Karst Depressions Research Articles

Paleoclimatic and anthropogenic impacts on the environment of Southwest China since 33 ka based on multiproxy analysis of karst depression deposits

Paleoclimate change and anthropogenic activities have fundamentally modified the Earth's environmental system since the late Quaternary. Southwest China has one of the largest continuous karst areas in the world, yielding geological records from karst depressions that can improve our understanding of changes in the dominant control on regional environmental conditions. In this study, we investigated a sediment sequence formed over 33,000 years in a typical karst catchment by using combined geophysical, geochemical and biological properties of the depression deposits. The results suggest that paleoclimate change was the main modulator of environmental changes prior to the Late Holocene, and that significant vegetation change related to paleotemperature influences played an important role during the Early (ca. 12–8 ka) and Middle (ca. 8–4 ka) Holocene. During the Late Holocene (ca. 4–0 ka), the impact of anthropogenic activities has overwhelmed paleoclimatic influences on the environment. This study demonstrates the utility of dated karst depression deposits for reconstructing Holocene climate change and the timing of anthropogenic impacts on the landscape.

Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content

Aluminium (Al)-rich palaeosols—i.e., palaeobauxite deposits—should have formed in karst depressions in carbonate sequences as a result of acidic solutions from oxidative weathering of sulfide minerals during the Great Oxidation Event (GOE), but no GOE-related karst-palaeobauxite deposits have so far been recorded. Here, we report results of in situ uranium–lead (U–Pb) dating of detrital zircon and spatially associated rutile from a metamorphosed Al-rich rock within a dolomite sequence in the Quadrilátero Ferrífero (QF) of Minas Gerais, Brazil, known as the Gandarela Formation. Rutile grains are highly enriched in thorium (Th = 3–46 ppm; Th/U ratio = 0.3–3.7) and yielded an isochron, lower-intercept age of ca. 2.12 Ga, which coincides with the final phase of the GOE—i.e., the Lomagundi event. The rutile age represents either authigenic growth of TiO2 enriched in Th, U and Pb during bauxite formation, or subsequent rutile crystallisation during metamorphic overprint. Both cases require an authigenic origin for the rutile. Its high Th contents can be used as a palaeoenvironmental indicator for decreased soil pH during the GOE. Our results also have implications for iron (Fe)-ore genesis in the QF. This study demonstrates that in situ U–Th–Pb-isotope analyses of rutile can place tight constraints on the age and nature of palaeosols.

Flood hazard assessment in a polje: the case of Mucille (Classical Karst Region, NE Italy)

The Mucille karst depression is one of the few examples of polje on the Italian side of the Classical Karst Region, a shared area between Italy and Slovenia. The polje is subject to frequent flooding, becoming more problematic since 2000, as swallow holes more frequently have affected housing and recreational areas, leading the population to believe that their ability in draining the area had stopped functioning. Climate changes play an important role as there has been an intensification in extreme events (30-day cumulative precipitation of more than 350 mm) within the considered time-period 1919–2020. The necessity to provide answers to the inhabitants required in-depth studies such as hydrogeological investigations, discharge measurements and Electrical Resistivity Tomography (ERT) acquisitions. Over the 3 years of monitoring (2017–2020), two flood events reached the Selz settlement. In total, four events have been analysed to build a hydrogeological model of the area in order to properly estimate its recharge and regression curve, and to define the functionalities of the swallow holes. The defined model allowed a better knowledge and a greater awareness in proposing the proper mitigation measures able to increase the drainage capacity of the area avoiding always more frequent future floodings.

A tropical soil (Lixisol) identified in the northernmost part of the Mediterranean (Istria, Croatia)

Terra rossa soils on the Istrian peninsula are generally considered polygenetic relict soils. They overlie and mark the youngest subaerial unconformity in the northwestern part of the Adriatic Carbonate Platform and are susceptible to erosional and redepositional processes. Terra rossa soils represent a valuable archive of information that can be used to understand present and past soil formation processes in the context of climate variability and landscape dynamics, especially in low-lying parts of the karst landscape where thicker deposits are typically found. We studied a 3 m thick terra rossa profile in a karst depression on the northernmost part of the southwestern Istrian planation surface and analysed the micromorphology, physical/chemical properties, geochemistry, bulk and clay mineralogy, and composition of heavy and light mineral fractions. The profile shows several signs of colluviation and polygenesis with clay distribution along the profile, clearly indicating lithic discontinuities. Kaolinisation, along with iron oxide formation, is the dominant process in the studied profile. Ferralitisation and lessivage processes were diagnosed throughout the profile, with intense clay translocation occurring despite the extremely stable soil microaggregates. In accordance with the WRB system, the profile is classified as Rhodic Lixisol (Clayic, Aric, Cutanic). This implies that some terra rossa soils, previously classified as e.g. Cambisols or Luvisols, might actually be Lixisols or other (sub-)tropical soils (e.g. Nitisols), in which relict properties are preserved in the present climate. The finding of a Lixisol in the study area indicates the old age of the surface of the karst depression and provides the first data on the burial history of the southwestern Istrian planation surface. We suggest the older Quaternary interglacials, the mid-Piacenzian Warm Period (Pliocene) and the Miocene Climatic Optimum as favourable periods for the formation of the studied soil.

Laboratory Model Test and Field In Situ Test of Distributed Optical Fiber Monitoring of Seepage in a Karst Depression Reservoir Basin

Karst depressions are ideal places for building reservoirs and stacking wastes, but karst depressions are mostly located in areas with strong karst development, and there is a problem of karst leakage. The study of karst depression monitoring methods can provide technical support for the construction of reservoirs in karst depressions, and leakage monitoring technology based on distributed temperature sensing (DTS) has the clear advantages of a large measurement range, high precision, continuity, wide distribution, large area, etc. In this paper, a temperature-sensing optical cable is first used to conduct leakage monitoring tests in different soil media, and the temperature change curve in different media is obtained, which verifies the feasibility of the heatable temperature-sensing optical cable to identify leakage in soil media with different moisture contents. Then, for the heatable distributed temperature-sensing optical cable, the variation law of the temperature eigenvalues of the sensing optical cable under different seepage velocities is studied using the layout method wrapped with saturated medium-coarse sand, and the relationship between the seepage rate and the temperature eigenvalues was analyzed. A regression formula for quantitative analysis of leakage velocity was established; finally, the manganese slag silo project in Songtao County in Guizhou Province, China, was used to simulate different leakage conditions on site for testing, and the data were compared and analyzed to obtain the route under different conditions. The temperature distribution law of the seepage section verifies the feasibility of the application of heatable temperature-sensing optical cables in the seepage monitoring of karst depressions.

Monitoring Methodology Study of a Reservoir Built on the Karst Depression: A Large Scale Laboratory Simulation Test.

The karst depression with a natural negative landform is favorable for reservoir construction with less excavation work. The underground air-water pressure would be complicated after the construction of a reservoir that blocks the natural air-water channel (sinkhole, karst channel, etc.). To develop a monitoring system for the reservoir, a large-scale laboratory simulation test was carried out to study the air-pressure evolution and water migration process during water table fluctuation after the blockage of the sinkhole in a karst depression. The results are as follows. (1) The positive pressure jacking effect and the negative pressure sucking effect were observed in the karst channel and inside the model during groundwater table fluctuation. (2) A water imbibition test on the rock-like material was carried out with layered resistivity and layered NMR measurement. The relationship of resistivity and water saturation was developed based on Archie's equation. (3) The resistivity evolution during the test was monitored using the high-density electronic resistivity imaging method. The resistivity of the measured profile was sensitive to the variation of the water level, and the water migration process was quantified by the resistivity to water saturation conversion. (4) The suitability of many techniques for monitoring a reservoir built on the karst depression was discussed. The resistivity monitoring technique combined with the air-water pressure monitoring technique could provide useful information for safety and reliability assessment.

Petrographic and Geochemical Inferences for Genesis of Terra Rossa: A Case Study from the Apulian Karst (Southern Italy)

Terra rossa is a reddish clay soil which is often present on the surface of limestone in regions with a Mediterranean-type climate. Its genesis is a controversial subject in terms of the origin of the parent material, from the residuum of underlying (carbonate/dolomite) bedrock in the absence/presence of an external silicate contribution (e.g., aeolian dust). Within this context the main goal of the present work was the understanding of the geochemical processes leading to the formation of the terra rossa starting from the carbonate bedrock. We report the results of a multi-method analysis on a terra rossa deposit occupying the bottom of a Quaternary karst depression on Mesozoic limestones exposed in the Murge area (Apulia Foreland, southern Italy). Geological, petrographic, textural, and chemical data were collected on karst products (reddish calcite incrustations and nodules, and fine-grained portion of terra rossa) by a detailed field mapping, optical microscopy, XRF and fusion ICP/MS analyses and by scanning electron microscope. New collected data show that the mineralogical composition of reddish incrustations and nodules is comparable, consisting of fibrous and impure calcite, detrital fragments of quartz, K-feldspar, zircon and authigenic minerals as (Mn, Ba, Ca) phases, (Al, Si, Mn, Fe, Mg, Ba, Ca) minerals, Fe-kaolinite and anatase. The prevailing minerals, instead, in the fine-grained portion of terra rossa are hematite, kaolinite, and goethite. Based on the chemical composition, and especially on REE patterns, a progressive interaction of silicate aqueous solutions (with Al, Si, Fe), containing pelite material, with the calcareous bedrock, as a source of carbonic acid, was the process driving the formation of terra rossa. Obtained results add new elements to the definition of the long-lasting question about the genetic processes responsible for the formation of terra rossa, corroborating their polygenetic origin, as result of limestone alteration in conjunction with the chemical interaction with allochthonous siliciclastic material.

Characterization of the palaeoenvironmental conditions in the Pannonian Basin during the last 34 mys related to the formation of haematitic and kaolinitic sedimentary rocks

The Pannonian Basin is a sediment-filled back-arc basin which was formed during the Late Paleogene–Neogene period. Such sedimentary rocks like bauxites or kaolinites, which are important with regard to industry in the region, are the products of former weathering processes. However, our knowledge about the palaeoenvironmental conditions which led to the formation of these mineral resources has not yet been integrated. For this purpose, the former biomes, climatic classes as well as karstification, weathering and paedogenic processes were investigated over the last 34 mys using the southeastern regions of China and the United States as climatically analogous regions. By the second half of the Oligocene, the conditions of intensive chemical weathering ceased to exist in the Pannonian Basin, which enabled the formation of lateritic bauxites during earlier periods. However, up to the mid-Serravallian Stage, intensive karstification as well as the formation of haematite-rich ultisols and alfisols continued when humid subtropical climatic conditions came about. The modelled biomes indicate that aridification during the Early to Middle Miocene led to the formation of less dense vegetation, exacerbating the re-deposition of weathering products. These conditions resulted in the accumulation of haematitic and kaolinitic sedimentary rocks in the fossilized karst depressions. The progressive cooling following the mid-Pliocene decreased the intensity of karstification, moreover, the cessation of humid subtropical conditions brought the sedimentation of haematitic and kaolinitic sedimentary rocks in the Pannonian Basin to an end.

Deciphering the water balance of poljes: example of Planinsko Polje (Slovenia)

Poljes are flat closed karst depressions prone to regular flooding. The floods can be several meters high, last for months and damage significantly human infrastructures. To predict the maximum level reached, the polje water balance needs to be implemented. This technique encounters the difficulty that important part of the inflow and outflow flowing through many poljes is ungauged, as it is challenging to measure accurately the numerous springs and ponors activating temporarily with the rise of water level. This work aims to see whether this problem can be handled and the polje water balance reconstituted. To do so, a typical Dinaric polje is equipped with several water level stations installed over its surface and in the nearby water active caves. Combining a 1*1m digital elevation model of the polje surface with water levels and inflow records of the main two springs allowed assessing the variation of flooded volume and reconstructing the water balance. The highest total inflow values reached during the observed period were of about 140-150 m3/s, with up to a third of it being ungauged. In addition, the effect of a large estavelles group on the polje inflow and outflow could be identified, and helped to characterize the outflow, with values comprised between 65 and 75 m3/s. Finally, intense rainfall over the polje flooded surface showed to be a temporary important source of inflow. The values found by the water balance analysis have been used as input and calibration data in a numerical model reproducing the flood dynamics in the polje and its surrounding aquifer. Results validated both polje water balance and conceptual hydrogeological model. They justify the significance of combining water level measurements with a digital elevation model to monitor the floods. The method can be applied to other poljes flooding in a complex way of superposed input and output signals. Finally, the places to be equipped in priority if the polje has no measurement network or if available funding is limited are discussed.

RETRACTED: A tropical soil (Ferralsol) identified in the northernmost part of the Mediterranean (Istria, Croatia)

Terra rossa soils on the Istrian peninsula are generally considered polygenetic relict soils. They overlie and mark the youngest subaerial unconformity in the northwestern part of the Adriatic Carbonate Platform and are susceptible to erosional and redepositional processes. Terra rossa soils represent a valuable archive of information that can be used to understand present and past soil formation processes in the context of climate variability and landscape dynamics, especially in low-lying parts of the karst landscape where thicker deposits are typically found. We studied a 3 m thick terra rossa profile in a karst depression on the northernmost part of the southwestern Istrian planation surface and analysed the micromorphology, physical/chemical properties, geochemistry, bulk and clay mineralogy, and composition of heavy and light mineral fractions. The profile shows several signs of colluviation and polygenesis with clay distribution along the profile, clearly indicating lithic discontinuities. Kaolinization, along with iron oxide formation, is the dominant process in the studied profile. Ferralitization and lessivage processes were diagnosed throughout the profile, with intense clay translocation occurring despite the extremely stable soil microaggregates. In accordance with the WRB system, the profile is classified as Lixic Rhodic Ferralsol (Clayic, Aric). This implies that some terra rossa soils, previously classified as e.g. Cambisols or Luvisols, might actually be Ferralsols or other tropical soils (e.g. Lixisols), in which relict properties are preserved in the present climate. The finding of a Ferralsol in the study area indicates the old age of the surface of the karst depression and provides the first data on the burial history of the southwestern Istrian planation surface. We suggest the older Quaternary interglacials, the mid-Piacenzian Warm Period (Pliocene) and the Miocene Climatic Optimum as favourable periods for the formation of the studied soil.

Interaction Mechanism of Fe, Mg and Mn in Karst Soil-Mango System

Manganese (Mn), an essential trace element for plants in which it is involved in redox reactions as a cofactor for many enzymes, represents an important factor in environmental contamination. Excess Mn can lead to toxicity conditions in natural and agricultural sites. Manganese toxicity is one of the most severe growth limiting factors in acid soil, which accounts for 21% of the total arable lands in China. The more significant part of Mn-toxicity is its interactions with other mineral elements, in particular with phosphorus (P), calcium (Ca) and iron (Fe). The application of P or Ca can be beneficial in the detoxification of manganese, whereas Mn seems to interfere with Fe metabolism. Manganese toxicity varies with plant species, nutrients, and the soil environment. Mango is the main economic fruit in the karst area of the subtropical region of China. The karst soil in the mango orchard is characterized by high Fe, Mn and Mg. In order to explore the interaction among Fe, Mg, and Mn in karst soil and mango systems under high Mn conditions, a typical mango orchard in the karst depression landform in Baise in southern China was selected to study the effects of Fe and Mg on the toxic expression of Mn in mango plants and the interaction mechanism of Fe-Mn-Mg in mango plants. The results show that: (1) the mango growth status is closely correlated with Fe2+ (active iron) and Mg under the same soil Mn concentration; (2) The black spots on mango leaves were mainly caused by Fe and Mn. There is a lot of Fe3+ and Mn3+ in the black spots, which accounts for more than 90% of the total; (3) In addition, the studies also showed that the Fe and Mg inhibited the expression of Mn toxicity in mango. Conclusively, the interaction effect of Fe, Mn, and Mg is an important factor that affects mango growth, which can indicate the status of the soil and plants.

Mineralogical and geochemical investigations on the Early Permian Yuxi karstic bauxite deposit, Central Yunnan, China

The metallogenic process of karstic bauxite in Central Yunnan Basin (CYB) remain obscure. Mineralogical, geochemical, and isotopic analyses of two profiles (profile A and B) in the Yuxi karstic bauxite deposit within CYB were conducted to explore the formation environment and metallogenic process of Early Permian bauxite. Profile A in the karstic uplifted area comprises clayey bauxite, claystone, and oolitic bauxite from the bottom to the top, while profile B in the karstic depression area contains oolitic bauxite, claystone, oolitic bauxite, and claystone. The oolitic bauxite mainly contains diaspore and chamosite, and a small amount of kaolinite and anatase, while the clayey bauxite primarily includes boehmite and kaolinite, and a small amount of goethite and anatase. The claystone is dominated by kaolinite and illite. The δ15N value of the oolitic bauxite in the top of profile B is close to zero (−0.8 ‰), indicating a reducing deposition environment. The δ13Corg value of profile A is negatively shifted between clayey bauxite and oolitic bauxite, while that of profile B is not significantly changed between the lower oolitic bauxite and the upper oolitic bauxite. This indicates that REDOX changes in the formation of clayey bauxite and oolitic bauxite in profile A are obvious, while REDOX changes in profile B are not evident. Mineral assemblages and isotopic results suggest that the mineralization conditions of profile A experienced a transition from oxidizing and weakly alkaline to reducing and alkaline, while the mineralization conditions of profile B were always reducing and alkaline. The presence of large-scale microorganisms, negative δ34S values (−42.2 to −10.1 ‰), and framboidal and cuboidal goethite provide evidence for microbial involvement in the entire mineralization process. The above results confirm two metallogenetic events of Yuxi karstic bauxite deposit. In the early metallogenetic event, the water table was low and most of the karstic terrains were exposed except some karstic depressions. Abundant boehmite and kaolinite thus formed in the karstic uplifted areas forming the clayey bauxite, and diaspore and chamosite in the karstic depressions forming the oolitic bauxite. In the late metallogenetic event, with the rise of water table, all karstic terrains were submerged, and large-scale diaspore and chamosite were widely deposited, thus forming oolitic bauxite.

Restoration of palaeogeomorphology and its petroleum-geological significance: Transitional-facies deposition during the early Permian in the southeastern Ordos Basin, North China

The identification of favourable shale-gas areas in marine-continental transitional shales are challenging because of the multiple thin producing layers and abrupt lateral pinch-outs. Using data obtained from drilling, logging, and 3D seismic and natural gamma spectroscopic logging, this paper investigates the control of early Permian palaeogeomorphology on the distribution of organic-rich shale in the southeastern Ordos Basin. Using the impression method and de-compaction correction of strata thickness based on seismic velocity reconstruction, we restore the karst palaeogeomorphology at the top of the Taiyuan Formation for the early Permian. Three geomorphic units including karst highland, karst slope, and karst depression are identified, and constitute a geomorphological pattern of interjacent depressions and highlands. In addition, based on the multi-lithological eigenvalue method, the spatial distribution of transitional shales in the Shan 2 lower sub-member of the lower Permian Shanxi Formation was inferred using post-stack seismic parameter inversion. Our results show that organic-rich shale is distributed mainly in the low section of Shan 2 lower sub-member, which is controlled by karst palaeogeomorphology mainly distributed in karst depressions. Organic-rich shale distributed in karst depressions are characterised by large cumulative thickness, high TOC, moderate thermal maturity, strong hydrocarbon generation intensity, and relatively high gas content, indicating that shale-gas sweet spots should be distributed mainly in karst depressions rather than in highlands or slope areas. Meanwhile, Th/U and Th/K ratios, which indicate paleo-environment during the period of shale deposition, show that the strength of reducing conditions was greatest in karst depressions, moderate in karst slopes, and lowest in karst highlands. Therefore, it is inferred that the distribution of shale-gas sweet spots may be related to the weakly reducing environment, which favoured the preservation of organic-rich sediments during sedimentation. The upper section of Shan 2 lower sub-member was dominated by a high-energy transitional sedimentary environment, and the paleo-redox conditions tended to be oxic, disfavouring the preservation of shale organic matter, and the organic-rich shale in the upper section is relatively poorly developed. This study shows that palaeogeomorphology and paleo-redox conditions affect the deposition and preservation of organic-rich sediments in lower section, the relationship between palaeogeomorphology and the distribution of organic-rich shale can provide reference for shale gas sweet spots prediction and promote the rapid exploitation of shale gas in eastern Ordos Basin.

Caves, sinkholes, and fractures in the eogenetic karst of Florida, a GIS-based spatial analysis

The correlation between surface and subsurface karst development was explored by comparing the directionality and spatial distribution of karstic depressions around twenty-two select caves in the eogenetic karst of Florida. Orientations of cave passages and major axes of depressions around cave centrelines imply varying degrees of correlation between them. Spatial distribution of karstic depressions was studied by standard deviational ellipses of sinkhole centroids and nearest neighbour orientations around caves using spatial statistics tools of ArcGIS. An overall analysis of the data shows close connections between some caves and the surrounding sinkholes in terms of their orientation and spatial distribution, suggesting the importance of fracture systems in the development of karst.

Soil organic carbon stock capacity in karst dolines under different land uses

The soil organic carbon (SOC) was determined in soils of enclosed karst depressions (dolines) (NW Dinaric Mts.) to define their potential for organic carbon sequestration. SOC was measured in the forest, succession (scrubland), and grassland plots at the bottom of dolines at four depths (0–40 cm) and for 40 cm soil layer SOC stock was calculated. We demonstrated that the prevailing fine soil fractions, the C/N ratio and soil thickness play a positive role in the storage capacity of SOC in dolines regardless land use type. Grasslands have the lowest SOC storage capacity (106 t/ha/40 cm), while the highest SOC storage capacity is in succession plots (130 t/ha/40 cm). The last are covered by shrub communities dominated by Prunus spinosa, forming dense communities, and are typical of abandoned croplands or meadows that have been impacted by high levels of nutrients during cultivation phase. At this stage, there is no additional nutrient input in studied plots, which lowers the nutrient content and increases the C/N ratio. C/N ratio is the highest in the forest, where SOC stock capacity is 116 t/ha/40 cm. Given the trend towards the abandonment of agricultural land at Kras Plateau (SW Slovenia), we can expect more overgrowth of dolines, and thus an increase in carbon stocks and stabilization of organic carbon in forest soils. In contrary, we noticed the alarming decrease in grasslands and increase in urban land. The SOC storage in 2020 was for 12,538 t/ha/40 cm lower than in 2002. Although grasslands showed the lowest SOC storage, their contribution to total SOC storage in dolines is very important. Since there is a lack of studies on carbon stocks in doline soils, our research is of great importance and a novelty and gives an important background for further research on SOC stock in karst landscapes worldwide.

The Characteristics of Soil Ca and Mg Leakage in a Karst Depression

The soil leakage characteristics of depressions in karst areas should be explored by investigating their soil Ca and Mg contents. As a starting point, in a karst landform typical of Guizhou Huajiang dry valley karst watersheds, we selected surface depression and performed a field investigation, soil profile excavation, and soil sample collection. The soil samples were pretreated and taken to the laboratory for soil physical and chemical analysis. The data were processed, and the Ca and Mg contents of the soils were analyzed to determine the spatial variation characteristics of soil leakage in the depression. We found that in the depression watershed, the Ca and Mg contents in the 0–30 cm soil profile generally increased with increasing soil depth, and the Ca and Mg contents in the 30–150 cm soil profile exhibited more variability with increasing soil depth. The Ca and Mg in the soil surface profiles from the top of the slope, middle of the slope, base of the slope, bottom of the depression, and sinkhole showed obvious characteristics of a sedimentary soil leakage migration path. In combination with analyses of the soil K and N contents, the soil Ca and Mg contents can be used as an index of the likelihood of soil and water leakage in karst depressions.

The Impact of Digital Elevation Model Preprocessing and Detection Methods on Karst Depression Mapping in Densely Forested Dinaric Mountains

Karst landscapes have an abundance of enclosed depressions. Many studies have detected depressions and have calculated geomorphometric characteristics with computer techniques. These outcomes are somewhat determined by the methods and data used. We aim to highlight the applicability of high-resolution relief laser scanning data in geomorphological studies of karst depressions. We set two goals: geomorphometrically to characterize depressions in different karst plateaus and to examine the influence of data preprocessing and detection methods on the results. The study was performed in three areas of the Slovene Dinaric Karst using the following steps: preprocessing digital elevation models (DEMs), enclosed depression detection, calculating geomorphometric characteristics, and comparing the characteristics of selected areas. We discovered that different combinations of methods influenced the number and geomorphometric characteristics of depressions. The range of detected depressions in the three areas were 442–491, 364–403, and 366–504, and the share of the depressions’ area confirmed with all the approaches was 23%, 29%, and 47%, which resulted in different geomorphometric properties. Comparisons between the study areas were also influenced by the methods, which was confirmed by the Mann–Whitney test. We concluded that preprocessing of high-resolution relief data and the detection methods in karst environments significantly impact analyses and must be taken into account when interpreting geomorphometric results.

A forested wetland at a climate-induced tipping-point: 17-year demographic evidence of widespread tree recruitment failure

Regeneration and survival of forested wetlands are affected by environmental variables related to the hydrologic regime. Climate change, specifically alterations to precipitation patterns, may have outsized effects on these forests. In Tennessee, USA, precipitation has increased by 15% since 1960. The goal of our research was to assess the evidence for whether this change in precipitation patterns resulted in shorter growing seasons and recruitment failure in common canopy trees for a forest wetland. In 2001 and 2018, the density of Quercus lyrata (overcup oak), Liquidambar styraciflua (sweetgum), Quercus phellos (willow oak), and Betula nigra (river birch) seedling, sapling and adult density were mapped in an area of 2.3 ha within a seasonally flooded karst depression. Overall, the percentage of the growing season experiencing inundation was 26% greater in the deep than in shallow areas between 2001 and 2018. Saplings and small adults of all four species were restricted to shallow areas, and their abundance has declined substantially. Overcup oak and sweetgum individuals that were recruited into the adult life history stage were repelled from the deep zone. Overcup oak and sweetgum adults experienced lower mortality across the 2.3-ha study area (11% and 26%, respectively) relative to willow oak (56%) and river birch (64%) over the 17-year study. Growing-season inundation showed no relation to mortality in adult sweetgum and willow oak, a positive relation to mortality among adult river birch across size classes and among small adult overcup oak, and an inverse relation to mortality among large adult overcup oak. In shallow regions, overcup oak and sweetgum adults had greater basal area increment relative to the intermediate and deep regions of the pond. Results of hydrologic modeling for the study area, based on rainfall and temperature records covering 1855–2019, show ponding durations after 1970 considerably longer than the historical baseline, across ponding-depth classes. Our results strongly suggest that climate change is a driving factor suppressing tree regeneration since 1970 in this seasonally flooded karst depression.

Late Holocene hydroclimatic changes inferred from a karst peat archive in the western Guizhou Plateau, SW China

The paleoenvironmental implications of Ca-related indices of peat from karst regions are poorly understood. In this study, we analyze a 450-cm peat core drilled from a sub-alpine karst mountain in the western Guizhou Plateau, SW China. This core is analyzed for carbonate contents and loss on ignition (LOI). High-resolution X-ray fluorescence (XRF) core scanning was also performed to see the variation of chemical compositions. Based on these measurements and 12 accelerator mass spectrometry (AMS)14C dates, we reconstructed the history of hydroclimatic shifts in SW China and explored the significance of Ca-related indices. The reconstructed hydroclimatic conditions were consistent with the stalagmite, lacustrine sediment, and peat records from the regions controlled by the Indian monsoon. Abrupt decreases in precipitation and temperature were clearly shown during the 4.2 ka and Little Ice Age (LIA) cold events. High carbonate contents in the Yejiping peat during the Medieval Warm Period (MWP) were linked to a warm and humid climate. Additionally, wavelet analysis shows that variations in Ca content have 500, 125, 103, 80, 58, 43, 20, and 12-a quasi-periods, which correspond to the Gleissberg, Pacific Decadal Oscillation (PDO), and Schwabe cycles. Our results highlight the importance of Ca-related indices in peat deposits from karst depressions and provide a detailed description of the MWP and the LIA.

Provenance and ore-forming process of Permian lithium-rich bauxite in central Yunnan, SW China

Permian lithium-rich karstic bauxites are newly discovered in central Yunnan province, western margin of the South China Block, with a reserve of 16 million tonnes, which has a great potential for aluminum and lithium exploitation. Provenance and ore-forming processes are still debatable. Mineralogical, geochemical, and geochronological analyses were conducted on two typical bauxite profiles to explore the genetic process of the Permian bauxites in central Yunnan province. Profile A contains three layers from bottom to top: clayey bauxite, claystone, pisolitic bauxite. Profile B contains five layers from bottom to top: claystone, pisolitic bauxite, claystone, pisolitic bauxite, and mottled claystone. Pisolitic bauxite mainly consists of diaspore and chamosite, with small amounts of goethite, anatase, and boehmite. Clayey bauxite dominantly consists of boehmite and kaolinite, with small amounts of diaspore and anatase. Claystone predominantly consists of kaolinite or illite. Three stages of bauxite formation are identified: (1) during weathering of regional sedimentary strata, boehmite, kaolinite, and illite were formed; (2) during bauxite deposition, diaspore, chamosite, anatase, pyrite, with small amounts of illite, kaolinite, and barite were formed; (3) during second exposure caused by paleo-uplift, goethite was formed. Correlation analyses demonstrate that kaolinite is the most likely host mineral of Li in the studied bauxites and the leaching process after bauxite deposition can significantly reduce the lithium contents. Two profiles’ differences in sequence can be attributed to their different locations in the paleokarst terrain. Topography of karst terrain controls the ore-forming conditions, thus affecting the sequence and mineralogy of bauxite profiles. In our case, during the bauxite deposition, the uplifted area was not submerged at first, then as the groundwater rose, the uplifted area was immersed. Therefore, the ore-forming conditions in uplifted areas changed from oxidizing to reducing and became more alkaline. In contrast, the karstic depression was submerged throughout the bauxite deposition. All layers in the karstic depression were formed under reducing and alkaline conditions. Data from U-Pb isotope measurements of detrital zircons from bauxites in central Yunnan province provides 3 main ages, which are 500–650 Ma, 800 Ma, and 980 Ma. There are several minor peak ages in the age spectrum, which are 288–310 Ma, 1500 Ma, 1800 Ma, 2000 Ma, and 2500 Ma. Age distribution pattern of bauxites is more similar to that of the Ordovician sedimentary units than other regional rocks in central Yunnan province, which indicates the major contribution of the eroded Ordovician sedimentary rocks to bauxites.