Pyrolysis-AMS Study of Age Structure of SOC in Volcanic Soils on Kohala Volcano, Hawaii

Abstract:

Soil organic carbon (SOC) is a complex substrate; the soil matrix has a wide array of chemical inputs, leading to a heterogeneous mixture of carbon compounds. In volcanic soils on Kohala Mountain, Hawaii Island, old organic carbon (OC) (>10,000 years) is associated with short-range order (SRO) minerals with large, reactive surface areas (Torn et al., 1997). Large variations in precipitation generate significant changes in soil pH, secondary minerals and soil redox state. A study to measure the age distribution in a given sample of SOC by the ramped pyrolysis-AMS technique was carried out at Woods Hole NOSAMS facility. Five soil samples, from three sites (precipitation = 2.4m/year, 2.3m/year, 1.78m/year, respectively) on a 350 ka volcanic (Pololu) substrate were analyzed. Samples were freeze-dried, homogenized, and combusted under a programed temperature pyrolysis regime from 25 to 900°C; evolved CO₂ was collected in fractions for ¹⁴C analysis. The abundance of SRO minerals is characterized through sequential extractions and total elemental analysis. Results include: 1.) The Pu’u Eke profile (2.4m/year), older carbon (bulk radiocarbon age: 2530 years) is deeper (63-74cm) in the soil profile, but it is thermally less stable (thermograph T_max: 314°C) than the younger carbon, which was in the associated with the 25-36cm sample (radiocarbon age: 1030yr) and T_max: 324°C. The top horizon of the profile (13-21cm) had a modern radiocarbon age, but a T_max: 396°C. 2.) The high precipitation site has significantly younger OC (2698 yr) than the drier site (7585 yr). This is coincident with changing redox conditions and loss of nano-crystalline iron oxide minerals (ferrihydrite) from the wet site. 3.) In a given sample, the age distribution is fairly uniform. The initial results support the hypothesis that nano-crystalline ferrihydrite acts to stabilize OC, as older carbon is found in the dryer, ferrihydrite rich sites, while at the wet, Fe-poor site ¹⁴C ages were substantially younger. Despite its younger radiocarbon age, the high precipitation site has substantially greater SOC (27.7%) compared to the drier sites with only 5.68% SOC, implying much faster turnover times at the Fe-poor site. This suggests that soil mineralogy, especially the presence of ferrihydrite, has a significant control on the turnover time of OC.