Amazonian Lowlands Research Articles

From Andes to Amazon: Assessing Branched Tetraether Lipids as Tracers for Soil Organic Carbon in the Madre de Dios River System

AbstractWe investigate the implications of upstream processes and hydrological seasonality on the transfer of soil organic carbon (OC) from the Andes mountains to the Amazon lowlands by the Madre de Dios River (Peru), using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids. The brGDGT signal in Andean soils (0.5 to 3.5 km elevation) reflects air temperature, with a lapse rate of −6.0 °C/km elevation (r2 = 0.89, p < 0.001) and −5.6 °C/km elevation (r2 = 0.89, p < 0.001) for organic and mineral horizons, respectively. The same compounds are present in river suspended particulate matter (SPM) with a lapse rate of −4.1 °C/km elevation (r2 = 0.82, p < 0.001) during the wet season, where the offset in intercept between the temperature lapse rates for soils and SPM indicates upstream sourcing of brGDGTs. The lapse rate for SPM appears insensitive to an increasing relative contribution of 6‐methyl isomer brGDGTs produced within the river. River depth profiles show that brGDGTs are well mixed in the river and are not affected by hydrodynamic sorting. The brGDGTs accumulate relative to OC downstream, likely due to the transition of particulate OC to the dissolved phase and input of weathered soils toward the lowlands. The temperature‐altitude correlation of brGDGTs in Madre de Dios SPM contrasts with the Lower Amazon River, where the initial soil signature is altered by changes in seasonal in‐river production and variable provenance of brGDGTs. Our study indicates that brGDGTs in the Madre de Dios River system are initially soil derived and highlights their use to study OC sourcing in mountainous river systems.

<p><strong><em>Magnolia napoensis</em></strong><strong> (subsect. <em>Talauma</em>, Magnoliaceae): a new species from the Amazonian lowlands of Ecuador and Peru</strong></p>

Magnolia napoensis, a new species from the Amazonian lowlands of Ecuador and Peru is described and illustrated. This species belongs to subsection Talauma; it is similar to M. rimachii in leaf shape but differs from the latter in being taller with a larger diameter and having fewer lateral leaf veins, more numerous hypsophylls, larger flowers, longer outer petals, more numerous stamens and fruits ovoid and ribbed vs. subglobose and smooth. The new species differs from M. neillii by its leaves with fewer lateral veins, glabrous petioles and terminal internodes, more numerous hypsophylls, fewer stamens and ovoid fruits of smaller size, with fewer carpels. Magnolia napoensis is assessed as endangered (EN B2ab(iii)) in accordance with the IUCN criteria.

An integrative approach to identify the impacts of multiple metal contamination sources on the Eastern Andean foothills of the Ecuadorian Amazonia

Currently, several concerns have been raised over metal contamination in the upper Amazon basin. Rivers that flow from the high Andes to the lowland Amazon are threatened by anthropogenic activities, which may, in turn, lead to increased metal concentrations in both water and sediments. In the present study, the impacts of multiple metal contamination sources in these ecosystems were identified. The degree of metal contamination was assessed in water and sediment and seed phytotoxicity analyses were carried out in samples taken from 14 sites located in upper Napo River tributaries, combining geochemical and ecotoxicological techniques. These tributaries were chosen based on their degree of anthropogenic contamination and proximity to known sources of relevant pollution, such as small-scale gold mining (MI), urban pollution (UP), fish farming (FF) and non-functional municipal landfill areas (LF). Our results suggest that anthropogenic activities are introducing metals to the aquatic ecosystem, as some metals were up to 500 times above the maximum permissible limits for the preservation of aquatic life established by Ecuadorian and North American guidelines. Sites located close to small-scale gold mining and sanitary landfills presented 100 to 1000 times higher concentrations than sites classified as “few threats”. In water, Cd, Pb, Cu, Zn and Hg were mostly above the maximum permissible limits in the samples, while Cd in sediment reached concentrations 5-fold above the probable effect level (PEL). Phytotoxicity was associated through the diffuse contamination present in urban and landfill areas. Overall, metal concentrations and phytotoxicity assessments suggest anthropogenic effects to environmental contamination, even though natural sources cannot be disregarded. Anthropogenic effects in the eastern Andean Rivers need to be constantly monitored in order to build a complete picture on how pollution sources may affect this strategic Amazon basin area.

Multispectral canopy reflectance improves spatial distribution models of Amazonian understory species

Species distribution models are required for the research and management of biodiversity in the hyperdiverse tropical forests, but reliable and ecologically relevant digital environmental data layers are not always available. We here assess the usefulness of multispectral canopy reflectance (Landsat) relative to climate data in modelling understory plant species distributions in tropical rainforests. We used a large dataset of quantitative fern and lycophyte species inventories across lowland Amazonia as the basis for species distribution modelling (SDM). As predictors, we used CHELSA climatic variables and canopy reflectance values from a recent basin‐wide composite of Landsat TM/ETM+ images both separately and in combination. We also investigated how species accumulate over sites when environmental distances were expressed in terms of climatic or surface reflectance variables. When species accumulation curves were constructed such that differences in Landsat reflectance among the selected plots were maximised, species accumulated faster than when climatic differences were maximised or plots were selected in a random order. Sixty‐nine species were sufficiently frequent for species distribution modelling. For most of them, adequate SDMs were obtained whether the models were based on CHELSA data only, Landsat data only or both combined. Model performance was not influenced by species’ prevalence or abundance. Adding Landsat‐based environmental data layers overall improved the discriminatory capacity of SDMs compared to climate‐only models, especially for soil specialist species. Our results show that canopy surface reflectance obtained by multispectral sensors can provide studies of tropical ecology, as exemplified by SDMs, much higher thematic (taxonomic) detail than is generally assumed. Furthermore, multispectral datasets complement the traditionally used climatic layers in analyses requiring information on environmental site conditions. We demonstrate the utility of freely available, global remote sensing data for biogeographical studies that can aid conservation planning and biodiversity management.

The Western Amazonian Richness Gradient for Squamate Reptiles: Are There Really Fewer Snakes and Lizards in Southwestern Amazonian Lowlands?

The lowland rainforests of the Amazon basin harbor some of the most species-rich reptile communities on Earth. However, there is considerable heterogeneity among climatically-similar sites across the Amazon basin, and faunal surveys for southwestern Amazonia in particular have revealed lower species diversity relative to sites in the northwestern and central Amazon. Here, we report a herpetofaunal inventory for Los Amigos Biological Station (LABS), a lowland site located in the Madre de Dios watershed of southern Peru. By combining active search and passive trapping methods with prior records for the site, we provide a comprehensive species list for squamate reptiles from LABS. We also estimate an “expected” list for LABS by tabulating additional taxa known from the regional species pool that we consider to have a high probability of detection with further sampling. The LABS total of 60 snake and 26 lizard taxa is perhaps the highest for any single site in the southern Amazon. Our estimate of the regional species pool for LABS suggests that the southwestern Amazonian lowlands harbor at least 25% fewer species of snakes relative to the western equatorial Amazon, a diversity reduction that is consistent with patterns observed in several other taxonomic groups. We discuss potential causes of this western Amazonian richness gradient and comment on the relationship between spatial diversity patterns in squamates and other taxa in the Amazon basin.

Gut Microbiota Features Associated With Campylobacter Burden and Postnatal Linear Growth Deficits in a Peruvian Birth Cohort.

Background Campylobacter infection is associated with impaired growth of children, even in the absence of symptoms. To examine the underlying mechanisms, we evaluated associations between Campylobacter infection, linear growth, and fecal microbial community features in a prospective birth cohort of 271 children with a high burden of diarrhea and stunting in the Amazonian lowlands of Peru.Methods Campylobacter was identified using a broadly reactive, genus-specific enzyme-linked immunosorbent assay. 16S rRNA-based analyses were used to identify bacterial taxa in fecal samples at ages 6, 12, 18, and 24 months (N = 928). Associations between infection, growth, and gut microbial community composition were investigated using multiple linear regression adjusting for within-child correlations, age, and breastfeeding. Indicator species analyses identified taxa specifically associated with Campylobacter burden.ResultsNinety-three percent (251) of children had Campylobacter present in asymptomatic fecal samples during the follow-up period. A 10% increase in the proportion of stools infected was associated with mean reductions of 0.02 length-for-age z scores (LAZ) at 3, 6, and 9 months thereafter (P < .01). We identified 13 bacterial taxa indicative of cumulative Campylobacter burden and 14 taxa significantly associated with high or low burden of enteroaggregative Escherichia coli, norovirus, or Giardia.Conclusions Campylobacter infection is common in this cohort and associated with changes in microbial community composition. These results support the notion that disruptions to the fecal microbiota may help explain the observed effects of asymptomatic infections on growth in early life.

A New Species of Terrestrial-Breeding Frog (Amphibia, Strabomantidae, Noblella) from the Upper Madre De Dios Watershed, Amazonian Andes and Lowlands of Southern Peru

We describe and name a new species of Noblella Barbour, 1930 (Strabomantidae) from southern Peru. Key diagnostic characteristics of the new species include the presence of a short, oblique fold-like tubercle on the ventral part of the tarsal region, two phalanges on finger IV, and an evident tympanum. The elevational distribution of the new species spans 1250 m (240–1490 m) from lowland Amazon rainforest to montane forest on the eastern slopes of the Andes.

The impact of language contact on the Quechua varieties of Northern Peru

Abstract Studies of language contact in the Central Andes of Peru and Bolivia have focused strongly on the present-day contact situation between Quechua and Spanish, and the intricate and multilayered contact relationship between the Quechua and Aymara lineages. There are fewer studies of the influence of Quechua on minor non-Quechua languages of the Andes, and still fewer studies which, conversely, explore the influence of non-Quechua languages on Quechua. Focusing on the lexicon, this article explores the impact of the complex linguistic ecology of Northern Peru on the five Quechua varieties of that region—Lambayeque, Cajamarca, Chachapoyas, San Martín and Ancash Quechua. The study identifies lexical items that lack clear Quechua etymologies in the relevant varieties and carries out external comparisons of these items with the vocabulary of the non-Quechua languages of Northern Peru to identify possible sources. Results show that borrowing is mostly localized: that is, whereas influence from Amazonian lowland languages is almost exclusively found in the eastern varieties of Chachapoyas and San Martín, highland Quechua varieties have typically borrowed from neighboring highland languages.

Spatiotemporal Variations of Riverine Discharge Within the Amazon Basin During the Late Holocene Coincide With Extratropical Temperature Anomalies

AbstractLate Holocene hydroclimate variations have been extensively recognized in Amazonia, but the effects of such changes on riverine discharge within the Amazon lowlands are still poorly understood. We investigated a sediment core covering circa 4,000 to 300 cal yr BP collected in the lower valley of the Xingu River (Xingu Ria) in an area under the influence of the Amazon River. Our results indicate a decrease in precipitation in the Amazon lowlands throughout the studied period and reduced input of coarser and potassium‐rich Amazon River sediments to the confluence from about 2,600 to 1,400 cal yr BP. We suggest that lower temperatures in the extratropical Southern Hemisphere weakened the South American Summer Monsoon and led to a decrease in the water discharge of the Amazon River during this period.

Historical Distal Lahar Deposits on the Remote Eastern-Drainage of Cotopaxi Volcano, Ecuador

Cotopaxi is an active stratovolcano, located 60 km SE of Quito. During historical times, Cotopaxi experienced at least five eruptive cycles triggering major lahars which descended along all three main fluvial systems: Pita (north), Cutuchi (south) and Tambo-Tamboyaku (east). Northern and southern lahar deposits have been studied for decades, due to their impact on large populated areas. In contrast, the eastern deposits have only been occasionally mentioned. However, historical reports suggest that the AD 1744, 1768 and 1877 eruptions triggered lahars in the eastern drainage, which caused damage in this remote zone. These lahars flowed more than 120 km downstream crossing the uninhabited Eastern Cordillera until reaching the Amazon lowlands where rural-indigenous settlements were located.In this study, two different historical lahar deposits were recognized and characterized at the eastern lowlands. Several samples were collected in order to perform geochemical analyses of the juvenile blocks, grain size distribution, and componentry analyses of the proximal and distal eastern zones. Finally, inundation areas, deposit volumes and peak discharges were estimated. The results allowed us to make a detailed characterization of the deposits, which is valuable information for future investigations and which allows a better understanding of the potential consequences of a similar future event.

Identifying the Molecular Signatures of Agricultural Expansion in Amazonian Headwater Streams

AbstractAgricultural impacts on aquatic ecosystems are well studied; however, most research has focused on temperate regions, whereas the forefront of agricultural expansion is currently in the tropics. At the vanguard of this growth is the boundary between the Amazon and Cerrado biomes in Brazil, driven primarily by expansion of soybean and corn croplands. Here we examine the impacts of cropland expansion on receiving lowland Amazon Basin headwater streams in terms of dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition via ultrahigh‐resolution mass spectrometry. Streams draining croplands had lower DOC concentrations and DOM molecular signatures enriched in N‐ and S‐containing formula in comparison to forested streams. Cropland streams were also enriched in aliphatic, peptide‐like, and highly unsaturated and phenolic (low O/C) compound categories in comparison to forest streams (enriched in polyphenolics, condensed aromatics, and highly unsaturated and phenolic [high O/C] compound categories) indicative of the shifting of sources from organic‐rich surface soils and litter layers to autochthonous and more microbial biomass. Distinct molecular assemblages were strongly correlated with cropland and forest catchments, highlighting headwater streams as sentinels for detecting change. On investigation of unique molecular formulae present in only cropland sites, four cropland markers provided the ability to track agricultural impacts in the region. Overall, these patterns indicate reduced organic matter inputs in croplands and greater microbial degradation at these sites leading to declining DOC concentrations, and DOM of more microbial character in receiving streams that is more biolabile, with clear ramifications for downstream ecology and biogeochemical cycles.

The role of Late Pleistocene-Holocene tectono-sedimentary history on the origin of patches of savanna vegetation in the middle Madeira River, southwest of the Amazonian lowlands

Savanna patches are features of the Amazonian landscape that have been long under intense debate, but there are still questions about the main factors that have determined their establishment and evolution within the rainforest matrix. In particular, their geological substrates were poorly documented. The aim of this work is to reconstruct the tectono-sedimentary history of four savanna patches in the middle Madeira River, southwest Amazonia, and discuss its potential control over the development of the savanna communities. The approach consisted in the integration of geomorphological, sedimentological, chronological and floristic data. The results revealed that the savanna substrates are sandy deposits aged between 118.9 and 35.6 ky, overlain by late Pleistocene to Holocene muddier successions <25,700 cal yr BP. Most of the geologically stable and topographically higher terrains of the geomorphological unit T1 is dominated by forests. These intermingle with open habitats formed by renewed sedimentation that were colonized by tree and shrub species of the Amazonian biome. By contrast, the ground of unit T2 subsided to a level that allowed the meandering of the main river. This process resulted in the replacement of the rainforest by open habitats dominated by savannas with lower richness and diversity than in the T1 unit, which were dominated by species with wide distribution in the cerrado biome of Central Brasil. The cerrado species might have expanded into this region during Pleistocene drier climatic episodes, but environmental filters shaped by geological processes determined the floristic contrasts between units T1 and T2. The lack of competition with Amazonian species probably constituted an important factor for the preferential colonization of arboreal and shrubby species from neighboring cerrados on meandering scroll bars, marginal levees and crevasse splays of unit T2.

Persistent Early to Middle Holocene tropical foraging in southwestern Amazonia.

The Amazon witnessed the emergence of complex societies after 2500 years ago that altered tropical landscapes through intensive agriculture and managed aquatic systems. However, very little is known about the context and conditions that preceded these social and environmental transformations. Here, we demonstrate that forest islands in the Llanos de Moxos of southwestern Amazonia contain human burials and represent the earliest settlements in the region between 10,600 and 4000 years ago. These archaeological sites and their contents represent the earliest evidence of communities that experienced conditions conducive to engaging with food production such as environmental stability, resource disturbance, and increased territoriality in the Amazonian tropical lowlands.

Corrigendum to: “Phylogenetics of Datureae (Solanaceae), including description of the new genus Trompettia and re‐circumscription of the tribe” [in Taxon 67: 359–375. 2018

After the publication of our article, some issues were brought to our attention that we would like to clarify. First, our study was based on Lockwood's (1973) taxonomy of Brugmansia, which recognizes five species. More recently, the work of Hay & al. (2012) recognizes two additional species (one described as a subspecies and the other as a hybrid in Lockwood, 1973), bringing the number of Brugmansia species to seven sensu Hay & al. (2012). Thus, our phylogeny contains the five species of Brugmansia sensu Lockwood (1973), but is not complete under the taxonomy of Hay & al. (2012). One of the two taxa not included in our study, B. vulcanicola (A.S.Barclay) R.E.Schult., was however sampled in another recent phylogeny (Ng & Smith, 2016) and appears sister to B. sanguinea (Ruiz & Pav.) D.Don. This species was formerly considered a subspecies of B. sanguinea (Lockwood, 1973) and was placed in the same section (sect. Sphaerocarpium J.M.H.Shaw) along with B. arborea (L.) Sweet by Shaw (1999). We would also like to correct several errors in the text. First, since we considered Datura quercifolia Kunth and Datura ferox L. synonyms (following Symon & Haegi, 1991), the species should have been listed as D. ferox (not as D. quercifolia) throughout due to nomenclatural priority. Also, the authority listed for Brugmansia arborea should appear as “(L.) Sweet”, and that for B. suaveolens should appear as “(Willd.) Sweet”. Also, although we state that Datura are generally referred to as annuals, some species, such as D. wrightii, can grow as perennials in warmer areas (Bronstein & al., 2009). Avery & al. (1959) describe section Dutra of Datura (that includes D. wrightii) as “annual herbs, occasionally forming perennial roots”, although D. wrightii is the only species described as forming “perennial thickened roots”. Luna-Cavazos & al. (2008) refers to all species in section Dutra (D. inoxia, D. lanosa, D. metel, D. wrightii) as “long-lived, tuberous rooted perennials”. Further, the text on p. 366 (left column, second paragraph) should state that the clade comprising D. stramonium and D. ferox is the only lineage of Datura that lacks elaiosomes, following Figure 4. Finally, the first paragraph on p. 368 stated incorrectly that Hay & al.'s (2012) treatment divides Brugmansia as in Lockwood (1973), when in fact, these authors used Shaw's (1999) formal infrageneric classification, which recognizes two sections (sect. Sphaerocarpium as described above and sect. Brugmansia containing the remaining taxa). This division proposed by Shaw (1999) mirrors the two clades of Brugmansia recovered in our phylogeny. In this same paragraph, we state that B. suaveolens is found in the Amazonian lowlands, when it is in fact native from the southern Atlantic forest in Brazil. We are grateful to Alistair Hay, Judie Bronstein, Dave Keil, and Julian Shaw for pointing out these issues.

SAR Backscatter and InSAR Coherence for Monitoring Wetland Extent, Flood Pulse and Vegetation: A Study of the Amazon Lowland

Synthetic aperture radar (SAR) data have been identified as a potential source of information for monitoring surface water, including open water and flooded vegetation, in frequent time intervals, which is very significant for flood mapping applications. The SAR specular reflectance separates open water and land surface, and its canopy penetration capability allows enhanced backscatter from flooded vegetation. Further, under certain conditions, the SAR signal from flooded vegetation may remain coherent between two acquisitions, which can be exploited using the InSAR technique. With these SAR capabilities in mind, this study examines the use of multi-temporal RADARSAT-2 C band SAR intensity and coherence components to monitor wetland extent, inundation and vegetation of a tropical wetland, such as Amazon lowland. For this study, 22 multi-temporal RADARSAT-2 images (21 pairs) were used for InSAR processing and the pairs in the low water stage (November, December) showed high coherence over the wetland areas. The three-year intensity stack was used for assessing wetland boundary, inundation extent, flood pulse, hydroperiod, and wetland vegetation. In addition to the intensity, derived coherence was used for classifying wetland vegetation. Wetland vegetation types were successfully classified with 86% accuracy using the statistical parameters derived from the multi-temporal intensity and coherence data stacks. We have found that in addition to SAR intensity, coherence provided information about wetland vegetation. In the next year, the Canadian RADARSAT Constellation Mission (RCM), will provide more data with frequent revisits, enhancing the application of SAR intensity and coherence for monitoring these types of wetlands at large scales.

Chronology of Terra Firme formation in Amazonian lowlands reveals a dynamic Quaternary landscape

In the Amazonian lowlands, the shift from a large wetland dominated by flooded forests (Várzea) to the modern incised valleys bounded by extensive areas of non-flooded forests (Terra Firme) is considered a key driver of the Amazonian mega-biodiversity. Dating the sedimentary beds covered by Terra Firme forest is crucial to constrain the timing of such landscape change. Here we determined the formation ages of widespread regions of Terra Firme substrates in central Amazonian lowlands combining optically stimulated luminescence (OSL) dating, magnetostratigraphy and palinostratigraphy. Our data suggest a very dynamic fluvial system leading to a recent expansion of Terra Firme forests and retraction of Várzea forests during the late Pleistocene. The shift from an extensively flooded to non-flooded surface resulted from river incision around 45-35 ka, which potentially influenced the distribution of taxa by expanding available habitat, creating new barriers to dispersal, or changing the permeability of previous barriers. Transitions between flooded and non-flooded landscapes may have driven diversification and caused changes to species distributions, contributing to the high species diversity and biogeographic patterns currently found in the region. The late Quaternary evolution of Terra Firme substrates reveals that physical landscape changes play a major role in shaping biotic habitats even in the 104–105 years time-scales.

Seasonal and spatial variability of CO2 in aquatic environments of the central lowland Amazon basin

Different sources and processes contribute to pCO2 and CO2 exchange with the atmosphere in the rivers and floodplains of the Amazon basin. We measured or estimated pCO2, CO2 fluxes with the atmosphere, planktonic community respiration (PCR), and environmental and landscape variables along the Negro and Amazon-Solimoes rivers during different periods of the fluvial hydrological cycle. Values of pCO2 ranged from 307 to 7527 μatm, while CO2 fluxes ranged from − 9.3 to 1128 mmol m−2 d−1 in the Amazon-Solimoes basin. In the Negro basin, pCO2 values ranged from 648 to 6526 μatm, and CO2 fluxes from 35 to 1025 mmol m−2 d−1. In a general linear model including data from Negro and Amazon-Solimoes basins, seasonal and spatial variation in flooded vegetated habitat area, dissolved oxygen, depth and water temperature explained 85% of surface pCO2 variation. Levels of pCO2 varied with inundation extent, with higher pCO2 values occurring in periods with greater water depth and inundation area, and lower dissolved oxygen concentrations and water temperatures. In a separate analysis for the Amazon-Solimoes river and floodplains, ecosystem type (lotic or lentic), hydrological period, water temperature, dissolved oxygen, depth and dissolved phosphorus explained 83% of pCO2 variation. Our results demonstrate the influence of alluvial floodplains and seasonal variations in their limnological characteristics on the pCO2 levels in river channels of the lowland Amazon.

White sand vegetation in an Amazonian lowland under the perspective of a young geological history.

What controls the formation of patchy substrates of white sand vegetation in the Amazonian lowlands is still unclear. This research integrated the geological history and plant inventories of a white sand vegetation patch confined to one large fan-shaped sandy substrate of northern Amazonia, which is related to a megafan environment. We examined floristic patterns to determine whether abundant species are more often generalists than the rarer one, by comparing the megafan environments and older basement rocks. We also investigated the pattern of species accumulation as a function of increasing sampling effort. All plant groups recorded a high proportion of generalist species on the megafan sediments compared to older basement rocks. The vegetation structure is controlled by topographic gradients resulting from the smooth slope of the megafan morphology and microreliefs imposed by various megafan subenvironments. Late Pleistocene-Holocene environmental disturbances caused by megafan sedimentary processes controlled the distribution of white sand vegetation over a large area of the Amazonian lowlands, and may have also been an important factor in species diversification during this period. The integration of geological and biological data may shed new light on the existence of many patches of white sand vegetation from the plains of northern Amazonia.

Neotectonics and tree mortality in a forest ecosystem of the Negro basin: Geomorphic evidence of contemporary seismicity in the intracratonic Brazilian Amazonia

Neotectonic activity was increasingly recorded over the Amazonian lowlands, even during historical times of a few tens of centuries. However, detailed analyses linking structures and styles of deformation are still few considering the large dimension of the region. Such events in a relatively recent time are expected to have caused conspicuous impacts on Amazonian ecosystems and forest disturbance. Changes in topography and hydrology by fracturing, faulting and folding during either subsidence or uplifting are direct effects on landscape that leave marks on the structure of the current forest, such as tree mortality and community succession. Two rectangular elbow-like shaped wetlands from the left margin of the middle Negro River valley in northwestern Amazonia were attributed to neotectonics. Remote sensing imagery was applied to test this hypothesis and also provide a tectonic model that can explain deformation dynamics in this area. The studied wetlands and adjacent river systems display various morphostructural anomalies compatible with a tectonic control, as well as lineaments paralleling main NW and NE-trending regional structures. The geomorphic analysis led to suggest that the wetlands are depressions formed by NW and NE-trending master boundary faults of horizontal displacements intercepted by various subsidiary faults. Transtensional strike slip regime is recorded by both left and right-lateral faults, with the wetlands corresponding to subsiding areas owned to conjugate strike slip faults. Habitat fragmentation and other ecological processes promoted by tectonic deformation would have impacted the overlying forest canopy by changing its structure due to tree mortality. The complex compartmentalization imposed by active tectonics would have exposed the ground to contrasting hydrological conditions, which controlled the rate of tree mortality within the wetlands. We pose that the tectonic disturbance and associated tree mortality documented in the middle Negro River evidence contemporary seismicity within the intracratonic Brazilian Amazonia.

Tropical soil profiles reveal the fate of plant wax biomarkers during soil storage

The waxy coating that protects the leaves and other soft tissues of plants includes n-alkane and n-alkanoic acid compounds that are commonly used as biomarkers to reconstruct past environment. Plant waxes have geological relevance given their persistence in soils and paleosols, as well as in lake and marine sediments, yet diagenesis may alter their molecular and isotopic signatures from synthesis to deposition. This study seeks to understand the fate of plant wax biomarkers in soils after leaf-fall as characterized by a series of tropical soil profiles. We investigate the changes in abundance, molecular distributions, and hydrogen (δD) and carbon isotopic compositions (δ13C) of plant waxes (n-alkanes and n-alkanoic acids) in six litter-to-soil profiles along a 2740 m elevation transect from the eastern flank of the Andes mountains down to the lowland Amazon floodplain in Peru. From litter to soil, we find acid/alkane ratios increase while absolute abundances decrease. In contrast, within each soil, acid/alkane ratios are roughly constant, and we find an equivalent exponential decline in concentration in both compound classes with depth, with molecular distributions indicating some new production. We observe a 4–6‰ 13C-enrichment from litter to deeper soils for both C29n-alkanes and C30n-alkanoic acids, of which the Suess effect accounts for ≤2‰. We infer that microbial degradation and production (or ‘turnover’) processes influence the δ13C of plant waxes that survive in soils; in contrast, no systematic change in δD values is observed. The plant wax signal in soils includes averaging of inputs and diagenetic effects, so this signature is particularly relevant for the interpretation of plant wax archives in paleosols and the plant waxes eroded from soils and exported to downstream sedimentary archives. We show that soils represent the major stock of plant wax under living ecosystems, suggesting that soils may be a quantitatively-important source of plant waxes available for fluvial erosion, with implications for studies of carbon cycling and paleoenvironmental reconstructions from downstream geological archives.