Abstract
Abstract. Recent earth system models predict a 10 %–20 % decrease in particulate organic carbon export from the surface ocean by the end of the 21st century due to global climate change. This decline is mainly caused by increased stratification of the upper ocean, resulting in reduced shallow subsurface nutrient concentrations and a slower supply of nutrients to the surface euphotic zone in low latitudes. These predictions, however, do not typically account for associated changes in remineralization depths driven by sinking-particle size. Here we combine satellite-derived export and particle size maps with a simple 3-D global biogeochemical model that resolves dynamic particle size distributions to investigate how shifts in particle size may buffer or amplify predicted changes in surface nutrient supply and therefore export production. We show that higher export rates are empirically correlated with larger sinking particles and presumably larger phytoplankton, particularly in tropical and subtropical regions. Incorporating these empirical relationships into our global model shows that as circulation slows, a decrease in export is associated with a shift towards smaller particles, which sink more slowly and are thus remineralized shallower. This shift towards shallower remineralization in turn leads to greater recycling of nutrients in the upper water column and thus faster nutrient recirculation into the euphotic zone. The end result is a boost in productivity and export that counteracts the initial circulation-driven decreases. This negative feedback mechanism (termed the particle-size–remineralization feedback) slows export decline over the next century by ∼ 14 % globally (from −0.29 to −0.25 GtC yr−1) and by ∼ 20 % in the tropical and subtropical oceans, where export decreases are currently predicted to be greatest. Our findings suggest that to more accurately predict changes in biological pump strength under a warming climate, earth system models should include dynamic particle-size-dependent remineralization depths.
Highlights
A key mechanism that controls the partitioning of carbon dioxide (CO2) between the atmosphere and ocean is the biological pump, in which CO2 is fixed into phytoplankton organic matter via photosynthesis and is exported from the surface to the deep ocean as sinking particles (e.g., Ducklow et al, 2001)
Because β and export are negatively correlated, export tends to be high when β is small and low when β is large. These empirical findings are in agreement with Cram et al (2018), who observed that large particles tend to comprise a larger fraction of the sinking flux where productivity and carbon export are high
We used remotely sensed data to show that sinking-particle size is empirically correlated with the rate of particulate organic carbon export out of the euphotic zone across the global ocean, such that larger particles tend to dominate when export is high
Summary
A key mechanism that controls the partitioning of carbon dioxide (CO2) between the atmosphere and ocean is the biological pump, in which CO2 is fixed into phytoplankton organic matter via photosynthesis and is exported from the surface to the deep ocean as sinking particles (e.g., Ducklow et al, 2001). Decomposition of this particulate organic carbon (POC) in the ocean interior maintains a reservoir of respired CO2 that is sequestered out of contact with the atmosphere, exerting an important control on long-term atmospheric CO2 concentrations and global climate (e.g., Martínez-García et al, 2014; Passow and Carlson, 2012; Sarmiento and Siegenthaler, 1992). Leung et al.: Variable particle sizes reduce export flux sensitivity
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