eThe iridium (Ir) spike was observed in freshwater sediments.
The iridium (Ir) spike was observed in freshwater sediments.
In samples taken from the Raton Basin in New Mexico, the authors observed anomalously high levels of Ir corresponding to the KPg boundary layer similar to Ir anomalies published in past literature. However, while previous reports of the Ir spike in the K-Pg layer were all in marine rock, the Ir anomaly found by Orth et al. occurred in sediment from freshwater and could not have been the result of marine enrichment processes or related to a mass marine extinction, as had been suggested. Additionally, the anomaly was found just underneath a coal bed, consistent with the point at which several Cretaceous plant species went extinct. Analysis of plant matter showed a sudden decline in the angiosperm pollen to fern spore ratio coinciding with the Ir spike; the ratio begins increasing while Ir becomes less abundant toward the surface.
eShocked quartz grains were found in the K-Pg layer.
Shocked quartz grains were found in the K-Pg layer.
In the Brownie Butte region of Montana, the authors found that a claystone layer at the K-Pg boundary contained Ir levels 200x greater than background levels. By this point, Ir spikes had been noted in a few dozen locations, spanning seven countries and two ocean basins, but only two of these occurred in continental deposits, both in the New Mexican part of the Raton Basin. Discovering the anomaly in non-marine rock at another site indicated that this global phenomenon was not due to oceanic processes, as some had suggested. Most of the nonclay portion was quartz. Dissolution and etching procedures revealed intersecting sets of lamellae in ~25% of the grains. Neither etched nor unetched quartz grains from the same area exhibited such features. The lamellae in the unetched quartz grains were planar and analogous to shock metamorphism in rocks associated with meteorite impact craters. Some grains had up to 7 distinct sets of planar features, while terrestrially deformed grains typically contain only one.
Bohor, B.F., Foord, E.E., Modreski, P.J., and Triplehorn, D.M.
eNanometer-sized diamonds containing a carbon isotope ratio similar to type C2 chondritic meteorites were found in K-Pg boundary layer rock.
Nanometer-sized diamonds containing a carbon isotope ratio similar to type C2 chondritic meteorites were found in K-Pg boundary layer rock.
The authors discovered nanoscale diamonds in K-Pg layer in Alberta, Canada. Previous studies found nanometer-sized diamonds in chondritic meteorites; following those studies’ dissolution/oxidation procedure, the authors extracted a mostly carbon residue from K-Pg layer rocks. The residue was absent from samples taken a few centimeters above and below the boundary. The residue grains were ~3-5 nm, similar to those of the chondritic meteorite diamonds, and their atomic spacings matched those of the diamond lattice. The authors also note that the diamond structures would have decomposed at in the temperature/pressure conditions associated with volcanic eruptions. The diamond to iridium ratio was 1.22:1, matching the ratios calculated for the Murray and Murchison C2 chondritic meteorites.
eZircon grains containing shock-specific crystallographic features were discovered in K-Pg layer rock samples.
Zircon grains containing shock-specific crystallographic features were discovered in K-Pg layer rock samples.
The authors discovered zircon grains containing planar deformation features (PDF), signs of shock metamorphism akin to those in shocked quartz, in the Raton Basin area of Colorado. Zircons containing such definitive signs of shock metamorphism had never been reported previously. Although zircons previously recovered from known extraterrestrial impact environments displayed certain other features consistent with shock, those particular features could also result from tectonic processes. They also observed a polycrystalline texture that had also not been reported before, as well as PDF and granular textures occurring simultaneously in the same area of a zircon grain. Some zircons exhibited progressions from PDF to combined PDF/granular to fully granular textures, which they suggested was due to increasing shock pressures and temperatures, such as those in an extraterrestrial impact environment.
The authors found stishovite, a very hard, dense polymorph of silica, in the K-Pg boundary layer at Raton Basin, NM. Stishovite forms at extremely high pressures and temperatures; at depths shallower than the lower mantle, the presence of stishovite indicates a shock event. While explosive volcanism is a possible source of such conditions, the authors argue that because stishovite is thermally unstable, decomposing at higher temperatures even at moderate pressure, the stishovite structure would not have survived the prolonged heat generated by a near-surface volcanic event.
McHone, J.F., Nieman, R.A., Lewis, C.F., and Yates, A.M.
e40Ar/39Ar dating places the deposition of the Deccan Traps basalt flows at 65-69 Ma.
40Ar/39Ar dating places the deposition of the Deccan Traps basalt flows at 65-69 Ma.
Using 40Ar/39Ar dating, Courtillot et al. determined that most of the Deccan eruptions occurred within a short window 65 to 69 Ma. Since 40Ar/39Ar dating measures age relative to that of a known standard, and distinct mineralogical phases retain argon differently, the authors reported ages only for pure plagioclase samples taken from Deccan Traps basalt along with one consisting of whole rock. One plagioclase sample, which was positioned directly above Maastrichtian (the final age of the Cretaceous Period) sediments of the Lameta formation, gave a plateau age of 66.4 ± 1.9 million years. Three other plagioclase samples returned ages of 67.6 ± 1.8, 65.1 ± 0.6, and 63.6 ± 0.5 million years, and the whole rock sample was dated to 63.6 ± 0.2 million years. These samples span most of the thickness of the Deccan flows, and their age distribution includes the absolute age reported for the K-Pg boundary layer. Even small-scale volcanic eruptions can potentially devastate local life and slow recovery via habitat loss, lingering toxic atmospheric conditions, and suppression of photosynthesis, and the latter two mechanisms in particular can spread far beyond the eruptions’ immediate area. The sheer size of the Deccan Traps would indicate that the eruptions had globally significant effects, so the coincidence of the K-Pg boundary with the bulk of the Deccan eruptions suggests that the K-Pg mass extinction event was connected to Deccan volcanism.
Courtillot, V., Féraud, G., Maluski, H., Vandamme, D., Moreau, M.G., and Besse, J.
eDeccan eruptions became much larger and less frequent for 500,000 years following the K-Pg boundary.
Deccan eruptions became much larger and less frequent for 500,000 years following the K-Pg boundary.
The authors discovered that the Deccan Traps underwent a significant behavioral shift within 50,000 years of the Chicxulub impact such that eruptions became less frequent but released much higher volumes of lava, which may have contributed to the K-Pg mass extinction and inhibited ecological recovery afterward. The most studied region of the Deccan Traps, the Western Ghats, is geochemically divided into the Wai, Lonavala, and Kalsubai Subgroups. High-resolution 40Ar/39Ar dating indicated that most of the Kalsubai Subgroup, the bottommost of the three strata, erupted prior to the K-Pg boundary; these eruptions were rapid and occurred over the course of 173 ± 84 ky. The K-Pg layer followed the Kalsubai Subgroup about 165 ± 68 ky later. Dating of the Wai Subgroup indicated a mean magma production rate double that of the Kalsubai eruptions. The Wai Subgroup vastly dominates the volume produced by the three strata; previous studies observed many more oxidized layers between lava flows in the Wai Subgroup than in older strata, suggesting either lower eruption frequency or greater weathering rates related to climate change following the K-Pg. The authors found that these infrequent but large eruptions continued for ~500 ky after the K-Pg boundary, which is consistent with the time interval between the K-Pg and the first stages of marine ecosystem recovery. Although the common consensus by this time was that there was in fact a massive bolide impact at 66 Ma (resulting in the Chicxulub crater), the authors suggested that the behavioral shift observed here had significant ramifications for ecosystems worldwide. In this case, terrestrial volcanism could have been more responsible for the K-Pg mass extinction than extraterrestrial events. Even if the shift itself resulted from the Chicxulub impact, as the authors speculated, the eruptions would have repeatedly placed massive stress on ecosystems without allowing enough time for recovery in between, reducing survivability and suppressing biological recovery.
Renne, P.R., Sprain, C.J., Richards, M.A., Self, S., Vanderkluysen, L., and Pande, K.
eMarine regressions/transgressions appear to have coincided with multiple Phanerozoic extinction events.
Marine regressions/transgressions appear to have coincided with multiple Phanerozoic extinction events.
Hallam demonstrated a strong correlation between six marine mass extinctions that occurred during the Pharenozoic Eon (at the ends of the Cambrian, Ordovician, Devonian, Permian, Triassic, and Cretaceous periods) and major marine regression events. Sea level histories were obtained via stratigraphic analysis by Hallam in 1984; the data suggested marine regressions followed by transgressions. Notably, the most severe regression coincided with the end-Permian extinction event approximately 252 Ma, the largest of the Phanerozoic Eon (541 Ma - present). Fossil records obtained by other groups also indicate a pattern of ammonoid diversity dropping to nearly zero then rebounding roughly consistent with the sea level regressions and transgressions. The early stages of marine transgressions tend to correlate with anoxia and resulting extinction of deep-sea aerobic organisms. Hallam specifically notes the discovery of black shales (organic-rich sediments deposited under anoxic conditions) at basal Tertiary layers in Denmark, suggesting a link between sea level changes and the K-Pg mass extinction event. Mass extinction events have usually involved both aquatic and terrestrial species; this is the case for the K-Pg mass extinction. However, while a regression would have only influenced the extinction of marine species, such events have been associated with shifts in terrestrial behavior. These include glaciation events and, importantly, varying levels of tectonic activity, which themselves result in volcanic episodes such as the Deccan Traps eruptions.
eMarine fossils indicate a rapid sea level drop across 17,000 years before the K-Pg boundary.
Marine fossils indicate a rapid sea level drop across 17,000 years before the K-Pg boundary.
The authors found concurrent changes in the frequency of sporomorphs (fossilized pollen grains and spores) and certain dinoflagellate cysts (a type of algae or marine plankton) that indicated a rapidly falling sea level during a 17-ky period leading up to the K-Pg boundary. The boundary was marked by the extinction of Cretaceous plankton foraminifera and the first appearance of Danea californica, a species of dinoflagellate cyst; this also appeared to correlate with lowered oceanic primary productivity. The authors considered the steep drop in sea level here as a peak regressive pulse and part of the well-documented end-Cretaceous marine regression. They propose that the lower sea levels would have minimized rates of deep water formation and suppressed the nutrient transfer resulting from the mixing of deep and shallower waters, ultimately leading to the extinction of highly depth-stratified planktonic foraminifera. Plankton comprise the basis of the marine food chain; without them, larger marine organisms would have been unable to survive. While marine regressions are seen more as a symptom of mass extinctions (which tend to involve land-based species as well as aquatic ones), they are generally associated with terrestrial developments like changes in tectonic behavior, which lead to significant volcanic activity.
eOcean environments were under high stresses concurrent with Deccan volcanism prior to the K-Pg boundary.
Ocean environments were under high stresses concurrent with Deccan volcanism prior to the K-Pg boundary.
The authors found that high stress conditions for the planktic foraminifera developed prior to the K-Pg boundary, suggesting the presence of taxing environmental conditions leading up to the K-Pg extinction event. They conducted investigations into foraminifera fossil records, stable isotopes, magnetic susceptibility, and carbonate dissolution in samples from Bidart, Gamsbach, and Elles (in France, Austria, and Tunisia, respectively). Faunal responses indicated warming during the late Maastrichtian age (72.1-66 Ma); low δ13C values and lower CaCO3 content coincident with the warm period led the authors to suggest the equilibration of ocean surfaces with higher amounts of volcanically generated CO2 as an extant environmental condition. Foraminifera fossil records and the observation of increased carbonate dissolution suggest acidification of surface ocean water immediately prior to the K-Pg boundary at Bidart and Gamsbach and coincident with a period of rapid cooling. Samples from Elles indicate a subsequent rapid warming at the time of the K-Pg boundary. Based on the timing and the relationship between ocean acidification and high CO2 levels, the authors attribute this sequence of events to main phase-2 Deccan volcanism.
Punekar, J., Keller, G., Khozyem, H.M., Adatte, T., Font, E., and Spangenberg, J.
The transition from the Mesozoic Era (252.2-66 Ma) to the Cenozoic Era is marked geologically by the Cretaceous-Paleogene (K-Pg) boundary, also referred to as the Cretaceous-Tertiary (K-T) boundary, which is notable for its association with a mass extinction event that ended the dinosaurs along with most other Mesozoic species. Prior to 1980, the K-Pg mass extinction was largely thought to have been caused by terrestrial events, such as climactic changes and volcanic activity. While extraterrestrial causes had also been suggested, it was the discovery of anomalously high levels of iridium (and other elements rare on Earth) at the K-Pg boundary that gave real credence to the asteroid impact theory. Widespread observations of shocked minerals in the following years appeared to support the hypothesis.