Diagenetic overprinting of the sphaerosiderite palaeoclimate proxy: are records of pedogenic groundwater δ¹⁸O values preserved?

Abstract

Meteoric sphaerosiderite lines (MSLs), defined by invariant δ18O and variable δ13C values, are obtained from ancient wetland palaeosol sphaerosiderites (millimetre‐scale FeCO3 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% FeCO3) compositions. Diagenetically altered sphaerosiderites typically exhibit corroded margins, replacement textures and increased crystal lattice substitution of Ca2+, Mg2+ and Mn2+ for Fe2+. 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 δ18O and δ13C values are preserved. All three units contain poikilotopic calcite cements with significantly different δ18O and δ13C 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 δ18O vs. δ13C 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 δ18O and δ13C 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.

Paleoenvironmental implications of early diagenetic siderites of the Paraíba do Sul Deltaic Complex, eastern Brazil

“…Similar cases occurred in the Warham concretions (Norfolk, UK) (Pye et al, 1990;Coleman et al, 1993) and with synthesized laboratory siderites removed from microbial cultures and submitted to posterior exposition to oxidation conditions (Mortimer et al, 1997). Another product of these conditions is the hydration and oxidation of pyrite, forming jarosite (KFe 3 +3 (SO 4 ) 2 (OH) 6 ), as demonstrated in (Ufnar et al, 2004), with strong meteoric influence. Negative δ 13 C vpdb values (− 12.68, − 5.61, − 5.22, − 4.33‰) are typical for porewaters in which dissolved carbon is derived from the alteration of organic matter under suboxic conditions (McArthur et al, 1986).…”

Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model

“…Using the precipitation flux values calculated from the model coupled with the paleotemperature estimates (Spicer and Corfield, 1992;Wolfe and Upchurch, 1987), modern mean annual temperatures, and modern, zonally-averaged mean annual precipitation rates, estimates of paleoprecipitation rates can be calculated for the KWIB (Ufnar et al, , 2004b. mm/yr, with a best scenario value of 3600 mm/yr.…”

Giant calcite‐cemented concretions, Dakota Formation, central Kansas, USA

“…Schoewe (1937) gave a brief petrographic description of the concretions and mapped the distribution of the majority of them at Rock City. Previous reports of trace elemental and isotopic analyses in poikilotopic calcite (non‐concretionary) in Cretaceous sandstones (including the Dakota) are found in Ufnar et al. (2004).…”

“…(2004). Those calcites occur in sandstones that are reported to be essentially uncompacted (Ufnar et al. , 2004, p. 139).…”