Deep Carbon Research Articles

Sluggish lithium isotopic response of continental intraplate basalts to recycled sedimentary carbonate

Recognizing the signal of recycled carbonate in the deep mantle and associated basalts is crucial for depicting deep carbon cycling. It has long been recognized that marine sedimentary carbonates have much heavier lithium isotope compositions (δ7Li) than the depleted mantle. However, Li isotope systematics are rarely employed to trace recycled carbonate. To investigate basalt Li isotopic response to recycled carbonate, we carried out whole-rock geochemical and Li isotopic studies on a series of the Cenozoic continental intraplate basalts from eastern China that sample a typical carbonated mantle. The basalts have been reported to have light magnesium (Mg) isotope compositions, but their δ7Li values vary in a limited range (0.9–5.7‰) around the depleted mantle (~3.8 ± 1.4‰). The decoupled LiMg isotopic records are closely related to recycled Mg-rich carbonates (dolomite) that have lighter Mg isotope compositions than the depleted mantle but show mantle-like δ7Li values. After modeling the Li isotopic effects of carbonate sediment, silicate sediment and altered oceanic crust (AOC) recycling, we find that even calcium-rich carbonate recycling is inadequate to shift basalt Li isotope composition. Collectively, Li isotopic responses of intraplate basalts to all types of recycled sedimentary carbonates are more sluggish than expected. Our modeling also suggests that recycling of AOC and silicate sediments is likely responsible for the basalts with δ7Li values higher and lower than the depleted mantle, respectively. In addition, seawater δ7Li values probably exert a first-order role in the Li isotopic response. Low δ7Li values of paleo-seawater (< 20‰) for most of the Earth's history have generated AOC with δ7Li values indistinguishable from the depleted mantle, which goes against the emergence of high δ7Li values for intraplate basalts. Although some limitations exist in Li isotopic applications, we propose that a combination of Li isotopic data and whole-rock element ratios is effective in tracing recycled crustal materials hidden in the intraplate basalts.

Geophysical and geological contribution to determining the neritic limestone aquifer structure of Hammam Bradaa – El Fedjoudj (Seybouse medium), Northeastern Algeria

Purpose. The neritic aquifer that extends between Hammam Bradaa and El Fedjoudj, despite its faulted and in places karstified structure, due toits capacity and lateral extension represent a strategic resource for the region. Its waters are used for drinking water supply in the neighboring towns: Heliopolis, Guelaat Bousbaa, Nechmaya and El Fedjoudj, as well as in part of the Annaba wilaya. These neritic limestones outcrop at Djebel Debagh and Bouzitoune. Methods. In order to better study the potential of this aquifer, it is essential to identify its geometry and structure by analyzing the geological data, mechanical drilling data and geophysical data analysis through an electrical survey campaign. Findings. The main results indicate that the study area has identified two important aquifer formations: a formation in the Mio-Plio-Quaternary alluvium consisting of clays, marls, gravel and sand; the second is a deep carbonate aquifer composed of fissured and karstified neritic limestone of the Cretaceous age of variable depth ranging within 50 and 350 m. Originality. The originality of the study is in the fact that the studied area is characterized by the presence of thick, fractured and karstified carbonate formations, which are intensively tectonized and have significant aquifer potential. Practical implications. The correlation of geological data with geophysical data made it possible to conclude that the stu-died area is a sedimentary basin bounded by faults predominantly oriented to the south-west and north-east, forming a highly fractured unit consisting of horsts and grabens. The significant water potential reservoir is formed essentially by carbonate geological formations, highly fractured with the presence of karst forms represented by resistant horizons.

Ultra-deep carbonate basement reservoirs formed by polyphase fracture-related karstification in the Offshore Bohai Bay Basin, China

The Palaeozoic carbonate basement of the Offshore Bohai Bay Basin (OBBB) presents considerable potential for hydrocarbon exploration. However, the multistage tectonism and complex superimposed palaeo-karstification in the area are unclear, which leads to a lack of understanding on the formation mechanism and distribution of the deep carbonate basement reservoirs. In this study, the occurrence of a fracture-vug network and its fillings in carbonate reservoirs were investigated based on borehole cores, thin sections, and image logs from the southwestern slope of the OBBB's Bozhong Sag. Then the diagenetic fluid properties of the carbonate matrix and fillings were analysed via the data of carbon, oxygen, and strontium isotopes, and major, rare elements from coring intervals. The results revealed that fracture-related karst reservoirs have lithologic selectivity inclined toward dolomite strata. The intersecting relationships, widths, and strikes of the fractures and the regional tectonic background indicate three structural fracture families: NW-, NNE-, and NNW- trending, related to the Indosinian, middle Yanshanian, and late Yanshanian orogeny, respectively. The Indosinian NW- and end-Mesozoic NNE-trending fractures produced by compressional tectonic stress mainly contributed to the formation of the basement reservoirs. The geochemistry of the calcite veins filling these fractures suggests two main types of diagenetic fluids. The fluid of autogenic recharge related to the earlier fills is karstification diffuse flow dominated by internal runoff from rainfall in the highland setting of the Indosinian thrusting orogenic belt. The other fluid of allogenic recharge related to the later fills is the main lateral freshwater flow dominated by external runoff from the catchment in the setting of the horst-lowland within the rifting basin, induced by the Yanshanian destruction of the North China Craton. Finally, the relationship between the three fracture families and two kinds of related fluids is revealed. This allows us to propose a model to understand the polyphase-superimposed fracture-related karst reservoir complexes within the deep carbonate basement of tilting fault blocks that neighbour the Bozhong hydrocarbon kitchen and predict the formation of potential plays with high accuracy.

Self-Generated Organic Acid System for Acid Fracturing in an Ultrahigh-Temperature Carbonate Reservoir.

As an important part of acid fracturing technology for the carbonate reservoir, the performance of the acid system directly affects the stimulation effect. In view of the current problems of an acid fluid system in an ultrahigh-temperature carbonate reservoir, such as fast acid-rock reaction, short effective action distance, and difficulty maintaining the conductivity of acid etching fractures, an experimental study on the self-generated organic acid system was carried out. The results showed that acetic anhydride and ethyl acetate, which had a strong acid generating ability, were suitable for the parent acid types of self-generated organic acids. Preferably, the peak temperature of 25% acetic anhydride is 160 °C, while the peak temperature of 30% ethyl acetate is 180 °C. The acid-rock reaction kinetics experiment shows that the order of activation energy is ethyl acetate > acetic anhydride > cross-linked acid. The reaction rate is arranged as follows: ethyl acetate < acetic anhydride ≪ cross-linked acid. The self-generated organic acid of ethyl acetate has the largest activation energy, the smallest reaction rate, and the best retarding effect. Acetic anhydride forms a strong nonuniform pitting morphology on the surface of the rock plate, with an initial conductivity of 225.4 μm2·cm. Ethyl acetate forms an uneven pitting morphology, and the initial conductivity is 53.1 μm2·cm. However, the ability of acid etching fracture formed by ethyl acetate to maintain fracture conductivity is stronger than that of acetic anhydride. Ethyl acetate is more suitable for a deep ultrahigh-temperature carbonate reservoir.

Depicting past stress history at passive margins: A combination of calcite twinning and stylolite roughness paleopiezometry in supra-salt Sendji deep carbonates, Lower Congo Basin, west Africa

The paleostress history and associated deformation mechanisms affecting passive margins are seldom studied, as access to offshore parts is rather limited. We analyze an offshore wellbore core of the Albian, post-rift carbonates of the Sendji Fm which directly overlies the salt of the Aptian Loeme Fm in the Lower Congo Basin. Paleopiezometry based on stylolite roughness inversion (SRIT) and calcite twin inversion (CSIT) was combined with fracture analysis, U–Pb geochronology of calcite cement and burial modeling to unravel the orientations and magnitudes of horizontal and vertical stresses affecting the Sendji Fm over time, with a focus on the impact of salt tectonics on stress records. The results of SRIT on bedding-parallel stylolites constrain the range of depths over which the Sendji Fm strata deformed under a vertical principal stress σ1 to 650–2800 m (median ∼1300 m). Once projected onto the burial model derived from TemisFlow™ software, the range of depths converts to a period of pressure solution activity, from 101 to 15 Ma. Calcite twin measurements within the early diagenetic cement (U–Pb age = 101 ± 1 Ma) reveal three main types of stress regimes: (1) extensional stress regimes with σ3 trending ∼ N–S and ∼E-W associated with local, thin-skinned salt tectonics (101–80 Ma), (2) strike-slip stress regimes with σ1 trending ∼ NW-SE to NNE-SSW and a compressional stress regime with σ1 trending NNW-SSE, reflecting possible intraplate stress transfer from the distant Africa-Eurasia plate boundary (67–60 Ma); (3) a strike-slip and a compressional stress regime with σ1 trending E-W likely related to the mid-Atlantic ridge push possibly combined with effects of variations in elevation and density of the lithosphere and sedimentary flexural loading (15 Ma to present). The paleostress sequence suggests that the sedimentary cover was decoupled from the crust during salt tectonics and then recoupled from the late Cretaceous onwards.

Tetracarbonates in silicate melts may be at the origin of a deep carbon reservoir in the deep Earth

Much of Earth’s carbon may have been stripped away from the silicate mantle by dense metallic-iron during core formation. However, at deep magma ocean conditions carbon becomes less siderophile and thus large amounts of it may be stranded instead in the deep mantle. Here, we describe the structure and compaction mechanisms of carbonate glass to deep mantle pressures. Our results, based on non-resonant inelastic X-ray scattering, X-ray diffraction and ab initio calculations, demonstrate a pressure-induced change in hybridization of carbon from sp2 to sp3 starting at 40 GPa, due to the conversion of [3]CO32- groups into [4]CO44- units, which is completed at ~112 GPa. The pressure-induced change of carbon coordination number from three to four increases possibilities for carbon-oxygen interactions with lower mantle silicate melts. sp3 hybridized carbon provides a mechanism for changing the presumed siderophile nature of deep carbon, becoming a possible source for carbon-rich emissions registered at the surface in intra-plate and near-ridge hot spots.

The Strike-Slip Fault Effects on Tight Ordovician Reef-Shoal Reservoirs in the Central Tarim Basin (NW China)

The largest carbonate condensate field in China has been found in the central Tarim Basin. Ordovician carbonate reservoirs are generally attributed to reef-shoal microfacies along a platform margin. However, recent production success has been achieved along the NE-trending strike-slip fault zones that intersect at the platform margin. For this contribution, we analyzed the strike-slip fault effects on the reef-shoal reservoirs by using new geological, geophysical, and production data. Seismic data shows that some NE-trending strike-slip faults intersected the NW-trending platform margin in multiple segments. The research indicated that the development of strike-slip faults has affected prepositional landforms and the subsequent segmentation of varied microfacies along the platform margin. In addition, the strike-slip fault compartmentalized the reef-shoal reservoirs into multiple segments along the extent of the platform margin. We show that fractured reef-shoal complexes are favorable for the development of dissolution porosity along strike-slip fault damage zones. In the tight matrix reservoirs (porosity &lt; 6%, permeability &lt; 0.5 mD), the porosity and permeability could be increased by more than 2–5 times and to 1–2 orders of magnitude in the fault damage zone, respectively. This suggests that high production wells are correlated with “sweet spots” of fractured reservoirs along the strike-slip fault damage zones, and that the fractured reservoirs in the proximity of strike-slip fault activity might be a major target for commercial exploitation of the deep Ordovician tight carbonates.

Hydrogen Energy: Development Prospects, Current Obstacles and Policy Suggestions under China’s “Dual Carbon” Goals

Hydrogen energy has an advantage over conventional fossil energy because of its clean and low-carbon features, while its stability gives it an advantage over renewable energy sources such as hydropower, photovoltaic (PV) power, and wind power. Using hydrogen as a fuel can achieve zero carbon emissions or even net zero emissions at the end of energy conversion. Moreover, with hydrogen as the carrier, an energy system of multi-energy complementarity will be established by coupling multiple energy systems such as power grids, gas grids and heat networks, which can realize the comprehensive utilization of renewable energy sources in an efficient, stable, and flexible manner. Based on the “dual carbon” goals, this paper targets sectors with high carbon emissions represented by electric power, steel and transportation, analyzes the feasible path of using hydrogen energy to promote deep carbon reduction, and points out the obstacles faced by hydrogen energy development in various sectors. On account of the current development and expected prospects, this paper proposes that the current focus of hydrogen energy industry is to coordinate the development of industrial sectors and regions, build a pattern that can facilitate the industry’s systematic and concerted development, address key technical challenges (such as green hydrogen production or hydrogen storage and transportation via pipeline), improve industrial standards, and promote the industry’s development in a balanced, coordinated and orderly way.

Deep SOC stock dynamics under contrasting management systems: Is the EPIC model ready for carbon farming implementation?

Process-based models have been recognized as cost-effective tools to assess carbon farming mechanisms through quantifying the C fluxes in the agroecosystems. A result-based approach is suggested however the wide variability of agricultural environments makes further model implementation necessary to limit the uncertainty of the results, especially on deep soil organic carbon (SOC) stock estimation and stratification and in agroecosystems characterized by a shallow water table. In this study, a comprehensive soil and crop dataset collected over a seven-year period from different pedoclimatic conditions across the Veneto Region (NE Italy) was used for EPIC model calibration and validation of SOC stock dynamics. Experimental data included yields from several crops (corn, winter and spring wheat, rapeseed, and soybean), continuous monitoring of soil water content, and SOC stocks (1872 total samples within the 0–50 cm soil profile) under conventional, cover crop and conservation agriculture systems. Modelling was performed by testing two N-C sub-models (CENTURY and PHOENIX), which differentiated in terms of mineralization/immobilization rates.Results showed that the procedure was able to obtain parameters valuable for most of the management system and pedoclimatic condition, reproducing well the tested variables. The EPIC model acceptably captured soil water dynamics (Nash-Sutcliffe coefficient – NSE – was up to 0.26), especially in the topsoil. Furthermore, simulation of weed—crop competition in conservation agriculture strongly contributed to properly explain the variability in crop production among the contrasting agricultural systems (R2 ranged from 0.51 to 0.71). Likewise, EPIC skillfully simulated SOC stocks within the 0–50-cm profile regardless of the sub-model used (NSE was up to 0.64). Moreover, the model acceptably captured the profile SOC stratification among the different management practices. This study highlights the EPIC robustness for predicting SOC stocks and assessing with high accuracy carbon farming results.

Sedimentary evolution of the late Cretaceous - Eocene deep carbonates series of the foreland Magrebian chain. The Tellian domain of northern Tunisia

This paper aims to give an overview of the Upper Cretaceous – Eocene in the Tellian domain of the Magrebian chain in Tunisia. The sedimentological studies of the carbonates of this time interval, in the Fernana - Ain Draham area, offer a better knowledge of the Tellian foreland. Stratigraphic analysis of ten sections located in the structural units (Autochthonous, Para-autochthonous, and the allochthonous, Ediss Unit, Ain Draham - Adissa Unit and Kasseb unit) brings new results. The facies analysis of the nappes zone reinforces the definition of the allochthonous units that have undergone planar displacements during late Tertiary phases. The depositional setting evolutions during the Late Cretaceous - Eocene period testify to an area in the process of deformation. Since the late Cretaceous (Maastrichtian), the abundance of older dismantling rocks in the allochthonous units is interpreted as a synchronous shortening of the basin. The clastic deposits attributed to mélanges chaotic deposits forming events layers testify two tectonic stages of the early foreland basin formation with details: the upper Cretaceous (Gansseri Biozone) and the early Eocene (Lutetian). The studied deposits outcropping in the Tellian domain (part of the Maghrebian chain) invoke the earliest stage of synchronous compressive syn-sedimentary tectonics of the subduction of Europe over the African plate southern branch, as observed in the Mediterranean rim.

The characteristics of strike-slip faults and their control on hydrocarbon distribution in deep carbonate reservoirs of the central Sichuan Basin

The past decade has witnessed a breakthrough in the gas exploration of deep marine carbonates of the central Sichuan Basin. Deep faults research has also attracted increasing attention, as faulting plays an important role in reservoir control. Previous studies have suggested a developed series of high-angle strike-slip fault systems in the central Sichuan Basin, but correlated exploration activities are limited, as distribution rules and dynamic mechanisms remain unclear. In this study, the spectral decomposition coherence method was used to describe the geometric and kinematic characteristics of these strike-slip faults. Using a comprehensive analysis technique to assess the strike-slip fault tectonic activity history, the formation and evolution processes of strike-slip faults and their control on hydrocarbon distribution were examined. The results showed that the deep strike-slip fault system, mostly distributed in the Dengying Formation, can be divided into four stages, three levels, and three groups of orientation, which controlled the structural framework and shape of the central Sichuan area, as well as the zoning from north to south, and blocking from west to east. The faults showed features of layered deformation and staged evolution in the vertical direction. The segmentation of strike-slip faults strongly controls the quality of fractured vuggy reservoirs. Reservoirs of the hard-linked zone of the strike-slip fault are the most developed, followed by those of soft-linked segments, with translational sections of the strike-slip fault being relatively undeveloped. Strike-slip faults are important hydrocarbon migration paths, and their multistage activities have different controlling effects on hydrocarbon accumulation.

Changes in deep Pacific circulation and carbon storage during the Pliocene-Pleistocene transition

The global climate has been gradually cooling over the Cenozoic and is punctuated by the intensification of Northern Hemisphere Glaciation (NHG) from the latest Pliocene to earliest Pleistocene (∼3.1–2.5 millions of years ago, Ma). A decline of atmospheric CO2 is supposed as a prerequisite for the NHG, but the associated carbon-cycle processes remain elusive. Here we combine foraminiferal records of neodymium isotope and boron-calcium ratio, and simulations of an Earth system model, to investigate changes in the water-mass composition and carbonate-ion concentration of the deep Pacific Ocean during the NHG. Our proxy records have revealed a significant expansion of southern-sourced waters with increased respired carbon storage into the deep Pacific during the NHG. These changes may be explained by strengthened deep-water formation and biological-pump efficiency in the Southern Ocean due to Antarctic sea-ice growth, as suggested by our model experiments and evidence from the Sub-Antarctic region. These results provide key clues for quantifying the role of the dissolved inorganic carbon content of deep Pacific waters in modulating atmospheric CO2 during the NHG.

Preface: Deep Carbon Cycle and Abiotic Methane in the Subduction Zone

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Effects of Spin Transition and Cation Substitution on the Optical Properties and Iron Partitioning in Carbonate Minerals

AbstractThe high‐pressure behavior of deep carbonate dictates the state and dynamics of oxidized carbon in the Earth's mantle, playing a vital role in the global carbon cycle and potentially influencing long‐term climate change. Optical absorption and Raman spectroscopic measurements were carried out on two natural carbonate samples in diamond‐anvil cells up to 60 GPa. Mg‐substitution in high‐spin siderite FeCO3 increases the crystal field absorption band position by approximately 1000 cm–1 but such an effect is marginal at &gt;40 GPa when entering the low‐spin state. The crystal field absorption band of dolomite cannot be recognized upon compression to 45.8 GPa at room temperature but, in contrast, the high‐pressure polymorph of dolomite exhibits a strong absorption band at frequencies higher than (Mg,Fe)CO3 in the low‐spin state by 2000–2500 cm–1. Additionally, these carbonate minerals show more complicated features for the absorption edge, decreasing with pressure and undergoing a dramatic change through the spin crossover. The optical and vibrational properties of carbonate minerals are highly correlated with iron content and spin transition, indicating that iron is preferentially partitioned into low‐spin carbonates. These results shed new light on how carbonate minerals evolve in the mantle, which is crucial to decode the deep carbon cycle.

Main controlling factors and genetic mechanism for the development of high-quality reservoirs in the mound-shoal complexes on the platform margin of the intra-cratonic rift: A case study of the fourth member of the Dengying Formation in the eastern limb of Deyang-Anyue intra-cratonic rift, Sichuan Basin, China

Reservoirs of large platform margin mound-shoal complexes of the fourth member of Dengying Formation (Deng 4 Member) are developed in the margin of the Deyang-Anyue intra-cratonic rift in the Sichuan Basin and it is the main pay horizon of the Anyue gas field. A clear understanding of the reservoir genetic mechanism of the mound-shoal complexes is the key to predicting the distribution of high-quality reservoirs and guiding the deployment of exploration. Based on the data of drilling, seismic, outcrop, and analytical data, this paper analyzes the reservoir characteristics and genetic mechanism of the mound-shoal complexes at the margin of the Deng 4 Member and obtains three new understandings: (1) Platform margin mound-shoal reservoirs are developed on the margin of Deyang-Anyue intra-cratonic rift in Sichuan Basin. The mound-shoal complexes are mainly composed of algal mounds and bioclastic shoals in multiple stages. The reservoir space is mainly dissolution pores, caverns, and fractures, with low porosity and low permeability in general. (2) The reservoir can be divided into three types, i.e., the fracture-dissolution pore type, the dissolution pore type, and the matrix pore type, and the reservoirs of fracture-dissolution pore type are high-quality reservoirs; the development of reservoirs is mainly controlled by the platform margin mound-shoal complexes, the penecontemporaneous interstratal karst, and two-stage weathering crust karstification as well as multi-stage disruptive actions; the upper part of the reservoir in the same stage is good, and the reservoir at the top of Deng 4 Member is good. (3) Before the Himalayan movement, the reservoir forming and the environments of the two platform margin mound-shoal complexes were the same, and the characteristics of the formation of the reservoir were similar. The Himalayan movement led to a great difference in the current buried depth of the reservoir, resulting in a host of fractures and retaining a host of dissolution pores and caverns. The whole platform margin mound-shoal complexes have large-scale reservoirs developed and have a good exploration prospect. The results enrich the theory of ancient and deep carbonate reservoir forming and its genetic mechanism and provide the geological basis for the deployment of exploration.

Variations in the Southern Ocean carbonate production, preservation, and hydrography for the past 41, 500 years: Evidence from coccolith and CaCO3 records

Changes in ocean alkalinity affect atmospheric pCO2 (i.e., higher alkalinity lowers atmospheric pCO2). Ocean alkalinity is partly determined by sedimentary burial of carbonates, which is primarily controlled by carbonate flux and the degree of deep ocean carbonate saturation. In this study, we investigate the factors determining the coccolith burial in subantarctic sediments and the surface ocean changes in the subtropical South Indian Ocean. The downcore coccolith records from the subantarctic region (SK200/22a) of the Indian sector of the Southern Ocean display low coccolith concentration during the glacial period. A possible explanation for this is, 1) the low glacial production of coccolithophores due to the competition from diatoms and 2) dilution by biogenic silica in the glacial sediments. Additionally, reduced carbonate burial owing to the low carbonate saturation of the deep-water accounts for the decline in glacial coccolith concentration. This also explains the low coccolith dissolution index and enrichment of the large dissolution-resistant coccolith species, Coccolithus pelagicus subsp. braarudii in the glacial sediments. The low carbonate saturation is attributed to, 1) the replacement of carbonate saturated, North Atlantic Deep Waters by the undersaturated southern sourced water masses and 2) increased storage of dissolved CO2 in the deep glacial Southern Ocean. Our study suggests that changes in coccolith production and the deep ocean carbonate saturation determine their burial in subantarctic sediments for the last 41,500 years. Other than these changes, the study region also records the changes in the Agulhas Return Current via variation in the proportion of tropical-subtropical coccolith assemblage.

CO2 leak detection threshold using vertical seismic profiles

At the Containment and Monitoring Institute Field Research Station in Alberta, Canada, CO2 is being injected into a saline aquifer at 300 m depth to simulate a shallow leak from a deep carbon sequestration operation. The injection interval is a 7 m thick sandstone with a porosity of 11%. The gas-phase CO2 plume was detected with time-lapse vertical seismic profiles (VSP) after 32 tonnes of CO2 had been injected. The VSP surveys had high repeatability with permanent borehole geophones and carefully repeated source locations. A data processing workflow was developed to produce directly comparable baseline and monitor amplitudes while preserving subtle amplitude changes caused by partial CO2 saturation. Seasonal variations in the physical properties of the near-surface glacial tills introduced spectral differences between baseline and monitor datasets that were successfully mitigated by deterministic deconvolution and selected band-pass filters. The 32 t detection threshold represents a lower limit of CO2 detectability with borehole geophones in this geological setting. These results provide a rare field data example of CO2 monitoring at shallow to intermediate depths and demonstrate that VSP surveys are capable of identifying loss of containment in industrial-scale CO2 sequestration operations.

Corrigendum to “Structural diagenesis of deep carbonate rocks controlled by intra‐cratonic strike‐slip faulting: An example in the Shunbei area of the Tarim Basin, NW China”

Corrigendum to “Structural diagenesis of deep carbonate rocks controlled by intra‐cratonic strike‐slip faulting: An example in the Shunbei area of the Tarim Basin, <scp>NW</scp> China”

Hydrogeological Study in Tongchuan City Using the Audio-Frequency Magnetotelluric Method

Tongchuan City, located in Shaanxi Province, northwest China, has limited groundwater resources. Rational planning and exploitation of groundwater are crucial to the sustainable development of the city, for which investigating the distribution of groundwater is the premise. Traditional resistivity sounding methods are often used to detect groundwater; however, these methods are not applicable in the study area where thick Quaternary loess is extensively distributed. In this study, we arranged five audio-frequency magnetotelluric (AMT) profiles to detect the deep clastic rock groundwater and carbonate karst fissure groundwater in Tongchuan. Firstly, we analyzed the electromagnetic interference (EMI) noises in Tongchuan City, revealing that the main EMI is power frequency interference (PFI). We used the dictionary learning processing technology to suppress the PFI. Secondly, the two-dimensional (2D) nonlinear conjugate gradient method was employed to invert a 2D electrical structure model for the area shallower than 1 km. We analyzed the characteristics of the electrical structure and its geological significance. Lastly, the three-dimensional (3D) electrical structure model of the study area was inverted using the 3D nonlinear conjugate gradient method, and the spatial distribution characteristics of the water-bearing strata were further analyzed. The results show that the PFI in urban environment can be suppressed by the dictionary learning processing technology. In Tongchuan city, the distribution of clastic rock fissure water is controlled by folds and faults, as well as the thickness of sandstone layers, and that of the carbonate karst fissure water is mainly controlled by faults. On this basis, we infer that the water-bearing areas are in the middle east and south of the study area.

Deep soil water depletion and soil organic carbon and total nitrogen accumulation in a long‐term alfalfa pasture

AbstractDeep soil resources are essential for improving the production and ecological functions of dryland ecosystems. Relationships between deep soil water depletion and carbon and nitrogen accumulation in long‐term dryland pastures were not well quantified and understood. This study aimed to quantify the changes in deep soil water, soil organic carbon (SOC), and total nitrogen (TN) in alfalfa pasture and identify their relationships with root density. We conducted a field experiment in the south Loess Plateau in 2020 and 2021. Soil water, SOC, and TN contents and alfalfa root length density to 1000‐cm‐depth were measured in alfalfa pastures and annual crop reference fields. Soil water depletion by alfalfa mainly occurred in the first 6 years with a depletion rate of 41.6, 49.1, and 62.1 mm yr−1 in the shallow (0–200 cm), middle (200–500 cm) and deep (500–1000 cm) soil layers, respectively. Total depletion after 6 and 19 years were 916.8 and 1049.4 mm, respectively. SOC and TN storages in the shallow and middle layers continuously increased and peaked at 19 years after planting, and those in the deep soil increased quickly in the early ages but showed few changes in the following years. Final SOC and TN storages in the 0–1000 cm profile in alfalfa pasture reached 61.1 and 5.4 kg m−2, respectively, 13.7% and 20.4% higher than the reference field. Soil in 200–1000 cm contributed to 60.7% and 47.8% of the total SOC and TN storage increments, respectively. The deep rooting system of alfalfa derived soil water depletion and SOC and TN increments, as indicated by the significant relationships between fine root length density and the changes in soil water, SOC and TN storages. Therefore, the deep soil contributed substantially to the carbon and nitrogen sequestration in alfalfa early ages but the contribution was limited by water shortage in the middle and late ages. We suggest stopping alfalfa early (≤6 years) to increase water sustainability and maintain carbon and nitrogen sequestration efficiencies in the alfalfa pasture and crop rotation system on Loess Plateau.