|29 Nov. 2011
GSA Release No. 11-80
Director - GSA Communications & Marketing
GSA Bulletin Highlights:
New Research Posted Ahead of Print
Boulder, CO, USA – Highlights for GSA BULLETIN articles published ahead of print between 1 and 21 Nov. 2011 are provided below. Highlights include two related articles on carbon isotopes and the evolution of C4 grasslands in the U.S. Great Plains, as well as a detailed reconstruction of paleoenvironmental conditions in the Asturian vertebrate track assemblage Spain's Vega Formation and the identification four distinct types of paleosols (fossilized soils) there.
Keywords: C4 grasslands, paleosols, Great Plains, Border Rift, Sonoran Bisbee basin, Vega Formation, Spain, Yanshan belt, China, 40Ar/39Ar isotopic dating, Vermont, Quebec, South America, precipitation, inland seaway, climate change, geodynamics, Urals, Russia, time scale.
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Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Carbon isotopes and the evolution of C4-dominated grasslands
David L. Fox et al., Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA. Posted online 1 Nov. 2011; doi: 10.1130/B30401.1.
The stable carbon isotope composition of calcium carbonate that forms in soil horizons is sensitive to the relative abundance of plants growing in the soil that use the C3 (trees, shrubs, cool-growing season grasses) or C4 (warm-growing season grasses, many sedges) photosynthetic pathways. David L. Fox of the University of Minnesota and colleagues use the carbon isotope composition of carbonates in fossilized soils (paleosols) to reconstruct the relative abundance of C4 grasses in the southern Great Plains over the past 12 million years. C4 grasses constituted 0 to 20% of plant biomass in southwest Kansas 12 million to 5 million years ago (i.e., during the Miocene epoch), then increased to an average of 40 percent of biomass with little secular change from 5 million to 2.5 million years ago (i.e., during the Pliocene epoch). After 2.5 million years (i.e., during the Pleistocene epoch), the abundance of C4 grasses increased steadily and first reached the modern abundance for the region at about 1.3 million years ago. Thus, the history of the modern C4-dominated grassland in the region is protracted and involved several discrete intervals of increased C4 abundance, in contrast to some other proxy records from the region and the patterns of C4 abundance in other regions.
Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Oxygen isotopes and paleoclimate during the evolution of C4-dominated grasslands
David L. Fox et al., Dept. Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA. Posted online 1 Nov. 2011; doi: 10.1130/B30402.1.
The stable oxygen isotope composition of calcium carbonate that forms in soil horizons is sensitive to both soil temperature and the oxygen isotope composition of soil water, which is derived from local rain water. Thus, the oxygen isotope composition of soil carbonates is a useful paleoclimate proxy for many terrestrial regions. The carbon isotope composition of fossilized soils (paleosols) in southwest Kansas records the increase in the local abundance of warm-growing season grasses that use the C4 photosynthetic pathway. Fox et al. use the oxygen isotope composition of those carbonates to examine the climatic changes in the past 12 million years associated with the evolution of the modern, C4-dominated grassland in the region. Modern C4 grasses are commonly associated with warm and arid conditions. Surprisingly, the oxygen isotope composition of paleosol carbonates decreases through the past 12 million years as the abundance of C4 grasses increases. This inverse correlation indicate that C4 grasses were increasing in association with a combination of cooler temperatures, increased soil moisture availability, or an increased fraction of soil water recharge from winter precipitation. These results suggest that the role of climate change in the history of C4 grasslands may be more complicated than previously assumed.
Lower Cretaceous strata of the Sonora Bisbee Basin: A record of the tectonomagmatic evolution of northwestern Mexico
T.C. Peryam et al., Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Posted online 1 Nov. 2011; doi: 10.1130/B30456.1.
The Border Rift was a large area of intra-continental extension spanning from the Gulf of Mexico to southern California during the Jurassic and Cretaceous periods. T.C. Peryam of New Mexico State University and colleagues accurately date and describe sedimentary deposits from the Mexican portion of this rift, the Sonoran Bisbee basin of northern Mexico. Their data record four separate volcanic and magmatic episodes in the region, from the Permian to the Cretaceous Period. Their data also provide evidence for a previously unrecognized uplift event along the coast of northwestern Mexico in the early Cretaceous (ca. 130-140 million years ago). The authors suggest that this event may be related to accretion of a magmatic arc to North America.
Paleoenvironmental reconstruction of Jurassic dinosaur habitats of the Vega Formation, Asturias, Spain
Karen Gutierrez and Nathan D. Sheldon, Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Posted online 1 Nov. 2011; doi: 10.1130/B30285.1.
Jurassic-aged strata of Asturias, Spain, contain trace fossils including sauropod, theropod, and ornithopod dinosaur footprints, but their paleoenvironmental context has been relatively unstudied. A coastally exposed continuous section at Playa de Vega shows a clear transition from the marine Middle Jurassic Rodiles Formation to the terrestrial Upper Jurassic Vega Formation. Within the more than 100 m of Vega Formation stratigraphy that was logged there, four distinct types of paleosols were identified: (1) Entisols, (2) Inceptisols, (3) Vertisols, and (4) composite or cumulative paleosols. The paleosol types and their features indicate a floodplain depositional setting with short stature, shrubby vegetation. Theropod and ornithopod tracks have been identified at the base of the section, indicating that a dinosaurian fauna was present at Playa de Vega during the Jurassic. The Asturian vertebrate track assemblage is remarkably similar to that of the Morrison Formation (western United States), but it has greater overall richness. The pattern is reversed for body fossils. The reconstructed paleoenvironmental and paleoclimatic conditions indicate similar depositional settings in both places, but with a cooler, wetter, more seasonal environment in Asturias. The greater seasonality indicated by the Vega Formation relative to the Morrison Formation may explain the observed differences in richness.
Multiple controls on rift basin sedimentation in volcanic settings: Insights from the anatomy of a small Early Cretaceous basin in the Yanshan belt, northern North China
H.-H. Wei et al., Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China. Posted online 1 Nov. 2011; doi: 10.1130/B30495.1.
This paper deals with a small Early Cretaceous rift basin on the northern periphery of the North China block and tries to decipher potential controls on synrift sedimentation in volcanic settings. The results show that vigorous volcanism took place before and during synrift period, and basin fills were dominated by lacustrine, fan-delta, braid delta, and volcaniclastic deposits. Underwater extrusions of magma and pyroclastic flows are common. A deep lake was established soon after basin initiation and persisted throughout synrift period, indicating a rapid subsidence rate with both adequate sediment and water supply. The high-angle border faults are thought to have played a critical role in creating large accommodation space. The high rate of basin subsidence might have been caused mainly by left-lateral transtensional faulting and enhanced by the superposition of basement downsag induced by underlying magma withdrawal. A humid paleoclimate not only provided enough water supply but also promoted weathering of volcanic rocks in source areas. H.-H. Wei of the Chinese Academy of Sciences and colleagues conclude that relatively small and isolated rift basins in volcanic settings can experience both rapid subsidence and receive thick lacustrine deposits if they are under the combined control of favorable structure, climate, and magmatism.
Timing of tectonometamorphism across the Green Mountain anticlinorium, northern Vermont Appalachians: 40Ar/39Ar data and correlations with southern Quebec
S. Castonguay et al., Geological Survey of Canada, 940 rue de la Couronne, Quebec, Quebec G1K9A9, Canada. Posted online 21 Nov. 2011; doi: 10.1130/B30487.1.
This article uses field-based structural and metamorphic observations and the 40Ar/39Ar isotopic dating method to propose a refined model for the tectonic evolution of the Appalachian belt of northern Vermont during the Early Paleozoic. The dating of muscovite, a type of white mica found in the metamorphosed rocks and in the rock fabric of faults and shear zones, and of amphibole has allowed S. Castonguay of the Geological Survey of Canada and colleagues to establish the precise timing of mountain-building processes. The results show that these processes (such as thrust faulting, extensional faulting, and burial metamorphism) are polyphased and have occurred from latest Cambrian to the Middle Devonian across the Green Mountains of Northern Vermont. Correlations with similar studies in southern Quebec further exemplify the along-strike diachronism of tectonism induced by the inherited irregular geometry of the Laurentian margin.
Impacts of Cenozoic global cooling, surface uplift, and an inland seaway on South American paleoclimate and precipitation delta18O
M. Louise Jeffery et al., Dept. of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Posted online 21 Nov. 2011; doi: 10.1130/B30480.1.
The past elevation of mountains provides a strong constraint on the timing and mechanisms of mountain belt deformation. In the present day Andes, the isotopic composition of precipitation is observed to show a strong correlation with surface elevation. Based on this relationship, soil carbonates that record precipitation can be used to estimate past changes in elevation. However, during the Cenozoic, global climate cooling and the waxing and waning of an inland seaway may also have modified precipitation in South America. M. Louise Jeffery of the University of Michigan and colleagues use global climate model simulations to explore the impact of global climate change and the presence of an inland seaway on precipitation in the Andes and reexamine existing South American stable isotope records. Changing climate conditions and seaway incursions lead to an uncertainty of up to 2 km in past elevations estimated from stable isotope records.
Foreland basin subsidence driven by topographic growth versus plate subduction
H.D. Sinclair and M. Naylor, School of GeoSciences, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, Scotland, UK. Posted online 21 Nov. 2011; doi: 10.1130/B30383.1.
The subsidence of foreland basins is classically thought to accelerate with time. However, a synthesis of data from numerous foreland basins recognizes a wide range of signals and rates. This study by H.D. Sinclair and M. Naylor of the University of Edinburgh investigates the influence of two broadly ubiquitous controlling factors: (1) the upward and outward growth of mountain topography and (2) the subduction velocity of the underthrust lithosphere. These two factors have contrasting subsidence signals: The former tends to be slow and decelerates, while the latter is more rapid and accelerates. The geodynamic setting of a foreland basin determines the degrees to which these components dominate the final signal of tectonically induced subsidence.
Quantitative radiometric and biostratigraphic calibration of the Pennsylvanian-Early Permian (Cisuralian) time scale and pan-Euramerican chronostratigraphic correlation
Mark D. Schmitz and Vladimir I. Davydov, Dept. of Geosciences, Boise State University, Boise, Idaho 83725, USA. Posted online 21 Nov. 2011; doi: 10.1130/B30385.1.
Mark D. Schmitz of Boise State University and colleague Vladimir I. Davydov present a quantitative biostratigraphic and radiometric calibration for the Pennsylvanian through Early Permian global time scale, based upon high-precision isotope dilution-thermal ionization mass spectrometer U-Pb zircon ages for interstratified ash beds in the parastratotype sections of the southern Urals of Russia.