Weathering and the global carbon cycle: Geomorphological perspectives
142
Citation
156
Reference
10
Related Paper
Citation Trend
Keywords:
Carbon fibers
Soil production function
Global Change
Carbon flux
How flowing water and organisms can shape Earth9s surface, the Critical Zone, depends on how fast this layer is turned over by erosion. To quantify the dependence of rock weathering and the cycling of elements through ecosystems on erosion we have used existing and new metrics that quantify the partitioning and cycling of elements between rock, saprolite, soil, plants, and river dissolved and solid loads. We demonstrate their utility at three sites along a global transect of mountain landscapes that differ in erosion rates – an "erodosequence". These sites are the Swiss Central Alps, a rapidly-eroding, post-glacial mountain belt; the Southern Sierra Nevada, USA, eroding at moderate rates; and the slowly-eroding tropical Highlands of Sri Lanka. The backbone of this analysis is an extensive data set of rock, saprolite, soil, water, and plant geochemical and isotopic data. This set of material properties is converted into process rates by using regolith production and weathering rates from cosmogenic nuclides and river loads, and estimates of biomass growth. Combined, these metrics allow us to derive elemental fluxes through regolith and vegetation. The main findings are: 1) the rates of weathering are set locally in regolith, and not by the rate at which entire landscapes erode; 2) the degree of weathering is mainly controlled by regolith residence time. This results in supply-limited weathering in Sri Lanka where weathering runs to completion in the regolith, and kinetically-limited weathering in the Alps and Sierra Nevada where soluble primary minerals persist; 3) these weathering characteristics are reflected in the sites9 ecosystem processes, namely in that nutritive elements are intensely recycled in the supply-limited setting, and directly taken up from soil and rock in the kinetically settings; 4) the weathering rates are not controlled by biomass growth; 5) at all sites we find a deficit in river solute export when compared to solute production in regolith, the extent of which differs between elements. Plant uptake followed by litter export might explain this deficit for biologically utilized elements of high solubility, and rare, high-discharge flushing events for colloidal-bound elements of low solubility. Our data and new metrics have begun to serve for calibrating metal isotope systems in the weathering zone, the isotope ratios of which depend on the flux partitioning between the compartments of the Critical Zone. We demonstrate this application in several isotope geochemical companion papers.
Regolith
Saprolite
Soil production function
Biogeochemical Cycle
Bedrock
Cosmogenic nuclide
Parent rock
Denudation
Parent material
Cite
Citations (17)
Evaluating conflicting theories about the influence of mountains on carbon dioxide cycling and climate requires understanding weathering fluxes from tectonically uplifting landscapes. The lack of soil production and weathering rate measurements in Earth's most rapidly uplifting mountains has made it difficult to determine whether weathering rates increase or decline in response to rapid erosion. Beryllium-10 concentrations in soils from the western Southern Alps, New Zealand, demonstrate that soil is produced from bedrock more rapidly than previously recognized, at rates up to 2.5 millimeters per year. Weathering intensity data further indicate that soil chemical denudation rates increase proportionally with erosion rates. These high weathering rates support the view that mountains play a key role in global-scale chemical weathering and thus have potentially important implications for the global carbon cycle.
Soil production function
Cite
Citations (205)
Over geologic timescales, chemical weathering in mountain landscapes may play an important role in regulating atmospheric CO2. Understanding the feedbacks between climate, tectonics, erosion rates, biota, and weathering has been a recent focus of research, but disentangling these complex relationships remains a challenge. One area of particular interest has been the potential for a kinetic limit to weathering and soil production. Studies in New Zealand's Southern Alps were among the first to clearly exceed proposed kinetic limits on soil production and demonstrate thresholds in the influence of precipitation on chemical weathering. Here we present a new dataset that addresses chemical weathering, soil production rates, and surface erosion rates, measured across an altitudinal transect in the Tararua Range on New Zealand's North Island. The transect spans a kilometer in relief, and receives 3.5-5.5 m of annual precipitation. Underlying bedrock comprises silty and sandy members of the same Cretaceous Greywacke, but subtle lithologic changes correspond to abrupt shifts in soil production rates and total weathering. Total weathering across the transect is roughly invariant for each lithology and reflects near-complete depletion of weatherable species, consistent with a previously proposed threshold in the influence of precipitation. However, spatial patterns in weathering differ markedly in saprolite and in soils. Deep weathering in saprolite decreases with elevation and makes up a large fraction of the total weathering. This pattern suggests that climate may influence saprolite weathering, even where the total weathering is supply-limited. Spatial patterns in saprolite and total weathering do not correlate with an abrupt vegetation transition from dense forest to alpine tussock, which may suggest that biota are more strongly affected by a temperature threshold or more complex biogeochemical cycling. We contrast these results with new and previously published data from the Southern Alps, which have a similar climate but experience rapid tectonic uplift. There, the fresh supply of minerals to soils provided by uplift and erosion may enable much faster weathering and soil production rates. Taken together, these observations suggest a strong lithologic and tectonic control on soil production and weathering rates in humid climates.
Saprolite
Soil production function
Bedrock
Lithology
Parent material
Cite
Citations (0)
Denudation
Soil production function
Cosmogenic nuclide
Bedrock
Cite
Citations (63)
Carbon fibers
Soil production function
Global Change
Carbon flux
Cite
Citations (142)
Bedrock
Soil production function
Parent material
Cite
Citations (88)
Saprolite
Cosmogenic nuclide
Soil production function
Denudation
Cite
Citations (343)
Abstract This chapter considers the various processes of rock weathering, starting with physical weathering processes (frost, wetting and drying, heating and cooling or insolation weathering, and salt weathering (haloclasty)). It then turns to a consideration of chemical weathering processes and to the remarkable increase of interest in biological (organic) processes. This is followed by a discussion of rates of weathering and the techniques that have been developed to study them, including the analysis of sediment loads and the measurement of surface changes with instruments like the micro-erosion meter. The global carbon cycle and the links between geomorphology and soils are also considered. Progress in the study of weathering forms such as alveoli, pits and tafoni is described. There is also a discussion of materials deposited as a consequence of chemical weathering, such as various types of duricrust. Finally, weathering hazards are addressed together with an assessment of the ways in which humans have accelerated some weathering processes.
Soil production function
Frost weathering
Cite
Citations (9)