The latest on

Loess A n unusual set of geological circumstances has led to the formation in North China of a remarkable deposit called loess (pronounced lerss) (Fig. 1.). Over the past few years there has been an increasing realiz- ation of just how significant these geological circum- stances were, and some of the notable aspects of the loess itself have really begun to be appreciated. Any- one who has studied geography at school knows a few things about loess; it’s yellow, it occurs in China, and it’s a wind-blown (aeolian) deposit. It consists mainly of silt-size particles in the range 20-60 pm, pre- dominantly of quartz but also including feldspars and mica with generally small amounts of clay mineral. Over the last 20 years investigators have come to appreciate that the thick loess successions in China contain perhaps the best terrestrial record of Quaternary events, and that they have an enormous resource value because the soils formed on them are among the most fertile in the world.

There is a great deal of interest in loess at the moment, and research on this subject is very vigorous and active. Several factors account for this state of affairs: the Chinese loess has become accessible and Chinese investigators are publishing some remarkable results; new dating methods such as thermo- luminescence analysis are enabling researchers to obtain absolute dates from loess material; the potential of loess stratigraphy is being realized and this stratigraphy is helping to establish a clear picture of the multiple-event nature of the Quaternary period; and the need for a continued high level of food pro- duction has brought the problem of loess soil con- servation to the forefront in several fields of research.

China The Xian Laboratory for Loess and Quaternary Geology was established in 1984. Its existence shows how seriously the study and exploitation of loess are taken in China. In 1985 an international loess con- ference held at Xian clearly demonsuated the enormous advances made by Chinese workers in virtually all aspects of loess research. Chinese investi- gators in the late 1950s and early 1960s had realized that the thick loess deposits in their country contained many palaeosols - ancient soil horizons denoting periods of warmer climate - and they published a few preliminary results (Fig. 2). In 1966, however, the Cultural Revolution caused a total shut-down of publication and an enormous reduction of scientific activity so that, as a result, the appreciation of the usefulness of the thick successions of loess as climatic indicators was delayed. In fact, the first announce- ments of the remarkable potential of loess stra- tigraphy came not from China but from the reports by George Kukla and Julius Fink on the Central European loess. Now the study of Chinese palaeosols is in full swing and many cooperative ventures have been started. George Kukla’s new INQUA’ Working

Group on Chronostratigraphy will concentrate on the thick Chinese loess sections.

The palaeosols are numbered from the top: S , , Sz, S3, etc. (Fig. 3). Palaeosol S5 is a thick multiple pro- file, well developed within the Lochuan loess section and about 460 000-560 000 years old. This palaeosol complex is composed of three palaeosol layers and two thin loess layers intercalated with them. Ss records a climatic optimum with a mean annual temperature of about 12.5-14.S”C and an annual precipitation of about 700-800 mm. It is tentatively correlated with oxygen isotope stages 13, 14 and 15 from deep-sea cores. The climatic event recorded by S5 is believed to be of worldwide significance. The Brunhes- Matuyama magnetic transition occurs in S 8 . This transition is about 730 000 years old and gives a very useful time marker in the loess system. Good cor- relations have, in general, been obtained between the climatic variations indicated by the Chinese loess and those suggested by deep-sea cores.

S 7 0 W

Fig. 1. A classic loess landscape in north China - the Baxie Valley in Southern Gansu.

INQUA: International Union for Quaternary Research.

Brunhes-Matuyama magnetic transition: the last major reversal of the Earth’s magnetic field, from its ‘reversed’ state to its present ‘normal’ state.

Fig. 2. The first indication of the effectiveness of multiple palaeosols in climate analysis - the loess section at Wucheng, Shanxi Province, China. ( 1 ) Malan loess, (2) Upper Lishih loess, (3) Lower Lishih loess, (4) Wucheng loess, (5) buried soils, (6 ) Lower Pliocene conglomerate, (7) Palaeozoic sandstones. This illustration is from the paper by Liu Tung-sheng and Chang Tsung-hu presented at the 1961 INQUA Congress in Lodz and published in 1964.

GEOLOGY TODAY May-yune 1989197

LM9

LMs

LMd

LMn

TL --- 20OOOie~ n Oh Rt

105000 BP P

M

BU

230000 BP

BLAKE EVENT

POLAND

: AW AK AW A ‘EPHRA

dT. CURL rEPHRA

SOUTHERN ENGLAND

KIMBOLTON SECTION NEW ZEALAND

B 730 000 BP

M

Fig. 3. Loess sections from Poland, England, New Zealand and China compared. The Brunhes(B)-Macuyama(M) magnetic transition (see glossary item) is in Sg. Ss is dated at about 500 000 years BP(bef0re present). Thennoluminescence (TL) dating of the English loess gives dates around 18 OOO years BP. The Blake Event at 105 000 years BP was a very short transition of the geomagnetic field from normal to reversed and back again.

Sl

s2

s3

s4

s5

s6

S8

LOCHUAN CHINA

Sedimentology The earliest ‘loess problem’, much discussed in the later years of the nineteenth century, was a sedi- mentological one. How was the loess emplaced? How did it arrive at the position in which it was observed? These questions were ‘solved‘ in China by Ferdinand von Richthofen, who proposed the ‘aeolian’ theory that is universally accepted today, although the Richthofen solution was, in fact, only a solution to part of the sedimentological problem. Other aspects were long neglected and it was not until the 1960s that the whole problem of loess sedimentology was surveyed. To give a satisfactory sedimentological description of a loess deposit we need to know how the essential 20-60 pm clastic particles were formed, how they were transported, how they were deposited, and what post-depositional changes have occurred. Within this framework there are sti l l many questions to be answered and problems to be solved.

We have made some progress towards an under- standing of how the fine primary mineral particles (usually quartz) comprising a loess deposit are formed, but there are some remarkable outstanding problems. It is relatively easy to relate the loess deposits of northern Europe and North America to the great continental glaciations of the cold phases of the last million years, but it is not easy to find an obvious source, and a source mechanism, for the

particles comprising the great desert-related deposits of China and Central Asia. The particles appear to be formed by rock-weathering in nearby mountains, transported by rivers into desert regions, and from there moved by aeolian transportation to the eventual place of deposition. Tracing all the significant events in the formation of a ‘desert’ loess deposit is still a considerable problem for geomorphologists and sedimentologists.

Dating The succession of palaeosols in a thick loess section clearly indicates the alternation of warm and cold climates, but to be really satisfying to the Quaternary scientist this observation needs to be supplemented with some absolute dates. These are now becoming available. The Chinese sections are so thick that they include the Brunhes-Matuyama magnetic transition (in palaeosol S8) and this gives a good absolute date of around 730 000 BP (before present). Various other absolute dates are available in other loess systems. For example, there is great promise in the New Zealand loess deposits because of the interspersed layers of datable volcanic ash. In the history of loess investi- gation in New Zealand, the early studies were all con- centrated in the South Island where the most obvious loess deposits occur, but latterly it has been realized that the North Island has extensive loess deposits. As these are close to the great volcanoes of the Ruapehu- Tongariro group there are plentiful ash layers.

One of the great leaps forward in loess dating has been the development of the thermoluminescence technique, which is particularly suitable for appli- cation to loess. The basis of the method is the ability of minerals such as quartz and feldspar to trap electrons produced by radioactive decay processes in the natural environment. These electrons are removed from their traps when the grains are exposed to light, as in aeolian transport. Thus the number of electrons in the minerals today represents the amount of time that has passed since the grains were last exposed to light - i.e. during deposition and burial. Heating the grains in the laboratory releases the electrons, which recombine within the crystals and produce light. This is the natural thermoluminescence signal, which is compared with that produced by laboratory irradiation and by this comparison yields the ‘equivalent dose’ used in the age equation. Deter- minations are being made in several laboratories in Britain.

Agriculture Draw on an outline map of the world the regions with the most productive soils, then draw on a similar base map the loess regions of the world, and compare the two. The match is remarkable, because most of the highly productive agricultural soils in the world are loess soils. Central North America is a classic area, an amazingly productive region of loess soils. However, it will be appreciated that good soils require sensible treatment and that when they are neglected and abused, problems follow. The traumatic years of the ‘dirty thirties’ (the dry years of the 1930s) demon- strated very clearly that the loess soils which had blown into position could blow away again.

98IGEOLOGY TODAY M ~ 7 y - 3 ~ 1989

Loess soils, by their very nature (high silt content, not much clay-mineral content) are very erodible (Fig. 4). Probably the world’s worst erosion problems at present are in the North China loess, where vast amounts are carried away each year by the Yellow River. This river derives its name from the amount of loess material suspended in it, and it carries an amazingly high suspension load (about 400 000 parts per million of solids). Ironically, however, the major problem with the North China loess lands is aridity. The uplift of the Himalayas and the Tibetan plateau has increased the rain-shadow effect, and the climate is becoming too dry far effective agriculture. The Chinese government has a number of schemes, including pumping water from the Yellow River to irrigate loess surfaces over 500 rn higher, and there is a particularly ambitious proposal to pump water from the River Yangtze northwards for hundreds of kilo- metres to irrigate the loess soils. This could restore fertility to the soils that provided the ‘good earth’ for the first emergence of Chinese civilization over 4000 years ago.

Organization Worldwide loess research is correlated and encour- aged by the Loess Commission of the INQUA. Policies and research targets are set a t major, four- yearly INQUA meetings, such as the one held in Ottawa, Canada, in August 1987. At that meeting it was decided that progress could best be encouraged by a series of working groups, eight being established. There are now groups devoted to China, North America, geotechnology, geochemistry, chrono- stratigraphy, palaeogeographic maps, geomorphology and land use, and documentation. These groups are based all around the world; the China group has its headquarters at the Xian Loess Laboratory, the North American group at the Illinois State Geological Survey. The geomorphology and land use and the documentation groups are based at the Centre for Loess Research and Documentation at Leicester University, although the major land-use project, directed by Professor E. Derbyshire, includes con- sideration of problems of slope stability in China. Twice a year the documentation group publishes Loess Lerrer, the newsletter of the INQUA Loess Commission, and copies can be obtained from Loess Letter, Geography Department, Leicester University, Leicester LEI 7RH. The next major INQUA Congress is in Beijing, China, in August 1991, and a significant part of this meeting will be devoted to discussing the problems of the Chinese loess. There will also be held in Beijing the John Hardcastle Cen- tenary Symposium celebrating 100 years of loess stratigraphy. John Hardcastle invented loess stra- tigraphy in 1890 at Timaru in New Zealand. We will thus have 100 years of stimulating intellectual debate and a century of progress to celebrate.

Suggestions for further reading Derbyshire, E. & Mellors, T. W. Loess. 1987.

Chapter 20 in A Handbook of Engineen’ng Geo- morphology (eds P. G. Fookes & P. R. Vaughan). Surrey University Press.

Fig. 4. Scanning electron micrographs of loess particles, largely in the 20-60 p diameter range: (a) Vertical section through Upper Pleistocene (Malan) loess from Jiuzhoutai Mountain, Lanzhou, China; particles angular and highly micaceous. (b) Vertical section through Upper Pleisrocene loess from Glos, near Lisieux, Normandy; particles mainly angular to subangular quartz.

Pecsi, M. 1974. Loess - entry in Encyclopedia

Pye, K. 1987. Aeolian Dust and Dust Deposirs.

Smalley, I. 1987. Loess Lerrer, nos 1-10 (reprints).

Britannia.

Academic Press.

Geo Books.

Loess Letter no. 18 (October 1987) - gives details of working groups and research programmes for 1987- 199 1.

IAN SMALLEY & EDWARD DERBYSHIRE Centre for Loess Research & Documentation

University of Leicester

GEOLOGY TODAY May-June 1989199