The oceans are expected to become less oxygenated and more acidic in relation to future warming and anthropogenic greenhouse gas emissions. These changes in seawater chemistry will negatively impact marine ecosystem and global biogeochemical cycles.
To understand the natural, longer-term cycling of seawater oxygen concentrations and pH across the geological record of environmental change, several novel proxy methods were recently developed. Until now individual projects have focussed on reconstructing either oxygenation or pH, even though deoxygenation and ocean acidification seldom occur in isolation.
This project will consolidate recent progress made in such methodologies, using novel trace element ratios to concomitantly reconstruct seawater oxygen and pH during key warm intervals of the last ca. 3.6 million years, allowing to assess environmental conditions during intervals that were warmer, similar to, and colder than today. This may include the Piancenzian (2.6 to 3.6 Ma), and more recent interglacials of the Pleistocene.
The project will utilise geochemical information contained in the calcite shells of microscopic fossil foraminifera (Fig. 1). Bottom water and surface water oxygen reconstructions will be based on of carbon isotope gradients between different species of benthic foraminifera (e.g. Hoogakker et al., 2015), complemented by morphology and trace element measurements, and planktonic foraminifera iodine/calcium ratios (Lu et al., 2016). Bottom and surface water pH will be reconstructed using the boron isotope composition (11B) of benthic planktonic foraminifera (Foster & Rae 2016). These methods have provided several high profile reconstructions (e.g. de la Vega et al., 2020, Hoogakker 2018).
The tropical Pacific holds the worlds’ largest oxygen minimum zone (Fig. 2), which are of low pH. Reconstructions will focus on this area, utilizing sample material from the International Ocean Discovery Program (http://www.iodp.org/).
The student will be trained in combining oxygen and pH reconstructions from critical periods with Earth System Model (cGENIE) simulations to identify and assess links between climate variability and ocean chemistry changes.
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Fig 1. Photographs of examples of planktonic (floating surface and subsurface waters) and benthic (seafloor dwelling) foraminifera: a – planktonic Globigerina bulloides, b – benthic Uvigerina.
Fig. 2. Top: Oxygen concentrations in the Pacific at 125 meters water depth. Bottom: Pacific surface water (top 50 meters) pH (adapted from Raven et al., 2005).