Description: Available online 29 December 2012 Publication year: 2012 Source: Earth and Planetary Science Letters The southern Patagonian Andes constitute the narrow, high-latitude end of the Andean orogen belt in South America, where inherited basin paleogeography, subduction processes, retroarc crustal thickening, and late Cenozoic glaciation have collectively influenced their unusual tectonic and physiographic evolution. New zircon and apatite (U–Th)/He thermochronology from the Patagonian Andes between 50°30′ and 51°30′S suggest concentrated exhumation across the retroarc (leeward) side of the orogen since early Miocene time. Zircon (U–Th)/He (ZHe) ages range from 44 to 10 Ma; oldest ages are recorded in the Patagonian batholith and along the far eastern frontal foreland monocline. Regionally-uniform ZHe ages between 22 and 18 Ma, located across a ∼75 km wide-zone of the Patagonian retroarc thrust-belt indicate widespread early Miocene cooling through the ZHe partial retention zone. Mesozoic sedimentary and volcanic rocks in this region have been exhumed from at least 5 to 6 km depths. Early Miocene denudation of the thrust-belt, deformation, and increased foreland sedimentation rates coincided with opening of the Scotia Sea, suggesting a causal response of the foreland to changes in plate dynamics. The apatite (U–Th)/He (AHe) ages from a similar region range from 11 to 3 Ma; the youngest ages (6–3 Ma) are spatially clustered within the more deeply-exhumed central thrust domain. We interpret these AHe ages to record 〉1–2 km of erosional denudation associated with late Cenozoic glaciation and fluvioglacial processes in Patagonia beginning ca. 7 Ma. Zircon results indicate that since ca. 22 Ma, long-term exhumation rates have been highest across the western and central thrust domains (0.22–0.37 mm yr −1 ), and significantly lower along the eastern thrust front (0.10–0.17 mm yr −1 ). Since ∼7 Ma, apatite results from these same regions suggest comparable and slightly higher denudation rates (0.14–0.46 mm yr −1 ), particularly within the eastern thrust domain, consistent with efficient erosional processes acting in the retroarc region. These results from the Patagonian retroarc region, particularly the predominance of 〈7–4 Ma AHe ages, provide a new understanding for regional orogenic erosion models that are largely based on existing data from the windward regions. We suggest that retroarc denudation was enhanced by widespread Miocene structural uplift and unroofing of the fine-grained siliciclastic rocks of the marine Cretaceous Magallanes/Austral foreland basin. Furthermore, its location at the southern tip of the orogen, may have allowed additional moisture to reach the leeward side, leading to sustained late Cenozoic erosional denudation. Highlights ► We conduct apatite and zircon (U–Th)/He thermochronology in the Patagonian Andes. ► We model time–temperature paths to characterize rock cooling and exhumation. ► The Patagonian retroarc region underwent deep exhumation (5–6 km) since 22–18 Ma. ► Onset of exhumation is coeval with deep-seated thrusting in the retroarc region. ► Up to 2 km of erosional denudation was synchronous with Patagonian glaciations.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 CO 2 sequestration is regarded as an important strategy for reducing anthropogenic CO 2 emissions. Both the nature and rate of fluid–mineral reactions in CO 2 –water–rock systems are crucial, yet poorly constrained, parameters in understanding the fate of CO 2 injected in geological formations. This study models reactions and reaction rates in an exhumed CO 2 -charged aquifer where CO 2 -rich brines have bleached red sandstones by dissolution of hematite grain coatings. We show that the vertical movement of the reaction front is dominated by diffusion and this allows calculation of reaction rates for the dissolution of hematite grain coatings and K-feldspar. Using mineral surface areas calculated from BET measurements, we estimate K-feldspar dissolution rates of 3.4 ± 2.0 × 10 − 13 mol m − 2 s − 1 ; and hematite reaction rates of 6.1 ± 5.0 × 10 − 16 mol m − 2 s − 1 . The rates for K-feldspar are lower than previous, experimentally derived, estimates of K-feldspar dissolution rates by 1–2 orders of magnitude, likely explained by the proximity of the natural system to equilibrium. The inferred hematite reaction rates are 5–6 orders of magnitude slower than laboratory experiments and appear to be controlled by the chemical gradients imposed by the more sluggish K-feldspar dissolution. As the majority of potentially mobile trace metals are hosted in iron-oxide grain coatings, we argue that the rate of contaminant mobilization by CO 2 -charged brines will be lower than suggested by laboratory experiments. Highlights ► We model dissolution rates of K-feldspar and hematite in a natural CO 2 reservoir. ► Vertical transport in the system is dominated by diffusion. ► Calculated kinetic constants are lower than predicted from laboratory experiments. ► Hematite reaction rates are inhibited by the rate of silicate dissolution. ► Contaminant release from oxide grain coatings will be slower than models suggest.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 From the Late Jurassic through the Cretaceous, collision between the North American and Farallon plates drove extensive thin-skinned thrusting and crustal shortening that resulted in substantial relief in the North American Cordillera. The elevation history of this region is tightly linked to the tectonic, climatic and landscape evolution of western North America but is not well constrained. Here we use an atmospheric general circulation model with integrated oxygen isotope tracers (isoCAM3) to predict how isotope ratios of precipitation would change along the North American Cordillera as the mean elevation of orogenic highlands increased from 1200 m to 3975 m. With increases in mean elevation, highland temperatures fall, monsoonal circulation along the eastern front of the Cordillera is enhanced, and wet season (generally spring and summer) precipitation increases. Simulated oxygen isotopic ratios in that precipitation are compared to those obtained from geologic materials (e.g. fossil bivalves, authigenic minerals). Quantification of match between model and data-derived δ 18 O values suggests that during the Late Cretaceous, the best approximation of regional paleoelevation in western North America is a large orogen on the scale of the modern Andes Mountains with a mean elevation approaching 4000 m and a north-south extent of at least 15° of latitude. Highlights ► Combined isotopic data and climate/isotope simulation can constrain paleoelevation. ► The Campanian highlands of Western North America were Andean in scale. ► Paleoelevation estimation can be practically approached geologically.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 Geochemical studies of primitive basalts have documented the presence of crustal-derived garnet pyroxenite in their mantle sources. The processes whereby melts with the signature of garnet pyroxenite are produced in the mantle are, however, poorly understood and somewhat controversial. Here we investigate a natural example of the interaction between melts of garnet pyroxenite derived from recycled plagioclase-rich crust and surrounding mantle in the Ronda peridotite massif. Melting of garnet pyroxenite at ∼1.5 GPa generated spinel websterite residues with MREE/HREE fractionation and preserved the positive Eu anomaly of their garnet pyroxenite precursor in whole-rock and clinopyroxene. Reaction of melts from garnet pyroxenite with depleted surrounding peridotite generated secondary fertile spinel lherzolite. These secondary lherzolites differ from common spinel lherzolite from Ronda and elsewhere by their lower-Mg# in clinopyroxene, orthopyroxene and olivine, lower-Cr# in spinel and higher whole-rock Al 2 O 3 , CaO, Sm/Yb and FeO ⁎ at a given SiO 2 . Remarkably, secondary spinel lherzolite shows the geochemical signature of ghost plagioclase in the form of positive Eu and Sr anomalies in whole-rock and clinopyroxene, reflecting the transfer of a low-pressure crustal imprint from recycled pyroxenite to hybridized peridotite. Garnet pyroxenite melting and melt–peridotite interaction, as shown in the Ronda massif, may explain how the signature of subducted or delaminated crust is transferred to the mantle and how a garnet pyroxenite component is introduced into the source region of basalts. The efficiency of these processes in conveying the geochemical imprint of crustal-derived garnet pyroxenite to extruded lavas depends on the reactivity of pyroxenite melt with peridotite and the mantle permeability, which may be controlled by prior refertilization reactions similar to those documented in the Ronda massif. Highly fertile heterogeneities produced by pyroxenite–peridotite interaction, such as secondary spinel lherzolite in Ronda, may nucleate magmatic channels that remain chemically isolated from the ambient mantle and act as preferential pathways for melts with the signature of recycled crust. Highlights ► Garnet pyroxenite in the Ronda massif derives from plagioclase-rich crustal gabbro. ► Garnet pyroxenite melting and melt–rock reaction induced peridotite refertilization. ► The geochemical imprint of crustal gabbro was transferred to refertilized peridotite. ► Refertilized peridotite may be the source of melts with the imprint of recycled crust.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 Water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation (i.e., fabric) in mantle rocks. The A-type olivine fabric is the most commonly observed olivine fabric in natural peridotites. Development of A-type olivine fabric (developed by the (010)[100] slip system) is known to be limited to the water-poor conditions of the shallow upper mantle (〈200 km depth). We have performed simple-shear deformation experiments of olivine at 7.2–11.1 GPa and 1400–1770 K. Here we show that A-type olivine fabric was developed under water-rich conditions (〉2130 ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) was observed under moderately wet conditions (750–2130 ppm H/Si). Developments of C-type (by the (100)[001] slip system) fabric was limited to water-poor conditions (〈220 ppm H/Si). We found that monotonic decrease in the seismic anisotropy V SH / V SV (the ratio of horizontally and vertically polarized shear waves) with depth in the global one-dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water-poor mantle. Only A-type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of V SH / V SV 〈1 in the deep upper mantle (〉200 km depth). A strong anomaly of V SH / V SV 〈1 observed in the deep upper mantle beneath the East Pacific Rise is well explained by the distribution of water-rich regions (in which A-type olivine fabric is dominantly developed) in the deep upper mantle and upwelling mantle flows. Highlights ► Developments of olivine fabrics under Earth's deep upper mantle conditions were investigated. ► Formation of the A-type olivine fabric was observed under water-rich conditions. ► B-type fabrics were formed under moderately wet conditions. ► Origin of the anomaly of V SH / V SV reported beneath the East Pacific Rise is discussed.

Description: 1 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 361 We present a speleothem stable oxygen isotope record for the last 12.5 ka based on two stalagmites from western Cuba. The δ 18 O signal is interpreted to represent past precipitation variability. Both stalagmites show a pronounced transition from higher δ 18 O values (indicating drier conditions) to more negative δ 18 O values (suggesting wetter conditions) between 10 and 6 ka. This transition is also visible in a planktonic δ 18 O record off Haiti. On orbital timescales, the δ 18 O value of Caribbean precipitation, thus, strongly resembles the oxygen isotope composition of Caribbean surface water. On millenial timescales, the speleothem δ 18 O record shows a high correlation to a North Atlantic sea surface temperature (SST) record off West Africa as well as a similarity with the Bond events. Periods of lower North Atlantic SST correspond to less precipitation in the Caribbean and vice versa. The potential teleconnection to the Caribbean may reflect the position of the Intertropical Convergence Zone because a further southward position of the ITCZ leads to reduced precipitation in the northern Caribbean. Highlights ► We reconstruct precipitation variability in the Caribbean during the last 12.5ka. ► Two stalagmite records show the transition from drier to wetter conditions between 10 and 6ka. ► On orbital timescales, recorded δ 18 O in speleothems resembles sea surface δ 18 O values. ► On millennial timescales, δ 18 O resembles a SST record off West Africa and shows Bond events. ► Mechanism: lower SST leads to southward movement of ITCZ and less precipitation in Cuba.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 We apply the carbonate clumped isotope thermometer (Δ 47 ) to macrofossils from the Baculites compressus (∼73.5 Ma) and the Hoploscaphites nebrascensis (∼67 Ma) ammonite zones of the Western Interior Seaway (WIS) of North America, and nearby coeval terrestrial and open marine environments. The carbonate clumped isotope thermometer is based on a single-phase isotope exchange equilibrium that promotes the ‘clumping’ of two heavy isotopes together within a single carbonate molecule as temperature decreases. Due to the thermometer's isotopic independence from water, coupled measurements of Δ 47 and the bulk oxygen isotopic composition of a carbonate ( δ 18 O c ) enable the reconstruction of both paleotemperature and the isotopic composition of the water in which the organisms grew. Before applying the technique to the aragonite shells of fossil marine organisms (mostly ammonites, but also some gastropods, bivalves, and one belemnite), we measure the clumped isotopic composition of modern nautilus and cuttlefish, two of the nearest living relatives to the Cretaceous ammonites. Modern cephalopods exhibit disequilibrium isotope effects with respect to Δ 47 , but not δ 18 O c , therefore a simple correctional scheme is applied to the Late Cretaceous macrofossil data before reconstructing paleotemperatures. Diagenesis is also assessed by visual preservation and previously measured Sr concentrations ( Cochran et al., 2003 ). Temperatures reconstructed for the Late Cretaceous Western Interior Seaway range from 16.4±3.5 °C for an offshore Interior Seaway environment from the H. nebrascensis zone to 24.2±0.4 °C for the B. compressus ammonite zone. The seaway itself has an isotopic composition of approximately −1‰ (relative to VSMOW), the expectation for an ice-free global ocean average, while a nearby freshwater environment has an isotopic composition approaching −20‰. We compare the attributes of the reconstructed climate to predictions based on Late Cretaceous climate models and previous reconstructions of the seaway, and also assess the sensitivity of our results to the modern cephalopods correction by comparisons to suitable modern analogs. Finally, our clumped isotope data are consistent with cooling between the Late Campanian and Maastrichtian, as also seen in benthic foraminfera δ 18 O. Highlights ► Clumped isotope thermometry is applied to Late Cretaceous macrofossils. ► Clumped isotope disequilibrium is found in modern nautilus and cuttlefish. ► Population average reconstructed temperatures are between 16 and 24 °C. ► The isotopic composition of the WIS approximates the ice-free global ocean average. ► The isotopic composition of a freshwater environment approaches −20‰.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 New paleomagnetic data from Early Miocene to Pliocene terrestrial sedimentary and volcanic rocks in Central Greece constrain the history of vertical-axis rotation along the central part of the western limb of the Aegean arc. The present-day pattern of rapid block rotation within a broad zone of distributed deformation linking the right-lateral North Anatolian and Kephalonia continental transform faults initiated after Early Pliocene time, resulting in a uniform clockwise rotation of 24.3±6.5° over a region 〉250 km long and 〉150 km wide encompassing Central Greece and the western Cycladic archipelago. Because the published paleomagnetic dataset requires clockwise rotations of 〉50° in Western Greece after ∼17 Ma, while our measurements resolve no vertical-axis rotation of Central Greece between ∼15 Ma and post-Early Pliocene time, a large part of the clockwise rotation of Western Greece must have occurred during the main period of contraction within the external thrust belt of the Ionian Zone between ∼17 and ∼15 Ma. Pliocene initiation of rapid clockwise rotation in Central and Western Greece reflects the development of the North Anatolia–Kephalonia Fault system within the previously extended Aegean Sea region, possibly in response to entry of dense oceanic lithosphere of the Ionian Sea into the Hellenic subduction zone and consequent accelerated slab rollback. The development of the Aegean geometric arc therefore occurred in two short-duration pulses characterized by rapid rotation and strong regional deformation. Highlights ► We present paleomagnetic remanence directions from Neogene rocks in Central Greece. ► Clockwise rotation occurred in two distinct episodes. ► Initial clockwise rotation occurred in Middle Miocene time. ► A second pulse of clockwise rotation initiated after Early Pliocene time. ► Pliocene start of clockwise rotation records growth of Central Hellenic Shear Zone and onset of differential trench retreat.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 The timing of uplift of the Tibetan Plateau remains controversial, with estimates varying from Eocene to more recent than Pliocene. In particular, the paleoaltimetry of the Qiangtang terrane, on the central Tibetan Plateau, is completely unknown. Here, we present new stable isotope results of fluvial/lacustrine carbonate cement, pedogenic carbonate and marl from the Kangtuo and Suonahu formations deposited between ∼51 and ∼28 Ma in the Heihuling–Bamaoqiongzong area of the northern Qiangtang terrane. The lithofacies associations indicate that the Kangtuo formation was deposited in alluvial fan and fluvial floodplain environments, and the Suonahu formation was deposited in near shore lacustrine, playa-lake and channelized fluvial environments. Carbon and oxygen isotope values, coupled with the sedimentary facies interpretations, point to evaporation and low respiration rates in the Eocene–Oligocene paleosols, suggesting an arid climate in the high Qiangtang area at the time. The δ 18 O psw values of paleo-surface water reconstructed from the lowest (i.e. the least evaporated) δ 18 O c (PDB) values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin. The paleoelevation of the northern Qiangtang terrane is reconstructed as above 5000 m by at least the middle Oligocene (28 Ma), similar to the present elevation in this area. The aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high Lhasa terrane established by the middle Oligocene blocked the northward transport of tropical moisture. Maintenance of high elevation (〉5000 m) from at least the Oligocene to the present suggests that the Qiangtang crust was already thickened by that time, and that underthrusting of India beneath Asia since then has continued to provide additional material while at the same time driving Asian lower crust eastward by crustal flow. Highlights ► Paleoelevation attained above 5000 m of Qiangtang by at least before 28 Ma. ► Paleoenvironment was dry in Qiangtang during Paleogene. ► Elevation maintained is balanced by northward underthrusting of Indian lithosphere.

Description: 15 January 2013 Publication year: 2013 Source: Earth and Planetary Science Letters, Volume 362 The strong increase in the consumption of rare earth elements (REE) in high-tech products and processes is accompanied by increasing amounts of REE released into the environment. Following the first report of Gd contamination of the hydrosphere in 1996, anthropogenic Gd originating from contrast agents has now been reported worldwide from river and estuarine waters, coastal seawater, groundwater and tap water. Recently, microcontamination with La, that is derived from a point source where catalysts for petroleum refining are produced, has been detected in the Rhine River in Germany and the Netherlands. Here we report the occurrence of yet another REE microcontamination of river water: in addition to anthropogenic Gd and La, the Rhine River now also shows significant amounts of anthropogenic Sm. The anthropogenic Sm, which enters the Rhine River north of Worms, Germany, with the same industrial wastewater that carries the anthropogenic La, can be traced through the Middle and Lower Rhine to the Netherlands. At Leverkusen, Germany, some 250 km downstream from the point source at Worms, anthropogenic Sm still contributes up to 87% of the total dissolved Sm concentration of the Rhine River. Results from ultrafiltration suggest that while the anthropogenic Gd is not particle-reactive and hence exclusively present in the truly dissolved REE pool (〈10 kDa), the anthropogenic La and Sm are also present in the colloidal/nanoparticulate REE pool (between 10 kDa and 0.2 μm). Though difficult to quantify, our data suggest that the Rhine River may carry up to 5700 kg of anthropogenic La, up to 584 kg of anthropogenic Sm, and up to 730 kg of anthropogenic Gd per year toward the North Sea. There exist no regulatory limits for dissolved REE in natural waters, but total REE and Y (∑REY) concentrations of up to 0.14 mg/kg in the plume downstream of and 52.2 mg/kg at the head of an effluent pipe at Rhine-km 447.3 at Worms get close to and well-above, respectively, the levels at which ecotoxicological effects have been documented. Because of the increasing use of REE and other formerly “exotic” trace elements in high-tech applications, these critical metals have now become emerging contaminants that should be monitored, and it appears that studies of their biogeochemical behavior in natural freshwaters might soon no longer be possible. Highlights ► First report of anthropogenic Sm anomaly in river water. ► Rhine River shows anthropogenic Sm, La and Gd. ► Ultrafiltration reveals anthropogenic Gd exclusively truly dissolved. ► Anthropogenic La and Sm also colloid/nanoparticle-bound. ► Rare earths have become emerging contaminants in the hydrosphere.