NESSC study: Missing link of the Asian monsoon riddle

Summer 2023. After three years in my PhD, I finally embark on my journey to China, with my supervisor and two colleagues from Utrecht University. Our main destination: the Chinese Loess Plateau, an over hundred meters thick pack of wind-deposited dust. Our very special study site,  Yuanbao, is situated in the western Chinese Loess Plateau. But what is the reason I want to see this place, what is it that makes this site so special?

For that, we need to talk about rain, and particularly about the rains of the East Asian Summer Monsoon. This is one of the strongest monsoon systems on Earth, supplying freshwater to over a fifth of the World’s population. The East Asian Summer Monsoon is characterized by warm and wet climate conditions, and it delivers about 80 percent of the annual precipitation to Chinese Loess Plateau.

The Plateau is regarded as one of the best continental paleoclimate archives on land. Succeeding cycles of glacials and interglacials have resulted in the deposition of alternating layers of loess and paleosols. The East Asian Monsoon dominates during interglacial periods of time, whereas a winter monsoon, transporting cold, dry air from the Siberian High from the northwest toward the equator dominates during glacial periods.

Löss-paleosol-opeenvolging uit het zuidelijke deel van het Chinese Lössplateau
Löss-paleosol-opeenvolging uit het zuidelijke deel van het Chinese Lössplateau (uit Rutter et al., 2009)

Variations in strength

The picture shows a loess-paleosol sequence from the southern part of the Chinese Loess Plateau. The darker layers represent paleosols which are deposited and formed during interglacial periods when the summer monsoon prevails. The lighter layers consist of loess. They are deposited during the cold glacial periods, when the winter monsoon is dominant. Records of past East Asian Monsoon climate have been derived from changes in sediment properties like grain size and magnetic susceptibility, which have been related to glacial-interglacial variations in the strength of the winter and summer monsoons, respectively.

There is another archive that records the East Asian monsoon climate: speleothems from caves located in southern China. The oxygen isotopic composition of the speleothem carbonate (δ18O) is related to the strength of the East Asian summer monsoon.

However, researchers have had difficulties reconciling both records. Although both loess magnetic susceptibility and speleothem δ18O supposedly reflect the same parameter (namely, the strength of the summer monsoon), the loess records are dominated by glacial-interglacial (100,000 years, i.e. 100 kyr) cycles, whereas the speleothem record contains a strong precession signal (23 kyr cycle). This mysterious paradox is known as the ‘the Chinese 100-kyr problem’. So which one of these records represents the most accurate signal and can tell us more about the climate conditions?

More info: NESCC website.