Table of Content

03 July 2024, Volume 30 Issue 03

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Reconstructing Humidity Using the Triple Oxygen Isotopes of Pedogenic Carbonates

DA Jiawei

2024, 30(03):  241-252.  DOI: 10.16108/j.issn1006-7493.2023069

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The secondary carbonate formed during the soil formation process (pedogenic carbonate) is a crucial material in
paleoclimate studies. In previous research, it has been widely used to reconstruct atmospheric CO₂ concentrations, paleoaltimetry, the evolution of regional paleovegetation (C₃, C₄ plants), precipitation, and temperature. However, like most other paleoclimate indicators, proxies based on pedogenic carbonate (such as stable carbon and oxygen isotopes, trace elements, etc.) are influenced by multiple climate factors, resulting in ambiguity. Therefore, obtaining reliable paleoenvironmental information requires the integration of multiple indicators. With the recent development of isotope testing methods, in addition to the traditional ¹⁸O/¹⁶O, triple oxygen isotope analysis (¹⁷O/¹⁶O) has been increasingly applied in paleoclimatology. Pedogenic carbonates inherit the oxygen isotope signal of soil water. Combined with the carbonate-clumped isotope temperature, the ¹⁷O anomaly of soil water can be reconstructed. The soil-water-Δ′¹⁷O is mainly controlled by the evaporation process, thus providing information about past humidity. This article provides a detailed introduction to the theoretical basis of pedogenic carbonate triple oxygen isotopes. It quantitatively discusses the impact of various climate factors on pedogenic carbonate triple oxygen isotopes using existing soil water triple oxygen isotope models.


Exploration of New Methods in Marine Strontium Isotope Stratigraphy

CAI Yue

2024, 30(03):  253-268.  DOI: 10.16108/j.issn1006-7493.2024015

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Strontium Isotope Stratigraphy (SIS) is an important tool for establishing age models for oceanic sedimentary deposits. It has wide applications in many fields including oil and gas exploration and paleoclimate reconstructions. SIS is based on the
premise that global seawater is homogeneous relative to Sr isotopes but with variations through time. A key assumption of SIS is that the samples retain the Sr isotope composition of the original seawater where they form. However, diagenesis and loosely attached impurities can alter the Sr isotope composition of the fossil samples, thereby undermining the meaningfulness of the SIS age. Therefore, sample selection and preparation are crucial for obtaining meaningful SIS ages. This article evaluates existing methods for SIS, including sample preparation and Sr isotope analysis. In recent years, to precisely extract the composition of ancient seawater, studies using Nd, Pb, and Li isotopes in carbonates have thoroughly investigated different methods of sample preparation. Based on these technical advances, this paper thoroughly investigated an oyster sample with independent age constraints from the Miocene shallow-marine deposits of the Gulf of Suez, Egypt. By comparing the elemental composition of sequential leachates of the oyster with those of present-day organisms, this paper proposes a new protocol for SIS sample preparation. This paper also compiles and compares the accuracy and precision of Sr isotope data measured using Thermo-ionizing mass spectrometry (TIMS) vs. Multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). While for samples with abundant Sr, the new generation of MC-ICP-MS can reach similar precision and accuracy as the old generation TIMS, for smaller samples and samples that require higher precision, the new generation TIMS is still the best choice for SIS research. Finally, this paper makes some suggestions for Sr analysis on the MC-ICP-MS. 

Oceanic Lithium Cycling and Implications for Carbon Cycle

CAO Cheng

2024, 30(03):  269-287.  DOI: 10.16108/j.issn1006-7493.2024022

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How the carbon cycle influences the atmospheric carbon dioxide level is of a major concern for Earth’s habitability.The oceanic lithium (Li) cycle can be used to trace carbon cycle because both cycles are controlled by chemical weathering,
hydrothermal-seafloor interaction, and marine clay authigenesis which also known as reverse weathering. The variations in
seawater Li isotope compositions posit changes in the sink and source processes such as continental weathering intensity and/or rate as well as reverse weathering rate. This paper reviews the global cycle of oceanic lithium and the mass balance for seawater lithium isotopes, focusing on the present-day fluxes and associated isotope fractionation mechanisms. Challenges still remain to better constrain the budgets as well as isotope fractionation factor especially in hydrothermal alteration and reverse weathering. The review also extends to the use of seawater li isotope records in tracing carbon cycle during climate events and critical time periods in the earth history. Collectively, this review highlights the potential as well as limitations of utilizing seawater Li isotope records to trace global carbon cycle in deep time. 


Changes in Oceanic Ba Cycle Driven by the Neoproterozoic Oxygenation Event

WEI Wei, SUI Peishan, CHEN Tingting, HUANG Fang

2024, 30(03):  288-296.  DOI: 10.16108/j.issn1006-7493.2024009

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The late Neoproterozoic witnessed an increase in the atmospheric and oceanic oxygen levels, namely the Neoproterozoic Oxygenation Event (NOE), likely resulting in the naissance and radiation of metazoans and the establishment of complex ecosystem. Oceanic oxygenation could change oceanic chemistry, such as species and valence states of Fe, C, and S, and the biogeochemical cycle of Ba in the ocean is strongly controlled by the S species and sulfate concentration. This review introduces how the NOE changed the oceanic Ba cycle: (1) Before the NOE, the oceanic sulfate concentration was low and the oceanic Ba cycle was conservative; (2) during the NOE, the oceanic sulfate increase led to excess Ba enrichments in sediments and formation of massive barite deposits; and (3) after the NOE, the ocean kept over-saturated relative to barite until the terminal Paleozoic and the Ba cycle was controlled by biological productivity afterwards. In addition, this review suggests to use Ba isotope system to reconstruct the oceanic Ba concentration, and indirectly to estimate the oceanic sulfate concentration (oxygenation extent) during the late Neoproterozoic.


Equilibrium Isotope Fractionation Effect between Zn-containing Minerals and Aqueous Solution in Contaminated Soils

HE Hongtao, GU Yifan, XING Lecai, WANG Yanfang, YANG Yang, CAI Xingping, HE Yuyang

2024, 30(03):  297-311.  DOI: 10.16108/j.issn1006-7493.2024006

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Zinc (Zn) isotope fractionation, resulting from multiple physicochemical processes under Earth’s surface conditions, confounds the source identification of pollutants. The determination of Zn isotope fractionation factors between typical Zncontaining minerals and aqueous solutions in contaminated soils is crucial to trace Zn sources using isotopic tools. In this study, we used density functional theory based first principles calculations to investigate the most stable geometries of Zn-containing species, including hydrated Zn²⁺ complexes, Zn in hydroxy-interlayered minerals (Zn-HIM), Zn-rich phyllosilicates (Talc), Zn-layered double hydroxide (Zn-LDH), and surface adsorbed Zn²⁺. Based on these optimized configurations, we calculated the equilibrium isotope fractionation factors between the aforementioned structures and Zn2+ in aqueous phases. Our results showed that adsorbed Ⅵ Zn²⁺ is slightly enriched in 64Zn (Δ66/64Zn=-0.29‰~-0.20‰ ), while adsorbed Ⅳ Zn²⁺ is enriched in ⁶⁶Zn (Δ^66/64Zn =0.45 ‰~0.73‰ ). Secondary mineral phases are evidently enriched in ⁶⁶Zn (Δ^66/64Zn=0.51‰~1.11‰ ), if Zn transferred to stable crystalline precipitates. Using obtained fractionation factors, we successfully simulated the evolution trends of Zn isotope composition under the influence of single pollution sources (electroplating waste, sphalerite ore, emissions and metallurgical sludge) in equilibrium and Rayleigh fractionation modes. By comparing with available isotope data, these trends facilitate to find out the main source of Zn in contaminated soils.

Zinc Isotopes in Environmental Geochemistry: A Review

CHENG Wenhan, WU Meng, ZHAO Yanli, ZHAO Junzhe

2024, 30(03):  312-321.  DOI: 10.16108/j.issn1006-7493.2024036

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Zinc is one of the essential trace elements for life activities, but in excess, it can cause environmental pollution, ecological toxicity, and harm to human health. Therefore, the study of the environmental geochemical behavior of zinc is the basis for the scientific and rational use of zinc, and it is also a research hotspot in related fields. Zinc has five stable isotopes, and the
isotopic composition of zinc varies from different sources. The determination of zinc isotope composition in different  environmental samples provides a new means for studying the environmental geochemistry of zinc. In recent years, with the development of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS), the study of zinc isotope environmental geochemistry has covered chemical weathering, pollution tracing, paleoclimate reconstruction, and biological processes, and new research fields are constantly expanding. The article provides a detailed overview of zinc isotope analysis methods and their current research status in environmental geochemistry, and looks forward to the future development of zinc isotope environmental geochemistry, in order to promote further development in this field.


Rate, Mechanism, and Geological and Geochemical Effects of Fungi Oromoting Silicate Mineral Weathering

LI Zibo, LU Xianca, TENG HuiHenry, LIU Lianwen, QIE Wenkun, PANG Ke, ZHANG Wenxuan, JI Junfeng, CHEN Jun

2024, 30(03):  322-335.  DOI: 10.16108/j.issn1006-7493.2024011

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Fungi are widespread and can be found from the Earth’s surface to depths of up to 1.4 km in the continental crust. Based on their ecological habits and nutritional modes, fungi can be categorized as saprotrophic, symbiotic, or parasitic. Hyphae are the basic structural units of fungi. Through their tip-elongated growth and robust metabolic capabilities, fungal hyphae play a unique role in the weathering of silicate minerals, especially those containing nutrient elements. This process regulates essential geological and geochemical processes such as soil formation, mineralization, and the biogeochemical cycling of elements. However, the role of fungi in natural silicate weathering has been relatively overlooked. Our review starts by examining fungal growth patterns, aiming to elucidate their impact on the rate and mechanisms of silicate mineral weathering, as well as their contribution to natural silicate weathering. Through a literature review and in the context of global change, we propose key areas of focus for future research: (1) further quantifying the contribution of fungi to silicate mineral weathering in natural environments, (2) clarifying the coupling of fungal-promoted silicate mineral weathering with geological and geochemical processes, and (3) leveraging functional fungi to improve the efficiency of terrestrial enhanced silicate weathering for carbon removal. These investigations will deepen our understanding of the role of fungi in key surface processes, provide important information for Earth system models (GEOCARB, COPSE, and SCION), enhance the accuracy of predictions regarding the interactions of different spheres in Earth systems, and offer new methods and scientific evidence for the effective carbon sequestration through enhanced silicate weathering.


Mountain Building and Silicate Weathering: A Review and Perspectives

LI Shilei, CHEN Yang, CHEN Jun

2024, 30(03):  336-344.  DOI: 10.16108/j.issn1006-7493.2024024

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The relationship between mountain uplift and climate change has been a prominent focus of research in recent decades. Since the emergence of the famous “uplift-weathering “ hypothesis in the 1980s, which suggests that tectonic uplift of
mountains drives climate change over millions of years through silicate weathering, significant attention has been directed towards this concept. Extensive continental denudation and weathering records have been established to test this hypothesis. Despite the majority of these records aligning with the hypothesis, the presence of alternative interpretations complicates direct hypothesis testing. Consequently, numerous studies have explored contemporary weathering processes to better understand this relationship. However, these studies have unveiled that weathering in mountainous regions is not primarily controlled by physical denudation processes and is unlikely to fluctuate in response to tectonic activities. This challenges the fundamental premise of the “upliftweathering” hypothesis. This paper conducts a comprehensive review and analysis to elucidate the reasons for this contradiction. Moreover, it examines the potential of non-local weathering in floodplains as a novel weathering mechanism to resolve this inconsistency. Additionally, it delves into the opportunities and challenges within the realm of non-local weathering research. 


Niobium Mineralization in the Miaoya Alkaline Complex, Hubei Province: Constraints from Rutile Mineralogy and Geochronology

YING Yuancan, CHEN Wei, LIU Jiajun, YANG Fan, JIANG Shaoyong

2024, 30(03):  345-361.  DOI: 10.16108/j.issn1006-7493.2024019

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The Miaoya alkaline complex is mainly composed of syenite and carbonatite enriched in Nb and REE, with a verified
Nb₂O₅ reserve of 0.93 Mt @ 0.12%. It is the second largest carbonatite-hosted Nb-REE deposit in China, second only to Bayan Obo. Previous studies have focused on the genesis of carbonatite and the process of REE enrichment, but the occurrence and enrichment mechanism of Nb in syenite are still unclear. Based on field geological surveys, detailed petrography, mineral chemistry, and U-Pb dating of the Nb-bearing minerals in syenite were obtained using TIMA, EMPA and LA-ICP-MS analyses. The results show that the main Nb-bearing mineral in the Miaoya syenite is rutile, which can be divided into magmatic and hydrothermal generations based on texture and chemical composition. Magmatic rutile is rare, while hydrothermal rutile is common in various syenites, mostly disseminated along the edges or fissures of primary rutile or closely associated with biotite and ilmenite. Magmatic rutile shows low contents of Nb2O5 (1.43%~2.56%), FeO (0.74%~1.01%), and other trace elements (e.g., Ta, Cr, V, W, Mo, Sb); while hydrothermal rutile has variable enrichments of Nb2O5 (3.48%~20.68%), FeO (1.18%~6.92%), and other trace elements. In-situ U-Pb dating of rutile illustrates that the formation age of magmatic rutile is 446±21 Ma, while the formation age of hydrothermal rutile is 240±19 Ma, indicating that Nb mineralization in syenite experienced initial enrichment during the early Paleozoic magmatic stage and secondary enrichment during the Triassic hydrothermal metasomatic stage. In summary, we believe that Nb enrichment and mineralization in the Miaoyao complex are controlled by both magmatic and hydrothermal processes. During the early Paleozoic magmatic stage, niobium within syenite is mainly hosted by rutile, biotite and Ti-bearing minerals (e.g., ilmenite); in the Triassic hydrothermal metasomatic stage, hydrothermal fluids can decompose biotite to form secondary Nb-rich rutile and columbite, or metasomatize primary rutile and ilmenite to generate hydrothermal Nb-rich rutile. 

Mechanism of Vital Effect of Chemical Composition of Foraminifera Calcite Shell

SUN Qianyuan, CHEN Tianyu

2024, 30(03):  362-370.  DOI: 10.16108/j.issn1006-7493.2024014

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The element and isotope compositions of calcite formed by calcification of foraminifera shells are widely used in reconstructing paleo-marine evolution. However, it has been found that the chemical composition of foraminiferal calcite is
significantly different from that of inorganic calcite precipitated directly in seawater, indicating that the chemical composition of foraminiferal shell is affected by the “vital effect”. In order to explore the controlling factors behind this “vital effect” and
get more reliable paleo-oceanic indicators, a series of studies have been carried out from the aspects of calcification process and element partition mechanism. This study first summarized the two main ways of foraminifera calcification: seawater vacuolization model and Ca2+ transmembrane model. In the enclosed or semi-enclosed calcified space, the composition of trace elements in the fluid produces a Rayleigh fractionation effect with the precipitation of calcium carbonate, which becomes a classical model to explain the mechanism of the “ vital effect” of trace element partition in foraminifera. However, this model is still difficult to explain quantitatively the mechanism of low magnesium in foraminifera calcite shells, the composition of calcium isotopes, and the sensitivity of Mg/Ca ratios to temperature. Bio-carbonates, including foraminiferal calcite, may be common exist metastable precursor carbonate. The partition effect of trace elements during the conversion of metastable precursor carbonates to calcite may be an important reason to the formation of low magnesium in foraminifera. The recent discovery of vaterite in living foraminifera supports this hypothesis, but the partition of trace elements and isotope effects during the conversion of carbonate precursors to calcite are rarely studied. This paper focuses on the partition model of trace elements when the precursor of vaterite in foraminifera transformed to calcite. This model can explain the phenomenon of low magnesium in foraminifera quantitatively. At the same time, combined with the calcium isotope data synthesized in the laboratory and the calcium isotope fractionation mechanism in the calcification process of coccolith, this paper attempts to further conjecture the mechanism of “vital effect” of the chemical composition of foraminifera calcite shells from the perspective of calcium isotope. Metastable precursors may contribute significantly to the “vital effect” of trace elements partition and isotope fractionation during foraminifera calcification, and this model needs to be further verified from the perspective of other trace elements and isotopes. 

Using Vanadium Isotopes to Constrain the Proportion of Lunar-forming Material

SHI Zhen, QI Yuhan, HUANG Fang, DING Xin

2024, 30(03):  371-378.  DOI: 10.16108/j.issn1006-7493.2023065

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The classic giant impact model is currently the most widely accepted hypothesis for explaining the lunar formation process. It posits a collision between a proto-Earth in its late accretion stage and a Mars-sized impactor named Theia. According to this model, the majority of the Moon’s material is derived from Theia. However, there is still a lack of precise constraints on the contribution percentage of the impactor to the lunar mass. In this study, high-precision measurements of mantle peridotites and komatiites are employed to reevaluate the V isotope composition of the Bulk Silicate Earth (BSE). Unlike previous studies, the new data indicate δ51VBSE=-0.91±0.02‰（2SE, n=18）. We incorporated this into a two-component mixing model for the Earth-Moon system, considering a system with pre-impact (proto-Earth, Theia) and post-impact (Earth, Moon, escaping mass) components. The best estimate for the mass fraction of Theia in the present Moon ranges from 73% for M_Theia=0.8M_Mars to 83% for M_Theia=0.45M_Earth This represents a reduction of approximately 5% in Theia’s contribution compared to earlier studies. These findings provide more reliable parameters for the classic collision model, contributing to a deeper understanding of the lunar formation process.