Boulder, CO, USA - GEOLOGY articles cover the minute to the grand, from calcite-producing earthworms, skeletal metazoans, and mineral discoveries, to Earth's highest coastal mountain range, a newly discovered extension of the Nile deep-sea fan, and a Canadian impact crater, and mark important events in time, from a 9-degree warming in Greenland only 14,700 years ago to Africa's Middle Stone Age. GSA TODAY goes farther back in time to analyze the formation and closure of the Rheic Ocean.
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Rapid exhumation of ice-covered rocks of the Chugach-St. Elias orogen, Southeast Alaska
Eva Enkelmann et al., Earth and Environmental Science Department, Lehigh University, 31 Williams Drive, Bethlehem, Pennsylvania 18015, USA. Pages 915-918.
The Chugach-St. Elias orogen in Southeast Alaska is the Earth's highest coastal mountain range, with peaks exceeding 5500 meters. Due to the high elevation, high latitude, and proximity to the Pacific Ocean, the mountain rage is mostly covered by ice. The thick ice cover hinders rock sampling at low-elevation areas, which are generally used for the study of the exhumation processes in mountain ranges. Enkelmann et al. studied the cooling ages of detrital zircons from glacial rivers to get information about the erosion processes underneath the ice. They present a new exhumation model for the Chugach-St. Elias orogen that starts about 30 million years ago.
South Atlantic interocean exchange as the trigger for the Blling warm event
Cristiano M. Chiessi et al., MARUM-Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany. Pages 919-922.
14,700 years ago, Greenland experienced a warming of 9 degrees Celsius within a couple of decades, the most striking temperature increase in the Northern Hemisphere high latitudes during the entire last deglaciation. The abrupt warming was probably linked to a sudden strengthening of the Atlantic Ocean circulation, but the ultimate mechanism responsible is not clearly understood. Chiessi et al. show new geochemical records from the South Atlantic that address this issue. According to the new records, the upper South Atlantic also underwent a strong increase in temperature and salinity synchronous to the temperature change in Greenland. While the warming in the South Atlantic is also a consequence of the strengthening of the Atlantic Ocean circulation, the increase in salinity points to a massive inflow of salty Indian Ocean waters into the Atlantic Ocean. High salinities in the upper Atlantic Ocean have been described as a key condition for a strong Atlantic circulation. Chiessi et al.'s records highlight the crucial role played by Indian-Atlantic interocean exchange as the trigger for the strengthening of the Atlantic circulation and the abrupt warming in the Northern Hemisphere high latitudes 14,700 years ago.
The eve of biomineralization: Controls on skeletal mineralogy
Andrey Yu. Zhuravlev and Rachel A. Wood, Area y Museo de Paleontologia, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, c/Pedro Cerbuna, E-50009, Zaragoza, Spain. Pages 923-926.
In the first quantitative analysis of the onset of biomineralization -- from the late Precambrian to Ordovician -- Zhuravlev and Wood show that changing seawater chemistry and climate exerted considerable control over the mineralogy of emerging skeletal metazoans. The earliest skeletons known were composed exclusively of aragonite or high-magnesium calcite, but these were replaced by lineages with low-magnesium calcite mineralogies. These changes were controlled by the changing magnesium-calcium ratio of seawater, and probably also by increasing partial pressure of CO2 in the atmosphere as a result of the onset of greenhouse conditions.
Newly recognized eastern extension of the Nile deep-sea fan
Yehoshua Folkman and Yossi Mart, Geological Survey of Israel, 30 Malkhei Yisrael St., Jerusalem 95501, Israel. Pages 939-942.
The Nile River deep-sea fan is currently a prime exploration objective in the Mediterranean Sea due to major gas discoveries in its channel complexes offshore Egypt. Folkman and Mart present a newly recognized easternmost segment of the Nile deep-sea fan that has been revealed recently in the deep Mediterranean Sea offshore Israel, based on high resolution, three-dimensional seismic imaging. This segment is vertically divided into two sequences: basal sand-rich sequence deposited in a basin floor setting, and an overlying younger mud-rich sequence, characterized by densely spaced seabed and sub-seabed distributary slope channel complexes.
Earthworms produce granules of intricately zoned calcite
Martin R. Lee et al., Department of Geographical and Earth Sciences, Gregory Building, Lilybank Gardens, University of Glasgow, Glasgow G12 8QQ, UK. Pages 943-946.
In the 19th century, Charles Darwin first found that common species of earthworms produce millimeter-sized clusters of the mineral calcite. Darwin was unable to explain how or why these calcite "granules" are formed, and this question has remained unanswered despite the fact that granules are very abundant in soils and so make an important, albeit poorly understood, contribution to the carbon cycle. Lee et al. have tackled this conundrum by analyzing the composition and internal structure of granules at very fine scales using several newly developed electron microscope techniques. Their results have revealed that the granules form in a complex sequence of events involving several episodes of crystallization of different minerals. What is more remarkable is that these reactions are all mediated by the earthworm and completed very quickly, probably in less than a day. These results illustrate very clearly how the granules form, but Darwin's other question of why they are produced, and in such numbers, can only be answered by further work.
Source of fluids forming distal Zn-Pb-Ag skarns: Evidence from laser ablation-inductively coupled plasma-mass spectrometry analysis of fluid inclusions from El Mochito, Honduras
Iain M. Samson et al., Department of Earth & Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada. Pages 947-950.
A significant proportion of the Earth's metal resources have been concentrated in the crust through the action of hot, saline (hydrothermal) fluids. In exploring for such metal resources, it is important to understand the processes that have led to metal concentration at particular locations. An issue that is especially important to address is the source of the fluids. Many zinc-lead-silver deposits are hosted by sedimentary rocks, yet the characteristics of some of these deposits point to the involvement of fluids derived from magmas. The chemical composition of the fluids responsible for metal deposits can reveal important information about the source of these fluids. This compositional data is only available from the analysis of the small aliquots of the fluids that are commonly trapped in minerals during their formation. Comprehensive chemical analysis of such fluid inclusions has recently been made feasible using laser ablation-inductively coupled plasma-mass spectrometry. Samson et al. have analyzed fluid inclusions from the El Mochito Zn-Pb-Ag deposit in Honduras using this technique and demonstrated that, despite being hosted by sedimentary rocks, the fluids responsible for mineralization were magmatic. Such data are also valuable for interpreting ore genesis in that they provide information on the metal contents of paleo fluids, allowing more accurate modeling of hydrothermal mineralizing processes.
Anatomy of a young impact event in central Alberta, Canada: Prospects for the missing Holocene impact record
C.D.K. Herd et al., Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Pages 955-958.
Based on the amount and frequency of meteorite falls and the formation of impact craters on the Earth, there should be over 20 impact craters in the < 100 m size range that formed within the past 10,000 years, yet only five such craters are known worldwide. Herd et al. report the discovery of a 36-m-diameter impact crater located in a forested area near the town of Whitecourt, Alberta, Canada. Although too overgrown to be seen in air photos or satellite images, the crater is revealed using a bare-Earth digital elevation model obtained through airborne Light Detection and Ranging (LiDAR). The crater formed in deglacial sediments, with impact ejecta burying a soil with a radiocarbon age of ~1100 years. Seventy-four iron meteorites (0.1-1196 g) have been recovered, most having an angular, shrapnel-like shape. These meteorites were buried at depths of < 25 cm and are interpreted to result from fragmentation of the original larger mass of the impactor, either at low altitude or during the impact event. Impact of the main mass formed the simple bowl-shaped impact structure associated with an ejecta blanket and crater fill. Herd et al. show that LiDAR may be a useful tool to look for additional small impact craters.
A landslide in Tertiary marine shale with superheated fumaroles, Coast Ranges, California
Robert H. Mariner et al., U.S. Geological Survey, MS 434, 345 Middlefield Road, Menlo Park, California, 94025, USA. Pages 959-962.
In August 2004, a National Forest fire crew extinguished a 1.2-ha fire in a wilderness area ~40 km northeast of Santa Barbara, California. Examination revealed that the fire originated on a landslide dotted with superheated fumaroles (gas vents). A temperature of 262 C (504 F) was measured in one of the superheated fumaroles. A temperature of 307 C (645 F) has been measured at a depth of about 3 meters (~10 feet) in a borehole near this fumarole. Temperatures in this borehole are decreasing by ~0.1 C/day. Gas collected from the fumaroles and boreholes is mostly air with 3% to 8% carbon dioxide and trace amounts of carbon monoxide, methane, ethane, and propane. The chemical and isotopic composition of the gas show that it is not associated with volcanic activity, a geothermal system, an oil field fire, or spontaneous combustion of modern organic material incorporated in the slide. Pyrite (iron sulfide) and fine-grained carbonaceous matter (essentially small fragments of low-grade coal) typically occur disseminated in this early Tertiary sandstone-shale sequence. However, local concentrations of secondary minerals normally produced when pyrite oxidizes show that high concentrations of pyrite do occur in this formation. Mariner et al. speculate that pyrite in the jumbled blocks of the Juncal shale, involved in the landslide, oxidized rapidly when air was introduced during the initial landslide movement, and this heated the rock enough to ignite dispersed solid carbonaceous matter present in the shale.
Diachronous dawn of Africa's Middle Stone Age: New 40Ar/39Ar ages from the Ethiopian Rift
Leah E. Morgan and Paul R. Renne, Department of Earth and Planetary Science, University of California-Berkeley, Berkeley, California 94720, USA. Pages 967-970.
The Middle Stone Age (MSA) of Africa is thought to be a period wherein human ancestors experienced significant advancement of cognitive abilities and physical dexterity, but the nature and timing of these advances is not yet well understood. MSA archaeology at Gademotta, in the Main Ethiopian Rift Valley, is dated to older than 277 thousand years ago, over 40 thousand years older than previously believed. The workmanship and sophistication of tool making at Gademotta and nearby Kulkuletti are markedly advanced for the antiquity of the sites' ages, suggesting that more primitive MSA archaeology should be even older, and that the MSA evolved at different rates in different places. If the emergence of the MSA is linked to the evolutionary appearance of Homo sapiens, as has often been suggested, then by implication early forms of our species may be 100 thousand years older than previously believed. The ready availability of high-quality geological materials such as obsidian may have played a role in rapidly advancing technology in certain areas such as Gademotta/Kulkuletti. The results are based primarily on new 40Ar/39Ar dating of volcanic ashes by Morgan and Renne at the Berkeley Geochronology Center.
Sulfur isotopes in sediment-hosted orogenic gold deposits: Evidence for an early timing and a seawater sulfur source
Zhaoshan Chang et al., CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 126, Hobart, Tasmania 7001, Australia. Pages 971-974.
Chang et al. report sulfur isotopic compositions of sulfides of various paragenetic stages in the giant Sukhoi Log sediment-hosted orogenic gold deposit. The overall mean value and the significant variability in early pyrite indicates that the sulfur was from the reduction of seawater sulfate. The later generations of sulfide have 34S values in successively smaller ranges, coincident with the mode that is around the median value of the whole data set. Together with textural evidence, sulfide trace element data, and gold occurrence, the data demonstrate that metamorphism has gradually homogenized the early sulfur, accompanied by the segregation of quartz and the release of gold from the lattice of early pyrite and its re-precipitation as inclusions in later pyrite. The sulfur isotopic compositions of sulfides in Sukhoi Log, and many other major orogenic gold deposits hosted in sedimentary rocks of various ages, show a pattern generally parallel to the seawater sulfate curve through geologic time, indicating that the sulfur in most sediment-hosted orogenic gold deposits was probably also originally from the reduction of seawater sulfate. Chang et al. conclude that sulfidation and gold mineralization in many sediment-hosted orogenic gold deposits was early during basin evolution when seawater was the principal active fluid, rather than later during or after basin inversion, as proposed in current models.
Multiple sulfur isotopes reveal a magmatic origin for the Platreef platinum group element deposit, Bushveld Complex, South Africa
Sarah C. Penniston-Dorland et al., Department of Geology, University of Maryland, College Park, Maryland 20742, USA. Pages 979-982.
Models for the formation of platinum group element (PGE) ore deposits commonly require that magma incorporate parts of the wall rock into that which they intrude to provide sulfur. This externally derived sulfur is thought to be necessary to concentrate PGEs into the high concentrations found in these ore deposits. Using a relatively new analytical tool, the analysis of multiple isotopes of sulfur (32S, 33S, and 34S), Penniston-Dorland et al. demonstrate that sulfur within the two-billion-year-old Platreef PGE ore horizon of the Bushveld Complex in South Africa was not derived from local wall rock, and instead must have been brought in by the intruding magma.
GSA Today Science Article
The Rheic Ocean: Origin, Evolution, and Significance
R. Damian Nance, Department of Geological Sciences, 316 Clippinger Laboratories, Ohio University, Athens, Ohio 45701, USA; and Ulf Linnemann, Staatliche Naturhistorische Sammlungen Dresden, Museum fur Mineralogie und Geologie, Konigsbruecker Landstrasse 159, D-01109 Dresden, Germany. Pages 4-12.
What was the most important ocean to adorn the face of the Earth over the past 500 million years? Damian Nance of Ohio University and Ulf Linnemann of Museum of Mineralogy and Geology in Dresden, Germany, would argue that it was the Rheic Ocean. The Rheic Ocean formed some 490 million years ago during a rifting event that tore apart the northern margin of the continents that together formed Gondwana, including Africa and South America. It was not the opening of the Rheic Ocean, but rather its demise that makes it so important. The closure of the Rheic Ocean at about 360 million years ago that gave rise to Earth's most recent supercontinent, Pangea. Ironically, the subsequent rifting of Pangea, giving rise to the Atlantic Ocean, tore apart the mountain systems that formed during the closure of the Rheic Ocean. Nance and Linnemann, based on careful correlation of structures and strata from the American Appalachians and the European Variscan mountain systems, have been able to reconstruct the tectonic life and death of the Rheic Ocean, and hence have provided us with insight into the processes that gave rise to and presaged the inevitable tectonic destruction of Pangea.
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