Masterarbeit

Did nickel depletion contribute to the Great Oxidation Event? A lab study with synthetic and natural Fe and S minerals

Kai Jansen (09/2018-09/2019)

Betreuer: Britta Planer-Friedrich

The exact timing and reason for the Great Oxidation Event (GOE) remain subject of speculation to this day. Recent theories suggested a decline in nickel (Ni) concentrations in the Precambrian oceans leading to a methanogen famine that enabled the rise of atmospheric oxygen. To date, mainly ferrihydrite has been used to reconstruct element cycles in the ancient oceans. Recently, the metastable mineral phases Green Rust (GR) and zerovalent sulphur (S0) have been proposed to exert an influence on Ni availability. In order to assess the role of these minerals for Ni depletion and therefore the GOE, sorption experiments with natural GR-containing material (“red flocs”, collected at the Arvadi Spring, a modern model habitat for Precambrian ferro-euxinic transition zones), synthetic GR(CO32-), and natural S0 (“white flocs”, collected at the Arvadi Spring) were carried out at different dissolved silica (Si) concentrations. Furthermore, sorption to more stable phases was studied to allow a first estimation on how Ni would behave during a mineral transformation. Synthetic GR(CO32-) was very effective at removing Ni from solution, much more so than its oxidation products goethite and lepidocrocite. A comparison with ferrihydrite was not possible as the results had to be excluded due to the unexplainably low sorption. Red flocs were found not to represent GR(CO32-) since Ni removal was largely determined by other mineral components. White flocs sorbed substantially less Ni than its transformation products mackinawite and pyrite. The observed increased sorption in the presence of dissolved Si could be caused either by a shift in the point of zero charge due to specific adsorption of Si or by precipitation of Si-containing minerals. It is suggested that GR(CO32-) could have had a significant contribution to Ni immobilization and thus the GOE. Zerovalent sulphur, if present in the form investigated in this study, is assumed to have played a minor role in scavenging Ni. More Ni removal is expected during the transformation of S0 to mackinawite and pyrite. Further studies are necessary to confirm the high affinity of GR(CO32-) for Ni and to investigate the fate of Ni during the transformation of GR(CO32-) and S0.