Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean

摘要

Most marginal seas in the North Pacific are fed by nutrients supported mainly by upwelling and many are undersaturated with respect to atmospheric CO_2 in the surface water mainly as a result of the biological pump and winter cooling. These seas absorb CO_2 at an average rate of 1.1 +- 0.3 mol C m~(-2)yr~(-1) but release N_2/N_2O at an average rate of 0.07 +- 0.03 mol N m~(-2)yr~(-1). Most of primary production, however, is regenerated on the shelves, and only less than 15% is transported to the open oceans as dissolved and particulate organic carbon (POC) with a small amount of POC deposited in the sediments. It is estimated that seawater in the marginal seas in the North Pacific alone may have taken up 1.6 +- 0.3 Gt (10~(15) g) of excess carbon, including 0.21 +- 0.05 Gt for the Bering Sea, 0.18 +- 0.08 Gt for the Okhotsk Sea; 0.31 +- 0.05 Gt for the Japan/East Sea; 0.07 +- 0.02 Gt for the East China and Yellow Seas; 0.80 +-0.15 Gt for the South China Sea; and 0.015 +- 0.005 Gt for the Gulf of California. More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas may act as conveyer belts in exporting 0.1 +-0.08 Gt C anthropogenic, excess CO_2 into the North Pacific Intermediate Water per year. The upward migration of calcite and aragonite saturation horizons due to the penetration of excess CO_2 may also make the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution, which would then neutralize excess CO_2 in the near future. Further, because most nutrients come from upwelling, increased water consumption on land and damming of major rivers may reduce freshwater output and the buoyancy effect on the shelves. As a result, upwelling, nutrient input and biological productivity may all be reduced in the future. As a final note, the Japan/East Sea has started to show responses to global warming. Warmer surface layer has reduced upwelling of nutrient-rich subsurface water, resulting in a decline of spring phytoplankton biomass. Less bottom water formation because of less winter cooling may lead to the disappearance of the bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200 AD.