Tag Archives: #sediment

What Causes Pools below Waterfalls to Periodically Fill with Sediment? (Earth Science)

Deep pools below waterfalls are popular recreational swimming spots, but sometimes they can be partially or completely filled with sediment. New research showed how and why pools at the base of waterfalls, known as plunge pools, go through natural cycles of sediment fill and evacuation. Beyond impacting your favorite swimming hole, plunge pools also serve important ecologic and geologic functions. Deep pools are refuges for fish and other aquatic animals in summer months when water temperatures in shallow rivers can reach lethal levels. Waterfalls also can liquefy sediment within the pool, potentially triggering debris flows that can damage property and threaten lives. Over geologic time, pools are importance because the energetic waterfall jet can erode the rock walls of the pool—slowly moving the waterfall upstream, while simultaneously creating a deep canyon in its wake.

Reporting in the journal Geology today, Joel Scheingross of the University of Nevada Reno, and Michael Lamb of the California Institute of Technology, provided a new theoretical framework to predict when plunge pools fill with sediment, and when they subsequently evacuate that sediment exposing the rock walls of the pool to erosion. They showed that waterfall plunge pools tend to fill with sediment during modest river floods when sediment is transported in the reach upstream of the waterfall, but the waterfall jet is too weak to move all the sediment it receives from upstream. In contrast, during large floods, a strong waterfall jet can more efficiently move sediment up and out of the pool, outpacing the delivery of sediment from upstream, and exposing bedrock to erosion.

Scheingross and Lamb showed that waterfall plunge pools are most likely to fill with sediment following landscape disturbances—such as wildfire or landslides—that cause large influxes of sediment to rivers, or during prolonged droughts when river floods are rare. This information is useful to scientists and land managers interested in maintaining habitat and mitigating natural hazards. They also showed that bedrock erosion in plunge pools likely occurs only during large, infrequent floods, which tend to happen every 10 years or even less frequently. Therefore, the slow march of waterfalls upstream over geologic time likely occurs by fits and starts at a cadence set by extreme flood events.

Images: Example of sediment filling and evacuation of sediment in a small waterfall plunge pool on Arroyo Seco, San Gabriel Mountains, California. Top: Plunge pool, free of sediment, in June 2006 with swimmer leaping. Bottom: The same plunge pool filled with sediment in March 2010 following a wildfire in 2009 that overwhelmed the pool with sediment. Top photo credit: Kelin X. Whipple, bottom photo credit: Michael P. Lamb.


Reference: Joel S. Scheingross, Michael P. Lamb; Mass balance controls on sediment scour and bedrock erosion in waterfall plunge pools. Geology 2021; doi: https://doi.org/10.1130/G48881.1


Provided by Geological Society of America

New Study Finds More than Half of Hudson River Tidal Marshes were Created Accidentally by Humans; Resilient Against Sea Level Rise (Geology)

UMass Amherst geologist and team studied marshes from Wall Street to Albany.

In a new study of tidal marsh resilience to sea level rise, geologist and first author Brian Yellen at the University of Massachusetts Amherst and colleagues observed that Hudson River Estuary marshes are growing upward at a rate two to three times faster than sea level rise, “suggesting that they should be resilient to accelerated sea level rise in the future,” he says.

Hudson River graphic. Courtesy of the Dams and Sediment in the Hudson project.

Writing in Earth Surface Processes and Landforms, Yellen and colleagues documented that more than half of Hudson River tidal marshes formed since 1850. That year, the channel was straightened and a riverside railroad, berms, jetties and human-made islands of dredged soil were built. This all trapped sediment and created backwaters that often – but not always – turned into marshes, an “unintended result of early industrial development,” they state.

“In one case, historical aerial photos document this transition occurring in less than 18 years, offering a timeframe for marsh development,” they point out. Yellen’s co-authors are colleagues Jonathan Woodruff, Caroline Ladlow and undergraduate Waverly Lau at UMass Amherst, plus Sarah Fernald at New York’s department of environmental conservation and David K. Ralston of Woods Hole Oceanographic Institution.

Yellen notes that for this “very collaborative” study, the researchers took advantage of “an experiment that has already happened over decades or centuries. Many of these accidental tidal marshes worked; they protect the shoreline and provide one of the richest ecosystems in terms of direct ecological and human benefits.”

Further, marshes are “really useful,” he adds – as a first line of defense against coastal flooding, essential habitat for juvenile commercial fish species, they store huge amounts of carbon that mitigates climate change, they provide habitat for migratory birds, filter nutrients coming off the land “and they’re beautiful.”

Yellen and colleagues write that these tidal wetlands “currently trap roughly 6% of the Hudson River’s sediment load. Results indicate that when sediment is readily available, freshwater tidal wetlands can develop relatively rapidly in sheltered settings. The study sites serve as useful examples to help guide future tidal marsh creation and restoration efforts.”

Their research involved seven sites spanning more than 100 miles of the Hudson Estuary, “from Wall Street up to Albany,” Yellen says. The bays where tidal marshes developed started out six to seven feet deep and have steadily grown vertically. “One concern for marshes globally is that they will be drowned by rising sea level, but this case study shows how marshes can be created and it gave us some timing benchmarks for what is considered a tricky ecosystem restoration,” he adds.

The researchers used two main methods to investigate the river’s history and resilience in the face of sea level rise – sediment cores that shed light on how fast sediment accumulated and historical maps, charts and aerial photos to determine how the sites have changed over time.

Yellen summarizes, “As long as there is space for sediment to accumulate, new marshes can develop. There is a community of land trusts up and down the river who are planning now for future sea level rise and considering where new marshes can form. This research will help them and state agencies guide land acquisition and land conservation strategies adjacent to the river to 2100 and beyond.”

The research, part of the Dams and Sediment in the Hudson (DaSH) project, was supported by a grant to Ralston from NOAA’s National Estuarine Research Reserve Collaborative, plus the U.S. Geological Survey and the Department of Interior Northeast Climate Adaptation Science Center at UMass Amherst.

Also, Lau received a Polgar Fellowship from the Hudson River Foundation for her senior thesis project. She took the lead on one of the sites and made a short film about tidal marshes around the world and the Hudson River marsh near her home in Queens.

References: Yellen, B., Woodruff, J., Ladlow, C., Ralston, D. K., Fernald, S., and Lau, W. (2020) Rapid Tidal Marsh Development in Anthropogenic Backwaters, Earth Surf. Process. Landforms, doi: Accepted Author Manuscript. https://onlinelibrary.wiley.com/doi/abs/10.1002/esp.5045 https://doi.org/10.1002/esp.5045.

Provided by University of Massachusetts Amherst

Researchers Discover Life in Deep Ocean Sediments At Or Above Water’s Boiling Point (Oceanography)

3 URI researchers part of international research team.

An international research team that included three scientists from the University of Rhode Island’s Graduate School of Oceanography has discovered single-celled microorganisms in a location where they didn’t expect to find them.

Microbial cells in sediment; microbial cells are green, sediment particles are yellow. (Image courtesy of JAMSTEC)

“Water boils on the (Earth’s) surface at 100 degrees Celsius, and we found organisms living in sediments at 120 degrees Celsius,” said URI Professor of Oceanography Arthur Spivack, who led the geochemistry efforts of the 2016 expedition organized by the Japan Agency for Marine-Earth Science and Technology and Germany’s MARUM-Center for Marine and Environmental Sciences at the University of Bremen. The study was carried out as part of the work of Expedition 370 of the International Ocean Discovery Program.

The research results from a two-month-long expedition in 2016 will be published today in the journal Science.

The news follows an announcement in October that microbial diversity below the seafloor is as rich as on Earth’s surface. Researchers on that project from the Japan marine-earth science group, Bremen University, the University of Hyogo, University of Kochi and University of Rhode Island, discovered 40,000 different types of microorganisms from core samples from 40 sites around the globe.

The research published in Science today focused on the Nankai Trough off the coast of Japan, where the deep-sea scientific vessel, Chinkyu, drilled a hole 1,180 meters deep to reach sediment at 120 degrees Celsius. The leader of the study is Professor Kai-Uwe Hinrichs of MARUM.

Spivack, who was joined by recent Ph.D. graduates, Kira Homola and Justine Sauvage, on the URI team, said one way to identify life is to look for evidence of metabolism.

“We found chemical evidence of the organisms’ use of organic material in the sediment that allows them to survive,” Spivack said. The URI team also developed a model for the temperature regime of the site.

“This research tells us that deep sediment is habitable in places that we did think possible,” he added.

While this is exciting news on its own, Spivack said the research could point to the possibility of life in harsh environments on other planets.

According to the study, sediments that lie deep below the ocean floor are harsh habitats. Temperature and pressure steadily increase with depth, while the energy supply becomes increasingly scarce. It has only been known for about 30 years that, in spite of these conditions, microorganisms do inhabit the seabed at depths of several kilometers. The deep biosphere is still not well understood, and this brings up fundamental questions: Where are the limits of life, and what factors determine them? To study how high temperatures affect life in the low-energy deep biosphere over the long-term, extensive deep-sea drilling is necessary.

“Only a few scientific drilling sites have yet reached depths where temperatures in the sediments are greater than 30 degrees Celsius,” explains study leader Hinrichs of MARUM. “The goal of the T-Limit Expedition, therefore, was to drill a thousand-meter deep hole into sediments with a temperature of up to 120 degrees Celsius – and we succeeded.”

Like the search for life in outer space, determining the limits of life on the Earth is fraught with great technological challenges, the research study says.

“Surprisingly, the microbial population density collapsed at a temperature of only about 45 degrees,” says co-chief scientist Fumio Inagaki of JAMSTEC. “It is fascinating – in the high-temperature ocean floor, there are broad depth intervals that are almost lifeless. But then we were able to detect cells and microbial activity again in deeper, even hotter zones – up to a temperature of 120 degrees.”

Spivack said the project was like going back to his roots, as he and David Smith, professor of oceanography and associate dean of URI’s oceanography school, where they were involved in a drilling expedition at the same site about 20 years ago, an expedition that helped initiate the study of the deeply buried marine biosphere.

As for the current project, Spivack said studies will continue on the samples the team collected. “The technology to examine samples collected from the moon took several years to be developed, and the same will be true for these samples from deep in the ocean sediments. We are developing the technology now to continue our research.”

References: Verena B. Heuer, Fumio Inagaki, Yuki Morono, Yusuke Kubo, Arthur J. Spivack, Bernhard Viehweger, Tina Treude, Felix Beulig, Florence Schubotz, Satoshi Tonai, Stephen A. Bowden, Margaret Cramm, Susann Henkel, Takehiro Hirose, Kira Homola, Tatsuhiko Hoshino, Akira Ijiri, Hiroyuki Imachi, Nana Kamiya, Masanori Kaneko, Lorenzo Lagostina, Hayley Manners, Harry-Luke McClelland, Kyle Metcalfe, Natsumi Okutsu, Donald Pan, Maija J. Raudsepp, Justine Sauvage, Man-Yin Tsang, David T. Wang, Emily Whitaker, Yuzuru Yamamoto, Kiho Yang, Lena Maeda, Rishi R. Adhikari, Clemens Glombitza, Yohei Hamada, Jens Kallmeyer, Jenny Wendt, Lars Wörmer, Yasuhiro Yamada, Masataka Kinoshita, Kai-Uwe Hinrichs, “Temperature limits to deep subseafloor life in the Nankai Trough subduction zone”, Science 04 Dec 2020: Vol. 370, Issue 6521, pp. 1230-1234. https://science.sciencemag.org/content/370/6521/1230
DOI: 10.1126/science.abd7934

Provided by University of Rhode Island

Microbial Diversity Below Seafloor Is As Rich As On Earth’s Surface (Earth Science)

For the first time, researchers have mapped the biological diversity of marine sediment, one of Earth’s largest global biomes. Although marine sediment covers 70% of the Earth’s surface, little was known about its global patterns of microbial diversity.

Microbial cells in sediment: microbial cells are green, sediment particles are yellow. Image courtesy of JAMSTEC.

A team of researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), the University of Hyogo, the University of Kochi, the University of Bremen, and the University of Rhode Island delineated the global diversity of microbes in marine sediment. For the study, published in the Proceedings of the National Academy of Sciences, Tatsuhiko Hoshino, senior researcher at JAMSTEC, and his colleagues including URI Graduate School of Oceanography Professor Steven D’Hondt analyzed 299 samples of marine sediment collected as core samples from 40 sites around the globe. Their sample depths ranged from the seafloor to 678 meters below it. To accurately determine the diversity of microbial communities, the authors extracted and sequenced DNA from each frozen sample under the same clean laboratory condition.

The 16S rRNA gene sequences (approximately 50 million sequences) obtained through comprehensive next-generation sequencing were analyzed to determine microbial community composition in each sample. From these 50 million sequences, the research team discovered nearly 40,000 different types of microorganisms in marine sediment, with diversity generally decreasing with depth. The team found that microbial community composition differs significantly between organic-rich sediment of continental margins and nutrient-poor sediment of the open ocean, and that the presence or absence of oxygen and the concentration of organic matter are major factors in determining community composition.

A deep-frozen sediment core which was frozen on board immediately after sampling for microbiological analysis. DNA was extracted from the frozen core in a clean room. Photo courtesy of JAMSTEC.

By comparing their results to previous studies of topsoil and seawater, the researchers discovered that each of these three global biomes–marine sediment, topsoil, and seawater–has different microbial communities but similar total diversity. “It was an unexpected discovery that microbial diversity in the dark, energy-limited world beneath the seafloor is as diverse as in Earth’s surface biomes,” said Hoshino.

Furthermore, by combining the estimates of bacterial and archaeal diversity for these three biomes, the researchers found that bacteria are far more diverse than archaea–microbes distinct from bacteria and known for living in extreme environments–on Earth.

“In this respect as well, microbial diversity in the dark realm of marine sediment resembles microbial diversity in the surface world,” said D’Hondt. “It’s exciting to glimpse the biological richness of this dark world.”

References: Tatsuhiko Hoshino, Hideyuki Doi, Go-Ichiro Uramoto, Lars Wörmer, Rishi R. Adhikari, Nan Xiao, Yuki Morono, Steven D’Hondt, Kai-Uwe Hinrichs, Fumio Inagaki, “Global diversity of microbial communities in marine sediment”, Proceedings of the National Academy of Sciences Oct 2020, 201919139; DOI: 10.1073/pnas.1919139117

Provided by University Of Rhode Island

It Was Matter Ejected From The Chicxulub Crater That Led To Impact Winter (Paleontology)

Shelby Lyons and colleagues in their study found that K-pg extinction event caused mainly by sedimentary carbon ejected from the impact crater, not from wildfires.

A large asteroid (~12 km in diameter) hit Earth 66 million years ago, likely causing the end-Cretaceous mass extinction. Credit: Southwest Research Institute/Don Davis

An asteroid impact in the Yucatán Peninsula set off a sequence of events that led to the Cretaceous–Paleogene (K–Pg) mass extinction of 76% species, including the nonavian dinosaurs. The impact hit a carbonate platform and released sulfate aerosols and dust into Earth’s upper atmosphere, which cooled and darkened the planet—a scenario known as an impact winter. Organic burn markers are observed in K–Pg boundary records globally, but their source is debated.

In this new effort, the researchers suggest that while some of the material in K–Pg boundary records is likely from such burnt material, most of it came from material ejected from the crater at the impact site.

The work involved analyzing sediment samples from within the Chicxulub crater and from other ocean sites near the crater. In their analysis, the researchers focused on polycyclic aromatic hydrocarbons (PAHs), which can provide evidence of a source of black carbon. In so doing, they found that characteristics of polycyclic aromatic hydrocarbons (PAHs) in the Chicxulub crater sediments and at two deep ocean sites indicate a fossil carbon source that experienced rapid heating, consistent with organic matter ejected during the formation of the crater. Furthermore, PAH size distributions proximal and distal to the crater indicate the ejected carbon was dispersed globally by atmospheric processes.

Molecular and charcoal evidence indicated wildfires were also present but more delayed and protracted and likely played a less acute role in biotic extinctions than previously suggested. Based on stratigraphy near the crater, between 7.5 × 10^14 and 2.5 × 10^15 g of black carbon was released from the target and ejected into the atmosphere, where it circulated the globe within a few hours. This carbon, together with sulfate aerosols and dust, initiated an impact winter and global darkening that curtailed photosynthesis and is widely considered to have caused the K–Pg mass extinction.

References: Shelby L. Lyons et al. Organic matter from the Chicxulub crater exacerbated the K–Pg impact winter, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2004596117 link: https://www.pnas.org/content/early/2020/09/22/2004596117