The terminal Cretaceous event: Circumterrestrial rings of tektite glass particles?

It is proposed to regard the terminal Cretaceous event as similar to the radiolarian extinction event in the late Eocene: the result of a volcanic eruption or series of eruptions on the moon. Some glassy ash, lapilli and blocks from these eruptions fell to the earth; some, in geocentric orbit, formed clouds around the earth. In accordance with current theory, it is found that the clouds in orbit would evolve into sets of rings, which would last a few hundred thousand to a few million years, and would perturb the climate of the earth. One such eruption apparently included iridium-bearing material, perhaps from the deep interior of the moon.The hypothesis permits a reconciliation between the evidence for the catastrophic intervention of extra-terrestrial masses in the earth environment, and the evidence for gradual (though rapid) change of flora and fauna at the Cretaceous/Tertiary boundary. The formation of the E-ring of Saturn by ejecta from the Saturnian satellite Enceladus may have been analogous. The theory might be tested by studies of diurnal layering in molluscan shells.     

Reconciling the Earth's stratigraphic record with the structure of our galaxy

The passage of our Solar System through the spiral arms has been implicated as a contributor to global environmental perturbations.The suggestion of a consistent structure within the arms,informed by density wave theory,raises the possibility of repeating patterns of events at each arm crossing.Here we test the hypothesis that the structure of the arms of our galaxy influences the stratigraphic record on Earth.We construct independent structural and temporal models and colbine these to compare the timings of arm tracers,materials from the earliest Solar System and events on Earth,including the largest extinctions.We find that a recurring sequence of events across the four arms emerges with an average arm-passing time of 188 million years.We suggest that the multiple temporal overlaps of events across arms,and their alignment with arm tracers and the earliest Solar System,presents an opportunity for a greater understanding of both Earth-based phenomena and galactic structure.     

Effects of Pleistocene environmental changes on the distribution and community structure of the mammalian fauna of Mexico

Biological communities in Mexico experienced profound changes in species composition and structure as a consequence of the environmental fluctuations during the Pleistocene. Based on the recent and fossil Mexican mammal checklists, we determine the distribution, composition, diversity, and community structure of late Pleistocene mammalian faunas, and analyze extinction patterns and response of individual species to environmental changes. We conclude that (1) differential extinctions occurred at family, genus, and species level, with a major impact on species heavier than 100 kg, including the extinction all proboscideans and several ruminants; (2) Pleistocene mammal communities in Mexico were more diverse than recent ones; and (3) the current assemblages of species are relatively young. Furthermore, Pleistocene relicts support the presence of biogeographic corridors; important refugia existed as well as centers of speciation in isolated regions. We identified seven corridors: eastern USA–Sierra Madre Oriental corridor, Rocky Mountains–Sierra Madre Occidental corridor, Central United States–Northern Mexico corridor, Transvolcanic Belt–Sierra Madre del Sur corridor, western USA–Baja California corridor, Tamaulipas–Central America gulf lowlands corridor, and Sonora–Central America Pacific lowlands corridor. Our study suggests that present mammalian assemblages are very different than the ones in the late Pleistocene.     

On the ages of flood basalt events

We review available data constraining the extent, volume, age and duration of all major Phanerozoic continental flood basalts (CFB or traps) and oceanic plateaus (OP), together forming the group of large igneous provinces (LIP), going from the smallest Columbia flood basalts at ～16 Ma to the as yet ill-known remnants of a possible trap at ～360 Ma in eastern Siberia. The 16 traps (CFB and OP) reviewed form a rather unimodal distribution with an initial modal volume of the order of 2.5 Mkm³. Most provinces agree with a rather simple first order model in which volcanism may have lasted of the order of 10 Ma, often resulting in continental break-up, but where most of the volume was erupted in about 1 Ma or sometimes less. This makes CFBs/OPs (LIPs) major geodynamic events, with fluxes exceeding the total output of present day hot spots and even possibly exceeding over short times the entire crustal production of mid-ocean ridges. The proposed correlation between trap ages and the ages of several geological events, including mass extinctions and oceanic anoxia, is found to have improved steadily as more data have become available, to the point that the list of trap ages may coincide with many major divisions in the geological time scale. The four largest mass extinctions in the last 260 Ma coincide to the best resolution available with four traps, making a causal connection between the two through some form of catastrophic climatic perturbations the most likely hypothesis. The time sequence of LIPs appears to have been random and there is no robust evidence for long time trends in the corresponding crustal production rate over the last 260 Ma.     

Correlation of the largest craters,stratigraphic impact signatures,and extinction events over the past 250 Myr

The six largest known impact craters of the last 250 Myr(≥70 km in diameter),which are capable of causing significant environmental damage,coincide with four times of recognized extinction events at 36(with 2 craters),66,and 145 Myr ago,and possibly with two provisional extinction events at 168 and215 Myr ago.These impact cratering events are accompanied by layers in the geologic record interpreted as impact ejecta.Chance occurrences of impacts and extinctions can be rejected at confidence levels of99.96%(for 4 impact/extinctions)to 99.99%(for 6 impact/extinctions).These results argue that several extinction events over the last 250 Myr may be related to the effects of large-body impacts.     

Trace-element patterns at the Cretaceous-Tertiary boundary—Consequences of a large impact

The earlier discovery of high concentrations of iridium and osmium at the very base of the Tertiary in marine pelagic sequences, has led to the hypothesis that the impact of a large projectile on Earth terminated the Mesozoic era.This paper describes the distribution of other trace elements in samples closely spaced around the Cretaceous-Tertiary boundary. We found an enrichment of Co, Cr, Ni, As, Sb and Se, and a depletion of the rare earth elements (REE), coupled with the anomalously high concentration of Ir and Os. Also, in the enrichment layer, problematic sanidine spherules occur which may be spheres, ablated from the impacting projectile or microtektites.The REE depletion could not only be demonstrated in the basal millimetres of the relatively thick (～ 10 cm) boundary marl bed of the Barranco del Gredero section, Caravaca, Spain, but also in the boundary clay of the Stevns Klint section—the “Fiskeler”—Denmark. In these boundary clay beds are also the highest concentrations of Ir recorded [44 and 86.7 ppb (1 ppb = 10^?9 g/g), respectively].This REE depletion is tentatively connected with the low REE concentration in common meteorites. In that case the basal millimetres are dominated by a meteoritic component and represent a “fall-out” layer of the impact event. This in turn would imply low terrestrial dilution at impact on Earth, suggesting that the projectile might have fallen into the ocean.Identification of the projectile remains speculative; especially the high As, Sb and Se anomalies are problematic. These latter may point to a projectile of uncommon composition, possibly a comet.An integrated scenario of a possible sequence of events is presented, chiefly based on the interpretation of micropaleontological, lithological, geochemical and stable isotope data of the studied sections.The direct and indirect effects of an impact leading to mass extinction constitute a complicated problem and may comprise a sudden temperature rise, a poisoning by cyanide or arsenic, and a few years suppression of sunlight, or preferably, a combination of these effects.