Size and abundance variations of Schizosphaerella across the Toarcian Oceanic Anoxic Event&#160;
Authors: Faucher Giulia , Visentin Stefano , Gambacorta Gabriele , Erba Elisabetta 
Published: March 2022

&lt;p&gt;The Toarcian oceanic anoxic event (T-OAE), dated as early Toarcian is considered one of the most extreme paleoenvironmental perturbations in Earth&amp;#8217;s history. It is characterized by global warming, accelerated weathering, sea level rise, oceanic anoxia and acidification and extensive accumulation of organic matter. In Jurassic times, calcareous nannoplankton was already a most efficient rock-forming group and therefore pelagic sedimentary successions preserve invaluable data to track changes across the T-OAE. In this work, we focus on &lt;em&gt;Schizosphaerella&lt;/em&gt; across the T-OAE recovered in the uppermost Pliensbachian&amp;#8211;lower Toarcian Sogno Core that consists of a fully pelagic, continuous, well-dated record from a deep plateau (~1500 m water depth) in the Lombardy Basin (northern Italy). The objective of this investigation is the quantification of changes in size and abundance of the micrite-forming schizosphaerellids to derive their biocalcification tempo and mode in response to the T-OAE perturbations, to assess the implications of &lt;em&gt;Schizosphaerella&lt;/em&gt; biocalcification changes, in terms of abundance and size, for the pelagic carbonate sedimentation. Absolute abundances and morphometric changes obtained for &lt;em&gt;&amp;#8220;&lt;/em&gt;small&lt;em&gt; Schizosphaerella punctulata&amp;#8221; &lt;/em&gt;(valve width &lt; 7 &amp;#956;m), &amp;#160;&lt;em&gt;S. punctulata &lt;/em&gt;(valve width &gt; 7 &amp;#956;m) and &amp;#8220;encrusted &lt;em&gt;S. punctulata&amp;#8221;&lt;/em&gt; (all specimens characterized by a crust surrounding the valve) revealed large fluctuations in the investigated interval. We identify an abundance fall caused by the failure of &lt;em&gt;S. punctulata&lt;/em&gt; and &amp;#8220;encrusted &lt;em&gt;S. punctulata&lt;/em&gt;&amp;#8221; during the core of the T-OAE, that along with the increased abundance of small specimens produced the reduction of average dimensions. Thus, the average size decline is not the result of a general valve reduction, but rather derives from the increase in abundance of small specimens (&lt; 7 &amp;#956;m). This is substantiated by absolute abundances of individual &lt;em&gt;S. punctulata&lt;/em&gt; morphogroups that unambiguously demonstrate that such a pattern is not an artefact of relative abundances (closed sum problem).&lt;/p&gt;&lt;p&gt;We hypothesize that the concomitant drop in abundance and shrinkage of valve size is related to hyperthermal conditions associated with excess CO&lt;sub&gt;2&lt;/sub&gt; and ocean acidification.&lt;/p&gt;&lt;p&gt;Finally, the co-occurrence in the same samples of &lt;em&gt;S. punctulata&lt;/em&gt; specimens (&gt; 7 mm) with and without a crust, is indicative of species-specific diagenetic effects. Based on the &lt;em&gt;S. punctulata&lt;/em&gt; ultrastructure we conclude that specimens without diagenetic crusts belongs to &lt;em&gt;S. astrea &lt;/em&gt;while encrusted specimens&lt;em&gt; &lt;/em&gt;are attributable to&lt;em&gt; S. punctulata &lt;/em&gt;and we&lt;em&gt; &lt;/em&gt;infer that the presence of the diagenetic crust could be taxonomically diagnostic to distinguish &lt;em&gt;S. punctulata&lt;/em&gt; from &lt;em&gt;S. astrea.&lt;/em&gt;&lt;/p&gt;