The stakes of this work are considerable. Today, humanity is pumping carbon dioxide into the atmosphere at a rate of roughly 10 petagrams of carbon per year, a pace that scientists say is unprecedented across the last 66 million years. The only comparable episode in the geological record is the late Paleocene–early Eocene, when atmospheric carbon dioxide climbed to levels even higher than today’s, albeit roughly nine to ten times more slowly, at about 1.1 petagrams of carbon per year. During the Paleocene–Eocene Thermal Maximum, or PETM, deep ocean and high-latitude surface temperatures rose by 6 to 8 degrees Celsius, triggering widespread extinction of benthic foraminifera and profound ecological disruption in both marine and terrestrial ecosystems. Understanding how that ancient greenhouse world responded is one of the best natural experiments available for anticipating our own future.

The new study focuses on bulk organic carbon isotopes, a technique that measures the ratio of the light isotope carbon-12 to the heavier carbon-13 in the organic matter preserved within sediments. When enormous quantities of carbon-12-enriched carbon, whether from volcanic methane, destabilised gas hydrates, or other reservoirs, flood the ocean–atmosphere system, the isotopic signature of everything that grows and dies within it shifts sharply negative. Geologists call these shifts negative carbon isotope excursions, and they are the primary diagnostic tool for identifying hyperthermal events in the rock record. At Jerain–Dauki, the team collected thirty-six stratigraphically constrained samples spanning coal seams, sandstones, shales, siltstones and carbonaceous horizons across a vertical profile stretching from just 5 centimetres to more than 16 metres above the base of the section.

The results are striking. The δ¹³C values of the organic matter range from −23.1 to −32.5 per mil, a total variability of 9.4 per mil, and within that spread the researchers identified two major negative excursions of roughly 6 to 6.5 per mil and two minor ones of about 3 to 4 per mil. The first major excursion appears abruptly at about 4.87 metres up the section, where values plunge by around 6 per mil before slowly recovering. A smaller shift of roughly 3 per mil follows at 8.10 metres, then a second major excursion of about 6.5 per mil strikes at 11.60 metres, and a final minor excursion of around 4 per mil appears at 15.50 metres. The pattern of sharp drops followed by gradual recoveries suggests a complex, multi-phase history of carbon release rather than a single catastrophic injection.

Correlating these excursions with specific global events remains provisional, but the stratigraphy offers tantalising clues. The major excursion recorded between samples JD-8 and JD-9 may broadly correspond to the PETM itself, while one of the excursions higher in the section may be comparable to ETM-2, a smaller early Eocene hyperthermal that followed roughly two million years later. The dinoflagellate cyst assemblages recovered from the section, though sparse, include species such as Apectodinium longispinosum and Homotryblium tasmaniense that support a late Paleocene to early Eocene age, placing the excursions squarely within the window of globally recognised hyperthermal activity. The researchers stress that the presence of multiple negative excursions separated by intervals of partial recovery points to repeated pulses of isotopically light carbon entering the exogenic reservoir.

One of the most surprising findings concerns where the biggest isotopic shifts occur. Intuitively, one might expect the most negative carbon isotope values in the coal seams and carbonaceous shales, where organic matter is most abundant and best preserved. Instead, the largest excursions, at samples JD-9 and JD-25, occur within sandstone-dominated intervals. The team tested whether this could be an artefact of lithology, reworking, or organic matter mixing, and found the evidence against such a explanation compelling. If lithological control were dominant, the carbonaceous units would show the most negative values, yet those units remain relatively stable at around −25 to −26 per mil. Moreover, a statistical cross-plot of δ¹³C against total organic carbon content reveals essentially no correlation, with a coefficient of determination of just 0.05. The excursions, in other words, reflect genuine changes in the isotopic composition of the global carbon reservoir, not local quirks of sedimentation or preservation.

The palynological evidence also paints a vivid picture of the ancient environment. The samples are dominated by terrestrial material, including plant cuticles, degraded debris, black oxidised plant fragments and charcoal, along with mangrove pollen of the genus Spinizonocolpites, which is associated with the Nypa palm that fringed tropical coastlines of the Tethys Ocean. Marine indicators, including dinoflagellate cysts such as Apectodinium, Diphyes colligerum, Homotryblium tasmaniense, Hystrichokolpoma rigaudiae and Spiniferites, appear only in low abundances and in a handful of horizons. Together, these assemblages indicate deposition in a restricted marginal-marine setting, likely a coastal swamp or lagoon that was episodically inundated by marine waters. Earlier work from the region had documented an Apectodinium acme, a burst of this warm-water dinoflagellate that marks the PETM worldwide, in equivalent rocks of the Shillong Plateau, lending further support to the regional correlation.

Placed in a global context, the magnitude of the Jerain–Dauki excursions is remarkable. Open-ocean carbonate archives typically record PETM-related shifts of only about 2.5 to 4 per mil, whereas the major excursions here reach 6 to 6.5 per mil. Comparable large values have been reported from the Tingri section of Tibet, where excursions reach about 7 per mil, from Arctic Ocean higher-plant n-alkanes at 6 to 8 per mil, and from bulk carbonate records on the Atlantic Coastal Plain. Within India itself, the new record exceeds the excursions documented from western Indian lignite successions such as Vastan, Tadkeshwar, Giral and Panandhro, as well as the roughly 3.4 per mil excursion recorded in the Sylhet Lakadong Limestone. This variability highlights how differently the same global perturbation can be expressed across depositional settings, latitudes and proxy types, with marginal-marine and terrestrial organic records often showing amplified responses relative to deep-marine carbonates.

The broader significance of the study lies in filling a conspicuous gap. Most hyperthermal records come from sub-tropical to temperate regions of the Northern Hemisphere and from deep-sea cores, while genuinely tropical archives remain scarce. The Jerain–Dauki section demonstrates that tropical continental-margin systems were highly sensitive to Paleocene–Early Eocene carbon cycle perturbations and capable of preserving high-amplitude isotopic signals. Recent paleoclimate modelling suggests that these carbon release events were accompanied by enhanced terrestrial methane emissions and an intensified hydrological cycle, with increased global precipitation and evaporation under elevated atmospheric carbon dioxide, and the new record is consistent with those inferences. The authors note that future compound-specific biomarker analyses could further disentangle organic matter sources and local environmental influences from the global signal.

For a world once again conducting an uncontrolled experiment with the carbon cycle, the message from the ancient swamps of Meghalaya is sobering. Fifty-six million years ago, a slower but still enormous release of carbon-12-enriched carbon reshaped climates, acidified oceans, reorganised ecosystems and left its isotopic signature in sediments from the Arctic to the tropics. The fact that a restricted coastal bay on the Indian plate recorded these events so clearly, and in some cases more dramatically than the open ocean, underscores both the global reach of hyperthermal carbon injection and the value of tropical marginal-marine archives in constraining the magnitude, structure and mechanisms of early Cenozoic climate disruption. As researchers continue to assemble the global picture, records like Jerain–Dauki will be essential for testing how Earth’s climate system responds when the carbon cycle is pushed far beyond its normal operating range.

Subject of Research: Paleocene–Early Eocene carbon isotope excursions recorded in the tropical marginal-marine Lakadong Sandstone of north-eastern India

Article Title: A tropical marginal-marine record of Paleocene–Early Eocene carbon isotope excursions from the Lakadong Sandstone, South Shillong Plateau, India

Article References: Sharma, A., Uddandam, P. R., Shukla, M. K., & Agrawal, S. (2026). A tropical marginal-marine record of Paleocene–Early Eocene carbon isotope excursions from the Lakadong Sandstone, South Shillong Plateau, India. Discover Geoscience, 4(1), Article 355. https://doi.org/10.1007/s44288-026-00713-0

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00713-0

Keywords: PETM, carbon isotope excursion, Paleocene–Eocene, hyperthermal events, Lakadong Sandstone, South Shillong Plateau, Meghalaya, dinoflagellate cysts, marginal-marine, paleoclimate, carbon cycle, tropics