Glacial to Holocene climate changes in the SE Pacific. The Raraku Lake sedimentary record (Easter Island, 27°S)

Abstract

Easter Island (SE Pacific, 27ºS) provides a unique opportunity to reconstruct past climate changes in the South Pacific region based on terrestrial archives. Climates in the mid-to lowlatitude region of the eastern South Pacific Ocean are controlled by fluctuations in the Westerlies winds, the South Pacific Convergence Zone and the South Pacific Anticyclone. Here we present a high-resolution reconstruction of lake dynamics, watershed processes and paleohydrology for the last 34000 years based on a sedimentological and geochemical multiproxy study of 8 cores from the Raraku Lake sediments constrained by 22 AMS radiocarbon dates. This multicore 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 to 28 cal kyr BP. Sedimentological and geochemical evidences support previously reported pollen datashowing a relatively open forest in the watershed during the Glacial period and a cold and relatively humid climate. 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 end of Glacial Period occurred at 17.3 cal kyr BP and was characterized by a sharp decrease in lake level conducive to the The main aim of the manuscript that we are submitting to Quaternary Science Reviews is to provide new insights about the environmental and climate evolution of Easter Island during the last 34000 cal years BP. We have achieved this aim through the high resolution study of new eight cores recovered in March 2006 at Lake Raraku and constrained by 22 AMS radiocarbon dates. This is the first time that a multicore strategy coupled with sedimentological, mineralogical and geochemical techniques are applied to Raraku Lake. As a result, we have been able to reconstruct the architecture of the lake sedimentary infilling and the limnological evolution of the lake. The reconstructed changes in terrigenous input, lake productivity, and lake level changes from Last Glacial to the present have been integrated with previous studies and the available pollen records to provide a paleoclimate reconstruction of the mid-to low-latitude southeastern Pacific region. The timing and characteristics of the main temperature and moisture changes in Easter Island show similarities with other terrestrial and marine records located in the south circum Pacific region. Moreover, our research provides new data on the impact in the lake ecosystem of human activities (deforestation and intensive cattle farming) during the last centuries.We suggest following reviewers because they are the main specialists in the topic of the manuscript: Mark

Paleoecology of Easter Island: Evidence and uncertainties

“…According to this author, humans (Polynesians) arrived on the island during a "climatic optimum", from a human point of view, occurring between about 1200 and 600 BP, at which time the island was already grass-covered with only a few trees and shrubs growing on protected sites. Recently, Mann et al (2008) and Sáez et al (2009), using pollen analysis and lake level reconstructions, respectively, proposed the existence of a mid-Holocene dry period characterized by a sedimentary hiatus. According to Mann et al (2008), this period was followed by forest clearance by humans.…”

“…This interpretation emphasizes the altitude as the differential factor between these situations, but there are other important environmental and paleoenvironmental differences. For example, according to the sedimentological interpretation, Rano Kao and Rano Raraku are lakes and have been so for most of the interval studied, while Rano Aroi has been a peat bog throughout this time (Sáez et al 2009). Therefore, differences in basin size and morphology, as well as in local hydrological conditions (i.e., the existence or not of a permanent water body, the connection or not with the groundwater system, etc.)…”

“…If this proposal is favored by further evidence, eventual climatically-driven oscillations in the water table position would determine changes in the position and extent of the assumed gallery and coastal forests. Therefore, changes in the hydrological balance (precipitation/evapotranspiration ratio) such as those suggested by Sáez et al (2009) over the last 34 000 years, and not only human disturbance, would emerge as important forcing factors for vegetation change on Easter Island.…”

“…The magnitude of ecological and cultural inferences about Easter Island's history contrasts with the limited number of cores studied, their fragmentary nature and the low resolution of the analyses, the few proxies used (mostly pollen), and the existence of a sedimentary hiatus within which important information 4 about the environmental shift is still hidden (Mann et al, 2008). At present, studies about the paleoecology of the island seem to have spring up again and new cores have been obtained and are being analyzed (Gossen, 2007;Azizi and Flenley, 2008;Mann et al, 2008;Sáez et al, 2009). We consider it appropriate to accurately review the existing evidence at this point, to provide a neutral perspective on the proposals erected so far.…”

“…This paper is also intended as a "hygienic" introduction to further paleoecological studies on new cores obtained from peat and lake sediments on Easter Island (Sáez et al, 2009). …”