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Controls on Long-Term Climate: Tectonics and Erosion

Group Members

M.J. Bickle, H.J. Chapman, A. Galy & N. Hovius

Previous Group Members

J. Bunbury (Cambridge), E.T. Tipper (Cambridge & ETH), J.A. Becker (Cambridge)

Collaborators

N.B.W. Harris (Open University), T. Ahmad (University of Dehli), I.J. Fairchild (University of Birmingham), J. West (Oxford)


Trisuli
Sampling the Trisuli River, central Nepal, April 2002
Chemical weathering of silicate minerals is thought to provide the feedback which moderates global climate on long timescales. Global climate may be perturbed by changes in any of the processes which control chemical weathering, particularly erosion, rainfall, vegetation as well as temperature. The feedback only operates in catchments in which erosion is sufficiently rapid that chemical weathering is incomplete. Thus, rapidly eroding mountain belts play a key role in controlling climate, both by providing the locations for ‘kinetically controlled’ chemical weathering and potentially forcing changes in climate by moderating physical erosion rates. We are studying the controls on chemical weathering rates in rapidly eroding catchments in the Himalayas, Tibet and Taiwan as well as sampling major rivers and their the flood plains in S.E. Asia. A major focus of the work is on controls on Sr-isotopic compositions because of the potential significance of the Himalayas to the Cenozoic increase in marine 87Sr/86Sr.

Multiple Controls on Silicate Weathering Rates From a Compilation of Global Catchment Data

West et al. (2002) compiled silicate chemical rates from a global set of small and large catchments. They were able to quantify the separate relationships between silicate chemical weathering and rainfall, temperature and erosion rate. The figure shows silicate chemical weathering rate versus physical erosion rate. A key observation is that in catchments with low erosion rates, chemical weathering is nearly complete (weathering rates in such catchments give a linear relationship with erosion rates) and chemical weathering rates in such ‘transport limited’ catchments do not respond to changes in climate.

Silicate chemical weathering rate versus physical erosion rate. A key observation is that in catchments with low erosion rates, chemical weathering is nearly complete (weathering rates in such catchments give a linear relationship with erosion rates) and chemical weathering rates in such ‘transport limited’ catchments do not respond to changes in climate
Silicate chemical weathering rate versus physical erosion rate. A key observation is that in catchments with low erosion rates, chemical weathering is nearly complete (weathering rates in such catchments give a linear relationship with erosion rates) and chemical weathering rates in such ‘transport limited’ catchments do not respond to changes in climate

Head Waters of the Ganges: Alaknanda River

We have been working in the Alaknanda and Bhagirathi catchments since 1996 with major collecting trips in 1996, 1997, 1998, 2003 and 2004. The major target has been to quantify the Sr inputs from the different geological units and to discriminate between inputs from carbonate and silicate rocks. The very high 87Sr/86Sr ratios of some carbonates (in excess of 1.0!) complicates this. We use the average composition of tributaries draining single geological units to characterise their outputs and the changes in mainstem chemistry to quantify relative inputs (Bickle et al., 2003). We have been attempting to distinguish carbonate from silicate inputs by modelling mixing between carbonate and silicate-derived components in Sr-Ca-Mg-Na space (Bickle et al., 2005) but precipitation of secondary calcite probably compromises these calculations (see results of Marsyandi work below).

(A) Geological map of the Alaknanda and Bhagirathi valleys showing the  geologically-restricted catchments used for input modelling. (B) The Alaknanda River where it emerges from the High Himalayan  Crystalline Series across the Main Central Thrust. Note the ~6 km  topographic difference between the river bed at ~1000 m and Nanda Devi  at 7817 m, only 30 km away.
(A) Geological map of the Alaknanda and Bhagirathi valleys showing the geologically-restricted catchments used for input modelling. (B) The Alaknanda River where it emerges from the High Himalayan Crystalline Series across the Main Central Thrust. Note the ~6 km topographic difference between the river bed at ~1000 m and Nanda Devi at 7817 m, only 30 km away.

 

(C) Modelling mixing relations in Sr-Ca-Na space.
(C) Modelling mixing relations in Sr-Ca-Na space.

 

(D) Projection into Sr/Ca to Ca/Na space. Note fit to two-component mixing  and extrapolated high Sr/Ca ratios of silicate and carbonate end-members.
(D) Projection into Sr/Ca to Ca/Na space. Note fit to two-component mixing and extrapolated high Sr/Ca ratios of silicate and carbonate end-members.

 

Flood Plains of the Ganges and Brahmaputra Rivers

Ganges flood plain
Ganges flood plain

We estimate that silicate material eroded from the High Himalayas undergoes up to six times as much weathering in the Ganges Plain as in the High Himalayas. We are currently studying weathering in the flood plain to try to better constrain these fluxes. The partition of weathering between the mountains and flood plains is important for understanding the climatic controls on weathering as the warm, wet densely vegetated flood plain is very different from the cold, rocky High Himalayan catchments in which erosion rates are highest.

Nepal: Marsyandi River

We have sampled the Marsyandi river catchment in Nepal in April (pre-monsoon) and September (late-monsoon) in 2002 and, in conjunction with Doug Burbank's Geomorphic Himalayan Project in Nepal sampled a number of major tributaries and mainstem sites at two-weekly intervals continuously for two years.

The  Marsyandi valley north of the Annapurna range where the river flows  through the Tibetan Sedimentary Series on the southern margin of the  Tibetan plateau. Despite, or perhaps because of, the cold and arid  climate we estimate that between 65 and 80% of dissolved Ca is  precipitated as secondary calcite within the catchment.
The Marsyandi valley north of the Annapurna range where the river flows through the Tibetan Sedimentary Series on the southern margin of the Tibetan plateau. Despite, or perhaps because of, the cold and arid climate we estimate that between 65 and 80% of dissolved Ca is precipitated as secondary calcite within the catchment.

 

The time-series samples show marked seasonal changes in the ratio of carbonate-derived to silicate-derived chemical fluxes which we think reflects more rapid carbonate dissolution during high runoff during the monsoon. Correlations between the Ca and Mg-isotopic ratios of the dissolved load are thought to relate correlated variations in the extent of weathering of biotite, the main Mg-containing mineral and precipitation of secondary carbonate. Comparison of Sr-Ca-Na ratios between the bedload and dissolved loads suggests that secondary a higher proportion of Ca is lost to secondary calcite during the dry season, and this coupled with the seasonal variations in carbonate to silicate weathering rotates Sr-Ca-Na correlations.

Seasonal variations in the fraction Ca and Mg derived from silicate (Tipper et al., 2006a)
Seasonal variations in the fraction Ca and Mg derived from silicate (Tipper et al., 2006a)

 

 srcanaca gam Figure 1: Array of time-series mainstem Marsyandi water compositions compared to bed-load compositions in Sr-Ca-Na space. We attribute displacement of water compositions to precipitation of low Sr/Ca secondary calcite in catchment. Preferential calcite precipitation in the dry season, when waters are more silicate dominated (higher Na/Ca), rotates water mixing array which means that extrapolation of the water mixing array no longer passes through carbonate or silicate end-member components.

 new 4 isot vectorsFigure 2: δ44/42Ca versus δ26Mg for mainstem Marsyandi waters drain the Tibetan Sedimentary Series catchment. The displacement of all the water analyses from the mixing array defined by carbonate and silicate rocks suggests fractionation by incongruent dissolution or precipitation reactions which is supported by the correlation in the magnitude of the Ca and Mg-isotopic fractionations (Tipper et al. 2006b).

 

Myanmar: Irrawaddy and Salween Rivers

In conjunction with the Department of Meteorology and Hydrology, Myanmar, we have been sampling and both the Irrawaddy and the Salween at two-weekly intervals since 2004 for analysis of major cations and anions, Sr and Sr-isotopic compositions. We hope to continue this sampling through 2008 to 2011 in collaboration with Ruth Robinson and Michael Bird (St Andrews).

myanmar2
Farmers in the upper Salween Valley, Myanmar
    
myanmar1
Mike Bickle sampling waters from the Salween River in Myanmar

Taiwan: Multiple Catchment Studies

This is a major NERC funded project which started in 2007. The objective is to sample waters from catchments in Taiwan to:

  1. determine the locations where weathering takes place
  2. determine the reaction mechanisms which control the weathering process; and
  3. provide robust (time-averaged) estimates of the silicate and carbonate weathering fluxes.

Taiwan has been chosen for this study because it is accessible, well studied and is one of the few locations in a rapidly eroding, high weathering rate regime where the major controlling parameters on weathering rates (principally temperature, rainfall, and physical erosion rate, but also vegetation, lithology and hydrology) are either well documented or can be determined as part of this study. The project will exploit a collaborative bi-weekly sampling of river waters from 14 catchments in Taiwan initiated in March 2005, the results of past and continuing work on erosion mechanisms and rates in Taiwan, as well as detailed ecological and hydrological work by our Taiwanese colleagues. The project builds on the research by the Earth Surface Processes group.


Our first field season was in September 2007 and currently we are starting analyses of the several hundred water samples collected since 2005.

Taiwan
Landslides, a common feature of Taiwan's rugged topography, contribute extensively to Taiwan's rapid erosion rates.
 
Tawian
Dr Neils Hovius (left) Professor Mike Bickle and Dr Josh West (right), filtering waters from a roadside borehole in Taiwan.

Himalayan Hot Springs

Rivers extract CO2 from the atmosphere. Metamorphic belts also produce CO2 by decarbonation reactions and in the Himalayas this CO2 is released from numerous hot springs. We (Becker et al., 2007) have quantified the hotspring output of CO2 in the Marsyandi catchment in Nepal by modelling the thermodynamics of the high level degassing and quantifying the flux of hot spring waters by the perturbation of Cl contents in the rivers (cf. Evans et al., 2002).

degassing
CO2 produced through metamorphic decarbonation and oxidation of organic matter (1) exsolves from metamorphic fluids due to decompression and temperature loss (2). Metamorphic CO2 dissolves in meteoric groundwaters (3). Degassing near the surface (4) drives d13C DIC to values in excess of +13 permil.
  
bahundanda
Bahundanda hot spring on the Main Central Thrust, in the Marsyandi Valley, central Nepal. Waters are heated by the elevated geothermal gradient, and mix with deep-sourced carbon dioxide before degassing at the surface.

 

Publications From This Work

  1. Becker, J.A., Bickle, M.J., Galy, A. and Holland, T.J.B. (2007) Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs, Earth Planetary Science Letters. doi:10.1016/j.epsl.2007.10.046
  2. Tipper, E. T., Galy, A. and Bickle, M. J. (2006a) Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle, Earth and Planetary Science Letters, v. 247, p. 267-279
  3. Tipper, E. T., Bickle, M. J., Galy, A., West, A. J., Pomies, C. and Chapman, H. J. (2006b) The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry, Geochimica et Cosmochimica Acta, v. 70, p. 2737-2754
  4. Tipper, E. T., Galy, A., Gaillardet, J., Bickle, M. J., Elderfield, H., and Carder, E. A. (2006) The magnesium isotope budget of the modern ocean: Constraints from riverine magnesium isotope ratios, Earth and Planetary Science Letters, v. 250, p. 241-253
  5. West, A. J., Galy, A. and Bickle, M. J. (2005) Tectonic and climatic controls on silicate weathering, Earth and Planetary Science Letters, v. 235, p. 211-228
  6. Bickle, M. J., Chapman, H. J., Bunbury, J. M., Harris, N. B. W., Fairchild, I. J., Ahmad, T. and Pomies, C. (2005) Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges, Geochimica et Cosmochimica Acta, v. 69 p. 2221-2240
  7. Bickle, M. J., Bunbury, J. M., Chapman, H. J., Harris, N. B. W., Fairchild, I. J. and Ahmad, T. (2003) Fluxes of Sr into the headwaters of the Ganges, Geochimica et Cosmochimica Acta, v. 67, p. 2567-2584
  8. Oliver, L., Harris, N., Bickle, M. J., Chapman, H. J., Dise, N. and Horstwood, M. (2003)
  9. Silicate weathering rates decoupled from the 87Sr/86Sr ratio of the dissolved load during Himalayan erosion, Chemical Geology, v. 201, p. 119-139
  10. Vance, D., Bickle, M. J., Ivy-Ochs, S. and Kubik, P. W. (2003) Erosion and exhumation in the Himalaya from cosmogenic isotope inventories of river sediments, Earth and Planetary Science Letters, v. 206, p. 273-288
  11. White, N. M., Pringle, M. S., Garzanti, E., Bickle, M. J., Najman, Y. M. R., Chapman, H. J. and Friend, P. F. (2002) Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits, Earth and Planetary Science Letters, v. 195, p. 29-44
  12. West, A. J., Bickle, M. J., Collins, R., and Brasington, J. (2002) A small catchment perspective on Himalayan weathering fluxes, Geology, v. 30, p. 355-358
  13. Bickle, M. J., Harris, N. B. W., Bunbury, J. M., Chapman, H. J., Fairchild, I. J. and Ahmad, T. (2001) Controls on the 87Sr/86Sr ratio of carbonates in the Garhwal Himalaya, headwaters of the Ganges, Journal of Geology, v. 109, p. 737-753
  14. Najman, Y. M. R., Bickle, M. J. and Chapman, H. J. (2000) Early Himalayan exhumation: isotopic constraints from the Indian foreland basin, Terra Nova, v. 12, p. 28-34
  15. White, N. M., Najman, Y. M. R., Bickle, M. J., Friend, P. F., Parrish, R. R., Pringle, M. S., Burbank, B. and Maithani, M. (1999) Constraints on Himalayan Eo-metamorphism, post-metamorphic cooling, exhumation and erosion provided by detrital monazite and white mica in the Dharamsala formation, N.W. Indian foreland basin, Terra Nostra, v. 99, p. 168-170
  16. Harris, N., Bickle, M. J., Chapman, H. J., Fairchild, I. J. and Bunbury, J. M. (1998) The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary, Chemical Geology, v. 144, p. 205-220
  17. Bickle, M. J. (1996) Metamorphic decarbonation, silicate weathering and the long-term carbon cycle, Terra Nova, v. 8, p. 270-276

 

Last updated on 02-Oct-14 12:35