The Stable Isotope and Minor Element Geochemistry of Iron- Rich Chemical Sediments

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Chemical sediments dominated by iron(III) oxides are a manifestation of the unique combination of abundant free oxygen and liquid water found in the surface and many near-surface environments of Earth. Research performed in this laboratory has shown that the stable isotope ratios of oxygen, hydrogen and carbon in certain iron(III) oxides contain information on ancient temperatures, sources of water and organic contributions to aqueous carbonate systems associated with iron(III) oxide precipitation. In addition trapped CO2 abundance data from geothites may be related to ambient CO2 partial pressures at the time of mineral formation. The oxygen isotope systematics of ubiquitous phosphate-iron(III) oxide pairs will be studied to determine if they contain information on paleotemperatures. Knowledge obtained from this and our previous studies of iron(III) oxides will be applied to isotopic, chemical and attendant petrographic investigations of Phanerozoic oolitic ironstones, recent bacterially mediated iron(III) oxide precipitate and Late Precambrian banded iron formation(BIF). Fe(III) carbonate abundance and carbon-13 variations in a Late Ordovician weathering surface could provide knowledge of ambient CO2 partial pressures and the role of organic matter in this ancient environment. Oxygen, hydrogen and carbon abundance and isotopic data from tropical oolitic ironstones that date from global "greenhouse" and "icehouse" intervals in the Phanerozoic will be obtained. The ancient tropical temperatures and sources of water will be compared for the deposits that represent these putative global climatic extremes. Oxygen isotopic measurements of a Late Precambrian quartz-hematite BIF should provide insight into the depositional environment of this deposit. Paleoclimatic data of the type to be obtained in this research are essential to an ultimate understanding of the possible link between global tectonics and the types of climatic regimes that could produce apparent tropical continental ice sheets in the Late Precambrian or polar ice sheets in the Late Ordovician.