&#948;11B and B/Ca ontogenetic variability within Globigerina bulloides
2020-03-10

&amp;lt;p&amp;gt;Understanding the atmosphere-continent-ocean carbon cycle and its associated oceanic carbon system is one of the keystones to face the Anthropocene&amp;amp;#8217;s climate change. Since the 1990s the isotopic ratio of boron (&amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B) in calcitic shells of planktonic foraminifera has proven to be a powerful geochemical proxy to determine the oceanic paleo-pH and its link to atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; level over geological times&amp;lt;sup&amp;gt;1&amp;lt;/sup&amp;gt;, whereas the ratio B/Ca as proxy of the seawater carbonate chemistry is still questionable&amp;lt;sup&amp;gt;2,3&amp;lt;/sup&amp;gt;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;However, the use of planktonic foraminifera in paleoclimatic reconstructions requires calibrations of the pH &amp;amp;#8211; &amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B relationships to correct what is known as &amp;amp;#171;&amp;amp;#160;vital effect&amp;amp;#160;&amp;amp;#187;&amp;lt;sup&amp;gt;4&amp;lt;/sup&amp;gt;: each species controls differently its calcification process and consequently slightly modifies the seawater chemistry during biomineralization&amp;lt;sup&amp;gt;5,6&amp;lt;/sup&amp;gt;. Moreover, shell size effect on &amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B has been reported for some symbiont-bearing species due to photosynthetic increase of pH&amp;lt;sup&amp;gt;7,8&amp;lt;/sup&amp;gt;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Calibrations for the symbiont-barren &amp;lt;em&amp;gt;Globigerina bulloides&amp;lt;/em&amp;gt; have been already determined&amp;lt;sup&amp;gt;9,10 &amp;lt;/sup&amp;gt;but sparse data have been reported so far for the test size effect on &amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B &amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Here we measured the &amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B of three different fractions (250-315, 315-400 and &amp;gt;400 &amp;amp;#956;m) of &amp;lt;em&amp;gt;G. bulloides&amp;lt;/em&amp;gt; sampled along the coretop PS97-122 from the Chilean margin (54.10&amp;amp;#176;S, 74.91&amp;amp;#176;W), by using a new protocol developed at IPGP and dedicated to small samples which couple a microsublimation technique and a micro-direct injection device (&amp;amp;#956;-dDIHEN&amp;lt;sup&amp;gt;12&amp;lt;/sup&amp;gt;). Our preliminary results show significantly higher &amp;amp;#948;&amp;lt;sup&amp;gt;11&amp;lt;/sup&amp;gt;B values for the large fractions compared to the small ones, as found for symbiont-bearing planktonic species such as &amp;lt;em&amp;gt;Globigerinoides sacculifer&amp;lt;/em&amp;gt;&amp;lt;sup&amp;gt;7&amp;lt;/sup&amp;gt; and &amp;lt;em&amp;gt;Globigerinoides ruber&amp;lt;/em&amp;gt;&amp;lt;sup&amp;gt;8&amp;lt;/sup&amp;gt;.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;ul&amp;gt;&amp;lt;li&amp;gt;(1) Pearson &amp;amp; Palmer, 2000, &amp;lt;em&amp;gt;Nature&amp;lt;/em&amp;gt; 406, 695-699&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(2) Yu et al., 2007, &amp;lt;em&amp;gt;Paleoceanography&amp;lt;/em&amp;gt; 22, PA2202&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(3) Allen et al., 2012, &amp;lt;em&amp;gt;EPSL&amp;lt;/em&amp;gt; 351-352, 270-280&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(4) Urey et al., 1951,&amp;lt;em&amp;gt; Soc. Am. Bull.&amp;lt;/em&amp;gt; 62, 399-416&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(5) Erez, 2003, &amp;lt;em&amp;gt;Rev. in Min. and Geochem.&amp;lt;/em&amp;gt; 54 (1), 115-149&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(6) de Nooijer et al., 2014, &amp;lt;em&amp;gt;Earth-Science Reviews&amp;lt;/em&amp;gt; 135, 48-58&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(7) H&amp;amp;#246;nisch &amp;amp; Hemming, 2004, &amp;lt;em&amp;gt;Paleoceanography&amp;lt;/em&amp;gt; 19, PA4010&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(8) Henehan et al., 2013, &amp;lt;em&amp;gt;EPSL&amp;lt;/em&amp;gt; 364, 111-122&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(9) Mart&amp;amp;#237;nez-Bot&amp;amp;#237;et al., 2015, &amp;lt;em&amp;gt;Nature&amp;lt;/em&amp;gt; 518, 219-222&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(10) Raitzsch et al., 2018, &amp;lt;em&amp;gt;EPSL&amp;lt;/em&amp;gt; 487, 138-150&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(11) Henehan et al., 2016, &amp;lt;em&amp;gt;EPSL&amp;lt;/em&amp;gt; 454, 282-292&amp;lt;/li&amp;gt; &amp;lt;li&amp;gt;(12) Louvat et al., 2019, &amp;lt;em&amp;gt;JAAS&amp;lt;/em&amp;gt; 8, 1553-1563&amp;lt;/li&amp;gt; &amp;lt;/ul&amp;gt;