BOULDER, Colo. - The Geological Society of America's February issue of GEOLOGY contains a number of newsworthy items. Highlights include new estimates of K-T impact dust that show it did not cause mass extinctions, a hypothesis about rare jellyfish fossils in Wisconsin, evidence from East Greenland fossil leaves of atmospheric carbon dioxide levels during the last (Eemian) interglacial, and two articles from different groups of geologists studying the Indian Vindhyan Supergroup that come to similar conclusions.
Highlights from GEOLOGY are provided below. (We do not have a science article for February’s GSA TODAY.) Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.
Impact dust not the cause of the Cretaceous-Tertiary mass extinction.
Kevin O. Pope, Geo Eco Arc Research, 16305 St. Mary's Church Road Aquasco, MD 20608. Pages 99-102.
A prominent theory for the cause of the mass extinction of life at the Cretaceous-Tertiary (K-T) boundary is that fine dust from a giant asteroid impact blocked out the sun, shut down photosynthesis, and caused a global collapse of the food chain. Many studies over the last decade have confirmed that a giant impact did occur at the K-T boundary, forming the 200-km-diameter Chicxulub crater in Mexico. However, recent research indicates that impact effects other than the dust apparently caused the extinctions. Most of the impact debris found in the global K-T boundary layer was deposited as droplets of glass that condensed from the impact vapor plume, and only a small fraction of this layer is dust (pulverized rock fragments). The global mass and size distribution of the dust indicates that stratospheric winds spread the debris from North America, over the Pacific Ocean, to Europe, with little debris reaching high southern latitudes. Theoretical calculations, coupled with observations of the coarse dust fraction, demonstrate that very little (less than 0.01%) of this dust is of the size (less than one micrometer) required to shut down photosynthesis for any significant length of time. These findings indicate that the original K-T impact extinction hypothesis is not valid, because it requires more than 100 times more fine dust than is estimated here. Furthermore, many predictions of future impact hazards are in error, since they are based upon the earlier, inaccurate dust estimates for the K-T impact. For example, the often cited 1:20,000 risk of death by impact, which assumes mass mortality during relatively small impacts, is greatly overstated.
Stranded on a Late Cambrian shoreline: Medusae from central Wisconsin.
James W. Hagadorn et al. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, 91125, USA. Pages 147-150.
This article describes one of the only fossil jellyfish deposits in Earth history. Because jellyfish have no skeleton, their tissues decay easily. In modern settings, when jellyfish wash up on the shore, they are often eaten by birds and other scavengers. Thus, to bury and preserve a jellyfish in the geologic record requires unique conditions in which there are few terrestrial scavengers, and few burrowing organisms to disturb buried carcasses. These conditions occur in the Mt. Simon -Wonewoc Sandstone, which was deposited ~495 Ma. In this deposit there are thousands of fossil jellyfish impressions, including the largest jellyfish known in the geologic record. Some of the fossil jellyfish may represent some of the largest early pelagic predators. This deposit is important because it indicates that large, soft-bodied pelagic animals were an important and abundant component of early marine ecosystems, despite the fact that we rarely see such organisms preserved in ancient marine rocks.
1.6 Ga U-Pb zircon age for the Chorhat Sandstone, lower Vindhyan, India: Possible implications for early evolution of animals.
Birger Rasmussen et al. Department of Geology and Geophysics, University of Western Australia, Crawley 6009, Australia. Pages 103-106.
Bedding-plane markings in the Chorhat Sandstone (lower Vindhyan) of central India were recently interpreted as burrows produced by worm-like animals. Because the rocks were thought to be more than 1000 Ma, these structures were regarded as the oldest fossil evidence of animal life. However, the biological origin of the markings has been questioned, as has their age. Age estimates for the structures are based on antiquated radiometric studies, though some contentious evidence suggests that the rocks may be only 540 Ma. In this article the authors provide the first robust age data for the lower Vindhyan using zircon crystals in silicified volcanic ash layers. Their results show that the sediments were deposited between ca. 1630 and 1600 Ma. If the Chorhat markings are burrows left by worm-like animals, then the data suggest that complex multicellular organisms had evolved before 1600 Ma, a staggering 1 b.y. before the Ediacara biota, an assemblage containing the first undisputed animal fossils. However, given the doubts expressed about the origin of the bedding-plane structures, as well as the surprisingly old age of the host rocks, further work is urgently required to provide supporting evidence for an animal origin of the structures.
U-Pb zircon dating and Sr isotope systematics of the Vindhyan Supergroup, India.
Jyotiranjan S. Ray et al.Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada. Pages 131-134.
The rocks of the Vindhyan Supergroup of India are believed to contain fossil clues to the emergence of animal life on our planet. A recent report of trace fossils of multicellular animals found in a Vindhyan formation believed to be 1100 Ma suggested that advanced life forms were nearly twice as old as the nearest well-dated multicellular organisms. Complications arose subsequently when animal body fossils (small shelly fossils and Ediacaran fossils), usually found in much younger rocks, were reported in rocks of comparable stratigraphic positions. In the midst of these fossil controversies, the main issue that remains unresolved is the age of these rocks. In this present study, with the help of U-Pb zircon dating and Sr isotope stratigraphy, the authors have resolved much of this controversy. The data suggest that these fossil-bearing rocks were deposited between ca. 1700 and 1200 Ma, and that the uppermost part of the supergroup is not younger than ~ ca. 700 Ma. These findings are clearly in conflict with the report of body fossils and suggest an age older than 1100 Ma for the formation that contains the alleged trace fossils.
Century-scale changes of atmospheric CO2 during the last interglacial.
Mats Rundgren, Department of Quaternary Geology, Lund University, SE-223 63 Lund, Sweden, and Ole Bennike, Geological Survey of Denmark and Greenland, DK-2400 Copenhagen NV, Denmark. Pages 187-189.
Because atmospheric carbon dioxide is an important amplifier in Earth's climate system, there is a need for data on past CO2 variations. Specifically, reliable records of past variations in climate and CO2 for the last (Eemian) interglacial, which may be a relevant analogue to the present (Holocene) interglacial, could give clues to future climate change. So far, the Vostok ice core has been the primary source of information on Eemian CO2 levels. In this study, the stomatal frequency of fossil Dwarf willow (Salix herbacea L.) leaves from East Greenland was used to provide independent Eemian CO2 estimates. This method has the potential to detect short-lived CO2 variations that may not be recorded in ice cores because of diffusion during air enclosure. The resulting estimates are in general agreement with Vostok data, but a few deviating (lower) estimates may reflect century-scale Eemian CO2 variability. Therefore, the Eemian carbon cycle may have operated more dynamically than previously believed.
Paradox of groundwater age.
Craig M. Bethke and Thomas M. Johnson, Department of Geology, University of Illinois, Urbana-Champaign, 61801, USA. Pages 107-110. (NB: PDF not available for this article at this time.)
Groundwater age is important for hydrologists to know because it reveals how quickly water flows through the subsurface. This quantity, typically figured from the concentrations of radioisotopes or marker molecules, therefore, is critical in estimating how quickly pollutants might migrate into a water supply, for example, or in determining the safe yield of an aquifer. Historically, groundwater age has been derived from the time required for the water to flow through the subsurface, but a rigorous analysis presented in this paper shows that instead age reflects the influence of confining layers. This result is of broad importance because it suggests that groundwater moves more rapidly than previously estimated.
Geomorphic evidence for prior earthquakes: Lesson from the 1999 Chichi earthquake in central Taiwan.
Yue-Gau Chen et al., Department of Geosciences, National Taiwan University, Taipei 106, Taiwan, Republic of China. Pages 171-174
An earthquake (MW7.6) occurred in central Taiwan on September 21, 1999. It destroyed several thousand buildings and caused more than two thousand fatalities. The earthquake occurred along the Chelungpu fault, a thrust fault located on the western flank of the Taiwan fold-and-thrust belt. The surface rupture was more than 90 km long, with vertical displacements ranging from 3 to 8 m. Although pre-existing scarps were identified along the Chelungpu fault, the fault had previously been categorized as a suspect active fault, due to the lack of geochronological evidence and the failure to recognize fault-related geomorphic features. Identifying geomorphic features at active faults in Taiwan will permit the delineation of future surface ruptures and will help to determine past magnitude, thus contributing to hazard assessment.
Creeping soil.
Arjun M. Heimsath et al. Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA. Pages 111-114.
“Soil creep” is a broadly applied term used to describe processes of soil movement that have not been well understood. Assumptions of the mechanisms for soil creep have been applied across many styles of landscape-evolution modeling, despite evidence that they are not always applicable. This paper offers new insight into how soil is transported off a hilly landscape in southeastern Australia and shows that individual soil grains are mixed throughout the soil column by biogenic processes. The authors coupled measurements of the optically stimulated luminescence of individual soil grains with measurements of in situ cosmogenic radionuclides from the bedrock beneath the mobile soil layer and were able to compare the two data sets with a Monte Carlo simulation of soil creep. Finally, the authors derived estimates of downslope soil-creep velocities by integrating soil- production rates downslope and by normalizing grain age profiles with soil depth, concluding that a significant fraction of downslope transport occurs by overland flow processes.
Deep-sea paleotemperature record of extreme warmth during the Cretaceous.
Brian T. Huber et al. Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20560, USA. Pages 123-126.
Deep-sea temperatures estimated from oxygen isotopic ratios in microscopic foraminiferal shells confirm that Earth’s poles were extremely warm during much of the Cretaceous. Analyses of foraminifera that lived below 1000 m water depth at locations around the world indicate that the deep ocean averaged 15 ºC from ca. 100 to 75 Ma and reached 20 ºC during a “supergreenhouse” interval between 95 and 93 Ma. Deep ocean temperatures averaged 11 ºC between 75 and 65 Ma and cooled to ~9 ºC ca. 66.5 Ma. At similar depths in modern oceans water temperature is less than 2 ºC. In addition to being warm, Cretaceous bottom-water temperatures are similar to estimates of contemporary sea-surface temperatures at high latitudes. This match suggests that, like today, Cretaceous surface waters tended to sink at high latitudes and flow toward the equator, although flow may have been reversed during the mid-Cretaceous supergreenhouse interval (when temperature differences between the equator and the poles were at a minimum). Regardless, the evidence for long-term polar warmth is difficult to reconcile with the existence of significant Cretaceous ice sheets and underscores the problem that greenhouse climate models that consistently underestimate Cretaceous polar temperatures.
Paleoproterozoic crust within the Great Falls tectonic zone: Implications for the assembly of southern Laurentia.
Paul A. Mueller et al. Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. Pages 127-130.
Approximately 1,800 Ma North America, as we know it today, was just taking shape as a result of the welding of many smaller continental masses, or microcontinents. Although much is known about when and where many of the various microcontinents came together, the welding of one of the oldest microcontinents, the Wyoming province, to the main mass of North America is poorly understood, and its location is debated. This paper presents new geochemical data obtained at the University of Florida, as well as new age determinations obtained by U/Pb [AB1]radiometric dating of the mineral zircon. The sensitive [AB2]high-resolution ion microprobe (SHRIMP) at the USGS[AB3]-Stanford microanalytical facility was used to obtain the U/Pb information. Through this research, it was discovered that rocks exposed in the Little Belt Mountains in north-central Montana contain a variety of rocks that have chemical similarities to igneous rocks produced in modern magmatic arcs, such as the Cascade Range of North America. Analyses of samples from this area strongly suggest that the welding of the Wyoming province began almost 1,900 Ma along a boundary called the Great Falls tectonic zone, a fault zone stretching from central Idaho to southern Saskatchewan, Canada. This interpretation contrasts with many previous hypotheses that place the boundary at the Vulcan structure, a buried magnetic feature that parallels the Great Falls tectonic zone, but lies approximately 700 km to the north along the U.S.-Canadian border.
Rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth.
J.E. Otamendi et al. Departamento de Geología, Universidad Nacional de Río Cuarto, 5800 Río Cuarto, Argentina. Pages 159-162.
Garnet is a mineral that is widely distributed in many metamorphic and igneous rocks and that is notably refractory, in the sense that it does not change composition easily. This latter characteristic is especially important when applied to heavy rare earth elements, which are trace elements for which garnet is particularly avid. As garnet crystals grow, they incorporate heavy rare earth elements. The spatial distribution of heavy rare earth elements in garnet reflects the growth processes and the composition of the enclosing rock materials. The authors have determined spatial concentrations and the composition of the enclosing rock materials as well as the spatial concentrations of heavy rare earth elements in garnet crystals from partially melted metamorphic rocks by means of a laser-ablation microprobe inductively coupled plasma -mass spectrometer. These data, together with theoretical models of the behavior of heavy rare earth elements, allow them to demonstrate that garnet growth in metamorphic rocks is a process that is capable of separating, or fractionating, the heavy rare earth elements from one another. Understanding how these elements, that have virtually identical geochemical behaviors, can be fractionated from each other is in turn crucial to understanding the origin and provenance of many magmas.
Sea-level fall below the shelf edge, without basin-floor fans.
Piret Plink-Bjorkland and Ron J. Steel, Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 115-118.
Most research of deep-water depositional systems has assumed that rapid fall of sea level to below the shelf edge, especially where the sediment is supplied by point-sourced, sandy, shelf-edge deltas, is sufficient to make canyons in the slope and ensure a sand supply to the basin floor. A new data set from large-scale clinoform outcrops on Spitsbergen demonstrates that repeated sea-level fall below the shelf edge, even with the forcing of shelf-edge deltas down onto the slope, can be insufficient to produce basin-floor, turbidite fans (where there are no canyons in the slope). This article urges caution in the use of “sea-level fall below the shelf edge” as a criterion in prediction of basin-floor sands.
Temporal changes in stable isotope composition of spring waters: Implications for recent changes in climate and atmospheric circulation.
Laura K. Rademacher et al. Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA. Pages 139-142.
Age tracers, including chlorofluorocarbons and tritium/3He, and the stable isotopes of water were measured in shallow groundwater emerging from 10 cold springs in the Sierra Nevada, California. The groundwater ages of the spring waters ranged from 10 -35 years old. The stable isotope composition of the spring waters varied significantly and correlated with spring age. This data indicates that precipitation on the central Sierra was more than 1 per mil lighter in delta O-18 in the 1960s than today. The documented change in the local mean annual air temperature is not large enough to explain the change in the stable isotope composition of the spring waters. Therefore, other mechanisms, such as changes in atmospheric circulation patterns, must be affecting the stable isotopic composition of precipitation in the Sierras.
Strontium isotope record of paleohydrology and continental weathering, Eocene Green River Formation, Wyoming.
Meredith K. Rhodes et al. Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 167-170.
Lake Gosiute existed in the Green River Basin of southwestern Wyoming ca. 50 Ma, during the warmest period of the Cenozoic. The study investigates changes in the paleohydrology of the lake throughout individual lake expansion-contraction cycles during its uppermost freshwater stage. The approach we have taken is to geochemically analyze the carbonate sediments deposited in the lake, because they record the geochemistry of the lake water in which they were deposited. This lake water in turn reflects the geochemical constituents of the rocks exposed in the drainage basin that fed the lake. Therefore, changes in the geochemical record of the lake sediments reflect changes in the paleohydrology of the lake. The authors have found that individual lake expansion-contraction cycles preserve a predictable geochemical trend, and they have interpreted that trend to suggest a climatic control for deposition on the lake-cycle scale.
Effects of orographic precipitation variations on the concavity of steady-state river profiles.
Gerard H. Roe et al., Quaternary Research Center, University of Washington, Seattle, Washington 98195-1360, USA. Pages 143-146.
In this article the effects of orographic precipitation variations on the concavity of steady-state river profiles are studied. Many actively uplifting mountain ranges result from the competition between uplift due to tectonic motions and erosion due to rivers cutting into bedrock channels. The profiles of such rivers are important indicators of the mechanisms of landscape evolution. As such, they have been the subject of intensive study, but such studies have generally not taken into account the strong gradients in rainfall typically seen in mountainous areas. By accounting for these gradients, the authors show that they change the basic form of the river profiles, and moreover are a fundamental control on the development of mountain ranges.
To view the abstracts for these articles, go to: http://www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org.