Multiple sulphur isotope records tracking basinal and global processes in the 1.98 Ga Zaonega Formation, NW Russia

Abstract

The exceptionally organic-rich rocks of the 1.98 Ga Zaonega Formation deposited in the Onega Basin, NW Russia, have refined our understanding of Earth System evolution during the Paleoproterozoic rise in atmospheric oxygen. These rocks were formed in vent-or seepinfluenced settings contemporaneous with voluminous mafic volcanism and contain strongly 13 C-depleted organic matter. Here we report new isotopic (δ 34 S, Δ 33 S, Δ 36 S, δ 13 Corg) and mineralogical, major element, total sulphur and organic carbon data for the upper part of the Zaonega Formation, which was deposited shortly after the termination of the Lomagundi-Jatuli positive carbon isotope excursion. The data were collected on a recently obtained 102 m drillcore section and show a δ 13 Corg shift from-38‰ to-25‰. Sedimentary sulphides have δ 34 S values typically between +15‰ and +25‰ reflecting closed-system sulphur isotope behaviour

Hydrothermal dedolomitisation of carbonate rocks of the Paleoproterozoic Zaonega Formation, NW Russia — Implications for the preservation of primary C isotope signals

“…Based on Fe speciation and Mo, U and Se isotopes, dominantly euxinic water column conditions have been inferred (Asael et al, 2013(Asael et al, , 2018Kipp et al, 2017;Scott et al, 2014). In contrast, Re concentrations imply oxic conditions (Sheen et al, 2018) and the multiple S isotope data of Paiste et al (2018) indicate that euxinia was not pervasive and that redox conditions in the basin may indeed have been episodically oxic.…”

“…However, those rocks have experienced complicated diagenetic and metamorphic overprinting and obtaining geochemical proxies that preserve original (i.e. depositional) signals requires careful, detailed assessment of the origin of the minerals hosting those proxies (e.g., Črne et al, 2014;Paiste et al, 2018). Here we provide new high-resolution C,…”

Iron Isotopes Reveal a Benthic Iron Shuttle in the Palaeoproterozoic Zaonega Formation: Basinal Restriction, Euxinia, and the Effect on Global Palaeoredox Proxies

“…Intermixed with these positive shifts in atmospheric O 2 fugacity is a retrograde redox shift-an "O 2 crash"-that follows the Lomagundi Event and the O 2 overshoot [9,10] (though see Ohmoto et al [11] for a contradictory view). The timing, extent, and environmental significance of this O 2 crash are contentious, largely due to conflicting interpretations based on different geochemical redox proxies [9,10,[12][13][14][15][16][17]. Resolving the redox dynamics of the post-Lomagundi time period requires new geochemical proxy data from the sedimentary rock record to be placed into a framework that takes into account both global and local elemental cycling [18].…”

“…Many of the interpretations in those previous studies rely on the assumption that the Zaonega Formation faithfully records chemical signals from the global ocean, so that inferences from these rocks may be extrapolated to assess global conditions. However, this assumption is being increasingly challenged on the basis that the Zaonega Formation was influenced by significant syn-depositional magmatic activity resulting in hydrothermal circulation, hydrocarbon generation and seepage, basinal reconfiguration and restriction, as well as local modulation of both the sulphur and carbon cycles [12,15,16,20,21,28,29].…”

“…Yet, recently published interpretations of the Zaonega Formation do not agree in this regard. On one hand, sulphur isotope modeling results [15,16] and persistent positive europium anomalies [12,21] seem to suggest a basin with poor access to the open ocean, but where anoxia and sulphidic conditions were restricted mostly to pore waters. On the other hand, selenium isotopes and trace metal abundances in the Zaonega Formation seem to require more anoxic and less restricted conditions [13,17,23,24].…”