||2 January 2007
GSA Release No. 07-01
Boulder, CO - Topics include: evidence linking Holocene climate change in the southwestern U.S. with variations in solar output; impacts of climate change on Pleistocene vertebrates in Australia; underwater exploration of an active seafloor volcano approximately 300 miles from the Oregon coast; a 45,000-year record of penguins and climate change in Antarctica's Ross Sea; and new insights into the Appalachian Mountains' aging process.
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- Solar forcing of Holocene climate: New insights from a speleothem record, southwestern United States
Yemane Asmerom, University of New Mexico, Department of Earth and Planetary Sciences, Radiogenic Isotope Laboratory, Albuquerque, NM 87131-1116, USA; et al. Pages 1-4.
- Although climate over the past 10,000 years has been relatively stable compared to previous interglacial periods, there has been significant enough variability in climate to cause major changes in ecology and cultural activities. The cause of climate change during the Holocene is not well understood. In this study, Asmerom et al. report the first high-resolution record of climate change for the southwestern United States for the entire Holocene. Asmerom et al. reveal a strong link between Holocene climate change in the Southwest and variations in solar output. Increased solar radiation correlates with decreased rainfall in the Southwest, the opposite to that observed in the Asian monsoon. They suggest that a solar link to Holocene climate occurs through changes in the atmospheric circulation of the tropical Pacific Ocean. The results from this study have direct relevance to the current debate on global warming. The effect of global warming on regional climate is of critical importance. Long-term data, such as that presented by Asmerom et al., can shed light on the issue. Based on this study, if increased greenhouse-driven global warming leads to conditions similar to periods of high solar activity, it would likely result in sustained extreme arid conditions in the desert Southwest and wetter conditions — and thus greater flooding events — in the monsoon regions of Asia.
- Dating young basalt eruptions by (U-Th)/He on xenolithic zircons
Madalyn S. Blondes, Yale University, Geology and Geophysics, New Haven, CT 06511, USA; et al. Pages 17-20.
- Accurate ages of young volcanic eruptions are important for geomorphic, tectonic, climatic, and hazard studies. Blondes et al. have developed a new technique for dating such eruptions using rocks that become entrained in the rising magma (xenoliths). The (U-Th)/He method is a radioisotopic thermochronometer, which Blondes et al. use to date ancient heating events from the 'low' temperatures of burial and exhumation of rocks, to the high temperatures of magma crystallization. Here, they use this method on zircon crystals found in xenoliths to date four different volcanic centers. In some cases, the eruptions can be dated precisely, but in others, the zircons were not entrained in the hot magma long enough to cause complete resetting of the thermochronometer. In these cases, Blondes et al. can estimate the duration of xenolith entrainment, and therefore the ascent rates of the magmas. Thus, the (U-Th)/He method on xenolithic zircons is a useful new tool for both dating eruptions and calculating magma ascent rates.
- Little Ice Age drought in equatorial Africa: Intertropical Convergence Zone migrations and El Niño-Southern Oscillation variability
J.M. Russell (corresponding author), Brown University, Geological Sciences, Providence, RI 02912, USA; +1-401-863-6330; and T.C. Johnson, University of Minnesota, Duluth, Large Lakes Observatory and Department of Geological Sciences, Duluth, MN 55812, USA. Pages 21-24.
- Climate researchers have long known that northern Europe experienced exceptionally cool conditions during much of 1500-1800 A.D. This time interval, often referred to as the 'Little Ice Age' also appears to have been marked by significant climate anomalies in tropical regions; however, scientists are only beginning to understand the mechanisms by which climate anomalies in these regions are linked. New research by Russell and Johnson, using geochemical analyses of well-dated sediments from Lake Edward (a central African rift lake), shows that central Africa experienced severe drought during the Little Ice Age. This finding contrasts with paleoclimate records from further east in tropical Africa, and suggests significant regional variations in climate during the Little Ice Age. By comparing paleoclimate records that extend through the Little Ice Age at sites throughout the tropics, Russell and Johnson argue that tropical climate anomalies during the Little Ice Age resemble the pattern of tropical rainfall and temperature anomalies experienced during an El Niño event in the Pacific Ocean. This finding suggests that coupled ocean-atmosphere variability in the tropical Pacific Ocean may play an important role in connecting tropical and polar climate anomalies. Moreover, the results of Russell and Johnson suggest that the climate of central equatorial Africa has been wetter and more stable over the past century than at any time during the preceding millennium. This finding implies that central Africa is overdue for a return to prolonged drought that would severely disrupt developing East African economies.
- Faster seafloor spreading and lithosphere production during the mid-Cenozoic
Clinton P. Conrad (corresponding author), Johns Hopkins University, Department of Earth and Planetary Sciences, Baltimore, MD 21218, USA, and Carolina Lithgow-Bertelloni, University of Michigan, Geological Sciences, Ann Arbor, MI 48109 USA. Pages 29-32.
- Earth's surface is composed of roughly 13 tectonic plates that move with average speeds of 2.5 centimeters per year. Spreading of these plates beneath the oceans creates new ocean floor at the rate of about 2.8 square kilometers per year. Using an analysis of the age distribution of the ocean floor, Conrad and Lithgow-Bertelloni show that this rate of seafloor production was 18 to 25% higher as recently as 20 million years ago. The additional seafloor production in the past was caused by a long, rapidly spreading ridge off the west coast of North America. This ridge, known as the Farallon ridge, began sinking beneath the western United States and Canada beginning about 30 million years ago. Faster seafloor production in the past explains the observed trends of decreasing sea level, decreasing atmospheric carbon (and associated cooling), and increasing Mg/Ca ratio in seawater during the Cenozoic (past 65 million years). Thus, these environmental trends may have been caused by the gradual disappearance of this Farallon spreading ridge.
- Mammalian responses to Pleistocene climate change in southeastern Australia
Gavin J. Prideaux, Western Australian Museum, Department of Earth and Planetary Sciences, Perth, WA 6000, Australia; et al. Pages 33-36.
- The differentiation of past climatic and human impacts is of central relevance in the quest to resolve the causes of the late Pleistocene megafaunal extinctions, arguably the most widely and consistently controversial topic in the Quaternary sciences. Until we establish how communities responded to climatic changes prior to the arrival of humans, separating the relative effects of humans and climatic changes during intervals of megafauna/human overlap will remain highly contentious. Australia lost 90% of its large animals within ~20,000 years of human arrival, but despite ever-expanding datasets from the overlap interval, recent analyses continue to present markedly different interpretations. Prideaux et al. provide the first evidence from Australia for the longer-term effects of glacial-interglacial cycling on a Pleistocene mammal fauna. The Naracoorte Caves World Heritage Area contains the richest assemblage of Pleistocene vertebrates anywhere in Australia, and is one of very few localities on Earth known to preserve a composite vertebrate record spanning more than 500,000 years. What makes this record more remarkable is that it can be directly compared to the long, detailed record of effective precipitation generated from speleothem (e.g., stalagmites) growth in these caves. The data reveal that although specie densities in the Naracoorte region fluctuated in concert with glacial-interglacial climatic cycling, the overall faunal composition remained remarkably stable during the half million years before the extinctions that occurred about 45,000 years ago. Because the Naracoorte megafauna perished under climatic conditions similar to those under which they had previously thrived (relatively wetter intervals), this finding undermines the argument for climatic change as the main cause of extinction, and simultaneously demonstrates that there was no long-term trend of faunal 'attenuation' or staggered extinction leading up to their disappearance. This discovery represents a fundamental advance in understanding the influence of climate change on terrestrial vertebrate communities.
- No change in the neodymium isotope composition of deep water exported from the North Atlantic on glacial-interglacial time scales
Gavin L. Foster, University of Bristol, Department of Earth Sciences, Bristol, Avon BS8 1RJ, UK; et al. Pages 37-40.
- Determining past patterns of ocean circulation is crucial for our understanding of the modern climate system. Proxy records, such as the Nd isotopic composition of marine precipitates, are very valuable in this regard because they allow the vigor and architecture of ocean circulation to be determined during times for which no observational records exist. Here, Foster et al. use laser ablation analysis of ferromanganese crusts to show that the Nd isotopic composition of the northern Atlantic end member has remained roughly constant over the last 500,000 years or so — an observation that greatly simplifies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past.
- Climate and vegetation in southeastern Australia respond to Southern Hemisphere insolation forcing in the late Pliocene-early Pleistocene
J.M. Kale Sniderman, Monash University, School of Geography and Environmental Science, Monash, Victoria 3800, Australia; et al. Pages 41-44.
- Glacial cycles during the late Pliocene and early Pleistocene (2.6-0.78 million years ago) were strongly dominated by the 41,000-year obliquity cycle, but the response of terrestrial vegetation to orbital climate forcing during this interval is poorly understood. Sniderman et al. present a pollen record indicating that cyclic changes in terrestrial vegetation were not primarily related to glacial cycles in southeastern Australia between ~1.8-1.5 million years ago, but were a direct response to local insolation, which varies with a 23,000-year period. Although the world's oceans bear the signature of the growth and decay of high-latitude ice sheets during the late Pliocene and early Pleistocene, this glacial cyclicity apparently did not yet control the climate development of mid-latitude to subtropical continents, at least in the Southern Hemisphere.
- Porosity of the upper edifice of Axial Seamount
Lisa A. Gilbert, Williams College and Mystic Seaport, Maritime Studies Program, Mystic, CT 06355, USA; et al. Pages 49-52.
- Using the deep sea submersible, Alvin, Gilbert et al. explored Axial Seamount, an active seafloor volcano in the North Pacific, approximately 300 miles off the coast of Oregon. Geologists know very little about the internal structure of seamounts. Gilbert et al. explored Axial's caldera floor, about a mile underwater, and made detailed measurements of seafloor gravity. Using the known relationships between gravity, density, and porosity, they found that the ocean crust atop this seamount is highly porous. On the caldera floor and bounding walls, Gilbert et al. observed cavities, lava tubes, cracks, and long fissures several meters wide. These types of water-filled cavities and cracks are inferred to comprise one-quarter to one-third of the upper 100 meters of Axial Seamont. This study has broad implications for the unknown number of hydrothermally active seamounts throughout Earth's oceans.
- Chronology of Miocene-Pliocene deposits at Split Mountain Gorge, Southern California: A record of regional tectonics and Colorado River evolution
Rebecca J. Dorsey, University of Oregon, Geological Sciences, Department of Geological Sciences 1272, University of Oregon, Eugene, OR 97403-1272, USA; et al. Pages 57-60.
- Dorsey et al. document the sedimentology, paleomagnetism, micropaleontology, and age of late Miocene to early Pliocene sedimentary rocks exposed in Split Mountain Gorge, a deeply eroded canyon in the western Salton Trough of Southern California. This location is important because it provides a stratigraphic record of deposition along the Pacific-North America plate boundary, and thus allows scientists to address long-standing questions about the timing and significance of regional tectonic and geomorphic transitions in southwestern North America. Dorsey et al. correlate magnetic reversals to the global chronology of reversals in Earth's magnetic field using information from microfossils and constraints from previous studies in overlying deposits. They find that deposition began at about 8.1 million years ago, and provides the earliest record of crustal extension and normal faulting in this region. Transgression of marine waters over late Miocene alluvial fan deposits occurred around 6.3 million years ago. Dorsey et al. believe this records acceleration of the relative motion between the Pacific and North American plates along the Gulf of California-Salton Trough corridor, an important tectonic change that may have been modulated by a highstand in global sea level at about the same time. The first appearance of Colorado River sand in this section is dated at about 5.33 million years ago. This timing suggests rapid propagation of the river into the Salton Trough, and supports a new hypothesis for initiation of the lower Colorado River via a series of downward-propagating catastrophic floods and lake spillover events.
- A 45,000 yr record of Adélie penguins and climate change in the Ross Sea, Antarctica
Steven D. Emslie, University of North Carolina-Wilmington, Biology and Marine Biology, Wilmington, NC 28403, USA; et al. Pages 61-64.
- The Adélie penguin is the most abundant penguin species found in Antarctica and does not exist outside this region. This species nests on ice-free coastal areas and uses pebbles to build nests where both parents raise the chicks. A colony of penguins may use the same nesting site for hundreds to thousands of years, resulting in an accumulation of nest pebbles, bones, eggshell, and food remains from guano. All this material preserves well in the cold dry climate that characterizes the Antarctic. Emslie et al. excavated and sampled 23 abandoned nesting colonies of Adélie penguins to determine how long the colonies had been occupied and why they were abandoned. Comparison of 62 radiocarbon dates on remains found in these sites with the paleoclimatic record from ice cores and marine sediments reveals that penguins periodically occupied and abandoned coastal regions in the Ross Sea for millennia, in response to changing sea ice conditions and the expansion/retreat of the Ross Ice Sheet. Emslie et al. report the oldest radiocarbon dates now known for Adélie penguins: ~45,000 years old. These dates document sites occupied by Adélie penguins prior to the last advance of the Ross Ice Sheet at the end of the Pleistocene, and indicate that open water conditions must have existed in the Ross Sea at that time. In addition, radiocarbon data reveal that most of the active penguin colonies today are no older than 1000-2000 years old, demonstrating how dynamic population movements in Adélie penguins are in this constantly changing region of the world. Finally, this study also provides insight into how this ice-obligate species may respond to current warming trends in Antarctica and the subsequent reduction in sea ice extent.
- Stalagmite evidence for the precise timing of North Atlantic cold events during the early last glacial
Russell N. Drysdale, University of Newcastle, School of Environmental and Life Sciences, Callaghan, NSW 2308, Australia; et al. Pages 77-80.
- The occurrence of North Atlantic ice-rafting events during the early part of the last glacial stage (about 115,000-110,000 years ago) is well documented in marine cores. However, the ages of these events have not been determined by radiometric dating. Drysdale et al. show that North Atlantic cold events C23 and C24 were recorded in a cave stalagmite from Italy. Uranium-thorium (U/Th) dating of the stalagmite reveals the first precise ages for these events. These new ages provide important new tuning points for studies of ice, marine, and pollen cores.
- Processes controlling water and hydrocarbon composition in seeps from the Salton Sea geothermal system, California, USA
Henrik Svensen, University of Oslo, Physics of Geological Processes, Department of Physics, Oslo 0316, Norway; et al. Pages 85-88.
- Research in basic energy and related sciences is becoming ever more important as industrialized societies expand their energy consumption. The Salton Sea (Southern California) geothermal area is a well-documented site of geothermal activity that is being exploited to generate electricity. Hot waters and gases leak out of the sediments, causing seepage at the surface in several places near the southeast shore of the Salton Sea. The gases are dominated by carbon dioxide and also include hydrocarbons derived from the rapid heating of organic matter at depth. Seep waters are a mixture between deep and shallow water sources. Svensen et al. evaluate the composition of the seep waters and also document the first known instance of liquid petroleum in these seeps. Its composition suggests hydrothermal formation. The results have relevance for understanding the formation of petroleum in sedimentary basins with magmatic intrusions and may also be applied to understanding submarine analogs such as the Guaymas Basin in the Gulf of California.
- Summit erosion rates deduced from 10Be: Implications for relief production in the central Appalachians
Gregory Hancock (corresponding author), College of William and Mary, Department of Geology, Williamsburg, VA 23187, USA; and Matthew Kirwan, Duke University, Nicholas School of the Environment and Earth Sciences, Durham, North Carolina 27708, USA. Pages 89-92.
- It has long been thought that the Appalachian Mountains (eastern United States) represent a Himalaya-like mountain range that were once more spectacular than the Himalayas, but are now showing signs of old age as they are slowly being erased by erosion. Past work has suggested that high summits in the unglaciated central Appalachians were rapidly worn down during the cold climates associated with the ice ages, leading to further reduction in the elevation and overall relief of the range, and further subduing this ancient topography. Hancock and Kirwan's research points to something quite different, however. They used the cosmogenic radionuclide 10Be to make the first direct measurements of erosion rates on a high summit in the central Appalachians, and have found the rate of erosion to be very slow. So slow, in fact, that the surrounding river valleys are eroding much more quickly. Contrary to the range becoming more subdued and less rugged, these results indicate an increase in the overall relief of the central Appalachians, as river valleys erode at rates faster than the adjacent hilltops. Hancock and Kirwan suggest that, rather than cold climates helping to bring down summits, the transition to a colder and stormier climate during the late Cenozoic may in fact be helping to make this ancient mountain range more rugged.
- Imaging the pulsing Iceland mantle plume through the Eocene
Craig J. Parkin and Robert S. White (corresponding author), University of Cambridge, Earth Sciences, Bullard Laboratories, Cambridge, Cambridgeshire CB2 5BG, UK; et al. Pages 93-96.
- Present-day Iceland consists almost entirely of volcanic rocks, and owes its existence to a hot mantle plume with a temperature of about 1400 °C extending hundreds of kilometers beneath it. By mapping the variations in thickness of the oceanic crust in the North Atlantic, Parkin et al. were able to map very small fluctuations in mantle temperature (as little as 25 °C) that happened up to 50 million years ago. Even these small mantle-temperature variations may affect global climate by periodically elevating the seabed near Iceland and choking the flow of the Arctic current over it, thereby limiting the return flow of the North Atlantic Drift, which provides a dominant control of climate in the Northern Hemisphere.
- John Perry's Neglected Critique of Kelvin's Age for the Earth: A Missed Opportunity in Geodynamics
Philip England, Department of Earth Sciences, Oxford University, Oxford OX1 3PR, UK; et al.
- How old is the Earth? James Hutton, the father of modern Geology, first posed this question in 1786 by noting that in the rock record "we find no vestige of a beginning and no prospect of an end." How, then, did we come to our present knowledge of the Earth having formed about 4.55 billion years ago? This is the question that is addressed in the January 2006 GSA Today science article by Philip England of Oxford and his colleagues, Peter Molnar of Colorado and Frank Richter of Chicago. They focus on the monumental battle waged between geologists and physicists throughout much of the nineteenth century. Physicists, led by Lord Kelvin, used physical first principals to arrive at estimates of the age of the Earth that were far too young. Geologists, on the other hand, adhered to a doctrine of a steady state Earth of indefinite age. England and his colleagues demonstrate that it was the resolution of this battle that sowed the seeds for today's multidisciplinary approach to understanding the Earth.
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