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Eleven years later, researchers announced evidence of a massive crater in the carbonate rock buried beneath Mexico’s Yucatán Peninsula.

These same records, combined with those of mammals from the well-studied Hell Creek Formation in Montana, show that many of Earth’s ecosystems faced on-again, off-again environmental stress, resulting in shifts in biodiversity and loss of species.Paleontologists have long recognized from the fossil record that more than half of the species inhabiting Earth perished at the end of the Mesozoic — the most emblematic of course being the remaining nonavian dinosaurs, like T. And for the last quarter-century, since the identity and approximate age of the roughly 200-kilometer-wide Chicxulub crater on Mexico’s Yucatán Peninsula were confirmed, the impact’s devastating blast, and resulting fallout, has been the favored explanation for the K-Pg extinctions.But before the impact’s ascendance in both the scientific and popular psyches, another prevailing hypothesis invoked to explain the mass die-off described an altogether different global calamity: massive volcanism that heaped layer upon layer of basaltic lava onto the Indian subcontinent and belched climate-modifying gases and aerosols into the atmosphere.And over the years, repeated studies of impact deposits have led to updated understandings of when the meteorite struck: Ages of 65 million years and then 65.5 million years were once favored, but current consensus puts the impact at about 66 million years ago.Meanwhile, the complex, tangled pile of Deccan lava flows defies “classical stratigraphic methods,” says Mike Widdowson, a volcanologist at the University of Hull in England.The scale of change leading up to the K-Pg boundary pales in comparison, however, to what happened during the main pulse of extinction, when an estimated 60 to 70 percent of Earth’s species died off in a geological instant.In addition to the dinosaurs, many other large land animals and terrestrial plants, as well as marine reptiles, mollusks and numerous ocean-dwelling microorganisms, succumbed.But as researchers have continued amassing relevant data — geochemical, geo­chronological, paleomagnetic, paleontological, sedimentary and stratigraphic — explanations for the extinction involving both the impact and volcanism have emerged.It’s not certain whether a definitive picture of exactly what happened is even possible.Possibly totaling more than 1 million cubic kilometers, these lava flows — known as the Deccan Traps — erupted over several million years, beginning before and ending after the mass extinction.The debate over the cause of the K-Pg extinction has continued to simmer through the years, boiling over at times as proponents of each explanation traded barbs in the literature and at scientific meetings.

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