Expanding the utility of the zinc and cadmium stable isotope micronutrient proxies, and reconstructing palaeo-productivity through the last deglaciation
The oceanic biogeochemical cycling of trace metal micronutrients is being progressively developed to assess the efficiency of the ocean’s ‘biological pump’, atmospheric carbon dioxide (CO2) sequestration, and ocean-atmosphere climate cycling due to their biological importance to phytoplankton growth and marine primary production. Technological advancements have facilitated investigations into the micronutrient biogeochemical cycling in the modern ocean using seawater samples, and in the past oceans using mineral archives that retain a seawater component at the time of formation. These developments have recently enabled the use of micronutrient stable isotope systems to reconstruct variations in the coupling between the ocean’s ‘biological pump’ and carbon-climate cycling throughout Earth’s history, with enhanced resolving power compared with concentration analyses alone. In this thesis, the utility and accuracy of the zinc (Zn) and cadmium (Cd) isotope micronutrient systems is vastly expanded for accurate reconstructions of past seawater and marine primary productivity. To achieve this, four primary objectives were targeted. First, a scarcely utilised ‘double-spike’ design was used to obtain enhanced analytical resolution, required to accurately document subtle but distinct isotope fractionation that is typical of the Zn isotope system in seawater. To this end, up to a factor of four improvement in measurement precision is routinely attained by implementing a 70Zn-67Zn double spike design, compared with traditionally used double spikes, while accuracy is maintained across a range of Zn load sizes and matrix types. New and refined Zn isotope values for a suite of certified reference materials and standards are provided for use across multiple emerging disciplines, and the analytical improvements could be essential for documenting previously unresolvable biogeochemical processes. Second, the utility of commonly used carbonate chemical cleaning protocols, and the effects of partial carbonate dissolution on recent Holocene (<11,000 year old) surface carbonate sediments and an ancient Mesozoic (~94 million year old) carbonate sediment, was interrogated for Zn and Cd isotope analysis. This study demonstrates that accuracy is improved by using carbonate chemical cleaning methods that selectively remove contaminating secondary iron-manganese (Fe-Mn) (oxyhydr)oxide phases. However, the ‘reductive step’ required to achieve this could induce anomalous zinc isotope fractionation and must be carefully monitored. Natural and laboratory-controlled partial carbonate dissolution is also shown to cause Zn and Cd isotope fractionation. This study provides an evidence-based approach to selecting the methodological protocols that can be used to obtain accurate Zn and Cd isotope analysis in carbonates. Furthermore, this study highlights that partial carbonate dissolution must be accounted for, or avoided, in Zn and Cd isotope palaeo-productivity reconstructions across all time periods. Third, a suite of seven Holocene surface carbonate sediments retrieved from the New Zealand and southeast African continental margins were used to construct, for the first time, full water column calibrations between the Zn and Cd isotope composition of carbonates, and modern seawater. In particular, an in-depth study of the Zn and Cd isotope systematics of individual species of foraminifera, and the zooplankton and phytoplankton fractions, as well as the typically used bulk sediment fraction, was conducted. These sediments were obtained from either side of the subtropical frontal zone in the Southern Ocean that provides a natural laboratory to investigate the systematics of Zn and Cd isotopes in past seawater across a wide range of physical and chemical conditions. Biogenic carbonates record a Cd isotope signature that is systematically offset from seawater towards lighter isotopic compositions and is largely insensitive to Cd concentration. Upon correction for these isotope effects, the seawater Cd isotope signature can be accurately reconstructed. The Cd isotope composition of secondary Fe-Mn (oxyhydr)oxide coatings precipitated on the primary foraminifera carbonate surface is unfractionated relative to the seawater, providing an estimate for the Cd isotope composition of the bottom water overlying each site. Carbonates record a systematically heavy Zn isotope composition relative to seawater and require correction for the concentration dependence of isotope fractionation associated with adsorption. Accurate correction for these Zn isotope effects allow the seawater signature to be reconstructed. Furthermore, particulate organic matter is shown to represent a sink for light Zn and Cd isotopes. The magnitude of this isotopically light sink is capable of balancing the Zn and Cd sources to the oceans from riverine and atmospheric inputs. Finally, the novel Zn isotope system was applied, for the first time, as a micronutrient palaeo-productivity tool in down-core carbonate sediment samples from the southeast African continental margin, a region that is highly sensitive to changes in southern and northern hemisphere climate change. The zinc/calcium and Zn isotope compositions of carbonates are used to demonstrate that the southeast Indian Ocean was depleted in Zn during the Last Deglaciation that occurred from ~21 to ~11 thousand years ago (ka), but variations in the regional hydroclimate during the following Holocene interval increased the supply of nutrients to the mixed layer increasing primary production. The position of the subtropical front, south of Africa, is shown to have shifted southwards since the Last Glacial Maximum at ~21 ka, which could have accelerated northern hemisphere deglaciation and strengthened Atlantic Meridional Overturning Circulation. Finally, periods of micronutrient depletion and repletion at the northern margin of the Southern Ocean correspond to abrupt ocean-climate reorganisations during the Last Deglaciation. The enhanced resolving power and high sensitivity of the Zn and Cd isotope micronutrient systems, compared with elemental concentrations alone, will make these palaeo-productivity proxies indispensable for constraining carbon and climate cycling mechanisms in Earth’s past that can be used to better predict possible future climate scenarios.
Advisor: Stirling, Claudine; Frew, Russell; Rolison, John
Degree Name: Doctor of Philosophy
Degree Discipline: Chemistry
Publisher: University of Otago
Keywords: Micronutrient; Palaeoclimate; Foraminifera; Climate change; Sediment; Proxy development; Palaeoproductivity; Geochemistry; Biogeochemistry; Phytoplankton; Marine; Stable isotopes; Zinc; Cadmium; Last Deglaciation; Biological pump
Research Type: Thesis