Oxygen, Sardines, and Jupiter’s Moons: What Oceans Can Tell Us About Our Past, Present, and Future

EarthzineOriginal, Writing Fellowship 2017

Want to know about life, the universe, and everything? Look to the ocean.

How did we get here? Where are we going? Are we alone? The ocean may provide us with clues to help answer questions humans have been asking since the dawn of time.

Oceans Past

Ariel Anbar, a professor at Arizona State University’s School of Earth and Space Exploration, has been working to answer that first question – how did we get here. His lab studies the Great Oxygenation Event, a time period about 1 billion years ago when the oxygen content of the Earth’s atmosphere suddenly increased.

Today, oxygen comprises about 20 percent of the Earth’s atmosphere; it is a necessity for complex life. Oxygen is highly reactive and used in the production, storage, and release of chemical energy. Through photosynthesis, cells use energy from sunlight to combine oxygen, carbon, and hydrogen into sugars called carbohydrates. Later, the carbohydrates are broken down into simpler molecules to release energy in a process called respiration. This same process occurs when animals eat and digest food.

The abundance of life in the extreme environments found near deep sea hydrothermal vents may unlock clues to finding life elsewhere in the universe. Image Credit: OET/Nautilus Live

Oxygen’s extreme reactiveness also means that it doesn’t last very long in a planet’s atmosphere. It rapidly reacts with other elements in the Earth’s crust, like iron, producing compounds such as rust. Since the development of more complex forms of life depends on having readily available oxygen, tracing the amount of free oxygen in the atmosphere can help us visualize what life was like at different stages in the Earth’s history.

But this can be a challenging prospect. “What you’re looking for are chemical fingerprints in the rock that are indicators of what the environment was in the past,” Anbar explains.

One common element that they look at is molybdenum. Molybdenum is present at the Earth’s surface in association with sulfide minerals, such as iron pyrite. These minerals weather faster if there is more oxygen present. When these minerals break down, water carries dissolved molybdenum to the ocean. Molybdenum’s solubility, meaning how easily it can dissolve in water, also is increased when oxygen is present.

“So the end result is that the amount of molybdenum in the water is much different in a world that has oxygen versus a world that has none,” says Anbar.

Some of this molybdenum then precipitates out of the water into sediments on the ocean’s floor. Over time, these sediments compact and form sedimentary rock. Since rocks undergo weathering and metamorphic processes, outcroppings of these primordial rocks are very rare. Anbar and his team travel to places like Western Australia to search for samples. They then grind them up to analyze their chemical composition.

From this and other analysis, Anbar and other researchers find that the Earth’s atmosphere had a very low amount of oxygen until about 1 billion years ago when the amount of oxygen in the atmosphere began to rise.

Cyanobacteria was and is responsible for the buildup of oxygen in Earth’s atmosphere. Image Credit: Wikimedia Commons

The leading theory is that single-celled organisms called cyanobacteria produced most of this early oxygen. These bacteria were able to harness the power of the sun, creating energy using photosynthesis. As the cyanobacteria multiplied and spread, they pumped more and more oxygen into the atmosphere, which just as quickly oxidizes rocks. Eventually the rate of oxygen production outstripped the rate of oxidation, leading to oxygen quickly building up in the atmosphere.

This buildup resulted in one of the largest mass extinctions the Earth has known. A large number of species at the time were obligate anaerobes, single-celled organisms that find oxygen very toxic. The rapid increase in the amount of free oxygen killed off most of these organisms and forced the remainder to evolve defenses against an oxygen-rich environment.

The Great Oxygenation Event greatly changed the chemical makeup of Earth’s atmosphere and its oceans. This took course over about 1 billion years. But not all changes happen that slowly. The ocean is a dynamic system, and its properties can vary over the course of a few months.

Oceans Present

Anbar’s research into the chemistry of the ancient oceans reveals something of the distant past, but other researchers use the oceans to find clues about what the future holds. Using our knowledge of how the oceans change over small time scales we can make near-term predictions to aid people whose livelihoods come from the ocean.

Samantha Siedlecki, a research scientist at the University of Washington’s Joint Institute for the Study of the Atmosphere and Ocean (JISAO), helps develop software to help predict these variations in ocean chemistry on a local scale. The forecasts, called the JISAO Seasonal Coastal Ocean Prediction of the Ecosystem (J-SCOPE), are designed to be used by fisheries, aquaculturists, environmental scientists, and anyone else connected to the ocean.

“These kinds of tools are really trying to help navigate those extreme events which are looking like they’re going to affect us more often in the future,” Siedlecki said. “There’s this background upward march of increase in temperature, decrease in pH, decrease in oxygen. But there’s a lot of variability on top of that.”

The ocean is a large environment, with many interlocking systems. To generate a prediction, Siedlecki’s group first examines the large-scale conditions, like major ocean currents, and applies them to the boundaries of their small area, the coasts of Washington and Oregon. They then run simulations on computing clusters housed at the University of Washington to calculate what will happen in the near future. These predictions are used as the new initial conditions for another round of simulations.

Thus, J-SCOPE is able to predict important environmental conditions such as sea surface temperature, pH, and amounts of oxygen and chlorophyll. Researchers can use this data to determine where certain species of fish and other animals are most likely to be. Different species of ocean animals have different tolerances of oxygen and temperature, so they will migrate accordingly.

An example of J-SCOPE’s predictions, these images show oxygenation levels, chlorophyll, temperature, and acidity. Image Credit: J-SCOPE

Currently, J-SCOPE predicts the locations of sardines. Siedlecki explains, “There’s a northern extent to which the sardines are observed. Sometimes it’s in U.S. waters and sometimes it’s in Canadian waters. You can see how that might be a good thing to know in advance.”

Siedlecki adds that they plan to work with crab managers along the coasts of Washington in the future.

She hopes these tools will connect people with basic science studied by researchers like her in a way that will allow them to make informed decisions. “Decision-makers are interested in what’s happening right in the middle of their backyard,” she adds.

J-SCOPE publishes forecasts twice a year, in January and April.

Oceans Elsewhere

Still other researchers use ocean clues to investigate if there could be life elsewhere in our universe. Cynthia Phillips, a planetary geologist at NASA’s Jet Propulsion Laboratory (JPL), has been applying principles of Earth’s ocean science to researching the potential for Europa to house life. Europa, Jupiter’s third largest moon, has a salty ocean hiding beneath an icy crust.

“On Europa, it turns out that there’s enough energy through this tidal heating to keep Europa’s ocean liquid over the age of the solar system, as long as it starts (as) liquid,” Phillips said. “So if there’s an ocean there today, it has likely been there for 4.5 billion years, since Europa formed. That means there’s a stable, clement environment for the formation of life. That’s part of what makes it a good place to search for life.”

Europa orbits 414,000 miles from Jupiter. As it goes around, Jupiter’s gravitational pull stretches and squishes Europa’s interior. This energy is converted into heat by frictional forces. The heat keeps the ocean liquid, and could serve as a potential energy source for life, similar to hydrothermal vent communities on Earth.

Phillips explains, “Is there a sufficient food source to drive a biosphere? That’s the sticking point for a world like Europa. Even if there is life on Europa, it will be severely energy limited. You won’t have the huge biodiversity of organisms on Earth.”

To determine whether Europa could host extraterrestrial life, scientists must find out more about this icy moon.

An artist’s rendition of the proposed Europa Clipper mission. Image Credit: NASA JPL

Phillips and other researchers at JPL are planning a mission, dubbed the Europa Clipper, to examine Europa close up. This spacecraft will study Europa from orbit, analyzing its surface and interior to determine its habitability. It will also map the surface, allowing researchers to search for the optimal location to send a lander on a future mission. Such a lander would search for traces of life in Europa’s water.

“One of the things we talk about for the detection of life is looking for chemical systems that are in disequilibrium,” says Phillips. Life often leads to the increased and imbalanced presence of certain chemicals such as seen in the oxygenation event in Earth’s past. However, detecting life in such a distant ocean is no easy feat.

Europa is a large moon, and its ocean is likely well mixed. It’s unlikely that there is life throughout the ocean, like there is on Earth. But there might be small environments where life has produced enough changes that we can detect it. A lander mission could use strategies similar to ones scientists use to find life in subglacial lakes.

According to Phillips, “On Earth, life is so intimately bound up with our geologic sphere that it is very difficult to disentangle what the effects of life are versus what the effects of just geology are.  It’s changed the chemistry and the physical structure of the Earth.”

Discovering life on Europa and studying how it may have changed the moon’s environment will provide insight into how life has altered Earth’s biosphere. These alterations are happening on both a global and local scale, over vast amounts of time and on a day-to-day basis. Changes such as the ones discussed here have huge consequences, like what the chemical makeup of Earth’s atmosphere is, as well as small ones, like what influences where sardines go at different times of the year.

Peter Sinclair is an IEEE Earthzine writing fellow and a graduate student in astrophysics at University of New Mexico.