Coral atolls indicate birth of Indian monsoon

15 December 2016, by Franziska Neigenfind

Photo: UHH/CEN

Prof. Dr. Christian Betzler is a geologist at Universität Hamburg’s Center for Earth System Research and Sustainability and head of research at the Control Station German Research Vessels.

News from Climate Science: Once a month, climate researchers report on their latest findings in the newspaper “Hamburger Abendblatt”. Prof. Dr. Christian Betzler is a geologist and head of research at the Control Station German Research Vessels. He explains how coral atolls indicate birth of Indian monsoon.

The Maldives mostly evoke images of bright sandy beaches, crystal clear water, and palm trees. Few people know, however, that the flat coral islands are merely the peaks of a limestone massif composed of sedimentary rocks and culminating at 3,000 meters. Their deepest strata hold plenty of information about Earth’s creation that is yet to be deciphered. In collaboration with my colleagues from Universität Hamburg’s Center for Earth System Science and Sustainability I have investigated and read this 50-million-year-old sediment archive. Our primary aim was to unravel the causes of past sea level changes in the Indian Ocean in order to draw conclusions about today’s sea level variations.

Within the context of our investigations, we repeatedly set off on expeditions to the Maldives aboard research vessels. By means of drillings, sonar, and seismic instruments we explored the geological structures reaching down several thousand meters beneath the atolls. The layers of the drill cores and seismic records provided us with a pretty clear picture of climate and sea level changes and also told us how strong or weak the ocean currents were at the time.

Suddenly, we made an astonishing discovery: layers built up 12.9 million years ago contained extraordinary sediments. Analyses showed they were drift sediments as accumulated only by strong ocean currents. This suggests an immense and abrupt increase of the interactive winds and ocean currents at that time. Currents in the formerly shallow ocean had evidently eroded sediments in some places and created characteristic drift sediments elsewhere. What did these unusual structures teach us? On further examination, they proved true what had been mere assumption at first: we had discovered the roots of the Indian monsoon system!

But what triggered those strong winds and the sudden monsoon onset? One crucial factor was the ongoing formation of the Himalayas. The earth’s crust lifted and unfolded. This changed atmospheric airstreams so they could build up the wind field responsible for the monsoon. Yet, this explanation alone does not suffice, for the wind increase was too abrupt; rather, we identified a whole series of determinants. For instance, the global climate changed, causing great temperature differences between the poles and the equator. Simultaneously, an existing sea passage between India and the Eurasian continent closed up. Desert dust sediments from the drill cores have revealed that the climate above the Asian mainland thus became very dry, strengthening the monsoon even more.

Now, further analysis of our samples and data is to shed light on the long-term development of the Indian monsoon. Monsoon rain, in particular, has a great impact on humans and nature. In numerous countries, agriculture largely depends on rains being neither too heavy nor weak. We cannot make exact prediction about such short-term changes before we interpret the monsoon system correctly.

Contact Prof. Dr. Christian Betzler