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1990s

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1990

  • Courtillot calculated that the volcanism that formed the Deccan Traps may have gradually released as much as two million cubic kilometers of lava spread over a two million square kilometer area. He also dated this volcanic activity paleomagnetostratigraphically from 30 normal to 29 normal. The K-T boundary itself lay at 29 reversed and Courtillot found this to apparently coincide with the peak of Deccan Trap volcanism.[1]
  • The Chicxulub Crater in Mexico's Yucatan Peninsula was rediscovered.[2]
  • Dodson performed a survey of dinosaur biodiversity and found no support for the hypothesis that the group was in terminal decline during the Late Cretaceous.[3]
  • Archibald and Bryant concluded that the pattern of extinction uncovered by his research showed that losses in biodiversity were not uniform across different groups and ecological niches.[4]
  • They performed a detailed census of the 150,000 fossils from the Hell Creek Formation and overlying Tullock Formation that were curated by the Museum of Paleontology at Berkeley.[5] Their census uncovered a total of 111 different terrestrial species in the two units, only 35 of which survived the boundary. They calculated that 68% of all terrestrial species went extinct at the end of the Cretaceous. They also observed that only 1 of the 28 mammal species they identified in the units managed to survive into the Cenozoic.[6]
  • Johnson and Hickey estimated that 79% of plant species went extinct at the end of the Cretaceous.[7] Johnson and Hickey observed an apparent series of four distinct floras preserved in the macroscopic fossil record of rocks spanning from the Hell Creek Formation to the lower Fort Union Formation. The passing of each distinct flora was accompanied by the extinction of some taxa. The first three floras lay within the Hell Creek and had respective extinction rates of 59%, 75%, and 79%. The last of these represents the losses incurred during the mass extinction at the Cretaceous-Tertiary boundary.[8]
  • Wolbach and others argued that there is a nearly global layer of rapidly deposited charcoal and soot that formed as a result of widespread forest fires ignited by an asteroid impact.[7]
  • Yanicke Dauphin examined the claims of Zhao Zi-Kui and others that increased quantities of trace elements in the environment during the Cretaceous led to dinosaurs concentrating these substances in their eggshells until so many dinosaur hatchlings died in the egg or were born with birth defects that the whole group went extinct. Dauphin's research refuted this hypothesis because his experiments with living birds showed that eggshell trace element concentrations are not always related to their dietary concentration.[9]
  • Yanicke Dauphin and J-J Jaeger published additional research on Zhao Zi-Kui's trace element hypothesis.[9]
  • Ursula Marvin argued that the asteroid impact explanation for the end-Cretaceous mass extinction was at odds with the idea of uniformitarianism and criticized those who attempt to reconcile the two as engaging in "newspeak".[10]
  • Alvarex and Asaro measured the iridium levels of a 57m span of rock near the K-T boundary at Gubbio once more. They estimate that it took roughly 10 million years for the sediments composing these rocks to be deposited. Their analysis found low iridium levels throughout the sampled interval of strata except at the K-T boundary, where there was a tremendous spike in iridium content accompanied by trivially elevated levels immediately above and below it. James Lawrence Powell characterized their results as consistent with those of the Rocchia team.[11]
  • May: Hildebrand and Boynton published the result of a literature search for craters that could have resulted from the end-Cretaceous impact event. They concluded that the best candidate was a buried crater on the seafloor north of Colombia, but noted that the nature of the ejecta preserved at K-T boundary sites around the world are inconsistent with a marine impact. They also briefly mentioned a potential crater reported from the Yucatan Peninsula, but did not examine the possibility in-depth.[12] However, in doing so they "scooped" Pope, Duller, and Ocampo who were completely unaware of Hildebrand and Boynton's work. Pope reached out to Hildebrand, who responded with an unpublished manuscript detailing his intent to name the crater Chicxulub.[13]
  • Keith Meldahl verified the Signor-Lipps effect experimentally by taking core samples of mud at a modern tidal flat in Mexico. His samples contained a total of 45 species, of which 35 disappeared from the sample at some point below the top, as if this tidal flat ecosystem was experiencing a gradual mass extinction when in fact every species in the sample was still alive.[14]
  • Michael Benton summarized the history of scientific research into the extinction of the dinosaurs. He described three major periods of this research project beginning with the recognition of dinosaurs as a distinct group in the 1840s. Dinosaur extinction attracted little interest because it was taken for granted that they were inferior to mammals and were simply outcompeted. Interest in the subject increased during the years from 1920-1970, a span of time that Benton called the "dilettante phase", but many of the proposed hypotheses lacked rigor or were even put forward only in jest. The following years Benton deemed the "professional phase" of dinosaur extinction research. During the early part of this period most paleontologists had concluded that dinosaurs were eliminated by the normal processed of extinction.[15]
  • U. B. Marvin attributed the earth science community's reluctance to accept bolide impact as a significant process in the history of the plant to philosophical uniformitarianism.[16]
  • Keller and Barrera published their research indicating for significant foraminiferan extinctions occurring hundreds of thousands of years before the Cretaceous-Tertiary boundary.[17]

1991

  • Hildebrand and Boynton declared the Chicxulub Crater to be the result of the impact that triggered the mass extinction at the end of the Cretaceous.[18]
  • Hildebrand and others estimated the diameter of the Chicxulub Crater at 170 kilometers.[2]
  • Sheehan and others collected dinosaur fossils from the lower, middle, and upper Hell Creek Formation in North Dakota and Montana. They found no evidence for a gradual decline in dinosaur biodiversity toward the end of the Late Cretaceous, nor did they find any evidence for a change in the proportions of various dinosaur groups composing the Hell Creek's magafauna. Sheehan and the other researchers concluded that a catastrophic extinction scenario best explained the results of their analysis.[19][20]
  • Zhao Zi-Kui and others observed that many fossil dinosaur eggs from Late Cretaceous rocks in China had shells that were too thick, too thin or suffering other kinds of structural deformities. They reported a 75% increase in deformed eggs of the oospecies Macroolithus yaotunensis approaching the boundary between the Cretaceous and Tertiary.[21] They analyzed the chemical structure of the eggshells and found a spike in trace elements about 90m below as well as an increase in heavier oxygen isotopes in shells preserved about 80 m below the K-T boundary.[22] They hypothesized that about 200-300,000 years before the end of the Cretaceous the world's climate suddenly became very dry, concentrating trace elements and heavy oxygen isotopes in the water supply. These substances were absorbed and concentrated by plants thereby entering and climbing the food web to the dinosaurs. As the dinosaurs ingested more and more trace elements their concentrations in the tissues of female dinosaurs led them to produce and ever greater proportion of deformed eggs and young causing the dinosaurs to gradually go extinct over this period.[23]
  • Carlisle and Braman reported the anomalous presence of tiny diamonds at the K-T boundsry in Alberta, Canada. Diamonds like these can form in explosions and are found in meteorites, so diamonds at the K-T boundary support the impact hypothesis.[24]
  • Carter and Officer reviewed many "enigmatic terrestrial structures" in the rock record that had been discussed in the scientific literature. They concluded that most, like the structures at Manson, Sudbury, and Vredefort, resulted from earth's internal geological forces.[25]
  • Robin and others measured iridium levels and observed spinel spherule distribution at the K-T boundary near El Kef, Morocco. They found that the iridium levels were spread out somewhat broadly, but the spinel spherules precisely demarcated the boundary. They concluded that the boundary itself formed very rapidly over a period of less than a century.[26]
  • Penfield published a letter in Natural History objecting to Hildebrand's claim to have identified the Chicxulub Crater as "ground zero" to the end-Cretaceous mass extinction. He pointed out that he proposed that very hypothesis back in 1981.[27]
  • Pope and others finally published their research that had been "scooped" by Hildebrand and Boynton.[13]
  • Izett and others radiometrically dated spherules from the K-T boundary of Haiti to an age of 64.5 million years. They found feldspar from the K-T boundary of the Hell Creek Formation to be 64.6 million years old.[28]
  • Hall and others confirmed the Izett team's radiometric date of the Haitian tektites at 64.75 million years.[28]
  • Sigurdsson and others analyzed the composition of the Haitian tektites, concluding that they mostly derived from continental granite.[29]
  • Huber published the first results of his research on the core sample extracted from the sea floor off the coast of Desolation Island, Antarctica.[30]

1992

  • Sigurdsson and others concluded that global mean temperatures dropped 2-3 degrees celsius across the Cretaceous-Tertiary boundary.[31] They also argued that evaporite material ejected from the impact site could have formed sulfuric acid in the atmosphere that would fall back to earth as acid rain.[7]
  • Johnson found that the position of the coal layers once thought to mark the Cretaceous-Tertiary boundary between the Late Cretaceous Hell Creek Formation and Paleocene Tullock Formation may deviate from the actual boundary "by as much as 5 m".[32]
  • Officer and others argued that the Chicxulub Crater was formed by volcanic activity rather than an impact event.[2]
  • Swisher and others dated the formation of the Chicxulub Crater almost exactly to 65 million years ago.[2] Specifically, they dated igneous rock from the Chicxulub crater to 64.98 million years ago.[28]
  • Sheehan and Fastovsky published a study based on Archibald and Bryant's 1990 paper about Cretaceous-Tertiary extinction event survivorship. They criticized Archibald and Bryant's treatment of the Hell Creek's elasmobranch taxa as victims of the extinction due to their absence from the lower portion of the overlying Tullock Formation because this stratigraphic interval was deposited in a terrestrial environment, so the absence of marine life is actually expected in that context.[4] Sheehan and Fastovsky also criticized Archibald and Bryant for considering some kinds of mammals as "survivors" of the Cretaceous-Tertiary extinction. Archibald and Bryant had justified treating these mammals as survivors because they were supposedly close evolutionary relatives of certain Paleocene mammals, but Sheehan and Fastovsky observed that none of the individual Hell Creek mammal species are found in rocks post-dating the extinction and should thus be considered extinctions.[33] Overall, Sheehan and Fastovsky found that roughly 50% of vertebrate species went extinct at the end of the Cretaceous. Archibald himself would find a similar percentage of extinctions in a later 1996 paper. Sheehan and Fastovsky found terrestrial vertebrates to be the primary victims of the end Cretaceous extinction event, with 88% of their biodiversity lost. Freshwater vertebrates only lost 10% of their biodiversity across the boundary[6] and the researchers found this divide in habitat preference to be the single greatest source of variation in survivorship rates among the taxa they studied.[34] They observed that the better survival rates among aquatic tetrapods as opposed to terrestrial ones was consistent with the idea of an extensive period of darkness following an asteroid impact. This is due to aquatic ecosystems being less dependent on primary productivity than terrestrial ones because many aquatic tetrapods would be able to subsist on detritus and scavenged remains until photosynthesis resumed.[35][6]
  • Yanicke Dauphin published additional research on Zhao Zi-Kui's trace element hypothesis. He concluded that the elevated trace element levels of Chinese dinosaur eggs studied by Zhao and his collaborators were not present in "life" but instead entered the egg dissolved in ground water during the fossilization process.[9]
  • Buck Sharpton and others reported another radiometric date for igneous rocks from the Chicxulub crater. They found the rocks to be 65.2 million years old.[28]
  • Smit and others reported the presence of another tsunami deposit at Arroyo el Mimbral, Mexico. Evidence that it formed as a result of a tsunami connected with the end-Cretaceous impact include elevated iridium levels, fossils of terrestrial plants, shocked minerals, and tektites.[36]
  • Clemens disputed Sheehan and Fastovsky's conclusion that dinosaurs were not in decline based on their census of Hell Creek Formation fossils and insisted that dinosaur extinction must have been gradual.[37]

1993

  • Sepkoski calculated that roughly 20% of families and 50% of genera went extinct during the Cretaceous-Tertiary extinction event.[38]
  • Lecuyer and others concluded that mean temperatures in some areas dropped as much as 8 degrees celsius following the Cretaceous.[31]
  • Venkatesan and others estimated the age of the Deccan Traps.[1]
  • Ivany and Salwitch argued that in the wake of the end-Cretaceous impact event as much as 25% of earth's terrestrial biomass was consumed in a nearly global conflagration.[7]
  • Sweet and others found evidence in fossil pollen and spores suggesting a gradual change in Canada's flora during the Late Cretaceous.[8]
  • Johnson saw no evidence for any "biotic upheaval" in the fossil pollen and spores of the latest Cretaceous of New Zealand.[8]
  • Raup and Jablonski observed that 63% of bivalves went extinct during the last 10 million years of the Cretaceous but could not determine how gradually or abruptly the pace of these extinctions were.[39]
  • Zhao Zi-Kui and others reported abnormal quantities of amino acids in deformed dinosaur eggshell from the Late Cretaceous of China.[40]
  • Dewey McLean accused the journal Science of bias favoring the impact hypothesis. He counted a total of 45 pro-impact papers published by the journal since the hypothesis was first proposed in contrast to only four anti-impact papers.[41] Dan Koshland, the journal's editor, denied showing favoritism to either hypothesis.[42]
  • Hildebrand argued that the K-T boundary layer must have been deposited extremely quickly through some means James Powell characterized as "independent of normal sedimentation processes". Hildebrand drew his conclusion from the boundary layer's ubiquity and uniformity of thickness across rocks that formed in different environments and at different rates.[43]
  • Izett and others radiometrically dated the Manson crater again, but found an age of 73.8 million years, too old for it to be the end-Cretaceous impact crater. To confirm this new measurement the team examined rocks of that age in South Dakota. Their fieldwork turned up a layer of shocked minerals, confirming that an impact ocurred in the region at that time and thus the revised date was the true age of the Manson crater.[44]
  • Blum and others compared the isotope ratios of neodymium, oxygen, and strontium found in the Haitan tektites with the igneous rock from the Chicxulub crater. Their results indicated that the crater and the tektites had identical isotope ratios and they concluded that the tektites and the rock "come from the same source".[29]
  • Stinnesbeck and others disputed Smit and others' attribution of the Arroyo el Mimbral deposits to a tsunami and supposed connection to a nearby impact.[36] Instead, they attributed the Arroyo el Mimbral deposits to "coastal sediments [that] slumped into deeper water", a completely mundane occurrence.[45]
  • Bohor and others reported the presence of zircon grains at the K-T Boundary in Colorado which exhibit similar shock deformation to that commonly reported in quartz grains from the boundary elsewhere. Shocked zircon had never been observed before.[46]
  • Krogh and others used Uranium-Lead dating to study zircons from the K-T Boundary in Colorado, Haiti, and the Chicxulub crater.[47] They found that the zircons first crystalized 545 million years ago, and experienced a loss of lead during an episode that occurred 65 million years ago. This loss of lead could have been caused by heat from the hypothesized impact event.[48]

1994

  • Prasad and others estimated the age of the Deccan Traps.[1]
  • Smith and others concluded that the Late Cretaceous drop in sea levels constituted the most severe marine regression of the entire Mesozoic Era.[32]
  • Smith and others concluded that the withdrawl of shallow seas from Earth's continents during the Late Cretaceous led to a greater area of exposed land during that time period.[49]
  • D'Hondt and others argued that an asteroid impact at the end of the Cretaceous would not have produced enough acid for acid rain to be a significant factor contributing to the mass extinction.[7]
  • Weil argued that the hypothesis of acid rain occurring in the wake of an asteroid impact contributing the Cretaceous-Tertiary mass extinction was a poor explanation for the which taxa actually survived or perished.[7]
  • Askin and others found no evidence for any "biotic upheaval" in the fossil pollen and spores of the latest Cretaceous of Antarctica.[8]
  • Popsichal concluded that the extinction of many foraminifera at the end of the Cretaceous occurred abruptly rather than gradually.[39]
  • A conference dedicated to the end-Cretaceous extinction event was held in Houston, Texas.[50] During the conference several expert attendees embarked on a field trip to the Mexican Arroyo el Mimbral site to assess whether or not the deposit formed rapidly, as in the tsunami hypothesis or gradually as in the sedimentary slumping hypothesis.[45] Personal accounts on which model was more widely supported among the attendees differ.[51]
  • Kerr reported in Science that most of the participants in the Arroyo el Mimbral field trip supported the rapid deposition model.[45]
  • Kerr responded in another Science article that most of the participants in the Arroyo el Mimbral field trip actually concluded that the deposit formed independantly of any impact event.[52]
  • Kerr responded once more maintaining that the vast majority of the Arroyo el Mimbral participants he interviewed supported a rapid deposition model.[52]
  • Macleod and Keller denied that an asteroid impact caused the extinction of foraminfifera at the K-T boundary.[53]
  • Fischer reported the results of an attempt to resolve the controversy surrounding the rate of formainiferan extinction at the K-T boundary by having new samples collected and examined by uninterested third parties. This project found that between 2 and 21% of Late Cretaceous foraminiferans had gone extinct before the boundary. Both feuding camps saw support in these data for their own hypotheses. Keller's gradual model was supported by the disappearance of some taxa before the boundary. Smith argued however, that these disappearances were statistical anomalies resulting from the Signor-Lipps efffect rather than actual extinctions. He demonstrated that every foraminiferan found in multiple samples survived up to the boundary and only the uncommon ones "disappeared". Keller argued in return that similar-looking species that died out stepwise may have been misidentified as single species that persisted up to the boundary before going extinct. Such a misidentification would have inflated the number of species that appeared to go extinct at the boundary itself.[54] Brian Huber subsequently disputed some of Keller's identifications and taxonomic opinions and rejected a gradualist model of Cretaceous foraminiferan extinctions.[55]

1995

  • Hurlbert and Archibald argued that the statistical analyses used by Sheehan and others in 1991 were not precise enough to reliably conclude that the make-up of the Hell Creek dinosaur fauna did not change over time. They also argued that the quality of the Hell Creek fossil record was too poor to determine whether or not the extinction of the dinosaurs was gradual or sudden.[19]
  • Keller and MacLeod concluded that the extinction of many foraminifera at the end of the Cretaceous occurred gradually.[39]
  • By the end of the year, 50 K-T boundary sites with anomalously high iridium levels had been identified.[56]
  • Peuckerehrenbrink and others studied osmium isotope ratios from sediments ranging in age from recent to 80 million years old. They found only the osmium at the K-T boundary to preserve an anomalous extraterrestrial-like ratio.[57]
  • N. Bhandari and others reported the discovery of the Cretaceous-Tertiary boundary in the Deccan Traps.[58] The Deccan Traps are a series of basalt layers released by intermittent volcanic activity across the Cretaceous-Tertiary boundary. During the periods between eruptions, normal sediments accumulated in deposits called intertrappeans. The basalt deposits can be dated with paleomagnetism and radiometric dating, so the intertrappeans can be dated fairly precisely. Bhandari and the other researchers found the third intertrappean to have been laid down at the K-T Boundary. This intertrappean proved highly significant because this layer alone among the traps contained elevated iridium levels, so the volcanic activity itself could not be the source of the iridium. Further, Intertrappean III preserves dinosaur eggshells, proving that they survived up to the very end of the Cretaceous.[59]
  • May: Dewey McLean retired due to ill health. He attributed his medical problems to stress caused by persecution from Luis Alvarez, who McLean claimed had been trying to destroy his career ever since McLean first voiced opposition to the impact hypothesis back in the 1980s.[60]
  • Kamo and Krogh published their analysis of zircons from the K-T Boundary in Saskatchewan.[47] They found the zircons to have crystalized 548 million years ago and to have lost lead 59 million years ago. As the latter date had a 10 million year margin of error, heat from the hypothesized K-T impact could have been the cause of the lead loss.[61]
  • Peter Ward criticized the perennial hypothesis that dropping sea levels at the end of the Cretaceous contributed to the extinction of the dinosaurs because there was no known explanation for how lower sea levels could lead to such an extinction.[62]
  • J. K. Rigby reported to a meeting of the Geological Society of America their discovery of a 300,000 to 500,000 year gap in the rock of the Hell Creek Formation. They identified this temporal gap by studying the formation's paleomagnetism.[63]
  • Rigby and others published the paleomagnetic research that led them to conclude that there was a 300,000 to 500,000 year gap in the rock record of the Hell Creek Formation.[63]
  • Ashraf and others reported the presence of possible Cenozoic dinosaur bones in China.[37]

1996

  • Keller and Stinnesbeck estimated that the lowest sea levels of the entire Late Cretaceous marine regression 100,000-300,000 years before the Cretaceous-Tertiary boundary.[32]
  • Officer and Page disputed the hypothesis that an asteroid impacted with the earth at the Cretaceous-Tertiary boundary.[64]
  • Schultz and D'Hondt argued that the age, geochemistry and location of the Chicxulub Crater all support its attribution to the hypothetical impactor that triggered the end-Cretaceous extinction event.[2]
  • Archibald observed that the more common ornithischian dinosaurs declined in diversity more sharply than the saurischians during the Late Cretaceous.[3] He observed that most of the vertebrate extinctions at the K-T boundary occurred in the following groups elasmobranchs, non-avian dinosaurs, metatherian mammals, and squamates. He also observed that terrestrial life experienced greater losses (72%) than freshwater life (24%). Likewise, warm-blooded animals (74%) experienced greater losses than cold blooded ones (39%) and large animals (67%) versus small ones (26%). He noted that the pattern of extinction uncovered by his research showed that losses in biodiversity were not uniform across different groups and ecological niches.[4] Archibald argued that the withdrawal of shallow seas from Earth's continents during the Late Cretaceous reduced the size of and fragmented the coastal plain habitats preferred by large dinosaur species and that this fragmentation may have driven some taxa extinct.[65]
  • Retallack found evidence for acidic conditions near the Cretaceous-Tertiary boundary in the Hell Creek Formation, but argued that the buffering capacity of the local environment would have protected its inhabitants by buffering the harsh pH values.[7] However, he argued that the local acidity may have been enough to remove iridium from the sediments and dissolve shocked minerals. This implies that the physical consequences of an impact event can actually destroy the same geological evidence needed to prove that such ever happened in the first place.[66]
  • Archibald criticized Retallack's paper on pH in the Hell Creek Formation because its focus on a single regional geographic unit prevented the reliable generalization needed to draw conclusions about a global extinction event. He also dismissed as unsubstantiated Retallack's "claims of a trauma for herbivorous vertebrates".[7]
  • D'Hondt and others reinterpreted the carbon isotope data Hsu and others had argued implied the existence of a "Strangelove Ocean" with no primary productivity at the Cretaceous-Tertiary boundary. This reinterpretation concluded that the data actually represented a cessation of carbon transport from the surface to deeper water at that time and that this cessation lasted up to three million years beyond the Cretaceous They also argued that the remains of Cretaceous foraminifera had been physically disturbed and redeposited in Paleocene sediments, creating an illusion of a more gradual extinction than had actually occurred.[39]
  • Huber also argued that the remains of Cretaceous foraminifera had been physically disturbed and redeposited in Paleocene sediments, creating an illusion of a more gradual extinction than had actually occurred.[39]
  • MacLeod concluded that the extinction of many foraminifera at the end of the Cretaceous occurred gradually.[39]
  • Popsichal concluded that the extinction of many foraminifera at the end of the Cretaceous occurred abruptly rather than gradually.[39]
  • Macleod and others observed that inoceramid bivalves suffered a significant worldwide episode of extinctions during the mid-Maastrichtian, although not all at exactly the same time.[39]
  • Marshall and Ward published a detailed examination of latest Cretaceous ammonite biostratigraphy at Zumaya, Spain. They tracked the survivorship of 28 different ammonite species. They found that of these 28, 6 went extinct significantly before the end of the Cretaceous, 12 survived up to the period's boundary with the Tertiary, and the rest may or may not have perished in between the other extinctions.[39]
  • Dypvik and others found shocked quartz and anomalous iridium levels in the undersea Mjolnir crater. This provided further evidence that these phenomena are the geological fingerprints of impact events.[67]
  • Anbar and others measured the iridium content of modern bodies of water. They found that the K-T boundary preserved 1,000 times as much iridium as is present in all of the world's oceans combined. Such high levels could not be accoundted for by normal sedimentary processes. Anbar and his colleagues also estimated that it would take 10,000 to 100,000 years for marine iridium to be incorporated into seafloor sediments. This provided a way to reconcile any rocks found to have a gradual increase and decrease in iridium around the K-T boundary with the impact hypothesis because there would have been a delay between the introduction of the iridium into the ocean water and its incorporation into the sediments that would later lithify into the boundary rocks.[68]
  • Birger Schmitz and Asaro re-examined volcanism as a potential source of elevated iridium levels in the rock record.[69] They verified "that some types of explosive volcanism" can release significant quantities of iridium, but argued that levels of other elements in these volcanic ashes distinguish them from impact material. Despite confirming volcanism in general as a potential iridium source, Schmitz and Asaro disputed the validity of certain specific reports of volcanic iridium that had supposedly called the impact hypothesis into question. The first of which was the report of high levels of iridium in volcanic ash from Russia.[70] When Schmitz and Asaro studied the same kind of volcanic ash, they were unable to find significant amounts of iridium. Secondly, they disputed the attribution of iridium reported from under Antarctic ice to volcanic activity and hypothesized that the elevated iridium levels there represented accumulations of meteoritic dust.[71]
  • Sharpton and others argued that the Chicxulub crater was actually about 300 km in diameter rather than about 170 km.[72] According to James Lawrence Powell, if this estimate is correct, the Chicxulub crater is one of the largest impact structures in the inner solar system.[73]
  • Ocampo and others reported the nearest known impact ejecta deposit to the Chicxulub Crater on Albion Island, Belize.[74]
  • July: Officer and Page published their book, The Great Dinosaur Extinction Controversy.[75]
  • The 20 paper anthology Cretaceous Mass Extinctions: Biotic and Environmental Changes was published. The volume was edited by Keller and Macleod who continued to argue that foraminifera were not victims of a catastrophic mass extinction at the end of the Cretaceous.[76]
  • Huber published further research on the core sample extracted from the sea floor off the coast of Desolation Island, Antarctica. He found evedience for Cretaceous foraminiferans having been reworked into Paleogene sediments.[30]
  • Fastovsky and Sheehan criticized the notion that a drop in sea level fragmented dinosaur coastal plain habitat, leading to their extinction as nonsensical because a drop in sea level would more likely expanded their available habitat.[77]
  • Marshall and Ward performed a statistical analysis on ammonite fossils collected at Zumaya to determine the rate and timing of their extinction. They found that some local species perished in time with a drop in sea level, but roughly 50% went extinct at the K-T boundary.[78]

1997

  • Fastovsky and Sheehan argued that there was no evidence for the kind of habitat fragmentation Archibald hypothesized to occur with Late Cretaceous marine regression.[79]
  • Ginsburg reported the results of a "blind test" of both sides in the controversy over whether or not foraminifera went extinct gradually or abruptly at the end of the Cretaceous.[39] However, even this blind test proved inconclusive and was unable to settle the controversy between Keller and Smit.[80]
  • Hallam and Wignall observed that all five of Earth's mass extinctions were associated with world-wide drops in sea level.[81]
  • Buffetaut published the results of his field work aimed at studying vertebrates near the K-T boundary. His findings were consistent with the impact hypothesis.[82]
  • Pope and others estimated that the impact which formed the Chicxulub Cater would have ejected 200 billion tons of sulfur dioxide and water into the atmosphere. They argued that the world would have suffered "a decade of impact winter" in the impact's aftermath.[66]

1998

  • Sullivan argued that dinosaur biodiversity experienced a marked decline over the last ten million years of the Cretaceous Period.[3]
  • Stromberg and others reported that fossil pollen from the Hell Creek Formation provided evidence for a gradual shift in the region's flora "from more open to more closed and moist habitats".[8]

1999

  • Norris and others concluded that the extinction of many foraminifera at the end of the Cretaceous was abrupt rather than gradual.[39]
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  23. ^ Cite error: The named reference enbd-reason2-251-252 was invoked but never defined (see the help page).
  24. ^ Cite error: The named reference powell-prediction7-64 was invoked but never defined (see the help page).
  25. ^ Cite error: The named reference powell-shock-81 was invoked but never defined (see the help page).
  26. ^ Cite error: The named reference powell-spher-83 was invoked but never defined (see the help page).
  27. ^ Cite error: The named reference powell-red-103 was invoked but never defined (see the help page).
  28. ^ a b c d Cite error: The named reference powell-age-109 was invoked but never defined (see the help page).
  29. ^ a b Cite error: The named reference powell-geochem-110 was invoked but never defined (see the help page).
  30. ^ a b Cite error: The named reference powell-forams-156 was invoked but never defined (see the help page).
  31. ^ a b Cite error: The named reference arch-fast-events-672 was invoked but never defined (see the help page).
  32. ^ a b c Cite error: The named reference arch-fast-regress-673 was invoked but never defined (see the help page).
  33. ^ Cite error: The named reference arch-fast-patt-678-679 was invoked but never defined (see the help page).
  34. ^ Cite error: The named reference arch-fast-patt-679 was invoked but never defined (see the help page).
  35. ^ Cite error: The named reference arch-fast-corolast-680 was invoked but never defined (see the help page).
  36. ^ a b Cite error: The named reference powell-ejecta-111 was invoked but never defined (see the help page).
  37. ^ a b Cite error: The named reference powell-triumph-174 was invoked but never defined (see the help page).
  38. ^ Cite error: The named reference arch-fast-intro-672 was invoked but never defined (see the help page).
  39. ^ a b c d e f g h i j k Cite error: The named reference arch-fast-marine-682 was invoked but never defined (see the help page).
  40. ^ Cite error: The named reference enbd-reason2-251 was invoked but never defined (see the help page).
  41. ^ Cite error: The named reference powell-volc-damage-93 was invoked but never defined (see the help page).
  42. ^ Cite error: The named reference powell-volc-damage-93-94 was invoked but never defined (see the help page).
  43. ^ Cite error: The named reference powell-volc-clues-99 was invoked but never defined (see the help page).
  44. ^ Cite error: The named reference powell-manson-100 was invoked but never defined (see the help page).
  45. ^ a b c Cite error: The named reference powell-ejecta-112 was invoked but never defined (see the help page).
  46. ^ Cite error: The named reference powell-zircon-118 was invoked but never defined (see the help page).
  47. ^ a b Cite error: The named reference powell-zircon-119 was invoked but never defined (see the help page).
  48. ^ Cite error: The named reference powell-zircon-116-119 was invoked but never defined (see the help page).
  49. ^ Cite error: The named reference arch-fast-corol-679 was invoked but never defined (see the help page).
  50. ^ Cite error: The named reference powell-alvpred-57 was invoked but never defined (see the help page).
  51. ^ Cite error: The named reference powell-ejecta-112-113 was invoked but never defined (see the help page).
  52. ^ a b Cite error: The named reference powell-ejecta-113 was invoked but never defined (see the help page).
  53. ^ Cite error: The named reference powell-forams-155 was invoked but never defined (see the help page).
  54. ^ Cite error: The named reference powell-forams-153 was invoked but never defined (see the help page).
  55. ^ Cite error: The named reference powell-forams-153-154 was invoked but never defined (see the help page).
  56. ^ Cite error: The named reference powell-prediction1-57 was invoked but never defined (see the help page).
  57. ^ Cite error: The named reference powell-prediction7-63-64 was invoked but never defined (see the help page).
  58. ^ Cite error: The named reference powell-volc-indian-91-92 was invoked but never defined (see the help page).
  59. ^ Cite error: The named reference powell-volc-indian-92 was invoked but never defined (see the help page).
  60. ^ Cite error: The named reference powell-volc-damage-94 was invoked but never defined (see the help page).
  61. ^ Cite error: The named reference powell-zircon-116-120 was invoked but never defined (see the help page).
  62. ^ Cite error: The named reference powell-theories-168 was invoked but never defined (see the help page).
  63. ^ a b Cite error: The named reference powell-andback-171 was invoked but never defined (see the help page).
  64. ^ Cite error: The named reference arch-fast-impact-673-674 was invoked but never defined (see the help page).
  65. ^ Cite error: The named reference arch-fast-corol-679-680 was invoked but never defined (see the help page).
  66. ^ a b Cite error: The named reference powell-hell-178 was invoked but never defined (see the help page).
  67. ^ Cite error: The named reference powell-prediction3-59 was invoked but never defined (see the help page).
  68. ^ Cite error: The named reference powell-hills-78 was invoked but never defined (see the help page).
  69. ^ Cite error: The named reference powell-volc-irid-86 was invoked but never defined (see the help page).
  70. ^ Cite error: The named reference powell-volc-irid-86-87 was invoked but never defined (see the help page).
  71. ^ Cite error: The named reference powell-volc-irid-87 was invoked but never defined (see the help page).
  72. ^ Cite error: The named reference powell-size-105-106 was invoked but never defined (see the help page).
  73. ^ Cite error: The named reference powell-size-106 was invoked but never defined (see the help page).
  74. ^ Cite error: The named reference powell-ejecta-110 was invoked but never defined (see the help page).
  75. ^ Cite error: The named reference powell-predictions-113 was invoked but never defined (see the help page).
  76. ^ Cite error: The named reference powell-forams-154-155 was invoked but never defined (see the help page).
  77. ^ Cite error: The named reference powell-theories-167-168 was invoked but never defined (see the help page).
  78. ^ Cite error: The named reference powell-ammonites-148 was invoked but never defined (see the help page).
  79. ^ Cite error: The named reference arch-fast-corol-680 was invoked but never defined (see the help page).
  80. ^ Cite error: The named reference powell-forams-154 was invoked but never defined (see the help page).
  81. ^ Cite error: The named reference arch-fast-multi-683 was invoked but never defined (see the help page).
  82. ^ Cite error: The named reference arch-fast-single-684 was invoked but never defined (see the help page).
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