Quantifying the contribution of biogenic iron sulfides to magnesite formation in the Basque Lakes, British Columbia, Canada

Abstract

Understanding terrestrial carbon cycling is important to develop carbon dioxide removal (CDR) technologies. Neutralization of carbonic acid (H₂CO₃) to make carbonate minerals provides a secure, long-term sink for CO₂ in natural and anthropogenic systems. In the sediment, dissimilarly sulfate reduction has the potential to generate alkalinity and induce biogenic precipitation of carbonate minerals, however, the proportion of Fe-sulfides to carbonate minerals is rarely quantified. Saline and hypersaline lakes containing carbonate minerals are excellent environments to examine closed system geochemical cycles of carbon, sulfur and iron. Here, we quantified the contribution of biogenic Fe-sulfide minerals and buried organic carbon to magnesite (MgCO₃) formation in the sediment of Basque Lake #2, a Mg-Na-SO₄ hypersaline lake near Ashcroft, British Columbia, Canada. While the overlying surface water contains over 2 M SO₄²⁻, the total solid iron sulfide content of the sediment was <0.4 wt%. The Basque Lake #2 sediment core had much more abundant carbonate minerals than sulfide minerals, suggesting the biogenic processes that form Fe-sulfides have a minimal role in magnesite formation (<1.0 wt%). Laboratory experiments were also conducted to study the relative influence of microbial sulfate and iron reduction on alkalinity generation and biogenic mineral formation with different organic carbon amendments. Filter-sterilized sulfate-rich waters (∼800 mM SO₄²⁻) from Basque Lake #1 were inoculated with anoxic Basque Lake sediment and microbial communities with or without the addition of ferrihydrite [Fe₁₀^IIIO₁₄(OH)₂]. Biogenic magnetite (Fe^III₂Fe^IIO₄) and mackinawite (Fe^IIS) formed when ferrihydrite was added to microbial experiments and siderite (Fe^IICO₃) precipitated in the lactate-amended experiment. The bulk analysis techniques used in these laboratory experiments suggest there is minimal to no additional magnesite precipitation. This study highlights that in closed systems, such as Basque Lake #2, the role of dissimilatory sulfate reduction in net carbonate precipitation will be limited by ferric iron input into lakes, which therefore inhibits biogenic Fe-sulfide formation, even with extremely high aqueous sulfate concentrations. Ultimately, it is unlikely that sulfate and iron reduction play important roles in Mg‑carbonate formation within Fe-limited environments.