Variations from dry to aquic conditions in Vertisols (Esplugafreda Formation, Eastern Pyrenees, Spain): Implications for late Paleocene climate change

Sedimentological studies of ancient fluvial systems commonly do not consider in detail palaeosols in floodplain deposits. This is the case of the upper Paleocene Esplugafreda Formation (Pyrenees, Spain), a 100–340 m thick alluvial succession, in which palaeosols represent more than 80% of its thickness. This unit closely predates the climate crisis of the Paleocene/Eocene transition, the PETM (Paleocene/Eocene Thermal Maximum). The purpose of this paper is to define the palaeoenvironmental conditions of formation of these palaeopedogenic conditions, and to establish possible changes during the late Paleocene directly prior to the onset of the PETM. This paper analyses the palaeosols in terms of variations in ‘aquic conditions’, concerning to water saturation, reduction and formation of redoximorphic features in soils.

The uppermost part of the Esplugafreda Formation consists of cumulative sandy mudstone palaeosols, laterally associated with sparse channel deposits. Two pedotypes were recognised: Pont d'Orrit and Areny. A mud content >60% (mainly montmorillonite), pedogenic slickensides, wedge-shaped aggregates, mukkara and blow structures indicate that both pedotypes belong to the Vertisol order. They reveal a climate characterised by strong seasonal moisture variations. The Pont d'Orrit pedotype, which dominates the lower portion of the succession, shows reddish brown horizons (Bss), calcareous nodules concentration (Bssk horizon) and scarce redoximorphic features, which indicate a semi-arid climate. The Areny pedotype, which prevails in the upper part, shows a yellow horizon (Bssg) containing goethite and abundant redoximorphic features, which reveal aquic conditions. There is no clear sedimentological evidence that river flooding became relatively more frequent in the upper part of the succession, suggesting that the aquic conditions were caused by an increase in precipitation. This study demonstrates that (i) changes in some pedogenic features within the Vertisols enable interpretation of palaeoenvironmental variations and (ii) this region experienced a changing climate in the late Paleocene, before the onset of the PETM.