FORMATION OF HISTORICAL PEDOGENIC SIDERITE IN PAH-CONTAMINATED ALLUVIAL CLAY SOILS, TENNESSEE, USA: PART I: INSIGHTS FROM FIELD RELATIONSHIPS AND MICROMORPHOLOGY

Siderite (FeCO₃) is a mineral that has become increasingly recognized in paleosol (fossil soil) deposits where it has been used to reconstruct paleotemperatures and paleolatitudinal temperature gradients, especially during the Cretaceous period; it has also been used to infer past reducing environments associated with persistent soil saturation. Unfortunately there is still little understanding of the physical, chemical, and biological conditions under which siderite precipitates in surface soils. Alluvial clay soil samples obtained from six boreholes advanced to depths of 400-450 cm (top of limestone bedrock) from the Chattanooga Coke Plant site, a former coal-gasification plant, were examined micromorphologically and geochemically in order to determine if pedogenic siderite was present and whether siderite occurrence was related to organic contaminant distribution. Samples from shallow depths were generally more heavily contaminated with polycyclic aromatic hydrocarbons (PAHs) than those at greater depth. The upper 1 m in most boreholes consisted of mixtures of anthropogenically remolded clay soil fill containing coal clinker and cinder grains, and limestone gravel; most layers of coarse fill were impregnated with creosote and coal tar. Most undisturbed soil (below 1 m depth) consisted of highly structured clays exhibiting fine subangular blocky ped structures, illuviated clay coatings on pore walls and ped surfaces, and Fe-Mn concretions. Pedogenic siderite was abundant in the upper 2 m of most cores and in demonstrably historical (< 100 year old) soil matrices, based on observations of siderite nucleation directly on the surfaces of coal slag and clinker grains. Two pedogenic morphologies were identified: (1) Sphaerosiderite crystals ranging from 10-200 μm in diameter, and (2) Coccoid bacterial siderite comprising 5-20 μm-diameter grape-like “clusters” of crystals. The sphaerosiderite form (1) is morphologically identical to that observed in paleosols preserved in the older rock record. The siderite, formed in both macropores and within fine-grained clay matrices, indicates development of localized anaerobic, low-Eh conditions, possibly due to microbial degradation of organic contaminants, which is examined further in a related talk presented by Ludvigson et al. in this session.