Where have all the craters gone?

(Press-News.org) Boulder, Colo., USA – Impact craters reveal one of the most spectacular geologic process known to man. During the past 3.5 billion years, it is estimated that more than 80 bodies, larger than the dinosaur-killing asteroid that struck the Yucatan Peninsula 66 million years ago, have bombarded Earth. However, tectonic processes, weathering, and burial quickly obscure or destroy craters. For example, if Earth weren't so dynamic, its surface would be heavily cratered like the Moon or Mercury.

Work by B.C. Johnson and T.J. Bowling predicts that only about four of the craters produced by these impacts could persist until today, and geologists have already found three such craters (larger than 170 km in diameter). Their study, published online for Geology on 22 May 2014, indicates that craters on Earth cannot be used to understand Earth's bombardment history.

Johnson and Bowling write, however, that layers of molten rock blasted out early in the impact process may act as better records of impacts—even after the active Earth has destroyed the source craters. The authors suggest that searches for these impact ejecta layers will be more fruitful for determining how many times Earth was hit by big asteroids than searches for large craters.

Other Geology articles (see below) cover such topics as 1. A meteor impact that hit Earth 3.24 billion years ago, remnants of which are preserved in South Africa.
2. The late Miocene Messinian Salinity Crisis (MSC)—This extraordinary geologic event is marked by massive salt accumulation, basin desiccation, and possible major sea-level drop as a consequence of the reduced water exchange with the Atlantic Ocean.
3. Provenance of volcanic ash found in Maya pottery from El Pilar, an archaeological site on the Yucatan Peninsula.

Geology articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geology in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

FEATURED ARTICLEs Where have all the craters gone? Earth's bombardment history and the expected terrestrial cratering record B.C. Johnson and T.J. Bowling, Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (Johnson), Dept. of Earth, Atmospheric, and Planetary Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, Indiana 47901, USA (Bowling). Published online 22 May 2014; http://dx.doi.org/10.1130/G35754.1.

RELATED ARTICLE Paleoarchean ocean crust and mantle excavated by meteor impact: Insight into early crustal processes and tectonics Alexandra E. Krull-Davatzes et al., Dept. of Earth and Environmental Science, Temple University, Philadelphia, Pennsylvania 19046, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35614.1.

Our understanding of how and when plate tectonics began on Earth is impeded by the rarity of rocks older than three billion years. In particular, we lack remnants of ocean crust forming at spreading centers or clear evidence of how much continental crust had formed. However, a meteor impact that hit Earth 3.24 billion years ago excavated and vaporized Earth's ancient crust, and remnants of this impact are preserved in South Africa. This meteor was 30 km in diameter and excavated down into the mantle. Our geochemical analysis determined that the meteor hit ocean crust and underlying mantle that is very similar to modern ocean crust and upper mantle. These deposits therefore give us a unique insight into the crust under ancient oceans. Because of the similarity to modern mantle, a significant amount of continental crust had to have formed. It is also dissimilar to the ancient terrains that remain today, implying that the site of impact was far from the location where the samples were collected, and that our knowledge of Earth's surface at this time is heavily biased.

Black Sea desiccation during the Messinian Salinity Crisis: Fact or fiction? A. Grothe et al., Marine Palynology and Paleoceanography, Laboratory of Palaeobotany and Palynology, Dept. of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD, Utrecht, Netherlands. Published online 16 May 2014; http://dx.doi.org/10.1130/G35503.1.

The connection between the Atlantic Ocean and the Mediterranean Sea became restricted about 5.3 million years ago. Called the late Miocene Messinian Salinity Crisis (MSC), this extraordinary geologic event is marked by massive salt accumulation, basin desiccation, and possible major sea-level drop as a consequence of the reduced water exchange with the Atlantic Ocean. The impact of this event on the regional and global climate is still an important open question. Erosional features interpreted from seismic profiles of the Black Sea margin, correlated by some to the Pebbly Breccia unit, were used to support this hypothesis. However, the age of the Pebbly Breccia is poorly constrained, and its origin and relevance to the MSC subject to controversy. In this study, Arjen Grothe of Utrecht University and colleagues present new biostratigraphic (dinoflagellate cyst) data from two key sedimentary successions located in a deep and a marginal setting of the Black Sea Basin. These new data demonstrate that the Pebbly Breccia unit is more than six million years old, older than the Mediterranean water-level drop during the MSC. Because the presumed desiccation phase of the Mediterranean occurred about 5.5 million year ago, Grothe and colleagues conclude that there is no evidence for simultaneous desiccation of the Black and Mediterranean seas during the Messinian Salinity Crisis.

Volcanic ash provenance from zircon dust with an application to Maya pottery Kevin T. Coffey et al., Dept. of Earth, Planetary, and Space Sciences, University of California–Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, California 90095-1567, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35376.1.

Maya pottery from El Pilar, an archaeological site on the Yucatan Peninsula, commonly contains abundant volcanic ash as an additive. The source of this ash is a mystery because ash deposits are not found in this region. It has been suggested that the ash was gathered from wind-blown fallout of Tierra Blanca Joven (TBJ), a widespread deposit formed approximately 1500 years ago by a major explosive eruption of Ilopango volcano in present-day El Salvador. The TBJ eruption coincides with the earliest use of volcanic ash in Maya pottery from the Yucatan Peninsula, and TBJ and pottery ash are broadly similar compositionally, but this correlation is tenuous, as the process of firing the pottery alters the composition of the ash. By comparing the crystallization ages of zircon in TBJ pumice from Ilopango and dust-sized zircon in the pottery and in TBJ ash collected 160 km from Ilopango, we conclusively demonstrate that the pottery ash is not derived from Ilopango. Instead, zircon dust in the pottery indicates at least two other sources. Their exact locations remain unidentified, but they must have yielded copious amounts of ash containing zircon with ages much older than the ongoing volcanism in Central America.

Stevensite in the modern thrombolites of Lake Clifton, Western Australia: A missing link in microbialite mineralization? R.V. Burne et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Published online 16 May 2014; http://dx.doi.org/10.1130/G35484.1.

Microbialites form when minerals accumulate around colonies of microscopic organisms. They are the earliest large-scale evidence of life on Earth, and still form today in sheltered environments such as coastal lagoons. Although formed by tiny organisms, microbialites can rival coral reefs in size. Much of the mineral content of microbialites is calcium carbonate. This is generally assumed to have been precipitated alongside living microorganisms. However, we have discovered that the first rigid structure of modern microbialites in Lake Clifton, Western Australia, forms not from carbonate, but from the magnesium silicate mineral stevensite. Carbonate crystallizes only later, replacing stevensite, and can actually eradicate the distinctive texture of the microbial filaments. Such two-stage mineralization may explain why many ancient microbialite carbonates lack textural evidence for their biogenic origin. Stevensite is usually assumed to require highly alkaline conditions to form, such as volcanic soda lakes. However, our stevensite microbialites grow in a lake less saline than seawater and with near-neutral pH. This discovery sheds new light on the paleoenvironmental conditions under which the extensive stevensite and microbialite-rich hydrocarbon-bearing deposits offshore of Brazil and Angola were formed during the Cretaceous.

Mobilizing salt: Magma-salt interactions Nick Schofield et al., Dept. of Geology & Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK. Published online 22 May 2014; http://dx.doi.org/10.1130/G35406.1.

Molton rock (magma) is a strange thing, and does some very strange things when intruding into the rocks underneath the surface. We've discovered that when magma cuts up through salt sequences, such as those underlying large parts of Germany, as well as other regions around the world, such as Afar in East Africa, the magma interacts in what can only be described as a slightly bizarre way with the salt. When the intruding magma cuts through certain salt sequences, in particular those ones which have water locked into their crystal structure (e.g., Carnallite), the heating by the magma (at approx. 1100 degrees Celsius or 2012 degrees Fahrenheit) causes the salt to begin to flow as a viscous fluid (a bit like treacle). The magma, which is also viscous, then begins to intrude and mingle with the salt. This creates some spectacular relationships which have not been documented before. This process happens without a fracture occurring. With little deformation of rocks above the intruding magma either, in places like the Afar, where current day volcanism is happening, magma could intrude hundreds of meters below the surface into the thick salt sequences without the earthquake and ground deformation monitoring networks even recording it happening.

Paleomagnetism reveals the emplacement age of tsunamigenic coral boulders on Ishigaki Island, Japan T. Sato et al., Dept. of Earth Science, Tohoku University, 6-3 Aoba, Sendai 980-8578, Japan. Published online 22 May 2014; http://dx.doi.org/10.1130/G35366.1.

You can see lots of tsunamigenic erratic boulders of coral along the eastern sea side of the Ishigaki Island, Japan. Previous radiocarbon dating of coral boulders tells us that the Island has been met by multiple tsunamis at the 200 to 400 year interval since 5000 years ago. These tsunamis may displace the boulders more than one, but we don't know how many times the boulder displaced to the present position. The latest one is Meiwa tsunami at A.D. 1771. Here the authors applied a paleomagnetic method to solve this problem. If corals have a tiny nanometer-sized magnetite as an inclusion in a skeleton, the coral can record the geomagnetic field direction as a remanence. When the boulder was displaced or rotated, the direction of remanence is also changed and a new remanence will be added as time goes by in the boulder. This is a paleomagnetic clock with a level. The result showed that a huge boulder (>200 tons) had displaced in the present position by ancient tsunami, but stayed there by the Meiwa tsunami. Therefore, the application of the paleomagnetic method to tsunami boulders unveils a displacement history of tsunami boulders.

Decoupling of the carbon cycle during Ocean Anoxic Event 2 J.S. Eldrett et al., Shell International Exploration and Production Inc., Shell Houston Technology Center, 3333 Highway 6 South, Houston, Texas 77082, USA. Published online 16 May 2014; http://dx.doi.org/10.1130/G35520.1.

The Cenomanian-Turonian boundary (93 to 95 million years ago) represents one of the most profound global perturbations in the past carbon cycle. This interval is characterized by widespread deposition of organic-rich fine-grained sediment marked by a globally recognized carbon isotope excursion (CIE) reflecting the widespread burial of organic matter in marine sediments under globally oxygen depleted [anoxic] ocean conditions. However, the exact nature and trigger of this inferred global phenomenon, termed Oceanic Anoxic Event 2 (OAE-2), is still debated. Here we present the first evidence for widespread and persistent oxygenation during OAE-2 based primarily on the distribution of oxic/anoxic-sensitive trace metals and biota preserved in sedimentary rocks from the Eagle Ford Formation, Texas, USA. Our data indicate anoxic conditions in the mid- to late Cenomanian, but improved bottom water oxygenation prior to and during the CIE. Trace metal enrichments support large volumes of volcanism possibly from the High Arctic- large igneous province (HA-LIP), which occur within the middle of the CIE indicating that the emplacement of a LIP was not the primary trigger of the Cenomanian-Turonian CIE. The apparent paradox of an oxygenated phase within OAE-2 suggests a much more complex carbon cycle during these global perturbations than previously thought.

Disentangling abrupt deglacial hydrological changes in northern South America: Insolation versus oceanic forcing J. Hoffmann et al., Institute of Geosciences, University of Frankfurt, 60438 Frankfurt, Germany. Published online 16 May 2014; http://dx.doi.org/10.1130/G35562.1.

Northern South America and the Caribbean hinterland react highly sensitive to climate variations. This is evident from climate records that show periods of pronounced dry and wet conditions during the last 15,000 years. These are triggered by shifts of the Intertropical Convergence Zone (ITCZ), a tropical rain belt located where northeast and southeast surface winds converge. However, the forcing mechanisms responsible for changes in the ITCZ remain controversial. In our study we reconstruct the amount and source of the Orinoco River outflow into the Atlantic during the last 15,000 years. We find that a very abrupt and distinct change in water chemistry occurred 10,800 years ago, suggesting a massive reorganization of moisture sources in northern South America. This reorganization is triggered by enhanced rainfall over the Venezuelan Andes due to an extreme northward position of the ITCZ. This situation is unique within the last millennia, as the ITCZ is usually located farther south, but it can be explained by the maximum of solar radiation received by the tropics during the early Holocene. Because the long-term changes in solar input are gradual, the abruptness of the hydrological changes over South America was unexpected and surprising.

A cool temperate climate on the Antarctic Peninsula through the latest Cretaceous to early Paleogene David B. Kemp et al., Environment, Earth and Ecosystems, Centre for Earth and Planetary Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Published online 22 May 2014; http://dx.doi.org/10.1130/G35512.1.

Understanding the climatic history of Antarctica is of great importance owing to the key role that this continent played, and continues to play, in modulating global climate. Nevertheless, our knowledge of ancient temperatures on Antarctica millions of years ago is relatively poor owing to a paucity of accessible study sites and inadequate dating of existing data. David Kemp of Open University (UK) along with colleagues from Oxford University, the British Antarctic Survey and the Danish Geological Survey, have employed a novel organic geochemical technique to calculate ancient land temperatures of Seymour Island on the Antarctic Peninsula across the end-Cretaceous mass extinction (66 million years ago). Their data reveal that a cool temperate climate of ~12 degrees Celsius prevailed on the Peninsula from ~67 to ~56 million years ago. These new data add important detail to an emerging picture of climatic evolution of the Antarctic continent, and emphasize the dynamic nature of climate in the circum-Antarctic region before the development of major ice-sheets around 34 million years ago.

Tibetan garnet records early Eocene initiation of thickening in the Himalaya Matthijs A. Smit et al., Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Published online 22 May 2014; http://dx.doi.org/10.1130/G35524.1.

The High Himalayas are the result of the India-Asia collision ca. 50 million years ago and provide the one place on Earth where active mountain building can be investigated. Decades of research provided a deep understanding of the past 25 million years of crustal deformation and mountain building. Looking further back in Himalayan history, however, has proven difficult. Similar to research on The Middle Ages, studies investigating early deep-crustal deformation were compromised by obscured or missing geological records. As a result, the 'when and why' of crustal thickening during the first half of Himalayan history has been a matter of great debate. To advance beyond the status quo, this study focused on dating garnet, a mineral that typically forms in the deep crust during early thickening. The approach showed that the deep crust of the central Himalaya began to thicken ca. 54 million years ago, thus confirming for the first time that thickening was as old as the continental collision itself. This result provides a key piece of information for the tectonic reconstruction of Earth's largest range, and improves the knowledge of mountain building in general.

A nanolite record of eruption style transition Mayumi Mujin and Michihiko Nakamura, Dept. of Earth Science, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aobaku, Sendai 980-8578, Japan. Published online 22 May 2014; http://dx.doi.org/10.1130/G35553.1.

Single volcanic eruptive events often exhibit transitions between eruption styles, yet the erupted magmas often possess similar characteristics despite their diversity in explosivity. Accordingly, it has proven difficult to extract information that could help determine the cause of these transitions. Mayumi Mujin and Michihiko Nakamura of Tohoku University studied the "nanolites" in the quenched products of the 2011 eruption of Shinmoedake, Kyusyu, Japan; these nanolites are the nanometer-scale components of the minute minerals in the erupted materials, and are higher number density than microlites (the micrometer-scale crystals). During the 2011 eruption, the volcanic activity transitioned from sub-Plinian eruption to Vulcanian explosion and intermittent effusion of lava. The authors found that the products of these different eruption styles can be distinguished clearly by their nanolite mineral assemblages, and these different assemblages are assumed to have resulted from differences in magma residence time near the surface. Thus, it has been proposed that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points between eruption styles.

Could microorganisms be preserved in Mars gypsum? Insights from terrestrial examples Kathleen Counter Benison and Francis J. Karmanocky III, Dept. of Geology and Geography, West Virginia University, P.O. Box 6300, Morgantown, West Virginia 26506-6300, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35542.1.

Acid saline shallow lakes in the extremely arid, high altitude volcanic terrain of the Andes Mountains of northern Chile are excellent analogs for Mars. Gypsum crystals that form in these lakes contain microfossils, including diatoms, green algae, and prokaryotes, some of which are known to become dormant when saline lakes dry and form salt. The microfossils were entrapped as solid inclusions and within fluid inclusions as the gypsum grew. There is a high probability that many of these microorganisms may remain alive within fluid inclusions for long periods of geologic time. Abundant similar gypsum has been documented on Mars, yet has not been examined for entrapped microfossils. Gypsum and other salt minerals on Mars should be examined with optical and UV petrography for such types of fossil entrapment. We propose that the long-term viability of microorganisms within fluid inclusions in gypsum suggests the possibility of a living, yet isolated and likely dormant, microbiological community on Mars today.

An iodine record of Paleoproterozoic surface ocean oxygenation Dalton S. Hardisty et al., Dept. of Earth Sciences, University of California–Riverside, Riverside, California 92501, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35439.1.

Despite hosting the earliest oxygen-producing photosynthetic organisms, the initial source of oxygen to the atmosphere, the first eukaryotic organisms, and ultimately the advent of animals, relatively little is known about the oxygen dynamics of the Precambrian surface ocean. Using a new approach, iodine geochemistry in shallow carbonate rocks, this study presents a novel temporal record of surface ocean oxidation from the Mesoarchean to the Paleoproterozoic spanning from roughly 3.5 to 1.9 billion years ago. Results suggest a dominantly anoxic surface ocean until the so-called Great Oxidation Event or GOE. The GOE, marking the first permanent accumulation of oxygen in the atmosphere, culminated with the dramatic Lomagundi carbon isotope excursion—a unique feature in Earth history that suggests surprisingly high early oxygen contents that fell again, equally dramatically, after a few hundred million years. Beyond the insights provided into early surface ocean oxidation and its relationship to early life and climate, this study acts as a proof of concept, revealing the potential of iodine-to-calcium ratios in carbonates to provide a robust record of oxygen dynamics in the early surface ocean with potential applications throughout Earth history.

Hot faults: Iridescent slip surfaces with metallic luster document high-temperature ancient seismicity in the Wasatch fault zone, Utah, USA James P. Evans et al., Dept. of Geology, Utah State University, Logan, Utah 84322-4505, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35617.1.

The Wasatch Fault in central and northern Utah is an active fault that poses significant seismic hazard to the most populous region of the state The Wasatch Range is uplifted along this fault, allowing geologists to decipher processes that are locked into the rocks at great depths. The rocks near the active trace of the Wasatch Fault near Willard, Utah, is characterized by hundreds of small, exceptionally reflective colorful fault surfaces. These small faults are extremely glossy and often exhibit multicolored bands of iridescence, similar to what is produced by the slip of cutting tools in a metal shop. We combine traditional geological analyses with new, nano-scale X-ray and electron beam methods to determine the composition of the fault surfaces. Our study suggests that high temperatures due to rapid slip on the faults produce these colored surfaces. Although elevated temperatures have been proposed along faults, there are few geologic indicators of the high temperatures predicted for rapid slip. This work adds to our ability to determine when and where ancient faults have been brought to the surface.

Thresholds for Paleozoic ice sheet initiation D.P. Lowry et al., Dept. of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/G35615.1.

The climate of the Paleozoic era (541-252 million years ago) was dynamic and transitioned from early-era ice-free conditions to a late-era ice age. This climate transition has been attributed to both a decrease in atmospheric carbon dioxide levels (pCO2) and continental drift of Gondwana over the southern pole. In this manuscript, we demonstrate using a coupled ice-sheet-climate model that atmospheric pCO2 was the primary driver of Late Paleozoic glaciation. Simulations that incorporate continental drift but constant pCO2 either do not grow ice sheets on Late Paleozoic continents or grow ice sheets on mid-Paleozoic continents -- predictions that are at odds with the geologic record. Our model only produces ice growth that matches the Paleozoic glacial record under falling Paleozoic pCO2. More broadly, our results also demonstrate that changes in landmass configuration and solar luminosity modify the pCO2 threshold for glacial inception -- pCO2 glacial thresholds may have been as low as 560 ppm in the late Paleozoic, but as high as 1120 ppm in the early Paleozoic.

Cycles of explosive and effusive eruptions at Kilauea Volcano, Hawai'i Donald A. Swanson et al., U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, Hawaii 96718, USA Published online 22 May 2014; http://dx.doi.org/10.1130/G35701.1.

For the past 2500 years, Kilauea Volcano (Hawai'i) has experienced alternating periods of mainly explosive eruptions and mainly lava-flow eruptions, each lasting several centuries. The popular view of Kilauea as almost continuously producing lava flows is misleading, based mainly on the past 200 years but overlooking a 300-year period of dominantly explosive eruptions (1500-1800 CE) and an even longer 1200-year period of explosive activity between 200 BCE and 1000 CE. If Kilauea returns to an explosive period -- a likely event sometime in the future -- society could expect several centuries of sporadic dangerous explosions. The explosions are driven by groundwater when the summit caldera drops to the water table; they differ from lava fountains, which typically accompany lava-flow eruptions. The supply of magma to Kilauea during an explosive period drops to only a few percent of that during lava-flow periods, suggesting that the caldera reflects deep processes and is not just a near-surface feature. This view of Kilauea, developed from more than 200 carbon-14 ages, provides a new perspective on the volcano, showing that it can be a more dangerous place for longer periods of time than was previously thought.

Mid-Cretaceous to Paleocene North American drainage reorganization from detrital zircons M. Blum and M. Pecha, ExxonMobil Upstream Research, Houston, Texas 77252, USA (Blum), Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA (Pecha). Published online 22 May 2014; http://dx.doi.org/10.1130/G35513.1.

From the abstract: Detrital zircons (DZ) from fluvial sandstones of the Western Canada Sedimentary Basin and the U.S. Gulf of Mexico (GoM) passive margin indicate mid-Cretaceous through Paleocene continental-scale drainage reorganization. DZ populations from the Early Cretaceous Mannville Group of Alberta represent a continental-scale system that routed sediment from the Appalachian Mountains and the eastern three-quarters of North America to the Boreal Sea. In contrast, DZ populations from the GoM coastal plain show that only the southern United States and Appalachian-Ouachita orogen contributed sediment to the GoM through the Late Cretaceous, whereas by the Paleocene, southern North America, from the Western Cordillera to the Appalachian Mountains, had been routed to the GoM. This continental-scale drainage reorganization reflects the culmination of an ~300 m.y. trajectory that began with Paleozoic Appalachian assembly, and broad east to west sediment routing, followed by assembly of the Mesozoic Western Cordillera, which resulted in west-derived rivers in the United States draining to the GoM in Texas, or to an ancestral Mississippi River in the Mississippi embayment, setting up the template for sediment routing that persists today.

Toward accurate numerical calibration of the Late Triassic: High precision U-Pb geochronology constraints on the duration of the Rhaetian J.-F. Wotzlaw et al., Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland. Published online 16 May 2014; http://dx.doi.org/10.1130/G35612.1.

From the abstract: Numerical calibration of the Late Triassic stages is arguably the most controversial issue in Mesozoic stratigraphy, despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end-Triassic mass extinction. Here we report new chemical abrasion-isotope dilution-thermal ionization mass spectrometry zircon U-Pb dates for volcanic ash beds within the Aramachay Formation of the Pucara Group in northern Peru that place precise constraints on the age of the Norian- Rhaetian boundary (NRB) and the duration of the Rhaetian. Our results end a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end-Triassic mass extinction.

INFORMATION: http://www.geosociety.org