Hydrogen and triple oxygen isotopic composition of the early Paleoproterozoic seawater as recorded by 2.43-2.41 Ga hydrothermally altered rocks from the Baltic Shield, Russia

The stable isotopic composition of seawater is controlled by the interplay of major geological processes such as tectonics, continental weathering and formation of continental ice, and is poorly constrained for the early Paleoproterozoic time. At 2.43-2.41 Ga the Earth's systems underwent rapid changes including the first emergence of free oxygen, a supercontinent break-up and a series of snowball Earth glaciations. To understand the influence of these changes on the stable isotopic composition of contemporaneous seawater, we use mineralogical, fluid inclusion, hydrogen, and triple oxygen isotopic analyses of submarine hydrothermally altered basalts from the Baltic Shield, Russia. Remarkably well-preserved mineralogical assemblages returned temperatures of isotopic equilibrium between 286 and 387 °C, which compares well to the homogenization temperatures measured for saline fluid inclusions hosted in vein quartz. The computed equilibrium fluids have average values δ¹⁸O = 0.9 ± 1.9 ‰ and δD = 6 ± 18 ‰ (mean ± σ) which correspond to the isotopic composition of Quaternary seawater. In addition, we present Δ¹⁷O values of modern hydrothermally altered oceanic crust as sampled by the ODP Hole 504B, eastern Pacific Ocean. The similarity between triple oxygen isotope analyses of the ancient and modern submarine hydrothermally altered rocks indicates that the 2.43-2.41 Ga seawater had Δ¹⁷O value close to that of modern-day seawater. Despite the drastic environmental changes at 2.43-2.41 Ga such as large fluctuations in fluxes from continental weathering and in the amount of ice, the δ¹⁸O, Δ¹⁷O, and δD values of the seawater were not drastically different from that of modern values reflecting dominant role of submarine hydrothermal alteration in the stable isotopic budget of seawater throughout the Earth's history. Together with our studies of 2.44-2.41 Ga ultra-low δ¹⁸O rocks from the Baltic Shield, these results confirm presence of active hydrologic cycle during snowball Earth glaciations and the possibility to attain isotopically depleted precipitation at low latitudes comparable to modern high-latitude glacial ice.