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in the Kilimanjaro record, which is dated ~4.0 ka B.P. (Figure
3c), was also the product of this
“
megadrought
”
that may
have lasted a few centuries and during which time the North-
ern Ice Field, presently the largest on the mountain, dramat-
ically decreased in size [Thompson et al., 2002]. In marine
sediments off the coast of Peru, Rein et al. [2004, 2005] found
that the lithic
plankton reached its lowest levels between ~5 and 2 ka B.P.,
and the percent of saline diatoms increased at ~6 ka B.P. (
14
C
age), reaching a maximum at ~3.6 to 4.0 ka B.P. (
14
C age)
[Baker et al., 2001]. Tapia et al. [2003] offer a more re
ned
time line for climate variation in this interval using the Lake
Titicaca record of percent saline planktonic taxa, which con-
ux was much lower between ~8.0 and 5.0 ka B.
P. than in both the earlier and later Holocene. These records
are interpreted to indicate reduced ENSO activity during what
was a dry period along coastal Peru.
Lake records from equatorial and North Africa [Gasse,
1977; Gillespie et al., 1983; Servant and Servant-Vildary,
1980] indicate that water levels had decreased greatly (Fig-
ures 3d
-
3f), as did levels in western Tibet [Gasse et al.,
1991, 1996], which are not shown. Other records also con-
tain evidence of a sudden and marked arid climate episode at
this time. A speleothem record from the Soreq Cave (location
in Figure 1) in Israel [Bar-Matthews et al., 1999; Bar-
Matthews and Ayalon, 2004] shows an abrupt decrease in
δ
firms that although water levels were low between 3 and 6 ka
B.P., the lowest levels were achieved at ~4.5 ka B.P.
In addition to paleoclimate records, archeological and
historical records indicate that during the third millennium
B.C.E., a sudden and severe drought occurred in many
regions that had previously been under the expanded in
u-
ence of the Asian, Indian, and African monsoon systems in
the early Holocene. As with the 5.2 ka B.P. cold reversal, this
arid event occurred at a time of widespread cultural disrup-
tions. It was contemporaneous with the decline of the Akka-
dian Empire in Mesopotamia [Weiss et al., 1993], the failure
of Nile River
floods that were contemporaneous with severe
crises in North African civilizations [Hassan, 1997], and the
decline of the urban Harappan civilization in the Indus valley
[Staubwasser et al., 2003]. In central China, this time also
marked the decline of Neolithic culture [Wu and Liu, 2004].
Whether the drought was instrumental in the collapse of
these societies is controversial among some archeologists
[Butzer, 1997], but the evidence is compelling that this
climatic anomaly occurred at or near the same time as these
historical events.
There are several potential mechanisms that could lead to
the mid-Holocene abrupt climate change events. For exam-
ple, several episodes of abrupt aridity in the Indian and Asian
monsoon regions have been linked to variations in solar
output as was discussed above. Mechanisms discussed by
Wanner et al. [2008] that were in
13
C, implying lower precipitation within the 4.2 to 4.5 ka B.
P. window. Palynological data from north central China also
suggest a cold, dry period from 3.95 to 4.45 ka B.P., which is
inferred by a sudden decrease in tree pollen concentration
[Xiao et al., 2004]. An oxygen isotope record from a spe-
leothem from Southern China shows evidence for multiple
dry periods that coincide with Bond (1.5 kyr) events in the
North Atlantic [Wang et al., 2005]. Some of them, including
the one at 4.2 ka B.P., occurred at a time of variations in solar
activity, which was also observed in the record from a marine
core off the Indus Delta [Staubwasser et al., 2003]. The
10
Be
concentrations measured in the Greenland Ice Core Project
icecore[Vonmoos et al., 2006] and in the radiocarbon
production rates [Müller et al., 2006] suggest lower solar
activity (higher cosmosgenic nuclide production) during
both the ~5.2 ka B.P. event as well as the ~4
-
4.5 ka B.P.
events. The lack of precise time scales makes direct compar-
isons dif
uential in mid- to late
Holocene climate change include orbital forcing and shorter-
term solar variations and volcanic eruptions, as well as
changes in greenhouse gas forcing, land cover, and modes
of climate variability. Complex feedback mechanisms among
vegetation, the atmosphere, oceans, snow, and ice may also
have been critical. While no evidence for volcanic forcing of
the mid-Holocene abrupt events discussed above have been
found in the ice core records, many of these mechanisms
occur gradually over long periods of time and thus would
require a nexus of events to push the climate system past
some threshold thus leading to rapid, large-scale changes in
the climate system.
final answer
on what role, if any, the relatively small changes in solar
forcing might have played in these events. This stems from
the lack of a clear physically based link between solar activ-
ity and solar forcing [Wanner et al., 2008].
It is not only in northern Africa, the Middle East, and Asia
that this middle Holocene drought appeared. Stable isotope
analyses of planktonic foraminifera from the Amazon fan
show that the highest
cult. Unfortunately, as yet, there is no
18
O values in the Holocene, sugges-
tive of reduced Amazon River
δ
ow, occurred at ~4.5 ka B.P.
[Maslin et al., 2000], almost contemporaneously with a
depletion of
18
O in the isotopic record from Lake Junin, Peru
[Seltzer et al., 2000]. Evidence for middle Holocene aridity
on the Altiplano comes from Lake Titicaca on the border
between Peru and Bolivia, where the percent of freshwater
3. CLIMATE CHANGE IN THE PERUVIAN ANDES
AND THE RISE AND FALL OF CIVILIZATIONS
We have the advantage of an increasing abundance of
written records from the late Holocene to help reconstruct
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