Carbon isotope studies and lignin analysis of plants and soil organic matter detect vegetation changes in the southern Brazilian highlands

GEOC 29

Alexander Dümig, duemig@wzw.tum.de1, Peter Schad1, Cornelia Rumpel2, Marie-France Dignac2, Heike Knicker1, and Ingrid Kögel-Knabner1. (1) Lehrstuhl für Bodenkunde, Department für Ökologie und Ökosystemmanagement, Technische Universität München, Am Hochanger 2, D-85350 Freising-Weihenstephan, Germany, (2) Laboratoire de Biogéochimie et Ecologie des Milieux Continentaux, CNRS-INRA, F-78850 Versailles-Grignon, France
The isotopic and biochemical composition of plants, organic surface layers and humic substances from 13 soils was characterized to discover the origin of the present mosaic of grassland (C4) and Araucaria forest (C3). 14C dating and δ13C values reflect a chronosequence of Araucaria forest expansion on grassland which started after 1300 yr BP. The δ13C signature of lignin oxidation products (alkaline CuO oxidation, GC/C-IRMS) indicate that the 13C depletion with depth in old grassland soils results partly from the relative accumulation of 13C-depleted lignins. Forest expansion on grassland is clearly shown by 13C enrichment with depth and changes the composition of soil organic matter towards higher amounts of aliphatic compounds and lower aromatic C and O-alkyl C contents as detected by 13C NMR spectroscopy. Thus, current grasslands represent relics from periods with drier climate than today in the early and mid Holocene and are not the result of recent deforestation.
 

Modern Calibrations and Terrestrial Applications of Stable Isotope Paleo Proxies
9:00 AM-11:20 AM, Monday, August 20, 2007 Boston Park Plaza -- St. James Rm, Oral

Sci-Mix
8:00 PM-10:00 PM, Monday, August 20, 2007 BCEC -- Exhibit Hall - B2, Sci-Mix

Division of Geochemistry

The 234th ACS National Meeting, Boston, MA, August 19-23, 2007