Southern Ocean Calcification Controls the Global Distribution of Alkalinity

Abstract

Biological processes in Southern Ocean surface waters have widespread impacts on global productivity and oceanic CO 2 storage. Here, we demonstrate that biological calcification in the Southern Ocean exerts a strong control on the global distribution of alkalinity. The signature of Southern Ocean calcification is evident in observations as a depletion of potential alkalinity within portions of Subantarctic Mode and Intermediate Water. Experiments with an ocean general circulation model indicate that calcification and subsequent sinking of biogenic carbonate in this region effectively transfers alkalinity between the upper and lower cells of the meridional overturning circulation. Southern Ocean calcification traps alkalinity in the deep ocean; decreasing calcification permits more alkalinity to leak out from the Southern Ocean, yielding increased alkalinity in the upper cell and low-latitude surface waters. These processes have implications for atmosphere-ocean partitioning of carbon. Reductions in Southern Ocean calcification increase the buffer capacity of surface waters globally, thereby enhancing the ocean's ability to absorb carbon from the atmosphere. This study highlights the critical role of Southern Ocean calcification in determining global alkalinity distributions, demonstrating that changes in this process have the potential for widespread consequences impacting air-sea partitioning of CO 2. Plain Language Summary Plankton living in the Southern Ocean affect the composition of seawater through biological processes. Due to the particular oceanic circulation in the Southern Ocean, these biologically driven changes in ocean chemistry can have widespread effects on the global ocean. Species of plankton that form shells of calcium carbonate remove alkalinity from surface waters through the process of calcification. The amount of alkalinity in surface waters is important because it affects how much CO 2 the ocean can absorb from the atmosphere. We show that Southern Ocean calcifying plankton affect the global distribution of alkalinity through the presence of a Southern Ocean "alkalinity trap." More Southern Ocean calcification yields a stronger alkalinity trap, with more alkalinity being retained in the Southern Ocean and in deep waters, away from the atmosphere. Reduced calcification in the Southern Ocean permits more alkalinity to escape the Southern Ocean and remain in the upper ocean globally. These changes affect oceanic CO 2 uptake from the atmosphere.

Simulations With the Marine Biogeochemistry Library (MARBL)

“…Sarmiento et al. (2007) demonstrated that the Southern Ocean effectively traps silica (see also Moore et al., 2018; Primeau et al., 2013), a phenomena we have also demonstrated operates in POP for CaCO 3 and alkalinity (Krumhardt, Long, et al., 2020; Krumhardt, Lovenduski, et al., 2020). In this vein, excessive silica leakage from the Southern Ocean in CESM2 may help explain why upper‐ocean SiO 3 concentrations are too high at the surface over much of the rest of the global ocean, with the exception of the North Pacific (Figure 8).…”

“…The CESM‐LE enabled a series of studies that explicitly separated natural variability from anthropogenic forced trends in ocean biogeochemistry (Eddebbar et al., 2019; Krumhardt et al., 2017; Long et al., 2016; Lovenduski et al., 2015, 2016; McKinley et al., 2016). BEC has also been used in Decadal Prediction experiments with CESM1 (Yeager et al., 2012), and a handful of studies have examined predictability of ocean biogeochemical dynamics in this framework (e.g., Krumhardt, Long, et al., 2020; Krumhardt, Lovenduski, et al., 2020; Lovenduski et al., 2019; Yeager et al., 2018).…”

Southern Ocean Phytoplankton Community Structure as a Gatekeeper for Global Nutrient Biogeochemistry

“…By additionally facilitating a more efficient transfer of POM to greater depths via the production of the ballasting agent calcite (see Figures 2b and S5 and e.g., Rosengard et al., 2015), coccolithophores directly contribute to setting the fraction of nitrate trapped in the SO, as opposed to being transported to lower latitudes. Altogether, acknowledging the impact of SO coccolithophores on carbonate chemistry (not assessed here, but see Krumhardt et al., 2020), this suggests that a change in the relative importance of diatoms and coccolithophores directly affects not only the Si‐to‐N ratio of exported waters, but also global carbon cycling.…”

“…North of the APF, where diatom growth is limited by silicic acid availability, coccolithophores are more efficient than other similarly sized phytoplankton in drawing down nitrate (Balch et al, 2016;Nissen et al, 2018), thus giving coccolithophores a vital role in setting the transition from negative Si* levels to zero Si* between 30 and  40 E S. By additionally facilitating a more efficient transfer of POM to greater depths via the production of the ballasting agent calcite (see Figures 2b and S5 and e.g., Rosengard et al, 2015), coccolithophores directly contribute to setting the fraction of nitrate trapped in the SO, as opposed to being transported to lower latitudes. Altogether, acknowledging the impact of SO coccolithophores on carbonate chemistry (not assessed here, but see Krumhardt et al, 2020), this suggests that a change in the relative importance of diatoms and coccolithophores directly affects not only the Si-to-N ratio of exported waters, but also global carbon cycling.…”

High and low latitude controls on Mid-Brunhes coccolithophore bloom and its implications on ocean carbon cycle

“…Similarly, a minimum in the phase lag between Antarctic temperature and obliquity, and ice volume and obliquity, between 450-350 ka has also been attributed to a 400 kyr forcing of CO2 43 . More intense coccolithophore production in the Sub-Antarctic zone of the Southern Ocean, has been modeled to decrease the buffering capacity of the surface ocean globally and therefore reduce the CO2 uptake capacity of the ocean 44 . Therefore, the strong enhancement of coccolithophore production in the Sub-Antarctic zone which we show between 500-400 ka, should be investigated as a potential additional mechanism for modulation of atmospheric CO2 on long eccentricity periods.…”