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Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand

Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand
Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand
Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget.
We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist-amphibolite facies boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3-/Ca2+ ~9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate.
The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be ~6.4 x 108 mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute ~10% and ~3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 x 108 mol/yr, which is 30-50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 x 109 mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.
CO2, mountain building, carbon isotopes, metamorphism, carbon cycle, chemical weathering
0012-821X
305–315
Menzies, Catriona D.
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Wright, Sarah L.
6f77f8df-a02c-4de5-8ddf-6ab3a50fb49a
Craw, Dave
a3a52d6d-2bc2-45cc-96ee-e37eb84055ce
James, Rachael H.
79aa1d5c-675d-4ba3-85be-fb20798c02f4
Alt, Jeffrey C.
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Cox, Simon C.
dfc97577-7816-4f6a-8d88-cbc594ee04ec
Pitcairn, Iain K.
dff344fe-2165-4934-8931-02a067f8d86d
Teagle, Damon A.H.
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Menzies, Catriona D., Wright, Sarah L., Craw, Dave, James, Rachael H., Alt, Jeffrey C., Cox, Simon C., Pitcairn, Iain K. and Teagle, Damon A.H. (2018) Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand Earth and Planetary Science Letters, 481, 305–315. (doi:10.1016/j.epsl.2017.10.010).

Menzies, Catriona D., Wright, Sarah L., Craw, Dave, James, Rachael H., Alt, Jeffrey C., Cox, Simon C., Pitcairn, Iain K. and Teagle, Damon A.H. (2018) Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand Earth and Planetary Science Letters, 481, 305–315. (doi:10.1016/j.epsl.2017.10.010).

Record type: Article

Abstract

Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget.
We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist-amphibolite facies boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3-/Ca2+ ~9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate.
The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be ~6.4 x 108 mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute ~10% and ~3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 x 108 mol/yr, which is 30-50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 x 109 mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.

Text Menzies et al IN PRESS Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps New Zealand- EPSL - Accepted Manuscript
Restricted to Repository staff only until 6 January 2019.
Text 1-s2.0-S0012821X17305691-main - Version of Record
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Accepted/In Press date: 6 October 2017
e-pub ahead of print date: 5 November 2017
Published date: 1 January 2018
Keywords: CO2, mountain building, carbon isotopes, metamorphism, carbon cycle, chemical weathering

Identifiers

Local EPrints ID: 414779
URI: http://eprints.soton.ac.uk/id/eprint/414779
ISSN: 0012-821X
PURE UUID: 322982c0-6bc2-401e-9c9e-105880c5b373
ORCID for Damon A.H. Teagle: ORCID iD orcid.org/0000-0002-4416-8409

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Date deposited: 11 Oct 2017 16:31
Last modified: 08 Nov 2017 17:30

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Contributors

Author: Sarah L. Wright
Author: Dave Craw
Author: Jeffrey C. Alt
Author: Simon C. Cox
Author: Iain K. Pitcairn

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