The ice core record of atmospheric CO2 variability during the Last Glacial Period: new insights from timing and isotopes
Authors: Bauska Thomas , Marcott Shaun , Brook Ed 
Published: March 2020

&lt;p&gt;Atmospheric carbon dioxide (CO&lt;sub&gt;2&lt;/sub&gt;) concentrations during the last glacial period (70,000 &amp;#8211; 23,000 years ago) fluctuated on millennial timescales closely following variations in Antarctic temperature. This close coupling has suggested that the sources and sinks driving millennial scale CO&lt;sub&gt;2&lt;/sub&gt; changes are dominated by processes in the Southern Ocean. However, recent work revealed centennial-scale increases in CO&lt;sub&gt;2&lt;/sub&gt; during abrupt climate events of the last deglaciation which may represent a second mechanism of carbon cycle variability.&amp;#160;&lt;/p&gt;&lt;p&gt;Here we analyze a high resolution CO&lt;sub&gt;2&lt;/sub&gt; record from the last glacial period from the West Antarctic Ice Sheet (WAIS Divide) that precisely defines the timing of CO&lt;sub&gt;2&lt;/sub&gt; changes with respect to Antarctic ice core proxies for temperature, dust delivery, and sea-ice extent down to the centennial-timescale. Although CO&lt;sub&gt;2&lt;/sub&gt; closely tracks all these proxies over millennia, peak CO&lt;sub&gt;2&lt;/sub&gt; levels most often lag behind all proxies by a few hundred years. This decoupling from Antarctic climate variability is most prominent during the onset of DO interstadial events when CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt; and Greenland temperature all increase simultaneously. Regression analysis suggests that the CO&lt;sub&gt;2&lt;/sub&gt; variations can be explained by a combination of two mechanisms: one operating on the time scale of Antarctic climate variability, and a second responding on the Dansgaard-Oeschger time scale.&amp;#160;&amp;#160;&lt;/p&gt;&lt;p&gt;Recent &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; data from the last glacial period support our finding that CO&lt;sub&gt;2&lt;/sub&gt; variability is the sum of multiple mechanisms.&amp;#160; The Antarctic climate variability is likely associated with the release of respired organic carbon from the deep ocean.&amp;#160; Superimposed on these oscillations are two types of centennial-scale changes: CO&lt;sub&gt;2&lt;/sub&gt; increases and &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; minima in the middle of Heinrich stadials and ii) CO&lt;sub&gt;2&lt;/sub&gt; increases and small changes in &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2 &lt;/sub&gt;that at the onset of DO interstadial event.&lt;/p&gt;&lt;p&gt;To provide a comprehensive and quantitative constraint on the mechanisms of CO&lt;sub&gt;2&lt;/sub&gt; variability during the last glacial period, we run a large suite of transient box model experiments (n = 500) forced with varying combinations of forcings based on proxy time-series (e.g. AABW formation, NADW formation, ocean temperature, dust delivery, and sea-ice extent).&amp;#160; Using data constraints from the ice core records of CO&lt;sub&gt;2&lt;/sub&gt;, &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; and mean ocean temperature, we arrive at an ensemble of scenarios that can explain a large amount of the centennial and millennial-scale variability observed in the ice core record. Parsing this into a series of factorial experiments we find that Southern Hemisphere processes can explain 80% of the observed variability and Northern Hemisphere processes account for the remaining 20%.&amp;#160; A further breakdown on the level of individual mechanisms is marred by the high degree of correlation between carbon cycle forcings likely operating in the Southern Hemisphere.&amp;#160; None-the-less, our results highlight how multiple mechanisms operating over multiple timescales may have interacted during the last glacial period to drive changes in atmospheric CO&lt;sub&gt;2&lt;/sub&gt;.&amp;#160;&lt;/p&gt;