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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Santa Cruz Electronic Theses and Dissertations (2025)

Carbon isotopes serve as powerful tools for understanding ocean carbon cycle processes, both past and present. This dissertation investigates two distinct applications of carbon isotopes in the context of ocean alkalinity enhancement (OAE): as tracers of natural geologic carbon and alkalinity release in the past, and as monitoring tools for future carbon dioxide removal. Using a combination of global and regional modeling approaches paired with geochemical data, this work provides new insights into both geologic carbon release during the last deglaciation and verification methods for future ocean-based carbon removal.

In Chapters 1 and 2, I investigate records of anomalously low 14C water in the eastern tropical North Pacific Ocean during the last deglaciation. First, through global carbon cycle modeling constrained by atmospheric CO2 and ∆14C records, I establish that large-scale release of neutralized geologic carbon (up to 2,400 Pg C) could have occurred without significantly disrupting the carbon cycle. Building on this, I develop a regional model of the eastern tropical North Pacific and combine it with new boron isotope (δ11B) data to directly simulate these anomalies, demonstrating that this carbon release must have been neutralized by alkalinity---representing a natural analog for OAE.

In Chapter 3, I shift focus to the present day, examining how carbon isotopes can support modern climate solutions. Using a high-resolution regional model of the California Current System, I evaluate the utility of stable carbon isotopes (δ13C) as a tool for verifying atmospheric CO2 uptake following OAE deployment. This work demonstrates that δ13C provides a diagnostic signal of CO2 removal that persists longer than traditional carbonate measurements, offering a robust verification method for marine carbon dioxide removal.

Together, these chapters advance our understanding of both past ocean carbon cycle processes and future carbon removal strategies, while highlighting the versatility of carbon isotopes as tools for studying natural and engineered perturbations to the marine carbon cycle.

    Cover page: Carbon Isotopes as Tools for Understanding Natural and Engineered Ocean Alkalinity Enhancement
    • Article
    • Peer Reviewed
    UC Santa Cruz Previously Published Works (2022)
    Ice cores and offshore sedimentary records demonstrate enhanced ice loss along Antarctic coastal margins during millennial-scale warm intervals within the last glacial termination. However, the distal location and short temporal coverage of these records leads to uncertainty in both the spatial footprint of ice loss, and whether millennial-scale ice response occurs outside of glacial terminations. Here we present a >100kyr archive of periodic transitions in subglacial precipitate mineralogy that are synchronous with Late Pleistocene millennial-scale climate cycles. Geochemical and geochronologic data provide evidence for opal formation during cold periods via cryoconcentration of subglacial brine, and calcite formation during warm periods through the addition of subglacial meltwater originating from the ice sheet interior. These freeze-flush cycles represent cyclic changes in subglacial hydrologic-connectivity driven by ice sheet velocity fluctuations. Our findings imply that oscillating Southern Ocean temperatures drive a dynamic response in the Antarctic ice sheet on millennial timescales, regardless of the background climate state.
      Cover page: Subglacial precipitates record Antarctic ice sheet response to late Pleistocene millennial climate cyclesCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2018)
      Globally averaged riverine silicon (Si) concentrations and isotope composition (δ30Si) may be affected by the expansion and retreat of large ice sheets during glacial-interglacial cycles. Here we provide evidence of this based on the δ30Si composition of meltwater runoff from a Greenland Ice Sheet catchment. Glacier runoff has the lightest δ30Si measured in running waters (-0.25 ± 0.12‰), significantly lower than nonglacial rivers (1.25 ± 0.68‰), such that the overall decline in glacial runoff since the Last Glacial Maximum (LGM) may explain 0.06-0.17‰ of the observed ocean δ30Si rise (0.5-1.0‰). A marine sediment core proximal to Iceland provides further evidence for transient, low-δ30Si meltwater pulses during glacial termination. Diatom Si uptake during the LGM was likely similar to present day due to an expanded Si inventory, which raises the possibility of a feedback between ice sheet expansion, enhanced Si export to the ocean and reduced CO2 concentration in the atmosphere, because of the importance of diatoms in the biological carbon pump.
        Cover page: The silicon cycle impacted by past ice sheets
        • Article
        • Peer Reviewed
        UC Santa Cruz Previously Published Works (2022)
        Using new and published marine fossil radiocarbon (14C/C) measurements, a tracer uniquely sensitive to circulation and air-sea gas exchange, we establish several benchmarks for Atlantic, Southern, and Pacific deep-sea circulation and ventilation since the last ice age. We find the most 14C-depleted water in glacial Pacific bottom depths, rather than the mid-depths as they are today, which is best explained by a slowdown in glacial deep-sea overturning in addition to a "flipped" glacial Pacific overturning configuration. These observations cannot be produced by changes in air-sea gas exchange alone, and they underscore the major role for changes in the overturning circulation for glacial deep-sea carbon storage in the vast Pacific abyss and the concomitant drawdown of atmospheric CO2.
          Cover page: Global reorganization of deep-sea circulation and carbon storage after the last ice ageCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Santa Cruz Previously Published Works (2017)
          During the Mid-Pleistocene Transition (MPT; 1,200-800 kya), Earth's orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.
            Cover page: Causes of ice age intensification across the Mid-Pleistocene TransitionCreative Commons 'BY-NC-ND' version 4.0 license
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