3.1 Peat bogs

Peat is a classic model for Quaternary pollen analysis (Birks and Birks 1980). However, peat bogs are rare in the Mediterranean region of Iberia, and not particularly numerous in its wetter, Eurosiberian, region (Turner and Hannon 1988; Peñalba 1994; Ramil-Rego et al. 1998; González-Sampériz et al. 2005). Long pollen sequences (e.g. more than 10m) obtained from Iberian peat deposits like Padul in Andalucía (Florschütz et al. 1971; Pons and Reille 1988), Quintanar de la Sierra in the northern Meseta (Peñalba et al. 1997), El Portalet in the Pyrenees (González-Sampériz et al. 2006), and Area Longa in Galicia (Gómez-Orellana et al. 2007) are exceptional.

Given that peats may occur in different wetlands (e.g. peat bogs, fens, swamps, marshes) resulting from a complexity of geomorphological and sedimentary situations, sediments obtained after coring peat deposits are not always peaty throughout. Inorganic layers within mires are problematic for pollen analysis (Moore 1986; Barber and Charman 2003). Several of the records considered here show pollen in sediments formed under continuous sedimentation processes in marshy and shallow lacustrine environments. However, they also show palynologically sterile levels in fluvial and marine depositional environments (Table 3). This could be the situation with the Torreblanca peat bog (Dupré et al. 1994), but is demonstrated most clearly with Navarrés (Valencia), a tectonic, flat-bottom valley from which only the uppermost 250cm of a 25m core were polliniferous (Carrión and Dupré 1996; Carrión and van Geel 1999) (Table 2). In particular, this is the section corresponding to the accumulation of peat in a waterlogged context (Dupré et al. 1998a). The rest of the Quaternary sequence, starting at c. 178,000 years BP, is dominated by high-energy fluvial facies. Samples between 166 and 145cm depth were also palynologically sterile, a hiatus corresponding with the Last Glacial Maximum, during which conditions were not favourable for biotic preservation over large areas of the basin (Carrión and van Geel 1999). A former study including two cores, taken 5m apart in another part of the basin, had reported pollen only in the uppermost 180cm (Carrión and Dupré 1996), and the prevailing minerogenic sediments of a Neolithic settlement site in the vicinity were poor in pollen, and totally sterile in sandy sediments (Dupré et al. 1985). A similar pattern of pollen occurrence is described in a pioneer study by Menéndez-Amor and Florschütz (1961).

In the Arroyo de los Monjes (La Rioja), a 90cm-depth peaty sand core was fully sterile (Table 3). The 8000-year pollen record from Comella peat bog (Asturias, near Covadonga lakes) was produced from the uppermost 5.7m of peaty sediment, while the underlying detritic, sandy silt section did not contain any pollen (Ruíz-Zapata et al. 2002). In the Alpiarça peat bog (Portugal), clayey levels were sterile (van Leeuwaarden and Janssen 1985). In several limnic deposits from Galicia, peats occurring between thick detritic, sterile layers were the only sediments successfully analysed (Ramil-Rego and Gómez-Orellana 1996).

The case of Cañada del Gitano in the Sierra de Baza (Granada) could also be related to the abundance of detritic materials in parts of the peat bog. Two sediment cores were collected from the head of this deposit, and coring stopped at 417 and 378cm on reaching bedrock (Carrión et al. 2007b). While the 378cm-deep core was fully polliniferous, the longer 417cm-deep core was discarded because of its poor pollen content, with total sterility in several layers. The polliniferous core was mainly peat and silty peat, while the sterile one consisted of clastic silt.

Alteration of the original sedimentary structure may lead to sterility in peats. This is perhaps what happened to one of the two cores from Villaverde (Albacete), a tufaceous peat deposit overlying a calcreted conglomerate bedrock (Table 3). A sediment core of 550cm depth obtained from the eastern part of the fan was useful for pollen analysis (Carrión et al. 2001b). However, another one of c. 490cm depth obtained in the northern area was almost completely sterile, showing signs of corrosion in the few pollen grains and spores observed. In this case, the sedimentary context, a detrital marl interbedded with peats and sapropels, was identical for both cores (Carrión et al. 2001b). An earlier study by Taylor et al. (1998) based on a 600cm-core from the western part of the fan had already pointed to the abundance of inorganic matter and the investigators complained about poor pollen preservation. After reviewing unpublished reports provided by the landowner, we observed that, over the preceding years, the northern and western parts of the basin had been subject to trench excavation for a peat exploitation project requiring drainage.


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