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

Author:

Smid, Elaine R.

Title:

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

Year:

2004

Volume:

M.S.

Subject:

Hawaii Scientific Drilling Project Volcanoes Hawaii Island Hydrology Carbon isotopes

Summary:

As part of the Hawaii Scientific Drilling Project (HSDP), an elevational transect on the southeast slope of Mauna Kea Volcano, Hawaii, was sampled for soil gas CO (sub 2) delta (super 13) C over a one-year study interval. Our objective was to determine the principle influences on the delta (super 13) C 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 deltaD/H and delta (super 18) O values of the water indicate that the average elevation of recharge is at 2000 m on Mauna Kea. It was postulated that this delta (super 13) C 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 delta (super 13) C values should have changed as a result of the conversion of the formerly forested Mauna Kea slopes to grassland in the past 2200 years. Results support this hypothesis. Present day delta (super 13) C values range from -12o/oo 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 -3% today. The conversion of this area to grassland approximaterly 200 years ago and the introduction of exotic C (sub 4) grasses are assumed to be the causes of the difference, as C (sub 3) trees have lighter values than C (sub 4) grasses. Other areas of the transect follow predicted patterns for vegetation type. There was a slight decrease in (super 13) C content from summer to winter, which is opposite of expected seasonal trends. This lighter value could be due to the very dry conditions in the winter (due to El Nino) or cool-season grasses taking over in production in the winter. In diurnal samples, daytime delta (super 13) C values were a few per mile lighter than nocturnal values. Samples taken in recently burned areas showed similar delta (super 13) C values but lower CO (sub 2)concentrations. The values can be divided into two groups, which follow the major photosynthetic pathways of the area. The lower 4 forested areas show no difference in values, but the upper two pasture lands show some difference, which is attributed to a greater proportion of C (sub 4) grasses in the lower elevation pasture.

URL:

http://hdl.handle.net/10125/10465

Collection:

Monographs