PEDOGENIC SPHAEROSIDERITES AS TRACES OF ANCIENT SATURATED SOILS: NEW INSIGHTS ON CHANGES IN THE HYDROLOGIC CYCLE DURING EARTH HISTORY

Sphaerosiderites are mm-scale siderite spherulites that form in the reducing groundwaters of saturated soils, and are common in coal-bearing sedimentary successions. Micromorphologic studies show that they frequently are arrayed along fossil root traces, and their concentration in ancient soils often results from reduction of earlier pedogenic ferric oxide mottles during subsequent rises in the local base level and groundwater table. The carbon and oxygen isotopic compositions of well-preserved sphaerosiderites within a given paleosol can be used to reconstruct the isotopic paleohydrology of ancient terrestrial landscapes, and the oxygen isotopic ratios preserve a record of the composition of the d¹⁸O of local mean annual paleoprecipitation during soil formation. Comparisons between the paleolatitudinal gradients of d¹⁸O values of sphaerosiderites during different periods in Earth History have yielded important new insights on major changes in the global hydrologic cycle. Specifically, sphaerosiderite d¹⁸O values extracted from mid-Cretaceous paleosols in North America are isotopically lighter than their theoretical modern counterparts by 5.5 per mil at 34°N to 13.6 per mil at 75°N. Oxygen isotopic mass balance modeling indicates that this difference probably resulted from a significant increase in mid-latitude precipitation rates on the order of 400-500 %. Modeled precipitation and evaporation fluxes along the equator-to-pole transect, based on this empirical data from 34° to 75°N, also imply mid-Cretaceous intensification of aridity between 10° to 30°N paleolatitude. Empirical data from the tropics are needed to further test these model results.