Isótopos radiogênicos como traçadores da origem do depósito sedimentar da Lagoa da Pata: implicações sobre o transporte eólico de poeira mineral para a Amazônia Ocidental

Abstract

Amazon site (Lagoa da Pata, located at the Morro dos Seis Lagos, in the State of Amazonas/Brazil) allowed to reconstruct the paleoclimate along the last ~7.5 kyr before the present, aiming identify potential atmospheric source in Holocene sediment deposits. The geographic location of this lake is such that, depending on the season, it can remain below the indirect influence of the Intertropical Convergence Zone ITCZ (receiving low influence of NE trade winds) or under the indirect influence of ITCZ receiving, in theory, part of the dust flow coming from the Sahara region as the literature suggests. The mineral dust can be used as a proxy for atmospheric transport patterns over time, by tracing its source region using radiogenic isotopes rations (87Sr/86Sr and 143Nd/144Nd). Many authors suggest that the Amazon Basin receives great amounts of African dust coming from the Sahara region, although most of these conclusions are based on satellite images observations and air-mass trajectories models. Core length was 118 cm long, sliced in 1 cm layers and dated for 14C method. Samples were previous treated for the organic matter removal and chemistry separation of the isotopes of interest for further insertion at the Thermal Ionization Mass Spectrometer (TIMS). We analyzed backward air mass trajectories using the HYSPLIT model (hosted at NOAA/NASA website). A database of most likely aeolian dust sources, as well as the rock that forms the lake, were computed for the radiogenic isotopes domains considering the ratios 87Sr/86Sr and 143Nd/144Nd. Lastly, we calculated the ITCZ average latitudinal positioning over the Atlantic Ocean for the last 10 years using the data for Interpolated Outgoing Longwave Radiation at the Equatorial Atlantic (NOAA data base).The theoretical data suggests that the trajectory pattern follows the temporal dynamics of the ITCZ position along the year, indicating that the origin of the trajectories that arrive at Lagoa da Pata are modulated by the ITCZ position: exclusive influence of North African (December to April - southern ITCZ), Southern South America and Southern Africa (May to September - northern ITCZ) and mix influence of Northern Africa and Southern Africa (October and November). The radiogenic isotopic ratios showed mixed contributions from the local rock (weathering), Southern Africa and Southern South America (mainly from the Southern Volcanic Zone - SVZ), instead of, surprisingly, the Saharan region. These results points to a different aeolian dust source reaching at Western Amazon: although the observational data suggests a huge contribution of the Saharan dust to the Amazon, we propose that maybe this influence is more valid to the Eastern Amazon. Lastly, we found an association of 143Nd/144Nd time variability with North Atlantic Bond Events, more clear for Bonds 1,2,3,4 and 5. Bond Events are likely to influence the ITCZ dynamics and, in this case, it seems to impact Amazon hydrological cycles and its aeolian dynamics, by determining periods with stronger or weaker local weathering and greater or lesser influence of aeolian sources from the South Hemisphere.The study of the variability of atmospheric flow over the Amazon rainforest, its future trends and possible teleconnections becomes valuable to a more reliable prognosis for climate forecasts in the Amazon