Calibration and paleohydrological application of compound-specific isotope analyses (&#948;13Cwax, &#948;2Hwax and &#948;18Osugar) in semi-arid/arid Mongolia
Authors: Struck Julian , Bliedtner Marcel , Strobel Paul , Daut Gerhard , Schumacher Jens , Bittner Lucas , Plessen Birgit , Glaser Bruno , Klaes Björn , Bazarradnaa Enkhtuja , Salazar Gary , Szidat Sönke , Zech Michael , Zech Roland 
Published: March 2020

&lt;p&gt;Several lake sediment studies have investigated the Holocene climate history in Mongolia using pollen, organic and inorganic elemental analyses. However, these studies come to very different conclusions. Isotope analyses, particularly compound-specific carbon, hydrogen and oxygen isotopic composition of leaf wax n-alkanes (&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;wax&lt;/sub&gt;, &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax&lt;/sub&gt;) and hemicellulose sugars (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt;) are increasingly used for paleoenvironmental and -hydrological reconstructions and might have great potential to address the controversies in Mongolia.&lt;/p&gt;&lt;p&gt;Here we present a regional calibration of &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;wax&lt;/sub&gt;, &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; on topsoils along a distinct climate gradient in semi-arid/arid Mongolia. &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;wax &lt;/sub&gt;significantly correlates with aridity indicating variations in water use efficiency. The apparent fractionation (&amp;#400;&lt;sub&gt;app&lt;/sub&gt;) of &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; is nearly constant at -131 &amp;#177; 13&amp;#8240; for &amp;#400;&lt;sub&gt;n&lt;/sub&gt;&lt;sub&gt;-C29/p&lt;/sub&gt;, -148 &amp;#177; 11&amp;#8240; for &amp;#400;&lt;sub&gt;n&lt;/sub&gt;&lt;sub&gt;-C31/p&lt;/sub&gt; and 40.8 &amp;#177; 1.9&amp;#8240; for &amp;#400;&lt;sub&gt;sugar/p&lt;/sub&gt;, respectively. &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax&lt;/sub&gt; (n-C&lt;sub&gt;29&lt;/sub&gt; and n-C&lt;sub&gt;31&lt;/sub&gt;) and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; thus, reflect the isotopic composition of precipitation, which in turn is controlled by atmospheric circulation systems bringing moisture to continental Mongolia, i.e. the interaction between the Westerlies and the Asian Summer Monsoon. Therefore, we applied regionally calibrated &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;wax&lt;/sub&gt;, &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; isotopes, as well as well-established sedimentological and geochemical proxies and &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;org&lt;/sub&gt;, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;carb&lt;/sub&gt;, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;carb&lt;/sub&gt; on a 160 cm long gravity core from Lake Telmen (Central Mongolia) that covers 4,110&amp;#160;&lt;sup&gt;+350&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;340 &lt;/sub&gt;cal.&amp;#160;a BP.&lt;/p&gt;&lt;p&gt;Low terrestrial input (e.g. low Al, Fe, K, Sr) suggests decreased runoff and points to overall dryer conditions in the area around Lake Telmen between 4,110&amp;#160;&lt;sup&gt;+350&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;340&lt;/sub&gt;&amp;#160;and 3,040&amp;#160;&lt;sup&gt;+610&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;400&lt;/sub&gt;&amp;#160;cal. a&amp;#160;BP. Those findings are in line with positive &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;n&lt;/sub&gt;&lt;sub&gt;-C23&lt;/sub&gt;, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;carb&lt;/sub&gt;, which indicate enhanced lake water evaporation. From 3,040&amp;#160;&lt;sup&gt;+610&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;400&lt;/sub&gt;&amp;#160;to 1,360&amp;#160;&lt;sup&gt;+230&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;220&lt;/sub&gt;&amp;#160;cal.&amp;#160;a BP, high terrestrial input and more negative &amp;#948;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;n&lt;/sub&gt;&lt;sub&gt;-C23&lt;/sub&gt;, &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;carb&lt;/sub&gt; values indicating more humid conditions. This is in line with seismic results which reveal distinct subaqueous cliffs and an extreme lake level rise beginning at ~ 2,000 cal. a BP. Drier conditions and low lake levels occurred between 1,360&amp;#160;&lt;sup&gt;+230&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;220&lt;/sub&gt;&amp;#160;and 700&amp;#160;&lt;sup&gt;+210&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;180&lt;/sub&gt; cal. a&amp;#160;BP and are indicated by low Ca/Mg ratios and a distinct enrichment in &lt;sup&gt;13&lt;/sup&gt;C&lt;sub&gt;wax, &lt;/sub&gt;&lt;sup&gt;2&lt;/sup&gt;H&lt;sub&gt;wax,&lt;/sub&gt;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;sugar&lt;/sub&gt; and &lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;carb&lt;/sub&gt;. From 700&amp;#160;&lt;sup&gt;+210&lt;/sup&gt;/&lt;sub&gt;&amp;#8209;180&lt;/sub&gt;&amp;#160;cal. a&amp;#160;BP onwards, drier conditions continue but the terrestrial input increases possibly reflecting anthropogenic impact.&lt;/p&gt;