Landscape response to Pleistocene-Holocene precipitation change in the Western Cordillera, Peru:¹⁰Be concentrations in modern sediments and terrace fills

Abstract

The landscape response to climate change is frequently investigated with models because natural experiments on geologic timescales are rare. In Quebrada Veladera, in the western Andes Mountains, the formation of alluvial terraces during periods of high precipitation presents opportunities for such an experiment. We compare drainage‐average erosion rates during Pleistocene terrace deposition with Holocene rates, using cosmogenic 10Be samples for seven pairs of quartz sand taken from the trunk and tributaries of Quebrada Veladera and adjacent terraces. Each pair consists of sediment collected from the modern channel and excavated from an adjacent fill terrace. The terrace fill was deposited at ~16 ka and preserved an isotopic record of paleoerosion rates in the Late Pleistocene. Modern sands yield 10Be concentrations between 1.68 × 105 and 2.28 × 105 atoms/g, corresponding to Holocene erosion rates between 43 ± 3 and 58 ± 4 mm/kyr. The 10Be concentrations in terrace sands range from 9.46 × 104 to 3.73 × 105 atoms/g, corresponding to paleoerosion rates from 27 ± 2 to 103 ± 8 mm/kyr. Smaller, upstream tributaries show a substantial decline in erosion rate following the transition from a wet to dry climate, but larger drainage areas show no change. We interpret this trend to indicate that the wetter climate drove landscape dissection, which ceased with the return to dry conditions. As channel heads propagated upslope, erosion accelerated in low‐order drainages before higher‐order ones. This contrast disappeared when the drainage network ceased to expand; at that point, erosion rates became spatially uniform, consistent with the uniformity of modern hillslope gradients.

Slow Long‐Term Exhumation of the West Central Andean Plate Boundary, Chile

“…More recent, shorter timescale denudation rates (<~10 6  years) based on catchment‐averaged 10 Be, 26 Al, and 21 Ne cosmogenic nuclide concentrations and exposure ages have been reported, as low as <1 m/Myr, up to several orders of magnitude lower than bedrock thermochronometric exhumation rates in the southern Peruvian and northern Chilean fore arc but range up to 300 m/Myr (~0.3 km/Myr) at higher elevations and slopes, particularly in the modern‐day Precordillera (Abbühl et al, 2010, 2011; Carretier, Regard, Vassallo, Aguilar, et al, 2015; Carretier, Regard, Vassallo, Martinod, et al, 2015; Kober et al, 2007, 2009; McPhillips et al, 2013; Placzek et al, 2010; Starke et al, 2017). Generally, higher rates are reported in southern Peru than in northern Chile but correlations with precipitation or hillslope angles are inconsistent (Abbühl et al, 2011; Kober et al, 2009; Reber et al, 2017; Starke et al, 2017) and extreme climatic events (e.g., El Niño) may be important (Abbühl et al, 2010).…”

Millennial-scale record of landslides in the Andes consistent with earthquake trigger

“…It extends continuously more than 20 km upstream from its confluence with the Pisco River, where it joins a contemporaneous fill terrace. Recent studies of this terrace and others along the Pisco River have indicated that they formed during periods of greater precipitation, which stripped regolith from hillslopes 9,10,17 .…”

“…In the Andes Mountains, such deposits have been interpreted in terms of El Niño/Southern Oscillation [5][6][7][8] . This study presents 10 Be data from two populations of granitic cobbles in the drainage called Quebrada Veladera: one population from the modern river channel and another from the adjacent fill terrace. The terrace was deposited ∼16 ka (refs 9,10), when the climate was much wetter than it is today [11][12][13][14] .…”

“…Cosmogenic nuclides such as 10 Be are routinely measured in detrital stream sediment; samples of tens of thousands of sand grains, each with a unique exposure history, are amalgamated to determine drainage-average erosion rates 18,19 . The probability distribution of cosmogenic nuclide concentrations assembled from individual sediment particles-clasts-contains additional information.…”

“…Such studies are possible because cosmogenic nuclide production depends on elevation, which constrains the upstream source-and therefore slope-for individual clasts. 10 Be production also declines rapidly with depth in the uppermost metres of Earth's surface. This property makes populations of single-clast concentrations sensitive to partitioning between shallow, grain-by-grain erosion and deep-seated landslides.…”