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An Andean ice-core record of a Middle Holocene mega-drought in North Africa and Asia


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An ice core from the Nevado Huascarán col in the Cordillera Blanca of northern Peru contains high-resolution time series of dust concentrations and size distributions since the end of the last glacial stage. A large dust peak, dated $4500 years ago, is contemporaneous with a widespread and prolonged drought that apparently extended from North Africa to eastern China, evidence of which occurs in historical, archeological and paleoclimatic records. This event may have been associated with several centuries of weak Asian/Indian/African monsoons, possibly linked with a protracted cooling in the North Atlantic. During the second half of the 20th century, high austral-summer dust concentrations in the Huascarán record are significantly correlated with atmospheric conditions, such as sea-level pressure and zonal wind velocities that are consistent with El Niño–Southern Oscillation (ENSO) and positive North Atlantic Oscillation (NAO) indices, and with aridity in North Africa, southwest Asia and the Middle East. Therefore, the dominant submicron fraction of the dust may have been transported by more intense northeasterly trade winds from the African dry regions across the tropical Atlantic during a period of frequent and/or intense ENSO activity. The proposed ENSO conditions that may have been linked with drought in the monsoon region may also have contributed to aridity in tropical South America, including the Cordillera Blanca.
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An Andean ice-core record of a Middle Holocene mega-drought
in North Africa and Asia
Mary E. DAVIS,
Byrd Polar Research Center, The Ohio State University, 1090 Carmack Road, Columbus, OH 43210-1002, USA
Department of Geological Sciences, The Ohio State University, Columbus, 155 South Oval Mall, Columbus,
OH 43210-1398, USA
ABSTRACT. An ice core from the Nevado Huascara
n col in the Cordillera Blanca of northern Peru
contains high-resolution time series of dust concentrations and size distributions since the end of the last
glacial stage. A large dust peak, dated 4500 years ago, is contemporaneous with a widespread and
prolonged drought that apparently extended from North Africa to eastern China, evidence of which
occurs in historical, archeological and paleoclimatic records. This event may have been associated with
several centuries of weak Asian/Indian/African monsoons, possibly linked with a protracted cooling in
the North Atlantic. During the second half of the 20th century, high austral-summer dust concentrations
in the Huascara
n record are significantly correlated with atmospheric conditions, such as sea-level
pressure and zonal wind velocities that are consistent with El Nin˜o–Southern Oscillation (ENSO) and
positive North Atlantic Oscillation (NAO) indices, and with aridity in North Africa, southwest Asia and
the Middle East. Therefore, the dominant submicron fraction of the dust may have been transported by
more intense northeasterly trade winds from the African dry regions across the tropical Atlantic during a
period of frequent and/or intense ENSO activity. The proposed ENSO conditions that may have been
linked with drought in the monsoon region may also have contributed to aridity in tropical South
America, including the Cordillera Blanca.
Compared to the last glacial stage (LGS) and the subsequent
deglaciation, the Holocene has been a period of relative
climatic calm during which human populations and civiliza-
tions have flourished and expanded over the Earth. However,
it has not been completely free of the kind of climatic
disruptions that have had detrimental effects on even
powerful societies. One such event, which was apparently
a far-reaching period of aridity, occurred 4000–4500 years
ago (4.0–4.5 kyr
BP), and left its mark on a variety of
archeological and geological records, including those from
excavations in Syria (Weiss and others, 1993), lake levels in
North Africa (Gasse, 1977; Servant and Servant-Vildary,
1980; Gillespie and others, 1983) and western China (Gasse
and others, 1991), speoleothems in Israel (Bar-Matthews and
others, 1999) and eastern China (Wang and others, 2005),
marine cores from the Gulf of Oman (Cullen and others,
2000) and the Indus delta region (Staubwasser and others,
2003), peat deposits in Mongolia (Xiao and others, 2004),
and an ice core from eastern equatorial Africa (Thompson
and others, 2002). According to many of these records, the
onset of the drought was abrupt (possibly within a decade),
lasted for up to several centuries, then ended abruptly.
Although the majority of the documentation for this dry
period comes from the Eastern Hemisphere, evidence for a
similar abrupt climatic episode has been found in tropical
South America. For example, deposits from Lake Titicaca on
the border between Peru and Bolivia (Baker and others,
2001; Tapia and others, 2003) and from the Amazon fan
(Maslin and Burns, 2000) show a Middle Holocene period of
desiccation. Finally, an ice core from a high-altitude Andean
tropical glacier contains a prominent dust event within the
Holocene, with particle concentrations that are several
orders of magnitude higher than all other levels since the
end of the LGS (Thompson, 2000; Thompson and others,
2000). The timing and nature of the dust peak in this ice
core, its possible linkages with the Asian/African climate,
and the possible Middle Holocene climate forcings for such
an event are discussed in this paper.
In the austral winter of 1993 (June–August) a team from the
Byrd Polar Research Center’s (BPRC) Ice Core Paleoclima-
tology Research Group drilled two ice cores to bedrock on
the col of Nevado Huascara
S, 77836
6050 m a.s.l.) in the Cordillera Blanca of northern Peru
(Fig. 1). The first core (core 1, 160.4 m long) was cut into
samples that were melted and bottled in the field, but the
second (core 2, 166.1 m long) was returned frozen to BPRC
for high-resolution analyses. This core was cut into three
equivalent sets, each with 4476 samples, and analyzed for
the ratios of
O), mineral dust concentration
and size distribution, and anion concentrations (chloride,
nitrate and sulfate) analyses. Core 1, on the other hand, was
analyzed only for d
Above the firn/ice transition (33.5 m below surface
(mbs)) the three sets of samples were cut from the center of
the core inside a class 100 clean bench in a laboratory
freezer maintained at –108C. The firn was handled using
several pairs of pre-cleaned latex gloves, and the cut
samples were placed in pre-cleaned polystyrene containers
which were transported to a class 100 clean room where
they were allowed to melt before analysis. Below the firn/ice
transition the dust and anion samples were rinsed with
Millipore reagent-grade deionized water to remove con-
taminants from the surface. The core was cut such that the
Annals of Glaciology 43 200634
sample lengths decreased from 17 to 6 cm in the top 55 m,
then from 5 to 3 cm in the interval 55–113 mbs, and finally
to 2 cm at 113 mbs, which was the size at which they
remained to the bottom of the core.
Dust concentrations and size distributions were measured
using two Coulter Counters (Model TAII), one equipped with
a30mm aperture tube to count particles with diameters
between 0.63 and 16 mm in 14 size ranges, and the second
equipped with a 100 mm aperture tube to count particles
with diameters between 2.0 and 40 mm, also in 14 size
ranges. The two datasets were integrated and the total
concentrations between 0.63 and 40 mm were normalized to
1 mL of melted ice. A Finnigan MAT Delta E mass
spectrometer was used for the analyses of d
O. The anion
concentrations, which were analyzed with a Dionex Model
2010 ion chromatograph, were presented initially in
Thompson and others (1995), but will not be discussed
further here.
The d
O and dust concentration data are plotted by
depth (1 m averages) in Figure 2a and b, respectively. One
obvious feature in the d
O profile is the 6.3% increase from
165.3 mbs to 163 mbs, which is consistent with the Last
Glacial Maximum to Early Holocene isotopic difference
found in other ice cores of similar age (Thompson and others,
1998, table 2). The dust record shows that above 165 mbs
this core is relatively clean, with an average concentration of
15 000 particles per mL. The exception is a large peak
around 157.7 mbs (marked with an asterisk in Fig. 2b) with
an average concentration of 82 000 particles per mL.
Since the 1 year snow accumulation between 1992 and
1993 was 3.30 m (1.74 m ice equivalent), and the 20th-
century net accumulation rate, or net balance, averaged
1.4 m ice equivalent (Henderson, 1996), the upper portion
of the core could be sampled at sub-annual resolution. The
dating for this section was determined by counting the peaks
in dust, nitrate and d
O that formed every dry season during
the austral winter from June to September (Thompson and
others, 1995). Thus the annual averages of the measured
parameters were calculated based on thermal years (dry
season to dry season). However, below
AD 1817
(119.26 mbs) the thinning of the annual layers from vertical
flow was such that seasonal resolution of the parameters was
difficult to maintain. However, annual dating was still
possible down to 125.2 mbs (corresponding to the 1719/20
thermal year).
The original Holocene timescale for the Huascara´n ice
core (Thompson and others, 1995) was constructed using an
empirical two-parameter model (Bolzan, 1985; Reeh, 1988),
as shown in Figure 3a. The rate of thinning with depth (p in
the model equation in Fig. 3a) was first calculated for the top
of the core where the timescale was annually resolvable,
using the dates of 1915 (78 years before 1993) at 86.47 mbs
and 1817 (176 years before 1993) at 119.26 mbs as pinning
points, and assuming steady-state conditions with a constant
accumulation of 1.74 m ice equivalent (b in the equation).
The lower 47 m of the core (119.26–166.07 mbs) were dated
by constraining the
O depletion at 164.1 mbs (marked by
an arrow in Fig. 2a) as the Younger Dryas (YD) event. This
was ascertained by matching the lowest 3 m of the ice core
with the calibrated
C dated record of d
OofG. bulloides
from the deep-sea marine core SU81-18, which was drilled
in the tropical North Atlantic off the coast of Portugal (Bard
and others, 1987; Fairbanks, 1989). The midpoint of the YD
event in the Huascara´n record was assigned an age of
12.25 kyr
BP, consistent with the YD age in the layer-counted
Fig. 1. Map of Peru depicting the Andes Mountains (areas above
4000 m a.s.l.) and the location of Nevado Huascara
Fig. 2. One-meter averages of (a) d
O(%) and (b) dust concen-
trations (0.63–40.0 mm diameters) (10
) from Huascara´n
core 2. The arrow in (a) and the asterisk in (b) indicate features
discussed in the text.
Davis and Thompson: Andean ice-core record of a Middle Holocene mega-drought 35
GRIP and GISP2 (Greenland) ice-core records (Johnsen and
others, 1992; Taylor and others, 1993). With this lower
chronological horizon established, the Holocene part of the
Huascara´n record was calculated with the two-parameter
model using the thinning rate (p ¼ 1.253) that was deter-
mined for the upper 119 m of the core (Fig. 3a). In this
chronology, the large Holocene dust event at 157.7 mbs
(shown by the asterisk in Fig. 2b) was calculated at 2 kyr
Since the LGS portion of the Huascara´n record was
matched with an absolutely dated marine core, the
confidence in the chronology of that part of the record
was reasonable. However, the confidence in the Holocene
dating was lower because of the lack of chronological
calibration between
AD 1817 and 12.25 kyr BP. Several years
later, the Holocene timescale was revised using the analyses
of the isotopic composition of O
in the air trapped in
glacier ice bubbles (d
), which were measured by
T. Sowers at Pennsylvania State University. Sowers and
others (1989) demonstrated that d
reflects changes in
the global atmosphere. Since the inter-hemispheric mixing
time is only 1 year, the d
is constant throughout the
atmosphere at any time, even though the turnover period is
1200 years (Bender and others, 1994b). The glacial/inter-
glacial transition (8.0–15.0 kyr
BP) is characterized by
large, virtually simultaneous changes in d
in Green-
land and Antarctic cores that are dated by independent
methods (Sowers and Bender, 1995), with an estimated error
of 600 years in the age–depth relationships. When the ages
derived from these d
measurements (Bender and
others, 1994a) were correlated with the d
data from
samples from the Huascara´n ice core (Fig. 3b), the range of
error for the tropical core was construed as being similar to
the polar records over this transition. The measurements of
the Huascara´n d
provided several time horizons for the
Early Holocene that allowed for the development of a new
age–depth relationship for the entire Holocene by passing a
third-order regression curve through the chronological
horizons from the layer-counted points at the top to the
match points towards the bottom (Fig 3a). The
match points above 9 kyr
BP were not used to calculate the
regression because they covered too large a range of
possible corresponding dates in the Greenland record
(Fig. 3b), but they are plotted on the regression curve as a
confirmation of the accuracy of the age–depth relationship.
An error of 5 years was estimated around the upper
pinning point of the model (
AD 1720), and the range
increased to 600 years at 8 kyr
BP, as determined by the
transfer of the d
-based timescale over the glacial/
interglacial transition as discussed above. The original and
the revised age–depth relationships are compared in
Figure 3a. The new chronology for the Huascara´n record
moved the lower (earlier) boundary of the large Holocene
dust peak (hereafter referred to as the MHDE, or the Middle
Holocene dust event) from its originally reconstructed date
at 2 kyr
BP to 4.5 kyr BP.
The dust and d
O data for the entire 19 kyr Huascara´n ice-
core record, which are recalculated according to the revised
timescale, are illustrated in Figure 4a and b, respectively, as
100 year averages. Other than the very high dust concen-
tration levels of the LGS (prior to 17 kyr
BP), the MHDE is the
most prominent feature of the dust record. During this time,
the d
O was decreasing from its high postglacial levels at
10 kyr
BP. The moisture source for the eastern Peruvian
Andes and for the northern Amazon Basin ultimately is the
tropical North Atlantic, from where the water vapor is
carried by northeasterly trade winds over the ocean, then
recycled over the Amazon forest (Grootes and others, 1989;
Thompson and others, 1995). The d
O curve over the last
10 kyr resembles the tropical Northern Hemisphere insola-
tion curve through the Holocene (Berger and Loutre, 1991),
and both the d
O and the sea surface temperatures (SSTs) of
the source region may be reflections of the insolation
The MHDE is prominent in Figure 4b, and detailed (i.e.
every sample) views of the dust concentrations and size
distributions between 3.7 and 5.0 kyr
BP are shown in
Figure 4c–h. In addition to the high concentration
(Fig. 4c), the sizes of the mineral dust in the MHDE also
demonstrate that this is a unique feature in the Huascara´n
Holocene record. Submicron particles (Fig. 4d) constitute
up to 70% of the total concentration between 0.63 and
40 mm in diameter, which is 30–40% higher than pre- and
Fig. 3. (a) Timescale development for Huascara
n core 2. The earlier
timescale, which was developed for Thompson and others (1995)
and is depicted by the dashed line, was calculated using the two-
parameter model formula, with h (total length of the core in ice
equivalent) ¼ 136.40 m, b (modern accumulation in ice equiv-
alent) ¼ 1.74 m, p (thinning parameter calculated from this model
for the top of the core where layer counting was possible) ¼ 1.253.
In addition, z is depth in the core, and T is its corresponding age in
BP. The revised timescale is depicted by the solid line, and is
discussed in the text. The SU81-18 match points were derived from
matching the d
O from the lowest 3 m of the Huascara
n core (i.e.
the LGS) with the d
O from a tropical North Atlantic marine core.
(b) Matching between d
in the GISP2 ice core and the
n ice core. GISP2 data can be downloaded from ftp://
Davis and Thompson: Andean ice-core record of a Middle Holocene mega-drought36
post-MHDE levels (Fig. 4e). However, the size fraction
>5 mm also increases during this event (Fig. 4f), and even the
concentration of giant particles (>16 mm) ranges over an
order of magnitude higher (Fig. 4g), although the per cent of
this size range is reduced from 3% of the total concentration
in the pre- and post-event levels to 1% within the peak
(Fig. 4h). The concentrations of the particles >5.0 mm
actually increase 100 years before the increase in sub-
micron dust, and remain high for several decades after the
fine-dust levels abruptly drop.
There is a large collection of Holocene proxy climate
records from the Andes, the Altiplano and the Amazon Basin
that demonstrate that an arid period occurred during the
time of the MHDE in the Huascara´n ice-core record. Stable-
isotope analyses of planktonic foraminifera from the
Amazon fan show that one of highest
O values in the
Holocene (inferring reduced Amazon River flow) occurred
4.5 kyr
BP (Maslin and Burns, 2000), almost contempor-
aneously with a depletion of
O in the isotopic record from
Lake Junı´n, Peru (Seltzer and others, 2000). Evidence for
Middle Holocene aridity on the Altiplano comes from Lake
Titicaca on the border between Peru and Bolivia, where the
per cent of fresh-water plankton reached its lowest levels
between 5 and 2 kyr
BP, and the per cent saline diatoms
increased at 6 kyr
BP (
C age) and reached a maximum at
3.6–4.0 kyr
BP (
C age) (Baker and others, 2001). Tapia
and others (2003) offer a more refined timeline for climate
variation in this interval using the Lake Titicaca record of per
cent saline planktonic taxa, which confirms that although
water levels were low between 3 and 6 kyr
BP, the lowest
levels were achieved at 4.5 kyr
The significant increase in very large dust particles in the
MHDE in the Huascara´n core is suggestive of aridity in the
Cordillera Blanca. A drier climate may have resulted in
decreasing snowfall which could not keep pace with
sublimation during the dry season, leading to ice recession
on the peaks. There is no evidence in the Huascara´n climate
record (in the stable isotopes or in the visible stratigraphy),
nor in any nearby climate record, that an anomalous
warming occurred during this time.
The MHDE is noteworthy not only because it is a unique
feature in the Huascara´n Holocene record, but also because
it appeared during a time when a global-scale climatic
disruption affected a large region from North Africa through
the Mediterranean area, to as far east as China. The
paleoclimatic, archeological and historical records pre-
viously listed indicate that during the third millennium
(before the Common Era) a sudden and severe event, which
was probably a protracted drought, occurred in many
regions that had been under the expanded influence of the
Asian/Indian/African monsoon during the first half of the
Holocene. This drought was contemporaneous with the
decline of the Akkadian Empire in Mesopotamia (Weiss and
others, 1993), the failure of Nile River floods that were
coincident with the decline of North African civilizations
(Hassan, 1997) and the end of the Harappan civilization in
the Indus valley (Staubwasser and others, 2003). Whether
the drought was actually instrumental in the collapse of
these societies is a controversial point among archeologists
(e.g. Butzer, 1997), but the evidence is very strong that this
climatic anomaly occurred at or near the same point in time
as these historical events.
Figure 5 shows some of the geological records that
document this Middle Holocene aridity, in addition to the
Huascara´n dust profile. A marine core from the Gulf of
Oman (Cullen and others, 2000) contains an abrupt spike in
carbonates (Fig. 5a) which have been chemically traced to
an archeological site (Tell Leilan) in Syria, the home of the
Akkadian culture (Weiss and others, 1993). The shape of this
profile is similar to that from Huascara´n (Fig. 5b), which
shows a sudden onset and then a more gradual decline
followed by a secondary peak. An ice-core record from
Kilimanjaro, Tanzania, (Thompson and others, 2002) con-
tains an unusually thick, black dust band that is dated
around 4.0 kyr
BP (Fig. 5c), and is indicative of a period that
may have lasted several decades to centuries during which
the Northern Ice Field, presently the largest on the moun-
tain, had dramatically reduced in size. Lake records from
equatorial and North Africa (Gasse, 2000) (Fig. 5d–f)
indicate that water levels had decreased greatly, as did lake
levels from western Tibet (Gasse and others, 1991, 1996),
which are not shown.
Fig. 4. (a, b) Huascara
n core 2 d
O (a) and dust concentration (b)
records for the last 19 kyr, shown as 100 year averages. The MHDE
is prominent and in (c–h) the data are shown as actual samples
(average length 2 cm). (c) Total dust concentrations from 0.63 to
40 mm; (d) the concentration of submicron dust; (e) the per cent of
submicron dust with respect to total dust; (f–h) concentrations of
large (5–16 mm) (f) and giant (>16 to 40 mm) particles (g) through the
MDHE, and the per cent of large particles with respect to total
dust (h).
Davis and Thompson: Andean ice-core record of a Middle Holocene mega-drought 37
Other records not shown in Figure 5 also contain
evidence of a sudden and marked arid-climate episode at
this time. A speleothem from the Soreq cave in Israel (Bar-
Matthews and others, 1999) shows an abrupt drop in d
implying that lower precipitation occurred within the 4.2–
4.5 kyr
BP window. Palynological data from north-central
China suggest a cold, dry period from 3.95 to 4.45 kyr
which is inferred from a sudden decrease in tree pollen
concentration (Xiao and others, 2004). Morrill and others
(2003) compiled paleoclimatic data from monsoon Asia,
particularly China, to produce a pattern of abrupt mid-
Holocene monsoon failure in this region.
The discrepancy in timing between the onset of the
MHDE in Huascara´n and the carbonate peak in the Gulf of
Oman core (4.5 kyr
BP vs 4.2 kyr BP, respectively) may be the
result of the different techniques used to develop the
timescales for the two records. The marine record was
dated with the aid of
C dates from carbonates, whereas the
Holocene timescale for the ice core was based on flow
modeling and curve matching, using d
match points
with well-dated polar cores as previously described. This
allowed for the revised age of the MHDE, but with a range of
uncertainty between 5 years and 600 years. In fact, of all the
records shown in Figure 5, Huascara´n is the only one with a
timescale that was not developed using calibrated
C dates.
Despite timing differences, the dust event has the same
general shape in the high-resolution records from Huasca-
ra´n, the Gulf of Oman and Kilimanjaro, i.e. a large, abrupt
dust increase, then a more gradual decrease, followed by a
smaller, secondary peak. It is interesting that the character of
a climatic event should be so similar in records that come
from sites located in different environments on opposite
sides of the Earth. If these records are recording the same
event, by what physical processes are they linked?
During the austral summer (December–March), which is the
wet season in the Southern Hemisphere tropics, the Inter-
Tropical Convergence Zone is at its southernmost position
and the northeasterly trade winds move across the equatorial
Atlantic from west Africa to northeastern South America.
The dust that is entrained off the west coast of Africa by
frequent windstorms is caught up by the trade winds, which
are connected to the southern limb of the subtropical Azores
high-pressure system, which in turn is the southern dipole of
the North Atlantic Oscillation (NAO). Moulin and others
(1997) statistically linked the transport of dust from the west
African coast to the strength of the NAO. Much of the dust is
blown back towards the northeast by the clockwise motion
of the subtropical high, but a smaller amount is carried with
the moisture across the tropical North Atlantic, and in fact
Saharan dust which has been transported during the austral
summer has been found in the Amazon Basin (Swap and
others, 1992).
Since the source of Huascara´n’s precipitation is likely the
tropical Atlantic, it is probable that some of the dust that
originates in North Africa is carried by the moist air masses
across the Amazon Basin to the Cordillera Blanca and wet-
deposited on glaciers there (Davis, 2002). The recent
atmospheric processes that facilitate the movement of dust
between the desert regions of North Africa and tropical
South America are illustrated in Figure 6, which shows
correlation fields between the wet-season dust concen-
trations in the Huascara´n core and 58 58 grids of US
National Centers for Environmental Prediction/US National
Center for Atmospheric Research (NCEP/NCAR) re-analysis
data (Kalnay and others, 1996) from December to March of
global mean sea-level pressure (SLP; Fig. 6a) and global
850 mbar zonal winds (Fig. 6b) from 1949 to 1992, the last
full year of the ice-core record. High SLP is associated with
dry conditions, so the significant two-tailed Pearson
correlations (R > 0.3, significant at the 0.05 level) between
SLP in the Sahara, the Sahel, the Bode´le´ region of north-
central Africa (Fig. 6a) and ice-core dust concentration
suggest that aridity here may provide a potent potential
source region for the dust that eventually finds its way to the
Cordillera Blanca. Furthermore, the significant negative
correlations (R < –0.3, significant at the 0.05 level) between
Fig. 5. Comparisons of (a) the Gulf of Oman Middle Holocene
carbonate record on a calibrated
C timescale (modified from
Cullen and others, 2000); (b) the Huascara
n Holocene dust record;
(c) the dust from the Kilimanjaro ice core (modified from Thompson
and others, 2002); and (d–f) tropical African Holocene lake level
records (modified from Gasse, 2000) from Lake Abhe (Gasse, 1977)
(d), Ziway–Shala system (Gillespie and others, 1983) (e) and Bahr-
el-Ghazal (Servant and Servant-Vildary, 1980) (f). The grey bar
marks the Middle Holocene arid period in the records.
Davis and Thompson: Andean ice-core record of a Middle Holocene mega-drought38
the dust and lower-level easterly winds from North Africa to
the eastern coast of South America (Fig. 6b) indicate that
zonal wind velocities from the potential source tend to be
higher during high-dust years. The correlation field is
negative because easterly winds are recorded in the NCEP/
NCAR database as negative values (Fig. 6c). This relationship
ends just inside the northeast coast of Brazil, perhaps
because the relatively ‘linear’ trade-wind circulation over
the ocean is disrupted by moisture recycling and convective
activity over the Amazon rainforest.
Other regions of significant correlation are also observed
in Figure 6a, such as the negative relationship between
Huascara´n dust and SLP in the far North Atlantic (in the
region of the Icelandic low). When this is viewed in
conjunction with the significant +R field over North Africa,
the Mediterranean and the tropical Atlantic, it contributes to
the evidence that austral summers of high dust concentration
are coincident with boreal winters with positive NAO
indices. In addition, the SLP grid in the tropical western
Pacific shows positive significant correlations with the dust
concentrations, indicating negative Southern Oscillation
Indices (SOI), which are symptomatic of El Nin
o conditions.
Thus, the easterly circulation across the tropical Atlantic
that is influenced by the subtropical node of the NAO
appears to be the link between the mineral dust in the
Huascara´n glacier and the climate in North Africa and the
Middle East. However, the interplay between the strength
and phase of the NAO and the circulation of the African/
Asian monsoon is connected with the climate of this
potential source region for Huascara´n dust. Today during
winters of high NAO indices, the Icelandic low and the
Azores high intensify and move northward, causing the
westerly storm tracks to shift from the Mediterranean region
and North Africa to northern Europe (Fig. 6b). For example,
low winter discharge from the Tigris and Euphrates Rivers is
significantly correlated with the positive phase of the NAO
(Cullen and deMenocal, 2000). Not only does this result in a
drier climate in southern Eurasia and southwest Asia, but the
vigorous northeasterly trade-wind circulation from the
Azores high transports more available dust across the
Atlantic to South America. Positive NAO is also associated
with a more intense Middle Eastern Jet Stream (MEJS) (Yang
and others, 2004), which is an antecedent signal for a
weaker Asian monsoon. When the MEJS is strong in the
winter, an ENSO-type warming appears in the equatorial
eastern Pacific (NINO3). Historically, increases in the
NINO3 SSTs have been significantly correlated with Asian/
Indian monsoon failure, although the correlation has
weakened in recent decades (Kumar and others, 1999).
The development of ENSO conditions during years of
positive NAO is also indicated in Figure 6, which shows
atmospheric circulation patterns indicative of negative SOI.
These observations are qualitative, since currently there is
no statistically significant year-to-year link that has been
noted between ENSO and NAO, although over decadal
timescales there does appear to be a relationship between
NAO and the ENSO-linked Pacific Decadal Oscillation
(PDO) (Readinger, 2003).
Possible teleconnections have been proposed between
the North Atlantic and the African/Asian monsoons during
the last glacial (Overpeck and others, 1996; Schulz and
others, 1998), during the glacial/Holocene transition (Sir-
ocko and others, 1996) and during the Holocene (Gupta and
others, 2003; Wang and others, 2005). Stated simply,
monsoon failures were contemporaneous with North
Atlantic cooling (associated with arctic ice discharge),
which in turn contributed to strengthened westerlies across
Eurasia. Comparisons between marine records from the
Arabian Sea and the North Atlantic show that these
conditions existed several times during the Holocene, and
one of the later episodes occurred between 4.0 and
4.5 kyr
BP (Gupta and others, 2003).
The MHDE therefore may have been connected to a
protracted interval of ENSO-linked African/Asian monsoon
weakening, which was coupled with strengthening of the
Icelandic low and Azores high. Cooling of the North Atlantic
accompanied an ice discharge event, and contributed to
more intense westerlies across northern Eurasia. Currently,
ENSO events are also linked to aridity in the Cordillera
Blanca, and if this was the case in the Middle Holocene, ice
recession on the Huascara´n peaks (which resulted in the
influx of large particles onto the col) may also have been
controlled by these conditions. Because the concentrations
of large dust particles in the MHDE increased before the
influx of the submicron particles, the ENSO conditions
responsible for aridity in the northeast Andes may have
Fig. 6. Significant correlation fields (–0.3 > R > +0.3, significant at
the 0.05 level) between Huascara´n wet-season (austral summer)
dust concentration and NCEP/NCAR re-analysis data (December–
March) for (a) SLP and (b) zonal wind velocity from 1949 to 1992.
Dark/light shading depicts fields of significant positive/negative
correlations. The correlation grids are 58 by 58. (c) A comparison
between the wet-season dust concentrations from 1949 to 1992
and the easterly wind velocities in the region of highest correlation
in north-central Africa illustrates why R is negative in the latitudes
dominated by easterlies.
Davis and Thompson: Andean ice-core record of a Middle Holocene mega-drought 39
preceded the monsoon weakening. In fact, the timing of the
interval of lake desiccation in tropical South America
suggests that it may have predated and postdated the Eastern
Hemisphere drought. The major elements of this near-
global-scale mechanism are illustrated in Figure 7, along
with the locations of some of the sites from where records of
this abrupt and prolonged aridity have been obtained.
The MHDE may have been the product of a protracted series
of ENSO events that possibly were associated with wide-
spread monsoon failures, which in turn may have been
linked with North Atlantic regional cooling and strength-
ened westerlies over Eurasia. The appearance of abrupt
climatic disruptions in such distant locations as the tropical
Andes, North Africa, the Middle East, southwest to east Asia
and the North Atlantic suggests strong linkages between
high- and low-latitude atmospheric processes. The questions
that remain to be addressed concern: (1) the underlying
cause for this near-global-scale episode, and (2) which
processes, i.e. those in the tropics (ENSO, monsoon
circulation) or those in the North Atlantic (ice rafting, ocean
cooling), might have been the triggers. The ultimate forcing
for abrupt climate change on orbital timescales is thought to
be non-linear responses (through oceanic, vegetative and
possibly cryospheric feedbacks) to linear insolation changes
(Ganopolski and others, 1998; deMenocal and others,
2000). In fact, a series of feedbacks between the low
latitudes and the arctic region may have sustained the
Middle Holocene drought in the tropics over several
hundred years.
We gratefully acknowledge the participants in the 1993
Huascara´n drilling expedition, B. Koci, V. Mikhalenko,
G. Seltzer, P. Kinder and Pin-Nan Lin, and the Peruvian
mountaineers F. Vicencio Maguina, M. Camones Gonzales
and M. Henostroga Zambrano. We also thank Pin-Nan Lin
for the d
O analysis, and K. Henderson whose efforts to
reconstruct the Huascara´n timescale were invaluable. The
Huascara´n program was funded by the US National Oceanic
and Atmospheric Administration. The comments of U. Ruth
and an anonymous reviewer greatly helped to improve this
paper and are very much appreciated. This is Byrd Polar
Research Center contribution No. 1322.
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... Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O 2 in air bubbles (δ 18 O atm ), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ 18 O ice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. ...
... Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O 2 in air bubbles (δ 18 O atm ), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ 18 O ice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δ 18 O atm , although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (O 2 ) by micro-organisms in the ice, which fractionates the δ 18 O of O 2 from the atmospheric value. ...
... The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δ 18 O ice are complex and temporally variable; however, Guliya δ 18 O ice values since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature. ...
Full-text available
Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O2 in air bubbles (δ18Oatm), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ18Oice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δ18Oatm, although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (O2) by micro-organisms in the ice, which fractionates the δ18O of O2 from the atmospheric value. Here, we make corrections for melt and respiration to address these complications. The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δ18Oice are complex and temporally variable; however, Guliya δ18Oice values since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature.
... A. Limites chronologiques et fondements scientifiques D'après les résultats des études de l'équipe de Thompson en 1993(Thompson et al. 1995Davis et Thompson 2006), sur le flanc occidental de la Sierra andine péruvienne (Cordillère Blanche) 18 , la transition Pléistocène -Holocène a eu lieu vers 10 000 ans BP 19 , tandis que l'Holocène moyen a débuté vers 8200 ans BP, pour finir vers 4200 ans BP (Walker et al. 2012 ;Davis et Thompson 2006). De ces trois périodes, la plus courte est l'Holocène ancien. ...
... A. Limites chronologiques et fondements scientifiques D'après les résultats des études de l'équipe de Thompson en 1993(Thompson et al. 1995Davis et Thompson 2006), sur le flanc occidental de la Sierra andine péruvienne (Cordillère Blanche) 18 , la transition Pléistocène -Holocène a eu lieu vers 10 000 ans BP 19 , tandis que l'Holocène moyen a débuté vers 8200 ans BP, pour finir vers 4200 ans BP (Walker et al. 2012 ;Davis et Thompson 2006). De ces trois périodes, la plus courte est l'Holocène ancien. ...
... 7 l'ensemble du continent américain peut paraître risquée, cela fonctionne bien pour notre aire spécifique d'étude et nous évite la tâche de développer une longue liste des chronologies régionales ou sous-continentales qui ne feraient qu'élargir infructueusement cette section. Ainsi, comme il était déjà évoqué, le cadre chronologique est tiré des deux sources suivantes : -pour l'Amérique du Sud, les études des carottes glaciaires du Huascaran (9°6' S, 77° 36 W, à 6050 mètres, dans la Cordillère centrale du Pérou), nous ont servi pour la reconstitution du paléoenvironnement et l'étude des changements climatiques pendant l'Holocène (études réalisées par l'équipe de Thompson en 1993: Thompson et al. 1995Davis et Thompson 2006). ...
Cette thèse porte sur l'étude technologique de matériel lithique provenant de trois contextes archéologiques situés dans la Puna péruvienne (la plus haute région écologique habitable des Andes, au-dessus de 4000 mètres). Il s'agit des sites inédits de Toromocho (département de Junin, puna central) et d’Yurac Corral (département d'Ayacucho, puna centre-sud).L'état actuel des recherches ne nous permet pas de dater les sites archéologiques. Par chronologie relative, nous désignons - dans son sens le plus large - la période de l'Holocène récent (fin des périodes préhistoriques, avec le passage aux périodes pastorales, jusqu’aux périodes précolombiennes, de 3000 av. J.-C jusqu’à 1532 ap. J.-C).Cette thèse dévoile certains aspects technologiques qui sont habituellement quasiment absents des études traditionnelles péruviennes. De même, ce travail de recherche apporte des contributions méthodologiques appliquées aux fouilles, qui ne sont pas pratiquées actuellement au Pérou. Cette thèse rassemble aussi une riche compilation bibliographique des principaux sites archéologiques préhistoriques des Andes péruviennes. Ainsi, dans l’annexe 1, ce travail de recherche propose une introduction à la problématique des premières populations du continent américain. Les aspects méthodologiques, couplés à la présentation de contextes archéologiques inédits ainsi que les sources bibliographiques se rapportant aux diverses problématiques, font l’originalité de cette thèse. L’objectif de nos recherches a considéré l’importance de réactiver et de valoriser les études lithiques du Pérou, en se fondant sur de nouvelles méthodologies de recherche.
... Increasing paleoclimatic data suggest that global drought conditions in the northern hemisphere were severe and prolonged (200-300 years) and began at around 4.2 ka b.p. (e.g., An et al. 2005;Arz et al. 2006;Dixit et al. 2014;Nakamura et al. 2016). The 4.2 ka b.p. event led to rapid climate change in the tropical Americas, as it is recorded as an abrupt change in speleothems (stalagmite) records from Peru and Brazil (Bustamante et al. 2016), in Andean ice cores (Davis and Thompson 2006), and in the biogeochemical composition of peat bogs in Peru (Schittek et al. 2015). With precise dating techniques, the synchronicity of these global changes is now widely accepted (Booth et al. 2005). ...
The fourth millennium b.p. in the Maya lowlands provides an interesting case, with mobile, aceramic peoples documented, while ceramic-using villagers lived in other parts of Mesoamerica. Rather than ask why ceramic containers and village life took so long to reach the Maya lowlands, the question can be inverted to posit that a mixed horticultural-foraging adaptation was so effective that it persisted longer than elsewhere. I propose that the so-called 4.2 ka b.p. event was the ultimate cause of increased sedentism and the first adoption of ceramic containers in a limited number of regions of Mesoamerica. My musings are grounded in the comparisons of data from the Soconusco region of southern Mexico and evidence from northern Belize at Colha and Pulltrouser Swamp, as well as the Freshwater Creek drainage. I assume that proximate behavior must account for local adaptations and different rates of change in each region of Mesoamerica. Therefore, regional adaptation in northern Belize during the Late Archaic period provides the evidence with which to reconstruct local adaptation. Excavations and regional reconnaissance document a distinctive orange soil horizon at Progresso Lagoon associated with patinated chert tools and an absence of ceramics. Stone tool assemblages from the preceramic components of three sites in the region indicate a spatial separation of tool use and resharpening at island versus shore. Starch grains recovered from these stone tools indicate that preceramic peoples in northern Belize harvested maize and several other domesticated plant species. These data are consistent with local paleoenvironmental studies that document an extended period of horticultural activity during the fifth and fourth millennia b.p. prior to the adoption of ceramics. Lithic assemblages and associated dietary information from multiple sites provide glimpses of the data necessary to reconstruct Late Archaic period adaptation from a single locale. Such data will be required to understand the proximate causes for the transition to a more settled, village life.
... A pesar de que las evidencias de este corto cambio climático abrupto, en un sentido diacrónico, no son siempre notorias en el registro paleoambiental y arqueológico, cada vez se conocen más investigaciones que correlacionan este evento climático del 4.2 ka cal BP con el potencial colapso de diversas civilizaciones en todo el planeta, lo que demuestra su carácter global antes señalado. Este es el caso de colapsos contemporáneos detectados en China (Wu y Liu, 2004;Gao et al., 2007), América del Norte (Booth et al., 2005;Menounos et al., 2008), Italia (Drysdale et al., 2005), Islas Británicas (Roland et al., 2014), Mesopotamia Cullen y de Menocal 2000;Kerr, 1998;Gibbons, 1993), valle del Indo (Staubwasser et al., 2003), Egipto (Stanley et al., 2003), Oriente Próximo (Riehl, 2008), África del Norte y Asia Occidental (Gasse y Van Campo, 1994;Davis et al., 2006), Mediterráneo oriental, central y occidental (Bar-Matthews et al., 1997;Bini et al., 2019;Di Rita and Magri, 2019), mar Rojo (Arz et al., 2006), Kilimanjaro (Thompson et al., 2002), o en Arabia (Parker et al., 2006). ...
Full-text available
RESUMEN Estudios llevados a cabo en los últimos años indican que la Cultura de las Motillas-asentamientos de la Edad del Bron-ce de La Mancha-pudieron constituir la primera ordenación del territorio con fines de abastecimiento hídrico, dando lugar a la que se podría denominar la primera cultura hidráulica de Europa. El análisis de los datos permite establecer una estrecha relación entre las características geológicas y la ubicación de las motillas. Fueron construidas durante el Evento Climático 4.2 ka cal BP, en un momento de estrés ambiental. La construcción de pozos para llegar al nivel freá-tico regional, y aprovechar el agua subterránea, constituyó una exitosa solución que pervivió casi un milenio y formó parte principal de los procesos de cambio hacia una sociedad más compleja y jerarquizada. El Holoceno constituye un periodo geológico muy dinámico en lo referente a las fluctuaciones climatológicas. Una de las más importantes, con repercusión a nivel mundial, es el mencionado evento climático 4.2 ka cal BP, relacionado con el colapso de diversas civilizaciones. Este evento, en nuestro territorio, ocurrió en la transición entre la Edad del Cobre y la del Bronce en La Mancha, y se caracterizó por una marcada aridez, con una fase más intensa, entre 2.000 y 1.800 cal BC, en que dismi-nuyeron de manera notable las precipitaciones y se incrementó la temperatura. La Cultura de las Motillas de la Edad del Bronce de La Mancha constituye una singular forma de adaptación de los pobladores del territorio a esta situación climatológica. ABSTRACT Recent investigations indicate that the culture of the "motillas"-the Bronze Age settlements of La Mancha-may be the oldest evidence for large-scale water management in Europe. The archaeological and paleo-environmental data suggest a close relationship between the location of the "motillas" and the geological landscape. "Motillas" were built during the 4.2 ka cal BP climate event, at a time of environmental stress. The construction of wells that reached the local water table to access groundwater was a successful solution that lasted almost a millennium and was an important technological development that shaped the emergence of more complex and hierarchical societies in the region. The Holocene is a dynamic geological period in terms of climatic fluctuations. One of the most important of these dynamics , with global impact, is the aforementioned 4.2 ka cal BP climate event, which has been related to the collapse of diverse civilizations around the world. This event, in the Iberian Peninsula, occurred at the transition between the Copper Age and Bronze Age in La Mancha (as well as in other regions of the Peninsula). It was characterized by marked aridity, with a more intense phase, between 2,000 and 1,800 cal BC, during which there was a decrease in rainfall and an increase in temperature. The Bronze Age culture of the "motillas" of La Mancha constitutes a unique adaptation of the inhabitants of the territory to this climatic situation.
... Concentrations of insoluble dust, major ions, and oxygen isotopes of glacier ice were analyzed as described previously [126]. The development of the chronologies for the two ice cores from which the samples were collected is discussed in Additional file 1: Table S3, where the ages of the samples were provided. ...
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Background Glacier ice archives information, including microbiology, that helps reveal paleoclimate histories and predict future climate change. Though glacier-ice microbes are studied using culture or amplicon approaches, more challenging metagenomic approaches, which provide access to functional, genome-resolved information and viruses, are under-utilized, partly due to low biomass and potential contamination. Results We expand existing clean sampling procedures using controlled artificial ice-core experiments and adapted previously established low-biomass metagenomic approaches to study glacier-ice viruses. Controlled sampling experiments drastically reduced mock contaminants including bacteria, viruses, and free DNA to background levels. Amplicon sequencing from eight depths of two Tibetan Plateau ice cores revealed common glacier-ice lineages including Janthinobacterium, Polaromonas, Herminiimonas, Flavobacterium, Sphingomonas, and Methylobacterium as the dominant genera, while microbial communities were significantly different between two ice cores, associating with different climate conditions during deposition. Separately, ~355- and ~14,400-year-old ice were subject to viral enrichment and low-input quantitative sequencing, yielding genomic sequences for 33 vOTUs. These were virtually all unique to this study, representing 28 novel genera and not a single species shared with 225 environmentally diverse viromes. Further, 42.4% of the vOTUs were identifiable temperate, which is significantly higher than that in gut, soil, and marine viromes, and indicates that temperate phages are possibly favored in glacier-ice environments before being frozen. In silico host predictions linked 18 vOTUs to co-occurring abundant bacteria (Methylobacterium, Sphingomonas, and Janthinobacterium), indicating that these phages infected ice-abundant bacterial groups before being archived. Functional genome annotation revealed four virus-encoded auxiliary metabolic genes, particularly two motility genes suggest viruses potentially facilitate nutrient acquisition for their hosts. Finally, given their possible importance to methane cycling in ice, we focused on Methylobacterium viruses by contextualizing our ice-observed viruses against 123 viromes and prophages extracted from 131 Methylobacterium genomes, revealing that the archived viruses might originate from soil or plants. Conclusions Together, these efforts further microbial and viral sampling procedures for glacier ice and provide a first window into viral communities and functions in ancient glacier environments. Such methods and datasets can potentially enable researchers to contextualize new discoveries and begin to incorporate glacier-ice microbes and their viruses relative to past and present climate change in geographically diverse regions globally. 8eoz6b1Gq7e8M2dM_SBXdyVideo Abstract
... They are often observed in the last glacial period (the Pleistocene) (Fawcett et al., 2011) and the postglacial period (the Holocene) (Forman et al., 2001), including the last millennium (Stahle et al., 2012). Scientific literature has reported such megadroughts for all continents, for example in Europe (Helama et al., 2009;Cook et al., 2016a) and North America (Acuña-Soto et Stahle et al., 2007;Seager et al., 2008) during the Middle Ages, in Asia and Oceania in the last millennium Sinha et al., 2011;Vance et al., 2015) and in Africa from the Holocene to the last millennium (Davis and Thompson, 2006;Scholz et al., 2007;Mulitza et al., 2008). ...
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Droughts have deep, widespread and underestimated impacts on societies, ecosystems, and economies. They incur costs that are borne disproportionately by the most vulnerable people. The extensive impacts of drought are consistently underreported even though they span large areas, cascade through systems and scales, and linger through time, affecting millions of people and contributing to food insecurity, poverty, and inequality. Climate change is increasing temperatures and disrupting rainfall patterns, increasing the frequency, severity, and duration of droughts in many regions across the globe. As we move towards a 2˚C warmer world, urgent action is required to better understand and more effectively manage drought risk to reduce the devastating toll on human lives and livelihoods, and ecosystems. The GAR Special Report on Drought 2021 explores the systemic nature of drought and its impacts on achievement of the Sendai Framework for Disaster Risk Reduction, the SDGs and human and ecosystems health and wellbeing.
... Because of the absence of charcoal fragments to accomplish 14 C dating, it is possible mudflows took place during the Neolithic Subpluvial period, from 9000 to 9500 to 5000-5500 years before present (YBP), which was one of the most wet and rainy periods of northern Africa and was preceded and followed by much drier periods (e.g., deMenocal et al., 2000;Bard, 2005). The following period known as the "4.2 ka aridification event" (started 4200 YBP) is one of the most severe arid periods of the Holocene for northern Africa, southern Europe, Arabia, and Asia (e.g., Bond et al., 2001;Arz et al., 2006;Davis and Thompson, 2006), and may have preserved the sediment deposited during the rainy period (Zielhofer et al., 2002). Because of the emplacement of different mudflows, the lower silt content in the CA and Bw1 horizons could not be interpreted as the result of actual deflation only, as it could be also due to the texture the former mudflow layers had. ...
Arid environments are fragile and the associated soils are subject to serious threats like water deficiency, erosion, salt accumulation, and loss of fertility. In this context, understanding the processes involved in soil genesis may contribute toward protecting land from degradation. This study highlights the interconnection between geomorphic and pedogenetic processes in soil formation of the Jeffara Plain, a pre-Saharan area of southern Tunisia. To reach this goal, one coastal oasis (Chenini Nahel) and two inland environments (Matmata Nouvelle and Menzel Habib) were studied. After geomorphological and pedological surveys, the soils were sampled by genetic horizons and characterized by physical, mineralogical, and chemical analyses, and by microscope observation. Field observations and laboratory data suggest that soil formation in the Jeffara Plain was a combination of additions and losses controlled by climate changes. At Chenini Nahel, the soils developed by accumulation of wind-blown sediments coming from a close area dominated by gypsum-bearing rocks. At Matmata Nouvelle, the soils mainly formed from sedimentation of repeated mudflows during a rainy period between 9000 and 5000 years before present, followed by drought periods. Finally, the soils of Menzel Habib developed from an early gypsum formation in the presence of a salt-rich water table and repeated cycles of sedimentation/deflation of wind-blown materials. The different genesis of these pre-desert soils produced characteristic B horizons: Byy horizons with poorly developed soil structure at Chenini Nahel, Bw horizons with a hard rupture-resistance at Matmata Nouvelle, and Bk horizons at depth due to intense sedimentation with CA + BC horizons at the surface due to the accretion of wind-blown materials at Menzel Habib.
... Since the initial publication the Holocene timescale has been fine-tuned using δ 18 O measurements of air from bubbles in the ice (Thompson, 2000;Davis and Thompson, 2006). A middle-Holocene dust event (MHDE), originally dated at ~2000 yrs BP, was determined to have occurred ~4500 yrs BP, which is contemporaneous with a mid-Holocene drought documented in many paleoclimate records throughout the world ( Figure 3). ...
En los Andes peruanos, el calentamiento de la troposfera media, potenciado por el reciente fuerte El Niño, está destruyendo las señales climáticas preservadas en los campos de hielo y acelerando la retirada de los glaciares. En ninguna parte está mejor documentada la pérdida de los glaciares tropicales y es más importante que en los Andes del Perú. El registro más largo de retirada de glaciares proviene de un estudio de 44 años realizado en el casquete de hielo Quelccaya en el sur de los Andes, que prueba la pérdida de un archivo climático muy importante y la pérdida acelerada de un recurso hídrico que alimenta el río Amazonas y el lago Titicaca. En la Cordillera Blanca, los glaciares por debajo de los 5400 msnm sufren tanto un deshielo estacional como el movimiento del agua de deshielo a través de las capas superiores porosas. Debido a su gran altitud, el nevado Huascarán es uno de los pocos sitios tropicales donde aún se conserva una historia climática prácticamente inalterada, que se extiende hasta la última etapa glacial. Sin embargo, todos los glaciares de Cordillera Blanca documentados por INAIGEM (en prensa) estar en retroceso. Dadas las tasas actuales de calentamiento en los Andes tropicales, solo es cuestión de tiempo para que los registros climáticos del hielo del Huascarán también se pierdan. El retroceso de los glaciares a lo largo de los Andes peruanos está contribuyendo a las crisis emergentes de los recursos hídricos y los peligros ambientales tanto para las poblaciones urbanas como rurales. Aunque actualmente la descarga de la estación seca está aumentando, no se mantendrá así a largo plazo. La mayoría de la población del Perú vive en el desierto de la costa oeste, que depende de los ríos alimentados por los glaciares para la agricultura y los medios de subsistencia. El derretimiento de los glaciares también agrava los peligros geológicos en esta región propensa a terremotos, formando lagunas glaciares con represas de hielo o morrenas, lo que puede dar como resultado estallidos de lagunas e inundaciones y flujos de escombros. Comprender el impacto de esta aceleración de la pérdida de glaciares en los recursos hídricos futuros requiere informaciónsobre los cambios del pasado en el balance de masas de los glaciares de alta elevación.
To determine the timing of megafaunal extinction in the high plains of Peru and also to determine if the timing was delayed in grasslands compared with previously published forested settings to dissociate the effects of succession from human or climatic impacts. The Junín Plateau, Peru. Flowering plants and Ascomycetes. The sediments used in this analysis were collected from the edge of Lake Junín, Peru. Eleven ages derived from 14C accelerator mass spectrometry provide a chronology. We provide a paleoecological reconstruction of past climate and vegetation change, fire history and megafaunal herbivore presence from c. 20,000–7000 years ago based on fossil pollen, charcoal and Sporormiella spores. Data were analysed using multivariate analysis and Bayesian change point analysis. Megafaunal populations appear to have been positively impacted by dry climatic oscillations until c. 15,200 years ago. A reduction in Sporormiella abundance between 13,000 and 12,300 years before the present coincides with increased charcoal abundance and is identified as the period of megafaunal population collapse leading to extinction. The timing of the extinction does not differ substantially from that observed in wooded Andean settings. The timing of the collapse of megafaunal populations in high grasslands was very similar to that of lower, now‐forested settings. Upticks in fire activity, during what is generally seen to be a wet period, formed the backdrop to extinction and are strongly associated with human activity.
In the study of global change, the relationship between the Holocene climatic disaster events and the rise and fall of ancient civilization is of considerable significance to a profound understanding of civilization evolution and human‐land harmony. During the geological survey in the western Wei River Basin, a debris‐flow event that would have destroyed an archaeological site was uncovered in the Holocene loesson the south bank of the Qian River. They were studied by field observations and laboratory analysis, including magnetic susceptibility, particle size distribution, carbonate content determination, and AMS14C dating. The debris‐flow event was dated at about 3702 BP to 4084 cal BP by using the AMS14C in combination with archaeological artefacts age determination. Mingling with cultural layer, whole pig bones, and covering ash pit and cellars, the debris flow incident was linked to the site destruction. Combined with the global climate background at about 4000 a BP, the palaeo‐floods records in the Wei River Basin, and the regional palaeo‐earthquakes, we infer that the debris‐flow was triggered by large earthquake and heavy rainfall and was a regional hydrological response to the 4.0 ka global climate event. Moreover, the debris‐flow incident and its resulting archaeological site ruin have essential scientific significance for exploring the climate environment evolution, the ancient civilization evolvement, and the human‐land relationship development in the Wei River Basin and surrounding area.
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Saharan dust is shown to enter the Central Amazon Basin (CAB) in bursts which accompany major wet season rain systems. Low-level horizontal convergence feeding these rain systems draws dust from plumes which have crossed the tropical Atlantic under the large-scale circulation fields. Mass exchange of air between the surface and 4 km over the eastern Amazon basin is calculated using rawinsonde data collected during storm events. Mean concentrations of dust observed by aircraft over the western tropical Atlantic are used to calculate the amount of dust injected into the Basin. Individual storm events inject some 480,000 tons of dust into the north-eastern Amazon Basin. Storm and dust climatology suggest that the annual importation of dust is in the order of 13 Mtons. In the north-eastern basin, this may amount to as much as 190 kg ha -1 yr -1 . Deposition of trace species, such as phosphate, associated with this dust ranges from 1-4 kg ha -1 yr -1 . Uncertainties in these estimates are not believed to be greater than ± 50% and may be as low as ± 20%. The deposition fluxes from Saharan dust are essentially identical to the CAB wet deposition fluxes from precipitation in the wet season; a result that implies that the major ionic composition of rain water in the CAB wet season may be strongly influenced by inputs of material originating on the African continent nearly 5000 km away. The total amount of Saharan dust calculated to enter the Amazon basin is 1/2 to 1/3 of that estimated to cross 60°W longitude between 10° and 25°N latitude. We conclude that part of the productivity of the Amazon rain forest is dependent upon critical trace elements contained in the soil dust originating in the Sahara/Sahel. This dependence should be reflected by expansions and contractions of the Amazon rain forest in direct relationship to expansions and contractions of the Sahara/Sahel. Turnover rates for nutrient species deposited with Saharan dust in the Amazon Basin suggest a time scale of 500 to 20,000 years. We believe the dependence of one large ecosystem upon another separated by an ocean and coupled by the atmosphere to be fundamentally important to any view of how the global system functions. Any strategy designed to preserve the Amazonian rain forest or any part thereof should equally concern itself with the inter-relationship between the rain forest, global climate and arid zones well removed from Amazonia. DOI: 10.1034/j.1600-0889.1992.t01-1-00005.x
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The glacial oscillations which dominate the Quaternary global climate are closely related to changes in incoming solar radiation, but the details of palaeoclimatic spectra suggest that other parameters, usually referred to as feedback, have significantly increased the climatic response to extraterrestrial forcing1. The last deglaciation appears to be one of the more characteristic examples of nonlinearity because its abruptness strongly contrasts with the smooth shape of the associated solar radiation maximum dated at ∼11,000 yr BP. In addition, the presence of the Younger Dryas cold event, concomitant with the solar radiation maximum, could be interpreted as a transitory phenomenon caused by the rapid meltwater influx linked to the first phase of the ice-sheet's disintegration2,3. Here, using accelerator mass spectrometry, we date precisely the movements of the polar front during the whole period of glacial retreat ∼15,000-8,000 yr BP. Our results show that the polar front has moved between 35° N and 55° N in < 1,000 yr for the earliest retreat ∼12,500-12,000 BP and moved almost 'instantaneously' (that is < 400 yr) during the two abrupt climatic changes associated with the Younger Dry as cold event.
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We review the current understanding of the Dole effect (the observed difference between the δ18O of atmospheric O2 and that of seawater) and its causes, extend the record of variations in the Dole effect back to 130 kyr before present using data on the δ18O of O2 obtained from studying the Vostok ice core (Sowers et al., 1993), and discuss the significance of temporal variations. The Dole effect reflects oxygen isotope fractionation during photosynthesis, respiration, and hydrologic processes (evaporation, precipitation, and evapotranspiration). Our best prediction of the present-day Dole effect, +20.8‰, is considerably lower than the observed value, +23.5‰, and we discuss possible causes of this discrepancy. During the past 130 kyr, the Dole effect has been 0.05‰ lower than the present value, on average. The standard deviation of the Dole effect from the mean has been only ±0.2‰, and the Dole effect is nearly unchanged between glacial maxima and interglacial periods. The small variability in the Dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant. Most periodic variability in the Dole effect is in the precession band, suggesting that changes in this global biogeochemical term reflects variations in low-latitude land hydrology and productivity or possibly variability in low-latitude oceanic productivity.
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In this study, the authors address several issues with respect to the antecedent signals of the large-scale Asian summer monsoon that were earlier identified by Webster and Yang. In particular, they revisit the changes in the subtropical upper-tropospheric westerlies preceding the monsoon, depict the detailed structure of the monsoon's antecedent signals, and investigate the physical processes from the signals to the monsoon. They also explore the teleconnection of these signals to various large-scale climate phenomena and emphasize the importance of the upstream location of the signals relative to the Tibetan Plateau and the monsoon. Before a strong (weak) Asian summer monsoon, the 200-mb westerlies over subtropical Asia are weak (strong) during the previous winter and spring. A significant feature of these signals is represented by the variability of the Middle East jet stream whose changes are linked to the Arctic Oscillation, North Atlantic Oscillation, El Niño–Southern Oscillation, and other climate phenomena. When this jet stream intensifies and shifts southeastward, cold air intrudes frequently from eastern Europe into the Middle East and southwestern Asia. As a result, in subtropical Asia, snow and precipitation increase, the ground wetness increases, and surface temperature decreases. A strengthening Middle East jet stream is also accompanied by increases in both stationary wave activity flux and higher-frequency eddy activities. The Tibetan Plateau acts to block these westerly activities propagating eastward and increase the persistence of the low-temperature anomalies, which in turn prolongs the atmospheric signals from winter to spring. A strong link is found between the persistent low-temperature anomalies and the decrease in geopotential height over southern Asia, including the Tibetan Plateau, in spring. The latter indicates a late establishment of the South Asian high, and implies a delay in the atmospheric transition from winter to summer conditions and in the development of the summer monsoon. The preceding scenario for a strong Middle East jet stream and a weaker Asian monsoon can be applied accordingly for the discussion of the physical processes from a weak jet stream to a strong monsoon. The current results of the relationship between the extratropical process and Asian monsoon resemble several features of the tropical–extratropical interaction mechanism for the tropospheric biennial oscillation (TBO). While the role of tropical heating is emphasized in the TBO mechanism, compared to the variability of the sea surface temperature related to El Niño–Southern Oscillation, the extratropical process examined in this study is more strongly linked to the Asian summer monsoon.
Electrical conductivity measurements from a new Greenland ice core are reported which confirm previous findings of relatively warm 'interstadial' periods during the last glaciation and short returns to colder conditions during the glacial to interglacial warming. The measurements also reveal a hitherto unrecognized mode of rapid climate variation. Fluctuations in ice conductivity on scales of less than five up to 20 years reflect rapid oscillations in the dust content of the atmosphere. The 'flickering' between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation.
The tropical hydrologic cycle affects atmospheric trace gases and global climate change, and thus records of hydrologic change encompassing a variety of time scales from the low latitudes are important in paleoclimatology. Isotopic analysis of calcite from Lake Junin, Peru, provides a record of hydrologic variability that spans the last glacial-interglacial transition in the southern tropics. The record reveals a 6‰ enrichment in δ18Ocalcite during the late glacial followed by a gradual depletion during the Holocene, which can be interpreted as a decrease followed by a long-term increase in effective moisture. Close agreement between δ18Ocalcite and rainy season insolation indicates that long-term changes in tropical hydrology were linked to orbital variations. Furthermore, hydrologic change was out of phase in the northern and southern tropics over this time period.
We calculate the temperature distribution with depth at the Dome C ice divide, East Antarctica, for a number of vertical strain rate functions; assuming two-dimensional flow. To solve the heat equation, we use the Dome C oxygen isotope profile and radar data to specify the surface and basal boundary conditions, respectively. Two different sets of simple, monotonic polynomials are used as strain rate test functions. For a net surface warming of about 9°C through the Wisconsinan-Holocene climatic transition, we find that the best agreement to the measured 800-m Dome C temperature profile results from a vertical strain rate of the form (1-z/H)p, with H the ice thickness and p~=2.5. Using dated horizons in the oxygen isotope profile obtained by cross correlation with a marine sediment core, we find virtually no change in accumulation rate at Dome C since the end of the Wisconsinan glacial.