Climatic and Vegetation Evolution during Early-Middle Holocene

In this paragraph I will provide a brief and general description of the changes occurred both on a climatic and vegetation level in the Central Pyrenees; however without entering in a detailed description of the different regions, considering the wideness and the environmental diversity of the studied area. Moreover, both climatic and vegetation evolution are extremely complex phenomena that could vary notably even on a small-scale, for example between the northern and southern slope of a same valley, or between two adjacent valleys. So, I will only describe some general trends for the analysed period, aware of the limits of such reconstruction.

At the broadest scale, Holocene is considered a period of relative climate stability, in contrast to the high variability observed during the last glacial period, occurred in the previous 60 ka yrs. (Fletcher et al. 2009). However, over the last decades, many paleoclimatic studies have highlighted the climate fluctuations of the last glacial interval, as well characterized by global abrupt changes, could events and climatic oscillations (Mayewski et al.

2004), that have been also evidenced in the study area (González-Sampériz et al. 2006).

The Early Holocene (around ca. 9500 to 5000 cal BC) is a period characterized by colder climatic conditions (González-Sampériz et al. 2006; Morellón et al. 2008; Pèlachs et al. 2011).

Precipitations were probably slightly higher in respect to the actual moment, at least at the end of the Younger Dryas; however a strong geographical variability between plain and mountain areas has been evidenced (Davis & Stevenson 2007; Morellón et al. 2008; Catalan et al. 2013). The vegetation cover of NE Spain during the Early Holocene also reflects humid conditions. In the Pyrenees and pre-Pyrenees, from about 9500 cal BC, vegetation passes from mountain steppe to a forested coverage (González-Sampériz et al. 2005; (Pèlachs et al. 2011; Catalán et al. 2013). Around 8000 cal BC the existence of arboreal mass is registered above the 2.200 m.a.s.l. (Miras et al. 2007; Pélachs et al. 2007; Cunill et al. 2013).

The lowlands of the Central Ebro Basin were probably characterized by Mediterranean forest formations (coniferous and evergreen Quercus), even if scarce steppe land proportions were probably present (González-Sampériz et al. 2005).

A first severe climate disruption has been worldwide registered around ca. 6300 to 5700 cal BC. The so-called ‘8.2 ka yr. cal BP event’, a short cooling moment of two or three centuries (Berger & Guilaine 2009), belongs to this interval. Such cold-event is considered producing a widespread decreasing of the arboreal masses, with a lowering of the treeline at

Introduction

higher altitudes and a period of increasing aridity at lower altitudes. In the Central Ebro valley a decrease in forest formation (Juniperus, Betula, Corylus, deciduous Quercus and other mesophytes) is detected in association with the 8200-yrs event, along with an increase in fire frequency (González-Sampériz et al. 2006; Davis & Stevenson 2007; González-Sampériz et al. 2008). After the 5600 cal BC, fire peaks have been also detected in the Axial Pyrenees, along with an increase in Poaceae. However, in the mountains the arboreal biomass remained at relatively high levels, suggesting that such fires consisted mainly in small opening of the forest (Miras et al. 2007; Cunill et al. 2013). Until the end of the Early Holocene, vegetation is mainly characterized by a deciduous forest in the valleys bottom and subalpine stages,

Fig.1.4. Hypothetical reconstruction of a vegetation transect between the Pyrenees and the Central Ebro Valley:

(i) before 20 ka yrs. BP, (ii) between 20-10 ka yrs. BP; (iii) around 10 ka yrs. BP and (iv) currently (present formations). Modified from González-Sampériz et al. (2005).

Introduction

while at alpine altitudes is gradually occupied by Pinus, with contributions of deciduous and evergreen Quercus together with Corylus and Betula (Catalan et al. 2013; Pérez-Obiol et al.

2012).

During Middle Holocene (ca. 5000 - 2000 cal BC), in most of the NE the Iberian Peninsula, a shift toward drier climate conditions and an increase in seasonality is detected (Pélachs et al. 2011; Catalán et al. 2013). In the Ebro Basin, sedimentary and palynological records shows the establishment of arid conditions, with a clear evidence of steppe herbs like Artemisia or Chenopodiaceae landscape, alongside a residual presence of coniferous forests.

In particular, an increasing aridity is observed in the Central Ebro Basin, in the Los Monegros area, where desiccation phenomena and the formation of the playa lakes are attested (Sancho et al. 2011). In the Pyrenean area, this phase corresponds to the establishment of an Abies forest at the subalpine stage, between 1.700 to 2.000 m.a.s.l., along with the appearance of Fagus at the same altitudes (from ca. 3200 cal BC) (Pélachs et al.

2007). Moreover, between ca. 4500-4000 cal BC are dated the first signs of anthropic impact over the landscape, in form of fires in the valleys bottom that probably correspond to clearances of forested areas (Galop et al. 2002; 2013). Throughout Middle Holocene Pinus percentages decrease and the increase of Poaceae becomes more important, along with an increasing of fire events in the mountainous ranges of both Axial Pyrenees and Pyrenees.

This landscape dynamic involves a general increase of human pressure on all Pyrenees areas (Galop 2006). From this moment onward, the human pressure over the environment will be constant, becoming the main factor in the shaping of the landscape.

Introduction

Fig.1.5. Geographical framework of the study-area. The red stars indicates the selected sites, from west to east:

1) Cueva de Chaves, 2) Espluga de la Puyascada, 3) Cova de Els Trocs; 4) Cova del Sardo. In green are indicated the other sites cited in the text, from west to east: 5) Cueva del Moro de Olvena, 6) Forcas II, 7) Cova Colomera, 8) Cova Gran, 9) Cova del Parco, 10) Bauma Margineda; In blue are indicated the rivers, from west to east: Ebro, Guatizalema, Alcanadre, Vero, Cinca, Ésera, Isábena, Noguera Ribagorçana, Noguera Pallaresa, Segre. In brown the main mountain ranges, from west to east: Sierra de Guara, Sierra Ferrera, Sierra del Castillo de Laguarres, Turbón, Sierra de Sis, Aneto, Besiberri, Sierra de San Gervàs, Sierra del Montsec. In black are indicated the states and provinces.

Guatizalema

Introduction