Paleoenvironmental reconstructions based on the clumped isotopes (Δ47) and traditional isotope (δ13C or δ18O) techniques are often problematic for carbonates that have undergone diagenetic ...alteration. One of the most common types of diagenesis is the transition between polymorphs, such as the replacement of aragonite by calcite. The isotope fractionation during such transitions in aqueous solutions remains unclear. We conducted a series of aragonite-to-calcite transition experiments in aqueous solutions of varying salinity and experiment durations at 25 °C and 90 °C to examine the variations of δ13C, δ18O, and Δ47 values for carbonates at varying degrees of the transition. The results confirm the retarding effect of Mg2+ and the catalytic effect of Na+ and Ca2+ on the transition, which are consistent with previous findings. Compared with the results from the transitions at 25 °C, the 90 °C experiments show a greater transition to calcite and a more depleted oxygen isotope composition. However, both clumped and carbon isotope values show no significant variation in the various experiments, suggesting that they are unaffected by mineralogical transition. Based on published equilibrium calibrations for δ18O and Δ47, the results demonstrate that kinetic effects in isotope systems are controlled primarily by the rate of polymorph transition, but there is a significant kinetic difference between the clumped isotope bond (13C18O) reordering and the 18O exchange within the dissolved inorganic carbon pool. This kinetic difference results in partial equilibration in δ18O with no significant reordering in Δ47 toward their equilibriums, which can be accounted for by the difference of equilibration rates between the oxygen isotopes bound to 12C and those bound to 13C. This study provides a clear observation of the response of isotope systems to carbonate polymorph transitions and sheds light on their reliability as paleoenvironmental proxies.
Carbon and oxygen isotopic exchange between external and internal fluids of carbonate minerals can reset the clumped isotope composition (Δ47) of paleotemperature archives. Understanding the nature ...of this exchange during carbonate diagenetic alterations is essential for correctly interpreting the paleoenvironmental significance of Δ47-derived temperatures. Through batch hydrothermal experiments on aragonitic coral carbonates, we examine the kinetic signatures of clumped isotope fractionation from the initial disequilibrium to final equilibrium states during the dolomitization of the aragonite. By analyzing oxygen isotope fractionation and Sr partitioning between carbonate and fluid, we simulate the effect of exchange reactions on the Δ47 values. As the resetting feature of Δ47 values in our aragonite samples disagrees with the fractionation pattern observed in previous aragonite solid-state reordering experiment, we rule out the effect of the solid-state reordering. Our findings demonstrate that isotope exchanges with internal or external fluids coexist, particularly during the early stage of aragonite self-recrystallization. The results of this study indicate that the Δ47 value of coral aragonite is susceptible to internal exchange reactions in Mg-bearing solutions at 160–220 °C. Additionally, by modeling the bulk recrystallization and internal fluid-mineral exchange and evaluating their relative importance, we determine that the internal exchange reaction results in a nonlinear correlation between δ18O and Δ47 values. Importantly, we highlight the previously unrecognized diagenetic scenario in which the internal fluid-mineral exchange effect on the Δ47 value can outweigh the bulk recrystallization effect with external fluid coexisting. Consequently, our findings emphasize the need for a comprehensive assessment on the reaction kinetics for different types of recrystallization effects, to support the application of clumped isotope proxies for altered carbonate samples in diagenetic environments.
Reliable temperature reconstruction by carbonate clumped isotope (Δ47) thermometry requires isotopic equilibrium during carbonate growth. However, many carbonate minerals grow at high rates and ...exhibit disequilibrium isotopic states. Carbonate Δ47 disequilibrium arising from kinetic isotope effects (KIEs) specific to carbonate growth still remains unclear, and requires experimental constraints. Here we present a series of rapid witherite (BaCO3) precipitation experiments intended to constrain bulk carbon, oxygen and clumped isotopic fractionation during unidirectional precipitation of BaCO3 from dissolved inorganic carbon solutions under various pH and temperature conditions. We found that rapid BaCO3 growth can lead to lower δ13C, δ18O, and Δ47 values in HCO3−‐dominated solutions, especially at low temperatures. Our experiments provide constraints on kinetic fractionation factors (KFFs) associated with the rapid carbonate growth and their temperature dependence. KFFs for δ13C and δ18O are broadly consistent with previous experimental estimates, although the Δ47 KFF for BaCO3 precipitation from HCO3−‐dominated solution is not consistent with an earlier theoretical estimate, necessitating a re‐evaluation of the current model of the KIE associated with carbonate growth. Our results clearly verify a preference for the CO32− pathway during carbonate precipitation, with important implications for new isotope fractionation models for natural carbonates. The fractionation relationship between Δ47 and δ18O found in this study allows more precise identification of KIEs associated with rapid carbonate growth.
Key Points
Unidirectional precipitation of BaCO3 in HCO3−‐dominated solutions leads to more negative carbon, oxygen, and clumped isotope values
Kinetic fractionation factors of oxygen and clumped isotopes between HCO3− and BaCO3 are correlated and temperature dependent
Isotopic analyses reveal a significant preferential incorporation for CO32− over HCO3− during rapid carbonate growth
It is crucial to understand the clumped isotope compositions (Δ47) of modern land-snail shells and their relationship with environmental parameters for palaeoenvironmental-studies. Previous studies ...have shown significant variations in relationships between snail-shell Δ47 and ambient air temperature, and the reliability of shell Δ47 in indicating air temperature remains uncertain. This study examined the shell Δ47 from China with mean annual temperatures ranging from 5 to 23 °C and the snail body fluid δ18O estimated from shell δ18O and Δ47-derived temperatures. For all snails studied in this study, site-averaged Δ47 values yield snail calcification temperatures ranging from 25 to 36 °C, which are higher-than-expected in either environmental temperatures or estimated snail activity temperatures. We suggest that snails up-regulate their body temperatures towards their preferred living conditions likely through behaviour or/and physiology adaptation, especially at low temperature environments, which is likely significant than observed previously. Δ47 disequilibrium associated with CO2 degassing during snail calcification is potential but it could possibly be corrected by using an empirical calibration based on natural land-snails. Varying degrees of δ18O enrichment (>8‰) relative to rainwater are observed in snail body fluids, possibly due to variable evaporation during shell calcification. Coupled with model calculations, the relationship between snail body fluid δ18O and rainwater δ18O could be improved by using Δ47-based temperature and δ18O thermometry, if snails prefer to be active at ~90% relative humidity. This study highlights the importance of land-snail shell Δ47 coupled with shell δ18O to indicate temperature and rainwater δ18O at micro-environments scales.
•Land-snail shell Δ47 is a snap shot reflecting micro-environmental temperature.•Snails possibly up-regulate body temperatures in colder environments.•Snail body fluids δ18O can better indicate rainwater δ18O at high relative humidity.
It is debated whether there was strong climate seasonality during the Eocene, which provides a close geological analogy for near‐future scenarios of greenhouse gas emissions. Lithological data ...suggest the existence of a broad arid zone centered around 30°N paleo‐latitude, while a humid climate was supported by palaeobotanic assemblages in East Asia. Here, we report the occurrence of massive primary lacustrine dolomite and magnesite in the central East Asia during the middle Eocene. We provide a novel perspective from magnesium isotopes to link the formation of Mg‐carbonates to seasonal dry‐wet cycles. Rapid magnesium input during the rainy season and intense evaporation in the dry season likely caused the formation of magnesium carbonates in an enclosed lake. These findings provide insights into hydroclimatic seasonality during the Eocene, contributing to our understanding of the hydrological cycle response to a greenhouse climate.
Plain Language Summary
The Eocene epoch serves as a valuable analog for future climates. While geochemical reconstructions and model simulations have illuminated lower thermal latitudinal gradients and seasonal variations, our understanding of Eocene precipitation patterns lags, encompassing wet‐dry conditions and seasonal dynamics. To enhance our understanding of Eocene precipitation patterns, we investigated a 158‐m‐thick primary dolomite and magnesite deposition in the middle Eocene lacustrine succession of the Lushi Basin, central China. From a novel perspective, we provide evidence from magnesium isotopes to link the formation of Mg‐carbonates to climate seasonality. Clumped isotopes (∆47) and Mg isotopes provide evidence supporting the formation under specific hydroclimatic conditions. A surge in magnesium input during the rainy season, succeeded by intense evaporation in the dry season, likely led to the development of extensive Mg carbonate layers in an enclosed lake. The prevalence of seasonal variations in precipitation in the central East Asia during the middle Eocene is further substantiated by a compilation of the occurrence of Eocene lacustrine Mg‐carbonates in this region. Our findings suggest that while Eocene temperature seasonal variability was weak, significant precipitation seasonality could have coexisted.
Key Points
Magnesium isotopes of Eocene lacustrine dolomites and magnesites provide insight into the presence of seasonal precipitation variation
Mg‐carbonate formation was linked to hydroclimatic seasonality characterized by alternation between heavy rainfall and strong evaporation
Weak temperature seasonal variation and significant precipitation seasonality could have coexisted in central East Asia during the Eocene
The fate of organic matter in deep-buried marine sediments determines the formation of hydrocarbon reservoirs, shapes the deep biosphere, and affects the global carbon cycle. Several geochemical ...processes such as silicate weathering, iron and manganese (hydr)oxide reduction, and sulfate reduction influence the remineralization of organic matter in buried sediments. However, little information on deep-seated organoclastic sulfate reduction is available and the involved geochemical processes are largely unknown. Here, authigenic carbonates recovered from Ship Shoal Block 296 (SS296) of the Gulf of Mexico, all dominated by low-magnesium calcite, were investigated to constrain organoclastic sulfate reduction in deep-buried sediments. The SS296 carbonates had been previously suggested to be of a deep origin and transported upward by salt diapirism. The presence of cone-in-cone textures in samples 3110R1 and 3110R2 as well as vitrinite reflectance values of 0.48% to 0.58% indicate carbonate formation at a relatively deep burial. Likewise, clumped isotope thermometry yielded formation temperatures from 45 °C to 53 °C for sample 3110R1, 60 °C for sample 3110R2, and 11 °C to 37 °C for sample 3110R3. In spite of formation at greater depth, the temperatures of samples 3110R1 and 3110R2 are still within the temperature range of microbial sulfate reduction, reflecting formation depths between 1 km and 2.7 km. The presence of framboidal pyrite and its relatively low δ34S values (−27.8‰ to +11.5‰) agree with the occurrence of microbial sulfate reduction during carbonate precipitation. The carbon isotopic composition of the carbonates (δ13C: −16.6‰ to −8.9‰) and enclosed organic matter (δ13Corg: −26.8‰ to −20.5‰) suggests that organic matter is the dominant carbon source of carbonate minerals. This study confirms that microbial sulfate reduction occurs in deep-buried sediments, driving the degradation of organic matter. Our findings suggest that organoclastic sulfate reduction might be common in deep-seated sedimentary environments, controlling the fate of organic matter in deep subsurface environments and affecting the global carbon cycle.
•Carbonates sampled at site SS296 in the Gulf of Mexico formed at about 60 °C.•Variable δ34S values of pyrite enclosed in carbonates reflect the presence of sulfate reduction.•The main carbon source of carbonates was sedimentary organic matter rather than hydrocarbons.•Carbonates reflect microbial-mediated organoclastic sulfate reduction in deep-buried sediments.
The carbonate clumped isotope (Δ47) thermometer has been widely used for paleoclimatic and paleo‐elevation reconstructions in sedimentary basins. Its reliable use requires that carbonates are free ...from later diagenetic alteration and retain their depositional Δ47 compositions, which is not always the case during the burial history of a basin. Therefore, identifying the alteration mechanisms that occurred during burial is essential for the correct interpretation of Δ47 signatures. In this study, we used the Δ47 thermometer to investigate possible carbonate alteration during burial of the Fenghuoshan Group (FHSG) in the Hoh Xil Basin, northern Tibetan Plateau, China. The Δ47 temperatures T(Δ47) of 30 carbonate samples through the ∼4.5 km thickness of the FHSG range from 51 to 132°C and exceed the surface temperature, implying the effects of thermal alteration on FHSG carbonates. The T(Δ47) values combined with the water‐rock exchange model, indicating that the alteration occurred under closed burial conditions at low water:rock ratios. The solid‐state reordering model fails to predict the Δ47 values, even with a modified time‐temperature history. In contrast, most measured Δ47 values are in agreement with predictions from recrystallization models. Therefore, the proposed alteration scenario is that FHSG carbonates underwent progressive microscale recrystallization during deep burial and subsequent exhumation, during which process recrystallized carbonate equilibrated with ambient burial temperature and therefore modified Δ47 values. This study highlights the potential of Δ47 thermometry and related models for evaluating the burial history and diagenesis of carbonates in sedimentary basins.
Plain Language Summary
Carbonate clumped isotope composition (defined as Δ47) refers to the abundance of 13C–18O bonds in the carbonate, which is solely temperature dependent. This temperature dependence makes Δ47 a useful tool that can be used to obtain the carbonate formation temperature. However, the original Δ47 composition could be altered by either solid‐state reordering of C–O bonds or carbonate recrystallization during the burial history of sedimentary basin. Discriminating which alteration occurred during burial process is essential for the correct interpretation of Δ47 signature and the burial history. In this study, we measured Δ47 values of Fenghuoshan Group (FHSG) carbonates in the Hoh Xil Basin, northern Tibetan Plateau, China, and investigated their alteration during the burial history of FHSG by using the solid‐state reordering and recrystallization models. Our results indicate that FHSG carbonates underwent progressive microscale recrystallization during deep burial history, and the recrystallized carbonates recorded modified higher Δ47 temperatures. This study highlights the potential of Δ47 thermometry and related models for evaluating the burial history of sedimentary basins.
Key Points
Clumped isotope temperatures of 51–132°C reflect thermal alteration during the burial history of the Fenghuoshan Group (FHSG)
The alteration of FHSG carbonate Δ47 occurred in a late‐burial environment at a low water‐rock ratio
FHSG carbonate underwent microscale recrystallization during burial and exhumation and eventually recorded high T(Δ47)
Climate change in the mid‐to‐late Holocene transition is very important for predicting future climate trends and understanding the relationship between abrupt climate change and the development of ...past human civilization. In this study, Sr/Ca ratios and δ18O records with the annual resolution extracted from four fossil corals that were growing during the Middle Bronze Age Cold Epoch (MBACE) were used to reconstruct sea surface temperature (SST) and seawater δ18O (δ18Osw) in the South China Sea (SCS) during the mid‐to‐late Holocene transition. The results indicate that the SCS experienced a rapid cooling and wetting event during the period of ∼3,500–3,800 years BP (before present year 1950). Specifically, the average SST and δ18Osw declined rapidly by ∼3°C and ∼0.65‰, respectively, over an interval of ∼100 years from ∼3,850 years BP to ∼3,750 years BP. This rapid climate change pattern recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records from adjacent land and ocean areas. Consistent with other records from the North Atlantic, this cold event in the Asia‐Western Pacific region that occurred during the MBACE and was originally identified in the North Atlantic and European regions should have occurred at the global scale, which might be caused by changes in the Asian summer monsoon linked with solar irradiance and/or the North Atlantic climate. In addition, this rapid climate change might support the occurrence and timing of the outburst flood event during the Xia Dynasty and might have led to the fall of the Xia Dynasty.
Plain Language Summary
Rapid and drastic climate changes will have serious impacts on the ecological environment and the development of human society. Studies on such climate events enable to better understand the relationship between climate change and human civilization, and provide valuable materials for predicting future climate trends. There was an extremely cold event during the period of ∼3,800–3,500 years BP (before present year 1950) in the Middle Bronze Age in North Atlantic and Europe region, which had a serious impact on European civilization. We carry out high‐resolution paleoclimate reconstructions during this period by using geochemical tracers in fossil corals from the South China Sea. The results indicate that the South China Sea and even the entire Western Pacific have experienced a rapid cooling and wetting event during the period of ∼3,800–3,500 years BP and the extremely cold event during the Middle Bronze Age was global scale. In addition, the consistency between the coral records and the stalagmite records from the inner land of China suggest that a rapid cooling and drying event during the Middle Bronze Age led to the collapse of the Xia Dynasty and Emperor Yu's success in taming floods was attributed to this event.
Key Points
A rapid cooling event over the western Pacific region during the Middle Bronze Age was identified
The cooling event recorded in coral archives broadly agrees with those in foraminiferal and stalagmite records
The rapid cooling and drying process in inland China might have led to the collapse of the Xia Dynasty
Major and trace elements, Sr and Nd isotopes of bulk particles from the SCS‐NW and SCS‐N‐03 traps were studied to trace the provenance of sinking particles in the northern South China Sea (SCS). ...About 63% of biogenic materials and most of lithogenic materials from the SCS‐NW trap may be contributed from resuspended sediments, and more biogenic materials are collected in winter than in summer. The immobile‐element discrimination diagrams also indicate that the lithogenic materials of particles are, to a large degree, from local seafloor sediments, and that the lithogenic materials of particles from SCS‐N and SCS‐W traps have seasonal variations. Sr isotopes (87Sr/86Sr and δ88Sr) of bulk particles are significantly influenced by biogenic materials and cannot be used in provenance tracing. In contrast, Nd isotopes are ideal tools to trace particle provenance. However, the traditional Nd isotope (143Nd/144Nd) shows no statistical difference on the particles of the SCS‐NW and SCS‐N‐03 traps, making it unable to identify their provenance. Fortunately, the stable Nd isotope, ε146Nd, shows different correlation trends to ε143Nd in these two traps, suggesting that stable Nd isotope can potentially identify the sources of lithogenic materials in the northern SCS. Our results indicate that distribution and transportation of sinking particles are controlled by currents of the SCS mainly driven by the East Asian Monsoon.
Plain Language Summary
As the largest semi‐enclosed marginal sea in the west Pacific, the South China Sea (SCS) is supplied with a large amount of suspended sediments from its surrounding continents and islands. Studies on sedimentary process and source of sediments in the SCS have long been carried out, but no consensus has been reached yet. This study presents elemental and Sr‐Nd isotope geochemistry of sinking particles from the northern SCS. According to fluxes and element concentrations of sinking particles, we find that resuspended sediments are an important contributor to sinking particles in the SCS. The relationship between trace elements of sinking particles and offshore surface sediments indicates that these resuspended sediments may be derived from continental shelves. The distribution and transportation of sinking particles are influenced by hydrological conditions of the SCS mainly driven by the East Asian monsoon. This study also reveals that stable Nd isotope could be a powerful tool in tracing sediment provenances in the northern SCS. Therefore, our results will improve the understanding of sediment transportation and sedimentary processes in the northern SCS.
Key Points
Resuspended sediments are an important contributor of the sinking particles collected by sediment traps, especially deep traps
Provenance and seasonal variation of sinking particles can be identified by immobile‐element discrimination diagrams
The stable Nd isotopic composition coupled with conservative trace elements is a potential tool in provenance tracing
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