Climate-induced speleothem radiocarbon variability on Socotra Island from the Last Glacial Maximum to the Younger Dryas

• Main Authors: S. Therre, J. Fohlmeister, D. Fleitmann, A. Matter, S. J. Burns, J. Arps, A. Schröder-Ritzrau, R. Friedrich, N. Frank
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Series: Climate of the Past
• Online Access: https://www.clim-past.net/16/409/2020/cp-16-409-2020.pdf

<p>In this study, the dead carbon fraction (DCF) variations in stalagmite M1-5 from Socotra Island in the western Arabian Sea were investigated through a new set of high-precision U-series and radiocarbon (<span class="inline-formula">¹⁴C</span>) dates. The data reveal an extreme case of very high and also climate-dependent DCF. For M1-5, an average DCF of <span class="inline-formula">56.2±3.4</span>&thinsp;% is observed between 27 and 18&thinsp;kyr&thinsp;BP. Such high DCF values indicate a high influence of aged soil organic matter (SOM) and nearly completely closed-system carbonate dissolution conditions. Towards the end of the last glacial period, decreasing <span class="inline-formula">Mg∕Ca</span> ratios suggest an increase in precipitation which caused a marked change in the soil carbon cycling as indicated by sharply decreasing DCF. This is in contrast to the relation of soil infiltration and DCF as seen in stalagmites from temperate zones. For Socotra Island, which is influenced by the East African–Indian monsoon, we propose that more humid conditions and enhanced net infiltration after the Last Glacial Maximum (LGM) led to dense vegetation and thus lowered the DCF by increasing <span class="inline-formula">¹⁴CO₂</span> input into the soil zone. At the onset of the Younger Dryas (YD) a sudden change in DCF towards much higher, and extremely variable, values is observed. Our study highlights the dramatic variability of soil carbon cycling processes and vegetation feedback on Socotra Island manifested in stalagmite DCF on both long-term trends and sub-centennial timescales, thus providing evidence for climate influence on stalagmite radiocarbon. This is of particular relevance for speleothem studies that aim to reconstruct past atmospheric <span class="inline-formula">¹⁴C</span> (e.g., for the purposes of <span class="inline-formula">¹⁴C</span> calibration), as these would rely on largely climate-independent soil carbon cycling above the cave.</p>