Spatiotemporal Δ17O variability in the rock record

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.