Modern precipitation anomalies in the Altiplano, South America, are closely
linked to the strength of the South American summer monsoon (SASM), which is
influenced by large-scale climate features sourced in the tropics such as
the Intertropical Convergence Zone (ITCZ) and El Niño–Southern
Oscillation (ENSO). However, the timing, direction, and spatial extent of
precipitation changes prior to the instrumental period are still largely
unknown, preventing a better understanding of the long-term drivers of the
SASM and their effects over the Altiplano. Here we present a detailed pollen
reconstruction from a sedimentary sequence covering the period between
4500 and 1000 cal yr BP in Lago Chungará (18
Detailed paleoclimate records are not only necessary to document past climate anomalies, but also to decipher regional drivers of atmospheric variability and explore teleconnections that extend beyond the period of instrumental climate measurements (Wanner et al., 2008; Loisel et al., 2017). Unfortunately, the number of reliable climate reconstructions is still small in many regions of the world, which hinders a contextualization of modern climate trends into longer-term natural variability. This long-term perspective becomes critical considering the brief period of meteorological measurements, the current scenario of unprecedented climate change, and the uncertainties about future trends (Deser et al., 2012; Neukom et al., 2015).
In the tropical Andes (5–13
Here we present a new pollen-based precipitation record for the Altiplano that covers the interval between 4500 and 1000 cal yr BP. The timing and direction of precipitation anomalies inferred from the pollen data are evaluated with the primary aim of exploring past drivers of precipitation change and assessing the evolution of teleconnections between the Altiplano, other regions in South America, and the Pacific and the Atlantic oceans. Our pollen data were therefore compared with proxy-based records from the Altiplano and the tropical Andes, along with reconstructions of past ITCZ variability and ENSO-like changes.
Physiography aspects of the study area.
The Altiplano (13–22
South American atmospheric features based on reanalysis data.
The climate of the Altiplano is cold and semiarid with mean annual
temperatures and precipitation ranging from 4 to 17
The vegetation of the Altiplano is rich and diverse, presenting a great number of different shrubland, grassland, peatland, and forest communities. The distribution of these communities varies locally according to elevation, soil, erosion, water availability, and drainage; it varies regionally by the gradients in precipitation and temperature described above (Brush, 1982). For the sake of the pollen–climate interpretations of this study and considering the outstanding diversity of the Altiplano vegetation, we restricted our characterization to the vegetation belts established across the western Andean slopes and the southern Altiplano around our study site. Our brief and simplified description of the regional vegetation communities is based on the much more comprehensive work of Villagrán et al. (1983), Arroyo et al. (1988), Rundel and Palma (2000), and Orellana et al. (2013). We followed the taxonomic nomenclature adopted by Rodrıguez et al. (2018).
The prepuna vegetation belt established above the upper margin of the
absolute desert from
The puna belt sits above the aforementioned formation, roughly between 3000
and 4000 m a.s.l. It is a floristically more diverse and more densely
vegetated shrubland–grassland community. The puna belt is dominated by
A gradual transition from a shrubland- to a grassland-dominated community
occurs at the upper limit of the puna belt. Above 4000 m a.s.l., high Andean
steppe tends to replace puna scrubs, forming a grassland belt characterized
by several species of the Poaceae family such as
Lago Chungará (18.24
Lago Chungará has been the focus of several paleolimnological and paleoclimatological studies over the last 20 years. An initial detailed seismic profile of the sediments was presented by Valero-Garcés et al. (2000). A 500-year reconstruction of hydrological variability around Lago Chungará was developed 3 years later based on sedimentological, geochemical, and stable isotope analyses from a 57 cm long sediment core (Valero-Garcés et al., 2003). In 2002, a field campaign retrieved 15 new sediment cores from different areas of the lake, and several publications have emerged from the analysis of those sediment sequences. Such studies include the following: a detailed lithological correlation of all cores (Sáez et al., 2007); an analysis of the physical properties of the sediments such as magnetic susceptibility, mineral composition, total organic carbon, and grey colour values (Moreno et al., 2007); a climate reconstruction based on the statistical analysis of mineralogical and chemical parameters (total organic carbon, an X-ray fluorescence (XRF) scanner, grey colour values, and total biogenic silica among others) and a detailed chronological framework (Giralt et al., 2008); the oxygen and carbon isotope composition of diatom silica and their relationship with the hydrological evolution of the lake, as well as solar and ENSO forcings (Hernandez et al., 2008, 2010, 2011, 2013); and the petrology and isotopic composition of the carbonate fraction (Pueyo et al., 2011). More recently, Bao et al. (2015) presented a long-term productivity reconstruction based on fossil diatom composition and geochemical data, assessing the interplay between regional precipitation, lake levels, and organic activity in Lago Chungará. Overall, these studies have provided a detailed paleolimnological and paleoenvironmental history of Lago Chungará that extends for at least 12 800 years, showing a sedimentation regime marked by diatom-rich deposits interbedded with multiple tephra and carbonate-rich layers (Pueyo et al., 2011). Volcanic input, more frequently recorded after 7800 cal yr BP, was most likely composed of tephra deposits erupted from Volcán Parinacota (Sáez et al., 2007). Relatively high lake levels are recorded before 10 000 cal yr BP, in overall agreement with a pluvial interval documented elsewhere in the Altiplano (Giralt et al., 2008). Dry conditions seem to have prevailed between 8000 and 3500 cal yr BP (Pueyo et al., 2011), with peak aridity probably occurring between 8000 and 6000 cal yr BP (Moreno et al., 2007; Giralt et al., 2008; Pueyo et al., 2011). Relatively high lake levels prevailed over the last 5000 years (Sáez et al., 2007), superimposed on several high-amplitude lake stand oscillations after 4000 cal yr BP (Sáez et al., 2007; Giralt et al., 2008). Humid and high lake stands have been recognized between 12 400–10 000 and 9600–7400 cal yr BP, while dry intervals prevailed between 10 000–9600 and 7400–3500 cal yr BP (Bao et al., 2015). The last 200 years are marked by overall dry conditions (Valero-Garcés et al., 2003). Despite all this information, the hydrological history around Lago Chungará and its regional drivers over the most recent millennia has not yet been addressed in detail (Hernández et al., 2010; Bao et al., 2015).
In November 2002, 15 sediment cores up to 8 m long were obtained from
different parts of the lake using a raft equipped with a Kullenberg corer
(Sáez et al., 2007). The fossil pollen sequence presented here was
developed from core 7, which was obtained from the northwestern border of
the lake at 18 m of water depth (Sáez et al., 2007; Fig. 1b). In
addition, we obtained pollen data from 26 surface samples collected along
a west–east transect on the western Andes slope at 18
The 26 surface samples plus 49 fossil pollen samples from core 7 were
prepared following standard procedures for palynological analysis (Faegri
and Iversen, 1989) in the Laboratory of Paleoecology and Paleoclimatology of
CEAZA, La Serena, Chile. Pollen data are expressed as a relative percentage,
which was calculated from the sum of a minimum of 300 terrestrial pollen
grains. The percentage of aquatic pollen was calculated from a sum that
included all terrestrial and aquatic pollen. Pollen accumulation rate (PAR;
particles yr
Core 7 comprises sections 7A-1K-1 (146 cm), 7A-2K-1 (151 cm), and 7A-3K-1 (51 cm) with a total length of 348 cm. These core sections correspond to Subunit 2b described in Sáez et al. (2007), composed of dark grey diatomite with
abundant macrophyte and four black tephra layers. These layers are made of
andesitic and rhyolitic material with the presence of amphibole (Moreno et al.,
2007). The chronological framework used in this study is based on the one
presented in Giralt et al. (2008). The chronology of core 7 is constrained
by three AMS radiocarbon dates obtained from Subunit 2b in cores 11 and 14,
which were translated into core 7 after direct correlation based on seismic
profiles and tephra keybeds identified as peaks in magnetic susceptibility
(Sáez et al., 2007). This chronology uses a modern reservoir effect of
Diagram with the altitudinal position of the 26 surface
pollen assemblages collected along a west–east transect on the western Andes
slope at 18
Our modern surface pollen transect reveals important changes in the
abundance of pollen type across prepuna, puna, and high Andean steppe
vegetation belts (Fig. 3). Between
Terrestrial and aquatic pollen percentages from core 7 in Lago Chungará. Pollen percentages are plotted against depth and age scales (Giralt et al., 2008). The diagram also shows the main zones of the records (right) determined with the aid of a CONISS cluster analysis.
Five general zones are recognized based on the major changes in pollen
percentages (Fig. 4) and PAR (Fig. 5), assisted by a CONISS cluster
analysis. Overall, the record is largely dominated by Poaceae (mean value
55 %), followed by Ast.
Terrestrial and aquatic pollen accumulation rate (PAR;
grain cm
The basal portion of the record (Zone CHU1; 4500–4100 cal yr BP) features
an above-mean percentage of Poaceae (67 %) and relatively low abundances of all
other major taxa, especially
The following interval (Zone CHU2; 4100–3300 cal yr BP) exhibits a downturn
of Poaceae (52 %), a rapid and sustained decline in
Zone CHU3 (3300–2400 cal yr BP) is characterized by a recovery of Poaceae
(59 %) and a notable decline in Ast.
Zone CHU4 (2400–1600 cal yr BP) features below-mean abundances of Poaceae
(49 %) and Ast.
Finally, Zone CHU5 (1600–1000 cal yr BP) exhibits a sustained recovery of
Poaceae (56 %), a rapid decline of Apiaceae (3 %), and a drastic drop of
about 50 % in
At present, Lago Chungará is situated within the high Andean steppe
vegetation belt and, consistent with this position, pollen assemblages are
primarily composed of high Andean pollen types. This composition suggests
that high-elevation steppe vegetation dominated the surroundings of Lago
Chungará between 4500 and 1000 cal yr BP. Nonetheless, the fossil pollen
sequence shows a significant representation of pollen types belonging to
vegetation belts situated at lower elevations on the western Andes slopes
(i.e. prepuna and puna belts). For instance, high Andean pollen types (e.g.
Relatively humid conditions are inferred prior to 4100 cal yr BP based on the
dominance of Poaceae along with below-mean abundances of all puna and
prepuna taxa. Above-mean abundances of
Between 4100 and 3300 cal yr BP, the record exhibits lower-than-mean Poaceae and
These changes are followed by a decline in Ast.
The climate trend described above finished abruptly with both a decline in
the percentage of Apiaceae and Ast.
Summary plot including key Lago Chungará climate indicators (black curves) and selected paleoclimate records from the tropical Andes and the Pacific Ocean.
Based on the modern pollen–climate relationships in the Chungará area
discussed in Sect. 5.1, we interpret the aforementioned precipitation
anomalies as responding to long-term variations in the mean strength of the
SASM, with a weakened SASM between 4100–3300 and 1600–1000 cal yr BP and a
strengthening between 2400 and 1600 cal yr BP. To identify potential drivers of
past SASM-related precipitation anomalies at Lago Chungará, Fig. 6 shows
two of our pollen–climate indicators (Fig. 6a–b) and a previously published
hydrological reconstruction from Lago Chungará (Giralt et al., 2008)
(Fig. 6c), along with a selection of paleoclimate reconstructions from the
Altiplano, the tropical Andes, and the tropical Pacific region (10
The centennial wet–dry anomalies experienced in Lago Chungará between
2400 and 1000 cal yr BP can be reconciled with some hydrological records from
the western Andes slopes and adjacent Atacama Desert. For instance, pollen
preserved in fossil rodent middens from Salar del Huasco (20
The SASM anomalies reconstructed from Lago Chungará do not correspond
to the
In sum, correlations with paleoclimate reconstructions from the Atacama
Desert and the Altiplano indicate that the centennial-scale precipitation
anomalies detected at Lago Chungará were regional paleoclimate events
caused by the strengthening and weakening of the SASM in those regions. On the other
hand, the timing, direction, and magnitude of our reconstructed anomalies are
considerably less replicated in SASM reconstructions from the tropical Andes
and further north. Such discrepancies could emerge from different climate
sensitivities, chronological uncertainties, and/or distinct sources of SASM
precipitation between northern and southern sites. Comparisons between
isotopic values in proxy records and modern rainfall time series, as well as
model outputs, suggest that the
A 2000-year record of ENSO-like changes based on several published
precipitation records in the western and eastern tropical Pacific Ocean (Yan
et al., 2011) offers an insightful comparison with our Chungará
precipitation anomalies. These Pacific records depict predominantly El
Niño-like conditions between 2000 and 1500 cal yr BP, followed by La
Niña-like conditions between 1500 and 950 cal yr BP (Yan et al., 2011) (Fig. 6j). Hence, increased SASM-derived summer precipitation at Lago Chungará
occurred predominately under El Niño-like conditions, whereas decreased
SASM precipitation occurred predominately under La Niña-like conditions.
In fact, the percentage of sands from El Junco Lake in the Galápagos Island
(1
Thus, what climatic mechanisms drove the observed SASM changes at
Chungará? We note that a latitudinal gradient in precipitation anomalies
over the Altiplano and tropical Andes has already been pointed out in a
recent speleothem
Our late Holocene pollen-based reconstruction from Lago Chungará reveals that significant changes in the altitudinal distribution of vegetation communities, terrestrial plant productivity, and lake levels occurred in the Altiplano. We interpret these changes as resulting from anomalies in precipitation associated with changes in the mean strength of the SASM at centennial timescales. In particular, we detected dry conditions suggestive of a weakening of the SASM between 4100–3300 and 1600–1000 cal yr BP and a conspicuous humid interval reflecting a strengthening of the SASM between 2400 and 1600 cal yr BP. Comparisons with multiple paleoclimate records from the Altiplano and the tropical Andes show spatially coherent changes in SASM intensity at centennial timescales, which were largely decoupled from records of the past positions of ITCZ and ENSO polarity. Hence, the close relationship between ENSO and precipitation in the Altiplano documented at inter-annual and decadal timescales cannot be extended to the centennial or multi-centennial domain. We further note a clear south–north gradient in the magnitude of precipitation responses, with southern records in the Altiplano and the Atacama Desert responding more markedly than northern sites in the tropical Andes. All this evidence is consistent with an extratropical source of moisture driving centennial-scale changes in SASM precipitation in the former regions, which is supported by modern climatological studies (Vuille and Keimig, 2004) and interpretations made from recent reconstructions (Apaéstegui et al., 2018). Finally, our results highlight (1) the lack of correspondence between past changes in the strength of the SASM at centennial timescales and climate components sourced in the tropics, which are the dominant drivers of the modern SASM, and (2) a strong teleconnection between the southern Altiplano and the extratropics during the most recent millennia. Hence, caution is required in assuming that the tropical drivers of precipitation in the Altiplano represent the exclusive forcings from which future conditions should be expected.
All data used to interpret the Lago Chungará record are provided in the
article. All the information from other records is given in their respective
reference in the main text. The pollen data from Lago Chungará will be
freely available on the Neotoma paleoecological database (
The supplement related to this article is available online at:
AS, AH, SG, RB, BVG, and AM carried out the field campaign and obtained the sediments. LG analysed the pollen samples. IAJ designed the study and analysed and interpreted the data. IAJ wrote the paper and prepared all the figures with the assistance of AM, AS, AH, SG, RB, and BVG.
The authors declare that they have no conflict of interest.
This investigation was funded by Fondecyt grant 1181829 and International Cooperation Grant PI20150081. The Spanish Ministry of Science and Innovation funded this research through the projects ANDESTER (BTE2001-3225), Complementary Action (BTE2001-5257-E), LAVOLTER (CGL2004- 00683/BTE), GEOBILA (CGL2007-60932/BTE), and CONSOLIDER Ingenio 2010 GRACCIE (CSD2007-00067). In addition, we acknowledge funding from the Spanish government through the MEDLANT project. BV-G is grateful for the support of project CGL2016-76215-R/BTE. IAJ would like to thank David López from CEAZA for his assistance in the drawing of Figs. 1–2 and Andrew BH Rees from Victoria University of Wellington, New Zealand, for his aid with the multivariate statistics. IAJ was supported by Fondecyt postdoctoral grant no. 3190181. AH was supported by a Beatriu de Pinós–Marie Curie COFUND contract within the framework of the FLOODES2k (2016 BP 00023) project.
This research has been supported by the Gobierno Español (MEDLANT Project).
This paper was edited by Hans Linderholm and reviewed by four anonymous referees.