The marine environments encompassing the Neoproterozoic glaciations: Evidence from C, Sr and Fe isotope ratios in the Hecla Hoek Supergroup in Svalbard

The Neoproterozoic Era records several important events in Earth history. The associations between BIF deposition, Snowball Earth events and the redox state of seawater are the key to explain the re-appearance of banded iron formations (BIF) during the Neoproterozoic. Unraveling ancient iron cycles provides important information about the linkage between the precipitation of BIF and the redox condition in the ocean, but current iron isotopic data are limited to sediments deposited only during the Sturtian glaciations and Ediacaran.We conducted a detailed geological survey of the Tonian to Ediacaran sedimentary succession in Nordaustlandet island, Svalbard. Our chemostratigraphies of δ¹³C_carb from pre-Sturtian to post-Marinoan sedimentary successions in the Polarisbreen Group are consistent with those in coeval sediments.We first analyzed iron isotope ratios (δ^56/54Fe) of individual pyrite grains with diverse shapes in carbonates, black shales, quartz arenites and diamictites in order to constrain the redox condition in seawater. We found large variations in iron isotope ratios; from ca. -2 to ca. +4‰. Positive δ^56/54Fe values, over +1.0‰, are only preserved in euhedral and aggregated pyrite grains in black shales of the Backlundtoppen Formation and the MacDonaldryggen Member of the Elbobreen Formation. A partial oxidation of ferrous iron in seawater is necessary to explain such high δ^56/54Fe values, which means that deep seawater was ferruginous (ferrous, iron-rich) before and after the Sturtian glaciation. This stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in Neoproterozoic seawater. This study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the Sturtian glaciation. If the deep ocean was intrinsically iron-rich, it is likely that Neoproterozoic BIFs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation.