Continental movement capable of throttling marine oxygen

A previously overlooked factor — the position of continents — helps fill Earth’s oceans with life-supporting oxygen. Continental movement could ultimately have the opposite effect, killing most deep ocean creatures. 

“Continental drift seems so slow, like nothing drastic could come from it, but when the ocean is primed, even a seemingly tiny event could trigger the widespread death of marine life,” said Andy Ridgwell, UC Riverside geologist and co-author of a new study on forces affecting oceanic oxygen.

The water at the ocean’s surface becomes colder and denser as it approaches the north or south pole, then sinks. As the water sinks, it transports oxygen pulled from Earth’s atmosphere down to the ocean floor. 

Eventually, a return flow brings nutrients released from sunken organic matter back to the ocean’s surface, where it fuels the growth of plankton. Both the uninterrupted supply of oxygen to lower depths and organic matter produced at the surface support an incredible diversity of fish and other animals in today’s ocean.

New findings led by researchers based at UC Riverside have found this circulation of oxygen and nutrients can end quite suddenly. Using complex supercomputer models, the researchers investigated whether the locations of continental plates affect how the ocean moves oxygen around. To their surprise, it does. 

“Many millions of years ago, not so long after animal life in the ocean got started, the entire global ocean circulation seemed to periodically shut down,” said Ridgwell. “We were not expecting to find that the movement of continents could cause surface waters and oxygen to stop sinking, and possibly dramatically affecting the way life evolved on Earth.”

Until now, models used to study the evolution of marine oxygen over the last 540 million years were relatively simple and did not account for ocean circulation. In these models, ocean anoxia — times when oceanic oxygen disappeared — implied a drop in atmospheric oxygen concentrations. 

“Scientists previously assumed that changing oxygen levels in the ocean mostly reflected similar fluctuations in the atmosphere,” said Alexandre Pohl, first author of the study and former UCR paleoclimate modeler, now at Université Bourgogne Franche-Comté in France.

This study used, for the first time, a model in which the ocean was represented in three dimensions, and in which ocean currents were accounted for.  Results show that collapse in global water circulation leads to a stark separation between oxygen levels in the upper and lower depths. 

That separation meant the entire seafloor, except for shallow places close to the coast, entirely lost oxygen for many tens of millions of years, until about 440 million years ago at the start of the Silurian period.

“Circulation collapse would have been a death sentence for anything that could not swim closer to the surface and the life-giving oxygen still present in the atmosphere,” Ridgwell said. Creatures of the deep include bizarre-looking fish, giant worms and crustaceans, squid, sponges, and more.

The paper does not address if or when Earth might expect a similar event in the future, and it is difficult to identify when a collapse might occur, or what triggers it. However, existing climate models confirm that increasing global warming will weaken ocean circulation, and some models predict an eventual collapse of the branch of circulation that starts in the North Atlantic.  

“We’d need a higher resolution climate model to predict a mass extinction event,” Ridgwell said. “That said, we do already have concerns about water circulation in the North Atlantic today, and there is evidence that the flow of water to depth is declining.”

In theory, Ridgwell said an unusually warm summer or the erosion of a cliff could trigger a cascade of processes that upends life as it appears today. 

“You’d think the surface of the ocean, the bit you might surf or sail on, is where all the action is. But underneath, the ocean is tirelessly working away, providing vital oxygen to animals in the dark depths,” Ridgwell said. 

“The ocean allows life to flourish, but it can take that life away again. Nothing rules that out as continental plates continue to move.”

Much of the matter in the universe remains unknown and undefined, yet theoretical physicists continue to gain clues to the properties of dark matter and black holes. A study by a team of scientists including three from Stony Brook University proposes a novel method to search for new particles not currently contained in the standard model of particle physics. Their method could shed light on the nature of dark matter. 

The three Stony Brook scientists include Rouven Essig, Ph.D., Professor in the C. N. Yang Institute for Theoretical Physics (YITP); Rosalba Perna, Ph.D.,  Professor in the Department of Physics and Astronomy, and  Peizhi Du, Ph.D., a postdoctoral researcher at the YITP.

Stars that pass close to the supermassive black holes located in the center of galaxies can be disrupted by tidal forces, leading to flares that are observed as bright transient events in sky surveys. The rate for these events to occur depends on the black hole spins, which in turn can be affected by ultra-light bosons (hypothetical particles with minute masses) due to superradiance. The research team performed a detailed analysis of these effects, and they discovered that searches for stellar tidal disruptions have the potential to uncover the existence of ultra-light bosons.

According to co-author Rouven Essig, the team demonstrated that due to the dependence of the stellar disruption rates on the black hole’s spin, the ultra-light boson uniquely affects such spins because of the superradiant instability, stellar tidal disruption rate measurements can be used to probe these new particles.

Additionally, the researchers suggest that with the enormous dataset of stellar tidal disruptions that is provided by the Vera Rubin Observatory, these data in combination with the researchers’ work can be used to discover or rule out a variety of ultra-light boson models over wide regions of parameter space.

Their analysis also indicates that measurements of stellar tidal disruption rates may be used to constrain a variety of supermassive black hole spin distributions and determine if close-to maximal spins are preferred.

“The potential implications of our findings are profound. The discovery of new ultra-light bosons in stellar tidal disruption surveys would be revolutionary for fundamental physics,” says Essig.

“These new particles could be the dark matter, and thus the work could open up windows into a complex dark sector that hints toward more fundamental descriptions of nature such as string theory. Our proposal may have other applications too, as measurements of supermassive black hole spins can be used to study the black hole’s formation history,” says Rosalba Perna.

“And ultimately, if these new particles exist they will affect how stars that get close to a supermassive black hole are disrupted by the black hole’s strong gravitational pull,” adds Peizhi.

The Stony Brook team worked with Dr. Daniel Egana-Ugrinovic, a  postdoctoral researcher at the Perimeter Institute, and Dr. Giacomo Fragione, a Research Assistant Professor at Northwestern University.

The Stony Brook research component was supported by the Department of Energy (Grant No. DE-SC0009854), the Simons Foundation (Simons Investigator in Physics Award 623940), and the National Science Foundation (Awards PHY-1915093 and AST-2006839), and the US-Israel Binational Science Foundation (Grant No. 2016153).

Stunning new images created by Keele researchers highlight the turbulent flow of energy inside distant stars. 

They were created using the 3D simulation software “PROMPI”, which scientists have been using to investigate stellar interiors to understand the science of stellar evolution and black holes.

For years scientists have used one-dimensional models to explain and understand how stars are structured and how they evolve. But these models are often limited in how well they can explain the structure of stars, as they take a very general view of the entire star rather than offering any detailed analysis.

But new research led by the University of Keele in the UK Ph.D. student Federico Rizzuti is helping to make these models more accurate, using 3D hydrodynamic simulations to look at the star’s layers and chemical composition in much greater detail than has previously been possible.

Much like planets, stars have multiple layers, and for this study, the researchers used existing data from previous 1D simulations and focused on a small section of the star in minute detail - in this case, a layer known as the neon-burning shell.

They conducted hydrodynamic simulations using the available data and looked at how fluids move about within the layer they are confined to, and how they drag along some material from the neighboring layers – a process known as entrainment, as well as how these chemicals move between the layer’s borders, known as convective boundaries.

These simulations offer an unprecedented degree of realism in recreating the environment within a star, but more importantly, they also highlight the limitations of the current 1D models. These findings, therefore, have a huge role to play in helping us improve the accuracy of these models, which in turn will help us understand how astronomical phenomena structured, such as black holes, supernovae, and neutron stars.

Lead author Federico Rizzuti said: “We live in exciting times, the computing resources to which we have access today make it possible to run simulations that were only a dream a few years ago. Understanding what is happening inside stars helps us to shed light on many aspects of the Universe we live in, from the dynamics of our Sun to the farthest black holes.”

Arteries can become thicker due to high blood pressure. However, the cause of this thickening is unclear. Eindhoven University of Technology, TU/e, researchers in the Netherlands, along with colleagues from Trinity College Dublin in Ireland have developed a new computer model to study arterial thickening in detail. The model shows that both mechanical changes in the artery due to higher blood pressure and cell communication involving so-called vascular smooth muscle cells could be critical for arterial thickening. The same model could be used to guide future approaches to therapeutic and regenerative treatments. 

The growth and changing of arteries in the body depend on many factors, such as blood pressure. Arteries are known to become thicker due to higher blood pressure.

“When the blood pressure increases, the artery stretches more and experiences higher forces. This leads to changes in the mechanics of the artery, and in response, the artery gets thicker,” says Jordy van Asten, a Ph.D. researcher in the department of Biomedical Engineering and the Institute for Complex Molecular Systems (ICMS). “But other factors might be important too, such as how the artery cells talk to each other.”

To gain a better understanding of the mechanisms underlying arterial thickening, van Asten along with fellow TU/e researchers co-first author Tommaso Ristori, Frank Baaijens, Cecilia Sahlgren, and Sandra Loerakker, as well as researchers from Trinity College in Ireland developed a computer model to study how the combination of stretching of the artery and cell signaling affects arterial thickening.

The significant mechanical challenge

One significant challenge for the researchers when developing the model was the need to capture the deformations of an artery when subject to high blood pressure.

“Arteries are pre-stretched, meaning that even if the load due to high blood pressure is removed, the arteries are still not in a fully relaxed state,” notes van Asten. “Including this in the model was difficult, and we achieved this by using a finite element analysis of the stretches in healthy in-vivo or living native arteries.”

Also, the researchers had to estimate the right values of the material properties of the artery tissue that best capture the mechanical behavior. “Arriving at these properties involved a combination of assumptions based on experiments and fits from previous experimental studies,” says van Asten.

Importance of cell chatter

Capturing the right mechanical behavior of the artery is just one part of the computational puzzle. The other part relates to how cell-cell signaling is affected by the mechanical changes and how this could be driving the growth and remodeling of the artery. So, van Asten, Ristori, and their colleagues looked at the cell-cell Notch signaling or communication pathway between vascular smooth muscle cells (VSMCs), which is known to play a key role in how vascular tissue develops and remains stable. And to model this, the researchers used a so-called agent-based model, previously developed by Sandra Loerakker and collaborators in the past.

“Using the model, we have learned how this Notch signaling pathway could be involved in the thickening of arteries due to higher blood pressure,” says van Asten. “We have shown higher blood pressure decreases the chatter among VSMCs which was predicted to change their behavior and, consequently, there is less growth or thickening of the artery.”

Besides high blood pressure, this finding on cell-cell communication (and artery growth) could be applied to other areas of research, such as tissue engineering. van Asten: “A lot of researchers are trying to create living, functional tissue (such as arteries) that could be used to replace diseased tissue in the body.”

Future hopes

In tissue engineering, tissues grow over time, either in the lab or inside the body. Extra control over the growth process would allow researchers to grow more precise replacement tissues, and the key to all of this could be cell chatter. “Our new findings on cell-cell communication in tissue as it grows could prove critical for future tissue growth studies,” says van Asten.

And van Asten has great aspirations for tissue engineering in the future. “I hope that tissue engineering continues to advance so that we can reliably produce replacement tissue, perhaps even organs, for patients suffering from cardiovascular diseases. This research is just a small part of the puzzle, and prospects of how it will be used are exciting and motivating for me personally.”

However, it will take several years before the findings of van Asten and his colleagues can be applied to grow new tissues. Van Asten: “First, we need to perform experiments and check if this is a feasible way to grow arteries, and if the resulting arteries are safe for patients.”