Marine Limestones Research Articles

2D nonlinear inversion of DC resistivity measurements, a case study; southeastern part of Ras El Dabaa, Northwestern coast, Egypt

The southern Mediterranean coast suffers from limited water resources as a result of exploitation of water supply, population growth, and climate change. Spatial lineaments and Seawater Intrusion (SWI) were detected at the southeast portion of Ras El Dabaa, on Egypt’s northwest coast, using the direct current resistivity (DCR) method. The Vertical Electrical Sounding (VES) data were acquired using Schlumberger array along four profiles and inverted both independently and jointly, aiming to obtain Two Dimensional (2D) geoelectrical images. The results of the one Dimensional (1D) inversion of VES data at each profile were stitched to form pseudo-2D sections on which the resistivity values and aquifer thickness in the southwest of the region appeared to be generally increasing, indicating a potential improvement in water quality.However, the results did not fully image the lateral variation but focused on the horizontal boundaries of the subsurface. On the contrary, the results of 2D inversion of the same data sets successfully managed to provide images that depicted resistivity distribution in both lateral and vertical directions. The detected sets of lineaments and fractured zones within the oolitic limestone and fossiliferous limestone units control the occurrence of groundwater in the region. The 2D inversion scheme revealed a low resistivity zone that indicated the presence of SWI and/or the dissolution of marine salts from the marine limestone bedrock of these aquifers in the northern portions of the studied area. Additionally, analysis of the 2D apparent porosity section shows how aquifers are connected by secondary porosity, which is defined by structures that resemble channels. The current approach offers valuable structural information for future planning and development of such complex geological coastal locations, taking into consideration the vulnerability of the groundwater system.

Case Study: Understanding H2S Occurrence: Bakken Production Using Sulfur Isotopes

_ Souring—or H2S generation—in the Bakken petroleum system (BPS) production is becoming problematic for oil producers and pipeline operators because of the sporadic occurrence and associated economic and HSE impacts. Paradoxically, crude oil produced from the tight Middle Bakken (MB) and underlying Three Forks (TF) reservoirs of the BPS has historically been light (40 °API to 50 °API) with low sulfur content (<0.2 wt.%), typically exhibiting nonexistent to low (<10 parts per million) H2S concentrations. Additionally, neither the MB nor the TF reservoirs have public records indicating the presence of H2S during drillstem tests (DST). However, based on previous work performed by the University of North Dakota Energy & Environmental Research Center (EERC), the frequency of H2S occurrence in BPS wells has increased from 2010 to 2023. Even though the observed H2S increases correlate with larger, multistage well stimulation and completion practices in the BPS, the unclear geographical and temporal trends make the exact mechanisms responsible for souring an open research question. At least five known mechanisms can potentially lead to H2S occurrence in the oil production stream of BPS wells, including: 1. Thermochemical sulfate reduction (TSR) 2. Bacterial sulfate reduction (BSR) 3. Source rock generation 4. Migration from deeper or shallower H2S reservoirs 5. Geochemical mechanisms related to reservoir stimulation work (Table 1). Understanding the mechanisms of H2S generation might reduce the risk of souring in new wells and enable more effective mitigation and management strategies in the wells already producing H2S. Samples Used in the Study This study is focused on oil and gas operations in the North Dakota portion of the Williston Basin and the H2S occurrences in the BPS, which includes two main oil-producing reservoirs: the MB and the TF. In the simplified stratigraphic sequence, the BPS comprises three distinctive lithofacies. From deepest to shallowest, those lithofacies are the Lower Bakken Shale (LBS), the MB reservoir, and the Upper Bakken Shale (UBS). Underlying the BPS is the TF, the upper part of which is the oil-rich TF reservoir, considered part of the BPS. The UBS and LBS are the source rocks for the oil in MB and TF reservoirs and consist of black, organic-rich, siliciclastic mudstones (i.e., shale). Directly overlying the BPS is the Madison Group, a carbonate-dominated Mississippian section that is subdivided into three formations—the Lodgepole (LP), Mission Canyon (MC), and Charles. The Birdbear Formation lies beneath the TF and on top of the Duperow Formation. Both the Duperow and Birdbear Formations produce oil and are dominated by carbonate deposits of shallow marine limestone and dolostones. A sampling program was developed to investigate areas of the BPS with varying levels of souring in the production stream and multiple organic-rich formations representing potential source rocks, including the UBS, LBS, LP, and MC. The samples were collected from locations and formations with different lithologies and maturity levels. Over 70 sour wells were identified for H2S sampling in western North Dakota.

Synchronous felsic volcanism and carbonate sedimentation as a setting for VMS deposits localization at the Salair terrane, NE Central Asian Orogenic Belt

The Salair terrane located in the northern part of the Central Asian Orogenic Belt (CAOB) contains many epithermal deposits including volcanogenic massive sulphide (VMS) deposits. The host rocks mainly consist of felsic volcanic rocks such as lavas, volcanic breccias and tuffs that associated with thick strata of carbonates. In this study, we present zircon U–Pb ages, whole rock geochemistry, and Nd isotope data from the volcanic rocks, and results of geochemical and isotope (Sr, C, O) studies of carbonates to constrain their age and petrogenesis, and to characterize the tectonic setting. Zircon U–Pb dating reveals that the ore-bearing felsic lavas have age of 519.3 ± 1.9 Ma, while the felsic tuffs have age of 516.0 ± 0.9 Ma. These volcanic rocks are characterized by high SiO2 and Na2O contents, enrichment in light rare-earth elements, remarkable negative Eu anomalies, and pronounced depletion in Nb, Ta, P, and Ti. They have depleted ɛNd(t) values ranging from +4.9 to +6.3, and young two-stage Nd model ages (from 0.82 to 0.64 Ga). The felsic volcanic rocks from the Salair terrane are interpreted as highly evolved I-type magmatic rocks that might be produced by high degree partial melting of juvenile lower crust without a significant contribution of ancient crust and without crustal reworking.Felsic volcanism was accompanied by the formation of thick strata of carbonates. These carbonates are marine limestones with Mg/Ca ratios less than 0.007, δ18O(SMOW) values from 17.1 to 23.8 ‰ and δ13C(PDB) values between –0.9 and +0.9. Their Sr isotope composition varies in a narrow range within 0.70844–0.70859 that interpreted as representing proxy for coeval seawater. These values are consistent with depositional ages of 520–510 Ma and confirms the synchronicity of the formation of carbonates and felsic volcanism. Based on the regional geology and geochemistry, the ore-host rocks of the Salair terrane were formed in the back-arc setting where the marine transgression occurred as a result of graben subsidence. It is important for better understanding epithermal deposits in the northern CAOB and might provide new insights about prospecting the VMS deposits in similar tectonic settings.

Boron isotopic compositions of middle Miocene to recent shallow-water carbonates from the South China Sea: Assessing diagenetic effects and implications for paleoclimate changes

The partial pressure of atmospheric CO2 (pCO2) significantly influences global climate change and biological evolution through geological history. Boron isotopic composition (δ11B) in carbonates has been used to reconstruct the paleo-pH of seawater, providing insight into atmospheric pCO2 levels. However, the fidelity of δ11B records in marine carbonates due to diagenesis remains uncertain. Here, to understand how diagenetic processes influence B isotopic records in marine carbonates, we examined B concentrations and δ11B values of modern corals (from Hainan Island), unconsolidated shallow pushcores (Jiuzhang A and Jiuzhang B from Nansha Islands, South China Sea), and a long drillcore (XK-1 from Xisha Islands, South China Sea) covering the Late Miocene to Holocene periods. Our coral samples show a uniform δ11B range (20.16 ± 0.85%), consistent with previously published values for modern corals in other open oceans. The δ11B values of the unlithified carbonate sediments from Jiuzhang A and B pushcores vary within a narrow range (18.70 ± 0.84%), yielding pH and CO2 concentrations consistent with the range between modern and pre-industrial values. Since we did not observe any statistically significant covariations between traditionally established diagenetic proxies (such as δ13C, δ18O, Mn/Sr, and Al/Ca) and δ11B values for non-dolomitized samples in the XK-1 drillcore, we suggest that δ11B values of bulk limestones are not significantly affected by typical diagenesis (meteoric, mixed-diagenetic, and marine diagenetic processes), likely due to limited post-depositional recrystallization of our study carbonates. In contrast, dolomitization significantly decreases δ11B values of bulk carbonates, rendering dolomites unsuitable as archives for reconstructing seawater δ11Bborate values. Our study supports that marine limestone with limited recrystallization or dolomitization have the potential to record ambient seawater pH values with high resolution. The established secular seawater pH variations based on XK-1 δ11B records provide information about ocean acidification over the past 5.1 million years.

Towards a better understanding of the geochemical proxy record of complex carbonate archives

Carbonate archives record a brief snapshot of the ambient Earth’s surface conditions at their deposition. However, the geologically reasonable extraction and interpretation of geochemical proxy data from ancient, diagenetically altered rock archives is fraught with problems. Three issues stand out: the dichotomy between petrographic and geochemical alteration; the lack of quantitative age constraints for specific diagenetic phases resulting in a poorly constrained admixture of local, basin-wide and over-regional (far-field) features; and an often insufficient understanding of the temperatures and compositions of diagenetic fluids. Here, the archive of Devonian marine limestones exposed to multiple far-field diagenetic events is used as an example to explore the above-listed issues. Methods applied include petrography, micro XRF, fluid inclusion data, clumped isotopes, δ13C and δ18O isotopes, 87Sr/86Sr ratios and quartz trace element data. Devonian limestones studied here were overprinted by two cross-cutting regional fault zones (T ≈ 230 °C) by multiple events between the Variscan Orogeny and the late Paleogene. The following processes are recorded: (i) protolith deposition and partial dolomitisation during rapid burial in the Middle/Late Devonian (T ≈ 180 °C); (ii) deep burial to ca 6.5 km and tectonic/hydrothermal overprint during the Variscan Orogeny in the Carboniferous (T ≈ 90–230 °C); (iii) rapid uplift to 1–2 km burial depth at the end of the Variscan Orogeny and hypogene karstification (T ≈ 50 to 100 °C) initiated by regional geology in the Permian/Triassic; (iv) tectonic/hydrothermal overprint during the opening of the Proto-Atlantic Ocean between the Early Jurassic and the Early Cretaceous (T ≈ 50 to 130 °C); (v) tectonic/hydrothermal overprint including renewed hypogene karstification and hydrothermal calcite cement precipitation (T ≈ 50 to 180 °C) during Alpine Orogeny between the Late Cretaceous and late Paleogene. Despite this complex series of diagenetic events, the protolith limestones largely preserved their respective Middle/Late Devonian dissolved inorganic carbon (DIC) and 87Sr/86Sr signatures. This study documents that geochemical proxy data, placed into their petrographic, paleotemperature, and local to over-regional context, significantly increases the ability to extract quantitative information from ancient carbonate rock archives. Research shown here has wider relevance for carbonate archive research in general.

Geochemical and Isotopic Characteristics of the Emir Geothermal Waters in Kula Area, Western Anatolia

In this study, we investigate chemical and isotopic characteristics of thermal waters of the Emir geothermal field in the Kula region, which hosts the youngest volcanism in Turkey. Studied thermal waters with temperature and electrical conductivity values of 21–63 °C and 3840 to 5210 μS/cm are of Na–HCO3 type and have neutral character. δ18O and δD of thermal waters are − 9.40 to − 8.41‰ and − 64.16 to − 56.38‰ (VSMOW) and indicate a meteoric source with local recharge. Tritium values of thermal waters in the Emir geothermal field are mostly < 1 TU signifying a deep circulation. Positive δ13C values (1.32–4.46‰ VPDB) imply that carbon is derived dominantly from marine limestone and dominantly from endogenic CO2. δ18O and δ34S of dissolved sulfate yield that marine limestone is the source of sulfur in thermal waters, which are partly affected by sulfide oxidation and bacterial reduction processes. At discharge temperatures, Emir thermal waters are oversaturated with respect to albite, aragonite, calcite, α-cristobalite, dolomite, gibbsite, illite, K-feldspar, kaolinite, muscovite and quartz but undersaturated for anhydrite and wairakite. The activity diagrams suggest that high Na+ and K+ concentrations in waters are attributed to dissolution of muscovite, K-feldspar and albite. Chemical and isotopic compositions of the Emir thermal waters are controlled by a combination of processes including water–rock interaction, dissolution/precipitation and ion exchange. Various chemical and isotopic geothermometers applied to the thermal waters yielded reservoir temperatures in the range of 80–125 °C. Paleo-temperatures estimated from δ18O values of travertines and thermal waters (isotope fractionation) are consistent with modern discharge temperatures.

The Early Jurassic sequence of Lyme Regis, Dorset, England and its place in the history of geology and palaeontology

Abstract The cliffs and foreshore at Lyme Regis on the Dorset coast expose a sequence of Early Jurassic marine limestones and mudstones of the Blue Lias Formation and the overlying Charmouth Mudstone Formation, the lowest units of the Hettangian–Aalenian Lias Group. Known for its fossils since at least the mid-seventeenth century, this coastal section became famous in the early nineteenth century for the bones of ‘fossil crocodiles’. Many of the most significant discoveries were made by a family of fossil dealers whose best-known member was Mary Anning (1799–1847). Anning and her family recovered the first scientifically described ichthyosaurs, the first complete plesiosaur and the first British pterosaur to be identified. Anning's finds from Lyme Regis formed the basis of the pioneering palaeoenvironmental reconstruction Duria antiquior . Her geologist friends, some with close personal associations to the town, did much to publicize her discoveries and contribute to both her fame and that of the locality. This famous coastal section, with its important historical associations with a key period in the development of geology and the source of so many significant discoveries in the early nineteenth century, now lies within the Jurassic Coast UNESCO World Heritage Site.

Controls on rapid rare earth element enrichment in sediments deposited by a continental-scale river system

Clay-hosted rare earth element (REE) deposits, generally considered products of extended subtropical weathering of REE-rich igneous protoliths, are a critical source of REEs globally.The Koppamurra deposit of southeast South Australia, in contrast, is developed in Pleistocene lacustrine clays overlying a REE-poor marine limestone. Here we examine the REE + Y (REY) sources and enrichment mechanisms of this unusual setting. We find that the greatest REY enrichment (up to 5000 ppm) occurs in lacustrine clays at the base of a ∼1–2 m thick contact zone with the underlying limestone. This zone features an increased proportion of leachable REYs (>50 %), increased smectite abundance and a sharp increase in pH from ∼5.0 to ∼8.3. Co-variation of HREEs with a suite of clay-specific elements demonstrates that HREEs are hosted mainly by Fe-rich smectite, with separate sorbed vs structurally bound HREE pools that are of similar magnitude. LREEs are mainly hosted in secondary LREE carbonates, these are preferentially intergrown with smectite but are only rarely present in kaolinite domains. Cerium occurs mainly as cerianite. The narrow range and overlapping εNd of samples within and above the REY enriched zone suggests that enrichment is related to internal redistribution within the clay unit. The clays are isotopically incompatible with local igneous or metasedimentary rocks, but 87Sr/86Sr and εNd isotopic fingerprinting shows that they were deposited by the Murray River, Australia’s largest river system. Murray sediments are enriched in REY due to the presence of material eroded from the Lachlan Orogen granites of southeast Australia. Deposition of these sediments occurred in a coastal lacustrine setting analogous to the modern-day Coorong, with tectonic uplift and shoreline progradation allowing the onset of subaerial weathering ∼1–2 Mya. We propose that vertical and lateral solute transport allowed for the concentration of REEs in local depressions in the karst-weathering surface which separates the limestone and overlying clay, facilitated by water table oscillation and pore water redox cycling driven by seasonally contrasted rainfall. Poor soil drainage, evaporative concentration of soil waters and alkaline pH adjacent to the limestone immobilized REEs in secondary carbonates and Fe-rich smectite. Our findings illustrate that REE mobilization and enrichment in clay deposits can occur more rapidly, over a larger area and in a broader range of host phases and geologic settings than is currently appreciated, with important implications not only for REE exploration but also for the use of REEs as tracers in the Earth Sciences.

Middle Aptian Orbitolinid limestones in Belgrade (Serbia): microfacies and depositional environment

Lower Cretaceous (Aptian) shallow-marine limestones with intercalated polymictic conglomerates were investigated with respect to their biostratigraphic age and their microfacies. They are the younger part of the generally carbonate-siliciclastic Lower Cretaceous deep-water (max. few hundred metres) turbiditic sequences (“Paraflysch”) of the so-called East Vardar zone in the Belgrade area. The biostratigraphic age of the limestones was determined by orbitolinid foraminifera: the co-occurrence of Dictyoconus? pachymarginalis SCHROEDER and Mesorbitolina texana (ROEMER) besides various other microfossils suggest a biostratigraphic age of this shallow- marine limestone succession as middle Aptian (Gargasian). Radiolarite components in the conglomerates are Triassic in age and were derived from the obducted Middle Triassic to Middle Jurassic Neo-Tethys ophiolites and/or their ophiolitic mélanges on the wider Adria plate. From both the first precise biostratigraphic age dating as middle Aptian combined with microfacies analysis of these shallow-marine limestones and the component spectrum in the intercalated conglomerates, it can be concluded that the Lower Cretaceous turbiditic “Paraflysch” succession was deposited on the eastern rim of the Dinarides. The results will allow a better comparison of the different Lower Cretaceous sedimentary successions deposited on the eastern margin of the Dinarides.

First report of the genus Thorectichthys (Ellimmichthyiformes) in America, from the Cenomanian deposits of the Cintalapa Formation, Chiapas, southeastern Mexico

The El Chango quarry is an outstanding paleontological site of Mexico. The marine limestones of this site represent a small outcrop of the Cintalapa Formation and bear a significant assemblage of Early Cenomanian continental plants plus marine crustaceans, decapods, mollusks, and fishes. This paper includes the description of a double-armored herring species from El Chango, which is named Thorectichthys fideli and belongs to the family Paraclupeidae, in the order Ellimmichthyiformes. This new species shares the diagnostic generic features with T. marocensis and T. rhadinus, two slightly younger North African species previously discovered in Cenomanian–Turonian deposits of the Akrabou Formation, Morocco. The new species has a couple of peculiar features never before observed in the ellimmichthyiforms; the caudal skeleton of this fish has four uroneurals, and the horizontal limb of the preopercle is dorsally notched allowing the superficial lateral exposure of a broad and massive posterior process of the quadrate. Including this species in a comprehensive phylogenetic analysis confirms the monophyletic nature of Thorectichthys and its inclusion as a basal clade of the family Paraclupeidae. Thorectichthys fideli sp. nov. adds to an increasing assemblage of Mexican Cretaceous ellimmichthyiforms that includes ancestral as well as more derivatives species that shows the broad Albian–Cenomanian distribution of the ellimmichthyiforms, along the epicontinental environments of the Tethys Sea.

A new broad-snouted fossil carettochelyid turtle from a previously unknown Caenozoic deposit in Sarawak, Malaysia

Fossils of pan-carettochelyid turtles are known from most continents of the world, except Antarctica. The fossils that have been described indicate a group of estuarine turtles that have little modified their body form since the Cretaceous. The only species for which ecological data exist is the extant Carettochelys insculpta, found in estuarine or fresh waters in Australia and New Guinea. Here we report the discovery of an incomplete skull of a previously unknown carettochelyid Carettochelys niahensis sp. nov. from an undated fossil deposit within or beneath a Miocene marine limestone formation in Niah Great Cave, Sarawak, Malaysia. The skull exhibits many anatomical features characteristic of this turtle group but differs from previously known taxa in that it has a broad, nasal orifice that is proportionately wider than in other carettochelyids and relatively shallow but wider temporal arches. Some aspects of the palaeoecology of this turtle are inferred.Arthur White [arfawhite@gmail.com] Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia;Michael Archer [m.archer@unsw.edu.au] Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia;Suzanne J. Hand [s.hand@unsw.edu.au] Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia;Henk Godthelp [hjgodthelp@gmail.com];Anna K. Gillespie [a.gillespie@unsw.edu.au] Earth and Sustainability Science Research Centre, School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney 2052, Australia.

Late Cretaceous arc magmatism in the Western Pontides (Turkey) – temporal and chemical changes

ABSTRACT During the Late Cretaceous, a magmatic arc extended from the Lesser Caucasus through the northern margin of the Pontides into Srednogorie, Timok, Banat, and Apuseni (ABTS) in the Balkans for a distance of 2700 km. We studied the arc volcanic rocks in three regions in the Western Pontides and reviewed the geological data on the Lesser Caucasus-Pontide-ABTS magmatic arc, which shows several common features. Prior to the start of the arc magmatism, the region underwent uplift and erosion. New and published geochronologic and biostratigraphic data show that magmatism in the Lesser Caucasus-Pontide-ABTS arc started in the Turonian (ca. 93 Ma) peaked in the middle Campanian (80–78 Ma) and became rare and sporadic after the late Campanian (ca. 75 Ma). Magmatism was dominantly calc-alkaline to high-K calc-alkaline and shows typical subduction geochemical signatures. Late Cretaceous volcanism took place in a submarine and extensional environment. In the Lesser Caucasus-Pontide-ABTS belt, the arc volcanic rocks are overlain by Maastrichtian to Palaeocene marine limestones and sandstones, marking the end of the main phase of arc magmatism. However, in the Kefken region in the Western Pontides, Maastrichtian limestone sequence includes a volcanic horizon with a U-Pb zircon age of ca. 71 Ma. The Maastrichtian volcanic rocks show a more varied geochemistry than the older arc volcanic rocks, and include alkaline basalts, calc-alkaline basalts, and adakites. The coeval start of arc magmatism in the 2700-km-long Lesser Caucasus-Pontide-ABTS belt is related to the inception of the Africa-Eurasia convergence at ca. 96 Ma, which is also independently indicated by the beginning of intra-oceanic subduction, inferred from the ages of sub-ophiolite metamorphic rocks in Anatolia. The end of magmatism is linked to a marked decrease in the convergence rate during the Campanian, which was also partitioned between the northern and southern Tethyan subduction zones.

Genetic Mechanism of Structurally Controlled Dolomites Derived from Seawater-Hydrothermal Mixed Fluids—A Case Study from Middle Permian, Central Sichuan Basin, South China

Dolomite bodies in the Middle Permian of the central Sichuan Basin have been reported as favorable natural gas reservoirs. The Middle Permian dolomite consists of three types of recrystallized dolomite (Rd1, Rd2, and Rd3) and one type of dolomite cement (Sd). Rd1 might be formed as the primary mineral along the calcite in the original sea-water. Its δ13C value and 87Sr/86Sr ratio, consistent with those of marine limestone and Permian seawater, support that the dolomitizing fluid for Rd1 was Permian seawater preserved in the strata. Rd2 consists of fine to medium (50 μm to 250 μm) and planar to curved crystals. Geochemical indicators (slightly high 87Sr/86Sr ratio, similar rare earth element patterns, negative δ18O, slightly high salinity) confirm that the dolomitizing fluid of Rd2 was mainly Permian seawater during shallow burial, with a small number of hydrothermal fluids. Rd3 and Sd are featured by very large (&gt;250 μm), curved crystals, and high-temperature, high-salinity, and obviously positive Eu anomalies, suggesting that their diagenetic fluids were mainly hydrothermal fluids from deep. Additionally, inherited carbon sources and the 87Sr/86Sr ratios of some samples fall within the range of Permian seawater distribution, confirming the contribution of Permian seawater. ELIP activity caused the formation of this dolomite through the mixing of seawater and hydrothermal fluids. The main fluid circulation channels were activated basement faults, epigenetic karst pores, and shallowly buried high-permeability strata. During the peak period of ELIP activity, the continuous upwelling of deep hydrothermal fluids led to the continuous formation of Rd2, Rd3, and Sd. The dolomitization fluid of Rd2 was mainly composed of seawater and featured a certain lateral extension, which was away from faults. Rd3 and Sd are mainly distributed along the fault system, and excessive dolomitization caused by the hydrothermal activity, to some extent, inhibited the lateral movement of hydrothermal fluids. This study provides a good example for exploring the genetic mechanism and distribution pattern of structurally controlled dolomites under a volcanic activity background.

Origin and hydrogeochemical formation processes of geothermal fluids from the Kibiro area, Western Uganda

The Kibiro geothermal field, located in the western branch of the East African Rift Valley System, has many geothermal surface manifestations. Several geochemical research has already been carried out in this area. However, the sources of dissolved ions in geothermal fluids have not been assessed so far. The hydrochemical and gas isotopic characteristics have been examined in this study to identify and determine the sources of the main dissolved components and the geochemical processes involved in the formation of the geothermal fluid. Methods applied in the analysis of the genesis and hydrogeochemical evolution of geothermal fluids include isotope tracers, major ion ratios, graphical methods (stability diagrams, durov diagrams, chloro-alkaline indices, piper diagrams, and HNC-plot), and hydrogeochemical simulations using PHREEQC software. The Kibiro hot springs are high salinity Na–Cl waters. Based on δ18O and δ2H values, the geothermal waters are found to be of meteoric origin. The assessments indicate that Na+ and Cl− are derived from halite dissolution. The dissolution of dolomite, calcite, gypsum, and anhydrite is responsible for the concentration of Ca2+, Mg2+, SO42−, and HCO3−. The main source of K+ is derived from the dissolution of silicate rocks (K-feldspar). The gas isotopes analysis indicates that 97.5% of He with isotope ratio of 0.217 Ra mainly originates from crustal sources and 2.47% is derived from the mantle. 51.39% of CO2 originates from marine limestone, 46.55% from sedimentary organic carbon and 2.06% from the mantle. This evidence further confirms that there are no direct inputs of volatiles from a magma chamber below the Kibiro geothermal field. The evolution of the fluids suggests that most of the common cations and anions found in the Kibiro hot spring waters originated from high-temperature water/rock interaction.

Warm acidified seawater: a dolomite solution

ABSTRACTThe “dolomite problem” is the product of two distinct observations. First, there are massive amounts of ancient marine limestone (CaCO3) deposits that have been replaced by the mineral dolomite (MgCa(CO3)2). However, recent (Holocene and Pleistocene) marine deposits contain relatively minuscule amounts of dolomite, although the occurrence of small quantities of dolomite is observed in many modern settings, from deep marine to supratidal. Second, low-temperature synthesis of dolomite in laboratory settings has been elusive, particularly in comparison to the ease with which common marine calcium carbonate minerals (aragonite and calcite) can be synthesized. Since low-temperature solid-state diffusion can be discounted as a method for Mg incorporation into calcium carbonate (as it operates on time scales too long to matter), the replacement of CaCO3 by dolomite is one of dissolution followed by precipitation. Therefore, an often overlooked but required factor in the replacement of limestone by dolomite is that of undersaturation regarding the original calcium carbonate mineral during replacement. Such conditions could conceivably be caused by rapid dolomite growth relative to aragonite and calcite dissolution–precipitation reactions, but laboratory studies, modern systems analyses, and observations of ancient deposits all point to this possibility being uncommon because dolomite growth is kinetically inhibited at low temperature. Pressure solution by force of dolomite crystallization is a second possible driver for CaCO3 undersaturation, but requires a confining stress most likely attained through burial. However, based on petrographic observations, significant amounts of ancient dolomite replaced limestone before burial (synsedimentary dolomite), and many such platforms have not suffered any significant burial. Because these possibilities of undersaturation caused by dolomite precipitation and crystal growth can be largely discounted, the undersaturation required for “dolomitization” to proceed is most likely to be externally forced. In modern natural systems, undersaturation and selective CaCO3 dissolution in marine porewaters is very common, even in warm-water environments, being forced by the breakdown of organic matter. Such dissolution is frequently attended, to varying degrees, by precipitation of a kinetically-less-favored but thermodynamically more stable phase of CaCO3. Laboratory studies as well as observations of modern systems show that when undersaturation is reached with respect to all common marine CaCO3 phases, dolomite assumes the role of this kinetically-less-favored precipitate. This degree of undersaturation is uncommon in modern shallow marine pore systems in warm-water settings, but it was more common during times of elevated atmospheric CO2, and ocean acidification. Furthermore, because oxidation of organic matter drives dolomite formation, near-surface organic-rich deposits such as the remains of microbial mat communities, were more predisposed to dolomite replacement in the acidified oceans of the ancient past relative to contemporaneous deposits that contained less organic matter. These observations lend to a more harmonious explanation for the abundance and occurrence of dolomite through time.

A 13 million-year record of Li isotope compositions in island carbonates: Constraints on bulk inorganic carbonate as a global seawater Li isotope archive

Lithium isotope (δ7Li) variations in marine carbonate sediments and rocks have been widely used to reconstruct global seawater Li isotope compositions and then trace the paleo-weathering processes. However, there are still debates on whether ancient carbonates can faithfully document original seawater δ7Li signals, as δ7Li values of shallow-water carbonates are tightly related to carbonate mineralogy and diagenetic alteration. In this study, we present high-resolution δ7Li, trace element and carbonate mineralogy data of shallow-water carbonate deposits from two shallow drillcores (Jiuzhang A and B) and a deep drillcore (XK-1) in the South China Sea. We compare these new δ7Li data to the ca. 13 Myr history of seawater δ7Li evolution from the middle Miocene to Pleistocene, in order to better constrain the effects of carbonate mineralogy and early diagenesis on δ7Li values of bulk inorganic carbonates. We observe that the δ7Li values of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores (24.6% ± 1.6%, n = 37, 1σ) are significantly lower than modern seawater δ7Li values (Δ7Liprimary-seawater = ∼–6.0%). In contrast, marine diagenetic carbonates in the XK-1 drillcore exhibit δ7Li values of 28.7% ± 0.7% (n = 54, 1σ), approaching coeval seawater values (Δ7Limarine diagenetic-seawater = ∼–2.0%) and much higher than those of primary carbonate deposits. Meteoric diagenetic carbonates in the XK-1 drillcore show δ7Li values of 22.4% ± 1.6% (n = 46, 1σ), close to that of primary carbonate deposits in Jiuzhang A, B and the uppermost XK-1 drillcores, but significantly lower than coeval seawater δ7Li values (Δ7Limeteoric diagenetic-seawater = –5.0% to –9‰ for modern and Miocene seawater). Such δ7Li variations in shallow-water carbonates are attributed to variations in carbonate mineralogy (i.e., aragonite, high-Mg calcite and low-Mg calcite) and diagenetic regimes (fluid- or sediment-buffered conditions of marine and meteoric diagenesis). By comparing δ7Li data from the South China Sea to those from the Bahamas, we suggest strongly fluid-buffered conditions for Li isotopes during marine diagenesis, resulting in δ7Li values of marine limestones and dolostones approaching ambient seawater δ7Li signals. In contrast, meteoric diagenetic carbonates, likely marked by sediment-buffered conditions, may inherit the original δ7Li signals of primary carbonate deposits. Hence, full considerations of the carbonate mineralogy and diagenesis facilitate a better use of bulk inorganic carbonate-archived δ7Li to reconstruct paleo-weathering evolution in deep time lacking the skeletal fossil records.

Stromatoporoids and their symbionts in the upper Silurian of Gotland: Implications for palaeobiological and palaeoecological analysis

Stromatoporoids in Silurian shallow marine limestones from Gotland are commonly symbiotic with syringoporid tabulates, rugose corals, shelly tubes and bioclaustrations (symbionts lacking shell walls). Syringoporid and rugosan symbionts occur almost exclusively in reefs, contrasting deeper facies where stromatoporoids have shelly tubes and bioclaustrations. In a Ludlow biostromal sequence at Kuppen, eastern Gotland, symbionts occur in stromatoporoids in both a marl unit and an overlying biostrome containing the same stromatoporoid taxa and symbionts, but in the marl, stromatoporoid growth forms appear more delicate. A minimum of 10% of a suite of 541 stromatoporoids contain symbionts: 1) two of the 16 stromatoporoid taxa (all Petridiostroma convictum and many “Stromatopora” venukovi specimens) contain abundant intergrown syringoporids; in others syringoporids are rare or absent. Rugosans occur sporadically in several taxa, commonly P. convictum. Some “S” venukovi specimens have syringoporids, rugosans and spiral shelly tubes close together; 2) symbionts mostly began growth after stromatoporoids established, rarely occurring at the base; 3) most stromatoporoid hosts and symbionts evidently had matched growth rates; 4) in some samples, symbionts terminated during stromatoporoid growth, reappearing later within the same host skeleton; and 5) at stromatoporoid growth interruption events, some coral symbionts survived and others did not. These results are interpreted to indicate that: a) symbionts discovered stromatoporoids by chance; b) in shallow locations, reasons for specific taxonomic relationships include suitable substrates for symbionts and a biological relationship; c) stromatoporoid hosts and symbionts responded differently to events; controls on growth were complex; d) there is little evidence that symbionts enhanced reef growth; e) in deeper facies, symbionts were largely overgrown by host stromatoporoids, thus appear unwelcome. In a-d, distinction between commensalism and mutualism remains unproven, but parasitism seems unlikely. The pattern found in this study may apply through stromatoporoid geological history and needs to be tested on other assemblages.

Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China

Hydrogeochemical characterization studies are regarded as an important method for determining the origin of hot springs. The major elements, trace elements, and stable isotopes of four groups of hot spring water samples and two groups of gas samples collected from the intersection of the Altyn Tagh fault zone and the East Kunlun fault belt were investigated in this study. The hot spring water temperature ranged between 6 °C and 14 °C. The water chemistry types of the hot springs were Na·Mg-Cl, Mg·Na-Cl·HCO3, Na-Cl·SO4, and Na-Cl·HCO3. The δD values ranged from −50.00% to −68.60%, while the δ18O values ranged from −6.90% to −8.60%. The hot spring water was recharged mainly by infiltrating precipitation, with a recharge elevation of 3390~3676 m. The heat storage temperature ranged from 66.7 to 164.9 °C. The circulation depth was estimated to range between 1043 and 2679 m. The strontium isotopic composition of the water samples in response to the main weathering sources comprised carbonate and sulfate. CO2 was the main component in the hot spring gas in the study region, and its content was over 95%. The 3He/4He-R/Ra relationship diagram revealed that the mantle-sourced helium from the Yitunbulake spring was 3.06%. In comparison, that from the Aiken spring was 7.38%, which indicated an intrusion of mantle-source material mixed into the hot springs in the study region. The crustal marine limestone contributed significantly to the carbon inventory of the hot spring gas samples (&gt;75%). The dissolution of the marl aquifer resulted in the release of CO2. Carbon was primarily obtained through metamorphism and hydrothermal reactions in the basement lithologies. The Yitunbulake and Aiken hot springs are found near the intersection of the Arjin and East Kunlun fractures, where the water–rock response is relatively strong and the depth of circulation and thermal storage temperature are both high. This causes relatively high ambient pressure to be released from the deep fluid, resulting in microseismic activity in this region. The continuous observation of Aiken spring water chemistry allows for the monitoring of fracture activity in the region. The results of the study could serve as a foundation for further exploration of the relationship between geothermal water and deep faults, shallow geological formations, hydrogeological conditions, and geothermal resource development in the region.

A tale of three fluids: Fluid-inclusion and carbonate clumped-isotope paleothermometry reveals complex dolomitization and dedolomitization history of the Latemar platform

Abstract This work focuses on an exceptionally complex natural laboratory, the Triassic Latemar isolated platform in the Dolomite Mountains of northern Italy. It explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization, and the dedolomitization of marine limestones. Rock samples were studied using dual fluid-inclusion thermometry and clumped-isotope thermometry. Independent of the spatial position at Latemar, Δ47 clumped-isotope and fluid-inclusion data provide contrasting paleotemperature estimates. An apparent lack of systematic patterns in fluid-inclusion data (homogenization temperature, salinity, density) results from analyses of micrometer-sized growth zones within a single crystal. The composition of the individual fluid inclusions represents a “snapshot” of fluid mixing with variable endmember elemental ratios. The bulk crush-leach data and slopes in Caexcessversus Nadeficit diagrams indicate different water–rock interactions and fluid signatures with evaporation sequences and crystalline rocks. The presence of three fluid types (crystalline basement brine, halite-dissolution brine, seawater) in all carbonates suggests that all fluids coexisted during contact metamorphism and dolomitization of Latemar carbonates. Non-equilibrium processes overruled thermodynamic controls on the precipitation of diagenetic phases. Fluid mixing resulted in the precipitation of two complex carbonate successions. The Δ47 data represent bulk temperatures, averaging the mixing ratio of fluids with different temperatures and their respective volume. Fluid-inclusions record patterns of remarkable complexity and shed light on the complexity of a multi-fluid system. Data shown here provide answers to the controversial interpretation of dolomitizing fluid temperature in the Latemar and exemplify the strengths of multi-proxy paleotemperature studies.

Ecosystem responses of two Permian biocrises modulated by CO2 emission rates

Carbon dioxide (CO2) emissions and associated climate change are thought to have caused a number of widespread marine anoxia and mass extinction events in the geologic past. However, how marine ecosystems respond to different CO2 emission patterns remains an important unresolved question. The geologic records of the Permian Period, which witnessed two mass extinctions associated with volcanic eruption (thus CO2 emissions) but with vastly different biological responses, provide a unique window to address this issue. Here, we present a long-term uranium isotope (δ238U) record using marine limestones covering the latest Early Permian through Middle to Late Permian. The δ238U values show two episodes of low values in the middle Capitanian and late Changhsingian, indicating two periods of expansion of marine anoxia during the Permian Period. We use a uranium isotope mass balance model to quantify the anoxic seafloor areas, and we further use a carbon cycle model (LOSCAR, Long-term Ocean Sediment Carbon Reservoir) based on observed δ13C of marine carbonates, sea surface temperature records, and ocean surface pH data to quantify the carbon emission rates across the two biocrises. The uranium isotope mass balance model reveals that the anoxic seafloor area is three times larger during the end-Permian mass extinction (EPME, covering ∼35% of the seafloor areas) than that during the end-Guadalupian event (EGE, covering ∼10% of the seafloor areas). The CO2 emission rates across the two biocrises modeled from the LOSCAR model show that the carbon emission rate across the EPME was at least five times faster than that during the EGE, with the best-fit δ13C values of the input sources ranging from −8 to −12‰, indicating a predominant volcanic CO2 source during the EPME, and close to −25‰ during the EGE. Comparing model results and observed proxy data led to the suggestion that the more severe ecosystem responses during the EPME, including higher extinction rate and larger extent of seafloor anoxia, are closely linked to the faster carbon emission rates compared to the EGE.