DENDROCLIMATIC ANALYSIS OF CLIMATE OSCILLATIONS FOR THE SOUTHEASTERN UNITED STATES FROM TREE-RING NETWORK DATA

Dendroclimatological research along a geographical gradient is important to understanding both spatial and temporal characteristics of climate influences on tree growth. In this study, three tree-ring width chronologies, obtained from field collection and previous research, were used to represent tree growth along a longitudinal transect from coast to inland in the southeastern U.S.: Hope Mills, located at the Atlantic Coastal Plain; Linville Mountain, located on the eastern side of the Appalachian Mountains; and Gold Mine Trail, located on the western side of the Appalachians. The variations of ring width indices in chronologies reflect extreme climatic events such as severe droughts or cold periods. Correlation and response function analyses were used to examine the climate-tree growth relationship at three sites. The temporal stationarity of climate signals was tested using moving interval analysis in DENDROCLIM2002. Winter temperature was the limiting climate factor for the western mountain site, while moisture was more important for tree growth in the eastern mountain and coastal area sites. However, all significant climate signals found in the trees were not stable over time. The tendency of a shift from precipitation signal to temperature signal is notable around the mid-20th century. Winter North Atlantic Oscillation (NAO) had positive correlations with radial growth at the two mountain sites, which might explain the winter temperature response by trees. The Atlantic Multidecadal Oscillation (AMO) showed an annual feature of associations with growth, and the multidecadal duration of significant correlations was also apparent. The Pacific-related Pacific Decadal Oscillation (PDO) and El Niño-Southern Oscillation (ENSO) also tended to influence tree growth. Along the coastal-inland transect, gradient features of climate oscillation signals did exist. Relationships changed with phase changes of the oscillations. Land-sea boundaries and high mountains may determine the climate response patterns in the Southeast. Other factors such as microenvironment, human disturbance, and biological reaction of trees to climate change also have influence. It is not reliable to use the composite chronology to study the effect of climate oscillations for the Southeast region. In the future, a large number of sample sites will be necessary to more extensively study the regional climate-tree growth relationship

Timing and Extent of the Little Ice Age Glacial Advances in the Eastern Tian Shan, China

Located in Central Asia, one of the most continental regions on Earth, the Tian Shan’s glaciers contribute critical fresh water to populated areas in the lowland. These glaciers are sensitive to climate change, and knowledge of contemporary glaciers and their changes in the past is of critical importance for sustainable development in this region. Constraining glacial fluctuations in recent centuries will fill a gap in numerical constraints on glacial history and paleoclimate information, and provide important evidence on the spatio-temporal changes of the climate systems in the Tian Shan. This doctoral dissertation investigates the timing and extent of Little Ice Age (LIA) glacial advances in the eastern Tian Shan. In particular, I conducted: 1) the mapping of glacial extents during the LIA and around 2010 using Google Earth high-resolution imagery and ArcGIS; 2) statistical analyses to examine relationships between local topographic/geometric factors and glacier change parameters; and 3) cosmogenic 10Be [beryllium 10] surface exposure dating of presumed LIA moraines. The major contributions of this dissertation include: 1) a total of 1173 contemporary glaciers with their corresponding presumed LIA extents were delineated in the eastern Tian Shan; 2) glacier area and mean elevation are the two major local factors that affect glacier area changes, but topographic/geometric factors cannot well explain changes in equilibrium line altitudes; 3) three major LIA advances occurred at 730±300 yr BP, 370±100 yr BP, and 210±50 yr BP were constrained based on 10Be surface exposure ages, and the maximum LIA extent (about 700–900 m beyond glacier termini) was reached asynchronously in different sub-regions; 4) presumed LIA moraines in front of small, thin glaciers yielded widely scattered and much older ages than LIA ages. The glacial deposits in front of such glaciers might have formed prior to the LIA and have been reworked during non-erosive glacial transport in LIA. This suggests that inheritance could be a more significant problem than degradation in the exposure age scatter of young glacial event. More dating work is needed to extend our knowledge on LIA glacial advances, and to better understand the influence of climate systems on glacier changes

Lattice and Charge Order in Layered Bi-Based Topological Insulators

Bi2X3 (X=Se/Te) is a topological insulator, as well as a layered dichalcogenide. The topological properties of Bi2Se3 have gained a lot of interest over the past decade. However, as a layered chalcogenide, much of its uniqueness has not been fully discovered, e.g. hosting Charge Density Wave as reported in most other chalcogenides. With intercalation of Nb, Cu and Sr, Bi2Se3 becomes an unconventional superconductor. Together with its topological properties, A-Bi2X3 (A=Nb, Cu and Sr) have been proposed to be potential Topological superconductors. However, the mechanism of the unconventional SC in these compounds is still under discussion. For my PhD research, I discovered charge density wave (CDW) order in self-doped Bi2Se3 and metal intercalated Bi2X3 together with superconducting transitions. Together with collaborators, I identified these phase transitions through studying and analyzing their crystal structures, electronic structures, and local nuclear environment. I further found that certain growth conditions (annealing and quenching temperatures) can help maintain the intercalation/defect phase and play as an important factor for observations of the intertwined electronic ground states. With intercalation or self-doped defects, the layered nature of Bi2X3 can easily be driven with a periodic lattice distortion. This lattice disorder can further lead to a local charge density distortion, with concomitant changes in the electronic structure. Depending on the relationship between the periodicity of the charge density and the underlying lattice constant, it can lead to I-CDW or CDW. In Cu doped Bi2Te2Se, I studied lattice and charge order due to different concentration of Cu into Bi2Te2Se. In self-doped Bi2Se3, I found CDW order with energy gap of 10meV. With Nb doped Bi2Se3, both superconductivity (SC) and CDW were found in this material. Both SC and CDW are broken symmetries at the ground state. The underlying relationship between these two states has long been under debate. Based on recent reports and my experimental observations, the fermiology seems play an important role for the electronic properties in A-Bi2X3. It is possible in doped Bi2X3 that both SC and CDW originate from the intercalation effect where the change of lattice structure symmetry leads to electronic symmetry broken. This work examines how lattice order leads to charge order in doped Bi2X3. It also discusses possible origins for the underlying electronic intertwined states and how they are all related to each other