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GEOLOGY
Alabama Gulf Coast groundwaters: 4He and 14C as groundwater dating tools
Anne Carey, The Ohio State University, Dept. of Geological Sciences, Columbus, OH 43210-1398, U.S.A., et al. Pages 289-292.
Researchers at The Ohio State University, Columbus, Ohio and the University of Rochester, Rochester, New York have completed a study of groundwaters in Gulf Coastal Baldwin County, Alabama. The groundwaters there are the sole source of fresh water in a major tourist destination on Alabama's Gulf Coast. The study used two different geochemical methods to determine the time since the groundwater was naturally recharged by rainfall. These two methods, determination of the stable isotope of helium, helium-4, and of radioactive carbon, carbon-14, agreed well. This is the first time these two methods have been used on the same groundwater samples and showed good correspondence between the results of the two methods. The groundwaters in this sandy coastal aquifer ranged in age of greater than 50 years to as old as 7500 years. The study was completed under funding from the U.S. Department of Energy's National Institute for Global Environmental Change and is contributing to the understanding of water resources in coastal regions and how these resources may change with future climate change.
Preeruption history of the Grande Ronde Formation lavas, Columbia River Basalt Group, American Northwest: Evidence from phenocrysts
Gautam Sen and Sedelia Durand, Florida International University, FCAEM, Earth Sciences, Miami, FL 33199, U.S.A. Pages 293–296.
Origin and eruption of basaltic lava floods is of interest to all planetary geologists. Columbia River Flood basalts erupted through the Columbia plateau in the northwestern United States ca. 17 Ma. There has been considerable debate about the origin of these basalt magmas. Although past authors have generally favored their origin by melting of the so-called Yellowstone plume, the very existence of this plume has been debated in recent years. One of the two recent hypotheses about the origin of Columbia River basalts has called for a rather unusual eclogite source in the plume, while the other has appealed to unusually water-rich magmas shooting up through the entire crustal column. Our paper uses mineral-chemical, textural, and phase equilibrium evidence to show that both of these hypotheses are incorrect in that the erupted lavas are not primary mantle melts but they all underwent efficient mixing and partial crystallization within the shallow crust.
Glacial-interglacial-scale paleoclimatic change without large ice sheets in the Oligocene of central Oregon
Gregory Retallack, University of Oregon, Department of Geological Sciences, Eugene, OR 97403-1272, U.S.A., et al. Pages 297–300.
A conventional explanation for the big swing in climate over the past 100,000 years is that the glaciers that encased Chicago and most of North America in a kilometer of ice amplified small changes in the amount of energy from the sun due to minute changes in Earth's orbit. Now it has been demonstrated that paleoclimatic swings of comparable magnitude occurred in central Oregon 30 million years ago when there were no ice sheets. The amplifier of changes in solar energy then is more likely to have been changing carbon balance of plant and animal communities than the effects of large ice sheets. Changing vegetation patterns may not be merely a passive result of paleoclimatic change, but can play a role in paleoclimatic change through greenhouse gases such as carbon dioxide. Our burning of fossils fuels may not be the first time that organisms have played a role in global climate change.
Large magnitude transient strain accumulation on the Blackwater fault, eastern California shear zone
Michael Oskin, University of North Carolina, Chapel Hill, Geological Sciences, Chapel Hill, North Carolina 27599, U.S.A., and Alexander Iriondo, University of Colorado at Boulder, Geological Sciences, Boulder, Colorado 80309, U.S.A. Pages 313-316.
A study of 3.6 million-year-old lava flows offset along the Blackwater Fault in the northern Mojave Desert reveals that recent earthquakes in the region may have increased the fault's earthquake hazard. The results also indicate the Blackwater fault has a much slower long-term deformation rate than previously thought. Rapid deformation rates in excess of 5 mm/yr (1/5 in/yr) across the Blackwater fault, located in the northern Mojave Desert of eastern California, have been attributed to either high geologic fault slip rates or accelerated fault loading. Modern satellite observations shed unprecedented detail on present-day deformation from loading of active earthquake faults. But recognizing accelerated fault loading requires context from longer-term geologic fault slip rate. This study documents much slower long-term slip rate of 0.5 mm/yr (1/50 in/yr) for the Blackwater fault from offset volcanic rocks northwest of Barstow, California. This evidence supports that high fault loading rates are transitory in the Mojave Desert, perhaps related to a cluster of earthquake activity that includes the 1992 Landers and 1999 Hector Mine earthquakes. This research also documents little geologic evidence for recent pre-historic earthquake activity on the Blackwater fault. Thus it appears that the Blackwater fault has not generated an earthquake during this most recent cluster of activity and that transient loading has increased its near-term earthquake hazard.
Holocene Climate in the Atlantic sector of the Southern Ocean: Controlled by insolation or oceanic circulation?
Simon Nielsen, Norwegian Polar Institute, Polar Climate, Polar Environmental Center, Tromsoe, Norway, et al. Pages 317-320.
The authors present a decadal-scale record of sea-surface temperature and sea-ice presence spanning the past 12,500 years from the Polar Front of the eastern Atlantic Southern Ocean. The record shows generally gradual climate change, with centennial-scale cyclic changes lasting 150–1220 years. The record shows good correlation to records from East Antarctica and the Atlantic sector of the Southern Ocean. But the record shows out-of-phase behavior with regard to climate records from West Antarctica and the Pacific sector of the Southern Ocean; such behavior hints at a climatic divide between the Atlantic and Pacific sectors of the Southern Ocean, and between West and East Antarctica.
Isostatic constraints on the assembly, stabilization and preservation of cratonic lithosphere
Rebecca Flowers, et al., Massachusetts Institute of Technology, Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, U.S.A., et al. Pages 321-324.
Heat production concentration and distribution may play an important role in the assembly, stabilization, and preservation of the earliest continental crust. Heat production exerts significant control on crustal temperature distributions. Density change is dominantly controlled by temperature change, so that geologic uplift or subsidence reflects the net temperature change. The modest erosion depth of most ancient crust is consistent with a relatively cool initial temperature distribution. Our thermal analysis of the Proterozoic orogen of the southwestern United States suggests that heat production differences may explain the distinct isostatic and thermal histories preserved in the region's geologic domains.
Hydrogeologic processes of large-scale tectonomagmatic complexes in Mongolia–southern Siberia and Mars
Goro Komatsu, IRSPS, Universita' d'Annunzio, Viale Pindaro 42, Pescara, Abruzzo 65127, Italy, et al. Pages 325-328.
Large-scale tectonomagmatic complexes, represented by uplifted areas, rift systems, and widespread volcanism, are common on both Earth and Mars. These complexes are often related to magma plumes that source from the interior of the planets. Tectonism, magmatism, and magma-water/water-ice interactions associated with the development of large-scale tectnomagmatic complexes, which include the Mongolian plateau/Baikal Rift and Tharsis regions of Earth and Mars, respectively, have tremendous implications on the evolution of water. We introduce landforms of the Mongolian plateau and Tharsis bulge, which are the manifestations of such interlinked processes, including their relations to the paleohydrology (subsurface and surface) and paleoclimate/atmosphere of both dynamic, water-enriched planets.
Tidal Cycles in the Sediments of Santa Barbara Basin
W. Berger, University of California, San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093-0524, U.S.A., et al. Pages 329-332.
The finely layered sediments of Santa Barbara Basin are a valuable source of information about the history of the California Current and the history of the regional climate in general. They have attracted worldwide attention in this respect, as they show climate change patterns that are also seen in the North Atlantic realm. Detailed statistical study of the record of the past 800 years suggests that one important factor in making this record has been overlooked, that is, ocean tides. The influence of tides is seen especially in the abundance patterns of organic matter within the sediment, which is commonly ascribed to changes in productivity. The abundances of fish scales, in contrast, seem to follow different rules; their patterns suggest a link to solar activity.
Magma degassing buffered by vapor flow through brecciated conduit margins
Alison Rust, et al., University of Oregon, Geological Sciences, Eugene, OR 97403, U.S.A. Pages 349-352.
Magma degassing plays an important role in generating signals of an impending eruption as well as modulating the explosivity of volcanic activity. Thus, understanding mechanisms of volatile escape from magma and distinguishing among styles of degassing during magma ascent could assist interpretations of data from volcanic monitoring. In addition to generating pumice (bubbly glass) and ash (bubble wall shards), explosive eruptions of rhyolitic magma often produce lesser amounts of dense (low bubble content) glass called pyroclastic obsidian. These obsidian clasts preserve a range of water and carbon dioxide contents that have been used to infer syneruptive conditions of magmatic degassing. Here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins. Obsidian clasts often contain rock fragments, sheared bands of rock powder, and textures consistent with magma fracturing as it flowed. These features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and re-anneals during ascent. We use these observations to develop a degassing model for pyroclastic obsidian from the A.D. 1340 Mono Craters eruptions. We suggest that degassing was buffered by continual flux of vapor through highly permeable, brecciated magma along conduit walls. Taken together, the chemical and physical evidence suggest that magma brecciation along conduit walls aids the degassing of ascending rhyolite.
High-resolution subducting-slab structure beneath northern Honshu, Japan, revealed by double-difference tomography
Haijiang Zhang, University of Wisconsin-Madison, Department of Geology and Geophysics, Madison, WI 53706, U.S.A., et al. Pages 361-364.
Using a new seismic imaging method, Zhang and coworkers have derived a model of the subducting Pacific plate beneath northern Honshu, Japan, that reveals features never seen before. In this region, there are two separate zones of earthquakes within the down-going plate, separated by ~30 km. Zhang et al.'s image of the plate shows that it has a layered structure, with the two zones of seismicity having different seismic velocity characteristics, which in turn are different from the characteristics of the region between the zones of earthquakes. The authors provide hypotheses for the mineralogical makeup of these zones and discuss the origin of the accompanying earthquakes.
How do spiral troughs form on Mars?
Jon Pelletier, University of Arizona, Department of Geosciences, Tucson, AZ 85721, U.S.A. Pages 365-367.
The polar ice caps of Mars contain deep troughs that spiral outward from the poles. These troughs are perhaps the most dramatic and enigmatic landforms in the solar system. Building upon earlier work by Alan Howard, University of Arizona geologist Jon Pelletier has constructed a three-dimensional numerical model for the origin and evolution of the spiral troughs based on melting of the ice by solar radiation and the transfer of heat energy within the ice cap. This model helps explain the spiral structure and makes specific predictions for the size and shape of the troughs that match recent satellite measurements.
GSA TODAY
Are plutons assembled over millions of years by amalgamation from small magma chambers?
Allen F. Glazner, Department of Geological Sciences, , University of North Carolina, Chapel Hill, North Carolina 27599, USA, et al.
A provocative new study suggests that huge volumes of magma at shallow depth in the Earth, similar to the source of the gigantic eruption that formed the Yellowstone Caldera, are ephemeral and rare. This study helps explain geologists' inability to locate large magma bodies under modern volcanic areas. Allen Glazner and his colleagues propose that large plutons -- igneous rock bodies that solidify beneath Earth's surface -- commonly are patchworks that grew over time from many small additions of magma. This is supported by recent field evidence showing that many plutons are more heterogeneous than previously believed. Mathematical models of these underground blobs of molten rock indicate that large plutons should solidify in less than a million years, but recent age determinations show that they can take up to ten million years to grow. Glazner and colleagues conclude that large, eruptible bodies of magma may be uncommon features of the Earth.
To view the complete table of contents for the April issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.