Paleoclimatic Applications and Modern Process Studies of Pedogenic Siderite

Abstract

Pedogenic siderite is a carbonate mineral that forms in the reducing groundwaters of poorly drained soils and paleosols in zonal climatic belts with strongly positive precipitation-evaporation balances. Microcrystalline and spherulitic forms of siderite are commonly recognized in micromorphologic studies of hydromorphic paleosols. Ancient paleosol sphaerosiderites commonly occur with diameters in excess of 1 mm, while modern pedogenic siderite crystal dimensions in excess of 100 lm are rare. Pedogenic siderites have been widely reported from Late Paleozoic, Mesozoic, and Cenozoic paleosols. The carbon and oxygen isotopic compositions of pedogenic siderites have been widely used as proxies for the oxygen isotopic composition of paleoprecipitation for their respective paleosols. Modern process studies of historic pedogenic siderites are yielding a more refined understanding of the stable isotopic systematics of low-temperature siderite. These works will lead to a future change in usage of published siderite-water 18 O fractionation equations.

Paleosols as Indicators of Paleoenvironment and Paleoclimate

“…Sphaerosiderite is a finely crystalline, millimeter-scale spherulitic form of siderite that occurs in a broad spatial and temporal range of paleosols (Ludvigson et al 2013). Ludvigson et al (1998) demonstrated that sphaerosiderites within a paleosol profile typically record a small range of δ 18 O values (less than ∼2 ) and a large range of δ 13 C values (up to >30 ); δ 13 C variability is interpreted to represent the range of microbial metabolic pathways present in poorly drained soils, whereas uniform δ 18 O values are interpreted to represent soil or meteoric water δ 18 O values during crystallization.…”

“…Ludvigson et al (1998) demonstrated that sphaerosiderites within a paleosol profile typically record a small range of δ 18 O values (less than ∼2 ) and a large range of δ 13 C values (up to >30 ); δ 13 C variability is interpreted to represent the range of microbial metabolic pathways present in poorly drained soils, whereas uniform δ 18 O values are interpreted to represent soil or meteoric water δ 18 O values during crystallization. Over the past two decades, a series of mid-Cretaceous sphaerosiderite lines have been generated over paleolatitudes ranging from the paleoequator to ∼70 • N (Ludvigson et al 2013), the results of which suggest a more pronounced greenhouse hydrological cycle (Suarez et al 2011b) than predicted by general circulation models (Poulsen et al 2007). …”

Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

“…The other form of rhodochrosite cement, microspherules, exhibits the same size and development as spherosiderites that form in humid continental settings within water‐logged soils, and are often associated with roots decomposing in phreatic environments (e.g. Ludvigson et al ., , ). The conditions in the Orava‐Nowy Targ basin were also characterized by a humid climate and water‐logged soils, as discussed above.…”

“…Both siderite and rhodochrosite exhibit a relatively small spread of δ 18 O values ( ca 1·5‰) which is probably related to temperature variations and, therefore, indicates formation during very shallow burial where seasonality is not buffered (cf. Ludvigson et al ., ). The δ 18 O values of rhodochrosite are slightly lower than those of siderite, suggesting a rather higher temperature of formation, probably at slightly shallower depth.…”

SIMS Bias on Isotope Ratios in Ca‐Mg‐Fe Carbonates (Part III): δ18O and δ13C Matrix Effects Along the Magnesite–Siderite Solid‐Solution Series

“…, , Gautier , Postma , Mozley , b, Curtis ); (b) as siderite nodules in wetland soils and sediments of the globe's humid climatic belts (Ludvigson et al . , , Ufnar et al . , Sheldon and Tabor , Tabor and Myers ); (c) as cements in sandstones and mudstones (Macquaker et al .…”