Easter Island, 27&Deg;S
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Quaternary Science Reviews 28 (2009) 2743–2759 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27S) Alberto Sa´ez a,*, Blas L. Valero-Garce´s b, Santiago Giralt c, Ana Moreno b, Roberto Bao d, Juan J. Pueyo a, Armand Herna´ndez c, David Casas e a Facultat de Geologia, Universitat de Barcelona, c/Martı´ Franque´s s/n, E-08028 Barcelona, Spain b Instituto Pirenaico de Ecologı´a – CSIC, Apdo 13034, E-50080 Zaragoza, Spain c Instituto de Ciencias de la Tierra ‘Jaume Almera’ – CSIC, c/Lluı´s Sole Sabaris s/n, E-08028 Barcelona, Spain d Facultade de Ciencias, Universidade da Corun˜a, Campus da Zapateira s/n, E-15071 A Corun˜a, Spain e Institut de Cie`ncies del Mar-CSIC, Passeig Marı´tim de la Barceloneta, 37, E-08003 Barcelona, Spain article info abstract Article history: Easter Island (SE Pacific, 27S) provides a unique opportunity to reconstruct past climate changes in the Received 18 February 2009 South Pacific region based on terrestrial archives. Although the general climate evolution of the south Received in revised form 23 June 2009 Pacific since the Last Glacial Maximum (LGM) is coherent with terrestrial records in southern South Accepted 25 June 2009 America and Polynesia, the details of the dynamics of the shifting Westerlies, the South Pacific Convergence Zone and the South Pacific Anticyclone during the glacial–interglacial transition and the Holocene, and the large scale controls on precipitation in tropical and extratropical regions remain elusive. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34 000 cal yrs BP based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This mul- ticore strategy has reconstructed the sedimentary architecture of the lake infilling and provided a stratigraphic framework to integrate and correlate previous core and vegetation studies conducted in the lake. High lake levels and clastic input dominated sedimentation in Raraku Lake between 34 and 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen data showing a relatively open forest and a cold and relatively humid climate during the Glacial period. Between 28 and 17.3 cal kyr BP, including the LGM period, colder conditions contributed to a reduction of the tree coverage in the island. The coherent climate patterns in subtropical and mid latitudes of Chile and Eastern Island for the LGM (more humid conditions) suggest stronger influence of the Antarctic circumpolar current and an enhancement of the Westerlies. The end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the development of major flood events and erosion of littoral sediments. Deglaciation (Termination 1) between 17.3 and 12.5 cal kyr BP was characterized by an increase in lake productivity, a decrease in the terrigenous input and a rapid lake level recovery, inaugurating a period of intermediate lake levels, dominance of organic deposition and algal lamination. The timing and duration of deglaciation events in Easter Island broadly agree with other mid- and low-latitude circum South Pacific terrestrial records. The transition to the Holocene was characterized by lower lake levels. The lake level dropped during the early Holocene (ca 9.5 cal kyr BP) and swamp and shallow lake conditions dominated till mid Holocene, partially favored by the infilling of the lacustrine basin. During the mid- to late-Holocene drought phases led to periods of persistent low water table, subaerial exposure and erosion, generating a sedimentary hiatus in the Raraku sequence, from 4.2 to 0.8 cal kyr BP. The presence of this dry mid Holocene phase, also identified in low Andean latitudes and in Patagonian mid latitudes, suggests that the shift of storm tracks caused by changes in the austral summer insolation or forced by ‘‘El Nin˜o-like’’ dominant conditions have occurred at a regional scale. The palm deforestation of the Easter Island, attributed to the human impact could have started earlier, during the 4.2–0.8 cal kyr BP sedimentary gap. Our paleoclimatic data provides insights about the climate scenarios that could favor the arrival of the Polynesian people to the island. If it * Corresponding author. Tel.: þ34 934034489; fax: þ34 934021340. E-mail address: [email protected] (A. Sa´ez). 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.06.018 2744 A. Sa´ez et al. / Quaternary Science Reviews 28 (2009) 2743–2759 occurred at ca AD 800 it coincided with the warmer conditions of the Medieval Climate Anomaly, whereas if it took place at ca AD 1300 it was favored by enhanced westerlies at the onset of the Little Ice Age. Changes in land uses (farming, intensive cattle) during the last century had a large impact in the hydrology and limnology (eutrophication) of the lake. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction ice cores and marine records (Stott et al., 2007), and (2) to understand the impact of the Northern Hemisphere (NH) abrupt Lacustrine sediments from Easter Island (27S) are the only climate events (e.g. Heinrich events) in the southern Hemisphere terrestrial records that can provide climate reconstructions for the (Baker et al., 2001; Kaiser et al., 2008). A strong influence of Late Pleistocene and Holocene in the mid- to low-latitude region of summer insolation changes in the South American climate has the eastern South Pacific Ocean. Easter Island is located thousands been documented through the influence in the summer monsoon of kilometers far from continental sites with potentially compa- (Cruz et al., 2005) and the westerlies location and intensity rable paleoclimatic records, including New Zealand (McGlone, during the glacial–interglacial transition and the Holocene in 2002; Newnham et al., 2006; Vandergoes et al., 2005, among terrestrial (Jenny et al., 2002; Valero-Garce´s et al., 2005; Moreno others), mid latitudes of the Andes (Moreno and Leo´ n, 2003; et al., 2007) and marine records (Lamy et al., 2001; Stuut and Valero-Garce´s et al., 2005; Heusser and Heusser, 2006; Maldonado Lamy, 2004). and Villagra´n, 2006; Haberzettl et al., 2007; Markgraf et al., 2007; The palaeoenvironmental record of Easter Island may Bertrand, et al., 2008, among others), and low latitudes of the Andes contribute to a better understanding of the relative influence of (Paduano et al., 2003; Tapia et al., 2003; Latorre et al., 2006; Giralt high- and low-latitude dynamics as drivers of climate change et al., 2008). Marine records are available in mid (Lamy et al., 2007) during glacial and interglacial periods. Sedimentary core records and low latitudes (Lea et al., 2006; Pena et al., 2008) of South of the Easter Island lakes have previously been studied mostly America and in the southern Pacific (ODP sites 1233, and 1240, using biological indicators as pollen and diatoms (Flenley and GeoB site 7139, and TR site 162-22) but not close to Easter Island King, 1984; Flenley et al., 1991; Dumont et al., 1998). These (Fig. 1). Climate of Easter Island is controlled by large scale atmo- studies focused on the deforestation, traditionally related to the spheric and oceanic patterns, and it is not biased by the continental human occupation of the island during the last millennium. Low effects on local convection as circum Pacific sites. The location of resolution pollen studies have also described the late Quaternary Easter Island in the subtropical latitudes, close to the northernmost paleoclimatic changes (Flenley et al., 1991; Azizi and Flenley, limit of the Southern Hemisphere (SH) westerly winds, is suitable 2008). The main aim of this paper is to provide new insights to reflect the timing and nature of the major oceanographic and about the environmental and climate evolution of Easter Island atmospheric changes affecting the southeast Pacific and the fluc- during the last 34 000 cal yrs BP through a high-resolution study tuations of the South Pacific Convergence Zone (SPCZ) during the of new eight cores recovered in March 2006 at Raraku Lake. This last glacial cycle. Particularly, the Easter Island lacustrine sequences is the first time a multicore strategy coupled with sedimento- can provide new insights on both the dynamics of the shifting logical, mineralogical and geochemical techniques are applied to westerlies and the SPCZ during the glacial–interglacial transition Easter Island. As a result, we have been able to reconstruct the (Lamy et al., 1998; Valero-Garce´s et al., 2005), and the Holocene architecture of the lake sedimentary infilling and the limnological (Jenny et al., 2002; Latorre et al., 2006). They could also help to evolution of the lake. The reconstructed changes in terrigenous understand the large scale controls on precipitation in tropical and input, lake productivity, and lake level changes from last glacial extratropical regions, and the connections between marine vari- to the present have been integrated with previous studies and ability and the hydrological response in the continents (Kaiser et al., the available pollen records to provide a paleoclimate recon- 2008). struction of the mid- to low-latitude southeastern Pacific region. More marine and terrestrial records are needed (1) to identify The timing and characteristics of the main temperature and the earlier timing of the deglaciation onset in the SH indicated by humidity changes in Easter Island show similarities with other 120ºE 150ºE 180º 150ºW 120ºW 90ºW 60ºW 30ºW 15ºN 600 600 1200 600 1200 Intertropical Convergence Zone 1200 600 S. American 0º 13 14 Summer Monsoon 600 South Pacific Convergence 15ºS 1200 <120 South Pacific 10 600 9 600 1200 Anticyclone 8 <120 1200 Zone Easter Island 1200 30ºS 12 7 storm track 6 600 4 11 5 1200 1 45ºS 1200 3 2 600 600 600 600 60ºS Fig.