Abundant sulphate in the Neoproterozoic ocean: implications of constant δ ³⁴ S of barite in the Aberfeldy SEDEX deposits, Scottish Dalradian

Abstract

The timing and extent of ocean oxygenation is controversial. Proterozoic sulphur isotope datasets often show marked fluctuations over small stratigraphic intervals, suggesting that oceanic sulphate concentrations were much lower than modern values. A large accumulation of Neoproterozoic sulphate (.8 million tonnes preserved), as stratiform barite rock, is located in the Grampian Highlands near Aberfeldy. Diagenetic/metamorphic alteration has caused pronounced d 34 S variations near bed margins. This aside, barite throughout the deposits shows a narrow range in d 34 S, mean 36 + 1.5‰. We infer that this is representative of contemporaneous seawater sulphate, and that d 34 S seawater was constant during deposition of a stratigraphical thickness .250 m of mostly fine-grained clastic sediments. Uniformity of d 34 S seawater during barite precipitation, even in thick (.10 m) beds and with the co-occurrence of abundant sulphides incorporating bacteriogenically reduced sulphur, implies no limit to availability of seawater sulphate during hydrothermal exhalative events. Our data, combined with previous d 34 S research on Dalradian metasediments, suggest a stability, abundance and constancy of ocean sulphate in the Neoproterozoic. This contrasts with isotopic data using trace sulphate in limestones. It appears that, around the time of the Marinoan glaciation (c. 635 Ma), the ocean, although stratified at least locally, comprised a substantial reservoir of sulphate-bearing oxygenated seawater.

How the Neoproterozoic S-isotope record illuminates the genesis of vein gold systems: an example from the Dalradian Supergroup in Scotland

“…34 S for the Dalradian sequence with the global curve shows a good fit for correlation 1, the only outlying points being the mineralized horizons of the Ben Eagach Schist, but given that this is a SEDEX horizon with a hydrothermal component to the sulphides (Willan & Coleman 1983;Moles et al 2014), it would not be expected to fit the global curve. The good fit of the Bonahaven Dolomite data to correlation 1 may be fortuitous as this is interpreted as being a local signature owing to closed-system reduction of evaporites, not a global signal.…”

“…The extensive dataset of d 34 S sulphide for Dalradian metasedimentary rocks (Appin-Southern Highland Groups) records large variations through the sequence (Figs 4 & 9;Willan & Coleman 1983;Scott et al 1987Scott et al , 1991Hall et al 1988Hall et al , 1994aLowry 1991) (Willan & Coleman 1983;Hall et al 1991;Moles et al 2014).…”

“…2), Bonahaven Dolomite, Ben Eagach Schist and Ardrishaig Phyllite (d 34 S ¼ 215 to +42‰; Willan & Coleman 1983;Hall et al 1987Hall et al , 1994aLowry 1991;Moles et al 2014) Willan & Coleman 1983;Hall et al 1994b;Moles et al 2014) and are probably the result of closedsystem reduction of evaporite sulphate (Hall et al 1994b).…”

“…The Aberfeldy Ba-Zn -Pb SEDEX deposits, Perthshire, are hosted largely in the Ben Eagach Schist Formation (Willan & Coleman 1983;Moles 1985;Moles et al 2014 Fig. 4).…”

“…The Neoproterozoic Era is recognized as a crucial time in Earth history, incorporating two snowball Earth events (Hoffman et al 1998) coupled with large fluctuations in seawater C-, O-, Sr-and S-isotope ratios (Halverson et al 2010). The increasing understanding of Neoproterozoic events and global isotope variations (Halverson et al 2010;Halverson & Shields-Zhou 2011) within the evolving Earth has impacted on the understanding of the Dalradian sequence and is beginning to refine the possible chronostratigraphies, particularly through the use of stable isotope stratigraphy (Thomas et al 2004;Prave et al 2009;Moles et al 2014).…”