whitemud Formation of south-central and southwestern Saskatchewan and southeastern Alberta consists of kaolinitic, metamorphic lithic sands and silts, and kaolinitic clays. The sands and silts are not highly feldspathic as was originally thought. The major constituent is metamorphic lithic grains with minor kaolinitic clay and vermicular kaolin, clear angular quartz, chert, muscovite, and minor volcanic lithic grains and feldspar. The upper part of the Upper Cretaceous Eastend Formation, which conformably underlies the Whitemud Formation, consists of non-marine sands, silts, and clays. Kaolin is very rare. The bulk of the sands are composed of volcanic lithic grains with minor metamorphic lithic grains, clear angular quartz, chert, feldspar, muscovite, and biotite.The contact is characterized by the following changes from the Eastend Formation upward into the Whitemud Formation: an abrupt decrease in volcanic lithic grains and increase in metamorphic lithic grains; the appearance of kaolin and the disappearance of biotite and apatite; a slight increase in clear angular quartz and muscovite and a decrease in feldspar; a general increase in metamorphic heavy minerals; and an increase in the percentage of ilmenite (both as solitary grains and intergrown with magnetite), which is altered to leucoxene.On the basis of mineralogy, the Whitemud Formation is definitely a correlative of the Colgate Member of the Fox Hills Formation in Montana and North Dakota.The upper Eastend and Whitemud Formations were derived from Upper Cretaceous volcanic rocks, Precambrian and Paleozoic metamorphic rocks, and Paleozoic carbonates all situated in Montana. Upper Eastend sediments represent fast mechanical weathering of mountains of freshly extruded volcanic rocks, whereas the Whitemud sediments represent slow chemical weathering and leaching, which predominated once the mountainous volcanic rocks were worn down. This deep chemical weathering altered the volcanic tuffs and flows into kaolinitic clay at the source area; the kaolin of the Whitemud Formation is not derived from the weathering of feldspars at the site of deposition.It is suggested that the Frenchman and Ravenscrag Formations were also derived from Upper Cretaceous and Lower Tertiary volcanic rocks in Montana.

scholarly journals Mechanical weathering and rock erosion by climate-dependent subcritical cracking

2017 ◽  
Vol 55 (2) ◽  
pp. 470-508 ◽  
Author(s):  
Martha-Cary Eppes ◽  
Russell Keanini
Keyword(s):  
Mechanical Weathering ◽  
Subcritical Cracking

Weathering regime and geochemical conditions in a polar desert environment, Haughton impact structure region, Devon Island, Canada

2008 ◽  
Vol 45 (10) ◽  
pp. 1139-1157 ◽  
Author(s):  
Denis Lacelle ◽  
Véronique Juneau ◽  
André Pellerin ◽  
Bernard Lauriol ◽  
Ian D. Clark
Keyword(s):  
Active Layer ◽  
Chemical Weathering ◽  
Surface Sediments ◽  
Impact Structure ◽  
Desert Environment ◽  
Polar Desert ◽  
Devon Island ◽  
Mechanical Weathering ◽  
Thermal Dilation ◽  
The Impact

This study examines the physical and geochemical properties of near-surface sediments, as well as the geochemical and stable O–H–C isotope composition of (ground)surface waters in and around the Haughton impact structure region (Devon Island, Nunavut) to determine the types of weathering (mechanical, (bio)chemical) and their relative contribution in this polar desert environment. The surface sediments collected from the Allen Bay and Thumb Mountain formations surrounding the impact crater are dominated by sand-sized particles; whereas the impact melt breccias inside the crater have a greater abundance of silt-sized particles. The subsurface sediments in the Allen Bay formation show a near equal amount of sand- and silt-sized particles. However, the micromorphologies of the sand-sized particles collected at the surface revealed that these grains, irrespective of the local geology, were heavily fractured. By contrast, fractures and rounded pits are observed on the surface of the sand grains located within the active layer; whereas those located just above the permafrost table have only rounded cavities on their surface. The (ground)waters also show variations in their solute concentration with depth; the highest concentrations being found in the groundwaters near the top of permafrost. Taken together, these observations suggest that there is a progressive evolution from a mechanically dominated weathering regime near the surface, to increasing chemical weathering with depth. The transition from mechanical weathering near the surface to increasing chemical weathering with depth can be attributed to the decreasing frequency and intensity of mechanical weathering processes (i.e., frost action, wetting–drying, thermal dilation) with depth, and to the presence of permafrost, which allows a greater availability of water for chemical aqueous reactions at the base of the active layer.

scholarly journals Biosphere studies supporting the disposal system safety case in the UK

2012 ◽  
Vol 76 (8) ◽  
pp. 3225-3232 ◽  
Author(s):  
R. C. Walke ◽  
M. C. Thorne ◽  
S. Norris
Keyword(s):  
Global Climate ◽  
Radioactive Wastes ◽  
Catchment Scale ◽  
System Safety ◽  
Near Surface ◽  
Geological Disposal ◽  
Safety Case ◽  
The Uk ◽  
Long Timescales ◽  
Surface Environment

AbstractHigher activity radioactive wastes remain hazardous for extremely long timescales, of up to hundreds of thousands of years. Disposing of such wastes deep underground presents the internationally accepted best solution for isolating them from the surface environment on associated timescales. Geological disposal programmes need to assess potential releases from such facilities on long timescales to inform siting and design decisions and to help build confidence that they will provide an adequate degree of safety. Assessments of geological disposal include consideration of the wastes, the engineered facility, the host geology and the surface and near-surface environment including the biosphere. This paper presents an overview of recent post-closure biosphere assessment studies undertaken in support of the Nuclear Decommissioning Authority Radioactive Waste Management Directorate disposal system safety case for geological disposal of the United Kingdom's higher activity radioactive wastes. Recent biosphere studies have included: (1) ensuring that the United Kingdom's approach to consideration of the biosphere in safety case studies continues to be fit for purpose, irrespective of which site or sites are considered in the United Kingdom's geological disposal programme; (2) updating projections of global climate and sea level, together with consideration of the potential importance of transitions between climate states; (3) considering geosphere–biosphere interface issues and their representation, including redox modelling and catchment-scale hydrological modelling; and (4) identifying key radionuclides and developing a series of reports describing their behaviour in the biosphere together with an evaluation of associated implications for post-closure assessment calculations.

scholarly journals Lithium and Sr isotopic composition of salar deposits in the Central Andes across space and time: the Salar de Pozuelos, Argentina

2021 ◽  
Author(s):  
Anette Meixner ◽  
Ricardo N. Alonso ◽  
Friedrich Lucassen ◽  
Laura Korte ◽  
Simone A. Kasemann
Keyword(s):  
Source Rock ◽  
Chemical Weathering ◽  
Isotope Composition ◽  
Central Andes ◽  
Hydrothermal Systems ◽  
Source Rocks ◽  
Surface Chemical ◽  
Near Surface ◽  
Magmatic Arc ◽  
Puna Plateau

AbstractThe Central Andes of South America host the largest known lithium resources in a confined area, but the primary lithium sources of the salar deposits and the mobilisation process of lithium are still a matter of speculation. Chemical weathering at or near the surface and leaching in hydrothermal systems of the active magmatic arc are considered the two main mechanisms of Li extraction from the source rock. The lithium and strontium isotope composition of typical salar deposits offer insights into the processes on how Li brine deposits in Andean evaporites are formed. Data from the Salar de Pozuelos indicate near-surface chemical weathering in a cold and dry climate as the dominant mobilisation process of Li, with evaporation being responsible for the enrichment. The Cenozoic ignimbrites are the favoured source rock for the Li, with subordinate additions from the Palaeozoic basement. The identification of the source rocks is supported by radiogenic Nd and Pb and stable B isotope data from salar deposits. A comparison with other Li brine and salt deposits in the Altiplano-Puna Plateau and its western foothills places the Salar de Pozuelos as an endmember of Li solubilisation by chemical weathering with only minor hydrothermal mobilisation of Li.

scholarly journals Assessment of Sulfate Sources Under Cold Conditions as a Geochemical Proxy for the Origin of Sulfates in the Circumpolar Dunes on Mars

Minerals ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 507
Author(s):  
Anna Szynkiewicz ◽  
Janice L. Bishop
Keyword(s):  
Atmospheric Deposition ◽  
Chemical Weathering ◽  
Sulfide Oxidation ◽  
Pond Water ◽  
Sulfur Cycle ◽  
Dry Valleys ◽  
Near Surface ◽  
Geochemical Model ◽  
The North ◽  
Sulfate Salts

Determining aqueous sulfate sources in terrestrial cold environments can provide an insight into the surface hydrological conditions and sulfur cycle on Mars. In this study, we analyzed sulfur and oxygen isotope compositions of secondary sulfate salts (e.g., gypsum, thenardite) in the surficial sediments and soils of the McMurdo Dry Valleys (MDV), Antarctica to determine contributions of sulfate from bedrock chemical weathering and atmospheric deposition under persistent dry polar conditions. The sulfate showed wider variation of δ34S (+15.8‰ to +32.5‰) compared to smaller ranges of δ18O (−8.9‰ to −4.1‰). In contrast, the δ34S of bedrock sulfide showed significantly lower and consistent values across the studied area (−0.6‰ to +3.3‰). Based on the δ34S trends, sulfide weathering may contribute up to 20–50% of secondary sulfate salts in the MDV. While the remaining 50–80% of sulfate inputs may originate from atmospheric deposition (e.g., sea aerosols, dimethulsulfide oxidation), the subglacial brines derived by relicts of seawater and/or lake/pond water influenced by microbial sulfate reduction could also be important sulfate endmembers particularly in the Antarctic lowland thaw zones. Additional field observations of frost, ponding water, and thin gypsum crusts on the terrestrial gypsum dunes at White Sands supports reactivity of gypsum on the surface of these dunes during cold winter conditions. Combined with our improved geochemical model of the sulfur cycle for cold Antarctic settings, we propose that transient liquid water or frost was available in near-surface environments at the time of gypsum formation in the north polar region on Mars. Ice and/or water interaction with basaltic sand of the basal unit (paleo-erg) would have enhanced leaching of sulfate from both sulfide oxidation and atmospheric deposition and resulted in formation of secondary gypsum salts.

Topographic controls on frost and thermal weathering processes and implications for rockwall erosion

2021 ◽  
Author(s):  
Daniel Draebing ◽  
Till Mayer
Keyword(s):  
Snow Cover ◽  
Swiss Alps ◽  
Rock Surface ◽  
Thermal Processes ◽  
Near Surface ◽  
Fracture Spacing ◽  
Weathering Processes ◽  
Erosion Rates ◽  
Frost Weathering ◽  
Mechanical Weathering

<p>Mechanical weathering by freezing and thermal processes are influenced by climate. Topography modulates this climatic influence due to altitudinal decrease of temperature, modifying insolation due to rockwall aspects and insulation by snow cover. In this study, we (i) quantify rock fracture damage in the field, (ii) monitor rock surface temperature and snow cover, (iii) model frost weathering processes, (iv) quantify fracture kinematics and (v) assess how these processes contribute to rockwall erosion. For this purpose, we conducted measurements on rockwalls with different aspects along an altitudinal gradient ranging from 2,500 to 3,200 m in the Hungerli Valley, Swiss Alps, between 2016 and 2019.</p><p>(i) The geology of the Hungerli Valley comprises schisty quartz slate with inclusions of aplite and amphibolite. We conducted Rock Mass Strength (RMS) measurements and used fracture spacing and uniaxial compressive strength (UCS) measurements as proxies for mechanical weathering. RMS ranges from 62 to 77 for schisty quartz slate rockwalls, up to 73 for aplite and 74 for amphibolite. Fracture spacing and UCS reflect lithological differences of the catchment area suggesting a geological control on weathering efficacy. </p><p>(ii) Rock surface temperatures (RST) were monitored using temperature loggers. RST decreases with elevation from 2,500 to 2,900 m, however, increases again at 3,150 m potentially due to higher insolation on ridges. Snow cover duration shows a similar altitudinal trend. Due to aspect, RSTs are 2 to 4 °C warmer on south facing rockwalls with significant shorter snow cover period.</p><p>(iii) We used measured RST to drive frost cracking models by Walder and Hallet (1985) and Rempel et al. (2016). Both models show near surface frost weathering at lower altitudes, which should results in lower UCS. The models show significantly higher frost cracking at higher altitudes with peaks at rock depths between 0.5 and 2 m suggesting a higher fracture spacing.</p><p>(iv) Rockwalls between 2,500 and 2,900 m were equipped with crackmeters and show higher daily temperature changes and crack deformation at lower altitudes or south facing aspects due to higher insolation compared to higher located rockwalls. Seasonal crack displacement depends on dipping of monitored blocks and is controlled by both thermal and cryogenic processes (Draebing, 2020).</p><p>(v) In summary, low-altitudinal rockwalls show a higher weathering at the surface due to a combination of thermal processes and near surface frost weathering resulting in release of small blocks and lower erosion rates. In contrast, rockwalls at higher altitudes reveal higher seasonal thermal changes propagating deeper into the rock in combination with frost cracking in higher depths, which results in larger blocks and higher erosion rates.</p><p> </p><p>Draebing, D.: Identification of rock and fracture kinematics in high Alpine rockwalls under the influence of altitude, Earth Surf. Dynam. Discuss., 1-31, 2020.</p><p>Rempel, A. W., Marshall, J. A., & Roering, J. J.: Modeling relative frost weathering rates at geomorphic scales. Earth and Planetary Science Letters, 453, 87-95, 2016.</p><p>Walder, J., and Hallet, B.: A Theoretical-model of the fracture of rock during freezing, Geological Society of America Bulletin, 96, 336-346, 1985.</p>

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