The changes that the Amazon River promotes in the tropical North Atlantic ocean water make an unfavorable environment for reef development. Every second, 175 million liters of water mixed with sediments are brought to the ocean. The result is low sunlight penetration, variability in nutrient concentration, salinity and pH, extensive moody bottoms and significant changes in temperature and oxygenation levels towards the bottom - conditions not associated with reefs. The plume generated by the Amazon River has 1.3 million km2 and flows mostly to the North, reaching areas as far as the Caribbean Sea.
Against all the odds, 39 scientists from nine Brazilian and one North-American universities mobilized two expeditions to the mouth of the river in other to map the bottom of the ocean in the outer shelf. Two previous studies - from the 1970s and 1980s - reported the findings of single samples of carbonatic structures. None suggested the existence of a reef system underneath the river plume. Surprisingly, the researchers found reefs in a complex as extensive as 1,000 km, in depths from 10 to 120 meters, with rhodolith beds, live calcareous algae, sponges, corals and hydrocorals colonies formed from 13,000 years ago till the present. The system is an habitat for 73 species of fishes and six species of lobsters.
New research led by Florida Institute of Technology shows that the impacts of indigenous people prior to European contact impacted riverside forests, but that such impacts were largely limited to an area within a day's walk from a river.
The findings by the international team of archaeobotanists, paleoecologists and ecologists will be published online in the paper "Anthropogenic influence on Amazonian forests in prehistory: An ecological perspective" on Oct. 28 in the Journal of Biogeography.
The new research, conducted using plant fossils, estimates of mammal density, remote sensing and human population modeling, reinforces that Amazonian forests may be very vulnerable to disturbance by logging , mining and other large enterprises. The study refutes an emerging theory from some archaeologists and anthropologists that Amazonian rain forests are the result of ancient managed landscapes - a notion that undermines the ecological view of these forests as fragile ecosystems.
"Nobody doubts the importance of human actions along the major waterways," said Mark Bush, professor of biological sciences at the Florida Institute of Technology and the lead author of the paper. "But whether humans had a greater impact on the ecosystem than any other large mammal has yet to be established in much of western Amazonia."
Dolores Piperno, curator of archaeobotany and South American archaeology at the American Museum of Natural History and co-author of the study, said the recent emphasis on Amazonia as a manufactured and domesticated landscape overstates the facts.
"At nearly the size of the continental United States, Amazonia is a vast landscape with considerable biotic and abiotic heterogeneity. Extrapolations being made from relatively few archaeological sites mainly located along water courses as to the overall effect of prehistoric human occupation must be tempered in the face of available and yet-to-be-accumulated empirical data."
A 2700 cal yr BP extreme flood event revealed by sediment accumulation in Amazon floodplains
Authors:
Moreira-Turcq et al
Abstract:
Climatic conditions are one of the most important factors affecting hydrological processes in fluvial systems. Higher discharges are responsible for higher erosion, greater transport, and also higher deposition. Consequently, sediment accumulation in Amazonia floodplain river-connected lakes can be directly related to hydrological patterns of the Amazon River mainstream. In this context, we analyzed five sediment cores taken in two floodplain systems situated in the lower Amazon River, to investigate sediment accumulation patterns during the Holocene. Our records show abrupt fluctuations in sedimentation rates in lakes that can reach more than 2 cm/yr during some periods. We find that in all cores, sediment stratigraphy is characterized by packages of sediments of uniform age, which are typically 10–80 cm thick and present a variegated color. The 14C age of the upper package is about 2700 cal yr BP. During this abrupt event, sediment accumulation rates in floodplain lakes can be at least 200 times higher than those of “normal” periods. This sedimentation event is interpreted as being the consequence of one or several successive extreme floods. The 2700 cal yr BP event has been also observed in other sites in South America and other regions in the world, although different impacts can be observed in each system. This event probably corresponds to a conjunction of favorable conditions for extreme Amazon discharge associated with the Middle to Late Holocene increase of austral summer insolation and shifts of the Intertropical Convergence Zone (ITCZ) from northern to southern positions. In this context, a marked negative peak in solar irradiance at 2700 cal yrs BP seems to have provoked cooling on the continents and a southward shift of the ITCZ associated with a probable reduction in the Atlantic Meridian Overturning Circulation.
Biogeochemical indicators of environmental changes from 50 Ka to 10 Ka in a humid region of the Brazilian Amazon
Authors:
Cordeiro et al
Abstract:
We present a geochemical record of a 113.6-cm sediment core (LPT V) from Lagoa da Pata, which is located in the forested upper Rio Negro basin. The record reveals significant changes in the environmental history of Amazonia during the late Quaternary. The results of biogeochemical analyses revealed three hydrological and climatic regimes from 50,000 to 10,000 cal yr BP. The first phase, between 50,000 and 26,300 cal yr BP, was characterized by a relatively wet climate as suggested by relatively high total organic carbon (TOC) and chlorophyll derivate concentrations, indicating high productivity linked to a high lake level. A decrease of the TOC and chlorophyll derivate accumulation rates between 43,100 cal yr BP and 26,300 cal yr BP marks a decrease in the productivity linked to a reduced lake level, indicating a decrease in moisture at the end of this phase. The second phase, between 26,300 and 15,300 cal yr BP, was characterized by a decrease in productivity, reaching a minimum at 21,950 cal yr BP, as indicated by a minima in sedimentary chlorophyll and TOC accumulation rates. Values of δ13C increased by 5‰ in relation to the preceding phase, indicating an influence of the C4 organic matter. High iron concentrations and accumulation rates, related to intense erosion of the lateritic crust in the watershed, were observed. All of the observations indicate a dry phase during this period. A third phase, from approximately 15,300 to 10,000 cal yr BP, was characterized by an increase in lacustrine productivity, as shown by an increased in TOC and chlorophyll derivate concentrations and accumulation rates. These increases likely correspond to a lake level rise due to a wetter climate.
Previously unknown archeological sites in forest islands reveal human presence in the western Amazon as early as 10,000 years ago, according to research published August 28 in the open access journal PLOS ONE by Umberto Lombardo from the University of Bern, Switzerland and colleagues from other institutions.
The study focuses on a region in the Bolivian Amazon thought to be rarely occupied by pre-agricultural communities due to unfavorable environmental conditions. Hundreds of 'forest islands'- small forested mounds of earth- are found throughout the region, their origins attributed to termites, erosion or ancient human activity. In this study, the authors report that three of these islands are shell middens, mounds of seashells left by settlers in the early Holocene period, approximately 10,400 years ago.
Samples of soil from these three mounds revealed a dense accumulation of freshwater snail shells, animal bones and charcoal forming the middens. The mounds appear to have formed in two phases: an older layer composed primarily of snail shells, and an overlying layer composed of organic matter containing pottery, bone tools and human bones. The two are separated by a thin layer rich in pieces of burnt clay and earth, and the uppermost layer of deposits was also seen to contain occasional fragments of earthenware pottery. Radiocarbon analysis of two middens indicates that humans settled in this region during the early Holocene, approximately 10,400 years ago, and shells and other artefacts built up into mounds over an approximately 6,000 year period of human use. The sites may have been abandoned as climate shifted towards wetter conditions later. Lombardo adds, "We have discovered the oldest archaeological sites in western and southern Amazonia. These sites allow us to reconstruct 10,000 years of human-environment interactions in the Bolivian Amazon."
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Neogene–Quaternary sedimentary and paleovegetation history of the eastern Solimões Basin, central Amazon region
Authors:
1. Afonso César Rodrigues Nogueira (a)
2. Rosemery Silveira (b)
3. José Tasso Felix Guimarães (c)
Affiliations:
a. Programa de Pós-Graduação em Geologia e Geoquímica, Universidade Federal do Pará, Rua Augusto Corrêa 1, Guamá, 66075-110 Belém, PA, Brazil
b. Departamento de Geociências, Universidade Federal do Amazonas, Av. General Rodrigo O. J. Ramos, 3000, Coroado, 69077-000 Manaus, AM, Brazil
c. Instituto Tecnológico Vale – ITV, Rua Boa Ventura da Silva, n° 955, 3° andar, Umarizal, 66055-090 Belém, PA, Brazil
Abstract:
Palynological and stratigraphical analyses were carried out on the outcrops of the Solimões River to present new information about freshwater paleoenvironments of a fluvial–deltaic and meandering river system, evaluate the vegetation changes in the upper Solimões and Içá Formations in the eastern Solimões Basin, and the role of the Purus Arc in the evolution of central Amazon during the upper Neogene. The upper Miocene to Pliocene Solimões Formation is related to a fluvial–deltaic system, with fine-grained sediments of the prodelta–lacustrine environment supplied by meandering distributaries and delta front environment. The lake and distributaries were surrounded by extensive deltaic and floodplains colonized by lowland freshwater forests under wet climate conditions, persisting until the Pliocene. The Içá Formation started to deposit unconformably on the Solimões Formation during the Pleistocene, following the development of extensive meandering channels surrounded by floodplains of an essentially fluvial system linked to development of the present eastward direction of the Amazon River until the Atlantic coast. These floodplains were colonized by few palm species and pteridophytes of lowland freshwater forests during the Pleistocene. Additionally, no algae and fungi were observed, which may be related to drier climate conditions and/or different morphological conditions than the upper Solimões Formation.
Influence of the Amazon River development and constriction of the Central American Seaway on Middle/Late Miocene oceanic conditions at the Ceara Rise
Authors:
1. S. Heinrich (a)
2. K.A.F. Zonneveld (a, b)
Affiliations:
a. Department of Geosciences, University of Bremen, P.O. Box 330 440, D-28334 Bremen, Germany
b. MARUM, Center for Marine environmental Sciences, University of Bremen, Leobener Straße, D-28359 Bremen, Germany
Abstract:
Sediment samples from ODP 154 Site 926A (Ceara Rise, western equatorial Atlantic Ocean) spanning the Neogene from 12.8 to 9.2 Ma were investigated on their calcareous dinoflagellate contents to better understand the oceanographic changes in relation to the closure of the Central American Seaway and the development of the Amazon River. Intervals with increased cyst accumulation rates and dissolution sensitive species occur from 12.4 Ma onward. They correspond to periods of enhanced North Atlantic Deep Water production indicating the presence of this water mass at the research site during these intervals. This suggests that pulses of North Atlantic Deep waters sporadically flew into the South Atlantic related to uplift phases of the Panama Sill.
At about 11.2 Ma the first appearance of Leonella granifera indicates river influence at the sample site. This first indication of river influence in the western equatorial Atlantic can be linked to the developing Amazon River. The cyst association changes at about 11.2 Ma from almost monospecific to highly diverse and the permanent presence and increased abundance of L. granifera suggests that river waters were able to reach the study site by now, probably as a result of the southward flowing North Brazil Current. After 10.5 Ma the cyst association indicates a decrease in Amazon influence at Site 926A. This change can be correlated to a reducing inflow of Pacific waters through the Central American Seaway leading to a reverse of the North Brazil Current to its modern northwards flow pattern.
The Amazon rain forest, popularly known as the lungs of the planet, inhales carbon dioxide as it exudes oxygen. Plants use carbon dioxide from the air to grow parts that eventually fall to the ground to decompose or get washed away by the region's plentiful rainfall.
Until recently people believed much of the rain forest's carbon floated down the Amazon River and ended up deep in the ocean. University of Washington research showed a decade ago that rivers exhale huge amounts of carbon dioxide – though left open the question of how that was possible, since bark and stems were thought to be too tough for river bacteria to digest.
A study published this week in Nature Geoscience resolves the conundrum, proving that woody plant matter is almost completely digested by bacteria living in the Amazon River, and that this tough stuff plays a major part in fueling the river's breath.
The finding has implications for global carbon models, and for the ecology of the Amazon and the world's other rivers.
"People thought this was one of the components that just got dumped into the ocean," said first author Nick Ward, a UW doctoral student in oceanography. "We've found that terrestrial carbon is respired and basically turned into carbon dioxide as it travels down the river."
Tough lignin, which helps form the main part of woody tissue, is the second most common component of terrestrial plants. Scientists believed that much of it got buried on the seafloor to stay there for centuries or millennia. The new paper shows river bacteria break it down within two weeks, and that just 5 percent of the Amazon rainforest's carbon ever reaches the ocean.
"Rivers were once thought of as passive pipes," said co-author Jeffrey Richey, a UW professor of oceanography. "This shows they're more like metabolic hotspots."
When previous research showed how much carbon dioxide was outgassing from rivers, scientists knew it didn't add up. They speculated there might be some unknown, short-lived carbon source that freshwater bacteria could turn into carbon dioxide.
"The fact that lignin is proving to be this metabolically active is a big surprise," Richey said. "It's a mechanism for the rivers' role in the global carbon cycle – it's the food for the river breath."
The Amazon alone discharges about one-fifth of the world's freshwater and plays a large role in global processes, but it also serves as a test bed for natural river ecosystems.
Richey and his collaborators have studied the Amazon River for more than three decades. Earlier research took place more than 500 miles upstream. This time the U.S. and Brazilian team sought to understand the connection between the river and ocean, which meant working at the mouth of the world's largest river – a treacherous study site.
"There's a reason that no one's really studied in this area," Ward said. "Pulling it off has been quite a challenge. It's a humongous, sloppy piece of water."
The team used flat-bottomed boats to traverse the three river mouths, each so wide that you cannot see land, in water so rich with sediment that it looks like chocolate milk. Tides raise the ocean by 30 feet, reversing the flow of freshwater at the river mouth, and winds blow at up to 35 mph.
Under these conditions, Ward collected river water samples in all four seasons. He compared the original samples with ones left to sit for up to a week at river temperatures. Back at the UW, he used newly developed techniques to scan the samples for some 100 compounds, covering 95 percent of all plant-based lignin. Previous techniques could identify only 1 percent of the plant-based carbon in the water.
Based on the results, the authors estimate that about 45 percent of the Amazon's lignin breaks down in soils, 55 percent breaks down in the river system, and 5 percent reaches the ocean, where it may break down or sink to the ocean floor.
