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.
