Extraterrestrial 3He shows that Mesozoic marl-limestone alternations are mainly driven by CaCO3 variations at the astronomical timescale
Authors: Blard Pierre-Henri , Suchéras-Marx Baptiste , Suan Guillaume , Mezine Taous 
Published: March 2022

&lt;p&gt;&lt;strong&gt;Introduction&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Marl-limestone alternations are well known rhythmical inter-bedded deposits that commonly occur in many hemipelagic to pelagic deposits of the Phanerozoic. It is quite well established that the origin of these lithological variations are astronomically-driven climatic variations (22, 41, 100 and 405 ka being the main periods) e.g. [1]. However, the exact sedimentological control is not clear: several models attribute these alternations to cyclic changes in the carbonate flux, whereas the terrigenous silicoclastic flux remained relatively constant. On the opposite, some models suggest that the carbonate flux was constant while the silicoclastic flux changed cyclically.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Material and methods&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;To disentangle these different scenarios, we collected marlstone and limestone samples from two sedimentary successions of Bajocian, Middle Jurassic&amp;#160;(3 marl-limestone- couplets over 3.5 m) and Valanginian, Lower Cretaceous (1 marl-limestone couplet over 1 m) age from the Southern French Alps (Barles). We measured their carbonate contents, the nannofossil proportion, as well as their extraterrestrial &lt;sup&gt;3&lt;/sup&gt;He (&lt;sup&gt;3&lt;/sup&gt;He&lt;sub&gt;ET&lt;/sub&gt;) concentrations in ~200&amp;#160;mg decarbonated aliquots.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Results and discussion&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;The carbonate content ranges from 45% in marls to 86% in limestones. Importantly, for all samples, measured&lt;sup&gt; 3&lt;/sup&gt;He&lt;sub&gt;ET &lt;/sub&gt;concentrations are constant in the silicoclastic fractions, within uncertainties. Hence, our results indicate that sedimentation rates at the astronomical timescale in the examined examples were mainly controlled by large changes in the CaCO&lt;sub&gt;3&lt;/sub&gt; net fluxes, leading to variable dilution of the terrigenous and &lt;sup&gt;3&lt;/sup&gt;He&lt;sub&gt;ET&lt;/sub&gt; fractions. Nannofossil counting shows that pelagic CaCO&lt;sub&gt;3&lt;/sub&gt; fluxes of coccolithophores are inversely correlated to the total CaCO&lt;sub&gt;3&lt;/sub&gt; along the marl-limestone alternations and represent less than 4% of the total carbonate content. Hence, in this setting, these marl-limestone alternations were more probably driven by variations in the CaCO&lt;sub&gt;3&lt;/sub&gt; supply from the nearby carbonate platform. Finally, assuming a constant &lt;sup&gt;3&lt;/sup&gt;He&lt;sub&gt;ET&lt;/sub&gt; flux of 0.1 pcc/cm&lt;sup&gt;2&lt;/sup&gt;/ka [2], and the whole thickness of Bajocian and Valanginian strata in this region, the measured &lt;sup&gt;3&lt;/sup&gt;He&lt;sub&gt;ET&lt;/sub&gt; concentrations imply sedimentation rates that are broadly compatible with current duration estimates of these two stages.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1]&amp;#160;Eldrett J. S. et al. (2015) &lt;em&gt;Earth. &lt;/em&gt;&lt;em&gt;Plan. Sci. Let., 423,&lt;/em&gt; 98-113.&lt;/p&gt;&lt;p&gt;[2]&amp;#160;Farley K.A. et al. (2012) &lt;em&gt;GCA&lt;/em&gt;, 84, 314-328.&lt;/p&gt;