Total Weathering Rate Research Articles

The Effect of Seafloor Weathering on Planetary Habitability

Abstract Conventionally, a habitable planet is one that can support liquid water on its surface. Habitability depends on temperature, which is set by insolation and the greenhouse effect, due mainly to CO2 and water vapor. The CO2 level is increased by volcanic outgassing and decreased by continental and seafloor weathering. Here, I examine the climate evolution of Earth-like planets using a globally averaged climate model that includes both weathering types. Climate is sensitive to the relative contributions of continental and seafloor weathering, even when the total weathering rate is fixed. Climate also depends strongly on the dependence of seafloor weathering on CO2 partial pressure. Both these factors are uncertain. Earth-like planets have two equilibrium climate states: (i) an ice-free state where outgassing is balanced by both weathering types, and (ii) an ice-covered state where outgassing is balanced by seafloor weathering alone. The second of these has not been explored in detail before. For some planets, neither state exists, and the climate cycles between ice-covered and ice-free states. For some other planets, both equilibria exist, and the climate depends on the initial conditions. Insolation increases over time due to stellar evolution, so a planet usually encounters the ice-covered equilibrium first. Such a planet will remain ice covered, even if the ice-free state appears subsequently, unless the climate receives a large perturbation. The ice-covered equilibrium state covers a large fraction of phase space for Earth-like planets. Many planets conventionally assigned to a star’s habitable zone may be rendered uninhabitable as a result.

Impact of the atmospheric deposition of major acid rain components, especially NH4, on carbonate weathering during recharge in typical karst areas of the Lijiang River basin, southwest China

We analysed precipitation and river water samples from the Lijiang River basin with the aim of understanding the important role of acid rain components, especially NH4, in chemical weathering of karst areas. First, analysis of the hydrochemical characteristics revealed the following: In the precipitation samples, the volume-weighted mean pH was 4.91, and the acid rain rate was 65%. The major ion composition was characterized by a dominance of NH4, Ca, SO4 and NO3, accounting for 40.0%, 35.3%, 46.3% and 30.5%, respectively, of the total cation and anion concentrations. In the river water samples, the pH ranged from 6.75 to 9.16, with an average of 7.55. The major hydrogeochemical species in the river water samples were Ca–HCO3 and Mg–HCO3. Second, by employing chloride-normalized ratios of the other major ions in precipitation, we estimated that the contributions of SO4 and NO3 from the atmosphere to the river were53.6% and 35.0%, respectively. Third, combining the hydrochemical index with the value of δ13CDIC and using the hydrochemical composition method showed that the proportion of carbonate weathered (R) by NH4, H2SO4 and HNO3 from acid rain accounted for 9.3%, 9.4% and 3.1% of the total weathering rate, respectively, and the proportions in the upper reaches were significantly higher than those in the lower reaches of the river. The average values of the carbonate weathering proportion (R) and the carbonate weathering rate (CWR) due to acid rain were approximately 22.0% and 2.1 t/km2/yr, respectively. Taken together, our results revealed that carbonate weathering by acid rain can significantly increase the carbonate weathering rate, which is essential information for accurate estimation of the carbon cycle (namely, inorganic carbon sources and sinks).

Colluvial deposits as a possible weathering reservoir in uplifting mountains

Abstract. The role of mountain uplift in the evolution of the global climate over geological times is controversial. At the heart of this debate is the capacity of rapid denudation to drive silicate weathering, which consumes CO2. Here we present the results of a 3-D model that couples erosion and weathering during mountain uplift, in which, for the first time, the weathered material is traced during its stochastic transport from the hillslopes to the mountain outlet. To explore the response of weathering fluxes to progressively cooler and drier climatic conditions, we run model simulations accounting for a decrease in temperature with or without modifications in the rainfall pattern based on a simple orographic model. At this stage, the model does not simulate the deep water circulation, the precipitation of secondary minerals, variations in the pH, below-ground pCO2, and the chemical affinity of the water in contact with minerals. Consequently, the predicted silicate weathering fluxes probably represent a maximum, although the predicted silicate weathering rates are within the range of silicate and total weathering rates estimated from field data. In all cases, the erosion rate increases during mountain uplift, which thins the regolith and produces a hump in the weathering rate evolution. This model thus predicts that the weathering outflux reaches a peak and then falls, consistent with predictions of previous 1-D models. By tracking the pathways of particles, the model can also consider how lateral river erosion drives mass wasting and the temporary storage of colluvial deposits on the valley sides. This reservoir is comprised of fresh material that has a residence time ranging from several years up to several thousand years. During this period, the weathering of colluvium appears to sustain the mountain weathering flux. The relative weathering contribution of colluvium depends on the area covered by regolith on the hillslopes. For mountains sparsely covered by regolith during cold periods, colluvium produces most of the simulated weathering flux for a large range of erosion parameters and precipitation rate patterns. In addition to other reservoirs such as deep fractured bedrock, colluvial deposits may help to maintain a substantial and constant weathering flux in rapidly uplifting mountains during cooling periods.

Pedon-scale silicate weathering: comparison of the PROFILE model and the depletion method at 16 forest sites in Sweden

Weathering of soil minerals is important for the recovery from acidification and for the sustainability of forestry. However, there is still substantial uncertainty about its absolute rate. This study presents a harmonized comparison of field weathering rates estimated with the mechanistic model PROFILE and the depletion method for 16 intensively sampled soil profiles across Sweden representing different site conditions. In general, a correspondence in total weathering rates was found between the two methods except in rare cases where either method yielded deviating results. The weathering rate was higher according to the depletion method than according to PROFILE for Mg, while PROFILE produced higher weathering rates for the other base cations. The Spearman rank correlation (ρ) between the two methods indicated significant correlation for Ca (ρ=0.44, p=0.04) and non-significant correlation for Mg (ρ=0.51, p=0.09), Na (ρ=0.25, p=0.34), K (ρ=0.07, p=0.80), and the sum of the base cations (ρ=0.11, p=0.67). The variation in weathering rates with depth showed opposite gradients in the upper 50cm, which reflects the conceptual differences between the methods. This study shows the potential of using multiple methods to identify a probable weathering rate, if harmonized input data are used. Furthermore, it highlights the importance of making comparisons for individual elements in order to interpret differences between methods. Regardless of the method used, weathering rates were below or at the same level as the losses caused by whole-tree harvesting, particularly in southern Sweden, indicating a risk of negative effects on soils and waters.

Weathering the escarpment: chemical and physical rates and processes, south‐eastern Australia

AbstractDifferences in chemical weathering extent and character are expected to exist across topographic escarpments due to spatial gradients of climatic and/or tectonic forcing. The passive margin escarpment of south‐eastern Australia has a debated but generally accepted model of propagation in which it retreated (within 40 Ma) to near its current position following rifting between Australia and New Zealand 85–100 Ma before present. We focus on this escarpment to quantify chemical weathering rates and processes and how they may provide insight into scarp evolution and retreat. We compare chemical weathering extents and rates above and below the escarpment using a mass balance approach coupling major and trace element analyses with previous measurements of denudation rates using cosmogenic nuclides (10Be and 26Al). We find a slight gradient in saprolite chemical weathering rate as a percentage of total weathering rate across the escarpment. The lowlands area, encompassing the region extending from the base of the escarpment to the coast, experiences a greater extent of chemical weathering than the highland region above the escarpment. Percents of denudation attributable to saprolite weathering average 57 ± 6% and 47 ± 7% at low and high sites respectively. Furthermore, the chemical index of alteration (CIA), a ratio of immobile to mobile oxides in granitic material that increases with weathering extent, have corresponding average values of 73·7 ± 3·9 and 65·5 ± 3·4, indicating lower extents of weathering above the escarpment. Finally, we quantify variations in the rates and extent of chemical weathering at the hillslope scale across the escarpment to suggest new insight into how climate differences and hillslope topography help drive landscape evolution, potentially overprinting longer term tectonic forcing. Copyright © 2009 John Wiley & Sons, Ltd.

Combining cosmogenic nuclides and major elements from moraine soil profiles to improve weathering rate estimates

Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine (∼ 21 ka) and the Bull Lake-age moraine (∼ 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g (oxide) m − 2 y − 1 for the Type Pinedale moraine and 4.78 ± 0.89 g (oxide) m − 2 y − 1 for the Bull Lake-age moraine, which are ∼ 2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.

Controls on rock weathering rates by reaction-induced hierarchical fracturing

Weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface, the hydrosphere and the biosphere. It is still poorly understood how this coupling affects weathering rates and patterns produced by weathering. Here, we show that the observed spheroidal weathering patterns in basaltic intrusions (dolerites) in the Karoo Basin in South Africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock, but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division. The fracture processes generates new reactive surface area in a self-accelerating manner, thus providing a first-order control on the total weathering rate.

160 m.y. record of marine sedimentary phosphorus burial: Coupling of climate and continental weathering under greenhouse and icehouse conditions

A compilation of all meaningful phosphate data published in the proceedings of the Deep Sea Drilling Project and the Ocean Drilling Program is used here for the extraction of a 160 m.y. marine burial record of bulk phosphorus (all sediment types; 5648 measurements) and a 100 m.y. marine burial record of biogenic phosphorus (pelagic, biogenic, sediment types; 1754 data). These records serve as estimates for total and dissolved (bioavailable) phosphorus flux rates, which in turn are highly dependent on total and chemical continental weathering rates, respectively. The similarity of both records indicates that in the past 100 m.y., changes in total weathering rates may have been tracked by changes in chemical weathering rates. Prior to 32 Ma, phosphorus burial and long-term sea-level change are positively correlated; from 32 Ma to the present, this correlation is inverse. This points to a fundamental change in feedback mechanisms between continental weathering, phosphorus, and climate which was probably linked to the onset of major glaciation.

160 m.y. record of marine sedimentary phosphorus burial: Coupling of climate and continental weathering under greenhouse and icehouse conditions

A compilation of all meaningful phosphate data published in the proceedings of the Deep Sea Drilling Project and the Ocean Drilling Program is used here for the extraction of a 160 m.y. marine burial record of bulk phosphorus (all sediment types; 5648 measurements) and a 100 m.y. marine burial record of biogenic phosphorus (pelagic, biogenic, sediment types; 1754 data). These records serve as estimates for total and dissolved (bioavailable) phosphorus flux rates, which in turn are highly dependent on total and chemical continental weathering rates, respectively. The similarity of both records indicates that in the past 100 m.y., changes in total weathering rates may have been tracked by changes in chemical weathering rates. Prior to 32 Ma, phosphorus burial and long-term sea-level change are positively correlated; from 32 Ma to the present, this correlation is inverse. This points to a fundamental change in feedback mechanisms between continental weathering, phosphorus, and climate which was probably linked to the onset of major glaciation.

The effect of acid deposition on cation fluxes in artificially acidified catchments in western Norway

Chemical weathering has been studied at two artificially acidified and two control catchments at a clean pristine area in western Norway. The gabbroic gneiss bedrock in the catchment area contains a Na-rich plagioclase which is a major constituent and the most important mineral undergoing weathering in the soils. Estimates for rates of chemical weathering combine several independent methods including input-output budgets, runoff chemistry under short-term acidic episodes, and laboratory weathering studies. The two manipulated catchments have a net flux of Ca ++, Mg ++, and silica which is about 50% higher than that of the control catchments, but the net flux of Na + is the same in all catchments. The increased flux of base cations in runoff waters at the two acidified catchments is the result of increased removal of base cations from exchange sites and increased chemical weathering of plagioclase and mafic minerals. The exchangeable Na-pool is small and its contribution to the fluxes in runoff waters is less than the fluxes related to precipitation and chemical weathering of plagioclase and mafic minerals. The flux of non-marine Na + in runoff gives an estimate of the plagioclase weathering rate at the catchments of about 27 kg Plag. ha −1 a −1. Given the assumption that the dissolution of soil minerals releases silica and base cations in amounts equivalent to the rates measured in mineral dissolution experiments and their weight fraction in the soil, the estimated total weathering rate of base cations at Sogndal is about 20 meq/m 2/a. The calculated total weathering rate agrees well with rates estimated by mass-balance studies ( Wright et al., 1988a) but is considerably lower than the total weathering rates reported from other ecosystems.

Towards a weathering model of mount lamington ash, Papua New Guinea

The Mount Lamington ash halo has been studied at six sites both close to and distant from the volcano. Negligible alumina occurs in the seepage waters from the ash and so alumina is assumed constant during weathering. In andesitic ashes, weathering to allophane, an approximation to a first order reaction was found in 1968 and is confirmed here. With given infiltration and silica concentration the weathering rate constant decreases with increasing content of weathering minerals and increasing thickness of ash being weathered. The rate constant also increased with increasing infiltration and decreasing median size of the ash. The total weathering rate is the product of the thickness, percentage of ash being weathered, and the weathering rate constant. The total weathering rate is the same for thick, less weathered, proximal ashes as it is for the thin, more weathered, distal ashes. First order kinetics imply a high rate of leaching decreasing with time but continual burial under new ash at a nearly constant rate leads to linear leaching of the upper part of the profile. The leaching rate decreases exponentially with depth and appears to ignore buried topsoil barriers. It is possible to construct weathering models exploring the rate constant and to link it with leaching rates. Two thirds of the loss on weathering is silica and there is a total leaching rate of 3–4 mg SiO 2 cm −2 year −1 as on basalt in Hawaii and on andesite in St. Vincent. Water analyses show silica concentrations in the ash of 17–35 ppm and in draining streamlets of 9–20 ppm. Measured concentrations agree with calculated concentrations from the weathering data. Silica concentrations are similar in other humid tropical rivers.