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Coral skeletons can preserve evidence of volcanic eruptions indirectly, but they do not yet provide a simple, definitive eruption detector. Two kinds of evidence are being studied: oxygen-isotope layers that record cooling after some eruptions, and trace-metal changes produced when living corals are exposed to volcanic ash.
The distinction matters. A historical coral record can show that cooling happened near the date of an eruption identified independently in ice cores, while a laboratory experiment can show that ash changes skeletal chemistry. Neither result, on its own, proves that a particular chemical pattern in a wild coral came from one specific eruption.
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What information is stored in a coral skeleton?
Reef-building corals add skeletal material in layers as they grow. Those layers can be sampled sequentially, dated, and analyzed for chemical and isotopic composition. In principle, a long-lived colony is a natural environmental archive: each growth interval captures something about the seawater and conditions surrounding the coral at that time.
Oxygen isotopes are especially useful for reconstructing past climate. Their ratios in carbonate skeletons respond to seawater temperature and to the isotopic composition of seawater. A volcanic eruption can inject particles and gases into the atmosphere, alter climate, and produce regional or global cooling. If that cooling affects the ocean around a coral, an oxygen-isotope record may preserve a time-aligned excursion.
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Trace elements offer a different route. Ash contains minerals and metals that can enter seawater. A coral and its associated tissue and symbiotic microorganisms—the coral holobiont—may take up some of those elements, changing the distribution of metals in newly formed skeleton. This is a response to ash-derived inputs, not necessarily a unique chemical fingerprint of the volcano that produced them.
Interpretation is complicated by “vital effects.” Corals do not all build skeletons in exactly the same way, and biological processes can modify the chemical signal expected from seawater alone. The USGS-hosted review by Robinson and colleagues (2014) describes both the value of coral archives and the diversity of biomineralization strategies that can affect their chemistry.
Two different kinds of volcanic evidence
Evidence on coral and eruptions currently falls into two complementary but non-equivalent approaches.
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| Approach | What is measured | How evidence is obtained | What it supports | Main attribution problem |
|---|---|---|---|---|
| Historical climate recording | Oxygen-isotope variation in dated coral layers | A natural coral archive is compared with eruption dates established from other records, such as ice cores | A possible cooling response occurring near an eruption | Cooling can have non-volcanic causes, including El Niño, and the signal is not direct ash detection |
| Experimental ash exposure | Trace-metal concentrations in skeleton, tissue, and symbionts | Living corals are exposed to volcanic ash under controlled laboratory conditions and compared with untreated controls | Evidence that ash can alter the coral holobiont’s metal distribution | One species and one treatment do not establish a universal field proxy for past eruptions |
What the 335-year New Caledonia record found
In a study published in Paleoceanography in 1997, Thomas Crowley, Terrence Quinn, Frederick Taylor, Christian Henin, and Pascale Joannot analyzed a 335-year oxygen-isotope record from a New Caledonia coral. The record spans 1657 through 1992, giving a long sequence against which to compare known volcanic events.
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The eruption chronology came from ice-core records rather than from the coral itself. Crowley and colleagues reported as many as 16 interannual-scale cooling events in the coral occurring within one year of eruptions identified in those ice cores. That timing is consistent with the idea that volcanic forcing can leave a climate signal in tropical coral growth layers.
The result is an association, not a chemical identification of ash. The coral isotope record indicates that local environmental conditions changed in a way compatible with cooling. It does not demonstrate that ash particles from each eruption reached New Caledonia or that the coral incorporated a diagnostic volcanic substance.
The authors also identified an important confounder: some of the cooling events coincided with El Niño. El Niño can cool this part of the western South Pacific, so the same isotope excursion could reflect internal ocean-atmosphere variability rather than an eruption, or a combination of causes. Crowley and colleagues therefore said that further work was needed to substantiate the conclusions.
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F. Förster and co-authors reported a separate line of evidence in Biogeosciences in 2025. They exposed the coral Stylophora pistillata to volcanic ash for six weeks, then measured metals in the coral skeleton, tissue, and symbiotic organisms. Untreated corals served as controls.
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The clearest changes occurred in the skeleton. Relative to controls, ash-exposed corals had:
| Element | Reported skeletal change | Qualification |
|---|---|---|
| Iron (Fe) | 2.4-fold increase | Relative concentration after the six-week laboratory exposure |
| Manganese (Mn) | 2.1-fold increase | Relative concentration after the six-week laboratory exposure |
| Nickel (Ni) | 1.26-fold increase | Relative concentration after the six-week laboratory exposure |
| Zinc (Zn) | 1.15-fold increase | Relative concentration after the six-week laboratory exposure |
| Vanadium (V) | 1.12-fold increase | Relative concentration after the six-week laboratory exposure |
| Copper (Cu) | Lower than controls | The study did not give a fold decrease in the cited results |
| Lead (Pb) | Lower than controls | The study did not give a fold decrease in the cited results |
These findings show that volcanic ash can alter the coral metallome—the distribution of metals across the holobiont—and that the skeleton can register some of those changes. They do not show that every eruption creates the same pattern, that the pattern survives unchanged for centuries, or that it can identify a volcano without supporting evidence.
The study’s own conclusion is explicit: “Although no definitive skeletal proxy for volcanic eruptions was identified, the metallomic shifts observed in coral skeletons in response to ash-derived metal inputs, and calculated metal distribution coefficients, may help interpret geochemical signals in corals near volcanic islands.”
Why a coral signal is difficult to assign to a volcano
Climate variability can mimic volcanic cooling
An isotope excursion records the conditions experienced by the coral, not the cause written on a label. El Niño is a demonstrated alternative explanation for some of the New Caledonia cooling events. Other oceanographic changes could also alter local temperature or seawater isotopic composition at the same time as an eruption.
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Biology modifies skeletal chemistry
The coral animal, its symbionts, and the process of calcification all influence which elements enter the skeleton. The vital-effects problem means that two coral species, or even two colonies of the same species, may not record identical seawater chemistry. A metal increase therefore has to be interpreted in the context of the coral’s biology and growth conditions.
Ash exposure is not the same as natural fallout
The Förster experiment used a controlled six-week treatment. Natural reefs experience variable ash loading, currents, dilution, particle settling, resuspension, and chemical weathering. The 2025 paper specifically calls for more work on ash loading, biological stress, and oceanographic dynamics before skeletal chemistry can be used confidently as an eruption indicator.
One pattern may have several sources
Metals can enter coastal water through more than volcanic ash, and a climate signal can have more than one driver. A credible attribution therefore requires multiple observations rather than a single elevated element or one unusual isotope value.
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Coral chemistry is most informative when treated as one part of a dated, multi-proxy investigation. A practical workflow is:
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- Build the coral chronology. Establish the age and growth sequence of the sampled layers using appropriate dating and annual or seasonal markers.
- Measure climate-sensitive isotopes. Analyze oxygen-isotope variation to identify intervals that indicate unusual cooling or seawater change.
- Measure trace metals separately. Test for the element shifts observed in ash-exposure work, while recording the coral species, growth conditions, and analytical uncertainty.
- Compare with an independent eruption timeline. Use dates from ice-core chemistry, tephra layers, historical observations, or other geological evidence instead of treating the coral as its own proof.
- Check climate alternatives. Examine whether El Niño or other oceanographic conditions could explain the same interval.
- Look for replication. Compare colonies, species, and locations. A pattern appearing in only one colony is weaker than a synchronous signal across several independently dated corals.
- Test the environmental pathway. Determine whether ash could have reached the site, how much was deposited, and whether currents and water chemistry would have delivered its metals to the coral.
This approach separates correlation from mechanism. Isotopes can indicate that cooling occurred; metals can indicate that ash exposure affected the coral; independent geological evidence is needed to connect those observations to a particular eruption.
What coral skeletons can—and cannot—tell us
| Question | What the current evidence supports |
|---|---|
| Can a coral preserve environmental history? | Yes. Layered growth and datable chemistry make corals useful archives, although biological vital effects complicate interpretation. |
| Can coral records show cooling near eruptions? | Possibly. The 1657–1992 New Caledonia record contained up to 16 cooling events within a year of ice-core-identified eruptions. |
| Does an isotope excursion prove an eruption? | No. Some of the same events coincided with El Niño, which can cool the western South Pacific. |
| Can ash change coral skeletal chemistry? | Yes, under the reported laboratory conditions. S. pistillata showed increases in several metals and decreases in Cu and Pb after six weeks. |
| Is there a definitive coral-skeleton eruption proxy? | No. The 2025 experimental study explicitly says that no definitive skeletal proxy was identified. |
| Can coral chemistry identify the source volcano by itself? | Not on the cited evidence. Source attribution needs independent tephra, ice-core, geological, or historical constraints. |
Why other archives remain essential
Coral records are one archive among several. A 2025 study in PNAS identified the source of the 1831 eruption as Zavaritskii caldera on Simushir Island in the Kurils by combining ice-core geochemistry with tephra correlation. That reconstruction illustrates how eruption histories are strengthened by matching volcanic particles and chemical signatures across independent records.
The 1831 result is not coral evidence, and it should not be presented as one. Its relevance is methodological: ice cores, tephra, geological deposits, historical observations, and coral climate archives answer different parts of the attribution problem. Coral isotopes may indicate how the ocean responded; coral metals may reveal exposure to ash-derived inputs; other archives can establish when and where the eruption occurred.
The evidence-based verdict
Coral skeletons can hold a history of volcanic effects, but the history is indirect and conditional. The New Caledonia oxygen-isotope record shows that coral archives may preserve cooling that occurs near eruptions, while also demonstrating how El Niño can create a competing explanation. The 2025 ash experiment shows that exposure can shift skeletal metals, including 2.4-fold higher Fe and 2.1-fold higher Mn under the study’s six-week treatment, but it did not establish a universal field signature.
For now, the strongest use of coral skeletons is as a climate and environmental archive that can contribute to a multi-proxy eruption reconstruction. They are not a standalone detector that can identify every past eruption from one chemical pattern.
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