Thursday, May 24, 2018

Ancient Mound Builders Carefully Timed Their Occupation of Coastal Louisiana Site

A study of ancient mound builders who lived hundreds of years ago on the Mississippi River Delta near present-day New Orleans offers new insights into how Native peoples selected the landforms that supported their villages and earthen mounds - and why these sites were later abandoned.

The study, reported in the Journal of Island and Coastal Archaeology, also offers a timeline of the natural and human events that shaped one particular site, said University of Illinois anthropology professor Jayur Mehta, who conducted the work with Vanderbilt University postdoctoral researcher Elizabeth Chamberlain while both were at Tulane University in New Orleans.



Hundreds of ancient mound sites, depicted here with yellow triangles, still survive in coastal Louisiana. A new study teases out the natural and human history of one of these mound-top villages, a site known as Grand Caillou, shown in red.
 
Graphic by Julie McMahon after Mehta and Chamberlain.


The site, now known as Grand Caillou, is one of hundreds of mound sites in coastal Louisiana, Mehta said. (Watch a video about the research and history of the site.)

"Louisiana is incredibly important in the history of ancient mound-building cultures," he said. "In what is now the United States, earthen monument and mound construction began on the Louisiana coast."

Ancient peoples began building mounds in North America as early as 4,500 B.C., Mehta said. They often situated their mounds near resource-rich waterways, which could support larger human settlements. As many as 500 people lived at Grand Caillou in its heyday. Some mounds also served ceremonial functions.

That so many mound sites have survived in coastal Louisiana is a testament to their careful construction, Mehta said. Neglect, however, and coastal subsidence - the result of engineered changes to the flow of the Mississippi River - are wearing away at the mounds.

"Louisiana loses about two ancient mounds and/or Native American villages a year," Mehta said.

The researchers used a variety of methods - sediment coring, radiocarbon dating, carbon-isotope analysis, the dating of ceramics found onsite and a method called optically stimulated luminescence - to figure out how and when the land underneath the Grand Caillou mound was formed by natural forces and when the mound builders arrived and established their settlement.

"We wanted to understand at a deeper level how Indigenous peoples of the coast were choosing where to build their villages," Mehta said.

The mound at Grand Caillou.

Photo courtesy Jayur Mehta

Grand Caillou is situated on a natural levee of the Lafourche sub-delta, one of several major lobes of the Mississippi River Delta near New Orleans. Fed by sediments deposited by the river, Lafourche expanded in size over a period of several hundred years, a process that ended at about 800 A.D., the researchers found. The mound builders set up their village around 1200 A.D., long after the site was stable and covered over with vegetation, the team found.

Core samples and excavations revealed that the mound was built in distinct layers, with clay on the bottom, looser sediments piled in the middle and a clay cap on top. This finding confirms earlier archaeological reports that ancient mounds were engineered in layers to withstand the elements.

"The way they were constructed contributes to their durability," Mehta said.

The Grand Caillou mound was built on top of a river deposit that was naturally higher than surrounding land.

"It's only a few feet higher than nearby areas," Mehta said. "But in a landscape where there's no topography, one or two feet can make a world of difference."

Ceramics found at the site date to between 1000 and 1400 A.D. Radiocarbon dating of charcoal found evidence that the site was abandoned by about 1400. By looking at ratios of carbon isotopes - carbon atoms with differing masses - the team saw changes over time that were likely the result of saltwater incursion into the area. These changes coincided with the ultimate abandonment of the village site.

The new study is a much-needed addition to research on threatened cultural sites in coastal regions, said University of Tennessee anthropology professor David G. Anderson, an expert on U.S. Paleoindian archaeology who was not involved in the research.

"We are facing the loss of much of the record of human settlement and use of coastal zones - and must take steps to address the challenge," Anderson said. "Mehta and Chamberlain's study exemplifies the kind of work that will be needed."
 



Contacts and sources:
Jayur Mehta
University of Illinois


Citation: "Mound construction and site selection in the Lafourche subdelta of the Mississippi River delta, Louisiana, USA" is available online

Earth's Temperature to Increase by 4 Degrees by 2084



A collaborative research team from China has published a new analysis that shows the Earth's climate would increase by 4 °C, compared to pre-industrial levels, before the end of 21st century.

To understand the severity of this, consider the Paris Agreement )\\ of the United Nations. It's a global effort to prevent an increase of 2°C. Nearly every country on the planet--the United States is the only country to withdraw--has agreed to work to prevent the catastrophic effects of two degrees of warming.

The study is selected as the cover article of Issue No. 8 of Advances in Atmospheric Sciences in 2018. The levels of future global warming relative to the pre-industrial period have been extensively addressed, in which 2 C and 1.5 C warming have attracted the most attention. A special report 'Turn Down the Heat: Why a 4°C Warmer World must be Avoided' by the World Bank in 2012 described a 4 C warmer world with significant changes in mean and extreme climates on the basis of earlier-generation climate models and emission scenarios.
Image by Advances in Atmospheric Sciences

The researchers published their analysis projecting a doubling of that increase in Advances in Atmospheric Sciences on May 18, 2018.

"A great many record-breaking heat events, heavy floods, and extreme droughts would occur if global warming crosses the 4 °C level, with respect to the preindustrial period," said Dabang Jiang, a senior researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences. "The temperature increase would cause severe threats to ecosystems, human systems, and associated societies and economies."

In the analysis, Jiang and his team used the parameters of scenario in which there was no mitigation of rising greenhouse gas emissions. They compared 39 coordinated climate model experiments from the fifth phase of the Coupled Model Intercomparison Project (), which develops and reviews climate models to ensure the most accurate climate simulations possible.

They found that most of the models projected an increase of 4°C as early as 2064 and as late as 2095 in the 21st century, with 2084 appearing as the median year.

This increase translates to more annual and seasonal warming over land than over the ocean, with significant warming in the Arctic. The variability of temperature throughout one year would be lower in the tropics and higher in polar regions, while precipitation would most likely increase in the Arctic and in the Pacific. These are the same effects that would occur under 1.5°C or 2°C increases, but more severe.

"Such comparisons between the three levels of global warming imply that global and regional climate will undergo greater changes if higher levels of global warming are crossed in the 21st century," wrote Jiang.

The researchers continue to investigate the changes associated with 4°C of global warming in extreme climates.

"Our ultimate goal is to provide a comprehensive picture of the mean and extreme climate changes associated with higher levels of global warming based on state-of-the art climate models, which is of high interest to the decision-makers and the public," said Jiang.





Contacts and sources:
Zheng Lin
Institute of Atmospheric Physics
Chinese Academy of Sciences

Researchers from the Chinese Academy of Sciences Center for Excellence in Tibetan Plateau Earth Sciences, the Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters at the Nanjing University of Information Science & Technology, the Joint Laboratory for Climate and Environmental Change at Chengdu University of Information Technology, and the University of Chinese Academy of Sciences contributed to this study.

This work was supported by the National Basic Research Program of China and the National Natural Science Foundation of China.