Temporal and spatial variability of isotopic and hydrochemical parameters in cave drip-water feeding stalagmites: a case study from SE Slovenia
Authors: Lojen Sonja , Zuliani Tea , Nagode Klara , Vreča Polona , Kanduč Tjaša , Tičar Jure , Zorn Matija , Lipar Matej 
Published: March 2022

&lt;p&gt;The accuracy and uncertainty of paleoclimate interpretations of geochemical and isotopic proxies from stalagmites depend critically on how accurately isotopic signals are transmitted through the vadose zone of the aquifer and how the elemental composition of the groundwater feeding the stalagmite changes as it passes through the aquifer.&lt;/p&gt;&lt;p&gt;Results of the first year (2021) of drip-water monitoring at 15 drip sites in the Jama v Dov&amp;#269;ku Cave in SE Slovenia are presented. Cumulative monthly samples were analysed for &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O of water, &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C of dissolved inorganic carbon (DIC), elemental composition of drip-water and concentration and &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C of CO&lt;sub&gt;2&lt;/sub&gt; in the cave atmosphere. While the seasonal variability of &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O of precipitation in 2021 at the nearest meteorological station exceeded 10 &amp;#8240; (from &amp;#8211;15.42 &amp;#8240; in January to &amp;#8211;5.28 in June), the intra-annual variability of &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O of drip-water was reduced to 0.18&amp;#8211;1.28 &amp;#8240; and showed no correlation with the thickness of the roof, which varied between &gt;1 and 49 m. A discernible annual cyclicity was observed at some drip sites, with the highest &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;18&lt;/sup&gt;O values determined in winter and the lowest in late summer. Dissolved inorganic carbon (DIC) &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values exhibited a wide range (between &amp;#8211;15.5 and &amp;#8211;5.0 &amp;#8240;), and drip sites could be divided into two groups: some drip sites exhibited large seasonal variability (up to 9.9 %) with low values in the warmer season, while the others varied within &lt;3 &amp;#8240; with no apparent seasonality. The CO&lt;sub&gt;2 &lt;/sub&gt;concentration in the cave atmosphere was significantly higher from May to October, about 4000 to 8000 ppm, while it fluctuated between 700 and 1500 ppm in the colder part of the year. The &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values of CO&lt;sub&gt;2&lt;/sub&gt; varied between &amp;#8211;23.8 and &amp;#8211;15.2 &amp;#8240; and decreased exponentially with CO&lt;sub&gt;2&lt;/sub&gt; concentration. Similar to CO&lt;sub&gt;2&lt;/sub&gt;, the &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values of DIC also decreased exponentially with increasing DIC concentration. The &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C value of &amp;#8220;added&amp;#8221; CO&lt;sub&gt;2&lt;/sub&gt; in the atmosphere obtained from the &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C x (C/C&lt;sub&gt;0&lt;/sub&gt;-1) &lt;em&gt;vs.&lt;/em&gt; (C/C&lt;sub&gt;0&lt;/sub&gt;-1) plot (Sayles &amp;% Curry, 1988) was &amp;#8211;23.4 &amp;#177; 2.6 &amp;#8240;, which is almost identical to the CO&lt;sub&gt;2&lt;/sub&gt; added to the drip-water estimated from DIC concentration and &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values of DIC (&amp;#8211;23.9 &amp;#177; 3.4 &amp;#8240;, considering the isotopic fractionation factor between CO&lt;sub&gt;2&lt;/sub&gt;(g) and HCO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; from Mook et al. 1974). Chemical analysis showed that drip sites with a large seasonality of &lt;em&gt;&amp;#948;&lt;/em&gt;&lt;sup&gt;13&lt;/sup&gt;C values have significantly lower saturation indices with respect to calcite in the warmer part of the year and that earlier calcite precipitation is most likely to occur at drip sites with lower drip rates.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Reference:&lt;/p&gt;&lt;p&gt;Mook et al., 1974, Earth Planet. Sci. Lett. 22,169&amp;#8211;186.&lt;/p&gt;&lt;p&gt;Sayles &amp; Curry, 1988, Geochim. Cosmochim. Acta 52, 2963&amp;#8211;2978.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;ACKNOWLEDGEMENT: We acknowledge the financial support of Slovenian Research Agency (J1-2478).&lt;/p&gt;