Tegler
Logan A.
Tegler
Logan A.
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ThesisFrom the atmosphere to the abyss: tracing organic carbon deposition, cadmium isotopes, and iron cycling using marine(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2023-02) Tegler, Logan ; Nielsen, Sune G. ; Horner, Tristan J.The marine biological pump refers to the formation and subsequent export of particulate organic carbon from the sunlit zone to the ocean’s interior. The magnitude and attenuation of this flux exert an important control over the air–sea balance of carbon dioxide. This thesis is focused on constraining this flux, the factors that control it, and developing novel tracers for it. First, I evaluate Holocene carbon depositional fluxes in margin sediment and shed light on seafloor OC deposition. I find that margins host 19.4 T mol yr-1 of marine OC and, contrary to the current paradigm, less than 4 % of the OC is buried in low-oxygen environments. However, in order to understand how the efficiency of the biological pump may have changed over time, it is necessary to use proxies. In Chapter 3, I examine cadmium isotopes as a potential paleonutrient proxy. I suggest that in addition to biological uptake, Cd isotopes may be influenced by local redox conditions, remineralization, and external Cd additions. In chapter 4, I measure Cd isotopes in the Mt. McRae shale (2.5 Ga) that was deposited across a purported ‘whiff’ of oxygen that is believed to reflect the onset of oxygenic photosynthesis. I find that the Cd isotopes are invariant and light during the ‘whiff’ interval. Rather than reflecting no changes in nutrient cycling, I suggest these compositions reflect a source–sink balance between Cd-depleted surface waters and external Cd inputs. Finally, in Chapter 5, we redirect our attention to the Fe cycle. Iron is a limiting nutrient in many ocean regions, which limits the efficiency of the biological pump. We use iron isotopes and Q-mode factor analysis to identify five sources of iron to sites in the South Pacific and Southern Oceans, including: dust, a ligand-bound background source, volcanic ash, and two hydrothermal sources. Taken together, this thesis examines elemental interactions and spans temporal scales, from ancient epochs to the modern era. While we leverage trace elements as proxies of past marine biogeochemical cycles, we also stress that careful work is needed to apply and analyze them.
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ArticleUp in smoke: most aerosolized Fe from biomass burning does not derive from foliage(American Geophysical Union, 2023-08-25) Tegler, Logan A. ; Sherry, Alyssa M. ; Herckes, Pierre ; Romaniello, Stephen J. ; Anbar, Ariel D.Iron (Fe) is a limiting micronutrient in many marine ecosystems. The lack of sufficient Fe can stunt marine productivity and limit carbon sequestration from the atmosphere to the ocean. Recent studies suggest that biomass burning represents an important Fe source to the marine environment because pyrogenic particles have enhanced solubility after atmospheric processing. We examined foliage representative of four distinct biomes subject to frequent burning events, including boreal/temporal forests, humid tropical, arid tropical, and grassland. We burned these samples in the absence of soil to isolate the Fe from the fine particle (PM2.5) fraction that is derived directly from the burning foliage. We find that <1.5% of the Fe in plant matter is aerosolized throughout the burn in the fine fraction. We estimate that between 2% and 9% of the Fe released from biomass burning can be attributed to the fine fraction of the foliage itself, and <50% from the foliage overall. Most of the Fe aerosolized during biomass burning is accounted for by soil-suspended particles.
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ArticleDistribution and drivers of organic carbon sedimentation along the continental margins(American Geophysical Union, 2024-08-17) Tegler, Logan A. ; Horner, Tristan J. ; Galy, Valier ; Bent, Shavonna M. ; Wang, Yi ; Kim, Heather H. ; Mete, Oyku Z. ; Nielsen, Sune G.Organic carbon (OC) sedimentation in marine sediments is the largest long-term sink of atmospheric CO2 after silicate weathering. Understanding the mechanistic and quantitative aspects of OC delivery and preservation in marine sediments is critical for predicting the role of the oceans in modulating global climate. Yet, estimates of the global OC sedimentation in marginal settings span an order of magnitude, and the primary controls of OC preservation remain highly debated. Here, we provide the first global bottom-up estimate of OC sedimentation along the margins using a synthesis of literature data. We quantify both terrestrial- and marine-sourced OC fluxes and perform a statistical analysis to discern the key factors influencing their magnitude. We find that the margins host 23.2 ± 3.5 Tmol of OC sedimentation annually, with approximately 84% of marine origin. Accordingly, we calculate that only 2%–3% of OC exported from the euphotic zone escapes remineralization before sedimentation. Surprisingly, over half of all global OC sedimentation occurs below bottom waters with oxygen concentrations greater than 180 μM, while less than 4% occurs in settings with <50 μM oxygen. This challenges the prevailing paradigm that bottom-water oxygen (BWO) is the primary control on OC preservation. Instead, our statistical analysis reveals that water depth is the most significant predictor of OC sedimentation, surpassing all other factors investigated, including BWO levels and sea-surface chlorophyll concentrations. This finding suggests that the primary control on OC sedimentation is not production, but the ability of OC to resist remineralization during transit through the water column and while settling on the seafloor.