Recreating the Pleistocene Coastline of Glacier Bay

By Greg Streveler (Icy Strait Environmental Services), Dan Mann (University of Alaska Fairbanks), Wayne Howell (National Park Service)

Is it possible that areas along Glacier Bay’s outer coast were occupied by humans 16,000 years ago? Where would we search for evidence?

Project Dates:

May 2012 - June 2014

two people stand in a field talking
Dan Mann and Greg Streveler ponder a landscape on the park’s outer coast for clues about its complex sea level history. The researchers must unravel the effects of several major forces that cause the land to rise and fall and thus affect “relative” sea levels.

Introduction

Anthropologists have proposed that a possible route of human colonization of North America during the late Pleistocene (15,000-20,000 years ago) was along the western continental margin, including the outer coast of what is today Glacier Bay National Park and Preserve. Those first immigrants would have traveled by watercraft and camped in coastal areas where needed resources could be found. If they left behind any archeological evidence, it would be found 1) along or near the coastline as it existed during those times, and 2) in areas that were attractive to humans for some reason. The researchers’ task is to make educated guesses about where to search for evidence of human habitation by addressing these two criteria.

To reconstruct the complicated history of the coastline, the researchers must consider the extreme rise and fall of relative sea level (RSL) that has occurred in the study area due to a complex combination of three major forces:

Tectonism: Landscapes west of the Fairweather Fault have a history of uplift (causing RSL fall), while landscapes east of the fault have been downthrust (causing RSL rise).

Isostasy: The tremendous weight of glacial ice actually depresses the land beneath it, causing RSL rise. When the ice load is removed and the land rebounds, RSL falls.

Eustatic (global) sea level change: Global sea level was substantially lower during periods when much of earth’s water was stored on the continents as glacial ice. As the ice melted, sea level rose.

In searching for places that were ice-free, near the coast AND attractive to humans, the researchers try to reconstruct ancient environments. They focus on places where important resources—food, firewood, good beaches for landing watercraft—might have been available. Known archeological sites are a good place to start. How long ago did this site acquire the attributes that made it attractive to more recent people?

a map of tectonic areas of glacier bay
The Lituya and Graves districts, though close together geographically, have very different tectonic and glacial histories.

Methods and Findings

The researchers have divided the study area into two distinct districts based on their tectonic histories. The Lituya District lies on the west side of the Fairweather Fault and has been upthrust relative to the Graves District, which lies on the east side of the fault. The districts also experienced very different glacial histories—the Graves District was much more heavily glaciated during the late Pleistocene. At the glacial maximum, ice filled Icy Strait to a depth of 1.5 km above present sea level and flowed to a calving terminus 15 km west of Cape Spencer. At the same time, glaciation in the Lituya District was relatively light. Considering the very different patterns of isostatic adjustments and tectonic movements in the two areas, the researchers expect to find two very different RSL histories.

Both districts contain intriguing landforms. Near Lituya Bay, ancient marine terraces that have not been glaciated for perhaps hundreds of thousands of years are of great interest. Near Graves Harbor, a unique lake system that might preserve a record of the area’s sea level and environmental history is being investigated. An archeological site dating to the late 1700s or early 1800s at a salmon “chokepoint” (where salmon were easy to catch) in this system shows that people traveled inland from the coast some distance to access the resource—perhaps ancient people did the same. How long has this lake system existed, and how long has it hosted salmon runs, and, perhaps, humans?

The field methods for attempting to answer questions such as these include:
1. Searching along stream cuts and other naturally exposed sites for traces of marine features, such as shells or micro-organisms called diatoms;

2. Coring low-elevation lakes to look for marine sediments;

3. Dating the retreat of glacial ice from an area and calculating isostatic adjustments to the removal of the ice load, to determine when the area became available for human occupation. In a previous study, the researchers concluded that late Pleistocene ice retreated from Icy Strait by about 14,000 years ago. It must have retreated from the Graves District somewhat earlier.

a map of alaska and the bering land bridge, with an arrow showing how people may have moved along the coast
People may have followed the coast to the New World from Asia 15,000-19,000 years ago. The Bering Land Bridge was open because much of earth’s water was locked up on the continents as glacial ice. But the ice blocked any land route to the “lower 48” until about 13,000 years ago.

Both districts contain intriguing landforms. Near Lituya Bay, ancient marine terraces that have not been glaciated for perhaps hundreds of thousands of years are of great interest. Near Graves Harbor, a unique lake system that might preserve a record of the area’s sea level and environmental history is being investigated. An archeological site dating to the late 1700s or early 1800s at a salmon “chokepoint” (where salmon were easy to catch) in this system shows that people traveled inland from the coast some distance to access the resource—perhaps ancient people did the same. How long has this lake system existed, and how long has it hosted salmon runs, and, perhaps, humans?

The field methods for attempting to answer questions such as these include:

1. Searching along stream cuts and other naturally exposed sites for traces of marine features, such as shells or micro-organisms called diatoms;

2. Coring low-elevation lakes to look for marine sediments;

3. Dating the retreat of glacial ice from an area and calculating isostatic adjustments to the removal of the ice load, to determine when the area became available for human occupation. In a previous study, the researchers concluded that late Pleistocene ice retreated from Icy Strait by about 14,000 years ago. It must have retreated from the Graves District somewhat earlier.