Almost 90% of the silt on the floor of the Indian Ocean consists of the remains of plankton that bloomed in the course of the summer monsoon. This has been discovered by earth scientists at the NWO's Netherlands Institute for Sea Research (NIOZ). Their study involved the use of automated sediment traps in which flows of marine material were caught while settling to the deep ocean floor. Because the blooming of plankton is greatly dependent on the monsoon climate, the analysis of oceanic sediments provides insight into climatic changes in the past.
Only the toughest parts of the plankton are preserved, for example flocs of resistent organic material and the tiny skeletons of the organisms. Most of the other material is recycled near the surface of the sea where it is formed or broken down during settling The researchers were surprised to find that 95% of the plankton's silicon skeleton dissolves just above the deep sea bed soon after arrival. Most of the organic carbon originating from dead plankton flocs is processed biologically in this layer. Only in areas with a low oxygen concentration closer to the shore, the organic carbon eventually ends up buried in the sea bed.
The summer blooming of plankton in the Indian Ocean turns out to be so enormous that despite the fact that a great deal of material is broken down at sea-bed level, the silt consists almost entirely of the remains of this living material. However, the lime which was found on the deep sea bed closer to the land turned out to be composed primarily of shell fragments and only partly of the remains of plankton skeletons. The shell fragments are produced in coastal waters and are washed into the deep ocean at the end of the summer monsoon. This is apparent from the large quantities of shell fragments which the researchers discovered in their deep-sea sediment traps in the late summer.
The cause of the summer plankton explosion is the south-western monsoon along the edge of the Indian Ocean. This pumps up the nutrients required for the plankton to bloom, including nitrate and phosphate, from deeper water levels into the well lit surface ocean. Conditions for growth are therefore at their best during the summer in the top 150 metre layer of the ocean.
This understanding of the most important sources of the various kinds of silt on the floor of the Indian Ocean provides earth scientists with a natural climatological "archive" covering the past several thousand years.
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