A new study finds that people at one well-situated spot in early Holocene Britain handled rapid climate swings remarkably well. Archaeologists and paleoclimate researchers combined sediment core data with dates from a major Stone Age site to reconstruct how hunter-gatherer communities adapted to a series of unpredictable, century-long cold spells around 11,000 years ago. And, it turns out, at least one group of hunter-gatherers seemed completely unbothered.

A chilly return

During the early Holocene, about 11,000 years ago, Northern Europe was emerging from nearly 100,000 years of cold storage under thick ice sheets, and rising sea levels hadn’t yet cut Britain off from the rest of Western Europe. As the last of those ice sheets collapsed into the sea, their effect on ocean temperature and circulation triggered several periods of regional cooling, which lasted for about a century each. Meanwhile, small communities of hunter-gatherers started drifting back into Northern Europe.

“These societies did not simply occupy northwest Europe, but were the earliest populations to attempt to recolonize this region after the Last Glacial Period,” wrote Simon Blockley of Royal Holloway University of London. Blockley noted they did so “against a backdrop of some of the most extreme abrupt climate events known from the Holocene.” You might expect communities in that situation—just gaining a foothold in a new land and at the mercy of an unstable environment—to be pretty vulnerable to the vagaries of climate.

So Blockley and his colleagues used their new study to look at how the unsettled early Holocene climate in Europe impacted those new arrivals. Some studies suggest that sudden bouts of cooling triggered massive population collapses in Britain and Ireland around 8,200 years ago, while others insist that early Holocene hunter-gatherers were much more resilient than that. To settle the debate, researchers need to understand when these climate shifts happened and compare those dates with the archaeological record.

But there aren’t many sites in northwestern Europe where archaeological materials have been dated well enough to make that possible. One of the exceptions is Star Carr, a Mesolithic archaeological site in what is now North Yorkshire, UK. Today, the site is surrounded by farmland; 11,000 years ago, it sat at the western end of a large lake, now known as Paleolake Flixton, and was surrounded by a reedy wetland. The layers of sediment that once lay at the bottom of Lake Flixton are still there, buried beneath the peat deposits that helped preserve the site’s huge collection of Stone Age artifacts.

A team of paleoclimate researchers led by Blockley took sediment cores from what was once the deepest part of Lake Flixton. Throughout the layers of sediment, tiny fossilized animal and plant remains still contain oxygen molecules from the late Holocene in their shells, which provide a way for paleoclimate researchers to estimate average temperatures at the time when each layer settled to the bottom of the lake. In general, rainfall contains more of the heavier oxygen isotope (oxygen-18) when temperatures are cooler, so oxygen isotope ratios in sediment layers contain information about ancient temperature fluctuations.

Using radiocarbon dating and deposits of volcanic ash that had drifted to Star Carr from distant eruptions, the team was able to date the layers. When they lined them up with radiocarbon dates from artifacts at Star Carr, the researchers had a side-by-side comparison of climate events and human activity in the area.

“Significant and extreme cooling events”

They identified two abrupt climate events in the early Holocene part of the record. The first, about 11,400 years ago, dropped summer temperatures by about 1.5 degree Celsius. That may not sound like much, but it can have a major impact on some plant and animal species. And winters during the hundred-year event seem to have been much harsher, since the team estimated a 10 degree Celsius drop in average annual temperatures.

People were just settling in the area of Star Carr around this time, and they set down a scattered mixture of worked and natural wood and other artifacts to help provide firmer footing at the muddy edge of the lake, like a very rudimentary boardwalk. Among the wood, they left behind animal bones, barbed projectile points, flint blades, deer antler headdresses, and other artifacts.

“We think the majority of materials there have been placed deliberately for ritual purposes,” said archaeologist Nicky Milner of the University of York, one of the study’s co-authors. An elk skull, placed among the wood debris in the earliest stages of its use, seems to have a similar placement to a set of elk bones found at a site in Denmark—both were carefully wrapped in skins before being placed. Modern hunter-gatherers around the world deposit animal remains and other artifacts at important locations in similar ways.

The century-long cold snap doesn’t seem to have stopped these post-Ice Age hunter-gatherers from living alongside Lake Flixton and its adjacent wetlands. In the archaeological record at Star Carr, layers of worked wood and artifacts kept being deposited right through the climate event 11,400 years ago.

It’s possible that the cold spell slowed down the process of settlement expansion, but archaeologists and paleoclimate researchers can’t yet say for sure. But even in a population that seems like it should have been very vulnerable to climate change, there’s no evidence that the community at Star Carr experienced a population crash or abandoned the area for some place warmer and more hospitable.

By 11,100 years ago, when the climate turned suddenly chilly again, people had settled here in greater numbers and started building large timber platforms around the edge of the lake, as well as structures further inland that archaeologists say were probably houses. These are the oldest built structures, and the oldest evidence of carpentry, in Britain. They seem to have been the community’s way of getting better access to Lake Flixton as its water levels were beginning to gradually drop.

Around this time, the average temperature dropped by about 4 degrees Celsius. The region around Star Carr was, like most of the rest of the British Isles, slowly shifting from brushy grassland to woodlands at the time, but there’s some evidence in the pollen record that the abrupt cooling halted that process here, at least temporarily. People at Star Carr stuck it out, though, and their construction projects seem to indicate they were doing a lot more than just hanging on with frostbitten fingers.

“They certainly had the skills to survive, and they certainly seemed to be eating the same kinds of animals as they did at other times,” said Milner.

Resilient hunter-gatherers

The hunter-gatherers of Star Carr took the wildly oscillating climate in stride, apparently. Both of the events recorded in the Lake Flixton sediment cores—which also show up in ice cores from Greenland—were at least as severe, and lasted at least as long, as an 8,200-year-old event that has been accused of wreaking havoc on early British populations. According to Blockley and his colleagues, that’s evidence that the unpredictable, generations-long cold spells of the early Holocene didn’t necessarily spell doom for hunter-gatherers, even with their rather tenuous foothold on the continent.

But it doesn’t mean that people everywhere weathered the cold as well as it seems they did at Star Carr. Environment and culture probably had a lot to do with that, Blockley and his colleagues say. The landscape at Star Carr was a mosaic of wetland, open grassland, shrub, and woodland, offering diverse sources of food for hunter-gatherers, as well as ecosystems that were less affected by cooling in the first place.

“Star Carr resources were broad and are likely a key reason for occupation at the site,” said Milner. “They may well have helped to buffer them against abrupt climate events.”

But as Lake Flixton gradually filled in with peat deposits, and the surrounding landscape shifted toward a broad expanse of swampy woodland, people gradually drifted away from Star Carr, seeking a better place to make a living. The area’s ecological diversity helped people survive a climatic roller coaster, but they couldn’t ride out the loss of that diversity.

People in less diverse environments may have had a harder time adapting if the cooler climate had more of an impact on the local flora and fauna. By the time the temperatures plunged again 8,200 years ago, most of the British Isles were covered with forests, which are more sensitive to changing temperatures.

The big picture

Milner says it’s important to examine similar sites at other locations in Northwest Europe to get a fuller picture of how early Holocene people in different places handled the unstable climate. And while it’s reassuring to think that even Stone Age hunter-gatherers were able to handle drastic temperature fluctuations with the classic British stiff upper lip, Milner cautions that modern societies face a very different situation.

As the Greenland Ice Sheet melts, the sudden flood of cold fresh water into the North Atlantic could cool down Western Europe, much like the melting ice sheets of the early Holocene did, but small groups of hunter-gatherers have more flexibility than large urbanized societies.

“Furthermore, the people of Star Carr were part of a tradition that had experienced dramatic climate shifts at the end of the last age. Extreme climatic instability was part of their way of life,” said Milner. But we’ve had thousands of years of stable climate. “Whether we have the potential to show the same resilience to sudden extreme climate change as the people of Star Carr is yet to be tested,” she said.

Nature Ecology & Evolution, 2017. DOI: 10.1038/s41559-018-0508-4  (About DOIs).

https://arstechnica.com/?p=1282071