What Lake Sediment in Arctic Alaska May Reveal About Climate, Fire and Early Humans

Imuruk Lake, in Bering Land Bridge National Preserve in northwest Alaska, may be more than 200,000 years old, according to recent evidence. If so, its sediments would offer the longest terrestrial sedimentary record of environmental change in Alaska from lakes and ocean sites, the authors say. Researchers from Brown University and the National Park Service have collected cores from Imuruk and from Whitefish Lake to learn how temperature, plant ecology and water chemistry changed on the Seward Peninsula in the geologic past.

Why look at lake mud

Arctic air temperatures have risen nearly two times faster than those at lower latitudes over the past 150 years. The authors tie that not only to greenhouse gas emissions from human activities but also to Arctic amplification, in which climate change is stronger at the poles. Alaska and its national parks are among the regions bearing the brunt, through permafrost thaw, sea ice loss, more severe wildfires, coastal erosion and changing habitat. To judge whether recent change is exceptional, and how landscapes respond to temperature, researchers turn to geological records such as lake and ocean sediments, which show how factors like greenhouse gases and solar insolation drove change in the past. Lake sediments pile up for thousands to hundreds of thousands of years, which makes them an environmental archive.

Two lakes that may reach far back

Both lakes are maars, basins formed when volcanic activity superheated ground ice or groundwater in permafrost terrain and the pressure built until it exploded. Their exact ages are debated. Imuruk's possible age of over 200,000 years would put its sediments beyond most nearby marine records: the ocean around the Bering Strait has no sediment from glacial times, because sea level dropped and exposed the seafloor, and to the authors' knowledge only one marine record, a drilling site on the Bering Sea shelf, holds sediments older than the last Glacial Maximum about 21,000 years ago, and even that one reaches back only 150,000 years. Preliminary dating of Whitefish Lake suggests it may have one of the highest sedimentation rates of any Alaskan lake, which would give one of the highest-resolution records in the region.

Reading a core

The team drills through ice, or works from a raft, and uses a piston corer to pull sediment from the lakebed. To date the layers, it uses the lead isotope content for sediments from the last 200 years or so, and radiocarbon in preserved organic matter for the last 45,000 years. Beyond 45,000 years it can use paleomagnetics, thermoluminescence and tephra, ash layers from volcanic eruptions of known ages. Once the layers are dated, the team analyzes the fossils and chemicals left by past plants and animals:

Charcoal is sieved out as particles larger than 125 microns, about the thickness of a human hair, and counted under a microscope.

Four questions

How has Alaska's climate varied? Reliable instrumental records in Alaska are far apart and brief. The authors hope alkenones will extend the region's temperature record past the earliest instruments, and comparing climate records with ecological ones will show how ecosystems respond to a changing climate.

How do fires respond to climate? The 2007 Anaktuvuk River Fire burned more than 250,000 acres (100,000 ha) of tundra on the North Slope and was thought to be unprecedented. Researchers later found two similarly sized fires there between 1880 and 1920. That suggests fire may have been more common in the past than thought, and that the temperature sensitivity of tundra fires may be lower than fire models predict. Fires are generally more frequent in the tundra of western Alaska, including Bering Land Bridge National Preserve, than on the North Slope, so the authors argue for studies of each region.

How do subsistence resources respond? Lakes are crucial fish habitat, and anadromous fish such as salmon are central to commercial and subsistence fishing across Alaska. Observation and modeling studies predict that subarctic fish may move into Arctic waters as temperatures rise, but the lack of long-term fish community records hinders conservation decisions, since that migration has not been seen in recent decades. Environmental DNA shed by fish can survive in sediment even when no visible fossils remain: in upstate New York, yellow perch DNA was recovered from a 2,200-year-old lake sediment sample. There are uncertainties about how well ancient eDNA keeps, but it could give a time series of fish communities in Alaska lakes.

When did the first Americans arrive, and what did they do to the environment? Beringia, the ancient landmass joining northeastern Asia and northwest North America when sea level was lower, is thought to be the route people took into the Americas, but the timing is hotly debated. The existing view puts arrival in the late Pleistocene, about 14,000 years ago. Recent genetic research suggests people reached Beringia much earlier, about 30,000 years ago at the height of the last ice age, and lived there for several thousand years before moving south, though no archaeological evidence unequivocally supports that. At other North Slope lakes the authors found sedimentary, not archaeological, evidence of ice-age humans as early as 32,000 years ago, in fecal sterols, PAHs and charcoal. Fecal sterols in the new cores may show when people reached the study area and, with the fire records, what effect they had.

Sources

• Richard S. Vachula and Karen J. Wang (Brown University), Jonathan A. O'Donnell (National Park Service) and Yongsong Huang (Brown University), New Approaches to Study Interactions Among Climate, Environment, and Humans in Arctic Alaska, Alaska Park Science 20(1), 2021, pp. 2-9, National Park Service; rewritten in hubnx's own words.