Soil gas delta-carbon-13 values and carbon dioxide concentrations on the southeast slope of Mauna Kea: Implications for palaeoclimate and hydrogeology

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 delta13C over a one-year study interval to determine the principle influences on the delta 13C values of dissolved inorganic carbon (DIC) in groundwater. Groundwater DIC from the HSDP drill hole, dated to 2200 B.P., had a carbon isotopic value of -12‰ at a recharge elevation of 2000 m.; Present day soil gas delta13C values range from -12‰ in grasslands at 2000 m ASL to -24‰ in the lower elevation forests. The recharge zone values translate to a value of -3‰ in DIC in groundwater today, a result of the conversion of forests to grasslands in the recharge zone in the past 200 years. Diurnal and seasonal trends were linked to El Nino and temperature variations.