Elsevier

Quaternary Science Reviews

Volume 46, 16 July 2012, Pages 46-56
Quaternary Science Reviews

Vegetation, climate and fire regime changes in the Andean region of southern Chile (38°S) covaried with centennial-scale climate anomalies in the tropical Pacific over the last 1500 years

https://doi.org/10.1016/j.quascirev.2012.04.016Get rights and content

Abstract

Pollen and charcoal analysis from Laguna San Pedro (38°26′S, 71°19′W), a small closed-basin lake located within the present-day distribution of Araucaria-Nothofagus forest in the Temperate-Mediterranean Transition zone in the Andes of Chile (35.5–39.5°S), reveal centennial-scale changes in vegetation, climate and fire regime since 1500 cal yr BP. We interpret periods of relatively low growing season (summer) moisture and increased fire activity between 1500–1300 and 1000–725 cal yr BP, the latter period is also characterised by remarkably rapid bulk sediment accumulation and we infer prolonged annual sedimentation resulting from a decrease in the duration of lake freezing under a warmer climate. Relatively moist conditions during summer and low fire activity occurred between 1300–1000 and 725–121 cal yr BP, with slow bulk sediment accumulation through the latter phase in particular implying a cool and wet climate. Our results suggest that the Medieval Climate Anomaly chronozone was relatively warm and dry, followed by a cool-wet climate during the Little Ice Age chronozone, before a substantial modification of the vegetation landscape by Europeans occurred in the mid 1800's. The timing and direction of changes in the Laguna San Pedro record bear a striking resemblance to multiple independent tropical Pacific precipitation reconstructions, areas where precipitation is governed by the El Niño-Southern Oscillation (ENSO), and with the modern influence of ENSO over the climate in the region. We conclude that ENSO was the main driver of changes in growing season moisture in this part of the Temperate-Mediterranean Transition in south-central Chile over the last 1500 years.

Highlights

► Low summer moisture and increased fire activity prevailed between 1500–1300 and 1000–725 cal yr BP. ► Moist summers and low fire activity prevailed between 1300–1000 and 725–121 cal yr BP. ► Relatively warm conditions prevailed between 1000 and 725 cal yr BP (the Medieval Climate Anomaly). ► Relatively cool conditions prevailed between 725 and 121 cal yr BP (the Little Ice Age). ► We posit a tropical ENSO driver for these trends.

Introduction

It is of vital importance that we understand the nature, timing, impacts and regional expression of past periods of abrupt climate change if we are to place current and future climate trends in to a meaningful historical context. A significant body of evidence now documents widespread evidence for decadal to centennial-scale climate intervals over the last 1500 years, most notably manifest in the so called Little Ice Age (LIA), Medieval Climate Anomaly (MCA) and the Dark Ages Cold Period (DACP) (Seager et al., 2007; Solomon et al., 2007; Mann et al., 2009; Graham et al., 2010). To date, there is a heavy bias towards northern hemisphere records and comparatively little empirical evidence for concurrent climate intervals from southern hemisphere landmasses. Available studies from the southern hemisphere indicate evidence of both regional-scale trends over the last 1500 years (e.g. Seager et al., 2007; Mann et al., 2009; Graham et al., 2010; Neukom et al., 2010a, 2010b) and a high degree of spatial heterogeneity in the nature, direction and timing of climate trends (Masiokas et al., 2009).

South America is the only landmass that virtually transects the entire meridional gradient from the tropics to the sub-Antarctic latitudes and is, thus, an important location for the discourse over global climate dynamics. Southern South America spans the entire zone influenced by the sub-tropical and extra-tropical climate domains and, as such, is well situated to track changes in two globally important components of the climate system: the sub-tropical South Pacific Ocean high pressure system and the extra-tropical southern westerly winds (SWW), as well as their leading inter-annual modes (the El Niño-Southern Oscillation (ENSO) and Southern Annular Mode (SAM), respectively). Moreover, the Andes Cordillera, a lofty and glaciated mountain range that runs in a north-south direction along the entire western sea-board of southern South America, intercepts the Earth's zonal tropospheric flow (Garreaud et al., 2009), resulting in sharp environmental gradients that make this region well suited for tracking the effects of past climate change on terrestrial environments. Despite this critical geographical setting, the plethora of fast sediment-accumulating lakes and long-lived climatically sensitive tree species, there are remarkably few continuous palaeoclimatic reconstructions of sufficient resolution to allow an adequate appraisal of change over the last 1500 years (e.g. Stine, 1994; Villalba, 1994; Jenny et al., 2002; Ariztegui et al., 2007; Boës and Fagel, 2008; Moy et al., 2009; von Gunten et al., 2009; Neukom et al., 2010b; Jara and Moreno, 2012).

Available evidence from sub- and extra-tropical southern South America suggests that the broadly defined LIA chronozone (ca 600-100 calendar years before present [cal yr BP]) was characterised by: (i) an expansion of virtually all studied Andean glaciers (Luckman and Villalba, 2001; Masiokas et al., 2009) and (ii) relatively low temperatures between 20 and 55°S (Villalba, 1994; Villalba et al., 2003; Neukom et al., 2010b). Despite these broad-scale trends, there is a substantial degree of spatial heterogeneity in the timing and nature of environmental responses to climate change through the LIA chronozone, with, for example, substantial differences in the timing of maximum ‘LIA’ glacier expansion across southern South America (Luckman and Villalba, 2001; Masiokas et al., 2009). Likewise, the MCA (ca 1050–600 cal yr BP) broadly encompasses a period of climatic change in southern South America that is variously manifested in both temperature and precipitation changes (Jenny et al., 2002; Bertrand et al., 2005; Mann et al., 2009; Moreno et al., 2009; Moy et al., 2009; Graham et al., 2010; Neukom et al., 2010a, 2010b; e.g. Villalba, 1994; von Gunten et al., 2009).

Here we present detailed pollen, charcoal and sedimentary data from a lake sediment core retrieved from Laguna San Pedro, south-central Chile, that span the last 1500 years. Laguna San Pedro is a small closed-basin lake located close to the precipitation dependent forest-steppe ecotone in the transitional zone between the sub-tropical and temperate regions of southern South America, the Temperate-Mediterranean Transition (TMT). The site, which was the focus of a previous palynological study of sediments extracted from a profile in an adjacent wetland (Rondanelli-Reyes, 2000), is located in Valle Lonquimay (Fig. 1), near several explosive stratovolcanoes. Precipitation in the TMT is derived entirely from the SWW and the main driver of inter-annual variability in the modern climate is ENSO (Montecinos and Aceituno, 2003; Garreaud, 2007; Garreaud et al., 2009). The Laguna San Pedro data allows assessment of the following questions: (1) what was the nature of environmental change in this region over the last 1500 years?; (2) are there discernible biological/climatic anomalies during the LIA, MCA and DACP chronozones?; and (3) what were the main driver(s) of environmental change in this region over the last 1500 years?

Section snippets

Physical setting

This study is centred on the mountainous Región de la Araucanía (36–39.5°S) in south-central Chile. The study area lies in the core of the Andes Cordillera which in this sector attains average elevation of ca 3000 m a.s.l., hosting several glaciers and active volcanoes. Here we sampled and analysed sediment cores from Laguna San Pedro (913 m a.s.l.; 38°26′S, 71°19′W; Fig. 1), a small closed-basin lake that lies within the formerly glaciated Valle Lonquimay that is situated 3 km east of the town

Field operations and sediment analysis

Two offset and overlapping cores were retrieved from the deepest part of Laguna San Pedro with a 7.5 cm diameter plexiglass piston corer that preserves the integrity of the water–sediment interface: MSF0907SC1(0–100 cm) and MSF0907SC2 (50–123 cm). The deepest part of the lake was located with the aid of a bathymetric map we created using an echo-sounder. We extruded and sub-sampled the cores at 1 cm-thick contiguous intervals on-site. Loss-on-ignition analysis was performed on contiguous

Bathymetry, stratigraphy and chronology

We produced a bathymetric map (Fig. 3) that reveals a simple and planar lake basin with a maximum depth of 450 cm. We matched the overlapping sediment cores (MSF0907SC1 and MSF0907SC2) on the basis of their loss-on-ignition data and produced a composite 123 cm-long record presented in Fig. 4. The sediments consist of brown organic lake mud (gyttja), pyroclastic material (tephra) and fine silt. The 123 cm composite core contains tephra layers at 78–88 cm and 98–114 cm, that are clearly evident

Palaeoenvironmental reconstruction

In-situ pelagic sedimentation of organic rich sediment has prevailed in Laguna San Pedro during the last 1500 years, punctuated by deposition of two thick coarse pyroclastic layers at ca 1350 and 1100 cal yr BP that reflect volcanic eruptions, and a silt layer at 121 cal yr BP. The pollen record from Laguna San Pedro reveals a dynamic vegetation landscape. Broadly speaking, the changes are characterised by either a trend towards increased forest importance (increasing N. dombeyi type pollen) or

Conclusion

Our palaeoenvironmental reconstruction from Laguna San Pedro in south-central Chile (38°S) suggest that periods of high relative growing season (summer) moisture and cool temperatures correspond well with evidence for persistent cool/La Niña ENSO states (1300–1000 and 725–121 cal yr BP; DCAP and LIA, respectively). We also infer periods of low relative growing season moisture and warmer temperature that correspond well with evidence for persistent warm/El Niño ENSO states (1500–1300 and

Acknowledgements

M.S.F. is funded by Fondecyt 3110180 and the Institute of Ecology and Biodiversity, Chile, and P.I.M by Fondecyt 1110612, and Iniciativa Científica Milenio P05-002, contract PFB-23. Our appreciation goes to Oscar Pesce and Bree Fletcher for help in the field and to Loreto Hernández for processing the palynological samples and for conducting LOI and macroscopic charcoal analyses, and to Carmen Miranda for drafting Fig. 1.

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