Landform evolution

Overview

LLNL researchers explore geomorphology—the shape of Earth’s surface—to understand how the land around us was shaped and how fast it’s changing. For example, they explore processes such as tectonic uplift and faulting, as well as flooding and sculpting by ice sheets and glaciers.

Scientists use the CAMS accelerator to measure cosmogenic nuclides, including beryllium-7 with a half-life of 53 days, chlorine-36 with a half-life of 300,000 years, aluminum-26 with a half-life of 705,000 years, and beryllium-10 with a half-life of 1.37 million years. The concentration of cosmogenic nuclides in rocks exposed by faulting or ice retreat enables scientists to estimate how long samples have been at Earth’s surface, accumulating rare radioactive isotopes. In addition, measuring the accumulation and decay of isotopes in soil, sediment, and bedrock provides insight regarding how long it has been since a surface was covered by material such as sand dunes or flood deposits. This information enables researchers to differentiate between (relatively) recent activity and changes that date back thousands or millions of years, providing insight regarding when events occurred and how rapidly changes unfolded.

Using these tools, researchers gain insight into how these changes might impact critical national security decisions, such as where to store radioactive waste, build a nuclear power plant, or place a military base.

Dynamic sands and human activity in the Kalahari Desert

Researchers collect samples in the Ugab Valley of Namibia. — LLNL collaborators collect samples in the Ugab Valley of Namibia, which are analyzed at CAMS to better understand the history of the Kalahari Basin. (Image courtesy of the Hebrew University of Jerusalem)

A long-term collaboration between CAMS scientists and researchers from the Hebrew University of Jerusalem (HUJI) has significantly revised the story of the Kalahari Desert sands, providing new insight regarding the migrations and evolution of hominins. Previously it was thought that the sands of the Kalahari had been accumulating for many millions of years, with some hiatuses and periods of erosion. However, research conducted over the last ten years by scientists from CAMS and HUJI has added a new chapter to this story about early human activity.

The team used the cosmogenic nuclide dating capabilities at CAMS to estimate the age of samples obtained from a site in South Africa by measuring the changing ratio of nuclides with different decay rates. Results showed that the well-known sequence of sediment layers at the site, called the Kalahari Group, were deposited over only a few hundred thousand years, approximately 1.2 million years ago. Initially the area was a shallow lake, and then it changed from an aquatic environment to a terrestrial setting, which was rapidly covered by Kalahari sand.

Through follow-on work, which included HUJI students, researchers determined that the sands across the southern Kalahari are extremely dynamic. The wind drives sand from place to place, and over the last 2 million years or so, the sand has been alternately exposed and buried in dunes. This time period coincides with the initial evolution of the genus Homo, so this new understanding gives critical context to rich archaeological sites in the region. Stone tools and other artifacts found in local sites provide important information about the presence, population sizes, and technological sophistication of hominins over the period, while understanding of the shifting landscape contributes information about changes in the availability of surface water and potential habitable sites over time.

Key collaborator: Hebrew University of Jerusalem

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Coastal erosion and changing sea levels

Researcher collects samples of the bedrock in Del Mar’s cliffs. — LLNL scientists collect samples of the bedrock in Del Mar’s cliffs, where homes are situated 70 feet above the beach.

LLNL scientists and their collaborators are studying the erosion of California’s coastal cliffs, hoping to gain insight regarding how to manage these disappearing coastlines. Their initial research focuses on Del Mar, California, a beach town in San Diego County with infrastructure built on its coastal bluffs, including homes and a major railroad. In recent years, cliff failures have resulted in several train derailments, and falling rock led to beach closures. Researchers are hoping to better understand the cliffs’ erosion rate, providing insight regarding how to mitigate risk associated with the infrastructure built on the cliffs. In addition, they are exploring how projected sea level rise and increased storminess exacerbates coastal erosion, due, in part, to wave energy delivered to the base of cliffs.

Researchers collected samples from the cliffs’ bedrock and used CAMS instruments to measure concentrations of beryllium-10 in the samples. The analysis enabled them to determine how long the bedrock has been exposed to cosmic radiation. They compared this data with recent studies of coastal erosion based on aerial photography, to better understand the cliff retreat rate.

LLNL scientists and their collaborators are also studying the history of changing sea-level patterns in Marguerite Bay, on the Antarctic Peninsula. They use CAMS instruments to measure concentrations of beryllium-10 in rock samples collected from raised shingle beaches, where the ice-free shoreline emerged after glacial retreat. These measurements show that sea level in the region has changed at different rates as the land slowly rebounded after being weighed down by ice. Results also reveal intervals when the sea level dropped more rapidly, matching time periods when glaciers melted or grew, helping researchers better understand how sensitive coastlines are to past ecosystem changes.

Key collaborators: Stanford University (coastal cliffs) and Istanbul Technical University (Antarctic Peninsula)