Boulder, CO, USA - Topics include: onset of plate tectonics on Earth; Arctic Ocean's role in the global climate system; cause of the end-Permian mass extinction; Olduvai Basin lake cycles and survival of early hominins; an ancient mantle plume and the break-up of Rodinia; relationship between earthquakes and melting glaciers; monitoring Mt. Etna gas composition and eruption forecasting; and cause of the end of the PETM. The GSA TODAY science article examines Colorado citizen involvement in geoscience research.
Subduction origin on early Earth: A hypothesis
Vicki L. Hansen, University of Minnesota, Duluth, Department of Geological Sciences, Duluth, Minnesota 55812-3036, USA. Pages 1059-1062.
How and when plate tectonics began has been enthusiastically debated for decades; since the advent of the plate tectonics theory in the early 1960s. Hansen’s paper presents a hypothesis for how plate tectonics might have begun on early Earth. According to this hypothesis, spatial and temporal overlap of internal mantle flow (“endogenic”, or internal process) and a large bolide impact (“exogenic”, or external) triggered two signature processes of plate tectonics: spreading, which leads to the formation of new crust, and Subduction, which leads to the recycling of crust. The spatial and temporal intersection of the triggering endogenic and exogenic processes could have occurred variably across early Earth; thus, plate tectonic mechanisms could have begun at different places and different times. Hansen’s hypothesis postulates that the ability of a terrestrial planet to develop plate tectonics results from a balance between the strength of its surface layer and the size of bolide impact it can survive—both of these factors can be related to planet size. Hansen’s hypothesis may allow us to predict if other planets could have evolved plate tectonics, and as such, might be Earth-like in character. The tectonic evolution of early Earth likely played key roles in concentrating Earth’s rich resources, and perhaps the formation and evolution of life.
Geologic origins of salinization in a semi-arid river: The role of sedimentary basin brines
James Hogan et al., University of Arizona, SAHRA / Dept. of Hydrology and Water Resources, Tucson, Arizona 85721, USA. Pages 1063-1066.
Rivers in semi-arid and arid regions exhibit increasing amounts of salt (salinity) downstream. Often this trend is attributed to the concentrating effect of evaporation and plant transpiration related to irrigated agriculture. In contrast, the results of Hogan et al.’s investigation of the Rio Grande River in Colorado and New Mexico suggest that geological sources of salt added by groundwater discharge have a greater impact than agricultural effects. Through detailed sampling of the surface water, Hogan et al. identified a series of salinity increases. These increases were typically located at the downstream end of sedimentary basins, where the regional groundwater flow system typically discharges to the river. Using geochemical methods, they show that these increases result from localized discharge of high-salinity groundwater, which is “fingerprinted” as a sedimentary brine source with an estimated age of millions of years. Hogan et al.’s calculations indicate that while these groundwater fluxes are very small relative to the overall flow in the Rio Grande, they are the dominant solute input and, when combined with downstream evaporation and transpiration, result in salinization. The recognition of a substantial geologic salinity source for the Rio Grande implies that alternative salinity management solutions, such as interception of saline groundwater, might be more effective in reducing salinity than changes in agricultural practices.
Nonlinear slope-dependent sediment transport in cinder cone evolution
Jon Pelletier and Michael L. Cline, University of Arizona, Department of Geosciences, 1040 E. Fourth St., Tucson, Arizona 85721, USA. Pages 1067-1070.
Geologists have long been interested in how hill slopes erode. In this paper, Pelletier and Cline use field observations and computer modeling of cinder cones to determine the precise relationship between sediment flux and slope steepness. Cinder cones allow this complex relationship to be unraveled because their initial shapes are known at precise times in the geologic past.
Tethyan oceanic currents and climate gradients 300 m.y. ago
Lucia Angiolini et al., Università degli Studi di Milano, Scienze della Terra A. Desio, Milano 20133, Italy. Pages 1071-1074.
Plants and animals adapt to the particular climatic conditions of an area and thus uniquely represent their habitat. Hence, the best tools to reconstruct past climate conditions are fossils. However, because of continental drift, the distribution of fossils must be associated with independent palaeolatitude estimates provided by palaeomagnetism. By adopting a multidisciplinary approach, Angiolini et al. reconstruct the oceanic circulation pattern of the Tethys Ocean 300 million years ago. A warm sea-surface gyre extended towards the southern tropics, but it was narrower than in modern times because it was delimited to the south by cold currents sweeping along the glaciated Gondwanan margin.
Effects of Arctic freshwater forcing on thermohaline circulation during the Pleistocene
Jochen Knies et al., Geological Survey of Norway, Surficial Deposits, Trondheim, Trøndelag 7491, Norway. Pages 1075-1078.
Knies et al.'s study from the Arctic Ocean highlights the impact of Arctic freshwater on the thermohaline circulation during the Pleistocene. The authors show that freshwater plumes released from collapsing circum-Arctic ice sheets reached the northern North Atlantic several times over the past 0.8 million years. As a consequence, the production of North Atlantic deep water slowed down, triggering severe cooling on the adjacent continents. The authors suggest that the Arctic Ocean is not only an important player in controlling global climate system in modern times, but also was in the recent geological past.
New insights into mid-ocean ridge volcanic processes from the 2005–2006 eruption of the East Pacific Rise, 9°46’–9°56’N
S. Adam Soule et al., Woods Hole Oceanographic Institution, Geology & Geophysics Department, Woods Hole, Massachusetts 02543, USA. Pages 1079-1082.
Digital seafloor imagery collected on 37 camera tows and 10 R/V Alvin dives, in which Soule et al. identify 186 contacts between new and old lava, are used to create the most detailed map of a mid-ocean ridge (MOR) eruption to date. Lava flows erupted in 2005–2006 at the East Pacific Rise (EPR) and covered an area of 14.6 square kilometers along approximately 18 km of the EPR crest between 9°46′ and 9°56′N. The 2005–2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors, and pillow forms at terminal flow fronts. Numerous lava channels approximately 10–50 meters wide and 1–5 meters deep trending approximately east-west served as distributory pathways. Eruptions were sourced from fissures within the EPR axial summit trough, as well as fissures located on an off-axis fissure mound approximately 600 meters east of the EPR axis between 9°52′ and 9°56′N. Portions of the lava flow reached as far as approximately 2 kilometers east of the axis near 9°51.2′N. Using a conservative estimate of 1.5 meters for the average flow thickness implies that the 2005–2006 eruptions produced approximately 22 × 106 meters3 of lava, 4–5 times larger than estimated volumes of 1991–1992 EPR lava flows. Estimated lava volume for the 2005–2006 eruptions represents less than 15% of the magma available in the axial magma chamber.
Changes in the global carbon cycle occurred as two episodes during the Permian–Triassic crisis
Shuceng Xie et al., China University of Geosciencess, Key Laboratory of Biogeology and Environmental Geology, Wuhan, Hubei 430074, China. Pages 1083-1086.
Earth witnessed its most severe mass extinction 250 million years ago. This extinction has been thought to be abrupt and probably caused by an extraterrestrial impact. However, Xie et al. present several lines of geochemical evidence from a South China section (an optimal section to study the biotic crisis) that indicates a two-episodic global change in association with the ecological crisis. The global carbon cycle, the enhanced terrestrial weathering, the marine photic zone euxinia, the faunal mass extinction, and the cyanobacterial expansion all occurred as two episodes, showing a close coupling among the ocean, the atmosphere, and the land system at that time. In particular, Xie et al. found that the first episode occurred before the presumed bolide impact. The temporal sequence of these two events suggests that the biotic crisis was a consequence of prolonged and episodic changes in the marine and continental systems, and argues against an extraterrestrial impact as the main cause.
Orbital rhythms, monsoons, and playa lake response, Olduvai Basin, equatorial East Africa (ca. 1.85–1.74 Ma)
Gail Ashley, Rutgers, The State University of New Jersey, Geological Sciences, 610 Taylor Road, Piscataway, New Jersey 08854-8066, USA. Pages 1091-1094.
Early humans evolved in Africa during times of increasing aridity and climate variability. Olduvai Gorge, Tanzania (near the equator) has been an important site for human origin research since its discovery in the 1930s. The Gorge exposes a 100,000-year-long record at the Plio-Pleistocene boundary that contains a rich vertebrate fossil record, two species of hominins, thousands of stone tools, and a detailed paleoenvironmental record. Ashley’s recent geological studies of Olduvai Basin revealed lake cycles that occurred on time scales of approximately 20,000 years. These wet and dry periods were likely forced by astronomical precession cycles that altered the intensity of the monsoon rains. A freshwater wetland fed by groundwater (on the basin margin) existed during even the driest times and may have been important for the survival of hominins in this harsh environment.
The flux of gold and related metals through a volcanic arc, Taupo Volcanic Zone, New Zealand
Stuart F. Simmons, University of Auckland, School of Geography, Geology and Environmental Science, Auckland 1142, New Zealand; and Kevin L. Brown, GEOKEM, Swanson, Waitakere 0653, New Zealand. Pages 1099-1102.
Using a breakthrough in sampling technology in order to understand how precious metal ore deposits form, Simmons and Brown determined how much. and how fast. gold and silver are transported by deeply circulating hot waters in a volcanically active belt, the Taupo Volcanic Zone, New Zealand. The chemistry of the hot fluids and the contributions from deep-seated magmas are the main controls of how much gold and silver are in solution, whereas fluid-flow rates dictate the gold and silver fluxes and how fast ore deposits form. Overall, enough gold and silver are transported through the Taupo Volcanic Zone in a geologically short period of time (approximately 50,000 years) to account for some of the largest precious metal deposits in the world.
Ca. 825 Ma komatiitic basalts in South China: First evidence for >1500 °C mantle melts by a Rodinian mantle plume
Xian-Hua Li et al., Institute of Geology and Geophysics, Chinese Academy of Sciences, State Key Laboratory of Lithospheric Evolution, Beijing 100029, China. Pages 1103-1106.
The breakup of the Neoproterozoic supercontinent Rodinia had important implications for extreme climate change and the first appearance of multicellular life on Earth. Although mantle plume activities have been invoked as a cause for the breakup of Rodinia, geological evidence, such as anomalously hot mantle-derived magmas (like picrite or komatiite), for the proposed mantle plume is rare. In this study, Li et al. present geochronological and geochemical data from a suite of pillowed lavas in central South China. Their work demonstrates that these rocks were typical komatiitic basalts generated approximately 825 million years ago by melting of an anomalously hot mantle source similar to that of modern mantle plumes. Thus, these komatiitic basalts provide the first solid petrological evidence for the proposed approximately 825 million-year-old mantle plume that played a key role in the breakup of the supercontinent Rodinia.
Postglacial slip-rate increase on the Teton normal fault, northern Basin and Range Province, caused by melting of the Yellowstone ice cap and deglaciation of the Teton Range
Andrea Hampel et al., Ruhr-Universitaet Bochum, Institut fuer Geologie, Mineralogie und Geophysik, Universitaetsstr. 150, Bochum, Nordrhein-Westfalen 44801, Germany. Pages 1107-1110.
The Teton Range in the northeastern Basin-and-Range Province is famous for its high relief, with peaks up to 13,770 ft high (4,197 m), and a spectacular glacial geomorphology with deep U-shaped valleys. The rapid rise of this rugged mountain range in the last few million of years is related to repeated earthquakes on the Teton normal fault, which runs north-south along the mountain front. Today, the Teton fault represents a prominent seismic gap in the greater Yellowstone-Teton region; however, up to 50-m-high fault scarps and paleoseismologic data document that multiple earthquakes ruptured the Teton fault in the last 15,000 years, i.e. after the last glaciation. Hampel et al.’s study shows that these earthquakes can be explained as a response to the melting of glaciers in the Teton Range and the large Yellowstone ice cap. More generally, their results imply that the melting of large volumes of ice stored on Earth's surface in glaciers and ice sheets can significantly influence the earthquake frequency on active faults.
Forecasting Etna eruptions by real-time observation of volcanic gas composition
Allesandro Aiuppa et al., University of Palermo, Palermo 90123, Italy. Pages 1115-1118.
Volcanic eruptions pose significant issues in terms of volcanic risk mitigation. By presenting an exceptional data set of volcanic gas observations performed at Etna before and during its recent eruption in 2006, Aiuppa et al. unambiguously show, for the first time, that real-time monitoring of volcanic gas composition can help to forecast volcanic eruptions.
Evidence from the Crow Creek Member (Pierre Shale) for an impact-induced resuspension event in the late Cretaceous Western Interior Seaway
Ryan Weber, Paleo-Data, Inc., Paleontology, New Orleans, Louisiana 70124, USA; and David K. Watkins, Department of Geosciences, University of Nebraska, Lincoln, Nebraska, USA. Pages 1119-1122.
The Crow Creek Member, a unique rock unit in the Upper Cretaceous Pierre Shale in South Dakota and Nebraska, has been thought to represent sea-level rise along the eastern margin of the Western Interior Seaway. However, Weber et al. show that the presence of impact ejecta from the Manson Impact Structure, found within the Member, suggests an impact-induced genesis. Current investigation of a reworked nannofossil assemblage occurring within the Crow Creek Member is consistent with an origin involving gravitational settling rather than sea-level rise. Additionally, the reworked assemblage decreases in abundance with increased distances from the Manson Impact Structure. These observations support the hypothesis that Manson Impact may have caused a resuspension event in the Western Interior Seaway, resulting in Crow Creek deposition.
Barite accumulation, ocean productivity, and Sr/Ba in barite across the Paleocene–Eocene Thermal Maximum
Adina Paytan et al., University of California-Santa Cruz, Institute of Marine Science, Santa Cruz, California 95064, USA. Pages 1139-1142.
55 million years ago, Earth went through a period of extreme global warming caused by rapid emission of greenhouse gases. It is unknown what ended this episode of greenhouse warming. Paytan et al. use the record of barite accumulation to show that high oceanic productivity over thousands of years was a factor in ending this warm period, by carbon removal from the atmosphere to the deep ocean. The chemical characteristics of barite indicate that records the water column productivity conditions, and not other unrelated processes.
Rapid fluid-rock interaction in oilfield reservoirs
Stephanie Houston et al., University of Leeds, Earth Sciences, Leeds, UK. Pages 1143-1146.
Important chemical reactions between water and rock in oilfield reservoirs can occur on a timescale of months. Seawater injection into a North Sea oilfield caused rapid reactions involving sulfate, carbonate, and especially silicate minerals. Houston et al.’s study indicates that CO2 injected into similar mature oilfields is likely to react with host pore-waters and rocks on a human timescale. This evidence greatly affects the safety case for CO2 storage (and sequestration) in oilfield reservoirs.
GSA TODAY Science Article
Earth history along Colorado’s Front Range: Salvaging geologic data in the suburbs and sharing it with the citizens
Robert G. Raynolds et al., Denver Museum of Nature & Science, Denver, Colorado, 80205, USA
What can past changes of climate teach us about the current warming of the Earth" In a recent GSA Today article, Bob Raynolds and his colleagues at the Denver Museum of Nature & Science described their research, which involved the mobilization of an army of volunteers, on the sedimentary rocks underlying the Denver region. These rocks are rich in dinosaur and plant fossils and provide us with a detailed record of climate change in the Denver region spanning tens of millions of years. Today’s dry, arid conditions contrast with the rainforests that covered the region 60 million years ago, and present us with a stark dilemma: the waters that flow through the aquifers beneath Denver are a testament to the wetter conditions that prevailed here in the past and constitute a non-renewable resource. Acknowledging the dependence of urban regions on the products of past climates is essential in order to make informed decisions about future development.
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