Geology and Geochemistry

▪ 2008

Introduction

Diamond inclusions in ancient terrestrial rock provided clues about the early history of Earth's crust. Scientists studied slow earthquakes and the crystalline structure of Earth's inner core. International scientific studies documented global warming, and new research anticipated climate-change effects in specific areas of the U.S.

      The oldest diamonds known in terrestrial rock were described in 2007 by Martina Menneken of the Institute for Mineralogy, Münster, Ger., and colleagues. The diamonds appeared as tiny inclusions in zircon crystals extracted from ancient metamorphosed sediments from Jack Hills in Western Australia. The scientists studied 1,000 zircon grains and found diamonds in 45 of them. Isotope dating of these zircons had previously established their extreme age, with some being as old as 4.25 billion years. Although geologic processes had destroyed all rocks from so long ago, the resistant zircons passed from one rock cycle to the next. The trapped inclusions were therefore the only known source of physical information about conditions on early Earth. The prevailing view had been that these early conditions were dominated by hot basaltic lavas. The geochemistry of a variety of silicate-mineral inclusions found in the zircon crystals had recently established that the inclusions had grown within water-bearing granitic magmas 4.25 billion–4 billion years ago and at temperatures as low as 680 °C (1,256 °F), which was a surprising indication that a continental crust was already present. An analysis of light scattered by the diamond inclusions (using Raman spectroscopy) revealed distinctive structural and chemical properties that were matched only by microdiamonds that had been found in zircon crystals of ultrahigh-pressure metamorphic rocks, and evidence had shown that these microdiamonds probably grew at depths of at least 100 km (62 mi). The authors' preferred interpretation of the origin of the Australian zircons with diamonds was that the zircons grew deep within a thickened continental lithosphere more than 4.25 billion years ago before they were caught up in the relatively cool granitic magmas in continental crust.

 

 Stromboli Island's volcano is spectacular for its explosive blasts of gas and lava, which typically occur every 10 to 20 minutes. The flying chunks of lava generally rise from shallow depth and fall within the crater, but occasional larger explosions from deeper sources threaten volcano visitors and nearby inhabitants. Mike Burton of the National Institute of Geophysics and Volcanology, Catania, Sicily, and colleagues published the results of gas analyses of measurements made in 2000–02 with geochemical remote sensing. They used an infrared spectrometer to take a series of continuous records from a distance of about 250 m (820 ft). The results demonstrated that gas composition and temperature changed abruptly during the explosive eruptions. In particular, the temperature and the ratios of carbon dioxide to water and to sulfur dioxide increased. The composition of the gas dissolved in the original magma was determined from analyses of glass inclusions that had been trapped in olivine at a depth of about 10 km (6.2 mi). Using the known solubility of gases as a function of pressure and temperature, a computer simulation of degassing during the rise of the magma helped explain the volcano's plumbing system. About 99% of the gases are released quietly as the magma rises to levels of reduced pressure. The other 1% coalesces into large bubbles, or slugs, that accumulate and intermittently clog the volcanic conduit until they rise rapidly and burst out explosively from about 250 m below the surface. The results showed that the gas slugs are generated at a considerable depth of about 3 km (1.8 mi) and are decoupled from the slower magma uprise and degassing process. Improved understanding of the mechanisms controlling strombolian explosive activity in Sicily and other areas was a high priority for civil defense.

 

 According to evidence published by Sanjeev Gupta of Imperial College, London, and colleagues, the folded chalk ridge that once formed a land bridge between England and France near the Dover Strait was disrupted twice, generating torrential floods that scoured the land surface that became the seafloor beneath the English Channel. High-resolution sonar mapping of the seafloor, supported by older charts from the U.K. Hydrographic Office, revealed an intricate array of features, including incised channels around elevated regions (former islands), scarps, cataracts, and hanging tributaries. Geomorphic interpretation of these features indicated the occurrence of two successive megafloods. During periods of maximum glaciation and lowest sea level, the floor of the English Channel was continuous low-lying land. The chalk ridge acted as a dam that retained a large glacial lake over part of what is now the North Sea, south of the edge of the Scandinavian ice sheet. The rising lake eventually broke over and through the chalk dam and rapidly drained in a catastrophic flood about 450,000 years ago. During a second glacial maximum about 250,000 years ago, a restabilized dam was ruptured, and the second megaflood incised its history across that of the first flood. The seafloor between England and continental Europe was alternately flooded and exposed as sea level rose and fell during the glacial cycles, and additional paleogeographic changes were associated with the huge glacial lakes, land bridges, and dam bursts. These changes greatly influenced subsequent plant, animal, and human migrations.

      Predictions about future climate change depend critically on knowledge about the timing of past climate changes. Progress in dating the fluctuations recorded in many geologic climatic proxies (indicators) was reviewed and evaluated at a workshop in 2007 on “Radiocarbon and Ice-Core Chronologies During Glacial and Deglacial Times.” Results were outlined by Bernd Kromer of Heidelberg (Ger.) Academy of Sciences and others. Accurate timescales with high resolution were essential for correlation of the various geochemical measurements made on cores from marine and lake sediments, ice, trees, caves, and corals. Radiocarbon dating furnished a common timescale for terrestrial and marine materials. Tree-ring analyses provided good calibration back to about 12,500 years ago, but the radiocarbon calibration curve that was extended back to about 26,000 years ago on the basis of studies of coral and foraminifera in marine sediment was less certain. There was no accepted older chronology because calibrations of the carbon- and oxygen-isotope records from ice cores (back to 650,000 years ago) and marine sediments (back to about 100 million years ago) had generated timescales with significant discrepancies. Important advances were presented on carbon-isotope measurements for tree-ring chronology. New carbon-isotope data from corals and uranium-thorium dating of stalagmites from Chinese caves provided the prospect of extending reliable radiocarbon dating beyond 26,000 years, well into the glacial period preceding the current interglacial period.

      Jean-Daniel Stanley of the Smithsonian Institution, Washington, D.C., and four coauthors reviewed the results of recent geologic, geochemical, and archaeological studies of seven sediment cores obtained from the east harbour of Alexandria, Egypt. Alexander the Great founded the city in 332 BC. A town had already existed at the site for at least seven centuries, but evidence of human activity was limited to periods later than about 400 BC. The sediments were classified and dated by radiocarbon analyses. Potsherds and ceramic fragments in the sediments that were dated to between 940 and 420 BC were typical of the cooking vessels, bowls, and jars used in the southeastern Mediterranean during the 9th to 7th century BC. The contents of lead, heavy minerals, and organic material in the sediments began to increase at about 900 BC, which provided signals of early human-related activity. Lead concentrations increased from less than 10 parts per million (ppm) up to about 60 ppm by 330 BC and then exceeded 100 ppm during the swift expansion of Alexandria. Heavy mineral and organic contents followed similar patterns, with abrupt increases through the three centuries after Alexander arrived. The heavy minerals were derived from imported construction rocks, and the organic material was derived from increased sewage runoff from the booming city.

 

Peter J. Wyllie

Geophysics

      On Aug. 15, 2007, an earthquake of moment magnitude 8.0 occurred off the coast of southern Peru, near the city of Pisco. The dimensions of the fault plane were about 100 km by 200 km (60 mi by 120 mi), and the relative movement between the two sides of the fault was 8 m (26 ft). Over 35,000 buildings were destroyed, and more than 500 persons were killed. Seismic waves from the earthquake were felt in all of the countries that border Peru, and a tsunami with wave heights generally ranging from 10 to 30 cm (4 to 12 in) was recorded throughout the Pacific basin. Large earthquakes are common in Peru because of its location next to a convergent plate boundary where the Nazca tectonic plate subducts (descends) eastward beneath the South American plate at an average rate of 7.7 cm (3 in) annually.

      Satoshi Ide of the University of Tokyo and colleagues discovered a new scaling law for what are known as slow earthquakes. In contrast to normal earthquakes, which last only tens to at most hundreds of seconds, these seismic events occur over a span of a few hours to a year. Slow earthquakes generally cannot be felt and were discovered only in the last decade because of the large numbers of broadband seismometers that had been deployed in Japan and the western United States. The researchers found that the size of slow earthquakes increased in direct proportion to the duration of the event, whereas for normal earthquakes the size was proportional to the cube of the event's duration. This observation unified a diverse group of slow seismic phenomena that were previously thought to be distinct. Although slow earthquakes do not pose a direct hazard to society, they do significantly affect the amount of strain at convergent plate boundaries and therefore influence when and where large damaging “normal” earthquakes occur.

      Since 2003 American seismologists had operated a network of 12 ocean-bottom seismometers along a 4-km (2.5-mi) stretch of the East Pacific Rise west of Central America where the Nazca and Pacific tectonic plates spread apart at an average rate of 11 cm (4.3 in) per year. During a data-recovery cruise in April 2006, the researchers were chagrined to find that only 4 of the 12 instruments could be recovered. Seismic data from 2 of the recovered instruments showed a gradual increase in microseismicity (tremors) with a large peak of activity about Jan. 22, 2006, followed by a sharp cutoff. In 2007 Maya Tolstoy of the Lamont-Doherty Earth Observatory, Palisades, N.Y., and colleagues reported that subsequent measurements of ocean-water temperature and light scattering, dating of rock samples, and seafloor digital images of new lava rock indicated that an eruption had taken place at the midocean ridge.

      A new volcanic island was recently born in the Pacific Ocean near Home Reef in the Vava'u island group of Tonga. These islands were formed by magma created during the westward subduction of the Pacific tectonic plate beneath the Australian plate. Evidence of the volcanic eruption that created the new island was first noticed by a passing ship in August 2006, and the eruption was subsequently monitored via satellite imagery. In early 2007 R. Greg Vaughan of the California Institute of Technology and co-workers reported on data from the Aqua and Terra satellites, which were used to monitor the size of the new island, the sea-surface temperature in the vicinity of the volcano, and the dispersal of pumice rafts (masses of floating pumice rock). The volcano had previously erupted in 1984, when it created a small island, but the island had eroded away prior to the 2006 eruption.

      Katrin Mierdel of the Institute for Geosciences, Tübingen, Ger., and colleagues reported the results of a series of mineral physics experiments that provided a novel explanation for the existence of Earth's asthenosphere, a layer in the upper mantle that is softer and less viscous than the lithospheric plates that override it. The scientists found significant differences in the water solubility of the two main minerals of the upper mantle, olivine and enstatite, as a function of temperature and pressure. Water solubility for olivine continually increases with depth, whereas for enstatite the water solubility sharply decreases with depth before gradually increasing. The combination of the two behaviours leads to a pronounced solubility minimum for the overall composition of the upper mantle at the depth of asthenosphere beneath both continents and oceans. The investigators suggested, therefore, that the partial rock melting prevalent in Earth's asthenosphere is likely caused not by volatile enrichment but by the inability of large amounts of water to chemically bind to the surrounding rock.

 

 An international team of geophysicists led by Leonid Dubrovinsky of Bayerisches Geoinstitute, University of Bayreuth, Ger., reported new evidence that the crystalline structure of Earth's solid inner core is body-centred cubic (bcc) as opposed to hexagonal close-packed (hcp). Scientists had traditionally believed hcp to be the stable phase of iron at the extremely high pressures and temperatures near the centre of the Earth. The researchers placed samples of an iron-nickel alloy that contained 10% nickel in heated diamond-anvil cells and used X-ray diffraction to image the internal structure of the samples as pressure was increased to more than 225 gigapascals (4.7 billion lb per sq ft) and as temperature was raised to more than 3,100 °C (5,600 °F). The team's results confirmed earlier suggestions that the presence of modest amounts of nickel alters the pressure-temperature stability of iron such that the bcc crystalline structure becomes the stable phase. On the basis of evidence from meteorites, scientists believed that Earth's core contains 5–15% nickel, so the new experiments strongly implied that bcc crystals exist within the core.

 

Keith D. Koper

Meteorology and Climate

 In 2007 the United Nations panel of experts on climate change issued its Fourth Assessment Report and concluded that the cumulative evidence since the previous assessment, released in 2001, more strongly indicated that human activities were affecting global climate. In the first part of the assessment, which covered the physical- science basis of climate change, the Intergovernmental Panel on Climate Change (IPCC) stated that a variety of observations had revealed that the warming of the climate system was “unequivocal” and that there was “very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming.” It indicated that 11 of the past 12 years (1995–2006) ranked among the 12 warmest years in the instrumental record of global surface temperatures, that mountain glaciers and snow cover had declined on average in both the Northern and Southern hemispheres, and that losses from the ice sheets of Greenland and Antarctica had “very likely contributed to sea-level rise over 1993 to 2003.” According to the report, however, not all aspects of climate were changing. For example, although the minimum extent of the Arctic ice cap during the summer months was shrinking—with the 2007 minimum shattering previous records—there were no significant trends in Antarctic sea-ice extent, a result consistent with the observation that atmospheric temperatures had not been warming when averaged across the Antarctic region. Also, there were no consistent trends in daily temperature ranges, since day and night temperatures had risen at about the same rate.

      As for the future, the IPCC projected a warming of about 0.2 °C (0.35 °F) per decade through the mid-2020s for a range of emission scenarios, but continued greenhouse-gas emissions “at or above current rates would cause further warming and induce many changes in the global climate system.” Estimated temperature increases for the end of the 21st century relative to 1980–99 global averages ranged from 1.8 °C (3.2 °F) for the low-emissions scenario to 4 °C (7.2 °F) for the high-emissions scenario. The IPCC narrowed the ranges for forecast sea-level rises in the current report compared with previous reports. For the years 2090–99 the rise varied from 18–38 cm (7–15 in) for the low-emissions scenario to 26–59 cm (10–23 in) for the high scenario, all relative to 1980–99 mean sea levels. (See Special Report.)

      In research news concerning climate change in the United States, a study by Martin Hoerling and colleagues from the National Oceanic and Atmospheric Administration Earth System Research Lab, Boulder, Colo., found that greenhouse gases likely accounted for more than half the above-average warmth experienced across the country in 2006. According to NOAA's National Climatic Data Center, mean U.S. temperatures in 2006 were the second highest since record keeping began in 1895, tying 1934 for second place and coming in slightly cooler than the record warm year of 1998. A study by Barry Lynn of NASA's Goddard Institute for Space Studies and colleagues suggested that greenhouse-gas warming might raise average summer temperatures by about 5.5 °C (10 °F) in the eastern part of the country by the 2080s. This conclusion was based on a climate simulation that used a weather-prediction model coupled to a global-climate model.

      A study published by a team of authors headed by scientists at the Lamont-Doherty Earth Observatory, Palisades, N.Y., suggested that more drought could be in store for the U.S. Southwest. A broad consensus of climate models indicated that during the 21st century the region would become drier than it had been and that it might already be undergoing the change. If the models were correct, the implication was that the levels of dryness seen in the droughts of the 1930s, 1950s, and 2000–04 could become the established climate in this region within years or decades. In a separate study by the U.S. National Academy of Sciences, researchers indicated that future droughts in the Colorado River Basin could be longer and more severe because of regional warming and that this would reduce the river's flow and the amount of water that it supplied.

      Most climate models did not initialize or take into account natural variability generated internally. Doug Smith and colleagues from the U.K. Hadley Centre for Climate Prediction and Research presented a new modeling system that took into account both internal variability and external forcing from such factors as solar radiation and human-related increases in greenhouse gases. The result was a decadal (10-year) forecast of global temperature fluctuations from 2005 that indicated that warming might be subdued for several years by internal variability but that the climate would continue to warm, so the average global temperature in at least one-half of the years from 2010 to 2014 would exceed that of the warmest year on record, 1998.

      On Nov. 2, 2007, a pilotless aircraft flew into a hurricane for the first time. The 1.5-m (5-ft)-long aircraft, with a wingspan of 3 m (10 ft), took off from Wallops Island, Virginia, and was guided by remote control into the eye of Hurricane Noel off the U.S. coast. The low-altitude flight allowed continuous observations in parts of the storm where a manned hunter-aircraft mission would have risked the lives of the crew.

 

Douglas Le Comte

 

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