Precipitation Of Secondary Calcite Research Articles

Origin and significance of ultra-slow calcite dissolution rates in deep sea sediments

Prediction of mineral-fluid reaction rates in geologic materials is subject to large uncertainties even though the rates are critical for many applications, from studies of past climate change to the engineering of carbon removal and storage. Deep sea carbonate sediments, which have been extensively cored and characterized at dozens of sites by ocean drilling programs, provide unique opportunities to examine how minerals and fluids react over multi-million-year timescales in natural conditions at low temperature. It has already been established that the calcite-fluid reaction in these systems is extremely slow, but the explanation for the ultra-slow rates, and the fact that non-zero rates persist for tens of millions of years, remains elusive. We extend the analysis of pore fluid Sr and Ca concentration data in the literature from ocean drilling archives to 21 drill sites in sediment sections where the fraction of carbonate is larger than 80% to systematically estimate the rates at which calcite dissolves and precipitates as a function of depth and sediment age. Pore fluid Sr/Ca used in this study, and Sr and Ca isotopes in other published analyses, provide estimates of the gross calcite dissolution rates, which are mostly balanced by secondary calcite precipitation. Pore fluid Ca concentration data provide key, previously underappreciated, information on the net calcite dissolution rates. Using these data we evaluate simple models for calcite reaction kinetics and pore fluid sediment evolution and conclude that (1) reaction rates are extremely slow because the reactive site density of the bulk sediment decreases continuously and systematically approximately as age−1, and (2) reaction persists to hundreds of meters burial depth and tens of millions of years because pore fluids never become closed due to diffusive communication with the oceans, and hence remain slightly undersaturated with respect to calcite. The systematic decrease in bulk reactive site density begins immediately upon deposition, greatly slowing the rate of dissolution and hence affecting models of calcite dissolution on the seafloor. The failure to reach equilibrium after millions of years and hundreds of meters of burial, is a consequence of extremely slow net reaction rates that cannot drive the pore fluids to equilibrium against the diffusive loss of Ca and DIC to the ocean. The existence of two or more calcite phases of different solubility is a likely source of a long-lived, but small, driving force for reaction, but its effect decreases with continued recrystallization and pore fluid evolution.

CO2 Release Driven by the Combination of Sulfide Oxidation and Carbonate Dissolution in the Upper Changjiang River: Effect of Erosion and Lithology on Chemical Weathering

AbstractThe positive link between weathering and orogeny suggests that the drawdown of atmospheric carbon dioxide (CO2) over the Cenozoic should be closely related to the enhanced silicate weathering caused by Cenozoic orogeny, especially the uplift of the Tibetan plateau. However, the uplift of orogens could also exhume carbonate and sulfide minerals, the coupling weathering of which can act as a source for atmospheric CO2 at the long‐term timescale and counteract the CO2 consumption by silicate weathering. In this study, the solute concentrations and isotopic ratios in the upper Changjiang River are determined to investigate the co‐weathering of silicate, carbonate, and sulfide minerals and its impact on the long‐term carbon cycle. An inverse mixing model incorporating elemental and isotopic ratios is established to quantitatively identify the sources of cations and weathering acids. The inversion of our samples reveals that carbonate weathering dominates the chemical composition of river water, even with the presence of secondary calcite precipitation. The sulfide oxidation widely occurs in the study area and the bulk of generated sulfuric acid is buffered by carbonate minerals. The erosion is decoupled from sulfide oxidation and chemical weathering, likely due to lithology and secondary calcite precipitation. Overall, we find that the CO2 release from sulfide oxidation coupled with carbonate dissolution exceeds the CO2 drawdown from silicate weathering in the Changjiang River basin, indicating the weathering will act as a source for atmospheric CO2 at the timescale between marine carbonate compensation (105 years) and sulfate reduction (<10 Ma).

Micro-karstification in a stalagmite, Küpeli Cave, southern Turkey

This article deals with micro-karstification forming abundant dissolution features in a stalagmite from Küpeli Cave in southern Turkey. Dissolution occurs when cave water enriched with CO2 from the atmosphere and soil seeps into the stalagmite. Water is transmitted from the surface of the stalagmite to the interior by the roughly vertical or diagonal notch-shaped pores formed by the enlargement of intercrystalline pores by dissolution. These slightly elongated pores appear embedded in different parts of the stalagmite and characterize different stages of dissolution during the stalagmite formation. Later, when this water reaches the relatively more permeable growth layer surfaces, it flows along these surfaces, and diffuse dissolution features form. These features include micro-scale pitted and etched surface structures, rounded and enlarged crystal boundaries and intercrystalline pores, and the breakdown of relatively large crystals into small crystals (micritization). When the percolating water is sufficiently saturated with calcium carbonate in the stalagmite, secondary calcite precipitation occurs as rim and pore-filling cements within the pores formed as a result of dissolution. In general, dissolution and calcite re-precipitation as cement are early diagenetic events and occur at different stages of stalagmite development due to seasonal variation in CO2 and CaCO3 contents of the water in the epikarst zone and within the stalagmite. These conditions were probably provided during the wet season.
  

CO2 storage in chalks: What are we afraid of?

Carbon Capture and Storage (CCS) has been recognized as an effective strategy to limit the temperature rise to 1.5 °C by 2050 under the Paris Agreement. As a result, more than 50 CCS pilot or large-scale projects have been commissioned in recent decades, targeting mainly sandstone reservoirs for their favourable petrophysical properties. Although many discussions and practical procedures have been developed, large-scale CO2 storage in carbonates has not been implemented due to practical risks associated with rapid geochemical interactions. This study attempts to evaluate the feasibility of CO2 storage in carbonate chalk formations by focusing on changes in storage capacity (porosity) and injectivity (permeability) over time. A series of laboratory tests were carried out on Stevns Klint chalk from Denmark after exposure to CO2 for 37 days at a fluid pressure of 15 MPa and a temperature of 50 °C. The results obtained indicated a large removal of Ca+2 ions from CO2 saturated water solution and strong precipitation of secondary calcite in the pore structure, which reduced the porosity and permeability of the samples. It seems that CO2 injection into chalk should be done very carefully, as the progressive dissolution of calcite and saturation of the formation water will initiate large secondary calcite precipitation in the long term, leading to a reduction in injectivity and storage capacity over time.

A micro-geoarchaeological view on stratigraphy and site formation processes in the Middle, Upper and Epi-Paleolithic layers of Sefunim Cave, Mt. Carmel, Israel

This paper presents a micro-geoarchaeological study carried out on the sedimentary sequence exposed at the entrance of Sefunim Cave, Israel, a sequence that spans from the Middle Paleolithic to the early Epipaleolithic periods. Using FTIR and micromorphological techniques, we investigated the stratigraphic sequence to reconstruct patterns of site use and archaeological formation processes. We identified formation processes that are common among Paleolithic caves sites in the Southern Levant, mainly the deposition of local terra rossa through colluvial sedimentation. Taphonomic disturbances of the deposits range from minimal to moderate, exhibited mainly by root and burrowing activity, but with no evidence for significant transport of archaeological materials. While the upper layers (II–III) are decalcified, the precipitation of secondary calcite results in increasing cementation of the sediments with depth in the lower layers (V–VII). We observed variation at the microscopic scale and identified an inverse correlation between human and carnivore activity throughout the layers. We observed human activity by the presence of micro-archaeological materials such as chert, bone, charcoal, rubified clay, burnt bone and shell, and wood ash. We observed carnivore activity by the presence of phosphatic grains and coprolite fragments as well as chewed and digested bones. We conclude that human activity at the site was characterized by episodes of varying intensity, based on the frequency of archaeological finds within the different layers. The alternating episodes of human and carnivore activity at Sefunim Cave may demonstrate the close-knit interactions and reciprocal relations that humans and carnivore shared at Paleolithic caves.

Neoformation of palygorskite in Calcids of Central Iran

Palygorskite has been reported in alluvial sediments and in soil clays in Central Iran, but it is not known if it is inherited or formed in situ. Here, we sampled Calcids developed from Triassic and Cambrian dolomitic formations in the southwestern part of Yazd and performed soil and clay characterization by X-ray diffraction and fluorescence, optical and scanning electron microscopy and energy dispersive X-ray spectrometer analyses. Our results suggested palygorskite neoformation in the calcic soil horizons in Calcids. The soils with aridic soil moisture regime and high Mg concentration, during the formation of a calcic horizon by precipitation of secondary calcite, conditions were suitable for authigenic palygorskite crystals of long size (>10–20 μm) and their stability. In addition, the occurrence of short-size (about 2 μm) palygorskite fibers in the lower gypsic horizon probably resulted from its translocation from the upper horizons. Therefore, such results suggested palygorskite neoformation in the calcic soil horizons in these Calcids of Iran.

A reactive transport approach to modeling cave seepage water chemistry II: Elemental signatures

Karst systems are useful for examining spatial and temporal variability in Critical Zone processes because they provide a window into the subsurface where waters have interacted with vegetation, soils, regolith, and bedrock across a range of length and timescales. These hydrologic pathways frequently include the precipitation of speleothems, which provide long-term archives of climate and environmental change. Trace element ratios in speleothems (Mg/Ca, Sr/Ca, Ba/Ca) have the potential to provide information about past changes in rainfall and infiltration, but controls on them can be complex and their interpretation must be based on an understanding of the modern cave system. Here we integrate observations of surface conditions, bedrock, soil, and drip water chemistry of Blue Spring Cave in Tennessee, USA with the reactive transport model CrunchTope, which we have calibrated for karst systems to investigate the primary controls on trace element variations in cave seepage waters. We find that measured drip water Mg/Ca and Sr/Ca are captured within the model through variable amounts of limestone dissolution followed by precipitation of secondary calcite that happens within the cave rather than the host limestone. However, strong spatial controls on drip water Mg/Ca and Sr/Ca likely reflect seepage water interactions with variable amounts of diagenetic phases in the host rock. In contrast, Ba/Ca values are consistent across the cave and vary with effective rainfall, suggesting that this parameter may be the most consistent metric for limestone dissolution and prior calcite precipitation and can act as a proxy for rainfall and infiltration in this cave system. Our findings emphasize the importance of evaluating spatial heterogeneity in cave drip waters and outline a novel modeling approach for determining the dominant controls on drip water chemistry in support of the interpretations of paleoclimate records.

The leaching mechanism of hydraulic mortars as part of autogenic self-healing process

Historic mortars have exhibited long term durability which has been largely attributed to their self-healing properties. The aim of this work is the study of the autogenic self-healing mechanism as observed in historic mortars as well as the simulation of the phenomenon in laboratory prepared lime-pozzolan and natural hydraulic lime mortars. One of the most significant parameters in autogenous self-healing phenomenon is the source of the calcium ions, which controls the formation of new healing phases inside cracks. In this study, an electrochemically accelerated leaching procedure was designed to examine the influence of mineralogy and curing time on the dissolution of Ca-bearing phases of lime-pozzolan and NHL mortars, which are characterized by high portlandite content. It was found that, apart from portlandite, Ca+2 are also leached from C2S and C–S–H phases and it was shown that this phenomenon takes place in two separate time-frames: an early period (t < 28 d), which is controlled by the portlandite content of the binder and, a later period (t > 28 d) where decalcification of C–S–H and dissolution of secondary portlandite are dominant. The phenomenon was further studied on mortar specimens where healing of micro cracks was observed by precipitation of secondary calcite inside cracks and voids, highlighting the role of leached Ca+2 on the autogenic self-healing mechanism. It is proved that, contrary to what is believed about the autogenic healing, besides portlandite, the contribution of hydraulic phases is also significant. Overall, this study confirms the significant role of the Ca-leaching process in the autogenic self-healing mechanism and highlights the different sources of calcium ions, as well as the different time frames of the events. Finally, the findings elucidate the high self-healing potential of traditional mortars which contribute to their prolonged service life.

Geochemistry of recent and fossil brachiopod calcite of Megathiris detruncata (Terebratulida, Megathyrididae): A modern baseline study to trace past environmental conditions

Modern and fossil calcareous shells of the terebratulide brachiopod Megathiris detruncata have been investigated for stable oxygen and carbon isotope and elemental (Mg, Sr, Mn, Fe, Ba) compositions to assess the possibility of using this species to trace past environmental conditions. Modern shells were collected from the Mediterranean Sea and the Atlantic Ocean, while fossil specimens were sampled from late Eocene to Pleistocene deposits in Europe. We mainly analysed bulk shells (i.e., primary and secondary layers together) but, in some cases, their single median and lateral septa, ventral and dorsal valves. The oxygen and carbon isotope compositions of the modern shells indicate that M. detruncata mineralizes the bulk of its calcite shell in equilibrium with seawater. In addition, their Mn, Ba, and Fe concentrations are low with few exceptions from the Atlantic Ocean where high Fe/Ca ratios can probably be related to locally enhanced Fe availability. The Sr/Ca ratio ranges narrowly between 1.4 and 1.7mmol/mol, while the Mg/Ca ratio has larger variability from about 23 to 45mmol/mol. The δ18O derived temperatures do not correlate with Mg/Ca, suggesting that Mg is likely affected by greater vital effects. Physiological control on biomineralization might also explain the somewhat larger spread of δ13C between different valves of the same individual and will require further testing by in situ analyses.The fossil M. detruncata shells have a compositional range much larger than modern specimens, with large intra-shell and intra-site variabilities indicative of partial or full alteration. Altered fossils, characterized by low δ18O and δ13C values, low Mg/Ca and Sr/Ca, and high Mn/Ca and Fe/Ca ratios are often associated with clear signs of re-crystallization and/or secondary calcite precipitation within the shells.We used fossil samples without obvious recrystallization features and chemical and isotopic compositions similar to modern specimens to discuss environmental parameters such as temperature and δ18Oseawater at the given time. We recognized variable local controls on seawater isotopic compositions in the Paratethys from the Oligocene until the Middle Miocene, and we propose somewhat warmer habitats for the M. detruncata in the Aquitaine Basin in southwestern France during the Late Oligocene. Isotope variation recorded in the Plio-Pleistocene Mediterranean shells are instead best explained by changing glacial-interglacial conditions.

Chemical weathering outputs from the flood plain of the Ganga

Transport of sediment across riverine flood plains contributes a significant but poorly constrained fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has important implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga, 31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water and 8 bedload samples from tributaries confined to the Ganga flood plain. The flood plain tributaries are characterised by a shallow δ18O - δD array, compared to the meteoric water line, with a low δDexcess from evaporative loss from the flood plain which is mirrored in the higher δDexcess of the mountain rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are dominantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided into two groups. Tributaries from a small area around the Kosi river have 87Sr/86Sr ratios >0.75 and molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87Sr/86Sr ratios ≤0.74 and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions of the carbonate and silicate components of the sediments were determined from sequential leaches of floodplain bedloads and these were used to partition the dissolved cation load between silicate and carbonate sources. The 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs were derived from the acetic-acid leach compositions and silicate Na/Ca and 87Sr/86Sr ratios derived from silicate residues from leaching. Modelling based on the 87Sr/86Sr and Sr/Ca ratios of the carbonate inputs and 87Sr/86Sr ratios of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average ∼ 50%) to precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sediments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate minerals. Because of significant evaporative loss of water across the flood plain, and in the absence of hydrological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best calculated by mass balance of the Na, K, Ca, Mg, Sr, SO4 and 87Sr/86Sr compositions of the inputs, comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical discharge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 41 and 63% of the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries.

Fossil travertines and quasi-travertine in the Minusa basin (West Siberia): structure, composition, and comparative analysis

We study a carbonate body looking like a classical fossil travertine which was discovered in the Chebaki-Balakhta basin within the Minusa trough (Khakassia, Russia) and called quasi-travertine. It is a thin layer sandwiched between a basalt-dolerite sill and calcareous siltstone. Comprehensive studies of the quasi-travertine and its comparison with Devonian fossil travertines located a few kilometers away in terms of structure and composition have made the basis for its formation model. According to this model, the quasi-travertine has had a two-stage history: deposition and subsequent hydrothermal metasomatism. Laminated limestone coexisting with calcareous siltstone of the Early Devonian Shunet Formation formed during the first stage and then experienced hydrothermal metasomatism with precipitation of secondary calcite, prehnite, and pyrobitumen (kerite).

Identifying geochemical reactions on wellbore cement/caprock interface under sequestration conditions

The integrity of wells, which are key components for CO2 sequestration, depends mainly on the seal between the wellbore cement and the geologic formation. To identify the reaction products that may alter the cement/caprock interface, batch experiments and computer modelling were conducted and analysed. Over time, the dissolution and precipitation of minerals alters the physical properties of the interface, including its tightness. One main objective of the simulation was thus to analyse the evolution of the porosity of cement and caprock over time. The alteration of the cement/caprock interface was identified as a complex problem and differentiated depending on rock type. The characteristic feature of a cement/shale contact zone is the occurrence of a highly carbonated, compacted layer within the shale, which in turn causes cement/shale detachment. In the case of a cement/anhydrite interface, the most important reaction is severe anhydrite dissolution. Secondary calcite precipitation takes place in deeper parts of the rock. The cement/rock contact zone is prone to rapid mineral dissolution, which contributes to increased porosity and may alter the well integrity. Comparison of computer simulations with autoclave experiments enabled the adjustment of unknown parameters. This enhances the knowledge of these particular assemblages. Overall, a good match was obtained between experiments and simulations, which enhances confidence in using models to predict longer-term evolution.

Sequestration of carbon as carbonate in the critical zone: insights from the Himalayas and Tibetan Plateau

The carbon pool stored in soil carbonate is comparable to the soil organic carbon. Therefore, secondary calcite precipitation in supersaturated catchment could be an important, yet poorly constrained, carbon sink within the modern global carbon cycle. The chemical analysis of some dissolved species transported by rivers, such as elevated Sr/Ca and Mg/Ca ratios but also heavy stable Ca isotopic compositions, witness the formation of secondary calcite in rivers draining arid regions. However, in areas affected by active tectonics and rapid physical erosion, co-variations in the fluvial Sr/Ca and Mg/Ca ratios could also be related to incongruent carbonate weathering processes. Here, we present a model to assess the roles played by incongruent carbonate dissolution and secondary calcite precipitation in modern weathering processes. We tested and applied the model to rivers draining the Himalayan–Tibetan region. The results suggest that regional aridity in the drainage basin promotes carbon sequestration as secondary carbonate but that for a given runoff, incongruent dissolution of carbonate possibly related to rapid physical erosion amplifies such sequestration. The isotopic compositions (13C/12C and 18O/16O) of detrital carbonate transported by the main rivers in South and South-East Tibet imply that around 1% of the suspended material transported by those rivers corresponds to secondary carbonate and can represent between 5% and 15% of the alkalinity flux. Most of these alkalinity transported as particulate material is, nevertheless related to the weathering of carbonate lithologies and is also subjected to dissolution prior its final storage in sedimentary basins. However, on glacial-interglacial timescale this will amplify the significant role of mountain weathering on climatic variations.

Effect of calcite precipitation on stable strontium isotopic compositions: Insights from riverine and pool waters in a karst cave

Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) serve as potential tracers for evaluation of weathering sources and associated physicochemical processes in terrestrial environments. Despite carbonate weathering sourcing two-thirds of the total continental Sr flux to the ocean, however, few attempts have been aimed at studying stable Sr isotope fractionation in carbonate-dominated aquifers. Here we present a monthly-resolved monitoring dataset of major ion concentrations and Sr isotope ratios for karst riverine and pool waters at the Luofang Cave in the Central China to understand the potential controls responsible for water δ88/86Sr compositions under various climatic conditions. With the exception of some pool waters in the dry period, significantly heavier δ88/86Sr were detected in the riverine and pool waters (0.20‰ to 0.42‰) compared to bedrock δ88/86Sr (0.24‰ to 0.28‰), which can't be interpreted solely by lithology. A sequential leaching experiment of the local soils suggests limited stable Sr isotope fractionation during incongruent silicate mineral dissolution. Local plant utilization and biological activity in the soil overlying the cave are possible processes for causing the observed heavier δ88/86Sr in cave water. However, air partial pressure of CO2 in cave, monitoring water pH, and Sr isotopic mass balance calculations suggest that plant utilization is unlikely to be the primary control on the heavier water δ88/86Sr. On the other hand, significant stable Sr isotope fractionation was found between the cave dripping waters and calcite precipitates, and the Δ88/86Srcar-aq was estimated to be −0.15‰±0.07‰ (2SD). Precipitation of secondary calcites might be a potential process responsible for the heavier δ88/86Sr in the waters. The negative correlation between δ88/86Sr and Sr/Na ratios in the cave waters is consistent with a scenario of preferential incorporation of the light Sr isotope by solid phase. Furthermore, by combining the calcite saturation states and water pH, our results suggest that calcite precipitation is the most likely process controlling δ88/86Sr compositions in cave water. We demonstrate that secondary calcite precipitation in carbonate-dominated catchments could be important in controlling the riverine water δ88/86Sr compositions.

Isotopic studies of the Upper and Middle Rio Grande. Part 2 — Salt loads and human impacts in south New Mexico and west Texas

Increasing groundwater and soil salinity is a threat to the land and water resources in arid regions. Global warming will likely increase salinity of dryland river systems. In order to characterize salt loading into the semi-arid portion of the Rio Grande in south New Mexico and west Texas, we sampled seasonally (2009–2011) the river, agricultural drains, and saline groundwater. In addition to major element chemistry, these samples were analyzed for sulfur and oxygen isotope compositions (δ34S and δ18O) of dissolved SO4 and in some cases for nitrogen and oxygen isotope compositions (δ15N and δ18O) of dissolved NO3. Uranium isotopes (234U/238U activity ratio) were also measured for selected samples. The natural inflow of basinal brines/groundwater (δ34S of +8 to +11‰) in the semi-arid Rio Grande study area was minor in the investigated seasons and could not be detected by the δ34S mass balance. However, we did find localized increases of δ34S (+2 to +5‰) in the Rio Grande that were attributable to salt loads from the intersections of agricultural drains with the water table of a natural salt flat and associated evaporative brine (δ34S of +12‰) in the shallow subsurface. In the areas, with higher water use for land irrigation, the δ34S of the river and drain water was relatively consistent (from ~0 to +2‰) compared to the δ18O (from ~+2 to +6‰). Most likely, this resulted from application of S-rich fertilizers (e.g., ammonium sulfates, elemental S, sulfuric acid) with low δ34S (−2 to +4‰) and high δ18O (+9 to +16‰). Additionally, we observed considerably lower δ18O (SO4) in the Rio Grande and agricultural drains (<7‰) compared to geologic and anthropogenic SO4 sources (+9 to +16‰), which likely resulted from microbial recycling of SO4 in soil of the irrigated land related to assimilatory sulfate reduction. Shallow recharge to the Rio Grande was also inferred from the lower 234U/238U activity ratios (1.62 to 1.88) compared to deeper groundwater (2.54 to 2.64) and the distinctive δ15N and δ18O values of nitrates (+5 to +25‰ and −5 to +15‰, respectively) typical for septic effluents that are undergoing denitrification. Agricultural practices during flood irrigation intensify evaporation of the Rio Grande surface water and considerably increase water salinity. This process is also important in the evolution of water chemistry toward a Na–SO4–Cl-rich composition and precipitation of secondary calcite in soil profiles.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

Selective dissolution of feldspars in the presence of carbonates: The way to generate secondary pores in buried sandstones by organic CO2

Carbonates are suggested to dissolve rapidly than feldspars by laboratory experiments. Petrography texture of selective dissolution of feldspars in the presence of carbonates, however, is widespread in buried sandstones and even shales, inspiring a revisit to the chemistry of burial secondary pores.Feldspar dissolution, precipitation of secondary minerals (quartz, clays), and carbonate cementation are common chemical reactions in the Eocene sandstones in the northern Dongying Sag. Petrography evidence demonstrates the selective dissolution of feldspars in the presence of carbonate minerals (both detrital and authigenic minerals) in these buried sandstones. The equilibrium constant of calcite leaching reactions is much smaller than that of K-feldspar leaching reactions. Numerical simulations of chemical reactions in K-feldspar-calcite–CO2–H2O systems utilizing the Geochemist's Workbench 9.0 (GWB) indicate that only a small amount of calcite was dissolved at the onset of simulation processes, while much K-feldspar was dissolved with precipitation of quartz, clays and some calcite for extended periods of time. Precipitation of secondary calcite could also promote feldspar dissolution. Simulation of reactions in a simplified sandstone system with constraints of present-day pore water and partical pressure of carbon dioxide (pCO2) in the northern Dongying Sag indicates that the pore waters are close to equilibrium with calcite. Petrography evidence and modeling results share consistence in confirming that only feldspar could be dissolved extensively, with precipitation of quartz, clays and some carbonate minerals.The organic CO2-leaching theory (Schmidt and McDonald, 1979) regarding the dissolution of mainly carbonates and, to a lesser degree, feldspars to form secondary pores has been questioned due to supposed extensive internal consumption of organic CO2 in source rocks by carbonate minerals. The CO2 in sandstones and shales and the isotopic composition of CO2 and carbonate cements in sandstones, however, suggest migration of organic CO2 from mudstones (even with large amounts of carbonate) to sandstones. Calculations in this study indicate that, with suppression of carbonate dissolution in sedimentary rocks, many secondary pores can be generated in sandstones through selective dissolution of feldspar by organic CO2 from thermal evolution of organic matter. Selective dissolution of feldspars, rather than carbonates, in sedimentary rocks is then likely a general mechanism to decipher the chemistry of secondary porosity.Other secondary porosity generation mechanisms were also reevaluated with numerical simulations. Results demonstrate that reverse weathering reactions of silicates may take place with increasing of temperatures, but cannot provide enough H+ to dissolve carbonates at elevated temperatures. Additionally, cooling of hot fluids can dissolve only insignificant amounts of carbonate unless significant amounts of water are provided.

Spectral and geochemical characterization of onshore hydrocarbon seep-induced alteration in the Dezful embayment, southwest Iran

The presence of hydrocarbon seeps at the surface is indirect evidence of the presence of mature source rocks within a geological system at depth. Chemical changes in the environment of surface rocks caused by hydrocarbon seeps cause mineralogical alterations. To determine the nature of the alterations and the influences of lithology and type of seep, rock samples were collected from altered and unaltered evaporite and marly limestone formations in the Dezful embayment, southwest Iran. Reflectance spectroscopy, bulk rock/wet chemical analyses, and sulfur, carbon, and oxygen isotopic analyses were used to delineate surficial alterations and relate alterations to hydrocarbons seeping from underlying reservoirs. In addition, the boosted regression trees (BRT) method was used to predict the presence of alterations from spectral indices. Comparisons of geochemical data and spectral data of altered evaporites and altered marly limestones showed that the minerals within alteration facies have distinctive spectral, chemical, and isotopic signatures. Gas-induced alterations were characterized by the formation of gypsum and native sulfur and depletion in . The released in natural gas reacted with gypsum in the evaporite sediments and calcite in the marly limestone formations, which led to precipitation of secondary gypsum and native sulfur. Oil-induced alterations were characterized by formation of secondary calcite and depletion in . The oxidation of seeping oil and reactions between this oil and host rocks caused precipitation of secondary calcite within both formations. The combination of fieldwork data and spectral-geochemical data showed a connection exists between surficial alterations and underlying petroleum reservoirs, which can be used in exploration campaigns.

Incorporation of trace metals into microcodium as novel proxies for paleo-precipitation

Trace element compositions of microcodium are applied for the first time as possible paleo-proxies based on a case study on the Chinese Loess Plateau (CLP). The Mg/Ca and Sr/Ca ratios of the microcodium picked from the Holocene paleosol across the CLP show distinct positive correlation over large range of nearly one order of magnitude. Higher Mg/Ca and Sr/Ca ratios of microcodium are recorded in the sites on the northwestern CLP where less monsoonal rainfall is received. Similar large variation of the positively correlated Mg/Ca and Sr/Ca ratios has also been observed for the stream water on the CLP with the same spatial pattern. The Mg/Ca and Sr/Ca ratios of the microcodium seem to be largely controlled by the composition of soil solution as reflected by stream water rather than partition coefficient. Rayleigh distillation, and thus evolving composition of soil solution as a result of progressive precipitation of secondary calcite, is responsible for the large variation and positive correlation of the Mg/Ca and Sr/Ca ratios for both the microcodium and stream water. We propose that the bio-remains in Chinese loess may become inactive when the soil water is extracted to a degree, and then be calcified into microcodium by the infiltration of freshwater that is mixed with the highly evolved soil solutes. Thus, the Mg/Ca and Sr/Ca ratios of microcodium may record the paleo-precipitation amount by reflecting the mixing ratio between the highly evolved soil solutes with higher Mg/Ca and Sr/Ca ratios and the fresh soil water with lower Mg/Ca and Sr/Ca ratios.

Granular disintegration of marble in nature: A thermal-mechanical origin for a grus and corestone landscape

Here we document for the first time in detail a natural landscape dominated by granular disintegration of marble and the associated production of marble grus sediment and corestones. We provide several lines of evidence for a thermal-mechanical origin for the observed granular disintegration and resulting corestones. In the San Bernardino Mountains of southern California, calcite marble outcrops whose average grain diameter is greater than approximately 0.5mm are weathering into grus sediment and exhibit a corestone morphology. Minimal soil development in thick grus colluvium combined with results from a simple erosion experiment indicates that grus production and erosion is rapid. Our data indicate that unlike granite, marble granular disintegration and corestone development is a uniquely sub-aerial process. Environmental Scanning Electron Microscope (ESEM) and micro-probe analyses of rock samples collected in recent quarry exposures indicate that detachment of grain boundaries occurs primarily within the upper 2–20cm of the outcrop surface. There is minimal evidence of secondary calcite dissolution or precipitation at grain boundaries below the upper-most few centimeters of the rock surface, despite widespread intergranular fracturing. At depths greater than 20cm below the natural ground surface, granular disintegration and corestone weathering are not evident, and dissolution and precipitation of secondary calcite is only observed within a few centimeters of joint planes. Corestone morphology appears to form through the erosion of loosened grus debris at sharp edges of subaerially exposed joints. The resulting form represents a morphologic equifinality to that of granite corestones and results from different processes than those which have been proposed for granite. An 8-month, once-per-three minute temperature record from a thermocouple embedded into a marble corestone indicates that the upper 2.5cm of the corestone regularly achieves temperatures greater than 40°C, the temperature which is thought to be sufficient to produce permanent strain in calcite. All of these results are consistent with theoretical and experimental studies that indicate that marble is susceptible to mechanical weathering by thermal processes. Overall, this study provides new documentation that solar heating and cooling can be an important driver of physical weathering in natural systems of the Earth's surface.