PARENT SESSION
Poster Session #46: Riparian Ecology.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Using changes in Tamarix tree-ring anatomy to date floodplain deposition events.

Shafroth, Patrick^*,1, Friedman, Jonathan¹, Vincent, Kirk², Steingraeber, David³, Auble, Gregor¹, Fleming, Julie⁴, ¹ U.S. Geological Survey, Fort Collins, CO² U.S. Geological Survey, Boulder, CO³ Colorado State University, Fort Collins, CO⁴ Johnson Controls World Services, Fort Collins, CO

ABSTRACT- Determination of sediment deposition rates from stratigraphy is typically limited by a scarcity of chronological information. We present a method for precise dating of sedimentary beds based on the change in anatomy of tree rings upon burial. When stems of tamarisk (Tamarix ramosissima) and sandbar willow (Salix exigua) are buried, subsequent annual rings in the buried portions become narrower and various aspects of the wood anatomy change, including vessel size and number, and ray width. Observation of these changes can be combined with tree ring counts to determine the year of deposition of sedimentary beds that are at least 10 cm thick. Using a backhoe we dug trenches across the flood plain at three locations along the arroyo of the Rio Puerco, New Mexico. At each cross section we prepared a detailed stratigraphic description and excavated several tamarisks to depths as great as 5 meters. From each excavated tree we cut and sanded 10-50 slabs for tree-ring analysis. We cross-dated slabs within and between plants and used the burial signature in the tree rings to date all sedimentary beds in the stratigraphic profile near each plant. We then used the trench stratigraphy to convert depths of sediment deposition around individual trees to areas of deposition in the cross section.

KEY WORDS: Tamarix ramosissima, tree rings, wood anatomy, sediment deposition rates