The Mobility Of Biologically Critical Transition Metals Following Simulated Iron Formation Diagenesis: Zinc and Nickel

English Abstract:

Banded Iron Formations (BIF) are iron- and silica-rich chemical precipitates deposited during the Precambrian. They have proven to be a key paleoenvironmental proxy, particularly with regards to the availability of biologically critical elements, such as zinc (Zn) and nickel (Ni), which play key roles in eukaryotic and prokaryotic metalloenzymes, respectively. Recent investigations into the BIF record indicate a stable paleomarine reservoir for Zn, while for Ni a dramatic decline is indicated immediately prior to the great oxidation event. However, what remains cryptic regarding the BIF record is the possible influences of post-depositional effects. Here, we examine the mobility of Zn and Ni from ferric oxyhydroxides, in the absence and presence of organic matter, during simulated pressure-temperature diagenesis (1.2 kbar and 170ºC). Following a two-week diagenetic treatment, that captures the critical mineral transformation from ferrihydrite to hematite, both metals were strongly retained, regardless of the presence of organic matter. Indeed, more than 90% of both metals were retained following diagenesis; hematite was the predominant iron mineral in the non-organic matter experiments, while siderite, magnetite and hematite formed in the carbon-bearing experiments. These results further support the use of BIF for examining the paleomarine trace elements concentrations, and that the authigenic records of Zn and Ni are faithfully recorded in well preserved BIF despite post-depositional alteration.