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

Abstract

Palaeosols of the Koluel‐Kaike Formation, a red colour‐banded, pyroclastic succession from southern Argentina, constitute a proxy for Eocene climate changes. Reticulated and vertically elongated ferric mottles, along with iron and manganese nodules are the most significant climate indicators, which originated by alternating cycles of waterlogging and drying conditions causing Fe‐Mn mobilization and fixation. Clay minerals vary from a kaolinite > smectite suite in the lower and middle sections, to a smectite > kaolinite one in the upper part. High concentrations of iron oxides/hydroxides and kaolinite, lack of exchangeable bases, absence of carbonate cement, presence of ironstone and redness of hue in most of the palaeosols suggest intense chemical weathering related to leaching and lateritization processes. Five pedotypes, ordered in a stratigraphic sense, were identified. Strongly developed, red to orange Chornk (Fragiaquult) and Kápenk (Plinthaquult) pedotypes display argillic horizons, abundant ferric nodules and slickensides; they are dominant in the lower and middle sections, and formed in seasonal humid and megathermic (tropical) conditions with a mean annual precipitation of 1200 to 1300 mm and a mean annual temperature of 15 °C. Weakly developed, less structured Ornek (Vitrand) and Pólnek (Placaquand) pedotypes occur in the middle and upper sections, and originated in sub‐humid and mesic‐megathermic conditions with a mean annual precipitation around 1000 mm and a mean annual temperature around 12 °C. The brownish Soorsh (Argialboll) pedotype exhibits a granular structure and is present at the uppermost part of the unit. It developed in sub‐humid/semi‐arid and mesic conditions, with a mean annual precipitation of 600 to 700 mm and a mean annual temperature around 10 °C. This pedotype succession and clay mineral distribution indicates a decrease in chemical weathering and degree of soil development with time. Koluel‐Kaike palaeosols from Central Patagonia are some of the first continental non‐palaeobiological data linked to the Early Palaeogene global warming in South America; they show an especially close relationship with the Early Eocene Climatic Optimum and the following long‐term cooling and drying initiated by Middle to Late Eocene time.

Quasi-static Eocene–Oligocene climate in Patagonia promotes slow faunal evolution and mid-Cenozoic global cooling

“…Although barren of fossils, paleosols of the Koluel-Kaike Formation provide useful constraints on precipitation (Krause et al, 2010). The age of the Koluel-Kaike Formation is loosely bracketed between ~42 Ma (the oldest overlying age from Gran Barranca; Ré et al, 2010a) and ~47 Ma (youngest detrital zircon ages from underlying sediments of the Las Flores…”

Inicio del enfriamiento global del eoceno medio y expansión de ambientes con vegetación abierta en la patagonia central.

“…Cooler conditions in NW Patagonia are clearly expressed by the Middle-Late Eocene expansion of Nothofagus (Melendi et al, 2003;Barreda and Palazzesi, 2007). This climate shift was not accompanied in central Patagonia by modifications in relief or sediment supply , Krause et al, 2010a. The first paleosol evidence of decreasing rainfall and temperature is preserved in the Koluel-Kaike Formation (Early-Middle Eocene), through the change from plinthite aquic Ultisols to vitric udic Andisols showing granular or near-mollic structure (Krause et al, 2010a).…”

“…This climate shift was not accompanied in central Patagonia by modifications in relief or sediment supply , Krause et al, 2010a. The first paleosol evidence of decreasing rainfall and temperature is preserved in the Koluel-Kaike Formation (Early-Middle Eocene), through the change from plinthite aquic Ultisols to vitric udic Andisols showing granular or near-mollic structure (Krause et al, 2010a). Mammal assemblage from this period ('Riochican' SALMA), mainly composed by the first time by notoungulates over marsupials, included the first high-crowned (protohypsodont) mixed-feeders, linked to initial grassland expansion (Ortiz Jaureguizar and Cladera, 2006).…”

“…These inferences are also supported by the phytolith assemblages from the upper section of the Koluel-Kaike Formation, where abundant palm and grass phytoliths were distinguished (A. Zucol, personal communication, 2012). Likewise, andic paleosols from this section showing granular peds, fine rhizoliths and cicada trace fossils are also compatible with dominant herbaceous communities (Krause et al, 2008(Krause et al, , 2010a.…”

“…4a). At southern Chubut localities (i.e., Gran Barranca, Cañadón Blanco) this boundary is transitional on the Koluel-Kaike Formation, which in turn lies upon the Las Flores Formation (Krause et al, 2010a, Raigemborn et al, 2010. This supports the interpreted unconformity between the Río Chico Group and the Sarmiento Formation at Cañadón Vaca.…”