From Shipwrecks to Snow: Exploring Applications of New Techniques in Dendrochronology

Abstract

Tree rings have been used to date archeological construction to study past climate, fire, icestorms, insect outbreak, human caused changes and many more environmental events. At the beginning it was only the ring width measurements as a traditional dendrochronology used to mark time or record environmental variability in the structure of the wood, but with the growing applications of this discipline in many interdisciplinary research, other aspects of tree ring and wood structure have become important and common to study. Many environmental variables can affect tree growth and different records can be derived from a variety of tree-ring proxies including ring widths, wood anatomy, blue intensity, and the isotopic composition of rings. Because tree growth is impacted by a wide variety of different environmental factors, analyzing more than one tree-ring proxy enhances our ability to understand how trees physiologically respond to these environment inputs allowing for a more precise climate response interpretation. The same principle is true in dendroarchaeology when, depending on the quality and quantity of available materials, no single method is able to answer all questions this is especially true in cases, like the one explored in this dissertation, when there are limitations in the size and quality of wooden samples. This dissertation presents how newer and less widely applied techniques in dendrochronology can be used to answer questions regarding the provenance of samples obtained iv from multiple shipwrecks from Bermuda and to explore the climate relationships in two high elevation conifers of Abies magnifica and Tsuga mertensiana in the Sierra Nevada. Using a multi proxy approach the first chapter explores the use of proxies such as, wood anatomy, stable isotope geochemistry and radiogenic isotope geochemistry to provenance small samples collected from shipwrecks in Bermuda. One of the challenges in identifying timbers is that wood anatomy in certain species is not precise enough to say separate European from North American species. For example oaks from the red oak and white oak groups have members from both continents that look very similar. The first chapter demonstrates that stable oxygen isotopes proved useful as a technique in differentiating oak timbers of probable European origin from samples that likely originated in northeastern North America. The results show that the oaks from three of these shipwrecks most likely originated in northeastern North America not Europe. This information is incredibly useful to archeologists who wish to understand if there were regional preferences in which wood was selected, or if there were architectural reasons or political reaons that would lead to wood from specific regions being selected over others. In this chapter further evidence is provided to show that the use of strontium isotopes to provenance shipwrecks is still not fully realized as a method. Finally, this chapter serves to demonstrate that even with samples smaller in both number and size than would be used in a traditional dendroprovenancing study using wood anatomy in conjunction with stable oxygen isotopes can still provide useful information regarding a samples likely origin. Using tree ring width, stable isotope analysis and blue intensity measurements from Abies magnifica and Tsuga mertensiana from the Sierra Nevada indicate that there is a stronger climate signal in blue intensity measurements than in ring width measurements. Stable isotope analysis reveals a correlation with climate variables in the prior fall is the strongest influence on the isotopic v value. Indicating that prior fall conditions may play a critical role in the physiologic response of trees. This signal is very different from that shown by the other proxies, August temperature in blue intensity and a weak correlation with cool-season precipitation for ring widths.