Atmospheric CO2 Consumption Research Articles

Coupled modeling of global carbon cycle and climate in the Neoproterozoic: links between Rodinia breakup and major glaciations

A coupled climate–geochemical model of new generation (GEOCLIM) is used to investigate the possible causes of the initiation of snowball glaciations during Neoproterozoic times. This model allows the calculation of the partial pressure of atmospheric CO 2 simultaneously with the climate at the continental surface with a rough 2D spatial resolution (10° lat. × 50° long.). We calculate that the breakup of the Rodinia supercontinent, starting 800 Myr ago, results in a global climatic cooling of about 8 °C triggered by enhanced consumption of atmospheric CO 2 resulting from increased runoff over continental surfaces. This increase in runoff is driven by the opening of oceanic basins resulting in an increase of soil moisture sources close to continental masses. This climatic effect of the supercontinent breakup is particularly strong within the 800–700 Ma interval since all continents are located in the equatorial area, where temperature and runoff conditions optimize the consumption of CO 2 through weathering processes. However, this effect alone is insufficient to trigger snowball. We propose that the efficient weathering of fresh basaltic surfaces that erupted during the Rodinia breakup, and were transported to the humid equatorial area through continental plate motion, contributed the necessary CO 2 sink that triggered the ca. 730-Ma Sturtian glacial event. Simulations of the GEOCLIM model for the ca 580-Ma Gaskiers ice age, where all continents are centered on the South Pole, shows that no snowball glaciation can be initiated. The calculated CO 2 partial pressure remains above 1000 ppmv, while a threshold of less than 80 ppmv is required to initiate a snowball glaciation. At that time, a polar configuration does not allow the onset of total glaciation. Nevertheless, a regional glaciation is simulated by the GEOCLIM when the climatic and geochemical (i.e. weathering related) effects of the Pan-African orogeny (∼600 Ma) are taken into account. Finally, the question of the role of the paleogeographic setting in the Marinoan snowball event (∼635 Ma) is still an open question, since no reliable Marinoan paleogeographic reconstruction exists due to the paucity of paleomagnetic data.

Basaltic volcanism and mass extinction at the Permo-Triassic boundary: Environmental impact and modeling of the global carbon cycle

The Siberian Traps represent one of the most voluminous continental flood basalt provinces on Earth. The mass extinction at the end of the Permian was the most severe in the history of life. In the present paper, these two major concurrent events that occurred are analysed and a geochemical model coupled with an energy balance model is used to calculate their environmental impact on atmospheric CO 2, oceanic δ 13C, and marine anoxia. The latitudinal temperature gradient is reduced relative to today, resulting in warmer temperatures at high latitudes. The warmer climate and the presence of fresh basaltic provinces increase the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO 2 through weathering. First, the eruption of the Siberian traps is accompanied by a massive volume of 13C depleted CO 2 degassed from the mantle and added to the ocean through silicate weathering, thus lowering marine δ 13C. Second, the rapid collapse in productivity induces a strong decrease in the global organic carbon burial. This too tends to increase the proportion of light carbon in the ocean. These two effects can explain the low δ 13C values across the PT boundary, and methane release need not be invoked to explain the δ 13C fluctuations. It is proposed that the phosphorus cycle, which drives primary production in the model, plays an important role on the recovery of productivity and the δ 13C variations.

Chemical weathering processes and atmospheric CO2 consumption of Huanghe River and Changjiang River basins

Rock weathering plays an important role in studying the long-term carbon cycles and global climatic change. According to the statistics analysis, the Huanghe (Yellow) River water chemistry was mainly controlled by evaporite and carbonate weathering, which were responsible for over 90% of total dissolved ions. As compared with the Huanghe River basin, dissolved load of the Changjiang (Yangtze) River was mainly originated from the carbonate dissolution. The chemical weathering rates were estimated to be 39.29t/(km2·a) and 61.58t/(km2·a) by deducting the HCO 3 − derived from atmosphere in the Huanghe River and Changjiang River watersheds, respectively. The CO2 consumption rates by rock weathering were calculated to be 120.84×103mol/km2 and 452.46×103mol/km2annually in the two basins, respectively. The total CO2 consumption of the two basins amounted to 918.51×109mol/a, accounting for 3.83% of the world gross. In contrast to other world watersheds, the stronger evaporite reaction and infirm silicate weathering can explain such feature that CO2 consumption rates were lower than a global average, suggesting that the sequential weathering may be go on in the two Chinese drainage basins.

Rivers, chemical weathering and Earth's climate

Abstract 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. To cite this article: B. Dupre et al., C. R. Geoscience 335 (2003).

Weathering the Alps

GEOCHEMISTRY Increases in atmospheric CO2 are assumed to increase weathering, because silicate minerals react to form water-soluble carbonates: for instance, CaSiO3 + CO2 → CaCO3 + SiO2. This reaction draws down atmospheric CO2 levels, and it has been suggested that mountain building contributes to global cooling. In turn, it is commonly assumed that mountain building and mechanical erosion, by increasing the exposed surfaces of minerals, accelerate this chemical weathering. Quantifying these processes is important for inferring the interaction between climate change and topography on Earth, and for predicting past and future CO2 levels. Jacobson and Blum worked to quantify these fluxes by examining stream chemistry in the Southern Alps of New Zealand. Here, the west side of the range is steep and wet (because of rapid uplift) as compared to the east side, despite the presence of similar types of rock. Although levels of dissolved Mg and Ca ions are much higher in the streams draining the western side of the Alps, most of this difference can be traced to the breakdown of carbonate minerals and not to silicate weathering related to CO2 consumption. Overall, mountain building seems to increase atmospheric CO2 consumption via weathering by only a factor of 2, less than previously thought. — BH Geology 31 , 10 (2003).

The Sturtian ‘snowball’ glaciation: fire and ice

The Sturtian ‘snowball’ glaciation (730 Ma) is contemporary with the dislocation of the Rodinia supercontinent. This dislocation is heralded and accompanied by intense magmatic events, including the onset of large basaltic provinces between 825 and 755 Ma. Among these magmatic events, the most important one is the onset of a Laurentian magmatic province at 780 Ma around a latitude of 30°N. The presence of these fresh basaltic provinces increases the weatherability of the continental surfaces, resulting in an enhanced consumption of atmospheric CO2 through weathering, inducing a global long-term climatic cooling. Based on recent weathering laws for basaltic lithology and on climatic model results, we show that the weathering of a 6×106 km2 basaltic province located within the equatorial region (where weathering of the province and consumption of CO2 are boosted by optimal climatic conditions) is sufficient to trigger a snowball glaciation, assuming a pre-perturbation PCO2 value of 280 ppmv. We show that the Laurentian magmatic province might be the main culprit for the initiation of the Sturtian ‘snowball’ glaciation, since the Laurentian magmatic province had drifted within the equatorial region by the time of the glaciation.

Relationship between mechanical erosion and atmospheric CO2 consumption in the New Zealand Southern Alps

To examine the influence of mountain uplift on the long-term carbon cycle, we used geochemical, hydrologic, and suspended-load data for 12 streams draining the New Zealand Southern Alps to quantify rates of erosion, weathering, and atmospheric CO 2 consumption. Rapid uplift in the western Southern Alps elevates mechanical erosion rates by a factor of ∼13 relative to those on the tectonically stable eastern side [125 × 10 8 vs. 9.4 × 10 8 g/(km 2 ·yr), respectively]. Similarly, the average chemical weathering rate is ∼5 times higher on the western compared to eastern side of the mountain range [9.8 × 10 7 vs. 2.0 × 10 7 g/(km 2 ·yr), respectively]. However, because the proportion of stream-water Ca 2+ and Mg 2+ from carbonate weathering increases as the rate of mechanical erosion increases, the long-term atmospheric CO 2 consumption rate on the western side is ∼2 times higher than that on the eastern side [14 × 10 4 vs. 6.9 × 10 4 mol/(km 2 ·yr), respectively] and only ∼1.5 times higher than the global mean value [∼9 × 10 4 mol/(km 2 ·yr)]. Data for major world rivers (including Himalayan rivers) provide a consistent interpretation regarding the relationship between mechanical erosion intensity and the ratio of silicate to carbonate weathering. Thus, we conclude that mountain building increases atmospheric CO 2 consumption rates by only a factor of ∼2, which is much lower than previous estimates.

Modelled glacial and non-glacial HCO 3−, Si and Ge fluxes since the LGM: little potential for impact on atmospheric CO 2 concentrations and a potential proxy of continental chemical erosion, the marine Ge/Si ratio

The runoff and riverine fluxes of HCO 3 −, Si and Ge that arise from chemical erosion in non-glaciated terrain, are modelled at six time steps from the Last Glacial Maximum (LGM) to the present day. The fluxes that arise from the Great Ice Sheets are also modelled. Terrestrial HCO 3 − fluxes decrease during deglaciation, largely because of the reduction in the area of the continental shelves as sea level rises. The HCO 3 − fluxes, and the inferred consumption of atmospheric CO 2 are used as inputs to a carbon cycle model that estimates their impact on atmospheric CO 2 concentrations ( atmsCO 2). A maximum perturbation of atmsCO 2 by ∼5.5 ppm is calculated. The impact of solutes from glaciated terrain is small in comparison to those from non-glaciated terrain. Little variation in terrestrial Si and Ge fluxes is calculated (<10%). However, the global average riverine Ge/Si ratio may be significantly perturbed if the glacial Ge/Si ratio is high. At present, variations in terrestrial chemical erosion appear to have only a reduced impact on atmsCO 2, and only little influence on the global Si and Ge cycle and marine Ge/Si ratios during deglaciation.

Present and past nonanthropogenic CO2 degassing from the solid earth

Global carbon cycle models suggest that CO2 degassing from the solid Earth has been a primary control of paleoatmospheric CO2 contents and through the greenhouse effect, of global paleotemperatures. Because such models utilize simplified and indirect assumptions about CO2 degassing, improved quantification is warranted. Present‐day CO2 degassing provides a baseline for modeling the global carbon cycle and provides insight into the geologic regimes of paleodegassing. Mid‐ocean ridges (MORs) discharge 1–3 × 1012 mol/yr of CO2 and consume ∼3.5 × 1012 mol/yr of CO2 by carbonate formation in MOR hydrothermal systems. Excluding MORs as a net source of CO2 to the atmosphere, the total CO2 discharge from subaerial volcanism is estimated at ∼2.0–2.5 × 1012 mol/yr. Because this flux is lower than estimates of the global consumption of atmospheric CO2 by subaerial silicate weathering, other CO2 sources are required to balance the global carbon cycle. Nonvolcanic CO2 degassing (i.e., emission not from the craters or flanks of volcanos), which is prevalent in high heat flow regimes that are primarily located at plate boundaries, could contribute the additional CO2 that is apparently necessary to balance the global carbon cycle. Oxidation of methane emitted from serpentinization of ultramafics and from thermocatalysis of organic matter provides an additional, albeit unquantified, source of CO2 to the atmosphere. Magmatic CO2 degassing was probably a major contributor to global warming during the Cretaceous. Metamorphic CO2 degassing from regimes of shallow, pluton‐related low‐pressure regional metamorphism may have significantly contributed to global warming during the Cretaceous and Paleocene/Eocene. CO2 degassing associated with continental rifting of Pangaea may have contributed to the global warming that was initiated in the Jurassic. During the Cretaceous, global warming initiated by CO2 degassing of flood basalts, and consequent rapid release of large quantities of CH4 by decomposition of gas hydrates (clathrates), could have caused widespread extinctions of organisms.

Carbon monoxide consumption in upland boreal forest soils

Biological consumption by aerobic soils is an important, but poorly understood sink for carbon monoxide, a chemically active atmospheric trace gas. We used laboratory experiments to characterize CO consumption in representative soils of the northern boreal forest. This ecozone may be important in atmospheric CO consumption because it occupies 13% of the world's landmass. Soils initially showing net CO consumption emitted CO following biocidal treatment (ethylene oxide or γ-irradiation), indicating that CO consumption was biologically mediated. The use of selective inhibitors (cycloheximide, streptomycin) suggested that both prokaryotes and eukaryotes were responsible. Soil profiles generally indicated net consumption of atmospheric CO to a depth of 15 cm (concentrations decreasing from ~150 to <20 nl l −1 with depth) and a dynamic equilibrium between CO production and consumption in deeper soils (concentrations nearly constant at <20 nl l −1). Soils to a depth of 30 cm showed vigorous CO-consuming activity, suggesting that local CO production provided necessary substrate beneath the 15 cm surface zone of atmospheric influence. Radiotracer experiments demonstrated that only 5–7% of assimilated 14CO was incorporated into biomass in 5 cm core sections taken to 30 cm and that <1% of assimilated 14CO was incorporated into cellular material by a methanotroph isolated from this soil. Collectively, these data point to nonutilitarian oxidation of CO by a diverse microbial community. Carbon monoxide oxidation increased with increasing temperature over the range 4–34°C, with a Q 10 of 1.8. Apparent half-saturation constants (8–36 (μl CO l −1), and maximum rates of CO consumption (0.7–2.7 μg g dry soil −1 h −1) were comparable to reports for diverse temperate soil environments, pointing to a fundamental similarity among CO-consuming microbial communities in aerobic soils.

Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river-water chemistry

Trace amounts of disseminated calcite were identified in gneiss, schist, and granite bedrock sampled from the Raikhot watershed and other locations within the Nanga Parbat massif of northern Pakistan. The calcite grains occur interstitially within individual silicate minerals, at grain boundaries, and as fracture fillings that transect the mineralogic fabric of the rock. Disseminated calcite composes is ≤ 0.29 wt% of silicate rocks sampled in the Raikhot watershed and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.878 to 5.33 and from 0.794 039 to 0.930 619, respectively. Elsewhere in the Nanga Parbat region, disseminated calcite composes ≤ 3.6 wt% of the silicate rock samples and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.535 to 3.33 and from 0.715 757 to 0.771 244, respectively. For all samples, the 87Sr/86Sr ratios of disseminated calcite are similar to the 87Sr/86Sr ratios measured in the silicate host rock. Within the partially glaciated Raikhot watershed, the rapid weathering of disseminated calcite with high Ca/Sr and 87Sr/86Sr ratios has a strong influence on the chemical composition of stream water and exceeds contributions from silicate mineral dissolution. Comparisons of disseminated calcite compositions with source-water chemistry throughout the Himalaya suggest that disseminated calcite may be a more important component of Himalayan silicate rocks than previously recognized. Therefore, calculations relating the Sr isotope geochemistry of Himalayan rivers to atmospheric CO2 consumption should consider this widespread and compositionally variable carbonate end member.

The role of carbonates in the evolution of early Martian oceans

Central to the question of life on Mars is whether there has been liquid water on the martian surface and how the planet could have evolved from possible initial warm and wet conditions to the cold and dry present state. Virtually all models for this climatic evolution rely strongly on the removal of an initial thick carbon dioxide atmosphere by precipitation of carbonate minerals from surface waters that may have been quite similar to those of Hadean Eon Earth's oceans. In order for this to occur, a hydrologic cycle would be necessary in which chemical weathering of silicate rocks consumes CO2 that precipitates as carbonates in an acidic martian ocean which probably had a very high alkalinity. The consumption of atmospheric CO2 by this process would result in a gradual decrease of the atmospheric greenhouse influence and cooling of the climate. Once the surface of Mars became cold enough so that freezing conditions prevailed, the hydrologic cycle would largely cease, and the uptake of CO2 by silicate rock weathering would greatly diminish. The alkalinity of the freezing seawater would probably be sufficient to result in the removal of all calcium as calcium carbonate. Some magnesium and sodium would also likely be removed as carbonates as well. The removal of these cations as carbonates has a major influence on the final temperature at which liquid brines would be able to persist on the surface of Mars. During the period of freezing, the oceans would act as a source of CO2 rather than a sink, further slowing the rate of climate change on Mars.

Enhanced chemical weathering of rocks during the last glacial maximum: a sink for atmospheric CO2?

It has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric CO2 on the continents may have at least partly contributed to the low CO2 concentrations during the last glacial maximum (LGM). Variations in continental erosion could thus be one of the driving forces for the glacial/interglacial 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 CO2 consumed by chemical rock weathering during that time. In this model, both the part of atmospheric CO2 coming from silicate weathering and the part coming from carbonate weathering are distinguished. The 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 CO2 consumption may have been greater than today (by about 20%), mainly because of greater carbonate outcrop area related to the lower sea level on the shelves. This does not, however, affect the atmospheric CO2 consumption by silicate weathering, which alone has the potential to alter atmospheric CO2 on the long-term. Silicate weathering and the concomitant atmospheric CO2 consumption decreased together with a global decrease of continental runoff compared to 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.

Soil erosion and atmospheric CO2 during the last glacial maximum: the rôle of riverine organic matter fluxes

Atmospheric CO2 is consumed both by organic matter formation and chemical rock weathering, and subsequently discharged as dissolved organic carbon, particulate organic carbon, and dissolved inorganic carbon to the oceans by rivers. In the long term, varying the ratio of the amount of atmospheric CO2 consumed by continental erosion and the amount of CO2 released during carbonate precipitation and organic matter respiration in the oceans can change the CO2 content in the atmosphere. The purpose of this paper is to determine whether riverine organic carbon fluxes during the last glacial maximum (LGM) may have been different from today in order to assess the potential impact on atmospheric CO2. Previous studies mainly focused on the role of the river fluxes of inorganic carbon in this respect, but none of them examined possible variations in the fluxes of organic carbon, although the erosion of organic carbon actually represents the bulk of the atmospheric CO2 consumption by continental erosion. We therefore applied a global carbon erosion model to a LGM scenario in order to determine the riverine fluxes of organic matter during that time. The climatic conditions during the LGM were reconstructed using a computer simulation with a general circulation model. It is found that during the LGM the riverine organic carbon input into the oceans was at least ˜10% lower than today. Most of the reduction of the total organic matter fluxes is due to the reduction of the fluxes of dissolved organic carbon. The fluxes of particulate organic carbon remained almost unchanged. The oceanic response to the lower carbon input was estimated on the basis of a present-day steady state budget for organic river carbon in the oceans, and implies that the reduction of the river fluxes were more than counterbalanced by lower burial rates due to the smaller shelf area during the LGM. This suggests that both the lower river carbon input and the relatively greater share of this carbon being subjected to oceanic respiration, acted as a negative feedback to the low atmospheric CO2 content during the LGM.

Atmospheric CO 2 consumption by continental erosion: present-day controls and implications for the last glacial maximum

The export of carbon from land to sea by rivers represents a major link in the global carbon cycle. For all principal carbon forms, the main factors that control the present-day fluxes at the global scale have been determined in order to establish global budgets and to predict regional fluxes. Dissolved organic carbon fluxes are mainly related to drainage intensity, basin slope, and the amount of carbon stored in soils. Particulate organic carbon fluxes are calculated as a function of sediment yields and of drainage intensity. The consumption of atmospheric/soil CO 2 by chemical rock weathering depends mainly on the rock type and on the drainage intensity. Our empirical models yield a total of 0.721 Gt of carbon (Gt C) that is exported from the continents to the oceans each year. From this figure, 0.096 Gt C come from carbonate mineral dissolution and the remaining 0.625 Gt C stem from the atmosphere ( F CO 2 ). Of this atmospheric carbon, 33% is discharged as dissolved organic carbon, 30% as particulate organic carbon, and 37% as bicarbonate ions. Predicted inorganic carbon fluxes were further compared with observed fluxes for a set of 35 major world rivers, and possible additional climatic effects on the consumption of atmospheric CO 2 by rock weathering were investigated in these river basins. Finally, we discuss the implications of our results for the river carbon fluxes and the role of continental erosion in the global carbon cycle during the last glacial maximum.

The fluvial geochemistry of the rivers of Eastern Siberia: I. tributaries of the Lena River draining the sedimentary platform of the Siberian Craton

The response of continental weathering rates to changing climate and atmospheric PCO 2 is of considerable importance both to the interpretation of the geological sedimentary record and to predictions of the effects of future anthropogenic influences. While comprehensive work on the controlling mechanisms of contemporary chemical and mechanical weathering has been carried out in the tropics and, to a lesser extent, in the strongly perturbed northern temperate latitudes, very little is known about the peri-glacial environments in the subarctic and arctic. Thus, the effects of climate, essentially temperature and runoff, on the rates of atmospheric CO 2 consumption by weathering are not well quantified at this climatic extreme. To remedy this lack a comprehensive survey has been carried out of the geochemistry of the large rivers of Eastern Siberia, the Lena, Yana, Indigirka, Kolyma, Anadyr, and numerous lesser streams which drain a pristine, high-latitude region that has not experienced the pervasive effects of glaciation and subsequent anthropogenic impacts common to western Eurasia and North America. The scale of the terrain sampled, in terms of area, is comparable to that of the continental United States or the Amazon/Orinoco and includes a similarly diverse range of geologic and climatic environments. In this paper the chemical fluxes from the western region, the very large, ancient, and geologically stable sedimentary basin, Precambrian to Quaternary, of the Siberian Platform will be presented and compared to published results from analogous terrains in the tropical basins of China. While the range in the chemical signatures of the various tributaries included here (∼60 sampled) is large, this mainly reflects lithology rather than the weathering environment. The areal chemical fluxes are comparable to those of the Chinese rivers, being dominated by the dissolution of carbonates and evaporites. The net consumption of atmospheric CO 2 by aluminosilicate weathering is minor, as it is in the tropical basins. It is much smaller than in active orogenic belts in similar latitudes, e.g., the Fraser and Yukon, but comparable to those of the Mackenzie tributaries that drain the eastern slope of the Rockies. Lithology exerts the dominant influence in determining the weathering yield from sedimentary terrains, and for a largely carbonate/evaporite terrain climate does not have a direct effect.

Isotopic constraints on the Cenozoic evolution of the carbon cycle

In the last few years, several models have been built to explore the Cenozoic evolution of the carbon and strontium cycles. Of particular interest is the study of the impact on the carbon cycle of major mountain uplifts such as the Himalayan orogeny. To explain the Cenozoic increase in the measured seawater strontium isotopic ratio, it was recently proposed that the Himalayan uplift could be responsible for an enhanced consumption of atmospheric CO 2 by continental silicate weathering. Here, a new model of the carbon cycle evolution over Cenozoic times is presented. It calculates the various fluxes involved in the organic and inorganic components of the carbon cycle from the seawater δ 13C, the biological isotopic fractionation in the ocean and the seafloor spreading rate. The model equilibrates the budgets of the carbon and alkalinity cycles on the million year timescale, assuming as many previous investigators that the system remains close to equilibrium. The validity of this equilibrium approximation is examined critically. Various sensitivity experiments are performed in order to test the impact of the model parameters on the results. The calculated history of the carbonate deposition rate is consistent with the available reconstruction. The continental silicate weathering rate calculated by the model appears to be widely insensitive to the model parameters, showing three distinct evolutions over the Cenozoic. The model indeed suggests a time of relative constancy of the silicate weathering flux before 40 Ma, followed by a period of slow decrease until 15 Ma and finally a marked increase up to the present. In a progressively cooler world, this evolution may be interpreted as a change from a `chemically' controlled to a `physically' controlled weathering regime. The evolution of continental silicate weathering thus partly appears decoupled from the increase in the observed seawater strontium isotopic ratio. For this reason, the evolution of the calculated riverine 87Sr/ 86Sr ratio shows a strong increase over the Cenozoic, from about 0.710 to 0.712. However, this increase may largely be reduced by considering the recycling of a pelagic carbonate reservoir increasing over the Cenozoic or by assuming that seafloor basalt weathering is a CO 2- or climate-dependent process.

Strontium Isotope Systematics of a Himalayan Glacial Chronosequence

Chemical weathering of silicate minerals consumes atmospheric CO2, and therefore, changes in global silicate weathering rates may act as a mechanism to drive major fluctuations in the Earth's climate and long-term global carbon cycle. Recent studies suggest that steep increases in the Cenozoic marine Sr isotope record correlate with periods of global cooling and enhanced silicate weathering rates, initiated by the onset of glaciation in the uplifting Himalayan mountains (Raymo and Ruddiman, 1992). However, due to the Himalaya's unique orogenic history, strontium with high 87Sr/86Sr ratios may reside in easily weathered carbonate minerals that do not act as a net sip& for atmospheric CO2 when undergoing chemical weathering (Edmond, 1992; Blum et al., 1998). Thus, the dissolved flux of Sr from the Himalayan Mountains may not be an ideal proxy for silicate weathering and atmospheric CO2 consumption. By studying the major element and Sr isotope chemistry of soils developed on a Himalayan glacial chronosequence, we are investigating how the dissolved flux of ions derived from carbonate versus silicate mineral weathering varies as a function of time following the exposure of fresh rock surfaces to the agents of chemical weathering. Geologic setting and methods. The -200 km 2 Raikhot Valley, located at an average elevation of ~4000 m on the northern slope of the Nanga ParbatHarimosh Massif (NPHM) within the Pakistani Himalayas, contains both the ~50 km 2 Raikhot Glacier as well as a clearly visible glacial chronosequence. Four moraines, thought to be roughly Little Ice Age to neoglacial in age, are nested near the modem terminus of the glacier, while a fifth moraine, believed to be last glacial maximum in age, forms a bench about 200 m from the glacial terminus. The bedrock of the watershed, considered to be representative of the High Himalayan Crystalline Series (HHCS) which stretches for 2000 km across the length of the Himalayan Mountains, primarily consists of quartzofeldspathic biotite gneisses, biotite schists, leucogranite dikes, and granitic stocks, as well as minor amounts (~1%) of interbedded amphibolite and calc-silicate schists. In order of increasing relative age, the sample numbers 96PK1, 96PK2, 96PK3, and 96PK4 refer to soil profiles collected from the four young nested moraines, while the sample numbers 96PK5 and 96PK5a refer to 2 separate soil profiles developed on the fifth moraine. This report presents results obtained from the analysis of the major element (Ca, Mg, Na, K, and Si) concentrations, minor element (Sr, Ti, and Zr) concentrations, and the Sr isotope ratios (87Sr/86Sr) contained in the bulk and exchangeable fractions of these soils. Ca to Sr ratios. The analysis of Ca/(1000*Sr) ratios provides a useful mechanism for characterizing the mineral provenance of the bulk soil and exchangeable ions. The Ca/(1000*Sr) ratios of the bulk soil and exchangeable ions represent a mixture between the average calcium carbonate and silicate endmembers for the watershed, as determined by sequentially leaching and digesting modem riverbed sediment with 4N acetic acid and HF-HC104, respectively (Blum et al., 1998). The average Ca/(1000*Sr) ratio of the exchangeable fraction in profiles 96PK1, 96PK2, 96PK3, and 96PK4 appears similar to the calcium carbonate endmember for the watershed (1.4), while the Ca/(1000*Sr) ratios contained in the entire suite of bulk soils as well as the exchangeable fraction in the C-horizons of profiles 96PK5 and 96PK5a appears more similar to the silicate endmember (0.19). These observations suggest that even though calcium carbonate represents only -1% of the rock in the watershed, calcium carbonate is the primary source of Ca and Sr provided to the dissolved groundwater flux during the early stages of weathering. In addition, the Ca/(1000*Sr) ratios in the C-horizons of profiles 96PK5 and 96PK5a demonstrate that the calcium carbonate reservoir in the soils is nearly depleted within the -15,000 to 20,000 years during which the soils have developed on the moraines. However, a reversal toward carbonate-like Ca/(1000*Sr) ratios in the uppermost horizons of the older profiles indicates that the continual addition of fresh weatherable material,

Modelling the Atmospheric CO2 Consumption and River Carbon Inputs to the Oceans by Continental Erosion

Modelling the atmospheric CO2 consumption and river carbon inputs to the oceans by continental erosion

The Role of Disseminated Calcite in Rates of Calcium Release and CO2 Consumption During the Weathering of Granitoid Rocks

Although Ca and Na reside predominately in plagioclase feldspar in granitoid rocks, recent studies have demonstrated that excess Ca often exits in surface and ground waters relative to plagioclase stochiometry. Explanations advanced to explain the Ca excess have included selective loss of Ca in soils due to watershed acidification (Hyman et al., 1998), selective weathering of the anorthite component of plagioclase (Clayton 1986), and the rapid weathering of trace mineral phases such as calcite (Mast and Drever 1990). Chemical hydrolysis of Ca-silicate rocks is recognized as impacting long term atmospheric CO2 consumption and global climate through silicate hydrolysis reactions (Bemer and Caldeira 1997). In contrast, carbonate mineral dissolution does not have a corresponding impact because the CO2 consumed in weathering is reintroduced back into the atmosphere by precipitation of carbonates in the ocean. Thus, the solute Ca/Na ratio in watersheds and rivers, reflecting proportions of silicate and carbonate weathering, is important in understanding long term global CO2 balances.