From the atmosphere to the abyss: Tracing organic carbon deposition, cadmium isotopes, and iron cycling using marine sediments

Abstract

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.