Boulder, CO, USA - Topics include: tree ring-based reconstruction of an extinct polar forest's seasonal environment; geological vs. seismological views of paleo-earthquakes; impact of Lake St. Martin bolide on groundwater quality; analysis of earthquakes at the Cascadia plate boundary; radar images of volcanic and impact deposits on lunar Aristarchus Plateau; dynamics of methane escape into the atmosphere; use of charred plants in modeling pyroclastic density currents; and the possibility we've left the Holocene and are in the Anthropocene.
Annual patterns within tree rings of the Arctic middle Eocene (ca. 45 Ma): Isotopic signatures of precipitation, relative humidity, and deciduousness
Hope Jahren, Earth and Planetary Sciences, 301 Olin Hall, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, USA; and Leonel Sternberg, University of Miami, Department of Biology, Coral Gables, Florida 33124, USA. Pages 99-102.
Jahren and Sternberg's study explains that the spectacular preservation of Eocene (approximately 45-million-year-old) tree rings allowed the reconstruction of the seasonal environment of an extinct polar forest. Deciduous conifer trees experienced high levels of humidity and a fluctuating water source during the short, intense (no darkness) Arctic winter.
Stretching of fluid inclusions in calcite as an indicator of frictional heating on faults
Kohtaro Ujiie et al., Institute for Research on Earth Evolution, Japan Agency for Marin-Earth Science and Technology (JAMSTEC), 3173-25 Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan. Pages 111-114.
The geological recognition of seismic slip on exhumed faults is one of those holy grails in the earth sciences. Overall, the structural geological community is conservative, with most only accepting frictional melts (pseudotachylytes as evidence of high-velocity slip). Ujiie et al. provide another line of evidence that can be invoked to recognize high-velocity slip, and begin to fill this ground between the geological and seismological views of paleo-earthquakes.
The Lake St. Martin bolide has a big impact on groundwater fluoride concentrations
Matthew I. Leybourne et al., GNS Science, Lower Hutt, New Zealand. Pages 115-118.
Meteor impact is popularly linked with catastrophic extinction events, but some effects can be harmful in a more subtle and insidious manner. As Leybourne shows, the shattering impact of a meteor on rocks results in increased groundwater-rock surface interaction. This has a direct effect on the quality of the groundwater that percolates through the fractured, melted rocks of the impact structure. A good example has been found at the Canadian town of Gypsumville, Manitoba (population 65), located near the Lake St. Martin meteor impact crater. Domestic wells in the town have elevated salinity, sulfate, and fluoride concentrations. The fluoride, which exceeds health limits, is of concern as excess intake causes mottling of teeth at moderate levels, to softening of bones and neurological damage at higher levels. The groundwater with elevated fluoride is shown to occur exclusively within the impact structure, and Leybourne's study is thought to be the first to document enhanced groundwater fluoride concentrations associated with impact structures.
Ridge reorientation mechanisms: Macquarie Ridge Complex, Australia-Pacific plate boundary
Sharon Mosher, Department of Geological Sciences, The University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254, USA; and Christina Symons, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093, USA. Pages 119-122.
Mosher and Symons analyzed a unique, complete seafloor record to document the processes associated with the transition of the Australia-Pacific tectonic plate boundary south of New Zealand from an oceanic spreading center at 40 million years ago into the present-day transform fault plate boundary. Although the geologic record shows that most plate boundaries have changed from one type of plate boundary to another over time, rarely is a record of the transition preserved. During this drastic 60°-90°change in spreading direction, individual spreading ridge segments propagated and failed while undergoing reorientation, resulting in the shortening of spreading segments and, in most cases, their complete disappearance. Moreover, newly created crust was modified and transferred between plates during the gradual reorientation of the spreading axes. The entire transition was accommodated by non-rigid plate deformation along the plate boundary prior to the cessation of magmatism.
Probable low-angle thrust earthquakes on the Juan de Fuca–North America plate boundary
Anne Tréhu et al., Oregon State University, CAOS Administration Building 104, Corvallis, Oregon 97331, USA. Pages 127-130.
In 2004, two clusters of earthquakes occurred in the central part of the Cascadia forearc. Tréhu et al.'s analysis suggests that both events, which had magnitudes of 4.9 and 4.8, resulted from low-angle thrust motion between the North America and Juan de Fuca plates in the zone that is generally thought to be locked. Paleoseismic data indicate that the locked part of the plate boundary ruptures in large earthquakes every several 100 years, most recently in 1750. Tréhu et al.'s observations represent the first instrumentally recorded events on the Cascadia plate boundary, with the possible exception of the Petrolia Earthquake of 1992 at the southern end of the subduction zone.
Elliptical mud volcano caldera as stress indicator in an active compressional setting (Nirano, Pede-Apennine margin, northern Italy)
Marco Bonini, Consiglio Nazionale Delle Ricerche, Istituto di Geoscienze e Georisorse, Via G. La Pira, 4, 50121 Firenze, Italy. Pages 131-134.
Active fluid release occurs through numerous vents located within an elliptical depression developed over the crest of an anticline associated with the active Pede-Apennine thrust front of the Northern Apennines (Italy). Bonini interprets this depression as a caldera collapse structure that may have developed in response to the deflation of a shallow mud chamber, triggered by eruption and sediment fluid evacuation events. Bonini concludes that the mud volcano caldera reflects the regional tectonic stress axes, and exhibits mechanical similarities to the igneous homologues.
Volcanic and impact deposits of the Moon's Aristarchus Plateau: A new view from Earth-based radar images
Bruce Campbell et al., National Air and Space Museum, MRC 315, 4th and Independence Avenue, SW, Washington, D.C. 20560, USA. Pages 135-138.
Ancient volcanic eruptions on the Moon produced deposits of fine-grained, often glass-rich, pyroclastic material. In some places, like the Aristarchus Plateau, these deposits can be 20-30 meters thick. The pyroclastics are of interest for their association with early lunar volcanic processes, and as possible sources of materials for lunar outposts. Campbell et al. used new radar images from Earth-based radio telescopes that penetrate the mantling layers to "see" the underlying terrain, to reveal details of the geologic events that shaped the Aristarchus Plateau. Radar data at longer wavelengths show the extent of lava flows that cover a significant portion of the plateau. When struck by relatively small meteorites, these lava flows are broken up into rocks and mixed into the fine-grained layers above. Such abundant rocks might complicate the use of the pyroclastics as a resource for future lunar explorers. The new radar data can be used to identify thick, rock-poor areas of the pyroclastic deposits best suited for resource recovery.
A new model linking atmospheric methane sources to Pleistocene glaciation via methanogenesis in sedimentary basins
Steven Petsch et al., Department of Geosciences, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA. Pages 139-142.
Natural gas is an important fossil fuel resource composed largely of methane. Methane is also a significant greenhouse gas and, like carbon dioxide, methane concentrations in the atmosphere have increased greatly as climate shifted from cold to warm at the end of the last Ice Age. Formolo et al. suggest how, in rocks from northern Michigan, fossil fuel methane may be connected to methane as a greenhouse gas. Many thousands of years ago, microorganisms were stimulated to produce methane deep underground through decomposition of ancient sedimentary rocks when those rocks were covered by large continental ice sheets. Formolo et al. show that this subsurface microbial activity is recorded in biodegraded oil-like compounds, as well as in the DNA of microorganisms still active deep within these rocks. As long as an ice sheet remained over Michigan, methane produced by these microorganisms remained trapped underground. Once the ice began melting, however, methane that had accumulated over thousands of years was rapidly released to the atmosphere, accelerating greenhouse gas warming of the planet. Natural gas currently recovered as a fossil fuel resource in Michigan and other locations may thus represent just a small fraction of methane produced by subsurface microorganisms, most of which long since escaped to the atmosphere and contributed to climate change.
Temperature proxy data and their significance for the understanding of pyroclastic density currents
Andrew C. Scott, Geology Department, Royal Holloway University of London, Egham, Surrey TW2O OEX, UK. Pages 143-146.
Scott et al. have shown that data from plants charred by contact with hot pyroclastic (ash) flows can be used to help in the modelling of pyroclastic density currents. Hot pyroclastic flows are one of the most dangerous volcanic phenomena and have been responsible for many fatalities. Famous examples include the eruption of Vesuvius with the destruction of Herculanium and Pompeii and the eruption of Mont Péle in Martinique that destroyed the town of St. Pierre, killing nearly all the inhabitants. Understanding how these flows behave is important in assessing volcanic hazards. One of the biggest problems has been obtaining temperature data from pyroclastic density currents. A major dome collapse of the Soufrière Hills volcano, Montserrat, on 26 December 1997, generated a devastating pyroclastic density current that destroyed vegetation and left a distinctive tar-like deposit on the surface that deposit included a range of charred, mainly herbaceous flowering plant axes and roots. Reflectance studies of these charcoalified plants provided minimum flow temperatures of 300-425 °C, providing an estimate of pyroclastic density current characteristics. Scott et al. estimate flow-front current densities near the ground of 1.8 to 3 kilograms per cubic meter, using constraints from a mean flow speed of 90 meters per second estimated from seismic data. The mean temperature of the ash component is estimated as 400-610 °C.
Caribbean chronostratigraphy refined with U-Pb dating of a Miocene coral
Rhawn Denniston et al., Department of Geology, Cornell College, Mount Vernon, Iowa 52314, USA. Pages 151-154.
Uplift of the Central American volcanic arc isolated the Caribbean and Pacific oceans approximately 3 million years ago, and changed the physiochemical characteristics of the Caribbean. In response, many shallow marine taxa experienced an increase in speciation. Quantifying these evolutionary rates requires having precise age constraints on the related sedimentary sequences, but such a well-defined chronology has been lacking. Denniston et al. refined the age of one sedimentary unit from the Dominican Republic that contains a diverse and well-preserved fossil taxa by obtaining a uranium-lead (U-Pb) age on the primary aragonite of a pristine Miocene age coral. The recognition that such ancient corals may be sufficiently well preserved to allow direct U-Pb dating opens doors for continued refinement of late Cenozoic sedimentary sequences.
Size of the earliest mollusks: Did small helcionellids grow to become large adults?
Mónica Martí Mus et al., Area de Paleontología, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain. Pages 175-178.
The oldest widely accepted mollusks are a group of cap-shaped fossil shells known as helcionellids. Helcionellids are usually very small (1-2 millimeters), which has contributed to the belief that ancestral mollusks were millimeter-scale animals. Mollusks are usually born with a shell which accompanies them their whole life. As they grow in size, so does the shell, but only at the margins; therefore, the juvenile shell always remains visible at the tip of the adult one. Martí Mus et al. have discovered a new, large (2 centimeter) mollusk from the Cambrian in Spain which has preserved, at the tip of its large shell, a tiny cap identical to the small helcionellids previously found only as "free" elements in Cambrian rocks. Considering the mode of growth of molluskan shells, Martí Mus et al.'s finding shows that at least some small helcionellids represent juvenile (or maybe even larval) stages of species that reached a centimeter-scale adult size. Therefore, contrary to a widely held wisdom, ancestral mollusks may have been relatively large animals.
Extinction and environmental change across the Eocene-Oligocene boundary in Tanzania
Paul Pearson et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK. Pages 179-182.
The transition between the Eocene and Oligocene epochs (between approximately 33.5 and 40 million years ago) was a time of extreme climatic upheaval as a warm "greenhouse" climate state, which had prevailed for many millions of years, gave way to a glaciated one in which a large ice cap appeared on Antarctica for the first time. It was also a time of widespread extinction, in the oceans and on land, as the world's biota adjusted to the new conditions. In their paper, Pearson et al. discuss a new, highly detailed record of these events that has been discovered in southern Tanzania. It comes from drill cores through ancient marine muds that originally accumulated on the sea floor but are now exposed on land. The muds contain abundant and exquisitely preserved microscopic fossils, which preserve a record of changes in tropical marine life and environments through the climate transition. The new records show that the marine extinctions peaked relatively early, well before maximum glacial conditions set in, and involved both the plankton and organisms from shallow-water reef environments. These new data will help geologists locate the epoch boundary more precisely than before.
Tenuous connection between high-silica rhyolites and granodiorite plutons
Allen F. Glazner et al., Department of Geological Sciences, CB# 3315, University of North Carolina, Mitchell Hall, 104 South Road, Chapel Hill, North Carolina 27599-3315, USA. Pages 183-186.
Rhyolite makes up some of the largest eruptions known on Earth. For decades it has been assumed that rhyolite (a magma that is rich in silicon and commonly forms explosive eruptions) forms as the last liquid in a crystallizing body of granite, and therefore rhyolite and granite are intrusive/extrusive equivalents of one another. In this paper, Glazner et al. geochemically show that late-stage liquids in granites have trace-element compositions that are quite distinct from erupted rhyolites. Thus, rhyolite must form by some other mechanism.
Dynamic adjustments in channel width in response to a forced diversion: Gower Gulch, Death Valley National Park, California
Noah Snyder and Lisa Kammer, Department of Geology and Geophysics, Boston College, 140 Commonwealth Avenue, Chestnut Hill, Massachusetts 02130, USA. Pages 187-190.
Snyder and Kammer's study makes use of an unusual experiment to make new observations about stream erosion processes. In 1941, Furnace Creek, the main desert river on the east side of Death Valley, California, was diverted into a small canyon (Gower Gulch) in an effort to alleviate flooding in the village downstream. This study analyzes aerial photographs from 1948 to 1995, and 2005 field and airborne laser elevation surveys. These images show that Gower Gulch is undergoing a variable, dynamic response to the diversion, with some areas incising and narrowing, and others widening. These findings have important implications to studies that seek to understand stream responses to changes in climate or tectonics.
GSA TODAY Science Article
Are we now living in the Anthropocene?
Jan Zalasiewicz et al., Department of Geology, University of Leicester, Leicester LE1 7RH, UK
"In 2002," write Zalasiewicz and colleagues in this revolutionary proposal, "Paul Crutzen, the Nobel Prize-winning chemist, suggested that we had left the Holocene and had entered a new epoch - the Anthropocene - because of the global environmental effects of increased human population and economic development." The authors document a radical yet compelling case for the idea that the appearance of humans has so physically changed Earth that there is no organic justification for linking pre- and post-industrialized Earth within the same epoch (the Holocene). With this article, Zalasiewicz and colleagues have laid the scholarly groundwork for the formal adoption by the International Commission on Stratigraphy of the Anthropocene as the youngest epoch of, and most recent addition to, the geological timescale.
To review the abstracts for these articles, go to www.gsajournals.org. View the complete table of contents for the current issue of GEOLOGY at www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.