Date of this Version
Our investigations in 2007 focused on continuing collection and analyses of tree rings from living trees and building a long ring-width series from interstadial wood. These interstadial forests were extensive and their preservation, although discontinuous and fragmented, is remarkable. In situ stumps, still rooted in growth position, and logs held within sediments remain from forests that existed at various times between glaciations over approximately the last 10,000 years.
We have analyzed previous and new collections of ring-width tree-ring series, including a multiple species collection from near Excursion Ridge in the southeast portion of the Park. These tree-ring series have a strong temperature signal and together with our other collections along the Gulf of Alaska are being used in three undergraduate theses. The trees show a divergence from expected growth within some sites and this decrease in growth is being investigated in Glacier Bay and along the Gulf of Alaska. We continue to share our tree-ring data with other researchers (Capps, Clague, Luckman) working on ice-dammed lakes of Brady Glacier who have been able to tree-ring date a significant lake damming event along the ice margin. Denny Capps, a PhD candidate from Simon Fraser University, worked at The College of Wooster Tree Ring Lab on tree cores from Brady Glacier during the fall of 2007, acquiring data that will be useful to both of our projects.
Our further sampling and tree-ring dating of logs from the southern end of Glacier Bay suggests a remarkably rapid advance of the tidewater glacier as it reached near its Holocene maximum in the early 1700s. This is an interesting find in that it meshes with the relatively well-documented oral history compiled by others for the area and the recent work of Streveler, Connor, Howell and Montieth. We built on our work in the previous year that developed calendar-dated ice advances about AD 800 and a relative tree-ring sequence spanning several hundred years tied to radiocarbon dating that suggests ice advance into forest about 3000 yr BP both from Geikie Inlet. During the summer of 2007, we targeted key stratigraphic sites within North Fingers Bay that had logs from both of these advances within glacial sediments, thus providing a context for the wood. We also concentrated on sampling in the Charpentier Inlet to target ~4500 yr BP wood. These collections are currently being analyzed.
We seek to continue documenting wood bearing exposures across the Park to gather sufficient data and wood sections for developing the potential 10,000-year record of paleoclimate and to place present glacier and ecological changes into a long-term context of climate changes that affected Glacier Bay and the North Pacific. Because of the complex regional and sub-regional climatic regimes, and the timing and location of glacial advance and retreat across the Park, numerous sites must be examined in detail to develop a full tree-ring chronology and glacial history. Thus to reconstruct the entire 10,000 year chronology, we must sample each location where forests grew prior to ice advancing across them: for example trees killed during the ice advance of 9000 years ago occur near the heads of inlets, whereas trees overrun by the same advancing ice 7000 years ago are located in mid-bay sites near Geikie and Adams Inlet. Each area is thus equally critical to developing a model of ice advance and retreat into lower Glacier Bay tied directly to the paleoclimate.