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Investigating the role of marine sediments in the global oceanic cycling of nutrient trace metals.

Dr Caroline Peacock (SEE), Prof Simon Poulton (SEE), Amy Atkins, Stefan Lalonde

Project partner(s): Dr Amy Atkins (ETH Zurich), Dr Stefan Lalonde (Brest CNRS)

Contact email: c.l.peacock@leeds.ac.uk

This project will investigate the mobility and fate of the nutrient trace metals during the diagenesis of marine sediments to determine whether marine sediments provide a net sink or a net source of these bioessential metals to seawater.

Project Background

The nutrient trace metals, including nickel, copper, zinc and cadmium, are required for photosynthesis, which is the main process regulating the short-term carbon cycle and the atmospheric concentration of CO2.  Over half the Earth’s photosynthesis takes place in the oceans by marine photosynthetic algae. Understanding what controls the concentration of nutrient metals in seawater therefore helps us understand the links between marine biology, seawater chemistry and ultimately the Earth’s carbon cycle and climate.

At the global scale most approaches to investigating the oceanic cycling of nutrient metals start by trying to identify the major metal reservoirs, for example seawater and marine sediments, and the major fluxes between these reservoirs, for example scavenging of metals from seawater onto particles that settle into the sediments. Then the concentration of the metals, and increasingly the isotopic composition of the metals, in these reservoirs and fluxes are either measured or calculated. This information is then used to construct a global oceanic metal cycle, where, if our understanding of the metal behaviour is complete, there will be a mass balance between the major sources and the major sinks of metals to seawater (Fig. 1a).

In reality however this is a hugely challenging task, in large part because we have only a very limited understanding of the detailed biogeochemical processes that go on inside the reservoirs (Fig. 1b). It is these biogeochemical processes that determine what parts of the environment act as metal reservoirs, whether these reservoirs act as sinks or sources of metals to other parts, and how large the associated fluxes of the metals are.

Despite our lack of detailed understanding however, there is one important fact that ties almost all of the biogeochemical processes together – they all involve metal interactions with freshly precipitated iron and manganese minerals.

Indeed for many of the nutrient metals, iron and manganese (hydr)oxide minerals present in marine sediments appear to be the primary reservoir for these nutrients in the modern oceans. These minerals are ubiquitous in pelagic muds, as dispersed nanoparticulate phases, and present throughout the oceans concreted into ferromanganese crusts and nodules (Fig. 2a,b).  And whilst they may only be present in very small amounts in pelagic muds, they are arguably the strongest naturally occurring metal scavengers in the environment (Fig. 2c ). This means that in both muds and crusts, these scavengers are able to control the concentration, isotopic composition and ultimately the bioavailability of nutrient metals in seawater.

Unfortunately however the freshly precipitated iron and manganese (hydr)oxides are transient phases and as they age during the diagenesis of marine sediments they transform into new, often more crystalline, minerals.

This leads us to a fundamental question that is crucial to understanding the oceanic cycling of the nutrient metals:

What is the mobility and fate of nutrient trace metals during the aging and transformation of iron and manganese (hydr)oxides?

Are the metals that are initially scavenged by the freshly precipitated minerals retained in the newly formed phases, and thus retained in the sediments?

Are the initially scavenged metals released during the mineral transformation into sediment porewaters, but then re-scavenged by other phases present in the sediments?

Or are the initially scavenged metals released during mineral transformation and escape from the porewaters into the overlying seawater?

These questions are of upmost importance because in the first two scenarios the sediments provide a net sink of metals from seawater, but in the final scenario they provide a net source of metals to seawater.  And in order to track metal cycling in the oceans, and link metal abundance to biological activity and carbon cycling, we must know which way the sediments behave.

Objectives

This project will investigate the mobility and fate of nickel, copper, zinc and cadmium during the aging of iron and manganese (hydr)oxides to determine whether marine sediments provide a net sink or a net source of these bioessential metals to seawater.

  1. Prepare synthetic iron and manganese (hydr)oxide minerals in the laboratory and dope these with nickel, copper, zinc and cadmium to produce synthetic samples that are analogous to iron and manganese (hydr)oxides found in marine sediments.
  2. Age these metal-mineral samples in laboratory experiments designed to simulate the diagenesis of marine sediments. During aging take samples of the experimental solution and the metal-mineral solids to form a time series of samples.
  3. Analyse the solution and solid time series samples with a suite of state of the art geochemical techniques, including ICP MS for the concentration of metals in solution, XRD, SEM and TEM for the concentration of metals in the solids, and MC ICP MS for the isotopic composition of the metals in both the solution and the solids, to determine the amount of the metals that are released into solution vs the amount that are retained in the newly formed mineral products.
  4. Analyse the solid time series samples in detail using synchrotron spectroscopy to determine how and why the metals are released or retained in the newly formed mineral products.
  5. Investigate metal concentrations and isotopic compositions in a suite of natural marine ferromanganese sediments deposited in a range of diagenetic regimes, to relate the experimental results to real-world environments.
  6. Conclude whether marine sediments provide a net sink or a net source of nutrient metals to seawater.

Training

You will work under the supervision of Dr. Caroline Peacock and Prof. Simon Poulton within the Cohen Geochemistry Group at Leeds, and, depending on your interests, you will have the opportunity to work with Dr. Amy Atkins at the Swiss Federal Institute of Technology and Dr. Stefan Lalonde at the European Institute for Marine Studies in Brest, where you will engage with a wide variety of Earth scientists.  You will receive specialist scientific training in state of the art geochemical, mineralogical, experimental and analytical techniques and computational geochemical modelling.  Specifically you will also have the opportunity to analyse your samples using world-leading synchrotron spectroscopy techniques at the UK synchrotron Diamond Light Source.

In addition, you will have the opportunity to be trained in a wide variety of key transferable skills within the SPHERES NERC DTP, from computer programming and modeling, to media skills and public outreach. You will also be encouraged and supported to present your research at national and international scientific conferences, for example the premier geochemistry conference Goldschmidt next year held in Japan.

Eligibility

The applicant must satisfy the requirements to register as a doctoral student at the University of Leeds, which involves holding appropriate Honours, Diploma or Masters Degree and having passed the appropriate English language tests. Applications are invited from graduates who have, or expect to gain, a good degree in chemistry, geology, environmental science, materials science, or another relevant science discipline. Relevant Masters level qualifications are welcomed. The applicant should have a good command of both written and spoken English.

Recommended Reading (copies available on request)

  • Atkins A.L., Shaw S., Peacock C.L. (2016) Release of Ni from birnessite during transformation of birnessite to todorokite: Implications for Ni cycling in marine sediments. Geochim. Cosmochim. Acta 189, 158-183.

  • Atkins A.L., Shaw S., Peacock C.L. (2014) Nucleation and growth of todorokite from birnessite: Implications for trace-metal cycling in marine sediments. Geochim. Cosmochim. Acta. 144, 109-125.

  • Gall L., Williams H.M., Siebert C., Halliday A.N., Herrington R.J. and Hein J.R. (2013) Nickel isotopic compositions of ferromanganese crusts and the constancy of deep ocean inputs and continental weathering effects over the Cenozoic. Earth. Planet. Sci. Lett. 317, 148-155.

  • Konhauser K.O., Pecoits E., Lalonde S.V., Papineau D., Nisbet E.G., Barley M.E., Arndt N.T., Zahnle K. and Kamber B. S. (2009) Oceanic Ni depletion and a methanogen famine before the great oxidation event. Nature. 458, 750-753.

  • Koschinsky A. and Hein J.R. (2003) Acquisition of elements from seawater by ferromanganese crusts: Solid phase associations and seawater speciation. Marine Geol. 198, 331-351.

  • Peacock C.L. and Sherman D.M. (2007) Sorption of Ni by birnessite: equilibrium controls on Ni in seawater. Chem. Geol. 238, 94–106.

  • Peacock C.L. (2009) Physiochemical controls on the crystal chemistry of Ni in birnessite: genetic implications for ferromanganese precipitates. Geochim. Cosmochim. Acta 73, 3568–3578.

  • Post J.E. (1999) Manganese oxide minerals: crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. U. S. A. 96, 3447–3454.

  • Usui A. (1979) Nickel and copper accumulation as essential elements in 10Å manganite of deep-sea manganese nodules. Nature. 279, 411-413.

  • Waychunas G.A, Kim C.S. and Banfield J.F. (2005) Nanoparticulate iron oxide minerals in soils and sediments: unique properties and contaminant scavenging mechanisms. J. Nanopart. Res. 7, 409-433.

Related undergraduate subjects:

  • Environmental science
  • Geology
  • Geoscience
  • Natural sciences