Intermediate-deep layers are well-ventilated in most modern oceans (Figure 1a), typically in the North Atlantic and Southern Oceans; this is primarily related to deep-water formation (Marshall & Speer, 2012; Weaver et al., 1999). This process plays a fundamental role in regulating global climate by affecting heat, gas exchange and nutrient cycles (Burls et al., 2017). In contrast, no new deep-water mass are formed in the modern North Pacific due to low-surface salinities and the existence of a strong halocline (Emile-Geay
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