HR: 1340h
AN: PP43A-0602    [Abstracts]
TI: Isotopic Evidence for C4 Grass Expansion During the Last Glacial Maximum and Younger Dryas in Northern Australia
AU: * Johnson, B J
EM: bjohnso3@bates.edu
AF: Department Geology, Bates College, 44 Campus Ave, Lewiston, ME 04240 United States
AU: Wakeham, S
EM: stuart@skio.peachnet.edu
AF: Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411 United States
AU: Gelinas, Y
EM: ygelinas@alcor.concordia.ca
AF: Chemistry and Biochemistry Department, Concordia University, 1455 de Maisonneuve Blvd West, Montreal, Que H3G 1M8 Canada
AU: Luly, J
EM: jonathan.luly@jcu.edu.au
AF: School of Tropical Environmental Studies and Geography, James Cook University, Townsville, QUE ACT 0200 Australia
AU: Miller, G
EM: gmiller@colorado.edu
AF: INSTAAR and Department Geological Sciences, University of Colorado, Boulder, CO 80303 United States
AB: In northern and central Australia, late Quaternary records of terrestrial environmental change are rare due to generally poor preservation of pollen grains and a derth of long-term, continuous lacustrine sedimentary deposits. The Wombe mound spring in the Keep River National Park, Northern Territory, is an organic mound and isolated patch of monsoon vine thicket thought to have formed tens of thousands of years ago. In an effort to obtain a record of paleovegetation and fire history from northern Australia, a 3.4 m sediment core was recovered from the Wombe mound spring and subject to multiple types of analyses. The core represents a continuous depositional sequence with radiocarbon ages spanning the last 35 ka cal years (hereafter referred to as 35 ka). Paleovegetation was reconstructed using a combination of pollen and carbon isotopes in bulk sediment and higher plant leaf wax (HPLW) lipid biomarkers. The fire history was reconstructed from paired graphitic black carbon (GBC) and polycyclic aromatic hydrocarbon (PAH) analyses of the core sediments. Between 35 and 11 ka, the bulk organic carbon (OC) isotope data fluctuate between -22 and -15%, with the most isotopically enriched values measured at 11.4 ka. Between 12.3 and 6.8 ka, OC isotope values decrease by 13%, and remain steady from 6.8 ka to the present at -28%. There are two distinct peaks of isotopic enrichment in the higher plant leaf wax biomarkers. These two peaks coincide with the Last Glacial Maximum (LGM; 21 ka) and the Younger Dryas (YD; 11.4 ka) and represent maximum increases in C4 grasses relative to C3 plants. Relative increases in C4 grasses during the LGM in other parts of the tropics (i.e., Sacred Lake, Mt. Kenya) have been attributed to the competitive advantage of C4 plants relative to C3 plants under reduced atmospheric pCO2 and is likely the cause for C4 expansion in northern Australia. The increase in C4 grasses during the YD is reflected in the bulk sediment and HPLW isotope data and is also documented in the Sacred Lake record and may result from an increase in intensity of the easterly trade winds. GBC and PAH analyses are correlated to each other with maximum concentrations of each occurring between 28 and 30 ka and between 13 and 18 ka. The fact that these records are correlated suggests that each is an independent proxy for fire frequency at the Wombe mound spring. These periods of increased fire frequency may be associated with excessive fuel build-up and burn, or increased vulnerability of the vegetation to periods of enhanced aridity. Low GBC and PAH concentrations at approximately 20 ka and 11 ka are attributed to relatively reduced fuel loads during extreme dry events of the LGM and YD. Low GBC and PAH values during the Holocene coupled with establishment of the C3 mound spring indicate that there is very little evidence of fire over the last 8 ka preserved at the site. This likely reflects the moist microenvironment of the mound spring and not the regional fire history of the area.
DE: 1045 Low-temperature geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting