Geoscience Reference
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23 and 19 ka precessional cycles, with the 41 ka obliquity cycles dominating from
around 2.5 to about 0.7 Ma, after which the 100 ka orbital eccentricity cycles became
dominant. As a result, the earlier portion of the Quaternary from 2.5 to 0.7 Ma
was characterised by high-frequency, low-amplitude climatic fluctuations, while the
last 0.7 million years were characterised by low-frequency, high-amplitude climatic
fluctuations.
21.5.1 Vegetation history
Deposition within the Bogota Basin has preserved a remarkably complete record of
river, lake and swamp sediments, as well as associated fossil pollen and spores,
allowing the reconstruction of the longest and most complete record of Quaternary
tropical vegetation changes for any place on earth (Helmens and van der Hammen,
1994 ; Hooghiemstra and Ran, 1994 ; van der Hammen and Hooghiemstra, 1995 ;
Wijninga et al., 2003 ). As a very broad generalisation, the glacial maxima were
cold and generally dry in the lowland tropical regions of South America, while the
interglacial maxima were warm and relatively humid. In the southern Andes and
parts of the central Andes, the LGM was cold but humid (Sylvestre, 2009 , fig. 1.2 ).
Complicating this interpretation were periodic episodes of rapid erosion and land-
slides that were linked, directly or indirectly, to the episodic uplift of the Andes.
Although many workers consider that even the lowland rainforests of Amazonia were
adversely affected by episodes of aridity (Iriondo and Latrubesse, 1994 ; Latrubesse
and Ramonell, 1994 ), it is not always easy to separate out the net effects of changes in
temperature, evaporation and precipitation, and Colinvaux and his co-workers have
always stoutly denied that the Amazon plains were ever arid (Colinvaux et al., 1996 ;
Colinvaux et al., 2000 ; Colinvaux, 2001 ).
The pollen preserved in lake sediments from Lago Condorito in north-west Patago-
nia provides a record of millennial-scale vegetation and climate changes for the past
15 ka (Moreno, 2004 ), with evidence of a cool-temperate, humid climate between
15 and 11 ka, followed by a very warm and dry phase between 11 and 7.6 ka, with
later cooling and then renewed aridity between 2.9 and 1.8 ka. Moreno ( 2004 ) sugges-
ted that the millennial-scale climatic changes evident in north-west Patagonia were
associated with changes of similar duration in the tropical Pacific, which were related
to corresponding changes in the mid-latitude South Pacific - a working hypothesis
for future testing.
21.5.2 Desert dust and loess
Iriondo ( 1993 ) has long argued that the presence of dunes in both the Amazon Basin
and in the vast tropical plains of the Chaco in Argentina, Paraguay and Bolivia is
indicative of previously drier climates, as are the thick loess deposits of the Chaco.
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