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Assessing bulk carbonates as archives for seawater Li isotope ratios

Assessing bulk carbonates as archives for seawater Li isotope ratios
Assessing bulk carbonates as archives for seawater Li isotope ratios
Silicate weathering is a primary control on the carbon cycle and therefore long-term climate. Tracing silicate weathering in the geological record has been a challenge for decades, with a number of proxies proposed and their limits determined. Recently lithium isotopes in marine carbonates have emerged as a potential tracer. Bulk carbonates are increasingly being used as a Li isotope archive, though with limited tests thus far of the robustness of this approach in the modern ocean. As the bulk composition of marine pelagic carbonates has changed through time and geographically, assessing the fidelity of bulk carbonate as proxy carrier is fundamental. To address the impact of compositional variability in bulk carbonate on Li isotopes, we examine 27 Bahamian aragonitic bulk carbonates and 16 Atlantic largely calcitic core-top sediment samples. Two core-tops only have trace (<10 %) carbonate, and are analysed to test whether carbonates in such sections are still a viable archive. We selectively extract the exchangeable and carbonate fractions from the core-top samples. The exchangeable fraction contains ∼2 % of the total Li and has a fairly constant offset from seawater of 16.5 ± 0.8‰. When leaching silicate-containing carbonates, acetic acid buffered with sodium acetate appears a more robust method of solely attacking carbonates compared to dilute HCl, which may also liberate some silicate-bound Li. Carbonates from samples that do not contain aragonite have the isotopic fractionation of seawater of Δ7Liseawater-calcite = 6.1 ± 1.3‰ (2sd), which is not affected by latitude or the water depth the sample was deposited at. The pure aragonite bulk carbonates from the Bahamas have a fractionation of Δ7Liseawater-aragonite = 9.6 ± 0.6‰. A sediment sample from the Galician coast that mostly consists of quartz is highly offset from seawater by ∼20‰ and also has relatively high Li/Ca ratios. These high values are not due to leaching of silicate material directly (Al/Ca ratios are low). We interpret this addition via cation exchange of Li from silicate during recrystallisation. Overall bulk carbonates from the open ocean are a reliable archive of seawater δ7Li, but care must be taken with carbonate mineralogy and low-carbonate samples. Overall, therefore, any examination of the palaeo-seawater δ7Li record must be reproduced in different global settings (e.g. multiple global cores) before it can be considered robust.
0009-2541
1-11
Pogge von Strandmann, Philip
32bd4ab8-8d22-442a-ac4b-5dc2aaf55c71
Schmidt, Daniela
da507c95-f27f-4702-ab9b-b2a06aa6b8b2
Planavsky, Noah
199f4559-506c-498f-9b7e-6cea4c75c016
Wei, Guangyi
b47cdce5-3163-42d3-a47a-821fb6e06b43
Todd, Chloe L.
f7162726-2238-4e55-b80d-9a1555d5d746
Baumann, Karl-Heinz
ed66d91b-4329-4db7-8fd4-16576abcecfc
Pogge von Strandmann, Philip
32bd4ab8-8d22-442a-ac4b-5dc2aaf55c71
Schmidt, Daniela
da507c95-f27f-4702-ab9b-b2a06aa6b8b2
Planavsky, Noah
199f4559-506c-498f-9b7e-6cea4c75c016
Wei, Guangyi
b47cdce5-3163-42d3-a47a-821fb6e06b43
Todd, Chloe L.
f7162726-2238-4e55-b80d-9a1555d5d746
Baumann, Karl-Heinz
ed66d91b-4329-4db7-8fd4-16576abcecfc

Pogge von Strandmann, Philip, Schmidt, Daniela, Planavsky, Noah, Wei, Guangyi, Todd, Chloe L. and Baumann, Karl-Heinz (2019) Assessing bulk carbonates as archives for seawater Li isotope ratios. Chemical Geology, 530, 1-11, [119338]. (doi:10.1016/j.chemgeo.2019.119338).

Record type: Article

Abstract

Silicate weathering is a primary control on the carbon cycle and therefore long-term climate. Tracing silicate weathering in the geological record has been a challenge for decades, with a number of proxies proposed and their limits determined. Recently lithium isotopes in marine carbonates have emerged as a potential tracer. Bulk carbonates are increasingly being used as a Li isotope archive, though with limited tests thus far of the robustness of this approach in the modern ocean. As the bulk composition of marine pelagic carbonates has changed through time and geographically, assessing the fidelity of bulk carbonate as proxy carrier is fundamental. To address the impact of compositional variability in bulk carbonate on Li isotopes, we examine 27 Bahamian aragonitic bulk carbonates and 16 Atlantic largely calcitic core-top sediment samples. Two core-tops only have trace (<10 %) carbonate, and are analysed to test whether carbonates in such sections are still a viable archive. We selectively extract the exchangeable and carbonate fractions from the core-top samples. The exchangeable fraction contains ∼2 % of the total Li and has a fairly constant offset from seawater of 16.5 ± 0.8‰. When leaching silicate-containing carbonates, acetic acid buffered with sodium acetate appears a more robust method of solely attacking carbonates compared to dilute HCl, which may also liberate some silicate-bound Li. Carbonates from samples that do not contain aragonite have the isotopic fractionation of seawater of Δ7Liseawater-calcite = 6.1 ± 1.3‰ (2sd), which is not affected by latitude or the water depth the sample was deposited at. The pure aragonite bulk carbonates from the Bahamas have a fractionation of Δ7Liseawater-aragonite = 9.6 ± 0.6‰. A sediment sample from the Galician coast that mostly consists of quartz is highly offset from seawater by ∼20‰ and also has relatively high Li/Ca ratios. These high values are not due to leaching of silicate material directly (Al/Ca ratios are low). We interpret this addition via cation exchange of Li from silicate during recrystallisation. Overall bulk carbonates from the open ocean are a reliable archive of seawater δ7Li, but care must be taken with carbonate mineralogy and low-carbonate samples. Overall, therefore, any examination of the palaeo-seawater δ7Li record must be reproduced in different global settings (e.g. multiple global cores) before it can be considered robust.

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Accepted/In Press date: 12 October 2019
e-pub ahead of print date: 21 October 2019
Published date: 30 December 2019

Identifiers

Local EPrints ID: 437143
URI: http://eprints.soton.ac.uk/id/eprint/437143
ISSN: 0009-2541
PURE UUID: 431b7729-f046-4525-a2cc-c81310b0f3f8
ORCID for Chloe L. Todd: ORCID iD orcid.org/0000-0003-0684-2404

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Date deposited: 17 Jan 2020 17:36
Last modified: 26 Nov 2021 03:20

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Contributors

Author: Philip Pogge von Strandmann
Author: Daniela Schmidt
Author: Noah Planavsky
Author: Guangyi Wei
Author: Chloe L. Todd ORCID iD
Author: Karl-Heinz Baumann

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