Evidence of Environmental Changes Recorded in River Delta Sediments

In a new study published in the Proceedings of the National Academy of Sciences, researchers say that the historical data for environmental changes can be gathered from the world’s large river deltas, and that this information is critical to better understanding of the results of human activity.

Photo: Sam Beebe / EcotrustRiver Delta

River Delta

University of Texas professor Thomas Bianchi and colleague Mead Allison have studied sediments from river deltas around the world, including the Mississippi in the U.S. and the Yangtze and Yellow rivers in China, and claim that the information contained in the sediment reveals data on numerous past events. Changes in nitrogen fertilizer application in the drainage basin and records of past hurricane and flooding events are among those recorded in the sediment.

“These deltaic sediments can serve as a history book of sorts on land-use change in these large drainage basins which is useful for upland and coastal management decisions as related to climate change issues. Although the information stored in these sediments can be altered during its transport from the upper drainage basin to the coast, we still find very stable tracers, both organic and inorganic, that can be used to document changes induced by natural and human forces.” – Bianchi

87% of the Earth’s land is connected to the ocean through river systems, and the sediments are present as a normal product of the freshwater cycle. Because 61 percent of the Earth’s human population lives along a coastal boundary (and is expected to climb to 75 percent by 2025), the activities along these river basins can have a huge environmental impact.

The authors state that 25 of the world’s largest rivers drain about half of the world’s surface, transporting 50% of the fresh water and 40% of particulate materials into the ocean, and the information gleaned from studying the delta sediments can inform us of the history of the area, including land usage. Some human activity that can be traced through studying the sediment in large river delta-front estuaries (LDEs) are the early civilizations in Mesopotamia and in China.

Hypoxic areas (areas with little or no oxygen) in the U.S. can be linked in some cases with deltaic regions that release large amounts of water and nutrients, and finding the causal relationship between human activities and the environment can help with models of possible future effects for these regions.

“Low oxygen in aquatic systems is clearly not good for the organisms in those systems, but not all aquatic systems respond in the same way. It affects marine life in some areas severely, while other areas seem unchanged. We need to find out why. Some LDE areas such as the Mississippi/Atchafalaya River system have had significant increases in the nutrient loading from fertilizers. We know we need to reduce the amount of these nutrients from draining into our rivers, but by how much? In this particular case, the linkages between excessive nutrients, hypoxia and their affects on aquatic life are not well understood.” – Bianchi

According to the study’s authors, China is also facing some major water issues due to human activity, with over-grazing, water shortages, and desertification among the big ones. The country’s manipulation of its natural water sources with dams and diversions, in its attempts to manage the water needs of a growing population, coupled with the fact China is the world’s largest user of fertilizers, mean significant changes for the river deltas. Both the Yangtze and Yellow rivers rank in the top five in the world for sediment discharge.

The processes that control the transport and transformation of carbon in the delta estuaries and in the sediment deposits are also a key to understanding carbon sequestration and exchange with the world ocean, which is an important objective in climate change research.

“Many scientists are expecting global temperatures to rise over the next 50 years due to climate changes, and how will these changes affect precipitation and soil erosion issues? We really don’t know now because in many cases, land-use change by growing populations can be very short-term and unpredictable, making modeling very difficult. These deltaic sediments might be able to give us some clues about what is ahead for us.” – Bianchi

The study, funded by NASA, the Department of Energy, the Office of Naval Research and the National Science Foundation, is published in the current issue of the “Proceedings of the National Academy of Sciences.”

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Author: Derek Markham

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