Behind the Science: Dissolved Oxygen

Author: Leah Chomiak

Hello from the middle of the Indian Ocean! The skies are blue, the seas are calm, the air is fresh and full of oxygen (O2)… and so is the water column! My name is Leah, one of the scientists onboard I07N running dissolved oxygen analyses along with my counterpart, Sam. Together, we are in charge of sampling, assessing, and quantifying the amount of gaseous oxygen dissolved in the seawater from every bottle of every station… coming to a grand total of 3,432 samples once we close out the transect in India. Wowza!

Dissolved oxygen, the same stuff you and I breathe each second of the day, is crucial to the marine environment; whether that be oceans, lakes, rivers, or ponds, for obvious reasons. Oxygen is essential for life and is taken up by organisms through a process termed cellular respiration where oxygen is taken in and carbon dioxide is spit out in return. The reverse process, photosynthesis, results in the generation of oxygen through the uptake of carbon dioxide. Just like on land, plant cells fill the oceanic environment in the form of plankton, where oxygen is produced. All remaining organisms of the sea, from fish and sharks to corals and giant squids, rely on the presence of dissolved oxygen in the water column just as we rely on oxygen in our atmosphere – to survive! We measure the dissolved oxygen to understand not only the biological and chemical aspects of seawater, but also for understanding the physical movement of currents and water masses too.

We start by first collecting a sample of seawater out of a Niskin bottle that is closed at a certain depth in the water column from a CTD rosette, yielding samples from 24 different depths spanning the surface to the ocean floor. Immediately after collecting the seawater sample, the sample is ‘fixed’ with two reagents that react to form a precipitate – in simplest terms, all of the dissolved oxygen reacts to form a solid and is suspended in solution. By performing this reaction, we essentially have the power to “freeze” all oxygen in the sample at that given moment, meaning photosynthesis and respiration are halted and no further O2 is produced or consumed. Once the entire CTD has been sampled, the O2 samples are brought back to our lab inside the ship, where we then give them a shot of strong acid to dissolve the precipitate (turning the sample a cool yellow color) and at last perform a titration to determine the amount of oxygen present in the sample.

Since the surface ocean and the atmosphere are in constant exchange with each other, oxygen from the atmosphere is assimilated into the surface waters, and we tend to see high oxygen near the surface for this reason. Due to the availability of sunlight near the surface, plankton aggregate and help to contribute to the surface oxygen levels through photosynthesis. Samples taken near the bottom of the ocean also tend to have high oxygen levels, one being because the extreme pressure and cold temperatures facilitate the dissolution of gas in seawater, and two, certain water masses, such as the cold and dense Antarctic Bottom Water are known to have high O2 content due to recent exposure with the atmosphere. Central in the water column is where the oldest water lies, and oxygen tends to be lacking due to uptake by biological organisms. Not only is oxygen useful in assessing biological productivity, its also a great parameter to use when observing global ocean circulation!

The outflow of the Arabian Sea/northern Indian Ocean is observed as one of the largest and most distinctive oxygen minimum zones (OMZ) in the world, where oxygen concentrations are extremely low due to high biologic consumption and other chemical factors. As we continue to head north along the I07N transit line, it will be very interesting to see how our oxygen profiles change as we enter the OMZ. It has been 23 years since this hydrographic line was last sampled; I wonder what we will observe this time around!

 

IO7N to provide novel dataset of DO14C in the Indian Ocean

Author: Christian Lewis

DO¹⁴C – What is it?

DO¹⁴C is an acronym for “Dissolved Organic Radiocarbon (¹⁴C).” The DO in DO¹⁴C refers to a complex cycling of dissolved organic matter throughout the oceans, which includes proteins, amino acids, and many other compounds that are primarily produced through plankton photosynthesis in the surface ocean. The ¹⁴C refers to radiocarbon. Radiocarbon is a radioactive isotope of carbon that decays through time with a known half-life. This known half-life (~6000 years) allows scientists to very accurately determine the age of something by measuring how much radiocarbon is left. Oceanographers and geochemists can use radiocarbon dating of seawater to understand the cycling of organic matter in the ocean.

What does it tell us?

DO¹⁴C measurements in the Atlantic and Pacific Oceans have shown that organic carbon in the deep ocean is 4000-6000 ¹⁴C years old. This is much older than the ~1000 years it takes the ocean to overturn. Therefore, DO¹⁴C measurements have shown us that the ocean can store organic carbon for much longer than inorganic carbon. This information is crucial for our understanding of the ocean’s carbon cycle and how it influences our climate on Earth.

DO¹⁴C has been measured more frequently in the Pacific and Atlantic Oceans, but this cruise will provide one of the first datasets of DO¹⁴C in the Indian Ocean, which is a unique location oceanographically.

How do scientists measure it?

Seawater for DO¹⁴C measurements are collected from the Niskin bottles on the CTD rosette into 1L glass bottles that have been acidified and baked prior to the cruise. Acidifying and baking out glassware removes extraneous carbon that could influence our measurements. All of our samples are frozen upon collection and stowed in a chest freezer on board where they await further processing on land at the University of California, Irvine.

Back in the lab, the seawater is oxidized to CO_2(g) using ultra-violet light, isolated, and reduced to graphite. The graphite is then measured for radiocarbon content in an accelerator mass spectrometer (AMS), which measures the ratio of ¹⁴C to ¹²C (the more common flavor of carbon). From there, a radiocarbon age can be calculated, and new discoveries can be made!

How do oceanographers measure and sample from the surface to the bottom?

Author: Denis Volkov

Figure 1. A rosette onboard the Research Vessel “Roger Revelle” during a research cruise in the Pacific Ocean in September 2016 (photo by D. Volkov). Note that the gray sampling bottles are open prior to the deployment.

The primary “workhorse” of sea-going oceanographers is a so called rosette – a framework with 12 to 36 sampling bottles (in our project we have 24 bottles with a volume of 12 liters each) clustered around a central cylinder, where a CTD and/or other sensor package can be attached. A CTD is an instrument that measures the conductivity, temperature, and pressure of seawater (the D stands for “depth,” which is closely related to pressure). The conductivity measurements are used to determine salinity. These are essential physical properties of seawater that determine its density and to a large extent ocean circulation. Usually, a rosette also houses Acoustic Doppler Current Profilers (ADCP) that measure the horizontal velocity, and oxygen sensors that measure the dissolved oxygen content of the water.

In order to take measurements, the ship stops and a CTD cast is carried out. The location where measurements are taken is called an oceanographic station. The rosette is lowered on a cable down to just above the seafloor with the sampling bottles opened at both ends, so that water can freely circulate through them. The CTD is connected to a computer onboard the ship, and scientists can monitor changing water properties in real time. When the instrument ascends, the sampling bottles are closed selectively at predefined depths by a remotely operated device.

During the I07N project, we are planning to complete 132 stations and collect water samples that will be analyzed for oxygen, nutrients, salinity, dissolved inorganic carbon, alkalinity, pH, chlorofluorocarbon, dissolved organic matter, dissolved organic radiocarbon, particulate organic matter, and some other parameters.

Figure 2. A rosette being lowered into the water by science technicians on board the Research Vessel “Ronald H. Brown” during the first I07N test station in the Indian Ocean on Apr. 25, 2018 (photo by D. Volkov).

What goes into turning the Ron Brown into a floating laboratory?

Author: Viviane Menezes

We are departing from Durban, South Africa to our amazing journey in the Indian Ocean. The crew and the scientific party have worked hard in the last couple of days to make the NOAA R/V Ron Brown into a floating laboratory.

Yes, we’re going on a cruise, but not the kind of cruise that most people imagine, including our family and friends. We don’t have fancy swimming pools, big restaurants, playground for kids or thousands of people. But, we have labs, many labs!

I would say that we’re a small group of explorers with a couple of labs that allow us to measure a lot of ocean properties. When you go to a doctor, he/she asks you a bunch of different medical exams to known about your health. We do the same to understand the ocean. But, we’re a kind delivery-doctor; we go to the sea to make the exams!

In this research expedition, we have labs to measure salinity, alkalinity, acidity, nutrients and many other chemical properties. We also have labs dedicated to biology. For example, we are going to get information about the phytoplankton, the tiny plants that drift at the sea surface and fabricate most of the air we breathe.  We have gadgets to measure physical properties such as ocean currents and temperature from the warmer sea surface to the colder and darker deep ocean.  As you may have realized we have a lot of stuff to make ahead and R/V Ron Brown will drive us on our journey.

Different from labs that you have in schools and universities, our labs are on the moving ship, so when we’re putting them together, we have to tie and strap every piece of equipment. We get so used to tie and strap that when we arrive at home, we want to do the same.

Of course putting together several labs in the ship far away from home can be challenging. In fact, “home” here is relative; we are from everywhere, a truly international community of explorers. Drama always happens, something essential is missing, some baggage has been lost, on and on. But, thanks to very kind people that we have met along the way, we have everything we need.

After working hard in the last couple of days, stripping and tying everything, our labs are secure and we are ready to go. We are just beginning our journey.

Why are we doing a research cruise in the Indian Ocean?

Author: Denis Volkov

Our technological ability to monitor the state of the ocean has greatly advanced over the last few decades. If in earlier days taking measurements from ships was the only available option, nowadays oceanographers are well equipped with Earth-orbiting satellites and autonomous devices. While ship-based measurements are very limited in space and time, satellites can see the entire Earth surface in several days, and today numerous profiling floats are sampling most parts of the ocean. So one may reasonably ask why oceanographers still go at sea if they can get plenty of information about the ocean remotely, literally sitting in front of a computer?

Indeed, there is an enormous amount of data gathered by satellites and autonomous devices, but these modern observing systems also have limitations. For example, satellites can see only the surface of the ocean. Floats are usually separated by distances of hundreds of kilometers. Autonomous devices can dive, but most of them, like Argo floats, sample only the upper 2000 m water column. Although deep Argo floats capable of reaching 6000 m depth already exist, their quantity is still very limited to provide global and sustained observations of the deep ocean. In addition, not all ocean variables can be measured by satellites and autonomous devices, and small sensors on autonomous instruments are usually less accurate. Therefore, ship-based hydrography still remains the only method for obtaining high-quality, high spatial and vertical resolution measurements of a full suite of essential physical, chemical, and biological variables over the full-depth water column. And in the end, speaking about autonomous devices, somebody still has to go at sea and deploy them in a preplanned location. This is something most research cruises do as a supplemental duty (“piggyback” projects), and during our cruise we will also deploy Argo floats and drifters.

This will be the first scientific occupation of line IO7N section since 1995. The scientists are eager to learn how the state of the Western Indian Ocean has changed over the last 23 years. Has the deep ocean warmed? Have the regional concentrations of dissolved oxygen, carbon dioxide, nutrients changed? Has the Western Indian Ocean become more acidic? These and many more questions will be addressed by scientists after the completion of the cruise.

One of the most climatically significant variables is the amount of heat that is stored in the ocean. Because the heat capacity and density of seawater are much larger than those of air (water can absorb more than 4000 times as much heat as air per unit volume), changes in oceanic heat content have profound and long-lasting effects on global and regional climate. Existing observations from different platforms show that the upper-ocean heat content for the World Ocean has been steadily increasing since 1970s. The Indian Ocean is the warmest ocean on our planet and its upper 2000 m heat content has also been increasing. But has the excess heat penetrated deeper than 2000 meters in the Western Indian Ocean? We do not know, but we are going to find out during the cruise.

Figure 1. The rate of sea level rise from 1995 to 2015 in mm per year calculated from satellite altimetry measurements.

As seawater warms, it expands, and the sea level rises, which is among the most challenging consequences of ocean warming. Since the early 1990s, the global mean sea level has been steadily rising at a mean rate of 3.3±0.4 mm per year. About one third of the present-day sea level rise is due to the thermal expansion of seawater and the remaining two thirds are due to melting ice sheets and glaciers. The latter contribution is an indirect effect of ocean warming on the sea level rise, because the warmer ocean may enhance basal melting and thinning of ice shelves and marine-terminating glaciers. As evidenced by altimetry satellites, the sea level rise is above the global average almost in all parts of the Indian Ocean (Figure 1). River deltas and small island states in the Indian Ocean are particularly vulnerable to sea-level rise. For example, Bangladesh, located in the Ganges-Brahmaputra delta with its low elevation and severe tropical storms, is among the most affected countries. The Maldives, which is the lowest country in the world, may in fact become uninhabitable by 2100 if the current rates of sea level rise remain the same.

Figure 2. Distribution of oxygen at 26.9 kg/m3 neutral density surface. From the WOCE Indian Ocean Atlas (http://whp-atlas.ucsd.edu/indian_index.html).

It is worth noting that our cruise is heading towards an oxygen minimum zone (OMZ) in the Arabian Sea (Figure 2), which is the thickest of the three oceanic OMZ, and it is of global biogeochemical significance. The oxygen deficient waters of the OMZ are important because in extremely low oxygen environments, denitrification is a prominent respiratory process that converts nitrate (NO3–), which is a form of nitrogen readily available to most plants, into free nitrogen gas (N2), which most plants cannot use. The OMZ appear to be increasing substantially and posing a threat to the marine ecosystems and fisheries. In addition, carbon-dioxide, phosphate, and nitrate all increase substantially to the north throughout much of the water column in the Arabian Sea. These variables include contributors to ocean acidification and important nutrients for phytoplankton growth in the ocean, which are important in this distinct biogeochemical province.