EGU2020-5752
https://doi.org/10.5194/egusphere-egu2020-5752
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Spatiotemporal Δ17O variability in the rock record

Matthew Warke1, Ross Pettigrew2, David Millward3, Robert Raine4, Stuart Clarke2, Yongbo Peng5, Huiming Bao5, and Mark Claire1
Matthew Warke et al.
  • 1School of Earth and Environmental Sciences, University of St Andrews, St Andrews, UK (mw438@st-andrews.ac.uk)
  • 2Basin Dynamics Research Group, Keele University, Keele, UK
  • 3British Geological Survey, The Lyell Centre, Edinburgh, UK
  • 4Geological Survey of Northern Ireland, Belfast, UK
  • 5Department of Geology and Geophysics, Louisiana State University, Baton Rouge, USA

The  Δ17O value of sedimentary sulfate can provide a direct, stable, geological archive of atmospheric-biospheric evolution. Negative Δ17O values in gypsum/anhydrite are inherited from the negative Δ17O value of atmospheric O2 which is transferred to sulfate during sulfide weathering. The magnitude of the O2 Δ17O value reflects pCO2, pO2 and gross primary productivity, hence modelling of the geological Δ17O record has led to estimates of changing atmospheric composition and primary productivity over Earth history. However, sulfate Δ17O values represent a conservative estimate of atmospheric Δ17O values as the magnitude of negative Δ17O in sulfate can be diluted (or erased) through sulfur cycling. As sulfate is transported away from the site of sulfide oxidation the likelihood of this happening increases.

Although this effect is acknowledged, the extent to which Δ17O values may vary within and between palaeoenvironments, and how evaporite sedimentology may affect stratigraphic interpretations of Δ17O values, remains unclear. We present the preliminary results of two case-studies probing the spatiotemporal variability of Δ17O values.

Case-study 1: temporally correlative Tournaisian (Lower Mississippian) evaporites within Carboniferous rift basins of Britain and Ireland were deposited in a range of settings: coastal wetland (Ballagan Fm.); supratidal sabkha on margin of a restricted basin (Ballycultra Fm.); and coastal sabkha on open ocean margin (Middleton Dale Anhydrite Fm.) All three settings plot on a positive slope in d34S vs Δ17O space with values ranging between δ34S ≈ +15 ‰, Δ17O ≈ -0.08 ‰ and δ34S ≈ +24 ‰, Δ17O ≈ -0.2 ‰. We discuss whether this trend (and intraformational trends) represents a spatial variability in sulfate Δ17O as controlled by fluctuating fluvial and marine dominance in evaporite depositional environments, or whether this might represent a temporal change in δ34S and Δ17O.   

Case study 2: non-marine evaporites of the early Permian Cedar Mesa Sandstone (CMS) Formation in Utah were deposited in continental saline pans in an erg-margin setting that fluctuated through arid and humid cycles. These evaporites record negative Δ17O values as low as -270 per meg, however δ34S values lie along the marine curve. We interpret the signal preserved in the CMS as recycling of the underlying marine evaporites of the late Carboniferous Paradox Formation which have been uplifted on the basin margin. Hence, we discuss how in non-marine settings the recycling of evaporites can decouple the age of the succession from the age of the atmospheric Δ17O signal.

How to cite: Warke, M., Pettigrew, R., Millward, D., Raine, R., Clarke, S., Peng, Y., Bao, H., and Claire, M.: Spatiotemporal Δ17O variability in the rock record, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5752, https://doi.org/10.5194/egusphere-egu2020-5752, 2020

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