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Constraining Nutrient Cycling in Modern Anoxic Lakes: Implications for Primary Productivity and Oxygenation on the Early Earth

Prof Simon Poulton (SEE), Dr Caroline Peacock (SEE)

Contact email: s.poulton@leeds.ac.uk

Project Background and Rationale:

Understanding the input and recycling of major nutrients (e.g. P, N) and bioessential trace metals (e.g. Fe, Ni, Cu, Mo) in the oceans has major implications for the regulation of primary productivity, organic carbon burial and oxygen production in both modern and ancient environments. One of the main geochemical processes that affects the bioavailability of these elements is uptake by reactive Fe minerals. Subsequently, the behaviour of Fe minerals and associated elements during early diagenesis exerts the dominant control on whether nutrients and bioessential trace metals are either sequestered in the sediment or released back into the water column. Clearly, the fixation of elements in the deposited sediment effectively limits their bioavailability, and thus it is crucial to understand the behaviour of nutrients during uptake by minerals under ferruginous conditions and during early diagenesis in order to evaluate implications for primary production and ultimately the rise of oxygen early in Earth’s history.

Fig. 1: Green rust particles

The modern ocean is largely characterized by an abundance of dissolved oxygen, and as a result, Fe oxides are the major group of Fe minerals that have been widely examined in terms of their uptake capacity for major and trace metal nutrients. However, recent research suggests that, considering the entirety of Earth history, the ocean has dominantly been free of oxygen (anoxic) and rich in dissolved Fe (ferruginous). Development of these anoxic non-sulphidic conditions in the water column is also a major concern with regard to the future impact of modern climate change on oxygen levels in the ocean. There is growing evidence that major initial Fe precipitates under ferruginous conditions may be iron phosphate (viviantite) or green rust (GR; Fig. 1), dependent on the precise chemistry of the anoxic waters. However, controls on the formation of these minerals, and on the behaviour of phosphorus and bioessential trace metals during their formation and transformation is poorly understood, particularly in terms of processes likely to operate under ferruginous oceanic conditions and during early diagenesis in anoxic porewaters.

Aims, Objectives and Key Hypotheses:

This project will focus on the uptake of phosphorus and trace metal nutrients to minerals formed under ferruginous water column conditions, and will investigate the subsequent potential for these elements to be fixed in the sediment or released to solution during early diagenesis. A dual approach will be taken, involving detailed laboratory experiments, coupled with studies of modern ferruginous lakes with differing P contents in Spain and other appropriate localities. The ultimate goal will be to build an understanding of the role of Fe minerals in the biogeochemical cycling of elements such as P, Ni (essential for methanogenesis), and Cu (essential for aerobic methanotrophy) under ferruginous conditions. This, in turn, will provide greater understanding of the potential for these elements to limit key biogeochemical processes in the ancient and future ocean. The main hypothesis to be tested is whether the multitude of experimental data that currently exists with regard to elemental adsorption to Fe oxides is really appropriate in terms of processes operating under anoxic, ferruginous water column conditions.

 
Fig. 2 Lake La Cruz, Spain.

Methodology:

The initial experimental phases will examine nutrient and trace metal uptake during the synthesis of green rust, in addition to examining controls on the formation of green rust versus vivianite under different environmentally relevant conditions. Controlled experiments will be performed to oxidise Fe(II) in the presence of different elements. These experiments will be conducted in different media, and will be compared to the results of separate adsorption experiments to pre-formed GR. The final experimental phase will be aimed at investigating the release and/or fixation of adsorbed elements during conditions encountered during early diagenesis. The field-based phase of the project will involving sampling and characterizing particulate and dissolved species in the anoxic and ferruginous Lake La Cruz and Lake Montcortés, Spain (Fig. 2), with the potential to expand this research to other lake systems. Geochemical extraction techniques (e.g. Fe and P speciation) will be combined with a range of mineralogical techniques available at Leeds (e.g. XRD, TEM), with the potential for further characterization of minerals at the Diamond Light Source Facility, UK.

Training:

The student will receive training in a wide variety of state-of-the-art experimental and sedimentary geochemical and mineralogical techniques, including techniques that the project supervisors have been personally responsible for developing. In addition, the student will be trained in a wide variety of key transferable skills within the Faculty Graduate School.

Opportunity for Travel:

The student will undertake fieldwork in Spain and potentially at other international sites. The student will also be encouraged to present their research at national and international conferences in Europe and North America (for example, the International V.M. Goldschmidt Conference).

References and Further Reading (copies available on request):

Zegeye, A., Bonneville, S., Benning, L.G., Sturm, A., Fowle, D.A., Jones, C, Canfield, D.E., Ruby, C., Maclean, L., Nomosatryo, S., Crowe, S.A., Poulton, S.W. (2012) Green rust formation controls nutrient availability in a ferruginous water column. Geology, 40, 599-602.
Poulton, S.W. and Canfield, D.E. (2011) Ferruginous conditions: A dominant feature of the ocean through Earth’s history. Elements, 7, 107-112.
Canfield, D.E., Poulton, S.W., Knoll, A.H., Narbonne, G.M., Ross, G.M., Goldberg, T. and Strauss, H. (2008) Ferruginous conditions dominated later Neoproterozoic deep water chemistry. Science, 321, 949-952.
Crowe, S.A. et al. (2008) Photoferrotrophs thrive in an Archean ocean analogue. Proceedings of the National Academy of Sciences, 105, 15938-15943.

Related undergraduate subjects:

  • Environmental science
  • Geology