When we think of the ocean, the first thing that comes to our mind is probably just water – billions of litres of water. But if we filter seawater through a filter disc with a mesh of smaller than 0.45 µm (or 0.00045 mm), we find some amount of particles. In some areas in the ocean we find lots of particles, such as near river mouths where several grams of sediment can be found in 1 litre of seawater Other areas however have almost no particles in seawater, for example in the deep Southern Ocean around Antarctica. Here the waters are so clear that visibilities of 50 m are very common.

There are many different types of particles in the ocean in all sorts of shapes and sizes. We can group these particle types into three broad categories. The first group are particles from sediments introduced by rivers and estuaries or by aerosols, such as dust or volcanic ash. These rock-derived particles are inorganic compounds (even though rivers and estuaries carry a significant load of organic material too). Although they are very important for the ocean chemistry, we haven’t quite understood yet how particles and seawater interact. For example much of the iron, which is an important nutrient, is vastly introduced to seawater by dust. However, these dust particles do not only introduce elements into the ocean, they also remove them by scavenging. The balance between elemental input and output by particles is one of the big challenges in modern chemical oceanography.

Saharan dust plume over the East Atlantic. Image: www.nasa.gov

Another type of particles are living and dead organisms – or particulate organic matter (POM). This type of particles can range in size between a couple of nanometres to up to tens of metres, — if we count the large animals as particles too! The deep ocean typically has lower POM than in the upper few hundred metres of the surface ocean (on average 19.2 and 192 micrograms per litre, respectively (1)). Organic particles are a very important component of the carbon cycle in the surface ocean, because POM consists of roughly 50% organic carbon. Many of the tiniest organisms, such as algae, take up atmospheric inorganic carbon, i.e. CO2, and reduce it to organic carbon by photosynthesis. These microorganisms can cover huge areas in the ocean during their bloom in spring or autumn.

Satellite image of algae activity in the Southern Ocean. Image from NASA’s Earth Observatory (www.nasa.gov).

The third type of particles are the ones that humans have introduced into the ocean, such as plastic or chemicals forming solid compounds. A recent estimate on how much plastic currently floats in the large ocean gyres of the global ocean accounts to about 5.25 trillion pieces of plastic weighing almost 270,000 tons (2). With 4.8 to 12.7 million tons estimates for 2010 (3) the mass of plastic waste we dump into the ocean is even greater. Plastic and microplastic in the ocean is a very problematic type of particles for marine life – and for us. Larger plastic pieces, such as shopping bags, are easily confused with jelly fish. Jelly fish are the main food source for sea turtles, but also sharks and other large fish. In 2013 a sperm whale shored in Spain – cause of death: 17kg of swallowed plastic.

The Great Pacific Garbage Patch. Image: Courtesy of the NOAA Marine Debris Program.

References:

Millero, F.J., 2005. Chemical Oceanography, 3rd edition. 520 pages, CRC Press.

Eriksen, M., Lebreton, L.C.M., Carson, H.S., Thiel, M., Moore, C.J., Borerro, J.C., Galgani, F., Ryan, P.G., Reisser, J., 2014. Plastic Pollution in the World’s Oceans: More than 5 Trillion Plastic Pieces Weighing over 250,000 Tons Afloat at Sea. PLoS One 9, e111913. doi:10.1371/journal.pone.0111913

Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R., Law, K.L., 2015. Plastic waste inputs from land into the ocean. Science (80-. ). 347, 768–771. doi:10.1126/science.1260352

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I love the ocean, studying it, and before joining the University of Southampton as a Research fellow, I had put much thought into the particular role of shelf seas in the global marine system.

In previous years I have put my focus on the deep ocean. I have been analysing trace metals in seawater to look at the big picture – how water masses with billions of liters per second are distributed along the ocean conveyor belt. I have looked at different tracers to understand where water masses come from and how they mix with each other. One particular tracer, neodymium, has been my focus for more than six years now – a study that involves collecting and processing thousands of liters of seawater from several expeditions.

Bad weather during a cruise to Antarctica.

Neodymium is a lithogenic element, which means it comes from land into the ocean via various weathering sources and it is only present in a few billionth of a gram per liter seawater. The cool thing about neodymium is that its composition in water masses gives direct information about their formation regions. For example, North Atlantic Deep Water has a distinct isotopic composition because its surrounding landmasses mix their isotope signal into the source region where this water mass forms. We can also reconstruct past ocean circulation to a certain degree with neodymium isotopes archived in marine sediments. The problem with this isotope system is that the observed values not always meet the expected ones. In other words: water mass mixing is not the only process that governs trace metal isotopic composition of seawater. Even though we have quite a good understanding on how water masses move and how they mix thanks to the help of reliable proxies, such as salinity, temperature and nutrients, there are processes involved, which we haven’t quite understood about neodymium, particular when it comes to sources and sinks of this element.

How do we analyse trace elements?

Their name already indicates the problem we have – they’re only present in traces in seawater. Most of our sample composition in seawater is dominated by major elements, like sodium, potassium or calcium. When analysing trace elements these vastly present elements need to be separated from the trace elements, we are interested in or they will disturb the analysis. This is particularly important when we look at the isotopic composition, which is an analysis sensitive to atomic mass differences of one particular element. Some elements share a very similar atomic masses, which the mass spectrometer (the instrument that measures these isotopic differences, e.g. a MC-ICPMS) can’t discriminate. So the presence of these interfering elements would cause an erroneous measurement and this is why we need to isolate the trace elements as good as possible.

Plasma torch of a Multi-Collector Inductively Mass Spectrometer (MC-ICPMS). Image: Torben Stichel

Why is that important for us?

I’m looking at ocean boundaries to better understand source and sink mechanisms that imprint the neodymium isotope signal on the water masses we are tracing. The shelf seas are potentially significant sources of neodymium and many other trace elements into the ocean. So connecting shelf seas’ processes with the global ocean conveyor belt will help us to better understand the cycle of trace metals in general in the ocean.

The climate of our planet has been changing on large (glacial to inter-glacial) and smaller scales (modern climate change). Much of these changes are closely linked with ocean circulation. Understanding proxies that trace water masses are therefore vital to reconstruct past, assess present, and predict future ocean conditions.

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I graduated as a geologist at Christian-Albrechts-University in Kiel, Germany, in 2010. Afterwards I carried out research for my PhD in the same city but a different institution: The Helmholtz Centre for Marine Research (GEOMAR). I have enjoyed this time very much and learned to manage a PhD project with all its caveats (sample collection, method development, analysis, publishing etc). Right after my PhD defence (just five days later!) I flew to the other side of the world to start my first Postdoc at the University of Hawaii at Manoa, USA. I spent two and half years in Hawaii working on trace metals collected in the North Atlantic Ocean —even though I was in the middle of the Pacific— and then returned to Europe to start a new post at the Imperial College London, where I focused on marine particles. Finally I joined the University of Southampton in 2014 as Research Fellow, where I work on the interplay between particulates and dissolved trace metals in seawater.

Since I was an undergrad, I have been interested in geochemistry – particularly in isotope geochemistry. I found fascinating how scientists are able to use geochemistry as a tool to explain so many parts of the Earth system from the processes that brought to its formation and the early Universe to how planktonic algae utilise nutrients and contribute to regulate climate.

Throughout my early career my research has focused on the marine cycle of trace metals including their isotope compositions and elemental concentrations. This has involved collecting and analysing thousands of litres of seawater!  Radiogenic isotopes have been widely used to reconstruct past ocean circulation and inputs from weathering. The specific input mechanisms and internal cycling, however, are not yet well understood mostly due to the current lack of data for the modern oceans, which is vital to understand paleo datasets. Geochemical processes in the ocean are of great importance for modern, paleo- and future applications. For instance tracing the modern ocean circulation and identifying the sources and sinks of trace metals allows for a better understanding of the connection between the global ocean and Earth’s crust as components of the climate system. Studying the modern ocean helps reconstruct the past ocean circulation and weathering inputs, because it enables detect the course of water masses and nutrient distribution throughout the different basins. Because the mean is the engine of Earth’s climate this kind of reconstruction provide insights into how the climate system has changed in the past and could change in the future. Marine geochemistry contributes to compose the big picture of how the ocean interacts with the global climate.

The post Torben Stichel: My research appeared first on Exploring our Oceans .

I did my undergrad and graduate studies in Kiel- just 100 km away from Flensburg and also at the sea, and during that time I participated in two oceanographic cruises, one of which for ten weeks in Antarctic waters. In that cruise especially I had several occasions to feel the glory of the ocean, its force and beauty. My first experience as a postdoc was in the University of Hawaii, where I moved right after my PhD defense. I lived in the middle of the Pacific Ocean for two and a half years and beside working on marine geochemistry, I also enjoyed the ocean as playground both from above the surface sailing with friends as well as below the surface scuba diving in the clearest waters surrounding the volcanic islands. My family and I now live in Southampton and I work at the National Oceanography Centre carrying out research in chemical oceanography. Living by the sea is what I keep doing, intentionally or by chance. The sea has always been a part of my life.

However, the ocean is still a big unknown for me. I find its power and secrets are overwhelming and it’s amazing how even its smallest components can tell us big stories. I study the ocean because there will always be something new to discover in all disciplines from biology and chemistry to physics. As a marine geochemist my field of research focuses on tiny amounts of trace metals that can tell us the origin and fate of trillions of litres of seawater moving through the ocean basins like on an enormous conveyor belt. I want to understand my “home” – I guess that’s why I do what I do as a marine scientist.

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