Carbonate-rich Sediments Research Articles

Carbon cycling during the India-Asia collision revealed by δ26Mg−δ66Zn−δ98Mo evidence from ultrapotassic volcanoes in NW Tibet

India-Asia continental collision−induced volcanic gas emissions are thought to have played an important role in driving Cenozoic atmospheric CO2 variations, yet the details of how the deep carbon cycle may influence volcanic CO2 degassing are not understood. We present a novel study employing Mg-Zn-Mo isotopic compositions of Cenozoic ultrapotassic lavas from NW Tibet. The negative Mg-Zn isotope correlation (δ26Mg = −0.39‰ to −0.19‰; δ66Zn = +0.27‰ to +0.36‰), bolstered by petrographic analysis of mantle-derived xenoliths from these lavas, demonstrates that the ultrapotassic magmas originated from a lithospheric mantle source that had been enriched by recycled carbonate-bearing sediments rich in calcite and dolomite. Very low δ98Mo values (−0.78‰ to 0‰) relative to the average continental crust (δ98Mo = +0.10‰ to +0.35‰) further indicate that the sedimentary components were derived from the subducted Indian continental crust after its dehydration. Monte Carlo modeling estimates that the input flux of carbon (elemental C) from such sediments into the lithospheric mantle is ∼5.6 Mt/yr, with a predicted CO2 emission rate of ∼15.5 Mt/yr. We suggest that the still ongoing subduction of the Indian tectonic plate has played a crucial role in introducing substantial quantities of carbonate-rich sediments into the Tibetan lithospheric mantle, leading to the sequestration of large amounts of CO2 via carbonatite metasomatism. Hence, partial melting of such a carbon-rich mantle reservoir in an orogenic setting provides the positive feedback mechanism that can explain the high flux of volcanic CO2 during India-Asia collision. These findings not only highlight the importance of continental subduction, sediment recycling, and mantle metasomatism by carbon-rich melts/fluids in the generation of Tibetan ultrapotassic volcanism, but they also show how the deep carbon cycle influences volcanic CO2 degassing.

Middle and Late Holocene paleolimnological changes in central Lake Tanganyika: Integrated evidence from the Kavala Island Ridge (Tanzania)

Middle and Late Holocene sediments have not been extensively sampled in Lake Tanganyika, and much remains unknown about the response of the Rift Valley’s largest lake to major environmental shifts during the Holocene, including the termination of the African Humid Period (AHP). Here, we present an integrated study (sedimentology, mineralogy, and geochemistry) of a radiocarbon-dated sediment core from the Kavala Island Ridge (KIR) that reveals paleoenvironmental variability in Lake Tanganyika since the Middle Holocene with decadal to centennial resolution. Massive blue-gray sandy silts represent sediments deposited during the terminal AHP (~5880–4640 cal yr BP), with detrital particle size, carbon concentrations, light stable isotopes, and mineralogy suggesting an influx of river-borne soil organic matter and weathered clay minerals to the lake at that time. Enhanced by the AHP’s warm and wet conditions, chemical weathering and erosion of Lake Tanganyika’s watershed appears to have promoted considerable nutrient recharge to the lake system. Following a relatively gradual termination of the AHP over the period from ~4640 cal yr BP to ~3680 cal yr BP, laminated and organic carbon-rich sediments began accumulating on the KIR. δ15Nbulk, C/N, and hydrogen index data suggest high relative primary production from a mix of algae and cyanobacteria, most likely in response to nutrient availability in the water column under a cooler and seasonally dry climate from ~3680 to 1100 cal yr BP. Sediments deposited during the Common Era show considerable variability in magnetic susceptibility, total organic carbon content, carbon isotopes, and C/N, consistent with dynamic hydroclimate conditions that affected the depositional patterns, including substantial changes around the Medieval Climate Anomaly and Little Ice Age. Data from this study highlight the importance of sedimentary records to constrain boundary conditions in hydroclimate and nutrient flux that can inform long-term ecosystem response in Lake Tanganyika.

Palynostratigraphy of the Mississippian-Pennsylvanian boundary in the Tyler Formation, Williston Basin, USA: Implications for organic matter-rich source rocks and paleoenvironmental reconstruction

The Carboniferous was a period of intense environmental perturbations, climate changes between greenhouse and icehouse, eustatic sea level change, and accumulation of organic carbon-rich sediments. At this time, the Tyler Formation was deposited in the midcontinent USA. A detailed palynological analysis of the Tyler Formation revealed a highly diverse assemblage of spores with minor pollen content, represented by 100 species belonging to 51 genera. Stratigraphically constrained spores and pollen grains were used to construct three interval zones of middle-late Chesterian (late Viséan-Serpukhovian) to early Morrowan (middle Bashkirian) ages. The stratigraphic position of the Mississippian-Pennsylvanian boundary was determined in the lower Tyler Formation based on the last appearance of typical forms of the late Chesterian, including Tripartites vetustus, Knoxisporites triradiatus, Knoxisporites stephanephorus, Densosporites diatretus, and Schopfipollenites acadiensis, compared to the first appearance of early Morrowan Crassispora kosankei, Cirratriradites saturnii, Radiizonates aligerens, and Raistrickia saetosa. Palynofacies analysis and statistical clustering of the Tyler Formation showed three palynofacies assemblages. PFA-1 showed moderate relative abundances of phytoclasts and AOM, suggesting deposition close to fluvio-deltaic and shallow marine environments, while PFA-2 exhibited high abundances of phytoclasts, mostly of opaque wood, reflecting deposition in active river-dominated delta plains. PFA-3 showed the highest abundances of AOM, suggesting deposition in a shallow marine environment. Organic petrography and geochemistry data indicate that the Tyler Formation is one of the best source rock intervals throughout the midcontinent USA. Based on organic matter richness, the Tyler Formation is subdivided into three groups. The first group has TOC contents higher than 10 wt% of kerogen Types III, mixed II/III, and II with excellent hydrocarbon generation potential. The second group has TOC content in the range of 2–10 wt% of kerogen Types III, mixed II/III, and II, and fair to excellent hydrocarbon generation potential. The third group shows organic matter richness with TOC content below 2 wt% with good organic matter richness and kerogen Types III to IV, and poor to fair hydrocarbon generation potential. The organic matter thermal maturity is evaluated based on Tmax and VRo% values, suggesting that all samples of Tyler Formation are in the early to late stages of the oil window. However, care should be considered when assessing a mature source rock because the kerogen typing and generation is based on present-day TOC, S2, and HI rather than their original values.

Paleoceanographic Significance of Calcareous Nannofossil Assemblages in the Tropic Shale of Utah during Oceanic Anoxic Event 2 at the Cenomanian/Turonian Boundary

Oceanic Anoxic Event 2 (OAE2) at the Cenomanian/Turonian Boundary (CTB: 93.9Ma) involved the global deposition of organic carbon-rich sediments, a distinctive positive shift in carbon isotope values, and significant species turnover, including changes in calcareous nannofossil assemblages. While it is thought that volcanism triggered organic C-rich sediment deposition during OAE2, it is unclear whether enhanced productivity, increased stratification, of some combination of the two increased organic matter preservation. Calcareous nannofossil assemblages have the potential to qualitatively assess changes in ocean nutrient and temperature conditions to disentangle such ecological dynamics during OAE2. Here we study an expanded section of the Tropic Shale in a drill core in southern Utah near the western margin of the Western Interior Seaway (WIS) to understand how circulation changed during the event and how this may have influenced primary productivity and organic carbon burial. Relative abundance data of well-preserved nannoplankton are complemented with measurements of trace metal, and organic carbon and carbonate concentrations to determine changes in temperature and water column structure, as well as controls on surface water productivity. Detailed statistical analysis helps refine species paleoecologies combined with information from planktic and benthic foraminiferal assemblages and organic biomarkers. Changes in calcareous nannofossil assemblages indicate that near the start of OAE2 the western WIS surface ocean actually cooled for a short time. Following this, surface waters became warmer and more stratified as a Tethyan water mass invaded the seaway. Assemblages suggest that warmth persisted for much of the OAE2 interval, while stratification waxed and waned. The local seaway cooled near the end of OAE2 as Boreal water masses streamed along the western margin. Variations, including the decrease in the abundance of Biscutum constans and short-lived peaks in the abundance of Eprolithus spp. are super regional or possibly global in extent. There is no correlation between calcareous nannofossil assemblages and trace metal concentrations, suggesting they were unaffected by volcanism-related nutrient inputs. Assemblages support other data that suggest increased stratification influenced organic carbon burial in the Western Interior Seaway, and possibly elsewhere, during OAE2.

Melting behavior of impure limestone under H2O-poor conditions: Implications for the contribution of carbonate-rich sediments to arc magmatic carbon output

Sedimentary carbon may be transferred from subducting slabs to the overlying mantle wedge as a component of sediment-derived melts and contribute to carbon output in subduction zones. To investigate the melting behavior of subducting carbonate-rich sediments at shallow depths and evaluate the consequent efficiency of carbon recycling, this study presents the phase relations of H2O-poor impure limestone at 1.5–4.5 GPa and 750–1300 °C. The onset of melting occurs between 750 and 800 °C at 1.5–2.5 GPa, between 950 and 1000 °C at 3.5 GPa, and between 1000 and 1050 °C at 4.5 GPa. The melting experiments yield silicate melts with CO2 contents below 5 wt% and H2O contents below 10 wt%, exhibiting compositional evolution from rhyolitic to dacitic and trachytic as the temperature increases. Combining these results with previous experimental studies on various sediment systems, we conclude that the position of the solidus and melt compositions are influenced by multiple factors, including pressure, bulk composition, and volatile concentrations. Calculations demonstrate that the subduction of carbonate-rich sediment leads to a near-surface recycling efficiency for carbon below 5%, notably lower than the recycling efficiency of water (>75%). This study also indicates that sediment melting in subarc regions is restricted to either hot subduction zones or to subducted slabs at subarc depths exceeding 150 km, which are uncommon among global subduction zones on the modern Earth.However, sediment diapirism may be an efficient way for subducting sedimentary carbon to migrate into source regions of arc magmas. Reaction experiments involving hydrous impure limestone and lherzolite were conducted at 2.5 GPa and 1000 °C, as well as 4.5 GPa and 1100 °C to constrain the melting behavior of carbonate-rich sediment diapirs in the mantle wedge. Silicate melts and a thin websterite reaction layer are generated at 2.5 GPa, while at 4.5 GPa carbonate melts are produced, resulting in the thorough metasomatism of lherzolite. These findings highlight distinct and diverse contributions that carbonate-rich sediment diapirs make to compositional heterogeneity in the mantle wedge at different depths, and also to carbon degassing in magmatic arcs.

Fractionation of Nb/Ta during subduction of carbonate-rich sediments

Fractionation of Nb/Ta during subduction of carbonate-rich sediments

Electrogenic sulfur oxidation by cable bacteria in two seasonally hypoxic coastal systems

Cable bacteria can reach high densities in coastal sediments, and as a result of their unusual electrogenic lifestyle and intense metabolic activity, exert a major and distinct impact on biogeochemical cycling, both locally in sediments and at the ecosystem level. This appears to be particularly true for seasonally hypoxic systems, but the driving force behind the proliferation of cable bacteria in these systems is not well understood. Moreover, the metabolism of cable bacteria induces strong acid production, which can be buffered through carbonate dissolution in sediments. A strong depletion of alkalinity in the pore water is therefore expected in carbonate-poor sediments. To evaluate the impact of cable bacteria metabolism on sediment geochemistry, we performed field sampling and laboratory sediment incubations in two seasonally hypoxic sites: one carbonate-poor site with low levels of free sulfide in pore water (Yarra Estuary, Australia) and one carbonate-rich site with high free sulfide (Lake Grevelingen, The Netherlands). Active cable bacteria populations were found in both field locations, with higher abundance and activity observed in spring compared to autumn. The sediment incubations tracked the metabolic activity of cable bacteria over time (maximum 84 days), and confirmed the fast development of an electric network (cell doubling time: ∼19 h). These results suggest that cable bacteria are widespread in seasonally hypoxic systems, supporting previous findings. Cable bacteria acidified the sediment by > 1.5 pH units in 6–13 days (differing per site) and their activity accounted for >70% of the oxygen uptake. A clear subsurface accumulation of Fe2+ was observed after 8 days of Yarra sediment incubations, indicative of increased FeS dissolution as e-SOx developed. The increased availability of sulfide from FeS dissolution promotes a positive-feedback loop that we infer allowed for a faster development of cable bacteria in the carbonate-poor sediments. A depletion of total alkalinity was observed in the deeper Yarra sediment, whereas, a higher alkalinity efflux was previously observed in the carbonate-rich sediments from Lake Grevelingen. These results suggest a differential pH and alkalinity dynamic due to the interaction between the local carbonate content of the sediment and cable bacteria activity.

Iron mobility in subduction zone fluids at forearc depths and implications for mantle redox heterogeneity

The mobility of redox sensitive elements such as iron, carbon and sulfur in subduction zone fluids is important for redox transfer between the subducting slab and mantle wedge. However, the speciation of Fe during its mobility in subduction zone fluids at forearc depths is still unclear. Here, we studied the Fe isotope compositions of high-pressure (HP) fluid-metasomatized rocks (Mg-rich leucophyllite and gneiss, which is mineralogically transitional between leucophyllite and metagranite) from the Eastern Alps and found evidence for two distinct types of Fe species during Fe mobility in subduction zone fluids. The protolith of both types of rocks is metagranite, which shows δ56Fe values of 0.12–0.25‰. Compared to the metagranite, the transitional gneiss shows slightly higher Fe2O3t contents, lower Fe3+/∑Fe ratios and significantly lower δ56Fe values of −0.03–0.16‰, whereas the leucophyllite exhibits much lower Fe2O3t contents, comparable or slightly lower Fe3+/∑Fe ratios and generally higher δ56Fe values of up to 0.55‰. These observations indicate that the granitic protolith has experienced two types of fluid metasomatism. Combined with previous results, we infer the mobility of Fe in subduction zone fluids at forearc depths via two kinds of Fe species, Fe2+-(CO3), and Fe2+-(HS) or Fe2+-Cl. The Fe mobility was dominant in the form of the Fe2+-(CO3) complex in a carbonate-rich fluid during the formation of transitional gneiss, consistent with reported radiogenic Sr and isotopically light Mg isotope compositions, whereas Fe loss in the form of Fe2+-(HS) or Fe2+-Cl complexes via dissolution of biotite and phlogopite can account for the formation of leucophyllites. According to the tectonic evolution of the Eastern Alps, both types of fluids possibly originated from the dehydration of the sediment-serpentinite mélange produced by the subducting oceanic slab of the Alpine Penninic unit. The diversity of ferrous iron species in the subduction zone fluids at forearc depths probably reflects the variation in the redox properties of fluids derived from carbonate-rich sediment and serpentinite under such conditions. Our results provide an excellent example of tracing Fe mobility with distinct Fe species in subduction zone fluids.

Miocene olivine leucitites in southern Tibet: Implications for the recycling of carbonates into the mantle during continental subduction

We present a comprehensive geochronological, mineralogical, and geochemical study on previously unknown olivine leucitites occurring in the Maiga area of southern Tibet to examine the possibility of carbonate recycling during continental subduction. The Miocene (∼16 Ma) Maiga olivine leucitites are characterized by low SiO2 (44.6–46.3 wt%), high MgO (14.0–15.7 wt%), and ultra-calcic (CaO/Al2O3 = 1.1–1.2) compositions, along with arc-like incompatible element patterns (e.g., enrichment in large-ion lithophile and light rare earth elements, and depletion in high-field-strength elements). These rocks also have extremely radiogenic 87Sr/86Sr(i) (0.715647–0.715844), unradiogenic εNd(i) (−11.6 to −11.2) and εHf(i) (−10.5 to −10.0), and moderately radiogenic 206Pb/204Pb(i) (18.44–18.46) isotopic compositions. The studied samples are also characterized by slightly heavy O isotope ratios (clinopyroxene δ18O = +6.7‰ to +8.7‰) and relatively low water contents (0.75–1.75 wt%). The geochemical features of the Maiga olivine leucitites are compatible with an origin from a carbonated peridotite mantle source. Additionally, based on the regional tectonic evolution and mid-ocean-ridge-basalt-like Zn isotopic compositions (δ66Zn = +0.26‰ to +0.30‰), we hypothesize these magmas formed from interactions between ambient mantle peridotite and recycled Ca-rich carbonate sediments during Indian continental subduction. The transition from the neighboring Miocene lamproite-like rocks in southern Tibet to olivine leucititic magmas in Maiga is assumed to reflect the tapping of two different mantle sources. The first was modified by silicate-rich sediments, which generated lamproite-like rocks, whereas the Maiga mantle source was modified by carbonate-rich sediments. This study and previous research on other K2O-rich subduction-related magmas indicate that non-negligible amounts of carbonates can be recycled deep into the mantle during continental subduction, which may play an important role in global carbon recycling.

Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc

Sediment contribution to the mantle is the key step for the generation of orogenic magmatism to produce its isotopic and geochemical inventory. Even though they are exceptional for the post-collisional settings, there are worldwide examples of arc-related ultrapotassic mafic magmas which require complex multi-stage processes along with sediment melting e.g. in Italy or Pontides of Türkiye. To understand the metasomatism leading mantle to produce ultrapotassic mafic melts, we simulated the reactions of depleted (harzburgite) and fertile (lherzolite) mantle with subducted carbonate-rich sediment at relatively cold (800–850 °C) and shallow (2 GPa, 60–80 km) slab-mantle interfaces. The melting of sediments can trigger the formation of immiscible and conjugate carbonatitic and silicic melts which flux the mantle to develop hydrous minerals and dolomitic melt. The metasomatic growth product is a wehrlite composed of clinopyroxene, phlogopite, carbonate minerals and amphibole, representing a source of choice for Si-undersaturated ultrapotassic lavas. The occurrence of conjugate carbonatitic and silicic melts and their potential physical separation, offer a possibility for fractionation of several canonical trace element ratios such as Th/La, observed in Si-saturated ultrapotassic lavas. The synergy between peridotite-melt interaction and the physical separation of the carbonatitic and extremely K-enriched silicic melts are essential for the compositional evolution of ultrapotassic orogenic magmas and their mantle sources.

Enhanced clay formation key in sustaining the Middle Eocene Climatic Optimum

The Middle Eocene Climatic Optimum (around 40 million years ago) was a roughly 400,000-year-long global warming phase associated with an increase in atmospheric CO2 concentrations and deep-ocean acidification that interrupted the Eocene’s long-term cooling trend. The unusually long duration, compared with early Eocene global warming phases, is puzzling as temperature-dependent silicate weathering should have provided a negative feedback, drawing down CO2 over this timescale. Here we investigate silicate weathering during this climate warming event by measuring lithium isotope ratios (reported as δ7Li), which are a tracer for silicate weathering processes, from a suite of open-ocean carbonate-rich sediments. We find a positive δ7Li excursion—the only one identified for a warming event so far —of ~3‰. Box model simulations support this signal to reflect a global shift from congruent weathering, with secondary mineral dissolution, to incongruent weathering, with secondary mineral formation. We surmise that, before the climatic optimum, there was considerable soil shielding of the continents. An increase in continental volcanism initiated the warming event, but it was sustained by an increase in clay formation, which sequestered carbonate-forming cations, short-circuiting the carbonate–silicate cycle. Clay mineral dynamics may play an important role in the carbon cycle for climatic events occurring over intermediate (i.e., 100,000 year) timeframes.

Paleoclimatic conditions during the accumulation of the Upper Jurassic and Lower Cretaceous organic carbon-rich deposits of the Ulyanovsk-Saratov trough (eastern Russian platform)

Relevance of the work. Black shale horizons may reflect abrupt paleoclimate changes. In the northeast of the Ulyanovsk-Saratov Trough (UST), two black shale horizons widely occur, the Middle Volgian Promzino Formation and the Lower Aptian Ulyanovsk Formation. Despite years of research, the role of paleoclimate in their accumulation is still under discussion. The aim of the research was to assess the climate as a factor of the accumulation of the Middle Volgian and Lower Aptian organic carbon-rich deposits in the northeastern UST. Methods. To reconstruct paleoclimate and to evaluate its impact on the black shale deposition, a comprehensive isotope study of calcitic and aragonitic shells of several groups of benthic and pelagic organisms was performed. Besides, the lithological and geochemical features of organic carbon-rich and host deposits were studied using optical microscopy, XRD, and pyrolysis. Results. Isotope study allowed to construct benthic and pelagic palaeotemperature curves and to reconstruct paleoclimate in the UST during the formation of black shales. Based on these data and data on lithological and geochemical features of sediments, the depositional mechanisms of the Promzino and Ulyanovsk Formations were clarified. Conclusions. No causal link has been revealed between climate changes and the deposition of the Middle Volgian black shales. At the end of the Middle Volgian, a short-term cooling has been recorded, probably related to the volcanic activity of the northern Tethyan margin. The Lower Aptian black shales were formed during the hyperthermal. This hyperthermal controlled the accumulation of organic carbon-rich sediments by increasing the input of nutrients and/or providing conditions for its stratification and stagnation.

New U–Pb geochronology for the Central Atlantic Magmatic Province, critical reevaluation of high-precision ages and their impact on the end-Triassic extinction event

The end-Triassic extinction (ETE) event represents one of the ‘big five’ episodes of mass extinction. The leading hypothesis for the cause of the ETE is the intrusion of voluminous magmas of the Central Atlantic Magmatic Province (CAMP) into carbon-rich sediments of two South American sedimentary basins, around 201.5 Ma. The timing of dikes and sills emplacement, however, must be considered in light of age models from CAMP rocks occurring in North America. In this work, we present new high-precision ages for critical samples in NE Brazil (201.579 ± 0.057 Ma) and Canada (201.464 ± 0.017 Ma), in order to evaluate how the South and North American magmatic events compare at the 100-ka level, and to the ETE timing. We also discuss inter-laboratory reproducibility of high-precision CAMP ages, including the 230Th disequilibrium corrections that are made to zircon U–Pb dates. Our findings in this newly discovered extension of the CAMP large igneous province in NE Brazil support the hypothesis that the CAMP may be responsible for the ETE through the triggering of greenhouse gas release from magma-evaporite interactions (contact metamorphism) in the South American basins.

Dynamic link between Neo-Tethyan subduction and atmospheric CO2 changes: insights from seismic tomography reconstruction

Volcanic arc degassing contributes significantly to atmospheric CO2 levels and therefore has a pivotal impact on paleoclimate changes. The Neo-Tethyan decarbonation subduction is thought to have played a major role in Cenozoic climate changes, although there are still no quantifiable restrictions. Here we build past subduction scenarios using an improved seismic tomography reconstruction method and calculate the subducted slab flux in the India–Eurasia collision region. We find remarkable synchronicity between calculated slab flux and paleoclimate parameters in the Cenozoic, indicating a causal link between these processes. The closure of the Neo-Tethyan intra-oceanic subduction resulted in more carbon-rich sediments subducting along the Eurasia margin, as well as continental arc volcanoes, which further triggered global warming up to the Early Eocene Climatic Optimum. The abrupt termination of the Neo-Tethyan subduction due to the India–Eurasia collision could be the primary tectonic cause of the ∼50–40 Ma CO2 drop. The gradual decrease in atmospheric CO2 concentration after 40 Ma may be attributed to enhance continental weathering due to the growth of the Tibetan Plateau. Our results contribute to a better understanding of the dynamic implications of Neo-Tethyan Ocean evolution and may provide new constraints for future carbon cycle models.

Clay authigenesis in carbonate-rich sediments and its impact on carbonate diagenesis

The Mg (δ26Mg), Ca (δ44Ca), and Sr (87Sr/86Sr) isotopic compositions of pore fluids, bulk carbonates, planktonic foraminiferal tests, and bulk clays from ODP Site 762 Hole B are presented, as are pore fluid and bulk carbonate δ26Mg and 87Sr/86Sr from ODP Site 806 Hole B and pore fluid δ26Mg from ODP Site 807 Hole A. The primary objective of the study is to elucidate the major processes controlling marine pore fluid δ26Mg, specifically the effects of calcite recrystallization and authigenic clay precipitation in sedimentary sections with relatively high carbonate contents. Such studies are critical for evaluating the potential of pore fluids in carbonate section to drive diagenetic alteration, which can compromise applications of geochemical proxies to the past. Pore fluid δ26Mg values at all three sites range from −0.83 to −0.13‰ and exhibit a systematic increase with depth. Bulk carbonate δ26Mg generally decrease with depth, ranging from −3.60 to −5.27‰, at Sites 762 and 806, while mixed species foraminiferal tests (∼250–500 μm) from Site 762 range between −5.08 and −4.36‰. Residual siliciclastics at depths of ∼105 to 145 mbsf at Site 762 have δ26Mg values (−0.09 to 0.27‰) that are markedly higher than carbonate and pore fluid δ26Mg values. Simple 1-D reactive transport modeling suggests that the general increase in pore fluid δ26Mg with depth, accompanied by a decrease in carbonate δ26Mg, is a result of calcite recrystallization (assuming an isotopic fractionation factor of ∼0.9955). However, subtle but significant deviations from the carbonate recrystallization-only scenario suggest that another process impacts δ26Mg at all three sites. Scanning electron microscope images document clay particles embedded in nannofossils and foraminiferal tests at Site 762, which suggest that clay authigenesis is active in carbonate sediments and could affect pore fluid δ26Mg. The formation of secondary clays preferentially sequesters isotopically heavy Mg (αclay-Mg2+≈1.0005), driving pore fluid δ26Mg to lower values. An increase in carbonate δ26Mg within the clay-rich layer at Site 762 and an increase in bulk carbonate Na/Ca supports the hypothesis that clay authigenesis also impacts the preservation of proxy archives. Multi-component reactive transport modeling suggests that authigenic rates of ∼1·10−13 mol/m3/s (∼3.15 µmol/m3/a; assuming that the authigenic clay is sepiolite) can generate deviations from the carbonate recrystallization-only case by several tenths of a permil, indicating that carbonate sediment-associated clay authigenesis (CSCA) may be more relevant in deep-sea carbonate sections than has been previously considered.

In situ trace element compositions and U[sbnd]Pb ages of cassiterite from tin-polymetallic deposits in the Dachang district, Guangxi, China: Implications for ore genesis and exploration

The Dachang tin-polymetallic district in southwest China is one of the largest tin ore fields in the world containing approximately 1.5 Mt. Sn, 6.8 Mt. Zn, 1.8 Mt. Pb, 1.4 Mt. Sb, 0.4 Mt. Cu and other critical metals. Both cassiterite-sulfide and ZnCu skarn mineralization occur as stratiforms, veins, and stockworks and are hosted in the Mid-Upper Devonian carbonate-rich sediments adjacent to the underlying Late Cretaceous Longxianggai granite pluton and/or the granite/diorite porphyry dikes. The Tongkeng and Huile deposits are typical stratiform- and vein-type tin-polymetallic deposits in the district. Previous studies suggest a genetic link between these tin-polymetallic deposits and the prolonged Longxianggai granite which was dated to have emplaced for multiple stages within ca. 91–97 Ma. However, existing constraints on the timing of tin mineralization display a board range at 90–96 Ma, whereas the accuracy and precision of these ages are less satisfactory. These ages are broadly overlapping with both the prolonged Longxianggai granite and the porphyry dikes (91 Ma), thus gives rise to question on their genetic association, which is a question of importance for ore exploration in the district.In this study, a large number of measurements of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace elements and UPb dating on cassiterites from the Tongkeng and Huile deposits was performed and their mineralization ages with hitherto the best accuracy and precision were obtained. LA-ICP-MS trace element analyses show that cassiterites from the Tongkeng and Huile deposits contain concentrations of Fe, W, Nb, and Ta characteristic of granitic-related skarn-type tin mineralization. UPb dating on cassiterites from 391 spots analyses yielded well-constrained ages of 91.13 ± 0.61 Ma to 89.46 ± 0.47 Ma for the Tongkeng deposit and 89.0 ± 1.3 Ma for the Huile deposit. These ages are apparently younger than the dominated equigranular phase of the Longxianggai pluton, but they agree well with the emplacement ages of the porphyritic granite represented either by the latest phase of the Longxianggai granite or the granite porphyry dikes (ca. 91 Ma). This suggests that the tin-polymetallic deposits in the Dachang district were most likely genetically associated with later, higher evolved granitic phases in the district. In this regard, the dike-hosting fault systems were most likely served as fluid conduits and their surrounding host rocks are favorable targets for ore exploration in the Dachang district.

Benthic Foraminifera and Productivity Regimes in the Kveithola Trough (Barents Sea)—Ecological Implications in a Changing Arctic and Actuopaleontological Meaning

The rapid response of benthic foraminifera to organic carbon flux to the seafloor makes them promising bioindicators for evaluating the organic carbon stored in marine sediments. Fjords have been described as hotspots for carbon burial, potentially playing a key role within the carbon cycle as climate regulators over multiple timescales. Nevertheless, little is known about organic carbon-rich sediments in Arctic open shelves and their role in global carbon sequestration. To this aim, four sites have been sampled along a W-E transect across the Kveithola Trough located in the NW Barents Sea. Living (stained) benthic foraminiferal density, biodiversity and vertical distribution in the sediment were analysed together with the biogeochemical and sedimentological data. We identified two main depositional environments based on the relationship between benthic foraminiferal assemblages and carbon content in the sediments: (1) an oligotrophic land-derived organic matter region located in the outer part of the trough influenced by the warm and saline Atlantic Water; and (2) a stressed eutrophic environment, with high-content of metabolizable organic matter in the inner part of the trough, which comprises the main drift and the Northern flank of the trough. The freshness and good nutritional quality of the organic matter detected in the inner region could be the result of the better preservation of the organic matter itself, basically driven by the rapid burial of fine-grained organic-rich sediments enhanced by the cold and less saline Arctic Water coming from the Barents Sea. We conclude that foraminifera provide a tool to describe the Kveithola depositional environment as a carbon burial hotspot in a changing Arctic area subjected to a pulse of fresh food intended as biopolymeric carbon.

Paleoenvironmental, paleoclimatic, and organic matter assessment of the hybrid Kharita Formation (Albian) in the Abu Gharadig Basin, Egypt: Integration between palynology, organic petrography, and organic geochemistry

The Early Cretaceous (Albian) was a time of equatorial to polar temperature warming and increased environmental perturbations of the global carbon cycle and oceanic anoxia, which resulted in the widespread accumulation of organic carbon-rich sediments. During the Albian, the southern margin of the Tethys Ocean was characterized by marginal to shallow water conditions with minor phases of enhanced stratification and bottom water anoxia in northern Egypt. At this time, accumulation of the thick siliciclastic succession of sandstones versus shale intercalations of the Kharita Formation took place in the Abu Gharadig Basin. The Kharita Formation is investigated for palynological composition and palynofacies analysis, organic matter characterization, hydrocarbon generation potential, and thermal maturity assessment based on 30 and 26 drill cuttings samples from the BED2-3 and BED2-1x wells, respectively. A rich palynological assemblage of herbaceous hygrophilous plants, including pteridophyte spores and water ferns, along with Elaterates and Afropollis pollen indicated warm humid climate conditions. Furthermore, abundant occurrences of Araucariaceae and Cheirolepidiaceae, including Araucariacites, Inaperturopollenites, and Classopollis, reinforcing warm climatic conditions and their climax in areas near coastal vegetation. Statistical cluster analysis identified two palynofacies assemblages (PFA) based on stratigraphic variation in the particulate organic matter (POM). The PFA-1 characterized all samples of the BED2-3 and some intervals of the BED2-1x wells and comprised moderate abundances of phytoclasts and amorphous organic matter (AOM) that defined a kerogen Type II, whereas the PFA-2 was recorded only in the BED2-1x well and is dominated by phytoclast fragments of kerogen Type III. The geochemical screening revealed, based on the low average values of TOC (1.2 wt%), S2 (1.8 mg HC/g rock), and HI (146 mg HC/g TOC), that the Kharita Formation is characterized by a kerogen Type III of gas-prone hydrocarbon but with low generation potential. Tmax and vitrinite reflectance measurements along with greenish-to golden-yellow liptinite macerals indicated that the thermal maturity of preserved organic matter is still in the immature to the early mature stage of the oil window. Inconsistency in kerogen types between PFA-1 and geochemical deductions was interpreted in terms of microscopic investigations of the recovered AOM. Morphologic characteristics observed that most AOM particles were of terrigenous origin due to the impact of biodegradation and transformation of phytoclast particles into AOM, confirming a kerogen Type III of gas-prone hydrocarbons instead of Type II that was elucidated by palynofacies ternary diagrams. The occurrence of coaly matter in the BED2-1x infers closer proximity to fluvio-deltaic sources during deposition of the lower part of the Kharita Formation than in the BED2-3.

Unravelling the fabrics preserved inside early diagenetic concretions: Insights for the distribution, accumulation and preservation of organic-rich mud in the interior of epicontinental basins

Fine-grained sedimentary rocks generally undergo severe mechanical compaction during burial, which complicates the recognition of primary mudstone fabrics and associated sedimentary features. Early diagenetic concretions, however, provide a rare glimpse of primary fabrics because cement filling the pore space prevents the collapse of original grain arrangements. Hand specimens of concretions collected from the basal condensed section of the Late Jurassic-Early Cretaceous Vaca Muerta Formation (Neuquén Basin, Argentina), allow for analysis of sedimentary processes responsible for the dispersal, accumulation and burial of organic carbon-rich sediment in an epicontinental sea. Representative samples from central basin depositional localities were examined by optical, scanning electron microscopy and energy dispersive X-ray spectroscopy. Petrographic observations were complemented with palynological and organic geochemical analyses. Close examination of uncompacted fabrics reveals a significantly more complex and dynamic depositional scenario than previously assumed (suspension settling). Although many of the component grains in the studied samples were originally delivered to the sediment–water interface by suspension settling processes (i.e., marine snow, hypopycnal plumes, pumice rafts), there is substantial evidence of episodic sedimentation controlled by punctuated events of seafloor disturbance and erosion. The common presence of muddy intraclasts indicate that the seafloor was frequently reworked by bottom currents that caused the widespread distribution of organic carbon-rich sediment across distal basin depositional environments. Bottom current circulation supplied oxygen to the sediment–water interface and created suitable conditions for benthic life, contravening the assumption of bottom water anoxia as a prerequisite for organic carbon preservation. The excellent preservation state of freshwater algae (Pediastrum complex) suggests that organic matter contained inside mud composite particles can travel long distances before being deposited in distal depositional settings. Encapsulation protects organic components from mechanical/biogenic degradation and provides an anoxic microenvironment for preventing the oxidation of the organic matter contained inside of mud composite grains. The study shows that organic carbon encapsulation may be an important mechanism for organic carbon preservation in relatively energetic and non-anoxic settings, calling for a critical reappraisal of the processes responsible for the sequestration of organic carbon from the biosphere and its long-term storage in organic-rich mudstone successions.

An integrated geochemical analysis, basin modeling, and palynofacies analysis for characterizing mixed organic-rich carbonate and shale rocks in Mesopotamian Basin, Iraq: Insights for multisource rocks evaluation

Shales and carbonate-rich sediments are crucial in characterizing the geological history of petroleum as source rocks. This emphasizes the global significance of carbonate source rocks, and it has sparked research interests and exploration of these strata as unconventional resources. This study presents inclusive organic geochemical characteristics, palynofacies analysis, and basin modeling of the carbonate source rocks in the Ajeel oilfield, Mesopotamian Basin, Iraq. This study used traditional and advanced geochemical techniques, as well as microscopic examination and basin modeling. A total of 109 core and cutting samples from the Middle Jurassic–lowermost Early Cretaceous succession were studied via geochemical and palynological methods to evaluate their thermal maturation, depositional conditions, and hydrocarbon (HC) generation potential by combining one-dimensional (1D) modeling of the burial/thermal history and the generation/expulsion timing in the Mesopotamian Basin, Iraq. The results of total organic carbon/Rock-Eval pyrolysis revealed that the Middle Jurassic–lowermost Early Cretaceous interval predominantly comprised carbonate-rich sediments with Type-II and mixed Type-II/III kerogen. Furthermore, the results showed that these potential source rocks were of fair to excellent quality (oil window) and were deposited mainly in an anoxic marine environment. Geochemical and palynological investigation suggested that the carbonate-rich source rocks were deposited mainly under a distal suboxic–anoxic condition and contained Type-II kerogen (oil prone), except for part of the Chia Gara Formation, which displayed distal dysoxic–oxic shelf depositional conditions. The 1D basin modeling results of the burial/thermal maturation history and the generation/expulsion timing showed that these sediments reached their peak generation potential (main oil window) during the Early Miocene, and this has continued until the present day. Subsequently, the Middle Jurassic–lowermost Early Cretaceous succession represents the main source of petroleum in the Mesopotamian Basin, Iraq.This work shows how an integrated palynofacies analysis, basin modeling, and geochemical study can be used for the precise assessment of multisource rock intervals of mixed organic-rich carbonate and shale rocks. These research findings will provide guidance for better resource exploitation in northern Iraq, which has higher source-rock quality in terms of organic richness and maturity than that in southern Iraq.