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Source of solutes to the coastal sabkha of Abu Dhabi.
Warren W. Wood et al. U.S. Geological Survey, Mail Stop 430 National Center, Reston, Virginia 20192, USA. Pages 259-268.
A new model is proposed for the origin of solutes and salts in the coastal sabkha (salt flats) of Abu Dhabi, United Arab Emirates. Hydrologic analyses of both the water and solute fluxes in this aquifer system demonstrate that most of the solutes are derived from ascending brines, while most of the water is from local precipitation. Isotopes and solute geochemistry are consistent with this model. This ascending brine model replaces two other widely published models that were based on an incomplete evaluation of the ground-water hydrology of the system.
A coupled fluid-inclusion and stable isotope record of paleofluids in the Monterey Formation, California.
Jonathan B. Martin and Richard A. Rymerson, Department of Geological Sciences, University of Florida, 241 Williamson Hall, P.O. Box 112120, Gainesville, Florida 32611-2120, USA. Pages 269-280.
The Monterey Formation contains some of the largest oil and gas reserves in the continental U.S. Exploration and production of these deposits depend on the nature of past flow of fluids through the formation. One way to study these ancient hydrologeologic systems is through the chemical record that is contained within authigenic minerals (minerals that precipitate chemically from water). Results from this study indicate that water with different chemistries, and thus from different sources, mixed at times that are shorter than required for the minerals to grow. Some of the water sources appear to be from the dehydration of detrital minerals (minerals that settle from water) as they were heated during burial to depths of several kilometers. This mixing of water in the subsurface gave the authigenic minerals a wide range of chemical compositions over very small distances (in some cases as little as a few millimeters). These small-scale variations complicate the study of ancient flow systems, but also provide unique clues about the systems. For example, flow appears to have been both episodic and continuous. The episodic flow could reflect tectonic driving forces such as earthquakes. Detailed studies of these small-scale variations could prove useful for understanding ancient flow systems.
Syntectonic deposits and punctuated limb rotation in an Albian submarine transpressional fold (Mutriku village, Basque-Cantabrian basin, northern Spain).
L.M. Agirrezabala et al. Estratigrafia eta Paleontologia Saila, Euskal Herriko Unibertsitatea, 644 postakutxa, 48080 Bilbao, Spain. Pages 281-297.
This paper describes a fossilized Cretaceous syncline (downward fold) associated with an active fault and formed in a deep-water marine environment. Very precise dating with ammonites (extinct mollusks similar in appearance to modern-day Nautilus) allows calculation of rates of uplift and limb rotation. Syncline growth was pulsating with intervening phases of tectonic calm during a period of time lasting about half a million years. Fold formation was related ultimately with rotation and translation movements of the Iberian plate by the opening of the Bay of Biscay.
Plate-boundary earthquakes and tsunamis of the past 5500 yr, Sixes River estuary, southern Oregon.
Harvey M. Kelsey et al. Department of Geology, Humboldt State University, Arcata, California 95521, USA. Pages 298-315.
In the last 5500 years, eleven large earthquakes of a magnitude greater than 8 have occurred along the southern Oregon coast. These earthquakes were triggered by the build-up of stress along the subduction zone between the Juan de Fuca plate and the North American plate. Each earthquake was accompanied by 0.5 to 2.5 meters of coastal subsidence and by tsunamis that extended as much as 3.5 kilometers up coastal valleys. Some tsunamis consisted of at least five distinct waves. The earthquakes are recorded by buried marsh soils in estuarine sediment of the Sixes River Valley in south coastal Oregon. The subsidence that accompanied each earthquake drown salt marsh soils along the coastline. Between earthquakes, the coast emerged from the ocean, only to be drowned by the succeeding earthquake. It is this record of buried soils that provides a history of earthquakes. The last large earthquake was about 300 years ago. Using radiocarbon dating of the buried soils, these large coastal earthquakes occurred roughly once every 510 years.
Seamounts at the continental margin of California: A different kind of oceanic intraplate volcanism.
Alicé S. Davis et al. Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA. Pages 316-333.
Volcanic seamounts located at the continental margin offshore central California may represent a previously unrecognized type of intraplate oceanic volcanism. Morphologically unlike typical ocean island volcanoes or near-ridge seamounts, they are complex, northeast-trending ridges that reflect the ridge-parallel structure of the underlying oceanic crust. Argon-based laser fusion ages of mineral separates indicate at least two episodes of volcanism between about 16 and 12 million years ago, younger by 7 to 11 million years than the underlying ocean crust. Episodes of volcanism are coeval with eruptions in coastal California. Although chemically distinct, the on-land and offshore volcanism apparently occurred in response to the same tectonic changes, related to movement along the major transform fault systems.
Vegetation invasions into absolute desert: A 45 k.y. rodent midden record from the Calama-Salar de Atacama basins, northern Chile (lat 22-24°S).
Claudio Latorre et al. Laboratorio de Palinología, Departamento de Biología, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago, Chile. Pages 349-366.
Nestled between the central Andes and the Pacific Ocean, the Atacama Desert of northern Chile is known as one of driest places on Earth. Extensive areas, the so-called Absolute Desert, are devoid of plants. A few plants eke out an existence along the upper margins of this Absolute Desert, at elevations between 2500 and 3000 meters, largely due to eastern tropical moisture that barely spills over the Andean crest. We have reconstructed past vegetation invasions across this hyperarid margin for the last 45,000 years using rodent middens. These middens, similar to those made by packrats in western North America, consist of plant and other debris encased in crystallized urine and preserved in rock shelters and crevices. Triggered solely by increases in summer rainfall, plant invasions into the hyperarid Atacama Desert have allowed us to infer variations in past South American monsoon intensity over the central Andes and Atacama. Our conclusions challenge conventional thinking, where solar heating over the continent and fluctuations in source moisture areas drive the monsoon at millennial timescales. Instead, we hypothesize that the tropical Pacific may play a large role in regulating monsoon intensity through upper air circulation over the Andes, similar in the way that El Niño-Southern Oscillation affects these features today.
Explosive silicic volcanism of the Yellowstone hotspot: The ashfall tuff record.
Michael E. Perkins and Barbara P. Nash, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 0111, USA. Pages 367-381.
This paper discusses the characteristics and significance of volcanic ash beds produced by large magnitude explosive eruptions from Yellowstone hotspot volcanoes in the western U.S. About 140 ash beds from these eruptions are recognized in this investigation. These ash beds are as old as 16 million years and include the three ash beds from geologically young eruptions (2 million years old and younger) in the region of Yellowstone National Park. Many of the 140 hotspot ash beds are widely distributed in the western U.S. and Great Plains. The wide distribution and great thickness of these ash beds attest to the large magnitude of typical Yellowstone hotspot eruptions. Changes in the abundance and chemical composition of these ash beds record systematic changes in the eruption frequency and magma temperature over the past 16 million years. Prior to 7.5 million years ago, eruption frequencies of 12 or more eruptions per million years where common, and magma temperatures were as high as 1800 °F. During the past 7.5 million years the large explosive eruptions have followed the pattern of the Yellowstone Park area over the past 2 million years, with one to two eruptions of magma per million years, with temperatures of about 1550 °F.
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