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Rio Grande Records past Climate Cycles

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Dr. Brad Sion examines the youngest terrace along the Rio Salado at a site where charcoal was collected for carbon-14 dating.
2016 Courtesy of Brad Sion

Socorro, NM
— October 28, 2020

A collaborative study between researchers in New Mexico Tech’s (NMT) Earth and Environmental Science Department and the New Mexico Bureau of Geology and Mineral Resources provided new age estimates for Rio Grande terraces near Socorro, New Mexico that indicate relationships between river dynamics and climate cycles.

The study, published in Geosphere, used a combination of field and photogrammetry-based mapping, soil characterization, and dating of terrace surfaces to evaluate a history of terrace formation and abandonment along the middle Rio Grande from San Acacia south to San Antonio. The techniques in this study were used to test previously proposed models for river dynamics.

“Primarily it involved looking at the geomorphic relationships, developing a conceptual model, and then testing that model with our field observations and quantitative techniques,” said lead author and former NMT doctoral researcher Dr. Brad Sion, now with the Desert Research Institute. “Can we recognize some sort of systematic spatial and temporal pattern preserved in the terrace record?”

Dating methods include cosmogenic nuclide dating using chlorine-36, which indicates the duration that sediments have been exposed at the land surface, in combination with radiogenic argon dating of lava flows in the valley and carbon-14 dating of organic material in river sediments. The dating allowed the researchers to identify the ages of five river terrace sequences and associated fans.

“There is an old saying in volcanology — “flows follow lows” — meaning that when a volcano erupts the lava flows are emplaced into topographic lows,” said Bureau of Geology Field Geologist II and co-author Dr. Matthew Zimmerer. “So the ages of the flows not only tell us when a volcano erupted, but also provide important information about the topography during the eruption.”

The lava flows dated in this study are Mesa del Contadero, which preserved the highest remaining Rio Grande terrace abandoned about 818,000 years ago, and a flow from the Jornada del Muerto volcano, which covered a lower river terrace approximately 78,000 years ago.

The new age results support a model proposed by Bureau of Geology Emeritus Senior Environmental Geologist Dr. John Hawley, which suggested periods of river aggradation correspond to the warmer and drier climates of interglacial periods, while periods of incision correspond to the colder and wetter climates of glacial periods.

“During glacial periods there was a high volume of water discharge and less sediment input, presumably because the landscape was better vegetated, so it introduced less sediment and the river incised,” explained NMT Emeritus Professor of Hydrology and co-author Dr. Fred Phillips. “And during the warmer drier periods you had more sediment because there’s more convective summer precipitation and you tend to get more erosion with intense summer precipitation events.” With less vegetation on the landscape, you have a greater sediment supply and less water to move it, so sediment enters the river valley raising the river bed.

In an unexpected result, the study also found that the rate of river incision changed over time. From about 610,000 to 135,000 years ago, the river incised on average 60 meters per million years. After 135,000 years ago, the river incision rate increased significantly to 300 meters per million years, a pattern seen in other western river systems. The reason for this change remains unclear.

“I think this is just the tip of the iceberg really,” said Sion. “I think there are additional questions that we can address by understanding the timing of the various processes recorded by the packages of terrace sediments. We might be able to understand for example, how fast is the system incising when it does incise? When does aggradation begin and what is the pace of aggradation?” These are just a few of the important steps to deciphering the relationships between river dynamics and external forcings, such as climate and tectonics.