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Diagenetic overprinting of the sphaerosiderite palaeoclimate proxy: are records of pedogenic groundwater δ 18 O values preserved?
Author(s) -
Ufnar David F.,
González Luis A.,
Ludvigson Greg A.,
Brenner Robert L.,
Witzke Brian J.
Publication year - 2004
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1046/j.1365-3091.2003.00615.x
Subject(s) - calcite , geology , diagenesis , meteoric water , phreatic , paleosol , geochemistry , mineralogy , cretaceous , overprinting , geochronology , stable isotope ratio , pedogenesis , authigenic , carbonate , paleontology , aquifer , metamorphic rock , groundwater , chemistry , hydrothermal circulation , loess , soil science , soil water , physics , geotechnical engineering , organic chemistry , quantum mechanics
Meteoric sphaerosiderite lines (MSLs), defined by invariant δ 18 O and variable δ 13 C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre‐scale FeCO 3 nodules), and are a stable isotope proxy record of terrestrial meteoric isotopic compositions. The palaeoclimatic utility of sphaerosiderite has been well tested; however, diagenetically altered horizons that do not yield simple MSLs have been encountered. Well‐preserved sphaerosiderites typically exhibit smooth exteriors, spherulitic crystalline microstructures and relatively pure (> 95 mol% FeCO 3 ) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca 2+ , Mg 2+ and Mn 2+ for Fe 2+ . Examples of diagenetically altered Cretaceous sphaerosiderite‐bearing palaeosols from the Dakota Formation (Kansas), the Swan River Formation (Saskatchewan) and the Success S2 Formation (Saskatchewan) were examined in this study to determine the extent to which original, early diagenetic δ 18 O and δ 13 C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ 18 O and δ 13 C values from the co‐occurring sphaerosiderites. The complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses. The Dakota and Swan River samples ultimately yield distinct MSLs for the sphaerosiderites, and MCLs (meteoric calcite lines) for the calcite cements. The Success S2 sample yields a covariant δ 18 O vs. δ 13 C trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters. The calcite cements in the Success S2 Formation yield meteoric δ 18 O and δ 13 C values. A stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end‐member compositions. Modelled hyperbolic fluid mixing curves for the Success S2 Formation suggest precipitation from fluids that were < 25% sea water.