Of Solar Flares, Cosmic Ray Physics and American Vikings

Exactly a thousand years ago this year an American Viking living in the Norse outpost on Straumfjord, on the northern tip of Newfoundland, took a metal axe and cut a tree.  The trimmed parts of the tree were cast away and, almost a thousand years later, were found by archeologists and stored for later study. What that study found was an exact date of the felling of the tree, in AD 1021.

How can that date be known to such precision?  The answer comes from a confluence of modern science: solar flares, cosmic ray physics, archeology, recent advances in dendrochronology, and the historiography of Icelandic sagas. The new findings were reported in the Oct. 20, 2021 issue of Nature.

American Vikings

Snorri Thorfinnsson was the first American Viking born in the Western Hemisphere.  He was born in Newfoundland sometime around AD 1007, the son of Thorfinn Karlsefni and his wife Gudrid Thorbjarnardottir, who were exploring the wooded coasts of Labrador and Newfoundland for timber to bring back to the Norse settlements in Greenland which had no wood for building.  Thorfinn and Gudrid traveled in a small fleet of Norse trading vessels known as knarrs.   

Knarrs were not the sleek long boats of Viking raiders, but were instead the semi-trailer trucks of the Viking expansion between AD 793 and 1066.  A knarr was an open planked boat about 50 feet long and 15 feet wide with a single mast and square-rigged sail.  It had a keel and could be rigged with a sprit to run close-hauled to the wind.  Its cargo was typically walrus ivory, wood, wool, wheat and furs with enough mid-ship room for a few livestock.

By using the technique of latitude sailing, that is by sailing to a known latitude and then keeping the North Star at a fixed angle above the horizon, knarrs could traverse the North Atlantic in a matter of weeks, sailing back and forth between Norway and Iceland and Greenland.  The trip from Greenland’s eastern settlement to Norway was 3000 km and took about 4 weeks (compare that to the two months it took the Mayflower to cross the Atlantic 600 years later).  Storms and bad weather put a crimp in this type of latitude sailing when the North Star could be obscured for days or weeks, and the sailors could end up somewhere they didn’t expect.  This is what happened to the merchant Bjarni Herjólfsson circa 985 when his ships were blown west in a terrible storm and he came upon a land of white beaches and green forests stretching to the horizon.  To get home, he sailed north along the new-discovered coast to the known latitude of Greenland and then headed east until he hit land. 

Map of the Norse voyages. Yellow: 3000 km between Greenland and Norway (about 4 weeks by knarr) was a “routine” voyage. Red: 3000 km between Greenland and the Norse outpost at Straumfjord in Newfoundland (about 4 weeks by knarr). Green: 2000 km from the northern tip of Newfoundland to Long Island Sound (about 3 weeks by knarr). Butternut wood remnants discovered at Straumfjord likely came from the southern coast of Maine or the coast of Connecticut.

Bjarni never set foot on the new land, but his tale inspired Leif Eriksson, the son of Erik the Red, to explore the new world.  Leif bought Bjarni’s knarr and with a small fleet sailed up the west coast of Greenland to where Bjarni had landed, then headed due west along the latitude of what is today the Davis Straight.  Leif made landfall on Baffin Island and sailed south down the Labrador coast to Belle Island in the Gulf of St. Lawrence, that he named Straumfjord, and then across to the northern tip of Newfoundland on the edge of a shallow bay where they could run their ships onto shore.  There, sometime around AD1000 they built a small settlement of wood houses that they used as a base for wider explorations of the land they called Vinland.  Later expeditions returned to the Straumfjord settlement and expanded it, including Thorfinn and Gudrid, where their son Snorri was born. 

View of the reconstructed Norse outpost at L’Anse aux Meadows in Newfoundland, Canada, and the Gulf of St. Lawrence (Straumfjord).

The voyage one-way between Newfoundland and Greenland took only 3 to 4 weeks, and each successive group repaired the damage from the ravages of the Newfoundland weather.  One of these repairs happened in the year AD 1021, long after Thorfinn and Gudrid and Snorri had resettled in northern Iceland, where their descendants crafted a saga of their exploits that was passed down by oral tradition through the generations until they were written down around AD 1400 and then almost forgotten…until the archeologist Anne Stine Ingstad with her husband Helge Ingstad found the remains of wood houses in 1960 buried under the turf at a place called L’Anse aux Meadows on Newfoundland’s northern tip. 

The Icelandic Saga of Erik the Red written around 1387-1394 and known as the Flateyjarbók (The Flatley Book).

The outpost at L’Anse aux Meadows was used on and off for decades as a base for the timber and fur trade. In addition to the dwellings, vast numbers of wood chips and discarded tree parts were uncovered, pointing to an active timber operation. Some of the wood is from the butternut tree which does not grow in Newfoundland nor anywhere along the shores of the Gulf of St. Lawrence. The modern areas of the butternut tree within range of Norse excursions are from the southern coast of Maine and the coast of Connecticut on Long Island Sound. Given how freely the Norse sailed their knarrs, making routine voyages of several weeks duration, the three-week trip from L’Anse aux Meadows to Long Island Sound seems easy, and there were plenty of bays to slip into for provisions as they went. Although there is no direct evidence for the Norse presence along the northeastern coast of the US, it seems highly likely that they plied these waterways and brought back the butternut timber to L’Anse aux Meadows.

Carbon 14 dating placed the age of the outpost at L’Anse aux Meadows at around AD 1000, consistent with the chronology of the Icelandic Sagas. But with an accuracy of plus or minus several decades it was not possible to know where it fit into the story…except for a lucky accident of solar physics.

Miyake Events and Solar Physics

In 2012, while studying tree rings from two cedar trees in Japan, Fuse Miyake of Nagoya University and his team from the Solar-Terrestrial Environment Laboratory made the unexpected discovery that a single tree ring, shared in common between the two specimens, had 20% more Carbon 14 than any of the other rings.  The ratio of Carbon 14 in nature to the usual Carbon 12 is very stable, with a variation of about 2% year to year, mostly due to measurement accuracy.  Therefore, the 20% spike in Carbon 14 was a striking anomaly.  By comparing the known ages of the cedars to the rings, using the techniques of dendrochronology, the date of the ring growth was pinpointed to the year 774-775.

A solar flare like this may generate a solar proton event (SPE).

Such a sudden increase in Carbon 14 over only a year’s time could only be caused by a sudden and massive influx of high-energy cosmic rays into the Earth’s upper atmosphere.  Carbon 14 is generated by the capture of 10-40 MeV neutrons by Nitrogen 14 followed by proton decay of the excited nitrogen nucleus.  The high-energy neutrons are generated as byproducts of even higher energy processes.  Miyake and his team considered high-energy gamma photons from a local super nova, but that was not consistent with the timing or amount of Carbon 14 that was generated.  They next considered a massive generation of high-energy solar protons when the sun spits out a massive surge of high-energy protons.    The exact cause of a solar proton event is still debated, but it is likely to be associated with solar flares that accelerate the protons to high energy.  The high-energy protons can penetrate the Earth’s magnetic field and cause particle cascades in the upper atmosphere.  They called it a Solar Proton Event (SPE), but it has since been renamed a Miyake Event.

Solar proton events may be associated with the Aurora Borealis. In the year of the Miyake event of 774 there were historical reports of unusual atmospheric lights and patterns. The Aurora is caused by electron currents which may be associated with the proton event.
High-energy protons from the sun cause high-altitude cosmic ray cascades that also produce high-energy neutrons. The neutrons are captured by Nitrogen 14 which decays rapidly into Carbon 14. Carbon 14 eventually decays back to Nitrogen 14 with a half life of about 5000 years.

Miyake Events are extremely rare.  There have been only about 3 or 4 SPE’s in the past 10,000 years.  By luck, another Miyake Event occurred in 993, about 8 years after Bjarni Herjólfsson was blown off course and about 7 years before Leif Eriksson began exploring the new world.  The excess Carbon 14 rained down on Earth and was incorporated into the fresh growth of juniper and fir trees growing near the northern Newfoundland shore.  Twenty seven years later, while repairing Leif Eriksson’s wood houses, a Viking felled the trees with a metal axe.  Chunks of the trees were discarded, with the traces of the metal axe blade as well as the outer bark of the tree intact.

The intact bark on the wood pieces was an essential part of the dating. Simply by counting the number of tree rings from the ring of 993, it was possible to know not only the year the tree was cut down, but even the season. Furthermore, the existence of the marks from the metal axe confirmed that the tree was felled by someone from the settlement because there were no metal tools among the indigenous people.

The Norse timber traders treated the indigenous people terribly from the very first expeditions, with tales of wanton murder recorded proudly in the later sagas. This was ultimately their undoing. Resistance from the local tribes could be fierce, and the Norse could spare few casualties in their small expeditions. Eventually, the Norse were driven off. The wood structures at L’Anse aux Meadows were burned before they sank beneath the turf, and human remains with arrow wounds have been uncovered from the site, hinting at how this bold tale ended.