Modulated orbital cycles in planktonic foraminifera &#948;18O
Authors: Bakker Pepijn , de Haas Auk , Metcalfe Brett , Roche Didier 
Published: March 2021

&lt;p&gt;&lt;span&gt;Measurements of &lt;/span&gt;&lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O on planktonic foraminifera represent an important geological archive. In the study of climate change on orbital time-scales, &lt;/span&gt;&lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O is used &lt;/span&gt;&lt;span&gt;either &lt;/span&gt;&lt;span&gt;as a proxy for temperature change, to construct age models through pattern matching, to deduce the drivers of climate change by looking at frequency power spectra &lt;/span&gt;&lt;span&gt;or all of the above&lt;/span&gt;&lt;span&gt;. The validity of these approaches hinges on a two-step assumption, namely that the signal of the orbital cycles is transferred unaltered from 1) &lt;/span&gt;&lt;span&gt;solar radiation that reaches&lt;/span&gt;&lt;span&gt; the top &lt;/span&gt;&lt;span&gt;of the atmosphere, insolation,&lt;/span&gt;&lt;span&gt; to the sea surface, and 2) from the sea surface &lt;/span&gt;&lt;span&gt;translated&lt;/span&gt;&lt;span&gt; into the &lt;/span&gt;&lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O &lt;/span&gt;&lt;span&gt;composition &lt;/span&gt;&lt;span&gt;of the foraminiferal shell. The complexity of these two steps make it difficult to validate the assumptions behind these major paleoclimatological approaches. In this research we &lt;/span&gt;&lt;span&gt;aim to&lt;/span&gt;&lt;span&gt; disentangle the problem, &lt;/span&gt;&lt;span&gt;here&lt;/span&gt;&lt;span&gt; focus&lt;/span&gt;&lt;span&gt;sing&lt;/span&gt;&lt;span&gt; only on the latter part: how do species-specific living habitats of foraminifera in the water column and throughout the year shape the &lt;/span&gt;&lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O response &lt;/span&gt;&lt;span&gt;were the Earth&amp;#8217;s surfce climate to&lt;/span&gt;&lt;span&gt; perfectly reflect the orbital forcing&lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;. To this end we combine an isotope-enabled climate model &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;&lt;em&gt;i&lt;/em&gt;&lt;/span&gt;&lt;span&gt;LOVECLIM)&lt;/span&gt;&lt;span&gt; with a foraminifera &lt;/span&gt;&lt;span&gt;growth &lt;/span&gt;&lt;span&gt;model and investigate the &lt;/span&gt;&lt;span&gt;response of&lt;/span&gt; &lt;span&gt;&amp;#948;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;18&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;O &lt;/span&gt;&lt;span&gt;from three species of foraminifera (&lt;/span&gt;&lt;span&gt;&lt;em&gt;Globigerinoides ruber&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, &lt;/span&gt;&lt;span&gt;&lt;em&gt;Neogloboquadrina pachyderma&lt;/em&gt;&lt;/span&gt;&lt;span&gt;, and &lt;/span&gt;&lt;span&gt;&lt;em&gt;Globigerina bulloides&lt;/em&gt;&lt;/span&gt;&lt;span&gt;) t&lt;/span&gt;&lt;span&gt;o obliquity and precessional cycles.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;Our results show that the planktonic foraminifera &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O response to astronomical forcings is dominated by annual mean changes in sea-surface temperature, thus corroborating a key assumption underlying many geological climate reconstructions. However, in various places changes in the depth habitat, temperature-dependent growth rates, seasonality and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of the sea water dominate the foraminifera &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O signal. Because of these modulations, planktonic foraminifera &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O time series can have very different characteristics compared to the orbital forcings, including limited spatial coherence as well as limited inter-species coherence at a single location.&lt;/p&gt;