Can tiny fossils disrupt global communications?

TALK LIKE A MARINE GEOLOGIST

Coastal upwelling — deep nutrient-rich water rising to the ocean surface near continents

Consolidation — volume reduction in sediments by the expulsion of fluids in response to a pressure increase

Diatom — a single-celled organism that produces an outer skeleton made of silica

Hydraulic — a system defined by moving fluid in a confined space under pressure

Hydrothermal — heated water

Impermeable — does not allow fluid to flow through

Pore fluid pressure — the pressure acting on fluids in rock or sediment pores (the voids between grains)

Resedimentation — a technique to simulate the natural process of sedimentation and burial under controlled lab conditions

Shear strength — a material property that describes a material’s maximum resistance against shear load before it fails

Shear load — a force that acts parallel to the plane of loading (like when both hands are aligned and moved past one another in opposite directions)

Submarine landslide — an underwater landslide in the ocean

Triaxial compression — compressing a sample in the vertical direction while it is subjected to pressure in all directions

Uniaxial consolidation — compressing a sample in the vertical direction while it is confined horizontally

When you hear the word ‘fossil’, what do you think of? The giant bones of T-rex and stegosaurus? The gracefully coiled spiral of an ammonite? These are the fossils that most of us are familiar with. But fossils come in all shapes and sizes, and many geologists would argue that microfossils are the greatest fossils of all – incredibly small and exquisitely beautiful, not only can microfossils teach us about past climates, but they are extremely useful in our everyday life. You can find them in toothpaste, water filtration systems and cat litter!

What are microfossils?

As their name suggests, microfossils are tiny. “They are often smaller than a grain of sand and can only be inspected with a microscope,” says Dr Julia Reece, a marine geologist at Texas A&M University. In the ocean, single-celled organisms (such as foraminifera, diatoms and radiolarians) produce outer mineral skeletons composed of calcite or silica. When the organisms die, their tiny skeletons sink to the bottom of the ocean where they join the sediment accumulating on the seafloor. Julia believes these microfossils in seafloor sediments may play a role in initiating submarine landslides.

What are submarine landslides?

Submarine landslides occur on the continental margin, the slope that exists between the shallow waters surrounding continents and the deep ocean. If sediment deposited on the seafloor becomes unstable, for example in response to an earthquake, it may collapse as a submarine landslide.

“Submarine landslides pose a significant risk to coastal communities and seafloor infrastructure, as they can generate tsunamis and destroy subsea cables and pipelines,” explains Julia. The seafloor is crossed by a network of cables and pipes, transporting digital data, electricity, oil and gas between countries. If you phone someone across the sea, or send them an internet message, the data will be sent through a cable lying on the seafloor. Despite living in a wireless age, 99% of all data that crosses an ocean travel through a seafloor cable, meaning submarine landslides can impact global connectivity and communication.

How might microfossils initiate landslides?

As microfossils are mineral skeletons that once held single-celled organisms, they are hollow. After the cell dies, the fossil fills with seawater. “Large quantities of microfossils can hold significant volumes of water,” says Julia. “If microfossils are buried by impermeable clay-rich sediments, the clay will trap the water in the layer of microfossils. This would increase the pore fluid pressure and reduce the shear strength of the sediment, potentially creating a weak layer that is more prone to failure.” To test this hypothesis, Julia and her team of researchers in the Sediment Mechanics Lab are conducting experiments and creating numerical models of sediment failure.

Combining lab experiments with numerical models

Julia uses a technique called resedimentation to create samples for her lab experiments. She mixes microfossil-free marine sediment with varying amounts of commercially available diatoms, allowing her to control the exact proportion of microfossils in each sample.

She conducts uniaxial consolidation tests on each sample by vertically compressing them to simulate the natural burial conditions of seafloor sediments. This allows her to determine how the physical, mechanical and hydraulic properties of sediments change as they are exposed to greater stresses. Julia then performs triaxial shear tests on each sample by applying pressure in all directions and loading it (uni)axially until the sample fails. “This tells us the shear strength of the sediment and diatom mixtures, which is a critical parameter in the stability of continental margin sediments,” she explains. Julia also uses high resolution imaging techniques to observe differences in sediment microstructure during these experiments.

Once Julia understands how microfossils influence the stability of sediments in her lab samples, her next challenge will be to scale up these results so they can be applied to continental margins. Using powerful computer software, she will create numerical models of continental margin sediments to analyse their stability. “The results from our lab experiments become the input parameters to these numerical models, as they define the physical, mechanical and hydraulic properties of all sediment layers,” Julia explains. Using her models, she will then test the impact of different factors, such as the thickness of a weak layer, on the stability of a continental margin. “Ultimately, we hope to better understand exactly where failure will occur and under which conditions.”

What data is Julia missing?

“The most accurate and realistic geological understanding is achieved when experimental, numerical and field observations are integrated,” says Julia. Her team is in the process of performing lab experiments to measure the properties of sediments and simulate natural processes on a small scale. Next, Julia will develop numerical models based on the experimental results, enabling her to test large-scale scenarios not possible in the lab. Field studies would allow Julia to compare her experimental and numerical observations with real-life data. A combination of these three approaches is best to understand geological processes.

“To really test our hypothesis, we therefore need to collect sediment cores from a submarine landslide deposit and its undisturbed counterpart that was not caused by an earthquake, but occurred in a coastal upwelling zone, where the abundance of microorganisms is high,” Julia says. During a submarine landslide, the failure plane that initiates the landslide will be destroyed. This means that as well as sampling the material deposited during the landslide, it is equally important to sample the undisturbed sediment on the continental margin, as this will contain the weak layers that may form future failure planes. “Recovering the sediment that comprises these weak layers is crucial for understanding their composition and role in the initiation of slope failure.”

How could Julia sample seafloor sediments?

There are several challenges to sampling seafloor sediments, not least that they are covered by hundreds to thousands of metres of water. This means there is no easy way to access them. Scientific ocean drilling is the only method that can collect them, as research ships can drill sediment cores from the ocean floor and return them to the surface for scientists to study. The second major challenge is that it is a difficult and long process to get scientific ocean drilling projects funded by the International Ocean Discovery Program (IODP). This is hardly surprising when it costs almost $300,000 per day to operate their research ship!

Although Julia has been involved with other IODP missions, the proposal to drill into submarine landslide deposits and continental margin sediments, proposed by her and other international scientists, has not yet been approved. For now, Julia must rely on her lab experiments using natural sediments and numerical models to draw conclusions about the role of microfossils in initiating submarine landslides, helping scientists understand when and where these potentially devastating underwater hazards may occur.

DR JULIA REECE
Sediment Mechanics Lab, Department of Geology and Geophysics, Texas A&M University, USA

Fields of research: Sediment Mechanics, Marine Geology

Research project: Investigating the role of microfossils in initiating submarine landslides

Funders: US National Science Foundation (NSF, Award # 1945011), US Science Support Office (USSSP). The contents are solely the responsibility of the authors and do not necessarily represent the official views of NSF

ABOUT THE INTERNATIONAL OCEAN DISCOVERY PROGRAM

The International Ocean Discovery Program (IODP) is a marine research collaboration to ‘explore the Earth under the sea’. The IODP uses research ships such as the JOIDES Resolution to drill into the seafloor to collect samples of sediments, the rocks below them, the fluids flowing through them and the organisms living in them. These expeditions generate data allowing scientists to uncover Earth’s geological, climatic and biological history.