Triple oxygen isotope fractionation of carbonate during carbonate precipitation and acid digestion
Authors: Roy Pallab , Laskar Amzad , Liang Mao-Chang 
Published: March 2022

&lt;p&gt;Stable oxygen isotopic composition (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) of CO&lt;sub&gt;2&lt;/sub&gt; produced from carbonates in natural archives is a useful proxy for paleo precipitation and paleo temperature reconstruction. However, there exist multiple factors controlling the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values, the applications of the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O alone for paleoclimate studies are thus limited. Anomaly in &lt;sup&gt;17&lt;/sup&gt;O in carbonates, expressed by &amp;#916;&amp;#8242;&lt;sup&gt;17&lt;/sup&gt;O=1000*ln(&amp;#948;&lt;sup&gt;17&lt;/sup&gt;O/1000+1)-&amp;#955;*1000*ln(&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O/1000+1) is another proxy to independently constrain aspects of climatic variables such as precipitation source variation and kinetic effects during carbonate precipitation. However, to use &lt;sup&gt;17&lt;/sup&gt;O anomaly for such studies, the triple oxygen isotope fractionation exponent (&amp;#952;= ln&amp;#945;&lt;sup&gt;17&lt;/sup&gt;/ln&amp;#945;&lt;sup&gt;18&lt;/sup&gt;) must be known precisely. Knowledge of this parameter is central to emerging applications of carbonate triple oxygen isotopes to paleoclimate and paleo-hydrology studies. Though a number of theoretical and experimental studies have been carried out in the last few years, there remains no consensus on &amp;#120563; value for carbonate-water system, likely due to kinetic isotope fractionation during precipitation.&lt;/p&gt;&lt;p&gt;Here, we measured &amp;#916;&amp;#8242;&lt;sup&gt;17&lt;/sup&gt;O in synthetic carbonates as well as in the water from which the carbonates are precipitated to check how reliable the &amp;#916;&amp;#8242;&lt;sup&gt;17&lt;/sup&gt;O value of the parent water can be reconstructed from the carbonates or carbonate-digested CO&lt;sub&gt;2&lt;/sub&gt;. To determine &amp;#952;&lt;sub&gt;carbonate_CO2-water &lt;/sub&gt;for precipitated carbonates, we synthesized carbonates in the laboratory at temperatures ranging from 10 &amp;#11824;C to 66 &amp;#11824;C using passive/active CO&lt;sub&gt;2&lt;/sub&gt; degassing method. Triple oxygen isotope compositions of the water were determined using water-CO&lt;sub&gt;2&lt;/sub&gt; equilibration followed by CO&lt;sub&gt;2&lt;/sub&gt;-O&lt;sub&gt;2&lt;/sub&gt; exchange method and of the carbonate (CO&lt;sub&gt;2&lt;/sub&gt; liberated by acid digestion) using CO&lt;sub&gt;2&lt;/sub&gt;-O&lt;sub&gt;2&lt;/sub&gt; exchange method. We analyzed our isotope data for their possible kinetic isotope effect and determined the &amp;#120563;&lt;sub&gt;carbonate_CO2-water&lt;/sub&gt; value for precipitated carbonates. We find that most of our synthetic carbonate samples did not attain the equilibrium. The &amp;#120563;&lt;sub&gt;carbonate_CO2-water &lt;/sub&gt;increases as the disequilibrium effect increases. We determined the &amp;#952;&lt;sub&gt;carbonate_CO2-water &lt;/sub&gt;from the samples precipitating in equilibrium. Furthermore, we do not find any differences in the &amp;#120563;&lt;sub&gt;carbonate_CO2-water &lt;/sub&gt;value for carbonate precipitated in equilibrium at 25 &amp;#11824;C and 35 &amp;#11824;C. An important issue of using &amp;#916;&amp;#8242;&lt;sup&gt;17&lt;/sup&gt;O in carbonates is to resolve the &amp;#120563;&lt;sub&gt;acid&lt;/sub&gt; for acid digestion which is resolved in the present study. Additionally, we determined the temperature dependent variation in &amp;#120563;&lt;sub&gt;acid&lt;/sub&gt; and find no significant changes between 0 &amp;#11824;C and 70 &amp;#11824;C.&lt;/p&gt;