Grain size and heavy minerals of the Late Quaternary eolian sediments from the Imbituba-Jaguaruna coast, Southern Brazil: Depositional controls linked to relative sea-level changes

The stratigraphic subdivision and correlation of dune deposits is difficult, especially when age datings are not available. A better understanding of the controls on texture and composition of eolian sands is necessary to interpret ancient eolian sediments. The Imbituba-Jaguaruna coastal zone (Southern Brazil, 28 degrees-29 degrees S) stands out due to its four well-preserved Late Pleistocene (eolian generation 1) to Holocene eolian units (eolian generations 2, 3, and 4). In this study, we evaluate the grain-size and heavy-mineral characteristics of the Imbituba-Jaguartma eolian units through statistical analysis of hundreds of sediment samples. Grain-size parameters and heavy-mineral content allow us to distinguish the Pleistocene from the Holocene units. The grain size displays a pattern of fining and better sorting from generation 1 (older) to 4 (younger), whereas the content of mechanically stable (dense and hard) heavy minerals decreases from eolian generation 1 to 4. The variation in grain size and heavy-mineral content records shifts in the origin and balance (input versus output) of eolian sediment supply attributable mainly to relative sea-level changes. Dunefields submitted to relative sea-level lowstand conditions (eolian generation 1) are characterized by lower accumulation rates and intense post-depositional dissection by fluvial incision. Low accumulation rates favor deflation in the eolian system, which promotes concentration of denser and stable heavy minerals (increase of ZTR index) as well as coarsening of eolian sands. Dissection involves the selective removal of finer sediments and less dense heavy minerals to the coastal source area. Under a high rate of relative sea-level rise and transgression (eolian generation 2), coastal erosion prevents deflation through high input of sediments to the coastal eolian source. This condition favors dunefield growth. Coastal erosion feeds sand from local sources to the eolian system. including sands from previous dunefields (eolian generation 1) and from drowned incised valleys. Therefore, dunefields corresponding to transgressive phases inherit the grain-size and heavy-mineral characteristics of previous dunefields, leading to selective enrichment of finer sands and lighter minerals. Eolian generations 3 and 4 developed during a regressive-progradational phase (Holocene relative sea level highstand). The high rate of sediment supply during the highstand phase prevents deflation. The lack of coastal erosion favors sediment supply from distal sources (fluvial sediments rich in unstable heavy minerals). Thus, dunefields of transgressive and highstand systems tracts may be distinguished from dunefields of the lowstand systems tract through high rates of accumulation (low deflation) in the former. The sediment source of the transgressive dunefields (high input of previously deposited coastal sands) differs from that of the highstand dunefields (high input of fluvial distal sands). Based on this case study, we propose a general framework for the relation between relative sea level, sediment supply and the texture and mineralogy of eolian sediments deposited in siliciclastic wet coastal zones similar to the Imbituba-Jaguaruna coast (C) 2009 Elsevier B.V. All rights reserved.

Holocene fires in East Amazonia (Caraja`s), new evidences, chronology and relation with paleoclimate

Past studies have evidenced the presence of charcoal in soils and lacustrine sediments of Amazonia region and suggested occurrences of widespread fires during the Middle Holocene. However, the available records do not indicate the changes in fire regime with enough time resolution. We quantified charcoal fragments in lacustrine sediments in a lake of North Carajas plateau in East Amazonia (5 degrees 50`-6 degrees 35`S and 49 degrees 30`-52 degrees 00`W). The charcoal quantification was compared to other sediment proxies, allowing a connection between paleofires and climate changes. Large variations in sediment characteristics led to distinct stages of sedimentation. From 11,800 (base of CSN 93/4) to 4750 cal yr B.P., low accumulation rates of organic matter are observed. Between 7600 cal yr B.P. (base of CSN 93/3 core) and 4750 cal yr B.P., this initial phase of sedimentation is characterized by low chlorophyll derivate accumulation rates and high accumulation rates of Botryococcus braunii, an alga resistant to episodic drought. The first phase of sedimentation would therefore correspond to, a low take level and a drier climate than today. Large biomass burning events occurred between 7450 cal yr B.P. and 4750 cal yr B.P., as indicated by the high charcoal particle concentration. From 4750 cal yr B.P. to 2800 cal yr B.P., accumulation rates of charcoal particles decreased, and the accumulation rate of chlorophyll derivate was low. From 2800 cal yr B.P. to 1300 cal yr B.P., the charcoal accumulation rates reached their lowest values in the core and a rapid increase in lacustrine production is evidenced by the increase in chlorophyll derivates and carbon accumulation rate. From 1300 cal yr B.P. to the last century, the charcoal accumulation rates increased. During the most recent period, the record is characterized by high accumulation rates of chlorophyll derivates while the charcoal particle accumulation rate decreased. This region is still unaffected by the current increase of anthropogenic fires. (c) 2007 Elsevier B.V. All rights reserved.