HR: 0830h
AN: B31D-0326    [PDF]
TI: Soil gas 13C Values and CO2 Concentrations on the Southeast Slope of Mauna Kea: Implications for Palaeoclimate and Hydrogeology
AU: * Smid, E R
EM: ers9w@soest.hawaii.edu
AF: University of Hawaii, SOEST, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Thomas, D M
EM: dthomas@soest.hawaii.edu
AF: University of Hawaii, SOEST, 1680 East-West Rd., Honolulu, HI 96822 United States
AB: As part of the Hawai'i Scientific Drilling Project (HSDP), an elevational transect on the southeast slope of Mauna Kea Volcano, Hawai'i, was sampled for soil gas CO2 13C over a one-year study interval. Our objective is to determine the principle influences on the 13C values of dissolved inorganic carbon (DIC) that is contributed to groundwater recharged in the area. Groundwater DIC from the HSDP drill hole, dated to 2200 B.P., had a carbon isotopic value of -12%. The D/H and 18O values of the water indicate that the average elevation of recharge is at 2000 m on Mauna Kea. It was postulated that this 13C was controlled mainly by the photosynthetic pathway of the predominant vegetation at this elevation at the time of recharge. If this is true, the current 13C values should have changed as a result of the conversion of the formerly forested Mauna Kea slopes to grassland in the past 2200 years. Preliminary results support this hypothesis. Present day 13C values range from -12% in grasslands at 2000 m ASL to -24% in the lower elevation forested areas. With the soil gas values of -12% in the grasslands, fractionation between soil gas and bicarbonate in groundwater would cause the DIC in groundwater to have values of -4% today. The conversion of this area to grassland approximately 200 years ago and the introduction of exotic C4 grasses are assumed to be the causes of the difference, as C3 trees have lighter values than C4 grasses. Other areas of the transect follow predicted patterns for vegetation type. Analysis of the data is continuing in order to elucidate diurnal and annual trends in the soil gas „13C values and CO2 concentrations.
DE: 0315 Biosphere/atmosphere interactions
DE: 1040 Isotopic composition/chemistry
DE: 1615 Biogeochemical processes (4805)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting