Boulder, CO, USA - Topics include: discovery of Sudbury impact event debris in Michigan; climate change and dispersal of early modern humans out of Africa; relationship of mantle plumes and supercontinent cycles; relationship of San Andreas fault system activity and the eastern California shear zone; and ramifications of sediment mixing in studying the Great Barrier Reef. An open-access Research Focus on paleoseismology addresses earthquake prediction. The GSA TODAY science article examines climate change, Ethiopian Plateau development, and human evolution.
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The Late Paleozoic ice age, one of the most severe glaciations in Earth history, came to an end in the Permian. Poulsen et al. use numerical climate-biome models to demonstrate that the deglaciation of high-latitude Gondwana likely had an enormous impact on tropical climate and environments. Climate model results indicate that the deglaciation of Gondwana may have led to warming, aridification, and desertification of equatorial Pangea. These model results are consistent with Late Paleozoic proxies of climate change, and imply a tight linkage between high- and low-latitude climate change.
Torrential rains associated with typhoon Mindulle in early July 2004 caused the Choshui River in western Taiwan to discharge to the Taiwan Strait more than 70 million tons of sediment in three days—about half the annual amount discharged by the Mississippi River to the Gulf of Mexico. Seismic and sedimentological data obtained before and after the typhoon indicate that the sediment was first deposited adjacent to the river mouth and then re-suspended and deposited as a discontinuous band of mud along northwestern Taiwan. Within one month most of the mud had been eroded (most likely transported northward by the Taiwan Warm Current), suggesting that although the immediate impact of floods off coastal Taiwan is considerable, flood-related deposits may last only a few weeks.
Sediment and pollutants are brought to oceans by rivers and transported through the coastal zone. Early work suggested that transport of material to deep-marine basins is less important during interglacial periods of high sea level, such as at present. Covault et al.’s work offshore Southern California, however, shows that the amount of sediment transported to deep-marine basins is approximately constant regardless of sea level, but the pathways of transport are different.
The Campi Flegrei area in southern Italy has been volcanically active for at least 50,000 years. Today Campi Flegrei is characterized by numerous low-magnitude earthquakes and continuous fumarolic activity. Associated with this activity is a slow, episodic meter-scale uplift and deflation of the ground surface that is referred to as bradyseism. Some researchers associate this ground movement with emplacement of new magma into the inferred magma chamber at depth, and suggest that magma emplacement (and uplift) may presage a future volcanic eruption in this area that is home to over two million people. Periods of uplift and deflation have been documented for over 2000 years, and only in 1538 did an eruption follow a period of uplift. Bodnar et al. propose that periods of uplift at Campi Flegrei are associated with the crystallization of a volatile-bearing magma at depth, and that the energy to lift the ground surface is provided by hydrothermal fluids that boil and expand in the deep subsurface. According to this model, uplift and volcanic activity are not linked, and periods of active bradyseism do not necessarily indicate that an eruption is imminent.
Determining the absolute timing of mineralization is a critical, but problematic, aspect for understanding the origin of metallic ore deposits—the ore minerals in these deposits are not amenable to direct isotope dating using common methods. Morelli et al used radioactive isotope dating, using the rare elements rhenium and osmium (Re-Os) in arsenopyrite, to establish the age of gold mineralization at the giant Muruntau gold deposit, Uzbekistan (>5100 metric tons gold) to be 287.5 ± 1.7 Ma (millions of years before present). These results suggest a possible link between gold mineralization and intrusion into the crust of ca. 290 Ma regional granitoid magmas. This temporal association was previously clouded because commonly used geochronometers are unable to accurately and/or precisely date the gold mineralizing event directly. Isotopic compositions of Os and helium provide important new constraints on potential metal and fluid sources to the deposit, including a small but detectable mantle contribution to the ore system. These new results provide a more comprehensive understanding of one of Earth’s giant gold accumulations, and rule out several current hypotheses for its origin.
Determining the concentration of carbon dioxide in the ancient atmosphere remains a critical hurdle to understanding Earth surface temperatures, compositional changes in atmospheric chemistry, and the evolution of Earth’s earliest biosphere. Kah and Riding report the finding of petrographic fabrics in 1.2-billion-year-old carbonate strata that suggest the process of cyanobacterial calcification. When ambient atmospheric partial pressure of CO2 (pCO2) concentrations fall below roughly 10 times present atmospheric levels (PAL), cyanobacteria begin to use a combination of dissolved carbonate species (CO32- and HCO3-) in the photosynthetic production of organic matter. The cyanobacteria, however, must perform a series of biochemical gymnastics to utilize HCO3- in this process. As a byproduct of these biochemical changes, the pH of the microbial sheath rises dramatically and induces the precipitation of calcium carbonate minerals. Identification of these calcified sheaths in the geologic record thus place an upper limit on pCO2. Atmospheric concentrations of CO2 <10 times PAL in the Mesoproterozoic are significantly lower than previous estimates, and indicate the requirement of additional greenhouse gasses, such as methane, in the atmosphere to keep the oceans from freezing.
Webb et al. present some surprising data that have implications for carbon dating of rocks. Such dating techniques are critical for understanding recent climate change, the extinction of Pleistocene mammals, and early archaeology. The technique works by measuring the amount of radioactive decay in some carbon atoms that are taken into an organism or a skeleton during the life of the organism. As carbon is no longer taken in after the organism dies, the amount of decay can indicate how much time has passed since the organic matter was alive - thereby dating the material. However, the technique requires that no additional carbon be added to the material after the death of the organism. For clam shells, carbon is commonly added in the form of subsequent cements, but the cements are easy to recognize because they are a different mineral than the original clam (i.e., calcite instead of aragonite). Webb et al. have found clams that contain cements that were formed in unusual groundwater with high magnesium contents, which caused the cements to form as aragonite, like in the original shell. Hence, they are very difficult to recognize by traditional methods. Thus, although the cements cause the radiocarbon ages to be erroneous, the presence of the cements could escape detection, allowing the erroneous dates to be accepted as genuine. Such clams should be vetted using tools such as scanning electron microscopy before radiocarbon dates are accepted uncritically.
Coral reefs, one of the world's most charismatic ecosystems, are currently threatened by a variety of human impacts, and biologists have been studying them for only a few decades. Thus, the sedimentary record is the best option for adding historical perspective to questions in reef conservation biology. Kosnik et al. used dating techniques to examine the accumulation of dead shells in shallow-water Great Barrier Reef sediments. The top 20 centimeters of sediment was dominated by living clams, while the meter of sediment below that was thoroughly mixed on a sub-century scale. Kosnik et al. also found that small shells were more likely to be destroyed quickly leading to a size-biased death assemblage and that large shells are more likely to be buried quickly creating a size-biased age structure within lagoonal sediments. This study provides an important context for paleoecological studies in shallow-water reef systems, and emphasizes the importance of sediment mixing in these environments.
A catastrophic extraterrestrial impact 1850 million years ago produced the Sudbury crater, the second largest known impact site on Earth. Pufahl et al.’s discovery of debris in northern Michigan, USA, produced from this impact has provided new information regarding the nature of this event. A prominent iridium anomaly in impact-generated tsunami deposits containing shocked quartz, spherules, tektites, and accretionary lapilli demonstrate that the extraterrestrial body was a meteorite and not a comet, as previously proposed. The Sudbury event was larger than those responsible for later major extinction events, and may prove important in the evolution of early life on Earth.
The African origin of early modern humans 200,000–150,000 years ago is now well documented, with archaeological data suggesting that a major migration from tropical east Africa to the Levant took place between 130,000 and 100,000 years ago via the presently hyper-arid Saharan-Arabian desert. This migration was dependent on the occurrence of wetter climate in the region. Whereas there is good evidence that the southern and central Saharan-Arabian desert experienced increased monsoon precipitation during this period, no unequivocal evidence has been found for a corresponding rainfall increase in the northern part of the migration corridor, including the Sinai-Negev land bridge between Africa and Asia. Passage through this “bottleneck” region would have been dependent on the development of suitable climate conditions. Vaks et al. present a reconstruction of paleoclimate in the Negev Desert based on absolute uranium series dating of carbonate cave deposits (speleothems). Speleothems only form when rainwater enters the groundwater system and vegetation grows above a cave. Today the climate in the Negev Desert is very arid and speleothems do not form, but their presence in a number of caves clearly indicates that conditions were wetter in the past. Vaks et al. dated 33 speleothem samples from five caves in the central and southern Negev Desert. The ages of these speleothems show that the last main period of increased rainfall occurred between 140,000 and 110,000 years ago. The climate during this time consisted of episodic wet events that enabled the deserts of the northeastern Sahara, Sinai, and the Negev to become more hospitable for the movement of early modern humans. The simultaneous occurrence of wet periods in the northern and southern parts of Saharan-Arabian desert could have led to the disappearance of the desert barrier between central Africa and the Levant. The humid period in the Negev Desert between 140,000 and 110,000 years ago was preceded and followed by essentially unbroken arid conditions; thus creating a climatic "window" for early modern human migration to the Levant. Vaks et al.’s study suggest that climate change had an important limiting role in the timing of dispersal of early modern humans out of Africa.
Deep-sea hydrothermal vents support communities of animals that depend on hydrogen sulphide, rather than sunlight, for their energy. Many of the most dominant animals in these communities have symbiotic relationships with bacteria, including giant tube worms. Ancient vent communities are very rare, but provide important insight into biological evolution in this challenging ecosystem. Little et al. decribe a newly discovered Cretaceous vent community from a metal ore deposit in the Republic of Georgia, which is the first to be identified from an ancient volcanic arc setting, and is very likely the shallowest ancient example yet found. The fauna consists of worm tube fossils and is one of three examples that occurred over a time span of roughly 6 million years in an area of the Neotethyan Ocean that was as tectonically complex as the present day west Pacific Ocean.
Supercontinent cycles are a poorly understood, but paramount, process in shaping Earth's surface. Geologic evidence strongly supports the existence of the supercontinent Pangea 200 million years ago. Alleged earlier supercontinents such as Rodinia (roughly 1 billion years ago) and Columbia (roughly 1.8 billion years ago) suggest that continents do aggregate and disperse in some cyclical fashion. However, there is currently no theoretical basis for the regularity of such cycles. Phillips and Bunge present numerical models of mantle convection with continents in a spherical geometry that show that periodic supercontinent cycles are unlikely to occur if hot plumes rising from the base of the mantle are of sufficient strength. This could have implications for the interpretation of global mountain building episodes, the stability of Earth's rotation axis, flood basalt generation, and even global climate events.
Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 years. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 years. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other.
Mann et al. used three-dimensional seismic data off the east coast of Trinidad to map the main South America–Caribbean plate boundary strike-slip fault that was known from previous onland GPS and fault-trenching studies in Trinidad. The offshore mapping shows that the fault has right laterally offset a buried fluvial channel by a distance of 322–506 meters. The channel is inferred to have formed when sea level was much lower during the Last Glacial Maximum at about 30,000 years ago. Combining the offset amount with the inferred age of the channel, a long-term slip of 17–19 mm/yr can be calculated. The earthquake hazard of this fault is significant because it accommodates most of the lateral strike-slip displacement between the South America and Caribbean plates.
Paleoseismology: Why can't earthquakes keep on schedule" Robert S. Yeats, Earth Consultants International, Tustin, CA, 92780, USA, and Oregon State University, Corvallis, OR 97331, USA. http://www.gsajournals.org/perlserv/"request=get-document&doi=10.1130%2Ffocus092007.1
Blue Nile incision on the Ethiopian Plateau: Pulsed plateau growth, Pliocene uplift, and hominin evolution
Nahid DS Gani, Energy and Geoscience Institute, University of Utah, Salt Lake City, Utah 84108, USA; et al.
More than three million years ago, early hominins evolved the ability to walk upright and in doing so started us along the evolutionary path that eventually gave rise to Homo sapiens. It was Darwin who first suggested that a change of climate, giving rise to vast, arid, savannahs, may have spurred on human evolution all those millions of years ago. But what caused that change of climate" Could the formation of one of Earth’s most spectacular landscapes, the Ethiopian Plateau, have been responsible for development of the great African grasslands" And if so, what were the geological processes that led to the formation of the plateau" To answer these questions, Nahid DS Gani of the University of Utah and colleagues turned to some of the most advanced technology that our species has yet produced - the Space Shuttle. By integrating radar topography data collected by the space shuttle with field observations, Gani and colleagues have placed tight constraints on the topographic evolution of the Ethiopian Plateau. Their results indicate that rapid uplift of the plateau began about six million years ago and was related to the development of large shield volcanoes that erupted great volumes of basalt. These findings suggest that plateau formation resulted from the presence of a hot plume of mantle pushing up against the base of the African continent. The timing of plateau formation coincides with and is therefore probably related to the change in climate that gave rise to the African savannahs and ultimately to human evolution. Ironically, Gani and colleagues had to turn to outer space to determine that the geological processes that spurred on human evolution lay deep within Earth’s mantle.
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