HR: 09:05h
AN: V21C-05    [Abstracts]
TI: Dendrochemical Dating of Tephra Layers
AU: * Ort, M H
EM: michael.ort@nau.edu
AF: Northern Arizona Univ., Environ. Sci., Box 5694, Flagstaff, AZ 86011 United States
AU: Sheppard, P R
EM: sheppard@ltrr.arizona.edu
AF: Univ. Arizona, Laboratory of Tree-Ring Research, Tucson, AZ 85721 United States
AU: Speakman, J
EM: SpeakmanR@missouri.edu
AF: Univ. Missouri, MURR, Columbia, MO 65211 United States
AU: Anderson, K A
EM: kirk.anderson@nau.edu
AF: Northern Arizona Univ., Environ. Sci., Box 5694, Flagstaff, AZ 86011 United States
AU: Elson, M D
EM: melson@desert.com
AF: Desert Archaeol. Inc, 3975 N Tucson Blvd, Tucson, AZ 85716 United States
AU: Siebe G., C
EM: siebe@servidor.unam.mx
AF: UNAM, Instit.. Geologia, Cd. Universitaria, Mexico, DF 04510 Mexico
AB: Dating eruptions in the past 1000 years can be difficult with $^{14}$C, as production rates have varied. Tree-ring dating has been used, with eruptions presumably causing thin rings. We are developing a dendrochemical dating technique that may give more confidence to such dates. When a tephra is deposited, soil chemistry may change and many components of the glassy matrix are mobile and available for leaching into the soil and, hence, to plants. If some components were scarce prior to the eruption, the content in tree rings may increase and serve as a chemical marker of the eruption onset. LA-ICP-MS analysis of tree cores allows earlywood from each ring to be analyzed. We are currently investigating whether acid dissolution will produce similar results and precision with small sample sizes. The 1943-1952 CE eruption of Paricutin, Michoacan, Mexico, deposited a thick tephra in a conifer forest. Analyses of tree cores show that pre-eruption P concentrations of a few ppm increased an order of magnitude within the first two years after the onset of the eruption and then dropped steadily during the rest of the eruption to 10-15 ppm, which was then maintained for decades. Ca and Sr dropped during the eruption but rebounded immediately afterward. These three elements appear immobile in the wood, as steep compositional gradients occur across rings. The Paricutin tephra has 0.2-0.4 wt% P$_{2}$O$_{5}$. Analyses of soils and tephra obtained by the Bray method (a weak HCl leaching intended to mimic leaching in slightly acid soils) yield values of 2-17 ppm available phosphorus, fairly high values for this area. The Sunset Crater eruption near Flagstaff, Arizona, was dated at 1064 CE based on the onset of thin tree rings in three beams from structures at Wupatki village, 20 km north of the volcano. Our analyses of this wood show pronounced increases in P, Ca, Sr, Mg, and Mn in the early 1080s CE. Currently, we are investigating Jeffrey pine reaction to the mid-1600s CE Cinder Cone eruption at Lassen Volcanic National Park, California.
DE: 8404 Ash deposits
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting