Evaluation of Stable Strontium Isotope Fractionation During Continental Weathering Processes

  • 劉 厚均

Student thesis: Doctoral Thesis


The relationship between global climate change and continental weathering is a cause-consequence feedback system Global climate may be perturbed by any of processes controlling weathering through changing atmospheric CO2 abundance and in turns to further affect the extent of weathering In this iteration the atmospheric CO2 fluctuation is controlled by the stoichiometric balance between the CO2 consumption by silicate mineral weathering and CO2 release by marine carbonate precipitation at long-term time scale Thus discriminating the terrestrial weathering sources from silicates and carbonates and determining the extent of weathering are important and the fundamental knowledge for comprehending the long-term climatic evolution In chapter 1 the application of newly triple Sr isotopes 87Sr/86Sr and δ88/86Sr ratios was introduced and suggested to serve as a powerful isotopic tracer rather than the 87Sr/86Sr alone for simultaneously determining the continental weathering fluxes to the ocean and quantifying the Sr burial flux by the carbonate precipitation in the marine system at glacial and interglacial climatic cycles Given that rivers are the largest Sr source to the ocean however there were only few attempts available for studying relevant controls on δ88/86Sr fractionation in riverine waters In order to have confidence in interpretation based on the recent developed isotopic proxy the following questions should be therefore clarified by the first order: (1) The variability of δ88/86Sr for lithological specimens i e silicates and carbonates (2) Potential physicochemical processes that controlling the δ88/86Sr fractionation in terrestrial environments (3) Lithology and/or fractionation control the variability of δ88/86Sr in riverine waters To address those fundamental questions the following chapters provide a comprehensive evaluation of stable Sr isotope fractionation during continental weathering processes by natural specimens and laboratory experimental works In chapter 2 we demonstrated a highly precise and accurate δ88/86Sr determination by using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) with empirical external normalization and standard sample bracketing (EEN-SSB) method for mass bias correction In this chapter we evaluated the fractionation behaviors of Sr and Zr isotopes during MC-ICP-MS determinations and established a modified procedure to apply 92Zr/90Zr instead of currently available 91Zr/90Zr for Sr isotopic mass bias correction A factor of 1 5 was improved in precision of our δ88/86Sr determination that is likely owing to the smallest instrumental mass-independent fractionation between the isotopic pair of 88Sr/86Sr and 92Zr/90Zr This new method allows 87Sr/86Sr and δ88/86Sr ratios to be measured simultaneously with high throughput and comparable precision and reproducibility with double spike (DS) applied MC-ICP-MS δ88/86Sr determinations In chapter 3 to evaluate the potential processes lead to δ88/86Sr fractionation during silicate mineral weathering an intense pedogenic paleosol sequence was sampled in the Chinese Loess Plateau (CLP) Sequential extraction procedure was applied to isolate different geochemical phases of the soils to study the stable Sr isotopic distributions in the extracted exchangeable reducible and silicate fractions The results suggested that heavier δ88/86Sr was preferentially released during incongruent silicate mineral weathering This is further evidenced by the acid leaching experiments using the soil silicate phase primary silicate minerals and clay standards In chapter 4 an annual time-series monitoring of chemical and Sr isotopic compositions on a carbonate-dominated catchment was performed to evaluate the influences of secondary calcite precipitation and silicate mineral weathering on water δ88/86Sr A seasonal systematic of karst cave stream water δ88/86Sr was detected showing coherent heavier δ88/86Sr and radiogenic 87Sr/86Sr ratios detected in the warm and rainfall period reflecting enhanced biological activities and intensive silicate weathering at that time On the other hand at least 30% to 55% of Ca was removed by the secondary calcite precipitation and lead to 0 1? heavier in water δ88/86Sr Those time-series data emphasizes the critical role of secondary calcite precipitation on the comprehensive heavy δ88/86Sr ratios in waters draining carbonate-dominated catchments and peak of related heavy δ88/86Sr ratios by intensive silicate weathering in wet season and extreme rainfall events In chapter 5 (conclusion) we compile all the currently published stable Sr isotope data from literature sources (2006 to early 2015) and this study for evaluating the potential mechanisms controlling the sensitivity and variability of δ88/86Sr of the continental weathering flux to the ocean and its implication for global Sr cycling The data suggests that the variability of δ88/86Sr of the terrestrial silicate materials and carbonates are statistically lighter than that of riverine waters In stead of lithology fractionation of Sr stable isotopes during and post-weathering processes such as secondary phase precipitated incongruent silicate weathering and vegetation uptake might be more important factors and by the first order lead to the enrichment of heavy δ88/86Sr in the waters The degree of δ88/86Sr fractionation in continental weathering environments may have significant implications to the oceanic Sr isotopic budget at glacial-interglacial time scales The variability of δ88/86Sr in modern rivers will still need to be better constrained for studying the global Sr cycling at geological time scales
Date of Award2015 Jun 10
Original languageEnglish
SupervisorChen-Feng You (Supervisor)

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