What role did Deccan Volcanism play in the end Cretaceous mass extinction?
There is widespread agreement that an asteroid struck what is now the Yucatan Peninsula region of Mexico 66.05 million years ago. The resulting environmental catastrophe precipitated an abrupt mass extinction, wiping out 70% of all species. In the rock record, a thin clay layer preserves evidence of the extraterrestrial origin of this impact event. It is referred to as the K-Pg boundary layer, K being Cretaceous and Pg referring to the Paleogene. The biota below the boundary layer is very different from fossil assemblages above it.
The earth at that time was also experiencing a major episode of basalt volcanism. This Large Igneous Province (LIP) is known as the Deccan Volcanic Province or the Deccan Traps. Volcanism occurred both before and after the impact event. Geologists have been divided on whether most the lava erupted after the asteroid hit or before it with different implications for the role of volcanism in the mass extinction. There was also an uncertain and incomplete assessment of the volumes of lava involved.
A new paper by Vivek S. Kale and coworkers titled “Spatio-temporal volume recalibration shows Deccan volcanism caused Terminal Cretaceous Mass Extinction” leaves no room for doubt about which side of the divide this group stands. The researchers examine over 80 lava sections across the Deccan Volcanic Province and recalculate the lava volume, assigning packages of lava a time bracket based of absolute dating, magnetic signatures, and fossils. They find that around 70% of the Deccan lava erupted in a time span of 300,000 years before the mass extinction.
Their results are summarized in this infographic. Source: Vivek S. Kale and coworkers, GSA Bulletin, November 2025.
The volume estimates of previous workers and their study is shown to the left. Also presented are marine paleo-environmental indicators represented by a paleo-temperature curve and carbon isotope curves spanning the mass extinction.
A short explanation of these curves will be useful.
For estimating temperature, geologists use the ratio of Oxygen 18/Oxygen 16 preserved in the shell (CaCO3) of planktonic organisms like foraminifera or calcareous nanoplankton. During warm phases, vigorous evaporation results in more of O18 escaping into the atmosphere, enriching the ocean slightly in the lighter O16. Calcium carbonate shell growth incorporate O18 and O16 without preference, maintaining the same O18/O16 ratio of sea water .
Warmer seas will result in shells having a lower O18/O16 ratio, than shells that grow in cooler water. The temperature is estimated from these measured oxygen isotope ratios.
The increase in sea surface temperature observed here, known as the Late Masstrichtian Warming Event (LMWE), has been linked to increased CO2 emissions during Deccan Volcanism.
Carbon isotope trends through time are presented by measuring the C13/C12 ratio of the target sample. ‘Bulk’ carbon means any carbon from hard parts as against carbon from organic material. It might mean just ground up limestone made up of a mixture of shells and lime mud. Jurassic onward, the proliferation of planktonic foraminifera has allowed geologists to select shells of just one or two species for their measurements. This makes it easier to screen for post depositional alteration which could reset the original oceanic C13/C12 ratio due to reaction with fluids of a different composition. It is more difficult to recognize these effects in a mixed sample, especially one with fine calcium carbonate mud.
Variation in C13/C12 ratio through time is generally regarded as an indicator of primary productivity. Photosynthesizing organisms have a strong preference for the lighter C12 to build organic tissue. The C13/C12 ratio of organic matter is strongly depleted compared to the sea water C13/C12 ratio. Similar to oxygen intake, growing shells preserve the C13/C12 ratio of the ocean. During environmental crises, reduced primary productivity enriches the ocean in the lighter isotope, also depressing the C13/C12 ratio of shells growing under these conditions.
The modest (relative to earlier Phanerozoic mass extinctions) carbon isotope excursion coincident with the K-Pg boundary suggests a decrease in primary productivity due to the extinction of some photosynthesizing groups. In contrast, the amplitude range of the isotope excursions seen during the Late Maastrichtian Warming Event can also be explained by a non-biological driver.
Carbon dioxide of volcanic origin is isotopically lighter than sea water. Pulses of volcanism may result in a large enough pool of lighter C12 dissolving in the ocean and shifting sea water C13/C12 ratio to the depleted values measured in Late Cretaceous shells.
Kale and coworkers argue that increased volume of lava eruption and emissions of CO2 and other gases in the Late Maastrichtian caused global warming and destabilized the biosphere. Ecosystems were already tottering when the asteroid delivered the knockout punch.
Just how stressed was the ecosystem? The most visible indicator of environmental changes impacting an ancient biosphere is the fossil record of that time period. Their paper however does not present any analysis of the Late Maastrichtian biota.
The several studies (Ref. 1, 2, 3) where from Kale and coworkers source the paleo-temperature and isotope record all unambiguously conclude that the LMWE had a limited impact on marine life. Species diversity did not change much. Fossil community structure remained stable, or where it did change, it reflected migration and range shifts. An example of this is L. Woelders and coworkers study of a Late Cretaceous section from the South Atlantic where they find fluctuating abundances of benthic foraminifera and dinoflagellates tracking climate and sea level cycles.
That is not to say that organisms were unaffected by this warming event. Other studies across the marine realm have documented biotic stress. Vincente Gilabert and coworkers find fragmentation of shells, abundance of low oxygen tolerant taxa, and dwarfing of some species, in their study of planktonic foraminifera of the Caravaca section in Spain. Similarly, Gerta Keller and coworkers study on planktonic foraminifera also reveals species dwarfism and abundance of high stress opportunistic species in the late Maastrichtian of the Indian Ocean and South Atlantic.
Nicholas Thibault and Dorothee Husson in a survey of the Late Cretaceous ocean point out that nanoplankton species numbers dropped during the LMWE, but diversity rebounded and increased with no significant extinction in the last 140,000 years of the Cretaceous. Michael Henehan and coworkers infer an increase in ocean acidity coinciding with the LTME based on reduced preservation of calcium carbonate shells on the sea floor. Shell preservation and sea water carbonate saturation return to pre event values in the last 200,000 years before the K-Pg impact event.
Do these observations on plankton species signal some sort of a beginning of the end of ecosystems across the entire marine biosphere? Since Deccan Volcanism spanned several hundred thousand years, could there have been a mass extinction if there had been no asteroid hit? It is conceivable that the prolonged volcanism could have resulted in a ratcheting of environment stress, eventually crossing tolerance thresholds.
This proposition has been hard to test because the asteroid crashed the Cretaceous party denying us an extended view of a volcanism only unfolding of history.
Still, we do have a good 300,000 year record before the impact of sedimentary deposition that coincided with the most voluminous phase of Deccan volcanism. Finer scale resolution of marine sections in the past few years allow us to examine global biotic trends from this time span.
As mentioned above, the impact of ocean warming seems to be quite limited to changes in shell size, temporary species abundance shifts, and adjustments by migration to favorable locales. Importantly, there is no signal of elevated extinction in diverse groups such as planktonic foraminifera, radiolarians, nanoplankton, ammonoids, bivalves, and gastropods in the final few hundred thousand years of the Cretaceous, when volcanism was at its peak (Ref 1,2,3,4).
A closer look at the environmental proxy data is revealing. After a warming phase, the paleo-temperature curve shows a 150 thousand year cooling trend in the terminal Maastrichtian until the K-Pg boundary. Mirroring this cooling trend is a distinct shift towards heavier carbon isotope values. This may be due to increasing primary productivity signaling a general amelioration of ocean conditions.
What could cause such a cooling despite ongoing voluminous volcanism and CO2 emissions? The answer may be enhanced weathering of all that fresh basalt which kept sequestering carbon dioxide during silicate weathering reactions.
There are two lines of observation that support increased weathering during end Cretaceous times.
The first observation is the recovery of calcium carbonate saturation level of the ocean. Weathering releases divalent cations like calcium and magnesium in to the ocean resulting in an excess positive charge (alkalinity) in sea water. The charge imbalance promotes dissociation of the poorly soluble CO2 gas (represented in equations as carbonic acid -H2CO3) and the formation of more stable negatively charged bicarbonate (HCO3) or carbonate (CO3) anions, raising carbonate saturation state. A decrease in dissolved gas leads to a draw down of atmospheric CO2 in to the ocean by air-sea gas exchange. Alkalinity can also be described as the buffering capacity of the ocean against excessive CO2 buildup.
Today, rapid atmospheric CO2 increase is a major concern for ecosystem and societal health. Research and small scale demonstrations to enhance ocean alkalinity are gaining importance as we try to replicate on human time scales what nature does over much longer time spans.
The other independent support for enhanced weathering comes from another isotope system, the osmium 187/osmium188 ratio. Continental crust is enriched in Rhenium187 which decays to Osmium187. Rocks like basalt derived directly from partial melting of the mantle have much lower Rhenium187 and consequently Osmium187. There is a progressive decrease in the Osmium187/Osmium188 ratio in Late Maastrichtian ocean sediments. This is consistent with weathering and increased supply of non-radiogenic Os188 from a fresh basalt source. Some of the decline can also be explained by a reduced supply of Os187 as large areas of radiogenic continental crust were blanketed by lava.
Late Cretaceous marine ecosystem health can also be assessed by looking at the difference in the C13/C12 ratio between surface dwelling planktonic and bottom dwelling benthic foraminifera shells. This isotope gradient is maintained by the biological pump, a transfer of organic carbon and nutrients from the sea surface to depths. Organic carbon is enriched in the lighter isotope compared to planktonic shells (as explained earlier). This organic matter sinks to the ocean depths and oxidizes, enriching the Dissolved Organic Carbon (DIC) pool in C12. Shells of benthic foraminifera (bottom dwelling) have a lower C13/12 ratio than planktonic calcifiers.
A noticeable C13/C12 gradient is present (Ref. 1, 2) in the latest Cretaceous ocean. It converges immediately above the K-Pg boundary, indicating a disruption of the marine carbon cycle only after the asteroid impact.
Environmental perturbations need not precipitate a terminal decline of ecosystems. My reading of the fossil record and the different proxies is that the effects of the LMWE were transient. Key biogeochemical cycles remained intact through the Late Maastrichtian implying that volcanism did not greatly diminish pelagic ecosystem function. The end Cretaceous marine biosphere shows recovery and resilience and not signs of a ‘critically damaged’ system.
David E. Fastovsky and Antoine Bercovici perceptively ask in their review of the terrestrial record of the K-Pg extinction- “how does one weaken an ecosystem in terms of its ability to withstand catastrophic, that is to say, very short-term events? In other words, this is not a ‘straw that broke the camel’s back’ situation, where preceding events play a big role in the eventual failure. Since large swaths were simply obliterated in the asteroid’s wake, and more distant regions experienced acute shock, it wouldn’t have mattered if ecosystems were weakened or hale and hearty.
I expressed a similar sentiment earlier in the post. To attribute a causal role to Deccan volcanism, there have to be signs that significant global extinction events occurred prior to the asteroid strike. The data just doesn’t point in that direction.
Earlier in earth history, the end Permian (251.9 million years ago) and the end Triassic (201.4 million years ago) mass extinctions have been more convincingly linked to massive environmental damage triggered by sustained igneous activity. The impact on the environment due to Deccan volcanism appears muted in comparison.
One reason could be that during the Permian and Triassic igneous episodes, magma intruded through thick sedimentary basins filled with limestone, organic rich shale, and sulfur bearing evaporites. Interestingly, the mass extinction coincided with the emplacement of thick sills, tabular bodies of magma injected parallel to the strata. As a result, a surge of volatiles emanating from baking sediment amplified volcanic emissions, overwhelming the earth’s natural buffering capacity. Both these mass extinctions are estimated to have unraveled in just tens of thousands of years.
In central Peninsular India, Deccan volcanism intersected older sedimentary basins only at the northern and southern fringes. Slow cooling shallow subsurface sills facilitating effective heat transfer to surrounding crust are absent. Instead, eruptions were fed by magma ascending rapidly along thin conduits (dikes) aligned in swarms. More relevant is that only sterile granitic rocks underlie the regions where the most voluminous eruptions took place.
Volatile bearing sedimentary rocks did play a role in the end Cretaceous mass extinction. But by a quirk of fate, it was the asteroid that found them. The Yucatan region of Mexico where the asteroid hit is underlain by hydrocarbon rich sediments, and sulfur rich salt. The impact released soot and sulfate aerosols in the atmosphere resulting in global cooling and reducing photosynthesis for weeks to months. Eventually, the dust settled upon a world utterly transformed. It was the beginning of a new world, one where flowering plants, song birds, and mammals diversified and flourished. From time to time, chance events have opened up entirely new avenues for evolution to explore.
It is pertinent to stress again the paramount importance of rates of processes in the context of our developing climate crises. Although in absolute terms humans may not emit more than past large volcanic events like the Cretaceous Deccan Volcanism or the Siberian Volcanism of late Permian times, we are emitting CO2 at a rate many times faster than these natural events. Carbon dioxide is building up in the atmosphere too quickly for weathering and ocean ecosystems to offset over the next few hundred years.
Lastly, Jurassic onward, the proliferation and evolution of plankton shell building organisms such as foraminifera and coccolithophores have played an important role in regulating ocean chemistry and as a buffer against atmosphere CO2 changes. This occurs by various processes. For one, the heavy shells act as ballast, exporting attached organic matter to depths before their degradation consumes oxygen in the uppermost oceanic layer. Much of this carbon gets buried, becoming a long term carbon sink.
The other important function is sustaining the ocean alkalinity balance. Precipitation of calcite and aragonite (CaCO3) removes positive charge from the ocean, in the form of divalent Ca cations, reducing alkalinity (see previous section on weathering). This alkalinity is recovered when calcareous skeletons, falling through the water column like an ocean snow, start dissolving when they sink below a threshold depth, releasing divalent Ca ions. This process known as carbonate compensation maintains the ocean’s capacity to absorb atmospheric CO2.
The cooling trend observed in the final 150,000 years of the Cretaceous suggests that the rates of emissions during Deccan volcanism likely remained relatively low. Silicate weathering on land and enhanced ocean alkalinity coupled to weathering runoff and carbonate compensation would have substantially offset Deccan CO2 emissions, moderating their environmental impact.
In the pre-Jurassic ocean, most carbonate skeleton producing organisms lived in the water column or on the sea bed of shallow continental shelves where the sea water was saturated with calcium carbonate. Skeletons did not dissolve in this environment and the biological contribution to ocean alkalinity was limited. This changed when the more geographically widespread plankton evolved biomineralization and their skeletons began settling down to depths where the ocean water is undersaturated with calcium carbonate. The calcareous plankton ecosystem has made post Jurassic oceans more resistant to external shocks, albeit over times scales of thousands of years.
By now readers may be wanting to ask- What about the dinosaurs? Did they persist right until the asteroid hit, or did they go extinct earlier? The avian branch of dinosaurs did survive. The following section is about the non-avian groups and the terrestrial fossil record.
There would have been an extirpation of local populations of dinosaurs and other fauna and flora living in areas directly affected by Deccan Volcanism. But what about the global record?
Terrestrial environments such as lakes and rivers which can preserve dinosaur fossils are patchily distributed. Frequent erosion also removes sediment, making the reconstruction of organic diversity and evolutionary trends much more difficult than for biota living in deep sea environments.
Despite these limitations, we do have some sedimentary sections which inform us about dinosaur life and the terrestrial biosphere of the Late Maastrichtian.
In their work on peat deposits from the Western Interior, U.S., Lauren O’Conner and coworkers estimate end Cretaceous paleo-temperatures based on configuration of preserved organic compounds. Besides the LMWE, they identify shorter warming and cooling phases in the final 100,000 years before the K-Pg boundary, likely driven by Deccan volcanism. However, the impact on the biota seems limited, with no decline in angiosperm diversity until the mass extinction.
Andrew J Flynn and coworkers study in New Mexico on the Naashoibito Member, a dinosaur rich sedimentary deposit, shows that dinosaurs were diverse and formed regionally distinct assemblages during the final few hundred thousand years before the asteroid strike.
The Hell’s Creek Formation spread over the States of North Dakota and Montana also offers insights in to the terrestrial ecosystems (Ref. 1, 2, 3) . Pollen and leaf assemblages attest to a healthy end Cretaceous flora. Both show a substantial and abrupt extinction at the K-Pg boundary. Vertebrate diversity also shows no decline throughout the Late Maastrichtian.
Mammals appear to the exception here. Gregory P Wilson and coworkers detailed survey of the Hell’s Creek Formation reveals two sequential changes in mammalian fauna 650,000 years and 200,000 years before the K-Pg boundary. Apart from changes in mammal assemblages, there is a decline in the abundance of metatherians. The authors link this decline to regional environmental changes, but don’t rule out the effect of Deccan volcanism.
Dinosaur fossils at the Hell Creek section have been found to within a few tens of cm of the K-Pg boundary, representing the last few hundred to a few thousand years of the Cretaceous. Their presence in numbers comparable to lower levels of the sedimentary section negates arguments that dinosaurs were either declining or had gone extinct well before the K-Pg impact event.
The icing on the cake is some recent work by Robert DePalma and coworkers on a debris layer in the Hell’s Creek section. These chaotic beds made up of boulders and fossils was deposited by a tsunami triggered by the asteroid hit. Among other organic remains, the researchers have found a fossil hind limb of an ornithiscian dinosaur. Their work was presented at the EGU 22 General Assembly 2022 in Vienna, Austria. The peer reviewed paper is yet to be published and some scientists have expressed skepticism and called for a more detailed examination of the fossils. But this initial announcement, along with other documented dinosaur fossil finds from North America, does strongly suggest that dinosaurs were well and alive until the very day the world changed 66.05 million years ago.
