Geochemistry of the Aptian bituminous limestones in Gümuü?hane area,Eastern Black Sea region:new insight into paleogeography and paleoclimate conditions

Aptian is characterized by widespread deposition of organic-rich sediment.The Aptian bitumen limestone horizon,which is thin decimetre-thick sequences,locally crops out in the Kircaova area,Eastern Black Sea Region(Eastern Pontides).They are well correlated with Aptian bitumen limestone in the other Tethys Reams.They are proposed as episodes of increased organic matter.However,background factors controlling organic matter enrichment are poorly known.In this study,we present new inorganic geochemistry,including trace elements,rare earth elements(REE),redox-sensitive elements(RSE),stable-isotopes(δ~(18)O andδ~(13)C),and total organic carbon(TOC).We integrated new geochemical data with existing stratigraphy,paleontology,and organic chemistry data to provide new insight into the depositional environment and paleoclimate conditions during Aptian.The lacustrine bitumen limestone(LBL)samples have variedδ~(13)C(ave.-1.45‰)andδ~(18)O(ave.-4.50‰).They possess distinct REE patterns,with an average of REE(ave.14.45 ppm)and Y/Ho(ave.35)ratios.In addition,they have variable Nd/YbN(0.28-0.81;ave.0.56)and Ce/Ce*(0.68-0.97;ave.0.86),and relatively high Eu^(*)/Eu(1.23-1.53;ave.1.35).They display seawater signatures with reduced oxygen conditions.The enrichment in RSE(Mo,Cu,Ni,and Zn)and the low Mo/TOC(0.70-3.69;ave.2.41)support a certain degree of water restriction.The high Sr/Ba,Sr/Cu,Ga/Rb,and K/Al records of the LBL facies suggest hot house climatic conditions.The sedimentary environment was probably an isolated basin that is transformed from the marine basin.In addition to depositional conditions,the regional parameters such as the climate,increased run-off period,nutrient levels,alkalinity level,and dominant carbonate producers favored the enrichment in organic matter of LBL facies.Thus,extreme greenhouse palaeoclimate conditions have an important role in organic matter enrichment in the isolated basin.Our results are conformable with the published data from marine,semi-restricted basin,and lacustrine settings in the different parts of the Tethys margin.Thus,this approach provides the first insight into the Aptian greenhouse paleo-climate conditions of the Eastern Black Sea Region,NE Turkey.

Meso-mechanical anisotropy and fracture evolution of reef limestones from the Maldives Islands and the South China Sea

Reef limestone is a biogenic sedimentary rock widely distributed in coral reef areas, acting as an important foundation for coast construction. Due to its special biogenic origin, reef limestone is different from conventional rocks both in terms of rock structure and mechanical properties. In this study, mesoscale uniaxial compression experiments with five different loading directions were conducted on two kinds of reef limestones from the Maldives Islands and the South China Sea, respectively. The real-time high-resolution videos and images of failure processes were recorded simultaneously to investigate the fracture evolution and fracture surface roughness of reef limestones. It demonstrated that the reef limestones belonged to extremely soft to soft rocks, and their uniaxial compressive strength (UCS) values fluctuated with high discreteness. The mesoscale mechanical properties of reef limestones were highly anisotropic and mainly controlled by pore structure. The occurrence of dissolution pores in reef limestone tended to intensify mechanical anisotropy. With the integration of the fracture initiation and propagation features of reef limestones, it is supposed that the intrinsic mechanism of anisotropy was probably attributed to the differences in coral growth direction and dissolution. Furthermore, the quantified fracture surface roughness was revealed to have a good consistency with density and UCS for the reef limestones from the South China Sea. The findings are helpful for providing theoretical and experimental references for engineering construction in coral reef areas.

Lithostratigraphy and Characterisation of Paleocene Limestones for Optimal Exploitation (Senegal, West Africa): Comparative Study of the Bandia and Popenguine Quarries

Limestones of different facies in the Senegal basin are most commonly used as aggregates, regardless of their petrographic properties. These differences may influence their behaviour. The aim of this paper is to study the stratigraphy and characterise the limestones of the Bandia and Popenguine quarries. Sampling and geochemical and geotechnical characterisation were carried out on these limestones. The results show the presence of several very different facies in the Paleocene formation. The geochemical study shows that the Bandia limestones (Bd2) have the highest CaCO3 percentages (98.54%) and the highest SiO2 percentages were recorded in the Popenguine limestones (Pop2). The study also gives MDE values of 22.38 and LA of 22, which are acceptable for use in road construction. It is important to carry out a complete study in order to select the right materials for optimal use.

New Evidence for Hydrothermal Sedimentary Genesis of the Ni-Mo Deposits in Black Rock Series of the Basal Cambrian, Guizhou Province: Discovery of Coarse-Grained Limestones and its Geochemical Characteristics

The molybdenum-nickel deposits in Shuidong District of Nayong County （Guizhou Province, Southwest China） are found mainly in black shale series of Lower Cambrian Niutitang Formation, which is another Mo-Ni-rich region besides Zunyi District （Guizhou province）. Our systematic study on the Mo-Ni deposits in Tangjiaba of Nayong reveals that layered coarse-grained limestones, spherical beaded limestones concretions are hosted at the lower seam of the Mo-Ni deposits. Its strong negative carbon isotope anomaly （the carbon isotope value of the coarse-grained limestones varies from -2.148‰ to 8.223‰） is similar to that in the modern submarine black smoker chimney. The carbon in the coarse-grained limestones from black rock series of Nayong County might be deep source inorganic carbon. The seams, coarse-grained limestones, ore-bearing coarse-grained limestones and the roof and floor of the deposits are characterized by co-variation on the trace element spider diagram, showing good homology. The extraordinary enrichment of Ag, As and Sb resembles hydrothermal sedimentation. Pro-Earth＇s core elements Se is strongly enriched in Ni-Mo ore-bearing coarse-grained limestones. The ore-bearing rock series has an extremely low Th/U value （0.012-0.19）; in the logU-logTh Cartesian Coordinates, the samples of the roof and floor of the deposits and ore-bearing coarse-grained limestones are found in the East Pacific tise; and the samples of coarse-grained limestones are found between the paleo-hydrothermal dedimentary area and the East Pacific tise. The chondrite-normalized rare earth element patterns of the Ni-Mo deposits show LREE enrichment, Ce negative anomaly, and Eu negative anomaly （which is supposed to be influenced by the deep magmatic processes in an extensional environment） resembles the rare earth element distribution patterns of the fluid and its sediments in modern submarine hydrothermal system. It proves that coarse-grained limestones is characterized by typical hydrothermal limestones, being closely related with the genesis of Mo-Ni deposits in Nayong County, which provides new evidence for hydrothermal sedimentary genesis of Mo-Ni deposit and negative carbon anomaly in the basal Cambrian on a global scale.

Rare Earth Elements and Yttrium(REY) Geochemistry of Reefal Limestones in the Ordovician, Tarim Basin, NW China and their Paleoenvironment Implications

This study examines the rare earth elements and yttrium(REY)concentrations of twenty-five samples from the reef outcrop exposed along the Lianglitage Mountain in the Ordovician,Tarim Basin in China.The concentration analysis provides constraints on the paleoenvironment during reef deposition.Based on the detailed sedimentology and petrographic work,we divide the reef facies into four sub-facies:the base facies,reef-core facies,reef-flank facies,and sealing facies.The geochemical data(such as major and trace elements,carbon and oxygen isotopes,and REYs)are further used to study the coeval seawater characteristics as well as potential diagenesis overprints.The result indicated that the diagenesis has little effect on the REY patterns of the reefal limestones.The REY concentrations of the reefal limestones are overall low(ranging from 3.69 to 19.60 ppm,arithmetic mean=10.22 ppm,SD=5.4).The PAAS-normalized REY patterns are consistently flat compared to the typical well-oxidized,shallow marine water patterns.However,the light REE(LREE)depletions,positive La anomalies,negative Ce anomalies and positive Y anomalies,suggest that these reefal limestones are likely an indicative of contemporaneous seawater REY signals.The seawater-like Y/Ho ratios(average at 37.51)further support that REY signals in these limestones are likely a reflection of seawater with little diagenetic modifications.The low Y/Ho ratios presented only in the reef-flank facies and sealing facies are likely a suggestion of detrital contamination.Hence,this study confirms that REY patterns of the limestones at the base facies and reef-core facies can record ancient seawater information,and reefs can be used as a potential geochemical proxy for paleoenvironment studies throughout the Earth’s history.

Magnetic Susceptibility and Natural Gamma Radioactivity as Indirect Indicators of Impurities in Limestones of the Sabinas Basin: Possible Relationships with the Sedimentation Environment

This study presents an analysis of magnetic susceptibility and natural gamma radioactivity as indirect indicators of impurities in Cretaceous limestones of the Sabinas basin, as well as of the possible relation of these physical properties with the changes in the sedimentation environment. Both of these physical properties indicate changes in the degree of impurities and the mineralogical composition of this rock, principally in its organic matter and detritus content. Considering that the sedimentation environment determines the impurities in these rocks, possible environmental conditions at different levels of the Cretaceous were suggested using the magnetic susceptibility, total gamma radiation and potassium values, as well as the uranium and thorium concentration in the limestones. The analyses suggest variations in the depth of water table of the basin, which are mainly related to transgressions and marine regressions.

Re-Assembly of Archaeological Massive Limestones Using Epoxy Resin Modified with Nanomaterials—Part 1: Experimental

Massive limestones were used in construction of ancient Egyptian tombs, temples, obelisks and other sculptures. These stones are always exposed to physico-mechanical deterioration and destructive forces, leading to partial or total collapse. The task of reassembling this type of artifacts represents a big challenge for the conservators. Recently, the researchers are turning to new technologies to improve the properties of traditional adhesive materials and techniques used in re-assembly of broken massive stones. The epoxy resins are used extensively in stone conservation and re-assembly of broken stones because of their outstanding mechanical properties. The adding of nanoparticles to polymeric adhesives at low percentages may lead to substantial improvements of their mechanical performances in structural joints and massive objects. The aim of this study is to evaluate the effectiveness of montmorillonite clay, calcium carbonate, and silicon dioxide nanoparticles for enhancing the performances of epoxy adhesives used in re-assembly of archaeological massive limestones. Scanning electron microscopy (SEM) was employed in order to investigate the morphology of the prepared nanocomposites, and the distribution of nanoparticles inside the composites. Artificial aging, tensile, compressive, and elongation strength tests were used to evaluate the efficiency of epoxy-nanocomposites. The results showed that the epoxy-clay nanocomposites exhibited superior tensile, compressive, and elongation strength, in addition to improving the mechanical properties of stone joints.

Petrography and geochemistry of Paleocene-Eocene limestones in the Ching-dar syncline,eastern Iran

The Ching-dar syncline is located to the west of the city of Birjand, in the east of han. The ca. 500 m thick studied section at the eastern flank of the syncline contains a sequence of almost continuous shallow- marine limestones that exhibit no major sedimentary breaks or evidence for volcanic activity. Skeletal grains consist of large benthic foraminifera and green algae whereas non-skeletal grains are mostly peloids and intraclasts. They were deposited on a shallow-marine carbonate ramp. The limestones have undergone extensive diagenetic processes with varying intensities, the most important of which are micritization, cementation, compaction （chemical and mechanical）, internal filling and stylolitization. Chemical analysis of the limestone samples revealed high calcium and low magnesium content. Major and minor element values were used to determine the original carbonate mineralogy of these lime- stones. Petrographic evidence and elemental values indicate that calcite was the original carbonate mineral in the limestones of the Ching-dar syncline. The elemental composition of the Ching-dar car- bonates also demonstrates that they have stabilized in a meteoric phreatic environment. Variation of Sr/ Ca vs. Mn values suggests that diagenetic alteration occurred in an open geochemical system.

Determination of C_((total))and O_((total))in the Calcite Rich Limestones by WD-XRF Spectrometry

Carbon and oxygen XRF spectra were studied using a WDXRF(WD X-ray fluorescence)spectrometer S8 TIGER(Bruker AXS,Germany).XRF spectra were obtained from the samples of the calcareous spar(CaCO3)(pure calcium carbonate),the limestones,the graphite,the microcrystalline cellulose,various organic compounds and prepared mixtures on the basis of the calcareous spar with addition of the graphite and the microcrystalline cellulose.The studied samples of the limestones are characterized by high concentrations of calcite(98.5-99%).Main objective of this investigation is to develop WDXRF technique for the determination of C(total)and O(total).Optimal instrumental measuring conditions were selected experimentally for quantitative determination of C(total)and O(total).The calculated values of the ILD(instrumental limit of the detection)vary from 0.0215%for C(total)to 0.0331%for O(total),respectively.The obtained values of the LDM(limit of determination of the XRF method)do not exceed 0.08%for C(total)and O(total).The RSDs(relative standard deviations)values do not exceed 1.0%in all cases.

Diffusion and reaction mechanism of limestone and quartz in fluxed iron ore pellet roasting process

The increase to the proportion of fluxed pellets in the blast furnace burden is a useful way to reduce the carbon emissions in the ironmaking process.In this study,the interaction between calcium carbonate and iron ore powder and the mineralization mechanism of fluxed iron ore pellet in the roasting process were investigated through diffusion couple experiments.Scanning electron microscopy with energy dispersive spectroscopy was used to study the elements’diffusion and phase transformation during the roasting process.The results indicated that limestone decomposed into calcium oxide,and magnetite was oxidized to hematite at the early stage of preheating.With the increase in roasting temperature,the diffusion rate of Fe and Ca was obviously accelerated,while the diffusion rate of Si was relatively slow.The order of magnitude of interdiffusion coefficient of Fe_(2)O_(3)-CaO diffusion couple was 10^(−10) m^(2)·s^(−1) at a roasting temperature of 1200℃for 9 h.Ca_(2)Fe_(2)O_(5) was the initial product in the Fe_(2)O_(3)-CaO-SiO_(2) diffusion interface,and then Ca_(2)Fe_(2)O_(5) continued to react with Fe_(2)O_(3) to form CaFe_(2)O_(4).With the expansion of the diffusion region,the sillico-ferrite of calcium liquid phase was produced due to the melting of SiO_(2) into CaFe_(2)O_(4),which can strengthen the consolidation of fluxed pellets.Furthermore,andradite would be formed around a small part of quartz particles,which is also conducive to the consolidation of fluxed pellets.In addition,the principle diagram of limestone and quartz diffusion reaction in the process of fluxed pellet roasting was discussed.

Hydration Behavior and Cementitious Properties of Calcium Carbonate-aluminate Minerals Composite

The purpose of this research is to investigate the hydration behavior and cementitious properties of the mixture of calcium carbonate and aluminate, and to explore whether it can be adopted as a new low-carbon cementitious material. The composite system of calcium carbonate and aluminate minerals is studied by measuring the component of hydration products, the hydration heat, setting time and compressive strength.The results prove that the composite system has certain cementitious properties and is feasible to prepare new low-carbon cement.

Competition between Coral and Algae in Tertiary and Quaternary Reefs: Greenhouse to Icehouse Transitions

The competition between coral and algae in marine reefs is pervasive through geologic time;that competition determines the structure and composition of reef communities, which we see in the fossil record. However, the relationships between coral and calcareous algae in reefs are poorly understood. To study this relationship, several hand samples and thin sections were examined from nine different foralgal reef localities around the world. Foralgal reefs typically extend from about 20 m depth or shallower on the seaward side of the reef. The first section is Salt Mountain, Alabama, which preserves a Paleocene reef. It contains a high percentage of red coralline algae with benthic foraminifera. The second section is IDOP-U1376, IIA Limestone, it is Middle Eocene, in the form of an isolated reef sandwiched between two igneous beds. The third section is the Utoe’ Limestone, New Caledonia, it is Middle Eocene in age and is composed mainly of grain-boundstone units with some igneous interlayered. The fourth section is the Darnah Formation in the West-Darnah roadcut section, Northeast Libya, it is Middle Eocene in age, it is composed of highly fossiliferous limestone (corals, red coralline algae, echinoids, mollusks, foraminifers, and bryozoans). The fifth section, the Al Bayda Formation (Algal Limestone Member) in Northeast Libya, is in the Drayanah—Al Abyar roadcut, Northeast Libya, it has several species of algae but also includes a high percentage of buildups of coral species. The sixth section is the Oligo-Miocene Al Faidiyah Formation (Al Fatayah Cement Quarry) limestone unit in Northeast Libya. The seventh section is (Core-core 20) late-early to middle Miocene Limestone Unit-Cicuco Field, NW Colombia. The eighth section is the Benghazi Formation at Benghazi Cement Quarry, in Northeast Libya, it is fossiliferous limestone, consisting of coral, algae, mollusks, and echinoids. The ninth and tenth sections are Quaternary reefs in the Bahamas and the Florida Keys, respectively. These reefs contain a high percentage of coral, red coralline algae, echinoids, mollusks, foraminifers, and ostracods. Based on the data and static analysis results on the thin sections and hand specimens, this study determines the occurrence and outcomes of coral-algal interactions among different coral growth forms (branching, upright, massive, encrusting, plating, and solitary). The Early Paleogene (Paleocene to Eocene) has the highest percentage of algae in two forms (crustose and frondose), which is a good indicator of a warm climate. In the Middle Eocene to Late Eocene, coral replaced algae in different localities in sections of that age. This change is an indicator of climatic cooling, especially in the western Lutetian Darnah section. In the Oligocene time, high-branching corals became abundant and escaped competition with the algae due to Icehouse conditions, as shown in the Al Bayda Formation. In the Miocene, coral species started to decline because of the return to Greenhouse conditions. Coral can lose its competitive edge when chemical and physical defense systems reduce growth and production due to warming. On the other hand, crustose-form algae attract the larvae of the coral. Algae induce them to get a more highly competitive frondose form, which is useful for corals as they decrease growth and production. Algae can quickly colonize the dead reef by using the firm substrate to rebuild themselves. This research may prove valuable when predicting the response of modern coral reef systems to future climatic warming conditions and provides a model for what future reefs may look like.

Emirati Limestones: Impact of Low Temperature Microstructure on the Industrial Applications

The main aim of this work is to study the possibility of using different Emirati limestones for the production of quicklime. Representative limestone samples have been collected to represent the Triassic, Jurassic, Cretaceous, Eocene and Oligocene ages. The limestone samples have been characterized for their microstructure, mineral and chemical composition, physico-mechanical characteristics, thermal behavior using polarizing and scanning microscopes together with X-ray micro-tomography, XRD-IR, XRF, Archimedes and Mercury intrusion methods and DTA-TGA, respectively. Post characterization, the samples were fired in electrical muffle furnace for calcination under the firing conditions (800℃ to 1100℃) for (0.25 – 2 h). Then the lime grains have been characterized for their hydration rate, free lime content, pore-distribution and microfabric.

Re-evaluation of several heavy metals removal by natural limestones

Different treatment technologies have been efficiently applied to remove heavy metals from waste- water. Efforts have been made to find out the most economic water treatment technology by using low cost and easily accessible natural materials. On the other hand, heavy metals are the most threatening groundwater contaminants because of their toxicity and harmful effects on human and biota. This review discusses the use of natural geological materials for heavy metal removal in aqueous systems. Special attention has been devoted to natural limestone through a systematic inventory of relevant published reports. The removal of toxic metals may include different mechanisms （e.g., physisorption, chemisorptions, precipitation, etc.）, depending on the physico-chemical properties of the material and the removed metal. Sorption of toxic metals （e.g., Pb, Cu, Cd, Zn, Cr, Hg, etc.） onto natural limestone involved precipitation of metal carbonate as a predominant removal process, but often subordinated by adsorption and ion exchange, depending on the physico-chemical properties of the studied limestone.

DIVIDING STAGE IN UPPER CARBONIFEROUS FUSULINID-BEARING LIMESTONES, SHANXI

There are 8 hayers of the Fusulinid-bearing limestone distributed widely and stably in the upper Carboniferous series of Shanxi, their unified names and sequence from bottom upward being Wuming Limestone, Wujiayu Limestone, Miaogou Limestone, Maogou Limestone, Xiedao Limestone, Dongdayao Limestone, Fucheng Limestone and Xiaodonggou Limestone (Table 1).

Hydrocarbon generation and organic matter enrichment of limestone in a lacustrine mixed sedimentary environment: A case study of the Jurassic Da'anzhai member in the central Sichuan Basin, SW China

The hydrocarbon generation effectiveness of lacustrine limestone has been gradually proven. The Da'anzhai Member limestone is the most important Jurassic oil-producing layer in the central Sichuan Basin, and the characteristics of limestone organic matter are often overlooked. 175 typical samples of different lithologies from 19 wells were systematically analyzed to determine hydrocarbon generation, controlling factors and formation models by analyses of organic matter, minerals, elements, isotopes and petrography. (1) Lacustrine paleoenvironments can be beneficial for the enrichment of organic matter in limestone. A favorable environment would be a quiet, low-energy zone in a warm and humid climate with an appropriate supply of terrestrial inputs. (2) Lacustrine limestone has a higher organic matter conversion rate, and a lower hydrocarbon generation threshold than argillaceous source rocks, and can be effective source rock. (3) The mud-bearing shell limestone from the forebeach to the lake slope is thick, with a relatively high abundance of organic matter, and its hydrocarbon generation is effective. This study can clarify the effectiveness and enrichment of the limestone organic matters in the study area, and contribute to an understanding of hydrocarbon generation for full-rock system in a lacustrine mixed sedimentary environment.

In situ infiltration-precipitation processes in some rock systems

We carry out a chemical treatment(acidization or basification)of typical rock specimens in-situ,and characterize the emerging pattern resulting from the infil-tration-precipitation scenario.Galena and limestone sam-ples were treated with sulfuric acid,while pyrite was reacted with sodium hydroxide.Various infiltration tech-niques were employed,after selection of the most feasible method for each rock separately.The patterns of anglesite(PbSO_(4)),anhydrite(CaSO_(4))and goethite(FeOOH)depo-sition presented different alteration modes of the bare rock textures.Among the three deposited minerals,only the anhydrite(CaSO_(4))displayed a band stratification.The formation of a Liesegang pattern in the rock of highest porosity indicates a plausible correlation between the band formation and a minimum porosity requirement.A banded rock of compact texture could then be formed by a cementation mechanism,governing the long time evolution of the rock.

Effect of High Content Limestone Powder on Microstructure and Mechanical Properties of Cement-based Materials

Compared with the control sample without limestone powder(LP), the mechanical properties of the sample with 30% LP can be significantly improved by using a small amount of water reducer to reduce the water-cement ratio, without significantly affecting the fluidity of the fresh mixture and increasing the economic cost. In addition, compared with the sole addition of limestone powder, dual addition of metakaolin and limestone powder can effectively improve the strengths. The reason of this phenomenon was investigated by means of XRD, TG-DTG, SEM, LF-NMR and isothermal calorimetry, etc. The reactive aluminum-rich phases in metakaolin react with limestone powder in the hydration process, and the formed calcium carboaluminate reduces the porosity and makes the hardened paste denser. The addition of ground granulated blast furnace slag can also improve the strength of the specimen added with limestone powder, whereas, the effect is inferior to that of metakaolin, for the ground granulated blast furnace slag contains less reactive aluminate phases, and accordingly, the amount of calcium carboaluminate generated is lower than that of metakaolin.

The formation of orthogonal joint systems and cuboidal blocks:New insights gained from flat-lying limestone beds in the region of Havre-Saint-Pierre(Quebec,Canada)

Vertical orthogonal joints are a common feature in shallow crustal rocks.There are several competing theories for their formation despite the ubiquity.We examined the exceptional exposures of orthogonal joints in flat-lying Ordovician limestone beds from the Havre-Saint-Pierre Region in Quebec,Canada(north shore of Saint-Lawrence River)to test conceptual models of joint formation in a natural setting.In the region,the spacing of cross-joints is consistently larger than the spacing of systematic joints by a factor of 1.5 approximately.The joint-spacing-to-bed-thickness ratios(s/t)are much larger in these beds(s/t=4.3 for systematic joints,and 6.4 for cross-joints)than those in higher strained strata along the south shore of the Saint-Lawrence River(s/t=1),highlighting the effect of tectonic strain in decreasing fracture spacing and block size.The high values of s/t indicate that cross-joint formation was unlikely caused by a switch from compression to tension once a critical s/t ratio for systematic joints was reached(as hypothesized in previous studies).We proposed a new model for the formation of orthogonal joint systems where the principal stress axes locally switch during the formation of systematic fractures.The presence of ladder-shaped orthogonal joints suggests a state of effective stress withσ_(1)^(∗)≫0>σ_(2)^(∗)>σ_(3)^(∗)and whereσ_(2)^(∗)-σ_(3)^(∗)is within the range of fracture strength variability at the time of fracture.This research provides a new mechanical model for the formation of orthogonal joint systems and cuboidal blocks.

Orchid diversity and distribution pattern in karst forests in eastern Yunnan Province,China

Global biodiversity loss and mass extinction of species have raised serious global concerns,especially in fragile ecosystems.Karst forest,a well-known fragile ecosystem,is served as a natural laboratory for biogeographical,ecological,evolutionary,and taxonomic research.However,species richness and distribution patterns in karst forests largely remain unclear.Orchidaceae has been regarded as the“flagship”group of plant diversity.In this study,we use Wanfengshan Nature Reserve in southeast Yunnan,China,as a model to determine the conservation status of orchids in the karst forest by conducting systematic field surveys in the past three years from 2019 to 2022.In total,we identified 78 orchid species in 35 genera in Wanfengshan Nature Reserve.The dominant species were Bulbophyllum andersonii,Eria coronaria,Nervilia mackinnonii and Paphiopedilum micranthum.Multiple life forms were also observed,including epiphyte(14.29%),facultative epiphyte(18.18%),lithophyte(23.38%),and terrestrial(44.15%).Orchid species richness showed a bimodal pattern varied with altitude,the higher richness was recorded at 1,500-1,600 and 1,200-1,300 m,while the minimum was below 1,100 m.Each independent hill is rich in orchid species including‘endemic’species.A total of 25(33.8%)orchid species were considered threatened by the Redlist of Chinese Vascular Plants(Qin et al.,2017),including 3 CR species,5 EN species and 17 VU species.We call for more research addressing the mechanisms of ecological adaption,mycorrhizal interactions,and pollination of orchids in karst forests,and effective ways for recovery and conservation of orchids.