Why would the S.S. Virginia, an oil tanker sunk by a U-boat in 1942 with the loss of 27 lives, be moving along the seafloor today? Since the shipwreck’s discovery in 2001, it has moved over 1,600 feet downslope.
Hint: It’s not the work of submarine ghosts, but rather landslide processes common in the Gulf of Mexico. What’s odd is that these landslides or mudflows have been moving the S.S. Virginia even in years without any major hurricanes to produce the large waves that are known to trigger submarine landslides.
The submarine edge of the Mississippi River Delta, known as the Mississippi River Delta Front, is located at the edge of the Gulf of Mexico's continental shelf. This region has long been known to be subject to large submarine landslides. The Mississippi River has historically dumped large amounts of loose sediments into this area. These sediments build up in large mounds, and eventually are prone to collapsing and producing landslides. The Mississippi River Delta Front is also an area with extensive oil and gas infrastructure, making these landslides problematic in many ways.
In 2004, waves from Hurricane Ivan were one of the primary triggers for a submarine landslide that toppled an oil platform. The landslide, which was roughly 100-feet thick, sheared the platform and left 16 oil wells leaking in the Gulf of Mexico. The recovery effort was massive, and a small amount of oil is still thought to be escaping from the seabed near the platform to this day.
“Every time there is a big hurricane that crosses the delta slowly, there are submarine landslides that can break pipelines and topple platforms,” said Sam Bentley, Professor of Geology & Geophysics, Billy & Ann Harrison Chair of Sedimentary Geology and Associate Dean of Research & Administration in the College of Science. “Usually the safety measures of shutting down the oil wells and pipelines reduces the environmental impacts of damage to these structures by landslides, but if the landslides run deep enough, they leave behind a huge mess.”
But the last time there was a large-scale regional study of the seafloor and landslide hazards in the Mississippi River Delta area of the Gulf of Mexico was in the late 1970s.
“The river has changed a lot since then, and our ability to study the sea floor has changed a lot,” Bentley said.
That’s why Bentley, who is also director of the LSU Coastal Studies Institute, is taking a new look at submarine landslides on the Mississippi delta front, in collaboration with the U.S. Department of the Interior (USGS) and the Bureau of Ocean Energy Management (BOEM), with grant funding from BOEM.
Submarine landslides have various triggers, including large waves produced during hurricanes. Waves produce pressure differences on the seabed, between areas that correspond to the crests and troughs of these waves. This process literally pumps the seabed. On the Mississippi River Delta front, where submarine sediments contain naturally occurring methane gas, this pumping causes gas bubbles in the seabed to contract and expand, liquefying the sediment and triggering a landslide of slushy mud and sediments.
Bentley’s team recently discovered that other activity in the absence of hurricanes, for example waves produced by winter storms and cold fronts and potentially even river floods, can trigger submarine landslides, but these are less well understood.
“With the S.S. Virginia, we found that it doesn’t take major hurricanes to produce slow movement due to landslide processes,” Bentley said. “That was a pretty cool find. It means that the risk of landslides hasn’t changed, as far as we can tell, even though the coastline is receding and the Mississippi River is depositing fewer fresh sediments in the Gulf.”
This summer, Bentley is teaming with scientists at the USGS, the Naval Research Laboratory, and marine geologists at the University of Bremen in Germany to map the seabed in areas of the Gulf that are known to be hotspots for landslides. The team is also taking core samples from mapped areas to better understand the composition of the seafloor in particular areas and how different compositions translate into landslides that are more easily triggered by any wave activity.
Bentley and Kevin Xu, an assistant professor in the LSU Department of Oceanography and Coastal Sciences, are leading the coring program starting this week. A total of 16 faculty members, students and postdocs traveled with Bentley and Xu out of Gulfport, Mississippi on Friday, June 2 to help drive cores into the seabed to learn more about landslide processes.
“We basically have a big pipe that we drop into the seabed, as deep as 33 feet, to pull up seafloor materials,” Bentley said. “We already know what the seabed is shaped like, from the recent mapping by the USGS, and we know what the layering subsurface looks like too. With physical samples, we will be able to measure the physical properties that control how easily these areas can produce landslides. We will be able to see if the seabed material in an area is very soft and gushy, or if it’s firm and compact.”
The researchers will bring core samples from the seabed back to the lab to analyze their physical properties. But they’ll also be able to glean some information about the relative gushiness of the seafloor on the spot, based on a unique device called a cone penetrometer.
“The cone penetrometer basically looks like a huge hypodermic needle,” Bentley said. “It’s essentially an 8 meter-long dart that we drop into the seabed as fast as we can. It has sensors inside of it that measure how fast it decelerates. You can imagine that if you drop a dart onto a rock, it decelerates instantaneously, so you get a sharp measurement spike. But if you drop it into mud, it decelerates slowly. So, you can tell something about the nature of the seabed just by the rate of the cone penetrometer’s descent. That data will give us a pretty good idea of how deep landslides in these areas could be triggered.”
By taking seabed core samples in the Mississippi River Delta area of the Gulf, Bentley and others will also find out more about how seafloor sediments are changing over time, as the leveed Mississippi River deposits fewer sediments in the Gulf. As Gulf shorelines retreat due to rising sea levels and reduced flow of fresh sediments from the Mississippi River, landslides may become less common given enough time.
As Bentley casts off his ties to land for his seabed coring expedition this week, he imagines reading the journal of a 19th century ocean explorer.
“When you are out there, you go by the ship’s clock and the ship’s routine, not anything else. There’s ‘water, water everywhere, and never a drop to drink.’ It’s a cool experience. It’s also exhausting, operating 24 hours a day as a crew. You can’t just park a 135-foot ship that costs $11,000 a day to operate, and rest. Getting up at 10 o’clock at night for a shift at midnight and working until lunchtime the next day is an adjustment!”
Follow Bentley's journey to uncover submarine landslide risks in the Mississippi River Delta Front via the LSU Coastal Studies Twitter account, @LSUCSI.