The Rano Aroi mire on Easter Island (also known as Rapa Nui; 27°09'S, 109°27'W, 430m above sea level) provides a unique non-marine record in the central South Pacific Ocean for reconstructing Late Pleistocene environmental changes. The results of a multiproxy study on two cores from the center and margin of the Rano Aroi mire, including peat stratigraphy, facies analysis, elemental and isotope geochemistry on bulk organic matter, X-ray fluorescence (XRF) core scanning and macrofossil analysis, were used to infer past water levels and vegetation changes. The chronology was based on 18 14C AMS dates for the upper 8.7m. The extrapolated age for the base of the sequence is 70kyr, which implies that this record is the oldest paleolimnological record on Easter Island. The recovered Rano Aroi sequence consists of a radicel peat formed primarily from the remains of sedges, grasses and Polygonaceae that have accumulated since Marine Isotopic Stage (MIS) 4 (70kyr BP) to the present. From 60 to 40kyr BP (MIS 3), high precipitation/runoff events were recorded as organic mud facies with lighter δ13C, low C/N values and high Ti content, indicating higher detritic input to the mire. A gradual shift in δ13C bulk organic matter from -14% to -26%, recorded between 50 and 45calkyr BP, suggests a progressive change in local peat-forming vegetation from C4 to C3 plant types. Post-depositional Ca and Fe enrichment during sub-aerial peat exposure and very low sedimentation rates indicate lower water tables during Late MIS 3 (39-31calkyr BP). During MIS 2 (27.8-19calkyr BP), peat production rates were very low, most likely due to cold temperatures, as reconstructed from other Easter Island records during the Last Glacial Maximum (LGM). Geochemical and macrofossil evidence shows that peat accumulation reactivates at approximately 17.5calkyr BP, reaching the highest accumulation rates at 14calkyr BP. Peat accretion decreased from 5.0 to 2.5calkyr BP, coinciding with a regional Holocene aridity phase. The main hydrological and environmental changes in Rano Aroi reflect variations in the South Pacific Convergence Zone (SPCZ), Southern Westerlies (SW) storm track, and South Pacific Anticyclone (SPA) locations.
Bibliographical noteFunding Information:
This research was funded by the Spanish Ministry of Science and Education through the projects LAVOLTER ( CGL2004-00683/BTE ), GEOBILA ( CGL2007-60932/BTE ) and CONSOLIDER GRACCIE ( CSD2007-00067 ) and an undergraduate grant JAE ( BOE 04/03/2008 ) to Olga Margalef. We thank CONAF (Chile) and the Riroroko family for the facilities provided on Easter Island, Anders Noren and Erik Brown for their valuable guidance in the process of obtaining XRF data at the LacCore/Limnological Research Center (LRC) and Large Lake Observatory (LLO) (University of Minnesota) and Hilary Birks, Torstein Solhøy and Heinrich Schwartz for their help in macrofossil identification. We also thank John Couwenberg, Martin Theuerkauf, Annie Wojatschke, Almut Spangenberg, Dierk Michaelis and René Dommain for their help and contributions to the peat facies descriptions and peatland characterization during the stay of OM at the University of Greifswald.
Copyright 2013 Elsevier B.V., All rights reserved.
- Last Glacial Maximum
- Marine Isotope Stage 3
- Peat geochemistry
- South Pacific paleoclimate
- Stable isotopes
- X-ray fluorescence core scanner