Spatiotemporal &#916;17O variability in the rock record
Authors: Warke Matthew , Pettigrew Ross , Millward David , Raine Robert , Clarke Stuart , Peng Yongbo , Bao Huiming , Claire Mark 
Published: March 2020

&lt;p&gt;The&amp;#160; &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O value of sedimentary sulfate can provide a direct, stable, geological archive of atmospheric-biospheric evolution. Negative &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values in gypsum/anhydrite are inherited from the negative &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O value of atmospheric O&lt;sub&gt;2&lt;/sub&gt; which is transferred to sulfate during sulfide weathering. The magnitude of the O&lt;sub&gt;2&lt;/sub&gt; &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O value reflects pCO&lt;sub&gt;2&lt;/sub&gt;, pO&lt;sub&gt;2&lt;/sub&gt; and gross primary productivity, hence modelling of the geological &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O record has led to estimates of changing atmospheric composition and primary productivity over Earth history. However, sulfate &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values represent a conservative estimate of atmospheric &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values as the magnitude of negative &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O 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.&lt;/p&gt;&lt;p&gt;Although this effect is acknowledged, the extent to which &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values may vary within and between palaeoenvironments, and how evaporite sedimentology may affect stratigraphic interpretations of &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values, remains unclear. We present the preliminary results of two case-studies probing the spatiotemporal variability of &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values.&lt;/p&gt;&lt;p&gt;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 d&lt;sup&gt;34&lt;/sup&gt;S vs &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O space with values ranging between &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S &amp;#8776; +15 &amp;#8240;, &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O &amp;#8776; -0.08 &amp;#8240; and &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S &amp;#8776; +24 &amp;#8240;, &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O &amp;#8776; -0.2 &amp;#8240;. We discuss whether this trend (and intraformational trends) represents a spatial variability in sulfate &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O as controlled by fluctuating fluvial and marine dominance in evaporite depositional environments, or whether this might represent a temporal change in &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S and &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O. &amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;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 &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O values as low as -270 per meg, however &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S 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 &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O signal.&lt;/p&gt;