What are TOC:TS ratios and the degree of pyritisation?
TOC:TS refers to the ratio of total organic carbon (TOC) percentages to total sulfur (TS) percentages in sediment. The degree of pyritisation (DOP) is the ratio %pyrite iron/(%pyrite iron + %reactive iron) in sediment .
Significance of sediment TOC:TS ratios and degree of pyritisation Values
The use of TOC:TS and DOP as environmental indicators arises from the process of biological sulfate reduction. Under anoxic conditions dissolved sulfate is reduced to hydrogen sulfide gas (H2S), which reacts with iron minerals to form iron sulfides. Iron monosulfides (FeS) form first, but are typically unstable, and are usually converted to pyrite (FeS2). Sediment TS comprises iron monsulfides and potentially a very small amount of elemental sulfur (So). TOC:TS ratios and DOP reflect the importance of sulfide reduction in the decomposition of organic matter, and thus give a qualitative indication of the redox status of the environment of deposition (Figure 1).
Figure 1. As dissolved oxygen status decreases, TS concentrations (S) in sediment increase and TOC contents (C) decrease
What causes TOC:TS ratios and degree of pyritisation values to change?
The amount of iron sulfide that can form and preserve in sediment is controlled by:
- sediment redox status — anoxic conditions are required for biological sulfate reduction (and pyrite formation) to occur;
- the availability of dissolved sulfate (SO42-) — this factor should not be limiting in the coastal zone because sulfate concentrations are usually very high in brackish waters;
- the loading of reactive iron minerals per unit surface area of sediment  – this factor is unlikely to be limiting in coastal waterways subject to fluvial input of terrigenous iron; and
- the amount of reactive organic matter — this is the most important control on the amount of TS that can form in normal marine sediments, and gives rise to a generally positive correlation between sediment TOC and TS (Figure 2) .
The above factors can be modified by:
- Bioturbation – mixing of sediments by benthic invertebrates continually adds dissolved oxygen, causing the oxidation of pyrite.
- Sedimentation Rates — when sedimentation rates are low, more iron sulfides can form because organic matter is exposed to dissolved oxygen in the water column for longer periods of time. However, if the water column is anoxic and contains H2S (e.g. under euxinic conditions) pyrite formation is maximised because H2S production is not hindered by reoxidation and because more time is available for iron minerals to react with H2S .
- Silicate Iron Reactivity – the reactivity of iron is influenced by the degree of crystallinity, the mineral assemblage and the grain size .
Figure 2. Carbon vs sulfur concentrations for some coastal waterways from around Australia (data extracted from the OzEstuaries database). Note that most sediments have TOC:TS ratios near 5.
Considerations for measurement and interpretation
Sediment organic carbon is best measured by high temperature oxidation methods (e.g. CHN analyser) . Appropriate standard reference materials should be analysed to check recovery. The loss on ignition (LOI) method for the determination of sediment TOC is not recommended because it can considerably overestimate TOC concentrations . Sediment sulfur (TS) can be estimated by X-ray fluorescence spectrometry.
Degree of pyritisation
Some different methods to determine pyritic sulfur are listed in Raiswell and others . Reactive iron is best measured by the 1 N HCl, 24-hour procedure of Leventhal and Taylor .
- Aerobic marine sediments typically have DOP values
- Marine sediments undergoing sulfate reduction under euxinic/inhospitable bottom conditions (e.g. anoxic bottom waters with high H2S concentrations) typically have TOC:TS ratios lower than 1.5 [1,5] and DOP in the range from 0.55 – 0.93 .
- Marine sediments undergoing sulfate reduction below an oxygenated water column typically have TOC:TS ratios in the range from 1.5 to 5.0 [1,3]. DOP values in the range from 0.46 – 0.80 correspond to poorly laminated sediments with sparse bioturbation , and probably to the same conditions.
*The DOP method should be used in preference to TOC:TS ratios if iron limitation is suspected, or when TS concentrations are low .
Existing information and data
Various data sets may be found in state agencies, Commonwealth institutions and universities. Some TOC and TS data was compiled during the National Land and Water Resources Audit  and is presented in summary form (e.g. medians and 25th and 75th percentiles) in the OzEstuaries database.
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- CSIRO Huon Estuary Study Team. 2000. Huon Estuary Study: Environmental Research for Integrated Catchment Management and Aquaculture. Project No. 96/284, Final Report to the Fisheries Research and Development Corporation, p. 285.
- Heap, A., Bryce, S., Ryan, D., Radke, L., Smith, C., Smith, R., Harris, P. and Heggie, D. 2001. Australian estuaries and coastal waterways: A geoscience perspective for improved and integrated resource management. A report to Theme 7 of the National Land and Water Resources Audit. AGSO Record 2001/07, p. 118.
- Raiswell, R., Buckley, F., Berner, R.A., and Anderson, T.F. 1987. Degree of pyritization of iron as a palaeoenvironmental indicator of bottom-water oxygenation. Journal of Sedimentary Petrology 58(5) 812-819.
- Haese, R.R. 2000. The reactivity of iron. In Schultz, H.D. and Zabel, M. Marine Geochemistry, Springer-Verlag Berlin Heidelberg, pp. 455
- Leventhal, J. and Taylor, C. 1990. Comparison of methods to determine degree of pyritzation. Geochimica et Cosmochimica Acta 54, 2621-2625.
David Heggie, Geoscience Australia
Graham Skyring, Skyring Environment Enterprises