Humans changed the ecosystems of Central Africa more than 2,600 years ago

Fields, streets and cities, but also forests planted in rank and file, and dead straight rivers: humans shape nature to better suit their purposes, and not only since the onset of industrialization. Such influences are well documented in the Amazonian rainforest. On the other hand, the influence of humans was debated in Central Africa where major interventions seem to have occurred there 2,600 years ago: Potsdam geoscientist Yannick Garcin and his team have published a report on their findings in the Proceedings of the National Academy of Sciences. The research team examined lake sediments in southern Cameroon to solve the riddle of the "rainforest crisis." They found that the drastic transformation of the rainforest ecosystem at this time wasn't a result of climatic change, it was humankind.

Farming activities in North-West Region of Cameroon [Credit: Better World Cameroon]

More than 20 years ago, the analysis of lake sediments from Lake Barombi in southern Cameroon showed that older sediment layers mainly contained tree pollen reflecting a dense forested environment. In contrast, the newer sediments contained a significant proportion of savannah pollen: the dense primitive forest quickly transformed into savannahs around 2,600 years ago, followed by an equally abrupt recovery of the forest approximately 600 years later. For a long time, the most probable cause of this sudden change, dubbed the "rainforest crisis," was thought to be climate change brought about by a decrease in precipitation amount and increase in precipitation seasonality. Despite some controversy, the origin of the rainforest crisis was thought to be settled.

High volumes of precipitation in the region (over 3,000 mm annually) have ensured that the lake has not dried out over
the millenia. This heavy rainfall has created large volumes of sediment, which are then washed into the lake.
These circumstances make it possible to perform sediment analyses with the utmost precision
[Credit: B. Brademann/GFZ]

Yet Garcin, a postdoctoral researcher at the University of Potsdam, and his international team of scientists from UP, CEREGE, IRD, ENS Lyon, GFZ, MARUM, AMU, AWI, and from Cameroon suspected that other causes could have led to the ecosystem's transformation. By reconstructing both vegetation and climate change independently -- through stable isotope analysis of plant waxes, molecular fossils preserved in the sediment -- the team confirmed that there was a large change in vegetation during the rainforest crisis, but indicated that this was not accompanied by a change in precipitation.

"The rainforest crisis is proven, but it cannot be explained by a climate change," says Garcin. "In fact, in over 460 archaeological finds in the region, we have found indications that humans triggered these changes in the ecosystem." Archaeological remains older than 3,000 years are rare in Central Africa. Around 2,600 years ago, coincident with the rainforest crisis, the number of sites increased significantly, suggesting a rapid human population growth -- probably related to the expansion of the Bantu-speaking peoples in Central Africa. This period also saw the emergence of pearl millet cultivation, oil palm use, and iron metallurgy in the region.

This floating platform can be completely taken apart and transported overseas. The platform enabled the collection of
sediment samples in the approximately 100-meter-deep Lake Barombi, which were then analyzed in the laboratory
[Credit: Y. Garcin/University of Potsdam]

"The combination of regional archaeological data and our results from the sediments of Lake Barombi shows convincingly that humans strongly impacted the tropical forests of Central Africa thousands of years ago, and left detectable anthropogenic footprints in geological archives," says Dirk Sachse at the Helmholtz Center Potsdam -- Research Center for Geosciences (GFZ). Sachse was one of the major contributors to the development of the method for analyzing plant wax molecular fossils (termed biomarkers).

"We are therefore convinced that it was not climate change that caused the rainforest crisis 2,600 years ago, but it was the growing populations that settled in the region and needed to clear the forest for exploiting arable land," says Garcin. "We are currently observing a similar process underway in many parts of Africa, South America, and Asia." But the work of Garcin and his team also shows that nature has powerful regenerative abilities. When anthropogenic pressure decreased 2,000 years ago forest ecosystems reconstituted, but not necessarily as before: as in the Amazonian rainforest, field studies show that the presence of certain species is very often related to past human activity.

Source: GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre [February 26, 2018]

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Stagnation in the South Pacific

A team led by geochemist Dr. Katharina Pahnke from Oldenburg has discovered important evidence that the rise in atmospheric carbon dioxide levels at the end of the last ice age was triggered by changes in the Antarctic Ocean. The researchers from the University of Oldenburg's Institute for Chemistry and Biology of the Marine Environment (ICBM), the Max Planck Institute for Marine Microbiology in Bremen and the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) were able to demonstrate that the deep South Pacific was strongly stratified during the last ice age, and could thus have facilitated long-term, deep-sea storage of the greenhouse gas carbon dioxide (CO2). The study, which has now been published in the academic journal Science, also indicates that in the course of the warming following the end of the last ice age the mixing of the deep water masses increased, releasing stored CO2 and enhancing global warming.

Snapshot from aboard RV Polarstern [Credit: Mario Hoppmann]

The Southern Ocean plays an important role in climate events because CO2 can be absorbed from the atmosphere into the ocean. When increased amounts of dust are deposited in the seawater, microscopic algae multiply because the iron contained in the dust acts as a fertilizer. When these single celled algae die, they sink to the ocean floor, taking the sequestered carbon dioxide with them. To ensure long-term removal of the CO2 from the atmosphere, however, it must be stored in stable conditions in deep water over long periods of time.

In order to find out how water masses in the deep South Pacific have developed over the last 30,000 years, the team recovered sediment cores from water depths of between 3,000 and more than 4,000 metres during an expedition of the research vessel "Polarstern" to the South Pacific. The geochemists Dr. Chandranath Basak and Dr. Henning Fröllje of the ICBM - the two main authors of the study - extracted tiny teeth and other skeletal debris of fossil fish from the sediment to analyse their content of isotopes of the rare earth metal neodymium.

"Neodymium is particularly useful for identifying water masses of different origin," said Pahnke, the head of the Max Planck Research Group for Marine Isotope Geochemistry based at the ICBM and the Max Planck Institute for Marine Microbiology in Bremen, explaining that each layer of water has its own characteristic neodymium signature. The isotope ratios of this element vary depending on which ocean basin the water comes from. For instance, the coldest and therefore deepest water mass in the Southern Pacific forms on the continental shelf of Antarctica and carries a distinct neodymium signature. Overlying this mass is a layer that combines water from the North Atlantic, the South Pacific and the North Pacific and hence is marked by a different signature.

View from RV Polarstern while collecting sediment samples used in the study by Basak et al.
[Credit: Dr. Katharina Pahnke]

Using fish debris in deep-sea sediments, the researchers were able to trace the variations in neodymium concentrations at different depths over the course of time. The result: at the peak of the last ice age approximately 20,000 years ago, the neodymium signature of samples taken from depths below 4,000 metres was significantly lower than at lower depths. "The only explanation for such a pronounced difference is that there was no mixing of the water masses at that time," said Fröllje, who currently works at the University of Bremen. He and his colleagues concluded from this that the deep waters were strongly stratified during the glacial period.

As the climate in the southern hemisphere grew warmer towards the end of the last ice age around 18,000 years ago, the stratification of the water masses was broken up and neodymium values at different depths converged. "There was probably more mixing because the density of the water decreased as a result of the warming," Pahnke explained. This then led to the release of the carbon dioxide stored in deep waters.

For some time now climate researchers have been speculating on why fluctuations in atmospheric CO2 levels followed the same pattern as temperature in the southern hemisphere whereas the temperature in the north at times ran counter to these fluctuations. One theory is that certain processes in the Southern Ocean played an important role.

"With our analyses we have for the first time provided concrete evidence supporting the theory that there is a connection between the CO2 fluctuations and stratification in the Southern Ocean," said co-author of the study Dr. Frank Lamy of the AWI in Bremerhaven. The current study supports the hypothesis that the warming of the southern hemisphere broke up stable stratification in the Antarctic Ocean, resulting in the release of the carbon dioxide that was stored in these waters.

Source: Alfred Wegener Institute [February 22, 2018]

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Moths in mud can uncover prehistoric secrets

Moth scales, preserved in the mud of a coniferous forest lake, have been used to identify outbreaks of these insects over the past 10,000 years. This groundbreaking new technique, reported in the open-access journal Frontiers in Ecology and Evolution, can tell us more about the frequency and intensity of past and future insect epidemics, their impact on the forest environment and how they are linked to climate change.

Dead Spruce Budworm moths (Choristoneura fumiferana) floating on the surface of a boreal lake
during an insect outbreak period in 2017. The lake is north of Lake Saint-Jean in Quebec, Canada
[Credit: Emy Tremblay, Karoline Pitre, Miguel Montoro Girona,
Lionel Navarro, Guillaume Grosbois]

"This is an exciting discovery, which will greatly increase our knowledge of prehistoric forest ecosystems," says Dr. Miguel Montoro Girona, joint-lead author of this paper, based at the Swedish University of Agricultural Sciences, Umeå, Sweden. "It is comparable to the fossil pollen and charcoal markers in sediments, which revolutionized prehistoric research to provide information on plants, climate and forest fires going back thousands of years. Our new method can be applied to many ecosystems where moths and butterflies have a marked influence on the landscape."

Moths are one of the most widespread and recognizable insects in the world. In the boreal forests of North America, which are mainly comprised of coniferous, evergreen trees, the larva of one moth species -- the Spruce Budworm -- can periodically cause severe and widespread damage. This leads to millions of dollars of lost revenue to the forest-based economy.

It was during a routine observation of one of these outbreaks that the researchers had a 'eureka' moment.

Thousands of scales cover the moth body and wings like shingles on a roof, arranged in rows and anchored in a socket.
Image taken using a x100 optical microscope from the Université de Québec à Chicoutimi. Scale size = 50 μ;
original colour has been modified [Credit: Emy Tremblay, Hubert Morin, Miguel Montoro Girona]

"Together with an insect specialist, we recently identified a 'strange' structure in a colleague's lake-water sample, which turned out to be a scale from a moth wing," says Dr. Hubert Morin, co-author, based at the Québec University, Canada. "Afterwards, during a Spruce Budworm outbreak, I noticed the lakes were covered in dead moths. Knowing they are made from a material that is likely to be well-preserved in the sediments of lakes, I realized they had the potential to provide information about these swarms going back thousands of years."

This hunch turned out to be true. Taking a 5-meter-long core of sediment from a forest lake near Québec, which represented 10,000 years' worth of sediment deposit, the researchers at the Québec University spent five years perfecting their method of extracting, examining and counting moth scales under the microscope from each sediment layer.

"Our analysis revealed peaks of moth scales that corresponded to known periods of insect outbreak. This means we can work out when these epidemics occurred before records began," says Dr. Lionel Navarro joint-lead author, based at the Québec University, who spent a lot of time in the laboratory processing these samples. "This new method will be of enormous help to future research in areas as wide as ecology and evolution, biodiversity conservation, climate change and forestry."

The diversity in shapes and colors makes moth scales a potential proxy to identify Lepidoptera species. Image taken
using an optical microscope from the Université de Québec à Chicoutimi. Scale size = 50 μ
[Credit: Emy Tremblay, Miguel Montoro Girona]

The authors explain, "Using the Spruce Budworm as an example, moths and butterflies can be one of the most important natural disturbances, changing the forest structure and species composition, with strong economic implications to forestry and agriculture. They are an 'umbrella species' -- the second most diverse animal group in the world -- so improving our understanding of these creatures will in turn provide a lot of information about other species and the state of the ecosystem they are in."

They continue, "Currently, climate change is one of the biggest challenges of our planet. Understanding our past ecosystems is essential to predicting the future. Based on climate-change predictions, insect outbreaks will be more frequent, with higher severity. Our new technique can help us understand how moths and butterflies react to different climatic scenarios and environmental changes, so we can predict their response to climate change, as well as their vulnerability and extinction risk."

Source: Frontiers [February 22, 2018]

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Extinct lakes of the American desert west

The vestiges of lakes long extinct dot the landscape of the American desert west. These fossilized landforms provide clues of how dynamic climate has been over the past few million years.

Erosional Pleistocene shorelines in Surprise Valley, California, USA [Credit: Anne Egger]

Identification of ancient lake shoreline features began with early explorers of the continent. The first detailed studies were conducted by pioneering American geologists such as G.K. Gilbert and I.C. Russell in the late 1800s, who studied Lake Bonneville, now the remnant Great Salt Lake in Utah, and Lake Lahonton, located in northwestern Nevada.

Through this long history of studying fossil shorelines and lake sediments, we know that these lakes existed during two periods with distinct environmental conditions during the geologically recent past. The first was during ice age maxima, such as the last ice age, 14 to 30 thousand years ago, when global temperatures were 4 to 6 degrees colder and continental ice sheets expanded into the continental United States.

The second time period was about three million years ago during the middle of the Pliocene epoch--a global climate characterized by warmer temperatures and atmospheric CO2 levels roughly equivalent to today's values, which has led many scientists to view the Pliocene as a potential analogue for future climate change.

These observations lead to an important question, says the study's lead author, Daniel Ibarra, "Why are there lake systems under both colder and warmer climates, but not today?" Of particular interest, he says, is the presence of lakes under warmer conditions, which, under a "wet gets wetter, dry gets drier" paradigm, goes against projections of future warming.

Shorelines of Lake Bonneville along the Oquirrh Mountains, Utah, USA. (H.H. Nichols [artist] and G.K. Gilbert,
in Gilbert, 1884) [Credit: H.H. Nichols [artist] and G.K. Gilbert, in Gilbert, 1884. Public domain]

To answer this question, Ibarra and colleagues looked at the competing influences of temperature and precipitation, and how they combine to allow for the existence of lakes under these dual climate states.

The authors compiled evidence for, and created models of, lakes during both colder and warmer than modern periods of the Pliocene-Pleistocene (the last 5 million years). During colder glacial periods, they found that increased precipitation and decreased evaporation combined to form large lakes that occupied the inward draining basins in the western United States, particularly in northern Nevada and Utah.

Increased precipitation also drove the formation of lakes, particularly in southern Nevada and southern California during the warmer middle Pliocene, outpacing higher temperatures and evaporation rates during that time. This increase in precipitation during the middle Pliocene and dominantly southwestern distribution of lake deposits is similar to the pattern of precipitation during modern El Niño years, corroborating previous hypotheses for mean "El Niño-like" conditions during the mid-Pliocene.

The team's interdisciplinary approach explains the conditions driving lake systems in mid-latitude regions today and over the geologic past. Further, notes Ibarra, "This work illustrates the importance of understanding how the El Niño Southern Oscillation drives precipitation patterns in arid regions, which is important for future water resources planning for the western United States."

The study appears in the Geological Society of America Bulletin.

Source: Geological Society of America [February 22, 2018]

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