Paez et al PAGES.cdr

VARIABILIDAD AMBIENTAL DEL HOLOCENO EN REGIONES ARIDAS Y SEMIARIDAS DEL
CENTRO-OESTE DE ARGENTINA.
Holocene environmental variability in arid and semiarid regions of west-central Argentina.
GRUPO DE INVESTIGACION DE
PALEOECOLOGIA Y PALINOLOGIA
RESUMEN
Paez, M.M.(1), M.A. Zárate(2,3),
L.Rojo(1,3), D. Navarro(1,3), A. Guerci(4), J. Chiesa(5), A. Srur(6,3)
La comprensión de la variabilidad natural del Holoceno radica en la resolución cronoestratigráfica y en la correlación de los eventos detectados en
diferentes tipos de registros (vegas, secuencias aluviales y lacustres y sitios arqueológicos). La respuesta diferencial de los proxies (estratigrafía,
sedimentología, polen, carbón vegetal, diatomeas, moluscos) es dependiente de la localización y de los registros en los diferentes gradientes
altitudinales y latitudinales. Los estudios multidisciplinarios realizados entre los 32°-38° S permiten discutir las hipótesis sobre los cambios
climáticos del Holoceno medio y tardío. La variación regional de la vegetación, a escalas de centurias y milenios, evidencia el incremento de
condiciones áridas desde ca. 9.500 14C años AP. Entre los ca. 5.000-6.000 14C años AP en alta montaña comienza la acumulación sedimentaria en los
ambientes de vega actuales, se inicia la excavación en afluentes del curso medio del río Tunuyán y ocurren cambios en los niveles lacustres de Salina
del Bebedero. Las asociaciones polínicas señalan vegetación Andina, del Monte y del Monte-Espinal y comunidades halófitas e hidrófitas que reflejan
la dinámica de los sistemas fluvial y lacustre. En el último milenio se evidencian fluctuaciones en el transporte sedimentario y de las asociaciones
polínicas en Precordillera y una degradación (excavación vertical) en la cuenca media de los ríos Tunuyán y del Atuel. La respuesta diferencial de los
ambientes se discute en relación con las ocupaciones humanas.
Facultad de Ciencias Exactas y Naturales, UNMdP
(1) Laboratorio de Paleoecología y Palinología. Departamento de Biología, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata. Funes 3250
(7600) Mar del Plata. [email protected]
(2) Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa. Santa Rosa, La Pampa. (3) CONICET. (4) Antorchas. Museo de Historia Natural de San
rafael, Mendoza. (5) Departamento de Geología, Universidad Nacional de San Luis. San Luis. (6) IANIGLA-CRICYT, Mendoza.
To compare the timing, direction and magnitude of climate changes and investigate
the spatial pattern of these changes during Pleistocene-Holocene transition and
Holocene using a multiproxy analysis with chrono-stratigraphic control on a network
of records of altitudinal gradients between 32° and 38°S.
The study area is subdivided in two latitudinal ranges, 32°-35°S and 35°-38°S, with different climate,
vegetation and geomorphological conditions.
The paleoenvironmental information comes from different types of records: peat/bogs, alluvial and
lacustrine sequences and arqueological sites. In order to understand the paleoclimatic variability is
essential a careful site-specific, archive-specific and time-scale-especific evaluation (sensu Grosjean
et al. 2003). Data collected from these different archives and environments reveal more localized
climatic information that can be used to identify the spatial pattern of climate change through time.
Salinas del Bebedero
Agua de La Cueva
33º32’S/66º39’W; 380 m asl
32º37’S/69º09’W, 3200 m asl
14
Stratigraphy
C
ye
s
ar
BP
De
h
pt
)
m
(c
r
St
i
at
ap
gr
hy
th
Li
o
ol
gy
0
100
Po
lle
n
At latitudes higher than 30°S the orientation of the Cordillera is perpendicular to the
atmospheric circulation. At latitudes lower than 35°S, the area is under the influence
of semipermanent anticyclones of Pacific and Atlantic Oceans. South of these latitudes,
the westerlies are predominant and the mean height of the Cordillera decreases.
Grassland
Hydrophitic
C
14
ye
sB
ar
200
Decrease
evaporation
(sulfate)
5000
300
7500
Lower level lake
High
11000
2100 +/- 25
High level lake
13000
rs
ye
a
Río Mendoza, Río Blanco
(Wayne, 1981; Espizúa, 1998, 2000)
600
2500 yrs BP?
700
Increase
evaporation
Holoceno 1
300
Holoceno II
2000
Holoceno III
4000
Till Almacenes
14000-12000?
8500
Travertino
9700
9500
Vallecito II
16000-12000?
Vallecito I-Los
Horcones
22000-18000?
sand
100
150
200
250
300
350
400
450
500
550
Gruta del Indio
600
34º45’S\68º22’W, 700m asl
BP
650
14C
Glaciers
yrs BP
n
le
Po
l
C
ye
ar
s
400
14
Río Valenzuela, Río Grande
(Espizúa, 1998)
Halophytic
2200
Riparian
3810
El Fierro
400
El Macho
2500-2200
B. Azufre
4700-5700
Valle Hermoso III
ca. 11.000
Valle Hermoso II
ca. 13600
S
?
8990
9650
0
y
5µ
t.
>1
2
Pa
r
t.
>2
5
n
lle
Po
ig
ra
p
ra
t
St
400
400
Paleosoil
500
500
10250 ± 40
Halophytic
Halophytic
600
Halophytic
s
s
s
700
3570 ±45
Paludal
s
3750 ±45
s
1100
800
3880 ±40
Paludal
900
1000
s
1200
1100
1200
paleosoil
silts \ clays
fine sands\
coarse sands
s
s
3570 ±20
Paludal
900
600
700
1000
Paludal: flooding plain vegetation
Halophytic: Chenopodiaceae
Hydrophytic: Cyperaceae-Typha
th
s
Halophytic
300
3780 ±45
Shrub steppe: Asteraceae subf. Asteroideae
ep
300
800
Andean-Patagonia: grassland, shrub steppe
D
le
s
s
Halophytic
3050 ±25
ARCHAEOLOGY: A mid Holocene archaeological hiatus was Bañados del Atuel
La Escala
widespread in southern America (Nuñez et al., 2001). In southern
Mendoza an archaeological hiatus occur between 6000 and 7000
14C yrs BP. The first occupations in the diferent envieonments
began at ca. 3800 14C yrs BP . The available data suggest that all
southern Mendoza has evidence of human use at ca. 2000 years
BP, but with differences in biogeographical phases (Gil et al,
2005, Neme et al., 2005).
Te p h r o c r o n o l o g y : A
100
540 ±25 200
2500 ±40
9610 ±70
9m
435 ±25
Paludal
100
200
Paludal
n
ra
ph
th
0
Charcoal
0
Po
l
0,3 m
Horizontal
Lamination
tig
+ Paludal
St
ra
9420 ±60
Modern analogous and pollen -climate calibration
Patagonia-Monte: grassland, shrub steppe, jarillal
hy
BP
rs
ye
a
C
14
7m
ep
Paez (2004)
Reinterpreted from
D´Antoni (1983)
0µ
Halophytic
11000
Gil et al. 2005
Monte-Espinal (jarillal, algarrobal, caldenal, grasslands)
S
4000 yrs BP?
Aggradational plain: homogeneous
ca 9.6 ka
sands (17 m), representing distal
paleosuelo
fan alluvial deposits (loess-like silt,
ca 10.2 ka
sensu Polanski, 1963) with several
tephra layers interbedded. The
pollen concentration is extremely
low suggesting low vegetation
density and abrupt deposition
events. A soil, traceable along 12 km of the Alluvial terrace: sand
arroyo La Estacada and its tributary the deposits at the lower
Anchayuyo, develops on the topmost part of the part grading upward to
alluvial sands. This paleosoil was buried by dominantly sand and sandy-silty
paludal deposits between ca. 10.000 and 9600 sediments (11 to 17 m). Several
14C yrs BP. The upper sedimentary section, diatomite layers with interstratified
overlying the paleosoil, is dominantly thin peat levels including
composed by alluvial deposits including carbonaceous vegetation remains are
diatomites with freshwater mollusk remains. interbedded in the lower sand
Very thin peaty and buried soils of ca. 7900 14C deposits. The upper part is
yrs BP, recording foodplain environments, are characterized by the occurrence of
interbedded in the alluvial deposits. The upper several buried soils (3800, 3000, 2500,
alluvial sediments are covered by an eolian 550 and 440 14C yrs BP) recording
superficial layer which is the parent material of brief stability intervals in swampy
present soils.
environments.
Puente El Zampal
La Escala
Brazo Abandonado
8690 ±70
±
D
B
P
m
)
s
(c
ar
pt
h
ye
C
14
De
50
sediments formed in the mid-Holocene when
the Atuel River was dammed by alluvial fans
downstream (Gosse and Evenson, 1994). The
charcoal analysis showed non-woody
charcoal concentration peaks (more than
1000 particles per cc) only in the peat levels
suggesting a lake marginal and flooding plain
vegetation for these stability intervals.
Psammophytic
18
33º30’S/69ºW; 900 masl
Grass >125u
Stratigraphy: alluvial
fan sediments of a
small basing, tributary
of the Atuel River,
interbedded with lake
Jarillal
Arroyo La Estacada
0
part./cc
Psammophytic
15
In VALLE DE UCO, the sedimentary material of both
the aggradational surface (Late glacial/Early
Holocene) and the alluvial terrace (Mid-Late Holocene)
was grouped altogether into two stratigraphic units
with remarkably different lithology and facies.
34º50’S/69º56’W; 2000 m asl
800
silt
1000
Agua Buena
(Stingl and Garleff, 1978) 750
14
19
Hypothesis: Intensified Westerlies are the main reason
for increased precipitation in Central Chile and Norte
Chico (Jenny et al., 2002). In lowlands, high
precipitation years correspond to El Niño years while
not all El Niño years correspond to high annual
precipitation (Aceituno, 1898).
700
High level lake
BP
(Villa-Martinez et al. 2003; Yenny et al. 2002)
900
salt
silt-salt
rs
(Maldonado and
Villagrán, 2003)
7500
Monte (jarillal, algarrobal, riparian)
S
17
ye
a
6100
part./cc
11
16
yrs BP
C
5700
Glaciers
14C
14
4200
200
Hidrophytic
C
3200
2000
Jarillal
500
Grassland
Hydrophitic
800
1000
9
14
1800
4080±75
Hidrophytic
10
(Garcia et al. 2003)
1300
2380±40
8
400
BP
BP
rs
ye
a
C
14
0
200
>125u
6
7
9200
200m asl
Hidrophytic
5
S
32°50’S/70°54’W; 350m asl
n
- Jarillal
3
Lower level lake
High evaporation
4
(chloride)
P
le
2
+ Jarillal
CENTRAL CHILE Ñague
Aculeo
31°50’S/71°28’W
l
Po
1
Stable
Summer 2005
Winter 2004
s
Pa
r
Pressure at sea level
pel
m
Sa
1300
5270 ±65
S
1400
sterile
1500
1600
20 40 60 80 100 20 40 60 80 100
Part cm-3
combination of luminescence
dating and geochemical
characterization of a series of
adjacent tephra layers exposed
in 18 m alluvial sequences was
used. The wider significance of
this study is that it is possible to
trace tephras in eolian deposits
of north and east as well as in
glacial and lacustrine
sequences located in the Andes.
CONCLUSIONS
# LGM stimated magnitude: temperature decrease of about 4.5-6.5°C - last large glacier extensions in the dry Andes to a much more humid
(and probably warner )- westerly stormtracks in the Southern America moved equatorwards no more than 5-10°S, is evidenciated the study
area . The paleoclimatic signal is opposite in relation to the inferred for the Pampa Interserrana, SE Buenos Aires, and are consistent with the
model of anticyclone latitudinal migration.
# Arid and semiarid conditions at different environments began to establish ca. 9500 14 C yrs BP. The Arid Diagonal (sensu Bruniard, 1982)
began to establish ca. 8000 14C yrs BP.
# Consensus exists about the occurence of a Mid-Holocene arid phase and the onset of more humid conditions about 3000 14C yr BP. In
general, humid conditions persisted during the last 2000 years compared to the entire Holocene. Most of the records at Central Chile and
Toms et al. 2004
the Altiplano suggest this tendency , however, the moisture sources in both region are very different (Jenny et al., 2002).
# How was the biogeographical vegetation history of the arid-semiarid region? How their spatial distribution varied during Mid-Late
An expansion of the study area may help to establish a regional litho-stratigraphic
Holocene?
framework for these alluvial sequences (Toms et al., 2004).
# Peats are recorded at high mountain environments ca. 6000 (Markgraf, 1983, Wingenroth, 2000). In Vega Las Herraduras (Precordillera)
ca.3.000 14C yrs BP, an stability interval in the agradation surface and an Andine grass-shrub steppe are recognized. Seasonal precipitation
are inferred from hydrophytic taxa.
# Sedimentary processes ocurred at Valle de Uco are also recognized at Atuel river (La Guevarina) ca. 7800 and 4000 14C yrs BP with a Monte and Monte-Patagonian associated vegetation respectively.
# Spacial distribution of xerophytic Monte ca. 6000 14C yrs BP suggest a wider extension of the Arid Diagonal.
# Low stimated magnitude of Mid-Late Holocene events, temperature decrease of about 0.5-0.6°C – Neoglacial extension ca. 50 km in the Cuyo, is registered in stratigraphyc sequences and in pollen spectra, with lower resolution.
# ENSO-event is one but not a definite criterion for high precipitation/high river runoff in Cuyo . It is important to note that records for the Mendoza river show that some years before or after an ENSO event, the runoff is less than normal
(Compagnucci, 2000).
# The snow accumulation rates on the highest peaks of the Cordillera are closely related with precipitation on its western slope as well as to the water content of the snow in the Cuyo river basin on the eastern slope (Compagnucci, 1969).
According to Llorens y Leiva (2000) while the fronts of many glaciers retreat, some glaciers advance significantly. A reliable chronology and precise study of the glacial formation and extension processes is still insufficient of area study.
# The Late Holocene was not a uniform, stable or predictable period. For example, at Salinas del Bebedero, water level and halophytic and hydrophytic taxa fluctuations are suggested from lithology and pollen spectra.
# Frecuent fluctuations in sediment transport and degradation (vertical excavation) are recognized in Tunuyán and Atuel basins during the last millenia.
# Is the relative synchronicity among ENSO - neoglaciation - high precipitation - high river runoff and among intensification of Easterlies – decreased precipitation - high temperature - Monte, a consistent interpretation? Where was
Arid Diagonal during the Mid-Late Holocene? Did the climatic variability actually affect the refuge availability and the human occupation hiatus?.
Project PIP 5819, UNMdP-EXA 275/03, 354/06, PICT03-04-14695, UNLPam 186