Compound-specific radiocarbon ages of soil and sedimentary leaf wax biomarkers in an arid high-altitude environment

Abstract

<p>Compound-specific radiocarbon analysis (CSRA) of leaf waxes has revealed significant lag times before compounds are deposited in marine and lacustrine sediments. No such data so far exist for a cold and arid high altitude lake system, where carbon turnover and biomarker fluxes to sediments are expected to be relatively low. To elucidate transport dynamics of terrestrial leaf waxes in such an environment (MAT: -4&#176;C, MAP <100mm), we determined CSRA-ages of selected long-chain <em>n</em>-alkanes in surface soil samples (0-10 cm), collected from alpine meadows in the catchment of Lake Karakul (Pamirs, Tajikistan), and in two sections &#160;of a well dated sediment core from the same lake. We aimed to answer the question if there is a potential bias in the interpretation of biomarker records, in case the leaf wax compounds are significantly older than the sediment age-model suggests.</p><p><em>n</em>C₂₉- and <em>n</em>C₃₁-alkanes in the soil samples exhibited variable ages, ranging from 105&#177;79 to 2260&#177;155 cal. yrs BP. In the two sediment core samples, three of the four obtained ages for <em>n</em>C₂₉ and <em>n</em>C₃₁ felt on the very lower ends of the 1&#1005;-uncertainty ranges of modelled ages (based on AMS ¹⁴C_TOC and OSL dating results).</p><p>The large span of CSRA-ages of soils gives evidence for heterogeneous decomposition and transport conditions in the lake catchment. We hypothesize that compounds with longest pre-aging contributed in lower proportions to the accumulated lake sediments and further suggest that sedimentary leaf waxes represent compounds with intermediate turnover time in soils, for example originating from alluvial plains close to the shores. Overall, the obtained results give evidence that sedimentary leaf wax compounds in this cold and arid high altitude setting are potentially older than the conventional age-model indicates. On the other hand, these findings need to be interpreted in context of the generally large uncertainty ranges of such age-models, which are further influenced by unknown factors for example changes of reservoir effects.&#160;</p>