|3 March 2009
GSA Release No. 09-07
Director - GSA Communications & Marketing
March Media Highlights
Boulder, CO, USA - GEOLOGY encompasses the vast (martian gullies) to the minute (the inner ear of teleost fish); from past (deep-sea corals and the Southern Ocean 17,000 years ago) to present (the disappearance Staghorn corals due to anthropogenic disruption of Caribbean-region ecosystems); from fire (coal fires in Wyoming and Montana; fire-fountain eruptions on Earth and the Moon) to water (flooding along the Silk Road). GSA TODAY addresses the vulnerability of inhabited Pacific atolls to global warming-driven sea-level rise.
Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Christa Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Southern Ocean evidence for reduced export of North Atlantic deep water during Heinrich event 1
Laura F. Robinson and Tina van de Flierdt, Dept. of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. Pages 195-198.
The polar regions are particularly susceptible to climate change; therefore, records of how they changed in the past are needed in order to understand their sensitivity. Deep-sea corals form unique, high-resolution archives of past ocean chemistry that can be dated using the decay of uranium to thorium. They are abundant in the Southern Ocean, and can provide unprecedented insights into ocean circulation and ocean chemistry on sub-millennial time scales in areas where we are lacking other, more conventional, archives to record the past. By analyzing the hard skeleton of deep-sea corals, Robinson and van de Flierdt show that water in the Southern Ocean had a different chemical composition 17,000 years ago than it does today. From other studies, we know that around that time, massive layers of material derived from icebergs were deposited in the North Atlantic Ocean, and many researchers have suggested that the freshwater provided by the melting of these icebergs slowed the production of deep water formation. Robinson and van de Flierdt’s geochemical data from the Southern Ocean are most readily explained by a twofold reduction in the flux of deep water leaving the North Atlantic Ocean, supporting the above hypothesis.
The terrestrial Permian-Triassic boundary event bed is a nonevent
Robert A. Gastaldo et al., Dept. of Geology, Colby College, Waterville, Maine 04901, USA. Pages 199-202.
The End Permian Mass Extinction is the largest known in Earth’s history. Both marine and land-based ecosystems were thought to be affected. The effects of the End Permian Mass Extinction on land are based on the stratigraphic record in the Karoo Basin, South Africa, and have been thought to be highlighted by a unique sedimentological interval previously termed the “event bed” or “dead zone.” This laminated interval overlies the last occurrence of the synapsid reptiles, Dicynodont, and has been used as a coeval marker bed capping the extinction of these reptiles across the Karoo Basin and other continents (e.g., Antarctica). Gastaldo et al. demonstrate that the event bed in the area where it was first described, Bethulie, Eastern Cape Province, is not found at the same stratigraphic position, and is separated by 8 m of stratigraphic section within 1 km on opposite sides of the valley. Several kilometers away, in the Tussen die Riviere game park, it is in fault contact with typical Upper Permian, green-gray siltstone. And, several hundred kilometers away at Lootsberg Pass, Eastern Cape Province, the last dicynodonts occur above a laminated interval rather than below it, as in Bethulie. There is no evidence to support a terminal extinction event in the continental record of the Karoo Basin based on previously published criteria of a terminal event bed or dead zone. This calls into question the credibility of hypotheses on the response of continental ecosystems to the marine mass extinction.
Unique chronostratigraphic marker in depositional fan stratigraphy on Mars: Evidence for ca. 1.25 Ma gully activity and surficial meltwater origin
Samuel C. Schon et al., Dept. of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, Rhode Island 02906, USA. Pages 207-210.
Since the discovery of gullies on Mars, a variety of formation hypotheses have been proposed to explain these enigmatic features, but it has been difficult to differentiate between groundwater release, dry granular flow, and top-down melting scenarios using past observations. Specifically, the age of Mars gullies, and thus their specific link to recent climate history, has been uncertain because the area of individual gullies themselves is too small to obtain reliable ages using crater size frequency distributions. Here, Schon et al. document a gully system whose depositional fan is composed of distinct lobes bracketing a datable chronostratigraphic marker: secondary crater clusters. They show that the source of these pervasive secondary craters is a nearby rayed crater which they have dated to 0.6-2.4 Ma. This places the gully activity above the stratigraphic marker in the most recent period of obliquity-controlled icy mantling of high latitude terrain on Mars, and suggests that a top-down source of meltwater is responsible for forming the gullies.
Isotopic analysis of coexisting late Jurassic fish otoliths and molluscs: Implications for upper-ocean water temperature estimates
G.D. Price et al., School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK. Pages 215-218.
Fish otoliths are the stato-acoustic organs of bony (teleost) fish and represent an under-exploited resource with respect to Jurassic isotope studies of climate and ecology. Price et al.'s oxygen isotope data from the UK provide upper ocean paleotemperature estimates in excess of 30 degrees Celsius that rival temperatures associated with the mid-Cretaceous thermal maximum. Price et al.’s carbon isotope data suggest that the otoliths examined in this study belong to fish with high metabolic rates.
Origin of basalt fire-fountain eruptions on Earth versus the Moon
Malcolm J. Rutherford and Paolo Papale, Dept. of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA. Pages 219-222.
In this study by Rutherford and Papale, the process of initial gas generation in ascending basaltic magmas on the Moon is compared with the related process on Earth for water-poor magmas. Because of the relatively low oxidation state on the lunar interior, carbon in the Moon is in the reduced state (graphite), below 8 km (40 MPa), in contrast with Earth, where carbon is oxidized at least in part, and carbon dioxide is dissolved in the melt. The gas generation in lunar basalts as a result of graphite oxidation (accompanied by iron oxide reduction) produces an iron-metal, and continues until graphite is consumed. The carbon-rich volcanic gas in terrestrial magmas is gradually exsolved during ascent, after the saturation depth is reached. Modeling the fluid dynamics of magma ascent following gas generation in lunar basalts shows that large gas volumes (40-80%) and high ascent velocities (10-40 meters per second) are reached in dike-shaped conduits (2-5 m wide) with as little as 50-100 parts per million graphite in the magma, not unlike terrestrial fire-fountain eruptions. Because of the very low viscosity of the lunar basalt magma, fragmentation of the lunar basalt is calculated to occur at the volcanic event upon eruption.
Direct dating of Fe-oxide-(Cu-Au) mineralization by U/Pb zircon geochronology
Peter M. Valley et al., Dept. of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland A1B 3X5, Canada. Pages 223-226.
Valley et al. present results for direct dating of iron-oxide (copper-gold) (IOCG) mineralization by uranium/lead zircon geochronology. Constraining the timing of mineralization has important geodynamic implications for the processes involved in the genesis of these types of deposits, and the tectonic evolution of rocks associated with these deposits. Hydrothermal zircon crystals were separated from four IOCG-type ore deposits associated with the Lyon Mountain Granite in the Adirondack Mountains (United States). Zircon grains from these low-titanium magnetite deposits reveal at least two periods of mineralization: one episode at about 1,039 million years ago and a second between about 1,015 and 1,000 million years ago. Previous age determinations of these deposits were constrained by the age of the altered host granitoids and the assumption that iron mineralization was directly related to pluton emplacement. Zircon crystals extracted from the ore yield ages that show ore mineralization was episodic and younger than pluton emplacement.
Early (pre-8 Ma) fault activity and temporal strain accumulation in the central Indian Ocean
K.S. Krishna et al., National Institute of Oceanography, Council of Scientific and Industrial Research, Dona Paula, Goa 403004, India. Pages 227-230.
Numerous researchers over the past 20 years have noted that there is an apparent agreement in age (6-8 million years ago) of ecological and climatic changes and tectonic processes surrounding the Tibetan Plateau. The uplift in Tibet is thought to have increased deviatoric compressive stresses around the plateau enough to imitate deformation in areas as distant as 4,000 kilometers south (central Indian Ocean). The 8.0-7.5 million year age of the onset of deformation in the central Indian Ocean has been considered to be the most accurate and reliable constraint on the onset of rapid uplift in Tibet. Krishna et al. show that compressional deformation within the central Indian Ocean started between 15.4 and 13.9 million years ago, much earlier than previously thought. The observation of this earlier deformation starting 15.4-13.9 million years ago presents an important new constraint to the role of Himalayan uplift in climate.
Remote quantification of methane fluxes in gassy marine sediments through seismic survey
A.W. Dale et al., Dept. of Earth Sciences-Geochemistry, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Netherlands. Pages 235-238.
In the organic-rich marine sediments of the continental shelf, methane is often present in dissolved and free gas phases. Calculation of the diffusive flux of dissolved methane flux toward the sea floor, required for improved budgeting of the global carbon cycle, typically requires sediment coring, pore-water and sediment sampling, and analyses. These steps are cost-, time- and labor-intensive and, ultimately, the spatial coverage and resolution achieved is limited. A new modeling-based approach combining geochemical and acoustic data to calculate methane fluxes in gassy sediments has been developed. The model by Dale et al. exploits the ready detection of free methane gas below the sea floor using shipboard seismic instruments to estimate the diffusive transport flux of methane upwards from this gas horizon. The approach represents a means to quantify regional methane flux budgets for gassy sediments in a cost-efficient manner.
A new look at old carbon in active margin sediments
Nicholas J. Drenzek et al., Dept. of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA. Pages 239-242.
Changes in climate and human activity may substantially alter the amount, type, and age of material weathered from the continents to the oceans. In order to develop a better record of these changes over the last century, several diagnostic characteristics of organic matter preserved in Eel River Margin sediments (California) were measured at the molecular level by Drenzek et al. Results reveal that the majority of this material comes from plant detritus that has been pre-aged in soils. And, while the total amount eroded from soils versus fossil rocks is sensitive to precipitation change, their proportion is not. These findings further underscore the urgent need to more fully explore the dynamic and complex link between climate and the global carbon cycle.
Rising springs along the Silk Road
Jian Sheng Chen and Chi-yuen Wang, State Key Laboratory of Hydrology, Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, China. Pages 243-246.
Despite its extreme aridity, the Hexi Corridor (Gansu, China), part of the ancient Silk Road, has recently been repeatedly flooded by rising springs, forcing about 1,000 families to abandon their homes. Chen and Wang use new isotopic and chemical data as tracers for waters collected from the Hexi Corridor and the Qilian Mountains to investigate the cause of the rising springs. The data show that the springs may have originated from the mountain slopes, where glacier melt mixes with the precipitation from a local convective system between the extensively irrigated Hexi Corridor and the Qilian Mountains. Accelerated glacier melting in the Qilian Mountains may have increased the recharge of groundwater in the mountains, which was released by recent earthquakes, to raise the water table in the valley. The result has potential implications on the impact of climate change on water resources in arid regions.
Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere
Jeremy P. Richards, Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Pages 247-250.
The bulk of the world's copper and a large amount of its gold are currently mined from ore deposits formed in response to the subduction of oceanic plates at convergent plate margins, such as the western cordilleras of the Americas. Release of fluids and soluble elements from the subducting oceanic plate hydrates and metasomatizes (introduces chemicals including metals into) the overlying mantle and causes partial melting. The magmas generated are water- and metal-rich, and upon emplacement in the upper crust they may exsolve these fluids again to form hydrothermal systems. Metals dissolved in these exsolved fluids at high temperatures may be reprecipitated during cooling in a sufficient concentration to form ore deposits, most commonly copper (porphyry copper deposits). After the cessation of subduction, the effects of previous hydration and metasomatism of the mantle and lower crust above subduction zones do not disappear, and may be re-mobilized during later tectonic events, such as collision or rifting. Magmas generated during these later events tend to be small in volume but relatively enriched in gold over copper, giving rise to a characteristic suite of gold-rich porphyry and epithermal (near-surface vein-type) deposits which can be of high economic value. Formulation of a coherent model for the formation of these relatively rare, but potentially valuable, deposits provides a basis for exploration in previously overlooked terrains, or for reassessment of terrains that have been explored only for large subduction-related copper deposits.
Simultaneous magma and gas eruptions at three volcanoes in southern Italy: An earthquake trigger?
T.R. Walter et al., Dept. of Physics of the Earth, Helmholtz Centre Potsdam, GFZ German Research Centre for Geoscience, Telegrafenberg, 14473 Potsdam, Germany. Pages 251-254.
In 2002, a significant earthquake took place in southern Italy and was followed by major eruptions at Mount Etna and Stromboli Island, and anomalous degassing at Panarea. Through observation, seismic investigation, and numerical modeling, Walter et al. suggest that the volcanoes were further activated by dynamic pressure fluctuations associated with the earthquake, having implications that are important for understanding clustered activity and hazards in southern Italy and elsewhere.
Climatic control on Quaternary coal fires and landscape evolution, Powder River basin, Wyoming and Montana
Catherine A. Riihimaki et al., Biology Dept., Drew University, Madison, New Jersey 07940, USA. Pages 255-258.
Climate likely affects the rates of processes that modify Earth’s surface through erosion, but this relationship has been difficult to document. Riihimaki et al. provide evidence of a robust link between long-term climate and landscape evolution through the occurrence of natural coal fires over the past 1 million years in Wyoming and Montana. They show that coal fires occurred preferentially at specific times in the past, correlating with the waxing and waning of glaciers and with changes in the intensity of incoming solar radiation. These coal fires have dramatically influenced this landscape, coloring the bedrock red (a characteristic described in Lewis and Clark's journals) and creating hard-to-erode plateaus and terraces. Riihimaki et al.’s results provide important constraints on the history of one of the most important coal mining regions in the United States, as well as a novel approach to assessing long-term changes in stream and hillside processes.
How vulnerable is Acropora cervicornis to environmental change? Lessons from the early to middle Holocene
Lisa Greer et al., Dept. of Geology, Washington and Lee University, Lexington, Virginia 24450, USA. Pages 263-266.
Staghorn coral (Acropora cervicornis) is rapidly disappearing from Caribbean coral reefs at a time of well-documented coral reef deterioration worldwide. Proposed causes for the decline of this species include both natural and anthropogenic factors. In the study by Greer et al., physical and geochemical data were used to explore environmental and climatic conditions during the growth of extensive early and mid-Holocene (around 9.4-5.4 thousand years ago) fossil Acropora cervicornis reefs from the southwestern Dominican Republic. The data indicate that the species thrived during large-scale climate and environmental changes that likely included high temperatures, variable salinity, hurricanes, and rapid sea-level rise in the Holocene. This study suggests that the current decline in Caribbean Acropora cervicornis is anomalous and likely tied to anthropogenic ecosystem changes.
Age constraints on alleged “footprints” preserved in the Xalnene Tuff near Puebla, Mexico
Joshua M. Feinberg et al., Institute for Rock Magnetism, Dept. of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, USA. Pages 267-270.
The history of human migration into the Americas is a controversial topic. Footprint-like impressions were recently found preserved in volcanic ash south of Puebla Mexico. The discoverer of these impressions reported an age for the ash of about 40,000 years, raising the possibility that humans migrated into the Americas far earlier than conventionally believed (about 13,000 years ago). Feinberg et al. report paleomagnetic and 40Ar/39Ar data that conclusively show the ash and the volcano from which the ash was generated to be 1.3 million years old. Thus, they believe that the impressions are not human footprints and are more likely to be marks left behind from quarrying processes that were later enhanced by weathering and erosion.
Deposition of highly crystalline graphite from moderate-temperature fluids
F.J. Luque et al., Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain. Pages 275-278.
Luque et al. present the first known evidence from a large graphite deposit of highly crystalline graphite precipitated from moderate-temperature fluids. The findings of this study of the historic Borrowdale graphite deposit in northwest England (United Kingdom) clearly contrast with previous work that argued against volumetrically large, highly crystalline graphite deposits being precipitated from carbon-bearing fluids at low pressures and low to moderate temperatures. In addition, this study sheds new light on the constraints controlling highly crystalline graphite precipitation from low- to moderate-temperature fluids, which could be of interest for laboratory, and even industrial, synthesis.
Magnetic and geochemical characteristics of Gobi Desert surface sediments: Implications for provenance of the Chinese Loess Plateau
Barbara A. Maher et al., Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, University of Lancaster, Lancaster, LA1 4YQ, UK. Pages 279-282.
Maher et al. address the ongoing controversy over the source(s) of the world’s largest body of windblown (eolian) sediment, the famous Chinese Loess Plateau. They show that the geochemical and magnetic properties of surface samples from what is currently thought to be the major source area, the Mongolian Gobi, display a serious mismatch with the loess sediments which accumulated across the Plateau during the last glacial climate stage. These data indicate that, contrary to the prevailing view, the Gobi area cannot have been the major source area for the extremely well-mixed, vast body of the Chinese Loess Plateau. Multiple sources, including especially the western deserts and the Himalayan/Tibetan regions, must instead be considered.
Pacific atoll living: How long already and until when?
William R. Dickinson, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA,
Pages 4-10, doi: 10.1130/GSATG35A.1.
The tropical atolls and islands of the western Pacific, home to about 180,000 people, are some of the most beautiful and idyllic places on Earth. The combination of coral reefs, which ring the islands, and white sand beaches give the atolls a feeling of timelessness. However, as demonstrated through March’s GSA Today article by William Dickinson of the University of Arizona, that sense of timelessness is an illusion. Sea level started declining in the southwest Pacific some 5,000 years ago, resulting in the development of stable islets capable of supporting habitation only 2,000 years ago. Unfortunately, sea level is no longer declining. Global warming is forcing an ever-accelerating rise in sea level. Between 2050 and 2160, sea level will over-top the solid rock foundation of the atolls, exposing the overlying unconsolidated sediments (those beautiful white sand beaches) to devastating wave attack. Those seemingly timeless atolls may, within 100 years, become uninhabitable.