||April 1, 2003
GSA Release No. 03-10
April Media Highlights: Geology
Boulder, Colo. - The Geological Society of America's April issue of GEOLOGY contains potentially newsworthy items. Topics include: paleoceanography and the dynamics of climate change; a challenge to prevailing explanations for the biodiversity of the Amazon Basin; a new slant on the origin of Tiger's-Eye and its cat-like glint; and possible causes of North American submarine slope failures at the time of the K/T boundary impact event.
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 at email@example.com for copies of articles and for additional information or other assistance.
- Holocene foraminiferal radiocarbon record of paleocirculation in the Santa Barbara Basin
E. Brendan Roark, Department of Geography, University of California, Berkeley, California 94720, USA, et al. Pages 379-382
- Differences in the apparent radiocarbon ages between small calcium carbonate shells (foraminifers) living at the sea surface (planktonic) and at the sediment-water interface (benthic) in the Santa Barbara Basin contain information about changes in the age and thus the source of water entering the basin. The Holocene (past 10,000 years) sequence indicates a greater variability in intermediate water circulation than previously detected. An unstable early Holocene at ca. 9,000 years ago is marked by rapid variations in the apparent Benthic-Planktonic (B-P) 14C age differences similar to rapid variations seen during the Younger Dryas and the last glacial episode. This unstable phase (ca. 9,000 years ago) likely represents a final reorganization in the strength or site of formation of North Pacific Intermediate Water in the final stages of deglaciation. The middle Holocene (8,500-2,000 years ago) and late Holocene (2,000 years ago to present) were apparently stable intervals for intermediate-depth water circulation but were marked by a rapid increase in the apparent B-P 14C age differences that likely represents two different modes of North Pacific circulation. This shift was concurrent with changes in regional climate patterns as well as with changes in temperature and accumulation rates measured in the Greenland Ice Sheet Project 2 (GISP2) ice core, suggesting large-scale changes in atmospheric circulation patterns. These results also point to the potential importance of intermediate water production or ventilation in climate change.
- Single foraminiferal test chemistry records the marine environment
Gert-Jan Reichart, Department of Geochemistry, Faculty of Earth Sciences, Utrecht University, P.O. Box 80021, 3508TA Utrecht, The Netherlands, et al. Pages 355-358.
- Reconstructing climate over geological times requires an indirect approach. Such an approach involves the use of so-called proxies, which relate chemical and biological changes to climate variability. The chemical composition of the microscopic tests of unicellular organisms, foraminifera, has proven to be a particular sensitive tool for such reconstructions. A new technique for measuring trace elemental composition, which greatly improves our ability to reconstruct past ocean and climate change, was applied to the microscopic shells of unicellular organisms, benthic foraminifera. This new technique, which involves micrometer scale sampling of these shells using an ultra violet laser and subsequent analyses with a mass spectrometer, allows the use of single foraminiferal shells for environmental reconstruction without time-consuming cleaning techniques. The possibility to analyze single shells opens the opportunity to avoid the usual averaging of larger multi-specimen samples, and to gain much more insight into the high-amplitude changes typical for periods of rapid climate change. On the basis of the measured trace elements incorporated in shells from different oceanographic regimes, we present a preliminary regression curve for ambient seawater temperature (based on Mg and Sr concentrations) and redox conditions (Mn concentrations).
- Constancy in the vegetation of the Amazon Basin during the late Pleistocene: Evidence from the organic matter composition of Amazon deep sea fan sediments
Thomas P. Kastner and Miguel A. Goni, Organic Geochemistry Laboratory, Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA. Pages 291-294.
- In this study, we characterized the composition of organic matter preserved in sediments from the Amazon Deep Sea Fan. The sediments in this large geological feature present in the offshore region of Brazil's central coast were deposited between 70,000 and 10,000 years ago during periods when sea level was significantly lower than it is today. Our results show that the organic matter remains exported by the Amazon River during that time were remarkably similar to the materials it currently carries as it drains the massive tropical rainforest of the region. This similarity includes the presence of organic compounds that are uniquely characteristic of the tropical angiosperm vegetation species that make up the Amazon rainforest today. From these findings, we conclude that the region of Amazonia must have been permanently and dominantly covered by angiosperm rainforest for the past 70,000 years. In order to maintain this constant vegetation coverage, the tropical wet climate that presently characterizes this region must have been a permanent feature in this part of South America over the last two glacial-interglacial cycles. These conclusions, which are consistent with other newly published studies, have important connotations regarding our understanding of past climate in the tropics since they suggest that equatorial South America must have received abundant moisture during both glacial and interglacial periods. Our findings also refute the theory of glacial refugia (islands of tropical rainforest surrounded by savannas) as an explanation for the extreme biodiversity in this ecosystem. Quite opposite, they raise the very plausible possibility that the constancy in the climate and vegetation may be responsible for the ecological diversity found in the Amazon Basin.
- Transtentional model for the Sierra Nevada frontal faulty system, eastern California
Jeffrey Unruh, William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA, et al. Pages 327-330.
- Recent measurements of the motion of Earth's crust using satellites has shown that the Sierra Nevada mountain range and a large part of central California are moving to the northwest relative to the stable interior of North America as a single rigid block or "microplate." Data presented in this paper show that the faults that form the spectacular eastern front of the Sierra Nevada range primarily accommodate the northwest motion of the Sierran microplate, rather than uplift of the range and simple east-west stretching of the crust east of the mountains as previously thought. The Sierran microplate is moving obliquely away from western Nevada, and the crust east of the range is being both pulled apart and dragged to the northwest in the wake of the microplate. This model accounts for the presence of active strike-slip faults and normal faults bordering the eastern Sierra Nevada, the origin of downdropped areas such as Lake Tahoe basin and Carson Valley, and styles of faulting in eastern California expressed by small to moderate background earthquakes."
- Geochemical confirmation of the Kula-Farallon slab window beneath the Pacific Northwest in Eocene time
K. Breitsprecher, Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada, et al. Pages 351-354.
- The subduction of an oceanic spreading ridge system beneath a continent leaves its fingerprint on the land surface. The fingerprint above the subducting spreading ridge consists in part of changes in composition of lavas produced by volcanoes, and the positions of volcanoes relative to the subduction zone, compared to either side of it. This happens because a gap forms between the descending oceanic plates, permitting rocks from the deeper mantle to be melted and mixed into the more regular lava magmas through the breach in the slabs. This paper finds such evidence in volcanoes that were active about 50 million years ago in south-central British Columbia and in the Pacific Northwest region, along the western edge of North America.
- New interpretation of the origin of tiger's-eye
Peter J. Heaney and Donald M. Fisher, Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA. Pages 323-326.
- The rich brown shimmer of tiger's-eye combines with a durable polish to make this lively gemstone a perennial favorite among jewelers and mineral collectors. Though tiger's-eye once was a highly valued rarity in western Europe, the discovery of abundant sources in South Africa in the 1870s sent its value careening downward; it now is one of the most affordable gems. For over a century, mineralogists believed that tiger's-eye was a kind of fossilized asbestos. Specifically, they assumed that tiger's-eye forms when a fibrous mineral called crocidolite is infiltrated with hot solutions rich in dissolved silica. Somewhat similar to the way petrified wood is formed, the solutions were presumed to dissolve the asbestos and replace it with quartz. Mineralogists call this process pseudomorphism-the exchange of one mineral with another while the shape of the first mineral is retained. Pseudomorphism serves as a record of a rock's "previous lives," and it is frequently exploited by geologists who are trying to unravel changes in Earth's ancient environment. Tiger's-eye has long been acclaimed in textbooks and museum displays as the archetype of pseudomorphic mineral replacement.
- Now those textbooks will have to be revised. Professors Peter Heaney and Donald Fisher at the Pennsylvania State University have examined tiger's-eye using a number of sophisticated techniques. Their analysis has revealed that pseudomorphism is not what underlies its elegant shimmer. Instead, they found that the chatoyancy, or cat's-eye reflection, seen in the gem is the result of tiny fibers of crocidolite asbestos that are completely encapsulated in quartz. The crocidolite and the quartz grew simultaneously inside microscopic cracks that opened as rocks experienced tectonic stresses during mountain-building events. The minerals were deposited in the cracks by solutions percolating through the fractured rocks.
- Interestingly, the quartz and the asbestos crystals nucleated on opposite sides of a single fissure and grew into each other as the cracks were sealed to make a quartz-crocidolite vein. Crocidolite is blue in color, and Heaney and Fisher found that the brown color of tiger's-eye is the result of tiny crystallites of iron oxide that formed more recently and decorate the surfaces of the crocidolite fibers. As the rocks were slowly rotated during repeated tectonic episodes, cracks were reopened but the direction of the asbestos growth changed. Since reflection of light from the crocidolite fibers is the cause of the cat's-eye effect, the zigzag chatoyancy that is a hallmark of tiger's-eye actually provides a history of the rock's displacement path during the collision of tectonic plates.
- 87Sr/86Sr test of the degree of impact-induced slope failure in the Maastrichtian of the western North Atlantic
Kenneth G. MacLeod, et al. Pages 311-314.
- Some investigators have suggested that the well-documented impact of an asteroid on the Yucatan peninsula approximately 65 million years ago (Cretaceous/Tertiary [K/T] boundary) caused earthquake activity which led to wide-spread submarine landslides, including locations near the east coast of North America. We have investigated several of the landslides or slumps of this age that are off the coast of North and South Carolina. Our Sr isotopic measurements and detailed observations of ocean floor rocks indicate that these slumps were not the result of impact, but were caused by gravity flow. This type of slope failure or slumping is considered to be a common feature throughout the geologic record.
To review the abstracts for these articles, go to www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns.
Review the complete table of contents for this issue of GEOLOGY.