Highlights from GEOLOGY and a summary of the science article for the January GSA TODAY are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.
Neoproterozoic glacial-rainout intervals: Observations and implications.
Daniel J. Condon et al. School of Geography and Geosciences, University of St.Andrews, St. Andrews, Fife KYI6 9AL, Scotland, UK. Pages 35–38.
The exact nature and severity of the glacial epochs that characterized the mid-to-late Neoproterozoic is the subject of debate. Some workers suggest that repeated cycles of “snowball Earth” existed for millions of years, while others favor the interpretation that although the glacials were severe (sea ice at low latitudes), they did not result in totally frozen oceans, hence the moniker “slushball Earth.” This paper presents observations from four separate Neoproterozoic glacimarine successions (on the Laurentia, Congo, and Kalahari cratons) and includes examples of both older and younger Neoproterozoic glacial epochs. These observations consist of dropstone intervals interbedded with dropstone-free beds that the authors interpret as recording episodic melting of debris-laden ice. This can be attributed to either calving of icebergs or undermelt of an ice sheet and thus suggest a dynamic glacimarine environment. The observation of what the authors interpret as an iceberg-dump structure associated with the dropstone-rich intervals of one succession (northeast Laurentia) suggests it records a series of ice-rafting events. Importantly, such a dynamic glacial environment is inferred for the Ghaub Formation (Namibia), which according to paleoclimate (snowball Earth) models, must have been deposited during glacial maxima and/or meltback. These observations suggest that Neoproterozoic glaciations were characterized by a dynamic glacial environment (with areas of unfrozen seas) rather than a totally frozen snowball Earth.
Nile River sediment fluctuations over the past 7000 years and their key role in sapropel development.
Michael D. Krom et al., School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK. Pages 71–74.
An understanding of the fluctuations in climate over the Holocene and late Quaternary and their effect on the environment is essential for us to understand the recent history of the earth as well as to predict the consequences of human induced climate change. The Nile River and its annual flood are a unique system in which to study such changes. It is the longest river in the world, spanning four major climatic belts. This study uses 87Sr/86Sr isotopes to determine the relative amounts of sediment from the Blue Nile/Atbara catchment in the Ethiopian Highlands and from the White Nile catchment in tropical East Africa. The authors show that when there was more rainfall in the Ethiopian Highlands, and more water in the Nile flood, there was less sediment. This is the opposite of most rivers. It was caused by changes in the amount of vegetation in the Ethiopian Highlands. The amount of rainfall has changed dramatically over the past 6500 years as a result of global climate change. The authors suggest that the changes in the type of mud brought down the Nile and discharged into the Eastern Mediterranean may have had a significant effect on environmental conditions in the Levantine Basin.
Widespread middle Holocene dune formation in the eastern upper peninsula of Michigan and the relationship to climate and outlet-controlled lake level.
Alan F. Arbogast, et al., Department of Geography, Michigan State University, East Lansing, Michigan 48824-1115, USA. Pages 55–58.
Michigan is located in the core of the Great Lakes region and is known for its dramatic glacial and coastal landscapes. Less well known are the prominent dune fields that are scattered throughout both peninsulas of the state. Historically, these dune fields were thought to have formed in a deglacial environment when poorly vegetated former (pro-glacial) lakebeds were deflated by strong winds. One of the largest of these dune fields is the Newberry dune field, which covers extensive parts of the Lake Algonquin lakebed in the eastern upper peninsula of Michigan. This dune field contains numerous parabolic dunes, with northwest orientations that are up to 8 m high. In order to test the age of these dunes, sands at five widely spaced sites were dated via optically stimulated luminescence (OSL) of quartz. The resulting ages establish that eolian sand was last mobilized in the region between ca. 7 and 5.5 ka. Although this interval corresponds to the drier Altithermal-Hypsithermal period, climate alone may not have been sufficiently arid to cause dune formation in this area. Instead, it appears that dune formation may also be linked to depressed water tables, associated with outlet-controlled low levels in Lakes Michigan and Superior. Following dune stabilization, peat began to accumulate in interdune areas ca. 4.4 ka. his study is significant because it demonstrates that interior landscapes in the core of the Great Lakes region may respond to the combined effects of climate change and base-level fluctuations.
Partial detachment of a lithospheric root under the southeast Carpathians: Toward a better definition of the detachment concept.
Zohar Gvirtzman, Institute of Earth Sciences, Hebrew University, 91904 Jerusalem, Israel. Pages 51–54.
The southeast corner of the Carpathian Mountains (Romania) provides a unique opportunity to study the final, short-lived stage of plate convergence in which a piece of subducted lithosphere finally detaches from the overriding plate and begins to sink into the mantle. This article emphasizes that the southeast Carpathians have an extremely low elevation considering their crustal thickness (the crust there is almost as thick as the crust of the Alps but the elevation is less than one-half that of the Alps). The author proposes that although seismologic studies indicate that the cold mantle body under the southeast Carpathians is already partly detached from the overriding lithosphere, it is still viscously coupled to the lithosphere and pulls it downward.
Late Pliocene age for the Atacama Desert: Implications for the desertification of western South America.
Adrian J. Hartley, Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK, and Guillermo Chong, Departamento de Ciencias Geológicas, Universidad Católica del Norte, Angamos 0610, Antofagasta, Chile. Pages 43-46.
The Atacama Desert, which fringes the west coast of South America, is one of the driest places on Earth. Many geologists consider the desert to have formed more than 10 Ma. It has been proposed that the desert was generated by the uplift of the Andean mountain range, which created a rain shadow along the west coast of South America. The authors’ work shows that this isn't true. There is good evidence for the presence of permanent lakes and rivers in the area occupied by the present day Atacama Desert until up to 3 Ma. Studies of deserts in Africa show that they also became more arid 3 Ma. This suggests that whatever caused the increase in aridity affected at least the Southern Hemisphere, and probably the world. Because the Southern Hemisphere appears to have become drier ca. 3 Ma, the authors think that the Atacama Desert developed because of global climate change, rather than because of the growth of the Andes.
Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the southwestern Pacific basin.
Roger L. Larson et al., Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, USA. Pages 67-70.
A geological structure called the "Tongareva triple junction" has been mapped for the first time across more than 3200 km (2000 miles) of southwestern Pacific Ocean seafloor using a high-resolution sonar system. Longer than the Appalachian Mountains, this structure would extend from the U.S.-Canadian border to Cuba if laid out along the eastern coast of the United States. The researchers located the structure at the intersection of two distinctly different trends of elongated abyssal hills that mark two of three former tectonic plate boundaries. They mapped this intersection by zigzagging the research vessel back and forth across it as the sonar system traced out the shapes of the elongated hills on the seafloor 5 km below. The triple junction trace contains the geological history of three tectonic plates that met in Cretaceous time ca. 120–85 Ma. The structure evolved from an initial point near the equator and between Samoa and Tahiti. Due to large differences in rates of motion across the three plate boundaries, this point then moved rapidly to the southeast and traced out the 3200-km-long structure during the 35 m.y. history. The researchers suggest that rates of oceanic crust formation were very fast on two of the three plate boundaries during this period. Rapid rates of oceanic crust formation have been suggested as a cause of elevated sea level and atmospheric temperature during the Cretaceous "greenhouse" period, and this tectonic system appears to have contributed to those effects.
Oceanic broad multifault transform plate boundaries.
M. Ligi et al., Istituto di Geologia Marina, Consiglio Nazionale Ricerche, Via Gobetti 101, 40129 Bologna, Italy. Pages 11-14.
Oceanic transform plate boundaries consist of a single, narrow (a few kilometers wide) strike-slip seismic zone offsetting two mid-ocean ridge segments. However, the authors define a new class of oceanic transform boundaries, with broad, complex multifault zones of deformation, similar to some continental strike-slip systems. Examples are the 750-km-long, 120-km-wide Andrew Bain transform on the Southwest Indian Ridge, and the Romanche transform, where the Mid-Atlantic Ridge is offset by a lens-shaped, ~900-km-long, ~100-km-wide sliver of deformed lithosphere bound by two major transform valleys. One of the valleys is seismically highly active and constitutes the present-day principal transform boundary. However, strike-slip seismic events also occur in the second valley and elsewhere within the deformed zone. Numerical modeling predicts the development of wide, multiple transform boundaries when the age offset is above a threshold value of ~30 m.y., i.e., in extra-long (>500 km) slow-slip transforms. Multiple boundaries develop so that strike-slip ruptures avoid very thick, strong lithosphere.
Biofilm architecture of Phanerozoic cryptic carbonate marine veneers.
Robert Riding, Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK. Pages 31-34.
Microscopically thin calcareous linings in fossil reef and other sediment cavities are identified as bacterial biofilms. These films were originally dense populations of bacterial cells surrounded by a protective and adhesive organic matrix. In these fossil examples, the original cells are not preserved, but the calcified biofilm retains distinctive morphological features such as internal channel systems and delicate external columns and plumes that are characteristic of modern biofilms. This discovery of recognizable biofilms in rocks up to 500 Ma provides a new way of identifying fossil bacteria, not only on Earth, but also in rock samples from other planets.
Ostracode turnover and sea-level changes associated with the Paleocene-Eocene thermal maximum.
Robert Speijer, Department of Geosciences, Bremen University, P.O. Box 330440, 28334 Bremen, Germany, and Abdel-Mohsen M. Morsi, Geology Department, Faculty of Science, Ain Shams University, 11566 Cairo, Egypt. Pages 23-26.
Many ostracode species (tiny bivalved crustaceans) live on the seafloor within rather narrow water-depth ranges. This feature enables paleontologists to infer changes in water depth through time by studying fossil ostracode assemblages enclosed in ancient rocks. An analysis of fossil ostracodes recovered from shallow marine sediments deposited in present-day Egypt ca. 55 Ma indicates a rapidly changing sea level that fluctuated by ~15-20 m. The combination of data from Egypt and indications from other continents suggests that sea level fluctuated worldwide, like it did during the more recent ice ages. These results point to significant variations in the size of ice caps during the globally warm period known as the Paleocene-Eocene thermal maximum. This is unexpected because it is generally assumed that there were no significant polar ice caps during this period of greenhouse climate.
Postdepositional tilt of the Miocene-Pliocene Ogallala Group on the western Great Plains: evidence of late Cenozoic uplift of the Rocky Mountains.
Margaret E. McMillan et al., Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 63–66. The authors evaluate the postdepositional tilt of the Ogallala Group in the Cheyenne Tablelands of Nebraska and Wyoming. They determine the slope during deposition and compare that to the modern slope of the unit. The change is significant, indicating tilt up to the west since deposition ceased. This tilt could be caused by subsidence, isostatic rebound following climate-induced erosion, or tectonic uplift. The authors’ results suggest that the tilt of the western Great Plains is most consistent with tectonic uplift centered under the Rocky Mountains since Ogallala deposition ceased ca. 5 Ma.
GSA TODAY
Eocene Meridional Weather Patterns Reflected in the Oxygen Isotopes of Arctic Fossil Wood.
A. Hope Jahren,* Johns Hopkins University, and Leonel Silveira Lobo Sternberg, University of Miami.
Paleoclimate studies are essential for understanding processes and rates of global climate change. This paper discusses a time in Earth’s history when climate is thought to have been unusually warm, in the Eocene, from 57.8–36.6 Ma. During this time period, the far northern latitudes of planet Earth were home to alligators and lavish vegetation, suggesting a highly tropical planet. The authors analyzed the spectacularly preserved Metasequoia trees excavated from the Fossil Forest site of Axel Heiberg Island in the Canadian High Arctic. These trees grew at paleolatitude 82°N. The cellulose of this fossil wood is still preserved, offering a unique window into the oxygen isotope composition of the Eocene groundwater. The isotopic data reveals that environmental water was strikingly depleted in heavy O isotopes. In modern ecosystems, such ratios would indicate annual average temperatures below 0 °C. However, the authors reject such cold paleotemperatures to explain the data because the lush forests could not have grown at these temperatures. Instead, they suggest that global-scale weather patterns were different: Moisture was progressively depleted in heavy oxygen isotopes as it traveled from low latitudes in the Pacific ocean across North America. These air circulation patterns delivered both warm air and isotopically depleted moisture to the Arctic during the summer months of continuous sunlight and explosive growth. Such a weather pattern, with no Arctic Front as we have today, is radically different from weather patterns thought to be in place before and after the Eocene and may partially account for the abundance of vegetation at high latitudes during the Eocene relative to any other period in Earth’s history.
* A. Hope Jahren, Assistant Professor of Geobiology and Climatology at the Department of Earth and Planetary Sciences, Johns Hopkins University, was awarded the GSA Young Scientist Award (Donath Medal) for 2001. The Young Scientist Award was established in 1988 to be awarded to a young scientist (35 or younger during the year in which the award is to be presented) for outstanding achievement in contributing to geologic knowledge through original research that marks a major advance in the earth sciences. The award, consisting of a gold medal and a cash prize, was endowed by Dr. and Mrs. Fred A. Donath.
To view the abstracts for these articles, go to: www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY or GSA TODAY article, contact Ann Cairns at acairns@geosociety.org.