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Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes

Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes
Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes
) to trace sources of Ca in Icelandic rivers. We report elemental and Ca isotope data for rivers, high- and low-temperature groundwater, basalt, hydrothermal calcite (including Iceland Spar), and stilbite and heulandite, which are two types of zeolites commonly formed during low-grade metamorphism of basalt. In agreement with previous research, we find that rivers have higher ?44/40Ca values than basalt, with a maximum difference of ?0.40‰. This difference may reflect isotope fractionation in the weathering zone, i.e., preferential uptake of 40Ca during clay mineral formation, adsorption, and other geochemical processes that cycle Ca. However, calcite ?44/40Ca values are also up to ?0.40‰?higher than bedrock values, and on a diagram of ?44/40Ca versus Sr/Ca, nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite, with glacial rivers plotting closer to calcite than non-glacial rivers. Calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Because heulandite ?44/40Ca values are ?1–2‰?lower than basalt, we suggest that 40Ca uptake by heudlandite explains the relatively high ?44/40Ca values of calcite and that calcite weathering in turn elevates riverine ?44/40Ca values. High mechanical erosion rates are known to facilitate the exposure and weathering of calcite, which explains the isotopic contrast between glacial and non-glacial watersheds. Using a mixing model, we find that calcite weathering provides ?0–65% of the Ca in non-glacial rivers and ?25–90% of the Ca in glacial rivers, with silicate weathering providing the remainder. Icelandic hydrothermal calcite contains mantle carbon. Noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon, we suggest that the contribution of basalt weathering to long-term CO2 drawdown and climate regulation may be less significant than previously realized.
Ca isotopes, Iceland, chemical weathering, long-term carbon cycle, isotope fractionation
0012-821X
132-142
Jacobson, Andrew
b825936d-34d4-4d99-91d8-74c2ab0a526d
Andrews, M. Grace
00f89dd1-fe94-418c-92d7-4aa648bb73f0
Lehn, Gregory
73b9500d-d72c-4ed1-ad33-5ab415cf7849
Holmden, Chris
94cb0813-1f52-4d09-9c52-364451d52a39
Jacobson, Andrew
b825936d-34d4-4d99-91d8-74c2ab0a526d
Andrews, M. Grace
00f89dd1-fe94-418c-92d7-4aa648bb73f0
Lehn, Gregory
73b9500d-d72c-4ed1-ad33-5ab415cf7849
Holmden, Chris
94cb0813-1f52-4d09-9c52-364451d52a39

Jacobson, Andrew, Andrews, M. Grace, Lehn, Gregory and Holmden, Chris (2015) Silicate versus carbonate weathering in Iceland: New insights from Ca isotopes. Earth and Planetary Science Letters, 416, 132-142. (doi:10.1016/j.epsl.2015.01.030).

Record type: Article

Abstract

) to trace sources of Ca in Icelandic rivers. We report elemental and Ca isotope data for rivers, high- and low-temperature groundwater, basalt, hydrothermal calcite (including Iceland Spar), and stilbite and heulandite, which are two types of zeolites commonly formed during low-grade metamorphism of basalt. In agreement with previous research, we find that rivers have higher ?44/40Ca values than basalt, with a maximum difference of ?0.40‰. This difference may reflect isotope fractionation in the weathering zone, i.e., preferential uptake of 40Ca during clay mineral formation, adsorption, and other geochemical processes that cycle Ca. However, calcite ?44/40Ca values are also up to ?0.40‰?higher than bedrock values, and on a diagram of ?44/40Ca versus Sr/Ca, nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite, with glacial rivers plotting closer to calcite than non-glacial rivers. Calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface. Because heulandite ?44/40Ca values are ?1–2‰?lower than basalt, we suggest that 40Ca uptake by heudlandite explains the relatively high ?44/40Ca values of calcite and that calcite weathering in turn elevates riverine ?44/40Ca values. High mechanical erosion rates are known to facilitate the exposure and weathering of calcite, which explains the isotopic contrast between glacial and non-glacial watersheds. Using a mixing model, we find that calcite weathering provides ?0–65% of the Ca in non-glacial rivers and ?25–90% of the Ca in glacial rivers, with silicate weathering providing the remainder. Icelandic hydrothermal calcite contains mantle carbon. Noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon, we suggest that the contribution of basalt weathering to long-term CO2 drawdown and climate regulation may be less significant than previously realized.

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Accepted/In Press date: 25 January 2015
Published date: 15 April 2015
Keywords: Ca isotopes, Iceland, chemical weathering, long-term carbon cycle, isotope fractionation
Organisations: Geochemistry

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Local EPrints ID: 405172
URI: http://eprints.soton.ac.uk/id/eprint/405172
ISSN: 0012-821X
PURE UUID: cb64d1d1-9079-4817-8fa2-05c641cc2823

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Date deposited: 27 Jan 2017 11:44
Last modified: 25 Nov 2019 19:36

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