Combined 14C and 230Th/U dating of fine-grained cryogenic cave carbonates from a permafrost cave in Greenland

Abstract

<p>Cryogenic cave carbonates (CCC) have become a valuable tool for providing evidence for past permafrost presence, particularly in low-elevation mid-latitude temperate locations (e.g. Germany and UK) and high-elevation mid-latitude periglacial environments (e.g. Austria and Spain). This study focuses on CCC from a low-elevation high-latitude site in the continuous permafrost of the high Arctic. Specifically, the fine-grained form of cryogenic cave carbonates (CCC<sub>fine</sub>), which precipitate from rapidly freezing thin water films on top of cave ice, are investigated from Eqik Qaarusussuaq (cave) in northeast Greenland (80.2°N). Under contemporary conditions, the sampling site in the interior of the cave is dry, cold (-14.7°C) and ice-free, thus water infiltration to facilitate CCC<sub>fine</sub> formation is not possible.</p><p>Previously, <sup>230</sup>Th/U dating efforts of CCC<sub>fine</sub> have suffered from poor age precision due to high detrital Th contamination. Similarly, <sup>14</sup>C dating has been hindered by the unknown reservoir effect (dead carbon fraction). To address these dating issues, we applied a multi-method dating approach to produce a unique dataset comprising eleven <sup>14</sup>C ages as well as six <sup>230</sup>Th/U ages from a single patch of CCC<sub>fine</sub>. An isochron indicates that the CCC<sub>fine</sub> formed synchronously and that the cleanest <sup>230</sup>Th/U age is representative for the whole patch. The dead carbon fraction is calculated based on this <sup>230</sup>Th/U age.</p><p>The results of <sup>230</sup>Th/U dating (97±34 a BP) agree with the calibrated <sup>14</sup>C age range (40-70 a BP (37.9%), 115-139 a BP (28.2%), 226-254 a BP (29.4%)) that the CCC<sub>fine</sub> from Eqik Qaarusussuaq most likely formed towards the end of the Little Ice Age or shortly after.</p>