A 70,000 year multiproxy record of climatic and environmental change from Rano Aroi peatland (Easter Island)

摘要

The Rano Aroi mire on Easter Island (also known as Rapa Nui; 27°09'S, 109°27'W, 430 m 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 8.7 m. The extrapolated age for the base of the sequence is 70 kyr, 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 (70 kyr BP) to the present. From 60 to 40 kyr BP (MIS 3), high precipitation/runoff events were recorded as organic mud facies with lighter δ~(13)C, low C/N values and high Ti content, indicating higher detritic input to the mire. A gradual shift in δ~(13)C bulk organic matter from - 14% to - 26%, recorded between 50 and 45 cal kyr 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 (39-31 cal kyr BP). During MIS 2 (27.8-19 cal kyr 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 17.5 cal kyr BP, reaching the highest accumulation rates at 14 cal kyr BP. Peat accretion decreased from 5.0 to 2.5 cal kyr 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.