Dissolution of Carbonate Rocks in a Laboratory Setting: Rates and Textures

Determining the dissolution rates of carbonate rocks is vital to advancing our understanding of cave, karst, and landscape processes. Furthermore, the role of carbonate dissolution is important for the global carbon budget and climate change. A laboratory experiment was setup to calculate the dissolution rates of two whole rock carbonate samples with different petrographic makeup (ooids and brachiopods). The carbonate rock samples were also explored under a scanning electron microscope to evaluate the textures that developed after dissolution The oolitic limestone dissolved at a rate of 1579 cm yr−1, and the pentamerous limestone (dolostone) dissolved at a rate of 799 cm yr−1. Both rocks did not dissolve evenly across their surface as indicated by scanning electron microscopy, it appears the allochems dissolved preferentially to the matrix/cement of the rocks and that some mechanical weathering happened as well. This work reports that the petrography and mineralogy of carbonate rocks is important to consider when exploring the cave, karst, and landscape evolution and that attention should be paid to the petrography of carbonate rocks when considering the global carbon budget.

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

<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>

Experimental Evaluation of Multiple Frost Heaving Parameters for Preflawed Granite in Beizhan Iron Mining, Xinjiang, China

Ice-driven mechanical weathering in cold regions is considered a main factor impacting the stability of rock mass. In this work, the response surface method (RSM) was employed to evaluate and optimize the multiple frost heaving parameters to seek the maximum frost heaving force (FHF), in combination with experimental modeling based on a specially designed frost heaving force measurement system. Three kinds of rocks were prepared with parallel flaws in it having different flaw width, length, and cementation type, and these factors were used to fit an optimal response of the maximum FHF. The experimental results reveal five distinguished stages from the frost heaving force curve, and they are inoculation stage, explosive stage, decline to steady stage, recovery stage, and sudden drop stage. The sensitivity analysis reveals the influential order of the considered factors to peak FHF, which is the rock lithology, flaw width, flaw cement type, and flaw length. For low-porosity hard rock, increasing flaw width, flaw length, and flaw cement strength can improve the probability of frost heaving failure. It is suggested that rock lithology determines the water migration ability and influences the water-ice phase transformation a lot.

Chemical Weathering, First Cycle Quartz Sand, and its bearing on Quartz Arenite

A self-consistent set of experimentally determined rates of mineral dissolution (Francke, 2009) has been used to estimate the relative loss of common constituents of plutonic igneous rocks that supply quartz to sands to be lithified as first-cycle quartz arenite. A first-order decay equation is used setting the decay constant of calcite to one, which renders the time-steps of loss dimensionless but keeps the relative loss of each mineral constant. Calculations show that 99.33% of pure calcite would dissolve only after 5 time-steps. On a relative scale, it would take about 1250, 900, and 1040 time-steps respectively to reduce the original compositions of quartz-bearing mafic plutonic, granodioritic, and granitic rocks to leave >95% undissolved quartz as residue that will qualify as quartz-sand, the precursor to first-cycle quartz arenite. These are very large numbers indicating that chemical dissolution alone, as in chemical weathering by itself, is not sufficient to generate first-cycle quartz sand; accompanied mechanical weathering is necessary. Therefore, it is necessary to re-evaluate many explicitly stated inferences of warm, humid climate in provenance studies of first-cycle quartz arenites.

Illite crystallinity index an indicator of physical weathering of the Sediments of the Tista River, Rangpur, Bangladesh

The Tista River is a tributary of the Brahmaputra River. The deposits that exposed along the both banks of the Tista River are characterized mainly by sand, sand laden with gravel and pebble with minor amounts of silt and clay. The X-ray Diffraction (XRD) of the clay sized sediments of the Tista River reveals that illite (and/or mica), chlorite, kaolinite, quartz and feldspar are the principal mineral constituents. The minor to trace amounts of lavendulan, glauconite lepidolite, enstatite, sekaninaite and ferrierite minerals were identified in the investigated area. Illite constitutes 36% of the total minerals of the Tista River basin. The values of the illite crystallinity index varies from 0.228 to 0.345, indicating that the illites are relatively well crystallized and derived from the mechanical weathering of pre-existing rocks. The presence of illite and kaolinite suggests their derivation from the crystalline rocks that contain feldspar and mica, as well as from the pre-existing soils and sedimentary rocks. Glauconite forms in the sediments of continental shelf in the marine environment. The accessory minerals like enstatite, sekanianite and ferrierite are derived from basic igneous rocks. The dissolution of copper arsenate mineral, lavendulan might be a source of arsenic in the sediments of the study area.

On the formation of desert loess

Sequences of quartz-rich coarse (20−63 μm) silt occur in many low- and midlatitude unglaciated arid and semiarid areas and have been termed “desert loess.” The processes by which these deposits are generated have been debated for decades. All hypotheses to explain their origin seek to provide mechanisms for the generation of silt-sized material without glacial grinding, which is the main process involved in the production of coarse silt at high latitudes. Possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones, mechanical weathering of silicate rocks, and abrasion of sand grains in active dune environments during intense transport events. Examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and/or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt. Improvements in the characterization of desert loess particle size, mineralogy, and geochemistry are needed, however, to identify sources and sinks of coarse silt, especially when combined with climatic back-trajectory analysis. Properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations.

Experimental microfracture propagation in gneiss through frost wedging

<p>Ice-driven mechanical weathering in mountainous environment is considered an efficient process for slow preconditioning of rockfalls. In this study (Musso Piantelli et al., 2020), we simulate with an innovative experimental approach subcritical fracture-propagation under frost-wedging conditions through pre-existing weaknesses of intact rock bridges. Two series of freeze-thaw experiments in an environmental chamber have been designed to investigate and monitor the propagation of artificially-induced fractures (AIF) in two twin gneiss samples. By employing 3D X-Ray Computed Tomography and a displacement sensor, an accurate characterization and new insights into the fracture-propagation mechanism are provided. Our results demonstrate that frost wedging propagated the AIFs of 1.25 cm2 and 3.5 cm2 after 42 and 87 freeze-thaw cycles, respectively. The experiments show that volumetric expansion of water upon freezing, cooperating with volumetric thermal expansion and contraction of the rock, plays a key role in fracture widening and propagation. Based on these results, this study proposes that: (i) frost wedging exploits intrinsic pre-existing weaknesses of the rock; (ii) the fracturing process is not continuous but alternates propagation stages to phases of tensile stress accumulation; and (iii) downward migration of “wedging grains”, stuck between the walls of the fracture, increases the tensile stress at the tip, widening and propagating the fractures with each freeze-thaw cycle. The experimental design developed in this study offers the chance to visualize fracture-propagation in natural joints quantifying the long-term efficiency of this process in near-natural scenarios.</p><p>REFERENCES</p><p>Musso Piantelli, F., Herwegh, M., Anselmetti, F.S., Waldvogel, M., Gruner, U., (2020). Microfracture propagation in gneiss through frost wedging: insights from an experimental study. Natural Hazards, 1-18. https://doi.org/10.1007/s11069-019-03846-3</p>