||February 27, 2003
GSA Release No. 03-04
Boulder, Colo.—The Geological Society of America’s March issue of GEOLOGY contains potentially newsworthy items. Topics include: first chemical analyses of permafrost ice from high elevations in Antarctica and an analogue for Mars; new isotopic evidence of extraterrestrial chromium in the spherule beds of Barberton Greenstone Belt, South Africa; investigation of asynchronous climate change between hemispheres; and new data from an Andean cloud forest environment in Bolivia and implications for climate history of the Amazon basin.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns for copies of articles and for additional information or other assistance.
- Asynchronous climate change between New Zealand and the North Atlantic during the last deglaciation
Chris S.M. Turney, School of Archaeology and Paleoecology, Queen's University, Belfast, Northern Ireland, U.K., et al. Pages 223-226.
- To successfully predict the effects of future potential global warming it is critical to understand the pattern of past extreme climate change throughout the world. Here we show that the climate in New Zealand was the opposite of that in the North Atlantic region in the recent geological past. At the end of the last ice age, when the North Atlantic region warmed to temperatures comparable to present, New Zealand experienced partial cooling. Conversely, around 200 years later, when the North Atlantic experienced a mini-glaciation, a warming phase took place in New Zealand, consistent with the climate operating as a see-saw between the hemispheres. It seems likely therefore that the effects of future climate change may be more complex than previously suspected.
- Testing for fullerenes in geological materials: Oklo carbonaceous substances, Karelian Shungites, Sudbury Black Tuff
David Mossman, Department of Geography, Mount Allison University, Sackville, New Brunswick, Canada, et al. Pages 255-258.
- Diverse ancient geologic materials have been tested by highly precise mass spectrometric methods by D. Mossman and colleagues in a search for the carbon molecules known as fullerenes. Results confirm the presence of fullerenes in the Sudbury Basin meteorite impact structure but not in shungite samples from Russian Karelia, or in samples from the fossil nuclear fission reactors of Oklo, Gabon. The presence of fullerenes in geologic samples is likened to finding stardust on Earth.
- Missing mollusks: Field testing taphonomic loss in the Mesozoic through early large-scale aragonite dissolution
V. Paul Wright, Department of Earth Sciences, Cardiff University, Cardiff, UK, et al. Pages 211-214.
- Clams and snails are among the most common fossils from Mesozoic times onward, and variations in their abundance in rocks have been interpreted as reflecting major evolutionary or environmental changes. However, re-study of the classic Lower Jurassic of South Wales suggests that the diversity and abundance of these groups have commonly been greatly underestimated. Many have aragonitic shells that in shelf environments are removed preferentially by microbially triggered acidity. The wider effects on the fossil record of early and large-scale dissolution of shell aragonite resulting in "missing mollusks" suggest that it is much more difficult to assess biodiversity and ecological patterns in the past than we previously suspected.
- Early Archean spherule beds: Chromium isotopes confirm origin through multiple impacts of projectiles of carbonaceous chondrite type
Frank T. Kyte, Center for Astrobiology, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA, USA, et al. Pages 283-286.
- The oldest known impacts on Earth formed recognizable layers of sediment that are now found in rocks from the Barberton Greenstone Belt, South Africa. These layers, which are 3.2 to 3.5 billion years old, are called spherule beds because they contain millimeter-size silicate spheres that formed from liquid silicate droplets produced by impacts. Several researchers have disputed the impact origin of these spherule beds, but in this paper Kyte et al. provide isotopic evidence confirming that three of these beds contain extraterrestrial chromium (Cr). The Cr in these beds contains an excess of the isotope 53Cr, a characteristic of all meteorite materials, relative to rocks typical of Earth. The beds also contain an excess of the isotope 54Cr, a characteristic of a group of meteorites known as the carbonaceous chondrites. The three spherule beds in this study were all deposited over a 20 million year interval from about 3.26 to 3.24 billion years ago. All three impacts are believed to have been larger that the 10-km-diameter asteroid that was responsible for the Cretaceous-Tertiary mass extinctions. It is possible that two of them may have been 30- to 70-km-diameter impactors. This suggests that ancient Earth at this time was subject to bombardment at a rate much higher than in the more recent geological record.
- Limestone-marl alternations: A warm-water phenomenon?
Hildegard Westphal, Institut fur Geologie und Palaontologie, Universitat Hannover, Hannover, Germany, and Axel Munnecke, Institut fur Paleontologie, Universitat Erlangen, Erlangen, Germany. Pages 263-266.
- Through geologic time and on a global scale, limestone-marl alternations are most abundant in settings where (in analogy to the modern world) aragonite production and accumulation was favored. This includes shallow-water settings in the global warm-water belt as well as deeper-water settings to which aragonite was exported. On active margins, in areas of upwelling, in restricted epeiric seas, and in cool- to cold-water settings, in contrast, they are scarce to absent. These observations make limestone-marl alternations a new tool for the reconstruction of paleoclimate.
- Last glacial maximum in an Andean cloud forest environment (Eastern Cordillera, Bolivia)
Philippe Mourguiart, Institut de Recherche pour le Developpement, Universite de Pau et des Pays de l'Adour, Departement d'Ecologie, Anglet, France, and Marie-Pierre Ledru, Institut de Recherche pour le Developpement, Departmento de Geologia Sedimentar e Ambiental, Universidade de Sao Paulo, Sao Paulo, Brazil. Pages 195-198.
- We show here new data from an Andean cloud forest in Bolivia, which today is strongly dependent on Amazonian moisture. Lacustrine pollen records attest to a strong opening of the forest during the last glacial maximum. These data are in favor of an Amazon basin that was drier during this time than it is today.
- The role of H20 in subduction zone magmatism: New insights from melt inclusions in high-Mg basalts from central Mexico
Pablo Cervantes, Department of Geology and Geophysics, Texas A&M University, College Station, TX, USA, and Paul J. Wallace, Ocean Drilling Program, Texas A&M University, College Station, TX, USA. Pages 235-238.
- Volcanic chains like the Cascades and Andes are associated with offshore deep-ocean trenches where oceanic crust is returned (subducted) back into the deep mantle of Earth. This recycled oceanic crust contains water that is released as the crust is slowly heated by the surrounding hot mantle rocks. When the water is released it causes the mantle to melt, creating the magmas that erupt in the overlying volcanic chains. In this paper, we measure the water contents of magmas by using tiny inclusions of trapped melt in crystals from volcanic rocks in central Mexico. Our results provide exciting new information about how water is recycled from the subducted oceanic crust back into the mantle, and how the water causes melting.
- Antarctic permafrost: An analogue for water and diagenetic minerals on Mars
Warren W. Dickinson, Antarctic Research Centre, Victoria University, Wellington, New Zealand, and Michael R. Rosen, Institute of Geological and Nuclear Sciences, Taupo, New Zealand. Pages 199-202.
- This paper presents the first chemical analyses of ice in permafrost from high (>1000 m) elevations in Antarctica. Interpretation of the data suggest that the ice originated from atmospheric water vapor and from downward diffusion of brine formed by weathering at the surface and in the ground. It explains not only how the ice in permafrost can form in the absence of flowing water, but also how minerals may precipitate in frozen ground. This provides an analog for Mars and is relevant to three problems on this planet: (1) the occurrence and composition of ground ice;(2) weathering, which requires an aqueous phase for the precipitation of red-colored ferric-bearing minerals; and (3) the occurrence of recently discovered lithification of sand dunes.
- Evidence of microbial consortia metabolizing within a low-latitude mountain glacier
R. Kramer Campen, Department of Geosciences and EMS Environment Institute, Pennsylvania State University, University Park, PA, USA, et al. Pages 231-233.
- The search for life beyond Earth is a quest that captures the imagination. Once we move outside of the realm of imagination, however, the first question we are faced with is where to look. It seems to make sense in this quest to use Earth as a template: it seems reasonable to hypothesize that the environments that nurture life on Earth might do so on other planets. A multitude of questions then arise, one of which is "What are the temperature limits of life on Earth?" In the work described in this paper, we measured the composition of a number of trace gases in an ice core extracted from a relatively dusty glacier perched on top of a South American volcano. The concentrations of the bioactive trace gases (carbon dioxide, nitrous oxide and methane) were found to be systematically different than multiple records of similar age from various high-latitude locales. We consider each of the possible causes of these surprising measurements in turn and find that only the active metabolism of a consortium of microorganisms living within the ice can explain them. This work adds an additional voice to prior theoretical and experimental arguments which suggest that life may not only survive, but may also flourish at temperatures below 0 °C.
- Carbon-reservoir changes preceded major ice-sheets expansion at the mid-Brunhes event
Pinxian Wang, et al., Laboratory of Marine Geology, Tongji University, Shanghai, China. Pages 239-242.
- The glacial cycles during the last couple of million years are, as generally agreed, caused by the geometric changes of Earth's orbit around the sun. However, the largest amplitude ice-sheet expansion at ca.430 ka, known as the mid-Brunhes event, cannot be explained by small changes in orbital forcing. The recent studies on the deep-sea sediments from the southern South China Sea show that this large ice-sheet expansion was preceded by a significant disturbance of the carbon reservoir in the global ocean, as evidenced by the recorded 13C/12C ratio in microfossils. Around 500 ka, the ocean water was enriched in 13C, when the low latitudes witnessed heavy precipitation giving rise to enhanced weathering. Most probably, this resulted in changes of the coccolith/diatom ratio in phytoplankton, and therefore in the organic/inorganic ratio in carbon deposition and the 13C/12C ratio in water. Through a series of events, the 13C/12C ratio decreased, and this disturbance in carbon reservoirs led to ice-sheet expansion. This new finding may help to explain the lag of ice-volume change behind that of CO2 in ice cores, but challenges the prevalent wisdom of Arctic control of glacial cycles.
- Crustal trace of a hot convective sheet
Suleiman Al-Kindi, Bullard Laboratories, Cambridge, UK, et al. Pages 207-210.
- For some time it has been suspected that large quantities of frozen melt or magma are trapped beneath the British Isles. This melt was generated 60 million years ago by the upwelling of hot material deep within the convecting Earth. There are two reasons why we suspected that this frozen melt existed: first, there are traces of 60 million year old melt activity on the surface, and second, the act of trapping the bulk of the melt caused rapid uplift and erosion. In this article, we show for the first time direct evidence for the existence of the trapped melt. An echo sounding experiment was used to map the geometry of the frozen melt body and its dimensions agree with earlier predictions.
- Reactivation, trishear modeling and folded basement in Laramide uplifts: Implications for the origins of intra-continental faults
Alexander P. Bump, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA.
- An understanding of the processes of reactivation of old faults during younger tectonic events is a very important issue in continental tectonics. Continental crust, unlike oceanic crust, records the cumulative history of multiple periods of tectonism, and continental deformation appears to get focused into large-scale fault networks that are repeatedly reactivated over long time scales. This paper uses the unique geometries of the Laramide uplifts in the southwestern United States as a field laboratory to explore the importance of reactivation versus new fault development in response to Laramide contractional deformation. Late Cretaceous-early Tertiary Laramide uplifts are bounded by reverse faults of enigmatic origin. Two end-member hypotheses have been proposed: (1) they formed during the Laramide orogeny as newly formed contractional features; and (2) they formed as normal faults at some previous time and were reactivated during the Laramide. This paper employs the trishear fault/fold model to test these ideas. Forward and inverse modeling suggests that the faults began propagating only a few kilometers below the basement-cover contact, too shallow for a regional detachment and too deep for a simple reactivated fault. It is suggested that these faults represent the formation of footwall shortcuts (i.e., lower angle, mechanically easier paths to the surface) to bypass the steep upper sections of reactivated listric faults. This idea unites the two end member hypotheses, allowing the large-scale map pattern of uplifts to be controlled by reactivated faults at depth while exposing either neoformed offshoots of those faults or reactivated faults at the surface.
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Review the complete table of contents for this issue of GEOLOGY.