Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Planavsky, Noah

Abstract

The existence of life on Earth for over 3.8 billion years implies continuously habitable conditions despite major changes to the planet’s climate and surface geochemistry. In this thesis, I explore two Earth surface processes – terrestrial weathering and marine bioturbation – and their roles in shaping environmental habitability across various timescales. Weathering has long been recognized as a fundamental component of the Earth system. On multi-million-year timescales, it is the primary source for limiting nutrients to the biosphere and primary sink for atmospheric CO2. This concept underlies recent interest in artificially accelerating global weathering rates to mitigate near-future warming. However, the extent to which natural weathering regulates global biogeochemical cycles both in modern and deep time remains actively debated, hinging on interactions between tectonics, hydrology, and biology at often highly local scales. In Chapters 1-3, I use the chemical composition of ancient soils (paleosols) and modern rivers to gain new insights into the weathering of the continents across Earth history. In Chapters 1 and 2, I investigate the nature of continental weathering prior to the origin of land plants using paleosols. In Chapter 1, I propose that aluminum mobilization in paleosols may provide evidence for the presence of organic acids in soils as far back as the Archean, and therefore a biological influence on weathering for much of Earth history. In Chapter 2, I analyze the lithium isotopic composition (δ7Li) of many of these same paleosols as a proxy for silicate weathering, specifically weathering congruency, the extent to which weathering products remain in solution rather than re-precipitating as secondary minerals such as clays. Reduced Li isotopic fractionation in Precambrian paleosols relative to their modern analogs indicates less clay formation, higher congruency, and more efficient removal of atmospheric CO2 per mass of rock weathered. At a global scale, higher weathering congruency may have counterbalanced increased CO2 outgassing from Earth’s interior. It would also imply a fundamentally different global hydrologic cycle, with faster runoff and shorter water-rock interaction times necessary to suppress clay formation within the weathering zone. In Chapter 3, I extend my work on Li isotopes to modern settings, using δ 7Li in river waters to investigate spatial variability in weathering congruency within tropical upland environments. These regions exhibit some of the highest observed weathering rates globally, yet river Li data remain sparse. I present new δ 7Li data from rivers in Thailand showing significant variability in upstream catchments, converging downstream to values near the global river average. Some of these catchments exhibit extremely heavy isotopic compositions (among the highest δ 7Li values reported globally) suggesting extensive clay formation over much shorter distances than typically expected for rivers. The strongest predictor of δ 7Li is catchment slope, with greater isotopic fractionation in lower-relief catchments, likely due to longer water-rock interaction times. This indicates a strong hydrologic and topographic control on Li isotope fractionation, supporting the potential use of δ 7Li as a proxy for continental water cycling in deep time. Likewise, marine bioturbation may also be a significant Earth system feedback, influencing the global carbon cycle on both geologic and shorter timescales by modulating the burial of organic matter, while also responding to changes in ocean temperature and chemistry driven by global climate perturbations. However, the magnitude and even direction of a potential bioturbation feedback on the global carbon cycle remains unknown. In Chapter 4, I investigate bioturbation’s potential feedbacks on ocean chemistry across an ancient warming event, the Permian-Triassic (P-Tr) transition ca. 252-247 million years ago, using the P-Tr sedimentary record in Svalbard, Arctic Norway. I find that higher bioturbation intensity corresponds with lower total organic carbon, total sulfur, and organic phosphorus content, as well as higher inorganic phosphorus content. These findings suggest that bioturbation primarily influenced sediment chemistry by enhancing organic matter oxidation, in contrast to some modern settings where downward mixing may promote organic matter preservation within the anoxic portion of the sediment pile. Across the P-Tr transition, the decline and subsequent recovery of bioturbation may therefore not only have been a symptom of environmental change, but also its driver, influencing nutrient exchange and reductant burial across the sediment-water interface, and thus water column oxygen availability and seafloor habitability more broadly. In summary, the outcomes of this dissertation clarify how terrestrial weathering and marine bioturbation shape Earth's surface geochemical cycles, both in modern and deep time. Chapter 1 demonstrates a remarkably consistent presence of biological weathering across Earth history, while Chapter 2 demonstrates a fundamental difference in terms of silicate weathering. Chapter 3 demonstrates that modern tropical weathering regimes exhibit highly variable congruency, challenging assumptions about their efficiency as a CO2 sink. Finally, Chapter 4 demonstrates that bioturbation may be a significant feedback on marine nutrient cycling and organic matter burial at regional scales across ancient warming events, justifying further research into its potential global impact under near-future ocean warming. Taken collectively, these findings help advance our understanding of the role of life in the Earth system.

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