Unexpected information about Earth’s climate history from Yellow River sediment

By meticulously examining sediments in China’s Yellow River,
a Swedish-Chinese research group are showing that the history of tectonic and
climate evolution on Earth may need to be rewritten. 

Their findings are
published today in the highly reputed journal Nature Communications.

To reconstruct how the global climate and topography of the
Earth’s surface have developed over millions of years, deposits of eroded land
sediment transported by rivers to ocean depths are often used. This process is
assumed to have been rapid and, by the same token, not to have resulted in any
major storages of this sediment as large deposits along the way.

However, knowledge gaps and contradictory data in research
to date are impeding an understanding of climate and landscape history. In an
attempt to fill the gaps and reconcile the contradictions, the researchers have
been investigating present-day and ancient sediment deposits in the world’s
most sediment-rich river: the Yellow River in China.

The researchers, from Uppsala University (led by Dr. Thomas
Stevens) and Lanzhou University (led by Dr. Junsheng Nie), China, analysed
Yellow River sediment from source to sink and determined its mineral
composition. They also determined the age of mineral grains of zircon, a very
hard silicate mineral that is highly resistant to weathering.

Zircon ages serve as a unique fingerprint that yields
information about the sources of these sediment residues from mountain chains,
according to Thomas Stevens of Uppsala University’s Department of Earth
Sciences, one of the principal authors of the study.

The Yellow River is believed to gain most of its sediment
from wind-blown mineral dust deposits called loess, concentrated on the Chinese
Loess Plateau. This plateau is the largest and one of the most important past
climate archives on land, and also records past atmospheric dust activity: a
major driver of climate change.

The scientists found that the composition of sediment from
the Yellow River underwent radical change after passing the Chinese Loess
Plateau. Contrary to their expectations, however, the windborne loess was not
the main source of the sediment. Instead, they found that the Loess Plateau
acts as a sink for Yellow River material eroded from the uplifting Tibetan
plateau.

This finding completely changes our understanding of the
origin of the Chinese Loess Plateau. It also demonstrates large scale sediment
storage on land, which explains the previously contradictory findings in this
area.

‘Our results suggest that a major change in the monsoon
around 3.6 million years ago caused the onset of Yellow River drainage,
accelerated erosion of the Tibetan plateau and drove loess deposition,’ Thomas
Stevens writes.

Weathering of this eroded material also constitutes a
further mechanism that may explain the reduced levels of atmospheric carbon
dioxide at the beginning of the Ice Age. The researchers’ next step will be to
compare terrestrial and marine records of erosion to gauge how far sediment
storage on land has impacted the marine record.

‘Only then will we be able to assess the true rates of erosion
and its effect on atmospheric CO2 and thus the climate in geologic time,’ says
Stevens.

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Posted on October 14, 2015, in Useful Information. Bookmark the permalink. Leave a comment.

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