Scientists use temperature records in bacteria to track US migration patterns

"When you think about what was happening in the Last Glacial Maximum, all these regions on Earth were super cold, with massive ice sheets, but this area was never fully glaciated," said Jennifer Kielhofer, Ph.D., a paleoclimatologist at DRI and lead author of the study.

"We're hypothesizing that if this area was comparatively warm, maybe that would have been an attractive reason to come there and settle," she added in the statement.

"We have to look to the past to try to better constrain how these areas have responded previously," said Kielhofer. "And how they might respond in the future under climate scenarios that we predict."

Using the brGDGT paleothermometry technique

Previous studies have used changes in flora and pollen to create rough temperature records. These data, however, can only give a broad impression of whether a place has been warming or cooling over time.

Kielhofer analyzed soil samples from the archeological sites to get a more exact history of temperatures. She studied temperature data preserved in bacteria using a method called brGDGT paleothermometry to establish a record of the mean yearly air temperature above freezing with a precision of roughly 2.8 degrees Celsius.

"Bacteria are everywhere," she said. "That's great because, in areas where you might not have other means of recording or assessing past temperature, you have bacteria. They can preserve for millions of years, so it's a great opportunity to look at pretty much anywhere on Earth."

Kielhofer and the team focusing on other historical documents, such as shifts in aridity, can clarify how the environment in this area affected early human groups.

Study abstract:

Aeolian deposits in the middle Tanana Valley of central Alaska offer a well-preserved record of paleoenvironmental change since the deglacial period (c. 16,000–11,000 cal yr BP). These deposits also contain some of North America's oldest archaeological occupations (c. 14,000–13,000 cal yr BP), making this region critically important for understanding human migration into the high latitudes and the Americas. Major research questions involve assessing the magnitude of deglacial climatic change and the influence of climate on early human groups. This study uses branched glycerol dialkyl glycerol tetraethers (brGDGTs) from six loess-paleosol sequences to develop a quantitative paleotemperature record within terrestrial locations in the Tanana basin that are close to archaeological sites. BrGDGT-derived temperatures demonstrate a lack of cooler temperatures associated with deglacial conditions, making this region relatively “warm” compared to other parts of the globe. Additionally, our brGDGT record shows little coherent temperature change associated with deglacial climate variability (e.g., Bølling-Allerød, Younger Dryas), and Holocene temperatures are relatively stable as well, indicating that temperature fluctuation was not the main driver of environmental or archaeological change over time. We recommend averaging data across multiple terrestrial exposures to produce regional temperature reconstrutions.