RECONCILING THE STABLE ISOTOPIC PALEOHYDROLOGY OF PEDOGENIC SIDERITES AND CALCITES

Pedogenic carbonates occur in zonal belts controlled by regional precipitation-evaporation balances. Pedogenic siderites (FeCO₃) accumulate in saturated soil groundwaters in regions of precipitation excess, and form along the equatorial humid belt, and at latitudes greater than 30°. Pedogenic calcites (CaCO₃) accumulate as soluble salts in vadose soils influenced by evaporation, and form at latitudes between 10° to 30°, but also at other latitudes in orographic rain shadows. Accumulation in phreatic and vadose groundwaters dictates that comparisons of isotope paleohydrology between these proxies must account for differing processes during formation, and how they are encoded during early diagenesis. Paleohydrologic processes can be distinguished by distinct diagenetic trends in C & O isotope space that are unique to each pedogenic carbonate. Pedogenic siderites produce diagenetic trends of meteoric sphaerosiderite lines (MSLs; sensu Ludvigson et al., 1998, Geology 26) that integrate time-averaged soil mean annual temperature (MAT) and groundwater δ¹⁸O recording paleoprecipitation. Pedogenic calcites produce diagenetic trends of positive linear covariant trends (PLCTs) that integrate variable ¹⁸O enrichments through evaporation, and variable ¹³C enrichments through changing soil respiration rates and kinetic effects from CO₂ degassing. Direct comparisons of isotopic paleohydrology between pedogenic siderites and calcites are complicated by the need for quantifying extents of vadose ¹⁸O enrichment in calcites. This problem is frequently overcome in ancient buried calcic paleosols, because of partial recrystallization of unstable CaCO₃ polymorphs, as ancient water tables ascended through the column with landscape aggradation. Phreatic overprinting of calcic paleosols produces diagenetic trends of meteoric calcite lines (MCLs; sensu Lohmann, 1988, Paleokarst) that integrate time-averaged MAT and δ¹⁸O of paleoprecipitation. Cretaceous paleoclimate studies show that MSL and MCL trends from 25-34 °N paleolatitude produce compatible groundwater δ¹⁸O estimates, but that PLCT trends in pedogenic calcites can produce ¹⁸O enrichments of up to 7 ‰ over that of coeval MCL trends. Failure to account for diverging MCL & PLCT trends can lead to erroneous paleoclimatic interpretations.