The role of the Southern Ocean in the global biogeochemical cycling of cadmium and zinc and their isotopes
Embargoed until 2021-05-01
- Doctoral Thesis
Trace elements, particularly transition metals, are critical for the growth of marine organisms, acting as micronutrients that fulfil vital functions in numerous biological processes. Over the past decades, a rapidly expanding body of research has shown that their oceanic distributions play a key role in controlling primary production in the upper ocean, influence the functioning of ocean ecosystems and impact the global carbon cycle. Specifically, the onset of the global GEOTRACES program has yielded important new insights into the marine biogeochemical cycles of trace elements and their isotopes at an unprecedented scale. The motivation for this work stems from the observation that the coupling of biogeochemical processes and physical circulation in the Southern Ocean plays a central role in determining the global distribution of trace metals such as cadmium (Cd) and zinc (Zn). This dissertation investigates in detail the effects of biogeochemical and physical process in the surface Southern Ocean on the distribution of dissolved Cd and Zn as well as the associated isotope systematics. Furthermore, it examines how distinct signatures are imparted during water mass formation and subsequently control Cd and Zn concentrations and their isotope distributions in the low latitude oceans. The first chapter examines to what extent distinct, southern-sourced signatures control the distribution of Cd and δ114Cd in the lower latitude South Pacific. Pre-formed signatures in intermediate waters, Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW), are strongly conserved as far north as 30°S, and overall, large-scale mixing of water mass endmembers with defined Cd and δ114Cd signatures can explain the subsurface distribution in the South Pacific remarkably well. Because, ultimately, all of the South Pacific subsurface water mass endmembers are Southern Ocean derived, this documents the importance of surface Southern Ocean processes for the global distribution of trace metals such as Cd. On a more regional scale, other process such as remineralization of Cd from sinking particles and Cd depletion (relative to phosphate) in low-oxygen subsurface waters, also can modulate the advected signatures. However, while such processes affect Cd concentrations on a basin wide scale, they appear to lack the leverage to significantly change deep ocean δ114Cd signatures. The second chapter investigates in detail the origin of the southern-sourced Cd signatures that set low latitude distributions. This is achieved by a high-resolution study of Cd cycling and associated Cd systematics in the upper ocean across all major zones of the Southern Ocean. In the upper Southern Ocean, Cd and δ114Cd are controlled by a complex interplay between biological uptake and regeneration, seasonal mixing, and upwelling while Cd signatures in deep waters remain largely unaffected by these processes. As a result, variations in Cd and δ114Cd are restricted to the upper Southern Ocean above homogeneous deep waters. In surface waters, Cd concentrations are depleted relative to the major nutrients nitrate and phosphate due to increased phytoplankton uptake. Consistent with preferential uptake of isotopically light Cd by phytoplankton, surface waters exhibit high δ114Cd signatures. Cadmium isotope systematics are consistent across dynamic-biogeochemical zones and different sectors of the Southern Ocean, and can be described well by a closed-system Rayleigh model. Signals in the shallow subsurface, however, vary throughout the Southern Ocean depending on the dominant process. Whereas regeneration results in a subsurface Cd maximum and a corresponding δ114Cd minimum, seasonal mixing brings heavy, low-Cd surface signatures to lower depths in the water column. The latter process is especially important in regions where the intermediate waters are formed, and, as a consequence, low-Cd, high δ114Cd signatures from the surface are imparted to SAMW and AAIW, and exported out of the Southern Ocean. The last chapter explores the biogeochemical cycling of Zn and its isotopes in the Southern Ocean in a manner similar to Cd. The Southern Ocean processes of biological uptake and regeneration, seasonal mixing, and upwelling are equally important in setting the Zn and δ66Zn distribution in the upper Southern Ocean without altering Zn signatures in the deep ocean. Even higher phytoplankton uptake for Zn, compared to Cd, results in lower surface Zn concentrations. Whereas this depletion also correlates with an elevated δ66Zn signal, preferential uptake of isotopically light Zn is associated with a significantly lower fractionation than for Cd. For large parts of the Southern Ocean Zn systematics are remarkably similar to those observed for Cd. However, due the combination of strong Zn depletion and the low fractionation, intermediate waters reflect the effects of surface Southern Ocean processes only in Zn concentrations but lack characteristic δ66Zn signatures. These findings document the extent to which biogeochemical and physical processes in the upper Southern Ocean, coupled with the large-scale circulation control the distribution of the bioactive trace metals Cd and Zn. Furthermore, it highlights the potential of trace metal isotopes in high-resolution studies to resolve complex relationships and unravel the impact of different processes. Show more
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ContributorsExaminer: Vance, Derek
Examiner: Conway, Timothy M.
Examiner: Henderson, Gideon M.
Examiner: de Souza, Gregory F.
Organisational unit03956 - Vance, Derek / Vance, Derek
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