The last 70 kyr of Rano Aroi (Easter island, 27ºS) peat record: New insights for the Central Pacific paleoclimatology

Abstract

The PhD entitled “Last 70kyr of Rano Aroi (Easter Island, 27ºS) peat record: New insights for Central Pacific paleoclimatology” presents an environmental and climate reconstruction of Easter Island during the last 70.000 years. To accomplish this purpose, two peat cores of 14 and 4 meters have been retrieved from the center and the margin of the Rano Aroi peatland. Facies description and age model allowed us to decipher that the peat started accumulating 70.000 years ago, what means that this is the oldest peat record recovered up to the moment over the Southern Pacific. To investigate changes in the type of accumulated peat we determined total carbon, nitrogen and sulfur (TC, TN, TS) of organic matter as well as stable isotope values of each element (delta13C, delta15N, delta34S). In order to decipher the composition of the mineral content, X-Ray Fluorescence measurements was performed over the cores, obtaining the quantity of Ca, Fe and Ti. Additionally, the concentration of the major, minor and trace elements was determined by spectrometry (ICP-AES). The geochemical approach was complemented by the study of macrofossil (plant or animal remains as seeds or insects) and pollen content, what permitted to reconstruct vegetation changes on the mire and the island. The combination of geochemical and biological proxies defined three main hydric conditions (flood, drought and baseline conditions) of the peatland that have been alternated along Late Pleistocene and Holocene. During MIS 4 (70 to 60 kyr BP in Rano Aroi), the hydric and environmental conditions in the mire were stable and resembled mire baseline conditions. Open grasslands covered Rano Aroi catchment area and C4 Poaceae dominated the mire. Early MIS 3 (60 to 40 cal kyr BP in Rano Aroi) was marked by the onset of wet events. During the first half of MIS 3 and probably driven by the wetter and warmer conditions Asteraceae and other small trees became gradually more abundant, forming small and scattered forests around the Terevaka volcano, while C3 peat forming plants colonized the Aroi mire. Late MIS 3 was a very dry period, which led to peat oxidation and exposure shortly after 39 cal kyr BP. These conditions changed on early MIS 2 (27.8–19 cal kyr BP). The water table recovered and peat accumulation resumed under C3 plant dominance. The evidence of the onset of deglaciation is concordant with Raraku Lake environmental development and was dated at 17.5 cal kyr BP in both sites. The regional mid-Holocene dry period is well characterized at Rano Aroi from 5 to 2.5 cal kyr BP. The correlation between Rano Aroi record and other circumpacific sites suggests important changes in the atmospheric or oceanic circulation on Central Pacific during Late Pleistocene. In particular, the Rano Aroi record indicates that the episodes of maximum precipitation over the island correlate with North Atlantic Heinrich stadials and other stadials. During these stadials, the Intertropical Convergence Zone (ITCZ) migrated to the south while the South Pacific Convergence Zone (SPCZ) shifted eastwards provoking the arrival of storm systems over the island. In accordance to this atmospheric and oceanic restructuration, Rano Aroi wet events also correlated with periods of a reduced Sea Surface Temperature (SST) gradient along the Equator. This fact indicates that more humid conditions over the Easter Island region occurred when the Walker circulation was reduced. The Central Pacific climate was also influenced by high latitude dynamics, like the changes in Intermediate Antarctic Water formation and the position of the Southern Westerlies.

Date of Award	14 Mar 2014
Original language	English
Awarding Institution	Universitat de Barcelona (UB)
Supervisor	Sergi Pla Rabes (Director) & Santiago Giralt (Director)