"South China Sea data do not support that interpretation," said Kip Hodges, a co-author of the paper and geologist at Arizona State University.
The pattern in the core suggests a steady increase in East Asian monsoon intensity from 23 to 10 million years ago, followed by a steady weakening until about 4 million years ago.
"After that, the intensity began to increase once more," said Hodges. "The implication is that either the development of the plateau was not as abrupt as we might have thought, or that an abrupt uplift of the plateau at 8 to 10 million years caused a change in precipitation patterns that was not recorded in East Asia."
Another controversy surrounds the degree of coupling between the South and East Asian monsoons. Could one have varied in intensity differently from the other?
The team compared the South China Sea record with less complete sedimentary records from the Arabian Sea and Bay of Bengal--which contain sediments that were eroded from the Himalaya, where the principal rainfall comes from South Asian monsoon storms--to argue for a linkage between the two monsoon systems over most of the past 23 million years.
The most interesting correlation was found when the team compared the sedimentary records to cooling age patterns in the Himalaya.
Compilations of the cooling ages obtained by Hodges and other researchers show that the periods of high East Asian monsoon intensity matched well with high frequencies of cooling ages, implying a relationship between monsoon intensity and erosion in the Himalaya.
"It implies, once again, that Earth is a complex system," said Hodges. "We cannot begin to fully understand mountain building without appreciating the roles of the hydrosphere and atmosphere in the evolution of mountain ranges.
|Contact: Cheryl Dybas|
National Science Foundation