at the beginning of the class we looked at evidence for the numerical age of the N Cretaceous Extinction when we discussed radiometric dating and in doing so we kind of took it for granted that the extinction was caused by Ebola impact will most find the evidence for an impact event as the cause for the extinction to be very strong other hypotheses are still actively debated in this segment we'll look at the two most frequently examined hypotheses for the cause of the extinction a bolide impact and a major volcanic event both catastrophic events have the potential to alter environments at a global scale and be the cause of a wide-ranging Extinction at the end of the Cretaceous more than just non-avian dinosaurs went extinct marine organisms also suffered in the extinction and we lost two major groups of shelled cephalopods belminites and ammonites which are closely related to today's Nautilus squid and octopi although this Extinction event gets a lot of public attention it was not the largest Extinction event our planet has ever experienced at the end Cretaceous approximately 50 percent of animal families went extinct across the marine and terrestrial realms at the species level approximately 75 percent went extinct however at the end Permian Extinction 60 percent of animal families and upwards of 96 of species went extinct determining the cause of the encryptation extinction is complicated because a lot was going on around 66 million years ago when we ceased to see the fossils of non-avian dinosaurs in the stratigraphic record one of the first things we do when trying to determine the cause of a past Extinction event is figure out what major events were occurring at the same time as the extinction only if the proposed caused Extinction is immediately preceding the extinction itself can we support a cause effect link however in this case two different major events occurred at about the same time complicating the story one event was a boli impact on the Yucatan Peninsula this map on the left shows location of the impact crater on a paleogeographic map where the brown is land and the greenish region is shallow epic Continental Seas on the right is a map showing the extent of the deckhand traps an extensive field of basalts that gives us evidence of a massive amount of volcanism occurring near the extinction event we know the age of the impact event because we found a globally extensive clay layer with up to 160 times more iridium than expected for background the discovery of this iridium layer was an example of scientific serendipity at the time of its Discovery we didn't yet have strong reasons to hypothesize that an impact event caused the extinction of the non-avian dinosaurs and no one was really looking for evidence of ebolite impact instead geologists were trying to figure out an easy way to determine how fast sediments accumulated over time they reasoned that if iridium only came to Earth from space and was generally delivered to our planet at a constant rate then we could use the concentration of iridium to calculate sedimentation rates if the sediment was accumulating rapidly the Iridium would be more diluted and if sediments were accumulating slowly their idiom would be more concentrated in that part of the stratigraphic record when they measure their iridium concentrations across the Cretaceous pathogene boundary they found an enormous concentration of iridium they could not be explained by a stop in deposition because other chronostratigraphic data showed that there was no extended Hiatus in sedimentation as similar analyzes were done on other well-dated sedimentary records this iridium layer was found to be globally extensive using the amount of iridium in the sediments and the amount of iridium known to be in other meteorites they reasoned that Eboli 10 kilometers in diameter must have impacted Earth to contribute that much iridium into the sedimentary record looking closer at sediments near the Iridium layer folks also noticed shocked quartz and impact sphericals that would be produced by the impact these particles lent further evidence that a bolide was the source of iridium these particles have been observed in deep sea sediment cores in this image we see a photograph of a sedimentary core that captures the Cretaceous paleogene boundary and a zoomed in image where the different sediment grains have been color-coded by their mineralogy the layer immediately after the kpg boundary is Sphero rich and contains a lot of shock quartz colored here in red so we had evidence of a bowline impact coincident with the extinction of the non-avian dinosaurs in the form of high concentrations of iridium shock cords and impact cereals but we didn't have a candidate crater until the 1990s back in the 1970s folks were exploring at Yucatan Peninsula for petroleum sources using geophysical techniques these scientists saw magnetic and gravity anomalies that outlined a circular bullseye-like object about 180 kilometers in diameter and they infer that this must be an impact crater however others were skeptical and uninterested and it didn't catch on and even though the lead author of The geophysical study told the scientists who discovered the Iridium anomaly about this potential crater right after they published their impact hypothesis based on the Iridium evidence years later a different group of scientists were looking for a potential crater to support the impact hypothesis and investigated sediments near the Brazos River here in Texas in those sediments they found tectite glass and jumbled sedimentary deposits suggesting that there had been an impact nearby that caused tsunamis throughout the region others found similar deposits in Haiti eventually collaborations among research groups formed and sediment cores from that original suspected crater were re-examined and found to have metamorphic rocks that could only be explained by the intense heat from a bullied impact we now have the impact crater the key piece of evidence that a large impact had occurred based on the size the crater the asteroid was estimated to have been 10 kilometers in diameter the same size as estimated from the Iridium anomaly As Time passed many places have been examined around the world for evidence of the impact and a fuller picture began to form impact debris or ejecta form thicker deposits near the Yucatan and became thinner with distance from the crater exactly as we would expect if it were indeed the Impact Center given the debris pattern we also became confident that this was the only site contributing to the end Cretaceous stratigraphic record and as we saw in the segment on radiometric dating absolute dating of minerals from Ash layers preserved near the Iridium layer show us that this impact occurred at about 66 million years ago coincident with the timing of the extinction of the non-avian dinosaurs all that is very strong evidence but not everyone is convinced that it was the impact that actually caused the extinction there was still the matter the Deccan traps that large igneous province that was active at the time and there are some analyzes that suggest that dinosaurs were going extinct gradually as we approached the entretations which would not be consistent with a sudden Extinction brought on by a giant rock falling out of the sky these estimates of dinosaur species diversity though are not based directly on the number of species found as fossils they are instead based on inferred Extinction and origination rates fossil preservation is also incomplete and for various reasons dinosaur fossils are rare in sediments just prior to the extinction so it is possible that the decline they see May reflect preservation more than what was actually alive at the time however the possibility of a gradual Extinction suggests that the cause of the extinction may be prolonged and millions of years of continual volcanic eruptions could be that long-term environmental disruption like the impact event the deckhand traps have been extensively dated using radiometric techniques these studies find basalts with ages ranging from about 65.5 to 66.5 million years ago showing that these massive basalts began to be deposited before the impact event and continued afterwards so how to resolve this we have two potential events that we are confident occurred at the time of the extinction but we want to know what actually killed the dinosaurs each of these events is a potential trigger a potential event that could start an Extinction but we will need to look at the kill mechanism to determine what the organisms actually die from to decide which of these events is truly responsible for the extinction because different organisms are more susceptible to different kill mechanisms the characteristics of which organisms go extinct the selectivity of Extinction can be really informative about the kill mechanism and thus help us determine the trigger to keep triggers kill mechanisms and extinctions selectivity straight I like to think of pots on shelves during an earthquake Imagine You observe that after an earthquake there are broken pots and unbroken pots inside a house the earthquake is the trigger that caused the pots to fall and break but the kill mechanism is actually hitting the floor the pots that hit the sofa instead of the floor survived they experienced the earthquake but did not interact with the kill mechanism more pods that are made of plastic may hit the floor but not break than do fragile clay pots that break easily when they hit the floor that is extinction selectivity both a giant rock falling out of the sky and hundreds of kilometers of lava and Associated gases oozing out of the ground can cause global changes but because they cause different changes the kill mechanisms the things that are actually killing the dinosaurs and other organisms will be different so as we prepare to weigh the evidence and see which hypothesis is best supported by the clues we find the rock record we need to do a bit of a thought experiment we need to First consider what kill mechanisms might be present during a bullet impact which might be present due to extensive volcanism and which kill mechanisms are unique to each such that observing evidence of those mechanisms would allow us to support one Extinction trigger over the other with a bullied impact we have some expectations immediately after the impact massive fires may be started and some organisms may die from heat stress the impact were alter the rocks and vaporize water and other materials directly at the impact site sending volatiles in the atmosphere that could cause a brief interval of ocean acidification the impact will also send lots of ejecta into the atmosphere causing a phenomenon sometimes called an impact winter because so much incoming solar radiation necessary for warming the planet can't reach the surface thus some animals might die because it is suddenly too cold for the physiology to function this lack of incoming solar radiation also impacts primary productivity which is reliant on light to dry photosynthesis this will reduce the amount of plants on land and phytoplankton in the ocean which typically form the base of food chains and terrestrial and Marine environments and the animals will die of starvation a massive prolonged period of volcanism on the other hand will provide other kill mechanisms volcanism releases a lot of carbon dioxide and that carbon dioxide interacts with the oceans to cause ocean acidification which is bad news for organisms that make their external skeletons out of calcium carbonate and other materials that aren't stable in acidic environments because the volcanism in this case is prolonged we would expect to see a long interval of ocean acidification and that organisms vulnerable to acidic conditions would preferentially go extinct if ocean acidification is the kill mechanism that carbon dioxide will also Drive higher temperatures and organisms could die of heat stress as temperatures become too warm for their physiology to function in the oceans we may also see oxygen stress because warm Waters can't hold as much oxygen as cold Waters and we will see marine animals that need high levels of oxygen suffocate with these potential kill mechanisms in mind we can now look to see who were the winners and losers of this Extinction to determine if the extinction is selective against certain organisms and infer the kill mechanism from that selectivity Plankton living in the upper ocean were severely impacted by the end Cretaceous Extinction the impacted groups included two that make calcareous shells including form andifera which are heterotrophs that eat existing organic matter and calcareous nanoplankton which photosynthesize and make their own food other groups of photosynthetic Plankton that do not make calcareous shells like Daniel flagellates also suffered an intense Extinction overall more than 90 percent of Plankton species were lost not only did the Plankton become less diverse heterotrophic Plankton that that cannot make their own food via photosynthesis became smaller before the extinction communities of planktonic form and effort were comprised of large individuals but after the extinction only species left were small Bodied during the Cretaceous giant strange-looking bivalves formed reefs the closest analog to this might be the oyster reefs we sometimes see in estuaries but the Cretaceous bivalves built reefs and warm tropical waters at shallow water depths where sunlight could still penetrate the water we think that like corals and giant clams today these bivalves have photosymbiotes in their tissues they were able to create sugars that benefited the clam during the end Cretaceous Extinction these went extinct and we no longer have representatives of this group of bivalves today we also saw the extinction of the ammonites these organisms are shelled cephalopods related to today's squid and octopi they were metabolically active Predators with well-developed eyes and are extremely common fossils in Cretaceous Marine sedimentary rocks the last appearance of multiple ammonite species occurs very near the impact clay layer indicating that this group was severely impacted by the end Cretaceous Extinction and of course all dinosaurs other than modern Birds went extinct at the end Cretaceous this group of modern birds that survived was distinct from other early birds and that they had beaks adapted to eat seeds and Grains other flying AV metatarsalia like the pterosaurs also went extinct this graph shows that during the Cretaceous the wingspans of pterosaurs were increasing and they were generally getting larger rather than showing signs of ecological decline thus it seems it was the extinction that was responsible for their demise and not some ongoing stress large marine reptiles such as mosasaurs and plesiosaurs also went extinct at the end Cretaceous ichthyosaurs however had gone extinct earlier in the lake Cretaceous and their demise was not associated with this Extinction event even though mammals as a whole did not go extinct if they had we wouldn't be here species of mammals did go extinct we can look at their teeth to determine what they ate because organisms with a plant-based diet versus a meat-based diet need very different teeth to process their food organisms that are omnivores that eat both plants and animals have a less specialized tooth shape that allow them to process both types of food before the extinction we see mammal teeth that are very specialized for a plant-based diet shown in the lower right corner of these plots as well as teeth that are very specialized for an animal-based diet shown near the top of the plot and towards the upper left corner after the extinction we only see teeth occupying the lower left quadrant of the plot where tooth morphology is consistent with a with an omnivorous diet fall out thus it seems that dietary Specialists were selected against during this Extinction and organisms that could eat a variety of foods were their survivors other winners of this event include fungi that were extremely abundant immediately after the extinction following that Global fungal Spike there was a global Spore Spike showing that ferns were very abundant in the earliest cenozoic next we see more fossils of flowering plants the angiosperms and other seed plants like the gymnosperms become more abundant this recovery of terrestrial plant communities happen faster than the recovery of the animal component of the ecosystems if we examine the winners and losers we can get a sense of what the kill mechanism might have been we see that decomposers and detritus feeders things that eat already dead organic matter did well omnivores many of which were likely scavengers that could eat many types of foods and birds that could eat seeds also made it through the extinction on the losing end were large-bodied animals on land and in the sea animals that had a strict specialized diet and oceanic plankton this pattern of Extinction selectivity points to a productivity collapse and a kill mechanism related to a lack of food and subsequent starvation herbivores and carnivores would starve quickly as their preferred Foods disappeared whereas omnivores that could make do with what they came across could survive smaller organisms that need less food preferentially survived whereas large metabolically active organisms that would need a lot of calories when extinct many plants have seed banks in the soil and even when the plant dies the next generation is waiting for growth conditions to return this may be why plants recovered so quickly it could also explain why dinosaurs with beaks specialize for eating seeds and Grains survived they could find food by eating from those seed banks starvation due to low productivity is a kill mechanism consistent with an impact event impact eject in the atmosphere would have diminished the sun's luminosity and affected the ability of primary producers to photosynthesize causing the collapse of food chains and the extinction of animals not able to find food blocking out the sun with impact ejecta would also cause an Abrupt Cooling using an organic biomarker preserving sediments called Tex 86 we can reconstruct past sea surface temperatures using this payload temperature proxy we see an Abrupt decrease in temperature consistent with decreased incoming solar radiation thus in addition to starvation some animals may have also died from low temperature stresses we can look at other geochemical proxies in Marine sediments to understand changes in Ocean productivity one thing we can look at is the carbon Isotopes preserved in the fossil shells of microorganisms that lived at the surface of the ocean and in the deep sea when photosynthesis is going strong at the surface there's a strong difference in the carbon isotope values in the surface ocean and in the deep ocean this is because organic matter produces the surface is exports the deep sea changing the chemistry of both environments however if productivity shuts down that export of organic matter stops and the chemistry of the surface and the Deep becomes more similar in these plots we see that near the extinction event the difference in carbon Isotopes goes to zero showing a collapse of marine productivity consistent with a lack of sunlight some studies estimate it took three million years to fully recover whereas others think recovery may have happened in a bit less than two million years these changes in the oceanic carbon cycle the decreases in temperature and the extinction of Plankton groups like planktonic formoneifera and calcareous nanofossils all happened at the same time as the impact event the environmental changes inferred from the geochemical proxies and the kill mechanism inferred from the extinction selectivity all point to a Sudden Impact event and are not consistent with an Extinction driven by Deccan trap volcanism perhaps the world the last non-avian dinosaur saw looked like this a red sky due to all the particulars in the atmosphere blocking the sunlight and created a cold wintry world with little primary production but why did this particular impact have such severe consequences on canvas there's an extra credit lab you can complete that examines the size of other impact craters on Earth and ask whether large impacts were always associated with mass extinctions I think you will find that there were a lot more impacts than there are extinctions as severe as the N Cretaceous so what was so special about this one it turns out that the particular place on Earth where the impact occurred was rich in hydrocarbons and sulfates much richer than most other places on the planet the impact would have heated and burned massive amounts of those hydrocarbons sending soot into the atmosphere we can analyze traces of that soot in the sedimentary record and see that they are full of pahs polyaromatic hydrocarbons and thus are derived from hydrocarbons rather than being sipped from forest fires or some other source if it wasn't for the location of the impact that soot would not have been sent into the atmosphere the authors of this study make an interesting point about the improbability of this Extinction happening because it was dependent on the asteroid not only being a sufficient size but also impacting in the correct location they say the history of life on Earth could have varied then according to impact sight and depend on my new differences in the orbital forcing of asteroids the probability of mass extinction was quite low even when an asteroid as large as the kpg Eboli because hydrocarbon rich and sulfide Rich sites are rare if the asteroid had hit a low medium hydrocarbon area on Earth mass extinction could not have occurred and the Mesozoic biota would have persisted beyond the kpg boundary it took Decades of work to amass the evidence necessary to support the hypothesis that a bolite impact was the trigger of the extinction event that killed off the non-avian dinosaurs and it wasn't until we had multiple sources of evidence and many distinct lines of reasoning that we became really confident that it was the impact and not some other trigger however scientists will continue to weigh alternative hypotheses as new Clues surface after mass extinctions we see ecological recovery the bounce back from the from the end Cretaceous was actually quite quick but with the loss of the non-avian dinosaurs there were a lot of ecological roles left unfilled until New tetrapod groups adapted to fill them in the next segment we'll look more closely at the survivors of the extinction and what made them unique and we'll see how the winners of the kpg extinction began to build cenozoic ecosystems