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 µm 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.

Abstract Two sections from Early Cretaceous lacustrine strata of the Xiagou Formation from the Changma Basin in Gansu Province, China, are correlated based on their carbon isotopic compositions of bulk sedimentary organic matter and carbonate, as well as carbonate oxygen-isotopic compositions. The samples were collected from fossiliferous strata, which contain well-preserved Cretaceous bird remains. The sections are primarily correlated based on a two-step increase in δ 13 C org with an overall magnitude of c. 12.5‰. The stratigraphic variations in carbon isotopes within the two lacustrine sections are correlated with global carbon isotope variations C3–C7 based on marine carbon isotope records. This correlation places the Xiagou lacustrine strata in this locality within the early Aptian Stage, specifically, the Selli Equivalent, which is associated with Ocean Anoxic Event 1a.