At the close of the last Ice Age, the North Atlantic climate lurched into chaos. The great Laurentide Ice Sheet was retreating across what is now Canada, vast meltwater lakes were swelling at its margins, and Europe had just emerged into a warmer, wetter phase known as the Bølling-Allerød. Then, around 12,870 years ago, the warming stopped abruptly. Temperatures plummeted, sea ice expanded, and the Younger Dryas cold period began. For more than a thousand years, the planet staggered back into glacial conditions before resuming its climb into the present Holocene warmth.
The cause of this sudden reversal has been debated for decades. The classic explanation invokes a catastrophic outburst of meltwater from glacial Lake Agassiz, flooding the North Atlantic and shutting down ocean circulation. In the early 2000s, another idea gained traction, the Younger Dryas Impact Hypothesis, which argued that a comet or meteor exploded over the ice sheet, throwing debris and soot into the atmosphere and triggering cooling. More recently, volcanism has entered the conversation, not as background noise but as a possible prime mover of the Younger Dryas shift.
The spark for this new debate came not from a smoking crater or an ash bed but from a thin section of Greenland ice. In 2013, Michael Petaev and colleagues at Harvard reported a startling signal in the Greenland Ice Sheet Project 2 (GISP2) core: an enormous spike in platinum concentrations exactly at the transition into the Younger Dryas. The anomaly stood out with platinum-to-iridium and platinum-to-aluminum ratios unlike any common terrestrial material. To Petaev’s group, the most plausible explanation was a rare type of iron meteorite with extremely high platinum and low iridium. That interpretation fit neatly with the impact hypothesis.
But the signal has never sat comfortably. No crater has been found of the right age and scale. Impact proxies like shocked quartz remain inconsistent. Meanwhile, other ice records tell a different story. The North Greenland Ice Core Project (NGRIP) record shows a colossal volcanic sulphate spike at almost the same time, around 12,870 years ago, with matching signals as far away as Antarctica. This pointed to a major eruption, one capable of blanketing the hemisphere in sulphur aerosols. The most obvious candidate was the Laacher See volcano in the East Eifel volcanic field of Germany, which erupted at nearly the same time, releasing more than six cubic kilometers of phonolite magma and as much as 15 teragrams of sulphur into the atmosphere, roughly twice the yield of Mount Pinatubo in 1991.
If Laacher See was indeed responsible, then perhaps its eruption explained both the sulphur and the platinum anomaly. That was the premise tested by Charlotte Green, James Baldini, Richard Brown, and colleagues, who returned to the tephra deposits of the Laacher See eruption with a new battery of geochemical analyses. They dug into outcrops around the crater, pulling pumice from three distinct stratigraphic horizons, and subjected the samples to fire assay and inductively coupled plasma mass spectrometry, pushing detection limits for platinum and iridium down to fractions of a part per billion.
The results were unequivocal. Platinum concentrations in the Laacher See Tephra were extremely low, often below detection, with the highest readings in the basal deposits barely reaching 0.2 milligrams per kilogram. Iridium was entirely absent. The ratios of hafnium to lutetium, platinum to lutetium, and other geochemical markers trended downward through the deposit but never matched the Greenland spike. The conclusion was unavoidable: Laacher See could not have been the source of the platinum anomaly.
That finding left scientists with two possibilities: either the spike represented an unusual extraterrestrial impact, or it came from a different volcanic source altogether. To discriminate, the team compared the Greenland ratios with published values for meteorites, volcanic rocks, and even secondary sediments. The platinum anomaly did not match any known meteorite group. It bore little resemblance to chondrites, iron meteorites, or achondrites. Nor did it align with typical volcanic rocks. Instead, the closest analogue came from an unexpected source: volcanic gas condensates collected at submarine eruptions.
Samples from the Niuatahi-Motutahi volcanic complex in the Tonga rear arc, for example, show platinum group element fractionation patterns strikingly similar to the Greenland anomaly. These condensates, stripped of ash and sulphur by interaction with seawater, concentrate platinum far out of proportion to iridium. Icelandic subglacial and submarine eruptions are known to generate similar volatile-rich plumes, carrying heavy metals upward while leaving little physical tephra in the atmosphere.
The ice itself strengthens this interpretation. The GISP2 platinum anomaly is not a sharp, instantaneous spike. Instead, elevated concentrations persist across a fourteen-year window, with the maximum peak around 12,822 years before present. This duration fits poorly with a bolide strike, which should leave a single pulse, but it matches the timescale of Icelandic fissure eruptions. Historical analogs exist. The Hrafnkatla eruptive episode in Iceland during the eighth century lasted twelve years, depositing a prolonged signal of bismuth and thallium in Greenland ice. Similarly, the Eldgjá eruption in the tenth century left heavy metal spikes traceable across multiple cores. These precedents suggest that the platinum anomaly may represent a sustained volcanic outgassing episode rather than a cosmic impact.
Timing is critical. The onset of the Younger Dryas, marked by sharp drops in oxygen isotope ratios in NGRIP and Seso Cave stalagmites in Spain, occurred at 12,870±30 years before present. The major sulphate spike in NGRIP aligns precisely with this cooling step. Mercury concentrations in the East Greenland Ice Core Project also peak at the same moment, supporting a large volcanic eruption. In contrast, the platinum anomaly appears about 45 years later, too late to have triggered the Younger Dryas itself. Svensson, Abbott, Holliday, and others have confirmed this offset on the GICC05 chronology. Green and colleagues independently verified the same result. Whatever caused the platinum spike, it was not the initial trigger of the cold event.
Yet volcanism remains central. Across the century bracketing the Younger Dryas onset, volcanic forcing calculated from sulphate fluxes exceeds anything in the last two millennia. Two major eruptions at 12,978 and 12,870 years ago stand out in ice records, ranked among the top twenty sulphur releases of the last hundred thousand years. Climate models suggest that such high-latitude sulphur injections would have cooled the northern hemisphere by ten watts per square meter, more than double the forcing from Pinatubo. The eruption or eruptions responsible remain unidentified, though Iceland is the leading suspect. The unloading of ice mass during deglaciation would have raised eruption rates by as much as a hundredfold.
The platinum spike, therefore, fits into a broader pattern. It may be the fingerprint of a fissure-style eruption beneath ice or sea near Iceland, one that spewed volatile-rich gases into the atmosphere for more than a decade. Chlorine-rich aerosols from such an event could have carried platinum preferentially over iridium, depositing it onto the Greenland ice sheet while leaving only muted sulphur signals thanks to dissolution in meltwater. The absence of visible tephra is consistent with subglacial eruptions where ash is scrubbed before reaching the open air.
Modern eruptions reinforce the plausibility. In 1991, Iceland’s Hekla and even the distant Mount Pinatubo caused platinum anomalies in Greenland ice. Antarctic snow shows similar spikes tied to Cerro Hudson in Chile. Gas emissions from Tolbachik in Kamchatka and Kudryavy in the Kuriles are enriched in platinum by factors of thousands relative to their parent magmas. At Erta Ale in Ethiopia, volcanic gases carried platinum and iridium at ratios far above crustal averages. The chemistry of the GISP2 spike, with its extraordinarily high platinum-to-iridium ratio, aligns best with such volatile-driven fractionation.
For advocates of the impact hypothesis, this poses a challenge. If volcanism alone can generate platinum anomalies in ice and snow, then the Greenland signal no longer demands a cosmic explanation. The lack of a crater, the absence of shocked minerals, and the timing mismatch with the Younger Dryas onset all weigh against an extraterrestrial trigger. Instead, the simplest explanation may be a long-lived volcanic eruption, probably Icelandic, that occurred shortly after the Younger Dryas began.
This interpretation reframes the Younger Dryas Event itself. Rather than a singular catastrophe, it may represent the cumulative effect of multiple large eruptions, including one around 12,870 years ago that injected sulphur and mercury into the atmosphere, driving rapid cooling, followed by another that enriched the ice with platinum. These eruptions occurred during a period of climatic fragility, when the Atlantic Meridional Overturning Circulation was sensitive to freshwater inputs and sea ice expansion. The volcanic veil could have tipped the balance, amplifying natural variability into a millennium-long stadial.
The Greenland cores preserve this sequence with remarkable fidelity. A sulphate spike, a drop in isotopes, a surge in platinum, all written in the layered snow of 13,000 years ago. Interpreting these signals requires careful alignment of timescales and cross-referencing with European lake varves, stalagmites, and marine sediments. As each dataset is synchronized, the narrative sharpens: volcanism, not impacts, holds the strongest claim.
Green and colleagues conclude cautiously. The Laacher See eruption can be ruled out. The platinum anomaly is not meteoritic. The closest match lies in volcanic condensates, probably Icelandic, released during a fissure eruption that lasted more than a decade. They recommend that future work replicate the platinum signal in other Greenland cores, search for additional platinum peaks across the deglacial interval, and analyze intervals of known volcanic heavy metal enrichment. Understanding the post-depositional mobility of platinum in ice will also be essential to confirm that the observed profile reflects primary deposition rather than migration.
As of September 2025, the platinum anomaly remains a singular feature of the GISP2 core. Its shape, chemistry, and timing all point to a volcanic source, most likely a fissure eruption under ice or sea near Iceland, occurring about 45 years after the Younger Dryas began. The investigation is not closed, but the case for impact has weakened considerably. In the stratified ice of Greenland, the record of ancient eruptions is etched not only in ash and sulphur but also in precious metals, waiting to be read.
Source:
Green, C. E., Baldini, J. U. L., Brown, R. J., Schmincke, H.-U., Edmonds, M., & Meisel, T. C. (2025). A possible volcanic origin for the Greenland ice core Pt anomaly near the Bølling-Allerød/Younger Dryas boundary. PLOS ONE, 20(9), e0331811. https://doi.org/10.1371/journal.pone.0331811
