The Elephant in the Room: Ocean Sulphate Control on the Marine Carbon CycleR.J.Newton@Leeds.ac.uk
Sulphate is the most important oxidant for organic matter in ocean sediments after dissolved oxygen. The continental shelves are the most significant location for marine carbon burial and sulphate can account for up to 80% of organic matter oxidation in these settings. Changes in the concentration of sulphate in the oceans are therefore likely to exert a profound effect on the cycling of carbon in ocean sediments. Specifically, ocean sulphate concentration plays a pivotal role in the ocean methane cycle: In the modern world high concentrations of sulphate cap methane emissions from sediments via the process of anaerobic methane oxidation. It’s known in a general way that ocean sulphate has been much lower at certain times in the past but these records only exist at low resolution, have large error bars, are sometimes poorly dated and different approaches produce different values. The reliance of oceanic methane emissions on sulphate concentration makes understanding past changes in marine sulphate a key goal of predicting atmospheric methane. The most direct method of estimating past ocean sulphate is from the chemistry of fluid trapped in halite crystals formed in evaporitic environments (fluid inclusions), but these records are sparse, and have large error bars in both concentration and dating (Figure 1; Holt et al., 2014; Horita et al., 2002). The latter is especially important as we are beginning to appreciate that ocean sulphate concentrations may have changed much more rapidly in the past than we previously thought (e.g. Wortmann and Paytan, 2012) with the potential to drive more rapid changes in oceanic methane emissions. The drivers for these rapid changes as well as their effects are also poorly understood. To address this knowledge gap, the student will explore two novel methods for deriving much higher resolution records of ocean sulphate concentrations from the beginning of the Jurassic to the present day based on the substitution of sulphate into phosphate and carbonate minerals. This time period is chosen because current records indicate it encompasses an approximately three-fold increase in ocean sulphate. In tandem, the student will use a various modelling approaches to explore the controls on the oceanic sulphur cycle and its effect on the marine carbon cycle and methane production. The records of sulphate concentration can then be used to ground truth the modelling efforts, the implications of which can then be explored across the geological timescale.
Figure 1. A compilation of Phanerozoic ocean concentrations of sulphate and calcium (left) derived from the chemistry of fluid inclusions. Note the scarcity of coverage. Black bars are new data from Carboniferous evaporite deposits. Error bars on ages are not shown but are on the order of a few million years. Primary fluid inclusions in primary halite shown to the right (both figures from Holt et al., 2014)
Francolite or hydroxyapatite are the main minerals in phosphorite deposits and phosphate nodules. Sulphate substitutes into the structure of these minerals in proportion to the concentration of sulphate in the solution from which they are formed (McArthur, 1985). Phosphate minerals can form in a variety of ways but often within the sediment in environments where pore-water sulphate is depleted by microbial sulphate reduction (MSR; Föllmi, 1996). This effect will be excluded by extracting and analysing the sulphate for its isotopic composition, which is strongly affected by MSR (Chambers and Trudinger, 1979; Piper and Kolodny, 1987). Additional information on the environment of formation will be derived from the isotopic composition of its structurally substituted carbonate (Benmore et al., 1983). Similar methods have only previously been applied to samples from the Cambrian (Hough et al., 2006) but pilot work by the lead supervisor has derived estimates for the Cretaceous that overlap those from fluid inclusion studies.
Experimental work with CASE partner GFZ will investigate the controls of sulphate incorporation into francolite under lab conditions to explore the interpretation of this record. This will involve a placement at GFZ to precipitate francolite in the laboratory under a range of conditions indicative of ancient seawater chemistry.
Figure 2. Left: Phosphorite S:P ratio plotted against the sulphur isotope composition of the sulphate substituted into the phosphorite lattice from the late Permian Phosphoria Formation (adapted from Piper and Kolodny, 1987). Where the sulphur isotope composition records seawater, the S:P ratio reflects the sulphate concentration of the ocean. Right: Scanning electron microscope image of the foraminifera Globigerinella siphonifera from the S.W. Indian Ocean.
Sulphate substituted into carbonate minerals is already widely employed as a proxy for the sulphur isotopic composition of ancient oceans, but the controls on the amount of sulphate contained in the mineral lattice are only understood from experimental work (e.g. Busenberg and Plummer, 1985). Recent work on cultured foraminifera suggest a clear relationship between S/Ca in the shell and the sulphate concentration of the water they were growing in (Paris et al., 2014). The student will explore the controls on the sulphate concentrations in foraminiferal calcite (e.g. Figure 2) first in the Holocene, then moving on to the rest of the Cenozoic. Factors to be assessed include species, temperature, growth rate, sulphate concentration and carbon chemistry.
Many of the samples necessary for the project are held in collections either in Leeds or with project partners. Additional sample collections will be made to improve the time resolution of the phosphorite records during fieldwork to Morocco and Bulgaria where phosphorite deposits are common in the Palaeogene, and Jurassic to Cretaceous respectively.
The student will integrate the results of the analytical phases of the project with other published information by developing a box model of marine sulphur cycle (Figure 3) and the sedimentary carbon cycle. The modelling will be based on well-established methods (see Garrels and Lerman, 1984) used to compute past seawater sulphate concentrations from variation in δ34S, and will draw on more recent approaches, which model fluxes of the major components of seawater under changing biogeochemical and tectonic processes (Arvidson et al., 2013). This will allow a quantitative appraisal of the various factors that may be influencing changes to the marine sulphur and carbon cycles on a range of time scales.
|Figure 3. A schematic representation of the global sulphur cycle illustrating the controls on ocean sulfate concentrations. The size of the solid boxes is proportional to the size of the reservoirs. Organic carbon is consumed during bacterial sulphate reduction (BSR). Only a small proportion of the sulphate that is reduced to sulphide goes on to be buried as pyrite. Dashed boxes and lines represent variable or poorly constrained reservoirs or fluxes. MOR – mid ocean ridge (Newton, unpublished)|
Aims and Objectives
The aims of the project will be to:
- Derive a high resolution record of sulphate concentrations across the last 200 million years
- Explore the controls on the preservation and interpretation of the new proxies
- Investigate the controls on marine sulphate concentrations and the implications for the sedimentary carbon cycle across the geological timescale.
This will be achieved by:
- Analysing a suite of phosphorite samples for their S:P ratio and the d34S of their structurally substituted sulphate. This will also entail SEM work and other techniques to constrain the presence of pyrite or other sulphur phases which may pose a risk of contamination.
- Undertaking experimental work to understand the controls of sulphate incorporation into phosphorite minerals at CASE partner Institution (GFZ, Germany).
- Undertaking fieldwork in Bulgaria and Morocco to supplement the pre-existing collections of phosphorite samples to improve the time resolution of the record. This work will exploit pre-existing links to help with local logistics and permissions.
- Selecting and analysing a suite of foraminiferal samples for their S:Ca ratio to test a range of possible controls on sulphate incorporation e.g. a range of species, cosmopolitan species from sites with a range of temperatures, etc. Possible diagenetic controls on the preservation of pristine S/Ca ratios will also be assessed using a range of standard techniques.
- Developing a box model for ocean sulphate concentrations and the marine sedimentary carbon cycle across the studied time interval constrained by the new information from 1) and 3) above and supplemented by previously published proxy records (e.g. marine sulphate isotope record).
Potential for high impact outcome
The project will address fundamental questions about the detailed chemical evolution of the oceans, one of the most important components of Earth’s biogeochemical cycles. It is also has the potential for significant impact on our understanding of the Earths atmospheric methane budget over time, a key component of the climate system. We therefore expect the project to result in a number of significant papers in the field with at least one or more being suitable for a high impact journal.
The student will be trained in the wet chemical techniques used to measure the element ratios of interest, isotope geochemistry, foraminiferal identification and box modelling. Techniques which will form an integral part of the project are stable isotope mass spectrometry, ion chromatography, ICP-MS, ICP-OES, SEM, and XRD. All of these techniques are available in the School. In addition, the student will gain expertise in experimental geochemical approaches via the link with Professor Liane Benning at the CASE partner institution, GFZ. The supervisory group is made up of expertise on geochemistry and stable isotopes (Dr Robert Newton), micropalaeontology (Dr Tracy Aze) numerical modelling (Dr Ben Mills) and experimental geochemistry (Professor Liane Benning) providing a high level of specialist training in all aspects of the project work. The student will become part of the Earth Surface Science Institute in the School and the Cohen Geochemistry and Palaeo@Leeds research groups. This organisational framework provides a broader supportive environment which allows the cross fertilisation of ideas and expertise. In addition to the bespoke high level training for the PhD the student will have access to a wide range of other training and support. Examples would include other useful scientific skills such as programming or statistics, transferable skills such as time management, writing and giving presentations, and skills specific to a PhD programmme such as managing your degree and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).
Busenberg, E., Plummer, L.N., 1985. Kinetic and thermodynamic factors controlling the distribution of SO42- and Na+ in calcites and selected aragonites. Geochimica et Cosmochimica Acta, 49(3): 713-725.
Horita, J., Zimmermann, H., Holland, H.D., 2002. Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites. Geochimica et Cosmochimica Acta, 66(21): 3733-3756.
Paris, G., Fehrenbacher, J.S., Sessions, A.L., Spero, H.J., Adkins, J.F., 2014. Experimental determination of carbonate-associated sulfate δ34S in planktonic foraminifera shells. Geochemistry, Geophysics, Geosystems, 15(4): 1452-1461.
Related undergraduate subjects:
- Earth science
- Earth system science
- Environmental science
- Geological science
- Natural sciences