Dark skies at the end of the Cretaceous

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

Thirty years ago, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that an asteroid collided with the Earth and caused one of the most devastating events in the history of life. The impact created the 180-kilometre wide Chicxulub crater causing widespread tsunamis along the coastal zones of the surrounding oceans and released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species while freshwater environments shows low extinction rates, about 10% to 22% of genera.

Recent studies suggest that the amount of sunlight that reached Earth’s surface was reduced by approximately 20%. Photosynthesis stopped and the food chain collapsed. The decrease of sunlight caused a drastic short-term global reduction in temperature (15 °C on a global average, 11 °C over the ocean, and 28 °C over land). While the surface and lower atmosphere cooled, the tropopause became much warmer, eliminate the tropical cold trap and allow water vapor mixing ratios to increase to well over 1,000 ppmv in the stratosphere. Those events accelerated the destruction of the ozone layer. During this period, UV light was able to reach the surface at highly elevated and harmful levels.

Gravity anomaly map of the Chicxulub impact structure (From Wikimedia Commons)

In 1980, Walter Alvarez and his father, Luis Alvarez ignited a huge controversy when they concluded that the anomalous iridium concentration at the K-Pg boundary is best interpreted as the result of an asteroid impact. They even calculated the size of the asteroid (about 7 km in diameter) and the crater that this body might have caused (about 100–200 km across). In 1981, Pemex (a Mexican oil company) identified Chicxulub as the site of a this massive asteroid impact. The crater is more than 180 km (110 miles) in diameter and 20 km (10 miles) in depth, making the feature one of the largest confirmed impact structures on Earth.

 

References:

Charles G. Bardeen, Rolando R. Garcia, Owen B. Toon, and Andrew J. Conley, On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections, PNAS 2017 ; published ahead of print August 21, 2017 DOI: 10.1073/pnas.1708980114

Brugger J.G. Feulner, and S. Petri (2016), Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the CretaceousGeophys. Res. Lett.43,  doi:10.1002/2016GL072241.

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Climate model simulations at the end of the Cretaceous.

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Artist’s reconstruction of Chicxulub crater 66 million years ago.

About thirty years ago, the discovery of anomalously high abundance of iridium and other platinum group elements in the Cretaceous/Palaeogene (K-Pg) boundary led to the hypothesis that a 10 km asteroid collided with the Earth and caused one of the most devastating events in the history of life. The impact created the 180-kilometre wide Chicxulub crater causing widespread tsunamis along the coastal zones of the surrounding oceans and released an estimated energy equivalent of 100 teratons of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere.

To model the climatic effects of the impact, a team of scientist from the Potsdam Institute for Climate Impact Research (PIK), use literature information from geophysical impact modeling indicating that for a 2.9 km thick target region consisting of 30% evaporites and 70% water-saturated carbonates and a dunite projectile with 50% porosity, a velocity of 20 km/s and a diameter between 15 and 20 km, a sulfur mass of 100 Gt is produced. This is about 10,000 times the amount of sulfur released during the 1991 Pinatubo eruption. Additionally, for a sulfur mass of 100 Gt, about 1400 Gt of carbon dioxide are injected into the atmosphere, corresponding to an increase of the atmospheric CO2 concentration by 180 ppm. There could be additional CO2 emissions from ocean outgassing and perturbations of the terrestrial biosphere, adding a total of 360 ppm and 540 ppm of CO2. The main result is a severe and persistent global cooling in the decades after the impact. Global annual mean temperatures over land dropped to -32C in the coldest year and continental temperatures in the tropics reaching a mere -22C. This model is supported by a migration of cool, boreal dinoflagellate species into the subtropic Tethyan realm directly across the K–Pg boundary interval and the ingression of boreal benthic foraminifera into the deeper parts of the Tethys Ocean, interpreted to reflect millennial timescale changes in the ocean circulation after the impact (Vellekoop, 2014).

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

A time-lapse animation showing severe cooling due to sulfate aerosols from the Chicxulub asteroid impact 66 million years ago (Credit: PKI)

References:

Brugger J., G. Feulner, and S. Petri (2016), Baby, it’s cold outside: Climate model simulations of the effects of the asteroid impact at the end of the Cretaceous, Geophys. Res. Lett., 43,  doi:10.1002/2016GL072241.

Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel (1980), Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, Science, 208 (4448), 1095{1108, doi: 10.1126/science.208.4448.1095.

Galeotti, S., H. Brinkhuis, and M. Huber (2004), Records of post Cretaceous-Tertiary boundary millennial-scale cooling from the western Tethys: A smoking gun for the impact-winter hypothesis?, Geology, 32, 529, doi:10.1130/G20439.1

Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan W. H. Weijers, Jaap S. Sinningh Damsté, and Henk Brinkhuis, Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary, PNAS (2014) doi: 10.1073/pnas.1319253111

A Brief Introduction to The Hell Creek Formation.

Hell Creek e Fort Union contact, as seen at Mountain Goat Lake Butte, southwestern North Dakota (Adapted from Fastovsky and Bercovici, 2015)

Hell Creek- Fort Union contact, as seen at Mountain Goat Lake Butte, southwestern North Dakota (Adapted from Fastovsky and Bercovici, 2015)

The Hell Creek Formation (HCF), in the northern Great Plains of the United States, is the most studied source for understanding the changes in the terrestrial biota across the Cretaceous-Paleogene boundary, because preserves an extraordinary record comprised of fossil flora, vertebrates, invertebrates, microfossils, a range of trace fossils, and critical geochemical markers such as multiple iridium anomalies associated with the Chicxulub impact event. The HCF is a fine-grained, fluvially derived, siliciclastic unit, that occupies part of the western Williston Basin, and overlies the Fox Hills Formation (Clemens and Hartman, 2014).
The history of research focused on the Hell Creek Formation and its biota started in October 1901, when William T. Hornaday, director of the New York Zoological Society, travelled to northeastern Montana and discovered three fragments of the nasal horn of a Triceratops in the valley of Hell Creek. He showed the fossils to Henry Fairfield Osborn who decided to include the valley of Hell Creek on the list of areas to be prospected by Barnum Brown the following year.
braun

Barnum Brown working in a quarry in 1902.

In July 1902, B. Brown arrived to Hell Creek. His field crew included Dr. Richard Swann Lull, and Phillip Brooks. Brown recounted that after their arrival, he found the partial skeleton that would become the type specimen of Tyrannosaurus rex. In 1904, William H. Utterback, preparator and collector for the Carnegie Museum of Natural History, collected a fragment of a jaw of Tyrannosaurus and two skulls of Triceratops. In the summer of 1906, B. Brown returned to Montana, and a year later he published a complete manuscript about the valley of Hell Creek. The field expeditions of 1908 and 1909 were crowned by the discovery of another skeleton of T-rex. Between 1902 and 1910, Osborn, Brown, and Lull published the analysis of some of the fossil vertebrates discovered in the Hell Creek Formation, including Tyrannosaurus rex, Triceratops, and Ankylosaurus.
Micrograph of Wodehouseia spinata and a specimenBisonia niemi, from the upper part of the Hell Creek Formation (Adapted from Fastovsky and Bercovici, 2015).

Micrograph of Wodehouseia spinata and a specimen Bisonia niemi, from the upper part of the Hell Creek Formation (Adapted from Fastovsky and Bercovici, 2015).

Plants are represented by fossil leaves, seeds and cones. Fossil wood is also commonly found in the HCF as permineralized fragments. The Hell Creek macroflora is largely dominated by angiosperms including palms, associated with several ferns, conifers, and single species of cycads and Ginkgo. The study of pollen and spores has played a very important role in the identification of the K/Pg boundary in the HCF. Palynologists were the first scientists to recognize that a major, abrupt change occurred at the end of the Cretaceous. Unlike the Permian-Triassic and Triassic-Jurassic boundaries, the palynologically defined K/Pg boundary is based on the extinction of Cretaceous taxa rather than the appearance of Paleocene taxa. Intimately associated with the K/Pg boundary globally, is the so-called “fern spike”, occurring exclusively at localities where the iridium anomaly is present. (Fastovsky and Bercovici, 2015; Vajda & Bercovici, 2014.)

 

References:

Fastovsky, D. E., & Bercovici, A., The Hell Creek Formation and its contribution to the CretaceousePaleogene extinction: A short primer, Cretaceous Research (2015), http://dx.doi.org/10.1016/j.cretres.2015.07.007
Clemens, W. A., Jr., & Hartman, J. H. (2014). From Tyrannosaurus rex to asteroid impact: early studies (1901- 1980) of the Hell Creek Formation in its type area. In J. Hartman, K. R. Johnson, & D. J. Nichols (Eds.), Geological society of America special paper: 361. The Hell Creek Formation and the Cretaceous-tertiary boundary in the northern great plains (pp. 217-245).
Husson, D., Galbrun, B., Laskar, J., Hinnov, L. A., Thibault, N., Gardin, S., & Locklair, R. E. (2011). “Astronomical calibration of the Maastrichtian (late Cretaceous)”. Earth and Planetary Science Letters 305 (3): 328–340.doi:10.1016/j.epsl.2011.03.008
Johnson, K. R., Nichols, D. J., & Hartman, J. H. (2002). Hell Creek Formation: A 2001 synthesis. The Hell Creek Formation and the Cretaceous-Tertiary Boundary in the northern Great Plains: Geological Society of America Special Paper, 361, 503-510.

The palynological record and the extinction events.

The main palynological provinces at the end of the Cretaceous (From Vajda and Bercovici, 2014)

The main palynological provinces at the end of the Cretaceous (From Vajda and Bercovici, 2014)

Pollen and other palynomorphs proved to be an extraordinary tool to palaeoenvironmental reconstruction. In 1921, Gunnar Erdtman, a Swedish botanist, was the first to suggest this application for fossil pollen study. Like spores, pollen grains reflects the ecology of their parent plants and their habitats and provide a continuous record of their evolutionary history. Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments. The morphology of pollen grains is diverse. Gymnosperm pollen often is saccate (grains with two or three air sacs attached to the central body), while Angiosperm pollen shows more variation and covers a multitude of combinations of features: they could be  in groups of four (tetrads),  in pairs (dyads),  or single (monads). The individual grains can be inaperturate, or have one or more pores, or slit-like apertures or colpi (monocolpate, tricolpate).

Since the 1980s, many fossil pollen data sets were developed specifically to reconstruct past climate change.

Aquilapollenites quadricretaeus and Nothofagidites kaitangata

Aquilapollenites quadricretaeus and Nothofagidites kaitangata

 

The palynological record across the Cretaceous–Paleogene (K–Pg) boundary  is a unique global  marker that can be use as template to asses the causal mechanism behind other major extinction events in Earths history. Four major palynological provinces have been recognized based on distinctive angiosperm pollen and fern spores of restricted geographic and stratigraphic distribution. The Aquilapollenites Province had a northern circumpolar distribution that extended from Siberia, northern China, Japon and the western North America. The Normapolles Province occupied eastern North America,  Europe and western Asia. The Palmae Province occupied equatorial regions in the Late Cretacic and included SouthAmerica, Africa and India. Finally, the Notofagidites Province that extended across southern South America, Antartica, New Zeland and Australia.

During the Late Cretaceous the global climate change has been associated with episodes of outgassing from major volcanic events, orbital cyclicity and tectonism before ending with the cataclysm caused by a large bolide impact at Chicxulub, on the Yucatán Peninsula, Mexico. Although, during the middle Maastrichtian, there was a short-lived warming event related to an increase in atmospheric carbon dioxide from the first Deccan eruption phase, the global climate cooled during the latest Maastrichtian and across the K–Pg boundary (Wang et al., 2014; Brusatte et al., 2014). The variations in floral composition reflect these paleoclimatic changes.

Fern spike adapted from Bercovicci

Fern spike adapted from Bercovicci

Mainly angiosperms, disappear at the boundary, as evidenced the palynofloral records of North America and New Zealand. Patagonia shows a reduction in diversity and relative abundance in almost all plant groups from the latest Maastrichtian to the Danian, although only a few true extinctions occurred (Barreda et al, 2013).  The nature of vegetational change in the south polar region suggests that terrestrial ecosystems were already responding to relatively rapid climate change prior to the K–Pg catastrophe.

The earliest Paleocene vegetation shows an anomalous concentration of fern spores just above the level of palynological extinction. R. H. Tschudy, in 1984,  was the first to recognize this very distinctive pattern when he analyzed samples from the K/PG boundary and observed that just after the extinction event, the palynological assemblages were dominated by a high abundance of fern spores.

Schematic illustration comparing the three extinction events analized (From Vajda and Bercovici, 2014)

Schematic illustration comparing the three extinction events analized (From Vajda and Bercovici, 2014)

During the end-Permian Event, the woody gymnosperm vegetation (cordaitaleans and glossopterids) were replaced by spore-producing plants (mainly lycophytes) before the typical Mesozoic woody vegetation evolved. At the end-Triassic event,  the vegetation turnover in the Southern Hemisphere  consisted in the replacement to Alisporites (corystosperm)-dominated assemblage to a Classopollis (cheirolepidiacean)-dominated one.

Despite their difference, these three extinction events are consequences of dramatic environmental upheavals that generated comparable extinction patterns, and similar phases of vegetation recovery but at different temporal scales. First, all these events share a similar pattern of a short-term bloom of opportunistic “crisis” taxa proliferating in the devastated environment. Second, there’s a pulse in pioneer communities (spore spike). Third , a recovery in diversity including the evolution of new taxa. Furthermore, the longer the extreme environmental conditions last the greater is the extinction rate and the extinction patterns between autotrophs and heterotrophs, and between terrestrial and marine faunas become more similar (Vajda and Bercovici, 2014).

 

References:

Vivi Vajda & Antoine Bercovici (2014); The global vegetation pattern across the Cretaceous–Paleogene mass extinction interval: A template for other extinction events; Global and Planetary Change (advance online publication) Open Access DOI: 10.1016/j.gloplacha.2014.07.014, http://www.sciencedirect.com/science/article/pii/S0921818114001477

Vajda, V., Raine, J.I., 2003. Pollen and spores in marine Cretaceous/Tertiary boundary sediments at mid–Waipara River, North Canterbury, New Zealand. New Zealand Journal of Geology and Geophysics 46, 255–273

Wang, Y., Huang, C., Sun, B., Quan, C., Wu, J., Lin, Z., 2014. Paleo-CO2 variation trends and the Cretaceous greenhouse climate. Earth-Science Reviews 129, 136–147.

Vanessa C. Bowman, Jane E. Francis, Rosemary A. Askinb, James B. Riding, Graeme T. Swindles, Latest Cretaceous–earliest Paleogene vegetation and climate change at the high southern latitudes: palynological evidence fromSeymour Island, Antarctic Peninsula, Palaeogeography, Palaeoclimatology, Palaeoecology, 408. 26-47. DOI 10.1016/j.palaeo.2014.04.018

Barreda VD, Cúneo NR, Wilf P, Currano ED, Scasso RA, et al. (2012) Cretaceous/Paleogene Floral Turnover in Patagonia: Drop in Diversity, Low Extinction, and a Classopollis Spike. PLoS ONE 7(12): e52455. doi: 10.1371/journal.pone.0052455

Brusatte, S. L., Butler, R. J., Barrett, P. M., Carrano, M. T., Evans, D. C., Lloyd, G. T., Mannion, P. D., Norell, M. A., Peppe, D. J., Upchurch, P., and Williamson, T. E. In press. The extinction of the dinosaurs.Biological Reviews

The Winter of Our Discontent: short-term cooling following the Chicxulub impact.

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The K-T impact by Don Davis.

The Cretaceous–Paleogene extinction that followed the  Chicxulub impact was one of the five great Phanerozoic  mass extinctions. The impact released an estimated energy equivalent of 100 teratonnes of TNT and produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Model simulations suggest that the amount of sunlight that reached Earth’s surface was reduced by approximately 20%.This decrease of sunlight caused a drastic short-term global reduction in temperature. This phenomenon is called “impact winter”.

Cold and darkness lasted for a period of months to years.  Photosynthesis stopped and the food chain collapsed. This period of reduced solar radiation may only have lasted several months to decades. Three-quarters of the plant and animal species on Earth disappeared. Marine ecosystems lost about half of their species while freshwater environments shows low extinction rates, about 10% to 22% of genera.

Three factors can be associated with the impact winter in marine and fresh water enviroments. First, starvation caused by the stop of photosynthesis. Second, the loss of dissolved oxygen. Third, the low temperatures. The flux of organic detritus to the sea floor also

A paleogeographic map of the Gulf of Mexico at the end of the Cretaceous (From Vellekoop, 2014)

Three factors can be associated with the impact winter in marine and fresh water environments. First, starvation caused by the stop of photosynthesis. Second, the loss of dissolved oxygen. Third, the low temperatures. Because the late Cretaceous climate was warm, a major challenge for aquatic organisms, especially in inland waters, may have been the persistence of low temperatures. Additionally, the vapour produced by the impact  could have led to global acid rain and a dramatic acidification of marine surface waters.

Fossil evidence for this impact winter was recovered in the Brazos River region of Texas.  The biostratigraphy of the section presents the Ir anomaly, and impact-related tsunami beds. The age of the outcrops was updated using  planktonic foraminifera and  dinocysts.

The “impact winter”  model is supported by a migration of cool, boreal dinoflagellate species into the subtropic Tethyan realm directly across the K–Pg boundary interval and the ingression of boreal benthic foraminifera into the deeper parts of the Tethys Ocean, interpreted to reflect millennial timescale changes in ocean circulation following the impact (Vellekoop, 2014).

References:

Johan Vellekoop, Appy Sluijs, Jan Smit, Stefan Schouten, Johan W. H. Weijers, Jaap S. Sinningh Damsté, and Henk Brinkhuis, Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary, PNAS (2014) doi: 10.1073/pnas.1319253111

Douglas S. Robertson, William M. Lewis, Peter M. Sheehan and Owen B. Toon, K-Pg extinction patterns in marine and freshwater environments: The impact winter model, Journal of Geophysical Research: Biogeosciences, JUL 2013, DOI: 10.1002/jgrg.20086.