Siderite transformation pathways in early diagenesis: Effects of phosphate, silicate, humic acid and sulfide

Abstract

Siderite (FeCO₃) is a widely distributed authigenic mineral found in marine and terrestrial sediments, playing a crucial role in benthic biogeochemical cycling and preserving key paleoenvironmental information. Despite its importance, the transformation pathways of siderite under complex sedimentary environments remain poorly understood. Here batch experiments were conducted to investigate the transformation of siderite (containing 10 mM Fe(II)) in the presence of phosphate (P/Fe ratios of 0.2, 0.5, and 1), sulfide (S/Fe ratios of 0.5, and 1), silicate (Si/Fe ratios of 0.5, and 1), and humic acid (C/Fe ratios of 0.5, and 1) over hours to 6 days under strictly anoxic conditions. Our results show that phosphate induced a rapid dissolution-reprecipitation process of siderite at pH 7.8, forming nanocrystalline vivianite first and gradually integrating into the lath-like structure, effectively sequestering phosphate. However, after transition into sulfidic conditions, vivianite rapidly converts into mackinawite, releasing nearly all immobilized phosphate, underscoring its transient role as a phosphate reservoir. Silicate and humic acid, though inert in directly transforming siderite under anoxic conditions, significantly influence sulfide-induced siderite transformation. Sulfide can rapidly transform siderite into FeS (e.g., mackinawite) while preserving the pseudomorph of siderite and generating a highly porous ultrastructure on the surface. The presence of silicate promotes mackinawite crystallization, while humic acid suppresses secondary iron mineral formation, resulting in smoother, less porous FeS structures. These findings highlight the dynamic interplay between siderite and environmental factors, offering new insights into phosphorus cycling mediated by siderite. Importantly, this study provides a refined understanding of the mineralogical and morphological evolution of siderite, enhancing the accuracy of applying this redox proxy for sedimentary history reconstructions, and informing biogeochemical models of the coupled Fe-P-S cycling in early diagenesis.