POM: The living and the dead

Author: Catherine Garcia

POM – What is it? – POM stands for “particulate organic matter”. Sea water particles could be living plankton, dead material, or even plastic bits that have made it out to sea. The dead material sometimes includes old plankton cells, plant matter, fecal pellets, airborne dust particles, river-borne soil particles, etc. The further the Ronald Brown moves away from the coast and into open water, a higher portion of the particles we capture is living phytoplankton, bacteria, and zooplankton.  Jenna and I are looking at the organic portion of the particulate matter, or the living/dead material. When this POM gets dense enough, it sinks out of the sunlit ocean layer and might make it to the ocean bottom if not consumed by bacteria on the way down. We call this the “Biological Carbon Pump”.

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Jenna and Catherine filtering POM samples.

POM – Why and how do scientists measure it? – Scientists care about POM because phytoplankton play a VERY large role in removing carbon dioxide (CO2) from the air in the upper ocean. To understand just how much carbon is being removed, scientists need to measure how much carbon is captured in particles and what percentage sinks into the deep ocean by the Biological Carbon Pump. To know how much is carbon, we measure the elemental composition of particulate organic matter to obtain its concentration. The most abundant elements in organic matter are carbon, hydrogen, nitrogen, phosphorus, oxygen, and sulfur.

In the Western Indian Ocean, we are collecting samples for particulate organic carbon, nitrogen, phosphorus, and oxygen. The NOAA/V Ronald Brown has a sea water pump constantly bringing sea water conveniently to our lab bench.  Once we collect ~6L of sea water, all of it is filtered onto glass fiber filters that let any particles smaller than 0.7um flow though.

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Glass fiber filter (on left) used to collect POM samples.

It really isn’t any different from than filtering the coffee grinds out of your coffee, except we keep what’s on the filter! Because larger plankton are rarer and a small fraction of the sea water in this area, we filter out any particles larger than 30um to avoid the occasional zooplankton. The filters are frozen after collection and stored frozen until analysis at Dr. Adam Martiny’s lab at the University of California-Irvine.  Once in the lab, the filters for carbon and hydrogen are eventually baked at more than 900oC in an Elemental Analyzer to turn all the carbon and hydrogen into a gas that can be measured. The phosphorus and oxygen are measured using chemical assays against a standard concentration curve.

POM – What does it tell us? – So, I mentioned why we cared about carbon, but why capture the other elements? Until recently the science community accepted a phytoplankton recipe of sorts: 106 parts carbon: 16 parts nitrogen: 1 parts phosphorus and so on. Mix together some carbon from photosynthesis and nutrients from the deep ocean to get your typical model phytoplankton. This ratio gives scientists a good estimate of how much POM is composed of carbon. Globally, an average phytoplankton does almost converge on this ratio of elements first described by Alfred Redfield in 1934. But not always.

Like us, plankton are what they eat. Phytoplankton in particular have extremely flexible elemental ratios. Just how flexible, and what causes them to change their ratios remains an open question. There are several theories that link elemental composition to environmental conditions, which group is present, or even how fast they grow. Jenna and I will try to track down this mystery on the IO7 cruise track.

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 CO2(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

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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.

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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.

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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!

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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.

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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.

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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.

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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.