Centennial- and orbital-scale erosion beneath the Greenland Ice Sheet

Erosion beneath glaciers and ice sheets is a fundamental Earth-surface process dictating landscape development, which in turn influences ice-flow dynamics and the climate sensitivity of ice masses. The rate at which subglacial erosion takes place, however, is notoriously difficult to observe because it occurs beneath modern glaciers in a largely inaccessible environment. Here, we present 1) cosmogenic-nuclide measurements from bedrock surfaces with well constrained exposure and burial histories fronting Jakobshavn Isbræ in western Greenland to constrain centennial-scale erosion rates, and 2) a new method combining cosmogenic nuclide measurements in a shallow bedrock core with cosmogenic-nuclide modelling to constrain orbital-scale erosion rates across the same landscape. Twenty-six 10Be measurements in surficial bedrock constrain the erosion rate during historical times to 0.4–0.8 mm yr-1. Seventeen 10Be measurements in a 4-m-long bedrock core corroborate this centennial-scale erosion rate, and reveal that 10Be concentrations below ~2 m depth are greater than what is predicted by an idealized production-rate depth profile. We utilize this excess 10Be at depth to constrain orbital-scale erosion rates at Jakobshavn Isbræ to 0.1–0.3 mm yr-1. The broad similarity between centennial- and orbital-scale erosion rates suggests that subglacial erosion rates have remained relatively uniform throughout the Pleistocene at Jakobshavn Isbræ.

KineDyn: Thermo-Mechanical Method for Validation of Seismic Interpretations with Dynamic Forward Models

<p>Rifts and rifted margins result from interaction of several physical processes, which produce a range of crustal structures, subsidence histories, and sedimentary architectures. Study of these processes in academia and industry includes kinematic modelling (i.e. cross-section restoration, backstripping) combined with simple thermomechanical models and dynamic modelling. In kinematic models, the thinning of the lower crust and mantle is kinematically imposed in the form of pure shear, which contradicts natural non-linear viscous behavior. Although, kinematic modelling can provide a crustal thinning profile, heatflow estimates, subsidence rates etc., imposed extension of the lower crust and mantle might strongly impact the result. On the other hand, a dynamic approach allows to model the whole range of possible physical processes, but it cannot be used to model particular extension histories.<br>Here, we show a new modelling technique, namely KineDyn, to combine the advantages of the above-mentioned approaches into a single modelling framework. Our method employs full non-linear visco-elasto-plastic rheology, surface process of erosion and sediment transport, decompression melting of the mantle, and serpentinization of mantle rocks. Faults are introduced as weak planes in the upper crust, in order to simulate faulting during the model run. In our approach, faults are initially controlled by prescribed initial locations, offsets and timings, while the rest of the model is resolved in a fully dynamic mode. Since fault planes are much weaker than the surrounding upper crust, extension of the model naturally leads to slip on the faults. We demonstrate that faults modelled this way reproduce a natural behavior, including rotation due to flexure and unloading of the fault plane. <br>In order to reconstruct the evolution of an existing rift or rifted margin we model extension of the lithosphere with controlled faulting. To do this we use the interpreted spatio-temporal evolution of the faulting from a seismic profile to guide the evolution of the dynamic model. After a trial-and-error process, where we correct the faults’ locations, the thicknesses of layers, surface process’s parameters, initial thermal gradient etc., we obtain the model that best fits the observations. Thus, KineDyn gives, in effect, the same results as existing section restoration techniques (i.e. the potential history of faulting) and forward modeling techniques (i.e. the likely history of sedimentation, thinning, heat flow and subsidence), while simultaneously taking into account non-linear interactions between processes occurring during rifting. <br>In this work we show the methodology, examples, tests and benchmarks of the technique. Finally, we present applications of KineDyn for the following rifts and rifted margins: Malawi Rift, East African Rift System, hyper-extended West Iberia Margin, and ultra-wide Santos-Benguela Rifted Margin.</p>

Method Quantifies Separator-Oil Shrinkage

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19775, “Quantifying Separator-Oil Shrinkage,” by Mathias Lia Carlsen, SPE, and Curtis Hays Whitson, SPE, Whitson, prepared for the 2020 International Petroleum Technology Conference, Dhahran, Saudi Arabia, 13-15 January. The paper has not been peer reviewed. Copyright 2020 International Petroleum Technology Conference. Reproduced by permission. In tight unconventionals, oil and gas rates often are measured daily at separator conditions. Consequently, converting these rates reliably to volumes at standard conditions is necessary in cases where direct stock-tank measurements are not available. Because of changes in producing-wellstream compositions and separator conditions, the separator-oil shrinkage factor (SF) can change significantly over time. The complete paper presents a rigorous and consistent method to convert daily separator rates into stock-tank volumes. Recommendations for developing field-specific shrinkage correlations using field test data also are proposed. SF and Flash Factor (FF) Separator-Oil SF. Separator-oil SF is the fraction of metered separator oil rate that remains (or transforms into) stock-tank oil after further processing to standard conditions of 1 atm and 60°F. Put simply, the SF quantifies the decrease in oil volume from separator conditions to stock tank. The magnitude can range from less than 0.65 to 0.99. Separator-Oil FF. Separator-oil FF is the ratio of liberated gas from metered separator oil after further processing to standard conditions of 1 atm and 60°F. The FF accounts for the increase in gas volume from separator conditions to stock tank and explains why oil is shrinking (i.e., gas is coming out of the solution). The magnitude of the FF can range from 5 to 1,000 scf/STB. Total producing gas/oil ratio (GOR) can be calculated easily when SF and FF are known. An SF always is associated with an FF and is literally the solution GOR of the separator oil. Both SF and FF are a function of the top-side surface process and an associated wellstream composition. Surface Process. The surface process represents the number of topside separation stages and the associated separator pressure and temperature of each stage. In shale basins, two- and three-stage separation trains are common. The number of separation stages typically is fixed throughout the lifetime of a well. However, the separator temperature and pressure may vary significantly. Wellstream Composition. The well-stream composition quantifies the relative amounts of different components flowing out of a well at a given day. This measurement is typically expressed in mol%. Tight unconventional basins contain many kinds of in-situ reservoir fluid compositions from dry gas to black oils. The produced-wellstream compositions from these systems tend to change considerably with time because of producing flowing bottomhole pressures below the saturation pressure, as seen in the field example presented in Fig. 1. In the figure, the shut-in period after approximately 330 days results in a transient period with large compositional changes.

Topographic evolution of mountain belts controlled by rheology and surface process efficiency

<p>It has been a long-standing problem how mountain belts gain and loose topography during their tectonically active growth and inactive decay phase. It is widely recognized that mountain belt topography is generated by crustal shortening, and lowered by river bedrock erosion, linking climate to tectonics. However, it remains enigmatic how to reconcile high erosion rates in active orogens as observed in Taiwan or New Zealand, with long term survival of topography for 100s of Myrs as observed for example in the Uralides and Appalachians. Here we use for the first time a tight coupling between a landscape evolution model (FastScape) with an upper mantle scale tectonic (thermo-mechanical) model to investigate the different stages of mountain belt growth and decay. Using two end-member models, we demonstrate that growing orogens with high erosive power remain small (<200 km), reach steady state between tectonic in- and erosional material eff-flux, and are characterized by transverse valleys. Contrarily, mountain belts with medium to low erosive power will not reach growth steady state, grow wide, and are characterized by longitudinal rivers deflected by active thrusting. However, during growth both types of orogens reach the same height, controlled by rheology and independent of surface process efficiency. Erosional efficiency controls orogenic decay, which is counteracted by regional isostatic rebound. Rheological control of mountain height implies that there is a natural upper limit for the steepness index of rivers on Earth. To compare model results to various natural examples, we quantify the degree of longitudinal flow of modeled rivers with river “longitudinality” in several active or recently active orogens on Earth. Application of the river “longitudinality index” gives information whether (parts of) an orogen is or was at steady state during orogenic growth.</p>