Sequential growth of early diagenetic freshwater siderites in the Holocene coastal deposits, Korea

Abstract

Spherulitic siderite grains ranging from 100 to 250 μm in diameter occur abundantly in a sediment layer, which is almost 8600 years old, in Namyang Bay, on the west coast of Korea. These large siderite grains consist predominantly of FeCO 3 (average 65%) and MnCO 3 (average 22%) and have low Mg/Ca and Mg/Fe ratios. Compositional scatter plots, together with the high Mn and low Mg contents, indicate that meteoric pore water was mainly involved in the siderite precipitation. Examination of the siderite grains with an electron microprobe reveals a systematic, micron-scale, center-to-margin compositional variation within individual grains. Three zones are recognized on the basis of the compositional variations: Zone I (center) in which Mn decreases and Fe increases, Zone II (intermediate) in which Mn increases and Fe decreases, and Zone III (outer) in which both Mn and Fe decrease and Mg increases. This zoning is interpreted as a result of depth-related early diagenetic reactions with different Fe, Mn, and Mg concentrations in the sediment pore waters. Meanwhile, compositional mapping of spherulitic grains implies that a siderite grain consists of numerous tiny siderite precipitates. Such dust-like tiny siderite precipitates appear to be adhered to the large spherulitic siderite grain. In some cases, large siderite grains are adjoined together and occurred as a siderite aggregate. We suggest that a spherulitic siderite grain is growing sequentially by adhering of numerous tiny siderite precipitates formed in each diagenetic zone onto the siderite grain like the formation of a snowball.