December Geology and GSA Today media highlights
Boulder, CO - Topics include: impact of melting glaciers on near-shore ecosystems; discovery of intact egg clusters from a Middle Cambrian oceanic invertebrate; biotic recovery from the End-Permian mass extinction; fossil evidence of unusual animal foraging behavior; dating the destruction of Herod the Great's harbor at Caesarea; El Niño-related landslides along the Big Sur coastline; and evidence of flowing water at Mars' Erebus crater. The GSA TODAY science article describes dynamics of ice sheet recession in East Antarctica.
Mid-Holocene El Niño–Southern Oscillation (ENSO) attenuation revealed by individual foraminifera in eastern tropical Pacific sediments
Athanasios Koutavas, College of Staten Island/CUNY, Engineering Science and Physics, 2800 Victory Blvd., Staten Island, NY 10314, USA; et al. Pages 993-996.
The El Niño–Southern Oscillation (ENSO) is a major mode of global climate variability, and much uncertainty surrounds its future evolution. Reconstructing past ENSO occurrences, particularly during the Holocene, can greatly advance scientific understanding of this phenomenon. However, progress toward this goal has been slow because geologic archives of ENSO activity are scarce. In this study, Koutavas et al. demonstrate that a statistical approach based on individual foraminifera from open ocean sediments has the ability to reveal changes in ENSO variability through time. The results indicate significantly reduced ENSO activity in the middle Holocene and a shift of the mean climate in the tropical Pacific toward the cold ENSO phase—La Niña. Because foraminifera are fairly abundant in open ocean cores, expanded application of this technique can generate continuous time series of ENSO variability directly from the marine locus of ENSO, which is presently lacking.
High-precision U-Pb zircon age from the Triassic of Italy: Implications for the Triassic time scale and the Carnian origin of calcareous nannoplankton and dinosaurs
Stefano Furin, Università degli Studi di Padova, Dipartimento di Geologia, Paleontologia e Geofisica, Padova PD 35137, Italy; et al. Pages 1009-1012.
Furin et al. present a single high-precision U-Pb radiometric zircon age for a volcanic ash bed enclosed in an almost continuous marine Triassic succession from southern Italy. The age is tightly controlled both by marine and terrestrial microfossils (conodonts and palynomorphs) and is correlated with Tethyan and Newark magnetostratigraphy. This biostratigrapy constrains the position of the ash bed to a Late Carnian age. Furin et al. use the U-Pb radiometric age as an independent confirmation of the orbital forcing of the Newark Basin's playa-like cyclicity and suggest a reconsideration of the duration of Late Triassic stages. The U-Pb age also suggests some intriguing considerations about the first appearance of dinosaurs and calcareous nannoplankton.
Extreme freshwater release during the late Paleozoic Gondwana deglaciation and its impact on coastal ecosystems
Luis A. Buatois, University of Saskatchewan, Department of Geology, Saskatoon, Saskatchewan SK S7N 5E2, Canada; et al. Pages 1021-1024.
New research in ancient fjord basins suggests that melting glaciers fundamentally altered near-shore ecosystems through a massive influx of freshwater that pushed back the edges of the saltwater marine environment. Buatois et al. examined trace fossils—the tracks left behind by fish, arthropods, and other animals—in the fine-grained rock formations in the Paganzo, Tarija, and Paraná basins of Argentina and Brazil. These formations have been the subject of some controversy, with researchers suggesting that they were formed in freshwater lake basins, estuaries, or even ordinary marine environments 290 million years ago. This time frame corresponds with the end of the 90-million-year-long Gondwanan Ice Age. At this time, most of the world's southern landmasses formed the supercontinent of Gondwana. While fossils of animals themselves are rarely formed in such environments, Buatois et al. interpret the trace fossils to reflect a marine environment that became inundated with freshwater fish and arthropod species as water from the melting glaciers rushed into the fjords of the ancient supercontinent.
A volcano bursting at the seams: Inflation, faulting, and eruption at Sierra Negra Volcano, Galápagos
William W. Chadwick Jr., Oregon State University, Hatfield Marine Science Center, Newport, OR 97365, USA; et al. Pages 1025-1028.
"A volcano bursting at the seams" is a fitting description of the Sierra Negra volcano in the Galapagos, based on behavior documented with Global Positioning System (GPS) monitoring. Chadwick et al. describe the largest inflation ever recorded before an eruption at a basaltic volcano (nearly five meters leading up to the October 2005 eruption). Periodic trapdoor faulting events associated with earthquakes punctuated this extraordinary uplift, each with approximately one meter of instantaneous surface displacement along a preexisting fault system. The faulting was due to increasing pressure in a shallow magma reservoir beneath the caldera. This process has clearly happened repeatedly in the past, as evidenced by parts of the caldera floor that have been uplifted over 100 meters and now stand higher than the caldera rim. Chadwick et al. show that the repeated faulting relieved strain caused by rapid inflation, and effectively postponed the eruption. Finally, the continuous GPS data reveal a pattern of deformation at Sierra Negra that could have predictive value in the future.
Survival of reactive carbon through meteorite impact melting
John Parnell (corresponding author) and Paula Lindgren, University of Aberdeen, Department of Geology and Petroleum Geology, Kings College, Aberdeen, Grampian, Scotland AB24 3UE, UK. Pages 1029-1032.
Until now, it has been assumed that when a meteorite strikes a planetary surface, any organic material in the ground will be volatilized and lost. Parnell and Lindgren, however, use research from an impact crater in Norway to illustrate that carbon is preserved after a meteorite strike in a way that allows it to be recycled by primitive life. This is important to scientific understanding of how life can survive meteorite impacts on a planetary surface.
Silicified egg clusters from a Middle Cambrian Burgess Shale-type deposit, Guizhou, south China
Jih-Pai Lin, Ohio State University, Geological Sciences, Columbus, OH 43210, USA; et al. Pages 1037-1040.
Fossil preservation of soft tissues is very rare, and the preservation of eggs is extremely rare in the fossil record. However, Lin et al. report, for the first time, intact egg clusters of an oceanic invertebrate from the Middle Cambrian (513 to 501 million years ago). The eggs from the Kaili Formation, Guizhou Province, China, represent a new mode of preservation associated with deposits of exceptional preservation in the Cambrian. The eggs are preserved three dimensionally in silica, and some embryos have early cleavage cells, clearly visible using high-end imaging techniques, such as synchrotron X-ray microscopy. This discovery provides additional information on both the reproductive strategy of early metazoans and a new type of fossil egg preservation. It also raises the possibility of discovering fossil embryos in similar environmental settings in both older and younger sediments.
Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China
Daniel J. Lehrmann, University of Wisconsin-Oshkosh, Department of Geology, Oshkosh, WI 54901, USA; et al. Pages 1053-1056.
The end-Permian extinction was the greatest mass extinction in Earth's history. Although studies of the extinction are numerous, fewer studies have focused on the biotic recovery following the extinction. Lehrmann et al. present constraints on the timing of the biotic recovery, employing a combination of high-precision uranium-lead radioisotope dates of single zircon crystals from volcanic ash layers, biostratigraphic correlation, and magnetic reversal stratigraphy from rock sections in southern China. The robust data set constrains the age of the biotic recovery horizon to 247.2 million years ago, and indicates that biotic recovery finally took place approximately 5 million years after the extinction.
Spiral-shaped graphoglyptids from an Early Permian intertidal flat
Nicholas J. Minter, University of Bristol, Department of Earth Sciences, Bristol BS8 1RJ, UK; et al. Pages 1057-1060.
Minter et al. have found evidence in the fossil record of unusual animal foraging behavior. Delicate spiral-shaped structures were uncovered in 280-million-year-old rocks that formed in an ancient intertidal zone in New Mexico, USA. Such structures, known as trace fossils, represent fossilized evidence of animal behavior, rather than their more familiar bodily remains, and provide a valuable resource for studying animal behavior through time. Similar structures are constructed today by worms in intertidal environments to trap microorganisms. The fossil examples are thought to have performed the same function. The distribution of trace fossils, and the behavior they represent, is controlled by various environmental factors. Similar structures do occur in the fossil record, but only from rocks that formed in deep marine settings, of which they were thought to be characteristic. The absence of such structures from ancient intertidal rocks led to suggestions that such sophisticated foraging behavior only evolved recently in such settings. Minter et al.'s study, however, confirms that such strategies were present in intertidal settings many hundreds of millions of years earlier and demonstrates that the distribution of such structures is not linked to water depth, but most likely to the predictability of resources. Minter et al. contend that the rarity of such structures in ancient intertidal rocks probably reflects a preservational bias due to more intense reworking of sediments. This discovery has implications for our understanding of the evolution of animal behavior, as well as the environmental factors that shape it.
The tsunami of 13 December A.D. 115 and the destruction of Herod the Great's harbor at Caesarea Maritima, Israel
Eduard G. Reinhardt, McMaster University, School of Geography and Geology, Hamilton, ON L8S 4M1, Canada; et al. Pages 1061-1064.
In 2003, Reinhardt et al. made an exciting discovery of shell and sediment deposits from an ancient tsunami during their underwater excavations of an ancient harbor in Caesarea, Israel. The harbor was built by Herod the Great at the end of the first century B.C., and since 1990 the authors have been conducting research to determine what destroyed it. There were many competing hypotheses regarding the timing and mechanism for the destruction, including seismic activity (subsidence) and natural decay, with some researchers contending it occurred early and others postulating that the harbor functioned for centuries. However, there was little evidence to prove any hypothesis until 2003, when Reinhardt et al. discovered part of the answer. Both radiocarbon and optically stimulated luminescence dating, as well as artifacts from the shell deposits, indicate that a tsunami struck sometime in the first or second century A.D. Reinhardt et al. also cite Talmudic sources that report a tsunami striking the shore on 13 December 115 C.E. The force of the tsunami impacted Caesarea and Yavne, and was likely caused by an earthquake that destroyed Antioch (in modern Syria). The tsunami no doubt affected the structural integrity of the harbor, and may have caused its total destruction a little more than a hundred years after it was built. This is the first clear physical evidence for a tsunami striking the Israeli coast, and presents a strong warning for modern development of the coast.
Coastal landslide material loss rates associated with severe climatic events
Cheryl J. Hapke (corresponding author), U.S. Geological Survey, Geosciences, University of Rhode Island, Kingston, RI 02881, USA; and Krystal R. Green, U.S. Geological Survey, Pacific Science Center, Santa Cruz, CA 95060, USA. Pages 1077-1080.
Frequent landslides along the Big Sur coastline of California produce large amounts of material that end up in the ocean waters adjacent to the coast. Landslides occur primarily during winter months when there are large waves and storms with significant rainfall. Such conditions are maximized during El Niño years. Hapke and Green describe a method of using aerial photography and state-of-the-art computer technology to produce detailed models of the terrain along the Big Sur coast. The terrain models were compared over three different time periods. Two of the three periods contained the severe El Niño winters of 1982–83 and 1997–98. The models were subtracted from one another, enabling the authors to measure the amount of material that moved via landslides into the ocean. The rate of material loss from landslides is up to 16 times higher during El Niño years than in non–El Niño years, and 75% of the total amount of material lost over the course of the study occurred during the 1997–98 El Niño period.
Sedimentary textures formed by aqueous processes, Erebus crater, Meridiani Planum, Mars
J. Grotzinger, Caltech, Geological and Planetary Sciences, Pasadena, CA 91125, USA; et al. Pages 1085-1088.
Evidence for water flowing across the surface of Mars has been discovered at Erebus crater by the Mars Exploration Rover Opportunity. Evidence includes centimeter-scale cross-lamination, which represents former sand ripples that were pushed along by water currents, and cracks in the sediments, which form when that water desiccates. Cross-lamination has been observed previously by Opportunity, but has never been this well developed. The desiccation cracks, however, have not been observed before, and provide further evidence for the extent of water across the surface of Meridiani Planum, the rover landing site. Considered collectively, this suite of sedimentary structures provides strong support for the involvement of liquid water during accumulation of sedimentary rocks at Meridiani, over an area of about 5 to 10 square kilometers. However, other data collected by orbiting spacecrafts have shown that the rocks studied by Opportunity are likely to extend over a much greater distance, suggesting that water may have extended across hundreds of square kilometers. This recent discovery is consistent with previous observations by Opportunity at outcrops at other craters. Each newly observed Martian crater provides a chance to test the hypothesis for water on Mars, and to date, all the observations continue to support it.
GSA TODAY Science Article
Marine sediment record from the East Antarctic margin reveals dynamics of ice sheet recession
Amy Leventer, Department of Geology, Colgate University, Hamilton, New York 13346, USA, [email protected], et al.
Tracking the Antarctic meltdown: The ice sheets of Greenland and Antarctica are two of the greatest concentrations of frozen fresh water on the planet. Understanding how these ice sheets behaved during past episodes of glaciation-deglaciation may help us understand how they will respond to contemporary climate changes. In the December issue of GSA Today, Amy Leventer and colleagues report on the results of bathymetric mapping and sediment cores taken off of the coast of East Antarctica. Their work shows that in short periods of time during deglaciation, fiord-like re-entrants formed in the ice sheet and helped foster extremely high seasonal biologic productivity. This episode of deglaciation, as constrained by radiocarbon dates, indicates that deglaciation between 10,500-11,500 yr B.P., considerably younger than the major meltwater pulse at 14,200 yr B.P., raises new questions about the source(s) of glacial water that drove sea level rise during the last glacial epoch.
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