Editing IPCC:AR6/WGI/Chapter-5 (section)

==== 5.3.1.2 Last Deglacial Transition ====

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The Last Deglacial Transition (LDT) is the best documented climatic transition in the past associated with a substantial atmospheric CO <sub>2</sub> rise ranging from 190 to 265 ppm between 18–11 ka ( [[#Marcott--2014|Marcott et al., 2014]] ). The amplitude of the deglacial CO <sub>2</sub> rise is thus on the same order of magnitude as the increase since the Industrial Revolution.

Boron isotope ( δ <sup>11</sup> B) data suggest a 0.15–0.05 unit decrease in sea surface pH ( [[#Hönisch--2005|Hönisch and Hemming, 2005]] ; [[#Henehan--2013|Henehan et al., 2013]] ) across the LDT, an average rate of decline of about 0.002 units per century compared with the current rate of more than 0.1 units per century ( [[#Bopp--2013|Bopp et al., 2013]] ; [[#Gattuso--2015|Gattuso et al., 2015]] ). Planktonic foraminiferal shell weights decreased by 40% to 50% ( [[#Barker--2002|Barker and Elderfield, 2002]] ), and coccolith mass decreased by about 25% ( [[#Beaufort--2011|Beaufort et al., 2011]] ) across the LDT. Independent proxy reconstructions thus highlight with ''high confidence'' that pH values decreased as atmospheric CO <sub>2</sub> concentrations increased across the LDT. There is, however, ''low confidence'' in the inferred rate of ocean acidification owing to multiple sources of uncertainties affecting rates estimates based on marine sediments ( [[#5.1.2.1|Section 5.1.2.1]] ).

Geochemical and micropaleontological evidence suggest that intermediate-depth OMZs almost vanished during the Last Glacial Maximum (LGM) ( [[#Jaccard--2014|Jaccard et al., 2014]] ). However, multiple lines of evidence suggest with ''medium confidence'' that the deep (&gt;1500 m) ocean became depleted in O <sub>2</sub> (concentrations were possibly lower than 50 μmol kg <sup>–1</sup> ) globally (Jaccard and Galbraith, 2012; [[#Hoogakker--2015|Hoogakker et al., 2015]] , 2018; [[#Gottschalk--2016|Gottschalk et al., 2016]] , 2020a; [[#Anderson--2019|Anderson et al., 2019]] ) as a combined result of sluggish ventilation of the ocean subsurface ( [[#Gottschalk--2016|Gottschalk et al., 2016]] , 2020a; [[#Skinner--2017|Skinner et al., 2017]] ) and a generally more efficient marine biological carbon pump ( [[#Buchanan--2016|Buchanan et al., 2016]] ; [[#Yamamoto--2019|Yamamoto et al., 2019]] ; [[#Galbraith--2020|Galbraith and Skinner, 2020]] ).

During the LDT, deep ocean ventilation increased as Antarctic Bottom Water (AABW) ( [[#Skinner--2010|Skinner et al., 2010]] ; [[#Gottschalk--2016|Gottschalk et al., 2016]] ; [[#Jaccard--2016|Jaccard et al., 2016]] ) and subsequently the Atlantic meridional overturning circulation ( [[#McManus--2004|McManus et al., 2004]] ; [[#Lippold--2016|Lippold et al., 2016]] ) resumed, transferring previously sequestered remineralized carbon from the ocean interior to the upper ocean, and eventually the atmosphere ( [[#Skinner--2010|Skinner et al., 2010]] ; [[#Galbraith--2015|Galbraith and Jaccard, 2015]] ; [[#Gottschalk--2016|Gottschalk et al., 2016]] ; [[#Ronge--2016|Ronge et al., 2016]] , 2020; [[#Sikes--2016|Sikes et al., 2016]] ; [[#Rae--2018|Rae et al., 2018]] ), contributing to the deglacial CO <sub>2</sub> rise. Intermediate depths lost oxygen as a result of sluggish ventilation and increasing temperatures (decreasing saturation). As the world emerged from the last Glacial period, OMZs underwent a large volumetric increase at the beginning of the Bølling-Allerød (B/A), a northern-hemisphere wide warming event, 14.7 ka ( [[#Jaccard--2012|Jaccard and Galbraith, 2012]] ; [[#Praetorius--2015|Praetorius et al., 2015]] ) with deleterious consequences for benthic ecosystems (e.g., [[#Moffitt--2015|Moffitt et al., 2015]] ). These observations indicate with ''high confidence'' that the rate of warming, affecting the solubility of oxygen and upper water column stratification, coupled with changes in subsurface ocean ventilation, impose a direct control on the degree of ocean deoxygenation, implying a high sensitivity of ocean oxygen loss to warming. The expansion of OMZs contributed to a widespread increase in water column (de)nitrification (Galbraith and Kienast, 2013), which contributed substantially to enhanced marine N <sub>2</sub> O emissions. Nitrogen stable isotope measurements on N <sub>2</sub> O extracted from ice cores suggest that approximately one-third (of the order of 0.7 ± 0.3TgN yr <sup>–1</sup> ) of thedeglacial increase in N <sub>2</sub> O emissions relates to oceanic sources (Schilt et al., 2014; H. [[#Fischer--2019|]] [[#Fischer--2019|Fischer et al., 2019]] ).

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