Isotopic evidence for a sizeable seawater sulfate reservoir at 2.1 Ga

The 2.1 Ga Tulomozero Formation, deposited on the Fennoscandian Shield, consists of shallow marine carbonate and siliciclastic rocks, dissolution breccias and widespread syndepositional marine evaporites, now mostly present as pseudomorphs after Ca-sulfates. This study investigates sulfates hosted in three different proxies to constrain the sulfur isotopic composition of seawater sulfate, acknowledging the presence of different types of sulfates (carbonate-associated sulfate (CAS), breccia-associated sulfate (BAS) and sulfate relicts (RS)) in a variety of lithologies of the Tulomozero Formation. A widely used and accepted proxy for the ambient seawater sulfate sulfur isotopic composition is carbonate-associated sulfate which was extracted from carbonate samples across the entire Tulomozero Formation. In addition, sulfate was extracted from a siliciclastic dissolution breccia, which is suggested to have formed after dissolution of sulfate evaporites. The sulfate sulfur isotopic composition from both lithologies was measured ex-situ (δ34SCAS = 10.9 ± 2.7‰, δ34SBAS = 9.0 ± 1.1‰). Ubiquitous pseudomorphs after Ca-sulfates occur in the Tulomozero Formation and contain μm-size anhydrite and barite relicts. Their sulfur isotopic composition was measured in situ by secondary ion mass spectroscopy (SIMS) (δ34Sanhydrite = 9.6 ± 1.0‰; δ34Sbarite = 11.0 ± 3.1‰). Sulfur isotope data from carbonate rocks, breccias and the sulfate relicts match the existing δ34S record of seawater sulfate during that time and show no distinct stratigraphic variations, indicating that the seawater sulfate sulfur isotopic composition remained unaffected from environmental changes during the time of deposition of the Tulomozero Formation.Physical evidence for widespread precipitation of sulfate (rather than abundant chloride) suggests a modern-style evaporite sequence with carbonates followed by sulfate and then chlorides. However, this requires a minimum sulfate concentration of 2.5 mM in the ambient seawater. Consequently, the evidence for persistent evaporitic sulfate deposition in the Tulomozero Formation implies a substantial seawater sulfate reservoir at 2.1 Ga. According to the estimated minimum depositional time from the thickness of the carbonate rocks of the formation, the residence time of seawater sulfate is expected to have been at least 5 Ma. The available data support the existence of a sizeable seawater sulfate reservoir in the Mid-Paleoproterozoic ocean, in fact more than 10% of the modern sulfate reservoir.

► We measured sulfur isotopic compositions in Paleoproterozoic strata.
► SIMS has been applied to measure sulfur isotopic compositions in sulfate relicts.
► Sulfate relicts, and carbonate and breccia associated sulfate reveal a homogenous sulfur isotope signature across the entire sampled section.
► The residence of seawater sulfate is estimated to have been at least 5 Ma.
► The results of the study indicate the presence of a sizeable seawater sulfate reservoir at 2.1 Ga