Geochemical implications for the mechanism of deep convection in a semi-closed tropical marginal basin: Sulu Sea

In 1996 and 2002, we measured the vertical profiles of potential temperature, salinity, dissolved oxygen, nutrients, tritium (only in 1996), and helium isotopes (only in 2002) in the Sulu Sea, a small semi-closed tropical marginal basin west of the Philippines with a maximum depth of >5000 m. Judging from these data in combination with those for the South China Sea adjacent to the Sulu Sea, we conclude that the deep convection in the Sulu Sea is controlled by a combination of the following three processes: (i) quasi-steady inflow of the South China Sea intermediate water through the Mindoro Strait (sill depth: ∼420 m); (ii) occasional inflow of denser intermediate water through the Mindoro Strait from the South China Sea when the thermocline in the South China Sea is uplifted; and (iii) formation of less dense bottom water by means of geothermal heating, which in turn causes overturning of the Sulu Sea bottom water below a depth of 3000 m. From 1996 to 2002, the bottom water of the Sulu Sea was stagnant, its temperature increased by 0.003 °C, and its dissolved oxygen concentration decreased by 5.8 μmol kg−1. The 6-years study results show that the temperature increase can be attributed to the terrestrial heat flow from the seafloor.