BOULDER, Colo. - The Geological Society of America's March issue of GEOLOGY contains a number of newsworthy items. Highlights include a discovery of a huge “cache” of nonmarine diatom fossils in northern Mexico; new data from the Arabian Sea that sheds light on erosion estimates for the Himalayas for the past ~30 my, one of the ramifications being the discovery that the monsoon may be radically older than thought--16 Ma rather than 8; an exploration of the role volcanic activity associated with rifting in the Central Atlantic had on the Triassic-Jurassic extinction event; and new evidence that the Greek Islands of Cyclades were once part of Africa. The Forum section this month includes a Snowball Earth spar with a comment from pro-Snowballist Paul Hoffman et al. on Martin Kennedy’s December article and a reply from Kennedy et al.
Highlights from GEOLOGY and a summary of the science article for the January GSA TODAY 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 at acairns@geosociety.org for copies of articles and for additional information or other assistance.
New perspective on Aptian carbon isotope stratigraphy: Data from δ13C records of terrestrial organic matter.
Atsushi Ando et al. Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan. Pages 227-230.
Carbon isotope (δ13C) analyses of detrital woody materials in Aptian (mid-Cretaceous) marine sediments of Hokkaido, Japan, exhibit a pronounced negative-positive excursion in the lower Aptian and a small positive anomaly in the uppermost Aptian. This δ13C profile is exactly in phase with that of marine carbonates on a Pacific guyot, indicating that δ13C compositions of Aptian marine and terrestrial carbon reservoirs changed simultaneously by the same amplitude within the ocean-atmosphere-biosphere system. Fluctuation patterns of δ13C curves of Tethyan pelagic limestones are slightly different from those of the Pacific and Hokkaido sections. Such differences in δ13C profiles may be attributed to local paleoceanographic setting of the Aptian Mediterranean Tethys.
70 Ma nonmarine diatoms from northern Mexico.
Elizabeth Chacón-Baca et al. Instituto de Geología, Universidad Autónoma de México, Circuito de la Investigación Científica, México D.F. 04510, México. Pages 279-281.
This manuscript reports for the first time the discovery of abundant and well-preserved diatoms in nonmarine sediments from a Late Cretaceous volcano-sedimentary sequence in Sonora, Mexico. This discovery nearly doubles the known fossil record of nonmarine diatoms. Further, given the morphological relationships of preserved forms to modern diatoms whose phylogenetic relationships have been ascertained by molecular analyses, the fossils show that by the time diatoms first became abundant constituents of the sedimentary record, they had already undergone considerable ecological and taxonomic diversification in both marine and nonmarine environments. This is evident in the Huepac chert where genera like Fragilaria, Tabellaria, Amphora, and Melosira are well represented in the fossil assemblage. In addition, this report will give some clues to early diatom evolution, which has been a debated issue. Insofar as diatoms are major primary producers in modern marine and lacustrine env! ironments, this report should be of interest to a wide spectrum of biologists and Earth scientists.
Accelerated mass flux to the Arabian Sea during the middle to late Miocene.
Peter Clift, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA, and Christoph Gaedicke, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany. Pages 207-210.
In this paper the authors present new geophysical data from the Arabian Sea that radically alter the present understanding of how rates of erosion in the western Himalaya have changed since ~30 Ma. This marine record parallels the believed time span of South Asian monsoonal strengthening ca. 8 Ma. However, erosion did not increase at that time, as might be expected during an interval when precipitation should have increased. Instead, it increased ca. 16 Ma, during a time of dramatic mountain building in the Karakoram, including the region of K2, and over much of southern and central Tibet. Thus the marine sediments provide a detailed and well-dated record of mountain building that has been eroded away onshore. Most importantly, because uplift of Tibet is believed to control the strength of the monsoon, this new study suggests that the monsoon may in fact be much older than typically believed today, i.e., 16 rather than 8 Ma. The new marine data reestablishes the proposed link between mountain building and climate change that had been cast into doubt by recent work suggesting a much older age for Tibetan uplift than monsoonal strengthening. Understanding what influence the solid earth has in triggering the major climatic systems of the planet is crucial if we are to understand the possible repercussions of human impact on climate evolution. Because the monsoon affects the lives of more than three billion people in Asia and East Africa and is directly tied to the development of the Himalaya and Tibet, it must be considered one of the most important examples of how the planet's evolution affects human society.
New geochemical evidence for the onset of volcanism in the Central Atlantic magmatic province and environmental change at the Triassic-Jurassic boundary.
Anthony Cohen and Angela L. Coe, Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK. Pages 267-270.
The 200 Ma Triassic-Jurassic boundary marks one of the “big five” extinction events in the history of life on Earth. As yet, the causes of the momentous environmental changes at that time remain unclear. The two main contenders are a meteorite impact, similar in scale to the one that caused the demise of the dinosaurs 65 Ma, or massive amounts of volcanic activity, or possibly both. The present study provides strong evidence, gathered through analysis of the record of seawater geochemistry at that time, suggesting that volcanic activity associated with rifting in the Central Atlantic was responsible for these sudden and major environmental perturbations at the Triassic-Jurassic boundary. Although the possibility of a meteorite impact is not excluded, a meteorite impact cannot explain the striking features of the data presented here.
Three-dimensional seismic imaging of Paleogene dike-fed submarine volcanoes from the northeast Atlantic margin.
Richard Davies et al. ExxonMobil Exploration Company, 233 Benmar, Houston, Texas 77060, USA. Pages 223-226.
In the 1980s much research was conducted to understand the structure, composition, and morphology of oceanic spreading centers using primarily sidescan sonar technology. This led to the discovery of groups of small circular volcanoes near to the spreading centers. Analysis showed that basaltic dikes that were extruding fragmental material onto the seabed were feeding the volcanoes. During the 1990s, high-quality 3-dimensional reflection data have been acquired over large areas of the continental crust in order to explore for hydrocarbons. In the Faeroe-Shetland Basin of the northeast Atlantic Margin, where these data have been acquired, there was significant basaltic magmatic activity at 55 Ma, when the proto-Icelandic plume appears to have been initiated. These 3-dimensional seismic data reveal unprecedented images of intrusive basaltic dikes and similar small extrusive volcanoes, which they fed. Vast swathes of 3-dimensional seismic data exist within the hydrocarbon indus! try. However, the common confidentiality of these data has meant that the immense potential for understanding geological processes and products, in this case igneous phenomena, is on the whole underutilized. This paper is one example of how these data can be used to address fundamental scientific issues.
Influence of soil development on the geomorphic evolution of landscapes: An example from the Transverse Ranges of California.
M.C. Eppes et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Pages 195-198.
This paper presents a new hypothesis for how soils and erosion, in addition to tectonic activity, can affect the expression of landforms that are related to earthquake-causing faults, particularly blind-thrust faults along the north flank of the San Bernardino Mountains in southern California. It was a blind-thrust fault that was responsible for the 1994 Northridge Earthquake in southern California that caused ~$20 million in damages. Until this and other recent earthquakes, these faults were not recognized as a significant seismic hazard risk, because, unlike other types of faults, they are not exposed at the surface of the earth. Instead, they typically manifest themselves as a warp or hill in the landscape. Workers have since learned to correlate the characteristics of these landforms (steepness, height, etc.) with degree of tectonic (potentially earthquake) activity and therefore to determine what hazard the underlying faults may pose. This paper describes an example of how erodibility of the soil being deformed by the fault, in addition to tectonic activity, affects the appearance of a blind thrust fault-related landform. Because this effect is in addition to tectonic activity of the fault, it can interfere with the interpretation of the seismic-hazard potential of the fault. For example, the paper describes a fault that was previously unrecognized, because the landform that was associated with it was forming in highly erodible granite soils. The idea that soil development and associated erosion can significantly affect the appearance of a tectonic landform will be of immediate use to workers endeavoring to characterize the seismic-hazard potential of blind-thrust faults.
Spatial coincidence of rapid inferred erosion with young metamorphic massifs in the Himalayas.
David P. Finlayson et al., Quaternary Research Center and the Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Pages 219-222.
A rate-of-erosion index based on models of bedrock river incision is used as proxy for long-term fluvial erosion throughout the Himalayas. The resulting erosion patterns inferred for the Himalayas reveal a correspondence between areas of high erosion and young metamorphic massifs and structural culminations. The heterogeneous distribution of erosion patterns in the Himalayas contrasts with the uniform convergence of the Indian subcontinent. If these patterns in erosion are persistent features of the Himalayas, then they support the emerging view of a positive feedback between rapid-erosion centers and lower crustal advection.
Rhizolith evidence in support of a late Holocene sea-level highstand at least 0.5 meters higher than present at Key Biscayne, Florida.
Carl R. Froede Jr., U.S. Environmental Protection Agency, Region 4, Atlanta, Georgia 30303-8960, USA. Pages 203-206.
Establishing sea-level position for south Florida over the past 5,000 yr has been controversial. Fossilized roots exposed at low tide on the northeastern side of Key Biscayne, Florida, have previously been interpreted as the remains of a former nearshore black mangrove forest. However, a recent reexamination of these fossilized roots suggests that they are actually the remains of sea grass. This new interpretation infers a sea-level position at least 0.5 meters higher than present at Key Biscayne, Florida, between 1 and 2 ka. Coastal dune ridges and paleo-Indian artifacts found at certain locations in south Florida also support this higher-than-present sea-level interpretation.
Paleomagnetism Indicates No Neogene Rotation of Qaidam Basin in Northern Tibet during Indo-Asian Collision.
Guillaume Dupont-Nivet et al. Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 263-266.
Formation of the Tibetan Plateau has been ongoing for the past 50 m.y., since the initial continent-continent collision between India and Asia. The Qaidam Basin, characterized by relatively smooth topography at elevations lower than the surrounding 5000-meter mountain ranges, covers a large part of the northern plateau. Although a key element in the development of the Tibetan Plateau, the motion history of the Qaidam Basin during the Indo-Asian collision is poorly constrained. In this study, the authors examined ancient geomagnetic field directions recorded in Qaidam Basin rocks during their formation ca. 30 Ma. At two separate locations within the basin, these directions were found to be similar to directions expected for rocks of this age at this location within Asia. These results indicate that the Qaidam Basin is behaving as a large coherent block that has not experienced rotation about a vertical axis during the past 30 m.y. These observations suggest that the strong crust of the Qaidam Basin is being translated eastward while weaker surrounding crust is being deformed into high mountains.
Baltica upside-down: A new plate-tectonic model for Rodinia and the Iapetus Ocean.
Ebbe Hartz, Department of Geology, University of Oslo, 0316 Oslo, Norway, and Trond H. Torsvik, Vista, c/o Geological Survey of Norway, 7004 Trondheim, Norway. Pages 255-258.
The authors propose that Baltica (Scandinavia and European Russia) was geographically inverted throughout the late Precambrian. This new paleoreconstruction contrasts with classic models suggesting that western Norway rifted away from East Greenland in the late Precambrian (ca. 570 Ma), along the same margins that collided during the Caledonian orogeny ca. 420 Ma. This classic model picturing an ancient Atlantic-like ocean that opened and closed long before the current Atlantic Ocean existed revolutionized plate-tectonic thinking in the 1960s and has represented one of the mainstays of plate reconstructions. However, several authors have noticed that the late Precambrian geology of East Greenland and Scandinavia does not match, and the new model thus proposes that it was the Russian side of Baltica (the continental margin that later became the Ural Mountains) that faced East Greenland in the Precambrian. Flipping Baltica upside-down can be justified using paleomagnetic data. Because Earth’s magnetic field reverses with time, we do not know if magnetic fabric recorded in rocks pointed north or south. The new reconstruction dismisses the need for a near instant 180° rotation of Baltica after rifting from Greenland and presents more plausible correlations of geologic events (opening and destruction of oceans) in Baltica, Laurentia (Greenland and North America) and Gondwana (Africa, South America, and parts of Europe and easternmost North America) than those accompanying previous fits.
African provenance for the metasediments and metaigneous rocks of the Cyclades, Aegean Sea, Greece.
Sue Keay, Department of Earth Sciences, University of Queensland, St. Lucia, QLD 4072, Australia, and Gordon Lister, Australian Crustal Research Centre, Monash University, Clayton, VIC 3128, Australia. Pages 235-238.
Researchers have found evidence that a group of Greek Islands, known as the Cyclades, were once part of Africa and that the rocks of these islands share a common history with those of North Africa. The Greek Islands are at the center of the ongoing continental collision between Europe and Africa that has produced the vast Alpine mountain chain. Understanding the exchange of material between these two huge continental landmasses is important if we are to reconstruct the configuration of the continents through time. New data on the age of the resistant mineral zircon in rocks from the islands not only link the islands to Africa but also fill in some of the gaps in our knowledge of the origin of this crustal material in the 3.5 b.y. of earth history before the formation of the Alpine-Himalayan mountain chain.
Folding within seconds.
Thomas Kenkmann, Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität Berlin, D-10115 Berlin, Germany. Pages 231-234.
Asteroids and comets larger than ~200 m diameter are capable of forming complex impact craters on Earth. Typical deformation patterns of craters of 5-15 km diameter are compiled and analyzed with respect to their kinematical development. Impact-induced deformation features, like folds and brittle fault zones, have many similarities to tectonic structures. Unlike their tectonic counterparts, deformation structures are formed in a single event that takes place in seconds to minutes. To understand the fast deformation processes, the microstructure of an impact-induced fold is investigated in detail.
Paleodeviations of radiocarbon marine reservoir values for the northeast Pacific.
D.J. Kovanen, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada, and Donald Easterbrook, Department of Geology, Western Washington University, Bellingham, Washington 98225, USA. Pages 243-246.
Differences in the radiocarbon (14C) ages of contemporaneous marine shells (which reflect surface ocean conditions) and wood (which reflects the 14C content of the atmosphere) from emergent glaciomarine sediments in the Fraser Lowland of southwestern British Columbia and northwest Washington indicate that a total marine radiocarbon reservoir value of ~-1100 14C yr is applicable in the region. This is 40% greater (300 yr) than the -800 yr value currently used to correct the ages of both modern and late Pleistocene marine shells from this area. The glaciomarine sediments were deposited rapidly between ca. 12,500 and 11,500 14C yr B.P. The ability to constrain the age between them and other records (such as ice cores and terrestrial data) is of fundamental importance. The paleodeviations of the marine reservoir value cannot be explained by changes in the production of atmospheric radiocarbon alone and are therefore thought to reflect changes in both deep and surface ocean circulation and hence climate. The increase of the reservoir value may indicate a rapid reordering of oceanic-atmospheric circulation in the region during the last deglaciation.
Tropical climates in the game of two hemispheres revealed by abrupt climatic change.
Marie-Pierre Ledru et al. Institut de Recherche pour le Développement, Universidade de São Paulo, Departamento Geologia Sedimentar e Ambiental, rua do Lago 562, 05422-970 São Paulo, São Paulo, Brazil. Pages 275-278.
The climatically sensitive equatorial regions provide important information for evaluation of the phasing between high- and low-latitude climate variability. A high-resolution pollen record from northern Brazil demonstrates a significant, abrupt, and rapid environmental change associated with the Northern Hemisphere Younger Dryas temperature reversal. This finding is consistent with the model in which the Younger Dryas had a stronger influence on temperature in the Northern Hemisphere than in the Southern Hemisphere because of the larger temperature gradient between pole and equator in the Northern Hemisphere. One consequence of the Younger Dryas changes would be the location of the Intertropical Convergence Zone in a southern position, so that even tropical regions would have been under Arctic influence.
Hydrologic controls on the morphology and mechanics of accretionary wedges.
Demian Saffer, Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA, and Barbara A. Bekins, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 271-274.
Fluid pressure is widely recognized to influence the strength of rocks and the mechanics of faulting in the brittle crust. At subduction zone plate boundaries in particular, the important role of fluids in affecting rock and sediment deformation is relatively well studied. As marine sediments are transported into a subduction zone, a portion of the sedimentary section is offscraped and incorporated into an accretionary wedge, and a portion is carried beneath the overriding plate with the subducting oceanic plate. Existing mechanical models show that significantly elevated pore-fluid pressures are required for accretionary wedges to maintain their observed geometry--that of a thinly tapered wedge. In general, increased pore pressure reduces both the taper angle of an accretionary wedge and the strength of the plate boundary fault. The model predictions are consistent with observations of subduction zones from ocean drilling and exposures on land. In this paper, the authors note that pore-fluid pressure is not a static quantity, but rather reflects a dynamic balance between geologic processes that act to increase pore pressure, and properties of the rock that allow pressures to dissipate. They propose a refined conceptual model for accretionary wedge morphology that incorporates the fundamental causes of excess pore pressure in subduction zones. The authors use numerical models of fluid flow to demonstrate that sediment permeability and plate-convergence rate are two important controls on subduction zone mechanics through their influence on pore pressure. Low permeability and rapid convergence sustain nearly undrained conditions and result in shallowly tapered wedge geometry, whereas high permeability and slow convergence result in steep geometry. The authors’ results are in generally good agreement with data from active accretionary complexes, but also illustrate the importance of other factors, such as incoming sediment thickness and stratigraphy. One key implication is that strain rate and permeability may strongly modify the strength of the crust in a variety of geologic settings by controlling pore pressure.
A plate tectonic control on the evolution of Cretaceous platform-carbonate production.
Thomas Steuber, Ruhr-Universität, Institut für Geologie, Mineralogie und Geophysik, Universitätsstrasse 150, D-44801 Bochum, Germany. Pages 259-262.
Changes in the mineralogy of carbonates that were deposited on Cretaceous carbonate platforms are related to varying rates of cycling of seawater through young oceanic crust, which changes the major ion composition of seawater. This emphasizes the control of plate tectonics on the type of biological carbonate deposition and provides an example of the linkage of Earth systems.
Tectonic implications for the along-strike variation of the Peninsular Ranges batholith, southern and Baja California.
Paul H. Wetmore et al. Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Pages 247-250.
This paper describes the Late Jurassic-Early Cretaceous tectonic evolution of peninsular southern and Baja California. Specifically, the western portion of the Peninsular Ranges batholith, the roots to an ancient arc that existed there in the middle Mesozoic, is interpreted to represent two distinct tectonic blocks that underwent vastly different tectonic evolutions. The two segments of this part of the arc, the Santiago Peak (north) and Alisitos (south), are juxtaposed by the Agua Blanca fault, an active strike-slip fault south of Ensenada Mexico. The Santiago Peak arc segment is interpreted to have developed in place on the western North American continental margin. Alternatively, the Alisitos arc segment originated as an island arc on oceanic crust not previously associated with North America and was subsequently accreted to the continental margin between 115 and 108 Ma. The Agua Blanca fault is inferred to have originated as a continuation of the fault(s) that joined the s! outhern arc segment to the continent, although here it joined the two arc segments.
Climate modulates the acidity of arctic lakes on millennial time scales.
Alexander P. Wolfe, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada. Pages 215-218.
This paper explores how the chemistry of lakes may be controlled by climate. Diatoms, microscopic algae with silica cell walls, are used to reconstruct 5000 years of lake-water pH from their distribution in sediment cores from two lakes on Baffin Island in the Canadian Arctic. The results suggest that intervals of cold climate engender more acidic lake waters, and vice versa. Because the lakes are chemically very pure, their acidity is primarily influenced by concentrations of dissolved inorganic materials, which are in turn regulated by photosynthesis and the duration of lake ice cover. Cold conditions with low attendant biological productivity and severe ice conditions favor buildup of carbon dioxide (and carbonic acid) in lake waters, leading to decreased pH. Warming has the opposite effect due to atmospheric escape and biological removal of carbon dioxide during longer ice-free seasons. Although the delicate balance between lake chemistry and climate suggests that pH reconstructions from fossil diatoms may be applied to climate change studies, it is also quite possible that human influences, such as atmospheric deposition, may disrupt the relationship.
Lipid and carbon isotopic evidence of methane-oxidizing and sulfate-reducing bacteria in association with gas hydrates from the Gulf of Mexico.
Chuanlun L. Zhang et al. Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. Pages 239-242.
The Gulf of Mexico has abundant gas hydrates and oil seeps. Geochemical evidence indicates that oxidation of hydrocarbons including methane plays an important role in carbon cycling and the development of biological communities in gas hydrate systems. An integrated lipid biomarker-carbon isotope approach reveals new insight to microbial methane oxidation in the Gulf of Mexico gas hydrate system. The phospholipid fatty acids suggest that total biomass is enhanced 11-30 fold in gas hydrate-bearing sediment compared to hydrate-free sediment. Lipid biomarkers indicative of sulfate-reducing bacteria are strongly depleted in 13C (-48‰ to -70‰) in the hydrate-bearing samples, suggesting that they are involved in the oxidation of methane (carbon isotope ratio is ~-47‰ for thermogenic methane and -70‰ for biogenic methane). In the hydrate-free sample, fatty acid biomarkers have 13C values of -27.6‰ to -39.6‰, indicating that crude oil (average ~-27‰) or terrestrial organic carb! on (average ~-20‰) are the likely carbon sources. Our results provide the first lipid biomarker-stable isotope evidence that sulfate-reducing bacteria play an important role in anaerobic methane oxidation in the Gulf of Mexico gas hydrates.
GSA TODAY
Structure and evolution of the lithosphere beneath the Rocky Mountains: Initial; results from the CD-ROM experiment. Karl Karlstrom, University of New Mexico, Albuquerque, and the CD-ROM (Continental Dynamics of the Rocky Mountains) Working Group. This paper presents the initial results from a multidisiplinary experiment that is studying the continental lithosphere beneath the Rocky Mountains. The paper presents a multiscale cross section of the continent in a transect from Wyoming to New Mexico. It integrates data from local scale geology of shear zones to continental reflection seismology to tomography of the deep mantle. The paper suggests that this part of the North American continent is thicker than previously thought, with mantle roots that extend greater than 200 km deep. The structure of the continent was largely "frozen in" at the time the continent was assembled by subduction-related collision of oceanic island arcs and fragments of ocean basins, but the Proterozoic boundaries and domains later strongly influenced mantle thermal modification during Cenozoic magmatism and uplift of the Rocky Mountains. At the crustal scale, the paper delineates old suture zones at the Cheyenne belt in southern Wyoming, Farwell-Lester Mountain zone in Colorado, and Jemez lineament in northern New Mexico, and identifies the Colorado Mineral belt as another possible paleosuture zone. As a result of their different Precambrian tectonic histories, the Proterozoic lithosphere of the Colorado Rockies has remained persistently weaker than the lithosphere of the Wyoming area. Seismic studies indicate the presence of a mafic lower crustal layer that is interpreted to have developed through time such that the Moho is a dynamic boundary that changed (in part by mafic underplating) during tectonic/magmatic events that reshaped the continent. The CD-ROM experiment collaborative research shows the utility of integrating geologic and geophysical studies to produce refined images of the nature and evolution of continents.
To view the abstracts for these articles, go to: http://www.gsajournals.org
To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org.
Geological Society of America
Release No. 02-10
Contact: Ann Cairns
303-357-1056
acairns@geosociety.org