Onset of Oceanic Anoxic Event 2 in the Lower Saxony Basin &#8211; Insights fromhigh-resolution stable isotope stratigraphy and geochemical modelling
Authors: Müller Pia , Heimhofer Ulrich , Ostertag-Henning Christian 
Published: March 2021

&lt;p&gt;The Oceanic Anoxic Event (OAE) 2 spanning the Cenomanian-Turonian boundary (93.5 Ma)&lt;br&gt;represents a major perturbation of the global carbon cycle and is marked by organic-rich&lt;br&gt;sediments deposited under oxygen-depleted conditions. In many studies the eruption of the&lt;br&gt;Caribbean LIP is considered to be the cause for rapidly increasing CO2 concentrations and&lt;br&gt;resulting global warming accompanied by widespread oceanic anoxia. In the Lower Saxony&lt;br&gt;Basin of northern Germany, the deposits of the OAE 2 are exposed in several industry drill&lt;br&gt;cores. In this study, the lower part of the OAE 2 has been studied in the HOLCIM 2011-3 drill&lt;br&gt;core. Sedimentary rocks are composed of limestones, marly limestones, marls and black&lt;br&gt;shales and have been analysed with a high-resolution stable isotope approach&lt;br&gt;(approximately one sample every 2 cm) combined with geochemical modelling. Using stable&lt;br&gt;carbon isotopes, bulk rock parameters and petrographic analysis, the onset of OAE 2 has&lt;br&gt;been investigated in detail. The high-resolution &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C curve exhibits overall stable values&lt;br&gt;around 3 &amp;#8240; before the onset of the Plenus event. This background level is interrupted by&lt;br&gt;three short-lived and small but significant negative carbon isotope excursions (CIEs) down to&lt;br&gt;&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values of 2.5 &amp;#8240;, 2.7 &amp;#8240; and 1.9 &amp;#8240;. Immediately before the main rise in the Plenus bed,&lt;br&gt;a longer-lasting negative CIE down to 2.8 &amp;#8240; is observed, preceding the large positive CIE of&lt;br&gt;the OAE 2 to values of 5.2 &amp;#8240; over 33 ka. Thereafter, the &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values decrease to 3.5 &amp;#8240; over&lt;br&gt;a period of approximately 130 ka. The results can be correlated with the lower-resolution&lt;br&gt;data set of Voigt et al. (2008) but enable a more accurate characterization of the subtle&lt;br&gt;features of the CIE and hence events before and during this time interval. Carbon cycle&lt;br&gt;modelling with the modelling software SIMILE using a model based on Kump &amp; Arthur (1999)&lt;br&gt;reveals that the negative excursion before the Plenus bed can be explained by a massive&lt;br&gt;volcanic pulse releasing of 0.95*10&lt;sup&gt;18&lt;/sup&gt; mol CO2 within 14 ka. This amount corresponds to only&lt;br&gt;81 % of the calculated volume of CO&lt;sub&gt;2&lt;/sub&gt; release during emplacement of the Caribbean LIP by&lt;br&gt;Joo et al. (2020). In the model the volcanic exhalation increases atmospheric CO&lt;sub&gt;2&lt;/sub&gt;&lt;br&gt;concentrations. This will increase global temperatures, intensify the hydrological cycle and&lt;br&gt;thus increase nutrient input into the ocean, resulting in an expansion of the oxygen minimum&lt;br&gt;zone, the development of anoxic conditions and an increase in the preservation potential for&lt;br&gt;organic material. In the model enhanced primary productivity and organic matter preservation&lt;br&gt;can be controlled by the implemented riverine phosphate input and the preservation factor for&lt;br&gt;organic matter. For the positive anomaly, the riverine phosphate input must be nearly&lt;br&gt;doubled (from 0.01 &amp;#956;mol/kg PO&lt;sub&gt;4 &lt;/sub&gt;to 0.019 &amp;#956;mol/kg) for the period of the increasing &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;br&gt;values (app. 33 ka), with a concomitant rise of the preservation factor from 1 % to 2 %. This&lt;br&gt;model scenario accurately reproduces the major features of the new high-resolution &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C&lt;br&gt;record over the onset of the OAE 2 CIE.&lt;/p&gt;