Washington, Nov 10 : A new research has suggested that variations in monsoon climate over longer time scales had influenced the evolution of the world's highest mountain chain, the Himalayas.
The climate over much of Asia is dominated by seasonal winds that carry moist air over the Pacific Ocean into East Asia and over the Indian Ocean into South Asia.
The East and South Asian monsoons are responsible for most of the rainfall in these regions.
Although the time when these monsoon patterns were first established is unknown, many lines of evidence suggest that they first came about at least 24 million years ago.
The new study has used geochemical data from an Ocean Drilling Project sediment core extracted from the seafloor of the South China Sea to establish a record of the East Asian monsoon climate over that time interval.
According to Peter Clift, lead author of the study and a professor of geology and petroleum geology at the University of Aberdeen in Scotland, "Sediments in this core were eroded from the drainage area of the Pearl River system in China, and their chemistry records the relative intensity through time of chemical weathering in an area that received the bulk of its precipitation from East Asian monsoon storms."
A controversy surrounds the degree of coupling between the South and East Asian monsoons. Could one have varied in intensity differently from the other?
Clift's 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 Himalayas, 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 really exciting moment in this research came when we began to compare patterns from one record to another and found broad agreement," said Clift.
The most interesting correlation was found when the team compared the sedimentary records to cooling age patterns in the Himalayas.
Compilations of the cooling ages obtained by Hodges' group 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 Himalayas.
According to Kip Hodges, a co-author of the research paper, "While it makes sense intuitively that heavy rainfall should be correlated with more aggressive erosion, it is important to see such direct evidence of the coupling between the processes that define the evolution of mountain ranges and climatic processes."