GEOLOGICAL SOCIETY OF THE PHILIPPINES
THE INFLUENCE OF THE MONSOONS AND EL NIŅO IN THE FLUX OF COCOLITHS IN THE SOUTH CHINA SEA
Alyssa M. Peleo-Alampay, Edward S. Lucero, Martin Weisner and Christian Gerbich
The distribution and cycling of carbon in the oceans have profound impacts on our understanding of global climate change. The effect of anthropogenic input of carbon dioxide into the closely linked ocean-atmosphere carbon system continues to be an area of active research (Holmen, 1992). Since the ocean is the major reservoir for carbon, an understanding of the complex processes involved in the cycling of carbon within this system is necessary. A critical part in the study of the oceanic carbon cycle involves the production, fate and cycling of biogenic matter (Steinmetz, 1994).
Coccolithophores (or calcareous nannoplankton) are unicellular haptophyte algae that live in the photic zone. Coccolithopores are the major primary producers that convert dissolved carbon dioxide to calcium carbonate by secreting calcareous outer hard parts called coccoliths that are preserved as ocean floor sediments. The vertical transport of the calcite hard parts (coccoliths) of these phytoplankton from surface waters to the seafloor is an important variable in the global carbon cycle. In light of growing concern regarding the anthropogenic input of carbon dioxide into the atmosphere, an understanding of the role of coccoliths in the overall carbon cycle is important. In recent years, devices called sediment traps that collect particles in the water column at different depths have been used to directly calculate time series fluxes of particles to the ocean floor.
The South China Sea is particularly ideal for seasonal variability studies because it is greatly affected by strong monsoons annually. This makes the South China Sea sensitive to climate change and results in a strong seasonal signal in primary production. At the same time, it is located between two major weather regimes of the world; the Indian Ocean monsoon area and the vast Pacific and The Australian and Asian Monsoon systems.
The fluxes of calcareous nannoflankton are analyzed from sediment trap samples collected during two periods, from December 1996 to November 1997 and from June 1998 to April 1999 in the South China Sea (SCS). The traps are located in the eastern (off Luzon), central and western (off Vietnam) sides of the SCS. Florisphaera profunda is the most dominant nannoflankton species in all of the trap stations, comprising most of the coccolith flux. Total coccolith flux was highest in the central trap site during both collection periods. In general, the flux of F. profunda is the highest during the intermonsoon months and the tail ends of the monsoon season. The decrease in flux of F. profunda can be explained by a shallowing of the nutricline as a result of a higher mixed layer which can occur during the main phase of the monsoon. An increase in lithogenic matter flux can also explain the decrease in F. profunda flux because of reduced water transparency. Among the other coccoliths, Gephyrocapsa oceanica has the highest flux. In general, G. oceanica flux is higher in the central deep northeast traps in the 1996-97 peiod. Coccolith fluxes are also compared between the two collection periods. The second collection period, 1998-99 are considered El Niņo years when considerably higher than the normal sea surface temperatures were present in the South China Sea. The variation in coccolith fluxes among the sediment trap depths and locations will also be discussed.
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