A Look at Biotic Events at High Southern Latitudes at the End of the Cretaceous
William Zinsmeister (Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana)
Ohio State University, Columbus, Ohio
19 November 1998
The Cretaceous-Tertiary (K-T) extinction is the smallest of the big 5 extinctions. 11% of all families go extinct at the K-T boundary. But, the K-T extinction is a favourite of many people, mainly because it included the extinction of the dinosaurs. There have been many theories proposed about why the dinosaurs went extinct, including mammals eating their eggs, the evolution of angiosperms interfering with their digestive systems, and other ideas.
But, all these dinosaurs extinction hypotheses are flawed because they only deal with dinosaurs. Many terrestrial and marine groups (plant and animal) went extinct at the same time.
Then, the impact hypothesis came along in the early 1980s by the Alvarez’. This idea explained the K-T extinction in both the terrestrial and marine realm - basically, the global food chain collapsed. Also, the impact theory caught people’s imaginations, and was immediately accepted as fact by the media and the general public. There is much debate among scientists, though.
Now, there is much compelling evidence for an impact. Lots of computer modelling has been done to see what an impact would do the the Earth. One idea is that debris would be thrown up high into the atmosphere and would fall back down as an enormous meteor shower, which would heat the atmosphere to 800˚ and everything on the surface was cooked. Other scary ideas: rainshowers of nitric acid and sulfuric acid, nuclear winter scenario, and global forest fires.
Problem: If things were so bad, how did anything survive? Where are the burned/charred dinosaur bones and trees if all this happened?
There are a few diehards who still say an impact didn’t happen.
Other than that, there are 2 hypothese: 1) Bad Day Hypothesis; 2) Impending Doom Hypothesis.
The Impending Doom Hypothesis says that the Earth’s biosphere had been under a long period of stress up to the time of the impact. The impact was the capstone to the extinction. This seems like a logical idea.
Seymour Island - is near the northeastern tip of the Antarctic Peninsula. The geology on Seymour Island is mostly a homoclinal succession of rocks - Cretaceous to upper Paleocene.
Post-impact scenarios - it has been difficult to assess post-impact scenarios because there are few areas in the world with Danian-age rocks. Danian rocks occur in northwestern Europe, a few other places, and on Seymour Island. The north part of Seymour Island is Tertiary in age.
There are about 70 km2 of outcrop to look at on Seymour Island (upper Cretaceous and lower Tertiary). The island has a desert-like topography similar to southwestern USA. These are good exposures, and they are packed with fossils with a high diversity and good preservation. About 800 species of fossils have been described from Seymour Island. Lots of ammonites are just lying on the surface. The ammonites are aragonitic. There are also good gastropods and bivalves (all fresh looking). There are also fossil echinoids (including 1 form with 5 brood pouches - the juveniles live in the parent up to a certain point). There are also plesiosaurs and mosasaurs. Ammonites are common.
A spectacular specimen of Diplomoceras was found last year - a specimen that was 1.5 meters long (but curved; uncurved, it would be 14 feet long). The animal itself was ~6’ long (a 6’ long living chamber). Smaller pieces of this fossil are relatively common, but this specimen was unusually preserved. It is the nicest, most remarkable specimen known.
Seymour Island is very muddy to work on - it is always above freezing during the field seasons.
An iridium anomaly does occur here on Seymour Island. It occurs in a unit referred to as the K-T glauconite, which is a greenish, glauconitic sandstone that occurs at the boundary. The glauconitic sandstone is easily correlatable across the island.
There are no stratigraphic or sedimentological signatures at the K-T boundary. There is no boundary clay, as seen at Gubbio, Italy.
Antarctica is ~8000 miles away from the Chicxulub Crater in Yucatan, Mexico, which is a significant point as far as considering the after-effects expected to be seen in Antarctica.
Rudist record - there is a gradual increase in rudist diversity in the Cretaceous. Most rudists went extinct at 68-67.5 my (~early-late Maastrichtian boundary). This pattern mirrors other groups’ diversity patterns as well - all mostly go extinct before the K-T boundary.
Seymour Island ammonites crash at the end of the Campanian. Seymour Island inoceramids are gone near the end of the Campanian. Seymour Island belemnites virtually disappear at the end of the Campanian. Seymour Island ammonites are quite diverse in the Campanian (35-36 species), but their diversity crashes to 10 species in the Maastrichtian, and they are gone at the K-T boundary. Several Campanian cosmopolitan ammonite families disappear in the Campanian in the Seymour Island area, but they extend to the K-T boundary in lower latitudes. So, their disappearance in the Seymour Island area indicates a general temperature decline (cooling).
A regression occurred at the mid-Maastrichtian.
Superimposed on the Maastrichtian temperature decline are several rapid warming spikes. 50,000 years before the K-T boundary event was a warming event, documented from some ODP sites.
Note the restricted occurrence to 1 horizon on Seymour Island of the ammonite Zealandites varuna in the Maastrichtian Lopez de Bertodano Formation. Its presence probably represents a warming spike.
The faunal transition across the K-T boundary on Seymour Island - no single extinction event seen - a gradual decline is seen instead. There is no marked extinction horizon.
Then, someone suggested that this pattern is due to the Signor/Lipps Effect, a phenomenon produced by collection/preservation biases. One can get a gradual extinction pattern purely due to collecting and preservation biases. So, Zinsmeister and others recollected the fossils on Seymour Island, and collected fossils spatially, doing detailed mapping.
The K-T glauconite (~5 meters thick) is actually 3 units - a lower glauconite, a middle fish bed/horizon, and an upper glauconite. The fish debris bed could represent a victim bed from the K-T impact.
The new diversity record from renewed collecting of fossils: the extinction is less gradual now, and more abrupt, but all groups are dying out just before the iridium anomaly. The K-T extinction is now more abrupt is high southern latitudes than previously thought.
Fish horizon - interpreted to be the effect of extreme ocean disruption; fish kills are not due to ocean poisoning (strangelove ocean), but by pulses of nutrients into the oceans. This is the only fish horizon in 1600 meters of section. It could be an interval of slow deposition, but this is not likely because fish degrade relatively quickly, and one needs special conditions to preserve fish. The fish debris bed represents conditions after the K-T event - lots of fish kills occurred.
The biosphere is far more robust than we give it credit for. Organisms have been able to survive truly catastrophic events in geologic history.
Example: the Millbrig Bentonite (Middle Ordovician, eastern USA and Scandinavia). This eruption resulted in no extinctions. Therefore, Earth’s biosphere is very robust. But, if the biosphere is alrady stressed, then a big event can push the biosphere into a mass extinction.