Holocene environmental variability in the Central Ebro Basin (NE Spain) from geoarchaeological and pedological records

Abstract

Slope deposits, in semiarid regions, are known to be very sensitive environments, especially those that occurred during the minor fluctuations of the late Holocene. In this paper we analyse Holocene colluvium genesis, composition, and paleoenvironmental meaning through the study of slope deposits at Peña Enroque (Ebro Depression, NE Spain). Two cumulative slope stages are described during this period in NE Spain. Both slope accumulations are superimposed and this has enabled an excellent preservation of the aggregative sequence and the paleosols corresponding to stabilisation stages. 14 C and TL dating, as well as archaeological remains, provide considerable chronological precision for this sequence. The origin of the accumulation of the lower unit is placed around 4295-4083 cal yr BP/2346-2134 cal yr BC (late Chalcolithic) and it developed until the Iron Age in a cooler and wetter climate (Cold Iron Age). Under favourable conditions, a soil A-horizon was formed on top of this unit. After a long erosive stage (Warm Roman Period, Medieval Climatic Anomaly), a new slope accumulation was formed during the Little Ice Age. Within the slope two morphogenetic periods are distinguished, both ending with A-horizons. Both periods can be related with the two main cold-wet climatic events in NE Spain.

Pyrogenic organic matter from palaeo-fires during the Holocene: A case study in a sequence of buried soils at the Central Ebro Basin (NE Spain)

“…In this way, it is most likely that the charcoal macrofragments recovered from the soil come from the remains of woody vegetation (Gerlach et al, 2012;Kaal et al, 2008b;Nocentini et al, 2010). Thus, the higher proportions of charcoal > 4 mm found in the oldest sediment layers (E12-E16) are in agreement with the abundant burnt plant biomass, mainly Juniperus and Pinus, as indicated by the anthracological and palynological analysis (Pérez-Lambán et al, 2018). On the other hand, the smaller sizes of the charcoal fragments (1-2 and 2-4 mm) could be related to a more open forest with a higher presence of shrubs and herbs in Units 2, 3, and 4.…”

“…Thus, charcoal from E4 is much younger (0.53-0.39 ky cal BP) than charcoal recovered from E12 (7.0-6.7 ky cal BP), a C horizon from fluvic materials. In addition, the anthracological analysis in E4 reveals a vegetation dominated by Juniperus, Phillyrea/Rhamnus and Rosmarinus officinalis, very similar to the current vegetation in the area, whereas in E12, where charcoal fragments have been transported from Unit 1 (Pérez-Lambán et al, 2018), the vegetation is arboreal and dominated mainly by Juniperus with the presence of Pinus halepensis. E6 and E9 are also different regarding their age (ca.…”

“…Unit 1 is formed by a soil developed on gypsum and marls located at the base of the profile that includes layers E13 to E18. Although the oldest 14 C date come from E16 (8.3 ky cal BP), this unit probably began to form before 9.5 ky cal BP (Pérez-Lambán et al, 2018), representing a period of ca. 2.5 ky (from 9.5 to 7 ky cal BP).…”

“…This caused intense soil losses from the surrounding slopes, resulting in processes of accumulation and incision of the valleys (Constante et al, 2011). This was the case for La Poza Valley, due to the combination of an increase of human activity during the Neolithic and a climatic change towards conditions of increasing aridity, mainly in the formation of the sedimentary Unit 2 (Pérez-Lambán et al, 2018). This is consistent with the anthracological and palynological studies made in Central and Southern Europe that point to an intense deforestation of most of the continent around 6.0 ky cal BP by the Neolithic populations, probably due to slash-andburn practices, which caused intense erosive processes and an accumulation of charcoal in soils and sediments (Knicker, 2011).…”

“…Given the lithological and climatic characteristics of the CEB, the preservation of this palaeosoil is considered exceptional. This site was recently subjected to an interdisciplinary study that included geomorphology and pedology analyses, which were combined with an anthracological and palynological studies and radiocarbon dating (Pérez-Lambán et al, 2018). The main goal of that work was to interpret the palaeoenvironmental evolution of La Poza Valley during the Holocene.…”