Earth warmed tremendously after the last ice age, but why did the climate reverse course and cool down in the middle of this warming?

In Bjorn Anderson's Footsteps

(placeholder section for Aaron/pair with Ellen interpretation of Bjorn's hand drawn maps)

I. Highland Plateaus

The highland plateaus around Lysefjord were a place where the ice could spread out without being hemmed in by the high fjord walls. This allowed for several moraines to be preserved when they would otherwise have been easily overprinted, giving us a chance to look in greater detail at ice fluctuations. Moraines are produced as a stable ice front sits in the same position for some time, churning out sediment and boulders that eventually build a ridge. By calculating the time that these moraines were deposited, we can pinpoint where the ice front was in space and time.

We scaled the over 500-meter plateau along the southern side of Lysefjord each day for a week, coming back down with packs full of rock samples. Our legs were quite tired! We collected samples from boulders that were deposited on the top of moraine crests, right when the ice pulled back from that location.

The views were incredible, with the exception of the day we spent entirely within a cloud, and the bogs were wet. We learned quickly to bring a fresh pair of socks and plenty of cookies for a midday break.

Ellen note: We can split these paragraphs/illustrations.

II. Moraines & Trolls

The area around the mouth of Lysefjord hosts several moraines bracketed by erratic boulders on either side. This geomorphological configuration offers a high resolution look at the timing of retreat and advance in this area.

Much of this region is pasture land for cattle and sheep. Most of the glacial features were left intact, however, special care was taken to avoid regions modified by humans. Very large and deeply embedded boulders are characteristics of untouched landforms.

This area also hosted sections of boulder moraines. These are formed in the same way as traditional moraines, but are lacking fine grained material. They can still be sampled and interpreted all the same, but are fascinating to observe! The famous Trollgaren moraine in Norway is translated to “Troll Fence” in English. Before a geologic explanation, locals once believed the strange features were created by trolls.

III. Erratic Boulders

Erratic boulders are deposited as the ice retreats rapidly, leaving boulders perched directly on glacially smoothed bedrock. We collected samples from erratic boulders from bedrock hills near the mouth of Lysefjord, the highland plateaus, and bedrock benches farther inland.

We combine deposition ages that we calculate from moraines with those from erratic boulders to get a fuller picture of when the ice was rapidly retreating versus standing still or even readvancing.

What did we learn?

The last ice age terminated ~18 thousand years ago, as the Earth began to warm drastically. This rapid warming caused massive ice sheets that were covering portions of North America and Scandinavia to retreat as ice melt increased. One of the big mysteries for researchers who study past climate is why this warming trend appears to have reversed itself ~14.7 thousand years ago, leading to a colder climate, before resuming the warming trend about 2 thousand years later.

Evidence for this cooling period in the middle of the ice age termination first was observed in Southern Hemisphere records, such as ice cores from Antarctica and mountain glacier moraine chronologies. Our big task in developing a chronology of the Lysefjord glacier system is to answer whether this warming reversal also happened in the Northern Hemisphere, such as in Norway. If so, this would indicate a global phenomenon.

Our initial results show that the ice rapidly retreated up Lysefjord, as it dropped erratic boulders, once the ice age termination began. But the ice appears to have made a dramatic comeback and surged back to the mouth of Lysefjord, where it deposited the moraines on the highland plateaus.

Why this cooling reversal happened is an open question, but understanding how this happened on a global scale will teach us about important mechanics of our climate system.