Tracking Carbon Cycle Behavior and Weathering Feedbacks through Earth’s History using Metal Isotope Proxies

Date of Award

Spring 2022

Document Type


Degree Name

Doctor of Philosophy (PhD)


Geology and Geophysics

First Advisor

Planavsky, Noah


The balance between carbon inputs – via degassing and breakdown of organic carbon – and outputs – via silicate weathering and organic carbon burial – on Earth dictate the concentration of atmospheric carbon dioxide (pCO2), which in turn, plays a major role in temperature and habitability in Earth’s past and future. Our understanding of Earth’s global carbon cycle and the processes that control it hinges, in part, on the tools and models that we use to track and reconstruct previous climates and environmental change. In this dissertation, I will present both empirical- and model-driven studies that contribute alternative and more precise explanations of existing isotope proxy records and provide novel data that tracks evidence for one of the major controls on atmospheric CO2.Chapter 1 focuses on the silicate weathering feedback as a carbon sink and the lithium isotope proxy, which is used to track this process. Our understanding of Earth’s habitability as a function of temperature depends on tracking atmospheric CO2 removal, which is driven in part by the silicate weathering feedback – the negative feedback process in which silicate weathering consumes CO2 on geological timescales. The end-Triassic mass extinction (ETE; ca. 201.3 Ma) is a period of global warming associated with significant release of CO2, and is therefore a prime case study for investigating the behavior of the silicate weathering feedback in the face of elevated atmospheric CO2. This increased source of CO2 is thought to have led to global warming, enhancement of the hydrological cycle, and therefore the enhancement of the silicate weathering feedback, eventually regulating atmospheric CO2. Yet, unambiguous evidence for an increase in weathering across the ETE has been scarce. In this chapter, we report the first lithium isotope dataset from two sections spanning the Triassic-Jurassic boundary (TJB) to track changes in the global silicate weathering feedback. Chapter 2 investigates the osmium isotope record from the last 65 million years and alternative explanatory mechanisms for the behavior of the empirical data. Osmium isotope ratios are a key tool to track changes in global weathering and carbon cycle evolution through time. Long-term changes in seawater Os isotope records over the Cenozoic have been used to argue for changes in weathering from increased uplift, leading to long-term global cooling. However, building from experimental results, we show that seawater chemistry dictates the amount of osmium coming from mid-ocean ridge (MOR) hydrothermal systems. Given that seawater chemistry has changed over the last 65 million years – the same period over which significant changes in the long-term Os isotope record are observed – we provide an alternative mechanism driving seawater Os isotope values, implicating changes in seawater chemistry rather than changes in uplift and weathering. This unconsidered player in the Os system has major implications for how we interpret Os data throughout Earth’s history and the conclusions drawn from them. Chapter 3 leverages the boron isotope system and models of the boron and lithium isotopic composition of seawater to constrain the magnitude of changes in seafloor weathering via low-temperature, off-axis hydrothermal basalt alteration. The seawater boron isotope system is strongly affected by the extent of low-temperature, off-axis hydrothermal alteration as it is the largest removal flux of boron and is associated with a large isotopic fractionation. The Li isotope proxy has several similarities with the B isotope system as they are a function of a variety of similar earth system processes given their similar fluid mobility, source and removal pathways, and associated isotopic fractionations. Therefore, coupling the two mass balances provides a way to evaluate different hypotheses surrounding the degree to which continental and marine processes have controlled the long-term carbon cycle, atmospheric carbon dioxide, and Earth’s climate evolution. Here, we use a mass balance modeling approach to show that – despite uncertainties of the boron isotope system and how it may have changed through time – current constraints on seawater boron isotope compositions across the Cenozoic are inconsistent with the hypothesis that there has been a dramatic change over the last 65 million years in the magnitude of off-axis hydrothermal alteration. Rather, modeling of the Cenozoic seawater boron isotope system affirms the view that continental processes have played the predominant role in shaping atmospheric pCO2 concentrations in Earth’s recent history. In sum, these chapters address solutions, uncertainties, and future directions related to multiple metal isotope proxy systems used for tracking various aspects related to the global carbon cycle and the silicate weathering feedback. This body of work has found proxy evidence for enhanced carbon removal across one of Earth’s ‘big five’ mass extinctions, identified alternative drivers of the osmium isotope system, and explored new means of estimating the off-axis basalt alteration flux over the last 65 million years. Our knowledge of the global carbon cycle is vital for understanding the history of habitability on this planet and others, while informing us what the future may hold – and results from this work will hopefully contribute to our growing understanding of Earth’s carbon cycle.

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