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Projections of the impacts of modern Relative Sea Level (RSL) rise on estuarine mangroves should be supported by coastal topographic data and records of mangrove dynamics under past RSL change. This work identified inland and seaward mangrove migrations along the Jucuruçu River (Bahia, Northeastern Brazil), during the Holocene based on sedimentary features, palynological and geochemical (δ13C, δ15N, C/N) data integrated with digital elevation models. During the Middle Holocene, in response to RSL rise, the estuary saw mangrove forest establish up to ~37 km inland. RSL stood between ‐1.4 (+0.36/‐2.2 m) and +1 (2.19/0.2 m) around 7400 cal yr BP, and rose to a highest position of +3.25 (4.22/2.45 m) reached around 5350 cal yr BP. That marine incursion caused the inland replacement of freshwater vegetation by mangroves on tidal flats. Since then, the estuary experienced RSL fall, reducing inland tidal water salinity towards the Late Holocene, making that the mangroves were replaced by freshwater floodplain vegetation. Today, in the seaward part of the estuary near its mouth, mangroves occupy an area of ~10 km2 along tidal channels. Considering a RSL rise of 98 cm up to the end of the 21st century, at a rate significantly higher than that of Middle Holocene RSL rise (1.5 mm/yr) and fall (0.6 mm/yr), the current mangrove substrates are expected to drown and/or eroded near the coast, while new mangroves may establish inland, at topographically higher tidal flats in nowadays freshwater‐tidal zones. Mangrove area could expand over 13 km2 of coastal and flood plain. Following the same interaction between RSL/climate changes and Holocene mangrove dynamics, such upstream mangrove migration may be attenuated or intensified by changes in fluvial discharge. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The origin of the transcontinental Amazon drainage system remains unrevealed. Sedimentary deposits formed from the Neogene in the Amazonas and Solimões Basins constitute natural archives for reconstructing this event in space and time. However, paleoenvironmental and chronological analyses focusing on these deposits, or even their basic mapping, are still scarce to allow such investigation. In this context, primary interests are fluvial strata related to the lithostratigraphic Içá Formation, mapped over a widespread area in western Amazonian lowlands. Although long regarded as Plio-Pleistocene in age, this unit has not yet been dated and its overall depositional setting remains largely undescribed. The main goal of the present work is to contribute for improving facies analysis and chronology of these deposits, approaching an area in southwestern Amazonia and another in northern Amazonia, which are located more than 1000 km apart. Despite this great distance, the sedimentological and chronological characteristics of deposits from these two areas are analogous. Hence, facies analysis revealed paleoenvironments including active channel, abandoned channel, point bar, crevasse splay and floodplain, which are altogether compatible with meandering fluvial systems. Similarly, optically stimulated luminescence (OSL) dating revealed thirty three ages ranging from 65.4 ± 16.9 to 219.6 ± 25.1 ky (in addition to three outliners of 54.0 ± 7.6, 337.3 ± 36.9 and 346.6 ± 48.6 ky), and nine 97.1 ± 9.9 to 254.8 ± 23.8 ky for the areas in southwestern and northern Amazonia, respectively. These data lead to establish that deposits mapped as Içá Formation over a vast area of western Brazilian Amazonia have a Mid-Late Pleistocene age, rather than the previously inferred Plio-Pleistocene age. It follows that if Plio-Pleistocene deposits exist in this region they remain to be dated and must be restricted to a narrow belt in western Amazonia, as well as isolated occurrences underlying the Mid-Late Pleistocene strata characterized herein. The combination of data from this work with previously published provenance studies supports main Andean sediment sources only in the Mid-Late Pleistocene. It is proposed that before this time, the Amazon River was restricted to eastern Amazonia, being separated from western Amazonian drainage basins due to the presence of the Purus Arch. Erosion and/or subsidence of this geological feature would have promoted the connection of these drainage systems, ultimately with the expressive record of the transcontinental Amazon pathway into the Atlantic Ocean in the Mid-Late Pleistocene. [ABSTRACT FROM AUTHOR]

Abstract: Late Quaternary sedimentary deposits from Amazonian lowlands are important geological archives for understanding the origin, evolution and controls of the largest fluvial drainage basin on Earth. Climate has been most often claimed as the main factor affecting river evolution in this region. An increasing volume of publications have also highlighted tectonics as a relevant control. This work investigates the late Quaternary evolution of the Madeira River, the biggest southern tributary of the Amazonas Basin and a major waterway in South America, with the goal of discussing the factor with highest potential of influence on its development in space and time. The approach consisted of carrying out a detailed morphological, sedimentological and chronological characterization of terrace deposits. Three terraces were recognized, which record sandy/pebbly channel, muddy channel/oxbow lake, point bar, floodplain, as well as crevasse channel, crevasse splay and levee deposits. The topographically highest terrace T1 formed in the time interval before 43,500 and 31,696–32,913calyrs BP, the intermediate terrace T2 between 25,338–26,056 and 14,129–14,967calyrs BP, and the lowest terrace T3 between 12,881–13,245 and 3158–3367calyrs BP. These terraces are the testimony of successive downcutting and sediment aggradation. Episodes of terrace erosion and deposition cannot be fully accommodated within the framework of fluctuating river base level tied to Late Pleistocene–Holocene global or regional climate fluctuations. Changes in tectonic subsidence rates might have played an important role in the equilibrium state of this particular fluvial system. [Copyright &y& Elsevier]

The Atlantic Forest and mangrove dynamics along the Brazilian coast have been affected by climate, relative sea-level (RSL), and anthropogenic forces. Individualizing the effects of these drivers on paleoecological records is a challenge. This was addressed by pollen analysis along three cores sampled from lakes on a coastal plateau, primarily influenced by rainfall, and from a fluvial valley, influenced by sea-level and fluvial discharge, in northeastern Brazil. Pollen records from the fluvial valley indicated the presence of mangroves along an estuary in the mid-Holocene. During the past ~5800 cal yr BP, mangroves, adapted to brackish water, were replaced by herbaceous vegetation adapted to freshwater in the fluvial valley. Concurrently (~5680–240 cal yr BP), expansion of Atlantic Forest and contraction of herbaceous vegetation occurred on the coastal plateau. This divergence of pollen records between two distinct sedimentary environments (lakes on a coastal plateau and fluvial valleys) was attributed to RSL and climate effects. The combination of RSL fall and higher fluvial discharge favored seaward mangrove migration and expansion of herbaceous vegetation along the fluvial valley during the mid-late Holocene. However, the increase in rainfall on the coastal plateau favored the expansion of arboreal vegetation and contraction of herbaceous vegetation. At ~1700 CE, an abrupt shift from arboreal to herbaceous vegetation occurred on the coastal plateau. This vegetation change is likely attributed to European colonization on the southern coast of the state of Bahia, which was colonized by Portuguese navigators ~1500 CE. Part of the Atlantic Forest has naturally regenerated and was preserved as a national park in recent decades. This study showed that although the sampling sites are close, pollen assemblages can differ considerably depending on the depositional environment. Therefore, pollen analyses in different depositional environments are necessary to assess individually driving forces impacts on coastal vegetation dynamics during the Holocene. • Atlantic Forest and mangrove in the Brazilian coast were studied by pollen analysis. • Mangroves along fluvial valley were replaced by herbs after 5800 cal yr BP. • Atlantic Forest expanded and herbs area shrank on coastal plateau (5700–240 cal yr BP). • Abrupt shift from arboreal to herbs occurred on coastal plateau at ~1700 CE. • Climate, sea-level, and European colonization effects on vegetation were identified. [ABSTRACT FROM AUTHOR]