The end of the Cretaceous period and the start of the Paleogene period 65.5 million years ago is marked by a global extinction event at the K-Pg boundary. The Paleogene was previously part of the Tertiary before the term was deprecated; the extinction event is thus sometimes referred to as the K-T boundary event. Principles from sedimentology and stratigraphy can be used to deduce the nature and extent of event or events that triggered this extinction.
Global Stratotype Section and Point
The official stratigraphic section to mark the K-Pg boundary is at El Kef, Tunisia. This section contains a continuous sedimentary record with excellent microfossils, clear geochemical and mineralogical marker horizons of the boundary, and no discontinuities or breaks across the boundary. The unit directly before the boundary (the Upper Maastrichtian) is uniform dark grey marls, marked by the extinction of all tropical and subtropical species. The boundary is a 2 millimeter thick dark red layer high in iridium, interpreted as impact ejecta (defined by the first presence of Danian planktic foraminifera). Above this is 50 centimeters of dark, organic rich material marking a crash in plankton populations. Unfortunately, the section is currently suffering from oversampling and agricultural encroachment, so alternate sections are being examined as possible replacements.
The Cretaceous ended with the extinction of the dinosaurs, along with many other species of land-based plants and animals. Marine species including calcareous plankton and tropical invertebrates also abruptly go extinct at the K-Pg boundary. The boundary is officially marked by the first arrival of Danian planktic foraminifera. The boundary is also marked by a sharp increase in disaster opportunist survivors, and freshwater species bouncing back within a decade of the K-Pg event.
The K-Pg boundary is commonly marked by the presence of shocked quartz. This is commonly thought of as an impact mineral, but it could also be the result of explosive interaction between a hot mantle plume and silicic crustal material. Along with shocked quartz, altered droplets have been recovered in Haiti, and in deep marine sediments off the coast of North Carolina. These droplets are typically interpreted as impact glass, as similar sediments produced by volcanic fire-fountaining are localized to the site of formation.
By the same fluid kinematics of turbidite flows, the settling time for either volcanic ash or impact ejecta out of the atmosphere is weeks to months, with anticipated identical sedimentation patterns for a given particle size and water depth. Both impacts and eruptions could plausibly lead to global cooling, extensive wild-fires, and airborne particles blocking sunlight (leading to a reduction of photosynthesis) and triggering acid rain. Soot associated with the boundary may be from global fires, or reworked influx from other events.
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. The clays contain shocked quartz, in a higher-than-average iridium, siderophile, and platinum-group elements. All the listed chemicals are are typically depleted on the Earth’s crust. The source of a peak in these minerals could either be from the impact of an extraterrestrial body or excessive volcanic activity, as asteroids, comets, and the Earth’s mantle are all rich in these elements.
A two-million year spike in seawater strontium is also associated with the boundary. The long residence time of strontium in seawater could be a response to a quasi-instantaneous input of anomalous strontium from acid rain, acidic volcanism similar to that observed in the Seychelles Islands, or from longer-term sea level regression. However, due to the gradual increase in the strontium ratio over extremely long timescales, and the difficulty in resolving times with strontium, the evidence for this spike is somewhat hazy.
Several marine sections have been identified around the world. The K-Pg boundary is marked by a 1.5 centimetre layer of limonitic clay within a 3 metre column of shales, sandy clays, and limestones indicative of a shallow marine environment. The shales below the boundary (from the Cretaceous period) are ash grey, while those above the boundary (from the Paleocene) are brownish grey.
What does this evidence suggest for possible causes of the K-Pg boundary and extinction event?
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