Estimating the Oxygen Isotopic Composition of Equatorial Precipitation During the Mid-Cretaceous

Abstract

Abstract Recent studies utilize the oxygen isotopic composition of pedogenic carbonates (siderite and calcite) to evaluate empirically the global hydrologic cycle during the Cretaceous. Pedogenic carbonates are used as a proxy for the isotopic composition of meteoric waters in order to estimate precipitation rates in the Aptian–Albian via a mass-balance model. These modeling studies have previously been limited to mid-latitude to high-latitude data, and thus a more extensive latitudinal data set is needed to produce a more globally constrained model. This study provides an equatorial (~ 2° S paleolatitude) data set from pedogenic sphaerosiderites of the Caballos Formation from the Upper Magdalena Valley, Colombia. In addition, macroscopic and microscopic morphological features of paleosols were used to identify ancient soil conditions. The gleyed appearance and presence of sphaerosiderites suggest that soils were dominated by water-saturated conditions, with occasional incursions of marine-influenced water resulting in pyrite precipitation. This observation constrains the sampling sites to paleoelevations near sea level. Paleosols are interpreted to have been Entisols, Inceptisols, and Ultisols similar to soils in coastal equatorial regions of modern Colombia. Relatively invariant δ18O values and more variable δ13C values produce vertical trends in δ13C vs. δ18O space called meteoric sphaerosiderite lines. The average oxygen isotopic composition for these trends is −4.41 ± 0.37‰ (VPDB). The average oxygen isotopic composition of meteoric waters precipitating the sphaerosiderites is estimated at −4.56 ± 0.38‰ (VSMOW). These are slightly more enriched than modern and modeled Cretaceous estimates of equatorial precipitation δ18O but are generally in agreement with expected oxygen isotopic compositions of precipitation for this paleolatitude.