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GEOLOGY
Simplifying the stratigraphy of time
Jan Zalasiewicz, Leicester University, Department of Earth Science, University Rd., Leicestershire, CO LE1 7RH, U.K.; et al. Pages 1-4.
Ever since the beginnings of organized geological study, the geological time scale has been a dual one, with a time interval (the Jurassic Period, say) on the one hand, and "time-rock" interval (the Jurassic System, which comprises all the strata laid down during the Jurassic Period) on the other. The distinction between the "time" and "time-rock" is subtle and has been misunderstood by generations of geologists. Now, Jan Zalasiewicz, Alan Smith, and their colleagues of the Stratigraphy Commission of the Geological Society of London question the need to retain both time scales, particularly given modern methods of establishing an effective framework of geological time by placing "golden spikes" at key horizons within rock strata. They suggest in a paper published by the journal Geology that, these days, the science of geology can operate perfectly well with only the "time" scale, within which all the events of the past (e.g., volcanic eruptions, meteorite impacts, the evolution and extinction of animal and plant species) can be placed. This suggestion will likely prove controversial, as it would mean dispensing with the long-established and well-entrenched "time-rock" units. So will it really be goodbye, Jurassic System, but stick around, Jurassic Period? Only time will tell.
Prolonged wet period in the southwestern United States through the Younger Dryas
Victor Polyak, University of New Mexico, Department of Earth and Planetary Sciences, 200 Yale Boulevard, N.E., Albuquerque, NM 87131-1116, U.S.A.; et al. Pages. 5-8.
The Pleistocene-Holocene transition was a crucial climatic period on Earth. It has been shown, based on marine and ice records, that during the time known as the Younger Dryas, the North Atlantic region plunged to near-glacial conditions for ~1300 years (ca. 12,940 to 11,640 yr B.P.) and recovered to normal conditions in less than a decade. How these large climatic shifts might have been expressed in continental interiors is of vital scientific and societal interest. In our work we show that in the southwestern United States climate change is not synchronous with the North Atlantic. The onset of wetter and likely cooler climate occurred after the start and persisted for a thousand years beyond the termination of the Younger Dryas in the North Atlantic. This period is also an important time of both cultural and faunal evolution of North America. There is much debate about the relative importance of climate and social factors for the large-scale faunal changes. For example, our data show that the middle Allerød to early Younger Dryas, which corresponds to Clovis occupation, was a time of drought that may have contributed to the extinction of some megafauna such as the mammoth. The transition from Clovis to Folsom seems to have occurred at the end of a drier period, but the Southwest became increasingly wetter through Folsom time and throughout much of the later Paleo-Indian period.
Subducted upper and lower continental crust contributes to magmatism in the collision sector of the Sunda-Banda arc, Indonesia
Marlina Elburg, Max Planck Institute for Chemistry, Geochemistry Division, P.O. Box 3060, Mainz, 55020, Germany; et al. Pages 41-44.
The Sunda-Banda arc (Indonesia) is in the process of changing from a subduction regime, where an oceanic plate sinks back into the mantle, to a collision regime, where the attempted subduction of the buoyant Australian continent halts the subduction process. New Pb isotope data show that the Australian continental crust is an important geochemical source for volcanism in the collision area. The data show contribution of both the upper and lower crust; the latter is mainly visible in the central collision zone. This reflects the break-off of the negatively buoyant oceanic part of the plate from the Australian continent in this area.
Linking continental slope failures and climate change: Testing the clathrate gun hypothesis
Mark Maslin, University College London, Department of Geography, 26 Bedford Way, London, WC1H 0AP, U.K.; et al. Pages 53-56.
A major debate in science is whether gas hydrate release or changes in tropical wetlands cause the massive variations seen in ice-core atmospheric methane record during the Pleistocene. Because submarine sediment failures can be up to the size of Jamaica, they have the capacity to release vast quantities of methane hydrates. One of the major tests of Jim Kennett's clathrate gun hypothesis is determining whether continental-slope failures and increases in atmospheric methane correlate. To test the clathrate gun hypothesis we have collated published dates for submarine sediment failures in the North Atlantic sector and correlated them with climatic change for the past 45 k.y. Our data support the clathrate gun hypothesis for glacial-interglacial transitions. The data do not, however, support the clathrate gun hypothesis for the rapid glacial-period millennial-scale climate cycles because the occurrence of sediment failures correlates with the cold Heinrich events. Thus sediment failure correlates with lows in sea level and atmospheric methane. A secondary use of our data set is the insight into the possible cause of continental-slope failures. Glacial-period slope failures occur mainly in the low latitudes and are associated with lowering sea level. This finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause. The Bølling-Ållerød sediment failures are predominantly low latitude, suggesting an early tropical response to deglaciation. In contrast, sediment failures during the Preboreal period and the majority of the Holocene occurred in the high latitudes, suggesting either isostatic rebound–related earthquake activity or reduced hydrostatic pressure, again caused by isostatic rebound, causing destabilization of gas hydrates. This correlation implies that the largest threat to continental-slope stability in the possible greenhouse future is melting of the ice-sheet margins and the resultant isostatic rebound. This is a significant worry, as there is already evidence that these processes are occurring in Antarctica and Greenland. Moreover, it should be noted that these continental slope failures would all be accompanied by large tsunamis.
Triggering of destructive earthquakes in El Salvador
Jose Martinez-Diaz, Universidad Complutense de Madrid, Geodinámica, Facultad de Geología, Antonio de Novais S/N, Madrid, Madrid 28040, Spain; et al. Pages 65-68.
El Salvador is a highly vulnerable area that has suffered at least 11 destructive earthquakes during the past 100 yr. These events caused >3000 deaths because of the effect of shaking and/or the subsequent landslides. We investigate the existence of a mechanism of triggering driven by the interaction of offshore normal faults in the Middle American subduction zone and continental strike-slip faults in the El Salvador volcanic arc. We modeled the static stress changes produced by the June 1982 and January 2001 offshore events on the continental area. The results support a causative relationship between these events and the catastrophic El Salvador February 2001 M6.6 earthquake. The results have also broad implications for future risk management in the region, as they suggest that likelihood of rupture was increased in a segment of the El Salvador fault zone.
Magmatic intrusion west of Three Sisters, central Oregon, USA: The perspective from spring geochemistry
William Evans, U.S. Geological Survey, Water Resources, 345 Middlefield Rd., M/S 434, Menlo Park, CA 94025, U.S.A.; et al. Pages 69-72.
The Three Sisters area in the central Oregon Cascade Range has come under intense scrutiny due to ongoing uplift of about an inch per year centered several miles west of the peaks. A survey of groundwater revealed a strong signature of magmatic carbon dioxide and helium in low-temperature springs located near the center of uplift. Based on pre-existing chemical data for springs in this area, it seems likely that the magmatic volatiles detected derive from a previous episode or multiple episodes of intrusion. If this interpretation is correct, it suggests that intrusions occur with some frequency in this area, even though all eruptive vents near the center of uplift are more than 10,000 years old. The findings demonstrate that, at least under certain conditions, low-temperature springs reveal information about intrusion location and time history that could be impossible to obtain by other means.
Slab detachment control on mafic volcanic pulse and mantle heterogeneity in central Mexico
Luca Ferrari, UNAM, Centro de Geociencias, Campus Juriquilla, Juriquilla, Qro. 76230, Mexico. Pages 77-80.
Based on geophysical and geological data the article suggests that the lower part of the Cocos plate subducted beneath central Mexico, broke off, and foundered in the upper mantle beneath central Mexico. This process initiated ca. 13–12 Ma in the Gulf of California and propagated to the east up to the Gulf of Mexico. The increase in the mantle temperature following the detachment produced a belt of basaltic volcanism with ages progressively younger to the east from 11 to 5 Ma. The model also explains the presence in volcanism usually associated with oceanic islands (such as Hawaii) in a continental arc setting.
Variable responses of western U.S. glaciers during the last deglaciation
Joseph Licciardi, University of New Hampshire, Department of Earth Sciences, Durham, NH 03824, U.S.A.; et al. Pages 81-84.
Advances and retreats of alpine glaciers are sensitive indicators of climate changes in western North America. The Wallowa Mountains in northeastern Oregon hosted one such glacier system during the last ice age, comprising a central ice cap drained by several large valley glaciers. The deposits left behind by these now-vanished glaciers preserve a record of their past fluctuations and eventual retreat. Application of cosmogenic nuclide surface exposure dating, an isotopic dating technique, to Wallowa Mountain glacial deposits identifies two glaciations at ca. 21,100 and 17,000 years ago, and a minor glacial event at ca. 10,200 years ago. This newly reconstructed glacial history, integrated with existing well-dated western U.S. glacial records, demonstrates substantial variability in the geographic pattern of western U.S. glacier responses to climate forcing associated with the global Last Glacial Maximum (ca. 21,000 years ago) and subsequent abrupt climate events originating in the North Atlantic region. Differences in the responses of alpine glaciers to climate change indicate significant changes in the relative contribution of regional versus global influences on the climate of the western United States during the last deglaciation.
GPR investigations along the North Anatolian fault near Izmit (Turkey): Constraints on the right-lateral movement and slip history
Matthieu Ferry, ETH-Zürich, Institute for Geophysics, ETH-Hönggerberg, Zürich, - 8093, Switzerland; et al. Pages 85-88.
We analyze ground-penetrating radar (GPR) profiles made across and parallel to the August 1999 earthquake ruptures of the North Anatolian fault. The profiles document cumulative right-lateral offset of stream channels and the successive faulting of a medieval (Ottoman) canal. Buried fluvial-channel deposits, exposed in some trenches dug to determine paleoseismicity, show lateral displacements, which suggest that the two previous events were similar to the 1999 earthquake. At the Ottoman canal site, GPR profiles complement a trench study and provide consistent results showing the occurrence of three faulting events after A.D. 1591, the date of canal construction. This study demonstrates that the use of ground-penetrating radar method in paleoseismology contributes to better identify cumulative slip along active faults.
Dynamical causes for incipient magma chambers above slabs
Taras Gerya, Ruhr-University Bochum, Institute of Geology, Mineralogy and Geophysics, Universitaetstrasse 150, Bochum, Northrhine-Westphalia 44780, Germany; et al. Pages 89-92.
Numerical computer simulations predict an intriguing natural phenomenon for subduction zones: the formation of large magma chambers below volcanic arcs from relatively cold, hydrated, partially molten wave-like structures rapidly propagating upward along descending slabs.
Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin
John Hopper, GEOMAR, Marine Geodynamics, Wischhofstrasse 1-3, Kiel, N/A D-24148, Germany; et al. Pages 93-96.
Rifting of the continents and the opening of the ocean basins results in melting of Earth's mantle, generating large-scale volcanism. Occassionally, however, this rifting occurs without any volcanism or very little volcanism. Such was the case when Iberia and Newfoundland broke apart ca. 130 million years ago. This breakup event has been well studied off the Iberian margin and has led to new paradigms about how Earth's crust and mantle respond to plate tectonic forces. This paper reports on key new data off the Newfoundland margin that have important implications for understanding this rift system. Predictions of large-scale asymmetry are confirmed, but evidence for a volcanic oceanic crust abutting thin continent crust suggests that there may have been more volcanism than believed based on the Iberia work.
GSA TODAY Science Article
Submergence of Ancient Greek Cities off Egypt's Nile Delta: A Cautionary Tale
Jean-Daniel Stanley, et al., Geoarchaeology-Global Change Program, E-205 National Museum of Natural History, Smithsonian Institution, Washington, D.C., 20560, USA
Submergence of coastal cities around the world has long been a hazard and persists today as human population growth and development continues to encroach on coastal areas. Jean-Daniel Stanley, from the Geoarcheology-Global Change program at the Smithsonian Institution, has teamed with colleagues to present the results of the discovery and geoarcheological analysis of drowned Greek cities of the Nile Delta. Formerly lying at estimated elevations of only 2 m above sea level, Stanley and colleagues used modern high-tech geophysical techniques and classic geologic studies in combination with written history to reconstruct the timing of the demise of the cities of Herakleion and Eastern Canopus. This crossover between geology and history integrates geologic studies on a human time scale and with written history. For example, visual records of the heights of buildings in the early seventh century in Eastern Canopus provided by the writings of Sophronius of Jerusalem. These cities were built around the 6th century B.C. as part of the infrastructure for Greek traders. Much of the team's archeological research took place under water, where divers helped to excavate key sites (presently located in 7 m of water) and complemented the extensive database provided by core samples and geophysics. Slide scan sonar was used to map the locations of the drowned cities and high resolution seismic reflection studies were used to document the geologic setting and history. Stanley et al. demonstrate that recent sea level is not enough to account for the sometimes sudden submergence of the cities, and additional subsidence that must have occurred was likely due to compaction and remobilization of unconsolidated sediment beneath the coastal cities. The drowned cities are testament to the consequences of construction in coastal areas without proper mitigation of potential hazards.
To view the complete table of contents for the January issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.