A Thousand Year History
of Glacial Change from Columbia Glacier,
Prince William Sound,
We spent two weeks collecting data at the Columbia
Glacier near Valdez,
Alaska. The Columbia Glacier is one of the largest tidewater glaciers
in the world and has been the subject of intense study for many
years. The Columbia Glacier has been retreating into the Chugach
Mountains for decades at a catastrophic rate (Figure 2).
Figure 1: The Columbia Glacier Group:
Austin, Parker, Greg, Aaron, and Kirk.
This retreat leaves the Columbia Bay choked by icebergs of all sizes (Figure 3). As this retreat continues, trees killed by the glacier during past advances are unearthed. These trees were the subject of our trip to Alaska. The group spent the two weeks collected samples from these trees in an effort to describe the conditions surrounding the movement of the Columbia Glacier (Figure 4).
Figure 4: A"kill site." This is an example
of the trees
run over by the Columbia Glacier during an advance.
Many of the trees found in the
area were partially buried, with roots in place. This ensures
that the trees were killed where they were found. Secondly, many
of the trees were found at any angle pointing down the Columbia
Bay, away from the glacier (Figure 5). This provides evidence
that the trees were run over by the Columbia Glacier as it advanced
down the fjord.
Using dendrochronology, I developed a thousand year tree-ring chronology from Columbia Glacier. Dendrochronology is a method of comparing tree rings from a group of trees in an effort to crossdate them and create a calendar date for their death. This provided the exact time when the Columbia Glacier advanced through an area of the Columbia Bay. Comparing this information to similar information collected for other glaciers in the area provided insights to the advance rates of calving glacier and contribute to the description of climate over the late Holocene.
Using sites throughout the Columbia Fjord (Figure 6), the Columbia Glacier tree-ring chronology was
Figure 6: Topographic map o the Columbia Fjord. Site locations are indicated by number. Both radiocarbon and tree-ring samples were taken from overrun trees within the fjord. The terminal moraine shoal near Heather Island shows the drastic change in water depth during the retreat since 1982 (modified after Post, A., Hallet, B. and Rasmussen, L.A., 1989).
Figure 7: Graph showing individual tree-ring dated series from the Columbia Fjord. Four distinct pulses of advance occurred during an overall advance from AD 1057 to 1778. Although the advance was continuous during this time, it is possible that stabilization and minor retreat may have occurred.
created. This preliminary chronology shows four distinct pulses of advance between AD 1057 and 1778 (Figure 7) during a maintained advance during this time with possible stabilization or minor retreat. Comparing these pulses of advance with those from surrounding land terminating glaciers within the Gulf of Alaska, it is clear that the Columbia Glacier has moved out of phase with land terminating glaciers. Possible causes of the asynchronous advance may include fjord geometry and water depth within the fjord. These aspects also play an important role in the ongoing retreat today. The initial retreat from the terminal moraine shoal may have been induced by climate change, however, the retreat has been maintained due to the large increase in the water depth behind the terminal moraine shoal.
The Tree Ring Lab of Lamont-Doherty Earth Observatory had information about many other dendrochronology research projects.
The Glacier Dynamics Group at the Byrd Polar Research Center, Ohio State University has excellent information about glaciers in general as well as the research they are currently working on.
This article co-written by Austin Post, who joined us on this research trip, discusses his work with the Iceberg Monitoring Project.
This news release from 1999 discusses the retreat of the Columbia Glacier.
This news release discusses the effect of global warming on the retreat of the Columbia Glacier.