Mountain ranges, climate and weathering. Do orogens strengthen or weaken the silicate weathering carbon sink?

doi:10.1016/j.epsl.2018.04.034

Earth and Planetary Science Letters

Volume 493, 1 July 2018, Pages 174-185

https://doi.org/10.1016/j.epsl.2018.04.034 Get rights and content

Highlights

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A weathering model taking into account erosion and climate influence is presented.
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Exploring parameter space does a test of model performances according to data.
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Two numerical experiments simulating a world with and without relief are conducted.
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The weathering model is run in both experiment by varying the parameters.
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A confidence interval of weathering change from a real to a flat world is deduced.

Abstract

The role of mountains in the geological evolution of the carbon cycle has been intensively debated for the last decades. Mountains are thought to increase the local physical erosion, which in turns promotes silicate weathering, organic carbon transport and burial, and release of sulfuric acid by dissolution of sulfides. In this contribution, we explore the impact of mountain ranges on silicate weathering. Mountains modify the global pattern of atmospheric circulation as well as the local erosion conditions. Using an IPCC-class climate model, we first estimate the climatic impact of mountains by comparing the present day climate with the climate when all the continents are assumed to be flat. We then use these climate output to calculate weathering changes when mountains are present or absent, using standard expression for physical erosion and a 1D vertical model for rock weathering. We found that large-scale climate changes and enhanced rock supply by erosion due to mountain uplift have opposite effect, with similar orders of magnitude.

A thorough testing of the weathering model parameters by data-model comparison shows that best-fit parameterizations lead to a decrease of weathering rate in the absence of mountain by about 20%. However, we demonstrate that solutions predicting an increase in weathering in the absence of mountain cannot be excluded. A clear discrimination between the solutions predicting an increase or a decrease in global weathering is pending on the improvement of the existing global databases for silicate weathering. Nevertheless, imposing a constant and homogeneous erosion rate for models without relief, we found that weathering decrease becomes unequivocal for very low erosion rates (below 10 t/km²/yr). We conclude that further monitoring of continental silicate weathering should be performed with a spatial distribution allowing to discriminate between the various continental landscapes (mountains, plains …).

Introduction

The continental topography is a key component of the Earth climate regulation. Large mountain ranges are thought to increase the weatherability of continental surfaces through enhanced physical erosion and sediment production. These processes promote chemical weathering and associated CO₂ consumption either in ranges or in the lowlands located at their feet (West et al., 2005, Moquet et al., 2011, Lupker et al., 2012, Goddéris et al., 2017). Mountain ranges would thus strengthen the negative feedback between silicate rock weathering and climate, avoiding extreme fluctuations in the atmospheric CO₂ concentrations (Maher and Chamberlain, 2014). In addition, enhanced erosion and sedimentation rates increase the efficiency of organic carbon burial (Galy et al., 2007, Bouchez et al., 2014), promoting further cooling.

The absence of mountains also exerts a topographic control on the global carbon cycle. Flattening of the continents is thought to decrease the global continental weatherability and the strength of the weathering feedback, allowing CO₂ to accumulate in the atmosphere and climate to warm, as it might have been the case in the early Eocene (Goddéris et al., 2008, Carretier et al., 2014, Froelich and Misra, 2014, Maher and Chamberlain, 2014, Vigier and Goddéris, 2015). The progressive shift from an Eocene world with potentially flat continents towards the present-day steeper topography is commonly invoked to explain the global climatic cooling of the Cenozoic.

However, this paradigm, linking the Cenozoic cooling to the uplift of large orogens, is still debated (Willenbring and von Blanckenburg, 2010, Norton and Schlunegger, 2017). Those contributions point towards a weak impact of mountain ranges on the global weathering and erosion rates. Mountains may also act as CO₂ producers, through the release of sulfuric acid by pyrite oxidation that will accelerate carbonate dissolution (Torres et al., 2016) and the metamorphic decarbonation (Girault et al., 2014). Furthermore, the impact of mountain building on global climate also requires knowledge of the size of the area covered by the uplifted domains, on the nature of the rocks exposed in the orogen (Brault et al., 2017), and on their latitudinal position (Goddéris et al., 2017).

In a simplified description of the world composed of flat and mountainous areas, can we quantify the contribution of mountains to the overall CO₂ consumption by silicate rock weathering? The answer to this question remains unclear. Mountains modify the local physical erosion, but mountain ranges are also impacting the atmospheric and oceanic global circulation. For instance, the presence of mountain ranges is one of the triggers of the Atlantic meridional overturning cell (Schmittner et al., 2011, Sinha et al., 2012, Maffre et al., 2018) and modern mountain ranges have been shown to lead to an aridification of continents interiors (Broccoli and Manabe, 1992, Kutzbach et al., 1993). The mountains heavily modify the global climatic pattern (including rainfall), and hence the weathering fluxes everywhere on Earth. Additionally, the response of weathering to continental surfaces may depend on our ability to simulate the climate and on the formalism chosen to describe the coupling between physical erosion and weathering. The response of weathering to the presence or absence of mountains will also depend on the quality of the available database for weathering that can be used for model calibration.

To address this issue, we focus on two end-members: the modern geography and an idealized mountain-free geography. We investigate how global weathering changes when the continents are assumed to be flat using a numerical modeling method designed for Earth's global scale. By comparing the true CO₂ consumption with that simulated for a flat world, we estimate the contribution of present-day mountain ranges.

Section snippets

General framework

The aim of this study is to provide constraints on the behavior of the global carbon uptake by silicate weathering in a globally flat world. Given that silicate weathering is a complex function of climate and physical erosion, a cascade of large-scale models describing the climatic conditions, the physical erosion, and the chemical weathering must be set up.

Our general methodology can be summarized as follows: a mathematical law linking chemical weathering to physical erosion and climate is

Weathering model performances at high resolution

In this section, we evaluate the performances of the weathering model (Eq. (1)) at present-day. To avoid any bias from other models, eq. (1) is applied to fields of temperature, runoff and erosion as close as possible to real ones. Runoff and temperature are thus taken from the Climate Research Unit (CRU) database. They were computed from 100 yr climate reanalysis. Such a database does no exist for erosion. We use the erosion chart of Ludwig and Probst (1998), which is a model locally corrected

Results

In this section, we first give details about the main results of the IPSL-CM5 climate simulations. We then use these outputs to calculate erosion rates for both CTRL and FLAT simulations. Finally, we quantify and compare the resulting global weathering rates using each of the different calibrations detailed in Section 3.

Discussion

A number of caveats might affect these results and are worth detailing:

Conclusions

In this contribution, we use an IPCC-class climate model and up to date formulations linking continental weathering rates to climate and to physical erosion, in order to explore the response of continental silicate weathering to a general flattening of the continents. We constrain the parameters of the models with published riverine geochemical data. Our results support a predominant effect of erosion, with globally lower weathering in a world without mountains. Nevertheless, the available data

Acknowledgments

This work notably relies on the model developed by Joshua West, and benefits from some conversations with him. Handling of geographic data (watersheds contours) was done with the help of Vincent Regard. We also thank Yannick Donnadieu for the proofreading.

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^☆: Abbreviations: CRU: Climate Research Unit; SPIM: Stream Power Incision Model; IPSL-CM5: Institut Pierre Simon Laplace Climate Model v. 5; LEM: Landscape Elevation Model; CTRL: Control experiment; FLAT: Flat Earth experiment; SRTM: Shuttle Radar Topography Mission.

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Mountain ranges, climate and weathering. Do orogens strengthen or weaken the silicate weathering carbon sink?☆

Highlights

Abstract

Introduction

Section snippets

General framework

Weathering model performances at high resolution

Results

Discussion

Conclusions

Acknowledgments

Geochim. Cosmochim. Acta

Geomorphology

Phys. Chem. Earth

Chem. Geol.

Geoderma

Geomorphology

Geochim. Cosmochim. Acta

Chem. Geol.

C. R. Géosci.

Chem. Geol.

Geochim. Cosmochim. Acta

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Slow rates of rock surface erosion and sediment production across the Namib Desert and Escarpment, Southern Africa

Am. J. Sci.

An exhumation history of continents over billion-year time scales

Science

The importance of terrestrial weathering changes in multimillennial recovery of the global carbon cycle: a two-dimensional perspective

Earth Syst. Dyn.

A simple model for regolith formation by chemical weathering: regolith formation

J. Geophys. Res., Earth Surf.

The effects of orography on middle latitude northern-hemisphere dry climates

J. Climate

Climate change projections using the IPSL-CM5 Earth System Model: from CMIP3 to CMIP5

Clim. Dyn.

Was the late Paleocene–early Eocene hot because Earth was flat? An ocean lithium isotope view of mountain building, continental weathering, carbon dioxide, and Earth's Cenozoic climate

Oceanography

A theoretical model coupling chemical weathering rates with denudation rates

Geology

Efficient organic carbon burial in the Bengal fan sustained by the Himalayan erosional system

Nature

Large-scale organization of carbon dioxide discharge in the Nepal Himalayas

Geophys. Res. Lett.

Onset and ending of the late Palaeozoic ice age triggered by tectonically paced rock weathering

Nat. Geosci

The new global lithological map database GLiM: a representation of rock properties at the Earth surface

Geochem. Geophys. Geosyst.