Late quaternary of the middle Caquetá River area (Colombian Amazonia)

The late Quaternary history of the middle Caquetá River area in Colombia, northwestern Amazonia is described, based on observations of river bank sections, radiocarbon dates and palynological analyses of organic layers in floodplain and low terrace sediments of the Caquetá River. It is shown that the Late Pleistocene and Holocene climatic changes that took place in the Andean Cordilleras, were related to the depositional and erosional history of the Caquetá River in the Colombian Amazonian lowlands. The low terrace sediments consist of sandy and gravelly deposits covered by clays that sometimes contain lenses of peaty material. From these organic low terrace sediments, seven finite radiocarbon dates were obtained of Middle Pleniglacial age, between 56 000 and 30 000 yr BP. The coarse textured basal deposits of the low terrace apparently stem from the early part of the Middle Pleniglaciai period, during which the effective rainfall in the Andes was relatively high and the Andean glaciers had a considerable extension. Palynological data from silty sediments with organic remains at one site, show an interval when drier and more open types of vegetation on poor soils must have covered a larger area than today, but Amazonian forest was still the dominating type of vegetation. This interval might correspond to one of the Middle Pleniglacial savanna intervals from eastern Amazonia (Carajas). No organic sediments from the Upper Pleniglacial period were found and hence radiocarbon dates were not obtained. In the Andes this period had a very cold climate with low effective rainfall and in the east Amazonian Carajas area it is characterised by the relative extension of open savanna vegetation. The river run-off and sediment transport must have been much lower than in the Middle Pleniglacial and the Caquetá River cut itself down in its own sediments. Two Late-glacial radiocarbon datings obtained at one site (ca. 12 500 yr BP) indicate the existence of a Late-glacial sedimentation phase, separated from the Holocene sequence by a minor erosional phase. Organic layers in the Holocene floodplain sediments yielded 28 radiocarbon dates between 10 000 and 355 yr BP. Holocene sedimentation started with the rapid deposition of (sandy) clay possibly in a partly permanently inundated Caquetá valley. During the major part of the Holocene (silty) clays were deposited, with a dominant seasonal inundation cycle.     

The scale of forest transition: Amazonia and the Atlantic forests of Brazil

This article adapts a general equilibrium model that provides a spatial solution for land use, labor allocation, and product markets in a two good economy. The adaptation, based on von Thünen, considers the multi-regional case, and solves for two regions, one industrial and the other, a newly opened agricultural frontier. The conceptual framework is considered with reference to Brazil, where forest recovery in the Atlantic Rainforest occurs simultaneously with forest losses in Amazonia. Simulation results of the theoretical model are given, demonstrating the impacts of comparative advantage in regional agriculture on the spatial system. The main theoretical interest of the article, aside from providing a formal spatial statement, is to define a distinction between aggregate forest transition (A-FT), when the area of all forests in a multi-regional system increases with the advent of trade relations, and regional forest transition (R-FT), when forest recovery is spatially constrained, and depends on forest losses elsewhere. Thus, the article addresses the role of scale in defining forest transition, and does so by representing spatial dynamics with a formal model. It also suggests that forest transition privileges one biome at the expense of others, and that a concept of landscape turnaround is more germane from a wildlands conservation perspective. The article closes with a discussion of Brazil, and how its forests in Amazonia and along the Atlantic will fare in the coming years.     

Hydrodynamics and suspended particulate matter retention in macrotidal estuaries located in Amazonia-semiarid interface (Northeastern-Brazil)

The aim of the current study was to determine the nature of the seasonal variability of the Suspended Particulate Matter (SPM) fluxes from the drainage basin to the estuary in a macrotidal region (Northeastern Brazil), and the estuarine response to a seawater intrusion regarding sediment deposition, which will support the understanding of the global transport of materials at the continent-ocean interface. Thermohaline structure data was acquired using a Conductivity, Temperature, and Depth (CTD) probe with a sampling frequency of 4 Hz. Suspended particulate material was measured by gravimetric measurements applied to exact filtered volume samples. The outflows were measured through the use of an Acoustic Doppler Current Profiler (ADCP) with frequency of 1.5 MHz. The horizontal thermal and saline gradients varied from warmer and less saline waters (2014) to cooler and saline waters (2015). The gradient behavior when linked to volume transport and SPM flows, suggests a minimization of the fluvial flows in 2015, easing the advance of coastal water (CW) towards the inner estuary, leading to an inversion of the baroclinic pressure gradient. The bottom saline front, generated by the entrance of coastal water masses, caused an increase in SPM concentrations due to increased fluid density, resuspension of previously deposited sediment, and erosion of banks. High concentrations of SPM indicate higher volume transport suggesting a hydraulic barrier due to the change/inversion of the baroclinic pressure gradient, resulting in water and material retention. Material deposition was observed during neap tide, while during spring tide the material is resuspended, increasing the concentration, generating cycles of deposition and erosion during the neap-spring tides. The sediment in suspension that reach the estuary, even with low fluvial volume, stay in this environment forming new islands because of deposition. High deposition rates or sediment cycling, if generated by the hydraulic barrier, may indicate that the flows of SPM from the continental drainage to the estuary and adjacent continental shelf are interrupted and the residence time is increased.     

GEOGRAPHICAL EXPLORATION AND THE ELUSIVE MAPPING OF AMAZONIA*

In this article I reexamine Alexander Hamilton Rice's seventh expedition to Amazonia (1924–1925) in order to highlight the ways in which the image of the modern explorer was constructed through various technologies of visualization, including maps, still photography, and film. The Rice expedition was equipped with the latest surveying instruments, among them a hydroplane especially adapted for taking aerial photographs, and it was the first to attempt shortwave radio communication in the Tropics. However, the expedition's efforts to reach the headwaters of the Rio Branco ended in failure. I investigate Rice's heavy investment in making a visual record of the expedition in the context of his academic ambitions. Despite his influence in learned societies in the United States and Europe, his questionable reputation within academic circles was difficult to overcome: His reliance on publicity to enhance his academic career had adverse consequences. The retelling of Rice's tale here also sheds light on the role of indigenous local knowledge and agency in the history of exploration. All the modern surveying technology employed notwithstanding, local support remained crucial for Amazonian exploration.     

Temporal relations between meander deformation,water discharge and sediment fluxes in the floodplain of the Rio Beni (Bolivian Amazonia)

The Andean Cordillera and piedmont significantly influence river system and dynamics, being the source of many of the important rivers of the Amazon basin. The Beni River, whose upper sub‐catchments drain the Andean and sub‐Andean ranges, is a major tributary of the Madeira River. This study examines the river in the south‐western Amazonian lowlands of Bolivia, where it develops mobile meanders. Channel migration, meander‐bend morphology and ox‐bow lakes are analysed at different temporal and spatial scales. The first part of this study was undertaken with the aim to link the erosion–deposition processes in the active channel with hydrological events. The quantification of annual erosion and deposition areas shows high inter‐annual and spatial variability. In this study, we investigate the conditions of sediment exportation in the river in relation to three hydrological parameters (flood intensity, date of discharge peak and duration of the bank‐full stage level). The second part of this study, focusing on the abandoned meanders, analyses the cutoff processes and the post‐abandonment evolution during 1967–2001. This approach shows the influence of the active channel behaviour on the sediment diffusion and sequestration of the abandoned meanders and allows us to build a first model of the contemporary floodplain evolution. Copyright © 2006 John Wiley & Sons, Ltd.     

HOME GARDENS IN AMAZONIAN PERU: DIVERSITY AND EXCHANGE OF PLANTING MATERIAL*

ABSTRACT. This article examines how peasant farmers build and maintain agrobiodiversity in home gardens found in two traditional peasant communities along the Marañón River in the Peruvian Amazon. Data were gathered through household and garden surveys as well as in‐depth interviews with garden tenders in an upland mixed agricultural village and a lowland fishing village. Substantial variations in cultivated plant diversity were encountered in gardens between and within the villages, which are found to be related to differential exchange of seeds, cuttings, suckers, and other planting material as well as to specific garden and household characteristics. Planting material flows along multiple pathways—from gift giving and purchase to inheritance and scavenging—to the gardens, reflecting a complex and often extensive network of exchange that enables the establishment and maintenance of home garden plant diversity in seemingly isolated and small communities. Further research is needed to identify broader geographical and sociocultural patterns of agrobiodiversity in Amazonia.     

Understanding Amazonian fluvial rias based on a Late Pleistocene–Holocene analog

Fluvial rias are elongated lakes at tributary mouths that can reach dozens of kilometers in length, constituting one of the most remarkable features in the Amazonian landscape. Thus far, definitive data which documents the genesis of fluvial rias have not been published. The main goal of this work was to integrate morphological, sedimentological and chronological information in order to characterize fluvial paleorias in the interfluve of the Purus and Madeira Rivers and discuss the most likely hypothesis for their genesis. These paleorias were first observed through remote sensing imagery as several elongated and interconnecting belts of open vegetation that are in sharp contact with the surrounding dense forest. The belts are branched and form a dendritic pattern similar to many modern drainage networks. The sedimentary record of these belts revealed the prevalence of sharp‐based sandstones and mudstones arranged into fining‐upward successions, which are compatible with deposition within channels. Active channel and abandoned channel deposits were recognized. These are topped by continuous mudstones related to rapid channel abandonment and formation of a low energy basin or ria environment. Radiocarbon dating of these deposits recorded only Late Pleistocene and Holocene ages ranging from 21 547–22 285 cal yr bp to 5928–6124 cal yr bp . This chronology for sediment deposition is not compatible with the hypothesis of Amazonian rias being formed by fluvial erosion during the Last Glaciation Maximum low sea level, with sediment accumulation during the subsequent Holocene transgression. Instead, the studied paleorias record previous tributaries of the Madeira River that became abandoned as the position of this river shifted southeastward and its interfluve tilted northward, inverting the drainage systems. Therefore, a neotectonic origin of some Amazonian paleorias seems most likely. This hypothesis should be considered in further investigations aiming at understanding the origin of numerous modern fluvial rias that typify the Amazonian landscape. Copyright © 2014 John Wiley & Sons, Ltd.     

Digital thermal monitoring of the Amazon forest: an intercomparison of satellite and reanalysis products

Remote sensing and climate digital products have become increasingly available in recent years. Access to these products has favored a variety of Digital Earth studies, such as the analysis of the impact of global warming over different biomes. The study of the Amazon forest response to drought has recently received particular attention from the scientific community due to the occurrence of extreme droughts and anomalous warming over the last decade. This paper focuses on the differences observed between surface thermal anomalies obtained from remote sensing moderate resolution imaging spectroradiometer (MODIS) and climatic (ERA-Interim) monthly products over the Amazon forest. With a few exceptions, results show that the spatial pattern of standardized anomalies is similar for both products. In terms of absolute anomalies, the differences between the two products show a bias near to zero with a standard deviation of around 0.2?K, although the differences can be up to 1?K over particular regions and months. Despite this general agreement, the proper filtering of MODIS daily values in order to construct a new monthly product showed a dramatic reduction in the number of reliable pixels during the rainy season, while for the dry season this reduction is only seen in Northern Amazonia.     

Is the current stress state in the Central Amazonia caused by surface water loading?

We present new fault data for the region of the Manaus, Central Amazonia, Brazil. Field measurements concentrate on the Miocene–Holocene sedimentary deposits exposed on the Amazonas River Basin, in order to investigate the development of this region in this time-interval. Two faulting events are distinguished since the Miocene. The oldest one is related to NW–SE extension during Miocene times and associated with paleoseismicity, while the younger is associated with NE–SW extension direction and seems to persist today. These two deformational events may be thereby considered Neotectonic. Moreover, the second extensional pulse with NE–SW orientation can be explained by the surface hydrological loading, which induces the Central Amazonia flexural subsidence and may promote extensional stresses in the upper crust.