Australia: The Land Where Time Began
This event forms the boundary between the Cretaceous, the last part of the Mesozoic, and the Palaeocene, the first part of the Tertiary. An iridium layer in sedimentary rocks from the boundary between the Mesozoic and the Tertiary, mass extinction of the marine microplankton, extinction of some marine invertebrates, the total extinction of Ammonites, and the apparent extinction of Dinosaurs, are the most obvious factors connected with this event.
For some time there has been a widespread belief that the mass extinction at the end of the Cretaceous was a single, cataclysmic event in which the most obvious feature was the extinction of the Dinosaurs. In recent times evidence has been accumulating and ages ascribed to fossil deposits have been refined. Evidence is now available that casts doubt on this simple cause and effect scenario the TCE.
The deposition of the iridium layer and the mass extinctions that are usually associated with it, appear to coincide, but the techniques used to date the iridium layer and the last known dinosaurs fossils are not precise enough detect a difference of a few million years. The result is that the correlation of these events is a major problem. A period as short as a few millions years make a difference when it come to saying the correlation is definite, so the mass extinction was caused by the event that deposited the iridium layer.
A lot of research has been carried out on the interval of time at the K-T boundary about 65 million years ago. This research has shown that there wasn't a single event, rather there were a number of events that occurred over a prolonged period of time and processes that had been going on for some time that reached their conclusion at this time.
The sea level dropped suddenly at the Cretaceous-Tertiary boundary. On a global scale the sea level had been high in the Early Cretaceous, resulting in extensive marine transgressions on the continents of the world. Later the epicontinental seas retreated as sea levels dropped, the sea levels then stabilised for some time. At this time the climate of the world cooled, though there is no known evidence for polar caps. Some authorities think the sea level dropped as a result of tectonic activity that led to the subsidence of vast systems of marine mountains in the Pacific Basin. The loss of shallow-water environments that resulted could explain the extinction of at least some of the marine invertebrates. It is suggested that the same mechanism may have contributed to the Permian Marine Collapse at the Palaeozoic-Mesozoic boundary.
The iridium layer consists of a thin layer of clay rich in iridium that has been found in sequences that date to about 65 million years ago. At the time of the discovery of the layer in Italy and Denmark it was claimed that it could only have been deposited from an extra-terrestrial source, as when a very large asteroid collided with the Earth, the dust cloud depositing the layer as it settled. It was suggested the cloud blocked out the sun for long enough for the vegetation to die, and the dinosaurs starved to death. It was assumed that the iridium layer coincided exactly with the mass extinction of the Terminal Cretaceous Event (TCE). When the impact crater was found in Mexico from about 65 Ma it was just the sort of impact that would have been required to cause the extinction by a single cataclysmic event.
It now seems that there may not have been an ubiquitous iridium layer, and the known occurrences are not necessarily contemporaneous. There are layers of iridium-rich clays that span 100,000-200,000 years in sedimentary rocks. Many iridium anomalies have been found in sections of America. It has been determined that the quantities of iridium and other elements in the clay layers are not inconsistent with them having originated in the Earth's mantle. The conclusion is that there is no single layer of the same age that is found everywhere and those that are known are compatible with an origin in the Earth.
It is now generally believed that the extinctions that occurred during the Late Cretaceous and the transition to the Tertiary were selective, which leads to the conclusion that the blocking out of the sun for long enough to kill plants is not tenable. No one has suggested another comprehensive scenario that incorporates an asteroid impact. At the K-T boundary there was an increase in volcanic activity that can explain the accumulation of layers of clay high in iridium that was deposited over a comparatively short time. There was believed to have been a relatively short period of intense volcanic activity worldwide, that is believed to have been connected with plate tectonic activity as the plates moved, leading to volatile emissions, acid rain, reduced alkalinity of surface ocean water, global cooling and depletion of the ozone layer. Cooling temperatures and habitat reduction, would already have caused stresses to the biota leading to their decline and the deteriorating conditions resulting from the volcanism would probably have finished off any lines that were already declining.
The situation with the dinosaur extinction has changed from the single mass extinction that was widely believed to have occurred. As more knowledge accumulated it had become clear that there had been a series of extinction events throughout the reign of the the dinosaurs, the species that died out being replaced by new forms. It is now believed there was actually a very high turnover rate of dinosaur species, in the order of a few million years each, though the dinosaurs as a whole continued to be abundant. The difference at the K-T boundary was that there was now a failure to replace the species that died out. Also, further research on dinosaur extinctions in North America has shown that the final extinction actually appears to have covered a period of about 7 million years leading up to the 65 million year boundary, with the process speeding up greatly in the last 100,000 years, during which it has been suggested there was apparent competition with immigrant mammals. And some dinosaur fossils that were thought to have been of Cretaceous age are now known to be of Palaeocene age, and apparently above the iridium layer.
The conclusion seems to be that there was no sudden cataclysmic extinction of Dinosaurs coinciding with the formation of the iridium layer, rather a decline that took several million years, and that was compounded by changing conditions causing their final demise. After such as long and successful period of dominance, the Dinosaurs appear to have simply faded away at the end.
Most vertebrate groups survived the K-T boundary, such as Fish, Amphibians, turtles, Crocodiles, Birds and Lizards. Outside Australia, marsupials were declining and placentals were diversifying before the final extinction of the Dinosaurs. No massive extinction is shown in the fossil record of plants, just a steady increase of Flowering Plants as they gained dominance.
At the K-T boundary there were extinctions of plankton, but it can possibly be explained by the large decrease in shallow-water environments, but the change in the acidity of surface ocean water is probably the main cause of their extinction. Acid rain results from the emission of sulphur by volcanoes and other elements. Submarine emissions, as well as the settling of ash and dust from the volcanoes, causes an immediate effect on the surface water. At the K-T boundary there are differences in extinction/recovery patterns among Foraminifera and Nannoplankton such as Algae with calcareous plates (Coccolithophoroids). At the time of the iridium anomalies both types are reduced considerably, but extinctions among the Nannoplankton continued well into the Tertiary.
No evidence has been found of extinctions among the flora of Australia and New Zealand during this event.
The Cretaceous-Tertiary, or K-T boundary, is now often called the Cretaceous-Palaeogene (K-Pa) event.
Geology, February 2002, V.30, No.2, Page 99-102.
Impact dust not the cause of the Cretaceous-Tertiary mass extinction
Kevin O. Pope
Most of the 3-mm-thick ejecta layer that was distributed globally from the Chicxulub impact at the K-T boundary was deposited as condensation droplets from the impact vapour plume. Less the 1 % is clastic debris. Theoretical calculations and observations of the coarse dust fraction was of submicrometer-sized dust. The grain-size and global mass of the clastic debris indicate that the debris was spread by stratospheric winds from North America over the Pacific to Europe, with little debris reaching high southern latitudes. These findings indicate that the original K-T impact extinction hypothesis with the shutdown of photosynthesis by submicrometer dust is not valid, requiring more than 2 orders of magnitude of fine dust than is estimated here. Furthermore, estimates of future impact hazards relying on inaccurate impact-dust loadings are greatly overstated.
Proposed by Dorrik Stow3 – trace metals
The clay-rich layer containing higher than normal concentrations of iridium has been used as evidence of a bolide impact being responsible for a mass extinction event at the close of the Cretaceous. Iridium has since been found to be a commonly produced by volcanic eruptions, such as the Hawaiian volcanoes and basalt flows from hotspots and rifting, so unusually high concentrations of iridium are now known to not be required to come from an extraterrestrial source. The 500,000 km2 of the Deccan Traps in India was produced by huge eruptions of flood basalt lava just prior to the KT boundary. This vast lava field is very similar to that of the Siberian Traps that were somewhat larger, that occurred about the time of the Permian extinction event.
Shocked quartz has also now been found associated with volcanoes, such as in the volcanic pipes of southern Africa. The author3 suggests they probably result from very violent deep-seated eruptions. He also suggests that if the volcanic pipes of the Deccan Traps go deep enough into the mantle shocked quartz may have been produced. It has also been found that microbial limestones scavenge iridium from seawater and concentrate it, thereby producing other iridium spikes, such as one that coincides approximately with the mass extinction event that happened in the Ordovician mass extinction event 443 Ma. Seawater has been found to contain traces of almost all known elements, among which a number of trace metals are known to be concentrated in particular sediments. The organic matter in black shales scavenges copper, nickel, vanadium, molybdenum, uranium, as well as others. Iridium, as well as other trace elements, also takes place as a chemical process by partial dissolution of limestone after deep burial and subsurface pressure and temperature are high enough. Limestone rocks commonly have dissolution clay seams in which such concentrations commonly occur.
While examining the place where Walter Alvarez first found the iridium layer that has been associated with a large bolide striking the Earth the author3 and some colleagues found that the strata indicated it was deposited on the seafloor in a mid-ocean location where the deposition was in the form of a gentle rain of tiny shell debris, mainly from coccoliths and foraminifera from the surface plankton. On its way to the seafloor the debris was mixed with small amounts of silt and clay carried to sea by rivers, blown from deserts, and periodically, volcanic ash. This sediment settled at the rate of centimetres per 1000 years, though in places he detected more rapid influx from a diluted turbidity current, or washing by other deep ocean currents. Previous studies of the area had not noticed either of these. Sediment had been deposited in a certain cyclical fashion, at times planktonic shells dominated, at others it was clay and ash that was dominant. This cyclicity had become accentuated into hard bands of pinkish limestone that were separated by thin bands of darker red-brown clay dissolution seams, after deep burial and the processes of dissolution. According to the author3 this was characteristic of the Scaglia Rossa Formation.
Preliminary geochemical analysis of the dissolution seams at the KT boundary, as well as others at the site, demonstrated the presence of trace metal concentrations that included iridium, but also many different trace metals.
On Leg 75 of the drilling program of the DSDP that drilled across the KT boundary in the South Atlantic, at time when it was a narrow arm of the Tethys, the author3 noticed a number of key problems and observations.
1. There was a problem of bioturbation (burrowing in, eating and excreting sediment on the seafloor, by many animals that are looking for food, a place to hide or rest), resulting in the disturbance of layers that were originally neat, and to smear out microfossils from one part to another, and in this case it was the KT boundary. A similar result can be obtained by currents scouring the seafloor.
2. Dissolution or partial dissolution of fossils by chemically aggressive waters, that as well as occurring after burial, as is seen in the Scaglia Rossa Formation in Italy, also occurs in the deeper parts of ocean basins. There is a higher proportion of carbon dioxide in deep ocean water than in surface water, effectively making the deep water weakly acidic. This results from the general ocean circulation (ocean conveyor belt). As the minute delicate calcareous shells of plankton drift down through the water column they eventually pass a chemical threshold, the carbonate compensation depth (CCD). Below this depth the shells are rapidly dissolved by the acidic bottom waters so that barely a trace of the shells actually reaches the seafloor.
3. The third problem that wasn’t encountered on Leg 75 of the drilling, but is typical of most sections across the KT boundary that is seen in exposures on land. It is the missing section, a gap or hiatus in the deposition record resulting from the total lack of deposition, intermittent deposition or erosion that occurs subsequently. As a result of this evidence of a transitional change may be misinterpreted as an abrupt change, a problem that needs to be considered when interpreting the geological record.
According to the author3 the main observation he made here from the evidence of micro-fossils across the KT boundary on Leg 75 is that ‘…there is much alteration of some poorly, some better preserved samples, and a transitional replacement of (Cretaceous) taxa by the newly evolving Tertiary ones.’ that he obtained from the micropalaeontologists. He says his own description of the sediments was that they are a normal suite from the deep ocean where there was slow, continuous sedimentation, with much bioturbation by burrowing animals as well as some dissolution of microfossil shells as the CCD was approached.
The Proposed model
The author3 suggests, in opposition to much scientific opinion, that the bolide impact crater at Chicxulub has still to be dated precisely. Also suggesting that all the other evidence for extraterrestrial influence, such as the iridium spike, shocked quartz, as well as osmium isotope ratios and global soot, can equally be explained by terrestrial processes. There is no doubt of the existence or scale of the supervolcano of the Deccan Traps that is indicated by dating to have been active just before the KT boundary event. He says the latest research results he obtained from Prof. Courtillot, a geophysicist at the University of Paris indicate that the superplume event at the Deccan Traps was several times larger than has been thought, occurring in 3 distinct events over a period of 3 My between 67.5 and 64.5 Ma.
He claims the real problem for either of these catastrophist theories, as well as others, is explaining the actual facts of extinction. Some organisms declined slowly while for others it occurred rapidly, though not abruptly. Many of the plants and animals were little affected, most terrestrial plants surviving. Among the animals that were apparently unaffected were most molluscs, sharks, bony fish, placentals and all amphibians, etc.
He suggests there is much evidence for a combination of environmental drivers. There was a large amount of volcanic activity just prior to the KT boundary, as also at the end Permian extinction event. Global environmental effects and increased stress would have affected certain groups of organisms. Throughout the Late Cretaceous the seas were extremely high and the climate was warm in all parts of the world, there was a prolonged state of ‘benign tranquillity’. One result of this was a lowering of the carbon dioxide levels in the oceans, and hence the atmosphere, the carbon being locked up in the calcium carbonate skeletons of marine plankton that eventually led to large concentrations of chalk on the seafloor, as many more of the skeletons of the planktonic organisms could reach the seafloor before they were dissolved by the less acidic bottom waters, and his would have affected the chemistry of the oceans. As sea levels fell the temperatures dropped progressively and dramatically, that can be quantified by oxygen isotope analysis of the fossil shells. Examinations of this period of time have repeatedly shown evidence of thermal stress among different groups.
Land bridges opened as coastal habitats diminished, and as a result of increased competition among animals would have led to the spread of diseases between animal groups.
According to the author3 it is difficult to avoid concluding that the extinction event at the KT boundary resulted from complex combinations of environmental factors triggered by biological stress. This didn’t occur instantaneously, but rather over a contracted period. The base of all the food chains were either altered or were badly affected, the competition between angiosperms and gymnosperms on land. The great reduction of coral reefs and then the rudists in the shallow seas, and a fundamental change in the primary producers of the sea, the plankton, resulted in turmoil that benefited some, such as the opportunists, but others were hard hit by it.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|