||26 January 2007
GSA Release No. 07-02
Boulder, CO - Topics include: a new technique for analyzing the relationship between climate and hurricane activity; discovery of fossilized embryos in advanced stages of development from South China; revised dispersal and extinction dating of the late Neogene "terror bird"; new data illustrating real-time changes beneath a large Antarctic glacier; evidence that mid-latitude North America's prevailing winds once blew from east to west; and recent formation of a mud volcano in an area of active petroleum exploration.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance.
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- Active deformation across the Sumatran forearc over the December 2004 MW 9.2 rupture
Donald Fisher, Pennsylvania State University, Department of Geosciences, University Park, PA 16802, USA; et al. Pages 99-102.
- The Sumatra earthquake and tsunami offshore survey (SEATOS) expedition off the coast of Sumatra collected seismic reflection data that provide an image of the shallow structures in the area of greatest slip during the December 2004 MW 9.2 rupture of the Sunda plate boundary. This image depicts folding of the sediment layers that once blanketed the upper plate, with continued infilling of isolated slope basins and evidence for ongoing shortening across the entire region. The shortening of this sediment blanket, combined with the structure of the deformation front, indicates that plate boundary slip, such as the 2004 event, likely occurs as the strong inner wedge surges forward and peels up the shallow portion of the sediment fill. Each event produces one shortening increment in the sedimentary blanket across the upper plate, with folds representing the accumulated shortening of many earthquakes. This permanent shortening thickens the sediments of the upper slope and, with the elastic rebound of the inner wedge, could contribute to the generation of tsunamis.
- Bedrock channel adjustment to tectonic forcing: Implications for predicting river incision rates.
Alexander C. Whittaker, University of Edinburgh, School of Geosciences, Grant Institute, Edinburgh, Scotland EH9 3JW, UK; et al. Pages 103-106.
- Rivers are a familiar feature of Earth's landscape and have a significant impact on human existence, either as a source of water or as a hazard through flooding or triggering of landslides and debris flows. Because they play a key role in shaping Earth's landscape, it is vital to understand how fluvial systems work. Although considerable research has focused on alluvial rivers that dominate lowland landscapes, much less attention has been paid to rivers that incise bedrock, which typify fluvial channels found in mountainous areas. Whittaker et al. focus on such channels that drain mountains in central Italy, where present-day earthquake activity is contributing to landscape instability. Current landscape models, which can be used to describe how this area might evolve as tectonic activity continues, make important assumptions about how rivers behave. Whittaker et al. challenge these assumptions by showing that rivers dynamically adjust to uplift on active faults and that traditionally used mathematical relations, which link river discharge to channel shape, do not apply in these settings. The results help to clarify the long-term erosion mechanics of bedrock rivers, explain how some rivers manage to cut across active faults, and challenge geomorphologists to improve their landscape evolution models.
- Fish tooth δ18O revising Late Cretaceous meridional upper ocean water temperature gradients
Emmanuelle Pucéat, Université de Bourgogne, UFR Sciences de la Terre, Dijon 21000, France; et al. Pages 107-110.
- The Cretaceous period is known as the warmest period of the past 250 Ma, with atmospheric CO2 levels that may have been as high as 5 to 10 times modern levels during the climatic optimum of the middle Cretaceous (ca. 95 Ma). This climatic optimum was followed by a long-term cooling through the end of the Cretaceous (ca. 65 Ma). Using the oxygen isotope composition of fossil fish teeth, a paleo-upper ocean temperature proxy exceptionally resistant to diagenetic alteration, Pucéat et al. show that low- to middle-latitude thermal gradients of the Middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to modern levels, despite a cooling of 7 °C between the two periods. These new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low- to middle-latitude thermal gradient supports an atmospheric CO2 decrease as the primary driver of the climate evolution recorded during the Late Cretaceous.
- Stalagmite stable isotope record of recent tropical cyclone events
Amy Benoit Frappier, Boston College, Geology and Geophysics, Chestnut Hill, MA 02467, USA; et al. Pages 111-114.
- Satellite and historical records of tropical cyclone events (tropical storms, hurricanes, and typhoons) are too brief to settle the scientific debate over possible links between climate change and hurricane activity. Paleotempestology, the study of ancient storms, aims to understand the causes of storminess by analyzing records of prehistoric storm activity. Frappier et al. present a new tool for paleotempestology that relies on evidence of hurricane rainfall preserved in caves. Although cave formations (stalagmites) grow relatively slowly, the researchers used a high-precision computer-controlled sampling device to extract an approximately weekly to monthly record of climatic variability from a Belize stalagmite, including individual historical hurricane events. This new technique has the potential to shed new light on hurricane-climate interactions by enabling the reconstruction of storm frequency and intensity even farther in the past than was previously possible.
- Rare helical spheroidal fossils from the Doushantuo Lagerstätte: Ediacaran animal embryos come of age?
Shuhai Xiao, Virginia Polytechnic Institute, Department of Geological Sciences, Blacksburg, VA 24061, USA; et al. Pages 115-118.
- Exquisitely preserved embryo fossils from 550-635 million-year-old rocks in South China represent some of the earliest known animals. However, their developmental trajectory and adult forms remain elusive. Previously described embryos from these rocks represent only early embryonic stages. Xiao et al. report a new group of more advanced embryos from these rocks, shedding light on the developmental biology of these early animals.
- A 10,000-year record of dune activity, dust storms, and severe drought in the central Great Plains
Xiaodong Miao, University of Wisconsin-Madison, Geography, Madison, WI 53706, USA; et al. Pages 119-122.
- Sand dunes and wind-blown dust deposits are used to reconstruct drought history; these deposits are useful because they preserve information on drought for much longer than observational data. Based on 95 optical ages, Miao et al. present a 10,000-year reconstruction of dune activity and dust production in the central Great Plains, USA. The integration of data from both wind-blown sand and dust deposits is an important new aspect of this record. Clusters of ages define periods of extensive dune activity and dust production, which Miao et al. interpret as a response to severe drought, at 1.0-0.7 ka and 2.3-4.5 ka (with peaks centered on 2.5 and 3.8 ka); sustained drought occurred from 9.6 to 6.5 ka. Parts of this record may be consistent with hypotheses linking drought of the past 10,000 years to sea surface temperature anomalies in the Pacific or Atlantic oceans or to the El Niño-Southern Oscillation phenomenon, but the record as a whole is difficult to reconcile with any of these hypotheses.
- Revised age of the late Neogene terror bird (Titanis) in North America during the Great American Interchange
Bruce J. MacFadden, Florida Museum of Natural History, Natural History Dept., Gainesville, FL 32611, USA; et al. Pages 123-126.
- The extinct terror bird Titanis walleri from North America was a flightless giant estimated to have stood 2 meters tall and weigh 150 kilograms. It is known from four Neogene fossil localities in Texas and Florida. Titanis walleri was previously thought to have dispersed from South America to North America across the Isthmus of Panama during the Great American Interchange about 2.5 million years ago. Titanis was also previously thought to have become extinct during the late Pleistocene megafaunal extinction. MacFadden et al. refine the age of the dispersal and extinction of Titanis walleri using rare earth elements (REE). REE fixed in fossil Titanis walleri bones during early stages of fossilization (diagenesis) are compared to those from fossil mammal bones within the same faunas in Texas and Florida. T. walleri from Texas has a REE pattern similar to 5-million-year-old horse fossils from the Nueces River, southern Texas. Given this similarity, Titanis walleri from Texas is now interpreted to be of similar age to these 5-million-year-old horses. This bird therefore dispersed northward during the early Pliocene, some 2.5 million years earlier than the major pulse of the Great American Interchange. In Florida, Titanis walleri has REE patterns similar to 2-million-year-old late Pliocene mammals. Given this similarity, the age of this last known occurrence of Titanis walleri is now interpreted to be from the late Pliocene. The extinction of Titanis walleri did not occur during the late Pleistocene as previously thought. MacFadden et al. demonstrate the utility of using distinctive REE patterns, taken up during early diagenesis, as a means of assessing relative ages of fossils; it potentially has widespread application at other fossil localities.
- Rapid erosion, drumlin formation, and changing hydrology beneath an Antarctic ice stream
A.M. Smith, British Antarctic Survey, Physical Sciences Division, Cambridge, CAMBS CB3 0ET, UK; et al. Pages 127-130.
- Smith et al. examine what is happening beneath a large, present-day Antarctic glacier. Measurements from the glacier surface show warehouse-sized mounds of sediment, known as drumlins, forming beneath two kilometers of ice. At other times, the ice is eroding parts of its bed at an exceptionally fast rate. Today, glaciated scenery is all around us, but normally all we can see is the end result — the ancient landscape left behind after the glaciers have gone. For the first time, Smith et al. show a glacier actually molding its bed in real time, which provides a new way of looking at how ice has formed Earth's landscape. It will help illuminate how glaciers behaved in the past and improve predictions of what might happen to them in the future.
- Tracing the impact of glacial-interglacial climate variability on erosion of the southern Andes
Dierk Hebbeln, Universität Bremen, MARUM — Center for Marine Environmental Sciences, Bremen 28359, Germany; et al. Pages 131-134.
- It is generally accepted that climate is a major factor in shaping the surface of Earth, primarily by its large impact on erosion. However, geologists still struggle to assess this climatic impact, as common methods to quantify erosion rates only give information on very long (106 years) time scales and do not resolve such major climate changes as the Late Quaternary glacial-interglacial cycles. Instead of analyzing such long-term erosion rates, Hebbeln et al. focus on the deposition of the erosion products, specifically marine sediments that accumulated off the Chilean Andes. Over the transition from the last glacial to the present warm phase (the Holocene), these sediments are well-dated by applying the radiocarbon method, thus enabling a 102-103-year resolution of the records. Hebbeln et al.'s data show that the Holocene sediment supply to the Chilean continental margin clearly correlates with the modern rainfall gradient along the western part of South America. Under glacial conditions, sediment supply and, thus, mountain erosion were substantially enhanced. At the same time, continental rainfall in the region was likewise higher due to a well-documented northward shift of the southern westerly wind belt. The observation that major changes in precipitation forcing appear to be transferred to offshore sedimentation rates by a rather fixed relation bears the potential for application of this approach at other continental margins lying offshore major mountain ranges.
- Abrupt glacial valley incision at 0.8 Ma dated from cave deposits in Switzerland
Philipp Haeuselmann, University of Applied Life Sciences, Institute for Applied Geology, Vienna 1190, Austria; et al. Pages 143-146.
- To the human eye, the landscape of the Alps seems to be eternal. However, like every feature on Earth, it evolves continuously. Mountaintops grow, valleys are deepened, and glaciations erode the rocks. However, the pace at which such processes take place is difficult to determine, because erosion tends to leave no marks that can be dated. Caves, fortunately, offer unique insight into geological processes because of several advantages. First, they are generated relatively rapidly, so that variations in valley deepenings are seen in cave levels. Second, the rapidity of their evolution leaves old sediments behind. These sediments can be dated by various methods. Combination of cave morphology, sediment sequences, and dating helps to decipher the evolution of today's landscape. Haeuselmann et al. used this approach in the Swiss Siebenhengste area, where extensive caves are found. Two dating methods, using in both cases radiogenic decay of nuclides, enabled Haeuselmann et al. to retrace the landscape evolution from 4.5 million years ago to the present. Additionally, the velocity of valley deepening could be calculated, as well as erosion of the surface above the cave system. Obtaining such data is not possible without caves.
- Sediment delivery after a wildfire
Steven L. Reneau, Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM 87545, USA; et al. pages 151-155.
- Wildfires have become increasingly common and destructive in the American West, aided by the buildup of fuel associated with land-use changes and possibly by global climate changes. Impacts to society have also intensified as the urban-wildland interface encroaches on fire-prone terrain. Reneau et al. document five years of post-fire sediment delivery to a small reservoir near Los Alamos, New Mexico, providing a relatively long and detailed record of erosion and sediment transport from a watershed that was burned in the May 2000 Cerro Grande fire. The transport of ash from burned slopes can have particularly negative effects on aquatic communities and water quality, and this study is the first to estimate ash content in post-fire sediment, using the fallout radionuclide 137Cs as a tracer. The watershed experienced major and rapid geomorphic responses to the fire, and erosion rates in the first year increased about 500-fold compared to pre-fire conditions. Maximum impacts from ash and other fine-grained sediment carried in suspension in floods occurred soon after the fire, declining rapidly after the first year. In contrast, coarse-grained sediment transported as bedload was first deposited along the channel close to the source in short-duration summer floods and subsequently remobilized by longer-duration snowmelt runoff. Substantial transport of coarse-grained sediment was still occurring five years after the fire, although at decreasing rates as the channel stabilized.
- Deep turbidity currents in shallow channels
David Mohrig, The University of Texas at Austin, Geological Sciences, Austin, TX 78712, USA; and James Buttles, MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, USA. Pages 155-158.
- Acoustical imaging of the seafloor and subsurface now commonly provides high-resolution maps of submarine landscapes dominated by submarine channels and their affiliated sedimentary deposits. Unfortunately, direct measurements of natural currents in these channels are still rare. Interactions between turbidity currents and channels can be studied in the laboratory at reduced scales. Mohrig and Buttles present the results from such an experiment; these results highlight how patterns of flow and sedimentation vary when turbidity currents are very thick relative to the depth of the guiding channel. Contrary to what is observed in rivers, the transition to unconfined submarine flow is not complete until current thickness is five times greater than the channel depth. This difference between rivers versus submarine channels is a consequence of the relative densities of the channelized flows found in the two environments. River water is much denser than air, so even the smallest fraction of overbank flow rapidly moves out over the floodplain. Turbidity currents are only slightly denser than seawater, so out-of-channel flow is towed downslope by a channelized portion of the current rather than rapidly spreading out over the unconfined surface. The experiment quantifies some aspects of these interactions, can be used to guide scientific interpretations of submarine landscape evolution, and — through systematic comparison with better studied river systems — will add to our general understanding of channelized landscapes on all planetary surfaces.
- Late Quaternary temperature record from buried soils of the North American Great Plains
Lee Nordt, Baylor University, Geology, Waco, Texas 76798, USA; et al. Pages 159-162.
- The first continuous high-resolution temperature record from the North American Great Plains grasslands documents unexpected trends during the past 12,000 years. Stable carbon isotopes of buried soil organic carbons contain an ecological record of the warm-season grass productivity of ancient landscapes, which according to modern analogs is directly related to mean July temperatures. This method shows that temperatures, which increased dramatically to near modern temperatures from 10,500 to 9600 years ago, were as much as 4 °C cooler than present during the well-known Younger Dryas. Two additional warm excursions were identified at 5000 and 1500 years ago when temperatures were as much as 1 °C warmer than present. Remarkably, the cooler intervals corresponded with ice rafting events in the North Atlantic, glacier meltwater pulse, and the movement of the intertropical convergence zone into the far northwestern Gulf of Mexico in response to 1500 solar irradiance cycles. The last warm-cool couplet corresponds to the Medieval Warm Period and Little Ice Age, respectively. These data offer for the first time a reliable record of temperature in a previously unknown region for calibration of general circulation models in the Great Plains, where fossil pollen preservation is rare.
- The changes in North American atmospheric circulation patterns indicated by wood cellulose
Xiahong Feng, Dartmouth College, Earth Science, Hanover, NH 03755, USA; et al. Pages 163-166.
- Today, the prevailing wind in the mid-latitude of North America (40-50°N) is westerly. In this zone of general atmospheric circulation patterns, most marine moisture is brought to the continent either from the western coast by westerly winds, or from both western and eastern coasts by extra-tropical storms. Using oxygen and hydrogen isotopic compositions of cellulose extracted from ancient wood, Feng et al. provide evidence that during the Last Glacial Period (14 to 36 thousand years ago), the prevailing wind in this zone was easterly, and marine moisture came predominantly from the eastern coast. The isotopic evidence comes in two interrelated observations. One is the relationship between ratios of deuterium to hydrogen and oxygen-18 to oxygen-16 in cellulose; the other is the variation of isotopic ratios in cellulose across the continent (as function of longitude). Feng et al. interpret the prevailing easterlies to be a result of intensification and enlargement of the northern circumpolar vortex under the powerful influence of the Laurentide Ice Sheet. Under this circulation regime, the jet stream shifted southward, carrying extratropical storm systems to the south of their current latitudes. As a result, the Pacific Northwest received much less marine moisture from the Pacific.
- Paleoecology reconstruction from trapped gases in a fulgurite from the late Pleistocene of the Libyan Desert
Rafael Navarro-González, Universidad Nacional Autónoma de México, Instituto de Ciencias Nucleares, Laboratorio de Quimica de Plasmas y Estudios Planetarios, México City DF 04510, México; et al. Pages 171-174.
- Lightning is a transient atmospheric event that can be petrified when it strikes the ground, heating, melting, and fusing the sand in soils to form glass tubes known as fulgurites. Fulgurites found in the Libyan Desert, the hyperarid core of the Saharan Desert, indicate that the region received rain in the past. The discovery of gases such as carbon dioxide (CO2), carbon monoxide (CO), and nitric oxide (NO) trapped in glassy bubbles of fulgurites has provided clues on the ecological environment at the time of lightning strike. The gases are not of atmospheric origin but rather are the result of the oxidation of organic matter present in the soil. Isotopic analysis of the gases further reveals that the organic matter was produced by plants that adapted to live in hot and arid zones. The timing of fulgurite formation was obtained for the first time by thermoluminescence dating, indicating that the event took place 15,000 years ago, in the late Pleistocene. The results imply that the semiarid Sahel region, which is currently located at 17°N, reached at least to 24°N at that time. The results also demonstrate that fulgurite gases and luminescence geochronology can be used in quantitative paleoecology.
- Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: A flat-slab subduction model
Zheng-Xiang Li, University of Western Australia, School of Earth and Geographical Sciences, Tectonics Special Research Centre, Crawley WA 6009, Australia; et al. Pages 179-182.
- In a soon-to-be dinosaur-infested continent covered by calm sea water, a mountain belt emerged out of water and swept from one end of the continent almost to the other like a weather front. Where the mountain belt had just swept through, newly emerged mountain ranges again submerged into a shallow sea, only to re-emerge soon afterward as volcanos started to erupt over the continent. All this happened in a time interval of 100 million years in South China. New evidence suggests that such dramatic continental-scale vertical movements were caused by horizontal subduction of an oceanic plateau, which may have occurred more often in Earth's history than people have realized.
- Seismicity preceding volcanic eruptions: New experimental insights
Luigi Burlini, ETH, Earth Sciences, Zurich 8092, Switzerland; et al. Pages 183-186.
- Burlini et al. describe the microseismicity associated with movement of melt inside a rock sample during high-temperature and high-pressure laboratory experiments. This microseismicity was observed to be very similar to earthquakes recorded in volcanic area. This suggests that the fundamental mechanisms producing the seismicity in the laboratory and nature are the same and therefore can be precisely calibrated in laboratory. That will have important consequences for the understanding of the volcanic activity, which in turn will improve volcanic hazard assessment.
- Mesoscale coastal behavior related to morphological self-adjustment
J.A.G. Cooper, University of Ulster, Environmental Studies, Northern Ireland BT52 1SA, UK; et al. pages 187-190.
- A study of a sandy beach in northwest Ireland during the past two centuries shows periods of rapid and spectacular erosion followed by periods of recovery when new dunes are formed. The erosion is of great concern to local residents, and there is much pressure to do something. The cycle between one erosion period, subsequent recovery, and the onset of the next erosion period takes about 50 years, which pushes the limits of human memory and is therefore difficult to interpret on the basis of human experience. The changes occur as a result of movements in an adjacent tidal inlet and involve the movement of sand within a rocky embayment. Despite the appearance of erosion, there is no net loss of sand: it is moved within the system between ebb deltas and the beach at various times. The movements of sand seem to occur independently of storms and are driven by natural thresholds in the beach system itself. This kind of shoreline behavior that produces massive erosion is not considered in projections of coastal response to future global climate change and sea-level change.
- Birth of a mud volcano: East Java (29 May 2006)
Richard J. Davies, Centre for Research into Earth Energy Systems (CeREES), Department of Earth Sciences, University of Durham, Science Labs, Durham DH1 3LE, UK; et al.
- Mud Volcanoes under Scrutiny: Mud volcanoes form from the eruption of a fluid-mud mix and represent a major way by which fluids are buried to depth and returned to Earth's surface (recycled). Mud volcanoes are not commonly recognized in the geologic past but are known in recent, active settings. The mechanisms of eruption are not well understood. In the February issue of GSA Today, Richard Davies and colleagues report on an unusual occurrence in eastern Java, where a new mud volcano (Lusi) has erupted in an area of active petroleum exploration so that the nature of the rock layers from which the mud was sourced can be constrained by drill data. The mud volcano (known as "pioneer") started to form on 29 May 2006 with the eruption of a very dilute mix of hot water, steam, and mud, and has been in a more or less continuous state of eruption. Because it is so dilute, it has buried a significant area, including four villages and 25 factories, and has contributed to 13 fatalities. The Lusi volcano adds new information as to how mud volcanoes form as well as demonstrating their potential as hazards.
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