Calcite Dissolution-Reprecipitation Reactions Are a Key Control on the Sr/Ca, Mg/Ca and δ88/86Sr Compositions of Himalayan River Waters

Differential weathering of basaltic and granitic catchments from concentration–discharge relationships

Cenozoic carbon cycle imbalances and a variable weathering feedback

Weathering of ultramafic rocks emplaced at low latitude during arc‐arc and arc‐continent collisions may provide an important sink for atmospheric CO2 over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ 36Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km−2 yr−1), chemical weathering fluxes are large (55–143 tons km−2 yr−1), and weathering intensities are among the highest yet reported for silicate‐rock weathering (up to 0.88). We use these data to parameterize power‐law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO2 consumption from the best‐fit power‐law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO2 consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time.

Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada

Silicate weathering and soil formation is critical to global - scale processes since silicates constitute about 83% of the rocks exposed at Earth's land surface. On geological time scales, atmospheric carbon dioxide content has been balanced by the weathering of Ca - Mg - silicate rocks, the burial and weathering of sedimentary organic matter and degassing of volcanoes. A large number of laboratory and fi eld - based studies have been performed to better understand the link between silicate weathering, climate and the long - term carbon cycle. In this chapter we summarize some of this past work and suggest directions for future research. ©2011 Blackwell Publishing Ltd. © 2011 Blackwell Publishing Ltd. All rights reserved.

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Abstract. The warmer early Pliocene climate featured changes to global sea surface temperature (SST) patterns, namely a reduction in the Equator–pole gradient and the east–west SST gradient in the tropical Pacific, the so-called “permanent El Niño”. Here we investigate the consequences of the SST changes to silicate weathering and thus to atmospheric CO2 on geological timescales. Different SST patterns than today imply regional modifications of the hydrological cycle that directly affect continental silicate weathering in particular over tropical “hotspots” of weathering, such as the Maritime Continent, thus leading to a “weatherability pattern effect”. We explore the impact of Pliocene-like SST changes on weathering using climate model and silicate weathering model simulations, and we deduce CO2 and temperature at carbon cycle equilibrium between solid Earth degassing and silicate weathering. In general, we find large regional increases and decreases in weathering fluxes, and the net effect depends on the extent to which they cancel. Permanent El Niño conditions lead to a small amplification of warming relative to the present day by 0.4 ∘C, suggesting that the demise of the permanent El Niño could have had a small amplifying effect on cooling from the early Pliocene into the Pleistocene. For the reduced Equator–pole gradient, the weathering increases and decreases largely cancel, leading to no detectable difference in global temperature at carbon cycle equilibrium. A robust SST reconstruction of the Pliocene is needed for a quantitative evaluation of the weatherability pattern effect.

If pressed to sum up this book with two phrases, we’d pick Silica is your friend and We owe just about everything to silicate weathering. Not only does silicate weathering liberate silica from silicate minerals so it can participate in the silica cycle and all the biomineralization, diatom blooming, and collagen production that goes along with that, it has maintained the habitability of Earth continuously over the last at least 3.8 billion years. Take a moment. Let that thought sink in. Without silicate weathering, the surface of the Earth would be inhospitable. Uninhabitable. Totally lacking the biosphere that has needed at least 3.8 billion climatically reasonable years to build up in mass, complexity, and diversity to be what it is today (glorious, incredible, and taken for granted by its cleverest denizens). How does silicate weathering manage it? Read on to learn how silicate weathering acts as a thermostat that, within a wide range of solar luminosity (today’s Sun being more than one-third brighter than its younger self that shone down upon the early Earth), has held Earth’s climate within reasonable bounds for at least as long as there has been life on Earth. What’s more, you’ll hear how silicate weathering will mop up the mess we’ve made of climate with our overzealous emission of greenhouse gases into the atmosphere.

This project investigates how isotope systems respond to changes in continental weathering processes and the consequences for the chemical composition of the oceans. Both experimental and natural data indicate that Li and Mg stable isotope systems preserve information on the mineral reactions controlling water chemistry. Dissolution experiments indicate that primary mineral dissolution has little effect on Li, but does fractionate the isotopes of Mg, whereas secondary mineral formation involves preferential uptake of the lighter isotopes of both Li and Mg. Glacial regions have low chemical weathering rates so their rivers should, in principal, have Li isotope compositions (δ⁷Li) that are similar to those of the underlying rock. In practise, glacial rivers in Greenland have δ⁷Li values that differ significantly from the source rock. Subglacial uptake of ⁶Li by iron oxyhydroxides appears to influence the riverine Li isotope composition. In contrast, the principal control on Mg isotopes in glacial rivers appears to be primary mineral weathering. Carbonate dissolution dominates the chemistry of glacial rivers, and this phase possesses a light Mg isotope composition imparting this signature to the Greenland rivers. Both the Li and Mg isotope compositions of glacial rivers are similar to their respective global riverine averages, suggesting that the impact of glaciation on the Li and Mg composition of seawater may be small. The Re-Os radiogenic isotope system is also a useful tracer of continental weathering, its composition in the oceans being sensitive to changes in the flux and composition of continental weathering. Laterites are widespread in tropical regions and are shown to contain high concentrations of highly unradiogenic Os. Consequently, their subsequent weathering and erosion has potential to significantly affect the Os isotope composition delivered to the oceans.

Silicate versus carbonate weathering in the Himalaya: a comparison of the Arun and Seti River watersheds

Li and δ7Li in Himalayan rivers: Proxies for silicate weathering?

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

Detailed observations of stratiform boundary layer clouds on 12 days are examined with specific reference to drizzle formation processes. The clouds differ considerably in mean thickness, liquid water path (LWP), and droplet concentration. Cloud-base precipitation rates differ by a factor of 20 between cases. The lowest precipitation rate is found in the case with the highest droplet concentration even though this case had by far the highest LWP, suggesting that drizzle can be severely suppressed in polluted clouds. The vertical and horizontal structure of cloud and drizzle liquid water and bulk microphysical parameters are examined in detail. In general, the highest concentration of r > 20 μm drizzle drops is found toward the top of the cloud, and the mean volume radius of the drizzle drops increases monotonically from cloud top to base. The resulting precipitation rates are largest at the cloud base but decrease markedly only in the upper third of the cloud. Below cloud, precipitation rates decrease markedly with distance below base due to evaporation, and are broadly consistent in most cases with the results from a simple sedimentation–evaporation model. Evidence is presented that suggests evaporating drizzle is cooling regions of the subcloud layer, which could result in dynamical feedbacks. A composite power spectrum of the horizontal spatial series of precipitation rate is found to exhibit a power-law scaling from the smallest observable scales to close to the maximum observable scale (∼30 km). The exponent is considerably lower (1.1–1.2) than corresponding exponents for LWP variability obtained in other studies (∼1.5–2), demonstrating that there is relatively more variability of drizzle on small scales. Singular measures analysis shows that drizzle fields are much more intermittent than the cloud liquid water content fields, consistent with a drizzle production process that depends strongly upon liquid water content. The adiabaticity of the clouds, which can be modeled as a simple balance between drizzle loss and turbulent replenishment, is found to decrease if the time scale for drizzle loss is shorter than roughly 5–10 eddy turnover time scales. Finally, the data are compared with three simple scalings derived from recent observations of drizzle in subtropical stratocumulus clouds.

Continental chemical weathering plays a crucial role in the regional and global carbon budgets. However, relevant research on tropical volcanic islands is still limited, particularly the influences of human activities on surficial chemical weathering, which remain scarce. This study analyzed the water geochemistry and dissolved inorganic carbon isotope (δ13CDIC) of the Changhua River draining Hainan Island, aiming to quantitatively elucidate the seasonal CO2 budgets at the catchment scale. We found that the major ions in the river were from atmospheric precipitation (6.79%), anthropogenic input (11.68%), carbonate weathering (41.96%), and silicate weathering (39.56%). The silicate weathering rate and the carbonate weathering rate were calculated as 16.91 t km–2 yr–1 and 30.68 t km–2 yr–1, respectively, resulting in CO2 consumption of 307.82 × 103 mol km–2 yr–1 and 400.95 × 103 mol km–2 yr–1. Considering sulfuric acid- and nitric acid-driven weathering reactions, the catchment became a net source of CO2 on a geological time scale. Furthermore, the drivers of chemical weathering and associated CO2 budget were deciphered. The significant positive correlations among chemical weathering fluxes, acids, and land uses quantitatively emphasized that the chemical weathering of CO2 budgets was largely disturbed by anthropogenic activities. This study showed that integrating river water geochemistry and geographical parameters could achieve a deep understanding of catchment weathering CO2 budgets and potential driving mechanisms.

Constraining seasonal active layer dynamics and chemical weathering reactions occurring in North Slope Alaskan watersheds with major ion and isotope (δ34SSO4, δ13CDIC, 87Sr/86Sr, δ44/40Ca, and δ44/42Ca) measurements