Spherulitic siderites in the Holocene coastal deposits of Korea (eastern Yellow Sea): elemental and isotopic composition and depositional environment

Siderite concretions are recovered from both freshwater (Unit II) and tidal (Unit I) deposits in the Holocene macrotidal flat of Kyunggi Bay, west coast of Korea. Siderites from both units show a spherulitic and well-rounded texture and well-developed equant rhombs on the external surfaces. The size of the concretions in Unit II (50–150 μm in diameter) is normally larger than that in Unit I (10–20 μm). The siderites in Unit II are chemically pure and highly enriched in Fe, whereas those of Unit I are relatively impure and distinctly zoned, demarcating high Mn (up to 69.1 mol%) in the core and high Fe (up to 75.9 mol%) in the margin. Within siderites of Unit I, substitution for Fe by Ca and Mg reaches up to 20.3 mol% (mean 15.1 mol%) and 16.1 mol% (mean 12.6 mol%), respectively. Bulk δ18O values (−0.7 to +1.3‰ PeeDee Belemnite (PDB)) for siderites in Unit I are relatively higher than those (−7.1 to +0.5‰ PDB) in Unit II. Regardless of units, siderite δ13C values (−21.3 to −11.2‰ PDB) are persistently low, indicating microbially-mediated precipitation at shallow burial depth. Extensive Mn, Ca and Mg substitution coupled with enrichment of 18O in the siderites of Unit I seems to reflect marine influence during the Holocene transgression. The occurrence of early diagenetic siderites provides supporting evidence on the depositional environment (marine vs. non-marine) of the Holocene muddy coastal deposits of Korea (eastern Yellow Sea).