Date of Award

Fall 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Planavsky, Noah

Abstract

In order to sustain habitability on our planet – Earth, a fine balance between the inputs and outputs of atmospheric carbon dioxide must be maintained. New steady states have emerged throughout the Earth’s history as the nature is dynamic. Yet, the carbon cycle seems to have been stabilized by the silicon cycle and the two cycles evolved concertedly together, contributing to the long-term stability of the Earth’s climate. Explicitly, silicate weathering, occurring both in terrestrial and marine environments, provides negative feedback to fluctuations in atmospheric carbon dioxide concentrations. Direct observations not being available, we often resort to geochemical proxies and mechanistic modelling to decipher the Earth’s past. One such promising geochemical proxy that tracks silicate weathering are the lithium isotope ratios as recorded in the rock archive. In this dissertation, I will present empirically focused studies, supported by modelling, that refine our understanding of the surface lithium cycle and contribute an extensive carbonate lithium isotope record ranging the last three billion years of Earth’s history that provides more precise evidence of the coevolution of the carbon and silicon cycles.Chapter 1 focuses on improving our knowledge of the modern lithium mass balance, particularly on understanding the controls on the riverine lithium isotope composition. The current view is that the interplay between denudation rates and silicate chemical weathering rates governs the lithium riverine influx to the ocean. The global estimate of modern averaged riverine lithium isotope value, however, is based on temporally sparse sampling from rivers. The paucity of high frequency monitoring could have potentially led to undersampling of noteworthy signal, especially if the riverine lithium isotope values and lithium concentrations vary significantly with time and discharge fluctuations over the course of major storm events. In this chapter, I report dissolved lithium isotope data from river water samples continually collected during low-frequency large precipitation events to monitor the propagation of the lithium signal. The results reveal that Ca, Mg and Li concentrations remain rather unaffected by the large discharges, exhibiting chemostatic behavior. More importantly, we found small (<2‰) variability in the riverine lithium isotopic composition during high precipitation events, with 7Li values at around the global modern riverine average. Therefore, this work bolsters the case that we have a relatively sound understanding of the global Li isotope mass balance and that Li isotopes are a promising tool for reconstructing weathering processes in the Earth’s history. Chapter 2 investigates the effect of early diagenesis on the recording of seawater lithium isotope composition in carbonate rocks. Multiple studies have explored the lithium isotope composition of ancient carbonates of various types with the intention to unravel the lithium isotope composition of paleoseawater and to better understand past carbon-silicon cycles and weathering regimes. In this study, I present new lithium isotope and elemental concentrations data of 260 samples from two shallow (49 and 70 cm) cores from the modern-day Bahamas carbonates, representing two different depositional environments. The first core is collected from a shallow mangrove inlet and is composed of Halimeda-rich rudstone to floatstone. The second core is collected from the Great Bahamas Bank flat and is composed of oolidic grainstone. Both cores were sliced into 1 to 2 cm thick segments, whereas each segment was further sieved into six grain size fractions; porewater was also extracted from the segments of one of the cores. Interestingly, the data reveal that there is not much variability of the carbonate δ7Li values with depth, while size fractions in the same core segments differ in their lithium isotope composition. The finest grain size fraction of carbonate sediments is the most susceptible to recrystallization and changes in the effective fractionation. The recorded lithium isotope fractionation in carbonates, thus, is between the intrinsic fractionation at deposition and nearly muted fractionation, representing contemporaneous seawater value. In Chapter 3 we argue that the lithium isotope system can be used to track processes controlling the long-term carbon and silicon cycles. I analyzed over 600 shallow water marine carbonate samples from 101 stratigraphic units from around the globe to construct a new carbonate lithium isotope record spanning the past 3 billion years. The results reveal that the carbonate lithium isotope values increase over time, which we suggest was driven by long term changes in the lithium isotopic composition of seawater rather than diagenetic alteration of older samples. The data, supported by a mass balance model and a reactive transport model, indicate that the observed trend in lithium isotopes reflects a transition from Precambrian carbon and silicon cycles – dominated by more congruent continental silicate weathering and extensive reverse weathering – to those more similar to the present day. This implies that profoundly different carbon and silicon cycles persisted for the majority of Earth’s history. The gradual transition to modern-like carbon and silicon cycles may be linked to a shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants. In sum, the findings of this study show that lithium isotopes can be used as quantitative constraints on weathering processes and, intrinsically, on the sources and sinks of carbon and silicon in the Earth’s surface environment.

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