Buried deep in Greenland's ice sheet lies a puzzling chemical signature that has sparked intense scientific debate. A sharp spike in platinum concentrations, discovered in an ice core (a cylinder of ice drilled out of ice sheets and glaciers) and dated to around 12,800 years ago, has provided support for a hypothesis that the Earth was struck by an exotic meteorite or comet at that time.
Author
- James Baldini
Professor in Earth Sciences, Durham University
Our new research offers a much more mundane explanation: this mystery platinum signature may have originated from a volcanic fissure eruption in Iceland, not space.
The timing matters. The platinum spike occurs near the beginning of our planet's last great cold period, the Younger Dryas Event. This lasted from about 12,870 to 11,700 years ago and saw temperatures plummet across the northern hemisphere.
This happened just as the planet had actually been warming up from the last ice age. Understanding what triggered this cold snap could help us understand how Earth's climate may change in the future.
We propose that this icy phase in Earth's climatic history was in fact caused either by a large volcanic eruption in Germany or by the eruption of an unknown volcano.
A climate mystery
Ice cores show that during the millennium-long Younger Dryas Event, temperatures across Greenland dropped to more than 15°C colder than they are today. Europe returned to near glacial conditions, with tundra replacing forests that had begun to flourish. Low-latitude rainbelts shifted to the south.
The traditionally accepted explanation involves a massive release of freshwater from melting North American ice sheets. This freshwater pulse disrupted the ocean circulation, affecting temperatures. However, other researchers have proposed that the event was triggered by a comet or asteroid impact over North America .
In 2013, researchers analysing ice cores drilled as part of the Greenland Ice Sheet Project (GISP2) discovered platinum concentrations that were well above normal levels . The ratio of platinum to a radioactive element called iridium was also unusual because space rocks usually have high levels of iridium, while the ice core spike does not. The ice core signature was very different from anything seen in known meteorites or volcanic rocks.
The authors of the space impact paper suggested that perhaps the unusual ice chemistry reflected the impact of an unusual asteroid made up of iron.
A subsequent paper proposed that the ice chemistry could reflect the German Laacher See volcanic eruption , which had an unusual geochemistry and occurred around that time. To test this idea, we collected and analysed 17 samples of volcanic pumice from deposits left behind by the Laacher See eruption. We measured platinum, iridium, and other trace elements to create a chemical fingerprint of the eruption.
Our results were clear: the Laacher See pumices contain virtually no platinum, with concentrations below or barely at detection limits. Even though some platinum may have escaped to the atmosphere before being trapped in the rock, the eruption was clearly not the source of Greenland's platinum spike.
Additionally, when we examined the timing carefully, using updated ice core chronologies, we found the platinum spike actually occurred about 45 years after the Younger Dryas began - too late to have triggered the cooling.
This result was arrived at independently but was consistent with previous research finding the same thing . Importantly, the elevated platinum concentrations lasted for 14 years, suggesting a prolonged event rather than an instantaneous asteroid or comet impact.
We compared the ice core's chemical signature with various other geological samples and found the closest match was with volcanic gas condensates (the products formed when gases released from a volcano cool from a gas to a liquid or solid state) particularly from submarine volcanoes.
Iceland's volcanoes can produce fissure eruptions lasting years or even decades, matching the 14-year duration of the platinum spike. During the melting phase that preceded the Younger Dryas, Iceland's volcanic activity increased dramatically as melting ice sheets reduced pressure on the Earth's crust.
Crucially, submarine or subglacial eruptions interact with water in ways that could explain the unusual chemistry. Seawater can strip away sulphur compounds while concentrating other elements like platinum in volcanic gases. These platinum-rich gases could then travel to Greenland and be deposited on the ice sheet, explaining the odd geochemistry.
Recent research on historical Icelandic eruptions supports this mechanism. The 8th-century Katla eruption produced a 12-year spike in heavy metals like bismuth and thallium in Greenland ice cores. The 10th-century Eldgjá eruption resulted in a cadmium spike within glacial ice. Although platinum was not measured in those studies, these examples show Icelandic volcanoes regularly deliver heavy metals to the Greenland ice sheet.
A smoking gun?
Because of the chronological mismatch, whatever mechanism was responsible for the platinum spike didn't trigger the Younger Dryas. Our research does, however, highlight previous results showing a massive volcanic sulphate spike in multiple ice cores coinciding precisely with the onset of cooling 12,870 years ago.
This eruption, whether from the Laacher See eruption or an unknown volcano, injected enough sulphur into the atmosphere to rival the largest eruptions in recorded history. Volcanic eruptions can trigger cooling by releasing sulphur into the stratosphere, reflecting incoming sunlight and potentially setting off a cascade of positive feedbacks including sea ice expansion, changed wind patterns and disruption of ocean currents, though future research needs to explore this further.
The substantial volcanic forcing around the Younger Dryas onset - a time when climate was already sitting between a glacial and an interglacial (the periods between cold snaps) - may have provided the nudge that tipped Earth's climate back into a cold state.
It is important to note that our research focused on the platinum spike and did not consider other evidence, such as spherules (spherical fragments of melted rock) and black mats (mysterious dark layers in soil), for an extraterrestrial impact . That said, based on our analysis of the new results and existing data, a large northern hemispheric volcanic eruption seems to be the most straightforward explanation for the Younger Dryas Event.
Understanding past climate triggers is vital for anticipating what lies ahead. Although the chance of a large meteorite impact or volcanic eruption in any given year is low, such events are virtually certain to occur eventually. Knowing how Earth's climate responded in the past is therefore crucial for preparing for the consequences of the next major event.
James Baldini does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
