T42A-01
Influence of Andean Plateau Rise on South American Climate Dynamics
Large mountain ranges exhibit a first-order control on climate. In South America, the modern Andes act as a barrier to atmospheric flow and control regional wind and precipitation patterns. However, it is unclear how climate may have changed over time as Andean topography developed. We present results from a regional general circulation model (RegCM3) to evaluate dynamical and physical atmospheric changes associated with variations in Andean plateau height during the Cenozoic. A series of five experiments were conducted with plateau topography systematically varying between 0 and 100% of the modern. Experiments were performed over a continental-scale domain with 60km horizontal resolution using the MIT-Emanuel convection scheme. Land surface characteristics, sea-surface temperatures and atmospheric boundary conditions were specified from modern NCEP reanalysis data. Model results show that large-scale upper-level (200 mbar) circulation characteristics are only weakly affected by the removal of the Andes with a slight weakening and eastward shift of the high pressure system over Bolivia. However, low-level (800 mbar) wind patterns change significantly and have a direct effect on precipitation in South America. The following features can be observed as the Andean topography decreases: (1) The dominant wind direction in the central Andes reverses with prevailing winds sourced from the Pacific Ocean. The Westerlies are characterized by low moisture content due to the presence of the cold Humboldt current along the west coast of South America. (2) A reduction in the surface pressure gradient between the Andes and the Amazon Basin reduces convergence over the plateau and suppresses the South American low level jet, reducing the southward moisture flux along the eastern flanks of the Andes that is the primary source for precipitation in the Chaco region. These changes lead to declines in precipitation over the Andes and decreased latent heat release. This results in a reduction in deep convection over the Andes, which is associated with reduced cumulus cloudiness and less evapotranspiration. (3) Moreover, the development of a low-pressure system and a narrow zone of strong convergence in southwest Brazil is accompanied by an intensification and eastward shift of advection of warm moist air along the South American Convergence Zone. These effects indicate that lowering the Andes has a profound impact on the climatology in South America. Significant changes in moisture availability and Andean regional climate have important implications for geomorphic processes and paleoaltimetry reconstruction of the plateau.
T42A-02
Stable isotope records of late Miocene precipitation and elevation patterns in the central Andean foreland of Bolivia
The pursuit of paleoaltimetry data from the world's highest plateau regions is generally motivated by the desire to provide previously unavailable boundary conditions for lithosphere scale tectonic models that aim at integrating mantle and lithospheric processes and the feedbacks of erosion, weathering and uplift of the Earth's surface. Recent stable isotope paleoaltimetry data indicate that a significant amount (1.5 to 2.5 km) of surface uplift of the Bolivian Altiplano occurred relatively rapidly during late Miocene times suggesting that delamination of the lithospheric mantle beneath the plateau may have triggered a direct uplift response at the Earth's surface. Here we present multi-isotope data from the Bolivian foreland (Chaco Basin) that records the sedimentary response of the uplift and climate history of the plateau and Eastern Cordillera over the last 12 Ma. Stable oxygen and carbon isotope data from pedogenic carbonate in fluvial deposits of the paleo-Pilcomayo river in southern Boliva suggest that a dramatic increase in sedimentation rates in the Chaco basin (130 to 628 m/a) at around 8 Ma is temporally correlated with a change in precipitation patterns that may have been the response to deflection of the South American Summer Monsoon to more southerly latitudes. In turn, focusing precipitation at the uplifting plateau margin and establishment of a more seasonal rainfall pattern will inevitably affect the amount (and isotopic composition) of precipitation that reaches the plateau interior. This amount effect needs to be taken into account when interpreting stable isotope patterns from the Altiplano. The temporal correlation between increases in sedimentation (and likely denudation) rates, changes in carbon and oxygen isotope compositions of pedogenic carbonate as well as the source of river waters indicates that during Miocene surface uplift rivers cut progressively deeper into the evolving orogen, due to higher erosion and transport capacity thus witnessing the combined effects of changing climate and surface elevation of the central part of the Altiplano.
T42A-03
Oxygen isotope evidence from Cenozoic paleosol carbonates on the Puna Plateau of NW Argentina: low or dry in the Neogene?
The uplift and climatic history of the Puna-Altiplano Plateau in the Central Andes has received increasing attention in the last decade. In particular, recent stable isotope studies from the Altiplano of Bolivia have proposed a ca. 2-4 km of surface uplift between 7 and 10 Ma (e.g. Garzione et al., 2008). This pulse of surface uplift has been attributed to lithosphere removal and subsequent rebound. The climatic evolution of the region is equally intriguing. An open debate exists as to whether arid climatic conditions were in place by the early Cenozoic or if aridity was only established in the Neogene as a result of plateau uplift, assuming that the Neogene uplift model developed in the Altiplano of Bolivia applies to the entire plateau. Preliminary stable isotope data from paleosol carbonate nodules from the southern Puna Plateau in NW Argentina are significantly different from data reported for the Altiplano Plateau. Carbonate nodules from fluvio-lacustrine and eolian strata spanning the entire Cenozoic show that δ18O(PDB) values are overall much higher (-5.9 to -4.3‰) than values reported for time equivalent strata in the Altiplano of Bolivia (ca. -11 to -13‰). In particular, ashes within the upper part of the section are 11Ma – late Pliocene in age and carbonate nodules from this part of the section show δ18O values of ca. -3.5‰ (PDB). Interpreted as a mere function of elevation and compared with values from equivalent strata in the Altiplano (ca. δ18O(PDB) = -12‰) these data indicate: i) a ca. 3km difference in elevation between the Altiplano and the Puna in the Miocene, and ii) over 3 km of surface uplift since the Miocene to reach present day ca. -12.4 18O values (water predicted values). However, the Cenozoic section samples in the southern Puna show a clear trend of increasing δ18O values upsection. The δ13C values show a similar and consistent pattern suggesting a change in environment from C3 to C4 plants and/or decreasing soil respiration rates. This would suggest that our data record increasing evaporation and aridification rather than a change in elevation and cannot be directly compared to present day values to calculate uplift rates. References Garzione, C., N., Hoke, G.D., Libarkin, J.D., Withers, S., MacFadden, B., Eiler, J., dGhosh, P. and Mulch, A., 2008. Rise of the Andes, Science 6 June 2008: Vol. 320. no. 5881, pp. 1304 – 1307 DOI: 10.1126/science.1148615
T42A-04
Stable isotope paleoaltimetry of high relief terrain: An atmospheric dynamics perspective
Stable isotope ratios in rain and snow from mountainous regions show a strong correlation with altitude. To the extent that these isotopic ratios are preserved in the geological record, they may provide a powerful constraint on the surface uplift history of mountain belts. Existing interpretive frameworks for paleoaltimetry are based on linear regressions of modern precipitation isotope transects or on a Rayleigh distillation model of air parcel ascent along a moist adiabatic temperature lapse rate. Neither of these frameworks accounts for the fully nonlinear dynamics of airflow over high-relief terrain, which predicts substantial deviations from the moist-adiabatic ascent model under common atmospheric conditions. The Weather Research and Forecast model (WRF), a numerical weather prediction model, has been modified to include a simplified isotope physics parameterization and has been used to explore the links between topography, atmospheric state, and precipitation isotopes. The controlling nondimensional parameter for atmospheric flow over terrain is Nh/U, where N is the Brunt-Vaisala frequency, a measure of atmospheric stability, h is the orogen- scale relief, and U is the horizontal wind speed. When Nh/U<1, winds can flow directly over topography and WRF precipitation isotopes match those predicted by the moist-adiabatic Rayleigh model. When Nh/U>1, the winds are blocked by the topography and are deflected around it. In these cases, the maximum elevation of condensation is much lower than the range crest, and precipitation isotopes are consequently substantially less depleted than predicted by the moist adiabatic Rayleigh model. Furthermore, the along-strike length of an orogen is shown to exert a strong influence on precipitation isotopes in blocked flow regimes because of the dynamical link between terrain length and atmospheric flow blocking. Terrain- blocked atmospheric conditions are common, especially in regions of high relief, but their impact on the geological record, especially on records used for paleoaltimetry studies is unknown. The model results show that these records are strongly influenced by the joint effects of changes in both climate and topography, and that interpretations in terms of changes in topography alone may be unwarranted.
T42A-05
Topographically-enhanced glacial erosion and the elevation of the Teton Range, Wyoming
Rapid rock uplift and significant glacial erosion of the Teton Range, Wyoming, have created some of the highest values of relief in the conterminous United States. We combine digital topographic analyses, fault slip, and climate data to explore patterns of topographically-enhanced glacial erosion, and implement a flexural-isostatic model to investigate the contribution of erosion to range elevations. Activity since ~5 Ma on the normal Teton Fault bounding the east side of the range accounts for the spectacular modern topography. A cross-range tectonic gradient results in a distinct topographic asymmetry, with the range crest (notably two high supra-elevated peaks, Grand Teton and Mount Moran) offset several kilometers east of the drainage divide. This strongly affects patterns of precipitation, insolation, and glacial erosion. Precipitation is focused at the high peaks and snow efficiently transferred to valley floors by avalanching. Slowly-falling snow would have been advected farther into the range by prevailing westerlies, which would also have redistributed snow deposits from the subdued topography typical of the headwaters of western-draining basins. Greater topographic shading and cover by rock debris would have mitigated ablation of glaciers bounded by high valley walls. Glacier size, ice flux and erosion were therefore enhanced in eastern-draining basins, generating the highest measures of relief, although only large catchments (>20 km2) appear to have held glaciers capable of eroding at rates that kept pace with rock uplift. A flexural-isostatic model accounting for relief production in the Tetons and sediment deposition in the adjacent Teton Valley shows that patterns of deflection correlate to the distribution of relief. However, isostatic uplift was unlikely to significantly outpace summit erosion, even for a relatively thin, flexible crust. The relatively small aerial dimensions of the Teton Range would have limited the total mass removed, despite the significant topographic-enhancement of erosion.
T42A-06 INVITED
Large spatial variations in glacial erosion detected with detrital thermochronology
Studies of drainage basin erosion and landform evolution are often limited by not knowing where sediment is sourced from and how climate change influences catchment erosion. Detrital thermochronometer cooling ages collected from Quaternary glacial moraines and modern river sediments provide a promising tool to address these problems. We present an application of detrital thermochronology to quantify spatial variations in alpine glacial erosion during the Last Glacial Maximum (LGM). Results are compared to the distribution of recent erosion recorded in samples from modern river sediments. The elevation dependence of detrital apatite (U-Th)/He (AHe) ages is used as a sediment tracer to track the elevations where glacially eroded sediment is produced from bedrock. We measured ~204 AHe single grain ages from three moraines located between 2.3 and 3.7 km elevation in the Lone Pine catchment, Sierra Nevada, California. Ages from the lowest elevation moraine were measured on fine (<250 um) and coarse (>250 um) grained fractions of the sample to assess potential variations in sediment supply from different erosional processes. Measured AHe age probability density functions (PDFs) were compared with predicted PDFs, calculated by convolving bedrock age-elevation relationships with catchment hypsometries clipped at different elevations to reflect variable source elevations of sediment. Statistical comparison of the PDFs using a Monte Carlo approach and Kuiper test are used to evaluate the spatial distribution of erosion in the catchments. Results from the lowest elevation moraine indicate sediment is produced from the lower ~50-70% of catchment elevations at the 95% confidence level, indicating erosion near the base and sides of the glacier proportionally outweigh erosion from higher elevation head wall retreat and rock fall onto the glacier. Furthermore, grain-age distributions from the fine and coarse grain fractions are virtually indistinguishable, suggesting either both size fractions are sourced from similar elevations, and/or a significant disaggregation of coarse grained material into finer material during transport. Finally, the intermediate to high-elevation moraines within the cirque indicate glacial erosion is possibly uniform and occurs over 70-100% of the elevations above the sample location. These spatial variations in glacial erosion are in stark contrast to previously published results from the fluvially dominated Inyo Creek where a uniform distribution of erosion is observed from detrital AHe analysis of modern river sediments. Taken together, these results demonstrate: (1) a high sensitivity of detrital thermochronology to spatial variations in glacial and fluvial erosion processes, and (2) an increase in topographic relief and/or significant valley widening during glaciation as supported by the abundance of sediment sourced from lower catchment elevations.
T42A-07
Numerical models of tectonics-erosion-climate couplings in a three-dimensional orogenic wedge
We present results of three-dimensional numerical models of the deformation within a three-dimensional orogenic wedge driven by subduction-like processes in the underlying mantle. The thermo-mechanical model has been coupled to a surface-processes model incorporating the effects of lateral tectonic advection on landform evolution and those arising from the erosional unloading on the force balance within the wedge. A simple orographic model has also been included. We present a wide range of model experiments in which we vary not only the wedge mechanical behaviour by altering the mechanical properties or the assumed thermal state, but also the rate and patterns of precipitation. We show that there exists a strong coupling between the various parts of the system at a wide range of scales, from the orogen-scale down to the scale of a single valley. We also demonstrate that the evolution of landform at the surface of an actively deforming orogenic wedge is strongly affected by tectonic advection and other effects arising, in parts, from the three dimensional nature of the deformation field that is brought upon by the assumed finite lateral width of the wedge. We will compare our results to a few geological cases to draw conclusions about the behaviour of the natural system.
T42A-08
The Influence of Climate Change and Uplift on Colorado Plateau Paleotemperatures From Clumped Isotope (Δ47) Carbonate Thermometry
The elevation history of Earth's surface is key to understanding the geodynamic processes responsible for the rise of continental plateaus and orogens, and topography's influence on the circulation of the atmosphere and global climate in the past. Yet few available tools can provide quantitative paleoelevation constraints independently of climatic conditions. 'Clumped isotope' carbonate paleothermometry - a new technique based on measurement of the 13C-18O bond enrichment in carbonates - independently constrains both the temperature and isotopic composition of ancient surface waters, offering a potentially powerful approach to reconstruct past elevations and climate. We investigate the timing of Colorado Plateau uplift by comparing measurements of both modern and ancient depositional temperatures of lake sediments that blanket the plateau interior and adjacent lowlands. In particular, we find that comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate on changes in temperature, and therefore more accurately assess the contribution from changes in elevation. Clumped isotope thermometry of modern lake calcite from 350-3300 m elevation in the southwestern United States reveals a lapse rate of 4.2°C/km, consistent with modern climatic data for the region. Lacustrine carbonates of the Late Miocene Bidahochi Formation (ca. 1900 m above sea level) record temperatures of 22-25°C, 8°C warmer than the temperature predicted by the modern trend. Temperature estimates for the low elevation (88-125 m above sea level) Late Miocene Bouse Formation suggest that a warmer Late Miocene climate can explain 2.6 to 5.8°C of the observed 8°C Bidahochi temperature anomaly, leaving approximately 2.2 to 5.4°C of the anomaly that can be attributed to elevation gain. Using our best- fit carbonate temperature-elevation slope of 4.2°C/km, the data suggest that this cooling was accompanied by 520 to 1300 m of uplift of the southern portion of the Colorado plateau since 6 Ma.