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Dissolved thorium, rare earth elements and neodymium isotopic composition in the Kerguelen Plateau : method development and application to quantify and trace lithogenic inputs

Perez Tribouillier, H ORCID: 0000-0001-5148-7355 2020 , 'Dissolved thorium, rare earth elements and neodymium isotopic composition in the Kerguelen Plateau : method development and application to quantify and trace lithogenic inputs', PhD thesis, University of Tasmania.

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The Southern Ocean (SO) is the largest high nutrient, low chlorophyll (HNLC) region in the global ocean, characterized by a minimum development of phytoplankton despite the abundance of macronutrients. The existence of such regions is caused by the absence of the bio-limiting trace element iron (Fe). In other ocean basins like the Atlantic and Pacific this limitation is overcome by the dissolution of aeolian dust. However, the SO is isolated from major dust sources. Here, the weathering and erosion of Subantarctic Islands, glacial run-off and resuspension of shelf-deposited sediments become the main source of lithogenic material. This lithogenic material naturally fertilizes surrounding waters with iron thus enabling the development of phytoplankton blooms, the largest of which occurs in the Kerguelen Plateau. The Kerguelen Plateau is the largest bathymetric barrier to the natural eastward flow of the Antarctic Circumpolar Current. Here, previous studies have demonstrated that the Heard and McDonald Islands located in the central part of the plateau and the Kerguelen Archipelago (northern part of the plateau) supply lithogenic material directly to the surface of the ocean. Additionally, the interaction between the ACC and the plateau causes dynamic conditions that can resuspend shelf-deposited sediments. These sediments release iron which can then be transported to the surface by vertical mixing or upwelling contributing with the natural fertilization over the plateau. The ACC can also transport Fe away from the plateau enabling the development of a plume with enhanced primary productivity that extends for several hundreds of kilometres east of the plateau. This bloom draws down atmospheric CO2 into the surface of the ocean and because this is a region of formation of intermediate and deep waters, this CO2 can potentially be incorporated into deeper layers of the ocean.
Understanding the conditions that drive these seasonal blooms is difficult first because of the remoteness of the SO. Iron also has a very complex oceanic biogeochemical cycle with several inputs and sinks. However, other trace elements like thorium (Th) and Rare Earth Elements (REE), which are not required by phytoplankton, can help trace lithogenic inputs, and represent a valuable toolbox for the study of ocean processes. Furthermore, Th and REE have a coherent chemical behaviour and constrained sources to the ocean. In the particular case of 232Th and REE, their sources are the same as for Fe. This makes them suitable to trace iron inputs to the ocean. However, these elements and their isotopes are present in such low dissolved concentrations that rigorous analytical procedures are required in order to measure them in seawater.
In this thesis I aim to: (1) Develop a new technique to simultaneously pre-concentrate Th (232Th, 230Th) and neodymium (Nd) isotopes using the Nobias chelating resin. (2) Adapt an existing technique to measure dissolved REE in seawater using the same resin. (3) Apply the above-mentioned analytical techniques to increase the knowledge about the factors that drive the seasonal phytoplankton bloom in the Kerguelen Plateau region. In particular, to better constrain the sources and pathways of iron in the plateau, and to provide a new estimate of the fluxes of lithogenic material using dissolved thorium data.
The results indicate that it is possible to accurately and precisely measure Th and Nd isotopes, as well as REE concentrations using the Nobias resin. This method drastically reduces sample processing time. The blank contribution of our techniques is comparable or less than previous studies. The analysis of different certified reference materials as well as intercalibration samples indicate an overall accuracy from both methods within 10% of the reported values and a long-term precision generally < 5%. We demonstrate that 230Th can be used to provide a scavenging residence time based on its disequilibria from its parent nuclide 234U. Following this approach, we calculate a residence time for the upper 500 m of the water column of ~ 260 days. We employ this residence time to calculate a dissolved 232Th flux. Using the 232Th concentration of the lithogenic material from the plateau and an estimated Th solubility of (1-20%) we obtain a dissolved lithogenic flux that ranges from 7 to 810 mg m- 2 day-1. This value is comparable to other studies in the Southern Ocean, and particularly similar to previous determinations of particulate fluxes in the Kerguelen Plateau using sediment traps (35-628 mg m-2 day-1).
The REE and εNd data confirm suggestions by previous studies that the predominant source of lithogenic material that fuels the bloom originates in the Heard and McDonald Islands, and surrounding shallow shelf. However, our results disagree about the relative importance to the region of material sourced from the Kerguelen archipelago. Europium and cerium anomalies, as well as the Nd/Yb normalized ratio and the εNd of our data, together with data from previous studies clearly indicate that the Polar Front acts as an effective barrier to the dispersal of lithogenic material (and likely Fe) sourced from the Kerguelen archipelago.
This thesis has improved constraints on the sources of Fe that allow the development of primary productivity around the Kerguelen Plateau region. However, some uncertainties remain. A more detailed sampling of the area between Heard and McDonald Island and the Kerguelen Islands will help to completely constrain the factors that drive the bloom in the region. Our results also indicate the need for more detailed studies that establish the solubility of Th not only from material from our area of study but in general of the lithogenic-sourced particles in the ocean. Finally, the method development in this study also represents a breakthrough in the way Th and Nd isotopes can be pre-concentrated from seawater. In particular, this method has the potential to be applied at sea reducing the amount of sample that needs to be brought back to land and eliminating the need for sample storage and transport.

Item Type: Thesis - PhD
Authors/Creators:Perez Tribouillier, H
Keywords: Southern Ocean, Kerguelen Plateau, thorium, neodymium, rare earth elements, phytoplankton bloom, GEOTRACES
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Copyright 2019 the author

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