Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka

Abstract

[1] Some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep, humid, tropical highlands of Sri Lanka. The total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites, bedrock from mountain crests, and bedrock from inselbergs. Denudation rates are in the range of 5-30 t km À2 yr À1(2-11 mm ky À1 ). These rates average denudation over the last 50-250 ky. Weathering exports in rivers draining the mountainous Central Highlands show that silicate weathering rates are also low, varying from 5 to 20 t km À2 yr À1 today (2-7 mm ky À1 ), but they represent a significant fraction of the total denudation. All these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief, temperatures, and precipitation. We speculate that the high relief in Sri Lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 Ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup. It is possible that throughout this history, hillslopes, where not exposing bare bedrock, were protected by thick weathered profiles. Such clay-rich layers would inhibit silicate weathering by shielding bedrock from weathering agents. In the absence of landscape rejuvenation, physical erosion rates are low, and fresh mineral surfaces are not being supplied. The observation that wet, steep, tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long-term controls of atmospheric CO 2 budgets: High temperature and precipitation, which are much invoked though controversial agents for silicate dissolution and CO 2 drawdown, become ineffective in promoting weathering in areas that are not tectonically active.

Cosmogenic nuclide‐derived rates of diffusive and episodic erosion in the glacially sculpted upper Rhone Valley, Swiss Alps

“…Soil samples used the 0·25-0·5 mm size fraction, however for two samples, Hil-I and Ober-R, multiple grain sizes were investigated in order to identify possible particle size infl uences on denudation rate. 10 Be was extracted using standard methods (von Blanckenburg et al, 1996;von Blanckenburg et al, 2004), 10 Be/ 9 Be ratios were measured in BeO targets prepared by evaporating and oxidizing Be together with AgNO 3 . Ratio measurements were performed with accelerator mass spectrometry at ETH Zurich as described by Synal et al (1997) relative to the standard S555 using a nominal 10 Be/ 9 Be ratio of 95·5 × 10 −12 which is based on a 10 Be half-life of 1·51.…”

A spheroidal weathering model coupling porewater chemistry to soil thicknesses during steady-state denudation

“…Regolith formation rates from bedrock are largely unknown, although rates on the order of 0.005 [1] to ~0.1 mm/y [2] have been estimated. Such rates are generally based upon the untested assumption of a steady-state soil thickness [3].…”

Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing

“…In turn, the rates of individual geochemical reactions contribute to the bedrock weathering rate, the flux of mineral nutrients to the biosphere, and the flux of solutes to the hydrosphere. Soils developed on granite, sandstone, and metamorphosed crystalline bedrock have often been presumed to be in steady state with respect to thickness (e.g., Brown et al, 1995;Heimsath et al, 1999;von Blanckenburg et al, 2004), regolith formation, erosion, weathering (e.g., Pavich, 1989;Brown et al, 1995;Riebe et al, 2003;Turner et al, 2003;Dosseto et al, Subm. ), elemental concentrations (e.g., Riebe et al, 2003), and yearly-averaged biomass and organic carbon (Buss et al, 2005).…”