Benoît Pasquier, Mark Holzer, Matthew A. Chamberlain, Richard J. Matear, Nathaniel L. Bindoff
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We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper-ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming-driven declines in solubility account for less than 10% of the total oxygen loss. 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引用次数: 0
摘要
海洋脱氧是气候变化的一个重要后果,对海洋生物和全球粮食安全构成了迫在眉睫的威胁。然而,我们对推动全球范围脱氧的环流、溶解度和呼吸作用变化之间复杂的相互作用的理解并不全面。在这里,我们考虑了理想化的生物地球化学稳态,这种稳态是在 2090 年代的气候模型模拟中构建的,并在时间上保持不变,其平衡状态是海洋温度永远较慢且较高。与对本世纪末瞬态的模拟不同,我们的理想化状态是深海严重脱氧,这与通风量永久减少和南极底层水形成节流是一致的。我们利用新颖的上游暴露时间概念,将脱氧驱动因素对预制氧和真实氧利用率(TOU)的影响区分开来,该概念将 TOU 与氧利用率和预制氧与通气量精确地联系在一起。在我们理想的稳定状态下,水深 2000 米以下的脱氧是由于 TOU 的增加,主要是由于较慢的环流使呼吸作用时间延长了大约 2-3 倍,从而压倒了呼吸速率降低的影响。在水深 500 米以上,呼吸作用减弱和环流速度减慢密切相关,导致上层海洋缺氧几乎没有扩大。大部分预形成的氧气损失是由于通气向赤道移动,使温暖的表层水含氧量减少。变暖导致的溶解度下降只占总氧气损失的不到 10%。我们的分析虽然是理想化的,但表明海洋氧气循环的长期变化可能主要是由环流变化而不是热力学或生物学驱动的。
Deoxygenation and Its Drivers Analyzed in Steady State for Perpetually Slower and Warmer Oceans
Ocean deoxygenation is an important consequence of climate change that poses an imminent threat to marine life and global food security. However, our understanding of the complex interactions between changes in circulation, solubility, and respiration that drive global-scale deoxygenation is incomplete. Here, we consider idealized biogeochemical steady states in equilibrium with perpetually slower and warmer oceans constructed from climate-model simulations of the 2090s that we hold constant in time. In contrast to simulations of the end-of-century transient state, our idealized states are intensely deoxygenated in the abyss, consistent with perpetually reduced ventilation and throttled Antarctic Bottom Water formation. We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper-ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming-driven declines in solubility account for less than 10% of the total oxygen loss. Although idealized, our analysis suggests that long-term changes in the marine oxygen cycle could be driven dominantly by changes in circulation rather than by thermodynamics or biology.