Geoscience Reference
In-Depth Information
2
WET
1
No
data
0
1
DRY
2
1
200
400
600
800
1000
1200
1400
1600
1800
2000
Year AD
Figure 2.7. Moon Lake (North Dakota) salinity record from AD 1 to 1980 (from Laird
et al ., 1996). Values below the zero line represent dry episodes.
drying of climate to desiccate the vegetation so it is susceptible to ignition.
Sediment cores derived from the rainforest floor in French Guiana suggest that
theforest here had been subject to intermittent palaeofires due to periods of
severe drought (Charles-Dominique et al . 1998)overthelast10 000 years. The
sedimentary and geological evidence, in combination with knowledge of pre-
historic farming dates and patterns, suggest that humans were not responsible
for these fires. Archaeological evidence of human agriculture is only evident in
this area after 2000 years BP but charcoal layers exist in sediments 10 000--8000,
6000--4000 and from 2000 years BP to the present (as recent as 200 years BP). Each
of these charcoal layers occurs in the absence of any evidence for agricultural
activity.
Layers of sand and gravel are also present within otherwise clay rich organic
sediments or peats in a Pino palm swamp in the area. These coarse-grained
sediments are assumed to have been transported into the swamp by surface
water. As the swamp does not receive coarse-grained particles during modern
heavy rain events, the sand and gravel layers were probably deposited during
extreme erosional events. Charles-Dominique et al .(1998)argue that these events
could only have occurred following extensive deforestation due to palaeofires as
the 30--45 m high stratified vegetation canopy and dense mat of surface roots
protect the forest floor from the impacts of heavy rains and surface erosion.
Furthermore, erosion in the earlier Holocene is likely to have been less than
at present because agriculture had not yet impacted the soils. It is possible for
waterdeficiency to be pronounced in the South American dry season causing
fire prone conditions. Historically, however, annual humidity and precipitation
levels have been well above the water stress exceedence threshold level required
forthe occurrence of fires. The record of palaeofires suggests that the area has
Search WWH ::




Custom Search