Partitioning riverine sulfate sources using oxygen and sulfur isotopes: implications for carbon budgets of large rivers
Partitioning riverine sulfate sources using oxygen and sulfur isotopes: implications for carbon budgets of large rivers
The weathering of carbonate rocks with sulfuric acid releases carbon dioxide (CO
2) to the atmosphere, offsetting the CO
2 drawdown from carbonic acid weathering of silicates thought to regulate global climate. Quantifying CO
2 release from sulfuric acid weathering requires the partitioning of riverine sulfate between its two main sources: sedimentary sulfate and sulfide. Although the sulfur (δ
34S
SO
4
) and oxygen (δ
18O
SO
4
) isotope ratios of sedimentary sulfates (gypsum and anhydrite) of different ages are well constrained, the δ
34S of sulfide minerals is highly variable, restricting the utility of δ
34S for partitioning sulfur sources. Here, we use oxygen isotope ratios in the river water (δ
18O
H
2O
) and sulfate molecules (δ
18O
SO
4
) to partition the fraction of sulfate and associated uncertainty delivered by the oxidative weathering of pyrite (f
pyr). The partitioning is illustrated using the Mekong River, one of the world's largest river basins, presenting new δ
18O
SO
4
, δ
18O
H
2O
and δ
34S
SO
4
data collected on 18 tributaries and 6 mainstem sites over two field seasons at peak flux. The geological, geomorphological and climatic diversity of the Mekong River basin make it an ideal field site to quantify the role of sulfuric acid weathering and its implications for the carbon cycle. There is a 12‰ range in both the difference between δ
18O
SO
4
and δ
18O
H
2O
(Δ
18O
SO
4−H
2O
) and δ
34S in the river waters of the basin. In the Mekong tributaries, sources of sulfate are highly variable with the fraction of sulfate derived from pyrite oxidation (f
pyr) ranging from 0.19 to 0.84. In the mainstem, f
pyr reflects the flux-weighted mean of these tributary inputs, with 56±7% (1σ) of the sulfate delivered to the ocean at the Mekong mouth being derived from the oxidative weathering of pyrite. As a result, we estimate that ∼70% of CO
2 consumed through silicate weathering in the Mekong basin is offset by the release of CO
2 via the dissolution of carbonates by sulfuric acid.
CO release, Mekong, chemical weathering, oxygen isotopes, pyrite oxidation, sulfuric acid
Relph, Katy
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Stevenson, Emily
724f51bf-ee45-48c8-a023-bfa555531ee0
Turchyn, Alexandra
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Antler, Gilad
c7f86fe4-7c3c-4a63-a522-ac33b02c7371
Bickle, Mike
fe310482-d764-498c-be04-baeceb88ec30
Baronas, Jotautas
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Darby, Stephen
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Parsons, Daniel R.
59f2673a-9c73-437a-8865-52d52830a3aa
Tipper, Edward
d16e0590-c55a-4ad2-ac48-a4088479dbb0
1 August 2021
Relph, Katy
fc912e1b-f6d3-4e2a-93b2-3afb59ebf947
Stevenson, Emily
724f51bf-ee45-48c8-a023-bfa555531ee0
Turchyn, Alexandra
d1d8cb2f-ec46-44c6-ae34-cca5844a67b6
Antler, Gilad
c7f86fe4-7c3c-4a63-a522-ac33b02c7371
Bickle, Mike
fe310482-d764-498c-be04-baeceb88ec30
Baronas, Jotautas
9c9473d9-6376-4dc7-b940-ee6d0427ef01
Darby, Stephen
4c3e1c76-d404-4ff3-86f8-84e42fbb7970
Parsons, Daniel R.
59f2673a-9c73-437a-8865-52d52830a3aa
Tipper, Edward
d16e0590-c55a-4ad2-ac48-a4088479dbb0
Relph, Katy, Stevenson, Emily, Turchyn, Alexandra, Antler, Gilad, Bickle, Mike, Baronas, Jotautas, Darby, Stephen, Parsons, Daniel R. and Tipper, Edward
(2021)
Partitioning riverine sulfate sources using oxygen and sulfur isotopes: implications for carbon budgets of large rivers.
Earth and Planetary Science Letters, 567, [116957].
(doi:10.1016/j.epsl.2021.116957).
Abstract
The weathering of carbonate rocks with sulfuric acid releases carbon dioxide (CO
2) to the atmosphere, offsetting the CO
2 drawdown from carbonic acid weathering of silicates thought to regulate global climate. Quantifying CO
2 release from sulfuric acid weathering requires the partitioning of riverine sulfate between its two main sources: sedimentary sulfate and sulfide. Although the sulfur (δ
34S
SO
4
) and oxygen (δ
18O
SO
4
) isotope ratios of sedimentary sulfates (gypsum and anhydrite) of different ages are well constrained, the δ
34S of sulfide minerals is highly variable, restricting the utility of δ
34S for partitioning sulfur sources. Here, we use oxygen isotope ratios in the river water (δ
18O
H
2O
) and sulfate molecules (δ
18O
SO
4
) to partition the fraction of sulfate and associated uncertainty delivered by the oxidative weathering of pyrite (f
pyr). The partitioning is illustrated using the Mekong River, one of the world's largest river basins, presenting new δ
18O
SO
4
, δ
18O
H
2O
and δ
34S
SO
4
data collected on 18 tributaries and 6 mainstem sites over two field seasons at peak flux. The geological, geomorphological and climatic diversity of the Mekong River basin make it an ideal field site to quantify the role of sulfuric acid weathering and its implications for the carbon cycle. There is a 12‰ range in both the difference between δ
18O
SO
4
and δ
18O
H
2O
(Δ
18O
SO
4−H
2O
) and δ
34S in the river waters of the basin. In the Mekong tributaries, sources of sulfate are highly variable with the fraction of sulfate derived from pyrite oxidation (f
pyr) ranging from 0.19 to 0.84. In the mainstem, f
pyr reflects the flux-weighted mean of these tributary inputs, with 56±7% (1σ) of the sulfate delivered to the ocean at the Mekong mouth being derived from the oxidative weathering of pyrite. As a result, we estimate that ∼70% of CO
2 consumed through silicate weathering in the Mekong basin is offset by the release of CO
2 via the dissolution of carbonates by sulfuric acid.
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Published date: 1 August 2021
Additional Information:
Funding Information:
This research was funded by a Natural Environment Research Council (NERC), UK, PhD Studentship to K.E. Relph ( NE/L002507/1 ) and by NERC Standard Grants (PI Tipper NE/P011659/1 , NE/M001865/1 , NE/K000705/1 , NE/N007441/1 ). We thank B. Savoeun, Nakarath Travel, J. Chen and Y. Liu for fieldwork logistics assistance, R. Robinson, T. Perkins, R. Santos and L. Feng for assistance in the collection of samples and discussions, J. Rolfe and H.J. Bradbury for analytical assistance and R.G. Hilton and C. Hackney for discussions. Chris Parish constructed all the field sampling equipment that made this possible. Lou Derry, Xin Gu and one anonymous reviewer are thanked for their thoughtful and astute comments which greatly improved this work.
Publisher Copyright:
© 2021 Elsevier B.V.
Keywords:
CO release, Mekong, chemical weathering, oxygen isotopes, pyrite oxidation, sulfuric acid
Identifiers
Local EPrints ID: 449410
URI: http://eprints.soton.ac.uk/id/eprint/449410
ISSN: 0012-821X
PURE UUID: ccf15458-b77e-4561-9ed5-4bfa294368bc
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Date deposited: 27 May 2021 16:31
Last modified: 17 Mar 2024 06:35
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Contributors
Author:
Katy Relph
Author:
Emily Stevenson
Author:
Alexandra Turchyn
Author:
Gilad Antler
Author:
Mike Bickle
Author:
Jotautas Baronas
Author:
Daniel R. Parsons
Author:
Edward Tipper
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