In an effort to understand how global sea levels and shorelines are changing, Julia Smith Wellner, an award-winning associate professor at the University of Houston’s Earth and Atmospheric Sciences Department, explained that many clues lie in modern depositional environments spanning from the U.S. Gulf Coast all the way to Antarctica.
Wellner shared her thoughts at the annual Michel T. Halbouty Lecture at the International Meeting for Applied Geoscience and Energy held recently in Houston in a presentation entitled, “Modern Sediment Transport as Analog for the Past and Future: From Houston to Antarctica.”
Although Wellner specializes in cold environments – focusing particularly on sediment facies, stratigraphic architecture, glacial history and sea-level change near Antarctica’s Thwaites Glacier – she has spent a significant amount of time with students collecting sediment samples on the Upper Texas Coast. The different projects she presented on were led by her graduate students at UH.
“We were interested in the San Luis Pass area and the tidal delta that forms through here. We wanted to study ripple height, grain size, water depth and flow velocity relationships because we are using these as an analog for paleo systems” of the past, she explained.
“A lot of times when you are working in an incised valley, you want to know – How big was the channel if you find the (sand) bar? Or, if you are working in a delta, what were the lows compared to water depth?” she said. “A lot of those relationships are defined by using modern sediments.”
Samples were analyzed for grain size using laser particle size analysis, microscopes with digital cameras to assess particle shape – so that thousands of grains could be studied in a statistical process – and a scanning electron microscope for grain textures.
Wellner and her team were able to show the grain-size distribution in the San Luis Pass area and how that was connected to ripple length and height and water depths.
“What we see for this microtidal environment is that ripple wavelength is the best proxy for looking at water depth,” she said.
Sediment and Serendipity
However, halfway through their sampling efforts, Hurricane Harvey barreled through the upper Gulf Coast in 2017 and cut their work short. Speaking, coincidentally, on the sixth anniversary of Harvey for the Houston area, Wellner reminded how Harvey dropped as much as 50 inches of rain in some parts of the region – approaching the area’s annual rainfall total in just five days.
Yet rather than see the storm as a crisis, Wellner and her team saw it at the time as “serendipitous” – an opportunity to study grain sizes pre- and post-Harvey for further clues into the area’s changing coastline.
“Regarding water depth corrected to the mean high tide, there is a dramatic change at each of the sampling spots. It (water depth) is getting deeper during Harvey,” she said. “And grain size is notably finer after Harvey.”
“This has allowed us to think not just about how to use this flood tidal delta as analog for microtidal environments, but it also allowed us to think specifically about – How did a storm impact this coast?” she explained.
The deepening came from wave erosion. The finer grains are a result of a plume of sediments from all across the Gulf Coast brought out by the storm.
A subsequent inland study in Houston revealed large mounds of sand built up along bike trails adjacent to Buffalo Bayou. Wellner and her students worked with retired geologist Jerry Kendall and his daughter, artist Kate Kendall, who made sand peels of the Harvey deposits.
“We wanted to use sand to teach the community about storm history,” she said. “The big (sand) peels are about a meter high. Bedforms begin to stick up when water recedes. The peels helped us learn a lesson about geology.”
It also prompted further research. Wellner and her students used LiDAR data both pre- and post-Harvey to study Houston’s 14 watersheds that drain directly into the Houston Ship Channel and ultimately into the Gulf Coast. LiDAR allowed them to calculate the change in sediment volumes before and after the storm, looking specifically at land loss and accumulation.
While Harvey brought a lot of rain to the area, a lot of sediment was displaced as well. In fact, the sediments discharge was not much less than the annual sediment discharge of the Mississippi River, Wellner said.
“Deposition is the overall result, not erosion, especially as we get down to the end of each of those streams (watersheds),” she said. “The erosion is happening across the entire area and the deposition is being focused into these distinct valleys.”
Harvey is actually a microcosm of what is happening all over the planet, as the Earth experiences an increase in alluvium sediment deposits from farming, building and other disruptive activities.
“That is reflected in this pulse of sediment that we see in Harvey,” Wellner said. Yet in the case of Harvey, she described the rapid sedimentation as a “vacuum cleaner model” as evidenced by the long-term incision and stranded overbank sediment and episodic delivery of sediment to the shoreline.
Changing her focus to Antarctica, Wellner explained that studying modern sedimentation can shed light on rising sea levels in the future.
Thwaites Glacier in western Antarctica is the area of the continent losing ice most rapidly.
Working on the Nathaniel B. Palmer icebreaker, Wellner and others spend 60-70 days at a time taking a variety of cores from 0.5 to 6 meters in length to study sediment deposits.
Wellner and other scientists work in multiple repositories studying the cores as well as at her laboratory at UH, which contains a gamma ray spectrometer for the radiometric dating of sediments. In this particular, case she is measuring two different isotopes: lead-210, which looks back about 100 years into the sedimentary record, and cesium-137, which, when present, allows a lookback to 1963.
Specifically, Wellner and others are studying marine-based ice in the Amundsen Sea, where oceanic studies have shown that warm water is affecting the ice. Unlike a glacier’s ice on land, marine-based ice is grounded – sitting on bedrock--that is below sea level.
However, “what does matter is the water temperature underneath the ice,” Wellner explained. “Most of the ocean that we work in there is around minus-1.5 degrees (Celsius) or so. It is salty so it doesn’t freeze at zero. But warm, deep water can come up the shelf and then melt this ice from the bottom, and that’s what destabilizes the ice sheet.”
Having performed CT scans on the collected cores and having looked at standard proxies, including density, magnetic susceptibility, pebble count and grain size, Wellner showed an image of a core and a lithographic log with various interpretations and diagrams that show water depth.
“As we go up through the core it’s getting gradually less proximal. The area that we had interpreted as having had grounded ice is retreating over time,” she said.
The results from this study were published in the Proceedings of the National Academy of Sciences this fall.
“We are able to say that … we have a period from the ‘40s to the ‘70s that we see this retreat,” she said, explaining that the warm water is now reaching more of the grounded ice.
“This major Antarctic glacier began to retreat at the same time as a lot of other warming events began to happen,” Wellner said. “This wasn’t just some … change in precipitation or destabilization because it lost a little bit of a sheer zone. If it is synchronous, it means it is globally connected and that’s what proving this did.”
Studying sedimentation along the Gulf Coast and in Antarctica allows geoscientists to measure how ice sheets, and thus sea levels, are changing and how shorelines retreat.
“In a lot of reports about what climate change is to come, you might hear about sea levels increasing. The truth of this matter is we don’t know very much about how much it is going to increase,” she said.
“While we know sea level is rising and will continue to rise, we don’t know which of the models is true. That’s why we need to understand how Antarctica is changing,” Wellner said.
Thwaites Glacier was given the nickname the “Doomsday Glacier” by a reporter after accompanying scientists to Antarctica. Unhappy about how the name took off, Wellner said it is an unfortunate and misleading misnomer.
“Where we are working is the area of Antarctica most subject to change,” she added, “but we also like to say, ‘Please stop calling it the Doomsday Glacier because we don’t know the answers yet.’”
Wellner reiterated the importance of sedimentation studies. “The present is the key to the past,” she said, and then referenced Shakespeare, adding that “the past is prologue” to the future.