Vegetation changes and human settlement of Easter Island during the last millennia: A multiproxy study of the Lake Raraku sediments

Canellas-Bolta N., Rull V., Saez A., Margalef O., Bao R., Pla-Rabes S., Blaauw M., Valero-Garces B., Giralt S. (2013) Vegetation changes and human settlement of Easter Island during the last millennia: A multiproxy study of the Lake Raraku sediments. Quaternary Science Reviews. 72: 36-48.
Enllaç
Doi: 10.1016/j.quascirev.2013.04.004

Resum:

Earlier palynological studies of lake sediments from Easter Island suggest that the island underwent a recent and abrupt replacement of palm-dominated forests by grasslands, interpreted as a deforestation by indigenous people. However, the available evidence is inconclusive due to the existence of extended hiatuses and ambiguous chronological frameworks in most of the sedimentary sequences studied. This has given rise to an ongoing debate about the timing and causes of the assumed ecological degradation and cultural breakdown. Our multiproxy study of a core recovered from Lake Raraku highlights the vegetation dynamics and environmental shifts in the catchment and its surroundings during the late Holocene. The sequence contains shorter hiatuses than in previously recovered cores and provides a more continuous history of environmental changes. The results show a long, gradual and stepped landscape shift from palm-dominated forests to grasslands. This change started c. 450 BC and lasted about two thousand years. The presence of Verbena litoralis, a common weed, which is associated with human activities in the pollen record, the significant correlation between shifts in charcoal influx, and the dominant pollen types suggest human disturbance of the vegetation. Therefore, human settlement on the island occurred c. 450 BC, some 1500 years earlier than is assumed. Climate variability also exerted a major influence on environmental changes. Two sedimentary gaps in the record are interpreted as periods of droughts that could have prevented peat growth and favoured its erosion during the Medieval Climate Anomaly and the Little Ice Age, respectively. At c. AD 1200, the water table rose and the former Raraku mire turned into a shallow lake, suggesting higher precipitation/evaporation rates coeval with a cooler and wetter Pan-Pacific AD 1300 event. Pollen and diatom records show large vegetation changes due to human activities c. AD 1200. Other recent vegetation changes also due to human activities entail the introduction of taxa (e.g. Psidium guajava, Eucalyptus sp.) and the disappearance of indigenous plants such as Sophora toromiro during the two last centuries. Although the evidence is not conclusive, the American origin of V. litoralis re-opens the debate about the possible role of Amerindians in the human colonisation of Easter Island. © 2013 Elsevier Ltd.

Llegeix més

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

Margalef O., Canellas-Bolta N., Pla-Rabes S., Giralt S., Pueyo J.J., Joosten H., Rull V., Buchaca T., Hernandez A., Valero-Garces B.L., Moreno A., Saez A. (2013) A 70,000 year multiproxy record of climatic and environmental change from Rano Aroi peatland (Easter Island). Global and Planetary Change. 108: 72-84.
Enllaç
Doi: 10.1016/j.gloplacha.2013.05.016

Resum:

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. © 2013 Elsevier B.V.

Llegeix més