K-Pg Extinction Event: Theories

The K-Pg boundary extinction event has several distinct characteristics that can be used to differentiate between possible causes.

Supernova

Cosmic radiation from a nearby supernova could be responsible for the mass extinctions and the iridium layer. However, the boundary should also contain a plutonium isotope which has not been found during sedimentary analysis.

Gradual Extinction

The least catastrophic scenario for extinction is a gradual species loss brought about by a change in the physical environment from the Cretaceous to the Paleogene. Long term cooling, major marine regression, and acid rain associated with volcanic activity were all proposed as contributors to a gradual species loss over a million-year span. Scant fossil records of dinosaurs in Europe leading up to the K-Pg boundary could be indicative of a gradual die-off, or merely of fossil loss from a spotty stratigraphic record. In contrast, shallow marine sections in Denmark demonstrate a large, diverse population of marine life up until the K-Pg boundary, followed by an abrupt reduction in quantity and diversity of species, gradually replaced by a new large and diverse population. Other authors insist that species abundance populations were dropping for the last 100,000 years prior to the K-Pg boundary, with the trigger event killing species already on the brink of gradual extinction.

Chemostratigraphy

Low strontium-calcium ratios (Sr}/Ca) in microfossil shells, and low potasium in clay mineraology leading up to the K-Pg boundary in Tunisia indicates a warm, humid climate with rising sea level. Immediately at the boundary, data suggests a maximum flooding event in an abruptly cooler and more arid climate, with decreased biotic productivity and an iridium anomaly. After the boundary event, oxygen isotope ratios suggest cooler temperatures, while low strontium-calcium ratios are indicative of high humidity and high sea level, with gradual drying over time.

Volcanic Eruption

A major eruptive sequence, like the flood basalts that formed the Deccan Traps in west-central India, could produce enough toxic outgas to destabilize climate conditions, leading to mass extinctions.

Chronostratigraphy

The age of the Deccan Traps can be determined through magnetostratigraphy and chronostratigraphy. The Deccan volcanic province locks in the magnetic reversal polarity epoch chron 29R, which encompasses the K-Pg boundary. Potassium-Argon (K-Ar) dates for whole-rock basalt samples in different sections of the traps fall in the range of 55-65 million years ago, but out of stratigraphic sequence which suggests post-crystallization loss of 40Ar* for some samples. Data from the low to intermediate temperature argon isotope (39Ar/40Ar) incremental heating studies are characteristic of fine-grained basalts exhibiting post-crystallization loss of 40Ar* and 39Ar recoil loss from internal redistribution or out of the sample entirely, producing ages greater than 70 million years old. Using the potassium data to constrain the higher-temperature argon isotope data produces ages of 64-66 million years old. Later studies confined the bulk of volcanism (~80%, 3.5 kilometers of lava flows) erupted in less than 800,000 years between 64.8-65.6 million years ago.

Biostratigraphy

Quarry outcrops in the Krishna-Godavari Basin in India contain two Deccan basalt flows (the Rajahmundry traps). The sediments directly on top of the lower flow contain Danian planktic foraminifera, the key fossil marking the K-Pg boundary, while the upper flow is deposited during chron 29N, before life bounced back from the mass extinction event.

Chemostratigraphy

The Takli interrappen sediments are a 2-meter thick shallow continental deposit from around the K-Pg boundary event in the Deccan province. The sediments are composed of lava ash, clay, and marl with Cretaceous fossils, all underlain and overlain by lava flows from chron 29R. Chemical analysis of these sediments indicate rare-earth elements concentrations similar to that of other basalts, with a few ash horizons with 2-5 times the base level concentration. The maximum observed iridium peak is two orders of magnitude smaller than the iridium peak observed in marine sediments a few hundred kilometers away in the Um Sohryngkew River section in Meghalaya, India, suggesting that the Deccan Traps are unlikely to have been the source of the iridium peak associated with the K-Pg boundary. However, fine volcanic ash from the Melbourne volcanic province recovered in Antarctica contained elevated iridium concentrations, suggesting it is plausible that some volcanic ash may be responsible for iridium anomalies.

Samples from the non-marine Raton Basin (Starkville South, Colorado, and Raton Pass, New Mexico) and marine Stevns Klint, Denmark contained two rare amino acids peaking at the K-Pg boundary, but present above and below it. The samples are otherwise typical of modern amino acid distribution. The samples specifically lack the amino acid distribution characteristic of the Murchison meteorite impact event. This suggests the exotic amino acids are a result of high-temperature hydrolysis of coal.

Computer Models

The atmospheric impact of SO2 released by volcanism over the formation of the Deccan Traps would produce an impact on climate conditions similar to a single asteroid impact. A sequence of such pulses over the period of formation would produce a runaway impact on climate that cannot be replicated by a single impact.

Extraterrestrial Impact

An asteroid impact would produce a massive crater (like the Chixulub Crater in the Yucatan Peninsula, Mexico), throwing ejecta and triggering a firestorm. The combination of direct trauma, indirect reduction in daylight, acid rain, and collapse of the food chain would result in an abrupt extinction event. A cometary impact would have similar results as an asteroid impact, but less severe due to the lower density of material involved, and spread over a longer time as the impact of the main nucleolus would be followed by the impact of much smaller cometary debris. The result would be a rapid extinction event, but less abrupt than the extinction event expected by an asteroid impact.

Chemostratigraphy

The SM-4 Sumbar river site in Turkmenistan\footnote{Formerly Turkmenia, USSR.} is an exposure of marine clay spanning the K-Pg boundary. The samples consist of detrital carbonaceous shale, and either a land plant or phytoplankton. Iridium and shocked quartz rise sharply at the boundary then quickly decline. Soot and charcoal rises sharply at the boundary, then continues to rise, suggesting fire started before the basal layer of ejecta fully settled, and continued to burn.

Boundary clay at geographically diverse locations including Denmark, New Zealand, the North Central Pacific Ocean, and Turkmenistan all share extremely similar chemical composition, which differs both chemically and mineralogically from proceeding and following clays in the strata. This suggests the clay is derived from a single unique source material: impact glass from a single-event impact.

Computer Models

A 10-kilometer diameter asteroid impacting the Earth would produce a crater of appropriate scale to the Chixulub Crater. The impact would release large amounts of water, dust, and climate-forcing gases — far more than would be released by large-scale volcanics. Flow models of atmospheric reentry of ejecta could cause pulses of thermal radiation, causing damage without triggering extensive fires. Detailed models of ejecta paths cannot provide the observed global distribution through purely ballistic motion; some of the ejecta must have been transported by atmospheric redistribution.

Multiple Impacts

A series of small impacts would produce local ejecta plumes instead of a global blanket of sediments; the uniformity of chemostratigraphy of marine clays thus suggest a single-impact scenario. However, the stratigraphic record is incomplete, with shallow marine sediments prone to erosion during transgression, and deep-marine stratigraphy condensed by low sedimentation rates.

Conversely, careful analysis of biostratigraphy with respect to impact glass ejecta with respect to the foraminifera that marks the K-Pg boundary indicates multiple impact events within 400,000 years. In addition to the Chixulub Crater in the Yucatan Peninsula, Mexico (120 kilometer diameter, 65.0-65.4 million years old), two other impact craters date to near the K-Pg boundary: the substantially smaller Boltysh crater in the Ukraine (24 kilometer diameter, 64.6-65.8 million years old) and the Silverpit crater in the North Sea (12 kilometer diameter, approximately 65 million years old). The Shiva structure west of India is also a potential crater of appropriate age (450-600 kilometer diameter, 66 million years old), and may have triggered the Deccan Trap eruptions. While only the Chixulub Crater and the potential Shiva structure are large enough to have created a global ejecta layer, the smaller impacts could have destabilized an already unsteady climate, enhancing extinction rates.

So, which is it?

The K-Pg boundary is a busy time, where climate change, low sea level, mass extinction, iridium anomaly, Chicxulub (and potentially Shiva) impact, and Deccan volcanism occurred within a few hundred thousand years. This is too fine of a time sequence for events to be distinguished with radiometric dates, so the law of superposition is the only way to determine the sequence of events, and untangle cause and effect. Unfortunately, the spotty stratigraphic record and potential sedimentary reworking over time make even that a fallible method. Attempts to declare the Chicxulub as the only cause of the K-Pg extinction event was immediately contradicted in letters to the same journal, which were rebutted by the authors of the original article. Thus, the debate as to how much each event contributed to mass extinctions between the Cretaceous and the Paleogene rages on to varying degrees of politeness as ever more data is analyzed and added to the puzzle.

My current backend doesn’t support in-line citation very well. Please see the bibliography for all papers used to research this topic.

This entry was posted in Geoscience and tagged , , , , , , , , , , , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *