Assessing time uncertainty and sediment mixing using three-dimensional high-resolution radiocarbon measurements from a marine boxcore
Authors: Zuhr Alexandra , Dolman Andrew , Groeneveld Jeroen , Ho Sze Ling , Grotheer Hendrik , Laepple Thomas 
Published: March 2021

&lt;p&gt;&lt;span&gt;Assessing&lt;/span&gt;&lt;span&gt; the effects of sediment inhomogeneity on &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;core&lt;/span&gt;&lt;span&gt; stratigraphy and on &lt;/span&gt;&lt;span&gt;proxy records is essential to perform reliable climate reconstructions from &lt;/span&gt;&lt;span&gt;marine &lt;/span&gt;&lt;span&gt;sediment&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Inhomogeneities can stem from s&lt;/span&gt;&lt;span&gt;ediment mixing &lt;/span&gt;&lt;span&gt;(e.g., bioturbation) &lt;/span&gt;&lt;span&gt;which &lt;/span&gt;&lt;span&gt;destroys the temporally layered climatic information stored in proxy carrier. &lt;/span&gt;&lt;span&gt;Thus&lt;/span&gt;&lt;span&gt;, in addition to the measurement error, the time-uncertainty in radiocarbon-dated sediments must be taken into account &lt;/span&gt;&lt;span&gt;for depth-age &lt;/span&gt;&lt;span&gt;modelling in order to obtain an unambiguous time scale. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;H&lt;span&gt;ere, we present a case-&lt;/span&gt;&lt;span&gt;study based on a boxc&lt;/span&gt;&lt;span&gt;or&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; (OR1-1218-C2-BC) &lt;/span&gt;&lt;span&gt;from the South China Sea &lt;/span&gt;&lt;span&gt;(2208 m water depth) &lt;/span&gt;&lt;span&gt;covering the last 20 k&lt;/span&gt;&lt;span&gt;yr&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;span&gt; The boxcore was divided into nine sub-cores by a grid of 3 x 3&lt;/span&gt; &lt;span&gt;(&lt;/span&gt;&lt;span&gt;each &lt;/span&gt;&lt;span&gt;sub-core is &lt;/span&gt;&lt;span&gt;8 x 8 cm &lt;/span&gt;&lt;span&gt;with a &lt;/span&gt;&lt;span&gt;length &lt;/span&gt;&lt;span&gt;of &lt;/span&gt;&lt;span&gt;34 cm), yielding a total surface area of &lt;/span&gt;&lt;span&gt;576&lt;/span&gt;&lt;span&gt; cm&lt;/span&gt;&lt;sup&gt;&lt;span&gt;2&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;. This sampling scheme offers the possibility for detailed, three-dimensional analyses on small spatial scales&lt;/span&gt;&lt;span&gt;. Radiocarbon measurements were performed in every sub-core for seven depth layers, each with a fraction of 200 crushed and well mixed foraminifera (&lt;/span&gt;&lt;span&gt;&lt;em&gt;Trilobatus sacculifer&lt;/em&gt;&lt;/span&gt;&lt;span&gt;,&lt;/span&gt;&lt;span&gt;&lt;em&gt; &lt;/em&gt;&lt;/span&gt;&lt;span&gt;250 &amp;#8211; 350 &amp;#181;m) to study the horizontal sediment heterogen&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;ity. In addition, small sample (5 specimen) replicate radiocarbon measurements from a single sediment sample allow to estimate the age heterogeneity &lt;/span&gt;&lt;span&gt;within a &lt;/span&gt;&lt;span&gt;1&lt;/span&gt;&lt;span&gt; cm sediment slice&lt;/span&gt;&lt;span&gt; and thus the vertical mixing from bioturbation. The replicate radiocarbon dates suggest a bioturbation depth of around 12 cm; &lt;/span&gt;&lt;span&gt;h&lt;/span&gt;&lt;span&gt;owever, the downcore radiocarbon dates show no clear sign of a well-mixed bioturbation layer. Using statistical analysis &lt;/span&gt;&lt;span&gt;(e.g., spatial correlation measures and &lt;/span&gt;&lt;span&gt;variance &lt;/span&gt;&lt;span&gt;analys&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt;s)&lt;/span&gt;&lt;span&gt;, we separate the errors from the radiocarbon measurements, the finite sample size and both the vertical and horizontal heter&lt;/span&gt;&lt;span&gt;o&lt;/span&gt;&lt;span&gt;geneity&lt;/span&gt;&lt;span&gt;. Comparing the radiocarbon dates in the sub-cores &lt;/span&gt;&lt;span&gt;indicates a small &lt;/span&gt;&lt;span&gt;horizontal heterogeneity compared to the vertical mixing. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;T&lt;/span&gt;&lt;span&gt;he thr&lt;/span&gt;&lt;span&gt;ee-dimensional&lt;/span&gt;&lt;span&gt; data &lt;/span&gt;&lt;span&gt;set &lt;/span&gt;&lt;span&gt;allows us to quantify the effect introduced by (post-depositional) sediment mixing on the age-estimate as well as on the proxy signal and to discuss the effects on low-sediment&lt;/span&gt;&lt;span&gt;at&lt;/span&gt;&lt;span&gt;ion climate records. &lt;/span&gt;&lt;span&gt;This will provide a better quantification of uncertainties with&lt;/span&gt;&lt;span&gt;in&lt;/span&gt;&lt;span&gt; proxy time series.&lt;/span&gt;&lt;/p&gt;