Is there a "continental shelf pump" for the absorption of atmospheric CO2?

Abstract

Based on the results obtained in the East China Sea, we propose a new term, Continental Shelf Pump, as a mechanism for the absorption of atmospheric CO2. We investigated the carbonate system of the East China Sea along a single observation line traversing its central part on 5 cruises in various seasons. The directly observed fugacity of CO2 dissolved in the surface water decreased with decreasing salinity and temperature as well as nutrient content. The relation has been expressed as a simple equation of these 3 parameters. Putting the observed data on the parameters in the various parts of the East China Sea in various months into this equation, we have obtained 55 ± 5 ppm as an annual mean fugacity deficit of CO2 in the surface water of the East China Sea, which nearly equals the directly measured mean fugacity along the observation line. The net absorption flux estimated from the fugacity deficit has agreed with the amount of carbonate transported out of the East China Sea calculated for the distributions of total dissolved carbonate and alkalinity. The distributions of density and total dissolved carbonate reveal the cause of this large deficiency, described as follows. The shallower shelf zone is more cooled than the open sea when heat is lost from the surface. This cooling produces denser water, which together with photosynthetic activity, accelerates the absorption of CO2 in the shelf zone. The absorbed CO2 is transformed to organic carbon and regenerated especially at the shallow bottom. Isopycnal mixing (advection and diffusion) transports the denser coastal water, especially the bottom water enriched in dissolved and particulate carbon, into the subsurface layer of the open oceans. The transport continues in the layer below the pycnocline even in the warm season and maintains the low fugacity of CO2 in the surface water of the shelf zone. This is the continental shelf pump. The pump would account for a net oceanic uptake of CO2 of 1 GtC/ yr, if the world continental shelf zone would absorb the atmospheric CO2 at the rate observed in the East China Sea.