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The article presents the author's response to comments about their article "Whole-genome analyses resolve early branches in the tree of life of modern birds." The comments were presented by K. J. Mitchell and others in this issue. A point of dispute is the dating of modern birds in the avian phylogeny. The study authors contend that most modern bird orders diverged prior to the Cretaceous-Paleogene mass extinction event, which happened 66 million years ago and that Mitchell and others incorrectly interpreted the fossil data used in the study.

Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time. [ABSTRACT FROM AUTHOR]

The article discusses a theory on the formation of the Isthmus of Panama by geologists Camilo Montes, Carlos Jaramillo, and team, which proposes that the land bridge between South and Central America arose 15 million years ago. It is believed that this bridge allowed for the migration of animal species such as porcupines, opossums, and horses between the continents. Scientists have debated the timing of the closure of the isthmus based on data from volcanic rock dating and oxygen isotope signatures of the oceans and foraminifera shells near Panama. The isthmus is also compared with the geology and species distribution of Indonesia. INSET: Salvage Paleontology on the Seaway.

The end-Triassic extinction is characterized by major losses in both terrestrial and marine diversity, setting the stage for dinosaurs to dominate Earth for the next 136 million years. Despite the approximate coincidence between this extinction and flood basalt volcanism, existing geochronologic dates have insufficient resolution to confirm eruptive rates required to induce major climate perturbations. Here, we present new zircon uranium-lead (U-Pb) geochronologic constraints on the age and duration of flood basalt volcanism within the Central Atlantic Magmatic Province. This chronology demonstrates synchroneity between the earliest volcanism and extinction, tests and corroborates the existing astrochronologic time scale, and shows that the release of magma and associated atmospheric flux occurred in four pulses over about 600,000 years, indicating expansive volcanism even as the biologic recovery was under way. [ABSTRACT FROM AUTHOR]

Soils in permafrost regions contain twice as much carbon as the atmosphere, and permafrost has an important influence on the natural and built environment at high northern latitudes. The response of permafrost to warming climate is uncertain and occurs on time scales longer than those assessed by direct observation. We dated periods of speleothem growth in a north-south transect of caves in Siberia to reconstruct the history of permafrost in past climate states. Speleothem growth is restricted to full interglacial conditions in all studied caves. In the northernmost cave (at 60°N), no growth has occurred since Marine Isotopic Stage (MIS) 11. Growth at that time indicates that global climates only slightly warmer than today are sufficient to thaw extensive regions of permafrost. [ABSTRACT FROM AUTHOR]

The Grand Canyon is one of the most dramatic features on Earth, yet when and why it was carved have been controversial topics for more than 150 years. Here, we present apatite 4He/³He thermochronometry data from the Grand Canyon basement that tightly constrain the near-surface cooling history associated with canyon incision. 4He/³He spectra for eastern Grand Canyon apatites of differing He date, radiation damage, and U-Th zonation yield a self-consistent cooling history that substantially validates the He diffusion kinetic model applied here. Similar data for the western Grand Canyon provide evidence that it was excavated to within a few hundred meters of modern depths by ~70 million years ago (Ma), in contrast to the conventional model in which the entire canyon was carved since 5 to 6 Ma. [ABSTRACT FROM AUTHOR]

The present-day 238U/235U ratio has fundamental implications for uranium-lead geochronology and cosmochronology. A value of 137.88 has previously been considered invariant and has been used without uncertainty to calculate terrestrial mineral ages. We report high-precision 238U/235U measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in 238U/235U values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional, trends. Variation between comagmatic minerals suggests that 238U/235U fractionation processes operate at magmatic temperatures. A mean 238U/235U value of 137.818 ± 0.045 (2δ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and "bulk Earth" 238U/235U composition. [ABSTRACT FROM AUTHOR]

The article discusses research into the chronology and possible causes of the mass extinction event at the end-Permian of the Phanerozoic era. Biodiversity data from sediments in South China was studied, including the mass spectrometric geochronological, isotopic, and biostratigraphic information from several sites. The results indicate that the mass extinction occurred in marine and terrestrial ecosystems 252.28 ± 0.08 million years ago over the course of 20,000 years or more, and point toward thermogenic carbon dioxide or methane emissions as possible causes of the extinction event.

The article discusses the Denisovans, which were a group of archaic humans that researchers hypothesize lived in the same area as both modern humans and Neandertals and whose range may have stretched into east Asia. Topics include an overview of the discovery of several fossils and tools in Denisova Cave in the Altai Mountains of Siberia, by anthropologist Bence Viola, and the results from a DNA analysis of the fossils, which revealed that the fossils were from both Neandertals and Denisovans. Also discussed is research by paleogeneticist Svante Pääbo and archaeologist Anatoly Derevianko mapping the trail of ancient humans, including the use of advanced methods in toolmaking and whether the Denisovans, Neandertals, and modern humans may occupied the Altai Mountains simultaneously. INSET: A Denisovan Legacy in the Immune System?.

We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 ± 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers. [ABSTRACT FROM AUTHOR]

Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower 142Nd/144Nd ratios than the terrestrial standard (ϵ142Nd = -0.07 to -0.15). Within a mafic amphibolite unit, 142Nd/144Nd ratios correlate positively with Sm/Nd ratios and produce a 146Sm-142Nd isochron with an age of 4280+53-81million years. These rocks thus sample incompatible-element-enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth. [ABSTRACT FROM AUTHOR]

Calibration of the geological time scale is achieved by independent radioisotopic and astronomical dating, but these techniques yield discrepancies of ∼1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40Ar/39Ar ages of tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine, the most widely used standard in 40Ar/39Ar geochronology. This calibration results in a more precise older age of 28.201 ± 0.046 million years ago (Ma) and reduces the 40Ar/39Ar method's absolute uncertainty from ∼2.5 to 0.25%. In addition, this calibration provides tight constraints for the astronomical tuning of pre-Neogene successions, resulting in a mutually consistent age of ∼65.95 Ma for the Cretaceous/Tertiary boundary. [ABSTRACT FROM AUTHOR]

Fossilized bones from extinct taxa harbor the potential for obtaining protein or DNA sequences that could reveal evolutionary links to extant species. We used mass spectrometry to obtain protein sequences from bones of a 160,000- to 600,000-year-old extinct mastodon (Mammut americanum) and a 68-million-year-old dinosaur (Tyrannosaurus rex). The presence of T. rex sequences indicates that their peptide bonds were remarkably stable. Mass spectrometry can thus be used to determine unique sequences from ancient organisms from peptide fragmentation patterns, a valuable tool to study the evolution and adaptation of ancient taxa from which genomic sequences are unlikely to be obtained. [ABSTRACT FROM AUTHOR]

Studies the application of cosmogenic isotope analysis for dating moraine landforms. Determination of quantitative exposure ages; Inconsistency with geological data; Consideration of the processes of erosion, deposition and weathering.

Discusses the biological diversification in the early Cambrian period. Difficulty in quantifying evolutionary rates during the Cambrian period; Uranium-lead zircon geochronology techniques for dating Cambrian fossils; Duration of the evolutionary explosion; Speed of faunal diversification and Cambrian turnover.

The article discusses research that directly dates fossils of the last survivors of the Australian megafauna which supports the hypothesis that they were extinct 40 thousand years ago and no sooner. According to the authors, giant marsupials, reptiles, and flightless birds once inhabited Australia, with most of them extinct by the late Pleistocene period. Most other species of megafauna appeared to have survived until 51 to 40 thousand years ago until hunting by humans or changes in vegetation drove them to extinction. Topics include an in-depth examination of fossils from Cuddie Springs, New South Wales, using electron spin resonance (ESR) and uranium-series methods, and what caused the extinction of Australia’s megafauna.

At geological time scales, the role of continental erosion in the organic carbon (OC) cycle is determined by the balance between recent OC burial and petrogenic OC oxidation. Evaluating its net effect on the concentration of carbon dioxide and dioxygen in the atmosphere requires the fate of petrogenic OC to be assessed. Here, we report a multiscale (nanometer to micrometer) structural characterization of petrogenic OC in the Himalayan system. We show that graphitic carbon is preserved and buried in marine sediments, while the less graphitized forms are oxidized during fluvial transport. Radiocarbon dating indicates that 30 to 50% of the carbon initially present in the Himalayan rocks is conserved during the erosion cycle. Graphitization during metamorphism thus stabilizes carbon in the crust over geological time scares. [ABSTRACT FROM AUTHOR]

The relative merits of molecular and paleontological dates of major branching points in the tree of life are currently debated. In some cases, molecular date estimates are up to twice as old as paleontological dates. However, although it is true that paleontological dates are often too young (missing fossils), molecular dates are often too old (statistical bias). Intense study of the dating of major splits in the tree of mammals has shown rapprochement as fossil dates become older and molecular dates become younger. [ABSTRACT FROM AUTHOR]

The article offers information on Misliya-1 fossil which is an excavated material which is represented by a partial upper jaw which includes some of the bone surrounding the tooth sockets, part of the cheekbone, the roof of the mouth, the bottom of the nasal cavity, and the complete upper left dentition. It mentions information on use of uranium-thorium dating for determine the age of tooth dentine samples. It states that Homo sapiens fossils represents excursions of species from Africa.

Discusses the dating of sedimentary rocks using SHRIMP, or sensitive high-resolution ion microprobe. Diagenetic xenotime common in siliciclastic sedimentary rocks; Ability to estimate age by in situ uranium-lead analysis of xenotime; Study of two Proterozoic sandstone units from north-western Australia, and their approximated diagenetic xenotime ages; Potential of method for dating sedimentary sequences of all ages; Suggested value for refining the Precanbrian time scale.

Presents research which studied the validity of using the mineral series glaucony in the geologic time scale for the dating of potassium-argon in bulk samples. Use of glaucony in the absolute-age database; Use of laser-probe argon-argon dating for comparison; Resulting different period of genesis; Modulation of formation times of younger grains in glaucony populations; Enclosing sediments in glaucony populations.

The article discusses the use of DNA to date the fossils of Denisovans found at the Denisova Cave, Siberia, Russia, including in regard to the dating of the cave's fossils and Neanderthals' fossils found in the cave. An overview of the genetic research conducted on the Denisovan girl, including the genetic sequencing of her finger, is provided.

The author discusses a study within the issue of ancient Andes river deposits found in northern Colombia by Montes et al. using a combination of geochronological analysis and geological mapping, which suggests the Central American Seaway, which separated South and North America, closed and formed what became the Panama land bridge more than 10 million years earlier than previously thought. The implications for ocean circulation, climate, and biotic exchange are discussed.

In this article the author discusses a study within the issue by J. Hiess and colleagues reporting the isotopic composition of uranium (U)-bearing accessory minerals using a novel ration of U isotopes. Hiess' team used a ratio of 238U/235U to facilitate a more accurate uranium-lead (U-Pb) geochronometer, reportedly providing fundamental insights into the mechanisms controlling 238U/235U fractionation. Topics include an overview of U, which is the heaviest naturally occurring actinide element on Earth consisting of three natural isotopes, two of which are the parent nuclides of the 238U- 235U- decay series chains that ultimately decay to stable lead isotopes, and U cycling in the environment.

The article presents science news briefs. A study by Chelton et al. notes that the source of ocean chlorophyll variability has been misattributed to Rossby waves. Research from Zhang et al. extended the self-assembly abilities of nanoscale materials. Waters et al. confirmed the 14,000 year old age of a mastodon fossil originally discovered in the late 1970s.

The article discusses a report within the issue which presents data suggesting that the assumption for the use of the uranium-lead (U-Pb) chronometer in geochronology may not be correct. According to the author, meteorites and their components are the only means to study the origins of the solar system. A single undifferentiated meteorite, which is known as a chondrite, contains matrix and inclusions that have formed over a wide range of conditions over time. Scientists can formulate models of the spatio-temporal thermochemical conditions that evolved to give rise to the solar system. Topics include an overview of the two types of chronometers used to measure the age of ancient meteorites and the assumptions regarding the fixed ratios of U and Pb which make chronometers inaccurate.

The author replies to two letters to the editor in the current issue, one from Giacomo Certini and Riccardo Scalenghe and one from Simon L. Lewis and Mark A. Maslin, both responding to his article "Defining the epoch we live in," which appeared in the April 3, 2015, issue.

The article reports on the recalibration of one of the two geologic clocks, the argon-argon radiometric dating, by geochronologists which is considered a great step forward in the field. Over the years, two of the workhorse timepieces have been differing by million of years. Mike Villeneuve of the Geological Survey of Canada in Ottawa, said that the synchronization of clocks contribute more support to a link between huge volcanic eruptions and mass extinctions. Argon-argon radiometric dating is based on the decay of potassium-40 to argon-40 while uranium-lead dating is based on the decay of uranium-238 to lead-206. Argon-argon dating was giving slightly younger ages for the same rocks.

The article discusses the evolutionary dynamics of latitudinal diversity gradients in reference to an analysis of the fossil record of the tropics documenting the patterns of origination, extinction, and migration of species. The expansion of geographic ranges of species contributed to its origination and extinction rates, and caused the variation for fossil records. In view of this expansion, tropics were described to be both a cradle and museum of diversity based on the extratropical aspects, taxonomic groups and molecular phylogenesis. In terms of taxonomic group, they are regarded as a museum which accounts from the retention of the primitive characteristics of the species and lack of derived features.

The article discusses the issue of origin and evolution of animals. A growing collection of exquisitely preserved fossils of soft-bodied animals from the Cambrian has highlighted the existence of Cambrian representatives of most of the living animal phyla. This has given rise to the "Cambrian Explosion" hypothesis that most animal phyla arose within a short period. Motivated by the desire to resolve the early evolution of animals, while accounting for insufficient data from some taxa, Rokas and his colleagues used an alignment of 12,060 amino acids, encoded by 50 genes, to infer the phylogeny of 16 representatives from nine animal phyla. This conclusion is consistent with a phylogeny based on fossil data, thus reconciling a key difference in opinion between evolutionary biologists and paleontologists. Rokas used nonparametric bootstrap and posterior probabilities to gauge support for the pattern and order of speciation events. However, the former is widely recognized to be statistically unwise. More research is required about the structure and function of gene products before models are developed that appropriately address the early evolution of animals.

The article presents information on Earth's "dark age," the mysterious time after accretion of the planet about 4560 million years ago. With the development of modern isotopic dating the extent of the dark age was established at about 800 to 1000 million years by the discovery of exceptionally old rocks in western Greenland. The discovery of still older grains of zircon in archean sedimentary rocks in Western Australia reduced the dark age to 400 to 300 million years and recently to less than 200 million years a mere 5% of Earth's life span. Some of the oldest zircon grains, however, plot well above or below the primitive mantle evolution line and require extreme fractionation of lutetium and hafnium and very early separation of their source rocks from the primitive mantle, probably around 4500 million years ago, less than 100 million years after the accretion of Earth. More reliable hafnium isotope data could be obtained from crystal domains that are shown to be homogeneous. However, analysis of these small crystal domains produces less precise data. Further progress in early Earth studies thus requires more sensitive and precise mass spectrometers for measuring Hf176/Hf177 ratios.

Archeological research from an Iron Age site in Tel Rehov, Israel suggests the origin of king Solomon. The dates estimated for the site support the traditional view that King Solomon was both real and powerful. Researchers led by Amihai Mazar of the Hebrew University ofJerusalem conducted experiments on olive pits and charred grain from one of Tel Rehov's three destruction layers. The results place the layer between 940 and 900 B.C. Archaeologist Israel Finkelstein of Tel Aviv University suggests that Solomon did not contributed to temples, palaces, and political structures usually attributed to him.

The article reports on the Fossil Calibration Database, an online information resource supported by the National Evolutionary Synthesis Center which will contain up-to-date information on the oldest fossil discoveries and is intended to facilitate paleontology research and fossil dating.

Presents news briefs related to science. United States legislation for permanent tax credit for research and development investments; Controversy over the Kennewick Man, found in the bank of Washington's Columbia River; Failure to date the remains; Claims laid by Native American tribes.

Focuses on the dating of sedimentary rocks. Report on page 78 of this issue by McNaughton et al, which describes the use of a large ion microprobe to measure lead isotopes in microscopic crystals of a rare-earth element phosphate mineral, Xenotime, which commonly grows during diagenesis; Use of radioactive decay sequences of elements and plate tectonics; Isotopic dating; Ability of McNaughton et al to date Xenotime overgrowths.

Examines discrepancies in radioisotopic dating. Decay rates and isotopes; Age relations in determining causality in science; Various radioactive decay schemes; Improvements in measuring isotope ratios; Lack of precision in many radioisotopic dating methods; Different values for a decay constant in different disciplines; Comparing ages based on different systems; Dating of meteorites; Calibration of the geologic time scale; Errors in geochronology.

Presents results from the meeting of the Geological Society of America, October, 1997, in Salt Lake City, Utah. Emphasis on change over geologic time; World's greatest mass extinction; Climate change in warm intervals between glacial epochs; Size change among mammals in past 80 million years.

The article presents abstracts for technical comments on the article "Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon," from the December 21, 2012 issue.

The article discusses a debate between two separate teams of researchers over the age of an upper jawbone found in Great Britain's Kents Cavern in 1927.

The article looks at further investigation into a mastodon fossil originally discovered in Washington state in the 1970s. Research done by Michael Waters and colleagues is described, noting they used DNA and radiocarbon dating to learn more about a spear point embedded in the mastodon's rib, which was determined to be made of bone. The date determined is said to date the mastodon before the Clovis people, strengthening beliefs that the Clovis people were not the first North American settlers.

The article discusses research on the botanical Asteraceae family, comprising daisies, sunflowers, and dandelions, and said to be the Earth's most diverse family of flowering plants. The capitulum, said to be the family's most prominent feature, is described. The authors discuss fossil capitulum from Asteraceae of the Middle Eocene that they discovered in Patagonia, southern South America.

The article discusses samples of the rock gneiss that are being sold by prospector and miner Mark Brown and remarks by geochronologist Samuel Bowring on dating of the gneiss.

The article discusses a report by researcher Jourdan and colleagues regarding age-dating of craters. There are 174 characterized impact structures on Earth ranging in size and age from South Africa to Arizona in the U.S. Topics include how understanding their age-size relationship could aid in the assessment of the impact and its relation to events such as extinctions. A brief overview of the difficulties in aging craters is presented.

The article is an abstract of a response by Jonathan O’Neil, Richard W. Carlson, Don Francis, and Ross K. Stevenson to technical comments on their paper "Neodymium-142 Evidence for Hadean Mafic Crust" in a 2008 issue. The full text of the response to comment is available on the journal's Internet website at www.sciencemag.org/cgi/content/full/325/5938/267-b.

The article is an abstract of technical comments that pertain to the paper "Neodymium-142 Evidence for Hadean Mafic Crust," by J. O'Neil, R. W. Carlson, D. Francis, and R. K. Stevenson in a 2008 issue. The full text of the comment is available on the journal's Internet website at www.sciencemag.org/cgi/content/full/325/5938/267-a.