|24 September 2008
GSA Release No. 08-53
Director - GSA Communications & Marketing
October Media Highlights
Boulder, CO, USA - GEOLOGY topics include the fossil conservation of organic tissues, including guts, gills, muscles, and eyes, as carbon; a habitable zone model for recovery after major extinction; the complex response of biodiversity to both climate and tectonics; the "celebrated weirdness of Ediacaran fossils" preserved in fine-grained carbonate sediments; and a powerful new way to concentrate gold. GSA TODAY compares present microbially induced sediment structures to microbial communities preserved in Earth history some three billion years ago.
Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.
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Cambrian Burgess Shale–type deposits share a common mode of fossilization
Robert R. Gaines et al., Geology Department, Pomona College, Claremont, California 91711, USA. Pages 755-758. NSF funding received.
Evidence for the proliferation of animals and the rise of complex ecosystems during the Cambrian period are best known from the Burgess Shale and a handful of other deposits like it. Whereas the typical fossil record is comprised of shells, teeth, bone, and other mineralized skeletal parts, Burgess Shale–type deposits preserve soft tissues, including guts, gills, muscles, appendages, and eyes, and offer an unparalleled record of early animal life. Significant disagreement in the paleontological community has persisted over exactly how these fossils were preserved and whether these fossils are preserved similarly or via different pathways in different deposits. Previous studies have focused on the Burgess Shale and the Chengjiang Biota, which have been compromised by metamorphism and intensive weathering, respectively. In this study, Gaines et al. present data from eleven other Burgess Shale–type deposits found worldwide that demonstrate unambiguously that a single mode of fossilization is common to all deposits; the conservation of organic tissues as carbon. These results significantly narrow the field of hypotheses that might explain how this enigmatic style of fossilization occurred and suggest that trophic web reconstruction may be possible via carbon isotope (delta 13-C) analysis.
Hydrothermal circulation at slow spreading mid-ocean ridges: The role of along-axis variations in axial lithospheric thickness
Fabrice J. Fontaine et al., IPGP-Boite 89, 4 place Jussieu 75252, Paris Cedex 05, France. Pages 759-762.
Hydrothermal circulation in the crust of mid-ocean ridges is one of the main mechanisms responsible for the cooling of Earth. One key question that needs to be addressed to better understand and quantify the spatial and temporal variability of heat and mass exchanges between the solid earth and the oceans is the distribution of seafloor hydrothermal sites. Fontaine et al. show, by means of numerical models, that variations in the depth of penetration of fluids in the crust along the ridge axis can explain the observed heterogeneous distribution of sites along ridge portions. Their models highlight the differences in the hydrothermal cooling regime between fast-spreading ridges, where the depth of penetration is constant, and slow-spreading ridges, where variations are expected.
Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma
James Hollingsworth et al., University of Cambridge, Department of Earth Sciences, COMET, Bullard Laboratories, Madingley Road, Cambridge, Cambridgeshire CB30EZ, UK. Pages 763-766.
Hollingsworth et al. examine how the region surrounding the eastern South Caspian Sea is deforming as a result of the collision of Arabia with Eurasia, using a combination of data from earthquakes, geology, and the shape of the landscape (which records fault movement over thousand-year time scales). The precise motions of sites within this collision zone have been previously measured using global positioning system (GPS) satellites. The data show the South Caspian is moving west relative to Eurasia and Central Iran and is therefore an example of extrusion tectonics, whereby tectonic plates move laterally to accommodate a colliding plate. Hollingsworth et al. identify major faults along the northern and southern margins of the eastern South Caspian region, which accommodate this westward extrusion. Estimates of the total lateral motion along these faults, at present-day rates of slip estimated from GPS data, show it could have been achieved in about 10 million years. The South Caspian is thought to be sliding beneath the North Caspian (a process called subduction). If subduction began with the westward extrusion of the South Caspian basin, it may also date from ~10 million years ago. This is significantly older than previous estimates for the onset of subduction, which range from 1.8 to 5.5 millions years ago. Furthermore, using GPS data for northeast Iran, the authors estimate the approximate rate of slip for major faults across the region. Knowledge of both the location and slip rate of active faults will allow a better assessment of the seismic hazard posed to communities (over 2 million people) that lie along these faults.
Anomalously diverse Early Triassic ichnofossil assemblages in northwest Pangea: A case for a shallow-marine habitable zone
Tyler Beatty et al., University of Calgary, Department of Geoscience, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. Pages 771-774.
The Permian-Triassic boundary interval (circa 252 million years ago) saw unparalleled species loss in the marine realm, and biotic recovery was delayed relative to other mass extinctions. A major unresolved question has been about where the marine organisms that recovered from the extinction were housed. Trace fossils, which preserve the activity of organisms, are recorded from Early Triassic rocks found in western Alberta, northeast British Columbia, and the barren landscapes of the Canadian Arctic; they provide evidence for refuges from ongoing extinction pressures in the immediate aftermath of latest Permian mass extinction. These fossils present a record of ocean-bottom dwelling organisms and indicate locally well-oxygenated conditions in an ocean otherwise characterized by widespread anoxia. The habitable zone model of Beatty et al. proposes that a colonization window existed along ancient western Canadian shorelines that was controlled by aeration through wave action and enhanced by frequent storms. Within the habitable zone, latest Permian extinction levels are reduced and the recovery time is minimized, supporting the interpretation that oxygen stress was a major cause of the recovery delay from Earth’s greatest extinction. It follows that the habitable zone provided the seed communities from which the marine realm was repopulated following a global return of well-oxygenated oceanic conditions.
Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea
Nadja Neubert et al., Universitat Bern, Institut fur Geologie, Balzerstrasse 3, 3012 Bern, Switzerland. Pages 775-778. NSF funding received.
Molybdenum is a trace metal that is sensitive to redox (reduction/oxidation reaction) changes in the water column. It is used by biogeochemists who study changes within the ocean system to track the rise of atmospheric oxygen. A rather new method is the measurement of the isotopic composition of molybdenum in sediments. Molybdenum isotope ratios act as fingerprints to identify changes in the oxygen content of the oceans through time -- i.e., the clearest and most direct geochemical trace of biological activity (life) through Earth's history. In order to quantify paleoceanic change, we need a fundamental understanding of the processes of the global molybdenum cycle. Neubert et al. has carried out a detailed study of surface sediment samples from the Black Sea from different water depths. The Black Sea is an archetype of recent marine anoxic (oxygen free) environments. The author’s isotope results confirm that molybdenum solubility is strongly related to the amount of dissolved sulphide in the water column. Further, molybdenum isotopes indicate that there is a well-defined threshold value of sulfide concentration above which the scavenging from molybdenum from the water column is complete. This determination of the threshold value provides a fundamental parameter for future studies of the oxygen history in the geological past by means of molybdenum isotopes.
Miocene tectonics and climate forcing of biodiversity, western United States
Matthew Kohn and Theodore Fremd, Department of Geosciences, Boise State University, 1910 University Dr., Boise, Idaho 83725, USA. Pages 783-786. NSF funding received.
Kohn and Fremd have found that the diversity patterns of carnivores and hoofed mammals in the western United States over the past 30 million years show changes related to crustal extension, global cooling, and increased climate seasonality. Increased diversities at 17 to 17.5 million years ago reflect increased topographic variability that resulted from extension. Decreased diversities circa 11 million years ago and the rapid increase in warm climate (C4) grasses at 7-8 million years ago reflect climate teleconnections, including global cooling and growth of the Tibetan Plateau. These interpretations imply that biodiversity responds complexly both to climate and to tectonics.
Capture of high-altitude precipitation by a low-altitude Eocene lake, western U.S.
Alan R. Carroll et al., Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, Wisconsin 53706, USA. Pages 791-794. NSF funding received.
The oxygen isotopic composition of minerals precipitated by surface waters has been used in many studies to estimate the magnitude of past uplift (paleoaltimetry), based on the decreased relative abundance of 18O versus 16O in precipitation falling at higher elevations. Such studies generally assume that these minerals were formed at, or at least near, the site of the original precipitation. However, mountain rivers can carry water hundreds or even thousands of kilometers away from its origination point (for example, the Columbia River in the U.S. Pacific Northwest). Eocene Lake Gosiute in the western United States appears to have captured such a river ~49 million years ago, based on a relatively sudden (circa 100 thousand year) change to sedimentation indicative of more open hydrology. This change is associated with a -5 ppm shift in delta-18O, indicating that the captured river originated at higher elevations than the lake. However, rather than reflecting a change in regional paleoaltitude, this large and rapid change appears to have resulted from stream piracy. Carroll et al.’s study therefore indicates that geomorphic processes alone may generate isotopic records similar to those sometimes interpreted as evidence for tectonic uplift.
Carbonate-hosted Avalon-type fossils in arctic Siberia
Dmitriy V. Grazhdankin et al., Institute of Petroleum Geology and Geophysics, Koptyug Avenue 3, Novosibirsk 630090, Russia. Pages 803-806.
Our present understanding of the origin of animals and Phanerozoic ecosystems depends critically on the ability to interpret impressions left behind by soft-bodied Ediacaran organisms, and to document their spatial and temporal distribution, which conceivably relate to strong environmental gradients in terminal Proterozoic seawater. To our disadvantage, ever since their first discovery in South Australia, the terminal Proterozoic Ediacaran fossils have been recurrently found in clastic sediments that are unfavorable for the preservation of soft tissues. To enhance paleobiological resolution and track environmental perturbations, however, requires the discovery of new taphonomic windows (sets of conditions for preservation) for the Ediacaran biota in relatively continuous successions dominated by chemical sediments. Yet, finding these fossils in alternative modes of preservation has been difficult. Grazhdankin et al. describe Ediacaran fossils uniquely preserved in fine-grained carbonate sediments, which promise a much-enhanced anatomical and paleoecological view of these enigmatic organisms and their taphonomic variants. Importantly, the appearance of Ediacaran fossils turned out to be not inferior or different to that seen in their counterparts found in clastic sediments, indicating that the celebrated weirdness of Ediacaran fossils is not due to unusual preservational circumstances. On the other hand, the Ediacaran fossils are excluded from a taphonomic window that otherwise favors preservation of diverse organic tissues, suggesting that certain Ediacaran tissues, specifically of rangeomorph and frondomorph organisms, had unique properties. Grazhdankin et al.’s results corroborate the hypothesis that the earliest macroscopic life forms were not ancestral to any Phanerozoic or modern organisms.
Naturaliste Plateau, offshore Western Australia: A submarine window into Gondwana assembly and breakup
Jacqueline A. Halpin et al., ARC Centre of Excellence in Ore Deposits, School of Earth Sciences, University of Tasmania, Private Bag 126, Hobart, Tasmania 7001, Australia. Pages 807-810.
Halpin et al. investigate rock fragments dredged from escarpments on the Indian Ocean floor off southwestern Australia and find evidence that a large submarine mountain range, the Naturaliste Plateau, is a fragment of an older supercontinent called Rodinia. This piece of crust formed over a billion years ago and is probably related to similarly aged rocks now exposed in parts of East Antarctica and southwestern Australia; it was then deformed when India collided with Australia and Antarctica to form the supercontinent Gondwana during Cambrian time (around 520 million years ago). This slice of crust, currently the Naturaliste Plateau, was exhumed to Earth's surface and left stranded underwater between India and Australia when Gondwana broke apart during Cretaceous time (around 83 million years ago). Evidence for all of these colossal events can be found preserved in tiny crystals from rocks dredged off the seabed, where they have been resting since the time of the dinosaurs.
Modeling of gold scavenging by bismuth melts coexisting with hydrothermal fluids
Blake Tooth et al., Geology and Geophysics, School of Earth and Environmental Sciences, The University of Adelaide, North Terrace, Adelaide 5005, Australia. Pages 815-818.
Native bismuth is molten down to 271 degrees Celsius, and polymetallic melts can occur at even lower temperatures if bismuth is combined with elements such as gold or tellurium. Such metallic melts are observed in many magmatic-hydrothermal gold deposits. Tooth et al. illustrate the importance of this additional liquid phase during ore deposit formation by presenting equilibrium thermodynamic modeling of a coexisting aqueous fluid and a bismuth-rich melt. The model's predictions match observations in a modern seafloor volcanogenic massive-sulphide (VMS) deposit (Escanaba Tough, Southern Gorda Ridge) and replicate bismuth and gold concentrations commonly observed in other deposit types, such as intrusion-related gold (IRG) and gold skarns. Scavenging of gold from aqueous fluids by bismuth-rich melts is therefore shown to be a powerful new way to concentrate gold. Furthermore, it is effective even where fluids are undersaturated with respect to gold minerals and precious metal concentrations in the parent fluid are low.
Submarine glacial landforms and rates of ice-stream collapse
J.A. Dowdeswell et al., Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. Pages 819-822.
The rate of deglacial ice-sheet retreat across polar continental shelves, and possible ice-stream collapse and sea-level rise, has been much debated. High-resolution imagery of seafloor morphology is available for many polar shelves and fjords. Dowdeswell et al. infer the rapidity of ice retreat from diagnostic assemblages of submarine landforms, produced at ice-stream sedimentary beds. These landforms, exposed by ice retreat across high-latitude shelves, demonstrate that deglaciation occurs in three main ways: (1) rapidly, by flotation and breakup; (2) episodically, by stillstands and/or grounding events punctuating rapid retreat; (3) or by slower retreat of grounded ice. Submarine landform assemblages imply, through the presence of grounding-zone wedges overprinting mega-scale glacial lineations on many polar shelves, that ice-stream retreat is more often episodic than catastrophic. These observations provide a robust test of the ability of numerical models to predict the varied response of ice-sheet basins to environmental changes.
View the complete table of contents for the current issue of GEOLOGY.
Turbulent lifestyle: Microbial mats on Earth’s sandy beaches—today and 3 billion years ago
Nora Noffke, Old Dominion University, Department of Ocean, Earth & Atmospheric Sciences, Norfolk, Virginia 23529, USA, p. 4-9.
A relatively new and important window on past sedimentary environments and processes is provided by microbially induced sedimentary structures (MISS). MISS are produced by the interaction between photosynthesizing bacteria (and the laminated mats they produce) and the physical sediment dynamics of siliciclastic, shallow-marine settings. MISS occur in active tidal and shallow shelf settings today, but have a continuous record extending back to Archaean times. For example, the 2.9 Ga Pongola Supergroup in South Africa includes MISS—representing the oldest known cyanobacterial community preserved in Earth history. Because MISS are present today and can be studied from the twin perspectives of process and spatial distribution, they provide valuable insights into the processes and sedimentary environments represented by ancient deposits.
To review the abstracts for these articles, go to www.gsajournals.org.