Geoscience Reference
In-Depth Information
and Harrison,
2001
) is consistent with alternating cold dry and warm wet climatic
phases, and has been linked to the marine isotope record in the Pacific. Initial dating
of the loess was based on the magnetic polarity time scale (see
Chapter 6
) supplemen-
ted by cross correlation based on magnetic susceptibility data (Kukla,
1987
;Evans
and Heller,
2001
; Maher et al.,
2010
). Subsequent work has used radiocarbon and
optical dating methods (Lu et al.,
2007
) for the upper part of the sequence, as well
as correlation with the marine oxygen isotope record for at least the last 1 million
years. Local dust sources inside China include the Taklamakan, Gobi, Badain Jaran,
Tengger and Mu Us deserts (
Chapter 8
,
Figure 8.12
; Ding et al.,
1999
; Pullen et al.,
2011
), with additional influxes from central Asia (Sun et al.,
2010
).
Traditional interpretation of the loess-paleosol succession invokes soil formation
during times of stronger summer monsoon and loess accretion during times of stronger
winter monsoon associated with a more intense Siberian high pressure system. Times
of strong summer monsoon were equated with interglacial and interstadial phases,
while times of strong winter monsoon were considered to be coeval with glacial and
stadial episodes. Roe (
2009
) has questioned this interpretation on the grounds that
present-day dust-storms in China occur mostly in spring, when the Siberian High is
already weakening. In Mongolia. dust-storms are also mainly in spring (Middleton,
1991
). However, soil formation requires a significantly wetter climate than that which
prevailed during accumulation of the parent loess. Soils within the loess sequence are
recognised on the basis of a wide variety of analyses, including grain size, magnetic
susceptibility and micromorphology. To be ranked as a soil, they need to be at least
as well-developed as the early Holocene soil at the top of the loess sequence.
Although the alternation between 'winter' and 'summer' monsoon may be some-
what oversimplified, it remains a useful model for future refinement. A more per-
plexing problem is that the loess sequence is not complete, and many sections from
different sites still need to be studied. Porter and An (
2005
) drew attention to this
issue after finding that interglacial phases often began with periods of severe gully
erosion on the Loess Plateau. Loess is peculiarly susceptible to this form of erosion,
so heroic efforts at hillside terracing are needed today to preserve the arable soils of
this region.
Comparisons between the Chinese loess records, the oxygen isotope record pre-
served in marine sediment cores from the East China Sea, Pacific and North Atlantic,
and the climatic record evident in Greenland ice cores (Liu et al.,
1985
; Kukla,
1987
;
Hovan et al.,
1989
; Porter and An,
2005
) have been fruitful. They confirm existing
climatic interpretations of the loess-soil couplets, with glacial maxima times of max-
imum dust deposition and interglacials with times of maximum chemical weathering
of the loess and soil development. The loess deposits of Russia and central Asia
show a similar sequence of alternating loess and soils, with loess accumulation during
times of cold, dry, windy climate and widespread frost action (Rutter et al.,
2003
).
The detailed studies of loess in China and elsewhere in Asia have demonstrated that
