Molecular carbon structure in a Meridional peat bog from Doñana National Park (SW-Spain).

Abstract

In less than 50 years the area occupied by peat bogs in the Gulf of Cádiz (SW-Spain) has been reduced drastically by more than 90% and the peat bog studied here may well be considered as relict and one of the last and more Meridional still in existence in the Northern hemisphere. The vertical molecular composition of a 85 cm peat bog from Ribetehilo lagoon (Doñana National Park; SW-Spain 37° 7'30.81"N; 6°37'50.19"O) is studied using analytical pyrolysis (Py-GC/MS) and solid state NMR (CP/MAS 13CNMR). The results obtained by the two techniques were similar and showed neat differences in the organic matter (OM) molecular structure with depth. In the shallow layer (0-10 cm) OM composition was dominated by carbohydrate a nd a conspicuous high relative amount of polyphenolic compounds were also detected in the pyrograms (c. 40% and 20% respectively). Down core the OM composition was found gradually dominated by lipids and unspecific aromatic compounds. These results suggest that while shallow OC reflects the molecular signature of a more or less fresh material OC in deeper bog amples underwent a severe chemical alteration resulting in a selective preservation of the most recalcitrant molecules lipids (polyesters) and aromatics both representing more than 80% of relative abundance of compounds in OM. This is probably the result of a high microbial activity in an anoxic environment supported by lower conductivity and basic pH values (~10 µScm-1 and ~8 respectively) as well as a lighter C isotope composition (δ13C < -29‰ VPDB) found in depth. The aromatic pool included PAHs from the middle of the core to the bottom layers this may be due either to the existence of older wildfires or to a transport (lixiviation) from upper layers. Our results show the occurrence of i) intense microbial activity combined with large water front fluctuations occurring along the year; and of ii) anaerobic processes. The lack of peat preservation in depth is probably a sign of bog degradation processes probably linked with climate changes to which Mediterranean wetlands are especially responsive.