||July 19, 2001
GSA Release No. 01-33
The Geological Society of America has just released the August issue of GEOLOGY
which contains a number of newsworthy items. Highlights from GEOLOGY and a summary
of the science article for the August 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 or GSA TODAY in stories published. Contact
Ann Cairns for copies of articles and for additional information or assistance.
- Demonstration of significant abiotic iron isotope fractionation in nature
Thomas D. Bullen, et al. Water Resources Division, U.S. Geological Survey, Menlo
Park, California 94025, USA. Pages 699-702.
- Iron is an abundant, bioreactive element and thus iron isotopes have the potential
to be extremely useful for detecting microbial processes in the geologic record.
Previous workers have suggested that iron isotopes on their own can be used as
a biosignature, based on the assumption that only biologic processes are capable
of fractionating these isotopes in nature. However, based on field and laboratory
studies of the formation of the common iron mineral ferrihydrite, the authors
present the first definitive evidence that abiotic processes are capable of fractionating
iron isotopes in nature to the same extent as microbial processes. The authors
also propose a plausible isotope fractionation mechanism, which promotes the preservation
of an abiotic iron isotope fractionation signal in the geologic record.
- How many Pacific hotspots are fed by deep-mantle plumes?
Valérie Clouard, Jeune Equipe Terre-Océan, Université de
la Polynésie Française, B.P. 6570, Faaa, French Polynesia, and Alain
Bonneville, UMR Géosciences Marines, Institut de Physique du Globe, 4 place
Jussieu, 75005 Paris, France. Pages 695-698.
- The hotspot volcanism theory of the 1960s proposes that the linear intraplate
volcanic chains that show an age progression in the direction of the plate motion
took their origin from deep-mantle plumes. The heads of such mantle plumes interact
with the lithosphere to form oceanic plateaus on the seafloor; the plume tails
then produce linear volcanic chains as the plate drifts over them. The authors
have carefully examined the relationship between oceanic plateaus, linear volcanic
chains, and hotspots on the Pacific plate. Among the fourteen hotspots on the
Pacific plate, only four might correspond to the classical theory, whereas the
majority exhibit short tracks and don't show any evidence of links with oceanic
plateaus. The authors conclude that deep-mantle plumes are unnecessary to explain
most, if any, volcanic chains on the Pacific plate. An updated compilation of
radiometric ages of Pacific seamounts and islands is provided as additional material.
- Metal leaching and inorganic sulfate reduction in volcanic-hosted massive
sulfide mineral systems: Evidence from the paleo-Archean Panorama 1district, Western
David L. Huston, et al. Australian Geological Survey Organisation, GPO Box 378,
Canberra, Australian Capital Territory 2601, Australia. Pages 687-690.
- Three-billion-year-old rocks in Western Australia are yielding new clues to
the working of modern-day "black smokers" (hydrothermal vents on the seafloor)
and the chemistry of ancient seawater. Geochemical studies of the Panorama district
copper-zinc deposits indicate that many of the metals including copper,
zinc, lead, barium, molybdenum, and sulfur were leached from volcanic rocks
below the seafloor by heated seawater, and that the rocks were oxidized during
the process. It seems that three-billion-year-old seawater was locally sulfate
bearing-perhaps more like modern seawater than generally thought.
- Paleoclimatic significance of Phanerozoic reefs.
Wolfgang Kiessling, Department of Geophysical Sciences, University of Chicago,
5734 S. Ellis Avenue, Chicago, Illinois 60637, USA. Pages 751-754.
- Reefs are thought to be very sensitive to climate change. In this article
the author demonstrates that long-term paleoclimatic change has affected the reef
ecosystem, but not in the expected way. In the past 540 m.y., the tropical reef
zone widened and contracted, apparently independent from paleoclimate. Long-term
paleoclimatic change only modulated biotic reef attributes, usually by favoring
algal growth during cool periods. Mass extinctions exerted an important control
on global reef distribution because they had the potential to contract the tropical
reef zone for an extended period of time. This is remarkable because most mass
extinction events are now seen to have occurred independent from long-term climate
- Low seismic-wave speeds and enhanced fluid pressure beneath the Southern
Alps of New Zealand.
Tim Stern, et al. School of Earth Sciences, Victoria University of Wellington,
P.O. Box 600, Wellington, New Zealand. Pages 679-682.
- This paper describes evidence for a 20 x 50 km region in the crust beneath
the Southern Alps of New Zealand where excess water under high pressure resides.
Low seismic wave speeds and high electrical conductivity are the principal data
presented as evidence. The source of the fluid is thought to be water released
by greywacke-schist rocks as they are carried to great depth and thus squeezed
in the collision zone. When fluid pressures in the crust approach the weight per
unit area of the overlying rock (this is called lithostatic pressure), the overlying
crust is effectively floating. In this circumstance faults can move under much
less stress-difference than if there were no fluid pressure present. Thus, this
study raises the question of how major faults in plate boundary zone such
as this one situated between the Australian and Pacific plates move? Do
they move because of large, relatively infrequent earthquakes, or do they move
in some other manner?
- New evidence for the geological origins of the ancient Delphic oracle (Greece).
J.Z. de Boer, et al. Department of Earth and Environmental Sciences, Wesleyan
University, Middletown, Connecticut 06459, USA. Pages 707-710.
- Ancient tradition linked the Delphic oracle in Greece to specific geological
phenomena, including a fissure in the bedrock, intoxicating gaseous emissions,
and a spring. Many modern scholars have dismissed these traditional accounts as
specious. This paper locates young faults that occur at the oracle site, corresponding
well to ancient reports of fissures. Moreover, the work identifies the vaporous
emissions supposedly associated with the prophecies as light hydrocarbon bases
generated in the underlying strata of bituminous limestone. The research confirms
the validity of the ancient sources in startlingly complete detail.
- Rock Varnish: Record of desert wetness?
Wallace S. Broecker and Tanzhuo Liu, Lamont-Doherty Earth Observatory, Columbia
- Rock varnish, a dark, shiny, and very thin (<200 microns) coating made
up of a cocktail of oxides rich in manganese, iron, and clay minerals, forms on
rock surfaces and is ubiquitous in desert regions. It has become the center of
a contentious controversy revolving around its use in dating geomorphic surfaces
and/or evaluating past climate conditions. This paper summarizes findings that
support the view that careful sampling and analysis of varnish "microbasins" can
potentially be useful in reconstructing past climates in deserts, an important
missing element in evaluating hypotheses for global-scale patterns of planetary
wetness. The authors have documented pronounced temporal variations in manganese
and barium that can be correlated over large regions and that appear to be related
to variations in paleo-wetness. They suspect, but cannot prove, that higher manganese
content is related to wetter climates. The mode of formation of varnish remains
uncertain, but this paper documents anthropogenic lead concentrated in outermost
varnish layers, indicating its continued formation. They also report on experiments
conducted at Biosphere 2 that used short half-life cosmogenic beryllium isotopes,
suggesting that while precipitation is a primary control, dust, dew, and aerosols
may also be important in delivering the ingredients for the growth of varnish.
Unfortunately, varnish is not directly datable and hence the timing of the potential
paleoclimate record is not yet well constrained. The authors suggest several steps
that may lead to the rejuvenation of and future breakthrough in varnish studies.
*To view abstracts and the complete table of contents of GEOLOGY, as well as
that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see www.gsajournals.org.
To obtain full text of these articles and articles from back issues, contact Ann