1.5 Dating and Rating Landscape Evolution with Geochemical Methods on Geomorphic to Geologic Time Scales
The Earth's surface is under constant change. Tectonic, climatic, biogenic and anthropogenic forcings have a measurable impact on erosion, weathering and surface uplift. Information on the interactions among these processes and their spatio-temporal distribution can be inferred from observations of the available geologic archives. These involve, among others, the morphology and the geochemical-mineralogical composition of exposed bedrock, sedimentary products and soils. In order to successfully predict future landscape responses, it is therefore important to investigate these archives and to quantify the timing and rate of past landscape changes in response to the different forcings. In this session, we gather contributions involving state-of-the-art applications of geochronologic, thermochronologic and geochemical methods, aiming to quantify dates and rates of landscape change. In particular, we welcome any field-, laboratory- and/or modeling-based study, covering a range of timescales (hundreds to millions years), spatial scales (hillslope, catchment, orogen) and techniques (e.g. cosmogenic nuclides, thermochronology, luminescence, isotopic dating, etc...).
9:00am - 9:30am
Non-linear forcing of climate on denudation in the Alps over the last 75 ka
1CRPG, CNRS - Université de Lorraine, Nancy, France.; 2Laboratoire de Glaciologie, DGES-IGEOS, Université Libre de Bruxelles, Bruxelles, Belgium; 3IFREMER, Laboratoire Géodynamique et Enregistrement Sédimentaire, Plouzané, France.; 4Laboratoire Géosciences Océan, Institut Universitaire Européen de la Mer, Plouzané, France.; 5Aix-Marseille Univ., CNRS, IRD, INRA, Coll. France, UM 34 CEREGE, Technopôle de l’Environnement Arbois-Méditerranée, Aix-en-Provence, France.
Reconstruction of denudation rates through time is an important task to quantify and understand the impact of climate on landscape evolution. Cosmogenic nuclides have been widely used as a tool to infer denudation rates at the watershed scale from both river sediments and past stratigraphic records. Here, we analyze the in-situ 10Be cosmogenic concentration over the last 75 ka in sediments cores that were collected offshore the Var River (Western Mediterranean Sea).
We present 26 10Be paleo denudation rates ranging from 0.15 ± 0.01 and 1.26 ± 0.16 mm yr−1. At the exception of the LGM period, the 10Be paleo denudation rates are similar to these of today in the Var (0.24 ± 0.04 mm yr−1). However, during the LGM, paleo denudation rates were 2 to 3 times higher than today, suggesting that glaciers may have played a role. To investigate this sharp increase in denudation rates, we use a mass balance approach to differentiate the glacial from the fluvial component of denudation rates. The resulting average glacial erosion rate during the LGM is 1.5 (+0.9 / -1.0) mm yr−1, roughly four times above the value of 0.4 (+0.4 / -0.5) mm yr−1 obtained during MIS 3-4 (29 - 71 ka) Our data suggest that climatic variations may only strongly affect denudation beyond a certain threshold, probably controlled by glacier dynamics, the duration of glacial advances, and temperature-driven processes such as frost cracking. Our study indicates that the denudation response to Quaternary glaciations is complex and nonlinear in glaciated areas.
9:30am - 9:45am
Recent headwall deglaciation and retreat from cosmogenic 10Be in medial moraine debris of a Swiss valley glacier
1GFZ German Research Centre for Geosciences, Potsdam, Germany; 2Institute of Geological Sciences, Freie Universität Berlin, Berlin, Germany; 3Institute of Earth Surface Dynamics, University of Lausanne, Lausanne, Switzerland
Debris-covered glaciers are fed from steep bedrock hillslopes that tower above the ice, so-called headwalls. Recent observations in high-alpine glacial environments suggest that rock walls are increasingly destabilized due to climate warming. An increase in debris delivery to glacier surfaces will modify glacial mass balances, as surface debris cover impacts on the melt behavior of the ice underneath. Consequently, we expect that the response of debris-covered glaciers to climate change is likely linked to how headwall retreat responds to climate change.
As debris is deposited on the ice surface along the sides of valley glaciers it is passively transported downglacier on and in the ice. Where glaciers join it is merged to form medial moraines. Due to the conveyor-belt-nature of glacier ablation zones, debris tends to be older downglacier and, hence, systematic downglacier-sampling of medial moraines holds the potential to assess rates of headwall retreat through time.
In order to quantify headwall retreat rates, we measured the concentration of in situ-produced cosmogenic 10Be in debris samples collected on downglacier profiles along parallel medial moraines from a partly debris-covered Swiss glacier. Our results indicate that indeed nuclide concentrations along the medial moraines vary systematically, being higher for older downglacier deposits and lower for younger upglacier deposits. This variation cannot be explained by additional nuclide accumulation during transport alone. Instead we propose that ongoing ice cover retreat across deglaciating headwalls since the end of the Little Ice Age and the exposure of newly eroding bedrock surfaces may explain recently decreasing 10Be concentrations.
9:45am - 10:00am
Quantifying carbonate denudation from cosmogenic 36Cl and climatic and tectonic controls on carbonate landscape evolution
1Department of Earth Sciences, ETH Zurich, Zurich, Switzerland; 2Earth Surface Geochemistry, German Centre for Geoscience Research, Potsdam, Germany; 3Department of Geosciences, Colorado State University, Fort Collins, US; 4Department of Soil and Water Sciences, The Robert H. Smith Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem, Rehovot, Israel; 5Advanced School for Environmental Studies, The Hebrew University of Jerusalem, Jerusalem, Israel
Quantifying carbonate denudation and the partitioning between chemical and mechanical surface lowering in karstic areas is challenging. Here we present a compilation of 36Cl denudation rates from alluvial samples in the Mediterranean and combine these with chemical weathering rates derived from water chemistry and satellite-dervied runoff data. We calculate mechanical erosion as the difference between the total denudation from cosmogenic 36Cl measurements and the chemical weathering rates. Our results show a dominance of mechanical erosion in Mediterranean carbonate regions. We observe a strong scaling between mechanical erosion, catchment steepness and total denudation rate, but a weak scaling with chemical weathering. This implies that slope dependent erosion gets progressively more important with increasing denudation rates and therefore is linked to tectonic uplift.
Significant amounts of chemical weathering can bias cosmogenic denudation rate measurements. Therefore, we investigate this potential bias for the calculation of 36Cl denudation rates in the reported Meditteranean sites, but find a limited influence.
These findings support a conceptual model of a dissolution speed limit in carbonates due to available water and acid such that areas of high local uplift require substantial mechanical erosion to balance uplift and form steep slopes. In contrast, areas experiencing low uplift rates with sufficient water availability (e.g. humid climate) can balance uplift entirely with dissolution resulting in subdued carbonate landscapes. This feedback explains why Meditterranean carbonate mountains are often high and steep compared to oher rock units, whereas carbonates in humid temperate climates form the subdued parts of the landscape.
10:00am - 10:15am
Co-variation of silicate, carbonate, and sulfide weathering drives CO2 release with erosion: Constraints from southern Taiwan.
1GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany; 2NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; 3Department of Geosciences, Colorado State University, Fort Collins, CO USA; 4Centre de Recherches Pétrographiques et Géochimiques, UMR7358, CNRS, Université de Lorraine, 54500 Nancy, France; 5Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA; 6Department of Geosciences, National Taiwan University, 10617 Taipei, Taiwan
The supply of fresh minerals to Earth’s surface by erosion is thought to modulate global climate by removing atmospheric carbon dioxide (CO2) through silicate weathering. In turn, weathering of accessory carbonate and sulfide minerals is a geologically-relevant CO2 source, which may dampen or reverse the effect of silicate weathering on climate. Although these weathering pathways commonly operate side by side, we lack quantitative constraints on their co-evolution across erosion-rate gradients. Using stream-water chemistry across a 3 order-of-magnitude erosion-rate gradient in shales and sandstones of southern Taiwan, here, we demonstrate that silicate, sulfide, and carbonate weathering are linked: Increasing sulfide oxidation generates sulfuric acid and boosts carbonate solubility whereas silicate weathering kinetics remain constant or even decline, perhaps due to buffering of the pH by carbonates. On timescales shorter than marine sulfide compensation, CO2 emission rates from weathering in rapidly-eroding terrain are more than twice the CO2 sequestration rates in slow-eroding terrain. On longer timescales, CO2 emissions are compensated, but CO2 sequestration rates do not increase with erosion, in contrast to assumptions in carbon cycle models. We posit that these patterns are broadly applicable to many Cenozoic mountain ranges that expose dominantly siliciclastic metasediments.
10:15am - 10:30am
Drivers of Topography in Fold-thrust Belts: A Perspective from Central Nepal
1University of Tübingen, Germany; 2University of Pittsburgh, USA
Topography in fold-thrust belts over geologic time reflects the development of an orogenic Coulomb wedge that represents a balance of tectonic and erosional forcings. The establishment of critically tapered topography is generally viewed under two contrasting mechanical frameworks: (i) shortening and rock uplift are occurring everywhere suggesting an orogenic wedge under mechanical failure everywhere; and (ii) rock displacement takes place along discrete fault planes, including the translation of uplifted topography laterally. Here we investigate whether the topography in central Nepal is maintained by a combination of rock uplift during sequential fault activity, the lateral translation of topography over ramp-flat subsurface geometries, and alternating phases of hinterland incision during out-of-sequence faulting and deformation front activity; if this is the case, then erosional efficacy dynamically varies along the orogenic wedge, in contrast to a wedge under mechanical failure everywhere. We test this hypothesis by employing a structural-kinematic model of the Neogene fold-thrust belt evolution of central Nepal and integrate this into a surface processes model applying end-member climatic scenarios, i.e., uniform precipitation and climatic change over geologic time. Model output is validated by comparing predicted geomorphic metrics with observed ones. Our results indicate a dynamic variability of erosional efficacy that promotes the interplay of two modes of orogenic wedge behaviour: phases of lateral translation of uplifted topography and in-sequence propagation of deformation fronts, and phases of hinterland incision during out-of-sequence fault activity.