Technical note: Lithium isotopes in dolostone as a palaeo-environmental proxy – an experimental approach
<p>Lithium (Li) isotopes in marine carbonates have considerable potential as a proxy to constrain past changes in silicate weathering fluxes and improve our understanding of Earth's climate. To date the majority of Li isotope studies on marine carbonates have focussed on calcium carbonate...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-04-01
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| Series: | Climate of the Past |
| Online Access: | https://www.clim-past.net/15/635/2019/cp-15-635-2019.pdf |
| Summary: | <p>Lithium (Li) isotopes in marine carbonates have considerable
potential as a proxy to constrain past changes in silicate weathering fluxes
and improve our understanding of Earth's climate. To date the majority of Li
isotope studies on marine carbonates have focussed on calcium carbonates.
The determination of the Li isotope fractionation between dolomite and a
dolomitizing fluid would allow us to extend investigations to deep times
(i.e. Precambrian) when dolostones were the most abundant marine carbonate
archives. Dolostones often contain a significant proportion of detrital
silicate material, which dominates the Li budget; thus, pretreatment needs to
be designed so that only the isotope composition of the carbonate-associated
Li is measured. This study aims to serve two main goals: (1) to determine the
Li isotope fractionation between Ca–Mg carbonates and solution, and (2) to
develop a method for leaching the carbonate-associated Li out of dolostone
while not affecting the Li contained within the detrital portion of the rock.
We synthesized Ca–Mg carbonates at high temperatures (150 to 220 <span class="inline-formula"><sup>∘</sup></span>C)
and measured the Li isotope composition (<span class="inline-formula"><i>δ</i><sup>7</sup>Li</span>) of the precipitated
solids and their respective reactive solutions. The relationship of the Li
isotope fractionation factor with temperature was obtained:
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<p>Competitive nucleation and growth between dolomite and magnesite were
observed during the experiments; however, there was no notable effect of their
relative proportion on the apparent Li isotope fractionation. We found that
Li isotope fractionation between the precipitated solid and solution is higher
for Ca–Mg carbonates than for Ca carbonates. If the temperature of a
precipitating solution is known or can be estimated independently, the above
equation could be used in conjunction with the Li isotope composition of
dolostones to derive the composition of the solution and hence make inferences about the
past Li cycle. In addition, we also conducted leaching experiments on a
Neoproterozoic dolostone and a Holocene coral. Results show that leaching
with 0.05 M hydrochloric acid (HCl) or 0.5 % acetic acid (HAc) at room temperature for 60 min
releases Li from the carbonate fraction without a significant contribution of
Li from the siliciclastic detrital component. These experimental and
analytical developments provide a basis for the use of Li isotopes in
dolostones as a palaeo-environmental proxy, which will contribute to further
advance our understanding of the evolution of Earth's surface environments.</p> |
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| ISSN: | 1814-9324 1814-9332 |