Sedimentary basins and archives of past climate change - loess

Basins in China - stable pieces of micro-continents between suture zones

They are areas of subsidence (depression) to cope with areas of uplift - act as sediment traps encouraging accumulation

Loess

Accumulates in basins - aeolian clastic silt (some sand clay) in situ

Found in stratified sequences with palaeosols - in China, mainly in CHinese Loess Plateau in the Ordos Basin

Asian monsoons believed to be responsible for deposition and pedogenic alterations - winter monsoon - sedimentation, arid climate/ summer monsoon = pedogenesis, humid climate (Porter, 2001)

• Due to direction in which the winds blow - from continent out to sea in winter, and from sea towards continent in summer
• Related to where pressure systems lie 

Shows dust dynamics - radiative forcing due to atmospheric dust - increase in dust in atmosphere reduces available sunlight therefore driving the climate

• forcing effects may cause changes in temps from 1-3oC (Harrison et al, 2001)

Controversy over the source of loess - from glaciers, mountains, deserts or rivers. Can be either or a combination - no concrete evidence to say which but most likely all contribute as source regions - see powerpoint

Uncertainty in sources

Samples always analysed in bulk analysis - comparison of Nd and Sr isotopes, areas of overlap denote source regions as isotopic values are similar to sample

Bulk analysis - not accurate as may be a variety of sources but no way to distinguish between them

Potentially, each grain could be of a different source - looking at indiv grains in future may resolve this, but need an extremely large sample pop

Stevens et al, 2010:

• By dating zircons, can tell age of seds ad how much was formed at that particular time
• Extract zircons from loess and use U-Pb testing to get distributions with ages - can then correlate to other zircons and assume a source region
• However, problem that areas may have more than one peak in zircon production - also ages can differ depending on testing methods and some may have more errors associated with
• Most dating carried out in loess, Eastern sandy lands and Northern Deserts…