PALEOSOL-BASED PALEOCLIMATE RECONSTRUCTION OF LATE PALEOCENE THROUGH MIDDLE EOCENE ARGENTINA

In the Salta Basin, northwest Argentina, the Maiz Gordo Formation has long been known to contain the Paleocene-Eocene (PE) boundary although the boundary’s precise stratigraphic position has not been well constrained. Here we refine the position of the PE boundary in the basin using carbon isotope chemostratigraphy: an amalgamated-paleosols marker zone is distinct from the calcic aridisols above and below it and is widespread in the basin across at least 200 kilometers in well-exposed sections. The paleosol zone also corresponds to coeval lacustrine highstand facies in the middle of the basin. A comparison is made to similar, well studied, and in many cases well dated, sections throughout North America and parts of Europe, and on a detailed literature compilation that includes sections on all continents except Antarctica. We present a conceptual paleoclimatologically based model for the development of a geosol (widespread coeval paleosol): the late Paleocene-early Eocene represents the warmest event during the past at least 70 million years of Earth history during which time tropical soil-forming conditions expanded poleward in both hemispheres.

Results from field measurements of depth to and thickness of calcic horizons, average diameter of calcic concretions, and an estimate of stage of calcic zone development are described for strata above and below the boundary geosol. We applied transfer functions relating depth to and thickness of the calcic horizon to mean annual precipitation and mean annual range of precipitation (Retallack, Geology, 33 (4), 333-336) respectively, to quantify changes in Paleogene atmospheric hydrology in northwest Argentina. Our records indicate an overall drying and decrease in seasonality leading up to the PE boundary, when maximum warmth and precipitation occurred, followed by fluctuating dry to seasonally wetter conditions in the Eocene. The Eocene record is punctuated by much wetter conditions high in our section which we preliminarily hypothesize is a record of the Early Eocene Climatic Optimum. In a very general sense our records appear to correspond to similar records generated from western North America (Retallack, see above), although our data indicate that Paleogene northwest Argentina was subjected to overall and episodically wetter conditions.