Chemical Weathering, Atmospheric CO 2 , and Climate (original) (raw)
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Enhanced chemical weathering of rocks during the last glacial maximum: a sink for atmospheric CO2?
Chemical Geology, 1999
It has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric CO on the 2 Ž . continents may have at least partly contributed to the low CO concentrations during the last glacial maximum LGM . 2 Variations in continental erosion could thus be one of the driving forces for the glacialrinterglacial climate cycles during Quaternary times. To test such an hypothesis, a global carbon erosion model has been applied to a LGM scenario in order to determine the amount of CO consumed by chemical rock weathering during that time. In this model, both the part of 2 atmospheric CO coming from silicate weathering and the part coming from carbonate weathering are distinguished. The 2 climatic conditions during LGM were reconstructed on the basis of the output files from a computer simulation with a general circulation model. Only the predicted changes in precipitation and temperature have been used, whereas the changes in continental runoff were determined with an empirical method. It is found that during the LGM, the overall atmospheric Ž . CO consumption may have been greater than today by about 20% , mainly because of greater carbonate outcrop area 2 related to the lower sea level on the shelves. This does not, however, affect the atmospheric CO consumption by silicate 2 weathering, which alone has the potential to alter atmospheric CO on the long-term. Silicate weathering and the 2 concomitant atmospheric CO consumption decreased together with a global decrease of continental runoff compared to 2 Ž . present-day both by about 10% . Nevertheless, some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known. The values we present refer to the ice-free continental area only, but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget. Although glacial runoff was considerably increased during LGM, weathering under the ice sheets seems to be of minor importance. q
Effects of carbon dioxide on mineral weathering rates at earth surface conditions
The weathering of silicate minerals is an important long-term control on the global carbon budget. While the rate of mineral weathering is influenced by the atmospheric variations in atmospheric carbon dioxide, the only measurements of those effects have occurred during dissolution experiments at temperatures much higher than earth surface conditions. Thus, any climate models that include such a relationship may not be able to fully couple variations in atmospheric carbon dioxide with the lithospheric sinks. Our study presents a relationship for the dependence of plagioclase dissolution rates on PCO2 based on field data from a site in the southeastern Sierra Nevada drainages. A series of canyons that have similar drainages show wide variability in water chemistry that is attributed to variations in PCO2 from geothermal sources. This setting allowed us to isolate the effect of PCO2 on weathering rates in conditions relevant to climate models. The results show that mineral dissolution rates are proportional to PCO2 0.45 when the observed variations are attributed solely to variations in PCO2. This relationship is likely to be more applicable to climate models than prior laboratory derived data.
Rivers, chemical weathering and Earth's climate
Comptes Rendus Geoscience, 2003
We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO 2 and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO 2 through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO 2 as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated.
American Journal of Science, 1999
We utilize predictions of runoff from two series of GENESIS (version 1.02) climate model experiments to calculate chemical erosion rates for 12 time slices that span the Mesozoic and Cenozoic. A set of ''control'' experiments where geography is altered according to published paleogeographic reconstructions and atmospheric pCO 2 is held fixed at the present-day value was designed to elucidate climate sensitivity to geography alone. A second series of experiments, where the (elevated) atmospheric CO 2 level for each time slice was adapted from Berner (1991), was executed to determine the additional climate sensitivity to this parameter. By holding other climate forcing factors (for example, vegetation) constant throughout the sequence of experiments we evaluate the effects of systematic/coherent paleogeographic changes on runoff and temperature, and thus on global rates of chemical weathering.
The Geobiology of Weathering: a 13th Hypothesis
arXiv: Geophysics, 2015
The magnitude of the biotic enhancement of weathering (BEW) has profound implications for the long-term carbon cycle. The BEW ratio is defined as how much faster the silicate weathering carbon sink is under biotic conditions than under abiotic conditions at the same atmospheric pCO2 level and surface temperature. Thus, a 13th hypothesis should be considered in addition to the 12 outlined by Brantley...(2011) regarding the geobiology of weathering: The BEW factor and its evolution over geological time can be inferred from meta-analysis of empirical and theoretical weathering studies. Estimates of the global magnitude of the BEW are presented, drawing from lab, field, watershed data and models of the long-term carbon cycle, with values ranging from one to two orders of magnitude.
Global carbon sequestration through continental chemical weathering in a climatic change context
Scientific Reports, 2021
This study simulates carbon dioxide (CO 2) sequestration in 300 major world river basins (about 70% of global surface area) through carbonates dissolution and silicate hydrolysis. For each river basin, the daily timescale impacts under the RCP 2.6 and RCP 8.5 climate scenarios were assessed relative to a historical baseline (1969-1999) using a cascade of models accounting for the hydrological evolution under climate change scenarios. Here we show that the global temporal evolution of the CO 2 uptake presents a general increase in the annual amount of CO 2 consumed from 0.247 ± 0.045 Pg C year −1 to 0.261 and 0.273 ± 0.054 Pg C year −1 , respectively for RCP 2.6 and RCP 8.5. Despite showing a general increase in the global daily carbon sequestration, both climate scenarios show a decrease between June and August. Such projected changes have been mapped and evaluated against changes in hydrology, identifying hot spots and moments for the annual and seasonal periods. Chemical weathering of rocks has a significant impact on long-term global climate regulation 1. It transforms soil CO 2 into inorganic dissolved carbon (such as HCO 3 − and CO 3
Does atmospheric CO2police the rate of chemical weathering?
Global Biogeochemical Cycles, 1998
A case is made that in the absence of an effective feedback control on the rate of delivery of CaO to the oceans, the CO2 content of the Earth' s atmosphere would have wandered over a large range threatening life either by overheating or by carbon dioxide starvation. In this paper, we defend the suggestion by Walker et al. [ 1981 ] that control is exerted by the interaction between the CO2 content of the atmosphere and the continental weathering rates. We contend that in spite of the arguments raised against it [Raytoo and Ruddiman, 1992; Edmond and Huh, 1997] the CO 2chemical weathering feedback is the dominant mechanism that stabilizes the atmospheric carbon dioxide content.
Modeling of continental weathering under high-CO 2 atmospheres during Precambrian times
Geochemistry, Geophysics, Geosystems, 2011
Batch experiments were conducted to simulate abiotic weathering of the continental crust under high-CO 2 atmospheric conditions during Precambrian times, i.e., corresponding to the general Archaean conditions as well as to the immediate aftermath of the Neoproterozoic Snowball Earth ice ages. Three types of rock (basalt, granodiorite and tonalite) representative of the Archaean to Proterozoic continental crust were reacted in the form of powders for 1 year at 40°C with pure water under various water/rock ratios, in oxic and anoxic atmospheres containing 10% CO 2 , that is, under conditions assumed to characterize the greenhouse effect which prevailed at that time. Chemical and mineralogical data collected during the course of the experiments reflect alteration phenomena occurring in two steps: (1) rapid leaching of the fresh surfaces, probably related to the proton-donor capacity of the CO 2 and (2) a steady state reaction under near-neutral conditions. These observed dissolution rates can be satisfactorily modeled by kinetic data available in the literature. Using a 1-D weathering model reproducing the two steps, we evaluate the atmospheric CO 2 consumption rate at the Snowball Earth aftermath, focusing on the relative contribution of surface leaching versus steady state reaction. The results show that the process is dominated by surface exchange.