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GEOLOGY
What controls gold distribution in Archean terranes?
Roberto Weinberg, Monash University, School of Geosciences, Melbourne, Victoria 3800, Australia, et al.
Pages 545-548.
Despite the long history of gold exploration and hundreds of years of investigation, determining a predictable pattern of gold distribution, which would simplify gold exploration, has remained elusive. We report on an unusually regular pattern of gold distribution in part of one of the richest gold provinces in the world: the 2.7 billion year old (Archean) Boulder-Lefroy fault zone of Western Australia. Four of the richest known gold deposits, each endowed with more than 100 tons of gold, and including the 1800 ton giant Golden Mile at Kalgoorlie, occur at 30–40 km regularly spaced intervals along this fault zone. Large gold deposits result from the converging of vast amounts of fluids carrying dissolved gold within Earth’s crust, into narrow paths within the fault system. This fluid flow inside the rocks is similar to a river drainage system in which a large catchment of water is drained to form a single potent river. We found that the major factor controlling the gold endo! wment is variations in the direction of the fault zone. When there was seismic activity along the fault zone, parts of the fault dilated while others contracted depending on variations in fault direction. During such events, fluid migrated into areas of dilation, and it is in these sections that rich gold deposition resulted. We speculate that the richness of this area is related to the existence of few, very effective zones of dilation along the fault zone, spaced at distances that maximized the catchment of these mineralizing crustal fluids.
Paleocene-Eocene carbon isotope excursion in organic carbon and pedogenic carbonate: Direct comparison in a continental stratigraphic section
Philip Gingerich, University of Michigan, Department of Geological Sciences, Museum of Paleontology, Ann Arbor, MI 48109-1079, U.S.A., et al. Pages 553-556.
The Paleocene-Eocene boundary 55 million years ago is marked by abrupt biotic change on land (first appearance of horses, dwarfing of some mammals, etc.) and in the sea (extinction of deep-water microfossils, etc.). Such changes were probably caused by massive influx of light carbon 12 into Earth’s atmosphere and carbon budget at this time, during the 100-thousand-year-long Paleocene-Eocene carbon isotope excursion or CIE. Massive influx of carbon 12 is attributed to dissociation of seabed methane hydrates during an important natural greenhouse thermal event known as the Paleocene-Eocene thermal maximum or PETM. This study shows that organic carbon dispersed in sedimentary rocks provides as faithful a CIE signal as biotically generated carbonate trace fossils and microfossils studied previously, which expands the range of times, places, and environments where we can investigate global greenhouse events.
New neodymium isotopic data quantifies Nile involvement in Mediterranean anoxic episodes
Derek Vance, University of Bristol, Earth Sciences, Bristol, BS8 1RJ, U.K., et al. Pages 565-568.
The river Nile has always fascinated scientists and historians alike. In historic times, this river provided the life-blood of the ancient Egyptian civilizations, providing water and fertile silt for crops. Moreover, the fortunes of those civilizations are thought to have waxed and waned with natural variations in the flow of the Nile. On longer, geological, timescales (10,000–1 million years), natural variations in Nile discharge are caused by changes in the strength of the East African monsoon, which feeds the Blue Nile, the tributary of the Nile that rises in Ethiopia. These fluctuations in discharge are thought also to exert a major influence on the Mediterranean Sea. The Mediterranean underwent periods in the past when deep water ran out of oxygen, with the result that deep sea sediments that record these episodes are devoid of fossils of deep-dwelling animals. The favorite mechanism for the development of this deep anoxia was the creation of a lid of light freshwater f! loating on top of the Mediterranean in regions where, normally, high salinity (and therefore heavy) oxygen-rich surface waters would sink to great depth. One obvious source of the freshwater was enhanced Nile floods, and in this paper we show that the central intense part of the anoxic events is associated with changes in the chemistry of the Mediterranean that are attributed to greater Nile discharge. However, the beginning and end of the anoxic events do not show an enhanced Nile signature and require a further source of freshwater—either from other rivers (now dry wadis) draining the Sahara further west or from increased rainfall over the European borderlands.
Volcano spreading controlled by dipping substrata
Luke Wooller, The Open University, Earth Sciences, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, U.K., et al. Pages 573-576.
Volcanic landslides can generate large avalanches of rock and debris reaching many dozens of miles. These failures most commonly occur in the downslope direction. This work studies the role that regional slopes play in the development of instability on volcanoes sited above weak basements such as clays or ash. The weight of a volcano causes the weak base to spread outward, in turn producing structures and areas of weakness in the volcanic edifice. Analogue modeling methods are used (sand and plaster cones on a silicone base), physically scaled to represent real-life examples of a volcano above a tilted weak base. It is demonstrated that underlying slopes as low as 1º cause structures that limit future failures to the downslope direction. This work explains the downslope collapse of many volcanoes, as well as highlights structures formed prior to collapse to help future identification of instability.
Evidence from Lake Lisan of solar influence on decadal- to centennial-scale climate variability during MIS 2
Sushma Prasad, GeoForschungsZentrum Potsdam, Climate Dynamics and Sediments, Telegrafenberg, Potsdam, Brandenburg D 14473, Germany, et al. Pages 581-584.
Annually laminated lake sediments, varves, have the potential of providing an extended and high-resolution record of past climate changes. In one such site, Massada in the Near East, such an initiative for reconstructing the past climate was undertaken by scientists from Israel, the U.S., and Germany. The studied sediments were deposited in palaeolake Lisan (the late glacial precursor of the Dead Sea) in the time interval 17.7–26.2 cal. k.y. BP. Counting of the laminae in combination with radiometric dates indicated that these sediments were varves. Each varve comprises a dark and light colored sublaminae, which were deposited during different seasons. The dark sublaminae include material brought in by the rivers and surface runoff during the winter season. During the summer season the mineral aragonite is precipitated from the surface waters in the lake. With a view to reconstruct the past climate, the mineralogy, grain size, and thickness of the seasonal sublaminae were st! udied. Our studies show that drier periods in the Near East, lasting a few decades or even centuries, were common in the studied time interval. A comparison of the drier events in the Lisan record with the oxygen isotope data from the Greenland ice cores (a proxy for past temperature) indicates that cooler temperatures in Greenland were coincident with drier climate in the Near East. Our studies also show that the discharge of large ice armadas into the North Atlantic (Heinrich events) were coincident with prominent droughts in the Near East. Time-series analyses of the data suggest a strong solar forcing of the Near East climate in the studied time interval, with drier events occurring during times of reduced solar activity.
Phasing of deglacial warming and Laurentide ice sheet meltwater in the Gulf of Mexico
Benjamin Flower, University of South Florida, College of Marine Science, St. Petersburg, FL 33701, U.S.A., et al. Pages 597-600.
Is heat retained in the low-latitudes during abrupt Northern Hemisphere cooling? The answer is yes, based on Gulf of Mexico sediment cores covering the last deglaciation. Sea-surface temperature (SST) estimates using on a new geochemical proxy (Mg/Ca in planktic foraminifera) indicate SST increased by >3 °C from ca. 17.2 to 15.5 ka (thousands of years ago) as Greenland cooled and North Atlantic Deep Water decreased during Heinrich Event 1. Paired δ18O and Mg/Ca data are also used to isolate δ18O of seawater, which reveals two intervals of meltwater flooding from the Laurentide Ice Sheet, one from ca. 16.1 to 15.6 ka and a second major episode from ca. 15.2 to 13.0 ka during peak Greenland warmth. Conversion to salinity indicates that Gulf of Mexico salinity decreased by 2‰–4‰ during these spikes. These results suggest that Gulf of Mexico SST warming preceded peak Laurentide Ice Sheet decay and peak Greenland warmth by >2 k.y., and that heat was retained in the subt! ropical Atlantic during an interval of reduced deep ocean circulation.
Crenarchaeotal membrane lipids in lake sediments: A new paleotemperature proxy for continental paleoclimate reconstruction?
Lindsay Powers, University of Minnesota, Large Lakes Observatory, Duluth, MN 55812, U.S.A., et al. Pages 613-616.
We have applied a recently developed method (TEX-86) for reconstructing sea-surface temperatures to lakes, which will allow reconstruction of continental temperatures over a wide range of timescales (tens to millions of years). This technique is based on analysis of microbial membrane lipids (from Archaea) that develop differentially only in response to growth temperature, making this an independent paleotemperature tool. This tool will be invaluable to paleoclimatologists, allowing more accurate temperature reconstruction, resulting in better climate model development.
Climate stability across the Eocene-Oligocene transition, southern Argentina
Matthew Kohn, University of South Carolina, Dept. of Geological Sciences, Columbia, SC 29208, U.S.A., et al. Pages 621-624.
The Eocene-Oligocene transition, ca. 33.5 million years ago, is believed to have been one of the largest climatic changes in the past ~65 million years, and is commonly described as the world’s transition from “greenhouse” to “icehouse” conditions. However, new isotope data from fossil teeth in southern Argentina show surprisingly little evidence for climate change across the transition, and instead are better explained by a constant climate. These observations call into question most views and models of the transition.
Metastability, mechanical strength, and the support of mountain belts
James Jackson, University of Cambridge, Department Earth Sciences, Bullard Laboratories, Cambridge, Cambridge CB3 0EZ, U.K., et al. Pages 625-628.
This paper makes a connection between deep earthquakes in mountain roots and the eventual destruction of those roots as they transform to denser material. It provides a link between observations of very old rocks exposed in the uplifted roots of ancient mountain belts, and geophysical observations than can be made today in modern mountains. These issues are fundamental to what controls the strength of the continents.
GSA TODAY Science Article
Greenhouse gas sequestration in abandoned oil reservoirs: The International Energy Agency Weyburn Pilot Project
D.J. White, Geological Survey of Canada, Ottawa, Ontario K1A 0E9, Canada, et al.
Although the role of carbon dioxide in forcing contemporary climate change remains a fiercely contested issue, Canadian researchers are not waiting around for the answer to test methods for sequestering carbon dioxide. In a novel approach that combines a team of geologic researchers, the International Energy Agency, and the oil company that owns the field (EnCana Corporation of Canada), White et al. report on the Weyburn project, a project designed to examine the economics and feasibility of greenhouse gas (carbon dioxide) sequestration in an active oil field in southern Saskatchewan. The Weyburn project is unique in that the research team has been able to piggyback onto a commonly used technique for enhanced oil recovery: the injection of carbon dioxide. In this case, the research team used a combination of geophysical and geochemical techniques to monitor the injection and spread of carbon dioxide into the subsurface. The results, presented in GSA Today, provide a much bet! ter understanding of the long-term fate of injected carbon dioxide and demonstrate the tools needed to effectively monitor such sequestration processes in the future.
To view the complete table of contents for the July issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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