Lateritized tephric palaeosols from Central Patagonia Eocene, Argentina: a southern high-latitude archive of Palaeogene global greenhouse conditions.

KRAUSE, J.M.; BELLOSI, E; RAIGEMBORN, M.S.

SEDIMENTOLOGY

Wiley-Blackwell

Lugar: Oxford, UK; Año: 2009

0037-0746

Palaeosols of the Koluel-Kaike Formation, a red colour-banded pyroclastic succession from southern Argentina, constitute a proxy of Eocene climate changes. Reticulated and vertically elongated ferric mottles, along with ferric and manganic nodules are the most significant attributes, originated by alternate waterlogging-drying conditions and ionic reorganization. Clay minerals vary from a kaolinite > smectite suite in the lower and middle sections to a smectite > kaolinite one in the upper part. High concentration of iron oxides/hydroxides and kaolinite, lack of bases, absence of carbonate cement, presence of ironstone and redness of hue in most of palaeosols suggest an intense chemical weathering related to leaching and laterization processes. Five pedotypes, ordered in stratigraphic sense, were identified. Strongly-developed, red to orange Chornk (Fragiaquult) and Kápenk (Plinthaquult) pedotypes show argillic horizons, abundant ferric nodules and slickensides. They are dominant at the lower and middle sections, and formed in seasonal humid and megathermic (tropical) conditions with a MAP of 1200-1300 mm and a MAT of 15°C. Weakly-developed, less structured Ornek (Vitrand) and Pólnek (Placaquand) pedotypes occur at the middle and upper sections, and originated in subhumid and mesic-megathermic conditions with a MAP around 1000 mm and a MAT around 12°C. Brownish Soorsh (Argialboll) pedotype shows granular structure and is present at the uppermost part of the unit. It developed in subhumid-semiarid and mesic conditions, with a MAP of 600-700 mm and a MAT around 10°C. This pedotypes succession and clay mineral distribution indicates a decreasing of chemical weathering and development degree of the soils with time. Koluel-Kaike palaeosols from central Patagonia are one of the first continental nonpalaeobiologic data linked to the Early Palaeogene global warming in South America. They particularly show a close relation with the Early Eocene Climatic Optimum and the following long term cooling and drying initiated by the Middle-Late Eocene.