The Cretaceous - Paleogene (K/P) boundary, 65 million years ago
What happened when a huge asteroid slammed into Mexico’s Yucatan peninsula?
Evidence of the cretaceous habitat, 145 - 65 million years ago Shell growth: Oxygen isotope ratios measured in this specimen’s shell indicate cyclical water temperature changes, demonstrating that the growth bands represent seasonal growth. The specimen added more than 5 cm (2 inches) of shell every year.
The dinosaurs are the most famous victims at the end of the Cretaceous, 65 million years ago, but they weren’t alone. Nearly half of the Earth’s species disappeared, including nearly 75% of ocean species. The scientific consensus indicates that an asteroid was to blame[1]. Dust and smoke from the impact blocked sunlight, disrupted photosynthesis and altered the Earth’s climate. “For many months, temperatures on the Earth’s surface would have dropped as the Sun’s light was obscured. Then, as layers of dust insulated the earth, creating a sort of greenhouse effect, temperatures would have risen once more. These wild temperature fluctuations were enough to wipe out many species of warm-adapted creatures. Modern birds, with their insulating layer of feathers, may be descendants of the few species of dinosaurs that survived the catastrophe of the late Cretaceous period”.[2]
These findings have recently been corroborated by a team under the leadership of Professor Paul Renne of the University of California, Berkeley. Using a new, more precise dating method relying on the decay rate of a radioactive potassium isotope, Renne’s team dated rocks from a well known dinosaur extinction site in Montana to a precise 66,043,000 years. The team then narrowed the time of the Chicxulub impact in the Yucatan Peninsula by dating impact debris, known as tektites, which can be found around the world for millennia after the initial collision. The results show the impact was just 32,000 years before dinosaurs died out, in lieu of the 300,000 years suggested in other studies. In other words, the impact was the final straw that pushed the earth past the tipping point, coming on top of the dramatic change in the Earth’s climate during the preceding million years, including long cold snaps, and a number of volcanic eruptions occurring about the same time. [3]
Recent research has revealed that the dinosaurs, or at least some species - the most abundant herbivores such as the horned and duckbill dinosaurs in regions characterised by declining diversity of species - were quite vulnerable at the time of the impact[4]. Their decline made ecosystems more vulnerable to collapse by destabilising the foundation of the food web, increasing the likelihood that the extinction of just a few species would cascade through the ecosystem.
This experience informs us that even the most dominant groups of organisms can go extinct and quite suddenly at that. The dinosaurs' extinction was made easier, "perhaps even enabled", by losses in biodiversity that preceded the asteroid impact. They had ruled the world for over 150 million years when they went extinct. Modern humans have been around for at most a few hundred thousand years, and as Stephen Brusatte reminds us "we are changing the environment at such a fast rate that so-called sixth extinction is occurring, with global diversity in rapid decline. Who knows how vulnerable we are making ourselves in the process?"[5]
Other species affected
Species in habitats little affected by the environmental changes, and those with greater tolerance for stressful conditions survived at the expense of those who couldn’t cope with disruptions to the food web. Survivors, by contrast, had other food sources, such as organic matter stored in sediments.
Microscopic fossils are plentiful in deep ocean core samples, so, the presence – or absence – of specific organisms is revealing. For instance, calcareous microplankton (those containing calcium carbonate) suffered in the extinction. Diatoms (single-celled predominantly microscopic algae which consist of two halves or valves) and dinoflagellates (marine plankton), which can go dormant until conditions improve, fared much better.
Rudist clams were one species which suffered. About 100 million years ago, they built community reefs, but when the asteroid impact changed the global climate rudists became extinct. Millions of years later, corals diversified, until soon they were the dominant tropical reef builders – and still are today. Modern clams are bivalves with two shells (valves) joined at a hinge. Rudist clams were bivalves too, but came in a remarkable array of shapes. They fell into two main groups:
- one group grew upright, like a big ice cream cone balanced on end. The bottom valve was anchored in the ocean floor. The top valve acted as a cover. Only the upper few inches poked above the sediment.
- another group “reclined”, lying on the ocean floor. Their horn-shaped shells kept them from washing away in strong currents.
[1] “An asteroid did kill the dinosaurs after all”: http://www.spacedog.eu/astronomy/historic-astronomy/an-asteroid-did-kill-the-dinosaurs-after-all.html, Tuesday, 30 November 2010 13:48. See also "Solving the Dinosaur mystery", on the Big History site at https://www.bighistoryproject.com/chapters/3#extinctions
[2] Christian, Maps of Time, 125. Or perhaps all dinosaurs might have had feathers. Previously only carnivorous dinosaur species were thought to have feathers, but the recent (2014) discovery of feather-like structures on fossils from a Siberian plant-eating dinosaur from a species named Kulindadromus zabaikalicus that lived during the Jurassic period shows that feathers were a more common feature among dinosaurs than at first thought: Nicky Phillips, “Feathers a common trait among dinosaurs”, SMH, 25 July 2014.
[3] “Dinosaur deaths: meteorite did do it”, SMH, 8 February 2013.
[4] Stephen Brusatte, "What killed the dinosaurs", Scientific American, December 2015, 46.
[5] Ibid, 51.
The extinction of 82% of planktonic foraminifera species (unicellular organisms with a complex cell) is apparent here[1]. The impact bed is composed largely of tektite apherites (glassy objects associated with meteor impacts) – evidence of the asteroid impact.
How do we know what caused the extinction? The impact ... left a clue – a chemical signature in the earth’s crust called the “iridium anomaly.” Iridium is rare in the earth’s crust but far more abundant in asteroids. That’s why, all over the world, scientists find unusually high concentrations of iridium in sediment layers at the boundary between the Cretaceous and Paleogene periods, otherwise known as the K-P Boundary by geologists[2].
Ocean floor sediment cores reveal that white layers are rich in calcium carbonate (chalk) from animal shells. Rust colour reveals the metallic dust containing iridium from an asteroid, and grey indicates clay with little calcium carbonate, an absence of shelled animals.
It is problematic whether such an event could recur. The 10 kilometres-across asteroid referred to here resulted in mass extinction. Yet 35.5 million years ago, a five-kilometre asteroid hitting Chesapeake Bay didn’t. Size and impact location matter. At present, there are probably fewer than five five-kilometres across asteroids in an orbit crossing our Earth, but certainly more than one.
[1] Foraminifera are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net. They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure. These shells are made of calcium carbonate (CaCO3) or agglutinated sediment particles. About 275,000 species are recognised, both living and fossil. They are usually less than 1 mm in size, but some are much larger, and the largest recorded specimen reached 19 cm: Source: Wikipedia
[2] Article: “An asteroid did kill the dinosaurs after all”. http://www.spacedog.eu/astronomy/historic-astronomy/an-asteroid-did-kill-the-dinosaurs-after-all.html, Tuesday, 30 November 2010 13:48. The article goes on to discuss an instance where fossils have been found above the iridium laden boundary. The geological opinion to explain this anomaly? the iridium “migrated”.
According to this gravity anomaly map, mapping variations in Earth’s gravitational field show the outline of the crater formed by the KP asteroid impact