Apparent Erosion Rate Research Articles

Delayed 10Be dilution in detrital quartz following extensive coseismic landsliding: A 2016 Kaikōura earthquake case study

Large earthquakes play a major role in the topographic evolution of active orogens. Earthquake induced landslides can alter the mass balance of mountain belts for decades to centuries depending on how landslide material is transported and stored. In this study, we analysed in-situ10Be concentrations in fluvial sediments to capture a time series of post-earthquake denudation rates of the Tūtae Putaputa|Conway River catchment, which experienced ∼13 × 106 m3 of landsliding in the 2016 Kaikōura earthquake. 10Be concentrations were determined for detrital samples from the Conway River at the rangefront of the Seaward Kaikōura Mountains, South Island, New Zealand, and near the catchment outlet over three sampling intervals, including after significant precipitation events, 1-2 yr following the earthquake. Our results indicate somewhat higher apparent erosion rates at the rangefront (0.41-0.58 mm/yr) when compared to erosion rates incorporating lower-relief sub-catchments at the river outlet (∼0.28 mm/yr). No changes were found over the same time period in the Hurunui River, a neighbouring catchment that experienced negligible coseismic landsliding during the Kaikōura event. Apparent catchment-averaged erosion rates in the Conway River have remained similar to long-term exhumation rates from previously published thermochronology studies. To explain our results, we explore the observed controls on catchment-wide 10Be concentrations in our study area, such as the spatiotemporal variability in landslide connectivity to the channel and storage within the catchment. Our results highlight that using in-situ cosmogenic 10Be in fluvial deposits as a proxy for changes in sediment fluxes of earthquake-affected landscapes is complex and highly site-specific.

Ice surface changes during recent glacial cycles along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica

Reconstructing past ice-sheet surface changes is key to testing and improving ice-sheet models. Data constraining the past behaviour of the East Antarctic Ice Sheet are sparse, limiting our understanding of its response to past, present and future climate change. Here, we report the first cosmogenic multi-nuclide (10Be, 26Al, 36Cl) data from bedrock and erratics on nunataks along the Jutulstraumen and Penck Trough ice streams in western Dronning Maud Land, East Antarctica. Spanning elevations between 741 and 2394 m above sea level, the samples have apparent exposure ages between 2 ka and 5 Ma. The highest-elevation bedrock sample indicates (near-) continuous minimum exposure since the Pliocene, with a low apparent erosion rate of 0.15 ± 0.03 m Ma−1, which is similar to results from eastern Dronning Maud Land. In contrast to studies in eastern Dronning Maud Land, however, our data show clear indications of a thicker-than-present ice sheet within the last glacial cycle, with a thinning of ∼35–120 m during the Holocene (∼2–11 ka). Difficulties in separating suitable amounts of quartz from the often quartz-poor rock-types in the area, and cosmogenic nuclides inherited from exposure prior to the last deglaciation, prevented robust thinning estimates from elevational profiles. Nevertheless, the results clearly demonstrate ice-surface fluctuations of several hundred meters between the current grounding line and the edge of the polar plateau for the last glacial cycle, a constraint that should be considered in future ice-sheet model simulations.

Cosmogenic soil production rate calculator

To understand the rates at which soils form from bedrock, it is important to know the rates at which the bedrock surface lowers (the apparent erosion rate, which is assumed to be constant). Previous models that calculate apparent erosion rates using measured concentrations of cosmogenic radionuclides rely on the assumption that the bulk density of the soil which forms as a product of bedrock erosion either equals that of the bedrock itself or is constant with depth down the soil profile. This assumption fails to recognise that soils have significantly lower densities that might not be constant with depth. The model presented here allows for the calculation of isotopically-derived soil production rates, considering the bulk density profile of the soil overlying the bedrock surface. This calculator, which can be run both in MATLAB® and GNU Octave©, represents a novel and significant contribution to the derivation of soil production rates.

Estimates of late Holocene soil production and erosion in the Snowy Mountains, Australia

Soil production in actively uplifting or high precipitation alpine landscapes is potentially rapid. However, these same landscapes are also susceptible to erosion and can be sensitive to changes in climate and anthropogenic activity which can upset the balance between soil production and erosion. The Snowy Mountains, southeastern Australia, are a tectonically stable, low relief, moderate precipitation mountain environment. The alpine area is extensively blanketed by soil that has been subjected to more intensive episodes of erosion during past periods of anthropogenic disturbance and under cold climate conditions of the late Quaternary. In this study, rates of soil development and hillslope erosion were investigated using radiocarbon dating, fallout radionuclides and sediment cores collected from lakes and reservoirs. Estimated Holocene soil development rates were 20–220t/km2/y. Erosion rates determined from the radionuclides 137Cs and 210Pb were equivocal, due to the inherent spatial variability of radionuclide inventories relative to apparent erosion rates. Estimated average erosion rates over the past 100years, determined from 210Pbex inventories, were 60t/km2/y (95% CI: 10, 90). Inventories of 137Cs observed at the same site implied that more recent erosion rates (over the past 60years) was below the detection limits of the sampling method applied here (i.e.<70t/km2/y). The upper estimate of 90t/km2/y is comparable to the mean erosion rate estimated using the radionuclide method for uncultivated sites in Australia and is significantly lower than that measured at sites were vegetation cover was disturbed by livestock grazing prior to its exclusion from the alpine area in the 1940s CE. Low erosion and high soil production rates relative to the lowland soils are likely related to extensive vegetation cover, which, in this context, protects soils against erosion and contributes to the formation of organic alpine soils, that rapidly accumulate organic matter by comparison to other soil types.

Decoupling of long-term exhumation and short-term erosion rates in the Sikkim Himalaya

Understanding the relative strengths of tectonic and climatic forcing on erosion at different spatial and temporal scales is important to understand the evolution of orogenic topography. To address this question, we quantified exhumation rates at geological timescales and erosion rates at millennial timescales in modern river sands from 10 sub-catchments of the Tista River drainage basin in the Sikkim Himalaya (northeast India) using detrital apatite fission-track thermochronology and cosmogenic 10Be analyses, respectively. We compare these rates to several potential geomorphic or climatic forcing parameters. Our results show that millennial erosion rates are generally higher and spatially more variable than long-term exhumation rates in Sikkim. They also show strongly contrasting spatial patterns, suggesting that the processes controlling these rates are decoupled. At geological timescales, exhumation rates decrease from south to north, with rates up to 1.2±0.6 mm/yr recorded in southwest Sikkim and as low as 0.5±0.2 mm/yr in the northernmost catchment. Long-term exhumation rates do not correlate with any geomorphic or climatic parameter. We suggest they are tectonically controlled: high rates in southwest Sikkim may be linked to the building of the Lesser Himalaya Rangit Duplex, whereas low rates in north Sikkim are consistent with cessation of extensional exhumation along the South Tibetan Detachment after 13 Ma. The highest apparent erosion rates recorded by cosmogenic nuclides (∼5 mm/yr) occur in catchments spanning the Main Central Thrust Zone, but these appear to be strongly influenced by recent landsliding. High millennial erosion rates (1–2 mm/yr) also occur in north Sikkim and may be climatically driven through strong glacial inheritance of the landscape, as attested by high channel-steepness values close to the maximum extent of glaciers during the Last Glacial Maximum. In contrast, variations in rainfall rate do not seem to strongly influence either millennial erosion or long-term exhumation rates in Sikkim.

Synchronous fluvial response to surface uplift in the eastern Tibetan Plateau: Implications for crustal dynamics

Synchronous response of river erosion to surface uplift is often assumed by studies using thermochronometer-constrained erosion histories to estimate changes in landscape and tectonics. We have tested this assumption by studying river incision histories at two sites (Heishui and Lianghekou) located in the westernmost inland part of the deeply incised eastern Tibetan Plateau. Apatite fission track age-elevation plots for the two sites show clear late Miocene inflection points, marking the time when apparent erosion rates increased fourfold to eightfold to 300–800 m/Myr. This timing is further refined as ~10–12 Ma by thermal history modeling and overlaps within analytical uncertainties with that recorded from the eastern Tibetan Plateau margin ~150–200 km to the east, demonstrating that river incision propagated rapidly from the margin to the interior in less than ~2 Ma. Such synchronous regional river incision is consistent with regional surface uplift generated by crustal extrusion along a detachment at depth.

Climate change and Late Pliocene acceleration of erosion in the Himalaya

Studies of active mountain ranges suggest that atmospheric and geodynamic processes may be strongly coupled through erosion — a hypothesis that has led to a debate over the relative importance of climate and far-field tectonic forcing in influencing erosion. We addressed this debate by developing the detailed long-term erosional history of a transect in the central Annapurna Range of Nepal for comparison with the climate and tectonic forcing histories of the region. Patterns of apatite fission-track and muscovite 40Ar/39Ar apparent ages with elevation indicate a five-fold increase in apparent erosion rate between 2.5 and 0.9 Ma ago. The time frame for this change corresponds to that of global climate destabilization associated with the onset of Northern Hemisphere glaciation and an intensification of the Asian monsoon. There is no evidence for important changes in the far-field tectonics of the Himalayan–Tibetan orogenic system over that interval, suggesting a largely climatic driver for enhanced erosion at the Himalayan range front.

A comparative study of detrital mineral and bedrock age‐elevation methods for estimating erosion rates

The age‐elevation pattern of thermochronologic data has been used for many years to estimate apparent erosion rates in orogenic settings. Recently, it has been suggested that thermochronologic data for detrital minerals from active river system sediments can be used as an effective proxy for bedrock age‐elevation distributions, serving as an alternative method for erosion rate studies. This “detrital mineral thermochronology” (DMT) method is easier, faster, and more cost‐effective than the traditional bedrock approach, and provides additional information about transients in topography and sediment delivery that may vary on diurnal to millennial timescales for a particular sampling site. However, two variants of the DMT method have been described in the literature, and they can yield very different erosion rate estimates. We tested both of these approaches against the traditional age‐elevation method using detrital and bedrock 40Ar/39Ar muscovite data sets from a single river catchment in the Annapurna Range, Nepal Himalaya. A nominal erosion rate estimate of ∼0.6 km/Myr for the 5.0–2.5 Ma period was calculated from the bedrock data, presented here for the first time. This result agrees with the ∼0.7 km/Myr (maximum) estimate from the detrital data set, which was derived from the DMT variant that emphasizes the range of single‐grain ages for a detrital sample, in this case, 11–2.5 Ma. However, the other DMT variant, which emphasizes the mean of the sample age distribution, yields an erosion rate estimate of ∼2.3 km/Myr. The simplest explanation for this discrepancy is that erosion rate increased significantly after ∼2.5 Ma, a scenario that is supported by apatite fission track data from the catchment.

Cosmogenic exposure ages of basalt flows: Lunar Crater volcanic field, Nevada

[sup 36]Cl and [sup 10]Be cosmogenic exposure age data for the Black Rock basaltic lava flow, Lunar Crater volcanic field, Nevada, imply an emplacement age of [approximately] 38 [+-] 10 ka. [sup 36]Cl data for the 600 ka flow north of the Lunar Crater maar are consistent with (1) an apparent erosion rate of [approximately] 3 m/m.y., (2) a model of desert pavement formation in which basalt clasts eroded from the flow remain on the surface of the eolian-derived sediments that mantle the flow, and (3) the early formation of the eolian-derived sediment mantle. 20 refs., 3 figs., 2 tabs.