�鶹��Ժ

After the fury, hurricanes can leave a lasting mark on deep ocean

The impact of hurricanes when they travel over land, or when they affect ships or oil-drilling platforms, are quite well understood. But these huge cyclones also stir up the ocean itself, with consequences that are relatively unknown and hard to study.

But a unique, subsurface experimental platform moored to the floor of the Sargasso Sea, about 47 miles southeast of Bermuda, is changing that. With collection points at increasing depths along the mooring line, the traps constantly collect the sinking particles of sediment, microplankton shells, detritus, and pollutants that drift down into the deep ocean, sampling every two weeks to provide a nearly five decades-long record of changes in the environment.

And now, that moored observatory, run by the (OFP) at the Marine Biological Laboratory (MBL), has provided detailed data that for the first time demonstrates how much of an impact hurricanes can have on this deep environment.

A team led by MBL Assistant Research Investigator Rut Pedrosa-Pamies studied the sediments that Hurricanes Fabian (2003) and Igor (2010) transported from the Bermuda carbonate platform—a shallow-water reef refuge for marine life—and deposited to the deep ocean. They found significant effects that lasted for weeks.

They published their data in the

Hurricane Fabian, it turns out, delivered as much sediment to the deep ocean in just two weeks as would normally take a full year to accumulate. These sediments—carbonate-rich material that forms in the thriving ecosystem on reef platforms—have major effects on the ocean environment.

If they get buried in deep sediments, they can sequester carbon for millennia or more. They can also provide a buffering effect to help offset ocean acidification, a consequence of rising atmospheric CO₂ concentrations.

"This is the first time that a study has demonstrated, in near real time, this hurricane-induced transport from a shallow carbonate platform to the deep ocean," Pedrosa-Pamies says. "And it's not just carbonate; [a hurricane] also transports a lot of other materials like phosphorus, lithogenic minerals, and also pollutants, such as lead."

Fabian and Igor in the deep sea

"I've been interested in extreme weather events for a long time now," Pedrosa-Pamies says. "When a hurricane passes through, there is an upwelling of cold, nutrient-rich waters" that nourish bacteria and plankton in the ocean's upper layers, stimulating their productivity.

But how hurricanes can impact the deep-ocean water column that surrounds shallow-water reefs has not been well studied.

The team also found that "not all hurricanes will trigger the same response," she says. "It depends a lot on the ocean depth of the area, the upper-ocean conditions, the hurricane characteristics, etc." Unlike Fabian, the carbonate platform particles resuspended by Hurricane Igor remained suspended for several weeks.

"That's a key finding, because it proves that particles that get suspended from these extreme weather events can last for a long time in the ecosystem and the water column," Pedrosa-Pamies says. "And I'm sure this has implications for the microbiome at different water depths, and also in terms of sedimentation rates and how the particles are aggregating."

Shallow-water reefs are distributed around the world, and over time they build up extensive platforms of carbonate. These platforms play an important role in ocean sedimentary processes and the carbon cycle, Pedrosa-Pamies says. They account for an estimated half of all shallow-water carbonate production, and more than a quarter of all the carbonate that gets buried in the deep ocean.

While the impact of Hurricanes Fabian and Igor are small at the global level, the knowledge gained from this work about the fundamental mechanisms of sediment transport during major storms should apply to the extensive carbonate platforms worldwide. It points out the important role that storms can play in carbon sequestration, and the buffering of ocean acidification.

From Bermuda to Woods Hole

The OFP has been running continuously since 1978 and is "the longest time series of its kind," Pedrosa-Pamies says. "Without having this time series, studying episodic events, like hurricanes in this case, would not be possible. You cannot sit out there when there is a storm passing and collect particles, while you have wind and big waves."

The proximity of the sampling to Bermuda is important, she explains, because "Bermuda is the northernmost subtropical coral reef and carbonate platform in the world, and it's frequently impacted by hurricanes."

The OFP recently published an analysis of the sinking particles at its moored platform off Bermuda over a 44-year period (1978–2022). The results are published in

The process of pulling up the OFP's deep mooring line aboard the ship and retrieving the samples for analysis, which is done every six months, is a challenging, day-long operation.

"I just cannot reinforce enough how important the team effort is in a time series like this. It would not be possible without the help of the entire crew," Pedrosa-Pamies says. "And they are all fantastic, and we've been working with them for a long time."