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Abstracts / Quaternary International 279-280 (2012) 233–345
identical shrublands occur at ca. 200 m elevation above the sampling site, suggesting an altitudinal displacement equivalent to a decline of 1.0-1.5 oC in average temperatures. Both records suggest dry climates during the YD, with respect to the early Holocene and the present. This evidence strongly supports the occurrence of the YD cold reversal in the tropics, at both high and mid altitudes, with different climatic regimes. PALEOECOLOGICAL STUDY OF A GRAN SABANA LAKE (SE VENEZUELA) DURING THE LAST MILLENNIA: IMPLICATIONS FOR MAURITIA COLONIZATION AND HUMAN SETTLEMENT Encarni Montoya. Universitat Autònoma de Barcelona, Spain E-mail address:
[email protected]
The southern Gran Sabana region (SE Venezuela) holds a particular type of savanna characterized by the local occurrence of morichales (Mauritia palm swamps), in a climate apparently more suitable for rain forests. We present a paleoecological analysis of a Late Holocene sequence from Lake Chonita based on physical and biological proxies. Savannas dominated the region during the last millennia, but a significant vegetation replacement occurred around the lake in recent times. From 3.6 to 2.2 cal kyr BP the site was covered by a treeless savanna with nearby rain forests, and water levels higher than today until about 2.8 cal kyr BP. From 2.2 cal kyr BP onwards, the area has been under high fire incidence, likely determining the retraction of forest and a dramatic increase in the extension of morichales. The simultaneous appearance and the coincident trends of charcoal and Mauritia pollen supports the hypotheses about the potential pyrophilous nature of this palm and its recent expansion, probably linked to human activities. Since the beginning of these local fires, the situation has not changed until today, suggesting that the present-day Pemón culture -in which fire is an essential component- may have established in the site around 2000 years ago. LATE QUATERNARY FIRE REGIMES OF AUSTRALASIA Scott Mooney. School of Biological, Earth and Environmental Scie, Australia E-mail address:
[email protected]
We have compiled 223 sedimentary charcoal records from Australasia in order to examine the temporal and spatial variability of fire regimes during the late Quaternary. While some of these records cover more than a full glacial cycle, here we focus on the last 70,000 years in an attempt to examine some persistent controversies about the relationship between fire, climate and humans. On orbital time scales, fire in Australasia predominantly reflects climate, with colder periods characterised by less and warmer intervals by more biomass burning. The composite record for the region also shows considerable millennial-scale variability during the last glacial interval (73.5 to 14.7 ka). We also demonstrate differences in the post-glacial composite charcoal records from the Australasian tropics and the region influenced by the austral subtropical high pressure belt. There is no distinct change in fire regime corresponding to the arrival of humans in Australia at 50 10 ka and no correlation between archaeological evidence of increased human activity during the past 40 kyr and the history of biomass burning. However, distinct changes in biomass burning in the last 200 years may have been exacerbated or influenced by humans.
changes in sedimentation constrains, which correspond to distinct sedimentary environments. The lower Unit I accumulated until ca 9550 cal BP and consists of coarse sandy sediments deposited in high-energy fluvial environment, contemporaneous of a lower sea-level. The earliest signature of marine influence was detected at 9550 cal BP (-17,14m MSL, base of Subunit IIA), indicating the first pulses of a rising sea level attempting to drown the preexisting valley, and promoting the deposition of muddy sediments in a low energy, confined environment (lagoonal/estuarine); towards the top of IIA, the contents of paleossalinity indicators (S, Cl and Br) and proportion of CaCO3 (marine bioclasts) increases. The maximum of marine influence took place between 6300 and 3670 cal BP (-5.35 and -2.14m MSL, Subunit IIB, consisting of mud) with the highest values of paleossalinity geochemical proxies and CaCO3 in the whole sequence. The intensity of the marine signal decreased afterwards, accompanied by increasing terrestrialization of this paralic system, which is clear at 2670 cal BP, where a marginal lowenergy environment developed, promoting accumulation of peat (Unit III). After the 12th century AD, deposition of fluvial materials in high terrestrial energy environment (Unit IV) occurred in result of progradation of the Alcôa River alluvial fan and plain sediments over the former lagoonal margin, contributing to the complete infilling of the lowland. RAPID PALEOHYDROGICAL HOLOCENE CHANGES IN THE AMAZON RIVER AND ITS IMPLICATIONS ON THE CARBON ACCUMULATION IN FLOODPLAIN LAKES Patricia Moreira Turcq. IRD LMTG, France E-mail address:
[email protected]
The Amazon basin provides 20% of the total fresh water worldwide and the reconstruction of its paleohydrologic changes is essential for the global paleo-environment. The Amazon River and its tributaries are followed along their upward and downward course by several floodplains lakes that cover a large area around 300 – 500 000 km2 (5 to 8% of the total surface area of the Amazon Basin). The floodplains lakes are built due to the fluctuations in the level of the rivers, which causes the formation of bars and accumulation of sediment carried by the rivers and its tributaries. Thus, significant quantities of organic matter can accumulate within these lakes that might represent important carbon sinks. The aim of this work is to evaluate the importance of past climatic changes on sedimentation process in Amazonia floodplain lakes and to determine the role played by the climatic driving forces on the carbon sink. Here, we present data from the sedimentary record of the greatest Amazon floodplain lake (Curuai floodplain) that reveal the presence of rapid and marked paleohydrological changes during the Holocene in the Amazon Basin. For this work, seismic profiles (3.5 khz) and eight sedimentary cores were studies in the floodplain system. The cores were analyzed using radiocarbon dates, mineralogy, apparent density, organic carbon concentration, C/N ratio and d13C. Our results point out two important events, at 5700 cal yr BP and 2700 cal yr BP, characterized by high sedimentation rates that indicate deep changes in the floodplain hydrology. Seismic and sedimentation data seem to indicate that these changes are due to abrupt climatic events. Both events provoked significantly high carbon accumulation in the floodplain system.
PALEOENVIRONMENTAL RECONSTRUCTION OF PEDERNEIRA LAGOON (WESTERN PORTUGAL)
HISTORIC TRENDS OF METALS ACCUMULATION AND HUMAN IMPACT RECORDED IN INTERTIDAL SEDIMENTS OF MORETON BAY, SOUTHEAST QUEENSLAND, AUSTRALIA
Sandra Moreira. Centro de Geologia, Faculdade de Ciências da Unive, Portugal
E-mail address:
[email protected]
Guia Morelli. The University of Queensland, Australia
E-mail address:
[email protected]
The Pederneira Lagoon formed in result of the Holocene transgression and the Nazaré alluvial plain corresponds to the remains of that paleolagoon. To reconstruct its paleonvironmental evolution in the last thousand years, a 29.3m-long core (NZS2) crossing the Late Pleistocene and Holocene was taken from the alluvial plain and studied using sedimentological and geochemical proxies. The interpretation of the data allowed the identification of four major lithostratigraphical units with different textures/compositions, reflecting
The extent of human impact across the intertidal shores of Moreton Bay (Australia) is investigated by evaluating temporal trends of trace metal concentrations preserved in four sediment cores. The short-lived radionuclides 210Pb and 137Cs are used to obtain geochronology of the past w 100 years. The 210Pb-derived sedimentation rates vary between 0.16 0.01 g/cm2/y and 0.71 0.30 g/cm2/y, showing high spatial variability across the Bay. Increasing sedimentation rates over the past years are in agreement with the period of major development, as a consequence of land use intensification after European settlement, as deforestation for agriculture
