Contemporaneous Seawater Research Articles

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.

Evaporite sequences as archives for Mg isotope compositions of seawater - Evidence from a Tethys marginal shelf basin in the Anisian

Evaporites have recently been suggested as a potential archive for recording the Mg isotope compositions (δ26Mg) of coeval seawater. However, episodic dolomitization during the deposition of massive evaporites could cause considerable Mg removal and isotopic fractionation. To constrain the hydrological changes and influence of dolomitization on ambient brine δ26Mg, we present petrographic and mineralogical features, as well as Mg-C-O-Sr isotope data extracted from carbonate phases of a middle Triassic (ca. 247 Myr) marine anhydrite-dolostone sequence from a drill core in eastern China. The drilled lithologies are characterized by massive dolostone layers in the lower part, followed by an upward decline in dolomite contents accompanied by a rise in anhydrite. Multiple lines of evidence consistently point to a syn-depositional origin for the dolostones in a marginal basin of the Tethys Ocean and a lack of diagenetic alteration since deposition. We reconstructed the dynamic changes of δ26Mg values of basin waters based on Mg isotope compositions in dolomite leachates. According to our findings, the δ26Mg values of the basin waters were remarkably high (about 0.38 ± 0.05‰) at the onset of evaporation, indicating a significant Mg sink of the massive dolomitization. The δ26Mg of brine in the basin then changed towards the value of coeval seawater (about −0.32 ± 0.05‰) starting with the deposition of evaporites. Concurrently, 87Sr/86Sr ratios of dolomites shift from radiogenic values towards contemporaneous seawater composition. Our results demonstrate that the evaporite basin was not strictly restricted, and water exchange with the open ocean never ceased. Modeling calculation reveals that, even when the seawater exchange rate is far below the average ocean circulation, δ26Mg of brine in the basin will reach the value of open ocean within 1 Myr, completely removing the influence of early dolomitization. We suggested that massive marine evaporite sequences have the potential to record the long-term evolution of seawater Mg isotopes.

Dolostone-barite-phosphorite sequence in the basal Doushantuo Formation: Origin and implications for post-Marinoan ocean chemistry and phosphogenesis

Barite (BaSO4) represents a major component of many post-Marinoan cap dolostones worldwide, recording important information on climate upheavals during the terminal Neoproterozoic. However, its origins and relations with other lithofacies remain to be explored. Here we report new occurrences of barite in the basal Ediacaran Doushantuo Formation at a major phosphate mine in Weng'an, South China, and present results of detailed petrographic and geochemical investigations on the barites and related phases. At the outcrop scale, barite is mainly present as a 1–2 cm-thick layer at the contact of the cap dolostones and overlying phosphorites. Detailed petrographic observations reveal a lithofacies transition from dolostone through a mixture of barite, dolomite and apatite to barite and finally to granular phosphorite. The barites can be categorized into three generations in stratigraphic order, which are present as loose, randomly-oriented blades (B1), dense aggregates (B2), and radially aligned crystal fans (B3), respectively. B1 is dispersed in a matrix of apatite and dolomite, which has an average δ13Ccarb of −0.61‰. This, along with the common presence of organic matter (OM) in this barite, point to a possible source of Ba from organic degradation. B3 forms a centimeter-scale layer of barite fans that laterally extends over the entire mining pit, whereas B2 is locally distributed between B1 and B3, and occurs as relatively unoriented aggregates. δ34S values of B3 show a narrow range between +25.9‰ and +30.9‰, which is similar to that of contemporaneous seawater and indicate a seawater source of sulfate in barite. Based on the present investigation and a compilation of published data, we propose that B1 is diagenetic barite with a main source of Ba from degradation of organic matter and possibly also from dissolution of previous pelagic barite, whereas B2 and B3 represent seafloor precipitates that resulted from interactions between deep, anoxic Ba-rich waters and oxic, sulfate-rich surface waters during the post-glacial transgression. This episode of barite precipitation was immediately followed by massive accumulations of phosphorites in Weng'an and many areas of South China. The consecutive deposition of cap dolostone, barite and phosphorite signifies the transition from an alkalinity-rich, stagnant anoxic ocean to a dynamic marine regime featuring oxygenated surface waters and upwelling currents. These pH and redox changes likely facilitated the cycling and enrichment of phosphate in the shallow waters that was necessitated by the unprecedented accumulation of Ediacaran phosphorites in South China.

Mesozoic orogenic gold metallogenesis at the Bangong–Nujiang suture belt, central Tibet: Constraints from pyrite textures, composition and sulfur isotope of the Shangxu gold deposit

Pyrite is a good carrier to provide gold within a hydrothermal mineralization system. Under prograde metamorphism, recrystallization of sedimentary and diagenetic pyrite and transformation of pyrite into pyrrhotite in a metamorphic terrane lead to the release of trace elements from the pyrite carrier into an upward migrating liquid phase carrier. In this case study, we report the in situ trace element and δ34S compositions of the pyrite from four different stages, which chronologically comprise sedimentary Py1, diagenetic Py2, metamorphic Py3a, Py3b and hydrothermal Py4a, Py4b, Py5, and the dominant sulfides from the hydrothermal stage at Shangxu, an Early Cretaceous turbidite-hosted orogenic gold deposit in the middle Bangong–Nujiang suture belt (BNSB) in central Tibet, in order to decipher the ore-forming material source and the metallogenesis of the Mesozoic orogenic gold mineralization in the BNSB. Concentrations of trace elements, i.e., Au, Co, Cu, Zn, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, that are easy to dissolve during pyrite recrystallization, show a general progressive decline from Py1 to Py4b (excepting in Py3b and Py4a, which have a sudden rise in trace element composition) and a plunge in Py5, indicating that Py1 and Py2 supply the parent source minerals for mineralization, and that the main ore-forming stage are contemporary with the deposition of Py3b and Py4a, ending up with the precipitation of Py5. The δ34S values of pyrite narrow from −46.7 ‰–19.3 ‰ (Py1, Py2), through −7.6 ‰–0.2 ‰ (Py3a, Py3b), to − 3.6 ‰–5.7 ‰ (Py4a, Py4b) and −7.7 ‰–5.6 ‰ (Cp, Gn), indicating that a probable coeval seawater sulfate sulfur source for Py1 and Py2 (by bacterial sulfate reduction), and an inherited sulfur source from Py1 and Py2 for Py3 and Py4. We highlight that contributions from the magmatic materials to the hydrothermal gold mineralization are limited at Shangxu, and the widely distributed Lower–Middle Jurassic pyrite-rich turbidite of the Mugagangri Group is a highly potential primary source rock for the regional gold mineralization.

Strontium Isotopic Variations of Authigenic Calcite in Clastic Strata Record Its Sediment Provenance and Fluid−Rock Interactions

Abstract Strontium isotopes of authigenic carbonate potentially record sediment provenance, fluid sources, and fluid–rock interactions, little was studied on this topic in clastic strata. This study investigated clastic rocks containing authigenic calcite in the Lower Triassic Baikouquan Formation in the Junggar Basin, northwestern China. Mineral compositional and fluid inclusion analyses were conducted to constrain the precipitation processes of authigenic calcite, and the Sr contents and isotope ratios of the calcite were also measured. The authigenic calcite was precipitated at 80–140°C as the final product of thermochemical oxidation of hydrocarbons and thus has high Mn contents and highly negative δ13CVPDB values (as low as −70‰). The calcite also exhibits anomalously low 87Sr/86Sr values (0.704827, 0.706612), which are lower than contemporaneous seawater and published 87Sr/86Sr values of carbonate cements in clastic sediments, and also much lower than 87Sr/86Sr values (0.722027, 0.736750) of alkali feldspar in the strata. These low 87Sr/86Sr values record the low 87Sr/86Sr of the dominant rocks in the provenance area, such as volcanic rocks. During diagenesis, especially mesodiagenesis, the charging of hydrocarbon-bearing fluids promoted abundant dissolution of orthoclase in the alkali feldspar detritus, releasing radiogenic 87Sr into the pore waters, and eventually increasing the 87Sr/86Sr values in the late-stage calcite that precipitated after this reaction. This inference is consistent with the positive correlation between the calcite 87Sr/86Sr ratios and the dissolution intensity of orthoclase. In regions that do not undergo hydrocarbon-charging and where orthoclase remains stable, the lower 87Sr/86Sr ratios of the calcite generally record the provenance. For authigenic calcite associated with intense fluid–rock interactions, the higher 87Sr/86Sr ratios reflect the enhanced dissolution intensity of 87Sr-rich minerals such as orthoclase. Therefore, combined with a petrological study, Sr isotopes of authigenic carbonate in clastic sediments can trace sediment provenance and intensity of fluid–rock interactions.

Genetic relationship between petroleum evolution and mercury mineralization: Insights from calcite U[sbnd]Pb dating, geochemical characterization, and solid bitumen TEM analysis

A spatial association between hydrocarbon reservoirs and metalliferous deposits is found worldwide. However, the genetic relationship between them is unclear. The associated fluids record information related to petroleum evolution and mineralization. Integrating UPb dating and geochemical analysis on calcite associated with either bitumen or cinnabar and transmission electron microscopy analysis of the bitumen along the mercury belt of the Xuefeng Uplift, South China, this work provides evidence supporting that hydrocarbons are critical in mercury mineralization. The UPb age for the calcite associated with either bitumen (395–402 Ma) or cinnabar (389–391 Ma) indicates that oil migration/accumulation and mercury mineralization were broadly coeval. The similar geochemical data (δ13C–δ18O and REE) on bitumen- and cinnabar-associated calcite indicate that they are cogenetic and derived from the dissolution of the limestone under low temperatures. High mercury abundance (0.20% to 0.36%) and the nanoscale mercury-bearing isolated aggregates within the solid bitumen indicate that the hydrocarbons could have functioned as a mercury carrier before mineralization. Furthermore, the similar evolution trend of the sulfur isotope composition among cinnabar, the contemporaneous seawater sulfate and gypsum, and the higher S/C atomic ratio (>0.03) of the bitumen indicate that petroleum participated in thermochemical sulfate reduction and could be a vital mechanism for mercury precipitation. After generating from the organic-rich shale, the hydrocarbons and ore-forming fluids migrated, and the hydrocarbons might have functioned as a carrier and reducing agent during mercury mineralization.

Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments: Sedimentological, petrographical and geochemical constraints

Carbonate concretions accompanied by elemental sulfur were found in an early Messinian (Late Miocene) marine succession of NW Italy. The rocks were studied with an integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses. Unlike other examples from Messinian strata of the Mediterranean area, the studied carbonate and sulfur concretions did not derive from the diagenetic replacement of sulfate minerals. Three lithofacies were distinguished: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The carbon and oxygen stable isotope composition of authigenic carbonate minerals and lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from below, possibly resulting from destabilization of gas hydrates, was not only responsible for the brecciation of the stromatolites, but also for the delivery of bicarbonate ions and the consequent precipitation of additional, 13C-depleted calcite (δ13C values as low as −52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM at the SMTZ and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S values and Δ33S values close to coeval seawater sulfate.

Robust sulfur and oxygen isotope evidence for a highly anoxic paleoenvironment in Late Devonian seawater: Insights from marine anhydrites in the Zaige Formation, South China

A global-scale mass extinction event occurred in the Late Devonian, and its triggering mechanisms have remained a subject of controversy. The key to resolving this controversy lies in elucidating paleoenvironmental characteristics and their coupling relationship with the mass extinction. Marine evaporites serve as sensitive indicators that can be used for reconstructing the geochemical history of ancient oceans. In this study, we present a dual sulfur and oxygen isotopic dataset of marine anhydrites in the Lunatian gypsum deposit (Yunnan, China), and investigate its paleoenvironmental significance within the framework of the Late Devonian mass extinction. These anhydrites originate in arid and hot continental margin seas near convergent plate boundaries, forming as a result of the dehydration of marine gypsums during burial processes. Anhydrites exhibit a conspicuous positive anomaly in δ34S values (23.6–25.1‰, averaging 24.3‰), primarily attributed to bacteria-mediated sulfate reduction. The δ18O values of the Lunatian anhydrites (12.2–15.2‰, averaging 14.0‰) indicate a slightly higher oxygen isotope composition for Late Devonian seawater (8.7–11.7‰, averaging 10.5‰) compared to those of contemporary seawater (9.5–10.1‰). Such an oxygen isotopic characteristic for the Lunatian anhydrites is intricately associated with the chemical processes that occur during the diffusion of sulfides into the upper seawater. The anhydrite-forming evaporating system has a noticeable brine layering where the surface seawater is less dense than the deeper brine, and bacterial processes gradually increase δ34S in the deep brine. When the deep brine and surface seawater rapidly mix, it leads to a significant positive shift in the sulfur isotope composition of the resulting evaporites. Our sulfur and oxygen isotope data from Lunatian anhydrites validate the existence of an exceedingly anoxic marine environment during the Late Devonian, potentially serving as a pivotal factor in the mass extinction event.

Ca isotopic gradients in epeiric marine carbonates: Diagenetic origins of and significance for Ca cycle reconstructions

Marine carbonates record geochemical information on the state of Earth’s surface environment in the geological past, but recrystallization during burial can make the records difficult to interpret. Here, we report shore to basin gradients in δ44Ca values in matrix limestone and burrow dolomite across a sequence of Late Ordovician (Katian Stage) burrow-mottled carbonates in the Williston Basin, an intracratonic epeiric marine basin in North America. Both gradients are interpreted to have diagenetic origins. The gradient in limestone δ44Ca values developed in a bathymetrically controlled sediment- to fluid-buffered diagenetic system operating between the center and the edges of the basin. Two dolomitizing events formed the gradient in dolomite δ44Ca values. The sediment-buffered end of the limestone gradient preserves the δ44Ca value of the original carbonate mud (∼–1.28 ± 0.10 ‰). The fluid-buffered end of the gradient conserves the δ44Ca value of contemporaneous seawater (–0.47 ± 0.10 ‰), because: (1) pore water exchange drivers operated with greater variety and intensity in shallow water settings, (2) shallow nearshore deposits exchanged more Ca with seawater during recrystallization to diagenetic calcite than deeper offshore deposits, and (3) diagenetic calcite formed with negligible fractionation under equilibrium conditions. The gradient conserves the local value of Δsed (–0.81 ± 0.20 ‰) despite its diagenetic origin, and the magnitude of Δsed is consistent with primary calcite precipitation in a ‘calcite sea’. Field assessment of the equilibrium Ca isotope fractionation factor between calcite and dolomite of –0.41 ± 0.17 ‰ at ambient temperature is applied to correct basin-centered dolomite for fractionation, yielding –1.62 ± 0.27 ‰, which is very different from the –0.25 ± 0.07 ‰ value of Ca in evaporatively concentrated seawater from which the dolomite originally formed. Applying the same fractionation factor to correct literature data on early diagenetic dolomite in the Great Basin, North America, yields –0.69 ± 0.22 ‰ for the δ44Ca value of seawater, which is ∼0.2 ‰ lower than ‘normal marine’ waters in stratigraphically equivalent deposits in the Willison Basin, and 0.44 ‰ lower than evaporatively concentrated seawater during a brief period of basin restriction and anhydrite deposition. The nearly 0.5 ‰ variation in the δ44Ca value of Katian seawater falls within the range of scatter in the brachiopod reconstruction of seawater Ca isotopes. Here, we propose that the true ocean record of secular change lies towards the bottom of the range of scatter, and that the higher values in the record reflect local Ca cycling and circulation restriction in epeiric seas. Epeiric seas cultivate the fluid-buffered diagenetic conditions required to promote Ca isotope exchange between reactive carbonate sediments and seawater through carbonate mineral dissolution/recrystallization, crystal coarsening by way of Ostwald ripening, and replacive dolomitization. It is not the apportioning of kinetically fractionated aragonite and calcite in the carbonate deposition flux that drove secular changes in global value of Δsed between ‘calcite’ and ‘aragonite’ seas, but rather the replacement of kinetic fractionations with equilibrium fractionations during fluid-buffered diagenesis with seawater.

Copper and zinc isotopic compositions of methane-derived carbonates: Implications for paleo-methane seepage and paleoenvironmental proxies

Copper and zinc isotopic compositions of methane-derived carbonates: Implications for paleo-methane seepage and paleoenvironmental proxies

Black shale LA-ICP-MS Rb-Sr and monazite SIMS U-Pb geochronology from the Cryogenian successions in the northern Yangtze Block

The late Cryogenian (possibly Marinoan-aged) sedimentary successions from the Shennongjia area in the northern margin of the Yangtze Block host important biological evidence for multicellular eukaryotes that have survived the Neoproterozoic glaciation. However, absolute constraints on the depositional age of these fossiliferous successions are lacking. Herein, we present direct dating of black shale and diagenetic monazite, which yield an Rb-Sr isochron age of 630 ± 27 Ma and a lower intercept U-Pb age of 626.4 ± 56.8 Ma from these fossiliferous successions. These dates are consistent with deposition during the Marinoan glaciation. A Marinoan-aged deposition is also demonstrated by the initial 87Sr/86Sr ratio (0.7084) of the ca. 630 Ma black shale sample, which is consistent with the initial 87Sr/86Sr ratio of contemporaneous seawater. In addition, petrographic observations coupled with REE distribution patterns and Th/Ce-Th and LREE-Y2O3 discrimination plots, facilitate identification of detrital and hydrothermal monazite. The former yields U-Pb dates that are older than the youngest detrital zircon peak at ca. 824 Ma and may have been recycled from igneous precursors. The later, in contrast, yields dates of 585.0 ± 95.1 Ma and 197.3 ± 31.2 Ma and likely records post-depositional hydrothermal events. Further evidence of an old hydrothermal event is recorded in the black shales, which yield Rb-Sr isochron dates of 554 ± 27 Ma and 545 ± 17 Ma that overlap the monazite date. Hydrothermal fluid circulation is associated with precipitation of minerals such as pyrite, sphalerite, and calcite, and potentially has genetic links with the large-scale Bingdongshan Pb-Zn ore deposit around the Shennongjia area, which has an Rb-Sr date of 571 ± 86 Ma.

REE and trace element mobility during the transformation of basalt to laterite and bauxite, Payas Province, Türkiye

The bauxites of the Payas area in the eastern Mediterranean region of Turkey occur as a stratigraphically continuous layer between Early and Late Cretaceous shallow marine carbonates. The laterites represent an in situ formation and mark a key stratigraphic layer in the region. Bauxite pockets are also present in the laterites, formed from the reworking of the laterites and accumulated in depressions in the karst terrain. Therefore, bauxite occurrences are found locally in this stratigraphic layer. The parental rock was a Ti-rich basalt that is compositionally similar to Hawaiian basalts, as indicated by petrographic features and distinctive trace element composition (Zr/TiO2 and Nb/Y). The behavior of elements during the lateritization and bauxitization processes was interpreted using the composition of the Hawaiian basalts with the iron laterite and Ti-rich bauxites of the Payas Region. During lateritization, Fe, Al, Ti, Cr, Nb, and Ta, were largely immobilie while REE and other trace elements, except for Rb, Ni, Co, and Pb were significantly removed. Approximately 75 % of the REE were removed at the end of the bauxitization and/or transportation of the lateritic soil into the karstic depressions. REE were not equally mobile with La to Ce and Lu to Yb having a relatively lower mobility than highly mobile middle REE. During the transformation of laterite to bauxite, low mobility elements such as Ti, Nb, Th, Cr, Hf, and Sn were enriched in the bauxite phase. In comparison to other elements, Rb, As, Pb, Mo, and Ni were strongly removed from the lateritic material during the bauxitization processes. The transfer of MREE during lateritization and bauxitization would have resulted in the enrichment of both light and heavy REE (concave pattern) in contemporaneous seawater. In other words, silicate weathering periods on land should be marked by a concave REE pattern with positive Eu anomaly in marine sediments throughout the geological period.

Magnesium isotopic compositions and origin of Neogene dolomites in Xisha Islands, South China Sea

A common consideration of dolomite-based records for paleoseawater geochemistry reconstruction is the process of dolomite precipitation, which is closely related to the source of relevant elements. “Island dolomite” has been proposed as having great potential for recording paleoseawater geochemical characteristics, however, dolomitization should be strictly constrained in advance. As a major element in dolomites, magnesium (Mg) and its isotopes are critically involved in dolomitization, which may serve as a tool for providing direct insights into this process. Here, we report the Mg isotopic compositions of reef dolomites from the well Xike−1 reef core in Xisha Islands, South China Sea. The geochemical and mineralogical characteristics suggest that these reef dolomites experienced limited diagenetic alteration, and retained primary geochemical characteristics after dolomitization was complete. These reef dolomites reached chemical equilibrium with contemporaneous seawater during dolomitization, which were buffered by coeval seawater. Our results suggest that the development of carbonate platform and the formation of reef dolomites in Xisha Islands were related to tectonic subsidence. These reef dolomites were deposited during periods of decreasing tectonic subsidence rate. Then, dolomitization model for these reef dolomites was established, which provides an effective basis for reconstructing the chemical characteristics of paleoseawater. The occurrence of near-invariant seawater δ26Mg value during the past ∼23 Myr was subsequently verified via cross-checks provided by different seawater δ26Mg archives in this and previous studies. Our results highlight the great potential of seawater-buffered dolomites with limited postdepositional diagenetic alteration for constructing paleoseawater geochemistry though the geological history.

Processes controlling rare earth element distribution in sedimentary apatite: Insights from spectroscopy, in situ geochemistry and O and Sr isotope composition

ABSTRACTIn phosphorites, the content and distribution of rare earth elements are linked to the environment of phosphogenesis. This paper focuses on the question of sources and processes controlling the rare earth element content of apatite from Belgian phosphorites formed during three major phosphogenic events in the Lower Palaeozoic, Late Cretaceous and Cenozoic. To constrain sources and processes, new data include petrological, mineralogical (including cathodoluminescence and Raman spectroscopy) and in situ trace element and Sr and O isotope analyses of apatite. Fluorapatite from Lower Palaeozoic P‐rich conglomerates has the greatest rare earth element enrichment. It is affected by metamorphism that led to deformation of apatite nodules and formation of garnet porphyroblast inclusions. The role of Fe‐oxyhydroxides in element scavenging is highlighted by some apatite nodules that maintain their primary middle rare earth element enrichment, while others are characterized by altered rare earth element patterns resulting from competition for these elements between co‐crystallizing minerals during deformation. A systematic shift towards lower δ18O and radiogenic Sr isotopic composition compared to contemporaneous seawater indicate interaction with 18O‐depleted meteoric fluids and a crustal component. By contrast, carbonate‐rich fluorapatite from the Late Cretaceous phosphatic chalk mostly keeps its primary trace element and isotopic signatures (close to seawater), although an external rare earth element addition is noted, as well as rare earth element redistribution induced by diagenetic alteration. Cenozoic carbonate fluorapatite nodules mostly present flat rare earth element patterns that are indicative of a detrital influence. Slight changes in rare earth element distribution are assigned to post‐depositional alteration, which also led to an increase in radiogenic Sr, with unchanged δ18O compared to seawater. The methodology followed here efficiently helps in deciphering the processes that modified the chemistry of apatite in the frame of major phosphogenic events.

The covariation between Mg[sbnd]C isotopes and trace elements in dolostones: Implications for reconstructing the seawater chemistry

Magnesium isotopes of marine dolomites have the potential to reconstruct the Mg isotopic signals of coeval seawater. However, a massive dolostone sequence can form via the coupling effects of multiple dolomitization events, which can potentially reset the Mg isotopic compositions of dolomites during each migration of the dolomitization fluid. How to distinguish changes of seawater chemistry and early diagenetic effects which have been recorded in dolomites is crucial for the reconstruction of seawater Mg isotopic signals. Here we studied the Mg-Sr-C-O isotope systematics and trace elements of a dolostone sequence from the Qiangtang Basin in the hinterland of Tibet. Multiple lines of evidence consistently indicate the lack of deep-burial diagenesis and high-temperature alteration in the carbonates. The stratigraphic variations of isotopic and elemental compositions of the dolostone profile are generally characterized by the co-variation between MgC isotopes and trace element contents. Our modeling results indicated that a single downward dolomitization can cause the large Mg isotopic variability of dolomites, ca. 1 ‰, within a dolomitization front. However, when the limestones in the profile were completely dolomitized, the coupling effects of multiple downward dolomitizations will homogenize the δ26Mg values of dolostones. Therefore, we attribute the observed co-variation in isotopic and elemental signals in the dolostone sequence to the changes in the chemistry of evolved seawater within the platform. The reconstructed seawater δ26Mg values from the Carnian to Early Jurassic are lower than previous model-based estimations. This result implies an overestimation of the production of shallow-water carbonate in literature during Late Triassic to Early Jurassic. Our study highlights the importance of utilizing multiple geochemical proxies to identify the controlling factors driving Mg isotope variability in dolomites.

Heavy Li isotopic compositions of the terminal Ediacaran dolostones in the Tarim Basin and their possible link to dolomitizations

Dolostones are promising archives for recording paleo-seawater Li isotope compositions, and are especially important for the Cambrian and Precambrian periods when limestones are scarce. However, the effect of dolomitization on carbonate Li isotopes is unclear, limiting their application in tracking (Pre-)Cambrian seawater Li isotopes. Research on Li isotopes in dolostone sections may resolve this issue. We present the Li isotope compositions of dolostones from the terminal Ediacaran Chigebrak Formation (NW Tarim Basin). The dolostone Li isotopes are ∼17 ± 5 ‰ heavier than contemporaneous seawater, indicating significant Li isotope fractionations during dolomite formation. Given that the formation of clay minerals may enrich the porewater with heavier Li isotopes and promote the dolomite formation, we argue that the clay-catalyzed dolomite formation would probably result in heavier Li isotope compositions of dolomites. Moreover, assuming a fixed fractionation factor (∼15 ‰) for the clay-catalyzed dolomitizations, we obtain the seawater δ7Li variations in the terminal Ediacaran, which show similar declining trends with the carbonate 87Sr/86Sr record. Using a simple ocean Li-Sr box model, we propose a reduced weathering scenario during the terminal Ediacaran (∼549 Ma) to explain the seawater Li-Sr isotope covariations. Our study suggests considerable fractionations of Li isotopes (∼17 ± 5 ‰) during clay-catalyzed dolomite formation and a reduced-weathering scenario during the terminal Ediacaran. This large Li isotope fractionation factor indicates the Li isotope fractionation factors during dolomitizations may vary widely and should be considered based on the type of dolomitization. More comparative studies on Li isotopes among coeval carbonates are expected to further explore the potential of dolostones in recording paleo-seawater Li isotopes.

Trace element evidence for diverse origins of superheavy pyrite in Neoproterozoic sedimentary strata

Sedimentary pyrite has long been used as an archive of marine environments in Earth history. To capture reliable paleoenvironmental signals, however, we need to first evaluate pyrite in sedimentary strata as it can be altered and masked by later diagenetic and/or hydrothermal processes. Here, we trained two supervised machine learning algorithms on a large LA-ICP-MS pyrite trace element database to distinguish pyrite of different origins. The analysis validates that two models built on the co-behavior of 12 trace elements (Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Au, Tl, and Pb) can be used to accurately predict pyrite origins. Further statistical analysis suggests four trace element clusters behaving differently among sedimentary (syngenetic and early diagenetic), synsedimentary hydrothermal (syngenetic hydrothermal), and post-sedimentary hydrothermal (epigenetic hydrothermal) pyrite, which is probably driven by chemical and physical properties of source fluids, interactions between elements, competition among coprecipitating minerals, and pyrite growth rate. Armed with this initial success and aided by new LA-ICP-MS trace element data from 9 samples, we then demonstrated the efficacy of this approach in identifying the origins of pyrite from two Neoproterozoic sedimentary successions in South China. The first set of samples contain isotopically superheavy pyrite (i.e., whose bulk-sample δ34S values greater than those of contemporaneous seawater sulfate and whose origins remain controversial) from the Cryogenian Tiesi’ao and Datangpo formations. The second set of samples contain pyritic rims (associated with fossiliferous chert nodules and thought to be critical in exceptional fossil preservation) from the Ediacaran Doushantuo Formation. For the superheavy pyrite, the models consistently show high confidence levels (mostly > 80 % probability) in identifying its genesis type, and three out of four samples were given sedimentary origins. For the pyritic nodule rims, the models suggest that early diagenetic pyrite was subsequently altered by hydrothermal fluids and therefore shows mixed signals. The study highlights the importance of pyrite trace elements in deciphering and distinguishing the origins of pyrite in sedimentary strata.

Uranium isotopes in non-euxinic shale and carbonate reveal dynamic Katian marine redox conditions accompanying a decrease in biodiversity prior to the Late Ordovician Mass Extinction

The Late Ordovician mass extinction (LOME) is the first major mass extinction event in the Phanerozoic. However, global biodiversity started to decline during the Katian (prior to the LOME) and coeval global ocean redox conditions are not well understood. The deposition of Katian organic-rich sedimentary rocks, namely the calcareous Collingwood Member and overlying siliciclastic Rouge River Member, was triggered by the concurrent Taconic Orogeny in southern Ontario (Canada) near the interface between the epicratonic Appalachian and Michigan basins. In this study, we used geochemical elemental proxies and uranium isotope compositions (δ238U) to reconstruct local and global ocean redox states during deposition of both units at seven drill core localities throughout southern Ontario. Local redox conditions are revealed by several proxies (redox sensitive trace metals, Fe speciation, and Corg:P ratios). Collectively, these proxies suggest spatiotemporal redox variations for the Collingwood Member: O2-deficient conditions (non-euxinic) for the deep waters of the Appalachian and Michigan basins and mostly oxic-suboxic conditions for the shallow waters. In contrast, predominantly oxic-suboxic conditions are suggested for the Rouge River Member.Global ocean redox conditions are inferred from the δ238U of both units. The coeval seawater δ238U value during Collingwood Member deposition is estimated to be from ∼−0.87‰ to ∼−0.64‰, based on both carbonate δ238U (samples leached with 1 N HCl) and δ238Ubulk-non-detrital data (corrected from the bulk δ238U). Particularly, the δ238Ubulk-non-detrital data of the Collingwood Member from multiple cores (both shallow and deep waters) are positively correlated with elemental redox proxies, suggesting a local redox control on δ238U offsets between sediments and water columns. In contrast, the δ238Ubulk-non-detrital of the suboxic Rouge River Member (average = −0.32 ± 0.12‰) from several cores do not correlate with local redox proxies. The coeval seawater δ238U during Rouge River Member deposition is estimated between −0.62 ± 0.12‰ and −0.42 ± 0.12‰ assuming a δ238U offset of 0.1–0.3‰ as observed between modern suboxic sediments and seawater. For this range of seawater δ238U, a three-sink U isotope mass balance model suggests a contraction of global euxinic seafloor area from deposition of the Collingwood Member (0.5–31.6%; median = 1.0–14.2%) to the Rouge River Member (0.2–2.0%; median = 0.3–1.0%), which could have been associated with the concurrent Taconic Orogeny. Collectively, global marine redox proxy data (i.e., δ98Mo, δ238U) from this and previous studies imply dynamic Katian ocean redox conditions during the decrease of metazoan biodiversity prior to the LOME. As a similar extent of global ocean euxinia during the LOME2 also occurred episodically during the Katian but did not result in LOME2-like mass extinctions, it is suggested that other factors (e.g., climate), besides expanded ocean euxinia, could have contributed to the second phase of the LOME.

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

AbstractMolybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction.

Not just range limits: Warming rate and thermal sensitivity shape climate change vulnerability in a species range center.

Climate change manifests unevenly across space and time and produces complex patterns of stress for ecological systems. Species can also show substantial among-population variability in response to environmental change across their geographic range due to evolutionary processes. Explanatory factors or their proxies, such as temperature and latitude, help parse these sources of environmental and intraspecific variability; however, overemphasizing latitudinal trends can obscure the role of local environmental conditions in shaping population vulnerability to climate change. Focusing on the geographic center of a species range to disentangle latitude, we test the hypothesis that populations from warmer regions of a species range are more vulnerable to ocean warming. We conducted a mesocosm experiment and field reciprocal transplant with four populations of a marine snail, Nucella lamellosa, from two regions in British Columbia, Canada, that differ in thermal characteristics: the Central Coast, a cool region, and the Strait of Georgia, one of the warmest regions of this species' range and one that is warming faster than the Central Coast. Populations from the Strait of Georgia experienced growth reductions at contemporary summertime seawater temperatures in the laboratory and showed stark reductions in survival and growth under future seawater conditions and when outplanted at their native transplant sites. This indicates a high vulnerability to ocean warming, especially given the faster rate of ocean warming in this region. In contrast, populations from the cooler Central Coast demonstrated high performance at contemporary seawater temperatures and high growth and survival in projected future seawater temperatures and at their native outplant sites. Given their position within the geographic center of N. lamellosa's range, extirpation events in the vulnerable Strait of Georgia populations could compromise connectivity within the metapopulation and lead to gaps across this species' range. Overall, our study supports predictions that populations from warm regions of species ranges are more vulnerable to environmental warming, suggests that the Strait of Georgia and other inland or coastal seas could be focal points for climate change effects and ecological transformation, and emphasizes the importance of analyzing climate change vulnerability in the context of regional environmental data and throughout a species' range.