In 1942, in the midst of World War II, the oil tanker Virginia was anchored off the mouth of the Mississippi River in the Gulf of Mexico, waiting to unload its cargo in nearby New Orleans. It never made it. Three torpedoes from a German submarine, the U-507, caused the ship to become engulfed in flames, sinking it and killing 27 of its crew.
For nearly 60 years, the Virginia rested forgotten on the sea floor. Its vacant decks became decorated with coral and its portholes housed small fish. Over time, a multibillion-dollar offshore energy industry boomed around it.
Then, in 2001, an oil and gas exploration crew discovered the wreck while using sonar to scan the sea floor. Now, the Virginia is once again serving as a workhorse: Instead of hauling oil, however, the tanker is helping archaeologists and geologists understand the Gulf’s underwater mudslides. The sometimes massive slides can threaten historic wrecks and can cause catastrophic damage to oil pipelines and wells.
In particular, the wreck’s movements over the past 16 years are helping researchers understand how even relatively modest changes in the weather, not just major events like hurricanes, are reshaping the seafloor.
A slide hotspot
The Virginia lies on the edge of the Gulf’s continental shelf, in about 200 meters of water just off the Louisiana coast. It’s a hot spot for underwater mudslides, according to geologist Sam Bentley of Louisiana State University (LSU) in Baton Rouge, because the Mississippi River deposits large amounts of loose sediment in the area. The sediments build up over time, forming giant lobes that can break apart and slide down the shelf.
Such slides worry regulators and the oil and gas industry for at least two reasons. One is they cause sediment to slip from beneath seafloor pipes, leaving them suspended in the water and more vulnerable to breaks. The other is that the slides—which can be 30 meters thick—can shear through pipelines and other infrastructure. In 2004, for example, Hurricane Ivan triggered a mudslide that violently damaged 16 oil wells and created leaks.
The wreck’s discovery has provided geologists with some unexpected insight into how fast these lobes can move. In 2006, archaeologists with the federal government—which manages areas more than 5 kilometers offshore—set out to survey the Virginia. What they found mystified them: the 10,000-ton wreck had moved 370 meters seaward from its location in a 2004 mapping effort.
The likely cause, the researchers believed, was hurricanes that violently stormed through the Gulf in 2004 and 2005. Geologists knew that severe storms create large waves that, in turn, produce pressure differences on the sea floor. This buffeting causes naturally occurring methane bubbles in the seabed sediments to contact and expand, liquefying the mud around them. The process unleashes mudslides—which can carry even a 150-meter-long wreck along for the ride.
In 2017, when another team of researchers from the U.S. Geological Survey (USGS) returned to survey the wreck, they found it had moved an additional 60 meters seaward. Again, the scientists puzzled: This time, there had been no major storms to propel the movement.
A recent study led by LSU’s Bentley might explain what is happening. In 2012, the federal Bureau of Ocean Energy Management (BOEM), which oversees offshore oil operations in federal waters, funded Bentley and researchers at the USGS to study mudslides in a 100-square-kilometer section near the Mississippi delta, which is dotted with numerous oil and gas wells and crisscrossed by pipelines.
The team mapped potential mudslide hotspots, extracted 9-meter-long cores from the sea floor, and measured movements of ocean floor sediments. They also examined weather and other records to see what might be driving sediment movements in the absence of hurricanes or other major storms.
One conclusion: Masses of cold air that regularly move over the gulf in the winter can stir up waves and pressure differences that can help push the sediment seawards at rates of up to a meter per year. The cold fronts could help explain the Virginia’s recent movements, Bentley says. “It would have to be something less energetic than a hurricane that was causing [the movement], so the next thing down from that are winter cold fronts.”
The findings have implication for both efforts to prevent oil spills and to protect historic wrecks. Under federal law, offshore oil and gas companies are required to protect cultural artifacts from industrial operations. In the Virginia’s case, for example, BOEM barred companies from drilling, running pipelines, or dropping anchors within 300 meters of the wreck. But a moving wreck complicates industry efforts to stay out of that buffer zone.
The rusting hulk could also pose a substantial threat to pipelines or other infrastructure that was originally far downslope. Luckily, researchers say there are no pipelines in the immediate vicinity of Virginia.
Researchers note, however, that there haven’t been systematic surveys of wrecks or slides in the Gulf of Mexico since the 1970s, so there could be unknown threats sitting in the deep. To fill that knowledge gap, Bentley is hoping BOEM will support expanding his mudslide study to a larger area covering some 2000 square kilometers. And archaeologist Douglas Jones, who works out of BOEM’s office in New Orleans, Louisiana, is proposing a study that would identify other wrecks in slide-prone areas.
In the meantime, researchers plan to keep an eye on the Virginia, which they say has opened a door to better understanding the complexity and instability of the Gulf’s sea floor. Where the wreck will go next, they say, will depend on the natural forces that can be difficult to predict.