New Geology science posted online Dec. 8-16 2011

December 16, 2011

Boulder, Colo., USA - GEOLOGY topics include an explanation for missing mid-Cenezoic sediments discovered in Lomonosov Ridge drilling; understanding position shifts of the Intertropical Convergence Zone; an analysis decoupling taxonomic and ecological severities of major mass extinctions; a Pleistocene reversal of British Columbia's Fraser River; quality comparison of land and sea fossil records; evidence of a large drop in the Gulf of Mexico's sea surface temperatures before major growth of the Antarctic ice sheet, and more.

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Tectonic subsidence of the Lomonosov Ridge

Alexander N. Minakov and Yury Yu. Podladchikov, Department of Earth Science, University of Bergen, Allegaten 41, NO-5007, Bergen, Norway (Minakov) and Faculty of Geosciences and Environment, University of Lausanne, CH-1015, Lausanne, Switzerland (Podladchikov). Posted online 8 Dec. 2011; doi: 10.1130/G32445.1.

The Lomonosov Ridge microcontinent (High Artic) drifted away from Eurasia in the early Cenozoic. Drilling revealed that more than 20 million years of the mid-Cenozoic stratigraphic section is missing. Minakov and Podladchikov propose that the missing sediments were removed during uplift and erosion of the Lomonosov Ridge, caused by combination of thermal expansion and metamorphic reaction in the upper mantle.

Paleoposition of the Intertropical Convergence Zone in the eastern Pacific inferred from glacial-interglacial changes in terrigenous and biogenic magnetic mineral fractions

T. Yamazaki, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Higashi, Tsukuba 305-8567, Japan. Posted online 8 Dec. 2011; doi: 10.1130/G32646.1.

The Intertropical Convergence Zone (ITCZ) is a low-pressure zone encircling Earth near the equator, where the northeast and southeast trade winds converge. The ITCZ is a key component of the global circulation system, and its position has been varied with global climate changes. Its position in the past was, however, not well understood. Yamazaki reveals from magnetic properties of sediments that the position of the ITCZ was more southward than today at and before about 250,000 years ago. Yamazaki used sediment cores obtained by the Integrated Ocean Drilling Program (IODP) in the east equatorial Pacific (Site U1337: 3 degrees 50′N, 123 degrees 12′W). The site is located near the southern boundary of the ITCZ, which ranges from about 4 degrees N to 11 degrees N there at present. Magnetic minerals in the sediments have two dominant sources: those of biogenic origin produced by magnetotactic bacteria and those of terrigenous origin transported as wind-blown dust from Asia and North and South America. Variations in contribution of these sources can be estimated using the rock magnetic technique. Yamazaki's interpretation is that contributions of Central and South American dust and biogenic magnetite have increased since about 250,000 years ago, which, since then, has been caused by a northward shift of the ITCZ.

Ecological ranking of Phanerozoic biodiversity crises: The Serpukhovian (early Carboniferous) crisis had a greater ecological impact than the end-Ordovician

George R. McGhee Jr. et al., Department of Earth and Planetary Sciences, Wright-Rieman Laboratories, Rutgers University, New Brunswick, New Jersey 08903, USA. Posted online 8 Dec. 2011; doi: 10.1130/G32679.1.

The familiar "big five" mass extinctions are usually ranked in terms of the severity of taxonomic diversity loss (either families or genera) that occurred in the crisis: the end-Permian, end-Ordovician, Late Devonian, end-Triassic, and end-Cretaceous, from greatest to least taxonomic diversity loss. McGhee et al.'s analysis of the ecological impacts of each of these crises reveals that the taxonomic and ecological severities of the events are decoupled. Ranking the ecological severity of the biodiversity crises that have occurred in the Phanerozoic reveals not only a different severity ranking, but a different big five as well: the end-Permian, end-Cretaceous, end-Triassic, Late Devonian, and Serpukhovian (end-Mississippian), from greatest to least ecological impact. The end-Cretaceous event, the least severe in terms of taxonomic diversity loss, is revealed to have been the second-most severe ecological event in the Phanerozoic. Most strikingly, the end-Ordovician crisis, which is the second-most severe event in terms of taxonomic diversity loss, had an ecological impact that was less than that of the Serpukhovian (end-Mississippian) biodiversity crisis. The end-Ordovician event was of minimal ecological impact, and does not occur in the ranking of the big five ecological-severity events.

Pleistocene reversal of the Fraser River, British Columbia

Graham D.M. Andrews et al., Earth Research Institute, University of California-Santa Barbara, Santa Barbara, California 93106, USA. Posted online 8 Dec. 2011; doi: 10.1130/G32488.1.

The Fraser River is the largest and most important river in British Columbia, Canada, forming a major delta upon which the Vancouver metropolitan area is built. The Fraser River flows through the Coast Mountains via a 1-km-deep canyon that also hosts the primary road and rail links between Vancouver and the rest of British Columbia and Canada. Recent studies by Andrews et al. of 1- to 3-million-year-old volcanic rocks in south-central British Columbia reveal that the Fraser River flowed northward at that time, presumably to either the Arctic Ocean via the Mackenzie River system, or to the Pacific Ocean via a longer and more tortuous path to the Columbia River system. Sometime after 1 million years, the Fraser River experienced a period of drainage reversal, presumably in response to one or more of the several glaciations to have affected the region in the past 1 million years. At the same time, the Fraser Canyon was carved, and, for the first time, sediment began to be transported into the Pacific Ocean basin, where the Nitinat Fan and Fraser Delta began to build up on the deep and shallow sea floor, respectively. Reversal of the Fraser River profoundly changed the landscape and hydrology of British Columbia.

Detecting earliest shortening and deformation advance in thrust belt hinterlands: Example from the Colombian Andes

Mauricio Parra et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Posted online 8 Dec. 2011; doi: 10.1130/G32519.1.

Chronological and structural geologic relationships identified in Parra et al.'s newly released reflection seismic data reveal a previously unrecognized zone of early shortening deformation in the northern Andes of Colombia. Low-temperature thermochronological data (apatite fission-track results) and thermal modeling help define the onset of rapid exhumation about 60-50 million years ago along the boundary between Magdalena Valley and the Eastern Cordillera. Subsurface angular unconformities above fold-thrust structures indicate sediment deposition above a deforming wedge with doubly vergent reverse faults. Restoration of a geologic cross section indicates early Cenozoic shortening and exhumation along a broad zone of east- and west-directed reverse faults, with later deformation focused on west-directed inversion structures. These relationships reveal that deformation did not proceed systematically from west to east, but varied spatially through time.

Comparative quality and fidelity of deep-sea and land-based nannofossil records

Graeme T. Lloyd et al., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. Posted online 16 Dec. 2011; doi: 10.1130/G32561.1.

Paleontologists have long been concerned with the quality of the fossil record and the degree to which we can use it to make statements about biological history. Here, Lloyd et al. compare two very different fossil records, that from the land against that from the deep sea, for the same group of open-ocean algae: the coccolithophores. Lloyd et al. show that face-value interpretation of species numbers give significantly different pictures of their history despite there being no plausible biological reason for this. Instead this indicates both the presence of major sampling biases and the different nature of such biases within the two environments. However, using established methods to correct for sampling, the two records do appear to converge on a similar result, indicating the data are not devoid of a common biological signal. Furthermore, Lloyd et al. show that the land-based record gives the more reliable estimate of this signal and appears to represent the better fossil record. This latter revelation is contrary to expectations, as the deep sea record exhibits some of the longest periods of continuous deposition of fossil-bearing rock and is more amenable to complete sampling through coring.

Dynamic growth of garnet in granitic magmas

Jade Star Lackey et al., Geology Department, Pomona College, Claremont, California 91711, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32349.1.

ABSTRACT: In order to improve understanding of how accessory garnet crystallizes in igneous rocks, and evaluate it as a mineral recorder of magma history, Lackey et al. analyzed garnets from the Hallowell and Togus plutons in south-central Maine (United States) by laser fluorination, and in situ by ion microprobe. Two types of garnet are recognized, magmatic and locally derived peritectic. Traverses of some single crystals show both gradual and abrupt changes of O-18, commonly >1‰, while other garnet grains are isotopically homogeneous. Rimward increase of O-18 in many crystals indicates that garnet grew while high O-18 metamorphic wall rocks were assimilated. Peritectic grains have a complementary record of the transfer of high O-18 melts to the plutons. In some rocks, O-18 varies among neighboring grains, evidence that crystals grew episodically or were juxtaposed from different sources during magma mixing. Garnet faithfully records changing magmatic O-18, and is a valuable tool to decipher magma petrogenesis.

Subglacial carbonates constrain basal conditions and oxygen isotopic composition of the Laurentide Ice Sheet over Arctic Canada

Kurt A. Refsnider et al., Institute of Arctic and Alpine Research (INSTAAR) and the Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32335.1.

Refsnider et al. have studied unique calcite mineral crusts deposited on bedrock surfaces on central Baffin Island in the eastern Canadian Arctic. Using a dating method based on the radioactive decay of uranium and thorium, they show that these calcite crusts formed 22-15 thousand years ago beneath the Laurentide Ice Sheet roughly coincident with the height of the last ice age. This reveals that melting of the basal ice of the ice sheet occurred at this time, but soon after, melting ceased and the ice sheet became frozen to the underlying bedrock in this region, allowing these delicate features to be preserved in the absence of glacial erosion. The oxygen isotopic composition of these calcite crusts also provides insight into the implications of such deposits for reconstructing the isotopic composition of past glaciers and ice sheets.

Multiproxy record of abrupt sea surface temperature cooling across the Eocene-Oligocene transition in the Gulf of Mexico

Bridget S. Wade et al., School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK. Posted online 16 Dec. 2011; doi: 10.1130/G32577.1.

The Earth today has a large ice sheet on Antarctica, but this was not always the case. The greenhouse-to-icehouse transition around 34 million years ago was a time of climatic change, with a major expansion of the Antarctic ice sheet and extinctions in marine and terrestrial organisms. But how much did temperature change at this time? Wade et al. studied a core from the United States Gulf Coast to gain information on the climate of the past. They reconstructed past sea surface temperatures using the geochemistry of organic and inorganic plankton preserved in marine sediments. They find that there was a large drop in sea surface temperatures of 3 to 4 degrees Celsius in the Gulf of Mexico before major growth of the Antarctic ice sheet. Temperature decline was accompanied by changes in seasonality. Previous research had suggested that temperature change in the Gulf of Mexico across the greenhouse-to-icehouse transition was very small. This research suggests that temperatures were not continuously warm or stable.

Subducted seamounts and recent earthquakes beneath the central Cascadia forearc

Anne M. Tréhu et al., College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32460.1.

ABSTRACT: Bathymetry and magnetic anomalies indicate that a seamount on the Juan de Fuca plate has been subducted beneath the central Cascadia accretionary complex and is now located approximately 45 km landward of the deformation front. Passage of this seamount through the accretionary complex has resulted in a pattern of uplift followed by subsidence that has had a profound influence on slope morphology, gas hydrate stability, and sedimentation. Based on potential field data and a new three-dimensional seismic velocity model, Tréhu et al. infer that this is the most recent of several seamounts subducted over the past several million years beneath this segment of Cascadia. More deeply subducted seamounts may be responsible for recent earthquake activity on the plate boundary in this region and for along-strike variations in the thickness of the subduction channel.

Preserved extent of Jurassic flood basalt in the South Georgia Rift: A new interpretation of the J horizon

David M. Heffner et al., Department of Earth and Ocean Sciences, University of South Carolina, Columbia, South Carolina 29208, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32638.1.

For several decades, Earth scientists have generally accepted that an areally extensive layer of volcanic rock lies buried at the base of the Coastal Plain of South Carolina, Georgia, Florida, and Alabama. This flood basalt province has subsequently been linked to the timing and process of opening of the Central Atlantic Ocean basin to address fundamental questions of how continents break apart. In this study, Heffner et al. present new integrated observations of subsurface (well and seismic) data to show that the preserved extent of this volcanic layer is much more limited than previously thought, indicating that either the original extent of the flood basalt was restricted, or that much of this layer was removed by erosion after opening of the Atlantic.

Abrupt changes in high-latitude nutrient supply to the Atlantic during the last glacial cycle

K.R. Hendry et al., Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Massachusetts 02543, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32779.1.

Diatoms, photosynthetic algae that contribute towards a major portion of marine carbon uptake, need dissolved silicon to make their opal shells. As a result, the cycling of dissolved silicon in the oceans plays a key role in controlling atmospheric CO2. This means that understanding the marine silicon system is of increasing interest given concerns over man-made carbon emissions. The surface waters of the Southern Ocean supply nutrients to a vast volume of the global ocean. Diatoms dominate production in the region where these waters sink to form intermediate waters, resulting in a low silicon supply compared to other nutrients feeding the Atlantic Ocean. Changes in intermediate water formation as a result of southern hemisphere warming, which have been documented in recent years, may result in significant changes in nutrient supply in future years. Here, Hendry et al. tested the sensitivity of the system to warming by investigating the silicon content of intermediate waters in the South Atlantic as the Earth came out of the last ice age. They used a newly developed measure of dissolved silicon concentration, the silicon isotopic composition of deep-sea sponges, to investigate the role of nutrient export in influencing global climate. Their results show that dissolved silicon exhibits peaks in concentration during abrupt climatic events and these variations are a result of changes in how intermediate waters are formed. Hendry et al. suggest that, with continued freshening and warming of the Southern Ocean, changes in nutrient supply could have significant consequences for algal populations and carbon cycling.

Evolution of middle to Late Cretaceous oceans--A 55 m.y. record of Earth's temperature and carbon cycle

Oliver Friedrich et al., Institut für Geowissenschaften, Goethe-Universität Frank furt, Altenhöferallee 1, 60438 Frankfurt, Germany. Posted online 16 Dec. 2011; doi: 10.1130/G32701.1.

Friedrich et al. report a new 55 million year global compilation of benthic foraminifera stable isotopes for the mid- to Late Cretaceous. In combination with the well-established compilation of the Cenozoic, our new compilation allows for a detailed comparison of the last 115 Ma of Earth's temperature and carbon cycle. Comparison with the Cenozoic clearly shows that the mid-Cretaceous exhibit the warmest temperatures of the last 115 million years. This mid-Cretaceous super-greenhouse ended when the Equatorial Atlantic Gateway between Africa and South America opened sufficiently to flood the deep North Atlantic with relatively cool polar waters formed in the Southern Ocean. The results also provide evidence for the widespread formation of bottom waters with temperatures greater than 20 degrees Celsius during the mid-Cretaceous. These bottom waters filled the North Atlantic and probably originated as thermocline or intermediate waters in the tropical oceans. This scenario is completely different than today's ocean circulation and is a precondition for the massive black shale deposition and source-rock formation during the mid-Cretaceous super-greenhouse climate.

Simulating Permian-Triassic oceanic anoxia distribution: Implications for species extinction and recovery

Cornelia Winguth and Arne M.E. Winguth, Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, Texas 76019, USA. Posted online 16 Dec. 2011; doi: 10.1130/G32453.1.

The Permian-Triassic boundary (about 252 million years ago) is characterized by the extinction of about 90% of all marine species and widespread devastation of terrestrial ecosystems. The recovery of species in the Early Triassic happened only slowly. Extensive volcanism occurred in Siberia at the end of the Permian, severely impacting the environment. While it is widely accepted that associated oceanic changes played a major role for the mass extinction and delayed recovery, the exact dynamics are still under debate. In this study by Winguth and Winguth, a fully coupled climate-carbon cycle model is used in order to investigate the effects of possible environmental changes on ocean chemistry. Model results suggest that around the Permian-Triassic boundary, the mid-ocean oxygen minimum zone expanded considerably as a result of volcanism-induced global warming, likely contributing to the end-Permian mass extinction. While the deep, global ocean remained ventilated at the Permian-Triassic boundary, widespread deep-sea anoxia were probably generated following a strong increase in weathering and nutrient input into the ocean in a warmer world. These adverse environmental conditions were likely a major cause for the delayed recovery of species in the Early Triassic.

Thermochronologic evidence for plateau formation in central Tibet by 45 Ma

Alexander Rohrmann et al., Institute für Erd- und Umweltwissenschaften, Universität Potsdam, 14476 Potsdam, Germany. Posted online 16 Dec. 2011; doi: 10.1130/G32530.1.

The time when the 5-km-high, low-relief Tibetan plateau came into existence is not known, despite its importance for evaluating models of continental deformation and climate. Rohrmann et al. conducted thermochronologic studies on rocks in Tibet to determine when and how fast they moved through the uppermost 5 km of crust. Their data indicate that very low erosion rates, and by inference low relief conditions, were established in large parts of central Tibet by 45 million years ago, shortly following the 55-50-million-years-ago collision between India and Asia. The results, when interpreted in the context of major documented Cretaceous to Eocene crustal shortening in Tibet and available paleoaltimetric data, are best explained by a scenario of plateau growth that began locally in central Tibet during the Late Cretaceous and expanded to encompass most of central Tibet by 45 million years ago with minimal later erosion on the order of less than 0.05 mm/yr. The resolved history of plateau formation is significantly older than what is widely thought or assumed.

Geological Society of America

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