Organic matter in anoxic marine pore water: oxidation effects

A series of cores were obtained from the Great Bay Estuary, New Hampshire during 1980 and 1981 in order to investigate the effects on the dissolved organic matter of exposure of anoxic marine pore water to atmospheric oxygen during laboratory handling. Sediment sections from these cores were homogenized and split into two equal parts, one of which was handled in a glove bag under a nitrogen atmosphere, and the other exposed to atmospheric oxygen during laboratory processing. Analysis of the pore water for dissolved organic carbon (DOC) indicated two effects: (1) in the top 15 cm, DOC was lost from the pore water of the oxidized subsections, probably as a consequence of co-precipitation with iron (III) oxi-hydroxides, whereas (2) deeper in the cores, the pore water from the oxidized subsections showed a net increase in DOC, possibly due to oxidation of sedimentary organic matter. In addition, structural changes in the dissolved organic matter in anoxic pore water following oxidation was indicated by high pressure liquid chromatography and ultrafiltration analysis.     

Mobility of arsenic in a Bangladesh aquifer: Inferences from geochemical profiles, leaching data, and mineralogical characterization

Aquifer geochemistry was characterized at a field site in the Munshiganj district of Bangladesh where the groundwater is severely contaminated by As. Vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer (depth >150 m) below a thick confining clay (119 to 150 m), a sandy upper aquifer (3.5 to 119 m) above this confining layer, and a surficial clay layer (<3.5 m). In the deep aquifer and near the top of the upper aquifer, aqueous As levels are low (<10 μg/L), but aqueous As approaches a maximum of 640 μg/L at a depth of 30 to 40 m and falls to 58 μg/L near the base (107 m) of the upper aquifer. In contrast, solid phase As concentrations are uniformly low, rarely exceeding 2 μg/g in the two sandy aquifers and never exceeding 10 μg/g in the clay layers. Solid phase As is also similarly distributed among a variety of reservoirs in the deep and upper aquifer, including adsorbed As, As coprecipitated in solids leachable by mild acids and reductants, and As incorporated in silicates and other more recalcitrant phases. One notable difference among depths is that sorbed As loads, considered with respect to solid phase Fe extractable with 1 N HCl, 0.2 M oxalic acid, and a 0.5 M Ti(III)-citrate-EDTA solution, appear to be at capacity at depths where aqueous As is highest; this suggests that sorption limitations may, in part, explain the aqueous As depth profile at this site. Competition for sorption sites by silicate, phosphate, and carbonate oxyanions appear to sustain elevated aqueous As levels in the upper aquifer. Furthermore, geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of Fe(III) oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved As levels in the upper aquifer. Microprobe data indicate substantial spatial comapping between As and Fe in both the upper and deep aquifer sediments, and microscopic observations reveal ubiquitous Fe coatings on most solid phases, including quartz, feldspars, and aluminosilicates. Extraction results and XRD analysis of density/magnetic separates suggest that these coatings may comprise predominantly Fe(II) and mixed valence Fe solids, although the presence of Fe(III) oxyhydroxides can not be ruled out. These data suggest As release may continue to be linked to dissolution processes targeting Fe, or Fe-rich, phases in these aquifers.     

Effects of iron on arsenic speciation and redox chemistry in acid mine water

Concern about arsenic is increasing throughout the world, including areas of the United States. Elevated levels of arsenic above current drinking-water regulations in ground and surface water can be the result of purely natural phenomena, but often are due to anthropogenic activities, such as mining and agriculture. The current study correlates arsenic speciation in acid mine drainage and mining-influenced water with the important water-chemistry properties Eh, pH, and iron(III) concentration. The results show that arsenic speciation is generally in equilibrium with iron chemistry in low pH AMD, which is often not the case in other natural-water matrices. High pH mine waters and groundwater do not always hold to the redox predictions as well as low pH AMD samples. The oxidation and precipitation of oxyhydroxides deplete iron from some systems, and also affect arsenite and arsenate concentrations through sorption processes.     

Modelling of modern mercury vapor transport in an ancient hydrothermal system: environmental and geochemical implications

Mercury has been used in some mineralizing hydrothermal systems as a significant pathfinder of Au and other elements. In addition, its geochemical behaviour reflects some characteristics related to the transport mechanism of metal-bearing fluids. In this work the authors analyse the geochemical aspects and the environmental impact of Hg mobilization from the old mining area of Valle del Azogue (VA), from smelting slags, mining waste and the partially exploited mineralization that remains a few metres underground. The main pathway of Hg is the transport of vapor from different foci, therefore a mathematical simulation of the process of Hg vapor transport was carried out in the Valle del Azogue (VA) mineral deposit. This is one of the two sites in Spain (the other being the famous district of Almaden), where Hg vapor emissions have been cited. The results obtained by means of mathematical simulation were compared with both the real Hg concentration values detected in the field (upper part of the mining area), and with the geochemical distribution patterns of Au and other elements related to the mineralization. A close relationship between the simulated Hg vapor distribution and soil gas concentrations was identified. It is proposed that the transport of Hg emissions from this old mining area could be the cause of present contamination of marine sediments in the area.     

Mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the Kuroko mine area, Japan: implications for the evolution of a Back Arc Basin hydrothermal system

Basalt in the Furutobe District of the Kuroko mine area in Japan is characterized by abundant chlorite and epidote. Fluid inclusion studies indicate that chlorite is formed at lower temperatures (230–250°C) than epidote (250–280°C). The seawater/basalt mass ratio for the early chlorite-rich alteration was high (max. 40), but that for the later alteration was low (0.1–1.8). The CaO, Na₂O and SiO₂ of the bulk rock correlate negatively with MgO, while FeO and Σ Fe correlate positively with MgO. These changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid-ocean ridge geothermal system accompanying basalt alteration.The MgO/FeO ratios of chlorite and actinolite and the Fe₂O₃ concentration of epidote from the basalt are greater than those of mid-ocean ridge basalt probably owing to the differences in the Fe₂O₃/FeO and MgO/FeO ratios of the parent rocks. The lower CaO concentration and the higher Na₂O concentration of the bulk rock compared with altered mid-ocean ridge basalt can be interpreted in terms of the difference in original bulk rock compositions.The Furutobe basalt, as well as other submarine back arc basalts, contains more vesicles filled with hydrothermal minerals (epidote, calcite, quartz, chlorite, pyrite) than do the mid-ocean ridge basalts. The abundance of vesicles plays an important role in controlling the secondary mineralogy and geochemistry of hydrothermally altered submarine back arc basin basalts.     

Mineralogical and geochemical characteristics of the hydrothermal illite from Hoam granite,South Korea: Implications for episodic fluid injections in the hydrothermal alteration system

Two stages of illite mineralization are recognized in the hydrothermal alteration zone of the Hoam granite. These illites are formed as a result of pervasive alteration by re-equilibration with high water/rock in a brittle environment below <2 km; the mineralization timing is middle Oligocene (26–27 Ma), coinciding with the timing of crustal deformation related to the opening of the East Sea (Sea of Japan). The mineralogical and geochemical characteristics of the clearly distinguished illites at each site indicate that they were mineralized from different fluid injections in distinct geological environments. Illites at the site-1 alteration zone are characterized by high-K content [K_0.84 per O₁₀(OH)₂], 2M₁ polytype of 99 %, hexagonal plate shape, and coexistence with pyrite. These observations indicate that the illites were formed in a slow cooling system (>250 °C), high fS₂, and a relatively acidic environment. The pseudomorphic replacement combined with matrix-filling texture indicates that the illites at the site-1 alteration zone recorded the changes in fluid conditions from low to high water-rock ratio. In contrast, the illites at the site-2 alteration zone show the coexistence of polytypes (2M₁, 1M, and 1M_d), high-K illites [(K_0.83 per O₁₀(OH)₂]/low-K illites [K_0.63 per O₁₀(OH)₂], platy/hairy shapes, and presence of magnetite. Furthermore, this alteration zone no longer exhibits primary textures because of pervasive alteration induced by the dissolution-precipitation process. These results indicate that they were formed in a rapid cooling system and were continuously under conditions of high water-rock ratio, as well as in a less acidic and fS₂ environment than that observed at the site-1 alteration zone. The behavior of trace elements for each illite primarily depends on the constituents of the hydrothermal fluid, which reflect different degrees of fluid evolution. The enrichment of high field strength elements (Nb and Ta), large ion lithophile elements (B, Be, and Cs), rare earth elements, and actinide elements (U and Th) in illite at the site-2 alteration zone shows that these elements formed by a more evolved fluid than that of the illite at the site-1 alteration zone. In addition, negative Ce anomalies at the site-2 alteration zone indicate that these crystallized in a reducing environment. Considering the mineralogical and geochemical properties of illites at the site-1 and site-2 alteration zones, the illite mineralization in the Hoam granite was likely generated by at least two episodes of hydrothermal illite mineralization, which originated from episodic injections of fluids, rather than continuously evolved fluids.     

Trace metal speciation in sea and pore water of the Gotland Deep,Baltic Sea, 1994

Studies on the speciation (particulate, colloidal, anionic and cationic forms) of trace metals (Cd, Co, Cu, Fe, Mn, Mo, Ni, Pb, Zn) in the water column and in pore waters of the Gotland Deep following the 1993/94 salt-water inflows showed dramatic changes in the total “dissolved” metal concentrations and in the ratios between different metal species in the freshly re-oxygenated waters below 125 m. Changes in concentrations were greatest for those metals for which the solubility differs with the redox state (Fe, Mn, Co) but were also noted for those metals which form insoluble sulphides (Cd, Pb, Cu, Zn) and/or stable complexes with natural ligands (Cu). Pore water data from segmented surface muds (0–200 mm) indicated that significant redox and related metal speciation changes took place in the surface sediments only a few weeks after the inflow of the oxygenated sea water into the Gotland Deep.     

高砷地下水的反向地球化学模拟:以中国吉林砷中毒病区为例

饮水型砷中毒分布在中国台湾、新疆、内蒙、山西、吉林等地。笔者采用GIS的空间数据叠加技术、化验测试与环境模拟技术,进行了地下水砷的反向地球化学模拟研究。研究表明,受构造运动控制,低洼地带堆积了巨厚的粉砂淤泥质沉积物和富含有机质的湖积物,为砷的赋存提供了空间。地下水砷的富集受水中Fe、Mn、pH、Cl-、PO34-、HCO3-、SO24-、Se的影响,其中,重碳酸钙型水中砷含量最低,氯化物重碳酸钠型水砷含量最高。臭葱石(FeAsO4:2H2O)等含铁、含锰矿物在进入地下水的溶解过程中,形成铁(锰)氧化物和砷化合物(砷酸盐或亚砷酸盐)。随着Eh降低,氧化物被还原形成更为活泼的离子组分,吸附在氧化物表面的砷化合物随之解吸,还原环境有利于砷从沉积物中向水中溶解、迁移。研究结果为实施安全供水提供了重要依据。     

Helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in Escanaba Trough

Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/R_A=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ¹³C(CH₄)=−43‰, δ¹³C(C₂H₆)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH₄. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.     

Groundwater contamination with arsenic in Sherajdikhan,Bangladesh: geochemical and hydrological implications

An integrated study has been carried out to elucidate the distribution and occurrence of arsenic in selected groundwater samples in the area of Sherajdikhan, Bangladesh. Arsenic and other parameters (T, pH, EC, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, NO₃ ⁻, SO₄ ²⁻, HCO₃ ⁻, PO₄ ³⁻, Fe, Mn and DOC) have been measured in groundwater samples collected from shallow/deep tube wells at different depths. Hydrogeochemical data suggest that the groundwaters are generally Ca–Mg–HCO₃ and Mg–Ca–HCO₃ types with bicarbonate (HCO₃ ⁻) as the dominant anion, though the other type of water has also been observed. Dissolved arsenic in groundwater ranged from 0.006 to 0.461 mg/l, with 69% groundwater samples exceeded the Bangladesh limit for safe drinking water (0.05 mg/l). Correlation and principal component analysis have been performed to find out possible relationships among the examined parameters in groundwater. Low concentrations of NO₃ ⁻ and SO₄ ²⁻, and high concentrations of DOC, HCO₃ ⁻ and PO₄ ³⁻ indicate the reducing condition of subsurface aquifer where sediments are deposited with abundant organic matter. Distinct relationship of As with Fe and Mn, and strong correlation with DOC suggests that the biodegradation of organic matter along with reductive dissolution of Fe–Mn oxyhydroxides has being considered the dominant process to release As in the aquifers studied herein.     

High resolution profiles of trace metals in pore waters of marine and riverine sediments assessed by DET and DGT

In polluted aquatic systems, toxic metals are often accumulated in bottom sediments. They are, however, not necessarily stored definitively because diagenetic processs can modify redox, pH and even the amount of complexing ligands, releasing the trace metals back into the pore waters and the water column. Especially the labile metal fraction in the pore waters is important since this is the bioavailable fraction determining the bio-toxicity of the sediments. The goal of our study was therefore to assess, with novel sampling techniques, this bioavailable metal fraction in the pore waters as well as the flux towards the overlying water column. High-resolution profiles of trace metals in pore waters of marine and riverine sediments were assessed by DET （diffusive equilibrium in thin films） and DGT （diffusive gradients in thin films） gel techniques. The DET technique uses a diffusive gel layer that equilibrates with the aquatic system and with this technique the concentrations of total dissolved trace metals are obtained directly. The DGT technique uses an acrylamide diffusive gel backed by a resin gel （Chelex） which binds trace metals. With the DGT technique only labile species of selected metals can be captured. According to the redox potential measurements, the marine sediments were suboxic （200 mV to -220 mV versus Ag/AgCl electrode）, while the riverine sediments were completely anoxic （-160 mV to -220 mV versus Ag/AgCl electrode）. This redox potential was apparently controlling the trace metals species in the pore waters： for example a strong correlation between Mn and Co was found in the riverine sediments （for DET and DGT sampling）, while in the marine sediments trace metals presented various behaviors.     

Petrographic and geochemical analysis of caliche profiles in a Bahamian Pleistocene dune

Two caliche profiles from a Pleistocene carbonate dune on San Salvador Island, Bahamas, were examined by petrographic and geochemical analysis. Profile A is an immature buried caliche profile characterized by caliche pisolites, a friable crust and abundant Cerion. Profile B is a more well-developed caliche profile at the top of the dune which contains abundant pisolites, rhizomorphs, laminated calcrete, a breccia and abundant Cerion. Geochemical changes in caliche profiles relative to the host rock are an increase in Al₂O₃, Fe₂O₃ and total organic carbon, a decrease in Mg and Sr, and a decrease in δ¹³C_carb, δ¹⁸O_carb and δ13C_org. The magnitude of these changes is probably a function of the duration of subaerial exposure and resultant colonization by dune plants and associated microflora. Abundance of calcified filaments and needle-fibre crystals in profile A attests to the importance of microbial processes in the early development of caliche profiles. Biogenic structures are largely destroyed in profile B due to recrystallization, but indirect evidence of biological activity is retained in the form of carbon isotope values.     

Aqueous and solid phase arsenic speciation in the sediments of a contaminated wetland and riverbed

Mobility of As in the environment is controlled by its association with solid phases through adsorption and co-precipitation. To elucidate the mobilization potential of As deposited in wetland and riverbed sediments of the Wells G & H wetland in Woburn, MA as the result of decades of industrial activity, As retention mechanisms were inferred from aqueous and solid phase geochemical measurements of sediment cores. Testing included a sequential extraction method designed for and standard-tested with As phases and pE/pH equilibrium modeling. The uppermost sediments in the Wells G & H wetland contain elevated concentrations of both dissolved and solid phase As (up to 2,000 μg/L and 15,000 μg/g, respectively) and a maximum concentration between 30 and 40 cm depth. Measurements obtained in this study suggested that As in the wetland sediments was predominantly adsorbed, likely onto amorphous Fe (hydr)oxide phases and mixed valence Fe phases. In the riverbed sediments, however, a relatively greater proportion of the solid As was associated with more reduced and crystalline phases, and adsorbed As was more likely associated with Al oxide or secondary reduced Fe phases. pH–pe modeling of the Fe–As–S system was consistent with observations. The association of As with more oxidized phases in the wetland compared with the riverbed sediments may result from a combination of plant activities, including evapotranspiration-driven water table depression and/or root oxygenation.     

东沙群岛海域沉积物游离烃和孔隙水特征及其地球化学意义

对东沙群岛海域HD170和HD196A两个站位，通过系统的顶空气和孔隙水离子取样测试，对柱状样沉积物和底层水中游离甲烷的含量和沉积物孔隙水的离子组成特征以及孔隙水的来源进行了分析。游离气的分析表明，多数沉积物样品中游离甲烷的含量小于20μL／kg，但在HD196A站位，随着沉积物在海底以下埋深的增加，其中的游离甲烷含量迅速增加，在754～774cm，沉积物中游离甲烷的含量达到了7468．66μL／kg，推测其下存在巨大的烃类供应源。孔隙水的^86St／^87Sr同位素测试显示，本文所研究的两个站位沉积物孔隙水来源于正常的海洋沉积过程，而δ^11B的特征表明，沉积物中存在着与海水交换的吸附水，并且HD170站位的交换更多。柱状样沉积物孔隙水中，Ca^2＋、Mg^2＋与SO4^2-浓度表现出随着深度增加而明显降低的趋势，其中HD196A站位的硫酸盐甲烷界面小于10mbsf，暗示了该站位深部很可能赋存天然气水合物。     

成都平原区水稻土成土剖面Cd形态分布特征及影响因素研究

成土过程中不同形态镉(Cd)迁移转化的影响因素研究对于了解Cd的地球化学行为具有非常重要的意义。以四川省成都平原区农田生态系统水稻土剖面中不同形态Cd分布特征为例,探讨成土过程中不同形态Cd的迁移转化影响因素及其生态危害性。结果表明:总量Cd、离子交换态Cd和碳酸盐Cd均表现出表层富集,深层含量稳定的特征;岷江流域和沱江流域水稻土Cd含量的背景值分别为0.147,0.215mg·kg-1。外源输入耕层土壤中Cd主要为活动态,其增加量占Cd总量增加量的60.71%~90.52%,向下垂向迁移能力非常弱。成土过程随着土壤中Cd总量增加,稳定态Cd含量显著增加,而活动态Cd含量明显降低。土壤的pH值主要对碳酸盐态Cd含量有影响,pH值越大,其含量越高;随着土壤成熟度增高,粉粒和粘粒含量增加的同时,残渣态Cd含量显著降低,而有机结合态Cd含量显著增高。耕层土壤中Cd的生物有效性系数高达0.59~0.65,已经严重地影响到农作物安全。     

P波入射下土饱和度对土层交界面运动的影响

给出波从基本饱水土层入射到弹性土层时在土层交界面上的水平位移和竖向位移分量的表达式。通过数值计算分析,研究了饱和度对交界面上反射、透射系数以及水平、竖向位移分量幅值的影响。     

Distribution and speciation of arsenic around roots in a contaminated riparian floodplain soil: Micro-XRF element mapping and EXAFS spectroscopy

Riparian soils are periodically flooded, leading to temporarily reducing conditions. Diffusion of O₂ through plants into the rhizosphere maintains oxic conditions around roots, thereby promoting trace element fractionation along a redox gradient from the reduced soil matrix towards the oxic rhizosphere. The aim of this study was to determine the distribution and speciation of arsenic around plant roots in a contaminated (170-280 mg/kg As) riparian floodplain soil (gleyic Fluvisol). The analysis of soil thin sections by synchrotron micro-X-ray fluorescence (μ-XRF) spectrometry showed that As and Fe were enriched around roots and that As was closely correlated with Fe. Arsenic contents of three manually separated rhizosphere soil samples from the subsoil were 5-9 times higher than respective bulk As contents. This corresponds to the accumulation of about half of the total As in the subsoil in Fe-enrichments around roots. The speciation of As in the soil was assessed by oxalate extractions at pH 3.0 as well as by X-ray absorption near edge structure (XANES) and extended X-ray fine structure (EXAFS) spectroscopy. More than 77% of the total As was oxalate extractable in all samples. XANES and EXAFS spectra demonstrated that As was predominantly As(V). For the accurate analysis of the EXAFS data with respect to the bonding of As(V) to the Fe- or Al-octahedra of (hydr)oxides and clays, all 3-leg and 4-leg multiple scattering paths within the As^(V)O₄-tetrahedron were considered in a fully constrained fitting scheme. We found that As(V) was predominantly associated with Fe-(hydr)oxides, and that sorption to Al- and Mn-hydroxides was negligible. The accumulation of As in the rhizosphere may affect As uptake by plants. Regarding the mobility of As, our results suggest that by oxygenation of the rhizosphere, plants attenuate the leaching of As from riparian floodplain soils during periods of high groundwater levels or flooding.     

Bottom sediments and pore waters near a hydrothermal vent in Lake Baikal (Frolikha Bay)

We discuss the redox environments and the compositions of bottom sediments and sedimentary pore waters in the region of a hydrothermal vent in Frolikha Bay, Lake Baikal. According to our results, the submarine vent and its companion nearby spring on land originate from a common source. The most convincing evidence for their relation comes from the proximity of stable oxygen and hydrogen isotope compositions in pore waters and in the spring water. The isotope composition indicates a meteoric origin of pore waters, but their major- and minor-element chemistry bears imprint of deep water which may seep through permeable faulted crust. Although pore waters near the submarine vent have a specific enrichment in major and minor constituents, hydrothermal discharge at the Baikal bottom causes a minor impact on the lake water chemistry, unlike the case of freshwater geothermal lakes in the East-African Rift and North America.     

Analytical solutions to 1-D horizontal and vertical water infiltration in saturated/unsaturated soils considering time-varying rainfall

Analytical solutions to 1-D horizontal and vertical water infiltration in saturated/unsaturated soils are developed that can consider the variation of rainfall with time. In this model, water content and the permeability coefficient are assumed to be exponential functions of the pressure head, and diffusivity is constant. By means of Fourier integral transformation, the analytical solutions are expressed as infinite series. The steady-state solutions for horizontal and vertical infiltration in unsaturated soils are then derived. The solutions can consider both flux and pressure head boundaries. The solutions are easy to implement compared with numerical solutions, although the restricted assumptions may limit their applicability in some ways. The analytical solutions provide a reliable tool for checking the accuracy of various numerical methods under the condition of constant diffusivity. Finally, the analysis carried out in a case study indicates that the pressure head differences caused by the transient infiltration in both the horizontal and vertical directions can be estimated using the steady-state solutions, and the effect of gravity on water infiltration mainly depends on the boundary conditions.     

Timing and genesis of early marine caymanites in the hydrothermal palaeokarst system of Buda Hills,Hungary

Sedimentological, biostratigraphic and magnetostratigraphic studies were carried out on five Late Eocene–Early Oligocene profiles in the Buda Hills, Hungary. The Szépvölgy Limestone Formation accumulated on the pre-Eocene basement. Basal strata of the limestone consist of a thin conglomerate followed by a coralgal limestone. The overlying limestone contains abundant Nummulites, Discocyclina. The contact between these two members is sharp. The Szépvölgy Limestone body, covering 60–65 km², is considered a carbonate bank. Above the limestone, the siliciclastic slope deposits of the pelagic and turbiditic Buda Marl, and the euxinic Tard Clay accumulated. A huge multiphase hydrothermal cave system developed in the Szépvölgy Limestone resulting in a long-term composite palaeokarstic evolution lasting from the Late Eocene to the Quaternary. The first palaeokarst phase during the Late Eocene is represented by two generations of early marine cavity filling sediments of caymanite-type, deposited at sea-level. The earlier, carbonate infilling is conformable while the younger, siliciclastic one is disconformable with the bedding of the host rock. Bio- and magnetostratigraphic studies indicate that deposition of the caymanites-bearing palaeokarst host sequence started in the Late Eocene, during Chron C15r (35.3 Ma) and terminated in the Early Oligocene during Chron C13n (33 Ma). Two marker horizons are present, the first is between the Szépvölgy Limestone and Buda Marl at ∼34.6 Ma, and the second horizon is between the Buda Marl and the Tard Clay at 33.5 Ma. The Szépvölgy Limestone, deposited on a mobile shelf, represents a deepening upward sequence, interrupted by two lowstand events. They can be correlated with the PHd event of Keller et al., 1987 (Global distribution of late Palaeogene hiatuses. Geology 15, 199–203) and resulted in marine palaeokarstification. The carbonate shelf with the infillings drowned at 34.6 Ma. The caymanites accumulated at about 35.2–35.0 Ma and represent short-term palaeokarstic events. Their deposition record was estimated in several thousand years.