Palaeoenvironmental variation of a sub-mountainous Holocene peat in North Spain based on biomarkers and FTIR proxies

The interest of peatlands as palaeoenvironmental records has increased in the recent years because they may represent a continuous record of vegetation growth with a limited number of species, peat profiles are in-land records close to populated areas and especially those fed only by rainfall (ombrotrophic peats), are not affected by external supplies (Barber et al., 2003). The North of Spain contains a number of peat bogs at littoral, submountainous and mountainous locations, in which increasing height also correspond to increasing distance to the coastline. Those at moderate height are known as cantabro-atlantic mires and characterized by the association Erico mackaianae-Sphagnetum papillosi (Fernandez Prieto et al., 1987). The Peat bog considered in this study is La Molina (UTM 29 T 716294 4806806, height 648 m above sea level=m.a.s.l), whose profile reached a depth of 295 cm. The profile consisted of ten top centimeters of living vegetation followed by brown-reddish bryophytic peat from 10-230 cm, turning darker in the deepest 40 cm. This part of the profile has a mineral matter content below 10%. The mineral matter content increases fast from 10 to 90 % in the following 40 cm interval and it is higher than 90% from the cm 270 downwards exhibiting an intermediate grey colour. The sample at 279 cm depth of the profile has yield an AMS 14 C date of 9384 ± 45 cal. yr BP (Arboleya, 2011). The biomarkers study by Gas Chromatography/Mass Spectrometry of the dichloromethane/methanol peat extracts has allowed the establishment of humid/dry periods on the assumption that herbs and heather dominate in dry periods with higher concentration of n-C29 and n-C27 and Sphagnum dominate in humid periods with higher concentration of n-C23 and n-C25 (Bass et al., 2000; Bingham et al., 2010). High values of n-C23/n-C29, associated to humid periods, have been recorded in two episodes in La Molina profile corresponding to 4200-4350 and 4500-5100 cal yr BP. These episodes have been also registered in other peats of the region at slightly different ages (Lopez-Dias et al., 2013). The combustion profiles of the Sphagnum-rich intervals indicate a dominance of the first combustion peak in the profiles (Borrego et al., 2011). This has been also observed during the humid episodes in La Molina profile. The Fourier Transformed Infrared (FTIR) spectra of peat provide an additional tool to find out about organic matter input considering that different peat-forming plants slightly differ in the relative intensity of their FTIR bands (Urbanczyk et al., 2012). The FTIR spectra exhibit bands at 2920 and 2850 cm -1 corresponding to stretching of antisymmetric and asymmetric CH2 groups in fats, wax and lipids; 1700 cm -1 band of carbonyl and carboxyl groups in carboxylic acids, aromatic esters and free acids; 1600 cm -1 band of aromatic C=C stretching; 1265 cm -1 band of C–O stretching of phenolic groups and arylmethylethers; 1080-1030 cm -1