Wednesday, March 25, 2020

Musings from the R/V Ron Brown (March-April 2020)

The R/V Ron Brown left port in Cape Town on March 21, 2020 and is heading back to
the United States. Most recent posts will be at the top.


April 14, 2020

After a long transit home, we’re just a few days away from port in Virginia. In certain ways, being at sea has prepared me for going into quarantine. I’m certainly familiar with life confined to small spaces. The social isolation, however, will be very new. Since everyone on the ship is healthy, we don’t have to worry about social distancing. I will miss the many conversations about science and life with all on board, which have been the highlight of the cruise.

A few weeks ago, we had a barbecue outside on the ship’s fantail to celebrate crossing the equator. The air was thick with moisture, but everyone was in high spirits. Some people were playing cornhole. Others were splashing around in an inflatable kiddie pool. I remember observing the whole scene from a picnic table and realizing: once I’m back on land, it could be weeks or even months before I’m at a gathering with this many people. It was a sobering thought.
The view from the fantail of the R/V Ron Brown
I’m sitting at that same picnic table now, as I write this, enjoying the late afternoon sun. The ocean extends to the edges of the horizon in every direction. I have (somewhat surprisingly) not gotten tired of this view. The seascapes have been varied, ranging from the heaving washed out gray waves of a storm, to gentle blue ripples that scatter morning light. On breezy days like today, I often find myself watching the wind blow across the sea surface and imagining the waves that are being generated beneath it, which mix the upper ocean. Or trying to visualize the complex topography on the seafloor, and how the deep currents are navigating it.

Sometimes I think about the SOCCOM floats, and picture what interesting processes they are observing, wherever they might be. Out here, the immensity of the ocean is perceptible, which makes it even more amazing to me that these tiny floats are changing our understanding of the climate system.
The blog author just before the final SOCCOM float deployment several weeks ago.
Thank you all for listening to my thoughts and reflections over the past few weeks. Being at sea during a global pandemic has been a strange and memorable experience. I hope that amidst all the uncertainty in the world, you’ve learned a bit about the SOCCOM floats and why the data they are collecting is so important. As I said in my first post, we need science now more than ever. Stay safe everyone!

- Channing


April 11, 2020

The ocean regulates the climate system by absorbing nearly one-third of global carbon dioxide (CO2) emissions. Once dissolved, CO2 causes chemical reactions that lower the pH of seawater, a measure of its acidity. This means that the ocean is becoming more acidic as it takes up the excess carbon emitted by burning fossil fuels, shifting the carbonate equation and causing a phenomenon known as ocean acidification. Ocean acidification is bad news for millions of tiny organisms, like the pteropod below, whose shells’ get corroded in these conditions. Therefore, monitoring pH levels in the ocean, for example by using the pH sensors on the SOCCOM floats, is necessary to determine how climate change will impact marine ecosystems.
Unhealthy pteropod with dissolving shell ridges showing the
effects of ocean acidification (NOAA Fisheries Collection)

In addition to telling us about ocean acidification, the pH of seawater can also reveal important information about the carbon cycle. Since pH levels are directly related to ocean carbon uptake through known chemical reactions, they can be used to calculate the amount of dissolved CO2 in seawater. This, in turn, allows scientists to estimate air-sea carbon fluxes, as described in a recent paper led by University of Washington professor Alison Gray. The Southern Ocean is typically thought to play an outsized role in the global ocean carbon uptake, but Alison’s study showed that it may not be absorbing as much CO2 as we thought. In fact, certain regions even released carbon into the atmosphere, acting as a source rather than sink for atmospheric CO2.

Much of this previously undetected ocean carbon release occurred in winter in the icy regions close to Antarctica, highlighting (as we’ve seen before) the importance of year-round sampling and expanded data coverage. Accurately quantifying air-sea carbon exchange, through studies like Alison’s, is essential to improve global climate models. Furthermore, combining information from all the different float sensors can help us untangle the complex set of physical, chemical, and biological processes that control the fluxes of carbon between the ocean and atmosphere.

While I’ve focused each post in this series on a particular sensor, the temperature, salinity, bio-optical properties, nitrate, oxygen, and pH of the ocean are all connected. In fact, we’ve seen that some of the most groundbreaking science happens when we consider how these properties interact and influence each other. Another key theme has been the unprecedented spatial and temporal resolution provided by the float array. The Southern Ocean is inaccessible, and numerous scientific discoveries resulted simply from having measurements during winter and in ice-covered regions. Only by continuing to observe these remote places can we hope to understand and predict how the climate will change in the future.
Map of the SOCCOM float array as of March 29, 2020,
including the 6 floats we deployed on this cruise! (SOCCOM)

And that is the power of SOCCOM! The new insights gained from this novel dataset are changing our understanding of the Southern Ocean and its impact on global biogeochemical cycles. Furthermore, the SOCCOM project has a team of world-renowned climate modelers using those findings to inform models and improve future climate projections. The breadth of work being done is truly remarkable, and the studies I’ve featured in this series are just the tip of the iceberg (more than 100 publications have already resulted from this program!). And as the size of the float array increases, so too will the number of questions that we’re able to answer about the ocean and its role in the climate system.

- Channing


April 8, 2020

In the last two posts, we talked about phytoplankton, microscopic algae that play a key role in marine ecosystems and the global climate. You’ve heard about how phytoplankton absorb carbon dioxide (CO2) through photosynthesis, but they also produce oxygen (O2) through this process. In fact, phytoplankton photosynthesis is responsible for roughly half of the oxygen in our atmosphere, which makes earth habitable. So be sure to thank these tiny organisms the next time you take a breath!

Oxygen is also central to the carbon cycle and can be used by scientists to partition ocean and land carbon sinks from atmospheric data. This is because terrestrial carbon uptake, by trees and other land plants, leaves an imprint on atmospheric oxygen levels in a known ratio based on the chemical reaction that takes place during photosynthesis. Ocean carbon uptake, on the other hand, occurs independently from air-sea oxygen exchange and thus does not affect atmospheric O2. The different influences of these processes can be used to separate the total global carbon uptake into land and ocean components from measurements of atmospheric O2 and CO2. The largest source of uncertainty in this calculation, however, is air-sea oxygen fluxes, which are poorly constrained due to lack of observations. Therefore, oxygen concentrations in seawater, which can be measured using the oxygen sensor on the SOCCOM floats, contain essential information about the climate system.
Schematic of the global carbon cycle showing both land
and ocean sinks (NASA Earth Observatory)
To illustrate how the SOCCOM float array can lead to new insights about the oxygen cycle, I’ll summarize some results from a recent paper led by University of Hawaii professor Seth Bushinsky. Seth’s study used float measurements to calculate air-sea oxygen fluxes, which revealed that the Southern Ocean is a larger oxygen sink than was thought based on sparse ship data. Most of this previously undetected ocean oxygen uptake occurred in winter in the regions closer to the pole (south of the about 60°S), where sea ice cover is common. It is extremely difficult to access these icy regions, particularly in winter, so this discovery relied on the SOCCOM floats’ ability to sample year-round and in hard-to-reach areas.

Using the float temperature and salinity data, Seth determined that this wintertime oxygen uptake was driven by ventilation, the process by which surface waters are transported into the ocean interior and away from their source region. These results highlight the value of the SOCCOM dataset, both by increasing the number of ocean oxygen measurements and by allowing us to relate that information to specific physical drivers. The improved estimates of ocean oxygen uptake, stemming from the float data, can reduce the uncertainty in the quantification of ocean and land carbon sinks from atmospheric O2 and CO2 measurements. This, in turn, will help reconcile differences between observations and models of the global climate.
Map showing location of SOCCOM float profiles (left) compared to all previously
available data collected by ships (right) (Bushinsky et al., 2017)
And that is the power of oxygen! This key element supporting life on our planet also provides important constraints on the carbon cycle due to the coupling of CO2 and O2 via photosynthesis (as well as respiration and combustion). SOCCOM floats equipped with oxygen sensors can help us refine estimates of air-sea oxygen fluxes, and in doing so, better understand the relative importance of ocean and land carbon sinks. Since ocean uptake of carbon and oxygen are independent, however, other methods must be used to calculate air-sea CO2 fluxes directly. But I’ll talk more about that in the next post.

- Channing


April 5, 2020

Last time, we talked about phytoplankton, microscopic algae that form the base of marine food webs and absorb carbon dioxide (CO2) from the atmosphere through photosynthesis. I previously referred to “forests” of phytoplankton in the ocean, but there are also “deserts”, expansive regions of sea with very little life. Why do phytoplankton thrive in certain places but not in others? Like plants, they need sunlight to grow. But they also require certain nutrients, including nitrate, phosphate, and iron, which they convert into proteins, fats, and carbohydrates. These nutrients are just like the ones contained in plant fertilizer that you might use in your garden, and without them, phytoplankton cannot survive. Therefore, measuring nutrient concentrations in seawater, for example by using the nitrate sensor on the SOCCOM floats, can help determine what controls patterns of biological productivity in the ocean.

Microscope image of diatoms, a major phytoplankton group
in the Southern Ocean (Wikimedia Commons

I mentioned before that phytoplankton sequester carbon in sediments when they die and sink to the seafloor. But that is not the fate of all phytoplankton, many get eaten by krill, copepods, and other organisms higher up on the food chain. In fact, only a fraction of the carbon produced by phytoplankton during photosynthesis gets stored in the ocean abyss where it is effectively removed from the atmosphere. To diagnose the impact of biological productivity on atmospheric CO2 levels, we need to know the amount of carbon that actually gets exported to the deep ocean. Since this is difficult to measure, scientists can estimate it using a number of different techniques, one of which relies on changes in nitrate. In a recent paper led by Monterey Bay Aquarium Research Institute scientist Ken Johnson, this method was applied to the nitrate measurements from the SOCCOM floats in order to quantify the biological contribution to Southern Ocean carbon uptake.

How do you get from nitrate to carbon storage? This method relies on the strong seasonality in phytoplankton growth, which peaks in spring and summer (just like flowers and other land plants across much of the US). For each float, Ken calculated the decrease in nitrate in the sunlit upper ocean over the course of a growing season, and then assumed that those changes were due to consumption by phytoplankton. The amount of nitrate utilized can then be converted to the amount of carbon produced by phytoplankton using the known ratio between nitrogen and carbon in their cells. In other words, based on the elemental composition of phytoplankton, we can infer the total annual carbon export to the deep ocean at a given location just by knowing the change in the near-surface nutrient inventory.

This innovative method requires year-round sampling of nitrate, which was scarce in the Southern Ocean before SOCCOM floats existed. The results from the full float dataset show that carbon sequestration by phytoplankton varies spatially, and is highest between 40° and 50°S. This is consistent with past studies, which required decades of data due to limited wintertime measurements. The SOCCOM floats now enable us, for the first time, to resolve this key process annually in locations around the Southern Ocean.
Floats used to estimate biologically-driven carbon export in Ken’s paper.
There are many more floats now than when the paper was published! (Johnson et al., 2017).

And that is the power of nitrate! This essential nutrient supporting all marine life can also help quantify carbon export to the deep sea associated with biological productivity. Taking advantage of the unprecedented spatial and temporal coverage provided by the SOCCOM float array, through studies like Ken’s, can lead to new insights about the impact of phytoplankton photosynthesis on atmospheric CO2 concentrations. This, in turn, improves models of the global climate system. Furthermore, this information can be combined with other parameters measured by the floats, such as oxygen, to provide even further constraints on the carbon cycle. But I’ll talk more about that in the next post.

- Channing


April 2, 2020

It is well known that expansive rainforests like the Amazon absorb carbon dioxide (CO2) from the atmosphere through photosynthesis. Similarly, the ocean has “forests” of microscopic algae called phytoplankton that take up atmospheric CO2 just like trees and other land plants. When these organisms die and sink to the seafloor, the carbon in their cells gets stored in deep-sea sediments. Therefore, determining the distribution of phytoplankton in the ocean can provide important constraints on the global carbon cycle.

Like flowers and other land plants, phytoplankton go through periods of rapid growth in spring called blooms. Blooms occur as a result of higher light levels and enhanced stratification, which increases the available nutrient concentrations near the surface where phytoplankton grow. Large green patches of ocean, marking regions with abundant phytoplankton, are even visible from space, and change the way the surrounding seawater reflects and absorbs sunlight. Scientists can exploit this fact to estimate phytoplankton biomass from optical measurements taken by satellites or by the bio-optical sensors on the SOCCOM floats.

To illustrate the importance of bio-optical data, as well as the science made possible by the SOCCOM float array, I’ll summarize some results from a recent paper that I led as part of my PhD thesis at Scripps Institution of Oceanography. In this study, we examined the drivers of the Scotia Sea phytoplankton bloom, which is the earliest and largest spring bloom in the open Southern Ocean. This can be seen from maps of satellite chlorophyll, a proxy for phytoplankton biomass calculated from optical measurements, which show high values in the Scotia Sea (outlined in red) while the rest of the Southern Ocean remains low.
October (spring in the southern hemisphere) satellite chlorophyll in the Southern Ocean showing
the signature of the Scotia Sea phytoplankton bloom. Image created by Channing Prend.

Although long-term satellite data clearly indicate that the bloom occurs in the same location around the same time each year, it’s difficult to know why since the satellites only measure surface properties and do not provide information about the physical processes that regulate phytoplankton growth. Therefore, SOCCOM floats, which simultaneously record physical and biological data throughout the upper 2000 meters of the water column, were essential in discovering what initiates and sustains the Scotia Sea bloom. Two floats, which captured the 2016 and 2017 bloom cycles in this region, revealed a close link between biological productivity and seafloor topography. The highest chlorophyll values were measured when the floats were trapped in a recirculating eddy that formed over an undersea mountain called Pine Bank. This is due to enhanced mixing when the current flows over the seamount, which supplies essential nutrients (in this case iron) from great depths to the sunlit upper ocean where phytoplankton can grow.
Schematic showing how vertical mixing at topography can deliver nutrients (in this case iron or Fe)
to the upper ocean and support phytoplankton growth. Image created by Channing Prend.

Although this result is specific to the local topography in the Scotia Sea, several other phytoplankton blooms in the Southern Ocean are located close to topographic features, suggesting that this process may be important in other regions as well. Since phytoplankton abundance varies considerably in space and time, understanding what controls bloom location, timing, and magnitude, through studies like this, is necessary to model Southern Ocean food webs and biological effects on atmospheric CO2 levels.

And that is the power of bio-optical data! The information about phytoplankton biomass inferred from these measurements, combined with the physical parameters recorded by the floats, helped introduce a new conceptual framework for a bloom system that scientists had known about for decades. This highlights one of the unique aspects of the SOCCOM data: its' ability to relate changes in biogeochemical properties directly to their physical drivers. These results also demonstrate, as we saw in the previous post, how just a few floats in the right place at the right time can lead to new dynamical understanding of phenomena observed by satellites. In the remaining posts, we’ll see how the entire float dataset taken together can be leveraged to uncover new insights about global biogeochemical cycles.

- Channing


March 30, 2020

This is the first post in a series about the different sensors on the SOCCOM floats and some of the recent scientific advancements that have been made using these data. We’re starting off with Conductivity Temperature Depth (CTD) sensors, which measure the temperature, salinity, and pressure (approximately equivalent to depth) of the water. These are the fundamental physical parameters in the ocean because they determine the density of seawater. Colder water is denser since the water molecules contract together at lower temperatures. Higher salinity water is denser since there is more stuff (salt) packed into it. Under high pressure (i.e. deeper) water gets compressed and thus denser. Changes in the temperature or salinity of seawater lead to gradients in density, which drive the currents in the deep ocean.

To illustrate the importance of temperature and salinity, as well as the science made possible by the SOCCOM float array, I’ll summarize some results from a recent paper led by University of Washington graduate student Ethan Campbell. Ethan’s study looked at the formation of polynyas, large holes in the winter sea ice, which formed over an undersea mountain called Maud Rise in 2016 and 2017. These were the largest such events to occur in the region since the 1970s (the holes in the ice were nearly the size of the state of South Carolina!), and scientists were stumped as to what caused the polynyas’ reappearance.
Satellite image of the 2017 polynya at Maud Rise (NASA Earth Observatory)

Collecting data within a polynya is extremely hard due to the remoteness of the formation regions and harsh weather. Therefore, SOCCOM floats provide an important source of information in ice-covered areas that are difficult to access by ship. In these icy locations, fresh water overlies warm, salty water. The cold surface water creates a barrier that prevents the ice from melting. During the polynya years, observations collected by SOCCOM floats show that the surface waters over Maud Rise were saltier and thus denser than usual. As a result, the density difference between the surface and deep ocean was small, allowing the water column to mix more readily. This led to an anomalously large heat transfer to the surface that melted the ice. Particularly strong storms during the polynya years also helped upwell warm water, establishing a feedback loop that prevented ice from re-forming and sustained the hole in the ice.
Schematic of the global overturning circulation, which is driven, in part,
by gradients in density due to temperature and salinity changes
(Robert Simmon via Wikimedia Commons)

Although the polynya formation is a local process, it could have large impacts on the climate system. For example, deep waters over Maud Rise are enriched in carbon (because they haven’t been in contact with the atmosphere for hundreds of years or more). This sequestered carbon can be released back into the atmosphere when the water gets drawn up by the polynya. By acting as a direct conduit between the surface and deep ocean, these holes in the ice can alter the exchange of properties at the air-sea interface. Therefore, accurately describing the dynamics of these systems, through studies like Ethan’s, is critical to improving future climate predictions.

And that is the power of temperature and salinity! These key ocean properties, measured by a few strategically placed SOCCOM floats, helped solve a decades-long puzzle about the drivers of open-ocean polynya events. Furthermore, the physical processes recorded by the floats can be linked to changes in ocean biogeochemistry using the properties measured by the other sensors. But I’ll talk more about that in future posts.

- Channing


March 27, 2020 

Our final floats have been deployed! These were Floaty McFloatface, named by the Monterey Bay Aquarium in homage to the infamous Boaty McBoatface, and Sylvia Whirl, named by the Scripps Polar Center in honor of the legendary oceanographer Sylvia Earle and in reference to the Southern Ocean’s active eddy field.
Floaty McFloatface ready to join the fleet of SOCCOM floats collecting data
in the Southern Ocean. - photo by Channing Prend

Shortly after being deployed, the floats will sink down to 1000 meters depth (that’s more than half a mile below the surface!) where they’ll live for the next few years, drifting with the ocean currents. Every 10 days they will go down to 2000 meters and then rise up to the surface, collecting data as they go (which they will send back to us via satellite). Floaty McFloatface and Sylvia Whirl are the newest additions to a fleet of more than 150 SOCCOM floats measuring the physical, chemical, and biological properties of the Southern Ocean.
The blog author enjoying sunrise over the Atlantic before deploying Sylvia Whirl
(photo by Susan Becker)

In a series of posts, I’ll talk about the different sensors on the floats, what they measure, and some of the scientific breakthroughs that have already been made using this data. So stay tuned! In the meantime, I hope everyone back on land is staying safe and adjusting to the changes to daily life. I’m thinking of you all from the middle of the Atlantic Ocean!

- Channing


March 26, 2020

It’s been a busy few days. Because of the ship’s orders to return to the US, all of our float deployments are occurring within the span of 3 days. Knight Drifter from Buckingham Brown and Nichols School and See Turtles from Winston Campus Elementary have both taken the plunge into the cold Southern Ocean waters.

The floats were deployed into calm seas. We have left behind the sea birds and dolphins near the coast. Here it seems desolate, empty. But we are only seeing the surface. Thousands of meters below us is an even more foreign world, where strong currents traverse undersea mountain ranges, and internal waves the size of skyscrapers break and mix the waters close to the seafloor. These are the forces that Knight Drifter and See Turtles will reckon with as they drift around collecting data.

The blog author standing proudly with Knight Drifter just before deployment. photo by Molly Martin.

See Turtles ready to begin its journey around the Southern Ocean. Photo by Channing Prend>

Now that the floats have been deployed, where will they go? This turns out to be a difficult question to answer since the ocean is immense, chaotic, and constantly in motion. Honestly, we don’t know exactly where the floats will travel; they are at the mercy of the waves and currents now. But we can predict where they are likely to go based on our knowledge of ocean circulation and statistics from floats that moved through this region in the past. Ultimately, only time will tell where the currents carry Knight Drifter and See Turtles. I’m excited to see where they end up though, because this information will help us determine the pathways by which the ocean transports heat, nutrients, and carbon around the world.

- Channing 


March 24, 2020

When I left home several weeks ago, I couldn’t have imagined the magnitude of the global pandemic that I would soon watch unfold from port in Cape Town and from the deck of the R/V Ron Brown. With all the uncertainty in the world right now, our ship has understandably been recalled to the US. Before heading home, we’ll be taking a slight detour to deploy six SOCCOM floats, the only portion of our initial science plan that will see completion.

There have been numerous times when I’ve wondered whether it’s appropriate to be conducting fieldwork under these circumstances. It seems frivolous to worry about float deployments given the challenges facing those back on land, and I feel guilty for being somewhat sheltered from the barrage of news (by the ship’s limited bandwidth). But I have come to think that now, more than ever, our society needs science and science-based policy. The data collected by these floats will provide new insights about the ocean and climate, and in doing so, ultimately contribute to a better world. When viewed through this lens, our science mission takes on a whole new urgency. And on a personal level, I am grateful for the sense of purpose that this has afforded me amidst all the turmoil.
Bobcat's Be-bopping Bobber on the deck of the R/V Ron Brown before deployment. Photo by Channing Prend
Gloria's Gulper takes the plunge into the cold Southern Ocean waters. Photo by Channing Prend

So today, a team of scientists worked together to deploy two SOCCOM floats: the Bobcat’s Be-bopping Bobber from Louisa County Middle School and Gloria’s Gulper from the Monterey Bay Aquarium Research Institute. It was not easy. We were battling large waves and high winds. Our boat was bobbing up and down in the swell like a toy sailboat, completely dwarfed by the immensity and sheer power of the ocean. Once the floats were in the water, we quickly lost sight of them. The storm seemed to have blurred the boundary between sea and sky. But as we sailed on to our next station, I had the distinct feeling that we’d done something important, not in spite of everything going on in the world right now, but because of it.

- Channing Prend


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