Seafloor nutrient factories: The use of silicon and germanium isotopes to investigate early marine sediment diagenesis

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The release of elements from shallow sediments through dissolution and other chemical reactions is an important source of key nutrients into the marine system that are essential for supporting biological production and carbon cycling. Dissolved silicon (Si) is one such nutrient, which is critical for the growth of diatoms, an important group of algae that are responsible for a significant proportion of organic carbon production in the oceans. Shallow sediments contain various reactive pools of silica and are an important source of dissolved silicon to overlying waters in various marine settings (e.g. Michalopolous & Aller, 2004), especially in areas lacking significant fluvial silicon input. Germanium (Ge) is the 'chemical twin' of Si, and its stable isotopes could be used together with Si isotopes to investigate these previously under-explored reactive silica pools, associated with non-biological and redox-sensitive oxides and hydroxides, as well as biological silica within continental margins and shelf-seas (Baronas et al., 2019; Guillermic et al., 2017; Ng et al., 2019; Pokrovsky et al., 2014).Project Aims and MethodsThe overall aim is to use paired silicon and germanium isotopes to investigate active processes within shallow sediments from continental shelves, to better understand the release of dissolved Si from sediments into overlying waters, and to better constrain the marine Ge budget. The specific objectives will include refining Ge isotope measurements for seawater, porewater, and sediment leach analyses using cutting-edge plasma mass spectrometry methods within the Bristol Isotope Group; analysis of Ge isotopes within samples reacted under controlled laboratory experiments carried out in collaboration with Jeffrey Krause at DISL; analysis of Si and Ge isotopes as well as inorganic macronutrients, and trace metals (e.g. iron and manganese) in natural sediment samples from contrasting high and low-latitude continental shelf seas. There is flexibility within the project to focus on specific aspects of Ge/Si analytical method development, laboratory or field analyses of a wider range of redox-sensitive isotope systems (e.g uranium isotopes, measured at Cardiff University), or geochemical modelling.