Ocean sediment controls of essential trace metals in the Arctic-Atlantic gateway (MetalGate)

Figure 1. The Research Vessel Pelagia of the Royal Netherlands Institute for Sea Research, supporting the MetalGate research expedition in 2020.

Summary

In this project you will pioneer new trace elemental analyses of dissolved metals in marine sediment porewaters collected from a key, sensitive and unexplored region of the ocean floor. You will examine how sediment composition impacts past and present ocean chemistry, life and climate, by testing emergent theories about the mechanistic controls of trace metal inputs to seawater that may govern primary and secondary food web productivity and regulate the oceanic uptake of carbon dioxide from Earth’s atmosphere.

 

Figure 2: Planned study sites (top)The positions of planned sampling stations for occupation in summer 2020 are indicated by yellow markers (circles). Colour scale bathymetry and an isobath (black line) at 1000 metres highlight the extent of sediment sills between Greenland, Iceland and mainland Europe. The yellow line joins a sampling transect through the Denmark Strait. Bathymetric transect through Denmark Strait (bottom). The Iceland-Greenland sill shoals to less than 500 metres and is a confluence for Greenlandic sediments, Icelandic sediments and the formation of NADW.

Background

Biologically essential trace metals in seawater can determine the timing and extent of marine primary production, and thereby influence earth’s climate and marine resources via the exchange of carbon between ocean and atmosphere (Bruland et al., 2014). It is widely observed that sediments can be both major sources and sinks for these dissolved micronutrient trace metals, but the nature and magnitude of these exchanges is little understood, so the extensive role that sediments play in setting ocean biogeochemical and climatic conditions remains unclear (Homoky et al., 2016). The Arctic-Atlantic outflow forms an important component of North Atlantic Deep Water (NADW); a water mass that subducts and journeys southward, carrying the pre-formed nutrient and trace metal inventories supplied to phytoplankton in the remote Antarctic circumpolar current systems. Prior to this component of NADW formation the Arctic outflow passes over the shallow barrier sills that extend from Greenland to Iceland and from Iceland to mainland Europe (Figure 2). Despite this profound physical interaction, the nature and extent of trace element exchanges in these regions is unknown. It is further suggested that sediments derived from young volcanic terrains, such as Iceland, ought to sustain larger fluxes of trace metals to seawater than older granitic rocks such as found in Greenland (Jeandel 2016; Homoky et al., 2013; 2016), but few field experiments have been conducted that are able to test this hypothesis or explore its wider implications for the ocean-earth-climate system. You will conduct such an experiment in an extensive and first of a kind assessment of porewater trace metal fluxes across the Arctic-Atlantic gateway in collaboration with the a GEOTRACES expedition MetalGate, due to set sail on the Research Vessel Pelagia of the Royal Netherlands Institute for Sea Research (NIOZ) in summer 2020.

Aim of project

Your aim is to define the nature and quantify the magnitude of trace metal exchanges at the seafloor between the Arctic and Atlantic oceans, and to calibrate boundary conditions used in ocean biogeochemical models. You will test and further develop two hypotheses:

Hypothesis 1: Sediment sills release bio-essential trace metal micronutrients to North Atlantic Deep Water.

Hypothesis 2: Igneous provenance has a determinant effect on the magnitudes of trace metal fluxes from sediments to the ocean.

Objectives

In this project you will:

  • Be invited to join a research expedition to the Arctic-Atlantic gateway on the Research Vessel Pelagia in summer 2020 (Figure 1), to co-lead collection of pristine surface sediments (Figure 2), perform high-resolution seafloor depth profiling of oxygen and pH and extract depth-resolved interstitial pore water samples for novel trace element analyses (Figure 3).
  • Characterise the mineralogical and bulk chemical composition of your marine sediment samples (e.g. using a carbon, sulphur and nitrogen analyser, and X-ray diffraction and electron microscopy for the concentration of elements in solids)
  • Purify and pre-concentrate key bio-essential elements (e.g. Fe, Mn, Co, Cu, Ni and Zn) that occur in trace quantities in marine sediment porewater, to discover their natural abundancies and nanoscale partitioning using a state-of-the-art ESI seaFAST pre-concentration and purification system and Inductively Coupled Plasma-Mass Spectrometer (ICP-MS).
  • Collaborate with an international team studying the physics, chemistry and biology of marine trace element cycling in the water column above your sample sites as part of a GEOTRACES project (MetalGate).
  • Quantify trace element relationships between porewaters and their solid carrier phases and develop a new model of benthic trace metal flux sensitivity to lithogenic composition, which will translate findings from the regional to global ocean scale.
Figure 3. From the seafloor to the clean laboratory. From Left to Right: Pristine recovery of ocean sediment-water interface; micro-sensor depth profiling of dissolved oxygen and pH; anoxic trace element separation; porewater purification for trace element and isotope analyses by ICP-MS.

Impact of the research

The regulation of global climate is a top priority interest for the Earth scientists with potential for impacts across all sectors of society. This project seeks a foundational insight into ocean-linked life and climate phenomenon in the present day. Its findings may have additional significance for understanding past North Atlantic Ocean nutrient regimes, and the controls of ocean nutrients  during formation new ocean basins in the geologic past. Published outputs in these subject areas regularly attract attention in top geoscience journals, as well as leading interdisciplinary journals. The study of sediments and ocean biogeochemistry is a crucial linkage between past and present planetary functions in the Earth and Environmental Sciences. Increasingly, such research has strategic relevance also for the governance of biological and mineral seabed resources. This project will address timely and important questions concerning the cycle of trace elements and their biological impact in the ocean motivated by an international working group at the Royal Society in 2016; Findings will be used to strengthen an identified weakness in ocean-coupled models of earth climate that are used in IPCC assessment reports and relied upon to inform climate-related policy decisions.

Training

There is scope within this project to study transformative ideas and acquire a wide set of specialist skills in solid and aqueous analytical and theoretical ocean biogeochemistry, both in the field and laboratory. There is further opportunity to broaden learning by participating in a multi-disciplinary and international GEOTRACES project (MetalGate). You will receive specialist scientific training at the practical and theoretical cutting edge of trace element and isotope cycling in ocean sediments and porewaters from your supersivors, Dr Will Homoky and Prof Caroline Peacock. Depending on your interest, you will also have the opportunity to visit a top marine analytical facility run by Dr Rob Middag – Principle Investigator of MetalGate and expert in the cycling of trace elements and their analyses in seawater – at Royal Netherlands Institute of Sea Research (NIOZ). Here you may receive supplementary training in trace elements purification and preconcentration techniques relevant to your samples.

You will also be trained in a wide variety of key transferable skills within the PANORAMA NERC DTP, from computer programming and modelling, to media skills and public outreach. You will be encouraged and supported to present your research at national and international scientific conferences, for example at Goldschmidt, the premier geochemistry conference due to be held in Hawaii in 2020.

Working environment

Under supervision of Dr Will Homoky and Prof Caroline Peacock you will join the Cohen Research Group and be based in the Earth Surface Science Institute (ESSI), within the School of Earth and Environment at the University of Leeds. Here you will benefit from an established world-class geochemical analytical laboratory suite, and new trace element clean working environment. The ESSI is a welcoming and collaborative research environment, home to many postgraduate analytical geochemists, biogeochemists, sedimentologists, and modellers of both climate and biogeochemistry.

Entry requirements

Applications are invited from graduates who have, or expect to gain, a good degree in chemistry, geology, ocean or environmental science, or another relevant science discipline, and enjoy practical and theoretical problem solving. Relevant Masters level qualifications and/or experience are also welcome. You must satisfy the requirements to register as a doctoral student at the University of Leeds, which involves holding an appropriate Honours, Diploma or Masters Degree and having passed the appropriate English language tests. You should have a good command of both written and spoken English. Participation in ship-based fieldwork is not an essential requirement for this project, but there is a unique opportunity for you to do so in the summer of 2020, which is subject to your availability and the necessary pre-requisite seafarer’s medical examination and Personal Sea Survival Techniques Training. For further enquiry, please contact Dr Will Homoky.

References and related reading (copy 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. 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. Cosmochim. Acta. 144, 109-125.
  • Bruland, K. W., Middag, R.& Lohan, M. C. Controls of Trace Metals in Seawater, in Treatise on Geochemistry
  • Delorme and Eddebbar (2017) Ocean Circulation and Climate: An Overview. ocean-climate.org.
  • Henderson G.M. et al., (2007) GEOTRACES—an international study of the global marine biogeochemical cycles of trace elements and their isotopes.  Erde-Geohem.67, 65–131.
  • Jeandel C. (2016) Overview of the mechanisms that could explain the ‘Boundary Exchange’ at the land–ocean contact.  Trans. R. Soc. A374, 20150287.
  • Homoky, W.B., Conway, T.M., John, S.G., Mills, R.A., (2013) Distinct iron isotope signatures and supply from marine sediment dissolution. Nature Communications 4:2143.
  • Homoky W.B., Weber, T., Berelson, W.M., Conway, T.M., Henderson, G.M.,Jeandel, C., Severmann, S.,Tagliabue, A. (2016) Quantifying trace element and isotope fluxes at the ocean–sediment boundary: a review. Trans. of the Royal Society A 374: 20160246.
  • Moore, C. M.et al.Processes and patterns of oceanic nutrient limitation. Nature Geoscience6, 701-710, doi:10.1038/ngeo1765 (2013).