Climatic variations during the Holocene inferred from radiocarbon and stable carbon isotopes in speleothems from a high-alpine cave
<p>Rapid and continuous analysis of radiocarbon (<span class="inline-formula"><sup>14</sup></span>C) concentration in carbonate samples at spatial resolution down to 100 <span class="inline-formula">µm</span> has been made possible with t...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-10-01
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| Series: | Climate of the Past |
| Online Access: | https://cp.copernicus.org/articles/17/2165/2021/cp-17-2165-2021.pdf |
| Summary: | <p>Rapid and continuous analysis of radiocarbon (<span class="inline-formula"><sup>14</sup></span>C) concentration in
carbonate samples at spatial resolution down to 100 <span class="inline-formula">µm</span> has been made
possible with the new LA-AMS (laser ablation accelerator mass spectrometry)
technique. This novel approach can provide radiocarbon data at a spatial
resolution similar to that of stable carbon (C) isotope measurements by
isotope ratio mass spectrometry of micromilled samples and, thus, can help
to interpret <span class="inline-formula"><i>δ</i><sup>13</sup></span>C signatures, which otherwise are difficult to
understand due to numerous processes contributing to changes in the C-isotope
ratio. In this work, we analyzed <span class="inline-formula"><i>δ</i><sup>13</sup></span>C and <span class="inline-formula"><sup>14</sup></span>C on the
Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria).
Both proxies respond in a complex manner to climate variability. Combined
stable carbon and radiocarbon profiles allow three growth
periods characterized by different <span class="inline-formula"><i>δ</i><sup>13</sup></span>C signatures to be identified: (i) the
period 8.5 to 8.0 ka is characterized by relatively low <span class="inline-formula"><i>δ</i><sup>13</sup></span>C
values with small variability combined with a comparably high radiocarbon
reservoir effect (expressed as dead carbon fraction, dcf) of around 60 %.
This points towards C contributions of host rock dissolution and/or from an
“old” organic matter (OM) reservoir in the karst potentially mobilized due
to the warm climatic conditions of the early Holocene. (ii) Between 8 and
3.8 ka there was a strong variability in <span class="inline-formula"><i>δ</i><sup>13</sup></span>C with values ranging from
<span class="inline-formula">−</span>8 ‰ to <span class="inline-formula">+</span>1 ‰ and a generally lower dcf. The <span class="inline-formula"><i>δ</i><sup>13</sup></span>C variability is most likely caused by changes in C exchange between
cave air CO<span class="inline-formula"><sub>2</sub></span> and dissolved inorganic carbon in drip water in the cave,
which are induced by reduced drip rates as derived from reduced stalagmite
growth rates. Additionally, the lower dcf indicates that the OM reservoir
contributed less to stalagmite growth in this period possibly as a result of
reduced meteoric precipitation or because it was exhausted. (iii) In the
youngest section between 3.8 and 2.4 ka, comparably stable and low
<span class="inline-formula"><i>δ</i><sup>13</sup></span>C values, combined with an increasing dcf reaching up to
50 % again, hint towards a contribution of an aged OM reservoir in the
karst. This study reveals the potential of combining high-resolution
<span class="inline-formula"><sup>14</sup></span>C profiles in speleothems with <span class="inline-formula"><i>δ</i><sup>13</sup></span>C records in order to
disentangle climate-related C dynamics in karst systems.</p> |
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| ISSN: | 1814-9324 1814-9332 |