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The Great Calcite Belt (GCB) is a band of high concentrations of suspended particulate inorganic carbon (PIC) spanning the subantarctic Southern Ocean and plays an important role in the global carbon cycle. The key limiting factors controlling coccolithophore growth supporting this high PIC have not yet been well‐characterized in the remote Pacific sector, the lowest PIC but largest area of the GCB. Here, we present in situ physical and biogeochemical measurements along 150°W from January to February 2021, where a coccolithophore bloom occurred. In both months, PIC was elevated in the Subantarctic Zone (SAZ), where nitrate was >1 μM and temperatures were ∼13°C in January and ∼14°C in February, consistent with conditions previously associated with optimal coccolithophore growth potential. The highest PIC was associated with a relatively narrow temperature range that increased about 1°C between occupations. A fresher water mass had been transported to the 150°W meridian between occupations, and altimetry‐informed Lagrangian backtracking estimates show that most of this water was likely transported from the southeast within the SAZ. Applying the observations in a coccolithophore growth model for both January and February, we show that the ∼1.7°C increase in temperature can explain the rise in PIC between occupations.

Biological processes in Southern Ocean surface waters have widespread impacts on global productivity and oceanic CO₂storage. 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 CO₂.

Interannual variations in marine net primary production (NPP) contribute to the variability of available living marine resources, as well as influence critical carbon cycle processes. Here we provide a global overview of near‐term (1 to 10 years) potential predictability of marine NPP using a novel set of initialized retrospective decadal forecasts from an Earth System Model. Interannual variations in marine NPP are potentially predictable in many areas of the ocean 1 to 3 years in advance, from temperate waters to the tropics, showing a substantial improvement over a simple persistence forecast. However, some regions, such as the subpolar Southern Ocean, show low potential predictability. We analyze how bottom‐up drivers of marine NPP (nutrients, light, and temperature) contribute to its predictability. Regions where NPP is primarily driven by the physical supply of nutrients (e.g., subtropics) retain higher potential predictability than high‐latitude regions where NPP is controlled by light and/or temperature (e.g., the Southern Ocean). We further examine NPP predictability in the world's Large Marine Ecosystems. With a few exceptions, we show that initialized forecasts improve potential predictability of NPP in Large Marine Ecosystems over a persistence forecast and may aid to manage living marine resources.

Anthropogenic CO₂emissions are inundating the upper ocean, acidifying the water, and altering the habitat for marine phytoplankton. These changes are thought to be particularly influential for calcifying phytoplankton, namely, coccolithophores. Coccolithophores are widespread and account for a substantial portion of open ocean calcification; changes in their abundance, distribution, or level of calcification could have far‐reaching ecological and biogeochemical impacts. Here, we isolate the effects of increasing CO₂on coccolithophores using an explicit coccolithophore phytoplankton functional type parameterization in the Community Earth System Model. Coccolithophore growth and calcification are sensitive to changing aqueous CO₂. While holding circulation constant, we demonstrate that increasing CO₂concentrations cause coccolithophores in most areas to decrease calcium carbonate production relative to growth. However, several oceanic regions show large increases in calcification, such the North Atlantic, Western Pacific, and parts of the Southern Ocean, due to an alleviation of carbon limitation for coccolithophore growth. Global annual calcification is 6% higher under present‐day CO₂levels relative to preindustrial CO₂(1.5 compared to 1.4 Pg C/year). However, under 900 μatm CO₂, global annual calcification is 11% lower than under preindustrial CO₂levels (1.2 Pg C/year). Large portions of the ocean show greatly decreased coccolithophore calcification relative to growth, resulting in significant regional carbon export and air‐sea CO₂exchange feedbacks. Our study implies that coccolithophores become more abundant but less calcified as CO₂increases with a tipping point in global calcification (changing from increasing to decreasing calcification relative to preindustrial) at approximately ∼600 μatm CO₂.