Coastal sabkha dolomites and calcitised sulphates preserving the Lomagundi-Jatuli carbon isotope signal

Palaeoproterozoic Tulomozero Formation carbonates of the Onega basin were targeted by ICDP FAR-DEEP Hole 10B. The recovered rocks contain abundant evidence for Palaeoproterozoic gypsum and anhydrite, particularly in the form of pseudomorphs of inclusion-containing nodules, swallow-tail twinned crystals and evaporite dissolution breccias. Similarly, sand-patch fabric indicates the former presence of surface-covering salt crusts in the Palaeoproterozoic. Atmospheric oxygen and seawater sulphate levels at that time must have been sufficient to allow such sulphates to form. Carbonate δ13C values are considered sedimentary or early diagenetic, predating greenschist facies metamorphism associated with the Svecofennian Orogeny. These carbonate δ13C values range from +7.7 to +15.7‰, with the highest (and lowest) values found in dolostones. The dolostones as well as calcitised calcium sulphates record the Lomagundi-Jatuli signal. However evidence for activity of sulphate-reducing and/or methanogenic bacteria seems to be lacking. Dolomite precipitation (or dolomitisation), calcium sulphate calcitisation and production of the high 13C values (processes which typically involve such bacteria in the Phanerozoic) are thus inferred to have proceeded without the direct influence of sulphate reducers and methanogens. The most plausible explanation for the Lomagundi-Jatuli excursion seems to remain significant sequestration of organic carbon in a location yet to be identified.

► Tulomozero Formation Palaeoproterozoic sediments were drilled by ICDP FAR-DEEP Hole 10B in Fennoscandian Russia.
► Rocks include shallow marine dolostones, tidal bundles and calcitised sulphates formed in a sabkha-like setting.
► Carbonate δ13C ranges from +7.7 to +15.7‰ (VPDB).
► But there is no evidence that bacteria either calcitised the sulphates or produced the heavy δ13C values.
► These dolostones and calcitised sulphates are therefore interpreted as displaying the global marine Lomagundi-Jatuli event signal.