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A puzzling chemical signal in Greenland ice
Buried in an ice core from central Greenland is a distinct, short-lived spike in platinum concentration dated to roughly 12,800 years ago. That anomaly has been invoked as forensic evidence for a catastrophic extraterrestrial event — a meteor or comet impact — that some researchers proposed triggered the abrupt cooling episode known as the Younger Dryas. New geochemical work, however, points to a far less exotic source: volcanism, likely from fissure or submarine eruptions in the North Atlantic region, especially Iceland.
The Younger Dryas was a dramatic, millennium-long reversal to near-glacial conditions that began about 12,870 years ago and lasted until ~11,700 years ago. Northern-hemisphere temperatures plunged, European forests retreated and expanded tundra took their place, and tropical rainfall belts migrated southward. Resolving the trigger for this rapid cooling matters because it illuminates how large external forcings — volcanic eruptions, impact events, or ocean-circulation changes — can tip a vulnerable climate system.
Scientific background and competing hypotheses
Two main classes of explanations have vied for acceptance. The long-standing hypothesis emphasizes a massive freshwater discharge from melting North American ice sheets that disrupted Atlantic meridional overturning circulation, weakening northward heat transport. Alternative proposals argue for an abrupt, externally forced perturbation: an extraterrestrial impact that injected dust, aerosols and melt products into the atmosphere, or an unusually large volcanic eruption that released sulfur into the stratosphere and produced hemispheric cooling.
A key piece of the impact argument was the 2013 report of a sharp platinum enrichment in a Greenland ice core recovered by the Greenland Ice Sheet Project (GISP2). The chemical fingerprint — a high platinum concentration combined with a low iridium-to-platinum ratio — did not match typical meteorite compositions, nor did it fit expected volcanic rock signatures. Some authors suggested an unusual, iron-rich impactor might explain the discrepancy.
New analyses: testing a volcanic origin
To test the volcanic alternative, researchers collected 17 pumice samples from deposits of the Laacher See eruption in Germany, a large volcanic event that erupted around the Younger Dryas interval and has an uncommon geochemical signature. The team performed trace-element analyses, focusing on platinum and iridium as well as other rare metals, to build a compositional fingerprint of Laacher See ejecta.
Sampling and geochemical methods
Analytical work used high-sensitivity techniques capable of detecting platinum at sub-ppm levels and quantifying iridium and other chalcophile elements. The study also re-examined ice-core chronologies to refine the temporal relationship between the platinum spike and the onset of Younger Dryas cooling.
The results were decisive: Laacher See pumices contained virtually no detectable platinum; measured concentrations were at or below analytical detection limits. That finding makes Laacher See an implausible direct source for the Greenland platinum anomaly. Updated ice-core dating further showed the platinum spike began roughly 45 years after the recorded start of Younger Dryas cooling — too late to be the primary trigger for the event.
Evidence points to Icelandic fissure or submarine eruptions
If Laacher See is excluded, what volcanic processes can produce a brief, high-platinum signal in Greenland ice and produce an anomalous platinum-to-iridium ratio? Comparative analyses found the best geochemical match between the ice-core signature and volcanic gas condensates derived from eruptions that interact with water — notably submarine or subglacial eruptions.
Icelandic volcanic systems are especially relevant. During deglaciation, reduced overburden pressure on Iceland’s crust increases melt production and can provoke long-lived fissure eruptions. Fissure systems in Iceland are capable of sustained activity lasting years to decades — comparable to the ~14-year duration of the platinum anomaly recorded in the ice core. When eruptions occur underwater or beneath ice, interactions with seawater and meltwater can strip sulfur from gas phases and preferentially concentrate certain metals in condensates and aerosols. Those metal-rich volcanic gases and fine particulates can be lofted into the atmosphere and transported to Greenland, where they are deposited on the ice sheet.
Historic precedents support this mechanism. Studies of the 8th-century Katla eruption show a 12-year spike in heavy metals such as bismuth and thallium in Greenland ice cores, while the 10th-century Eldgjá eruption produced a cadmium anomaly in glacial records. Although those investigations did not measure platinum specifically, they demonstrate that Icelandic eruptions can deliver episodic pulses of heavy metals to Greenland.
The volcanic-condensate hypothesis also accounts for the unusual iridium-to-platinum ratio: seawater and water–magma interactions can alter the partitioning of chalcophile elements in volatile phases, producing a composition distinct from both typical subaerial volcanic rocks and ordinary meteorites.
Implications for the Younger Dryas trigger
Because the platinum spike post-dates the initial Younger Dryas cooling by several decades, it is unlikely to have been the causal mechanism for that abrupt climate shift. However, independent ice-core data show a large sulfur-rich volcanic signal exactly at the Younger Dryas onset (around 12,870 years ago), consistent with an injection of stratospheric sulfate capable of producing short-term global cooling. A large eruption — whether from Laacher See, an as-yet-unidentified Northern Hemisphere volcano, or a cluster of high-latitude eruptions — could have provided the additional radiative forcing needed to nudge a marginal climate system back toward glacial conditions.
Volcanic aerosols can reduce incoming solar radiation, expand sea ice, modify atmospheric circulation, and perturb ocean–atmosphere heat exchange. These cascading feedbacks, acting on a climate already poised between glacial and interglacial states, could amplify a volcanic pulse into a prolonged cold interval. While the new findings challenge an extraterrestrial explanation based on the platinum spike, they do not address other purported evidence for impacts, such as spherules or so-called "black mats." A comprehensive account of Younger Dryas causation may require integrating multiple lines of evidence — geochemical, sedimentary, and paleoecological — across different sites and continents.
Expert Insight
"This study underscores how complex signatures in the geologic record can be interpreted several ways," said Dr. Laura Hayes, volcanologist and paleoclimate researcher. "High-precision trace-element work and refined ice-core chronologies are fundamental for distinguishing between an exotic impactor and terrestrial volcanic processes. Icelandic fissure and submarine eruptions are credible sources for short-lived metal anomalies in Greenland ice — and they remind us that volcanism can produce regionally large climatic effects when timing and background state align."
Conclusion
Re-evaluation of the Greenland platinum spike suggests a volcanic origin related to Icelandic fissure or submerged eruptions is the most parsimonious explanation for the anomalous chemistry recorded ~12,800 years ago. Laacher See pumice lacks the platinum signature needed to link that eruption to the ice-core anomaly, and the platinum peak occurs decades after the Younger Dryas began, making it an unlikely trigger for the initial cooling. Nonetheless, other volcanic sulfate evidence aligns with a volcanically driven perturbation at the Younger Dryas onset, supporting the view that large Northern Hemisphere eruptions — rather than a single catastrophic impact — remain a leading candidate for the abrupt climate reversal. Continued high-resolution ice-core work, broader trace-element surveys of suspected volcanic deposits, and integrated modeling of volcanic aerosol forcing will be needed to fully resolve the sequence of events that tipped Earth into the Younger Dryas.
Source: sciencealert
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