Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater

Abstract

The evolution of the seawater oxygen isotopic composition (δ18O) through geological time remains controversial. Yet, the past δ18Oseawater is key to assess past seawater temperatures, providing insights into past climate change and life evolution. Here we provide a new and unprecedentedly precise δ18O value of −1.33 ± 0.98‰ for the Neoproterozoic bottom seawater supporting a constant oxygen isotope composition through time. We demonstrate that the Aït Ahmane ultramafic unit of the ca. 760 Ma Bou Azzer ophiolite (Morocco) host a fossil black smoker-type hydrothermal system. In this system we analyzed an untapped archive for the ocean oxygen isotopic composition consisting in pure magnetite veins directly precipitated from a Neoproterozoic seawater-derived fluid. Our results suggest that, while δ18Oseawater and submarine hydrothermal processes were likely similar to present day, Neoproterozoic oceans were 15–30 °C warmer on the eve of the Sturtian glaciation and the major life diversification that followed.

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

“…Our Ediacaran results from Oman are also in agreement with other clumped-isotope studies from the Phanerozoic that indicate no change in ''ice-free'' seawater composition from shallowly buried, well-preserved carbonates and apatites (Came et al 2007;Finnegan et al 2011;Bergmann et al 2018;Henkes et al 2018). Finally, such seawater d 18 O reconstructions agree with an independent reconstruction of Neoproterozoic seawater d 18 O value of -1.33% from fossil black smokertype abyssal hydrothermalism (Hodel et al 2018). While the results from one location will not conclusively solve the decades-long debate about Precambrian-early Phanerozoic climate and seawater composition (Knauth and Lowe 2003;Kasting et al 2006;Robert and Chaussidon 2006;Jaffrés et al 2007;Gaucher et al 2008;Veizer and Prokoph 2015), based on the above evidence from the Ediacaran Nafun Group of Oman we argue our data does not support the evolution of seawater d 18 O values over the last 600 million years to d 18 O values as depleted as -6% VSMOW in the Ediacaran accommodated by changes in hydrothermal cycling at midocean ridges (Kasting et al 2006;Jaffrés et al 2007;Veizer and Prokoph 2015).…”

Hydrothermal-sedimentary dolomite — a case from the Middle Permian in eastern Junggar Basin, China

“…Therefore, we used 0.82‰ as the minimum value of δ 18 O SMOW for the ore-forming fluid to calculate the lowest formation temperature of the dolomite in this study. The average δ 18 O SMOW value of modern black smoker fluids (0.91‰) compiled by Hodel et al (2018) supports the validity of this hypothetical value. Using this hypothetical δ 18 O SMOW value for the parent water of both the penecontemporaneous dolomite in the dolomitic rocks and dolomite in this study, their formation temperatures were estimated ( Table 9).…”

“…We adopted 0.82‰, the δ 18 O SMOW value of the water in the surface mud at the bottom of Qarham Salt Lake (Xiao 1995), which has an environment similar to that of the Permian Lucaogou palaeolake in the Jimusar Sag (Zhang et al 2018), as the hypothetical value for the palaeolake water. The palaeolake also received intermittent seawater input as it was an offshore lacustrine environment (Bian et al 2010;Kuang et al 2012;Yang et al 2019); therefore, the palaeolake water is expected to have had an oxygen isotopic composition similar to that of seawater, whose δ 18 O SMOW value has remained at~0‰ since the Neoproterozoic (Muehlenbachs 1998;Hodel et al 2018;Ryb and Eiler 2018). Thus, we suggest that 0.82‰ is a reasonable hypothetical δ 18 O SMOW value for the parent water of the penecontemporaneous dolomite for such an estimation.…”

Magnetic Properties of Ferritchromite and Cr‐Magnetite and Monitoring of Cr‐Spinels Alteration in Ultramafic and Mafic Rocks

“…However, the North Aït Ahmane serpentinites, which endured much lower temperatures (below 300°C, Hodel et al, 2017Hodel et al, , 2018, host highly altered spinels showing a wide range of ferritchromite and Cr-magnetite compositions (Figures 2g-2l and 3a). As presented in detail in Hodel et al (2017Hodel et al ( , 2018, serpentinites from the North Aït Ahmane unit were variously impacted by abyssal hydrothermalism leading to important Cr-spinel alteration. Interestingly, T c of Cr-magnetite measured in these serpentinites is highly correlated to the intensity of their hydrothermal signature as quantified by the (Eu/Eu*) N ratio (see Hodel et al, 2018) (Figure 12).…”

“…As presented in detail in Hodel et al (2017Hodel et al ( , 2018, serpentinites from the North Aït Ahmane unit were variously impacted by abyssal hydrothermalism leading to important Cr-spinel alteration. Interestingly, T c of Cr-magnetite measured in these serpentinites is highly correlated to the intensity of their hydrothermal signature as quantified by the (Eu/Eu*) N ratio (see Hodel et al, 2018) (Figure 12). Such correlation is likely to be due to enhanced Fe-Cr substitutions, promoted by warm (ca.…”