Abstract
Abstract. During the Last Interglacial period (LIG), the transition from 125 to 115 ka provides a case study for assessing the response of the carbon system to different levels of high-latitude warmth. Elucidating the mechanisms responsible for interglacial changes in the ocean carbon inventory provides constraints on natural carbon sources and sinks and their climate sensitivity, which are essential for assessing potential future changes. However, the mechanisms leading to modifications of the ocean's carbon budget during this period remain poorly documented and not well understood. Using a state-of-the-art Earth system model, we analyze the changes in oceanic carbon dynamics by comparing two quasi-equilibrium states: the early, warm Eemian (125 ka) versus the cooler, late Eemian (115 ka). We find considerably reduced ocean dissolved inorganic carbon (DIC; −314.1 PgC) storage in the warm climate state at 125 ka as compared to 115 ka, mainly attributed to changes in the biological pump and ocean DIC disequilibrium components. The biological pump is mainly driven by changes in interior ocean ventilation timescales, but the processes controlling the changes in ocean DIC disequilibrium remain difficult to assess and seem more regionally affected. While the Atlantic bottom-water disequilibrium is affected by the organization of sea-ice-induced southern-sourced water (SSW) and northern-sourced water (NSW), the upper-layer changes remain unexplained. Due to its large size, the Pacific accounts for the largest DIC loss, approximately 57 % of the global decrease. This is largely associated with better ventilation of the interior Pacific water mass. However, the largest simulated DIC differences per unit volume are found in the SSWs of the Atlantic. Our study shows that the deep-water geometry and ventilation in the South Atlantic are altered between the two climate states where warmer climatic conditions cause SSWs to retreat southward and NSWs to extent further south. This process is mainly responsible for the simulated DIC reduction by restricting the extent of DIC-rich SSW, thereby reducing the storage of biological remineralized carbon at depth.
Highlights
The Last Interglacial (LIG, or Eemian) is composed of a warm onset around 125 ka characterized by warmer temperature at high latitudes relative to the present and a progressive cooling toward 115 ka when the last glaciation initiated (Otto-Bliesner et al, 2006; Masson-Delmotte et al, 2010)
Our model simulates a global and annual increase of sea surface temperature (SST; +0.27 ◦C) at 125 ka relative to the 115 ka experiment. This warming is mainly simulated at high latitudes (Fig. 1a–d) where higher SSTs are simulated throughout the year in the Southern Ocean (SO), south of Greenland, the Norwegian Sea, and the northern part of the Pacific Ocean
We focus on the differences in ocean carbon cycle that occur at 125 ka in comparison to 115 ka, the differences at global and basin scales
Summary
The Last Interglacial (LIG, or Eemian) is composed of a warm onset around 125 ka characterized by warmer temperature at high latitudes relative to the present and a progressive cooling toward 115 ka when the last glaciation initiated (Otto-Bliesner et al, 2006; Masson-Delmotte et al, 2010). A. Kessler et al.: Ocean carbon inventory under warmer climate conditions trends (Lourantou et al, 2010; Schneider et al, 2013). Kessler et al.: Ocean carbon inventory under warmer climate conditions trends (Lourantou et al, 2010; Schneider et al, 2013) Most of these studies provide a better understanding of the land carbon budget, highlighting the importance of temperature changes on the land vegetation and slow processes of CO2 change such as peatland carbon dynamics and CaCO3 shallow-water accumulation (Schurgers et al, 2006; Kleinen et al, 2016; Brovkin et al, 2016). There are numerous studies that have analyzed the role of the ocean carbon cycle in regulating the atmospheric CO2, especially for the interglacial-glacial transition period (Ridgwell, 2001; Sigman and Boyle, 2000; Menviel et al, 2012), to the authors’ knowledge there is no study that investigate in details changes in marine carbon and nutrient cycling during the Eemian period of the LIG (125–115 ka)
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