Boulder, Colo., USA - Precipitation reconstructions are essential for predicting impacts of future climate change and preparing for potential changes in terrestrial environmental conditions, such as shifting amounts of regional rainfall, which in turn impact water resource availability and crop growth patterns. Reliable proxy records of paleoprecipitation, especially from past warm periods, are a valuable tool for assessing and modeling future soil and plant moisture and local water availability. However, current terrestrial proxies are limited in their applications, and as a result, a wide range of paleoenvironments are underrepresented in the geologic record.
In their GSA Bulletin study, published online ahead of print on 3 Feb. 2015, Ethan G. Hyland and colleagues present a new relationship between the magnetic properties of soils and precipitation, and use this new proxy to describe changes in paleoprecipitation during past periods of major climate change. This work doubles the potential range of terrestrial paleoclimate applications, and has great potential for robustly describing hydrologic conditions in the deep past. Improvements in our understanding of these conditions can lead to more accurate predictions of water availability and ecosystem stability in a warmer world.
Because soil formation occurs in most terrestrial ecosystems and is controlled strongly by climatic factors, paleosol-based proxies provide an important archive of terrestrial climate. These records are ideal because they are applicable to significantly longer time scales than records from ice cores, and they give greater spatial fidelity and temporal coverage than records from sources such as lake cores or botanical (pollen) records.
Understanding precipitation in the present and during past episodes of climate change is important for determining and preparing for impacts of changes in the hydrological cycle on global environmental systems in the future. Soil magnetic properties, specifically the ratio of pedogenic goethite to hematite in modern soils, can be related quantitatively to modern precipitation regimes worldwide via a robust linear regression model. This newly derived relationship serves as a precipitation proxy that is applicable to a wide range of soil types and climatic regimes worldwide, and the resulting climo-function has been successfully applied to paleosols in order to estimate paleoprecipitation during the early Eocene climatic optimum, an interval of rapid global climate change.
A new paleoprecipitation proxy based on soil magnetic properties: Implications for expanding paleoclimate reconstructions
E.G. Hyland et al., University of Washington, Seattle, Washington, USA. Published online ahead of print on 3 Feb. 2015; http://dx.
Other GSA Bulletin articles published online in February 2015 are highlighted below.
GSA Bulletin articles published ahead of print are online at http://gsabulletin.
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Detrital zircon record of Neogene exhumation of the central Alaska Range: A far-field upper plate response to flat-slab subduction
Patrick R.K. Brennan and Kenneth D. Ridgway, Purdue University, West Lafayette, Indiana, USA. Published online ahead of print on 18 Feb. 2015; http://gsabulletin.
The Alaska Range is the highest mountain belt in North America, yet it lies ~500 km from the modern plate boundary. Recent studies have attributed the uplift of the central Alaska Range to flat-slab subduction processes in Alaska. Results of this study provide a chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes in south-central Alaska.
Syndepositional forced folding and related fluid plumbing above a magmatic laccolith: Insights from outcrop (Lower Cretaceous, Basque-Cantabrian Basin, western Pyrenees)
Luis M. Agirrezabala, Euskal Herriko Unibertsitatea, Bilbo, The Basque Country. Published online ahead of print on 3 Feb. 2015; http://dx.
Deep-water growth deposits at an outcrop of the Basque-Cantabrian Basin (north Iberia) record the coeval formation and evolution of an Early Cretaceous forced fold and related clastic dyke system above the igneous Larragan laccolith. A multidisciplinary study of the growth deposits reveals that the shallow emplacement of the laccolith created a dome-shaped fold, which had at the palaeoseabed a final relief of 18 to 36 m and a width of about 850 m. Simultaneously, in the hinge zone of the fold, local tensional stress induced the formation of a fracture system, which subsequently acted as a conduit for overpressured ascending fluids (methane and water) and fluidized sediments originating from the metamorphic aureole of the laccolith. This plumbing system reached the palaeoseabed, where extruded sediments were deposited and methane-derived carbonates precipitated. A comprehensive model is proposed, in which incremental emplacement of magma sheets induces pulsating contact metamorphism, folding and fracturing of the overburden and episodic activity of the fluid plumbing system
Active mountain building along the eastern Colombian Sub-Andes: A folding history from deformed terraces across the Tame Anticline, Llanos Basin
G. Veloza et al., University of Kansas, Lawrence, Kansas, USA. Published online ahead of print on 18 Feb. 2015; http://dx.
Quantifying deformation and fault slip rates is crucial to understand not only the presence and distribution of natural resources in developing countries, but also for earthquake risk assessment and hazards mitigation. Active deformation rates over millennial time scales are for the first time quantified in the Colombian Sub-Andes, utilizing a combination of tectonic geomorphology, reflection seismic interpretation, and structural modeling, and terrestrial cosmogenic nuclide dating to understand and quantify the kinematic evolution of the Tame Anticline. Key findings from the analysis are (1) millennial-scale shortening rates are in agreement with GPS measurements in other locations of the thrust front; (2) shortening rates are slightly slower than long-term geologic rates at the same latitude, probably due to slower convergence between Nazca and South America; (3) thrust-front migration utilizes basement heterogeneities to focus deformation and invert under contraction previously extensional features, as observed in other thrust belts; and (4) based on the modeled kinematic evolution of the fold, internal strain under the resolution of reflection seismic is required before faulting occurs, represented for instance in strain hardening, parasitic and disharmonic folding, among other plausible mechanisms.
Provenance of the early Permian Nambucca Block (eastern Australia) and implications for the role of trench retreat in accretionary orogens
Uri Shaanan et al., University of Queensland, Brisbane, Queensland, Australia. Published online ahead of print on 18 Feb. 2015; http://dx.
The paper presents new geochronological results and morphological classification of detrital zircons from the Nambucca Block, eastern Australia. Data show a genetic link between back-arc basin formation and oroclinal bending in the New England Orogen, and suggesting that both were controlled by trench retreat. These relationships may explain how accretionary orogens were able to obtain an anomalous width without a substantial contribution of accreted exotic terranes.
Clumped isotope thermometry in deeply buried sedimentary carbonates: The effects of bond reordering and recrystallization
B.J. Shenton et al., Texas A&M University, College Station, Texas, USA. Published online ahead of print on 18 Feb. 2015; http://dx.
We present clumped isotope temperatures from exhumed brachiopods, crinoids, cements, and host rock that are expected to be altered via solid-state C-O bond reordering due to heating during deep burial in the Palmarito Formation, Venezuela, and the Bird Spring Formation, Nevada, USA. We observe that while the different carbonate components experienced the same thermal history, they can yield significantly different clumped isotope temperatures. We also present new high-resolution thermal histories constrained by independent thermal history indicators and couple these with kinetic bond reordering models to predict the expected extent of C-O bond reordering. This integrated approach allows for direct comparison of clumped isotope temperatures and kinetic model predictions. Guided by the data-model comparison, we refine the thermal histories and present hypotheses on sample diagenetic histories and bond reordering kinetics.
Magnetostratigraphy, biostratigraphy and chemostratigraphy of the Pignola-Abriola section: New constraints for the Norian/Rhaetian boundary
M. Maron et al., University of Padova, Padova, Italy. Published online ahead of print on 3 Feb. 2015; http://dx.
A statistical correlation based on magnetostratigraphy has been applied between the basinal Pignola-Abriola section and the Newark APTS to provide numerical age constraints for the Rhaetian Stage. This correlation solved the chronological issue about the duration of the Rhaetian Stage (Late Triassic), illustrated as two options (Long-Tuvalian and Long-Rhaetian) in the Geological Time Scale 2012. The Norian/Rhaetian boundary, identified by both chemo- and biostratigraphical constraints, has been placed at 205.7 million years ago, defining a duration of the Rhaetian of ~4.4 million years.
Late Cretaceous evolution of the Coqen basin (Lhasa terrane) and implications for early topographic growth on the Tibetan Plateau
Gaoyuan Sun et al., Nanjing University, Nanjing, 210023, China. Published online ahead of print on 18 Feb. 2015; http://dx.
The Tibetan Plateau is the largest mountain belt in the world, and its geologic history is not well known. This study concentrates on the tectonic evolution of the Late Cretaceous Coqen basin in the Lhasa block (Southern Tibetan Plateau). An ~1700-m-thick terrestrial sandstone and conglomerate sequence (named the Daxiong Formation) recorded the quick and short-distance transported braided and alluvial fan sedimentation. By combining paleocurrent data, sandstone petrology, and detrital zircon isotope analyses, this paper demonstrates that the northern Lhasa was the source area for the Daxiong Formation. Together with the regional structural restoration, two thrust systems with opposite vergence were responsible for transforming the northern Lhasa area into an elevated mountain range during Late Cretaceous time. This process resulted in the evolution from a shallow marine environment (late Early Cretaceous) into a terrestrial depositional environment (Late Cretaceous) on the southern margin of the northern Lhasa. Given the regional paleogeographic context, the Daxiong Formation in the Coqen basin records the early topographic growth of the northern Lhasa prior to the Cenozoic India-Asia collision.